Research On Process Control System For Small-Hole Electrical Discharge Machining

The structures of small-holes are widely used in auto industry, aerospace, medical science and micro electronics. Electrical Discharge Machining (EDM) is capable of machining all conductive materials with neglectable forces and irrespective to the material hardness and strength. These characteristics of EDM establish its predominant situation in the fields of small-hole machining. A stable and high efficiency EDM process depends on a robust process control system, including the servo control, the pulse control and other control units for the EDM processes. The fundmental part of the process control of EDM is the on-line gap status detecting.The design target of the pulse generator is determined based on the analysis of the mechanism of small-hole EDM. The main discharge circuit is improved by separating the charge and discharge processes of the capacity. In order to address the difficulty of debris evacuation, the sweep pulse, which has a short pulse duration and a high discharge energy, are applied to improve the machining efficiency remarkbly. A pulse generator is developed based on the pulse control technology. To shut off harmful pulses and apply sweep pulses, the control for pulses is achieved.In order to detecte the gap status accurately, the approach of the wavelet transform EDM discharge status detecting method is introduced by combining with digital signal processing technology. The experimental results show that the pulses can be distinguished and identified well by using the wavelet transform detecting method. This approach is capable of being applied in small-hole EDM process control together with a proper control strategy. In order to realize the on-line gap status detecting with wavelet transform method, a real-time signal acquisition and processing system is developed. A small-hole EDM servo control system is designed to adapt the time-varying and non-linear characteristics of small-hole EDM process. A self-tuning regulator is used to adjust the reference voltage, and a fuzzy neural network is used to realize the reference voltage following. The experimental results show that the percentage of the harmful pulses in small-hole EDM process is reduced and the machining efficiency is improved. The stability of the fuzzy neural network is also studied and validated by the experimental results.A series of experiments for deep small-hole drilling have been carried out and the effectivness of the process control system developed in this paper is verified. In order to make further efforts on machining efficiency improving, a voice coil actuator vibration platform is developed. The experimental results show that the machining efficiency of vibration assisted small-hole EDM is improved significantly with proper driving frequency and amplitudes.