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

Diamond turning has become an important fabrication technique for producing 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 their axis motions and the limited update speed of their controllers. Although a fast tool servo overcomes the bandwidth limitation, the problem of integrating a fast control process (for the servo axis) and a slower control process (for the slide axes) remains. This article describes the computer hardware and software required to integrate a high-speed, low-amplitude fast tool servo into a conventional T-based diamond turning machine. This system can machine 0-dependent features, synchronized to the radial and axial position of the tool, up to the displacement range of the servo. A set of interface boards have been designed and built that pass the position feedback data from the laser interferometer to both a high-speed servo controller and a slower slide axes controller. This design allows the fast tool servo to be an independent add-on accessory to the diamond turning machine and successfully incorporates nonrotationally symmetric fabrication capability. As an example 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).