Challenges in Closed-loop Compliant Motion Control for Planetary Robotics

While tool frame force control has been deployed widely on terrestrial articulated arms, planetary robotic systems from NASA’s Jet Propulsion Laboratory have thus far only employed simplistic open loop strategies for regulating their forceful interactions with a robotic arm. This proved perfectly sufficient for surface sampling operations. However, the Mars Sample Return Campaign’s Sample Retrieval Lander will need to execute comparatively delicate operations when inserting sample tubes into the Mars Ascent Vehicle for return to Earth. This paper describes some of the challenges that have arisen during development of the first continuous, closed loop force controller on a robotic arm for this purpose. Constraints including computational power and radiation hardened force sensing fidelity are explored, with an emphasis on rigorous assessment of stability and wrench control performance for the flight model. The controller’s performance is then characterized using these models and shown to conform to present system requirements.