Focusing servo system and focus servo acquisition enable method

摘要

A focusing servo system comprises a signal generating unit (7), a focusing servo unit (9), a zero crossing detecting unit, a search signal generating unit and a system controller (11). The signal generating unit (7) generates a focus error signal on the basis of the position of an objective lens (3a), through which a light beam is projected on an optical recording medium (1), with respect to a direction perpendicular to the recording surface of the optical recording medium (1) by using the reflected light beam from the recording surface of the optical recording medium (1), a read signal representing information read from the optical recording medium (1) on the basis of the reflected light beam and a focus acquisition enable range signal representing a focus acquisition enable range on the basis of the read signal. The focusing servo unit (9) receives the focus error signal from the signal generating unit (7) and executes a focusing servo control operation for focusing the objective lens (3a) on the basis of the focus error signal. The zero crossing detecting unit receives the focus error signal from the signal generating unit and detects a zero crossing point in the focus error signal. The search signal generating unit generates a down-search signal and an up-search signal respectively for driving the objective lens (3a) for movement toward and away from the recording surface of the optical recording medium (1). The system controller receives the focus acquisition enable range signal from the signal generating unit and a zero crossing detection signal from the zero crossing detecting unit, selectively supplies the search signals generated by the search signal generating means and the focus error signal generated by the signal generating unit (7) to the focusing servo unit, and executes a control operation on the basis of the focus acquisition enable range signal received from the signal generating means (7) and the zero crossing detection signal received from the zero crossing detecting unit to supply the focus error signal provided by the signal generating means instead of the search signal provided by the search signal generating unit in a predetermined time after the start of the movement of the objective lens (3a) away from the recording surface of the optical recording medium (1) during the movement of the objective lens (3a) from its initial position away from the recording surface of the optical recording medium (1).