Servomechanism using traction drive (Improvement of servo accuracy at high-speed rotation)

The authors attempted to apply traction drive to a constant-speed servomech anism for Numerically Controlled (NC) machines. High-precision servomechani sms require high-resolution encoders to detect rotary position, whereas con ventional high-resolution encoders cannot rotate at high speed. Therefore, the authors attempted to solve the problem of resolution and maximum rotary speed using a nonsinusoidal-wave quadrature Phase-Locked Loop (PLL). Using this nonsinusoidal-wave quadrature PLL for interpolating the tracking erro r, a very high-resolution detection of rotary angle at high-speed rotation was achieved even when using inexpensive encoders whose output is nonsinuso idal wave. Traction drive damps the high-frequency torque ripple, but trans mits torque ripple of low frequency. Most of the torque ripple of low frequ ency is due to the eccentricity of rollers. As the rotary speed of rollers becomes high, the torque ripple has a frequency at which it cannot be elimi nated either by traction drive or feedback control. Because the mechanism o f torque ripple is very complicated, it is difficult to compensate hy detec ting roller eccentricity. Therefore, the authors used repetitive control to reduce the position error associated with roller rotation, and obtained hi gh-accuracy rotation in the experiments. In this paper, the interpolation m ethod of tracking error using a nonsinusoidal-wave quadrature PLL and the r eduction method of tracking error using repetitive control are presented.