Control of an Orbital Manipulator with Reaction Wheels for On-orbit Servicing

Abstract

In on-orbit servicing missions, a spacecraft equipped with a manipulator arm grasps a client satellite. Afterwards, the momentum-dumping phase follows and the manipulator can be utilized for servicing tasks. Once the momentum has been dumped, a desired manipulator configuration and base attitude may be needed for initializing a servicing phase such as berthing. The proposed approach prioritizes the attitude maintenance of the servicer spacecraft, and reconfigures the manipulator arm with the grasped client satellite in the nullspace of the servicer's attitude. The approach utilizes the redundancy in rotational degrees of freedom, provided by reaction wheels, to decouple the manipulator reconfiguration task from the attitude of the servicer-base. The proposed controller shows convergence of the attitude and joint-level task, independent of kinematic singularities of the manipulator arm. Additionally, the problem of speed saturation of the reaction wheels is tackled by increasing the damping torques as a function of the wheel speed. Simulation results verify the effectiveness of the proposed control approach.