Information storage apparatus and method for storing data

Details Information storage apparatus and method for storing data Information storage apparatus and method for storing data

1999-09-01

2002-05-28

Hindi, Nabil (Department: 2753)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044350

Reexamination Certificate

active

06396780

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field Of The Invention
The present invention generally relates to information storage apparatuses and methods for storing data, and more particularly to an information storage apparatus and a method for storing data in which tracking error signals are generated by the differential push-pull method so as to control tracking.
2. Description Of The Related Art
In an information storage apparatus such as an optical disk device, it is preferable to perform a precise tracking operation in order to write and read data on a preformed track. One of conventional tracking methods is a tracking servo method that employs the differential push-pull method using a main beam and two sub-beams for generating a tracking error signal.
FIG. 1
shows a diagram illustrating an optical system employing the differential push-pull method.
Generally, in the differential push-pull method, a main beam, the first side beam preceding the main beam and the second side beam following the main beam are emitted onto an optical disk.
For example, such an optical system as shown in
FIG. 1
can realize the differential push-pull method.
A laser beam emitted by a laser
1
is collimated by a collimator lens
2
and is split into three parallel beams by a diffraction grating
3
. The three split beams are supplied to an objective lens
5
through a beam splitter
4
.
The objective lens
5
condenses each of these three beams and focuses them on the optical disk
6
. These three beams correspond to the main beam MB, the first side beam SB
1
, and the second side beam SB
2
.
The three beams are reflected at three spots on the optical disk
6
and are diverted to a condensing lens
7
by the beam splitter
4
. The diverted beams, the first side beam, the main beam and the second side beam, are condensed by the condensing lens
7
and supplied to corresponding detectors
8
a
,
8
b
and
8
c
, respectively.
From three signals detected by the detectors
8
a
,
8
b
and
8
c
, three differential signals of the main beam, the first side beam and the second beam are obtained by a detecting circuit so as to generate three tracking signals.
FIG. 2
shows a block diagram illustrating a detecting circuit based on the differential push-pull method.
The detector
8
a
, which detects a reflected beam of the first side beam SB
1
, has two segmented detecting areas E
2
and F
2
. The detector
8
b
, which detects a reflected beam of the main beam MB, has two segmented detecting areas E
1
and F
1
. The detector
8
c
, which detects a reflected beam of the second side beam SB
2
, has two segmented detecting areas E
3
and F
3
.
The detecting area E
2
of the detector
8
a
is connected to a non-inverting input terminal of a differential amplifier
9
a
and the detecting area F
2
of the detector
8
a
is connected to an inverting input terminal of the differential amplifier
9
a
. The differential amplifier
9
a
generates a tracking error signal TE
2
corresponding to the difference between two signals from the detecting areas E
2
and F
2
. That is, the tracking error signal TE
2
corresponds to a tracking spot of the first side beam SB
1
.
The detecting area E
1
of the detector
8
b
is connected to a non-inverting input terminal of a differential amplifier
9
b
and the detecting area F
1
of the detector
8
b
is connected to an inverting input terminal of the differential amplifier
9
b
. The differential amplifier
9
b
generates a tracking error signal TE
1
corresponding to the difference between two signals from the detecting areas E
1
and F
1
. That is, the tracking error signal TE
1
corresponds to a tracking spot of the main beam MB.
The detecting area E
3
of the detector
8
c
is connected to a non-inverting input terminal of a differential amplifier
9
c
and the detecting area F
3
of the detector
8
c
is connected to an inverting input terminal of the differential amplifier
9
c
. The differential amplifier
9
c
generates a tracking error signal TE
3
corresponding to the difference between two signals from the detecting areas E
3
and F
3
. That is, the tracking error signal TE
3
corresponds to a tracking spot of the second side beam SB
2
.
The tracking error signal TE
2
generated by the differential amplifier
9
a
and the tracking error signal TE
3
generated by the differential amplifier
9
c
are supplied to an adder
10
that generates an added tracking error signal, which is the result of adding (TE
2
+TE
3
) the tracking error signals TE
2
and TE
3
. The added tracking error signal (TE
2
+TE
3
) from the adder
10
is supplied to an inverting input terminal of a differential amplifier
11
.
The signal generated by the differential amplifier
9
b
, that is, the tracking error signal TE
1
of the main beam MB, is supplied to a non-inverting input terminal of the differential amplifier
11
. The differential amplifier
11
generates another tracking error signal, which is a signal {TE
1
−(TE
2
+TE
3
)}, by subtracting the added tracking error signal (TE
2
+TE
3
) from the tracking error signal TE
1
.
As mentioned above, the final tracking error signal is generated by applying the differential push-pull method to the main beam and also the first and the second side beams. It should be noted that the same offset, which is caused by objective lens shift or radial skew, occurs in the original three tracking error signals TE
1
, TE
2
and TE
3
for the main beam MB, the first side beam SB
1
and the second side beam SB
2
because the same objective lens
5
is used. Thus, the final tracking error is without the offset caused by objective lens shift or radial skew.
Referring to
FIG. 2
, a spot
10
b
focused on by the main beam MB is positioned on a track TR
0
, a spot
10
a
focused on by the first beam SB
1
is positioned on a guard band GB
10
that is not used except as a space between the track TR
0
and a track TR
1
, and a spot
10
c
focused on by the second beam SB
2
is positioned on a guard band GB
20
that is not used except as a space between the track TR
0
and a track TR
2
.
In a conventional tracking servo method employing the above-mentioned differential push-pull method, it is assumed that the optical system in
FIG. 1
operates on recorded tracks on which many pits P are formed on tracks TR
0
, TR
1
and TR
2
. In this case, reflectance of the spot
10
a
nearly equals reflectance of the spot
10
c
because both spots
10
a
and
10
c
include parts of pits. Accordingly, there is no unbalance between two reflected beams from the spots
10
a
and
10
c
so that the offset caused by objective lens shift or radial skew is eliminated.
However, the conventional tracking servo method has a disadvantage that will now be explained.
FIG. 3
shows a diagram illustrating a case of causing an offset that should be eliminated.
Referring to
FIG. 3
, it is assumed that the optical system in
FIG. 1
is operating around a boundary between a recorded area and a blank area. That is, the first side beam SB
1
focuses on a spot
11
a
on the guard band GB
10
in the blank area in which there is no pit and the second side beam SB
2
focuses on a spot
11
c
on the guard band GB
20
in the recorded area in which there are many pits P. Accordingly, the presence of a pit P in the spots
11
a
and
11
c
affects both reflectances of the first side beam SB
1
and the second side beam SB
2
. Thus, unbalance between the reflectances occurs. Although the unbalance does not cause an offset directly, the offset can not be eliminated when objective lens shift or radial skew occurs simultaneously. In practice, the optical disk
6
is not flat and is not always mounted rigidly parallel to the focal plane.
Disadvantageously, a center of a peak value and a bottom value of a tracking error signal including the offset does not indicate a center of a track. Hence, data is recorded away from the center of the track because a tracking servo adjusts tracking by the offset so that a de-tracking is caused.
SUMMARY OF THE INVENTION
It is a general object of the pres

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