Attitude Stabilization of Spacecraft with Flexible Appendages by L1 Adaptive Feedback

Abstract

This paper presents an L1 adaptive control system for the attitude stabilization of a spacecraft with flexible appendages. The spacecraft model includes uncertain parameters as well as external disturbance torques. The modified Rodrigues parameters are used here to describe the orientation of the spacecraft. The objective is to control the three attitude angle trajectories to the origin, using control torquers, located on the central rigid body, despite uncertainties in the spacecraft dynamics. Based on the L1 adaptive control theory, a new attitude control law is derived. The control system includes a state predictor in the feedback loop to generated estimates of the unknown parameters and lumped unknown functions. The control signals are generated by passing estimated stabilizing control signals through a low-pass filter. The control law is synthesized using only the attitude angles and angular velocity of the spacecraft. Interestingly, the control law is independent of the elastic dynamics. In the closed-loop system, the attitude angles are controlled to the origin and flexible modes are stabilized as well. The designed control law achieves quantifiable performance bounds by the choice of large adaptation gains. Simulation results show that the L1 adaptive law accomplishes precise attitude control and vibration suppression, despite parameter uncertainties and external disturbance moments.