Robust tracking control for fast steering mirror based on iterative learning enhanced 2-DOF disturbance observer

This paper proposes a composite robust tracking control strategy for a fast steering mirror (FSM) with model uncertainty and repetitive external disturbances. A two-degree-of-freedom disturbance observer (2-DOF DOB) is proposed to achieve real-time estimation and compensation of disturbances. Compared with traditional DOB, the 2-DOF DOB enhances the gain of the outer-loop controller and improves the performance of the feedforward controller. To further enhance the estimation performance of the 2-DOF DOB for repetitive disturbances, an iterative learning law is incorporated into the design process. Subsequently, a trajectory tracking controller based on feedforward and $H_{\infty }$ method is proposed for the FSM system. Some comparative experiments are carried out on the FSM. The experimental results indicate that the proposed composite robust tracking control strategy demonstrates superior disturbance immunity and trajectory tracking performance compared to the traditional FSM control strategy.