32nd  Annual  International  Conference  of  the  IEEE  EMBS Buenos  Aires,  Argentina,  August  31  -­  September  4,  2010

Identification of intracellular calcium dynamics in stimulated cardiomyocytes A. Vallmitjana, M. Barriga, Z. Nenadic, A. Llach, E. Alvarez-Lacalle, L. Hove-Madsen and R. Benitez calcium may cause anomalies in the heart function such as T-wave alternans, ventricular fibrillation or conduction problems [6]. In particular, previous studies have established an interrelation between ventricular fibrillation and an overload in the intracellular calcium [7]. Similarly, the presence of spatially discordant alternans, characterized by an out-of-phase activity in different regions of the cell, is known to be related to the apparition of lethal arrhythmias [8]–[10].

Abstract— We have developed an automatic method for the analysis and identification of dynamical regimes in intracellular calcium patterns from confocal calcium images. The method allows the identification of different dynamical patterns such as spatially concordant and discordant alternans, irregular behavior or phase-locking regimes such as period doubling or halving. The method can be applied to the analysis of different cardiac pathologies related to anomalies at the cellular level such as ventricular reentrant arrhythmias.

I. INTRODUCTION There is an increasing number of studies that aim to establish relations between clinical conditions and physiological activity at the cellular level. This kind of research requires an interdisciplinary approach that combines knowledge and methods from different fields. Since most of the information at the cellular level is obtained by means of cell imaging techniques, novel image processing methods are needed in order to analyze, quantify and classify spatial and temporal patterns observed in life science areas such as neuroscience or cardiology [1]. In this context, calcium imaging is particularly relevant because calcium dynamics is a cell regulatory mechanism that plays an important role in many cellular processes such as muscle activation, gene expression or fertilization [2], [3]. In this work we present an automatic image processing method to analyze confocal calcium images of isolated cardiac myocytes. Cardiac myocytes are heart muscle cells that exhibit a variety of dynamical patterns due to the intracellular calcium dynamics [3]. The spatial and temporal distribution of intracellular calcium in cardiac myocytes determines the excitation-contraction coupling of the myocardium and is therefore a basic mechanism underlying heart function [4]. In particular, it is well known that high-frequency pacing of ventricular myocytes leads to the emergence of complex spatiotemporal patterns in the distribution of the intracellular calcium. The apparition of these complex dynamical regimes is a consequence of the nonlinear interplay between different cellular Ca2+ control mechanisms [3], [5]. Irregular distribution of intracellular

The purpose of this work is to present an analysis method that processes a sequence of fluorescence images of stimulated isolated myocytes and automatically identifies the spatiotemporal dynamics exhibited by the cell. The objective is to distinguish physiologically relevant regimes such as spatially concordant and discordant alternans, phase-locking oscillations or irregular patterns. The method uses a feature extraction technique that permits an effective characterization of the experimental sequence allowing for a robust identification of each regime. More specifically, an approach based in the Principal Component Analysis (PCA) is presented to detect the presence of spatial alternans in the experiment. Similar study that addresses this problem in the context of cardiac tissue patterns can be found in the recent literature [11]. The paper is organized as follows: In Section II we introduce the experimental data and provide a detailed description of the processing method. The main capabilities of the technique are described in Section III, where we evaluate its performance and report on several examples of the correct identification of different regimes. Finally, the potentialities of the method and an exposition of further improvements are discussed in Section IV. II. MATERIALS AND METHODS A. Data acquisition A total of 22 atrial myocytes were loaded with 2.5 µM fluo-4 for 15 minutes followed by wash and de-esterification for 30 minutes. The myocytes were stimulated intracellularly with an EPC-10 patch-clamp system (HEKA, Germany) as described in [12]. The sequences of confocal images were acquired at a frame rate of 100 Hz with a resonance scanning Leica SP5 AOBS confocal microscope. Ionic currents were recorded simultaneously with a HEKA EPC-10 amplifier. Synchronization of confocal images and current recordings was achieved using a Leica DAQ box and HEKA patchmaster software. Patch-master was used to design electro-

A. Vallmitjana and R. Benitez are with the Automatic Control Department, Universitat Politecnica de Catalunya (UPC), Barcelona, Spain. [email protected],

[email protected]

Z. Nenadic is with the Department of Biomedical Engineering, University of California, Irvine (USA). [email protected] E. Alvarez-Lacalle is with the Applied Physics Department (UPC).

[email protected]

L. Hove-Madsen, A. Llach and M. Barriga are with the Cardiovascular Research Center CSIC-ICCC and Cardiology Department, Hospital de Sant Pau (Barcelona, Spain). [email protected]

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physiological protocols and to generate triggers for confocal image acquisition and event marking in the stimulation protocols. Local and global changes in cytosolic Ca2+ levels were detected by quantifying fluo-4 fluorescence in selected regions of interest. The cardiomyocytes were analyzed at different stimulation rates with frequencies ranging from 0.25 to 2 Hz. This resulted in a set of 101 experimental sequences, each consisting in a sequence of N images of 512 × 140 pixels with a physical pixel size of 0.28µm. All the processing and analysis steps have been implemented in MATLABTM (The Mathworks, Natick MA). The original fluorescence images (24-bit truecolor) are converted to grayscale intensity images by using a weighted sum of the R, G, and B components with weights [0.2989, 0.5870, 0.1140]. We refer to an experimental sequence of grayscale images k as {Xij }, where k = 1 . . . N indexes the frame in the sequence and i = 1 . . . Nx , j = 1 . . . Ny specify a particular pixel in the image. In order to avoid the presence of static heterogeneities in the spatial distribution of the fluorescence, each pixel is normalized by subtracting its time average activity in the experiment.

The distribution of amplitudes is considered homogeneous if the variability of the peaks σa is four times smaller than the noise in the signal σ 1 . When the distribution of amplitudes is not homogeneous, alternating and irregular regimes are distinguished by testing for the presence of sustained oscillations in the peak amplitude. Similarly, an irregular behavior is identified when the variability in the inter-peak intervals exceeds a certain heuristic threshold σi /mi > 0.6. Finally, the auto-correlation function of Fk is used to determine the n:m correspondence between the calcium peaks and stimulation pulses. The previous procedure results in a set of four features, namely amplitude homogeneity, presence of alternance, irregularity of inter-peaks intervals and the n:m stimulation response. 2) Identification of spatial alternans: When the peak detection procedure detects the presence of an alternance in amplitude, an additional method is used in order to distinguish between spatially concordant or discordant alternans. To this extent, Principal Components Analysis (PCA) was used to identify the basic spatial modes in the experiment and to identify the existence of regions with an out-of-phase activity [13], [14]. In order to process the data, each image in the sequence k Xij , i = 1 . . . Nx , j = 1 . . . Ny was subtracted from its temporal mean and arranged as a d-dimensional column vector zk = [zk1 , zk2 , . . . , zkd ]T where d = Nx Ny . The whole experimental sequence was then represented by the d × N matrix A = [z1 , z2 , · · · , zN ]. The principal components are obtained by diagonalizing the d × d covariance matrix AAT . In our case, since the dimension of the data d is much larger than the number of observations N (typical values are d ∼ 7 × 104 , while N ∼ 2 × 103 ), we reduce the computational cost by using the fact that the largest N eigenvalues of AAT are the eigenvalues {λ1 , λ2 , . . . , λN } of the N × N matrix AT A [15]. The eigenvectors of AAT representing the spatial modes w can be then obtained from w = Av, where v are the eigenvectors of AT A. The main spatial mode in the experiment is found by reconstructing from the eigenvector w1 associated with the largest eigenvalue λ1 . PCA reconstruction is achieved by projecting w1 to the data matrix A, which results in an image representing the main spatial variability of the experimental sequence. The histogram of the reconstructed image is then divided in two regions A and B defined by the pixels above and below the average pixel intensity outside the cell (i.e. without calcium activity). The ratio between the pixel count in each region ρ = nB /nA defines a quantity that allows to identify the existence of regions presenting an out-ofphase activity in the sequence. Indeed, in the absence of spatial alternance the first order PCA reconstruction is homogeneous and the number of pixels in region B is low due

B. Feature extraction Fig. 1 describes the basic steps of the method, which includes feature extraction and classification. Feature extraction consists of two parts: On the one hand, we determine the temporal properties of the oscillations in the average fluorescence and its correspondence to the stimulation times. On the other hand, we analyze the experimental sequence in order to determine if the images present out-of-phase spatial heterogeneities. These two steps constitute a basis for peak detection and spatial analysis methods detailed below.

Fig. 1.

Schematic description of the method.

1) Peak detection: We first compute the average fluorescence cell activity in each frame Fk = � k X /(N N ), k = 1 . . . N , and we identify sequential x y ij i,j pairs of local extrema corresponding to the peaks and valleys of Fk . We then compute the mean and standard deviation of the peaks amplitude ma , σa and of the intervals between consecutive peaks mi , σi (inter-peak intervals).

1 Noise is robustly estimated by the median absolute deviation of s σ k ˆ = 1.4826 · median(|sk − median(sk )|), where sk = Fk − Fkd is a residual constructed from a denoised version Fkd obtained by applying a wavelet schrinkage method to the signal Fk (Symmlet order 8, soft heuristic SURE threshold).

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to background fluctuations in fluorescence (nB � nA , i.e. ρ ∼ 0). When the sequence includes a spatially discordant alternant, the PCA projection captures the spatial variability by setting the pixels of the discordant region to negative values, therefore increasing the relative size of region B and consequently the value of ρ. A heuristic threshold as low as ρ = 0.1 is proven to be sufficient to detect small spatial discordances.

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The information obtained from the peak detection and PCA analysis provide a set of features that allow us to classify the experimental sequences into one of the following cases: 1) Normal dynamics: Normal cell response is characterized by a 1:1 stimulation response showing homogeneity in the peak amplitude and a spatial distribution of calcium activity. An example of this behavior is represented in Fig. 4a. As it can be seen, the cell responds to a train of stimulation pulses applied every 4 seconds by generating a calcium transient. This regime is the typical response of a healthy cell and is normally observed at low pacing frequencies. 2) Spatially concordant alternans: An example of spatially concordant alternans is depicted in Fig. 2, which shows a 1:1 stimulation response presenting an alternance in peak amplitudes. This temporal alternance appears in the whole cell without spatial inhomogeneities.

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Fig. 3. Analysis of spatial alternance with PCA. A reconstruction from the most relevant eigenvector allows to identify two different regions A and B with alternating activities.

of a calcium signal with a frequency different from the frequency imposed by external pacing. Fig. 4b shows an example of period-halving of the calcium signal with respect to the stimulation pulses, whereas Fig. 4c depicts a case in which every other stimulation pulse is blocked and evokes no calcium transient. a) 15 10 5 0 0

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Fig. 4. Normal cell response and phase-locking at pacing frequency 0.25 Hz. a) Normal dynamics b) Example of phase-locking 2:1 (period halving): The cell responds with two Ca2+ transients every stimulation pulse. Note the correspondence between stimulation marks and signal peaks. c) Example of phase-locking 1:2 (period doubling): The cell responds with one Ca2+ transient every two stimulation pulses (blocking).

3) Spatially discordant alternans: Spatially discordant alternans present different regions with out-of-phase activity in response to different stimulation pulses. In Fig. 3a out-ofphase regions A and B are presented. The corresponding average calcium signal of each region is shown in Fig. 3c, exhibiting an alternating behavior in the activity of each zone. The use of the PCA method becomes necessary since this regime cannot be distinguished from a spatially concordant alternant from the average cell activity (see Fig. 3b). 4) Phase-locking regimes: Phase-locking is a dynamical regime in which there is a n:m phase synchronization between stimulation pulses and peaks in the signal. In such cases, a nonlinear interaction between stimulation and calcium regulation mechanisms results in the appearance

5) Irregular dynamics: Irregular dynamics occur when either inter-peak intervals present significant variability (i.e., non-periodic behavior) or when peak amplitudes are highly heterogeneous presenting no alternance. In such cases, we observe dynamical regimes as the ones shown in Fig. 5. B. Performance evaluation To quantify the performance of the method, we analyzed the 101 experimental sequences and compared the classification results to those obtained by an expert. True and false positive rates (TPR, FPR) were computed for each of the four classification groups (Normal, phase-locking, Alternans -both concordant and discordant- and Irregular) as FPR =

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with the alternating spatial modes. This might improve the overall method since PCA only identifies uncorrelated modes which are not necessarily statistically independent. Moreover, further dynamical information about the sequence may be obtained by using temporal and spatial phase synchronization techniques [5], [17], [18].

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Fig. 5. Examples of irregular Ca2+ transients at stimulation frequency of 1.33 Hz.

[1] J. Rittscher, R. Machiraju, and S. Wong, Eds., Microscopic Image analysis for life science applications, ser. Bioinformatics and Biomedical imaging. Artech House, 2008. [2] M. J. Berridge, M. D. Bootman, and H. L. Roderick, “Calcium signalling: dynamics, homeostasis and remodelling,” Nat Rev Mol Cell Biol, vol. 4, no. 7, pp. 517–29, Jul 2003. [3] J. P. Keener and J. Sneyd, Mathematical physiology, 2nd ed., ser. Interdisciplinary applied mathematics. New York, NY: Springer, 2009, vol. 8. [4] D. Bers, “Cardiac excitation-contraction coupling,” Nature, vol. 415, no. 6868, pp. 198–205, 2002. [5] S. H. Strogatz, Nonlinear Dynamics And Chaos: With Applications To Physics, Biology, Chemistry, And Engineering, 1st ed. Westview Press, 2001. [6] A. Karma and F. G. Jr, “Nonlinear dynamics of heart rhythm disorders,” Physics Today, vol. 60, no. 3, pp. 51–57, 2007. [7] E. Chudin, J. Goldhaber, A. Garfinkel, J. Weiss, and B. Kogan, “Intracellular ca(2+) dynamics and the stability of ventricular tachycardia,” Biophys J, vol. 77, no. 6, pp. 2930–41, Dec 1999. [8] M. A. Watanabe, F. H. Fenton, S. J. Evans, H. M. Hastings, and A. Karma, “Mechanisms for discordant alternans,” J Cardiovasc Electrophysiol, vol. 12, no. 2, pp. 196–206, Feb 2001. [9] D. Sato, Y. Shiferaw, A. Garfinkel, J. N. Weiss, Z. Qu, and A. Karma, “Spatially discordant alternans in cardiac tissue: role of calcium cycling,” Circ Res, vol. 99, no. 5, pp. 520–7, Sep 2006. [10] J. G. Restrepo and A. Karma, “Spatiotemporal intracellular calcium dynamics during cardiac alternans,” Chaos, vol. 19, no. 3, p. 037115, Sep 2009. [11] Z. Jia, H. Bien, and E. Entcheva, “Detecting space-time alternating biological signals close to the bifurcation point,” IEEE Trans Biomed Eng, vol. 57, no. 2, pp. 316–24, Feb 2010. [12] L. Hove-Madsen, C. Prat-Vidal, A. Llach, F. Ciruela, V. Casad´o, C. Lluis, A. Bayes-Genis, J. Cinca, and R. Franco, “Adenosine a2a receptors are expressed in human atrial myocytes and modulate spontaneous sarcoplasmic reticulum calcium release,” Cardiovasc Res, vol. 72, no. 2, pp. 292–302, Nov 2006. [13] R. O. Duda, P. E. Hart, and D. G. Stork, Pattern Classification, 2nd ed. John Wiley and Sons, Inc., 2001. [14] B. Ghanem and N. Ahuja, “Phase PCA for dynamic texture video compression,” in IEEE International Conference on Image Processing, 2007. [15] G. Blanchet and M. Charbit, Digital signal and image processing using MATLAB. ISTE-Wiley, 2006. [16] J. V. Stone, Independent Component Analysis: A tutorial introduction. The MIT Press, 2004. [17] M. Palus, “Detecting phase synchronization in noisy systems,” Physics Letters A, vol. 235, no. 4, 1997. [18] M. G. Rosenblum, A. S. Pikovsky, and J. Kurths, “Phase synchronization of chaotic oscillators,” Phys. Rev. Lett., vol. 76, no. 11, pp. 1804–1807, 1996.

TABLE I P ERFORMANCE OF THE IDENTIFICATION METHOD Index TPR FPR Sample size

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ratio of false positives over number of negatives and TPR = ratio of true positives over number of positives. Within the alternans group, the technique correctly distinguished all the cases presenting spatially discordant activity. IV. CONCLUSIONS AND FUTURE WORK A. Conclusions We have developed an automatic method for the identification of spatiotemporal regimes in a sequence of calcium fluorescence images in stimulated cardiomyocytes. The method distinguishes between spatially concordant and discordant alternating patterns and is able to identify phase-locking dynamics such as period doubling or halving as well as the presence of an irregular behavior. The technique can be used to obtain quantitative information about the dynamical response of the stimulated myocyte. In particular, it might be useful to characterize the sequence of bifurcations that the system undergoes as the pacing frequency is increased. Although the proposed method has been successfully applied to real experimental sequences, it would be necessary to quantify its performance and robustness under different signal-to-noise conditions. One of the straightforward improvements of the method is to substitute the PCA technique used for the identification of spatial alternans by an approach based on the use of Independent Components Analysis (ICA) [16]. This method would allow the decomposition of an experimental sequence in a set of statistically independent source signals associated

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Conditioning Data for Condition Assessment of a Power Transformer Roberto Villafáfila-Robles1, Marta Rodríguez 1, Pau Lloret1, Andreas Sumper1,2, Samuel Galceran-Arellano1 1

Centre of Technological Innovation in Static Converters and Drives (CITCEA) Universitat Politècnica de Catalunya (UPC) E.U. d’Enginyeria Tècnica Industrial de Barcelona, Electrical Engineering Department Comte d’Urgell, 189. 08036 Barcelona (Spain) e-mail: [email protected] 2

Catalonia Institute for Energy Research (IREC) Barcelona (Spain)

Abstract — Utilities have to guarantee a proper condition of network components in order to meet with regulatory and society demands regarding reliability and quality of power supply while optimizing costs. Maintenance strategies have evolved to cope with this issue. Condition Based Maintenance (CBM) strategy permits to adapt the maintenance actions to condition of the equipment. It is mainly used for critical equipment like power transformers. If an on-line monitoring system is used, the actual condition of the assets can be estimated. Such system consists of a set of sensors for acquiring condition related parameters and techniques/tools that process and analyze the data in order to assess its condition. However, anomalous data may appear due to a malfunction of monitoring system and may lead to errors when in data interpretation. Then, in order to overcome this issue, conditioning of such data is needed previously to analyse them. When the monitored data is refined, the condition can be estimated through models. A conditioning data process is presented for a case study of a power transformer in service. Furthermore, data mining process for obtaining behaviour patterns is also introduced. Keywords: Condition monitoring system, Condition assessment, Conditioning Data, Condition Based Maintenance

I.

INTRODUCTION

Asset management has become one of the main activities for utilities due to liberalization of electric sector. This environment needs for new strategies in operation and maintenance activities in order to reduce their costs while improving reliability and quality of power supply in order to meet with regulatory frames and society demands. Furthermore, the risk is likely to increase when optimizing technical and economical resources if financial interests are above the actual condition of the assets and not at the same level. The condition of assets is guaranteed through maintenance actions. Such actions can be grouped in different strategies depending of the criticality of the asset, its cost and available spare parts. There are four main maintenance strategies with the following characteristics [1]:  Corrective maintenance (CM): there is no inspection or maintenance until breakdown.  Time Based maintenance (TBM): there is a fixed time intervals for inspections and maintenance.  Condition Based maintenance (CBM): there is continuous or occasional monitoring and the maintenance is performed when required.  Reliability Centred maintenance (RCM): there is a priority list obtained from a connection of condition and failure effects that permits risk management. Utilities have been mainly performing maintenance of their assets as a combination of CM and TBM strategies, depending on the network component. However, a CM plan will have a significant impact on power system operation if critical component failures. On the other hand, TBM plans might over-maintain young equipment whereas infra-maintain ones close to their end-of-life. Thus, there is a shift towards a CBM approach for critical equipment, like power transformers, in order to avoid damages of network components by means of detecting faults at incipient stage. As it is no possible to measure directly the time to failure of any network component,

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such time is predicted by means of monitoring parameters that can provide an approximation of actual condition and ageing process after the corresponding analysis. A step forward in maintenance strategies after CBM is RCM. This last plan considers, apart from actual condition, other facts like resource constraints and power quality indices to prioritize the maintenance orders. However, in order to set up last two maintenance strategies, utilities require a high financial effort for deploying the related systems, and qualified and experienced staff able to manage and take advantage of such systems. The monitoring of condition related parameters of equipment can be done through both on-line and off-line methods. In order to carry out an on-line monitoring system, it is necessary to install sensors that continuously acquire the data from the monitored network component and information and communication system that transmits and storages such data. Then, these data are accessible for a later analysis. However, the installation of sensors represents an important drawback for equipment in service. Off-line monitoring methods can overcome this problem by checking equipment that should be out of service. However, such measurements might be done too late for preventing damages or might not provide useful results. Any action within a CBM strategy, like alarms, maintenance or replace orders, depends on assess of the condition of the equipment and later diagnosis. On one hand, some monitoring techniques use monitored data in standard models, like thermal models defined at IEEE Std. C.57.91 and IEC-354. However, such models consider parameters that have to be calculated for each one. On the other hand, other monitoring methods require power transformer’s fingerprint that is used as reference in later analysis to determine the evolution, like Frequency Response Analysis (FRA). However, these techniques need qualified staff to perform the test and assess the results. Thus, determining the condition of a power transformer and the limits to raise the alarm is a cumbersome task. Power transformers are a key component in power systems and utilities are doing huge efforts for avoiding damages in such equipment by deploying CBM plans for them. The techniques used for condition monitoring and condition assessment for power transformers can be found in [2]. As it has been already mentioned, on-line monitoring for a CBM implies two steps: a data acquisition system that gets the value of condition related parameters of the equipment and techniques/tools that process and analyze such data in order to assess its condition. The main outcome is to detect incipient faults and perform proper actions to reduce the damage and recover a good state-of-health of the equipment However, conditioning the data from monitoring system is needed in order to get useful information and remove erroneous data. If this process is not performed properly, it could conduct to wrong results. A mistake in the assessment of condition can lead to loss of both the equipment and significant amount of money. This paper deals with conditioning monitored data for estimating the condition of a transformer based on a case study.

II.

PILOT PLANT

A condition monitoring pilot plant has been deployed according to methodology described in [3]. The pilot plant is shown in Figure 1. It consists in 66/25 kV 30 MVA power transformer and substation circuit-breakers. The description of the condition monitoring system: sensors, data acquisition and warehouse systems, and communication systems are described deeper in [4] and [5]. The parameters of the active part of the power transformer that are monitored and the sensors are listed in Table I. The values of such parameters are acquired continuously and a pre-process is done before they are stored in the data base. The storage of these parameters is synchronous: instant values are aggregated in the average every 15 minutes and such average and the maximum and the minimum for each quarter of an hour are recorded. Date and time are recorded with each measure. Table I. Power transformer monitored variable Monitoring parameter Upper oil temperature Gases dissolved in oil Oil humidity Lower oil temperature High-voltage 3-phase currents High-voltage 3-phase voltages

Sensor Pt100 Hydran M2 Vaisala MMT318 Pt100 Current transformer Voltage transformer

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The data base stores data that can be used for assessing the condition of power transformer. A first step is to plot such data. Figure 2 shows monitored data of transformer temperature from the monitoring system for the same month in two different years. It can be seen that some data is missed or present a value equal to zero. Therefore, a conditioning procedure is needed to identify the cause of this situation and extract accurate information that permits estimate the condition of the power transformer in order to specify the proper maintenance actions if needed. The proposed methodology is shown in Figure 3 and is described next. It has two parts: finding wrong data and generate patter of behaviour. The objective is to obtain the set of data free of anomalous values and create for each monitored parameter a behaviour pattern to identify changes or trends that conduct to an unwanted situation Evolution of mean temperature values

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a. IDENTIFYING WRONG DATA On-line monitoring systems might have some malfunction that cause that anomalous data is inserted in the data base. Such abnormal data have not to be taken into consideration for assessing the condition. The origin of these inaccuracies is misoperation of some of its components like sensors and disfunction of communication and software systems, as for example a damaged sensor, loss of communication between sensors and data base due to the cable is broken, and writing failure when inserting in data base. In order to cope with these sources of errors, stored values of monitored parameters are asked next questions:

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Is the number of data expected? This doubt discovers missing intervals as the storage of monitored data is carried out in constant time intervals. o Is there a date without measure? This enquiry notices that a measurement is not recorded in the data base. o Are there data with zero value? This issue detects an error in stored data, although a null value in current and voltage could mean that the transformer is out of service. As a result of each question, a list with the detected wrong data is created and stored. The exact cause of misoperation of the on-line monitoring system can be determined by analysing the data lists generated after the question. After the whole set of data goes through the questions, the appropriate data is available for obtaining behaviour pattern of each parameter. o

Figure 3. Algorithm for conditioning monitored data

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b. GENERATION OF BEHAVIOUR PATTERNS The behaviour patterns are found through data mining process applied to free error set of monitored data. Data mining has been selected due to it is an efficient technique to obtain useful information from cleaned large amount of data. There are different techniques for performing data mining: neuronal networks, decision trees, genetic algorithms, clustering, linear regression, statistics, etc. Statistical analysis has been selected to derive the behaviour patterns for watching the evolution of the condition of the power transformer. This technique consists of adjusting the data to a statistical distribution model. A data distribution fit-test determines the suitable model. Before performing such tests, the influence of the season and time of day have to be considered. Then, refined data are separated in winter (from December to February), spring (from March to May), summer (from June to August) and autumn (from September to November); and for each season, the data is considered hourly. The fit-tests have been carried out according to previous conditions and the normal distribution fits with the refined data, as Figure 4 shows for top-layer temperature.

Figure 4. Distribution fit-tests (with Minitab®) for top-layer temperature: exponential (upper left), Weibull (lower left), normal (upper right) and log-normal (lower right)

Therefore, each parameter has four behaviour patterns that each one consists of a daily model made of 24 normal distributions, one for each hour of the day. Figure 5 depict the behaviour pattern of top-layer temperature for spring, where hourly means are connected by a continuous line, and the upper and lower lines limit the confidence interval of 95.44% (±2). For deriving this pattern, the wrong data that Figure 2 shows in April 2009 has not been taking into account and do not affect it. The models are stored in the data base and when the monitoring system acquires new raw data, such data is firstly refined and later is used for updating the corresponding pattern. The behaviour patterns permit to assess the evolution of monitored parameters and evaluate the condition of the power transformer by means of comparison and correlation between the parameters.

Figure 5. Spring top-temperature behaviour pattern. Continuous line: mean. Dot-point line: upper limit. Dot line: lower limit

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CONCLUSIONS

Power transformers are an important asset in power systems. Monitoring of power transformers permits to estimate their condition and life expectancy. Although degradation process of insulation materials and failure modes are known, the assessment of their ageing and time to failure is hard difficult. On-line monitoring systems help to estimate current condition of power transformers. However, raw data might present anomalous values due to malfunction of the monitoring system and if these are not identified, incorrect conclusions could appear in later analysis. In order to overcome such situation, a refining stage previous to condition analysis is needed. A conditioning monitored data process for an on-line monitoring pilot plant system has been described. This process permits to derive behaviour patterns to identify changes or trends that might conduct to an unwanted condition of power transformer. The patterns have been derived from cleaned monitored data using statistics data mining techniques, namely normal distribution considering season and each hour of the day.

REFERENCES [1]

[2] [3] [4] [5]

Joachim Schneider, Armin J. Gaul, Claus Neumann, Jurgen Hografer, Wolfram Wellow, Michael Schwan, Armin Schnettler, Asset management techniques, International Journal of Electrical Power & Energy Systems, Volume 28, Issue 9, Selection of Papers from 15th Power Systems Computation Conference, 2005 - PSCC'05, November 2006, Pages 643-654, ISSN 0142-0615, DOI: 10.1016/j.ijepes.2006.03.007. Ahmed E.B. Abu-Elanien, M.M.A. Salama, “Asset management techniques for transformers”, Electric Power Systems Research, Volume 80, Issue 4, April 2010, Pages 456-464, ISSN 0378-7796, DOI: 10.1016/j.epsr.2009.10.008. Velasquez, J.L.; Villafafila, R.; Lloret, P.; Molas, L.; Galceran, S.; "Guidelines for the implementation of condition monitoring systems in power transformers," Advanced Research Workshop on Transformers 2007, ARWtr2007, vol., no., pp.1-6, 29-31 Oct. 2007. Velasquez, J.L.; Villafafila, R.; Lloret, P.; Molas, L.; Sumper, A.; Galceran, S.; Sudria, A.; "Development and implementation of a condition monitoring system in a substation," International Conference on Electrical Power Quality and Utilisation, 2007, EPQU 2007, vol., no., pp.1-5, 9-11 Oct. 2007. Lloret, P.; Velasquez, J.L.; Molas-Balada, L.; Villafafila, R.; Sumper, A.; Galceran-Arellano, S.; "IEC 61850 as a flexible tool for electrical systems monitoring," 9th International Conference on Electrical Power Quality and Utilisation, 2007, EPQU 2007, vol., no., pp.1-6, 9-11 Oct. 2007.

ACKNOWLEDGEMENT The pilot plant project has awarded with Endesa’s R+D+i international prize NOVARE 2005 on distribution networks in the category of Power Quality and Reliability by the project: 'Substation monitoring for predictive maintenance'.

6

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

Formación y evaluación de la competencia en habilidad espacial Jordi Torner, Francesc Alpiste Penalba, Miguel Brigos Hermida Urgell 187, Barcelona, 934137398, 934017800 [email protected]

Resumen Diversos estudios señalan la habilidad espacial como una variable clave en los estudios de Ingeniería Industrial. Es fundamental para la actividad proyectual del ingeniero ya que resulta vital en el diseño de proyectos. Entre otros factores, se correlaciona con buenos resultados académicos y con facilidad de aprendizaje de sistemas de información y herramientas informáticas. Asimismo, el nuevo escenario creado por el EEES nos conduce a la definición y medición competencias, entre las cuales la integraremos. En el presente artículo planteamos la estrecha relación que une el desarrollo de esta habilidad con el trabajo con software de modelado de sólidos en 3D. El estudio se realiza con 812 estudiantes de 1er año de Ingeniería Industrial de la Universidad Politécnica de Catalunya, analizando la evolución de las puntuaciones obtenidas a través de los test DAT-SR y MRT, antes y después de la asignatura de diseño asistido por ordenador. Palabras Clave: competencias; EEES; habilidad espacial. Abstract Many studies show that spatial ability is a key factor in engineering studies. It is essential for the engineer in sketching activity and vital on projects design. Among other factors, it is correlated with brilliant academic results and capability on learning information systems and software. Besides, the new scenario created by EEES drives us to the competences definition and evaluation. In this paper we show the big relationship between this ability development with 3D solid modelling software. This study is made with 812 first year Engineering students at UPC-Barcelona Tech, analyzing the scores evolution over DAT-SR and MRT tests, before and after computer aided design subject. Keywords: competences; EEES; spatial ability.

1. Introducción La inteligencia humana se pone de manifiesto en el nivel de desarrollo de ciertas habilidades (verbal, numérica, espacial, etc.). Diversos autores destacan la importancia de la habilidad espacial (HE) en los procesos de diseño en Ingeniería y proponen estrategias didácticas para favorecer su desarrollo entre los estudiantes. El desarrollo de la habilidad espacial forma parte del currículum de la Ingeniería Gráfica desde hace largo tiempo [1]. En los últimos años, el interés ha

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

ido creciendo debido a las novedades y el impulso tomado desde la informática gráfica. Su valor reside básicamente en la relación entre la HE con el diseño y con la comunicación gráfica. El concepto de HE cubre un amplio abanico de funciones cognitivas. En la actualidad existen multitud de tests y pruebas que permiten abordar los diferentes componentes de dicha habilidad. Este hecho provoca que el concepto quede fragmentado en múltiples sub-factores y resulta complicado encontrar una definición aceptada de forma unánime por toda la comunidad científica. No obstante, encontramos 2 componentes básicos de la habilidad del que derivan los demás, aceptados por la comunidad científica [1]: Visión espacial: habilidad de manipular un objeto en un espacio 3D imaginario creando representaciones del objeto desde diferentes puntos de vista. Orientación espacial: se refiere a la capacidad para controlar el espacio de nuestro entorno y predecir el movimiento y la posición de los objetos. Un ingeniero debe ser capaz de resolver gráficamente la representación de estructuras y sistemas complejos en el desarrollo de su trabajo. Por lo que necesariamente la HE es útil y puede llegar a ser clave en el desarrollo de proyectos de ingeniería, tal y como apuntan diversos estudios [2,3]. En las primeras fases del diseño de proyectos es fundamental solventar con rapidez problemas en los que el razonamiento espacial juega un papel decisivo, por ejemplo, en la fase de croquización. Por otra parte, la HE se ha reconocido como factor determinante en la predicción de éxito en diversas áreas, especialmente en las áreas tecnológicas [4]. Es decir, se han establecido correlaciones positivas entre la HE y los resultados académicos de los estudiantes en ingeniería. Se han establecido correlaciones positivas con la capacidad de aprendizaje de aplicaciones informáticas, herramientas de CAD, en el diseño de Bases de Datos o en el desarrollo de estructuras moleculares [5]. El aprendizaje de una herramienta profesional de CAD en los estudios de Ingeniería Industrial se hace cada vez más necesaria debido, entre otros factores, a la demanda

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

del mercado laboral. En consecuencia la gran mayoría de universidades y escuelas técnicas utilizan una herramienta de CAD en los primeros cursos de las ingenierías. Varios autores [6,7] han demostrado que el uso de herramientas CAD puede potenciar el desarrollo de la visión espacial. En resumen, la HE se configura como: 

Competencia básica en el currículum del ingeniero.



Fundamental para la actividad proyectual: Resulta vital en el diseño y desarrollo de proyectos.



Se correlaciona con buenos resultados académicos y con facilidad de aprendizaje de sistemas de información y herramientas informáticas



Necesarias para resolver gráficamente la representación de estructuras y sistemas complejos en el desarrollo de su trabajo.



Factor determinante en la predicción de éxito en diversas áreas, especialmente en las áreas de ciencias y tecnológicas (correlaciones positivas con resultados académicos de los estudiantes en ingeniería).



Se han establecido correlaciones positivas con la capacidad de aprendizaje de aplicaciones informáticas, herramientas de CAD, en el diseño de BBDD o en el desarrollo de estructuras moleculares.



Relación entre la HE i la habilidad para trabajar con sistemas de información informáticos (navegación por menús jerárquicos y bases de datos, portales de e-learning, sistemas de almacenamiento de información y en general todo tipo de espacios web).

Debido a todos estos condicionantes, este trabajo pretende desarrollar un modelo que permita evaluar la HE de los estudiantes de ingeniería de la asignatura de primer curso “Expresión Gráfica y diseño asistido por ordenador” .

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

El modelo contará con los procedimientos y con los indicadores necesarios, permitirá ponderar las principales variables y orientar sobre las acciones a introducir de mejora de la práctica docente. Nuestro objetivo es comprobar si el uso de una herramienta de modelado de sólidos 3D, como Solidworks, desarrolla la HE en los estudiantes. Para ello se realiza la pasación de 2 tests de HE al inicio y final del cuatrimestre y se comprobará si existen diferencias significativas entre las puntuaciones obtenidas antes y después de las clases. De esta manera podremos estudiar si la intervención realizada en las clases de la asignatura produce un entrenamiento de la HE. El estudio se realiza en un momento de cambios importantes ya que se ha procedido a la adaptación de la asignatura Expresión Gráfica y Diseño asistido por ordenador al modelo acordado en Bolonia. La integración de las universidades en el Espacio Europeo de Educación Superior (EEES) nos conduce a modificar la estructura, los contenidos y el modelo de enseñanza-aprendizaje de nuestros programas de formación. Este hecho nos abre un nuevo eje en la investigación. Este escenario nos lleva a la definición de competencias: Las actividades formativas se orientan a la adquisición de competencias específicas de cada asignatura, adoptándose un enfoque formativo-práctico. Y a la evaluación de resultados: Se requiere la evaluación de los resultados obtenidos en el proceso en términos de competencia, intentando acercar el perfil profesional al académico observando los conocimientos y habilidades necesarios en el mundo laboral. La integración al espacio Europeo nos conduce a definir las competencias específicas que definirán la asignatura de Expresión Gráfica y DAO. Tal y como se ha comentado una de las competencias más importantes en la figura del Ingeniero y de la asignatura es la HE. Por lo tanto definir y evaluar dicha competencia se convierte en otro objetivo importante de nuestra investigación.

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

2. Objetivos y metodología El objetivo último del estudio es desarrollar un modelo que permita evaluar la HE en los estudiantes de Ingeniería Industrial y que permita a su vez evaluar las estrategias y los métodos de la programación y su relación con la HE.

Figura 1. Esquema de investigación

En el desarrollo de

este trabajo se ha analizado la asignatura de primer curso

Expresión Gráfica y Diseño asistido por ordenador de la UPC. Mediante la utilización del modelo propuesto, se quiere comprobar si la metodología didáctica

y las actividades realizadas en la asignatura colaboran en un desarrollo

significativo de la HE en los estudiantes. En referencia a la detección de HE mediante test se estudiaran las principales soluciones utilizadas. De entre ellas, se elegirá la tipología de test que se adapte mejor al objetivo de nuestro estudio.

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

Con este objetivo se pasarán 2 test (DAT-SR y MRT) de HE al inicio y final del cuatrimestre y se comprobará si existen diferencias significativas entre las puntuaciones obtenidas antes y después de las clases mediante técnicas estadísticas.

Figura 2. Rotación de figuras (basado en MRT)

3. Competencia en HE Con el EEES la definición y evaluación de competencias adquiere un papel relevante. La competencia es la habilidad aprendida para llevar a cabo una tarea, deber o rol adecuadamente. Un alto nivel de competencia es un prerrequisito de buena ejecución. Navío [8] apunta que las competencias profesionales son un conjunto de elementos combinados que se integran atendiendo a una serie de atributos personales tomando como referencia las experiencias personales y profesionales y que se manifiestan mediante determinados comportamientos o conductas en el contexto de trabajo. Destacan entre otros el trabajo de Moon [9] para la programación de la asignatura y el de Urraza [10] que nos propone un modelo de competencias de la asignatura en el que la HE queda integrada:

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

Tabla 1. . Competencias específicas de Expresión Gráfica y DAO y relación con las competencias trasversales implicadas.

C. COMPETENCIAS ESPECÍFICAS

TRANSVERSALES

Instrumentales;

T.I: T.P:

Interpersonales T.S: Sistémicas COMPETENCIAS RELACIONADAS CON LOS CONCEPTOS Y CONOCIMIENTOS BÁSICOS T.I.2. Capacidad de análisis y C.1 Comprender, gestionar y aplicar un soporte de síntesis

T.I.3.

Capacidad

de

conocimientos sobre los fundamentos y normalización del gestión de la información T.I.5. Dibujo de Ingeniería Industrial, plataforma necesaria para Conocimientos básicos de la abordar los problemas de ingeniería gráfica.

profesión

T.S.2.

Aprendizaje

autónomo C.2 Aplicar con destreza los programas de DAO, que hacen que el ordenador se constituya en una herramienta didáctica, precisa y rápida, para la confección de la base documental

de

los

objetos

que

deben

de

ser

representados desde la perspectiva de los conocimientos

T.I.6.

Conocimientos

de

informática T.S.2. Aprendizaje autónomo

del Dibujo de Ingeniería. COMPETENCIAS RELACIONADAS CON EL APRENDIZAJE CONSTRUCTIVISTA C.3 Gestionar y aplicar la capacidad espacial utilizando como soporte la croquización, dentro de un marco de T.I.1. Resolución de problemas desarrollo estrategias cognitivas que ayuden a la T.S.2. Aprendizaje autónomo visualización tridimensional de los objetos técnicos. C.4 Interpretar y realizar planos normalizados del Dibujo T.I.1. Resolución de problemas de Ingeniería Industrial.

T.S.1. Capacidad de aplicar los

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

conocimientos

a

la

práctica

T.S.2. Aprendizaje autónomo

C.5

Aplicar

el

conocimiento

procedimental

en

la

resolución de los problemas de la Geometría Constructiva orientados a la representación de superficies.

T.I.1. Resolución de problemas T.S.1. Capacidad de aplicar los conocimientos

a

la

práctica

T.S.2. Aprendizaje autónomo T.I.1. Resolución de problemas T.S.1. Capacidad de aplicar los

C.6 Aplicar las habilidades de investigación y creatividad en la introducción al diseño industrial.

la conocimientos T.S.2.

a

la

Aprendizaje

T.S.3.

práctica autónomo

Creatividad

T.S.5.

Habilidades de investigación C.7 Gestionar las fuentes de información, exponiendo y T.I.4. justificando de forma gráfica, oral y escrita los aspectos organización relacionados

con

las

ideas

de

diseño

y

con

Capacidad y

de

planificación

la T.I.7. Comunicación gráfica, oral

interpretación y realización de los documentos de y escrita Ingeniería. C.8 Trabajo en equipo que facilite el desarrollo de los T.P.1. Trabajo en equipo T.P.2. conocimientos con un intercambio cultural crítico y Capacidad responsable.

de

autocrítica

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

crítica

y

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

4. Modelo Definimos un Modelo para el desarrollo de la HE en la Expresión Gráfica. El objetivo del modelo es disponer de recursos de mejora docente a partir del estudio de la HE. El modelo permite el control de variables que afectan la HE y facilita su medida Pre y Post curso. Además, el modelo establece relaciones entre las metodologías didácticas, los resultados académicos

y la satisfacción de los

estudiantes.

Figura 3. Modelo para el desarrollo de la HE en la Expresión Gráfica

La actividad se centra en la programación de la asignatura de Expresión Gráfica y Diseño Asistido por Ordenador (EGDAO) y en el estudio de las habilidades espaciales que se desarrollan en ella. XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

Se aporta un modelo para medir la mejora de la habilidad espacial, qué es una competencia básica de los ingenieros. Para ello, se describen las variables que afectan la HE y se propone un sistema de depuración de las mismas que, además, orienta en las acciones didácticas a tomar para mejorar la HE Los estudios estadísticos realizados permiten obtener valores cuantitativos que pueden ser utilizados como referencia para los indicadores de calidad. Además contribuyen en la determinación de la fiabilidad de las encuestas realizadas. Se determina la correlación entre los valores de HE y los resultados académicos obtenidos a partir de las evaluaciones de las principales actividades didácticas realizadas. Esta correlación nos permite determinar la influencia de las metodologías docentes utilizadas en la mejora de HE y nos orienta acerca de la selección de actividades más eficaces. Finalmente, todos los datos analizados revierten en la toma de decisiones para incidir en la mejora de la calidad docente, toda vez que disponemos de un conjunto de métodos y herramientas con las que obtener y comparar los registros con los indicadores de referencia utilizados en un proceso de mejora continua.

5. Conclusiones y líneas futuras de investigación De todas las variables analizadas en el estudio, se identifican mediante el análisis de los resultados, las siguientes variables determinantes en las puntuaciones de HE: Uso de software de CAD: se aprecian diferencias significativas en los alumnos con experiencia en este tipo de programas. Especialidad:

encontramos

diferencias

importantes

entre

especialidades,

especialmente en química, que obtiene las medias más bajas. La relación más fuerte se encuentra entre del DAT inicial y la prueba DAO2 dedicada a la geometría del espacio. Por lo tanto, se propone potenciar las actividades relacionadas con la geometría del espacio para maximizar el desarrollo de la HE. XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

ETS de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria

DAT parece ser un buen indicador de éxito en la asignatura ya que muestra los valores más altos para las correlaciones. MRT no resulta un instrumento interesante ya que no nos aporta diferencias significativas en ninguna de las comparativas realizadas.

A partir de aquí, algunas de las posibles líneas de acción futuras serían: 

Utilizar los indicadores obtenidos comparar los registros con los indicadores en un proceso de mejora continua: Resultados de los test: Incrementos en el DAT, comparaciones internacionales, nacionales, años, rendimiento académico y la relación del DAT con la Nota Final, DAO1 y DAO2.



Completar el modelo con la medición de otras competencias fundamentales en el campo de la ingeniería y con su aplicación en la mejora de la práctica docente mediante la incorporación y evaluación de nuevas metodologías relacionando los resultados académicos y la adquisición de competencias.

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

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5. Referencias 1. Miller, C. L., & Bertoline, G. R. Spatial visualization research and theories: their importance in the development of an engineering and technical design graphics curriculum model. Engineering Design Graphics Journal 55 (3), (1991). 5-14. 2. H Jerz, R. “Redesigning engineering graphics to include CAD and sketching exercises” ASEE Annual Conference Proceedings, Montreal, Canada (2002). 3. J Strong, S. and Smith, R. “Spatial visualization: fundamentals and trends in engineering graphics”. Journal of Industrial Technology, vol. 18, no. 1, (2001). 4. Strong, S., & Smith, R.. Spatial visualization: Fundamentals and trends in engineering graphics [Electronic version ]. Journal of Industrial Technology, 18(1), (2001-2002), 1-5. 5. Norman, K. L. Spatial Visualitzation. A gateway to Computer Based Technology. Journal of Special Educational Technology, XII, (3), 195-206 (1994). 6. Devon, R., Engel, R.S., Foster, R.J., Sathianathan, D, and Turner, G.F.W. “The effect of solid modelling on 3D visualization Skills”. Engineering Design Graphics Journal, vol. 58, no. 2, 4-11 (1994). 7. Sorby, S.A., “Improving the spatial skills of engineering students: impact on graphics performance and retention”. Engineering Design Graphics Journal, vol. 65, no. 3, pp. 31-36 (2000). 8. Navío Gámez, Antonio. “Las competencias del formador de formación continua. Análisis desde los

programas de formación de formadores”. Tesis Doctoral.

Universidad Autónoma de Barcelona (2001). 9. Moon, J. Linking Levels, Learning Outcomes and Assessment criteria: the Design of Programmes and Modules in Higher Education. unpublished paper, Staff Development Unit, University of Exeter (2000). 10. Urraza, Guillermo. Evaluación de competencias en el diseño curricular de la asignatura de Expresión Gráfica y DAO. XVII Congreso Internacional de Ingeniería Gráfica, (2006).

XVIII Congreso Universitario de Innovación Educativa en las Enseñanzas Técnicas Escuela Técnica Superior de Ingenieros Industriales y de Telecomunicación. Universidad de Cantabria Santander, 6 a 9 de julio de 2010

61st International Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights reserved.

IAC-10-A1.8.4 SMALL MEDICAL EXPERIMENTS IN INNOVATIVE AEROBATIC SINGLE-ENGINE PARABOLIC FLIGHTS: PROVIDING DATA AND INSPIRATION FOR THE EXPLORERS OF TOMORROW Prof. Antoni Pérez-Poch EUETIB, Escola Universitària d’Enginyeria Tècnica Industrial de Barcelona; UPC, Universitat Politècnica de Catalunya, Spain, [email protected] Daniel Ventura González Aeroclub Barcelona-Sabadell, Barcelona, Spain, [email protected] Gloria García-Cuadrado BAIE Barcelona Aeronautics & Space Association, Spain [email protected] Recent research undertaken by the joint venture led by the Universitat Politecnica de Catalunya, with its partners, the Aeroclub Barcelona-Sabadell and BAIE, Barcelona Aeronautics Space Association, has shown that it is possible and safe to obtain zero-gravity conditions for up to 8 seconds with single-engine aerobatic planes. The quality of the microgravity is comparable to that obtained by conventional parabolic flights. The main advantage of this technique is that a lower cost-to-time of microgravity ratio, during the parabola is obtained. Small life science experiments that require no more than this short period of time and cannot be run in drop towers, benefit from an easy access to the experimental platform. We present here how data of small medical experiments which had own with our platform are thereafter used for the first time as an educational tool. Experiments were aimed at validating a numerical model (NELME) that has been developed in our research group, which is intended to suggest what actual changes in the cardiovascular system can be expected when the human body is exposed to reduced gravity. An educational tutorial was developed, based on these experiments, containing an introduction to space physiology, how the data was obtained and why it was useful, and a hands-on material where students can actually use a simulation software to see what changes may happen to the human body when exposed to long-term scenarios, like a long expedition to the Moon, or a trip to Mars. The material was tested by engineering students, who had nearly no previous understanding of medical concepts, but it can easily used also for life sciences students with no knowledge of simulation techniques. A final survey, and an evaluation of the students work results was conducted, in order to assess the impact of this activity. Students of our University also have the opportunity to design their own experiment, and actually build it and fly it in zero gravity at Sabadell Airport (Catalonia, Spain), very near to our Faculty premises in Barcelona. Students from the International Space University Space Studies Program 2010 have designed a number of experiments which will likely be flown by us this year, and an international contest led by the Space Generation Advisory Council is just being started. In conclusion, we believe that this innovative microgravity platform will open new doors to inspire students around the world to get an interest on space medicine and research, and we look forward to expand this opportunity in the upcoming years. I. INTRODUCTION Parabolic flights are a common way nowadays to obtain microgravity. About 20-30 seconds of microgravity can be obtained during parabolic flights. Jet airplanes such as the KC135(NASA) and the Caravelle or the Airbus A300(ESA) or the Ilyushin IL76 MDK (Gagarin Cosmonaut Training Center, Moscow) are used with their interiors completely empty and padded with foam rubber [1]. These planes are operated in professional or student experimental campaigns involving a number of different teams and experiments on-board, and typically require months of preparation.

IAC-10-A1.8.4

The flight profile is the following (see Figure 1): coming from a steady flight profile an introductory pull-up maneuver is performed at increased acceleration (roughly 2g for these planes), pilot reduces thrust and, with throttle or idle engines the airplane follows the parabolic trajectory of a free-flying body. As a consequence, after a short phase of transition, microgravity is obtained for about 20-30 seconds. After the recovery maneuver at increased acceleration (2g), the airplane flies again horizontally to the ground level for some minutes before introducing the next parabola. During one flight mission about 20 parabolas are performed.

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Figure 1: AIRBUS 300 parabolic maneuver (Credit: ESA/Novespace) Due to flight perturbations and the presence of many crew members, however, there is a comparatively low microgravity level of only about 0.01g. The utilization of such procedures ranges from testing of technology and procedures to qualification of experiments and subsystems to astronaut training. ESA has used since 1984 six types of airplanes to conduct its parabolic flight campaigns [2]: the KC135, the Caravelle from CNES, the Russian Ilyushin Il76 MDK, the Cessna Citation II, and the Airbus A-300 'zero-g' from CNES, all of them with 2 or 4 engines. An important number of physical and life sciences experiments have been conducted showing the success of this kind of access to microgravity. Our approach is different from the successfully previously reported parabolic flights as we propose the use of a tiny 2-passengers aerobatic plane. This kind of aircraft is already certified to sustain this maneuver and could also be used for professional experiments and testing technology. The advantage of this approach is the immediate preparation and saving cost as the budget of the flight is significantly small than those parabolic flights with bigger and more complex airplanesajor headings are capitalized, underlined and centred in the column. II. CALIBRATION AND OPERATIONS We first reported the implementation of parabolic maneuvres for professional experimentation in microgravity with a CAP-10B aerobatic airplane, certified to make aerobatic maneuvers at IAC in Glasgow 2008, after our maiden flight in November 2007 with the first calibration data [3]. The plane is a 2-passenger light model of airplane (Figure 2) and can be flown easily from an aerodrome with little preparation apart from the usual procedures in private flying.

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Figure 2: CAP-10B plane owned by ACBS. (Credit: Aeroclub Barcelona-Sabadell). The only limitation of this approach is that no huge equipment can be loaded into the cockpit as this was designed to be smart for aerobatic sport, but it is quite adequate for rapid testing and prototyping of technology subsystems, as well as physical or life science experiments that don't need a huge space to be stowed. We conducted in this mission six parabolic flights from the Sabadell Airport in November 2007 with an experiment on board. Every parabola was carefully planned and coordinate between the pilot (Ventura) and the mission specialist (Perez-Poch) of this mission. Timing of every part of maneuvre, g acceleration, and a number of parameters regarding the experiment on board were recorded through a laptop on board. As this is a 1-engine plane with a limited capacity of thrust, the power available from the plane engine to perform the parabola was less than those available from the other planes reported to have undergone parabolic maneuvres. As a result of this limitation, a more intense acceleration is needed in the pull-up entry reaching 3.8 g instead of the usual 1.8g found when using the Airbus 300 zero-g. With this approach we report six series of 4.5 to 6.8 seconds of microgravity during the parabola zero-g phase. Again, a nearly 4g pull-out maneuver is performed by the pilot to recover horizontal flight. We repeated the maneuver every two minutes with the experiment on board. The quality of the gravity attained is comparable to that obtained with earlier parabolic experiments, although we didn't control the z-axis so precisely as other planes do as the control of this plane is totally manual. However it can be estimated that the order of magnitude is comparable to that of 0.01 g obtained in bigger airplanes with more precise and strict control of the balance. The pilot of these maneuvers is an experienced aerobatic pilot (Ventura), who trains regularly as a sport aerobatic aviator. The mission specialist is a private pilot (Perez-Poch) with no previous experience in aerobatic flight, but did not require any medical treatment previous, during nor after the parabolas. No

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motion sickness symptoms were reported by any of us in this mission, although it is advisable to be fit enough to sustain the nearly 4g pull-up and pull-out experience. After the maneuvers the plane was conducted from the surrounding area to Sabadell Airport, 20km from Barcelona, and safely landed with no incidences to report. During the manual performance of the maneuver by the pilot (Ventura) the mission specialist (Perez-Poch) was carrying on himself the payload intended to in-flight validate the NELME model proposed and developed by the same author [4]. The equipment consisted of a laptop with an RF receiver, a blood pressure monitor with RF emitter, and a state-ofthe-art pulsometer able to register heart rate.. Analysis of these results were found to be reliable as their variations were minimal for every one of the six parabolas performed. The numerical model predicted the variations in blood pressure and heart rate when applying 3.8g , then zero g, and back to 3.8g of the subject. The experimental findings were fully compatible with the model in spite of the few seconds available in microgravity. More detailed results can be found in [4] as well as the whole description of the model. The total cost of this mission was estimated in less than 300 euros including the cost of hiring a professional aerobatic pilot, the same plane, essence and airport taxes. This is less than a thousand than what can be estimated for a usual parabolic campaign, thus resulting in a very advantageous time of microgravity/cost ratio. The preparation of the mission was reduced to a series of breafing and debreafing sessions as no special requirements were needed for this life sciences experiment. Therefore, the access time to microgravity was also significantly reduced from that need in a usual parabolic campaign which may last for months. Since then, an extensive number of flight tests have been carried out in order to improve the proficiency of the manual manuevre. Thanks to this optimization the quality of g attained has been significantly improved, and the likelihood of g jitter lessened. Thanks to these efforts, the technique was optimized in order to be able to provide a reliable source of microgravity to the European space research community, and also to provide with flight opportunities to the students. A joint venture between the Aeroclub Barcelona-Sabadell, UPC Barcelona Tech and BAIE Barcelona Aeronautics & Space Association has started early this year. This joint venture is able to provide flight opportunities and a legal framework for the researchers and students who wish to take advantage of this platform. An Announcement of Opportunity was released this year [5,6] by the institutions funding and leading this endeavour.

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III. EDUCATIONAL OPPORTUNITIES III.I Medical Data as a motivational tool During the calibration process, from November 2007 to the current data, data for the validation of the NELME model of the cardiovascular system under variable g conditions were collected. An educational tutorial was developed, based on these experiments, containing an introduction to space physiology, how the data was obtained and why it was useful, and a hands-on material where students can actually use a simulation software to see what changes may happen to the human body when exposed to longterm scenarios, like a long expedition to the Moon, or a trip to Mars. The material was tested by engineering students, who had nearly no previous understanding of medical concepts, but it can easily used also for life sciences students with no knowledge of simulation techniques. A final survey, and an evaluation of the students work results was conducted, in order to assess the impact of this activity. The students had to work out what changes were important, what implications have the data for the hypothesis of the experiment, and propose future lines of research. Students had a one-hour tutorial workshop introduction, two hours of class work, and 4 days to submit their work. All student teams presented their work on time, and the evaluation was fairly good to excellent for all teams. Students qualified with a 3.8 +/0.4 the activity (1 being boring, 5 exciting) and provided some quotes as ‘the activity was the most original of my studies’ or ‘I wish to also take part in the experiments’. A limited number of UPC graduate research collaborators, and UPC undergraduate students have also been invited by us to actually take part in these inflight tests during the calibration processes. In these selected motivational flights, which were also funded and directed by UPC, and operated by the Aeroclub Barcelona-Sabadell with Mr D.V. Gonzalez as pilot-incommand performing the maneuvres, these students could, as a result of these operations, make some proposals of in-flight experiments [7]. III.II International Space University Summer Students Program 2010 “Fly-your-experiment” Barcelona Campaign The International Space University is the leading University in the space sector providing top education under its three lines of inspiration: International, Interdisciplinar and Intercultural. As part of its educational curricula it organizes every year an

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intensive 9-week Summer Space Program . This program is attended every year by more than 100 graduate and undergraduate students from all over the world. During the program, they are exposed to a number of fundamental core lectures, workshops, and departmental activities. The last three weeks are dedicated entirely to the development of a Team Project in a topic related to space activities [8]. During the recent SSP10 one of the proposed activities was to design and actually build an experiment to be flown with our platform. A 1-hour workshop was conducted by us, in which the students were introduced with the basics of parabolic flight, the special features of our platform, and then they were challenged with the possibility to actually fly their designs with us. The students were given the detailed requirements that had to be taken into account, as well as safety mandatory requirments. A 1-hour guided work time was granted, during which the students formed their teams and began making their experiment designs, with the mentoring of experienced professors of this particular field. Finally, the students, had to develop and submit a detailed form, in a professional way, detailing all aspects concerning their experiment, with the endorsement of an expert professor in the space field. A selection process is currently underway based on this form, with the three best experiments to be chosen and entitled to fly with our platform. We are particularly impressed by the quality of some of them, and will be most likely to provide high-quality meaningful data. Students during all the prrocess of design, build and fly the experiment, clearly benefit from the interaction of a leading university in the space sector, and an innovative challenge to actually experiment in zero-g their ideas. The ISU SSP10 Barcelona Aerobatics Challenge students’ flight campaign will probably take place in the October-November timeframe in Sabadell Airport, Barcelona,Spain.

After this calibration period, a non-profit joint venture has been set up by the three institutions in Barcelona (Catalonia, Spain) involved in the development of the technique. Researchers in Europe can benefit from this opportunity thanks to the opening of a continously open call for proposals. Educational activities have been from the beginning an essential part of our motivation, and have provided meaningful results and a number of flight opportunities for students experiments, as well as tutorials after data collection. The Space Generation Advisory Council, a leading group of space enthusiast students and youth professionals has been invited by us to lead an international Challenge for students all over the world, in order to design and fly their experiments in zero-g. We expect this competition to start with their involvement before the end of this year. We are convinced that this innovative microgravity platform is already making an impact, and inspiring students around the world to get an interest on space medicine and research. Therefore, we certainly look forward to expand these activities in the upcoming years.

IV. CONCLUSIONS We first reported a successful series of parabolas performed with a light aerobatic plane with a life sciences experiment on board. Between 5 to 8 seconds of microgravity were attained with a very small cost. The optimization of the tecnique has made possible to provide a quality of g between 0.1g and 0.01g with a g jitter reduction depending on the strength of wind gusts. Very limited time is needed to prepare and perform the experiment so this approach is specifically suited for those kind of rapid prototyping technology tests, or simple experiments that do not need huge or sophisticated equipments.

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REFERENCES

[1] Messerschmid E, Bertrand R, Space Stations. Systems and Utilization. pp 300-310. Springer Verlag. ISBN 3-540-65464-X Berlin 1999. [2] Pletser V, Short duration microgravity experiments in physical and life sciences during parabolic flights: the first 30 ESA campaigns. Acta Astronautica, 55(10) 829854. 2004. [3] Pérez-Poch A., González, D.V. “Aerobatic flight: an innovative access to microgravity from a centennial sport”. 58th International Astronautical Conference, Glasgow, 2008. Conference Paper # IAC-08-A2.3-12 [4] Pérez-Poch A. "On the role of numerical simulations in studies of reduced gravity-induced physiological effects in humans. Results from NELME.". Proceedings of the 38th COSPAR General Assembly,Bremen, July 2010. Submitted to Advances in Space Research. [5] BAIE Announcement of Opportunity for European researchers/students: http://www.bcnaerospace.org/public/new.php?id=51 , retrieved on 30th August 2010. [6] CRAE-UPC Announcement of Opportunity for European researchers/students: http://recerca.upc.edu/crae/news/acrobatic-flight-aninnovative-access-to-microgravity-from-a-centennialsport-announcement-of-opportunity-for-upc-studentsresearchers , retrieved on 30th August 2010. [7] Schroeder, J.W. , Zurita D. “Aerog- the portal to weightlessness. Aerobatic flights as an educational platform for microgravity experiments” 60th International Astronautical Conference, Daejon 2009. Conference Paper # IAC-09-E1.4.6 [8] International Space University, Summer Space Program 2010 official website: http://ssp10.isunet.edu , retrieved on 3rd September 2010.

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IAC-10-D5.2.10 A NEW INFORMATION SYSTEM ARCHITECTURE FOR A NEW SPACE EXPLORATION PARADIGM: USING STAKEHOLDER ANALYSIS TO REENGINEER THE VALUE CHAIN Prof. A.Pérez-Poch EUETIB Escola Universitària d’Enginyeria Tècnica Industrial de Barcelona, Software and Information Systems Department, UPC, Universitat Politècnica de Catalunya, Barcelona, Spain, [email protected] An information system architecture is proposed in order to take into account the map of stakeholders and relations into a space human exploration venture led by private entrepreneurship and commercial ventures. The model is based on a stakeholder analysis that enables us to capture the main mechanisms that add value to nowadays' visions for space exploration. Current visions for human space exploration have turned their focus into commercial and private companies, rather than in public funding. Cancellation of the Constellation may deeply affect the development of technologies for human space exploration. Nevertherless, this decision will in fact open new opportunities for private entrepeneurships to participate in space exploration. In order to ensure success of these new ventures, requirements should be rewritten from the beginning, allowing main stakeholders to produce benefits in the value chain. A detailed value chain ow model is proposed, with an adaptive neural network of value propagation discussed. We conclude that advances in the modeling of information systems architecture, are useful not only for identifying key stakeholders in global enterprises such as human exploration, but also a tool for reengineering the whole process and adapting it into new vision paradigms. I. INTRODUCTION Stakeholder analysis has gained importance in the last decade, as a key process to perform corporate analysis. However, its implentation in large companies and in particular, large public enterprises has proven to be difficult, not to mention a combination of both. Requirements analysis is a well-known technique, that is widely used in many management project routines. Usual requirements analysis tend to select a particular set of architectures based on techical merit, rather than on any other topic. Stakeholders analysis is only taken account in a later step of the design process, with only minor consideration to its importance. However, space exploration is basically a human endeavour. Rationale to venture into space is not based on technical reasons, but on to the will of the human mind to get further and explore the unknown. Space anchorman Walter Cronkite defined during the 2002 IAC Opening Ceremony the arrival of men to the Moon as ‘the most important moment in human history’. He was not quoting this enterprise just because of its difficulty in technical terms. Indeed, he was referring to it as its value to mankind, as an accomplished that produced enormous value to a wide rage of human beings. Not without the social and political support of those times for putting a step on the moon would it had been possible to begin thinking in economic and technical terms. Therefore, it is obvious that stakeholders are not only important for any space endeavour, but a

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fundamental one. Without a wide and public support from a number of space actors it is impossible to even think in a large investment for a space activity. Space is a particular field in which stakeholders are key vectors of the full enterprise. It has been difficult in large government space projects, to identify and analyse the role of every stakeholder. The involvement of the private sector is a growing trend in space activities, with large public funded projects being cancelled, and private companies incoming into the development of new space technologies. The private sector involves a bigger role for a group of space stakeholders, with starring characters that were unthinkable decades ago. In spite of these considerations, the information system architectures currently in consideration in the space area are mostly atomized and do not take into account the relevant role of the stakeholders that creat value and momentum to the space activities. We first propose that the value chain vector should be considered in order to identify which stakeholders are most relevant to the space endeavours. We state that from a strategic point of view, the identification and analysis of stakeholders adding value to the process should be the core of the design process, and not a secondary addition to technical considerations. Those design solutions with a proper understanding of the system’s stakeholders will be those with early and clearly defined roles for them, that involve later decisions in accordance of their presence in the value chain of the project. The fundamental aim of this paper

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is to provide a general framework that reduces the gap between the stakeholders identification process and their technical considerations. We begin in Section 2 considering stakeholders, their needs and relationships between them. In section 3 we address the value chain, as a vector to reengineer space activities, as private business take a major role. In section 4 we provide with a basic tool with metrics to optimize the process of an organizational change. A discussion of the pratical implications of such a framework is taken into account in the latter section.

II. SPACE STAKEHOLDERS Stakeholders are defined as those individuals, entities or organizations that have a role in a definite process. The stakeholders analysis is usually aimed at finding which is the best organization design that optimizes its effectiveness. The work is performed by focusing in the stakeholders that take a substantial role on the value chain of the company. Basic needs and identification of the main relationships are most relevant for the public sector, where the concept of ‘added value’ is more difficult to identify. If we would like to identify the key stakeholders at the space area, the question should be: Who are the stakeholders of space exploration that will make value grow? A review of the literature [1-5] will show us that the major characters had already been identified. Science, Security, International Partners, Economic Area, Executive & Congress, People, Educators and Media are the main groups of people and organizations that typically add value in the United States, according to the latter references. Some of them, like Educators and Media are mainly intermediaries with the People. Finally, the major public space agency in the US, NASA is noted, to which the private sector should be added in an emerging growing role. Exploration missions require that people involved in these areas make flow the benefit, tangible or intangible that emerges from the space activity. The overall process of identifying stakeholders and assigning them a proper role and interrelationships between the different systems involved, are known to be the design of the stakeholders model. III. VALUE CHAIN AS A VECTOR TO REINGEENER THE PROCESS Once the basic process of modeling is done, we will have a detailed map of the connections between the different stakeholders involved. The process model is a dynamic one, although only a steady-state photograph of the whole system is considered.

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At this point of time, we introduce the concept of value chain coming from the industry and information systems architecture. Value chain is a collection of value flows which are connected by stakeholders, relevant to the process. Major white papers and requirements standards [6] refer to these concepts in the space area as well as others. The chain has the responsability to change and add a definite value onto the system. Only stakeholders that form part of input-output flows are the ones relevant to the reengineering process. By reengineering we define a major organizational change, that aims to optimize the creation of value within the system. A reengineering process based on the value chain, should follow the next steps: 1- Defining value for our system 2-Modeling the stakeholders matrix, 3-Identifying the key stakeholders which contribute to the value chain, and 4-Rearrenging the value flows in the organization to reinsert key stakeholders into the value chain. Individual flows according to [5] are categorized into six groups: Policy, Money, Workforce, Technology, Knowledge and Goods and Services. In the process of creating a value flow model framework, a number of decisions have to be made in order to simplify the value loops, and make the model easily understood. Value loops are defined as value chains that return to the starting stakeholder. Simplification of this map has no standard procedure, and is dependant on the level of detail needed in the reeengineering system. The overall system is then redesigned in order to help the value chain grow, and to lessen interferences and expenditure of resources on to areas that do not really add value in the system. IV. QUANTIFICATION AND METRICS The process of reengineering an organization is often regarded as an holystic one. Different levels of detailed among authors are observed, but it is somewhat difficult to quantitize what the necessary changes in a process of optimization. are. A framework to help reorganize and optimize the value chain should be composed of: -

Stakeholders matrix. Value flow model. Metrics. Optimization application tool. A feedback process

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The stakeholders matrix and the value flow model have been described in previous section. Metrics are a part of the reengineering design process. Qualitative variables can be quantitized in ordinal terms. The exact number of individual people, little institutions (like schools) has to be estimated and the number of people involved in the space activity driven from it. The output-input flow is then derived from the value map. It is somewhat difficult to assess science or education results in terms of ‘what space exploration inspires’. Usual quality terms in education or science evaluation can be used, such us number of degrees attained in the space area, number of papers published in peer-reviewed journal, etc. An important design decision is the weight that is associated with every single indicator in the model. The optimization algorithm should be one of easy implementation, and classic effectiveness. In our particular example we decided to parametrize the influence of the emerging private sector. We added to the model introduced by [5] a group of private companies as a block interconned as the pubic space agency was. The number of people involved in these activities was estimated to be a fraction of the overall workforce. We then constructed a flow map duplicating the input-output chain of the public sector, mantaining the rest of stakeholders like the public or science intact. We included a restriction that every increased step for the variables in the private sector should be accompanied by a decreased step for the public ones. For our particular study we chose as value the overall public understanding of space science, including space exploration outcomes, space science, and the increase of education and public understanding outreach. An adaptive neural network was chosen as the optimizing algorithm, and a process of iteration was conducted until value loops reflected a nearly steady-state. The system evolutioned to a significant part of the private sector taking over, and a pruning of the number of public organizations interconnetions. The independent variable was optimized, and in doing so a number of interconnexions had their values decreased nearly to zero, which suggest they should disappear were others began to grow. The overall results suggest that a significant part of the value chain could be taken over for the private sector, gaining value for the system, while reducing the overall costs. A significant result was that the risk of downgrading the benefits was higher while keeping the public funding low, or decreasing the public workforce under a certain deadline.

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DISCUSSION AND CONCLUSIONS We have presented the concept of stakeholders analysis, in relation with the value chain. We have considered the reenginering process, as a vector for organizational change, that allows to focus workforce and economic efforts into the process that add value to the system. Based on literature space stakeholders models, we have added a more relevant private sector into the system, and thought of what implications may it have on the effectiveness of the variables involved. Quantization of the variables involved in the value map allows to implement an optimizing method that visualizes possible changes that may arise from the new involvement of the private sector into the stakeholders matrix. Preliminary results show consistent findings with what is expected by new directions in the major components of the US space program. More work needs to be done in order to define more precisely the optimum metrics into the stakeholders map and the neural network architecture to optimize it. The process of quantifying and optimizing the map has proven to be successful in order to propose longterm organizational changes in the space arena, that will make space exploration more plausible and costeffective in human and economic terms.

Acknowledgements We are greateful to Mr Angel Linares-Zapater, Serinfo Information Systems CEO, for fruitful insights and discussions. EUETIB School of Engineering, from UPC Universitat Politècnica de Catalunya ‘Barcelona Tech’ has funded the study hereby presented.

REFERENCES [1] E. Rebentisch, E. Crawley, G. Loureiro, J. Dickmann, J. Catanzaro, Using stakeholder analysis to build Exploration sustainability, in: 1st Space Exploration Conference: Continuing the Voyage of Discovery, Orlando, Florida, January 30-1, 2005, AIAA-2005-2553. [2] NASA Exploration Systems Architecture Study. NASATM-2005-214062 http://www.sti.nasa.gov, November 2005, pp. 194, 541 [3] M.C. Jensen, Value Maximization, Stakeholder Theory and the Corporate Objective Function. Harvard

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Business School (Negotiation, Organization and Markets Unit), Working Paper no. 01-01, October 2001. URL: _http://papers.ssrn.com/abstract-220671_ (cited 6 November 2004). [4] Hoffman, Edward J, “NASA System Engineering Handbook”, SP610S, June 1995.

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[5] Cameron B.G., Crawley E.F., Loureiro G., Rebentisch E. Acta Astronautica 62 (2008) 324-33. [6] NASA Systems Engineering Processes and Requirements, NPR7123.1, Effective March 13, 2006.

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IAC-10-E1-3.15 PROGRAMAESPACIAL.COM: A DREAM COME TRUE Prof. Antoni Pérez-Poch EUETIB, Escola Universitària d’Enginyeria Tècnica Industrial de Barcelona. Universitat Politècnica de Catalunya, ‘Barcelona Tech’. Spain [email protected] Claudio Javier Mariani, Sergio Ezequiel Taleisnik [email protected] Programaespacial.com is a collaborative venture, a totally non-profit space which outreaches an educational web-site and broadcasts live Shuttle launches and other space events in the Spanish language. Back in the year 2006, two Argentinean friends who had already met in an online spaceflight forum felt they could not find any place inside the internet to share their love and passion for spaceflight the way they wished any more. That's why they decided to create their own spaceflight website: programaespacial.com. The main goal of the project is to educate and promote spaceflight and science interest on Spanish speaking communities. In order do so, project collaborators work voluntarily to inspire people by means of the different features the project consists on. Throughout these four years, the website has grown a lot, evolving from a simple spaceflight website to an educational venture. Many people have joined the project, and contribute to it every day. The Broadcasting Centre is one of the main features the website has. The centre consists of a webpage where the visitor can watch NASA TV in English and at the same time read live updated Spanish texts which are uploaded by the broadcaster. The broadcasts are carried out during major spaceflight events, mainly in Space Shuttle launches and landings. The broadcasts have included live updates by a website's correspondent, present at the Kennedy Space Centre (KSC), and who is becoming the only Latin American media in attendance at the spaceport. Last year, the Centre started to broadcast both text and audio; the audio broadcast implied a more fluid and enriching transmission, allowing speakers to explain concepts more deeply and to make live interviews. Another important project which is carried out by programaespacial.com members is “RDH" or Hondareyte Digital Reconstruction. It consists of the digitalization of old spaceflight radio broadcasts in Spanish, recorded by a professional radio operator called Luis Hondareyte. The website also includes a forum, where members of the community can exchange their opinions, experiences and doubts. I. INTRODUCTION Space exploration has been a matter of interest ever since the dark ages. Our ancestors tried to explain what they saw up in the skies by means of theories and myths. The invention of the telescope, together with further scientific breakthroughs, performed by brilliant minds such as Kepler, Newton, Euler, Lagrange and many other, offered people a better understanding of the universe. Despite all the scientific advances, it was not until the mid twentieth century when the human race saw the technical possibility to fulfill that dream that Jules Verne had seeded throughout the world with his 1865 novel From the Earth to the Moon: the dream of not only watching the stars, but flying to them. The first ones to travel were machines, followed by plants, animals, and finally, on 1961, the first human orbited the Earth. From that day on, spaceflight provided further knowledge about the universe that surrounds us, together with a battery of new technologies developed to perform those journeys.

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But it was not only scientific knowledge that space exploration offered us: the possibility of finally traveling to space became a source of interest and inspiration to millions around the globe. Just as former NASA engineer Homer Hickam relates in his autobiographical novel how he got interested in science after watching the Sputnik fly over his coal mining town, hundreds of thousands of people find in spaceflight a source of motivation and encouragement to fulfill their personal objectives. Spaceflight bounds science with adventure. It represents the accomplishment of massive challenges by means of hard work. Hickam’s story makes us think how to motivate more and more people not only into spaceflight but into scientific-related subjects. Unfortunately, we live in a world where space exploration does not represent big news any more. Fortunately, communication technologies are offering, day after day, new tools to inform and teach spaceflight

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news and concepts. During the late 60s and early 70s, Apollo missions were broadcasted to the entire world by international broadcasting companies. Nowadays, small groups organized by enthusiasts carry the flag of spaceflight to space-isolated communities. That is the case of programaespacial.com, a nonprofit educational venture created to inform, motivate and inspire Spanish-speaking people throughout the entire world. II. BIRTH AND EARLY DEVELOPMENT OF PROGRAMAESPACIAL.COM Mr. Claudio Mariani is an Argentinean graphic designer who has been a passionate for spaceflight since his early years. Back in the year 2005, Mr. Mariani met a young Argentinean physicist called Mr. Pablo Traverso in an online forum. They became best friends and in short time, they started to envision a project in which they could canalize their passion for spaceflight by sharing their knowledge with other people and, at the same time, learning from them, creating a true spaceflight cooperative learning community. The original idea evolved and resulted into an educational venture designed to educate and promote spaceflight and science interest on Spanish speaking communities. The project was called programaespacial.com and it officially began in the year 2006. On its early months, the site presented merely a news blog but, as time went by, both friends gave birth to the main features of the project which include: a broadcasting centre, an enhanced news section, a personal blogs section and an online forum. Ever since its creation, many people have joined the project. Some of them contributed to a particular feature, and others are still participating. III. THE BROADCASTING CENTER Public affairs policies in some of the busiest space agencies, such as the European ESA, the Japanese JAXA or the Russian RKA are clearly different with respect to NASA’s. This can be easily deduced by watching the American’s NASA TV. Originally created to provide the agency's Space Shuttle Program, managers and engineers with real time video of Shuttle operations, it evolved into a 24/7 informational and educational programming on space exploration, space science, earth science, and aeronautic research provided to the media and U.S citizens.

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NASA TV is available in Unites States only through cable operators and in the rest of the world through the use of the Internet. Unfortunately for Spanish-speaking communities, NASA TV is only broadcasted in English. The scientific importance of Space Shuttle missions, together with the milestone significance and visual impact of Shuttle launches and landings, make those human spaceflight events the perfect occasions to interest general public. Not so long, after the birth of programaespacial.com, both Mariani and Traverso decided to begin a series of broadcasts in Spanish on the website during Shuttle launches and landings. They managed to create an interface where the visitor could watch NASA TV’s live broadcast and at the same time read a board with comments in Spanish so as to be able to understand the activities that were being carried out; the board was updated every 30 seconds in order to keep visitors constantly updated. The feature was named Broadcasting Centre and ever since its creation, it has covered every single shuttle launch and landing. The broadcasts inform visitors about updated launch/landing activities, flight data and mission information such as objectives, crew and schedule. The length of the broadcast depends on the event and the occurrence of delaying issues: normally, launch broadcasts lasted approximately 5 hours and landing broadcasts no more than 2 hours. Landing delays due to weather constraints extend broadcasts about 3 or 4 hours. To increase participation and interest, the Broadcasting Centre includes a live conversation board where visitors can express their opinions or ask questions. One of the most important features the Broadcasting Centre included was the correspondent present at NASA’s Kennedy Space Center in Florida. Martín Juárez collaborated with the broadcasts in numerous occasions by providing an up-close personal look of the entire activities that were being carried on. Mr. Juárez has also attended pre and post launch/landing press conferences and had the possibility to be present at numerous shuttle processing milestones like rollout from OPF to VAB, rollover from VAB to Launch Pad, and RSS retract hours before launch. Each activity he performed in the KSC was further reported in articles. Audio broadcasts Starting with launch of Shuttle mission STS-129 on late November 2009, the Broadcasting Centre began transmitting audio while keeping the traditional text board.

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61st International Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights reserved.

Just as with traditional broadcasts, the speakers are not located in the same place; on the contrary, they transmit from different locations: Mr. Sergio Taleisnik from Cordoba City (Argentina), Mr. Traverso from Buenos Aires (Argentina) and Mr. Mariani from Quilmes City (Argentina) are the main speakers. Speakers communicate throughout a multi-part conference using IP telephony. The dialogue converges into Mr. Taleisnik’s computer, who mixes the conversation, equalizes it, and then uploads it to the Internet utilizing free online broadcasting services. The audio broadcasts implied a more fluent communication with visitors: it transformed the Broadcasting Centre into a very real audio show. The first transmissions consisted basically of reading what the broadcasters were used to post on the broadcasting board, but as time went by, the program evolved into a complete radio show including news reporting, live interviews and discussions about relevant spaceflight topics. The audio broadcast allowed speakers to perform live communication with Juárez who was at the KSC, where he could express more deeply his experience of being in the place where the events were taking place. Together with the main speakers, recent audio transmissions have included permanent participation of science writer/producer, Angela Posada-Swafford. Angela lives in Miami and has uncountable experience with spaceflight. Her participation in the Broadcast Centre has attracted numerous listeners from Latin America, who delight themselves with Angela’s stories and experiences related to human spaceflight. Angela is usually at the KSC for shuttle launches and landings, so she habitually speaks from the spaceport. IV. THE RDH PROJECT Back in July 2008, an interview that Mariani gave to a local newspaper caught the attention of Mr. Luis Hondareyte, a radio broadcaster who had recorded the transmissions in Spanish of NASA’s Mercury, Gemini, Apollo and early STS missions of a radio known as “The Voice of America”. Hondareyte contacted Mariani and offered him those recordings. They both decided to digitalize them in order to save them from potential wearing away which would end in total loss of the recordings. The final objective of the project would be to publish the digitalized work in the Internet so people around the world could gain access to it. The project was named “Reconstrucción Digital Hondareyte (RDH)” or “Hondareyte Digital Reconstruction,” after Luis who was the one to record the original tapes.

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In order to perform the digitalization, Mariani contacted his friend Mr. Damián Ferroso, who had lots of experience on digital and analog recordings and was a space enthusiast as well. Ferroso was in charge of the entire digitalization process. First he had to find an apparatus that could process the old tapes; then he had to ensure that playing the tapes would not destroy the originals, and finally he had to find the way to bond the playing appliance to a digital recording device and perform the digitalization. After digitalizing, Mr. Osvaldo Pulqui, a professional speaker and friend of both Mariani and Ferroso, was contacted to record a series of messages that would be included in each recording. The project received the collaboration of Mr. Jorge Cartès, one of the official designers of Space Shuttle and International Space Station Expedition patches and active member of programaespacial.com who accepted the community’s proposal of him designing an official RDH patch. His design was performed and improved with feedback provided by programaespacial.com members on the site’s forum. In a first stage of complete digitalization, the RDH project opened its website www.proyectordh.com, through which visitors can listen to different recordings every week. V. NEWS ARTICLES AND BLOGS On a daily basis, website story editors upload spaceflight articles. The articles are mainly pieces of news related to rocket launches and human spaceflight missions. However, there are also articles about spaceflight history, astronomy and technology. The articles are product of individual research and production carried out by each editor. Internet is the main source of information: official websites of the space agencies and specialized spaceflight websites are the preferred sources. NASA TV is also quoted in articles about broadcasted events. In order to make the articles interesting for a considerable group of potential readers, the information acquired has to be translated and processed. The sources are never found written in Spanish and the level of complexity is usually excessively high for notfamiliarized public. Programaespacial.com was not envisioned as a mere spaceflight news website in Spanish: its objective has always been to educate and promote spaceflight to Spanish-speaking communities and in order to achieve that goal, the information must be adapted to match a more popular level of knowledge. Programaespacial.com articles also include special reports and interviews carried out by members of the

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61st International Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights reserved.

project. Last year, Mr. Mariani and Mr. Traverso interviewed in the US Embassy of Buenos Aires NASA astronaut Chris Cassidy during an official visit to Argentina. Articles published in programaespacial.com are quoted in several Internet Blogs and forums. Last year an article published by Mr. Taleisnik regarding the upcoming Ares I-X flight was quoted in an article Ms. Posada-Swafford wrote for Spanish science magazine Muy Interesante. Apart from the news articles, programaespacial.com also features a special blog section where Claudio Mariani, administrator of programaespacial.com and space memorabilia collector, Angela Posada-Swafford, science writer/producer, and Martín Juárez, correspondent at the KSC, write about their experiences. This section contributes to the project with subjective information: personal experiences of people somehow involved with space science motivate, inspire and are of extreme interest to the general public.

VI. ONLINE FORUMS The explosive expansion of the Internet since the mid-1990s has fostered the proliferation of virtual communities. In this context, programaespacial.com built its own via an online forum which was called “Comunidad Espacial” (Space Community). Just as any standard Internet forum, Comunidad Espacial helps members of the community and regular visitors to communicate by allowing them to express their opinions, exchange their knowledge and ask questions. The forum also acts in an integration zone, where incoming visitors can introduce themselves so as to begin participating both in the forum and in the entire project.

The biggest challenge that the project will be facing in the forthcoming years is to achieve a higher level of organizational sustainability. The path to achieve this is to expand and ensure the continuity of its volunteer crew, fortify their commitment with the project, and carry out new educational features. Funding is a key factor in sustainability. Nowadays, the project is supported solely on Mr. Mariani’s contributions. Unfortunately, finding support on such project is extremely hard, taking into account the low popularity of space science on the general public and the relative importance of this subject in local politics. Nevertheless, demonstrating the importance of informal science education and acquiring external support to enhance further activities is one of the challenges the project will be facing the next years. Considering the imminent retirement of the Space Shuttle, one of the challenges will be to create new shows on the Broadcast Centre in order to replace the frequent Shuttle launch and landing broadcasts. Several ideas are already being considered, such as weekly radio programs including news, interviews and live debate. On the other hand, just as the transition from text to audio transmissions was the big first step, the upcoming challenge will be to broadcast audio and video together, transforming the Broadcast Centre into a true Space TV in Spanish. Finally, one of the ultimate goals of the project is to engage into formal education by means of habitual visits to academic institutions. As schools have regular science education, the objective of the project will be to inspire the younglings throughout interactive presentations. The main target will be elementary schools, but plans also include kindergarten and high schools. Mr. Mariani already did several presentations in schools of Buenos Aires.

VII. FUTURE. CHALLENGES AND OPPORTUNITIES

More than four years have passed since the beginning of the project. A lot of work has been done and still much is left to do. The project has evolved just as space exploration did, and both will continue to do so. In the next 12 months the Space Shuttle will be retiring. In the upcoming years the International Space Station will be engaging into full international scientific capability, China will be facing new human space challenges, commercial entrepreneurs will see themselves proving their competence and developing space agencies will enhance their own space exploration ventures.

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Electrochemical analysis of peptidefunctionalized titanium dental implant surfaces D.Rodríguez1,2,3, P.Sevilla2,3, G.Vidal1, F.J.Gil2,3 [email protected] 1E.U.

Enginyeria Tècnica Industrial de Barcelona (EUETIB), Technical University of Catalonia (UPC); C.Urgell,187, 08036-Barcelona (Spain). 2Center for Research in NanoEngineering (CRnE), UPC. 3Biomaterials, Biomechanics and Tissue Engineering Group (BIBITE), UPC.

Objective

Methods

Analysis of the functionalization of titanium surfaces with peptides1 is not immediate. This study compares standard analysis techniques with electrochemical techniques. Peptide

Clean titanium surfaces were plasma-activated and silanized with APTES (Ti+APTES samples). Silanized samples were immersed in peptide solution (4Morpholineethanesulfonic acid (MES) buffer, pH 6.0, with EDC NHS) overnight at room temperature. GGRGDSGG peptides (RGD: cell adhesion motive)2 linked to the silane through the carboxilate group. Clean titanium samples (Ti samples) and Ti samples with adsorbed RGD peptide (Ti+RGD samples) were as used as control group. Replicas were made to allow statistical analysis of data. Samples were characterized with XPS, ToF-SIMS, contact angle and FTIR-DR techniques. Electrochemical characterization of the samples was done with a ParStat 2273 potentiostat (medium: HBSS at 37ºC; reference electrode: KCl electrode; counterelectrode: graphite bar). Tests included a free potential measurement, cyclic voltammetry and Electrochemical Impedance Spectroscopy (EIS) with a sweeping range of 64kHz-2mHz and a signal of 50mV.

N-Terminus

Amide bond H

3-aminopropyl triethoxy silane (APTES). EtO EtO

Si

N O

H

C

H

H

C

H

C

H

C

H

H

C

H

H

N Si

H

EtO

Titanium functionalized with APTES silane + peptide.

TiO2

Surface chemical analysis

Results

Cyclic voltammetry

Chemical composition of the samples (at%).

Results of XPS, contact angle (not shown) and ToF-SIMS (not shown) confirm that the samples were silanized and functionalized as expected, detecting presence of silane and silane+peptide, respectively, as shown for XPS (presence of Si on silanized samples and N on functionalized samples).

FTIR

Measured values of free potential showed some differences between samples (Ti: 0.102V, Ti+APTES: -0.141V, Ti+APTES+RGD: -0.168V). Cyclic voltammetry showed significant differences in current intensity for the Ti+APTES+RGD samples compared to other samples.

EIS

FTIR was used to analyze the presence of silane and peptide on the titanium surface. The FT-IR spectra indicate the presence of the covalent bonding of the silane. No clear peak related to the presence of the peptide in the Ti+APTES+peptide sample was detected.

Conclusions The presence of APTES silanes and peptides such as RGD sequences on the surface of titanium can be detected and studied with electrochemical measurements.

The EIS models assume the presence of a TiO2 layer (Rb, Qb), and a double layer of silane (Rp, Qp) and peptide (Rs, Qs). The best-fitting parameters present significant differences between functionalised samples and the rest of the samples.

References 1 Chollet C. et al, Biomaterials. 2009; 30:711-20. 2 Ruoslahti E. et al, Annu Rev Cell Dev Biol. 1996;12:697-715.

Acknowledgements The authors would like to thank prof. Carlos Aleman and Dr. Elaine Armelin (CRnE, UPC) for the use of the electrochemical equipment. Supported by the Spanish Ministry of Science and Innovation through Project MAT2008-06887-C03.

Detailed Study of the Rotor-Stator Interaction Phenomenon in a Moving Cascade of Airfoils Alfred Fontanals1, Miguel Coussirat2, Alfredo Guardo2 and Eduard Egusquiza2 1

Fluid Mechanics Department. EUETIB. Universitat Politècnica de Catalunya. Compte d’Urgell 187, Barcelona, 08036, Spain, [email protected] 2 Centre de Diagnòstic Industrial i Fluidodinàmica. Universitat Politècnica de Catalunya. Av. Diagonal 647, Pab. D+1, Barcelona, 08028, Spain, [email protected], [email protected], [email protected]

Abstract In turbomachinery the Rotor-Stator Interaction (RSI) is an important phenomenon that has a strong influence on the machine behavior. These interactions can have a significant impact on the vibrational and acoustical characteristics of the machine. Unsteadiness and turbulence play a fundamental role in complex flow structure and the use of Computational Fluid Dynamics (CFD) is becoming a usual requirement in design in turbomachinery due to the difficulties and high cost of the necessary experiments needed to identify RSI phenomena. The flow inside a turbomachinery working under design condition is complex but apparently, when working under off-design conditions, it becomes more complex due to the boundary layer separation phenomena. Therefore, the choice of an appropriate turbulence model is far from trivial and a suitable turbulence modeling plays a very important role for successful CFD results. In this work the RSI generated between a moving cascade of blades and fixed flat plate located downstream were studied by means of CFD modeling and compared against experimental results. Design and off-design conditions were modeled and a detailed comparison between them has been made. To analyze in detail the flow pattern, mean velocities in the boundary layer were obtained and compared against experimental results. Furthermore, results concerning to turbulence intensity were compared against an experimental database. It was observed that for each operating condition, the flow in the cascade show special features. For flow inside the turbomachine under design conditions there is no separation, the wake is thin and the characteristic length of the eddies is small. For off-design conditions, there is a large separation and the wake is thick with large eddies. The results obtained can be used to obtain a deeper insight into the RSI phenomena. Keywords: Turbomachinery, Rotor Stator Interaction, Computational Fluid Dynamics, Turbulence

1. Introduction Due to its complexity, the blade and vane design in turbomachinery was currently based on the assumption that the flow in both the impeller and the diffuser is turbulent but steady. The steadiness however implies that the radial gap between impeller discharge and diffuser inlet is large so that no flow unsteadiness of any kind due to blade row interaction would occur [1,2]. However if the rows are closely spaced, there may be a strong interaction that influences both the aerodynamics and structural performance of blades and vanes. In some cases, this has led to vane failure. This phenomenon is called rotor-stator interaction (RSI). Nowadays, computational fluid dynamics (CFD) is broadly used to help the design of turbomachinery and it is frequently used to perform computations to solve RSI problems. From the viewpoint of the numerical prediction of RSI phenomena, it is not an easy task to model this type of flow due to its complexity. The geometrical complexity of the impeller and the diffuser, the turbulence of the flow and the unsteadiness phenomena play an important role in the RSI phenomena. The RSI may be divided into two different mechanisms: potential flow interaction and wake interaction [3,4]. If the impeller and the diffuser are closely spaced, both mechanisms will occur simultaneously. Potential interaction strongly depends on the machine’s geometry and the relative movement between fixed and moving parts. In spite of that complexity, a theoretical analysis is possible, allowing to compute its influence by means of a mathematical expression. Rodriguez et al. [5] carried out a theoretical analysis to predict and explain in a qualitative way the frequencies and amplitudes of the potential interaction in turbomachinery. The theoretical analysis incorporates the number of blades, the number of guide vanes, the RSI non-uniform fluid force and the © Eduard Egusquiza, 2010. Published in Engineering Conferences Online (ECO): http://eco.pepublishing.com DOI: to be inserted by the publisher

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DOI: to be inserted by the publisher sequence of interaction. This analysis gives a resulting force over the turbomachine taking into account the frequencies of interaction between blades and the relationship between amplitudes of pressure fluctuations. Wake interaction is related with the wake behind the blades. This wake induces structural vibrations due to the vortex shedding and extends farther downstream. The turbulence cascade process induced by these vortices strongly influences the turbulence state of the flow, enhancing the energy dissipation increasing the rate at which the vortices are dissipated [6]. The wake convects downstream and arrives to the gap between the fixed and moving blades, generating a periodic flow structure as a result of the interaction between the wake and the blades due to their relative movement. These structures are convected and affect the boundary layer of the blades situated downstream of the interaction zone, generating unsteadiness in the structure of its boundary layer [7]. Wake characterization is a subject of main importance in the study of RSI. Experimental results from Tsukamoto et al. [8] show the behavior of pressure fluctuations in a radial pump working in design conditions using semi-conductor type pressure transducers installed in a guide vane, Values of instantaneous pressure were obtained in order to study the interaction between impeller and diffuser vanes. Results obtained show that the pressure at a stationary point in guide vanes fluctuates with the basic frequency of the impeller blade passing frequency and higher harmonics. The maximum values are observed near the leading edge in the suction side of the guide vane. Results also show that the pressure fluctuation can propagate in circumferential direction. Depending on the frequency of the harmonic the propagation direction could be opposite to impeller rotation. Wang et al. [9] did a similar experiment but in off-design working conditions. Results obtained show that the impeller blade passing frequency and its higher harmonics are always dominant in the pressure fluctuations downstream of the impeller for the whole flow range because of the RSI phenomena, and that there exist some lower dominant frequencies in the pressure fluctuation downstream of the impeller for unstable flow range because of the effects of the complex flow structures such as separating flow, rotating stall and reverse flow. These lower dominant frequencies are dependent on the flow rate, and the unsteady pressure fluctuation is chaotic in these unstable flow ranges. Experimental results from Uzol et al. [10] and Chow et al. [11] using particle image velocimetry (PIV) in an axial compressor show that the flow is composed by a lattice of wakes and the resulting wake-wake and wake-blade interactions cause major turbulence and flow non-uniformities, showing that these interactions are dominant contributors to the generation of high deterministic stresses and tangential non-uniformities in the rotor-stator gap near the blades and in the wakes behind them. These non-uniformities in the flow structures have significant effects on the overall performance of the machine. The non-uniformities are mainly composed by localized regions with concentrated mean vorticity and elevated turbulence levels. At this zones the wake is chopped-off by the downstream blades. Due to difference in the mean velocity field, the wake segment on the pressure side of the upstream blades is convected faster than the segment on the suction side (using an absolute frame of reference) creating discontinuities in the stator wake trajectory, causing non-uniformities in the velocity field downstream. These non-uniformities are “hot spots” with concentrated vorticity, high turbulence level and high shear stresses. Although the “hot spots” diffuse as they are convected downstream, they still have an elevated turbulence level compared to the local turbulence levels around them. It is also clear that the turbulence plays a fundamental role in the flow structure. Soranna et al. [1] studied a rotor working downstream of a row of inlet guide vanes. Results show that the wake impingement significantly modifies the wall-parallel velocity component and its gradients along the blade downstream. Due to spatially non-uniform velocity distribution, especially in the suction side, the wake deforms while propagating along the blade, expanding near the leading edge and shrinking near the trailing edge. Turbulence levels here become spatially non-uniform and highly anisotropic. Experiments form Henderson et al. [2] are focused on the influence of the free-stream turbulence on the wake dispersion and boundary layer transition process. Results show that increments in the free-stream turbulence level strongly enhance the dispersion of inlet guide vanes wake. This fact modifies the interaction between stator and rotor wakes, leading to a significant decrease in the periodic unsteadiness experienced by the downstream stator. These observations have important implications for the prediction of the flow behavior in multistage turbomachines. From the viewpoint of numerical simulation, all effects should be taken into account for a suitable modeling of RSI. The first step is the suitable characterization of the boundary layer along the blade and the wake behind the blade. Several authors have attempted to obtain both experimental and numerical results for boundary layers along bodies (e.g. cylinders and blades) and the wake flows behind them, focusing in the wake structure. The most extensively studied case is the wake of cylinders, (see e.g. [1215]), and experimental databases of the wake generated by airfoils are also available in literature (see e.g. Nakayama [16], Wang [17] and Ausoni [18]). The second step is the characterization of the wake interaction between moving and fixed blades. Experimental data of the wake related with RSI in real turbomachines is more difficult to obtain. The main problem in these cases is to know the setup geometry details (see e.g. [3][4][10] and [11]), but some interesting results in linear cascade of moving cylinders and airfoils are available in the literature (see e.g. [7-19]). In this work the database from Gete and Evans [19] experiment was selected in order to obtain results for this characterization step. After these steps, application for design of real turbomachines follows. The main goal of this work is to obtain reliable and detailed numerical results that complement the experimental data on the RSI in a multistage turbomachine. Suitable CFD modeling is critical for understanding the physical mechanism of the RSI and its consequences in the turbomachine performance. This evaluation will contribute to the better understanding of this phenomenon. Results obtained are directly applied to the turbomachinery RSI modeling

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2. Numerical Modeling 2.1. Geometry and Grid Generation A turbomachinery stage was represented with a two-dimensional rotating rig and a flat plate arrangement in a wind tunnel (fig. 1). The moving mechanism comprises seven NACA0024 airfoils (rotor blades), with chord length of 50 mm an exit angle of 57.7° relative to input free-stream flow velocity, attached to rotating synchronized gear belts thereby generating travelling periodic wake disturbance in the oncoming air-flow. The blade spacing is s = 0.1 m and the distance between the trailing edge airfoil and the leading edge of flat plate is 40 mm. The flat plate (stator) has a 0.8 mm diameter trip wire located 20 mm from the leading edge. Complete details of experiments have been reported in [19]. The model consists in two parts: the moving rotor blades and the stator plate. Unstructured meshing technique is adopted establishing sliding mesh configuration as the analysis is unsteady as per CFD code [20].

 velocity 

Ur

periodic periodic

rotor  velocity 

rotor rotorblade foil

inlet  velocity  inlet 

wall wall

U 0  U 0 

y s

plate x 

pressure  outlet 

interface wall periodic  Fig. 1. General drawing of the setup and boundary condition imposed in the numerical modeling For evaluating the mesh sensitivity three 2D grids were used. The boundary layer around the rotor blades and the stator plate was modeled using the two layer model scheme, with a y+ = 1 (table 1). Table 1. Mesh sensitivity test mesh 1 2 3

Steady cells 9E+04 1E+05 1.3E+05

+

y 1 1 1

Unsteady cells y+ 2.9E+05 1 3.2E+05 1 4.3E+05 1

2.2. Unsteady Calculations Setup Two-dimensional, unsteady Reynolds-averaged Navier-Stokes equations were solved by means of a commercial CFD code [20]. To obtain the boundary layer-mean velocity, a constant velocity of U0 = 3 m/s was applied at the inlet, and a rotor transverse velocity Ur = 2, 3 and 4 m/s are employed. A non-slip condition was specified for the flow at the wall boundaries of the rotor blade, the stator plate and the wind-tunnel walls. A periodic condition was applied to the rotor fluid and to the external wind tunnel fluid, and a static pressure condition was imposed at the outlet of the stator plate. The ratio of the transverse rotor blades speed to the blade spacing determines the frequency of the wake disturbances passing in front the stator flat-plate, f = Ur/s. For the rotor transverse velocity Ur = 2, 3 and 4 m/s, the corresponding frequencies are f = 20, 30 and 40 Hz. The turbulence was modeled using the SST k-ω model, since it is a good option due to its accurate performance both in boundary layer and as in wake flow modeling [21]. Experimental turbulence intensity of 0.07% was applied at the inlet velocity boundary condition. The unsteady formulation used was a second-order implicit velocity formulation, and a pressurebased solver was chosen. The SIMPLE pressure-velocity coupling algorithm was used, and second order scheme discretization was selected for the numerical experiments. The interface between the rotor and the stator plate was set to a sliding mesh, in which the relative position between the rotor and the stator is updated every time step. The time step was set to 1 x 10-4 s. The maximum number of iterations for each time step was set to 40 in order to reduce all computed residuals under 1 x 10-5. Due to the unsteady nature of the flow, it is required that the whole flow domain is affected by the unsteady fluctuations. In order to check the aforementioned, a flow rate monitor was recorded at the domain outlet. Pseudo-steady flow behavior was reached after 40 and 80 25th IAHR Symposium on Hydraulic Machinery and Systems, September 20-24, 2010, Timisoara, Romania

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DOI: to be inserted by the publisher cycles of the rotor blades for f = 20 and 40 Hz respectively, due to the length of the stator plate. Boundary layer velocities vs. time were recorded for different locations of the stator plate (x = 0.1, 0.3, 0.5 and 0.7 m). 2.3. Validation of the model A comparison between the numerical results obtained for the boundary layer velocities and the experimental results obtained by Gete and Evans [19] was established. A steady state analysis without the effect of the moving rotor blade (f = 0 Hz) and an unsteady state analysis considering the RSI at different rotor frequencies (f = 20, 30 and 40 Hz) was developed. Figures 2 to 5 show the comparison between numerical and experimental results. The validation shows a close agreement between the implemented numerical model and the experimental results. Naca0024-placa-f=00Hz, model=g, x=03

Naca0024-placa-f=00Hz, model=g, x=05

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Fig. 4. Unsteady turbulent velocity in the boundary layer on stator plate (f=30Hz) at x=0.1, 0.3, 0.5, 0.7 m N a ca 0 02 4 -p la ca -f=4 0 H z, m o de l=g , x=0 3

N a ca 0 02 4 -p la ca -f=4 0 H z, m o de l=g , x=0 5

N aca0024-placa-f=40H z, m odel=g, x=07

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Fig. 5. Unsteady turbulent velocity in the boundary layer on stator plate (f=40Hz) at x=0.1, 0.3, 0.5, 0.7 m. 25th IAHR Symposium on Hydraulic Machinery and Systems, September 20-24, 2010, Timisoara, Romania

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3. Results and Discussion 3.1. Velocity profiles As a first approach, a steady state analysis without RSI effects generated by the rotor blade (f = 0 Hz) was performed. Figure 2 shows the results for steady turbulent boundary-layer velocity profiles for several meshes and different longitudinal locations on the stator plate. At the first monitor point (x = 0.1 m from the plate’s edge) the experimental boundary-layer is a transitional flow type (Rex = 2 x 104), and the SST k-ω turbulent model predict a fully developed flow. This is because the eddy viscosity models (EVM) do not capture adequately the transitional flow. Downstream a fully developed flow is present and the results are in good agreement with experiments. For the RSI unsteady cases (f = 20 and 30 Hz), the numerical boundary-layer mean velocity profiles are in good agreement with experimental data (figs. 3, 4), and it can be observed that the obtained results are mesh-dependant. For f = 40 Hz, there is a good fit between the experimental and the numerical data in the logarithmic zone of the boundary layer velocity profiles, while in the transition sub-layer, between the logarithmic and the viscous layers, velocity values are underestimated. In this case, the obtained results do not show mesh dependency. 3.2. Turbulence intensity For all the unsteady state cases modeled a numerical study of the flow behavior in the wake generated by the moving rotor blades and its corresponding interaction with the stator plate by means of modeling the flow turbulence intensity on the wake. Figure 6 shows that the behavior pattern of the wake is different for each rotor blade frequency analyzed. It can be observed that the turbulence intensity pattern is related to the relative velocity at the rotor blade outlet, reaching near-design operating conditions at f = 40 Hz. For f = 20 and 30 Hz, the turbulence intensity patterns clearly show off-design operating conditions. For off-design operating conditions there is boundary layer flow separation, and the wake presents a vortex shedding with large eddies. For design conditions the wake is thin and the characteristic length of eddies is smaller than for off-design conditions.

Fig. 6. Computed turbulence intensity contour fields at several frequencies (f = 20, 30 and 40 Hz) Computed values for turbulence intensity are shown in Table 2. It can be seen that for off-design conditions (f = 20 and 30 Hz) computed values are higher than those experimentally obtained, and that for design conditions (f = 40 Hz), computed values are lower than the experimental results. Previous work developed by our research group [21] showed that EVM are not able to capture coherent fluctuations in the lift coefficient for thin trailing edge foils and for a small angle of attack. Table 2. Turbulence intensity values at x = 0.1 m Turbulence intensity I (%) at x = 0.1 m f (Hz) Experimental [19] Numerical 0 0.7 1 20 4 10 30 6 8 40 8 4 In order to check the influence of the wake over RSI vortex shedding, a simulation set of the moving rotor blade without the stator plate was performed. Figure 7 shows the computed vortex shedding at off-design operating conditions (f = 20 Hz) with and without considering the potential flow interaction effects generated by the stator plate. Under the aforementioned operating conditions, it can be observed for both situations that the characteristic length of the eddies is similar, and that this length is close to the characteristic length of the wind tunnel (equal to the distance between the stator plate and the tunnel´s wall). Under these operating conditions the flow pattern is influenced by the boundary conditions of the system, and the computed values for the turbulence intensity are overestimated if compared with the experimental results reported by Gete and Evans [19].

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f = 20Hz Fig. 7. Computed turbulence intensity with RSI effect and without stator plate (f = 20Hz) For near-design operating conditions (f = 40 Hz), figure 8 compares the computed vortex shedding with and without considering the potential flow interaction effects generated by the stator plate. For this situation it can be seen that in absence of the stator plate, the computed wake does not show fluctuations. The potential flow interaction effects are visible when the stator plate is included in the geometrical model. The movement of the rotor blade in front of the stator plate induces the onset of the vortex shedding. At this frequency, the used EVM is only able to capture the potential flow interaction effects. The incapability of the tested EVM in capturing the vortex shedding due to wake interaction effects leads to an underestimation of the turbulence intensity levels when compared to the experimental data. In this case, the non-accurate estimation of the turbulence intensity may justify the velocity underestimation in the transition sub-layer of the boundary layer shown in figure 5.

f = 40Hz Fig. 8. Computed turbulence intensity with RSI effect and without stator plate (f=40Hz)

4. Conclusions CFD has been applied to the study of RSI. A turbomachinery stage was represented with a two-dimensional rotating rig and a flat plate arrangement in a wind tunnel .Boundary layer mean velocities at various distances from the leading edge and the turbulence intensity over the stator plate were computed. A comparison between the numerical results obtained for the boundary layer velocities and the experimental results obtained by Gete and Evans [19] was established. A steady state analysis without the effect of the moving rotor blade (f = 0 Hz) and an unsteady state analysis considering the RSI at different rotor frequencies (f = 20, 30 and 40 Hz) was developed. For near-design conditions (f = 40 Hz) there is no flow detachment on the blade, and the vortex shedding flow pattern is thin and with small eddies. Under these operating conditions, computed turbulence intensity is underestimated when compared to the experimental results. For off-design conditions (f = 20 and 30 Hz), there is flow detachment in the blade, and the vortex shedding flow pattern is wide and with large eddies. Under these operating conditions, computed turbulence intensity is overestimated when compared to the experimental results. It was possible to corroborate that EVM are not able to model coherent fluctuations in the lift coefficient for thin trailing edge foils and small attack angles when the potential flow interaction effects are not present in the computed model, as previously shown in [21]. When the potential flow interaction effect is present in the computed model, EVM are capable of modeling the vortex shedding.

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DOI: to be inserted by the publisher Results obtained in this work lead to the conclusion that the choosing of a suitable EVM for modeling the RSI phenomena is strongly dependent on the operating conditions in the cascade blades (design/off-design) due to the characteristic flow features for each case. The flow structures in each case present challenges of different nature, and a suitable EVM for modeling a off-design flow condition is not good enough to model another case. The results obtained can be used to obtain a deeper insight into the RSI phenomena.

Acknowledgements Funding from the Spanish Ministry of Science and Innovation (Grant No. DPI 2009 – 12827) is appreciated. A travel & congress registration grant from EUETIB – UPC for A. Fontanals is also acknowledged.

Nomenclature C dt f I L Re s u

Chord of foil [m] computational time step [s] Frequency [Hz] Turbulence intensity (=u’/Uref) Characteristic length [m] Reynolds number (=U0L/) Blade spacing [m] Velocity [m/s]

Uref, Uo Ur u’ x, y y+ δ ν

Free-stream flow velocity [m/s] Rotor velocity [m/s] Turbulent velocity fluctuations [m/s] Coordinates Non-dimensional wall distance Boundary layer thickness [m] Dynamic viscosity [m2/s]

References [1] Soranna, F., Chow, Y., Uzol, O. and Katz, J., 2006, “The effect of inlet guide vanes wake impingement on the flo w structure and turbulence around a rotor blade”, J. of Turbomachinery, No. 128, pp. 82-95. [2] Henderson, A., Walker, G. and Hughes, J., 2006, “The influence of turbulence on wake dispersion and blade row in teraction in an axial compressor”, J. of Turbomachinery, No. 128, pp. 150-165. [3] Dring, Joslyn, Hardin and Wagner, H., 1982, “Turbine Rotor-Stator Interaction”, J. Eng. for Power, No. 104, pp. 72 9-742. [4] Ardnt, Acosta, Brennen and Caughey, 1989, “Rotor-Stator Interaction in a Diffuser Pump”, Journal of Turbomachinery, No. 111(3), pp. 213-221. [5] Rodriguez, C., Egusquiza, E., and Santos, I., 2007, “Frequencies in the Vibration Induced by the Rotor Stator Interaction in a Centrifugal Pump Turbine”, Journal of Fluids Engineering, No. 129, pp. 1428-1435. [6] Coussirat, M. G., 2003, “Theoretical/Numeric Study of flows with strong Streamlines Curvature”, Ph. D. Thesis, Department of Fluids Mechanics, UPC, Barcelona. [7] Holland, R. and Evans, R., 1996, “The effect of periodic wake structures on turbulent boundary layers”, Journal of Fluids and Structures, No. 10, pp. 269-280. [8] Tsukamoto, H., Uno, M., Hamafuku, N., And Okamura, T., 1995, “Pressure fluctuations downstream of a diffuser pump impeller”, The 2nd Joint ASME/JSME Fluids Engineering Conference, Forum of unsteady flow, FED Vol. 216, pp. 133-138. [9] Wang, H. and Tsukamoto, H., 2003, “Experimental and numerical study of unsteady flow in a diffuser pump at off-design conditions”, J. Fluid Engineering, No. 125, pp. 767-778. [10] Uzol, O., Chow, Y., Katz, J. and Meneveau, C., 2002, “Experimental investigation of unsteady flow field within a two-stage axial turbomachine using particle image velocimeter”, J. of Turbomachinery, Vol. 124 pp. 542-552. [11] Chow, Y., Uzol, O. and Katz J., 2002, “Flow nonuniformities and turbulent “hot spots” due to wake-blade and wa ke-wake interaction in a multi-stage turbomachine”, J. of Turbomachinery, No. 124, pp. 553:563. [12] White, F, 1974, “Viscous fluid flow”, McGraw-Hill, New York. [13] Cantwell, B., and Coles, D., 1983,” An experimental study on entrainment and transport in the turbulent near wake of a circular cylinder”, J. Fluid Mechanic, No. 136, pp. 321-374. [14] Hwang, R., and Yao, C., 1997, “ A numerical study of vortex shedding from a square cylinder with ground effect”, J. of Fluid Eng., No. 119, pp. 512-518. [15] Jordan, S., and Ragab, S., 1997, “A large-Eddy simulation of the near wake of a circular cylinder”, J. of Fluid Eng., No. 120, pp. 243-252. [16] Nakayama, 1985, “Characteristics of the Flow around Conventional and Supercritical Airfoils”, Journal of Fluids Mechanics, No. 160, pp. 155-179. [17] Wang, H., 2004, “An experimental study of bubbly hydrofoil wakes”, MsC Thesis, University of Minnessota. [18] Ausoni, Farhat, and Avellan, 2005, “Fluid-Structure Interaction Induced by Karman Vortices in the Wake of a Truncated 2D Hydrofoil at Fixed Incidence Angle”, Hydrodyna Project report, delivery 3.2 part 1, LHM, Lausanne.

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DOI: to be inserted by the publisher [19] Gete, Z. and Evans, R., 2003, “An experimental investigation of unsteady turbulent wake boundary layer interaction”, Journal of Fluids and Structures No. 17, pp. 43-55. [20] Fluent Inc., Fluent 6.3. User’s guide, 2006. [21] Coussirat, M., Fontanals, A., Grau, J., Guardo, A. and Egusquiza, E. 2008, “CFD study of the boundary layer influence on the wake for turbulent unsteady flow in rotor-stator interaction”. IAHR 4th. Symposium on Hydraulic Machinery and Systems. Foz do Iguassu (Brazil).

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SIDO Buck Converter with Independent Outputs H. Eachempatti (1, 2), S. Ganta (1), J. Silva-Martinez (1) and H. Martínez-García (1)

Analog and Mixed Signal Center Electrical and Computer Engineering Department (ECE) Texas A&M University College Station, TX, 77843-3128, USA [email protected] and [email protected]

(2)

Qualcomm Incorporated 5775, Morehouse Drive, San Diego, CA, 92121, USA [email protected]

Abstract— The portable electronics market is rapidly migrating towards more compact devices requiring multiple high-integrity high-efficiency voltage supplies for empowering the systems. This paper demonstrates a single inductor used in a buck converter with two output voltages from an input battery with voltage of value 3V. The main target is low cross regulation between the two outputs to supply independent load current levels while maintaining desired output voltage values well within a ripple that is set by adaptive hysteresis levels. A reverse current detector to avoid negative current flowing through the inductor prevents possible efficiency degradation.

I.

(3)

(3)

College of Industrial Engineering of Barcelona (EUETIB) Department of Electronics Eng. Technical Univ. of Catalonia (UPC) C/ Comte d’Urgell, 187. 08036 Barcelona, Spain. [email protected]

windows ensures the minimization of cross-regulation that arises from sharing the inductor between the outputs.

INTRODUCTION

The typical buck converter is the most frequently used switching converter in portable applications. Since multiple voltage rails are required on a Power Management IC (PMIC), several such converters are normally used in a device for obtaining different voltage levels. If a PMIC supplies N independent voltage rails, N such converters are required. The costliest and most area consuming component on the board of a SMPS design is the inductor. A solution for this issue is to use a single-inductor serving to multiple outputs [1-8]. A singleinductor dual output (SIDO) buck converter is shown in Fig. 1. Two independent outputs V1 and V2 are obtained from a single inductor L. C1 and C2 are output capacitors that maintain average load voltages V1 and V2 respectively, and provide output current when the inductor is serving the other output. The voltages V1,2upDC, V1,2lowDC, V1,2up(t) and V1,2low(t) are described in the following subsections. For the SIMO buck, T1, T2, … and TN are the time windows for which L is connected to the outputs V1, V2, ... VN, respectively. The timing diagram in Fig. 2 shows the different phases of operation of a SIDO buck converter. The slopes of inductor charge and discharge depend on the output that L is connected to for regulation. Since the inductor is shared, the minimization of cross-regulation is highly desirable to maintain the regulator’s outputs independent of each other for a wide range of load values[1]. The time frames T1 and T2 depend on the load demanded at each output and are adjusted interactively by the feedback dynamic level comparator. The control of the time

Páginas: 719– 722 ISBN: 978-84-95809-75-9

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(a)

(b) Fig. 1 SIDO Buck Converter SMPS and Timing Diagram

SAAEI’10 Bilbao, 7 – 9 de julio de 2010

II.

CONTROL METHODOLOGY

In order to support low cross-regulation across independent load levels and achieve high output voltage accuracy, variable frequency control is chosen, for which hysteresis comparison levels are used. In this paper, the hysteretic control is slighlty different from the conventional one since the dynamic hysteresis levels that contain information about the slopes of the output voltages are used for providing an indication of the voltages and load currents. The first derivative of the voltage indicates the amount of load present at the outputs. For each output, two dynamic levels V1,2up(t) and V1,2low(t) are properly defined and serve as thresholds against which the SIDO buck outputs are compared so that the voltage ripples are limited to within a set percentage of the reference voltages under all loading conditions. Let us define dynamic thresholds V1,2upDC and V1,2lowDC as the bounds for the SIDO buck’s outputs V1,2 as follows: dV (t ) (1) V1,2up (t )  V1,2upDC  K z  1,2 dt dV (t ) (2) V1,2low (t )  V1,2lowDC  K z  1,2 dt The derivative of the output voltages is a measure of inductor current; hence the threshold levels are dynamically adjusted according to IL. The value of the coefficient KZ determines the sensitivity of the dynamic levels. A very large value of KZ causes high swing in the upper and lower dynamic levels and their possible overlap whereas a small value desensitizes the threshold levels to load current variations increasing the output voltage ripple. Confinement of the output voltages to well within the dynamic levels described by (1) and (2) helps achieve the low cross-regulation and well-defined ripple levels Thus, the proposed controller for the SIDO buck converter is able to supply the output at full load as well as the output at stand-by simultaneously without the undesirable drop or rise respectively in voltage levels at either output. A. Ripple Control and Cross Regulation The output voltages V1,2 are limited to the hysteresis bands by comparing them with the dynamic levels to control the switches SP and SN. As shown in Fig. 2, by monitoring the output voltage and its first derivative, the transient response is improved and the ripple is limited around the desired DC value. In Fig. 2, during T1, when L is connected to one of the outputs, SP is activated and its voltage variation is positive due to the current injected by the inductor. Based on the speed of the variation of the output voltage, the dynamic levels (1) and (2) are adjusted; large load current leads to large steps in the dynamic levels. Since IL is positive, the threshold voltage V1,2up decreases thus preventing significant overshoot at the end of the SP phase even in the presence of control circuit delays. In the following SN phase one of the inductor terminals is grounded but continues to serve the output if L is sized sufficiently to support the total DC load. During T2, the output voltage discharge at a rate given by I1,2/C1,2. This causes a step increase in the dynamic levels, making it move closer to the output voltage profile. When switching from one output to the other, switch S1 is closed if the voltage V1 discharges to below V1low(t); i.e. S1 and S2 are

controlled by load levels in the outputs. This guarantees that V1 and V2 stay well within the static bounds V1,2upDC and V1,2lowDC. Thus the ripples of the output voltages are stronger functions of the static levels V1,2upDC and V1,2lowDC than of the external LC tank. This is an advantage from form factor reduction point of view. Lower ripple requires closer static bounds, and also increases the frequency with which S1, S2, Sp, Sn switch. SP O N

SN O N

L ser vin g th e oth er ou tp u t v 1 ,2 u p (t)

V 1 ,2 u p D C v 1 ,2 (t) V ref1 ,2

C 1 ,2 d isch a rg in g p h a se

M a in d ecision s ta k en v 1 ,2 lo w (t)

V 1 ,2 lo w D C

t T1

T2

Fig. 2 SIDO’s steady state output and corresponding dynamic thresholds

The time periods T1 and T2 are adjusted by the controller such that the average loads at both outputs are delivered over one time period when the converter is in steady state; i.e. T2

T1



 i (t)dt

il (t)dt

0

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 i2average

.

(3)

The issue of cross-regulation arises from the sharing of boundary conditions of inductor current between the output branches. This causes coupling between the sub converters. If the inductor were to discharge to a state of zero current at the end of every time window, then independent load supply can be achieved at each output without undesirable rise or fall in voltage [2-5]. However, the disadvantage of operating the SIDO buck converter in this mode of discontinuous conduction for all load conditions is the rise in peak currents flowing through the inductor, increasing current stress of the switches and conduction losses as well as loss in system efficiency due to the full charge and discharge of additional parasitic capacitors. To decrease the peak inductor currents the inductor might be reset to a constant value Idc instead of zero [6-7]. A variation of this technique is to reset the inductor to different current values that are dependent on the individual loads. This technique requires an additional low-resistance switch across the inductor. Further, current sensing circuits that are sensitive to high frequency noise are required. Control methodologies like Adaptive Delta Modulation [8] and Ordered Power Distributive Control [9] use digital algorithms and analog signal processing circuits to control the voltages. These solutions have a fixed frequency of operation which leads to the inability of the SIMO SMPS to regulate with widely varied load ranges at both outputs. In order to meet the average load current requirements of both the outputs as set by (3), the time multiplexing of L is controlled. In Fig. 2, when S1,2 is ON during T1, C2,1 discharges, causing V2,1 to droop with a rate that is directly proportional to its load current. This causes an upward shift in V2,1low(t). At time

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T2 when V1 hits V2,1low(t), S2,1 is turned ON. Thus the output with higher load current takes priority since the transient voltage profile is monitored. L is connected for a longer time to the output with the higher load. The absence of any averaging circuits or any form of linear compensation leads to better transient performance. B. Architecture and control flow Fig. 3 shows the topology and control architecture of the SIDO buck converter. Eqns. (1) and (2) are implemented using analog differentiators with DC offsets. V1 and V2 are compared with the dynamic levels to generate the control signals for switches Sp and Sn. S1 and S2 are controlled by the comparator that sets the priority of the outputs based on the voltage error and load current, accordingly setting the flag ‘M’ to 1 or 0. The delays due to the control gates, drivers and comparators are negligible compared to the output time constant, leading to very small control loop delay. Reverse current is detected by monitoring voltages across S1 and S2 in order to avoid the flow of negative current flowing through the inductor. When the reverse current detector ‘R’ is high the switches Sn and Saux are closed while all the other switches are immediately turned OFF. This switching action grounds the inductor terminals avoiding negative current flow through it and also prevents efficiency degradation. The flowchart shown in Fig. 4 summarizes the sequence of operations implemented by the digital controller. The flag ‘M’ and reverse current flag ‘R’ control the states of S1 and S2. Once the inductor is connected to a particular output, the voltage is compared to corresponding dynamic levels for manipulation of Sp and Sn. Sp is switched ON when the voltage goes below the lower dynamic threshold and Sn is turned ON when the voltage overshoots the upper dynamic threshold. Thus the two loops that control the battery side and load side switches work independently of each other, except in the event of reverse current flow. This independent operation guarantees the reliable operation of the control system.

1.26 KZ

dV1t  dt V1 t 

1.14  KZ

dV1t  dt

V2 V 2low

V1 V 1low

V2 V 2low

V1 V 1low

V2 V 2up

V1 V1up

Fig. 4 Logical flow of operations in the SIDO Buck Converter

III.

SCHEMATIC SIMULATION RESULTS

In this work, the value of L is1 µH and the values of the output capacitors are 4.7 µF each. The controller was designed and simulated at transistor level using a conventional 0.5 m CMOS technology. When maximum load i.e. 300 mA is present at each output, the transient response of V1(t) and V2(t) with corresponding dynamic levels are shown in Fig. 5. Overshoots or undershoots about their designated static bounds are due to the response time of the loop control.

V1 t 

V2 t 

Fig. 5 Steady state V1 and V2 when load currents I1 = I2 = 300 mA

V2 t 

In Fig. 6, the load at V1(t) is 10 mA and at V2(t) is 300 mA. In Fig. 6, S1 stays ON for just as long as the capacitor C1 gets charged to over V1low. Once the light load output V1 is charged over its acceptable lower limit, the inductor is immediately connected to the output V2 with the heavier load, and the switches Sp and Sn continue to get manipulated according to the corresponding dynamic thresholds. Hence both outputs with widely varied load values are served by the inductor thus minimizing the cross regulation. Fig. 7 shows two families of curves; the dotted traces correspond to the output voltages with dynamic hysteresis, while the solid lines indicate the outputs with static hysteresis. When dynamic levels are used the voltage ripple is better controlled

dV2 t  dt

V1t 1.2

1.425 KZ

dV2 t  dt

V2t 1.5

1.575  K Z

V2 t  V1 t 

Fig. 3 SIDO system Overview

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within the permissible bounds. In the case of static hysteresis, a large output capacitor would be required to effectively control this ripple value.

depicted in Fig. 9 that is desirable in many applications in order to optimize the performance of the power supple over a wide range of loads. Higher values of efficiency are achievable with more sophisticated technology, leading to lower switch on-state resistance and parasitic capacitances.

Fig. 6 Steady state V1 and V2 when load currents I1 = 10mA and I2 = 300 mA

Fig. 9 Overall efficiency vs. total load current I1 and I2

IV.

Fig. 7 Comparison of static and dynamic hysteresis

Fig. 8 shows the load regulation of V1(t) and cross regulation of V2(t). There is a step increase in the load current I1 from 10mA to 300mA leading to an increase in the ripple frequency of V1(t) at the instant of the load step. This increase in the switching frequency ensures that the sudden load step is handled by the inductor energy, thus helping the loop recover without any undesirable dips in the output voltages.

CONCLUSION

A single-inductor two-output switching regulator with low cross regulation, high accuracy and 2.5% ripple limits has been described. These achievements are a result of the proposed non linear hysteresis control applied to the SIDO buck converter leading to superior transient performance and disturbance rejection. Using the proposed dynamic hysteresis control methodology, constant efficiency values at different load combinations that is essential for optimum performance is achieved. The downside of the proposed method is the variation of the operating frequency with load. Nevertheless, the switching frequency can be controlled to stay within a band of acceptable frequencies by tuning the width of the hysteresis loop using an auxiliary feedback loop. The principles presented in this paper can be extended to a multiple output (n>2) buck converter. REFERENCES

Fig. 8 Time response of V1 and V2  to load step from 10 mA to 300 mA.

Hence from simulations it can be concluded that frequency of operation is “adaptive” leading to faster switching during high loads and slower switching during low loads, leading to improved efficiency. Also, the problem of negative inductor current during discontinuous conduction mode is solved, with no ringing transients, due to the presence of the auxiliary switch. The efficiency of the system is almost constant over the individual load ranges. This leads to an almost flat efficiency-load curve as

[1] Wing-Hung Ki and Dongsheng Ma, “Single-Inductor Multiple-Output Switching Converters”, in Proc. IEEE PESC, Vol. 1, pp. 226-231, June 2001. [2] Dongsheng Ma, Wing-Hung Ki, Chi-Ying Tsui, and Philip K.T. Mok, “A Single Inductor Dual-Output Integrated DC/DC Boost Converter for Variable Voltage Scheduling”, in Proc. of the 2001 Conference on Asia South Pacific Design Automation, pp. 19-20, 2001. [3] Massimiliano Belloni, Edoardo Bonizzoni, and Franco Maloberti, “On the Design of a Single-Inductor-Multiple-Output DC-DC Buck Converters” in Proc. Of 2008 International Symposium on Circuit and Systems, Vol. 3, pp. 3049-3052, May 2008. [4] Dongwon Kwon, and Gabriel A. Rincón-Mora, “Single-Inductor–MultipleOutput Switching DC–DC Converters” in IEEE Transactions on Circuits and Systems-II: Express Briefs, Vol. 56, Nº. 8, Aug. 2009. [5] Suet-Chui Koon, Yat-Hei Lam, and Wing-Hung Ki, “Integrated Charge Control Single-Inductor Step-Up/Step-Down Converter” in International Symposium on Circuit and Systems, Vol. 4, pp. 3071-3074, May 2005. [6] Ming-Hsin Kuang, Ke-Horng Chen, “Single Inductor dual-output (SIDO) DCDC converters for minimized cross regulation and high efficiency in soc supplying systems”, in IEEE International Midwest Symposium on Cicuits & Systems, Vol.1, pp. 550-553, 2007. [7] Anmol Sharma, and Shanti Pavan, “A single inductor multiple output converter with adaptive delta current mode control” in Proc of IEEE International Symposium on Circuit and Systems, pp. 5643-5646, 2006. [8] Hanh-Phuc Le, Chang-Seok Chae, Kwang-Chan Lee, Se-Won Wang, Gyu-Ha Cho, and Gyu-Hyeong Cho, “A single-inductor switching DC-DC converter with five outputs and ordered power-distributive control” in IEEE Journal of SolidState Circuits, Vol. 42, Issue 12, pp. 2706-2714, December 2007.

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Current-Steering Switching Policy for a SIDO Linear-Assisted Hysteretic DC/DC Converter Herminio Martínez (1), Jose Silva-Martínez (2), Eduard Alarcón (3) and Alberto Poveda (3) (1)

College of Industrial Engineering of Barcelona (EUETIB) Department of Electronics Eng. Technical Univ. of Catalonia (UPC) C/ Comte d’Urgell, 187. 08036 Barcelona. SPAIN [email protected]

(2)

Analog and Mixed Signal Center Electrical and Computer Engineering Department (ECE) Texas A&M University College Station, TX, 77843-3128, USA [email protected]

Abstract— This paper proposes the use of linear-assisted switching power converters in the context of single-inductor dual-output (SIDO) applications. By combining a DC/DC ripplecontrolled switching power converter with the respective voltage linear regulators at each output, improved performance in terms of load and line regulations is obtained. To achieve that aim, a current-steering switching policy is proposed, together with a resource-aware circuit implementation. The ripple-based hysteretic control results in variable switching frequency to guarantee critical conduction mode (boundary of CCM and DCM).

I.

INTRODUCTION

Multiple regulated supply voltages are becoming a need in many applications that require different supply voltages for different subsystems. Possible applications include mobile phones, personal digital assistant (PDAs), microprocessors, wireless transceivers, etc. [1]. In order to obtain these output voltages, switching converters and voltage linear regulators are the main alternatives at the core of power-management systems. As all designers put effort into size reduction, a converter with different output voltages cannot stay out of that trend, forcing designers to find a method to shrink the size in both on-chip and off-chip implementations [2]. Of all of the approaches, single-inductor single-input multiple-output (SIMO) converters come to prevail. SIMO converters can support more than one output while requiring only one off-chip inductor, promising many appealing advantages, in particular the reduction of bulky power devices, including inductors, capacitors and control ICs [1], [2]. In this way, the cost of mass production is remarkably reduced. Therefore, the SIMO topology appears as the most suitable and cost-effective solution in the future development of power management systems, attracting many manufacturing companies with different applications in portable devices. However, it is still a notable challenge to find the best topology and control for the implementation of this type of converter. In order to obtain multiple outputs, two main alternatives have historically been used: (1) voltage series linear regulators, that have been widely used for decades [3]-[6], and (2) DC/DC switching converters, thanks to which high-

Páginas: 805– 810 ISBN: 978-84-95809-75-9

(3)

School of Telecommunications Engineering of Barcelona (ETSETB) Department of Electronics Eng. Technical Univ. of Catalonia (UPC) C/ Gran Capitán s/n, Ed. C4, 08034 Barcelona. SPAIN [email protected]

efficiency power supply systems can be obtained [7]-[9]. Linear-assisted DC/DC converters (also known as linearswitching hybrid converters) are circuit topologies of strong interest when designing power supplies concurrently requiring as design specifications both: (1) high slew-rate of the output current and (2) high current consumption by the output load. This is the case of the systems based on modern microprocessors and DSPs, where both requirements converge [10], [11]. This interest is also applicable to wideband adaptive supply of RF power amplifiers. Linear-switching hybrid converters are compact circuit topologies that preserve the well-known advantages of the two typical alternatives for the implementation of DC/DC voltage regulators, namely, achieving both moderately high efficiencies –by virtue of the switching regulator- together with fast wideband ripple-free regulation –by virtue of the linear regulator-. In this paper, the linear-assisted strategy is applied to SIMO converters. II.

TOPOLOGY OF A HYSTERETIC LINEAR-ASSISTED DC/DC CONVERTER

The basic schematic of a single-input single-output (SISO) linear-assisted converter is shown in figure 1.a [12], [13]. This structure consists, mainly, of a voltage linear regulator in parallel with a step-down switching DC/DC converter. In this type of converters, the value of the output voltage, theoretically constant, is fixed with good precision by the voltage linear regulator. The current through the linear regulator is constantly sensed by the current sense element Rm. Based on this sensed signal, the controller activates the output of comparator CMP1 which controls the switching element of the DC/DC converter. Notice that the current flowing through the linear regulator constitutes a measurement of the error of the power supply. The power stage (this is, the switching converter) supplies to the output the current required to force to a minimum value the current flowing through the linear regulator. As a consequence, it is obtained, altogether, a power supply circuit in which the switching frequency comes fixed, among other parameters (such as the possible hysteresis of the analog comparator), by the value of the current flowing through the linear regulator. In the linear-assisted converter shown in

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SAAEI’10 Bilbao, 7 – 9 de julio de 2010

figure 1.b, a step-down (buck) switching converter [14], [15] is used. On the other hand, the linear regulator consists of a push-pull output stage (transistors Q2a and Q2b). In this approach, the main objective of the DC/DC switching converter is to provide most of the load current in steady-state conditions (to obtain a good efficiency of the whole system). Thus, in steady state, the linear regulator provides a small part of the load current, maintaining the output voltage to an acceptable DC value. Vin

level, turning OFF the DC/DC switching converter. Thus, the current through inductor L1 will be zero (figure 2). Therefore, the voltage linear regulator supplies the required output current (Ireg=Iout). I (A)

iL(t)

L1

DC-DC Switching Converter

Iout

ireg(t)

IH

I IL

0 Driver

Vref

Rm Ireg

Voltage Linear Regulator

Iout

Vout RL

IQ (a) L1

Q1 R3 D1 R2

IL

Vin

R1 +

VC

– + –

CMP1

I 

Rm

Voltage Linear Regulator

Iout

Vin +

OA1

Vout

RL

Q2a

VZ

Vref

(1) Rm It is important to emphasize that reducing the value of the power dissipated in the pass transistor of the linear regulator increases the efficiency of the set, even for significant output currents. Therefore, it is important to fix the current limit Iγ to an appropriate value between a maximum border to limit the maximum power dissipation, and a minimum border to operate the regulator properly, without penalizing its good characteristics of regulation. Thus, Iγ must be set at a value such that: (a) It does not significantly increase the power dissipation of the pass transistor in the linear regulator and does not excessively diminish the efficiency of the linearassisted converter. (b) It does not significantly deteriorate the regulation of the output voltage.

Current Sensing

Vref

TOFF

However, when the current demanded by the load is above this current limit Iγ, the output of the comparator will automatically toggle to high level. As a consequence, the current flowing through the inductance L1 will grow linearly. Considering that the output current Iout=Ireg+IL is assumed to be constant (equal to Vout/RL), the linear regulator current Ireg will also decrease linearly, until the time instant in which it will become slightly smaller than Iγ. At this moment, the comparator will change its output to low level, turning OFF the switch transistor Q1 and causing the current trough the inductor to decrease. When the inductor current decreases to a value in which Ireg>Iγ, the comparator changes its state to high level, thereby repeating the complete switching cycle. Without hysteresis in the comparator, the switching instant of the DC/DC converter is controlled by Iγ. This control signal can be adjusted to a given command thanks to the gain of the current sensing element, Rm, and the reference voltage Vref, according to the expression:

Current Sensing

Vin

TON

Fig. 2.- Principle of operation of the proposed linear-assisted DC/DC converter.

VC

+

Both linear and switching blocks enabled

IL

CMP1 –

+ –

t Linear block enabled Switching block disabled

Ireg

– Q2b

Fig. 1.- (a) Block diagram of the proposed linear-assisted converter.(b) Basic structure of the proposed linear-assisted DC/DC converter.

If the current demanded by the load Iout is below a maximum current threshold, denominated switching threshold current, Iγ, the output of comparator CMP1 will be at low

Thus, we can denominate this type of control as a strategy control with non-zero average linear regulator current. For load currents below 10 A, it can be concluded through circuitlevel characterization that the suitable value of Iγ that fulfills the two previous conditions is between 10 mA and 50 mA. The proposed linear-assisted DC/DC converter is suitable to any kind of converter, in particular to SIMO linear-assisted

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DC/DC converters. Next sections are devoted to the extension of a single-output linear-assisted converter to obtain a SIDO converter. III.

SIDO LINEAR-ASSISTED DC/DC CONVERTER

Based on figure 1.b, the structure of the SIDO linearassisted DC/DC converter is obtained as shown in figure 3. In this topology, two voltage linear regulators (A and B), one for each output, are used and one buck DC/DC switching converter (without the output capacitor) provides part of the output current for the two outputs. In the presented topology the SIDO linear-assisted DC/DC converter operates at the boundary of continuous conduction mode (CCM) and discontinuous conduction mode (DCM) with variable switching frequency, as it will be justified in the next section.

converter until IL=0. It should be evident that information from both subconverters is needed to determine which of the two output currents is the largest, and any change in one phase necessarily affects the other two phases, this rendering the control of the two outputs interdependent. Notice that the aforementioned topology can easily be extended to implement different algorithms and generate multiple output voltages. SW1

Vin

VC1 VC5

Controller

IL1

VCout1

IL2

SW3 SW4

Linear Regulator A

Q12a

VZ1 +

Vout1 Iout1

Vin

RL1 Ireg1

OA1

– Q12b

SWITCHING POLICY FOR THE SIDO LINEAR-ASSISTED DC/DC CONVERTER

Linear Regulator B

The concept of SIMO converters control algorithms has been disclosed in different papers [16]. In classical approaches, the control and timing scheme is a form of time division multiplexing. This time multiplexing can be extended from two outputs (SIDO converter) to N outputs, and each output should occupy a time slot for charging and discharging the inductor. In all cases, the structure can work with constant or variable switching frequency.

An important component of the proposed SIDO structure shown in figure 3 is the switching control of the four switches that determine the operation phases of the DC/DC converter. In this topology the SIDO linear-assisted DC/DC converter operates at the boundary of CCM and DCM with variable switching frequency. In the proposed control algorithm considered in this work (figure 4), each period is divided into three phases, not necessarily of equal duration. In phase 1, the inductor is charged from 0 A to the larger of the two output currents (Iout1 in the case under discussion). In phase 2, the inductor discharges into the first converter until IL becomes smaller than the lower output current (Iout2 in our case). Finally, in phase 3, the inductor drains IL into the second

SW2

VSO2 VSO1 VSL1

Due to the current sensing circuit, the controller generates the control signals for the four switches of the SIDO linearassisted DC/DC converter as a current-steering switching policy. In this particular application, it is necessary to sense the two output currents (sensing signals VSO1 and VSO2). On the other hand, the current flowing through the inductor of the switching converter (sensing signal VSL1) has to be sensed as well.

For a multiple-output converter with stable outputs, each output should be independently regulated. If the output voltage of a subconverter is affected by the change of load of another subconverter, cross regulation occurs. This is an undesired effect that, in the worst case, could make the system unstable [1], [16].

IL

VCout2

On the other hand, four switches, which determine the operation phases of the DC/DC converter, steer the inductor current of the switching converter to the appropriate output. Note that synchronous rectification is considered as unavoidable in a low-voltage chip-compatible scenario.

IV.

L1

Iout2

Vin Q22a

VZ2 +

Vout2

RL2 Ireg2

OA2

– Q22b

Fig. 3.- Basic structure of a SIDO linear-assisted DC/DC converter.

V.

CONTROLLER IMPLEMENTATION FOR THE SIDO LINEAR-ASSISTED DC/DC CONVERTER

The control algorithm considered in this paper is shown in figure 4. It is necessary to sense the two output currents (VSO1 and VSO2 in figure 3) and the current flowing through the inductor of the switching converter (sensing signal VSL1). As a consequence, four control signals are obtained in order to control the four switches of the SIDO linear-assisted DC/DC converter, namely: control signal VC1 for the switch SW1, VC5 to control SW2, VCout1 for the switch SW3 and VCout2 for the switch SW4. In order to implement the control algorithm presented in figure 4 it is necessary to obtain which of the two output currents is the largest. In addition, it is necessary to compare the inductor current with these two output currents, generating internal control signals. The scheme presented in figure 5.a

807

shows the circuit that implements this part, obtaining three internal threshold levels: VT1, VT2 and VT3. Notice that the output of the comparator CMP1 provides the intermediate control signal VS that indicates which output current (Iout1 or Iout2) is the largest one. These three levels (VT1, VT2 and VT3) are the intermediate or internal signals that control a state machine, consisting of three R-S latches (figure 5.b). The state machine generates the control signals VC1, VC5, VCout1 and VCout2 for the switches SW1, SW2, SW3 and SW4, respectively. Finally, in figure 5.c, it is shown the block which, in the inductor discharge interval, decides which output (switch SW3 or SW4) is selected first. Note that this decision depends upon the signal VS provided by comparator CMP1 (figure 5.a). Thus, the largest of the two currents is selected in the subinterval TOFF1 and the lower in the interval TOFF2. Iout1

From 1.67 A to 0.83 A at t=250 s and vice versa at t=500 s, being Ireg2=1.33 A. VII.

In this paper, the design and performance characterization of a SIDO linear-assisted DC/DC converter has been described. A current-steering switching policy, in combination with a linear-assisted hysteretic DC/DC regulator in the context of single-inductor dual-output (SIDO) converters, allows to provide two independent outputs with suitable load and line regulations. In the proposed topology the SIDO linear-assisted DC/DC converter operates at the critical conduction mode with variable switching frequency by means of a hysteretic control, thereby restricting the inductor current ripple. Finally, note that different control algorithms can be implemented in the proposed SIDO structure in order to obtain the appropriate and accurate load and line regulations. Final experimental results corroborating the previous simulation results will be included in the definitive version of the article.

IL Iout2

ACKNOWLEDGMENT This work has been partially funded by project TEC2007– 67988–C02–01/MIC from the Spanish MCYT and EU FEDER funds.

t Iout1 Ireg1

IL

REFERENCES [1]

Ireg2

Iout2 [2]

0 TON Inductor charge

TOFF1

TOFF2

t [3]

Inductor discharge

[4]

Fig. 4.- Current waveforms of the SIDO linear-assisted DC/DC converter with control strategy A: through the load 1 and load 2 (red color traces), inductance L1 (blue trace), linear regulator 1 (discontinuous green trace) and linear regulator 2 (discontinuous violet trace).

VI.

CONCLUSIONS

[5]

BEHAVIOUR CHARACTERIZATION OF THE SIDO LINEAR-ASSISTED DC/DC CONVERTER

[6]

In order to validate the presented structure for the SIDO linear-assisted DC-DC converter depicted in figure 3, its controller shown in figure 5 and the control algorithm presented in figure 4, circuit level characterization has been obtained for system specifications requiring 5.0 V at Vout1 and 2.0 V at the output Vout2, being Vin=9 V. Figure 6 shows the most representative waveforms when the SIDO linear-assisted converter provides 1.67 A at the output 1 and 0.67 A at the output 2. In order to validate the controller operation under variations of the maximum of the two output current, figure 7 shows the current waveforms of the structure of the SIDO linear-assisted DC/DC converter when the output current Ireg1 changes from the largest value to a value lower than Ireg2:

[7] [8] [9] [10]

[11] [12] [13]

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D. Ma, W-H. Ki, C-Y. Tsui. “A Pseudo-CCM/DCM SIMO Switching Converter with Freewheel Switching”. IEEE Journal of Solid-State Circuits, vol. 38 (nº 6), pp. 1007-1014, June 2003. H-P. Le, C-S. Chae, K-C. Lee, S-W. Wang, G-H. Cho, G-H Cho. “A Single-Inductor Switching DC-DC Converter with Five Outputs and Ordered Power-Distributive Control”. IEEE Journal of Solid-State Circuits, vol. 42 (nº 12), pp. 2706-2714, December 2007. C. K. Chava, J. Silva-Martínez. “A Frequency Compensation Scheme for LDO Voltage Regulators”. IEEE Transactions on Circuits and Systems–I: Regular Papers, vol. 51 (nº 6), pp. 1041-1050, June 2004. R. J. Milliken, J. Silva-Martínez, E. Sánchez-Sinencio. “Full On-Chip CMOS Low-Dropout Voltage Regulator”. IEEE Transactions on Circuits and Systems–I: Regular Papers, vol. 54 (nº 9), pp. 1879-1890, September 2007. R. K. Dokania, G. A. Rincón–Mora. “Cancellation of Load Regulation in Low Drop–Out Regulators”. Electronic Letters, vol. 38 (nº 22), pp. 1300– 1302, 24th October 2002. V. Grupta, G. A. Rincón–Mora, P. Raha. “Analysis and Design of Monolithic, High PSR, Linear Regulator for SoC Applications”. Proceedings of the IEEE International SoC Conference, pp. 311–315, 2004. R. W. Erickson, D. Maksimovic. “Fundamentals of Power Electronics”. 2nd edition, Ed. Kluwer Academic Publishers, 2001. J. G. Kassakian, M. F. Schlecht, G. C. Verghese. “Principles of Power Electronics”. Ed. Addison–Wesley, 1991. N. Mohan, T. M. Underland, W. P. Robbins. “Power Electronics: Converters, Applications and Design”. Ed. John Wiley & Sons, 1989. V. Yousefzadeh, E. Alarcon, and D. Maksimovic, “Band Separation and Efficiency Optimization in Linear-Assisted Switching Power Amplifiers”. 37th IEEE Power Electronics Specialists Conference, 2006 (PESC’06), pp. 1-7, 18-22 Jun. 2002. B. Arbetter, D. Maksimovic. “DC–DC Converter with Fast Transient Response and High Efficiency for Low–Voltage Microprocessor Loads”. IEEE Applied Power Electronics Conference, pp. 156-162. 1998. P. Midya, F. H. Schlereth. ‘Dual Switched Mode Power Converter’. IECON’89. Industrial Electronics Society, pp. 155–158, 1989. F. H. Schlereth, P. Midya. ‘Modified Switched Power Convertor with Zero Ripple’. Proceedings of the 32nd IEEE Midwest Symposium on

[14] [15]

Circuits and Systems (MWSCAS’90), pp. 517–520, 1990. H. Martínez, A. Conesa,. “Modeling of Linear-Assisted DC–DC Converters”. European Conference on Circuit Theory and Design 2007 (ECCTD 2007), 26th-30th August 2007. A. Conesa, H. Martínez, J. M. Huerta. “Modeling of Linear & Switching Hybrid DC–DC Converters”. 12th European Conference on Power Electronics and Applications (EPE 2007), September 2007.

[16]

D. Ma, W-H. Ki, C-Y. Tsui, P. K. T. Mok. “Single-Inductor MultipleOutput Switching Converters with Time-Multiplexing Control in Discontinuous Conduction Mode”. IEEE Journal of Solid-State Circuits, vol. 38 (nº 1), pp. 89-100, January 2003.

SWA VC4 SWA1

VCout1

–

VT2

+ VSO1 + VSO2

VS

–

SWA2 – SWB1

CMP1

SWB

VS

CMP2

SWC

VT1

VC3

+

VCout2

CMP3

–

VT3

+ CMP4

SWB2

VSL1

SWD (c)

(a)

VC1 VC4 VC2 VC5 VT2 VT3

VT1 S

Q

VT3 S

Q

R

Q

VT1

VC3

VC5

VC3 S

R

R

Vres (b) Fig. 5.- Structure of the controller block for the SIDO linear-assisted DC/DC converter: (a) Generator of the internal threshold levels for the state machine. (b) State machine that generates the control signals VC1, VC5, VCout1 and VCout2 for the switches SW1, SW2, SW3 and SW4, respectively. (c) Block to decide which of the two outputs (switch SW3 or SW4) is selected first within the inductor discharge interval.

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6.0V

Vout1 4.0V

Vout2 2.0V

0V 0s

50us V(Vout2)

V(Vout1)

100us

150us

200us

250us

300us

350us

400us

Time

(a) 2.0A

Iout1 1.5A

Ireg1

IL

1.0A

Iout2 Ireg2

0.5A

0A 0s I(L1)

50us I(RL21) I(RL11)

100us I(RL1s)

150us

200us

250us

300us

350us

400us

I(RL2s) Time

(b)

Fig. 6.- Current waveforms of the structure of the SIDO linear-assisted DC/DC converter: (a) Output voltages Vout1 and Vout2. (b) Currents of interest in the circuit: IL, Ireg1, Ireg2, Iout1 and Iout2. 2.0A

Iout1 Ireg1

1.5A

Iout2

IL 1.0A

Ireg2 0.5A

0A 0s I(L1)

I(RL21)

100us I(RL11)

I(RL1s)

200us I(RL2s)

300us

400us

500us

600us

Time

Fig. 7.- Current waveforms of the structure of the SIDO linear-assisted DC/DC converter when the output current Ireg1 changes from 1.67 A to 0.83 A at t=250 s and vice versa at t=500 s: (a) Output voltages Vout1 and Vout2. (b) Currents of interest in the circuit: IL, Ireg1, Ireg2, Iout1 and Iout2.

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III Congreso Nacional de Pulvimetalurgia Valencia, 13 y 14 de junio de 2010 XXX-XXX

ESTUDIO DE LA INFLUENCIA DE LAS VARIABLES DE MOLIENDA EN LAS PROPIEDADES DEL POLVO DE ALUMINIO NANOCRISTALINO J. Solà, J. Llumà, J. Jorba Dep. Ciència de Materials i Enginyeria Metal•lúrgica, EUETIB, Universitat Politècnica de Catalunya, Comte d’Urgell 187, 08036 Barcelona. [email protected], [email protected], [email protected]

RESUMEN En la última década ha habido un creciente interés y esfuerzo de investigación centrado en el desarrollo de materiales nanocristalinos debido a la notable mejora de sus propiedades químicas, eléctricas, magnéticas, ópticas y mecánicas. Estos materiales pueden ser producidos mediante varios procedimientos. Aunque algunas de estas técnicas, como la molienda mecánica (BM), producen materiales en polvo que requieren una consolidación posterior para tener una aplicación estructural, permiten obtener los menores tamaños de grano. Por esta razón, se han realizado amplios estudios sobre la dinámica de los procesos de molienda y de su influencia en los cambios microestructurales producidos en esos materiales. No obstante, el principal esfuerzo se han centrado en materiales férricos, y son pocos los trabajos en aluminio puro. En la presente comunicación se presenta la evolución de la dureza y tamaño de partícula de aluminio nanocristalino obtenido por molienda mecánica y el rendimiento del proceso de producción en función de los parámetros de molienda.

ABSTRACT In the last decade or so there has been an increasing interest and research effort focused on nanocrystalline materials due to the remarkable improvement of their chemical, electrical, magnetic, optic and mechanical properties. These materials can be produced by several processes. Although some of those techniques as ball milling (BM) produce powdered materials that need to be consolidated in a further process to be useful in structural applications, they are able to obtain the smaller grain size. For this reason, the dynamics of the milling process and its influence on the microstructural changes produced in these materials has been extensive studied. However, the main efforts have focused on ferrous materials, and there are few studies of pure aluminium. This paper presents the evolution of hardness and particle size of nanocrystalline aluminium powder obtained by mechanical milling and the efficiency of production processes versus milling process parameters.

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III Congreso Nacional de Pulvimetalurgia, Valencia, junio 2010

Palabras clave (Keywords): aluminio, materiales nanocristalinos, propiedades mecánicas, técnicas de producción de polvos.

1. INTRODUCCIÓN La notable mejora en las propiedades químicas, eléctricas, magnéticas, ópticas y mecánicas que se obtiene en los materiales nanocristalinos ha propiciado una eclosión en su estudio durante la última década [1,2]. Los procesos que permiten obtener estos materiales son muchos y diversos, pero aquellos que permiten obtener un menor tamaño de grano cristalino suelen producir el material en forma de polvo, el cual precisa de una posterior sinterización para producir componentes aptos para aplicaciones estructurales. Entre estas técnicas se cuenta la molienda mecánica, que ha sido objeto de numerosos estudios metodológicos sobre la influencia de los parámetros de molienda en la evolución del material molido [3,4,5]. Ello ha permitido obtener tamaños de grano nanocristalino en diversos metales [6] y aleaciones [7]. No obstante, el principal esfuerzo se ha centrado en materiales férricos, y son comparativamente pocos los trabajos de molienda mecánica en aluminio puro [ 8], aunque no con sus aleaciones [9]. Este hecho no ha impedido detallados estudios de la evolución de la microestructura del aluminio al acumular trabajo en frío mediante otras técnicas de deformación [10], que en grandes líneas son extrapolables al caso de la molienda mecánica. Tal vez por este motivo, la mayoría de trabajos se centran en la influencia del tiempo de molienda en las propiedades mecánicas del polvo obtenido y deja de lado la influencia de otras variables como la energía de los impactos (consecuencia de la velocidad de giro) o el número de impactos por unidad de tiempo (consecuencia del número de bolas). El presente trabajo está enfocado a describir la influencia de estas variables, no sólo en las propiedades mecánicas del material molturado, sino de aquellas variables que influenciarán en el posterior proceso de sinterización en etapa industrial, es decir, tamaño de partícula del polvo y rendimiento del proceso de molturación. 2. MATERIALES Y PROCEDIMIENTO EXPERIMENTAL El material utilizado en este estudio es polvo de aluminio puro suministrado por la empresa ECKA Granules® con la especificación ECKA Aluminium AS 51 que ha sido tamizado en el laboratorio. La fracción utilizada corresponde al intervalo 72-100 µm. En la figura 1 se muestra la distribución de tamaño de partícula de la fracción tamizada que ha sido determinada mediante análisis de imagen de la superficie proyectada de las partículas [11]. En las figuras 2a y 2b se muestran imágenes de la morfología de las partículas y de la sección de una de las partículas obtenidas, respectivamente, mediante SEM y microscopía óptica. En la tabla 1 se muestra la composición química del material original y del polvo molturado en las condiciones extremas ensayadas determinada mediante espectrometría de masas con fuente de plasma de acoplamiento inductivo (MS-ICP). La dureza del polvo original es 38,9 ± 2,7 HV 0,025 determinada siguiendo la metodología descrita posteriormente. 2

Estudio de la influencia de las variables de molienda en las propiedades del polvo de aluminio nanocristalino

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Porcentaje de partículas

6 5 4 3 2 1 0 1E-3

0,01

Tamaño de partícula (mm2)

Figura 1. Distribución del tamaño de partícula de la fracción tamizada expresada como área de la superficie proyectada.

Figura 2. Morfología de las partículas de polvo utilizado y del interior de una partícula. El polvo de aluminio ha sido molturado en molino planetario de bolas Pulverisette 5 de la firma Fritsch® utilizando contenedores cilíndricos de 250 ml fabricados con acero X 5 Cr Ni 18 10 y bolas de 10 mm de diámetro fabricadas con acero 100 Cr 6. Se han ensayado distintas condiciones de tiempo de molienda, relación masa de bolas-masa de polvo (BPR) y velocidad de giro del molino (rpm) sin agente de control. También se ha determinado la influencia de Licowax C de Clarient®, cera tipo amida (EBS), como agente de control.

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III Congreso Nacional de Pulvimetalurgia, Valencia, junio 2010

Se determinado el rendimiento de cada condición de molienda como la diferencia entre la masa de polvo inicial y de polvo obtenido después de la molienda, y se expresa como porcentaje de variación respecto a la masa inicial. El tamaño medio y la distribución de tamaños de las partículas de polvo obtenidas en cada molturación han sido determinados mediante análisis de imagen de la superficie proyectada de cada partícula realizado sobre muestras no inferiores a 400 partículas. En algunos casos se han desestimado algunas partículas que, por su forma aplanada y tamaño, pudieran sesgar la medida. En todos los casos se ha ajustado una distribución log-normal y se han calculado la moda de esa distribución. Las barras de error representadas en las correspondientes gráficas delimitan los intervalos de tamaño de grano que incluyen el 90% de la población de partículas de la muestra. La dureza del polvo original y del material obtenido en las distintas moliendas se expresa en escala Vickers (HV 0,025) y ha sido determinada en un microdurómetro Buehler 5114. Todas las muestras utilizadas han sido embutidas en caliente (150ºC) en resina epoxi de alta resistencia (Epomet de Buehler®) y pulidas siguiendo el procedimiento metalográfico habitual hasta la mitad de su diámetro, aproximadamente. Se ha utilizado el criterio Cauchy para admitir o descartar en el cálculo los valores experimentales de dureza obtenidos El valor medio de dureza corresponde al promedio entre 15 medidas válidas de dureza y la barra de error representa la dispersión entre los valores de dureza válidos asumiendo un nivel de confianza del 95%. Tabla 1. Composición química del polvo del material inicial y del polvo molturado en las condiciones extremas ensayadas. Condiciones de molienda Material original 50 h/20 BPR/160 rpm 20 h/30 BPR/160 rpm 20 h/20 BPR/220 rpm

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