Predefined‐time impedance control of free‐flying flexible‐joint space robots for force sensor‐less target capturing with prescribed performance

Abstract Aiming at safely capturing faulty satellites on orbit, a novel predefined‐time impedance controller is designed to address the control challenges of free‐flying flexible‐joint space robots (FFSR) considering output constraint. The FFSR system model is transformed into a singularly perturbed form consisting of both fast and slow subsystems. For the slow subsystem, an adaptive predefined‐time sliding mode observer is developed to obtain the contact torque between the end‐effector and the target. To mitigate overshooting and enhance tracking precision, a predefined‐time prescribed performance function is proposed, and the output constraint issue is reformulated as a coordinate transformation problem involving the trajectory tracking errors. Based on these, a predefined‐time impedance controller is designed to achieve the compliant capture of the target. For the fast subsystem, a new non‐singular fixed‐time controller is proposed to rapidly overcome the vibration of the flexible joints. Stability analysis proves predefined‐time stability of the FFSR system and the tracking errors can be maintained within a predefined region. Finally, numerical simulations indicate the feasibility and validity of the presented control strategy.