Static Friction in a Robot Joint – Modeling and Identification of Load and Temperature Effects, Report no. LiTH-ISY-R-3038

Friction is the result of complex interactions between contacting surfaces in a nanoscale perspective. Depending on the application, the di erent models available are more or less suitable. Static friction models are typically considered to be dependent only on relative speed of interacting surfaces. However, it is known that friction can be a ected by other factors than speed. In this paper, the typical friction phenomena and models used in robotics are reviewed. It is shown how such models can be represented as a sum of linear and nonlinear functions of relevant states, and how the identi cation method described in [1] can be used to identify them when all states are measured. The discussion follows with a detailed experimental study of friction in a robot joint under changes of joint angle, load torque and temperature. Justi ed by their signi cance, load torque and temperature are included in an extended static friction model. The proposed model is validated in a wide operating range, considerably improving the prediction performance compared to a standard model.