ESA GNC Conference Papers Repository

The current tendency to employ spacecraft with large, complex, lightweight and therefore extremely flexible structures, together with the necessity to maintain high pointing accuracy during fast manoeuvring, poses the problem to reduce structural vibrations to a very low level. In fact, this kind of structures presents lowfrequency fundamental vibration modes which can be excited by a variety of tasks such as slewing and pointing manoeuvres. Therefore a complex control strategy for active suppression of the induced vibrations is required for achieving the desired pointing requirements. Two different control techniques for active structural vibration suppression have been investigated and compared: the open loop input shaping (IS) and the closed loop model predictive control (MPC). They have been applied to the linearized dynamics model of the 25 meter long IXO space telescope and simulation results are presented to show the effectiveness of the proposed control techniques. The IS technique produces an almost complete multiple mode vibration suppression while minimizing the sensitivity towards modelling errors (e.g. eigenfrequency and damping), the rest-to-rest slew distance and the transient deflection amplitude; besides, it reduces the open loop steady-state attitude error by almost two orders of magnitude. Although the MPC technique is not specifically intended for vibration suppression, a careful tuning of its control parameters allows achieving performances of the same order than those obtained with the IS technique. Nevertheless, MPC presents a much larger number of tuning parameters and a longer settling time compared to IS. For these reasons, the input shaping technique has been applied to a nonlinear dynamic model of the Euclid spacecraft for suppressing the vibrations caused by propellant sloshing during Euclid science observations. Results show that as long as displacements from initial conditions are small and the system is slowly varying, the input shaping technique applied to nonlinear systems continues producing an almost complete vibration suppression as in linear conditions.