Fixed-Time Refined Disturbance Observer-Based Composite Control for Periscope-Type Coarse Pointing Assembly

Attaining microradian control accuracy within seconds is essential for the coarse pointing assembly (CPA) to establish and maintain the intersatellite laser link under multiple disturbances. In response to the control requirements and challenges, this article proposed a fixed-time refined disturbance observer-based composite control (FTRDOBC) scheme. First, a periscope-type CPA model is established, incorporating gimbal coupling torque, friction, and satellite-transmitted vibration to characterize the operational environment. Second, by utilizing the partially known information of the multiple disturbances, a fixed-time refined disturbance observer (FTRDO) is presented, featuring a state-dependent variable exponent coefficient auxiliary input. Combining the FTRDO with the presented time-varying fixed-time controller, an FTRDOBC scheme is proposed, aiming for high precision control in predefined time while circumventing the singularity problem. Experimental results are provided to demonstrate the effectiveness of the proposed method.