AUTOMATIC FOCUS LOCKING DEVICE

摘要

PURPOSE:To attain sure focus servo by transmitting a signal cancelling the effect of face deflection or warp to an optical disc having defects larger than the detecting range of a focus error signal so as to move the optical system into the vicinity of the focal point. CONSTITUTION:When the focus error signal having changed in the order of D' D B C' reaches a position E to eliminate the face deflection of an optical disc, a search voltage of an objective lens is held by a signal hold control section 19. A switch SW2 is closed at the same time and a step voltage sufficiently faster than the relative speed of the information plane to the optical system due to the face deflection of the optical disc and corresponding to a distance from the point E to the point A is fed from a voltage generator 24 so as to move the optical system in the vicinity of the focal point A forcibly. Then sure servo locking is applied and accurate focus servo is conducted further so as to attain stable and sure operation.