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

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.