Frequency response data-based feedforward control strategy for ultra-precision tool servo diamond turning of freeform surfaces

Abstract

Tool servo diamond turning faces significant challenges when machining freeform surfaces at relatively high speeds, primarily due to tracking errors caused by the limited bandwidth of the servo axis. Integrating an additional feedforward controller with existing controllers is often necessary to enhance tracking performance. However, the design of a closed-loop inversion-based feedforward controller generally relies on an accurate transfer function of the controlled system, which is frequently compromised by unavoidable modeling errors. To address this issue, this paper proposes a novel frequency response data (FRD)-based feedforward control strategy for trajectory modification in tool servo diamond turning. This strategy directly utilizes the FRD to design a flexible-order finite impulse response filter that approximates the inverse behavior of the servo axis. A constrained optimization problem is formulated to obtain a flat amplitude and phase frequency response within the interested bandwidth, with the differential evolution algorithm adopted to determine the optimal filter parameters. Experimental validation on a commercial ultra-precision lathe confirms the effectiveness of the proposed strategy. Comparative results reveal a substantial enhancement in the form accuracy of the machined freeform surfaces, achieving a 70% reduction in the PV error and a 96% reduction in the RMS error compared to those obtained without the feedforward controller. The simplicity and efficiency of this strategy make it well-suited for industrial ultra-precision machining applications.