Concurrent Proximity Control of Servicing Spacecraft With an Uncontrolled Target

This paper investigates the six-degrees-of-freedom (6-DOF) rendezvous and docking issue with an uncontrolled target for a servicing spacecraft under thruster misalignment and external disturbances. Initially, a 6-DOF relative motion dynamic model is established to depict the relative position-attitude between the servicing spacecraft and uncontrolled object (freely tumbling spacecraft). Then, an adaptive fixed-time control scheme with a novel sliding manifold is employed to accomplish the 6-DOF rendezvous (close-range) and docking mission for the servicing spacecraft. Unlike existing methods, the sliding-mode surface is designed by using the new sufficient condition to achieve the system's fixed-time convergence, and this condition reduces the conservativeness of the traditional ones with less dominated terms. Moreover, the settling time of relative translation-rotation tracking errors is guaranteed irrespective of initial motion conditions. Accordingly, the fixed-time stability of the closed-loop system is guaranteed by the rigorous Lyapunov theory analysis. Finally, numerical simulation results with various cases are implemented to verify the superiority of the presented sliding-mode control strategy.