Adaptive variable structure and active vibration reduction for flexible spacecraft under input nonlinearity

This paper is concerned with vibration control of a flexible spacecraft in the presence of parametric uncertainty/external disturbances as well as control input nonlinearity through distributed piezoelectric sensor/actuator technology. To satisfy pointing requirements and simultaneously suppress vibrations, two separate control loops are adopted. The first uses piezoceramics as sensors and actuators to actively suppress certain flexible modes by designing suboptimal positive position feedback (SOPPF) compensators which add damping to the flexible structures in certain critical modes. The problem of determining the SOPPF gain is formulated as static output feedback problem. The second feedback loop is designed based on an output feedback sliding mode control (OFSMC) design where control input nonlinearity is taken into consideration. The controller has the ability to reject the disturbance, deal with uncertainty and to ensure that the system trajectories globally converge to the sliding mode. Furthermore, an adaptive version of the proposed controller is achieved through releasing the limitation of knowing the bounds of the uncertainties and perturbations in advance. Simulation studies for the proposed control strategy on a flexible spacecraft have been carried out which demonstrate the effectiveness of the proposed approach.