Control of three-axis stabilized flexible spacecraft using variable structure strategies subject to input nonlinearities

This paper proposes a robust control algorithm for stabilization of a three-axis stabilized flexible spacecraft in the presence of model uncertainty, external disturbances and control input nonlinearities. This control algorithm is based on a variable structure output feedback control design technique, and explicitly accounts for the control input nonlinearities in the stability analysis. The proposed variable structure output feedback controller ensures the global reaching condition of the sliding mode of the spacecraft dynamics system. Moreover, in the sliding mode, the investigated dynamic system still retains the robustness with respect to the uncertainties and disturbances it possesses for a system with linear input. An attractive additional feature of the control method is that the structure of the controller is independent of the elastic mode dynamics of the spacecraft, since in practice the measurement of flexible modes is typically difficult or impractical. It is also shown that an adaptive version of the proposed controller can be achieved by removing the need to know the bounds of the uncertainties and perturbations in advance. Numerical simulations show that the precise attitude control and vibration suppression can be accomplished using the derived controller for both cases with and without adaptive control.