Tracking servo circuit

Details Tracking servo circuit Tracking servo circuit

1999-11-15

2002-02-26

Edun, Muhammad (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044320, C369S044350

Reexamination Certificate

active

06351437

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a tracking servo circuit, and more particularly, to a tracking servo circuit for controlling the movement of a pickup device relative to a disk type recording medium.
A compact disc (CD) is mainly used as a digital audio recording medium, but it can also be used as a read only memory (CD-ROM) for storing various types of digital data read by computers.
FIG. 1
is a schematic block diagram showing a conventional disk reproduction apparatus. A disc
1
has a spiral recording track formed on at least one of its surfaces. Digital data, which is in a predetermined format, is recorded along the recording track. The disc reproduction apparatus includes a pickup
3
to read the data recorded on the recording track. The disc reproduction apparatus further includes a servo mechanism for controlling the position of the pickup
3
relative to the disc
1
so that the pickup
3
traces the recording track properly.
As shown in
FIG. 2
, digital data is recorded as a plurality of pits (bumps) that are formed on the recording track of the disc
1
. The digital data undergoes EFM modulation to generate an EFM signal. The pits are formed having a predetermined dimension in correspondence with the EFM signal. The disc
1
is rotated by a spindle motor
2
. The spindle motor
2
rotates the disc
1
at a predetermined speed in accordance with a drive signal SD generated by a servo controller
6
.
The pickup
3
is arranged opposite the recording track of the disc
1
. An actuator
4
, which is operated in accordance with a drive signal TD, moves the pickup
3
in the radial direction of the disc
1
. The pickup
3
includes laser beam sources and sensors. As shown in
FIG. 2
, the pickup
3
generates a main reading beam P and a pair of auxiliary reading beams T
1
, T
2
which are radiated toward the recording track. The main reading beam P is used to detect pits on the recording track surface. The auxiliary reading beams T
1
, T
2
are used to detect when the pickup
3
moves away from the recording track. The reading beams P, T
1
, T
2
radiated against the pits of the disc
1
are reflected toward the sensor as weak lights. The reading beams P, T
1
, T
2
radiated against portions other than the pits of the disc
1
are reflected toward the sensor as strong lights. When the sensor associated with each of the reading beams P, T
1
, T
2
receives the corresponding reflection beam, the sensor generates a voltage having a value which corresponds to the intensity of the reflected light.
A voltage signal having a value corresponding to the main reading beam P is sent to a signal processor
5
from the pickup
3
. The signal processor
5
conducts a waveform shaping process and a binarizing process on the voltage signal to generate an EFM signal. The EFM signal repetitively goes back and forth between a low level and a high level in accordance with the existence of pits.
The signal processor
5
generates a tracking error signal TE from the difference between the voltage values of the auxiliary reading beams T
1
, T
2
and an off track signal OT from a low frequency component of the EFM signal. The voltage value corresponding to the auxiliary reading beam T
1
is substantially the same as the voltage value corresponding to the auxiliary reading beam T
2
when the pickup
3
is accurately tracking the recording track (i.e., when the pickup
3
is at the proper position). Under these conditions, the tracking error signal TE is maintained at a null level. When the pickup
3
is not accurately tracking the recording track (i.e., when the pickup
3
is not at the proper position), for example, when the position of the pickup
3
is offset inward from the recording track, the voltage value corresponding to the auxiliary reading beam T
1
becomes smaller than the voltage value corresponding to the auxiliary reading beam T
2
and causes the tracking error signal TE to take a negative value. On the other hand, if the position of the pickup
3
is offset outward from the recording track, the voltage value corresponding to the auxiliary reading beam T
2
becomes smaller than the voltage value corresponding to the auxiliary reading beam T
1
and causes the tracking error signal TE to take a positive value. When the pickup
3
is properly tracking the recording track of the disc
1
, the signal processor
5
continuously outputs the EFM signal. Thus, the EFM signal does not include a low frequency component. Accordingly, the off track signal OT is maintained at a low level when the pickup
3
is properly tracking the recording track.
As shown in
FIG. 1
, the signal processor
5
sends the tracking error signal TE and the off track signal OT together with the EFM signal to the servo controller
6
. The servo controller
6
generates the spindle motor drive signal SD and the actuator drive signal TD based on the tracking error signal TE and the off track signal OT. The spindle motor drive signal SD controls the spindle motor
2
so that the frequency of the EFM signal is maintained at a predetermined value. The actuator drive signal TD controls the actuator
4
so that the tracking error signal TE has a null level and the off track signal OT is maintained at a low level. The spindle motor drive signal SD and the actuator drive signal TD servo control the spindle and tracking.
FIG. 3
is a chart showing the waveforms of the signals detected when the pickup
3
moves across the lines of the recording track on the disc
1
(when a so-called track jump is performed). The horizontal axis represents time.
FIG. 3
shows a state in which the pickup
3
gradually decelerates.
As shown in
FIG. 3
, the code (polarity) of the pickup
3
is inverted each time the pickup
3
moves across the recording track. Thus, the waveform of the tracking error signal TE is a sine wave during the track jump. A track jump signal TJ is generated by digitizing the tracking error signal TE using a null level of the tracking error signal TE as a threshold value. The track jump signal TJ falls or rises when the center of the pickup
3
is located at the center of the recording track. Furthermore, the EFM signal has a predetermined amplitude when the pickup
3
is located above the recording track. If the pickup
3
moves away from the recording track, the EFM signal maintains a. constant value. Accordingly, the off track signal OT rises or falls when the center of the pickup
3
reaches an end of a pit. Normally, the phase difference between the off track signal OT and the tracking error signal TE is ±90°. Accordingly, the number of times the pickup
3
crosses the recording track is detected by counting the tracking error signals TE or the off track signals OT. The moving direction of the pickup
3
is detected from the difference between the phase of the tracking error signal TE and the phase of the off track signal OT. The movement of the pickup
3
is controlled based on the two detection results.
When moving the pickup
3
in the radial direction of the disc
1
, the pickup
3
is accelerated when the movement starts. When stopping the pickup
3
at a target position, the pickup
3
is decelerated just before reaching the target position. The acceleration and deceleration of the pickup
3
are controlled by the drive signal TD. Normally, to control the stopping of the pickup
3
, a tracking error signal TE having a positive polarity or a negative polarity is acquired. An electromotive force acting in a direction opposite the moving direction of the pickup
3
is applied to the pickup
3
in accordance with the value of the acquired tracking error signal TE. For example, if the pickup
3
moves in an outward direction of the disc
1
and the track jump signal TJ is high, a tracking error signal TE having a positive polarity is acquired and a counter electromotive force, or braking force, is applied to the pickup
3
. When the pickup
3
moves toward the center of the disc
1
and the track jump signal TJ is low, a tracking error signal TE having a negative polarity is acq

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