Optical memory system having improved positioning and focusing control circuitry

Claim 1

In an optical data storage system including a recording medium having a large plurality of tracks containing stored data, the combination comprising: means providing a radiation beam for reading data recorded in a track; means including a movable focusing means for focusing said beam on said medium; driving means for controlling the movement of said movable beam focusing means; focus error detecting means including a plurality of photodetector elements for providing photodetector output signals indicative of the focusing of said beam on said medium; first signal combining means responsive to said photodetector output signals for providing a focus error signal indicative of beam focus error; focus error correcting means responsive to said focus error signal for causing said driving means to move said movable beam focusing means so as to substantially maintain said beam in focus on said medium so long as the beam focusing error remains substantially within a predetermined capture range; second signal combining means responsive to said photodetector output signals for providing an output signal capable of indicating when the beam focusing error changes from being within said capture range to being outside of said capture range; and capture range return means operative in response to said output signal indicating that the beam focusing error has changed from being within said capture range to being outside of said capture range for causing said driving means to move said movable beam focusing means in a manner such that the beam focusing error is brought back within said capture range; means providing for relative movement of said radiation beam along said track; track following means for causing said beam to accurately follow a selected track on said medium; and track seeking means for moving said beam from a current track being followed to a new track, said track seeking means including short seek moving means and long seek moving means, said short seek moving means being operative to move said beam when the track-to-track distance over which said beam is to be moved is equal to or less than a predetermined value and said long seek moving means being operative to move said beam when the track-to-track distance over which said beam is to be moved is greater than said predetermined value.