Friction in Space – Analysis of Robotic Joint Friction in Space Conditions

Robotic joints in space behave in a different way as they do on ground. Although ground testing in thermal and vacuum chambers offers an insight into the behavior of the joints under simulated space conditions, long-term experiments under real conditions provide a more valuable insight into the expected behavior. The German Aerospace Center (DLR) carried out the Robotics Component Verification on the ISS (ROKVISS) experiment from 2005 until 2011, where a two-joint robotic arm was mounted outside the International Space Station (ISS) exposed to temperatures between -20°C to +40°C. In this work, we retrieve the original experimental data and use viscous-Coulomb friction models for both joints in order to quantify the temperature dependence of each parameter. Our new analysis shows that for the evaluated data most of the friction can be modeled with static and viscous friction, which are both proportional to temperature. For load-dependent Coulomb friction both the temperature influence and the contribution to the total friction are small. We further present estimated friction model parameters over temperature with confidence bounds, quantifying the differences in joint friction for both joints.