Sliding mode attitude control for flexible spacecraft based on modal observer

To solve the attitude control problem of spacecraft with flexible appendages, sliding mode control law based on modal observer is presented. Hybrid coordinate method is adopted to model the dynamic of flexible appendages, and the modal displacements and their derivatives arc observed by the modal observer. A kind of sliding surface is selected to obtain the sliding mode control law based on the observer and the stability proof is given using the Lyapunov stability theory. Simulations are done under variable angular velocity tracking, constant angular velocity tracking and zero angular velocity tracking respectively, and the results show that compared with the conventional sliding mode control law, the control law presented here could improve the steady state control accuracy and diminish the effect of modal vibration on the attitude control.