Fixed-time anti-saturation compensators based impedance control with finite-time convergence for a free-flying flexible-joint space robot

Aiming at the compliant control problem of a free-flying flexible-joint space robot (FFSR) when it comes in contact with targets or obstacles, a nonsingular composite impedance controller with finite-time convergence is developed by backstepping technique. To handle the possible input saturation induced by the constraints of control moment gyroscope (CMG) and joint motors, two fixed-time anti-saturation compensators are designed respectively to ensure the stability of the saturated FFSR system and drive the system out of the saturated region rapidly. To avoid ’differential explosion’ arising from the multiple derivatives of virtual control signals in backstepping, a novel dynamic surface control (DSC) method is employed. Particularly, the singularity problem in the process of using the novel DSC is also considered. Theoretical analysis shows that the compensated control system can be semi-globally fixed-timely uniformly ultimately bounded (SGFxTUUB) and the desired impedance model can be achieved in finite time. Finally, the simulation results demonstrate the feasibility and validity of the suggested impedance controller.