U.S. patent application number 10/700460 was filed with the patent office on 2004-05-13 for optical disk apparatus having compensation for objective lens dislocation.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Kouno, Kazuhiko, Matsubara, Akira, Soma, Yasuhito.
Application Number | 20040090878 10/700460 |
Document ID | / |
Family ID | 11969625 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040090878 |
Kind Code |
A1 |
Soma, Yasuhito ; et
al. |
May 13, 2004 |
Optical disk apparatus having compensation for objective lens
dislocation
Abstract
The present invention provides an optical disc apparatus which
solves a problem that the tracking control takes off when an
objective lens is displaced in the tracking direction due to its
weight. In this optical disc apparatus, a spot position detector
108 generates a spot position signal which indicates a position of
a light spot on a light receiving element 105, and supplies the
signal to a traverse loop filter 111 and a spot position loop
filter 109. A controller 115 supplies an output from the spot
position loop filter 109 to a tracking actuator 104, before
supplying an output from the traverse loop filter 111 to a traverse
motor 113, previously moves the light spot near the center of the
light receiving element 105, and starts the traverse control in a
state where a value of the spot position signal at starting of the
traverse control is reduced.
Inventors: |
Soma, Yasuhito;
(Hirakata-shi, JP) ; Matsubara, Akira;
(Kameoka-shi, JP) ; Kouno, Kazuhiko;
(Takatsuki-shi, JP) |
Correspondence
Address: |
PARKHURST & WENDEL, L.L.P.
1421 PRINCE STREET
SUITE 210
ALEXANDRIA
VA
22314-2805
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
Kadoma-shi
JP
|
Family ID: |
11969625 |
Appl. No.: |
10/700460 |
Filed: |
November 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10700460 |
Nov 5, 2003 |
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10238587 |
Sep 11, 2002 |
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10238587 |
Sep 11, 2002 |
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09492269 |
Jan 27, 2000 |
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6473373 |
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Current U.S.
Class: |
369/30.16 ;
369/44.29; 369/53.28; G9B/7.066 |
Current CPC
Class: |
G11B 7/0941 20130101;
G11B 7/0901 20130101 |
Class at
Publication: |
369/030.16 ;
369/044.29; 369/053.28 |
International
Class: |
G11B 007/085 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 27, 1999 |
JP |
11-018363 |
Claims
What is claimed is:
1. An optical disc apparatus which applies a light spot to an
optical disc, thereby to record or reproduce information on or from
the optical disc, comprising: first moving means for moving the
light spot applied to the optical disc, in a radial direction of
the optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; first control means for subjecting the
spot position signal to a first processing by a spot position loop
filter, and outputting the spot position signal to the first moving
means; second moving means for moving the optical head in a radial
direction of the optical disc; second control means for subjecting
the spot position signal to a second processing by a traverse loop
filter, and outputting the spot position signal to the second
moving means; and system operation control means for operating the
first control means, and thereafter operating the second control
means.
2. An optical disc apparatus which applies a light spot to an
optical disc, thereby to record or reproduce information on or from
the optical disc, comprising: first moving means for moving the
light spot applied to the optical disc, in a radial direction of
the optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; first control means for subjecting the
spot position signal to a first processing by a spot position loop
filter, and outputting the spot position signal to the first moving
means; second moving means for moving the optical head in a radial
direction of the optical disc; second control means for subjecting
the spot position signal to a second processing by a traverse loop
filter, and outputting the spot position signal to the second
moving means; spot position signal monitoring means for receiving
the spot position signal as an input, and outputting a first signal
which indicates that the spot position signal comes to a value
smaller than a prescribed value; and system operation control means
for operating the first control means when the first signal is
input, and operating the second control means after or
simultaneously with the operation of the first control means.
3. An optical disc apparatus which applies a light spot to an
optical disc, thereby to record or reproduce information on or from
the optical disc, comprising: first moving means for moving the
light spot applied to the optical disc, in a radial direction of
the optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; correction signal generation means for
receiving the spot position signal as an input, and generating a
correction signal for correcting the spot position signal;
subtracting means for subtracting the correction signal from the
spot position signal; second moving means for moving the optical
head in a radial direction of the optical disc; and second control
means for subjecting an output from the subtracting means to a
processing by a traverse loop filter, and outputting the output to
the second moving means.
4. An optical disc apparatus which applies a light spot to an
optical disc, thereby to record and reproduce information on or
from the optical disc, comprising: first moving means for moving
the light spot applied to the optical disc, in a radial direction
of the optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; second moving means for moving the
optical head in a radial direction of the optical disc; second
control means for subjecting the spot position signal to a
processing by a traverse loop filter, and outputting the spot
position signal to the second moving means; and a coefficient
multiplier for reducing a coefficient for the control by the second
control means to a value smaller than that in a normal operation
time, at starting of the operation of the second control means.
5. The optical disc apparatus of claim 1 or 2 wherein the first
processing subjected by the first control means is a phase-lag
compensation.
6. The optical disc apparatus of claim 5 wherein the first
processing subjected by the first control means includes
compensation for reducing an open-loop gain at a primary resonance
frequency of the first moving means, in addition to the phase-lag
compensation.
7. The optical disc apparatus of claim 1 or 2 wherein the first
processing subjected by the first control means is a phase-lead
compensation and a phase-lag compensation, and the phase-lead
compensation is started from a frequency lower than a primary
resonance frequency of the first moving means.
8. An optical disc apparatus which applies a light spot to an
optical disc, thereby to record or reproduce information on or from
the optical disc, comprising: first moving means for moving the
light spot applied to the optical disc, in a radial direction of
the optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; tracking error detection means for
generating a tracking error signal which indicates a positional
dislocation between the light spot and a track on the optical disc;
first control means for subjecting the spot position signal or the
tracking error signal to a first processing by a phase compensation
loop filter, and outputting the signal to the first moving means;
second moving means for moving the optical head in a radial
direction of the optical disc; second control means for subjecting
the spot position signal to a second processing by a traverse loop
filter, and outputting the signal to the second moving means; and
system operation control means for operating the first control
means to perform a phase-lag compensation and a phase-lead
compensation by the phase compensation loop filter to the spot
position signal, thereafter switching the spot position signal to
the tracking error signal to perform the phase-lag compensation and
the phase-lead compensation to the tracking error signal, and
operating the second control means after operating the first
control means.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an optical disc apparatus
which applies a light spot to an optical disc, thereby to record or
reproduce information.
BACKGROUND OF THE INVENTION
[0002] In recent years, optical disc apparatuses which applies a
light spot to an optical disc thereby to record or reproduce
information, such as a CD (Compact Disc) or MD (Mini Disc) have
been developed. The optical disc has tracks on/from which the
information is recorded/reproduced and the optical disc apparatus
records/reproduces the information by making the light spot follow
the tracks. The tracks are arranged concentrically or helically at
intervals of several micrometers (1.6 .mu.m in the case of CD or
MD) in its radial direction. The optical disc apparatus has light
spot moving means for moving the light spot in a radial direction
of the optical disc at high speed and with high precision, to
follow these microscopic tracks. As examples of the light spot
moving means, a tracking actuator for radially moving an objective
lens which focuses a light spot, and a galvanomirror for changing
an angle of an incident light to the objective lens are cited.
However, only with such light spot moving means, a movement range
of the light spot is limited by the size of the tracking actuator
or the objective lens or the like and accordingly the movement
range is small. Therefore, the optical disc apparatus usually
includes traverse moving means for radially moving an optical head
itself which internally contains the objective lens. Generally,
control for making the light spot follow the tracks by using the
light spot moving means is called "tracking control" and control
for making the optical head follow the movement of the light spot
by using the traverse moving means is called "traverse
control".
[0003] In addition, in the conventional traverse control, a
difference between the objective lens position and the center of
the optical head is generated as an error signal, thereby to
perform the control by using this error signal. A method in which a
low-band component of a tracking driving signal supplied to the
tracking actuator is used as the error signal is widely adopted.
The low-band component of the tracking driving signal shows
displacement of the objective lens by the tracking control, on the
basis of a position where weight and gravity of the objective lens
is balanced. When the moving direction of the tracking actuator is
horizontal, it is a signal corresponding to a relative position of
the objective lens and the center of the optical head, because the
displacement of the objective lens by the gravity from the
operation center of the tracking actuator in contrast to the moving
direction of the tracking actuator is approximately "0" in this
case. However, when the position of the apparatus is set up so as
to have a vertical moving direction of the tracking actuator, i.e.,
when the optical disc apparatus is positioned "longitudinally
(vertically)", the objective lens is displaced downwardly due to
its weight, which is referred to as "self-weight dislocation". The
position of the objective lens displaced due to the self-weight
dislocation is the position where the weight and gravity of the
objective lens is balanced, and the traverse control is executed
with this position as a center. Therefore, in the conventional
optical disc apparatus, when the self-weight dislocation occurs,
the movable range of the tracking actuator is narrowed accordingly,
whereby the follow-up characteristics of the tracking control are
deteriorated.
[0004] Japanese Published Patent Application No. Hei. 9-223320
discloses an optical disc apparatus which solves such problems. The
optical disc apparatus disclosed therein comprises spot position
signal generation means for generating a spot position signal which
indicates relative displacement of a light receiving element in the
optical head and a light spot, and has a structure of using the
spot position signal as an error signal of the traverse control.
According to this structure, the traverse control of this optical
disc apparatus has a point where the spot position signal is zero
as a control target and moves the optical head such that the light
spot is positioned at the center of the light receiving element. In
constituting the optical head, the center of the light receiving
element and the operation center of the tracking actuator are
previously arranged so as to coincide with each other. Therefore,
the objective lens is always moved around the operation center of
the tracking actuator, thereby avoiding the deterioration of the
follow-up characteristics of the tracking control.
[0005] However, so constructed optical disc apparatus has a problem
in stability of the operation at the starting of the operation of
the traverse control means. FIG. 14 is a waveform chart showing
waveforms of a spot position signal and a traverse driving signal
in the conventional optical disc apparatus when the self-weight
dislocation occurs. In FIG. 14, at an operation start time 1301 of
the traverse control means, the objective lens is displaced due to
the self-weight dislocation and the spot position signal has a
large value A. When the spot position signal is input to the
traverse control means in such a state to start the traverse
control, a driving signal 1302 applied to a traverse motor has a
high amplitude and becomes oscillatory. In the worst case, when the
optical head is moved by the traverse motor, the tracking control
is taken off. This is because the frequency band of the traverse
control is usually limited to several Hertz or less so as not to
follow the eccentricity, the traverse motor has large inertia and
it is difficult to be moved or stopped, and the like.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide an
optical disc apparatus which allows operation of traverse control
means by a spot position signal, even when the self-weight
dislocation of an objective lens occurs due to variation in the
position of the optical disc apparatus.
[0007] Other objects and advantages of the present invention will
become apparent from the detailed description and specific
embodiments described are provided only for illustration since
various additions and modifications within the spirit and scope of
the invention will be apparent to those of skill in the art from
the detailed description.
[0008] According to a 1st aspect of the present invention, an
optical disc apparatus which applies a light spot to an optical
disc, thereby to record or reproduce information on or from the
optical disc, comprises: first moving means for moving the light
spot applied to the optical disc, in a radial direction of the
optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; first control means for subjecting the
spot position signal to a first processing by a spot position loop
filter, and outputting the spot position signal to the first moving
means; second moving means for moving the optical head in a radial
direction of the optical disc; second control means for subjecting
the spot position signal to a second processing by a traverse loop
filter, and outputting the spot position signal to the second
moving means; and system operation control means for operating the
first control means, and thereafter operating the second control
means. Therefore, even when the self-weight dislocation occurs in
the objective lens, the traverse control using the spot position
signal can be performed with stability, thereby realizing an
optical disc apparatus which is considerably effective in practical
use, particularly as a portable optical disc apparatus.
[0009] According to a 2nd aspect of the present invention, an
optical disc apparatus which applies a light spot to an optical
disc, thereby to record or reproduce information on or from the
optical disc, comprises: first moving means for moving the light
spot applied to the optical disc, in a radial direction of the
optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; first control means for subjecting the
spot position signal to a first processing by a spot position loop
filter, and outputting the spot position signal to the first moving
means; second moving means for moving the optical head in a radial
direction of the optical disc; second control means for subjecting
the spot position signal to a second processing by a traverse loop
filter, and outputting the spot position signal to the second
moving means; spot position signal monitoring means for receiving
the spot position signal as an input, and outputting a first signal
which indicates that the spot position signal comes to a value
smaller than a prescribed value; and system operation control means
for operating the first control means when the first signal is
input, and operating the second control means after or
simultaneously with the operation of the first control means.
Therefore, the execution time for the spot position control
precedent to the traverse control can be optimized and minimized,
thereby realizing an optical disc apparatus which is considerably
effective in practical use, particularly as a portable optical disc
apparatus.
[0010] According to a 3rd aspect of the present invention, an
optical disc apparatus which applies a light spot to an optical
disc, thereby to record or reproduce information on or from the
optical disc, comprises: first moving means for moving the light
spot applied to the optical disc, in a radial direction of the
optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; correction signal generation means for
receiving the spot position signal as an input, and generating a
correction signal for correcting the spot position signal;
subtracting means for subtracting the correction signal from the
spot position signal; second moving means for moving the optical
head in a radial direction of the optical disc; and second control
means for subjecting an output from the subtracting means to a
processing by a traverse loop filter, and outputting the output to
the second moving means. Therefore, the traverse control can be
stabilized with a simple structure without using the spot position
control, thereby realizing an optical disc apparatus which is
considerably effective in practical use, particularly as a portable
optical disc apparatus.
[0011] According to a 4th aspect of the present invention, an
optical disc apparatus which applies a light spot to an optical
disc, thereby to record and reproduce information on or from the
optical disc, comprises: first moving means for moving the light
spot applied to the optical disc, in a radial direction of the
optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; second moving means for moving the
optical head in a radial direction of the optical disc; second
control means for subjecting the spot position signal to a
processing by a traverse loop filter, and outputting the spot
position signal to the second moving means; and a coefficient
multiplier for reducing a coefficient for the control by the second
control means to a value smaller than that in a normal operation
time, at starting of the operation of the second control means.
Therefore, the traverse control can be stabilized with a simple
structure without using the spot position control, thereby
realizing an optical disc apparatus which is considerably effective
in practical use, particularly as a portable optical disc
apparatus.
[0012] According to a 5th aspect of the present invention, in the
optical disc apparatus of the 1st or 2nd aspect, the first
processing subjected by the first control means is a phase-lag
compensation. According to the 5th aspect, the first processing to
be performed to the spot position signal comprises only the
phase-lag processing, i.e., low-band compensation processing,
whereby the structure of the spot position filter can be
simplified. Therefore, even when the self-weight dislocation occurs
in the objective lens, the traverse control using the spot position
signal can be performed with stability, or the execution time of
the spot position control precedent to the traverse control can be
optimized and minimized, thereby realizing an optical disc
apparatus which is considerably effective in practical use,
particularly as a portable optical disc apparatus.
[0013] According to a 6th aspect of the present invention, in the
optical disc apparatus of the 5th aspect, the first processing
subjected by the first control means includes compensation for
reducing an open-loop gain at a primary resonance frequency of the
first moving means, in addition to the phase-lag compensation.
Therefore, even when the open-loop gain at the primary resonance
frequency exceeds 0 dB, the phase margin can be secured and the
spot position control is stabilized.
[0014] According to a 7th aspect of the present invention, in the
optical disc apparatus of the 1st or 2nd aspect, the first
processing subjected by the first control means is a phase-lead
compensation and a phase-lag compensation, and the phase-lead
compensation is started from a frequency lower than a primary
resonance frequency of the first moving means. According to the 7th
aspect, both of the phase-lead compensation and the phase-lag
compensation, i.e., phase compensation and low-band compensation
are performed as the first processing to be performed to the spot
position signal. Accordingly, the oscillations can be suppressed in
a low frequency band in the servo of the spot position control,
according to the spot position signal having a sensitivity which
cannot be increased. Therefore, even when the self-weight
dislocation occurs in the objective lens, the traverse control
using the spot position signal can be performed with stability, or
the execution time for the spot position control precedent to the
traverse control can be optimized and minimized, thereby realizing
an optical disc apparatus which is considerably effective in
practical use, particularly as a portable optical disc
apparatus.
[0015] According to a 8th aspect of the present invention, an
optical disc apparatus which applies a light spot to an optical
disc, thereby to record or reproduce information on or from the
optical disc, comprises: first moving means for moving the light
spot applied to the optical disc, in a radial direction of the
optical disc; an optical head having converging means for
converging the light spot on the optical disc; spot position
detection means for generating a spot position signal which
indicates a positional difference in a radial direction of the
optical disc, between a center of the optical head and the light
spot on the optical head; tracking error detection means for
generating a tracking error signal which indicates a positional
dislocation between the light spot and a track on the optical disc;
first control means for subjecting the spot position signal or the
tracking error signal to a first processing by a phase compensation
loop filter, and outputting the signal to the first moving means;
second moving means for moving the optical head in a radial
direction of the optical disc; second control means for subjecting
the spot position signal to a second processing by a traverse loop
filter, and outputting the signal to the second moving means; and
system operation control means for operating the first control
means to perform a phase-lag compensation and a phase-lead
compensation by the phase compensation loop filter to the spot
position signal, thereafter switching the spot position signal to
the tracking error signal to perform the phase-lag compensation and
the phase-lead compensation to the tracking error signal, and
operating the second control means after operating the first
control means. Therefore, even when the self-weight dislocation
occurs in the objective lens, the traverse control using the spot
position signal can be performed with stability. Besides, a loop
filter can be commonly used for performing the phase-lead
compensation and the phase-lag compensation in performing the spot
position control and the tracking error control. Therefore, an
optical disc apparatus which is considerable effective in practical
use, particularly as a portable optical disc apparatus, can be
obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a block diagram illustrating an optical disc
apparatus according to a first embodiment of the present
invention.
[0017] FIG. 2 is a waveform chart showing operations according to
the first embodiment.
[0018] FIG. 3 is a flowchart showing processings by a controller
116 in the first embodiment.
[0019] FIG. 4 is a block diagram illustrating an optical disc
apparatus according to a second embodiment of the present
invention.
[0020] FIG. 5 is a waveform chart showing operations according to
the second embodiment.
[0021] FIG. 6 is a flowchart showing processings by a controller
116 in the second embodiment.
[0022] FIG. 7 is a block diagram illustrating an optical disc
apparatus according to a third embodiment of the present
invention.
[0023] FIG. 8 is a waveform chart showing operations according to
the third embodiment.
[0024] FIG. 9 is a flowchart showing processings by a controller
604 in the third embodiment.
[0025] FIG. 10 is a chart showing waveforms of a correction signal
in the third embodiment.
[0026] FIG. 11 is a block diagram illustrating an optical disc
apparatus according to a fourth embodiment of the present
invention.
[0027] FIG. 12 is a waveform chart showing operations according to
the fourth embodiment.
[0028] FIG. 13 is a flowchart showing processings by a controller
1002 in the fourth embodiment.
[0029] FIG. 14 is a waveform chart showing operations according to
a conventional optical disc apparatus.
[0030] FIG. 15 are diagrams showing loop characteristics of spot
position control, FIG. 15(a) being a diagram showing open-loop gain
characteristics without a spot position loop filter, FIG. 15(b)
being a diagram showing gain characteristics of the spot position
loop filter, and FIG. 15(c) being a diagram showing open-loop
characteristics of the whole spot position control.
[0031] FIG. 16 is a block diagram illustrating an optical disc
apparatus according to another example of the first embodiment.
[0032] FIG. 17 are diagrams showing loop characteristics of spot
position control, FIG. 17(a) being a diagram showing open-loop gain
characteristics without a spot position loop filter, FIG. 17(b)
being a diagram showing gain characteristics of the spot position
loop filter, and
[0033] FIG. 17(c) being a diagram showing open-loop characteristics
of the whole spot position control.
[0034] FIG. 18 are diagrams showing loop characteristics of spot
position control, FIG. 18(a) being a diagram showing open-loop gain
characteristics without a spot position loop filter, FIG. 18(b)
being a diagram showing gain characteristics of the spot position
loop filter, FIG. 18(c) being a diagram showing open-loop
characteristics of the whole spot position control, FIG. 18(d)
being a diagram showing gain characteristics of a changed spot
position loop filter, and FIG. 18(e) being a diagram showing
open-loop characteristics of the whole changed spot position
control.
[0035] FIG. 19 are diagrams showing loop-characteristics of spot
position control, FIG. 19(a) being a diagram showing open-loop gain
characteristics without a spot position loop filter, FIG. 19(b)
being a diagram showing a gain characteristics of the spot position
loop filter, FIG. 19(c) being a diagram showing open-loop
characteristics of the whole spot position control, FIG. 19(d)
being a diagram showing gain characteristics of a spot position
loop filter, and FIG. 19(e) being a diagram showing open-loop
characteristics of the whole spot position control.
[0036] FIG. 20 are diagrams showing loop characteristics of spot
position control, FIG. 20(a) being a diagram showing open-loop gain
characteristics without a spot position loop filter 111, FIG. 20(b)
being a diagram showing gain characteristics of the spot position
loop filter 111, FIG. 20(c) being a diagram showing open-loop
characteristics of the whole spot position control, FIG. 20(d)
being a diagram showing open-loop phase characteristics without the
spot position loop filter 111 and phase characteristics of the spot
position loop filter 111, in the spot position control loop, and
FIG. 20(e) being a diagram showing phase characteristics of the
whole spot position control loop.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] Hereinafter, embodiments of the present invention will be
described with reference to the drawings.
[0038] Embodiment 1.
[0039] A first embodiment of the present invention will be
described with reference to FIGS. 1, 2, 3, and 15.
[0040] The first embodiment corresponds to claims 1, 5, 6 and 7,
and it allows the traverse control by executing spot position
control beforehand, even when the self-weight dislocation occurs in
an objective lens in a radial direction of an optical disc.
[0041] FIG. 1 is a block diagram illustrating an optical disc
apparatus according to the first embodiment. FIG. 2 is a waveform
chart showing operations according to the first embodiment. FIG. 3
is a flowchart showing processings by a controller 116 in the first
embodiment.
[0042] In FIG. 1, reference numeral 100 denotes a mechanism unit.
In this mechanism unit 100, numeral 101 denotes an optical disc
having concentric or helical tracks, on or from which information
is recorded or reproduced. Numeral 102 denotes an optical head,
internally containing an objective lens 103 as converging means for
converging a light spot on the optical disc 101, a tracking
actuator 104 as first moving means, for moving the objective lens
103 in a radial direction of the optical disc 101, and a light
receiving element 105 for converting a light reflected on the
optical disc 101 into an electric signal. Numeral 106 denotes a
traverse motor as second moving means, for moving the optical head
102 in a radial direction of the optical disc in accordance with an
output from a switch circuit 114 which is described later.
[0043] In addition, numeral 107 denotes a control unit. In this
control unit 107, numeral 108 denotes a tracking error detection
circuit for generating a tracking error signal which indicates
positional displacement between the track on the optical disc 101
and the light spot, from an output of the light receiving element
105. Numeral 109 denotes a tracking loop filter which receives the
tracking error signal as input, and outputs the signal after
performing phase compensation processing. Numeral 110 denotes a
spot position detection circuit as spot position detection means,
for generating a spot position signal which indicates a positional
difference between the center of the optical head and the light
spot on the light receiving element 105, from an output of the
light receiving element 105. Numeral 111 denotes a spot position
loop filter which receives the spot position signal as input, and
outputs the signal after performing the phase compensation
processing. Numeral 112 denotes a selection circuit for selecting
either of an output of the tracking loop filter 109 and an output
of the spot position loop filter 111, in accordance with an
instruction of a controller 116, which is described later, and
supplying the selected output to the tracking actuator 104. Numeral
113 denotes a traverse loop filter which receives the spot position
signal as input, and outputs the signal after performing the phase
compensation processing. Numeral 114 denotes a switch circuit for
switching on/off an output of the traverse loop filter 113, in
accordance with an instruction of the controller 116, which is
described later. Numeral 116 denotes a controller which receives a
detection result of the spot position detection circuit 110 as
input, and controls the selection circuit 112 and the switch
circuit 114, on the basis of the detection result. This controller
116 constitutes first control means, together with the selection
circuit 112. In addition, the controller 116 constitutes second
control means, together with the switch circuit 114. Further, the
controller 116 constitutes system operation control means for
operating the second control means after operating the first
control means.
[0044] Next, a description is given of an operation of the so
constructed optical disc apparatus according to the first
embodiment.
[0045] When the objective lens 103 experiences gravity in a radial
direction of the optical disc, the objective lens 103 is displaced
due to the self-weight dislocation. The spot position detection
circuit 110 generates a spot position signal on the basis of the
displacement amount of the light spot displaced from the center of
the light receiving element 105, and outputs the signal. At an
initial timing when no control is imposed (see 21 in FIG. 2), the
spot position signal has an offset of a value A (A.noteq.0). The
optical disc apparatus of the first embodiment controls the
position of the objective lens 103 in the following procedure so as
to make the value of the spot position signal approximately
"0".
[0046] The controller 116 opens the switch circuit 114, switches
the selection circuit 112 to the side for the spot position loop
filter 111 in a state where driving of the traverse motor 106 is
stopped, and supplies an output from the spot position loop filter
111 to the tracking actuator 104 (step S301 in FIG. 3). The spot
position loop filter 111 receives the spot position signal, and the
spot position loop filter 111 controls the position of the
objective lens 103 via the tracking actuator 104, so as to have
approximately "0" spot position signal, i.e., to position the light
spot at the center of the light receiving element 105. Hereinafter,
this control which is newly performed is referred to as "spot
position control". The displacement amount of the objective lens
103 due to the self-weight dislocation is determined by hardness of
a resilient support supporting the objective lens 103 and weight of
the objective lens 103. Further, a time period from when the spot
position control for the determined displacement amount is started
until the spot position signal has a value of approximately "0" is
determined by characteristics of a servo loop which is constituted
by the tracking actuator 104, the spot position detection circuit
110 and the spot position loop filter 111, and it can be previously
calculated. The controller 116 waits for the previously calculated
time period (see 22 in FIG. 2) until the spot position signal has a
value of approximately "0" (step S302 in FIG. 3), switches the
selection circuit 112 to the side for the tracking loop filter 109,
and closes the switch circuit 114 (step S303 in FIG. 3). When the
selection circuit 112 is switched to the side of the tracking loop
filter 109, the tracking error signal detected by the tracking
error detection circuit 108 is subjected to phase compensation
processing by the tracking loop filter 109, then supplied to the
tracking actuator 104, and thereby the light spot is controlled so
as to follow the tracks on the optical disc through the objective
lens 103. When the switch circuit 114 is closed, the spot position
signal which is subjected to the phase compensation by the traverse
loop filter 113 is supplied to the traverse motor 106, and thereby
the optical head 102 is moved to a position where the spot position
signal has an approximately "0" value. At starting of the traverse
control (see 23 in FIG. 2), since the spot position signal has an
approximately "0" value because of the spot position control
executed beforehand, the traverse driving signal does not oscillate
unlike that shown in the prior art. Therefore, the traverse control
having a control object of positioning the light spot near the
center of the light receiving element 105 (.apprxeq. near the
center of the optical head 102) can be started with stability.
[0047] A more detailed description is given of the spot position
loop filter 111. The spot position control in the first embodiment
is realized by a control loop comprising the tracking actuator 104,
the spot position detection circuit 110, the spot position loop
filter 111, and the selection circuit 112. This control system is a
closed-loop feedback control system. FIG. 15(a) schematically shows
gain characteristics of elements except the spot position loop
filter 111. As shown in FIG. 15(a), it does not have frequency
characteristics at frequencies lower than a primary resonance
frequency f0 and has characteristics that the gain is decreased by
-40 dB/dec at frequencies higher than f0. The characteristics shown
in FIG. 15(a) are seen in a control target having a structure for
supporting the objective lens 103 by a resilient support such as a
spring. When the control is to be performed to such a control
target, it is effective to perform phase compensation by a
differential operation, i.e., phase-lead compensation, in addition
to the phase-lag compensation. That is, as shown in FIG. 15(b),
when the spot position loop filter 111 has characteristics of 20
dB/dec (phase-lead compensation) and -20 dB/dec (phase-lag
compensation) and the inclination in the vicinity of the gain
crossover frequency fc in the whole open-loop characteristics (see
FIG. 15(c)) is -20 dB/dec, the control system can be
stabilized.
[0048] Since the control target is the same tracking actuator 104,
characteristics required in the tracking loop filter 109 are
approximately similar to those shown in FIG. 15(b). Therefore, loop
filters can be commonly used by using the spot position loop filter
111 and the tracking loop filter 109 exclusively.
[0049] FIG. 16 is a block diagram illustrating an optical disc
apparatus according to the first embodiment, which is constructed
as described above. In FIG. 16, numerals 100-108, 110, 113, and 114
denote elements of the same numerals as those shown in FIG. 1.
Numeral 120 denotes a selection circuit for selecting either of an
output from the tracking error detection circuit 108 and an output
from the spot position detection circuit 110 in accordance with an
instruction of a controller 116, which is described later, and
supplying the selected output to a phase compensation loop filter
121. Numeral 121 denotes a phase compensation loop filter which has
V-shaped gain characteristics and performs phase-lag compensation
in a low band and phase-lead compensation in a high band to the
tracking error signal or spot position signal selected by the
selection circuit 120. Numeral 116 denotes a controller which
receives a detection result of the spot position detection circuit
110 as an input, and controls the selection circuit 120 and the
switch circuit 114 on the basis of the detection result. This
controller 116 constitutes first control means together with the
selection circuit 120. In addition, the controller 116 constitutes
second control means together with the selection circuit 120.
Further, this controller 116 constitutes system operation control
means for operating the first control means, and thereafter
operating the second control means.
[0050] According to the structure shown in FIG. 16, a loop filter
having the same gain characteristics as the spot position loop
filter 111 as shown in FIG. 15(b) is provided as the phase
compensation loop filter 116. Then, the controller 116 executes the
control such that the selection circuit 120 inputs the spot
position signal initially to the phase compensation loop filter
116, and switches the input for the phase compensation loop filter
116 to the tracking error signal when the spot position control is
almost completed. Accordingly, the spot position loop filter 111
and the tracking loop filter 119 in FIG. 1 can be commonly
used.
[0051] When sensitivity of the spot position signal and the
tracking error signal is extremely different from each other, the
characteristics of the spot position loop filter 111 and the
tracking loop filter 109 should be differentiated. In a CD or MD,
while the track pitch is 1.6 .mu.m, the movable range of the
objective lens 103 is hundreds .mu.m. Therefore, in the case of the
same signal amplitude, the sensitivity of the spot position signal
is less than one hundredth of that of the tracking error signal.
When the open-loop characteristics shown in FIG. 15(c) are to be
realized in both of the tracking control loop and the spot position
control loop, the gain of the spot position loop filter 111 is
required to be more than one hundred times as large as that of the
track position loop filter 109, and this is not realistic.
Therefore, in such a case, it is effective to make the gain
crossover frequency fc lower than the primary resonance frequency
f0 and make the spot position loop filter have the characteristics
shown in FIG. 17(b). In this case, as shown in FIG. 17(c) showing
the whole open-loop characteristics, the gain characteristics in
the vicinity of the gain crossover frequency can be set at -20
dB/dec. For example in a stationary optical disc apparatus, since
the direction of the self-weight dislocation is fixed, the control
band of the spot position control is not required to be enhanced
and therefore this method is particularly effective. Further,
advantageously, this method can be easily realized by a simple
filter of -20 dB/dec.
[0052] When the spot position loop filter 111 is constituted using
the filter of -20 dB/dec, attention should be given to a gain at
the primary resonance frequency f0, i.e., a resonance point. When
the gain at the resonance point is large, a part having the gain
more than 0 dB occurs at frequencies higher than the gain crossover
frequency fc as shown in FIG. 18(c), whereby the spot position
control becomes unstable. To solve this problem, it is effective to
make the spot position loop filter 111 have the characteristics of
reducing the gain at high frequencies, as shown in FIG. 18(d).
Accordingly, the open-loop gain in excess of 0 dB at the resonance
point can be avoided as shown in FIG. 18(e), whereby the spot
position control can be stabilized.
[0053] When the spot position control is executed using the spot
position signal having a low sensitivity, by using the phase-lead
compensation as described with reference to FIG. 17, it is
effective to start the phase-lead compensation from a frequency
lower than the primary resonance frequency f0, as shown in FIG.
19(d). By doing so, even when the open-loop gain exceeds 0 dB at
the resonance point, a phase margin can be secured, thereby
stabilizing the spot position control.
[0054] With regard to this point, a detailed description is given
with reference to the drawings. FIG. 20(a) shows open-loop gain
characteristics without the spot position loop filter 111, FIG.
20(b) shows gain characteristics of the spot position loop filter
111, FIG. 20(c) shows open-loop gain characteristics of the whole
spot position control loop, FIG. 20(d) shows open-loop phase
characteristics without the spot position loop filter 111 and phase
characteristics of the spot position loop filter in the spot
position control loop, and FIG. 20(e) shows phase characteristics
of the whole spot position control loop. In the figures, f0 denotes
a primary resonance frequency and fB denotes a frequency at which
phase-lead compensation is started. Numeral 181 denotes open-loop
phase characteristics without the spot position loop filter 111.
Numeral 182 denotes phase characteristics of the spot position loop
filter. Numeral 183 denotes a phase margin for the spot position
control.
[0055] The phase characteristic 181 of elements except the spot
position loop filter 111 is 0 degree at a frequency band lower than
the primary resonance frequency f0 and thereafter abruptly
decreased up to -180 degree, passing through a point of about -90
degree at the primary resonance frequency f0. On the other hand,
the phase characteristic 182 of the spot position loop filter 111
is about 0 degree at the phase-lead compensation starting frequency
fB, -90 degree at a frequency band sufficiently lower than fB, and
+90 degree at a frequency band sufficiently higher than fB. The
phase characteristic of the whole loop can be obtained by adding
these characteristics, which is shown in FIG. 20(e). That is, the
phase-lead compensation is started from the frequency band lower
than that in the vicinity of the primary resonance frequency f0
where the phase is abruptly decreased to -180 degree (i.e., phase
margin of 0), whereby the phase margin 183 can be secured and the
spot position control can be stabilized.
[0056] While this method slightly complicates the structure of the
spot position loop filter 111, it is resistant to variation in the
open-loop gain at the resonance point, more specifically, variation
in the gain of the tracking actuator 104 at the primary resonance
frequency f0, and it can enhance the gain crossover frequency fc
with relative to that in a case where the phase-lead compensation
is not performed, thereby obtaining a quicker response. Generally,
the gain of the tracking actuator 104 at the primary resonance
frequency f0 varies according to the weight of the objective lens
103, hardness of a spring which supports the objective lens,
hardness of adhesive which is used for fixing the spring to the
optical head 102, and the like. Therefore, it is difficult to
reduce the variations. Accordingly, the present method which can
accommodate the variations is effective in practical use. In
addition, by providing a mechanism for switching the frequency
where the phase-lead compensation is started, for example by
switching a tap to the spot position loop filter 111, the spot
position loop filter 111 can be commonly used with the tracking
loop filter 109. In this case, the complicated structure of the
spot position loop filter 111 presents no problem any more.
[0057] Here, it is desirable that the switching of the selection
circuit 112 to the side for the tracking loop filter 109 is
completed before or simultaneously with the closing of the switch
circuit 114. As described above, the output of the tracking loop
filter 109 has a function of fixing the position of the light spot
to the track on the optical disc 101. However, when the switch
circuit 114 is closed in a state where this control is not
performed, while the optical head 102 is moved according to the
spot position signal, the light receiving element 105, the tracking
actuator 104 and the objective lens 103 are moved together with the
optical head 102. Therefore, the spot position signal is not
reduced and thus there is a risk of the optical head 102
mechanically continuing to move until the movable limit. In this
first embodiment, since the light spot is fixed to the track as
described above, the traverse control using the spot position
signal is realized.
[0058] As described above, the apparatus according to the first
embodiment performs the spot position control, corrects the
self-weight dislocation of the objective lens 103 occurring in the
radial direction of the disc, and thereafter performs the traverse
control. Thereby, even in a case where the self-weight dislocation
occurs according to the position of the apparatus, the traverse
control using the spot position signal can be performed with
stability.
[0059] Embodiment 2.
[0060] Hereinafter, a second embodiment of the present invention
will be described with reference to FIGS. 4, 5, and 6.
[0061] The second embodiment corresponds to claims 2, 5, and 6.
[0062] In the first embodiment, the fixed time previously
calculated (see 22 in FIG. 2) is set as the time period from the
starting of the spot position control until the starting of the
traverse control. However, the hardness of the resilient support
which supports the objective lens 103 or the sensitivity of the
tracking actuator 104 for input signals usually have variations.
Therefore, in order to operate the apparatus with stability on all
conditions, a time period including a longer margin should be set
to satisfy the worst condition.
[0063] In this second embodiment, a monitoring circuit 115 is
provided as shown in FIG. 4 and the spot position loop filter 111
is operated during the shortest possible time when oscillations do
not occur at the starting of the traverse control, whereby the
whole control time can be optimized.
[0064] FIG. 4 is a block diagram illustrating an optical disc
apparatus according to the second embodiment. FIG. 5 is a waveform
chart showing operations according to the second embodiment. FIG. 6
is a flowchart showing processings by a controller 116 in the
second embodiment.
[0065] In FIG. 4, reference numeral 100 denotes a mechanism unit.
In this mechanism unit 100, numeral 101 denotes an optical disc
having concentric or helical tracks, on or from which information
is recorded or reproduced. Numeral 102 denotes an optical head,
which internally contains an objective lens 103 as converging means
for converging a light spot on the optical disc 101, a tracking
actuator 104 as first moving means, for moving the objective lens
103 in a radial direction of the optical disc, and a light
receiving element 105 for converting a light reflected on the
optical disc 101 into an electric signal. Numeral 106 denotes a
traverse motor as second moving means, for moving the optical head
102 in a radial direction of the optical disc, in accordance with
an output from a switch circuit 114, which is described later.
[0066] Numeral 107 denotes a control unit. In this control unit
107, numeral 108 denotes a tracking error detection circuit for
generating a tracking error signal which indicates position
displacement between the track on the optical disc 101 and the
light spot, from an output of the light receiving element 105.
Numeral 109 denotes a tracking filter which receives the tracking
error signal as input, and outputs the signal after performing
phase compensation processing. Numeral 110 denotes a spot position
detection circuit as spot position detection means, for generating
a spot position signal which indicates a positional difference
between the center of the optical head and the light spot on the
light receiving element 105, from the output of the light receiving
element 105. Numeral 111 denotes a spot position loop filter which
receives the spot position signal as input, and outputs the signal
after performing the phase compensation processing. Numeral 112
denotes a selection circuit for selecting either of an output from
the tracking loop filter 109 and an output from the spot position
loop filter 111, in accordance with an instruction of a controller
116, which is described later, and supplying the selected output to
the tracking actuator 104. Numeral 113 denotes a traverse loop
filter which receives the spot position signal as input, and
outputs the signal after performing the phase compensation
processing. Numeral 114 denotes a switch circuit for switching on
or off the output of the traverse loop filter 113, in accordance
with an instruction of the controller 116 which is described later.
Numeral 115 denotes a monitoring circuit as spot position signal
monitoring means, for judging the size of the spot position signal.
Numeral 116 denotes a controller which receives a monitor result of
the monitoring circuit 115 as input, and controls the selection
circuit 112 and the switch circuit 114 on the basis of the monitor
result. This controller 116 constitutes first control means
together with the selection circuit 112. In addition, the
controller 116 constitutes second control means together with the
switch circuit 114. Further, the controller 116 constitutes system
operation control means for operating the first control means when
it receives a signal indicating that the spot position control is
statically determined from the monitoring circuit 115, and
operating the second control means after or simultaneously with the
operation of the first control means.
[0067] Next, a description is given of an operation of the so
constructed optical disc apparatus according to the second
embodiment.
[0068] When the objective lens 103 experiences gravity in a radial
direction of the optical disc 101, the objective lens is displaced
due to the self-weight dislocation. The spot position detection
circuit 110 generates a spot position signal in accordance with the
displacement amount of the light spot from the center of the light
receiving element 105, and outputs the spot position signal. At an
initial timing when no control is imposed (see 41 in FIG. 5), the
spot position signal has an offset of a value A (A.noteq.0). The
controller 116 opens the switch circuit 114 thereby stopping the
driving of the traverse motor 106, and switches the selection
circuit 112 to the side for the spot position loop filter 111 to
make the value of the spot position signal approximately "0",
thereby executing the spot position control (step S501 in FIG. 6).
Then, the controller 116 receives the input from the monitoring
circuit 115 (step S502 in FIG. 6). The monitoring circuit 115
monitors whether the value of the spot position signal goes into a
previously determined range (see 42 in FIG. 5), and judges that the
spot position control is statically determined at a timing when the
value goes into the predetermined range. The controller 116 judges
whether the output of the monitoring circuit 115 indicates that the
spot position control is statically determined (step S503 in FIG.
6). When it is judged that the output indicates the static
determination, the selection circuit 112 is switched to the side
for the tracking loop filter 109 as well as the switch circuit 114
is closed (see 43 in FIG. 5 and step S504 in FIG. 6). At this
timing, since the spot position signal has a sufficiently small
value, the traverse control is performed without being oscillated.
In addition, the execution time of the spot position control (see
44 in FIG. 5) can be shortened with relative to a case where the
monitoring circuit 115 is not used. When it is judged that the spot
position control is not statically determined, the controller 116
successively receives the output from the monitoring circuit 115,
and judges whether it is statically determined.
[0069] It is desirable that a value compared with the spot position
signal, which is used by the monitoring circuit 115 to judge the
static determination of the spot position control is set to have a
value smaller than an operation unit of the traverse motor 106.
When the traverse motor 106 is constituted by a motor with brush,
this traverse motor 106 often operates with "Cogging unit" as a
unit. For example, in a structure where the optical head 102 is
moved 40 .mu.m per 1 Cogging, assuming that the spot position
control is statically determined at a timing when the spot position
signal goes into a range of about .+-.40 .mu.m, this is equivalent
to a situation where the error is approximately "0" in the traverse
control performed later. Therefore, the turbulence in the traverse
control can be avoided.
[0070] As described above, the apparatus according to the second
embodiment includes the monitoring circuit 115 in addition to the
apparatus of the first embodiment and compares the spot position
signal with the previously set range, whereby the spot position
control can be ensured with stability. Further, the apparatus can
optimize and minimize the time period for executing the spot
position control (see 44 in FIG. 5) for each execution of the
traverse control, thereby obtaining the stable and high-speed
operation of the optical disc apparatus.
[0071] In this second embodiment, the monitoring circuit 115 judges
that the spot position control is statically determined, at a
timing when the spot position signal goes into the predetermined
range (see 42 in FIG. 5). However, any method, such as a method for
detecting that a time period when the spot position signal is
within a predetermined range is longer than a predetermined time
period, or a method for detecting that an average of the spot
position signals in a predetermined time period is smaller than a
predetermined value, can be utilized, as long as the method
substantially detects that the size of the spot position signal is
reduced and judges the static determination of the spot position
control.
[0072] Embodiment 3.
[0073] A third embodiment of the present invention will be
described with reference to FIGS. 7, 8, 9, and 10.
[0074] The third embodiment corresponds to claim 3. This third
embodiment controls the size of a spot position signal input to a
traverse loop filter also in an initial state, thereby obtaining
the stability of the traverse control.
[0075] FIG. 7 is a block diagram illustrating an optical disc
apparatus according to the third embodiment. FIG. 8 is a waveform
chart showing operations according to the third embodiment. FIG. 9
is a flowchart showing processings by a controller 604. FIG. 10 is
a diagram showing waveforms of correction signals in the third
embodiment.
[0076] In FIG. 7, numerals 100-114 denote elements of the same
reference numerals as those shown in FIG. 1. Numeral 601 denotes a
correction signal generation circuit as correction signal
generation means, for generating a correction signal for correcting
a spot position signal. Numeral 602 denotes a subtracter as
subtracting means for subtracting the correction signal from the
spot position signal (hereinafter referred to as "before-correction
spot position signal) output from the spot position detection
circuit 110. The spot position signal input to the traverse loop
filter 113 after being corrected by using the correction signal is
hereinafter referred to as "after-correction spot position signal".
Numeral 603 denotes a second switch circuit for switching on/off
the supply of the signal to the tracking actuator 104, in
accordance with an instruction of a controller 604 which is
described later. Numeral 604 denotes a controller for controlling
the switch circuit 114, the second switch circuit 603, and the
correction signal generation circuit 601. This controller 604
constitutes second control means together with the switch circuit
114. The second control means subjects the output of the subtracter
602 to the processing by the traverse loop filter 113, and outputs
the output to the traverse motor 106 as the second moving means,
via the switch circuit 114.
[0077] Next, a description is given of an operation of the so
constructed optical disc apparatus according to the third
embodiment.
[0078] When the objective lens 103 experiences gravity in a radial
direction of the optical disc 101, the objective lens 103 is
displaced due to the self-weight. The spot position signal has an
offset of a predetermined value A (A.noteq.0), as shown by numeral
71 in FIG. 8. The controller 604 controls the correction signal
generation circuit 601 to store the value of the spot position
signal at that time in prior to the execution of the traverse
control and output the value as the correction signal (see 72 in
FIG. 8 and step S801 in FIG. 9). The subtracter 602 subtracts the
correction signal from a before-correction spot position signal
output by the spot position detection circuit 110. Accordingly, an
after-correction spot position signal output from the subtracter
602 has a value of approximately "0" (see 73 in FIG. 8). Then, the
controller 604 closes the second switch circuit 603 thereby
supplying the output of the tracking loop filter 109 to the
tracking actuator 104, controls the light spot to follow the track
on the optical disc 101, further closes the switch circuit 114
thereby supplying the output of the traverse loop filter 113 to the
traverse motor 106, and thereby moves the optical head 102 so as to
locate the light spot in the vicinity of the center of the optical
head 102 (step S802 in FIG. 9). At this time, since the
after-correction spot position signal input to the traverse loop
filter 113 is approximately "0", the output from the traverse loop
filter 113 is also approximately "0". Therefore, the traverse motor
106 scarcely operates.
[0079] Then, the controller 604 instructs the correction signal
generation circuit 601 to gradually increase the correction signal
up to near "0" (step S803 in FIG. 9), and waits for the completion
of the correction signal changing by the correction signal
generation circuit 601 (step S804 in FIG. 9). When the correction
signal is changed, a difference is generated between the
before-correction spot position signal and the correction signal.
Since the traverse loop filter 113 and the traverse motor 106
operate so as to make this difference, i.e., the after-correction
spot position signal "0", the before-correction spot position
signal is changed so as to have almost the same waveform as that of
the correction signal (see 74 in FIG. 8). When the correction
signal which is to be output has "0" value, the correction signal
generation circuit 601 keeps the correction signal in "0" (see 75
in FIG. 8), and notifies to the controller 604 of the completion of
the correction signal changing. When the controller 604 receives
the notification of the completion of the correction signal
changing, it completes the starting of the traverse control.
[0080] The spot position signal can be changed by the correction
signal linearly with a predetermined time constant (see 91 in FIG.
10) or sinusoidally (see 92 in FIG. 10). However, the latter is
more desirable because the spot position control can be followed
more smoothly in that case. Further, it is desirable that the
changing speed of the correction signal is lower than the control
band of the traverse control. Accordingly, the traverse control can
follow the change in the spot position signal with stability.
[0081] As described above, the apparatus according to the third
embodiment stores the spot position signal at the starting of the
traverse control, subtracts the stored signal as a correction
signal from the spot position signal, and supplies the obtained
difference to the traverse loop filter 113. Therefore, the input to
the traverse loop filter 113 can be always kept in a small value,
whereby the unstableness in the traverse control due to the large
initial error signal can be avoided.
[0082] In this third embodiment, the correction signal is
subtracted using the subtracter 602. However, any unit, such as a
unit for holding a correction signal after performing the inversion
and using the adder to subtract the correction signal from the
before-correction spot position signal, can be utilized as long as
it can substantially subtract the correction signal from the
before-correction spot position signal.
[0083] Embodiment 4.
[0084] A fourth embodiment of the present invention will be
described with reference to FIGS. 11, 12, and 13.
[0085] The fourth embodiment corresponds to claim 4. In this
embodiment, a loop gain of the traverse control is set to have a
small value at the starting of the traverse control, and thereafter
the gain is changed to have a desired value, thereby stabilizing
the operation at the starting of the traverse control.
[0086] FIG. 11 is a block diagram illustrating an optical disc
apparatus according to the fourth embodiment. FIG. 12 is a waveform
chart showing operations according to the fourth embodiment. FIG.
13 is a flowchart showing processings by a controller 1002.
[0087] In FIG. 11, numerals 100-114 denote the elements of the same
reference numerals as those shown in FIG. 1. In addition, numeral
603 denotes the element of the same reference numeral as that shown
in FIG. 7. Numeral 1001 denotes a coefficient multiplier for
coefficient multiplying an output from the traverse loop filter
113, and outputting the same. Numeral 1002 denotes a controller for
controlling the switch circuit 114, the coefficient multiplier
1001, and the second switch circuit 603. This controller 1002
constitutes second control means together with the coefficient
multiplier 1001 and the switch circuit 114. The second control
means subjects the spot position signal to the processing by the
traverse loop filter 113 and outputs the processed signal to the
traverse motor 106 as the second moving means, as well as controls
a coefficient of the coefficient multiplier 1001 at the starting of
the operation to have a value smaller than that at the normal
operation.
[0088] Hereinafter, an operation of the optical disc apparatus
according to the fourth embodiment will be described.
[0089] When gravity is applied to the objective lens 103 in a
radial direction of the optical disc 101, the objective lens 103 is
displaced from the center of the optical head 102 due to the
self-weight. Accordingly, the spot position signal has an offset of
a value A (see 1101 in FIG. 12). The controller 1002 opens the
switch circuit 114 and the second switch circuit 603, and stops the
driving of the tracking actuator 104 and the traverse motor 106
(step S1201 in FIG. 13). Next, the controller 1002 sets a
coefficient of the coefficient multiplier 1001 at B times
(0<B<1), for example, 0.1 times (see 1102 in FIG. 12 and step
S1202 in FIG. 13). Then, the controller closes the switch circuit
114 and the second switch circuit 603, supplies the output of the
tracking loop filter 109 to the tracking actuator 104, and supplies
the output of the traverse loop filter 113, which is coefficient
multiplied by the coefficient multiplier 1001, to the traverse
motor 106, thereby executing the tracking control and the traverse
control (step S1203 in FIG. 13). In this case, the spot position
signal input to the traverse loop filter 113 remains having the
value A. However, since the value B is set in the coefficient
multiplier 1001, the voltage supplied to the traverse motor 106 is
reduced with relative to a case without the coefficient multiplier
1001. Therefore, the operation of the optical head 102 by the
traverse motor 106 is slowed. The controller 1002 makes the
coefficient of the coefficient multiplier 1001 closer to "1" with
the lapse of time (see 1103 in FIG. 12 and step S1204 in FIG. 13).
In this way, the coefficient of the coefficient multiplier 1001 is
initially set to have a smaller value in the state where the spot
position signal is large and the coefficient is increased with the
lapse of time, whereby the optical head 102 can be moved smoothly.
The controller 1002 judges that the coefficient has a value of "1",
and completes rising processing of the traverse control in the
state of the coefficient of "1" (step S1205 in FIG. 13).
[0090] As described above, the apparatus according to the fourth
embodiment includes the coefficient multiplier 1001, sets the
coefficient of the coefficient multiplier to have a smaller value
at the starting of the traverse control to decrease the loop gain
of the traverse control, and gradually increases the coefficient,
thereby obtaining the desired gain. Therefore, the operation at the
starting of the traverse control can be stabilized with a simple
structure using the coefficient multiplier, without providing the
new control system which performs the tracking control by the spot
position signal as described in the first and second
embodiments.
[0091] In this fourth embodiment, the coefficient multiplier 1001
is provided in a next stage of the traverse loop filter 113.
However, this can be provided between the spot position detection
circuit 110 and the traverse loop filter 113, or between the switch
circuit 114 and the traverse motor 106. Or, any unit including a
unit for changing the gain of the traverse loop filter 113 can be
used as long as it changes the loop gain of the servo loop
constituting the traverse control.
[0092] Further, in the fourth embodiment, the example where the
coefficient of the coefficient multiplier 1001 is changed linearly
is described. However, as similar to the correction signal in the
third embodiment, the coefficient can be changed in any manner
including a manner of changing the coefficient sinusoidally, as
long as the coefficient is set to have a small value at first and
finally have the desired gain for the traverse control loop.
[0093] In the foregoing description, the spot position signal is
generated from the output of the light receiving element 105 in the
optical head 102. However, a lens position sensor for detecting a
position of the lens can be provided separately from the light
receiving element. In such a structure, the lens position can be
used as a spot position.
* * * * *