Development and test of a robotic arm for experiments on close proximity operations

Future spacecraft will be provided with special features to enable On-Orbit-Servicing missions. Such features include capture interfaces, proximity sensors, and algorithms for autonomous RVD (Rendezvous and Docking). The reliability standards of the space industry require all these technologies to be tested in relevant environment; to this purpose, several ground testing facilities have been built during the last years. They reproduce many aspects of the free-falling motion of a spacecraft often exploiting two technologies: robotic systems and low friction tables. The first use a virtual environment to compute the effects of the external forces and reproduce the motion of a spacecraft with a robotic arm; the second unbind vehicle mock-ups from friction effects, allowing them to experience a free motion at least in three degrees of freedom. Merging together these two types of facilities gives the possibility to further extend their capabilities. This work presents a series of tests conducted on a custom robotic arm, intended to be integrated with a low friction table. The tests focus on two main aspects: (i) the accuracy of the positioning capability and (ii) the dynamic behaviour of the robotic arm. Both aspects are directly connected with the final purpose of operating in conjunction with a vehicle floating on a low friction table. The paper also presents some improvements of the robotic arm suggested by the results of the tests to fit the requirements on the positioning accuracy. These are represented by three main achievements: (i) the accuracy is comparable with the principal navigation sensor; (ii) the vibrations are below the error of the navigation sensor, and (iii) the ability of the robotic arm in performing a capture task.