Research On Wire Constant Tension Control And Workpiece Height Estimation In HS-WEDM

Lower surface quality problem has always bothered the further development of High Speed Wire Electrical Discharge Machining(HS-WEDM),which is difficult to satisfy the surface quality demand of modern industry especially for the structural parts with a complex shape or thickness variation,however widespread HS-WEDM machine tool has been applied across industries as a typical WEDM equipment in our country.In order to enhance the machining precision of HS-WEDM,by reducing wire electrode vibration and improving processing environment of variable thickness workpiece,critical technologies of wire electrode constant tension control and online real-time workpiece thickness identification were studied in this dissertation.The major factors affecting wire electrode vibration of HS-WEDM and theirs influence rules on wire electrode tension variation from two aspects of the theoretical analysis and experimental study,especially focusing on the effect of explosive force generated by spark discharge on the wire electrode vibration,were investigated comprehensively and deeply on the basis of the anal ysis of domestic and overseas wire electrode tension control technologies.In order to further comprehend the effect of discharge force on wire electrode vibration,aiming at the present situation that discharge force loaded in wire electrode vibration simulation is constant loading,the impact force generated by spark discharge was simulated by explicit dynamic analysis software Autodyn taking into account the modification of explosive material attributes,which is based on the underwater explosion theories.A physical model of the discharge process in WEDM was established,according to the actual dimensions and material attributes of WEDM,and its simulation results indicated the surface pressures at different positions of the wire electrode and the changing regularity of discharge force on wire electrode with time.In view of the problem of most researches on wire electrode vibration adopting the static simulation method,a physical model of continuous discharge processing was established by utilizing ANS YS Workbench software.A method which dynamic discharge force load at a random location on discharge area had been provided.Transient dynamics simulation analysis was conducted to study on the displacement of wire electrode at different positions by loading a dynamic discharge force in each pulse period,which is on account of the previously obtained changing regularity of the dynamic discharge fore of wire electrode.Trend of displacement for different position on X-axis and Y-axis was obtained.The changing regularity of the maximum displacement of wire electrode surface along its entire length was also investigated,and on this basis,the effect of discharge force on the oscillation amplitude was investigated in this dissertation.A wire electrode constant tension control device of HS-WEDM was developed in order to restrain wire electrode tension variation,reduce wire electrode vibration,and improve the quality of WEDM.On the basis of existing research on the structure characteristics of wire drive device and the tension sensor,wire electrode tension detecting modular structure and driving module driven by direct current servomotor of one-dimensional motion platforms were optimized,which can not only achieve automatic control function of electrode wi re tension,but also achieve wire broking protection and automatic wire tightening function.A physical model of wire electrode tension established considering the effect of frictional force with forward and reverse of wire electrode had been established.The dynamic tension characteristics of reciprocating wire electrode in wire drive device were researched.And the structure parameters of wire system optimized for making the same wire tension with the movement of wire electrode in forward and reverse of wrapping.It was optimized to improve the processing stability of WEDM.This paper provided a control strategy to remove the constant difference of wire tension which caused by tension sensor position.On account of the characteristic of the fluctuation of wire electrode tension,a constant tension control system based on the PID control strategy with dead zone was proposed to avoid oscillation caused by too frequent for motor control action.And a transfer function model of constant tension control system was established to study the fast responsiveness of the control system.It was found that the time for responding steady state was 5.5ms when dead zone was 0.2.In order to realize high precision machining of variable thickness workpiece in HS-WEDM,the online real-time identification technique of workpiece thickness was researched primarily in this dissertation to study the main factors affecting workpiece thickness identification,such as energy distribution coefficient,slit width,average voltage and current,servo feed rate of machine tool,etc.Aiming at the problem of the measurement of the slit width in WEDM is difficult to measure directly;a mathematical forecasting model for slit width was established based on the grey system theory,which correctly predicted the slit width with different machining parameters.On the basis of this,a mathematical model for online real-time workpiece thickness identification was established by utilizing multivariate nonlinear regression method of MATLAB software to real time prediction of workpiece thickness with different machining parameters of HS-WEDM.The dynamic response characteristics of the wire constant tension control device were studied by means of the WEDM experiment platform.T he control accuracy whose minimum value is ±0.25 N for constant tension control system had been obtained in different wire speed and different wire tension.Research had shown that the application of constant tension control system could reduce the roughness 10% in fist cutting and multi-cutting.Moreover,the influence of wire tension on machining efficiency and machined surface quality at different stages in multi-cutting of WEDM was investigated.The predicted value of process width and workpiece height had been forecasted.And the error between true walue and predicted value of process width was less than 2.9%.The error for workpiece height was almost less than 12%.These results proved the feasibility and validity of the process width forecasting model and the online real-time workpiece thickness identification model.The better machining parameters for workpiece machining with different height had been carried out to ensure the same surface roughness in different workpiece height.At the same time,the same discharge gap ensured the same allowance for machining,which could improve finish machining environment.