End-effector Position/Attitude Control of a Tethered Robot Around an Equilibrium Point at the Tether End

Abstract

We introduce a new type of space robot system, called a tethered robot system. The tethered robot system comprises a tethered robot, i.e. a body equipped with manipulator arms, which is attached to a main spacecraft through a tether. By operating the manipulator arms, the tethered robot can grasp an object which is located far away from the spacecraft. Task performance, however, is complicated due to the presence of external forces, such as gravity, centrifugal and tether- tension forces. In this paper, we propose decoupled control of tether tension and robot motion around an equilibrium point at the tether end. First, we examine rotational and translational motion of the tethered robot caused by the external forces. Then, it is noted that the tethered robot can be controlled in a way similar to a free-flying robot, by proper operation of the tether attachment point. Therefore, the control of the tethered robot with respect to its mass center should be composed of two subtasks: the end-effector motion subtask and the tether attachment point motion subtask. The effectiveness of the proposed approach is confirmed by computer simulation.