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 non-axisymmetric 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).

(cont.) 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 [mu]m 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 the proof-of-principles from the first machine and is the vehicle for developing the hybrid rotary/linear electromagnetic actuator used in it.

(cont.) The actuator is a normal-stress variable reluctance machine with a demonstrated order of magnitude increase in the peak torque and in the ratio of peak torque divided by the electrical power at its terminals, when compared to the actuator used in the 2 kHz FTS. By integrating the tool holder directly to the moving mass of the actuator to form a single rigid body, the overall torque-to-inertia ratio for the system and the frequency of the first uncoupled-mass resonance are both increased. The following closed-loop performance is demonstrated for the 10 kHz rotary fast tool servo: -3dB bandwidth of 10 kHz, 400 g tool tip acceleration at 5 kHz, 870 g tool tip acceleration at 10 kHz (aided by a stable mechanical resonance), maximum tool travel of 70 [mu]m PP, and tool position noise level of 1.4 to 2.5 nm rms (depending on the magnitude of the bias flux used). The hybrid rotary/linear electromagnetic actuator utilizes a constant bias magnetic flux, which linearizes the torque versus drive-current relationship for the actuator and provides up to half of the torque-producing magnetic flux in the rotor/stator air gaps. The actuator is similar to the rotary actuators used to drive and sustain a resonance in a mechanical oscillator in certain electric engraving heads.

(cont.) This research is distinguished from the prior art by the ability to generate closed-loop arbitrary trajectories for the tool tip. Using a separate current-mode amplifier for each stator half allows demonstrating closed-loop control of the rotary and linear degrees of freedom that are inherent in this class of actuators. This research is further distinguished from the prior art by a magnetic circuit that substantially decouples certain magnetic flux paths when a coil is used instead of a permanent magnet to provide the bias magnetic flux. This reduces the complexity of the actuator electrical dynamics from a MIMO system to a SISO system, and allows using loop-shaping techniques with classical control theory to design the control systems. Torque control for the hybrid rotary/linear actuator in the 10 kHz FTS is independent of force control, but force control requires a torque-generating current to act as an operating point. Alternate magnetic circuit topologies that fully decouple torque and force control are described and compared. Future work that utilizes the linear mode as an active suspension for improving the performance of a predominantly rotary system is considered. Using the experience gained by designing, building, and testing the 10 kHz FTS and hybrid rotary/linear actuator, future work involving alternate concepts for the actuator is suggested for a follow-on rotary fast tool servo, and a high bandwidth steering mirror.