A CONTROLLER ARCHITECTURE FOR INTEGRATING A FAST TOOL SERVO INTO A DIAMOND TURNING MACHINE

Diamond turning has become an important fabrication technique for prod ucing reflective optics. However, the generation of a general optical surface requires the capability to fabricate nonrotationally symmetric surfaces. Current commercial diamond turning machines cannot perform this task at production rates because of the limited bandwidth of thei r axis motions and the limited update speed of their controllers. Alth ough a fast tool servo overcomes the bandwidth limitation, the problem of integrating a fast control process (for the servo axis) and a slow er control process (for the slide axes) remains. This article describe s the computer hardware and software required to integrate a high-spee d, low-amplitude fast tool servo into a conventional T-based diamond t urning machine. This system can machine 0-dependent features, synchron ized to the radial and axial position of the tool, up to the displacem ent range of the servo. A set of interface boards have been designed a nd built that pass the position feedback data from the laser interfero meter to both a high-speed servo controller and a slower slide axes co ntroller. This design allows the fast tool servo to be an independent add-on accessory to the diamond turning machine and successfully incor porates nonrotationally symmetric fabrication capability. As an exampl e of the surfaces possible with this system, an off-axis segment of a parabolic mirror has been machined on-axis. The peak-to-valley figure error of this 125-mm optic is less than 1.1 waves (0.7 mum).