A novel long-stroke fast tool servo system with counterbalance and its application to the ultra-precision machining of microstructured surfaces

This paper presents a novel long-stroke fast tool servo (FTS) system with counterbalance and its application to the diamond machining of microstructured surfaces. The FTS system is driven by a voice coil motor and guided by air bearings. A hybrid control algorithm which combines PID control, sliding mode control and feed-forward control was specifically designed for the system and ensures that it has less than 1% tracking error and achieves +/- 1 mm stroke and 105 Hz bandwidth. The counterbalance is achieved by the symmetric arrangement of two of the same FTS systems. System vibration decreased significantly from +/- 3 mu m to +/- 0.145 mu m when the counterbalance was working. Machining experiments were conducted using the FTS system on a diamond turning machine. A microstructured surface simulation model was built and information about the workpiece surface tested and collected. When the counterbalance was working, error in the machine axes decreased from micron-level to nano-level and the surface roughness of the microstructured surface was also reduced. The system achieves +/- 1 mm (+/- 0.5 mm) stroke at 30 Hz (40 Hz) with 0.35% (0.68%) tracking error during machining processes, and the depth error of the machined microstructured surface was measured at 1.4% (2.3%).