Servo control system for optical system which focuses a converging light beam on a recording disc

摘要

A servo control system includes an error amplifier for deriving an error signal representing the displacement of a focus point of laser beam from different planes of an optical record disc, a differentiator for differentiating the error signal to detect its rate of variation, and a comparator for detecting the maximum peak of the differentiated signal which corresponds to reflections from the plane on which information is recorded. The focus point is moved in a direction toward the disc in an open loop mode until the maximum peak of the differentiated signal is detected, whereupon the focus point is controlled in response to the error signal in a closed loop mode to keep track of the recording plane.