High-precision velocity control of direct-drive systems based on friction compensation

Friction is a complex nonlinear behavior and a significant factor that limits the performance improvement of servo systems. Drawing inspiration from the particular prestiction friction phenomenon exhibited by direct-drive systems upon sudden emergency stops, this paper introduces a dynamic and continuous friction model that includes pre-sliding and gross-sliding regimes. By analyzing the friction dynamics when the system velocity briefly reaches zero, a concave function related to the previous state of the system is used to describe the transition of friction in the pre-sliding regime. The Stribeck model is employed to represent the friction behavior in the gross-sliding regime, ensuring stationarity during friction regime switching. Based on the established friction model, a friction compensation method is developed in velocity control mode. The superior performance of this proposed friction compensation method is confirmed through sine-tracking experiments. Compared with the proportional integral controller and the Stribeck friction compensation method, the peak-to-peak value of the proposed method is reduced by up to 61.1 %, and the root-mean-square (rms) value is reduced by up to 81 %, with the smallest rms value reaching 0.13 mrad, significantly improving the dynamic tracking performance of the system.