A new strategy for capturing a noncooperative spacecraft by a robotic arm

In robotic arm-based noncooperative spacecraft-capture missions, the gripper of the robotic arm collides with the target’s surface and graspable structure. The former can cause the graspable structure to collide with the gripper multiple times or escape from the gripper. The adverse of the latter is that the momentum transfer can result in a sudden change in motion of the service spacecraft. If the service spacecraft is required to be in the desired configuration, the control system needs to generate a considerable control output. This paper designs a new capturing strategy to avoid these adverse phenomena, which divides the capture process into two phases: colliding and locking. In the colliding phase, the gripper will actively contact the target’s surface and should maintain the contact, i.e., form a stable contact. This study proposes an improved damping-control scheme for forming a stable contact to realize this objective. Unlike the existing control scheme, this one allows the base of the service spacecraft to move, which helps to reduce the control force acting on the base, saving fuel. After establishing the stable contact, the gripper begins to close. The locking phase starts after forming a force-closure grasp. In this phase, the gripper will continue to close in a controlled manner. However, the states of the service spacecraft’s base and robotic arm should transform from the controlled state to the uncontrolled state. A significant benefit is that the control system does not need to produce large control outputs to resist the colliding impact. Meanwhile, since the service spacecraft has a large inertia, the momentum transfer caused by the collision does not cause significant changes in its configuration. Numerical simulations are performed to evaluate the validity of the proposed control scheme and capturing strategy at the end of this paper. The results indicate that the proposals allow the space robot to grasp the target spacecraft with less control cost.