Beam position control system of an optical disk unit

Abstract

This invention enables one, or both, or none of position servo control and position lock control of a galvanomirror, to control a read beam to an optimum position under any of on-track state, seek state, position setting state after the completion of a seek operation, and positional deviation existing state between a write beam and the read beam. The invention locks the position of the galvanomirror when a unit is energized or retried, reads a galvanomirror position signal provided at this moment, finds an offset signal for zeroing the galvanomirror position signal, and adds the offset to the position locked galvanomirror during a seek operation with the write beam, thereby removing an offset of the galvanomirror position signal. The invention determines that the galvanomirror has been abnormally deflected due to a malfunction of a tracking control servo system, upon detecting an abnormality that a tracking error signal used for the tracking control of the read beam exceeds a threshold value and an abnormality that a galvanomirror position signal used for locking the galvanomirror at a central position exceeds a threshold value. The invention removes an offset by adding the galvanomirror position signal to the tracking error signal of the read beam. It is possible to add a lens position signal as well to remove the offset. (FIG. 1B)