Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-10-24
2002-08-13
Edun, Muhammad (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044280, C369S053120, C369S053190
Reexamination Certificate
active
06434096
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical information recording and reproducing apparatus for recording or reproducing signals on optical discs by using a light spot.
BACKGROUND ART
In optical information recording and reproducing apparatuses, typified by DVD-R and DVD-RAM apparatuses, a light spot of a diameter of about 0.5 &mgr;m is applied to information tracks formed at a pitch of 0.74 &mgr;m on a disk to record and reproduce information. When the light spot is applied to the face of the disk, focus control is carried out so that the light spot traces the disk face at an accuracy of about ±0.5 &mgr;m or less in a direction perpendicular to the disk face. In addition, tracking control is carried out so that the tracking displacement of the light spot from the information track is about ±0.1 &mgr;m or less.
A one-beam push-pull system is available as a conventional method of making a light spot to follow tracks.
A conventional optical information recording and reproducing apparatus by use of this one-beam push-pull system will be described below referring to FIG.
41
.
FIG. 41
is a block diagram showing the configuration of the above-mentioned conventional optical information recording and reproducing apparatus. In
FIG. 41
, a disk
2
having helical or concentric information tracks
1
, shown magnified, is configured so as to be rotated by a spindle motor
3
. Recording and reproduction on the information tracks
1
of the disk
2
are carried out by an optical pickup
4
. The optical pickup
4
is provided with a semiconductor laser
5
, a collimator lens
6
, a beam splitter
7
, an object lens
8
, a two-part split PD (photodetector)
9
and a tracking actuator
10
used as a lens movement means. The tracking actuator
10
comprises a coil
11
secured to the object lens
8
and a magnet
13
secured to the housing of the optical pickup
4
, and the coil
11
is connected to the magnet
13
via a spring
12
. When a current flows through the coil
11
, an electromagnetic force is generated between the coil
11
and the magnet
13
, and the object lens
8
moves in the tracking direction. When the optical pickup
4
is not operating, that is, when no input signal is supplied to the tracking actuator
10
, the object lens
8
is designed to stop at the neutral position of the optical pickup
4
.
A tracking error detection circuit
14
generates a tracking error signal
101
, i.e. the displacement amount of the object lens
8
from the center of the information tracks
1
, on the basis of the difference between the outputs of the PD sections of the two-part split PD, and outputs the signal to a tracking control circuit
15
. The tracking control circuit
15
outputs a tracking drive signal
103
to a tracking drive circuit
16
on the basis of the tracking error signal. The tracking drive circuit
16
supplies a current depending on the tracking drive signal
103
to the coil
11
of the tracking actuator
10
. This circuit loop for the tracking control is referred to as a tracking control loop
201
.
Concrete operations of the optical information recording and reproducing apparatus will be described next. The light beam emitted from the semiconductor laser
5
is made parallel light by the collimator lens
6
and converged to the information track
1
of the disk
2
through the beam splitter
7
and the object lens
8
. By using part of the light reflected from the disk
2
, focus control for moving the object lens
8
vertically is carried out so that the focus position of the light beam focused by the object lens
8
meets the face of the disk
2
at all times. However, the focus control is not described in detail since it does not directly relate to the present invention.
Part of the light reflected from the disk
2
enters the two-part split PD
9
through the object lens
8
and the beam splitter
7
. The two-part split PD
9
outputs voltages indicating the amounts of light detected by the respective PD sections. The tracking error detection circuit
14
calculates the difference between the outputs of the two-part split PD
9
and outputs the tracking error signal
101
indicating the positional displacement amount between the focus of the light beam and the information track
1
to the tracking control circuit
15
. By the operation of the tracking control loop
201
, the tracking control circuit
15
outputs the tracking drive signal
103
to the tracking drive circuit
16
so that the amplitude of the tracking error signal
101
becomes zero, that is, so that the light beam positions at the center of the information track
1
. The tracking drive circuit
16
flows a current through the coil
11
to generate an electromagnetic force thereby to move the object lens
8
. With this configuration, even if the disk
2
is eccentric, the object lens
8
can be controlled so that the light spot follows the center position of the information track
1
during the rotation of the optical disk
2
.
FIG. 42
is a graph showing the change of the tracking error signal with respect to time at a moment when the tracking control loop
201
is switched from its OFF state (a state wherein the tracking control loop
201
does not operate) to its ON state (a state wherein the tracking control loop
201
operates). When the tracking control loop
201
is in the OFF state, no drive force is applied to the tracking actuator
10
shown in
FIG. 41
; therefore, the object lens
8
is stationary at the neutral position of the optical pickup
4
. If there is an eccentricity caused by the spindle motor
3
on which the disk
2
is mounted, or if the disk
2
itself has a slight eccentricity, the information track
1
also has an eccentricity with respect to the rotation center. Therefore, the information track
1
crosses the focus position of the object lens
8
, and the tracking error signal
101
having a sine waveform is output in synchronization with the rotation of the disk
2
. When the tracking control loop
201
is set the ON state, the object lens
8
is controlled so that the light spot follows the center position of the information track
1
by the operation of the tracking control loop
201
as described above, whereby the amplitude of the tracking error signal
101
becomes nearly zero.
In the configurations shown in the above-mentioned
FIGS. 41
to
44
, if the center of the object lens
8
is displaced from the neutral position of the optical pickup
4
because of some reasons, the center of the object lens
8
is displaced from the center of the,light beam emitted from the semiconductor laser
5
, thereby causing an offset. As a result, the distribution of the incident light from the disk
2
to the two-part split PD
9
varies, and the waveform of the tracking error signal
101
changes, whereby the follow-up control for the information track becomes unstable.
A first problem owing to the displacement of the center of the object lens
8
is an offset in the tracking error signal
101
. FIG.
43
(
a
) is a side view showing the positional relationship between the light beam of the reflected light and the two-part split PD at the time when the optical axis of the object lens
8
is displaced from the center position of the light beam. When the object lens
8
is displaced from the position indicated in solid lines to the position indicated in two-dot chain lines, the center position A of the light beam is displaced to position B; as a result, the center of the light beam reflected by the optical disk
2
and made incident upon the two-part split PD is displaced by distance x. Consequently, regarding the amounts of incident light at the detection areas a and b of the two-part split PD
9
, the amount of incident light in the detection area a is less than that in the detection area b, whereby an imbalance occurs therebetween. The tracking error detection circuit
14
detects the tracking error signal
101
by calculating the difference between the amounts of light at the detection areas a and b. Therefore, if an imbalance occurs between the
Akagi Noritaka
Kai Tsutomu
Kashiwagi Yasuo
Masaki Kiyoshi
Naka Teruyuki
Edun Muhammad
Matsushita Electric - Industrial Co., Ltd.
Pearne & Gordon LLP
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