Device for controlling the movement of a focal point of a light beam used for detecting recorded information

A device for controlling a focal point of a light beam for detecting recorded information, to be incorporated in an optical system for recording and reading out information comprising:

• focal lens for focusing the light beam on a recording medium;
• resilient means for supporting said focal lens and providing a balancing force substantially following the equation force =.beta.x, where .beta. is a coefficient and x is the position of the focal lens from a position of rest;
• focus actuator means including a drive coil connected to said focal lens, for moving said focal lens in accordance with a drive signal;
• photo detector means for detecting said light beam from said recording medium and producing a focus error signal;
• drive amplifier means for producing a drive current for said focus actuator means in proportion to the level of an input signal;
• starting signal generating means responsive to a focusing-on signal for generating a starting signal whose level increases with time after receipt of said focusing-on signal;
• comparator means for receiving said focus error signal and a reference level and producing an output signal when the level of said focus error signal passes said reference signal;
• and
• switching means connected to an input circuit of said drive amplifier means, and directly connected to and receiving the output of said comparator means, for selecting said starting signal when said output signal of said comparator is not present, and selecting said focus error signal when said output signal of said comparator is present.