Friction Modeling and Compensation for Feed Drive Motions of CNC Milling Machines

Friction exists in the feed drive servomechanisms of CNC milling machines and usually deteriorates their motion. Although many studies have demonstrated good motion results by applying friction compensation, the simplified model of friction still limits the motion accuracy of feed drives of CNC machine tools. In this study, breakaway and constant velocity experiments were performed to obtain data for analyzing the static and velocity-dependent friction of a feed drive servomechanism in CNC milling machines. Moreover, different payload conditions were considered in developing friction compensation lookup tables and velocity-dependent friction models for reducing the adverse effects induced by static and velocity-dependent friction. An integrated friction model that considers the payload effects and position dependency of friction was thus developed in this study for friction compensation, and a feedforward friction compensation structure was further developed for controlling the motions of the feed drives of CNC milling machines. Several experiments and motion tests were performed on a three-axis CNC milling machine to illustrate the feasibility of the friction compensation method developed in this study. The experimental results indicate that the friction-compensated motion control system with the integrated friction model developed in this study achieves a reduction of 29.13% (on average) for the maximum value and a reduction of 42.58% (on average) for the root-mean-square value of the contouring errors.