High accuracy motion control of linear motor drive wire-EDM machines

This paper aims to develop and investigate a permanent magnet linear-motor-driven table system for a wire-EDM machine. A dynamic model and system identification of `the linear motor system have been derived and analyzed. The linear motor drive system has nonlinear and time-varying behaviors because of the effect of irregular friction of the sliding surface and cogging force. Therefore, a conventional digital controller may not suffice to provide a high contouring accuracy as well as adequate disturbance rejection and parameter variation robustness. An indirect adaptive controller (IAC), combined with a neural network-based feedforward controller (NNBFC) is proposed to improve the contouring performance of the linear motor system. Experimental results not only indicate that the proposed control scheme can achieve a high contouring accuracy of +/- 0.3 mu m, they also demonstrate that the developed linear motor drive wire-EDM machine can meet the requirement for micro machining. The maximum contour error of circular trajectory at a feed-rate of 0.1 mm/min was significantly reduced from 8 mu m to 0.5 mu m in comparison with proportional-integral-derivative (PID) controller.