Sliding-mode controller and algorithm for improving servo control of discharge gap in precise fast hole EDM

Abstract

Fast hole electrical discharge machining (EDM) is widely used in drilling circular holes with sub-millimeter diameters on metal alloy materials. As a kind of non-contact machining process, the discharge gap between tool electrode and workpiece significantly affects the machining efficiency. Ordinarily, this process can achieve high machining efficiency based on large discharge energy and high pressure inner flush. However, in case of demanding high shape precision and surface quality, such as film cooling holes, the discharge energy needs to be reduced. Thus the electrode wear ratio is increased and the removal of machining debris becomes more difficult. Consequently, the control of discharge gap gets hard and then the machining efficiency is significantly decreased. In this research, a nonlinear velocity controller based on sliding mode control (SMC) method is proposed to improve servo control of discharge gap in precise fast hole EDM. The dynamic model of fast hole EDM is firstly established to provide the physical basis for the controller design. Then a sliding mode algorithm is designed based on Lyapunov stability theory to improve the anti-interference of the control system, and the dynamic characteristics are analyzed through simulations based on the state equation and control algorithm. Furthermore, the performance of the proposed method is discussed by comparing the tool electrode movement, gap average voltage and utilization ratio of pulses measured in experiments. The experimental results showed that the proposed method can increase machining efficiency by 38.1%.