A direct linear drive system with a laser interferometer and digital state-variable feedback control

Abstract

A linear driving technique with a small degree of wobbling for an optical disc cutting machine is discussed. The track-pitch in the order of one micrometre causes low-velocity linear driving, by means of which the stage holding the exposure optics is carried in the disc radius direction, while the disc is rotating. In order to obtain low velocity, a calculating technique using the displacement output from a laser interferometer was adopted. A voice-coil motor drove the moving part directly as an actuator, and, for maintaining the accuracy of the calculated digital velocity, a digital state-variable feedback control was applied. This paper describes the main features of the mechanical system and its mathematical model in the form of a pulse transfer function, the low-velocity detection using a laser interferometer and the stability of the laser interferometer, the digital state-variable feedback control, and the simulation and experimental results. Finally, a small amount of wobbling, less than ± 0.13 μm with 80 μm/s velocity was achieved, and the validity of the proposed technique was confirmed.