Velocity matching compliant control for a space robot during capture of a free-floating target

Space robotics provides a reliable solution to accomplish manipulation tasks in orbit without the presence of astronauts, an endeavor that is both dangerous and costly. A particular field of application is the servicing of a defective target satellite by a servicer satellite equipped with a robotic arm. Considering the phases a servicing mission may have, one of them is particularly critical: the contact phase between the robot and the target. The large forces involved as well as the short time-scales during which these forces are acting represent a challenge for the design of such missions. Therefore it is required to determine the most appropriate control strategy before, during and after the contact. This work aims to address the issue of finding the aforementioned strategy, under the condition that, after the contact, the servicer and the target satellites have the same velocity. In this way the capture of the target is guaranteed. In this paper, a novel method for tuning an impedance control scheme is presented, which ensures post-impact velocity matching between the servicer and target satellites. The method is based on the assumption that no knowledge on the position and velocity of the target is available. Lacking information on the contact duration, a passively compliant element is introduced in order to prolong the contact. The resulting dynamical system is analyzed in time domain with a simplified model. As a result, optimal parameters for the controller as a function of the target mass and the passive compliance parameters are obtained. Simulations are carried out to validate the theory which are confirmed by experiments on a one degree of freedom robot.