High Bandwidth Rotary Fast Tool Servos and a Hybrid Rotary/Linear Electromagnetic Actuator

Abstract

This thesis describes the development of two high bandwidth short-stroke rotary fast tool servos and the hybrid rotary/linear electromagnetic actuator developed for one of them. Design insights, trade-off methodologies, and analytical tools are developed for precision mechanical systems, power and signal electronic systems, control systems, normal-stress electromagnetic actuators, and the dynamics of the combined systems. A fast tool servo (FTS) is a high-speed auxiliary servo axis that is added to a diamond turning machine (ultra-precision lathe) to allow generating free-form nonaxisymmetric or textured surfaces on a workpiece. A rotary fast tool servo produces an in-and-out motion of the tool relative to a workpiece by swinging the tool along an arc having a fixed radius. The rotary fast tool servos developed in this project were designed for diamond turning prescription textured surfaces on small spherical workpieces (diameters in the range of 10 mm or less), and are suitable for generating free-form non-axisymmetric surfaces on similar-sized workpieces. Straightforward modifications would allow them to be used on larger workpieces. These rotary fast tool servos set new benchmarks for demonstrated closed-loop bandwidth (2 kHz and 10 kHz) and tool tip acceleration (400 g). The first machine, referred to as the 2 kHz rotary fast tool servo, uses a commercially available moving-magnet galvanometer as the actuator (Lorentz force), and provides proof-of-principles for a flexure bearing, small diamond tool and mounting method, circuit topology for a high bandwidth current-mode amplifier, and control system design. The following closed-loop performance is demonstrated for the 2 kHz rotary fast tool servo: -3dB bandwidth of 2 kHz, 20 g tool tip acceleration at 2 kHz, maximum tool travel of 50 μm PP, and tool position noise level of 10 nm PP. The 2 kHz FTS is integrated with a diamond turning machine and used to produce optical quality textured surfaces on the face and outside diameter of aluminum workpieces while operating at 2 kHz. The machining tests validate that a rotary-type fast tool servo can be used to produce optical quality surfaces on a spherical workpiece from its pole to its equator. The second machine, referred to as the 10 kHz rotary fast tool servo, incorporates