Stable and Fast Electrical Discharge Machining Titanium Alloy with MIMO Adaptive Control System

Abstract

In machining titanium alloy by electrical discharge machining (EDM), low heat conduction of titanium alloy often results in the temperature in the gap between electrode and work-piece rising so fast that the deionization of dielectric liquid after pulse discharging tends to be incomplete and thus induces large amount of arcing pulses in the gap burning the alloy surface. In this paper, we suggested to solve the issue of gap temperature rising fast from two aspects. First, as short ratio quantifies the extent of dielectric deionization in the gap, the electrode discharge time, which determines the amount of input energy to machine titanium alloy, should be adaptively regulated in terms of short ratio; this control action performs the function of inhibiting the heat accumulation. Second, as the heat conduction of titanium alloy is quite low, gap distance decided by servo voltage should also be tuned adaptively in terms of discharging ratio which is influenced by the heat in the gap. In this approach, two control variables were designed to form a multiple-input, multiple-output (MIMO) adaptive control system for EDM in which a two-step-ahead prediction control strategy, based on the real-time estimation of EDM process model, was employed to compute the two control variables. Confirmation experiments demonstrated that this control system in machining titanium alloy can adjust electrode discharge time and servo voltage simultaneously with respect to different machining situations to drive both the gap state, fluctuating smoothly with small amplitude around gap state expectation and the discharge ratio following discharging ratio expectation. Compared with EDM without the assisting of adaptive control systems, this MIMO EDM adaptive control system displays superior both the machining stability and the machining efficiency.