This paper exploits the robotic capabilities of an orbital manipulator equipped with an actuation module at its end-effector to perform close-proximity robotic operations. The proposed control strategy enables repositioning the system's center-of-mass by reconfiguring the manipulator configuration and using the end-effector-mounted thrusting mechanism to achieve displacement. The key advantage of the proposed method is that the plume impingement due to thruster firing of the servicer satellite in close-proximity operations towards the client is mitigated. This is achieved by regulating the internal motion of the manipulator such that the thrust firing does not occur near the space asset. The effectiveness of the controller is verified through a multibody dynamic simulation of an orbital manipulator.

The on-ground validation of orbital manipulators is a challenging task because the robot is designed for a gravity-free operational environment, but it is validated under the effect of gravity. As a consequence, joint torque limits can be easily reached in certain configurations when gravity is actively compensated by the joints. Hence, the workspace for on-ground testing is restricted. In this paper, an optimal strategy is proposed for achieving gravity compensation of an orbital manipulator arm on ground. The strategy minimizes the joint torques acting on the manipulator by solving an optimization problem and it computes the necessary forces to be tracked by an external carrier. Hence, full gravity compensation is achieved for the orbital manipulator. Experimental results validate the effectiveness of the method on the DLR CAESAR space robot, which uses a cable suspended system as external carrier to track the desired gravity compensation force, resulting from the proposed method.

In this paper, we propose a detumbling strategy that stabilizes the motion of a tumbling client satellite using an orbital servicing manipulator, which is the goal of the post-grasp phase. One of the critical aspects in this phase is ensuring that excessive contact forces are not generated at the grasp interface. In addition, space mission requirements might demand a nominal manipulator configuration that is suitable for further manipulation/servicing activities. The proposed strategy allows the detumbling of the client motion while ensuring that the contact forces developed at the grasp interface do not violate a safety threshold. Further, it allows the reconfiguration of the manipulator arm by exploiting the full actuation capability of the manipulator-equipped servicing spacecraft. The controller guarantees joint task convergence in the nullspace of the manipulator's end-effector, and is also valid for kinematically singular configurations of the manipulator. It is further augmented using a quadratic programming based approach to optimally constrain the contact forces. Finally, simulation results for a post-grasp detumbling scenario are shown to validate the effectiveness of the proposed method.