Hybrid dual-stage fast tool servo for integrated machining and measurement of optical microstructures

Abstract

This study presents the development of a dual-stage force sensor integrated fast tool servo (FS-FTS) for the integrated machining and measurement of optical microstructures. The modeling, design and control of the dual-stage FS-FTS are described in detail. Specifically, the normal-stressed electromagnetic actuator (NSEA) drives the primary stage, which provides a large stroke. The piezoelectric actuator (PEA) drives the secondary stage, which achieves a high bandwidth. The mechanical-electromagnetic model and mechanical-piezoelectric model of the two stages are established, based on which a simultaneous electro-mechanical parameter optimization method is proposed for the parameter design of the FS-FTS. Then, the closed-loop position controller and force controller are both carefully designed for the ultra-precision machining and in-process inspection of optical microstructures. Finally, a process that integrates the machining and measurement is developed and a 2-dimensional (2-D) water drop surface serving as the target surface is fabricated to demonstrate the performance of the developed system. The experimental result suggests that the stroke of the primary stage achieves 288 μm and the first-order resonant frequency of the secondary stage achieves 8520 Hz. The root-mean-square (RMS) tracking error is decreased from 1.0009 μm to 0.1521 μm by using the proposed primary-secondary coordinated position controller when tracking the toolpath of the 2-D water drop surface. Furthermore, the RMS error of the fabricated surface is only 125.7 nm when the spindle speed is 1600 r/min and that of the in-process inspection process is only 55.7 nm. These results demonstrate the superiority of the developed system in terms of integrated machining and measurement.