Research On Gap Discharge Characteristics And Adaptive Energy-saving Pulse Power Of Electrical Discharge Machining

Electrical discharge machining(EDM)is one of the most widely used methods of machining parts,especially in aerospace industry,mold industry and other fields.EDM is a complex process affected by many factors,so it has randomness and uncertainty.But currently research on discharge gap characteristics,theoretical basis and machining mechanism of EDM are insufficient,which have become bottleneck of restricting its further development.Pulse power as heart of EDM tool determines energy utilization rate,processing efficiency,stability of machining process,automation level and other indicators of EDM.Especially with the development of green and intelligent manufacturing,research on adaptive energysaving pulse power technology of EDM is the priority.Therefore,study on discharge gap characteristics and adaptive energy-saving pulse power technology of EDM has important theoretical significance and application value.Aiming at the problem of discharge gap characteristics of EDM,formation of spark discharge maintaining voltage and its oscillation,discharge gap characteristics of different modes pulse power are studied.Forming conditions of spark discharge maintaining voltage and its oscillation cause are analyzed.Through experiments and analysis of discharge main circuit of different modes pulse power,it is thought that spark discharge maintaining voltage exists in transistor inductor pulse power,transistor resistor pulse power and RC pulse power of EDM,but their manifestations are different.Then a method of spectrum analysis of spark discharge maintaining voltage by Fast Fourier Transform(FFT)is proposed.Influence of current-limiting resistance and open-circuit voltage on oscillation frequency of spark discharge maintaining voltage is studied by adopting this method in transistor resistor pulse power.For transistor inductor pulse power,effects of breakdown delay time,inductance and open-circuit voltage on spark discharge maintaining voltage and current are studied.Influence of capacitance on spark discharge maintaining voltage is studied in RC pulse power.For TRC pulse power,effect of capacitance on gap discharge energy per unit time is also studied.Through above research,it provides guidance for reasonable selection of electrical parameters and establishment of spark discharge gap equivalent circuit model.On the basis of research on discharge gap characteristics of EDM,spark discharge gap equivalent circuit model is studied.A method of determining spark discharge gap equivalent circuit model by pure capacitor discharge is put forward.The rationality and universality of the equivalent circuit model are verified by simulation,experiments and theoretical analysis.Based on this,mathematical model of gap spark discharge control system is established.Input and output of the control system are voltage of pulse power and gap voltage,respectively.Influence of pulse power parameters on stability and response speed of spark discharge is analyzed theoretically.It provides theoretical guidance for design of pulse power and matching optimization of processing parameters.In order to improve energy utilization rate of energy-saving pulse power,energy distribution of discharge main circuit of transistor inductor energy-saving pulse power,energy loss suppression technology of power switch,energy fast feedback circuit and suppression measures of energy loss of high frequency wire are studied.Switching characteristics of insulated gate bipolar transistor(IGBT)and causes of energy loss are analyzed.Based on this,push-pull output driving circuit and snubber circuit of IGBT are designed.A novel passive lossless snubber circuit of IGBT suitable for transistor inductor energy-saving pulse power of wire cut electrical discharge machining(WEDM)is put forward,which realizes zero current turn-on and zero voltage turn-off of IGBT.Energy fast feedback circuit of transistor inductor energy-saving pulse power is designed.A method of reducing energy loss of high frequency wire by adopting multi-strand silver-plated high frequency shielded wire is proposed.Above measures could improve energy utilization rate of energy-saving pulse power.To improve discharge state detection accuracy,spark discharge rate,energy utilization rate of energy-saving pulse power and machining stability of WEDM,pulse statistical method for relative time rate of discharge states,long short-circuit state suppression and adaptive control technology of discharge state are studied.A method of pulse statistics for discharge state signal by high frequency pulse modulation is proposed,which improves detection accuracy of open-circuit rate,spark discharge rate and short-circuit rate and lays the foundation for adaptive control of discharge state.A method of combining small pulse modulation for long short-circuit and machine tool table back is proposed to suppress long short-circuit state.Adaptive control system for discharge state is established based on floating threshold discharge state detection and proposed method of pulse statistics for discharge states.The adaptive control system adopts recursive least squares parameter identification with forgetting factor and minimum variance self-tuning control algorithm.Process input and output of the adaptive control system are feed frequency of machine tool and difference between open-circuit rate and short-circuit rate,respectively.The effectiveness of proposed energy-saving technology and adaptive control technology is verified by wire electrical discharge machining(WEDM)experiments.The stable processing with high spark discharge rate is realized.Energy utilization rate of energy-saving pulse power and processing speed are improved.