On-line Workpiece Height Estimation and its Application in Servo Feed Control of WEDM Process

On-line estimation of workpiece height is essential for wire breaking prevention in WEDM process. In addition, it can provide the reference of machining parameters setting for increasing machining speed and stability. A new on-line WEDM workpiece height estimation system for the iso-energy type wire EDM machine is presented. Based on the proposed specific discharge energy (SDE), a mathematical model relating workpiece height, feed rate and discharging frequency by a proportional constant is derived. This constant coined thickness identification coefficient (TIC) is found a function of workpiece height. Hence, a simplified approach is proposed. The value of TIC of a known workpiece height is taken to estimate the workpiece height during the WEDM process. The actual workpiece height is obtained by multiplying the estimated value by the correction factor of the associated workpiece height. Experimental results show that the system works satisfactorily. The estimation error of workpiece height is within 1 mm while the response time is less than 1 second in general. In the practical application of machining a workpiece with dynamic variations in thickness, a model relating optimum servo voltage and workpiece height that leads to maximum machining speed is established in advance. The servo voltage is adjusted according to the estimated height. It is found that the process can be maintained very stable at maximum machining speed without wire breaking.