Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2001-10-31
2003-12-09
Edun, Muhammad (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044260, C369S275400
Reexamination Certificate
active
06661751
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-333581, filed Oct. 31, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a track-following method in an optical disc apparatus and an optical disc apparatus that adopts this method and, more particularly, to a method of moving an optical pickup to seek a target track and guiding a light beam spot to the center of the track upon tracking the target track on an optical disc in an optical disc apparatus that can record data on an optical disc such as a DVD-RAM disc which has a header field having staggered pre-pits (so-called CAPA), and an optical disc apparatus that adopts this method.
2. Description of the Related Art
In recent years, specifications of a DVD-RAM (Digital Versatile Disc of Random Access Memory Type) disc as an optical disc have been provided. This DVD-RAM disc is provided with a phase-change type recording layer that allows overwrite, and adopts a single-track format. In the single-track format, data can be recorded not only on grooves but also on lands between neighboring grooves for the purpose of increasing the recording capacity, and in which a land and groove alternately appear per round. Therefore, since a recording track continues from a land to a groove, and then from a groove to a land, data can be continuously recorded. Since the DVD-RAM disc adopts a scheme for recording marks on lands and grooves, crosstalk is suppressed by adjusting the groove depth. That is, each groove is formed to have a predetermined depth (60 to 70 nm) so as to obtain an identical reflection intensity from a track independently of the presence/absence of marks on a neighboring track.
Furthermore, in the DVD-RAM disc, the track is segmented into a plurality of sectors, each of which has a header field, data recording field, and mirror fields located between these two fields. The header field located at the head of a sector has an embossed header formed by pre-pits, where fields PID
1
to PID
4
are formed by writing ID (identification data) four times. The fields PID
1
and PID
2
, and fields PID
3
and PID
4
, are offset to the left and right with respect to the center of the track. The left and right offset fields PID
1
and PID
2
and fields PID
3
and PID
4
are called CAPA (Complementary Allocated Pit Address).
It is very important for an optical disc apparatus that drives an optical disc such as the DVD-RAM disc to improve seek performance for seeking a target track. The optical disc apparatus has a track access mechanism for accessing a target track, and this track access mechanism normally uses two stages of mechanisms, i.e., coarse movement and fine movement. A seek sequence is executed in the order of coarse seek, center guide
1
, fine seek, and center guide
2
. Note that the track-following process is a control process for setting the relative speed between the disc and light beam spot formed on the disc in the radial direction of the disc to be zero, and setting the beam spot coming from an objective lens at the track central position, and relative motion between the disc and light beam spot is detected by a tracking error signal (T SUB signal) which is also called a tracking error signal (TE signal).
It is not so easy to realize a stable track-following process of the objective lens since the track-following process depends on the actuator sensitivity of a lens fine movement system, and the tracking error (TE) signal has nonlinear periodicity. For example, even when tracking-servo is suddenly started in the vicinity of a target track, since the deceleration performance of a lens actuator is limited, a track-following failure such as “tracking-servo failure” (the lens fine movement system repeats acceleration/deceleration) may occur.
As a method of realizing a stable track-following process, various devises are proposed. For example, in a DVD-ROM drive or the like, a so-called “hysteresis track-following” scheme for executing a hysteresis process of a tracking error (TE) signal on the basis of a track sum signal having a 90° phase difference from the tracking error (TE) signal to attain a stable track-following process is known, and this scheme is adopted.
Since the DVD-RAM disc adopts land-and-groove recording, the groove depth is determined to obtain equal amounts of light reflected from the land and groove and, hence, a sum signal having a 90° phase difference from the tracking error (TE) signal cannot be generated. As a result, the “hysteresis track-following” scheme cannot be applied to the DVD-RAM disc.
The DVD-RAM disc has pre-pits called CAPA, as described above, and upon detecting the CAPA before the end of track-following, noise is produced in the tracking error (TE) signal due to the CAPA, and the track-following process of the DVD-RAM disc drive becomes unstable.
In the conventional track-following process of the DVD-RAM disc drive, the control waits until the relative speed between the disc and lens becomes sufficiently small, e.g., until it is confirmed that the relative speed becomes 7 mm/s or less, and then switches to tracking-servo. However, the wait time required until the relative speed between the disc and light beam spot becomes sufficiently small disturbs a decrease in seek time.
As a processing method that allows to guide the beam spot to the center of a track even when the relative speed between the disc and light beam spot becomes higher than the current upper limit, a method of decelerating the lens using brake pulses may be used. In the brake pulse deceleration method, when the phase of a tracking error signal has been reached, maximum pulses are supplied to the lens fine movement driving system for a predetermined period of time so as to reduce the relative speed, thus achieving a nearly zero relative speed state near the center of a track within the shortest time, and tracking-servo is then started. As the control system, since the relative speed is abruptly decreased by an open lens actuator command, the track-following process can be done even when the relative speed is high at the beginning of tracking-servo. In this case, if relative motion (relative acceleration, relative speed, relative position) is constant at the beginning of tracking-servo, no problem is posed. However, if relative motion is not constant at the beginning of the track-following process, the light beam spot cannot be stably guided to the center of a track.
FIGS. 1A and 1B
show the time responses of a track-following command to a lens actuator and a tracking error (TE) signal upon guiding a light beam spot to the center of a given track by the method of decelerating the lens using brake pulses. In
FIGS. 1A and 1B
, reference symbol I denotes a tracking error (TE) signal; and II, an output (DSPout) from a digital signal processor (DSP).
FIG. 1A
shows a case wherein the initial thrust speed of the lens is 14 mm/s, and the acceleration is zero (0 m/S
2
). In this example, brake pulses are generated before an elapse of time “0.3 ms”, and tracking-servo is then started to make the tracking error (TE) signal converge to zero. In the example shown in
FIG. 1A
, the light beam spot is reliably guided to the center of the target track. By contrast,
FIG. 1B
shows a case wherein the initial thrust speed of the lens is 9 mm/s smaller than 14 mm/s, and the acceleration is zero (0 m/S
2
). In this example, brake pulses are generated after an elapse of time “0.25 ms”, and tracking-servo is then started. However, the tracking error (TE) signal cannot be converged to zero, and the light beam spot cannot be guided to the center of the target track, thus producing so-called “tracking-servo failure”.
As can be seen from
FIGS. 1A and 1B
, the method of decelerating the lens by brake pulses can successfully guide the light beam spot to the center of a track but may cause “tracking-servo failure” depending on the s
Edun Muhammad
Kabushiki Kaisha Toshiba
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