Attitude Stabilization of Tug–Towed Space Target by Thrust Regulation in Orbital Transfer

This paper studies the stability control of a target towed by a space-tug via a tether in orbital transfer. The attitude of the towed target is controlled by thrust regulation at the tug with the consideration of the coupling effect between the attitudes of the tug and towed target and the motion of the flexible and elastic tether. A model-based control scheme is proposed, where the tug and towed target are assumed as rigid bodies and the tether is approximated as a series of extensible but incompressible rods connected by spherical hinges. The governing equations of motion of the coupled rigid–flexible multibody system are derived based on the recursive dynamics algorithm. The attitude motion of the tug with the presence of tether tension disturbance is controlled by a simple proportional–derivative torque controller. The attitude motion of the towed target and the elongation oscillation of the tether are stabilized simultaneously via the tether tension by regulating the thrust at the tug. The thrust control is achieved by first designing an optimal trajectory considering the system constraints based on a reference model, and then the trajectory is tracked by a robust global terminal sliding-mode controller with the consideration of thrust saturation and tether slackness avoidance. Finally, numerical simulation is carried out to demonstrate the effectiveness of the proposed attitude control law based on the thrust regulation.