A rotary fast tool servo for diamond turning of asymmetric optics

Abstract

This thesis presents analysis, design, and control techniques developed for machines that diamond-turn asymmetric surfaces. Examples of this type of surface include spectacle and contact lenses, human lens implants, elements for laser eye surgery and camera lenses, as well as image-train elements in semiconductor processing equipment, laser printers, and other image processing equipment. The asymmetric features in such parts require a fast tool servo to synchronize the motion of the cutting tool with the motion of the workpiece spindle. At present, elements with low-amplitude asymmetry ( < 500[mu]m) can be fabricated on commercially-available machines. However, no device currently exists which can achieve appreciably larger stroke lengths along with the necessary accelerations and positioning accuracy to fabricate optics with significant asymmetry. The prototype machine developed as part of this thesis is specialized for diamond turning astigmatic spectacle lenses. This machine can turn 100 mm diameter parts having feature amplitudes of up to 3 cm and with 500 m/s2 peak accelerations and with micron-level form error. The fast tool servo uses a novel rotary arm design that carries a cutting tool through a circular path instead of along a straight line. Only the tool tip itself thus undergoes the highest translational accelerations. In contrast, all elements of a linear-axis fast tool servo accelerate at the same rate, and so contribute equally to the inertia. This feature allows a rotary fast tool servo design to achieve higher tool accelerations with reduced transmission of reaction forces and torques into the machine base than is possible with a comparable linear system. Associated with the rotary fruit tool servo are new approaches to tool path generation and control system design. The tool motion is periodic on the spindle angle, and thus can be represented as a summation of sinusoidal terms. The controller supplies high dynamic stiffness to the system mainly at integer multiples of the spindle frequency through a form of repetitive control. This existing technique has been further developed in this thesis to incorporate known system frequency response information. The new fast tool servo and associated algorithms developed herein have great promise for machining asymmetric surfaces with large-amplitude surface features.