Study on Adaptive Servo Control Strategy of WEDM Based on Discharge State Detector

Abstract

In wire electrical discharge machining (WEDM), with changes in the gap state or in workpiece thickness, the servo feedrate often fluctuates and the cutting process becomes unstable, thereby resulting in a decrease in machining efficiency and a degradation in surface roughness. In order to address the problems of unstable and inefficient cutting in WEDM, the servo control system in WEDM is studied in this paper. Firstly, the discharge state detector is designed and implemented based on a programmable System-on-Chip (PSoC), in which every single discharge pulse is discriminated by preprogramed FPGA logical circuit with adjustable thresholds. The thresholds of short circuit pulses and open circuit pulses are adjusted according to parameters of the pulse power supply. As each single pulse is precisely identified and counted, this device provides more accurate feedback information to the servo control system through a CAN bus. Secondly, based on the WEDM model obtained by online identification, a two-degree-of-freedom controller and an adaptive controller are designed respectively. With adaptive control, servo feedrate is adjusted based on the information provided by the newly designed discharge state detector in real time. The two-degree-of-freedom controller and adaptive controller are implemented in the Microcontroller Unit (MCU) of the WEDM numerical control system to realize the servo feedrate control in WEDM processes. Finally, experiments are conducted on the WEDM machine for verification. Compared with the conventional servo control methods, the proposed adaptive control with discharge state detector improves the cutting stability and efficiency significantly.