Constrained dual-loop attitude control design for spacecraft

A constrained dual-loop controller is designed for the attitude control of rigid spacecraft with inertia uncertainties, assigned angular velocity and control constraints. The design is based on a modular approach that the design of the control law and the online estimator leads to an important “separation property”. The control law is designed based on a two-loop structure in which the outer-loop control law implements by sliding mode control technique to stabilize the attitude under angular velocity constraints, and the inner loop control law is designed to track the limited command of outer loop under uncertainties and control input constraints. To improve the robustness of the attitude controller, an extended state observer is developed for online estimation of the lumped disturbance. The constraints on control input and angular velocity are implemented by second-order command filter. The stability of the subsystem during saturation is guaranteed by using a modified tracking error definition, in which the effect of the constraints has been filtered and removed. Simulation results are obtained to illustrate the effectiveness of the proposed attitude controller.