Intelligent proportional-derivative control for flexible spacecraft attitude stabilization with unknown input saturation

A novel attitude stabilization control scheme is presented for flexible spacecraft, based on the semi-globally input-to-state stable (ISS) concept. The modified Rodrigues parameters described attitude system is considered in the presence of unknown parameter uncertainties, external disturbances and even control input saturation. A simple and nonlinear proportional-plus-derivative (PD) controller is first developed through a special Lyapunov function construction. Sufficient condition under which the proposed nonlinear PD-type control law can render the system semi-globally input-to-state stable is provided such that the closed-loop system is robust with respect to any disturbance and uncertain inertia parameters, if the control gains are designed appropriately. The derived controller is then extended by adding a feed-forward compensator to reduce the effect of the unknown control input saturation on the system, while the compensator is derived by using radial basis function neural networks to approximate the unknown nonlinearity. It is shown that the extended control law can guarantee that the resulting closed-loop attitude system is ISS despite of actuator input saturation. The result is constructive in the sense that given any restriction on the initial conditions, it is possible to design large enough control gains that can render the system semi-globally ISS within the given restriction. Simulation studies have also been conducted to confirm and verify those highly desirable features of the proposed attitude control in comparison with the conventional control schemes.