UDC 537.226.86.681.327.634
Development of Shear-Mode Piezoelectric
Microactuator for Precise Head Positioning
vShinji Koganezawa vTakeyori Hara
(Manuscript received September 7, 2001)
We have developed a novel piezoelectric micro-actuator for dual-stage actuator
systems in magnetic disk drives. This microactuator is based on the shear deforma-
tion of piezoelectric elements and drives the head suspension assembly. The actuator
is suitable for thin devices and is easily manufactured because of its simple stack config-
uration.
We installed the microactuator in one of Fujitsu s 3.5-inch commercial drives to evalu-
ate the servo system of a dual-stage actuator. The dual-stage actuator system achieved
a non-repeatable position error (NRPE) 3 of 0.036 µm. The dual-stage servo system
reduced the NRPE by 35% compared with the conventional single actuator system,
even in HDDs with a high rotational speed of 10 000 rpm.
1. Introduction manufacture, they are expected to be used in
Over the last several years, the areal densi- HDDs in the near future in spite of their poor
ty of magnetic disk drives has increased by 100% mechanical characteristics compared with the
every year. Based on this trend, the track densi- slider and head element driving types. General-
ty is expected to increase at an annual rate of 40%. ly, the mechanical characteristics improve as the
In 3.5-inch high-performance hard disk drives microactuator is positioned closer to the head.
(HDDs), the rotational speed of the spindle motor Therefore, the MEMS (Micro-Electro-Mechanical
has reached 10 000 rpm and is expected to go even Systems)-based microactuator is expected to be
higher. This high rotational speed causes a large used in the future to achieve a very high track
windage disturbance and disk flutter, which are density.
serious obstacles to increasing the track density We have developed a piezoelectric microac-
of hard disk drives. To achieve a higher track den- tuator for dual-stage actuator systems that uses
sity, we will need to increase the servo bandwidth. the shear mode of piezoelectric elements to drive
For the single actuator system, the servo the head suspension assembly.4),5) This paper
bandwidth is limited by the mechanical resonanc- describes the structure of our shear-mode piezo-
es of the carriage, coil, and ball bearing pivot. electric microactuator and the positioning
Some types of microactuators have been proposed accuracy of a dual-stage servo system installed in
as possible ways to attain a wider servo band- a 3.5-inch high-performance commercial HDD.
width. Current research on microactuator design
may be divided into three types: driving a head 2. Piezoelectric actuator types
suspension assembly,1)-6) driving a slider,7)-9) and Our objective was to design a microactuator
driving a head element.10),11) Because head sus- from piezoelectric elements that has a small and
pension driving microactuators are easy to simple structure. For magnetic disk drives, so far,
212 FUJITSU Sci. Tech. J.,37,2,p.212-219(December 2001)
 S. Koganezawa et al.: Development of Shear-Mode Piezoelectric Microactuator for Precise Head Positioning
Figure 1 (b). When a voltage is applied to the
Expansion
outside faces of the elements, both elements con-
tract as shown in the figure. This kind of actuator
is suitable for thin structures, but its structure is
too complicated for head mounting blocks. An-
other problem is that the force transfer loss is
large, because the middle-layer stainless steel
sheet to which the piezoelectric elements are bond-
(a) Stacked type ed prevents the elements from deforming, but if
Elements are vertically polarized.
the stainless steel sheet is not used, the fragile
piezoelectric elements are easily damaged.
Contraction
The novel actuator we developed exploits
the shear deformation of piezoelectric elements
(Figure 1 (c)). Here, the piezoelectric element is
horizontally polarized. When a voltage is applied
to the faces of the element, it becomes sheared.
The displacement of the element is estimated by:
(b) Planar type
Elements are vertically polarized
L = n d15 V, (1)
Shear deformation
where L is the element displacement, n is the
number of layers, V is the applied voltage, and
d15 is the shear mode piezoelectric constant.
The displacement depends on the shear mode
piezoelectric constant and the number of layers
(c) Shear type
and is independent of the dimensions of the ele-
Element is horizontally polarized.
ment. The piezoelectric element, therefore, can
Figure 1
be designed to be small and thin providing it is
Piezoelectric actuators.
not thinner than the thickness required by the
coercive electric field. Therefore, this actuator is
the use of stacked-type and planar-type actuators suitable for thin structures. Another advantage
has been proposed. of our piezoelectric microactuator is that it has a
Stacked-type piezoelectric actuators high shock resistance (see Section 4 for details).
(Figure 1 (a)) are well known and are common-
ly used in various fields.1) They are made by 3. Microactuator structure and
stacking piezoelectric elements on top of each oth- design
er. When a voltage is applied to both sides of the 3.1 Structure
elements, they expand as shown in the figure. One Our shear mode piezoelectric microactuator
problem with these actuators is that their com- is shown in Figure 2. The actuator is comprised
plicated structure makes them difficult to of a stator plate, a head mounting block, and a
assemble. Furthermore, their relative thickness head suspension with a 30% pico-slider (Pico-
makes them unsuitable for use in thin devices. CAPS).12) The head suspension is spot-welded onto
The planar-type actuator2) has a sandwich the microactuator. The piezoelectric elements are
structure like the bimorph actuator shown in polarized in opposition to each other and glued to
FUJITSU Sci. Tech. J.,37, 2,(December 2001) 213
 S. Koganezawa et al.: Development of Shear-Mode Piezoelectric Microactuator for Precise Head Positioning
Table 1
Specifications of microactuator.
Mass
62 mg
(with head suspension)
Stroke 0.5 µm (Ä…30V)
Resonant frequency 9 kHz
Capacitance 650 pF
Shock resistance > 950 G, 1 ms half-sine
Terminals for R/W signal
Terminals for R/W signal Terminals for microactuator
Terminals for microactuator
(a) With head suspension
Head suspension assembly
has a hinge structure that amplifies the displace-
(30% pico-slider)
ment of the piezoelectric elements. The
microactuator assembly has two bonding areas
that are connected by a flexible printed circuit.
Head mounting block
One is for the read/write signal lines, and the oth-
er is for the two power lines for driving the
Electrode
microactuator. The head suspension is electrical-
Hinge
ly grounded, and the microactuator s leads are
Single-layer
electrically isolated from the head suspension so
piezoelectric element
that the control voltage of the microactuator does
not affect the head signal.
3.2 Actuator design
Stator plate We used single-layer piezoelectric elements
in order to simplify the manufacturing process.
The piezoelectric elements were made of a PZT
material which has a d15 constant of 8.45 ×
10-10 m/V. The element can generate a displace-
Piezoelectric
actuator
ment of Ä…25 nm with Ä…30 V applied. The hinge
structure was designed to amplify the displace-
ment of the piezoelectric elements by about
20 times. Therefore, we estimate a displacement
of about 1 µmp-p at the head.
Carriage arm The element is 2.2 mm long, 1.3 mm wide,
and 0.15 mm thick.
We designed the microactuator to have a
(b) Schematic view of microactuator
small mass of 62 mg so that the resonances of the
Figure 2
Piezoelectric microactuator. carriage arm are not excited when the microactu-
ator is driven. Its characteristics are shown in
the electrodes. They become sheared in opposite Table 1.
directions to each other when a voltage is applied, The stress in the hinge structure was esti-
which causes the head suspension assembly to mated by FEM (Finite Element Method) analysis.
swing. The maximum stress occurs in the notches and
As shown in Figure 2 (b), the microactuator equals Ä…8.4 MPa for a Ä…0.5 µm head stroke. This
214 FUJITSU Sci. Tech. J.,37, 2,(December 2001)
 S. Koganezawa et al.: Development of Shear-Mode Piezoelectric Microactuator for Precise Head Positioning
0.6 Microactuator
Disk Sensor
Arm
0.4
0.2
0
Rigid shaft
Rigid base
-0.2
Drop
-0.4
Hammer
-0.6
-40 -30 -20 -10 0 10 20 30 40
Voltage (V)
Figure 5
Impact tester.
Figure 3
Displacement vs. applled voltage.
gain at resonance is approximately 20 dB. This
180
resonance is the coupled mode of the head sus-
0
pension assembly and the microactuator. The
-180
torsion mode of the head-mounting block appears
0
at 20 kHz. These resonant frequencies are high
-20
enough for this device to be used as a tracking
-40
actuator for magnetic disk drives.
-60
-80
4.2 Shock resistance
-100
Microactuators are required to have high
200 1k 10k 100k
shock resistance so they can be handled easily.
Frequency (Hz)
We evaluated the microactuator s shock resistance
Figure 4
using an impact tester (Figure 5). The microac-
Compllance frequency response of the microactuator.
tuator and a disk are attached to a rigid base. The
assembly was dropped on a hammer, and the ac-
is less than 5% of the fatigue limit of stainless celeration was measured using an accelerometer
steel (172 MPa). Thus, repeated stress can be ig- attached to the tip of the arm. To evaluate the
nored. collision damage of the microactuator, we mea-
sured and compared its frequency response before
4. Mechanical characteristics and after the impact. We submitted the microac-
4.1 Mechanical response tuator to a 1 ms half-sine acceleration of 950 G
Figure 3 shows a Lissajous plot of a Ä…30 V, five times (950 G is the maximum value the tester
1.1 kHz driving signal and the corresponding head can generate). We found that the microactuator s
displacement. The range of movement at Ä…30 V frequency response was unchanged by the impact
is Ä…0.5 µm. The hysteresis observed in the figure and concluded that the actuator s shock resistance
causes the phase lag in the compliance frequency is better than 950 G.
response. Although this is not a fatal fault, it can
be a factor that decreases the control performance. 5. Prototype hard disk drive with
The microactuator s compliance frequency re- piezoelectric microactuator
sponse is shown in Figure 4. The microactuator We installed the piezoelectric microactuator
has a high resonant frequency of 9 kHz. The peak in a Fujitsu 3.5-inch commercial drive having a
FUJITSU Sci. Tech. J.,37, 2,(December 2001) 215
Displacement (
µ
m)
Amplitude (dB)
Phase (deg)
 S. Koganezawa et al.: Development of Shear-Mode Piezoelectric Microactuator for Precise Head Positioning
high rotational speed of 10 025 rpm. This drive We put resistances in series between the am-
uses 3-inch magnetic disks, which reduces the plifiers and the microactuator. The microactuator
power loss due to windage and also reduces the is electrically capacitive, so the combination of the
tracking error caused by disk vibration. The sam- microactuator and the series resistances formed
pling frequency of the prototype HDD was 20 kHz. an analog low-pass filter. We set the cut-off fre-
The specifications of the prototype drive are quency of the low-pass filter at 8 kHz to eliminate
shown in Table 2. any high frequency component in the microactu-
Figure 6 shows a photograph of the prototype ator s driving voltage.
HDD. The microactuator driver IC, which has volt-
age amplifiers and a DC/DC converter, is indicated 6. Servo system of dual-stage
on the printed circuit board. The DC/DC converter actuator
produces Ä…18 V from 12 V using a charge pump to 6.1 Servo system
supply positive and negative high voltages to the The block diagram of the dual-stage actua-
amplifiers. The differential amplifier applies a
voltage of Ä…30 V to the microactuator. In our de-
Microactuator driver IC
Microactuator driver IC
sign, the microactuator s leads are electrically
isolated from the head suspension. Therefore, we
can use a differential drive amplifier and reduce
the supply voltage of the amplifiers.
Table 2
Specifications of prototype hard disk drive.
TPI (tracks per inch) 13 500
BPI (bits per inch) 275 000
Rotational speed of spindle motor 10 025 rpm
Disk diameter 3.3-inch
Number of disks 3
Figure 6
Sampling frequency 20.05 kHz
Prototype hard disk drive installed with new microactuators.
DC/DC Ä…18 V
converter
d
Cm
Pm
+
Ä…30
Micro-
Displacement
Reference
PES +
Micro-
actuator
DAC
+
+ actuator
controller
+

Differential
voltage amplifier
Coarse Coarse
actuator DAC actuator
controller (VCM)
Current amplifier
Cv
Pv
Figure 7
Block diagram of dual-stage actuator system.
216 FUJITSU Sci. Tech. J.,37, 2,(December 2001)
 S. Koganezawa et al.: Development of Shear-Mode Piezoelectric Microactuator for Precise Head Positioning
tor servo system used in our experiments is shown
PES
S
in Figure 7. In this servo system, the microactu- (3)
d.k
ator follows the position errors, while the coarse
actuator follows the estimated relative displace- where k is the displacement constant of the mi-
ment between the microactuator and coarse croactuator (displacement per unit voltage).
actuator.5) Figure 8 shows that up to about 2 kHz the dual-
To evaluate the effect of the dual-stage actu- stage actuator system can reduce the positioning
ator on positioning accuracy, we compared the error better than the single actuator. We calcu-
position error signal (PES) of the dual-stage ac- lated the open-loop 0 dB crossover frequency of
tuator system with that of a single actuator. We the dual-stage actuator system from both the S
designed the single-actuator controller to have a characteristics and the measured Pm, and ob-
high crossover frequency of 1 kHz. tained a value of approximately 2 kHz.
The sensitivity function of the single actua-
tor and the approximate sensitivity function of the 6.2 PES evaluation
dual-stage actuator are shown in Figure 8. Be- The PES waveforms of the single and dual-
cause the open-loop characteristics could not be stage actuators are shown in Figure 9, and the
measured directly in the dual-stage actuator sys- power spectra are shown in Figure 10. Accord-
tem, we measured an approximate sensitivity ing to the power spectra, the low-frequency
function: the transfer function to the PES from component of the PES was effectively suppressed
disturbance d, which is added to the control volt- by the dual-stage actuator system. The high-
age. This transfer function is given by:
0.2
PES = Pm
(2)
d 1 + Cm (Pm + CvPv)
0.1
where Pm and Pv are transfer functions of the
0
microactuator and the voice coil motor (VCM), re-
spectively, and Cm and Cv are their controllers.
-0.1
In the frequency region where the microactuator s
compliance gain is flat (< 3 kHz), the sensitivity
-0.2
02468
function is given by:
Time (ms)
(a) Single actuator
0.2
20
:Single actuator
:Dual-stage actuator
0.1
0
0
-20
-0.1
-40
-0.2
02468
-60
50 100 1k 5k Time (ms)
Frequency (Hz)
(b) Dual-stage actuator
Figure 8
Figure 9
Sensitivity function.
PES waveforms.
FUJITSU Sci. Tech. J.,37, 2,(December 2001) 217
PES (
µ
m)
PES (
µ
m)
Sensitivity function (dB)
 S. Koganezawa et al.: Development of Shear-Mode Piezoelectric Microactuator for Precise Head Positioning
Table 3
-40
Positioning accuracies of single and dual-stage actuators.
(a) NRPE (3Ã)
Zone Single actuator Dual-stage Reduction
(VCM) actuator
-60
Outer 0.073 µm 0.046 µm 36%
Center 0.073 µm 0.047 µm 35%
Inner 0.059 µm 0.036 µm 39%
Dual-stage actuator Single actuator
-80
0 5000 10 000
(b) TPE (3Ã)
Frequency (Hz)
Zone Single actuator Dual-stage Reduction
(VCM) actuator
Figure 10
Outer 0.089 µm 0.069 µm 23%
Power spectra of PES.
Center 0.091 µm 0.071 µm 22%
Inner 0.093 µm 0.074 µm 21%
0.08
Single
0.07
Dual-stage
0.06
times the standard deviation) values of NRPE
0.05
were within 0.047 µm, and the 3Ã values of the
0.04
total position error (TPE) were within 0.074 µm
0.03
for the dual-stage actuator, even in HDDs with a
0.02
high rotational speed of 10 000 rpm. The effect of
0.01
the dual-stage actuator on reducing NRPE was
0
about 35% in every zone. The reduction rate of
Ball bearing Disk flutter Other NRPE
the TPE was approximately 22%. The reduction
Figure 11
rate of the repeatable run-out (RRO) was only 10%,
PES components as calculated from the power spectra.
because we used the RRO compensator in the VCM
controller of both actuator systems to reduce the
frequency vibration, however, remained. The peak RRO. The low-frequency RRO was compressed
at approximately 9 kHz is the microactuator s dom- enough by the RRO compensator, even in the sin-
inant resonant frequency, and the peak at 7.5 kHz gle actuator system. Therefore, the effect on the
is the vibration of the carriage arm. These reso- TPE was small in this experiment.
nances were excited by the wind caused by the
disk s rotation rather than by the microactuator s 7. Conclusion
control voltage or VCM s current. We have developed a piezoelectric microactua-
Figure 11 shows the components of the PES tor that uses the shear deformation of piezoelectric
as calculated from the power spectra. The ball elements. It has a high resonant frequency of 9 kHz.
bearing vibration and disk vibration were reduced The range of movement is Ä…0.5 µm at Ä…30 V. We
by 56% and 59%, respectively. However, other fac- installed the microactuator in a Fujitsu 3.5-inch
tors, for example, the windage disturbance, noise, high-performance hard disk drive having a high
and arm resonance, were not reduced much. The rotational speed of 10 025 rpm. We achieved a
resonances at 7.5 and 9 kHz decreased the reduc- servo bandwidth of approximately 2 kHz with a
tion ratio of the non-repeatable position error dual-stage actuator system. The dual-stage actu-
(NRPE) in the case of the dual-stage actuator. ator system had a 35% lower non-repeatable
The positioning accuracies of both actuator position error than the single actuator system and
systems are compared in Table 3. The 3Ã (three a 22% lower TPE. In the dual-stage actuator, the
218 FUJITSU Sci. Tech. J.,37, 2,(December 2001)
Power spectrum (dBrms)
NRPE (
µ
m)
 S. Koganezawa et al.: Development of Shear-Mode Piezoelectric Microactuator for Precise Head Positioning
3Ã values of the NRPE were within 0.047 µm and tem. IEEE Trans. on Industrial Electronics.,
the 3Ã values of the TPE were within 0.074 µm, even 42, 3, p.222-233 (1995).
in HDDs with a high rotational speed of 10 krpm. 9) W. Tang, V. Temesvary, R. Miller, A. Desai, Y.
C. Tai, and D. K. Miu: Silicon Micromachined
References Electromagnetic Microactuators for Rigid
1) K. Mori, T. Munemoto, H. Otsuki, Y. Yamaguchi, Disk Drives. IEEE Trans. Magn., 31, 6,
and K. Akagi: A dual-stage magnetic disk p.2964-2966 (1995).
drive actuator using a piezoelectric device for 10) T. Imamura, T. Koshikawa, and M. Katayama:
a high track density. IEEE Trans. Magn., 27, Transverse Mode Electrostatic Microactua-
6, p.5298-5300 (1991). tor for MEMS-Based HDD Slider. IEEE,
2) T. Imamura, S. Hasegawa, K. Takaishi, and MEMS, p.216-221 (1996).
Y. Mizoshita: Piezoelectric Microactuator 11) S. Nakamura, K. Suzuki, M. Ataka, and H.
Compensating for Off-Track Errors in Mag- Fujita: An electrostatic microactuator for a
netic Disk Drives. ASME Adv. Info. Storage magnetic head tracking system of hard disk
Syst., p.119-126 (1993). drives. Proc. of International Conference on
3) S. Koganezawa, K. Takaishi, Y. Mizoshita, Y. Micromechatronics for Information and Pre-
Uematsu, T. Yamada, S. Hasegawa, and T. cision Equipment, Tokyo, p.58-63 (1997).
Ueno: A Flexural Piggyback Milli-Actuator 12) T. Ohwe, T. Watanabe, S. Yoneoka, Y. Mizoshita:
for over 5 Gbit/in2. Density Magnetic Record- A New Integrated Suspension for Pico-Slid-
ing. IEEE Trans. Magn., 32, 5, p.3908-3910 ers (PICO-CAPS). IEEE Trans. Magn., 32,
(1996). 5, p.3648-3650 (1996).
4) S. Koganezawa, Y. Uematsu, T. Yamada, H.
Nakano, J. Inoue, and T. Suzuki: Shear Mode
Shinji Koganezawa received the B.S.
Piezoelectric Microactuator for Magnetic
and M.S. degrees in Mechanical Engi-
neering from Tokyo Institute of Technol-
Disk Drives. IEEE Trans. Magn., 34, 4,
ogy, Tokyo, Japan in 1989 and 1991,
p.1910-1912 (1998).
respectively. He joined the research
staff at Fujitsu Laboratories Ltd. in 1991
5) S. Koganezawa, Y. Uematsu, T. Yamada, H.
and moved to Fujitsu Ltd. in 1993. His
primary research interest is the actua-
Nakano, J. Inoue, and T. Suzuki: Dual-Stage
tor system for precise positioning, es-
Actuator System for Magnetic Disk Drives pecially dual-stage actuator systems for
magnetic disk drives. He is a member
Using a Shear Mode Piezoelectric Microac-
of the Japan Society of Mechanical Engineers (JSME). He re-
ceived the Outstanding Presentation Award from the JSME In-
tuator. IEEE Trans. Magn., 35, 2, p.988-992
formation, Intelligence, and Precision Equipment Division in 1996.
(1999).
E-mail: [e-mail]
6) R. B. Evans, J. S. Griesbach, and W. C.
Messner: Piezoelectric Microactuator for
Takeyori Hara received the B.E. degree
Dual Stage Control. IEEE Trans. Magn., 35,
in Mechanical Engineering from the Uni-
2, p.977-982 (1999).
versity of Tokyo, Tokyo, Japan in 1993.
He joined Fujitsu Ltd., Kawasaki, Japan
7) Y. Soeno, S. Ichikawa, T. Tsuna, Y. Sato, and
in 1993, where he has been engaged
in design and development of servo
I. Sato: Piezoelectric piggyback microactua-
controllers for hard disk drives. From
tor for hard disk drive. IEEE Trans. Magn.,
1996 to 1998, he was a Visiting Indus-
trial Fellow at the Mechanical Engineer-
35, 2, p.983-987 (1999).
ing Department, University of California,
Berkeley. He is a member of the Japan
8) L. -S. Fan, H. H. Ottesen, T. C. Reiley, and R.
Society of Mechanical Engineers (JSME).
W. Wood: Magnetic Recording Head Position-
E-mail: [e-mail]
ing at Very High Track Densities Using a
Microactuator-Based, Two-Stage Servo Sys-
FUJITSU Sci. Tech. J.,37, 2,(December 2001) 219