Attitude control of towed space debris using only tether

Tethered towing for Active Debris Removal (ADR) is promising but vulnerable to entanglement and collision due to the debris rotation. This paper proposes a novel way to control the attitude of debris towed with off-centered capture using only tether. For this purpose, a tethered manipulator (TM) on the bottom of gripper is employed to move the tether attachment point. Since the tether cannot yield a torque along itself and the TM motions are constrained, the underactuated problem subject to input saturation arises as a critical control challenge. This is tackled by using backstepping-based hierarchical sliding mode control (HSMC) with anti-windup technique. And a disturbance observer is developed to render the controller robust to the dynamics uncertainty and flexible appendages vibration. To avoid collision, by designing an impedance controller the tether tension is stabilized with a reel mechanism in platform for changing the tether length. Finally, the proposed method is verified numerically by the first impulsive maneuver of Hohmann transfer.