Research On Prediction And Compensation Control Methods Of Friction For The Feed System In CNC Machine Tools

The motion accuracy of the feed system is an important technical index to evaluate the performance of machine tools.It determines the machining precision,the surface quality and the production efficiency to a great extent.Therefore,the prediction and compensation control of friction has attracted attention of many scholars.The motion precision of the feed system has been greatly improved by the compensation control of friction.The paper took CNC lathe feed system as the research object,the idling and cutting tests were conducted on two CNC lathes,in which the position of the worktable and the servo motor torque current are collected,friction calculation method was deduced,feedforward compensation controller was designed.The following research were completed specifically.(1)Respectively,the idling and cutting tests were tested in two different types of CNC lathe feed system,FOCAS2 functions from the FANUC CNC system was used to obtain the position of the table and the servo motor torque current with time,the feed system friction was calculated.The results show that the frictional force fluctuates around the instantaneous mean value with the change of position,and the instantaneous mean value changes with the position,and then the algorithm of instantaneous mean polynomial fitting was proposed and its influencing factors are analyzed.(2)Using the experimental data,two friction models named Coulomb + viscous friction model and Stribeck friction model were compared,the results show that the two friction models can describe the variation of the average friction force with the feed rate of the whole feed stroke well.However,there is a big difference while describing the instantaneous mean value variation with the velocity at certain positions,and then a double polynomial fitting model was proposed to predict friction varing with position and velocity.(3)The mathematic model of the position control system of CNC linear motion table was established,and the compensation controller was designed,and then the friction controller based on Stribeck friction model and double polynomial fitting model were compared,the results show that the feed-forward compensation based on position-velocity friction model can effectively eliminate the unfavorable factors caused by friction and improve the precision of the feed system.