Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Patent
1990-01-02
1992-05-12
Envall, Jr., Roy N.
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
369 4438, 369109, 2502015, G11B 700
Patent
active
051133784
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an optical recording device which radiates laser beams onto an optical recording medium by an optical head for writing and reading data.
BACKGROUND OF THE INVENTION
In an optical recording device of the prior art, a socalled three-beam method is adopted in which laser beams are made into three diffraction beams by a diffraction grating as shown in FIG. 15. The diffracted beams are converged by a convergent lens whereby the diffracted beams are made main beams used for writing and reading the zero-order diffraction beams and the first-order diffraction beams are made tracking beams.
Tracking is performed in such a way that these two tracking beams are radiated on lines, several microns in width, which are positioned on both sides of an information unit row called a track guide 51 on an optical recording medium and a servo, which makes the amount of reflected light of both tracking beams equal, is then applied, as shown in FIG. 16.
As described above, the above-mentioned optical recording device of the prior art performs tracking by detecting light reflected from the optical recording medium for two tracking beams by using a photo detector. However, it has a drawback that in such a case where there are defects in the track guide 51 of an optical recording medium, tracking cannot be performed on those portions.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical recording device which can perform tracking by switching tracking beams even when there are defects on a track guide, thereby improving tracking reliability. An optical recording device of the present invention which attains the above-mentioned objects is arranged as follows. A diffraction grating is provided on the above-mentioned optical head and a plurality of laser beams are generated. Some of the laser beams are positioned on a track guide on the abovementioned optical recording medium and some other portions are positioned on another track guide, thus constituting a plurality of tracking beams. The optical recording device is also provided with a tracking detection circuit for outputting a tracking error signal indicating the difference between the amounts of reflected light to a photo detector which receives light reflected from the above-mentioned optical recording medium for the tracking beams and for switching the abovementioned tracking beams used for tracking. In a preferred mode, the tracking detection circuit is characterized in that when the value of a tracking error signal of one of the tracking beams exceeds a reference value, the tracking beams are switched to the other tracking beams.
In particular, for example, the tracking system of the present invention selects recording and reading of data by directing laser beams to an optical reading medium with at least two sets of track guides. A main laser beam is generated by an optical head and directed to the recording medium for the selected recording and writing of data. At least two sets of tracking beams are generated by the optical head, and a first set of the tracking beams is positioned on a first set of track guides. A second set of tracking beams are positioned on a second set of track guides for increasing reliability of operation of the tracking system. The tracking detection circuit has an output with a tracking error signal when the first set of track guides have a defect detected by the tracking detection circuit. The tracking detection circuit has a switching arrangement for switching from a tracking error signal detected from the first set of tracking beams to a tracking error signal detected from the second set of tracking beams when the first set of tracking guides have a defect determined from a change in reflected light of the first tracking beams. A tracking error signal detected from the second set of tracking beams becomes operative only after the tracking detection circuit detects a defect on the first set of track guides. A tracking error signal detected from the first set
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patent: 4532522 (1985-07-01), Tsunoda et al.
patent: 4598393 (1986-07-01), Pierce et al.
patent: 4660191 (1987-04-01), Maeda et al.
patent: 4720825 (1988-01-01), Kokado
patent: 4787075 (1988-11-01), Matsuoka et al.
Envall Jr. Roy N.
Fogiel Max
Kabushiki Kaisha CSK
Tran Thang V.
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