Robust adaptive attitude control of flexible spacecraft using a sliding mode disturbance observer

A composite control scheme based on a disturbance observer is presented in this paper to deal with the attitude control problem of flexible spacecraft. Specifically, a sliding mode disturbance observer (SMDO) is developed to attenuate the unmodeled system dynamics, parameter uncertainties, and multiple external disturbances. The key feature of this approach is to relax the assumption imposed on disturbance to be constant or changing at a slow rate, which is a typical assumption in this class of problems involving a disturbance observer. Then, an exclusive adaptive integral sliding mode controller is combined with the SMDO to get the improved closed-loop control performance of the system. The underlying benefit of the proposed control scheme is improved robustness against time-dependent external disturbances and system model uncertainties. The stability analysis of the closed-loop system is provided using Lyapunov's stability theory. Simulation results are provided to show the effectiveness of the proposed control scheme, especially in comparison with existing results.