Optical disc apparatus

Details Optical disc apparatus Optical disc apparatus

2000-09-19

2002-12-24

Tran, Thang V. (Department: 2653)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044350, C369S044320, C369S053140

Reexamination Certificate

active

06498772

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disc apparatus having a tracking control function for causing an optical beam to accurately follow an eccentricity of a track of a rotating disc-shaped information carrier (hereafter referred to as an “optical disc”) having tracks each comprising a pit or a guide groove.
2. Related art of the Invention
To reproduce a signal, a conventional optical disc apparatus irradiates an optical disc with relatively faint optical beams of a fixed light quantity to detect reflected light modulated for intensity by the optical disc. To record a signal, the device modulates the light quantity of optical beams depending on a recorded signal to write information to a recording material film on the optical disc (for example, Japanese Patent Laid-Open No. 52-80802).
A reproduction-only optical disc has pit-based information spirally recorded therein. A recordable and reproducible optical disc is produced by using evaporation or the like to form an optically recordable and reproducible material film on a base material surface having spiral tracks of a relief structure. Recording information on the optical disc or reproducing the recorded information requires focus control for controlling optical beams in a perpendicular direction (hereafter referred to as a “focus direction”) of the optical disc surface in such a manner that the optical beams are always in a predetermined converging state on the recording material film, and tracking control for controlling the optical beams in a radial direction (hereafter referred to as a “tracking direction”) so that the beams are always located on a predetermined track.
A retrieval operation by the conventional optical disc will be described with reference to FIG.
16
. An optical head
10
has a semiconductor laser
11
, a coupling lens
12
, a polarizing beam splitter
13
, a quarter wavelength plate
14
, a focus actuator
16
, a tracking actuator
17
, a detection lens
18
, a cylindrical lens
19
, and a photodetector
20
mounted therein. Optical beams generated by the semiconductor laser
11
are made parallel beams by the coupling lens
12
, so that the parallel beams pass through the polarizing beam splitter
13
and the quarter wavelength plate
14
and are converged on a disc-shaped optical disc
1
by a converging lens
15
.
The thus reflected beams pass through the converging lens
15
and the quarter wavelength plate
14
, are subsequently reflected by the polarizing beam splitter
13
, and then pass through the detection lens
18
and the cylindrical lens
19
onto the photodetector
20
, which is divided into four. The converging lens
15
is supported by an elastomer and is electromagnetically moved in the focus direction when current flow through the focus actuator
16
or in the tracking direction when current flows through the tracking actuator
17
. The photodetector
20
transmits a detected light quantity signal to a focus error generator
30
(hereafter referred to as an “FE generator
30
”) or a tracking error generator
40
(hereafter referred to as a “TE generator
40
”).
The FE generator
30
uses the light quantity signal from the photodetector
20
to calculate an error signal (hereafter referred to as an “FE signal”) indicating how the optical beams are converging on an information surface of the optical disc
1
and transmits the signal to the focus actuator
16
via a focusing filter
31
(hereafter referred to as an “Fc filter
31
”). The focus actuator
16
controls the converging lens
15
in the focus direction so that the optical beams converge on a recording surface of the optical disc
1
in a predetermined state. This is the focus control.
The TE generator
40
uses the light quantity signal from the photodetector
20
to calculate an error signal (hereafter referred to as a “TE signal”) indicating the positional relationship between the optical beams and tracks on the optical disc
1
and then transmits the signal to the tracking actuator
17
via a tracking filter
41
(hereafter referred to as a “Tk filter
41
”) and an adder
42
. The tracking actuator
17
controls the converging lens
15
in the tracking direction in such a manner that the optical beams follow the tracks. This is the tracking control.
A driving signal from the Tk filter
41
is transmitted to an memory
60
. A motor
50
rotates the optical disc
1
to transmit 1,000 encoder pulses per one rotation to a rotation phase detect device
51
. The rotation phase detect device
51
counts rising edges in the encoder pulses from the motor
50
, and clears the count value to zero when 1,000 pulses corresponding to one rotation are counted, thereby to transmit the resulting rotation phase information to the memory
60
. An eccentricity compensation controling signal
62
transmits a control signal to the memory
60
.
If the control signal from the eccentricity compensation controling signal
62
is at a high level, the memory
60
stores a signal from the Tk filter
41
at an address corresponding to rotation phase information from the rotation phase detect device
51
, and continues transmitting zero to a lowpass filter
61
. If the control signal from the eccentricity compensation controling signal
62
is at a low level, the memory
60
transmits a value stored at an address that is based on the rotation phase information from the rotation phase detect device
51
, to the adder
42
via the lowpass filter
61
. The adder
42
adds a signal from the Tk filter
41
and a signal from the lowpass filter
61
together and then transmits the resulting signal to the tracking actuator
17
.
The operation will be explained with reference to FIG.
17
.
FIG. 17
a
shows rotation phase information from the rotation phase detect device
51
,
FIG. 17
b
shows a control signal from the eccentricity compensation controling signal
62
, and
FIG. 17
c
shows a signal from the lowpass filter
61
. In
FIG. 17
, before t
0
, eccentricity correction is not working, between t
0
and t
1
, eccentricity correction learning is being executed, and after t
1
, the eccentricity correction is working. Between t
0
and t
1
, the memory
60
stores the signal from the Tk filter
41
at the address that is based on the rotation phase information as shown in
FIG. 17
a.
At the time t
1
, the memory
60
completes storing eccentricity correction driving for one rotation of the optical disc. After t
1
, the memory
60
outputs the stored eccentricity correction driving and passes it through the lowpass filter
61
to obtain a driving waveform such as that shown in
FIG. 17
c.
Since a driving waveform for causing to following the eccentricity shown in
FIG. 17
c
is applied in addition to the perpendicular tracking control, the tracking actuator
17
accurately follows the eccentricity of the tracks.
Not only the signal from the Tk filter
41
in the tracking control state but also the TE generator
40
in a tracking non-control state may be used for measurements for generating the eccentricity correction driving (Japanese Patent Laid Open No. 3-272030).
Eccentricity is caused by the deviation of a rotation center or track waves; it is mostly caused by the deviation of the rotation center. The components of the rotation center deviation comprises only the rotation frequency components of the optical disc.
With the tracking control, eccentricity correction corrects a main component of the rotation center deviation to enable the optical beams to accurately follow the tracks. When a signal waveform is to be recorded in the memory
60
, the result of the recording is affected by noise or the like. As shown in the left figure in
FIG. 18
a,
the waveform generally includes components other than the rotation frequency components of the optical disc, so that the result of the recording in the memory
60
has an error with respect to a target sine wave as shown in the right figure in
FIG. 18
a.
To eliminate such an error, there has been a method for recording waveforms for two or more rotation

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