Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-04-04
2003-10-14
Dorvil, Richemond (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044110
Reexamination Certificate
active
06633523
ABSTRACT:
The present invention relates to an optical memory apparatus to conduct various processes in the recording and reproduction of information using a laser beam. This invention further relates to the manufacturing process of optical memory devices and their focus servo control method.
BACKGROUND OF THE INVENTION
As an optical memory medium, a magneto-optical recording medium represented by a magneto-optical disk has been proposed. A magneto-optical disk has a substrate and a recording layer consisting of magnetic material formed on the substrate. Information can be recorded by heating an optical beam and changing a magnetic field. On the occasion of reproducing information from the magneto-optical disk, the magneto-optical effect is utilized.
In this magneto-optical disk, data tracks for recording/reproducing data are provided. In general, a spiral groove (tracking guide groove) is provided on the substrate of the medium and parts called “lands” sandwiched between adjacent grooves are formed as the data recording and reproducing tracks.
As explained above, in order to record/reproduce information to/from the recording surface of the recording medium, a laser diode (LD) beam is condensed to the recording surface through an objective lens. This condensing condition must be maintained to always keep a “just focusing” condition. This control is called the focus servo. The process of obtaining the focus servo condition from the non-focus servo condition, namely a series of operations performed for focus servo pull-in, is called “focus entry.”
Moreover, in order to record the data to the data track (land) explained above and reproduce such data, the LD beam in the just focusing condition has to follow the data track. This tracking control is called the “track servo.”
FES (focus error signal) and TES (track error signal) can be obtained by applying the reflected beam of the LD from the medium surface to a detector for servo, and then processing the signal from the detector. The FES indicates the focusing condition on the medium surface of the LD beam. Namely, deviation between the medium surface and the focal point is indicated as a voltage.
However, if deviation of the focal point to the medium surface is large, FES is approximated to the voltage of the servo center. Behavior of this FES is called “Character S of FES”.
For example, when the objective lens is moved to a considerably far position from a position considerably nearer to the medium, the FES voltage is displaced to a single side from the area near the servo center voltage. It is suddenly approximated to the servo center voltage from a certain level and it passes such voltage and is displaced to the opposite side. It is then displaced again to the servo center side for approximation.
Moreover, when the LD beam crosses the data track while focus servo is effectuated, TES changes like a sine wave conforming to such crossing. One period of the sine wave of the TES corresponds to the movement of the LD beam as much as one data track. In the ordinary recording and reproducing condition, the LD beam follows the data track. Namely, since the track servo is engaged, TES shows the voltage near the servo center.
When the medium is rotated under the condition that the LD beam is always in the just focusing condition on the medium surface, namely the track servo is searching when the focus servo is engaged, the LD beam crosses the data track due to eccentricity of the medium and the sine wave TES can be observed. Here, eccentricity of the medium causes deviation between the rotation center of the spindle motor and the spiral (circular) virtual center of the spiral data track on the medium. Due to this eccentricity, when the medium is rotated, the beam reflected from a fixed LD beam behaves as if the data track were moved in the radial direction of the medium.
TES reveals the LD beam crossing the data tracks. As explained above, when the focus servo is engaged but the track servo is searching across the data tracks of the medium, TES changes like a distorted sine wave due to the existence of eccentricity. However, since the sine wave of TES is generated because the LD beam crosses the data tracks, if the data track (land group) does not exist on the medium, such sine wave does not appear. Usually, the inner most circumference and outer most circumference have an annular area where the data track (land group) does not exist. This area is called the “mirror surface” area.
Here, when focus entry is performed, the objective lens is moved upward and downward and thereby the just focusing condition (on the recording surface on the medium) is attained at the point where the FES crosses the voltage at the servo center. Therefore, the focus servo can be effectuated by closing the control loop of the focus servo.
When focus entry is conducted, namely when the control loop of the focus servo is closed, it is impossible to continuously effectuate the focus servo due to axial deviation of the medium as it rotates, and unstable elements such as moving velocity of the objective lens cause the focus servo condition to be lost in some cases.
Focus servo maintains the focusing of the LD beam on the surface of the medium. Since the focal distance of the objective lens is fixed, the relative distance between the medium surface and objective lens may be kept constant by this control. When axial deviation of the medium is large and the moving speed of the objective lens is also large, though, change in the relative distance between the medium surface and objective lens becomes large, and it becomes a factor in failure of focus pull-in.
Therefore, since the focus servo is not always effective in the stable condition after focus entry (when the focus servo loop is closed), a determination is required to check whether stable focus servo is effective. If it is determined that stable focus servo is not effective, focus entry is conducted again, namely retry is conducted.
In order to judge whether normal focus entry has been conducted or not in known systems, namely whether stable focus servo is effective or not, the following has generally been specified:
1. FES shall not exceed a certain level, namely the FES level shall be located near the voltage of the servo center.
2. TES shall be changed at a certain amplitude (behavior of the sine wave).
3. LPOS (lens position signal) shall not exceed a certain level.
In item 2 above, in order to vary TES within a certain amplitude when focus servo is effective, the LD beam must be located at the groove (guide groove for effectuating track servo) on the medium surface.
In item 3, a mechanism for generating LPOS to be used for testing is necessary. LPOS will be explained below.
In these years, a movable optical system (head) for holding an objective lens as explained above has a mechanism called a carriage. A two-dimensional actuator mounted on the mechanism is moved in the radial direction with a VCM (Voice Coil Motor).
The two-dimensional actuator is formed of a focus actuator for controlling the objective lens in the focus direction with the focus coil, namely the focal position of the LD beam in the focus direction, and a track actuator for controlling the objective lens (or LD beam) in the track direction with the track coil. LPOS indicates the amount of displacement of the track actuator in the carriage, namely the objective lens.
In order to realize high speed seek by reducing the weight of the movable optical system, and to realize low cost by reducing the number of parts, the focus actuator, in the carriage consisting of only VCM (or track actuator) and an actuator of so-called single shaft type are employed in some cases.
In single shaft type actuators, the track actuator of the older art type is not present. LPOS is a signal indicating displacement of the carriage of the track actuator of the related art type. In the case of the single shaft type actuator, LPOS does not exist because the track actuator of the other type does not exist. Therefore, it is impossible to check success of focus entry by utilizing the LP
Ikeda Toru
Masaki Takashi
Dorvil Richemond
Fujitsu Limited
Greer Burns & Crain Ltd.
Olson Jason
LandOfFree
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