Unwinding-free composite full-order sliding-mode control for attitude tracking of flexible spacecraft

In this paper, the attitude tracking control problem is investigated for flexible spacecraft. A novel feedforward-feedback composite control scheme is proposed based on the combination of a generalized proportional integral observer (GPIO) and the full-order sliding-mode control technique. To estimate the lumped disturbances composed of the flexible appendages' vibration, external environmental disturbance and model uncertainty, a generalized proportional integral observer is firstly constructed. Then, a two-layer recursive full-order sliding-mode surface is developed, where the inner layer is designed as linear sliding-mode surface and the outer layer is designed as a terminal sliding-mode surface. With the feedforward compensation of the GPIO's estimates, a continuous unwinding-free composite attitude tracking controller is derived to achieve the finite-time arrival of the full-order sliding-mode surface and further ensure the asymptotic set convergence of the attitude tracking errors. Rigorous set stability analysis of the closed-loop attitude tracking error system is given. Comparative simulations are presented to demonstrate the effectiveness and superiority of the proposed composite control scheme.