Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2002-01-30
2004-04-13
Edun, Muhammad (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044280, C369S112010
Reexamination Certificate
active
06721244
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a focusing servo pull-in apparatus for pulling a focusing servo in a storage medium such as optical disk.
BACKGROUND OF THE INVENTION
In an optical disk unit, heretofore, a focusing servo pull-in operation, which means that light beams output from a laser light source are converged by the use of an objective lens, and a focusing position of the light beams is allowed to coincide with a film surface in an optical disk on which data is to be recorded/reproduced (hereinafter referred to as “recording surface”) has been made. Furthermore, a relative position between a focusing position of light beams and a recording surface of an optical disk varies generally due to surface deviation in the optical disk. Accordingly, such focusing servo control that a focusing actuator for driving an objective lens in a direction of the optical axis of an optical system is used to keep the relative position constant is conducted in an optical disk unit.
On the other hand, reduction in focal length with increase in an numerical aperture of an objective lens advances in optical disk unit in recent years. As a result of reduction in focal length, a working distance of an objective lens in case of focusing servo controlling on a recording surface of an optical disk is reduced from a conventional distance of around 1 mm to around 0.1 mm, so that it is indispensable for providing a measure for avoiding a collision of the objective lens with the optical disk. Such measure for avoiding collision is particularly important in case of focusing servo pull-in controlling for a focusing actuator at the time when a degree of risk in collision is high, because a distance between the objective lens and the optical disk is indefinite.
Conventional focusing servo pull-in manners are classified roughly into a first to a third three manners. The first manner is such that a focusing search operation is made to obtain two focusing positions detected on a recording surface of an optical disk and a surface of a substrate layer (hereinafter referred to as “substrate surface”) of the optical disk, from which the focusing position detected on the recording surface is extracted, and based on the extracted focusing position, a focusing servo is pulled in the recording surface.
For instance, as the first manner described above, there are Japanese Patent Laid-Open Nos. 150147/1982, 150148/1982, 129525/1988, 144425/1988, 223384/1994, 231583/1997, etc. wherein a manner for pulling a focusing servo in a recording surface by means of detecting the recording surface dependent upon appearing orders in moving directions of an objective lens, thereby focusing positions detected is disclosed. Moreover, for example, Japanese Patent Laid-Open No. 302427/1995 discloses such a manner that a recording surface is detected on the basis of appearing orders in moving directions of an objective lens and focusing positions detected as well as of differences in reflectivity of the recording surface and a substrate surface, whereby a focusing servo is pulled in the recording surface.
Besides, such a manner that a substrate surface is distinguished from a recording surface in an optical disk based on differences in intensity of light reflected by the optical disk, whereby a focusing servo is pulled in the recording surface is disclosed in Japanese Patent Laid-Open Nos. 12032/1991, 37828/1991, 225628/1991, 217177/1993, and 186532/1992. In Japanese Patent Laid-Open No. 272286/1995, such a manner that a difference in levels of focusing error signals in a recording surface and a substrate surface is detected by a difference in velocity signals of the focusing error signals, whereby a focusing servo is pulled in the recording surface has been disclosed. Japanese Patent Laid-Open No. 143875/1998 discloses such a manner that a recording surface is detected by the use of a high-frequency reproduction signal reproduced from the recording surface, whereby a focusing servo is pulled in the recording surface. In Japanese Patent Laid-Open No. 96829/1989, such a manner that a substrate surface is discriminated from a recording surface based on an existence of track modulation superimposed on focusing error signal, whereby a focusing servo is pulled in the recording surface has been disclosed.
In addition, Japanese Patent Laid-Open No. 48134/1986 discloses such a manner that a recording surface is detected on the basis of an appearing level of track error signals, whereby a focusing servo is pulled in the recording surface. Further, Japanese Patent Laid-Open No. 155425/1988 discloses such a manner that a time required for moving a distance extending from the lowermost point in an objective lens to a substrate surface in an optical disk is measured by pulling once a focusing servo in the substrate surface of the optical disk, and thereafter, the measured time is utilized in the case when the focusing servo pull-in operation is made again from the lowermost point of the objective lens, whereby the focusing servo is pulled in a recording surface by masking a focusing position detected in the substrate surface.
Next, as the second manner described above, there are, for example, Japanese Patent Laid-Open Nos. 28034/1985, 166195/1993, 112035/1998, 191223/1999, 120599/1999, etc. wherein a manner for preventing a collision of an objective lens with an optical disk by applying a voltage to a focusing actuator in a method for distancing an objective lens from an optical disk has been disclosed as a manner for retreating the objective lens in case of detecting abnormal pull-in or deviation of a focusing servo, or further in case where a focusing servo is in OFF state.
Moreover, as the third manner, for example, Japanese Patent Laid-Open No. 339276/1999 discloses such a manner that a focusing position of light beams is retreated to a substrate surface or a recording surface in a direction along which an objective lens comes away from an optical disk by means of focusing jump control in case of detecting deviation of a focusing servo, and after elapsing of a predetermined period of time, the focusing position of light beams is returned to the recording surface by the focusing jump control.
However, any of the first manners described in these Japanese Patent Laid-Open gazettes applies such manner that a focusing detection position was searched with respect to a recording surface as a target in case of pulling a focusing servo in the recording surface, and then, the focusing servo is pulled in the recording surface. Accordingly, decrease in working distance of an objective lens with reduction in focal point of the objective lens as mentioned above could not be compensated. Furthermore, the second manner involved such a problem that a focusing actuator collided with a supporting base of the focusing actuator, so that the focusing actuator was broken.
In this respect, an explanation will be made by referring to
FIG. 1
wherein an amount of surface vibration in an optical disk is around 0.3 mm at the maximum. In order to retreat an objective lens under a condition wherein a working distance of the objective lens is around 0.1 mm and an amount of surface vibration in optical disk is less than the maximum value without accompanying any contact of the optical disk with the objective lens, it is necessary that a retreating speed of the objective lens is faster than an approaching speed in a relative distance defined between the objective lens and the optical disk with rotation of the disk during a period until the upper end of the objective lens shifts to a position where it is below the lower end of surface vibration in an orbiting of the optical disk (a period wherein the objective lens shifts a distance: &Dgr;y
1
in FIG.
1
). In the second manner, a manner for avoiding collision by means of applying a step-like voltage in a direction away from the optical disk to a focusing actuator in case of retreating the objective lens is adopted.
However, since focusing actuator has generally resonant characteristics, when only a voltage
Edun Muhammad
NEC Corporation
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