SmArT (Scientific Microsatellite for Advanced Research and Technology) is a project of the two Universities of Naples, financed by Regione Campania, aimed at designing and developing a multi-mission microsatellite for remote sensing applications. Since remote sensing requires fine attitude control, the microsatellite is three-axis stabilized with three small reaction wheels. Three magnetic torquers, for initial attitude acquisition and wheel momentum unloading, complete the attitude control subsystem. In this paper the microsatellite attitude control in the case of wheel failure is analyzed. In order to save on-board mass and power, control techniques are developed to automatically re-distributed the required control torque among the operating wheels and the magnetic torquers in the case of failure of one or two wheels. The proposed control is designed in order to maintain the required pointing accuracy. To this end, the control techniques are developed using a mathematical model of the microsatellite attitude dynamics, which considers the variation of the Earth magnetic field magnitude and direction along the orbit. Numerical simulations demonstrate the effectiveness of the proposed techniques.
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