Friction Analysis of the Motion Suspension System for Improved Space Robot Testing

Future space missions will require the ability to manipulate satellites for repairing, refueling, and de-orbiting. This makes orbital and planetary space robotic arms a critical technology. Space robotic arms are designed to operate in zero gravity, but are tested on Earth. However, many space robotic arms are non-gravity-bearing and require a gravity support system. For this reason, the Institute of Robotics and Mechatronics at the German Aerospace Center (DLR) and the University of Duisburg-Essen have developed the Motion Suspension System (MSS), a space robot test facility. It is based on a cable-driven parallel robot that allows the space robot arm to operate on ground in the full three-dimensional workspace. To support the robot arm at arbitrary angles, the coupling interface between the MSS and the space robot arm is equipped with two passive joints with angle sensors. Their values are necessary to reconstruct the direction of the applied force, which is used for controller feedback and dynamics analysis of the space robot. As the joints are affected by friction, the measured angle might differ from the real one. This study proposes a experiment-based method to identify the friction. It reveals that the two joints are affected by viscous and static friction (stiction). However, the friction's influence on the performance of the MSS is minor in comparison to other error sources, such as controller and calibration errors. This finding contributes to a more detailed knowledge about the accuracy of the MSS which is important for using it as a verification and validation device for advanced non-gravity-bearing space robot arms.