Unified Control of an Orbital Manipulator for the Approach and Grasping of a Free-Floating Satellite

In robotic on-orbit servicing (OOS) missions, an orbital manipulator approaches and grasps a faulty client satellite. The approach phase and postgrasp phase pose different challenges and hence impose different requirements on the design of a controller for the orbital manipulator. In the approach phase, the foremost requirement is to track the client satellite with the end-effector in Cartesian space. In addition, it is desirable to have the servicing satellite and arm in a suitable safe configuration during grasp and stabilization. In the postgrasp phase, a crucial requirement is to limit the interaction forces at the manipulator's end-effector. This is to ensure that the grasping interface is not damaged during stabilization. In this article, we develop a unified control framework for an orbital manipulator in the approach phase and postgrasp phase. The controller hierarchically fulfills various requirements of each phase. In the approach phase, the proposed controller tracks the Cartesian pose of the grasp point on the client. It also simultaneously tracks a joint-space trajectory in the nullspace to achieve a suitable servicer pose for grasping. The proposed postgrasp controller stabilizes the client with limited interaction forces while bringing the servicer to a safe configuration with respect to the client. Furthermore, the unified controller redistributes torques between thrusters and reaction wheels so as to save thruster energy in the approach phase and reduce external momentum in the postgrasp phase. Results of simulation and experiments performed on the hardware-in-the-loop facility OOS-Sim at DLR validate the proposed method.