Design and control of a fast long range actuator for single point diamond turning

This dissertation focuses on the design and control of a fast long range actuator to machine non-rotationally symmetric (NRS) optical surfaces with millimeters of sag at high production rates. The goal is to retain the surface quality (form error of less than 200 nm PV and surface finish of less than 5 nm RMS) of existing diamond turning machines while moving the tool over a range of 4 mm at a frequency of 20 Hz.;The actuator in this dissertation features a light-weight slide supported by an air bearing, the tool feed motion is controlled by a linear motor, a linear encoder and real-time control platform. The first actuator prototype was built and tested in 2004-2005, but its performance was found to be unacceptable due to various deficiencies. This dissertation research has developed a methodology for the design and control of this type of actuator to optimize its performance. It essentially takes a three-step approach: investigate the characteristics of the tool motion control in the diamond turning in terms of tool motion trajectories, disturbances to the tool motion, and the required tool positioning precision; develop an actuator system to meet motion control requirement; integrate the actuator with a diamond turning machine and conduct precision machining and precision metrology for performance validation. In this research, the original actuator is modified and upgraded with new system components including a linear amplifier, two control platforms, two linear encoders and an add-on counterbalance drive. Effective feedback and feedforward control techniques for profile tracking and disturbance rejection are investigated and implemented. Critical implementation issues for motion planning are resolved to improve the quality of motion trajectory generation and the quality of motion synchronization while machining. Modification of the first prototype has pushed the limits of performance on both tool motion control (+/-30 nm for position holding error and +/-70 nm for 2 mm 20 Hz sinusoidal tracking error) and machining results (plated copper flat with 7.4 nm RMS surface finish for, non-rotationally symmetric PMMA tilted flat with 50.8 mm diameter and 4 mm excursion with 0.9 mum PV flatness error and 17 nm RMS surface finish). Further improvement performance depends on a total redesign of the actuator. The desirable system component characteristics to achieve required tool positioning quality, of the slide piston, the air bearing, the amplifier, the motor and counterbalance, are proposed and analyzed. Critical system configuration issues regarding the amount of moving mass, the size of the motor, the sampling rate of digital control system and the necessity of physical damping are also addressed.;Finally, by creating a biconic mirror with fiducial features for kinematic coupling, this dissertation has also proven the feasibility of fabricating real-world optics with this type of actuator.