Design and control of an electromagnetic fast tool servo with high bandwidth

This study presents the design, control and implementation of a high bandwidth electromagnetic fast tool servo (FTS) that has the potential for use in diamond-turning of non-rotationally symmetric surfaces. The FTS is driven by a normal stress electromagnetic fast linear actuator and a custom current amplifier. It is controlled by a host-target real-time computer system. To achieve the high tracking and disturbance rejection performance required for the non-rotationally symmetric turning process, the controller is designed through a combined active disturbance rejection control and a feed-forward scheme. This combination exploits the unique disturbance estimation and compensation concept as well as the known reference acceleration signal. The controller design does not need an explicit mathematical model of the plant, and it employs an extended state observer to estimate the unmodelled plant dynamics, non-linear cutting force load, machine tool dynamics and other uncertainties. A digital first-order inertia filter is specially designed to reduce the effect of the position feedback signal noise. A prototype of the electromagnetic FTS with a travel of 50 mu m is developed using the design approach. The experimental results are presented to illustrate the design procedures and demonstrate that the FTS achieves 10 kHz closed-loop bandwidth, 0.011 mu m positioning error and 0.03 mu m maximum tracking error.