Computer Science – Performance
Scientific paper
Jan 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002iaf..confe.746o&link_type=abstract
IAF abstracts, 34th COSPAR Scientific Assembly, The Second World Space Congress, held 10-19 October, 2002 in Houston, TX, USA.,
Computer Science
Performance
Scientific paper
This project has developed on the background of ongoing researches within the Control Technology Unit (TOS-OSC) of the Special Projects Division at the European Space Operations Centre (ESOC) of the European Space Agency. The purpose of this research is to develop and validate an Artificial Neural Network tool (ANN) able to model, simulate and predict the Cluster II battery system's performance degradation. (Cluster II mission is made of four spacecraft flying in tetrahedral formation and aimed to observe and study the interaction between sun and earth by passing in and out of our planet's magnetic field). This prototype tool, named BAPER and developed with a commercial neural network toolbox, could be used to support short and medium term mission planning in order to improve and maximise the batteries lifetime, determining which are the future best charge/discharge cycles for the batteries given their present states, in view of a Cluster II mission extension. This study focuses on the five Silver-Cadmium batteries onboard of Tango, the fourth Cluster II satellite, but time restrains have allowed so far to perform an assessment only on the first battery. In their most basic form, ANNs are hyper-dimensional curve fits for non-linear data. With their remarkable ability to derive meaning from complicated or imprecise history data, ANN can be used to extract patterns and detect trends that are too complex to be noticed by either humans or other computer techniques. ANNs learn by example, and this is why they can be described as an inductive, or data-based models for the simulation of input/target mappings. A trained ANN can be thought of as an "expert" in the category of information it has been given to analyse, and this expert can then be used, as in this project, to provide projections given new situations of interest and answer "what if" questions. The most appropriate algorithm, in terms of training speed and memory storage requirements, is clearly the Levenberg-Marquardt one. The ANN used is a three-layer one (2-4-1) with four inputs and one output. Having established all the ANN parameters and calculated all the input/target training data the ANN has been trained and validated. Afterwards, various simulations have been performed with BAPER to validate the performance of the software and test new alternative battery cycling strategies. Taking into account the small number of available training data for the ANN, and that the simulations have been carried out over a fairly extensive time frame (i.e. one year) the results obtained from the prototype tool must be considered more than satisfactory. It is found that the deliverable discharge capacity can be maintained circa 20% higher than the one obtained with the nominal cycling strategy if the batteries are left discharged for a longer period of time and the storage temperature is decreased. This ANN model has its limitations when asked to predict the discharge capacity deterioration that would be obtained with extraordinary cycling conditions (e.g. extremely low storage temperatures and continuous cycling). Hence, these results must be considered only approximate, as it is impossible to exactly state whether the ANN turn out to give extremely accurate realistic values or not, failing to extrapolate a correct pattern. One way to overcome the problem would be to do some parallel experiments in the laboratory, using the same battery and similar environment conditions (temperature, charge and discharge cycles) to the ones to be encounter in the spacecraft.
Donati Alessandro
Ottavianelli Giuseppe
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