Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Patent
1996-01-16
1999-11-09
Young, W. R.
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
369 4434, G11B 7095
Patent
active
059827220
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a tracking servo apparatus and a track counting apparatus, and more particularly to a tracking servo apparatus, etc., and more particularly to a tracking servo apparatus and a track counting apparatus for (an optical disc unit using) optical recording medium which can reduce offset quantity of the tracking error signal in the so-called push-pull method.
BACKGROUND ART
In optical disc apparatuses (units) using an optical recording medium, e.g., optical disc, the push-pull method is known as one of the methods for detecting tracking error signal used in the tracking servo control.
FIG. 1 is a view for explaining the principle of the push-pull method. On the base (substrate) of an optical disc 100, grooves 101 and lands 102 are formed in advance as shown in FIG. 1, for example. The push-pull method is a method of detecting, e.g., a light reflected and diffracted at the groove 101 by means of, e.g., a bisected photodetector, 115 of which divided portions are symmetrically disposed with respect to the center of the track to take out an output difference between these detector sections 115L, 115R as a tracking error signal.
In more practical sense, as shown in the FIG. 1 mentioned above, a reflected light from the optical disc 100 mainly consists of (rays of) the 0-th order diffracted light and (rays of) the +.+-.1-th order diffracted light in a direction perpendicular to the groove 101. In view of the above, respective detector sections 115L, 115R of the bisected photodetector 115 are disposed symmetrically with respect to the center of the track to respectively detect, by means of these detector sections 115L, 115R, sum total of intensities of the 0-th order light +(+1)-th order light and sum total of the 0-th order light +(-1)-th order light, i.e., sum total of intensities of the area S.sub.0 where only the 0-th order diffracted light exists and the area S.sub.1 where the 0-th order diffracted light and the 1-th order diffracted light exist and sum total of intensities of the above-mentioned area S.sub.0 and the area S.sub.1 where the 0-th order diffracted light and the (-1)-th order diffracted light exist. Then, a difference between respective outputs of the detector sections 115L, 115R is determined by, e.g., a differential amplifier, thereby making it possible to obtain a tracking error signal.
Meanwhile, in the push-pull method, as shown in FIG. 2, for example, any offset takes place in the tracking error signal resulting from movement of an objective (object lens) 114 followed by tracking (so called fluctuation (change) of the visual field of the objective) and/or inclination in the radial direction of the optical disc, etc.
In more practical sense, as shown in FIG. 3, for example, light beams emitted from a semiconductor laser 111 of the so-called optical pick-up 110 are changed into rays of parallel light by a collimator lens 112, and are then reflected on a beam splitter 113. The (rays of) reflected light thus obtained is converged onto the optical disc 100 by the objective 114. The light beams reflected on the optical disc 100 return through the light traveling (incoming) path in a manner opposite to the above and are transmitted through the beam splitter 113. Thereafter, these light beams are received by the bisected photodetector 115. In this case, in the system of moving only the objective 114 in a direction perpendicular to the optical axis to carry out tracking, when the objective 114 is assumed to be moved to the position indicated by reference numeral 114a, spot of diffracted light would deviate with respect to the center of the bisected photodetector 115. As a result, any d.c. offset might take place in the tracking error signal.
Moreover, as shown in FIG. 4, for example, when the optical disc 100 is assumed to be inclined as indicated by reference numeral 100a, spot of the diffracted light would deviate with respect to the center of the bisected photodetector 115 in a manner similar to the above. Thus, any offset is produced in the tracking error
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patent: 5140573 (1992-08-01), Ando
patent: 5202870 (1993-04-01), Aoki et al.
patent: 5289447 (1994-02-01), Kobayashi et al.
patent: 5416766 (1995-05-01), Horimai
patent: 5487056 (1996-01-01), Wachi
Sony Corporation
Young W. R.
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