Analysis Randon Causes Repeatability Errors Inducted by Friction at Joints in Industrial Robots

The present study was carried out to investigate and analyze the positionning repeatability introduced by friction variations based on stochastic ellipsoids. A mixed friction model has been developed with improved properties compared to existing standard models. The contact is presented as a multitude of micro contacts whose nature can be of two types: lubricated and solid. This model is experimentally tested on a reciprocating tribometer under extreme friction conditions, with sliding speed varying from 0.1 to 3 m/s and load modified from 40N to 150N to discuss the effect of speed, the effect of nominal contact pressure and the effect of sliding distance on friction parameters. The results showed how this model can be represented as a sum of functions of the relevant states, which are linear and nonlinear in the friction parameters. Thus, these results were used to evaluate the covariance matrix in order to locate the different ranges of errors which have an impact on the repeatability of position.