4軸アクチュエータを用いたラジアル・タンジェンシャルチルトサーボシステムの開発

4軸アクチュエータを用いたラジアル・タンジェンシャルチ
ルトサーボシステムの開発
Radial and Tangential-Tilt Servo Using Four-Axis Actuator
川崎 俊之*
赤沼 悟一*
前田 育夫**
北澤 智文***
Toshiyuki KAWASAKI
Goichi AKANUMA
Ikuo MAEDA
Tomofumi KITAZAWA Shunichi ANDOH
笠原 亮介
安藤 俊一***
*
Ryohsuke KASAHARA
要
旨
青紫レーザを用いた次世代光ディスクシステムにおいて,対物レンズをラジアル・タンジェン
シャル方向にチルトさせる,対物レンズチルトアクチュエータを開発した.このアクチュエータ
は,対物レンズをフォーカシング,トラッキングさせると同時に,ラジアル・タンジェンシャル
方向にチルトさせることができる.また,このチルトアクチュエータを4軸独立に制御するため
の,チルトサーボシステムを開発した.本報告では,ディスクが傾斜したときに生じる記録面上
のスポットのコマ収差を補正し,2軸チルトサーボのジッタに与える効果を実験的に検証してい
る.これらの実験の結果,対物レンズチルトアクチュエータを用いた2軸チルトチルトサーボに
より,最大でラジアルチルトが0.47°,タンジェンシャルチルトが0.43°のディスクチルトを,
0.1°の残留チルトまで減らすことができることがわかった.
ABSTRACT
A two-axis tilt servo using a four-axis object-lens actuator was developed. The four-axis actuator
can tilt the object lens in the radial and tangential directions, and can move it to allow axial tracking or
radial tracking. The tilt servo was applied on an optical disc system for next-generation Digital Versatile
Discs using a blue-violet laser, and the effect of disc-tilt compensation was determined experimentally.
These experiments show that the tilt servo with the four-axis object lens actuator reduces tilt by 0.1 deg
in the cases of a disc tangential tilt to 0.43 deg and a disc radial tilt to 0.47 deg.
*
研究開発本部 先端技術研究所
Advanced Technology R&D Center, Research and Development Group
**
研究開発本部 実用化開発センター
Advanced Prototyping Center, Research and Development Group
***
研究開発本部 オフィスシステム開発プロジェクト
Office System Development Project, Research and Development Group
Ricoh Technical Report No.31
47
DECEMBER, 2005
acceleration and reducing second-harmonic resonance. The
1.Introduction
arrangement of the moving coil and fixed magnets, and the
In an optical recording system using a blue-laser diode, a
magnetized direction of the magnets are shown in Fig.1. The
wavelength of 405 nm, an object lens with a numerical aperture of
mass of the moving part, including the object lens and the coil, is
0.65, and a 0.6-mm-thick substrate, compatibility between three
406 mg, and the moving part is held by eight flat wires extending
generations of optical memories, including Compact Discs (CDs)
from both sides of the same plane. This flat wire arrangement is
and Digital Versatile Discs (DVDs), is necessary. In this optical
effective for increasing the mobility in the tangential tilt direction
disc system, the disc-tilt margin decreases; thus, a system for
with a weak cross action between the focus and the tangential tilt.
compensating this disc tilt is required. Accordingly, we have
The mechanical and electrical characteristics of the actuator are
developed an object-lens tilt actuator for use in such a system.
shown in Table I. Using the actuator, we constructed a radial
Using this actuator, we constructed a radial and tangential-tilt
and tangential-tilt servo (see Fig.2). The two-axis optical tilt
servo and estimated the disc-tilt compensation effect in a blue-
sensor on the optical pickup detects the two-axis disc and object
laser optical disc system.
lens tilts independently.
The object-lens-tilt signals are
subtracted from the disc-tilt signals in both directions, and the
tilt-error signals for each direction are composed of the
2.Actuator/Servo
subtracted signals. For stabilizing the tilt servo, we developed
1)
The four-axis actuator is shown in Fig.1. It can independently
phase compensators with first-order lead-lag filters and dc-gain
tilt the object lens radialy and tangentially, and can also move the
enhancers.
object lens in the axial or radial direction. The motor structure
around 20 dB.
consists of a moving-coil system, which is useful for improving
An advanced four-axis actuator is shown in Fig.3, and the
Focus
Track
Radial Tilt
S
N
N
S
Object
lens
Track
Axial
Direction
Magnet
Coil
Radial Tilt
Tangential
Tilt
Magnetized
Direction
Eight Flat
Wires
Moving Part
Focus
With these compensators, the dc servo gain was
Radial
Direction
Radial Tilt
Fig.1
Structure of four-axis actuator.
Table I Characteristics of four-axis actuator.
Focus
Track
Sensitivity
Resonance
Sensitivity
Second-harmonic
Gain margin
Mobile distance
at 10 Hz
mm/V
frequency
at 200 Hz
frequency
at 3 kHz
range
Hz
m/(s^2・V)
kHz
dB
mm
1.55
49
129
25.0
33
±0.8
±0.3
1.17
45
84
60.2
43
rad/V
Hz
rad/(s^2・V)
kHz
dB
Radial tilt
0.046
96
15500
26.8
27
Tangential tilt
0.098
133
102000
70.7
44
Ricoh Technical Report No.31
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DECEMBER, 2005
Object Lens
Optical Disc
Disc
Tilt Sensor
Actuator Driver
4-axis
Actuator
Object Lens
Tilt Sensor
-+
Fig.2
Phase
Compensator
Servo on
Block diagram of tilt servo.
mechanical and electrical characteristics of the actuator are
is easy. The assembly cost of the advanced actuator can be
shown in Table II. The actuator was developed using printed
kept low because wire springs do not have to be soldered and
induction coils and laminated suspensions as eight flat arranged
rolling wired coils do not have to be bonded in the manufacturing
springs. For a four-axis moving coil, spiral patterns are printed
process.
on boards, and those are stacked and assembled as an induction
coil. Laminated suspensions are formed from a sheet of metal
3.Evaluation
plate, which is laminated for the eight flat arranged springs.
We evaluated the effect of the disc-tilt compensation by the
Both the printed coils and laminated suspensions form monolithic
Using these parts and this
servo in a blue-laser optical disc system.2) The error signals for
assembly technique, reducing the total cost of parts and assembly
the four axes with and without the activated tilt servo are shown
moldings using resin materials.
S
N
Magnet
Radial Tilt
N
S
Object
lens
Magnetized
Direction
Moving Part
Laminated
Suspensions
Molding
Printed Coils
On Printed Boards
Fig.3
Eight Flat
Wires
Tangential
Tilt
Advanced four-axis actuator.
Table II Characteristics of advanced four-axis actuator.
Focus
Track
Sensitivity
Resonance
Sensitivity
Second-harmonic
Gain margin
Mobile distance
at 10 Hz
mm/V
frequency
at 200 Hz
frequency
at 3 kHz
range
Hz
m/(s^2・V)
kHz
dB
mm
1.73
40
89
87.1
35
±0.8
±0.3
0.77
58
90
51.2
36
rad/V
Hz
rad/(s^2・V)
kHz
dB
Radial tilt
0.048
86
12800
85.1
39
Tangential tilt
0.071
116
47500
33.0
35
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DECEMBER, 2005
Rad tilt [deg]
Tan tilt [deg]
Tangential Tilt
Tr error [㱘m] Fo error [µm]
Radial Tilt
0.47 deg
0.11 deg
0.43 deg
0.06 deg
Focus
0.18 um
Track
0.015 um
Servo ON Time 10 ms
Servo OFF
Servo OFF
Cross act ion
0.1
Servo ON
Crossover
frequency
Frequency [Hz]
Tangential tilt
Frequency [Hz]
1000
Servo ON
Servo OFF
Cross act ion
Crossover
frequency
Decreased tilt error
0.01
Second
harmonic
Spindle
frequency
10
10
1
1000
0.1
10
Decreased t ilt error
0.01
Spindle
frequency
0.001
Tilt error [deg]
0.01
0.1
Second
harmonic
1
0.001
1
Radial tilt
Tilt error [deg]
0.01
0.1
0.001
Observed error signals.
Frequency [Hz]
Fig.5
1000
0.001
1
Fig.4
10
Frequency [Hz]
1000
㩷
Frequency analyses of error signals.
in Fig.4. Every signal was stabilized at around the base level.
The crossover frequency of each direction of the tilt servo is 500
This stabilization demonstrates that the servo can adequately
Hz and the disc tilt in the control band is almost completely
compensate the tilt. That is, the servo system can reduce the
eliminated. When the loop of the tilt servo was closed, the disc
radial and tangential tilts by about 14% to 25%.
tilt at each frequency decreased and the tilt error was
The results of frequency-domain analysis from each time-
compressed to less than 0.001 deg.
The radial tilt of the
domain wave using the Fourier transformation are shown in Fig.5.
fundamental frequency becomes 0.01 deg from the uncontrolled
The radial and the tangential-tilt characteristics are shown in the
tilt of 0.2 deg. This indicates the compression of the tilt by
upper and lower figures, respectively, and the tilt-error signals
more than 95%. In the case of the tangential tilt, the fundamental
obtained when the servo is on and off are shown in the right and
frequency becomes 0.02 deg from the uncontrolled tilt of 0.2 deg.
left figures. In the cases of both radial and tangential tilts, the
This indicates the compression of the tilt by more than 90% by
fundamental frequencies induced by spindle rotation and second
the tilt compensation system.
harmonics are larger than those induced by other components.
The signal jitter versus tilt characteristics are shown in Fig.6.
In the frequency range of 100 to 200 Hz, the tilt signal shows a
The right and left figures show the radial and tangential tilts,
few peaks induced by the cross action between the four axes.
respectively. The evaluation conditions used are a groove width
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DECEMBER, 2005
20
20
Radial tilt
18
Servo OFF
Servo OFF
16
14
Jitter 㱟/T[%]
16
Jitter㱟/T[%]
Tangentia
Tangentiall tilt
18
DVD
12
10
12
䇭
2.5%
8
DVD
14
4.5%
10
ON
8
ON
6
6
4
-0.6
4
-0.6
-0.4
-0.2
0
0.2
Pickup radial tilt [deg]
Fig.6
0.4
0.6
-0.4
Groove width 䋺 0.3 㱘m
Mark length 䋺0.24 㱘m
Line velocity 䋺 2.1 m/s
-0.2
0
0.2
0.4
0.6
Pickup tangential tilt [deg]
Jitter vs. tilt characteristics.
of 0.3 µm, a mark length of 0.24 µm, and a line velocity of 2.1
lower than that in the case of the radial tilt.
m/s.
The signal jitter was measured as the pickup tilt was
compensation, the tilt margin of ±0.1 deg is the same as the
applied when the tilt servo was on or off. To compare the DVD
radial tilt margin, but the jitter aggravation induced by the pickup
tilt margins, the characteristics are shown in both figures without
tilt change is intense. With this tilt compensation, the tangential
tilt compensation. In the case of DVD tilt characteristics, the
tilt margin varies by 0.3 deg during the servo operation.
bottom jitter is around 5%. If a jitter aggravation of around 3%
Accordingly, an identical tangential tilt margin is used for DVDs
were permitted, the radial and tangential tilts of ±0.4 deg and
operated without tangential tilt compensation.
Without tilt
±0.3 deg, respectively, would be permitted. In the case of a
blue-laser optical system, bottom jitter is around 7%.
If the
4.Conclusion
jitter aggravation of around 3% were permitted, the radial and
tangential tilts of ±0.2 deg would be permitted. Accordingly, the
To compensate for the disc tilt in the radial and tangential
tilt margin of the blue-laser optical system decreased sharply.
directions, we developed an object-lens tilt actuator with four
In particular, the tangential tilt margin decreased significantly;
axes.
thus, the disc tilt could not be permitted without tilt
tangential-tilt servo and estimated the disc-tilt compensation
compensation. In this state, if the tilt servo is on, the bottom
effect in a blue-laser optical disc system. The tilt-error signals
jitter is also around 7% and does not change with or without tilt
were reduced by more than 90%, indicating that a tilt servo
compensation. If the tilt servo is on, the sensitivity of the jitter
system using this four-axis actuator can effectively reduce tilt-
induced by the pickup tilt decreases and the signal jitter can be
error signals.
We used this actuator to construct a radial- and
reduced even if the pickup tilt increases with the disc tilt. If the
system requires a low jitter of less than 8%, as for a blue-laser
References
optical system without tilt compensation, the radial tilt margin will
1)
only be ±0.1 deg. With this tilt compensation, the radial tilt
T. Kawasaki, et al.: International Symposium on Optical Memory
2004 Technical Digest, (2004), pp. 176-177.
margin becomes±0.4 deg during the tilt servo operation.
2)
T. Kawasaki, et al.: Radial and Tangential-Tilt Servo Using Four-
Accordingly, an identical radial tilt margin is used for DVDs in
Axis Actuator, Jpn. J. Appl. Phys., Vol. 44, No. 5B (2005), pp.
the case of no radial-tilt compensation.
3402-3404.
In the case of the tangential tilt, the margin for the jitter is
Ricoh Technical Report No.31
51
DECEMBER, 2005
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