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IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control , vol. 60, no. 4, April 2013

Efficiency Improvement of Hybrid
Transducer-Type Ultrasonic Motor
Using Lubricant
Wei Qiu, Student Member, IEEE, Yosuke Mizuno, Daisuke Koyama, Member, IEEE,
and Kentaro Nakamura, Member, IEEE
Abstract—Ultrasonic motors have hit a bottleneck caused
by low efficiency and short life, which limits their applications
to some niche areas. We believe that lubrication is a promising
candidate to solve these problems. In this paper, we clarify,
both analytically and experimentally, that the performance ofthe hybrid transducer-type ultrasonic motor (HTUSM), including the transduction efficiency, can be drastically improvedat large static preloads if appropriate lubricant is applied.
First, simulation was performed using an equivalent circuitin dry and lubricated conditions, and the HTUSM characteristics were shown to be more desirable at high static preloads
in the lubricated condition than in the dry condition. Then,we experimentally investigated the mechanical performance ofthe HTUSM, verifying the effect of improving the motor performance at high preloads using lubricant, which was in good
agreement with the simulation results. The maximum transduction efficiency of the HTUSM was significantly enhanced
from 28% in the dry condition to 68% in the lubricated condition.

I. Introduction

B

ecause of their attractive features, such as hightorque at low speed, simple structure, and precise positioning capability [1], [2], ultrasonic motors have been
extensively studied for more than two decades, replacing
electromagnetic motors in some special applications. However, because ultrasonic motors are driven by the friction
force between the rotor and the stator in most cases, they
inherently suffer friction loss and the wear of contact materials, causing their low efficiency and short life. Reducing the friction loss to enhance the efficiency is, therefore,a significant issue for broadening the application areas ofultrasonic motors.
In ultrasonic motors, a half-cycle of the primary vibration is extracted and transmitted to the rotor with the assistance of the other vibration orthogonal to the primary
vibration. The secondary vibration modulates the preload
acting on the contact interface between the transducer
and the rotor; then, the friction force is varied synchronously with the primary vibration. Consequently, the priManuscript received May 28, 2012; accepted January 16, 2013.
W. Qiu, Y. Mizuno, and K. Nakamura are with the Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Japan(e-mail: qiu@sonic.pi.titech.ac.jp).
D. Koyama was with the Precision and Intelligence Laboratory, Tokyo
Institute of Technology, Yokohama, Japan. He is now with the Faculty of
Science and Engineering, Doshisha University, Kyoto, Japan.
DOI http://dx.doi.org/10.1109/TUFFC.2013.2627

mary vibration is transformed into the rotary motion of
the rotor. If the modulation depth in the friction force by
the secondary vibration is so large that the traction to therotor in the positive half-cycle of the vibration is muchhigher than the negative half-cycle, the transformation
efficiency from the primary vibration to the rotary motion of the rotor is expected to exceed 80% [3]. However,in most of the previous motors, the friction control was
not ideal, resulting in the limited efficiency. Some efforts
have been made to improve the motor efficiency from theviewpoint of friction control by optimizing the vibration
systems [4]–[6], selecting appropriate friction materials [7],[8], and developing drive/control techniques [9], [10], but
the motor efficiency has not yet been sufficiently enhancedwith these methods. Some new driving mechanisms forultrasonic motors have also been developed, including a
trial to change the coefficient of friction instead of modulating the preload. This was, however, extremely difficult
to perform well at ultrasonic frequency [11].
Meanwhile, with the development of the traction drive
techniques [12], employing lubricant, which can vary thecoefficient of friction during the operation, in ultrasonicmotors has been proposed [1]. Large traction force, i.e.,high coefficient of friction, is obtained during the period ofhigh preload, whereas the coefficient of friction decreases
for the period of low preload, which can reduce the reverse
torque without losing the traction force if the preload ishigh enough during the driving period.
In this paper, we investigate the mechanical characteristics of the hybrid transducer-type ultrasonic motor(HTUSM) using lubricant, and verify the effect of improving its motor efficiency. After explaining the lubrication
mechanism in the HTUSM, we theoretically show that the
motor characteristics in dry and lubricated conditions are
distinctively different: for instance, the motor efficiency
and the no-load speed are drastically enhanced at highstatic preloads in the lubricated condition compared withthose in the dry condition. We then experimentally clarify
the improvement of the motor transduction efficiency using
lubricant; the maximum efficiency is enhanced from 28% in
the dry condition to 68% in the lubricated condition.
II. Principles
HTUSMs comprise two types of piezoelectric transducers composed in one stator. The torsional vibration

0885–3010/$25.00 © 2013 IEEEqiu et al.: efficiency improvement of hybrid transducer-type ultrasonic motor

generates the mechanical rotational force, whereas thelongitudinal vibration controls the friction force between
the contact surfaces. Because of this independent driving mechanism of the two orthogonal vibration components, high controllability is achieved. The contact of the
HTUSMs is not distributed, as in traveling-wave ultrasonic motors, but is area contact.
Fig. 1 describes the operating principle of an HTUSM.
A rotor is pressed to a torsional-longitudinal hybrid transducer using a coil spring with a static preload Fc. In theideal case, the longitudinal vibration is so strong that thecontact duration between the rotor and the stator is lessthan a quarter of one vibration cycle, and the rotor andthe stator are detached when their motional directions
are reverse, resulting in relatively low friction loss. In theactual motors, however, it is difficult to reduce the contact
duration even below one half of the cycle. In some cases,
the rotor is in contact with the stator for the whole cycle.
Thus, friction loss becomes high when the elliptical trajectories of the particles in the contact surface of the stator
are opposite to the moving direction of the rotor, leading
to the low efficiency and the wear of contact surfaces.
Employing the functionality of lubricant is one of themethods to solve the preceding problems, because it canvary the coefficient of friction during contact as a result of
the change of the load on the interface, which can be explained by the Stribeck curve [13] given in Fig. 2. If no lubricant is used, i.e., in the dry condition, the coefficient offriction is constant and reverse torque occurs. In contrast,
as shown in Fig. 3, the coefficient of friction varies accord-

787

Fig. 2. Stribeck curve [13].

ing to the dynamic preload and the relative velocity whensuitable lubricant is applied. The coefficient becomes highat the instance of a small slip between the rotor and thestator, or at a high preload, whereas it decreases for theperiod of a large slip or at a low preload. As a result,because the reverse torque is significantly suppressed, themotor efficiency is enhanced. We predict that this concept
can be realized by boundary lubrication for high preloads,
and hydrodynamic lubrication for low preloads.
III. Simulation
A. ModelingTo numerically analyze the lubrication mechanism andits effect in the HTUSM, an equivalent circuit model thatsimulates the torque transmission mechanism was employed, as shown in Fig. 4. The electrical and the mechanical terminals are connected by the torque factor Aτ: i m = AτΩ T, τ 0 = AτV T,

Fig. 1. Operating principle of the hybrid transducer-type ultrasonicmotor.

where ΩT, im, τ0, and VT are the angular vibration velocity, the motional current, the open-circuited torque,and the driving voltage, respectively. A constant current
source represents the rotor, assuming that the momentumof its inertia is infinitely large and the rotational velocity
is constant. The friction characteristics in dry and lubricated conditions were separately investigated. In the dry
condition, a Coulomb friction model was used, in which
the torque τ is limited to the product of the coefficient offriction μ, the dynamic preload fc, and the contact radiusr, as shown in Fig. 5. On the other hand, if lubricant isapplied, the coefficient of friction varies according to thechange of the relative velocity between the angular vibration velocity of the stator ΩT and the rotor rotational
speed ΩR, and the dynamic preload fc, as corresponds to
the Stribeck curve. To simulate some basic characteristics
in the case in which the lubricant behaves ideally, themodel for the lubricant is simplified. The fluid viscosity
was assumed as a constant value because the rheology of788

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control , vol. 60, no. 4, April 2013

Fig. 5. Coulomb friction model used in calculation for the dry
condition.

Fig. 3. Lubrication mechanism in the hybrid transducer-type ultrasonicmotor.

the lubricant was not taken into account in this model.
The parameters in the Stribeck curve were assumed according to the range of friction levels described in [14], asshown in Fig. 6. In this simulation, we simply defined thatthe time-averaged dynamic preload fc equals the static
preload Fc (see Fig. 7), following the method described in
our previous papers [3], [15]. The other parameters used inthis calculation are summarized in Table I.
B. Instantaneous Waveforms
The instantaneous waveforms of the HTUSM using lubricant were investigated. Fig. 8 illustrates the instantaneous variables: the torsional vibration velocity ΩT, therotor rotational speed ΩR, the dynamic preload fc, and theFig. 4. Equivalent circuit of the hybrid transducer-type ultrasonic motor
for torque transmission mechanism.

torque τ. The contact duration ϕ was equal to 2π. The
positive torque was large during the driving period because the coefficient of friction was high as a result of thehigh preload and the boundary lubrication was dominantduring this period. The instantaneous torque τ became
negative when ΩT was lower than ΩR, but was followed
by a sudden reduction of the reverse torque resulting fromthe sharp drop of the coefficient of friction in the Stribeck
curve, and the hydrodynamic lubrication began to work.
Through this lubrication mechanism, the reverse torquewas drastically reduced without losing the traction force.
C. Motor Characteristics Versus Contact Duration
The dependence of the motor characteristics on thecontact durations ϕ was examined, as shown in Figs. 9(a)–9(c). In this simulation, the static preload was assumed tobe constant while the amplitude of the dynamic preload
was changed to vary the contact duration. The efficiency
is defined as the transduction efficiency of the HTUSM in
this paper, which is the ratio of the motor output power
to the input electrical power of the torsional vibrator. The
maximum motor efficiency was high under short contact
duration, irrespective of whether the lubricant was appliedor not, because the reverse torque was extremely small
as a result of the separation of the rotor and the stator.

Fig. 6. Stribeck curve used in calculation for the lubricated condition.

qiu et al.: efficiency improvement of hybrid transducer-type ultrasonic motor

Fig. 8. Instantaneous waveforms of torsional vibration velocity ΩT, rotational speed ΩR, torque τ, and dynamic preload fc.

Fig. 7. Waveforms of dynamic preload.

However, it is not often possible to achieve such shortcontact duration in actual experiments because the longitudinal vibration system is not strong enough comparedwith the high static preload. With the increase of the contact duration, the motor efficiency substantially decreased
because large reverse torque occurred and hence the friction loss increased. In contrast, the motor efficiency wasmaintained at a relatively high value if lubricant was employed. The effect of the critical Stribeck number SC on
the motor performance was also investigated. In general,a large value of critical Stribeck number is not desirable
for improving the motor performance as the period of thehydrodynamic lubrication becomes shorter. However, themotor efficiency decreases if SC is extremely small because
the period of the boundary lubrication is too short to keep
sufficient traction force. Similar trends were found in the
no-load speed and the maximum torque/contact duration
curves as well.
D. Motor Characteristics Versus Static Preload
Figs. 10(a) and 10(b) indicate the effect of changing
the applied voltage on the motor efficiency at various
static preloads in dry and lubricated conditions. In thissimulation, the amplitude of the longitudinal vibration
was assumed to be constant while the static preload waschanged, and the critical Stribeck number SC was set to0.001η. In the dry condition, the optimal static preload
for the motor efficiency clearly changed at different applied voltages to the torsional vibrator. Generally, the best
motor efficiency is obtained at low static preloads if the
TABLE I. Parameters Used in Calculation.
Torque factor Aτ
Free admittance Ym0
Mechanical Q
Torsional resonance fT0
Clamped capacitance CdDielectric loss Rd
Contact radius r
Coefficient of friction in the dry condition µ789

0.00377
1.27 mS
117
22.1 kHz
2.3 nF
100 kΩ
10.85 mm0.15

applied voltage is low, whereas it shifts to higher static
preloads if high voltage is applied. The highest value ofthe motor efficiency is achieved at low voltage, because
the maximum efficiency is obtained under shorter contact
duration. When lubricant was applied, the trends of the
motor efficiency at different applied voltages were similar
to those in the dry condition. However, the motor can be
operated at extremely high static preloads, and the maximum efficiency at each applied voltage was higher thanthat without lubrication.
Figs. 11(a) and 11(b) provide the trends of the maximum torque and the no-load speed with respect to thestatic preload, when 500 and 300 Vp-p were applied,
respectively. We set the critical Stribeck number Sc to
0.005η. The maximum torque with lubrication at low
static preloads was smaller than that in the dry condition,
because the motor was operated mainly in the hydrodynamic lubrication and the traction force was small as a result of the low coefficient of friction. However, large maximum torque was obtained at the region of high preloads,
which indicates that high contact pressure is required in
the lubricated condition to maintain the traction force.
As for the no-load speed, with lubrication, the motor was
operated at high preloads, where the motor did not rotate
without lubrication.
IV. Experiments
A. Motor Structure and Materials
The HTUSM used in the experiment was 25 mm in diameter; it contained two 4-mm-thick torsional PZT disks
and six 1-mm-thick longitudinal PZT disks, as shown in
Fig. 12(a). The contact side of the rotor had a triangularcross-section and was truncated for 0.3 mm in width forcontact, as shown in Fig. 12(b). Four deeply truncatedparts were made as oil reservoirs to keep sufficient lubrication. The total contacting area was 5.64 mm2. The friction materials for the stator and the rotor were alumina

790IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control , vol. 60, no. 4, April 2013

Fig. 10. Calculated motor efficiency as a function of the static preload in(a) dry and (b) lubricated conditions.

B. Experimental Setup

Fig. 9. Calculated dependences of (a) motor efficiency, (b) no-load speed,and (c) maximum torque on contact duration.

and silicon nitride, respectively, because severe wear still
occurred even with lubrication, and wear debris was noteasily transported away from the contact area when metals were used. High-traction fluid (HTRF) with 100 cSt
viscosity was selected as the lubricant.

Fig. 13 depicts the schematic diagram of the experimental setup. A function generator with a phase shifter was
employed to drive the HTUSM. The driving power was
amplified with two amplifiers and transformers. Different
weights were employed to obtain the load characteristicsof the motor. The rotor rotational speed was measured
by a high-speed digital camera (M5, Integrated Design
Tools Inc., Tallahassee, FL). The electrical power to thetorsional PZT disks was calculated from the definition of
the effective power with the voltage and current waveforms, and also verified by a digital power meter (PowerHiTESTER 3332, Hioki, Ueda, Japan).
C. Results and Discussion
The results of the experiments conducted at different
static preloads in dry and lubricated conditions are shownin Figs. 14 and 15.

qiu et al.: efficiency improvement of hybrid transducer-type ultrasonic motor

791

Fig. 11. Calculated dependences of (a) maximum torque and (b) no-load
speed on static preload.

During the experiment, the longitudinal resonance frequency was adjusted to be close to the torsional resonancefrequency. The applied voltage to the longitudinal PZTdisks was fixed at 450 Vp-p to avoid the effect of the longitudinal vibration on the transduction efficiency. Prior
to the experiment, we usually added a small quantity of
lubricant, which was apparently sufficient for the dailyexperiment. The effects of the average contact pressure
on the motor efficiency at different applied voltages tothe torsional PZT disks are shown in Figs. 14(a) and (b).
The average contact pressure was defined as the ratio of
the static preload to the contacting area. The experimental results were in good agreement with simulation results indicating that the maximum efficiency was achieved
at low static preloads if low voltage was applied. In thedry condition, the motor efficiency reached its maximum
at low static preloads, where the efficiency was low withlubrication. However, without lubrication, the motor almost stopped at 13.5 MPa contact pressure even whenhigh voltage was applied, so that the efficiency drasticallydecreased. In contrast, the motor efficiency was enhanced

Fig. 12. (a) Dimension of the hybrid transducer-type ultrasonic motor used in the experiment and (b) design of the contact side of therotor.

with the increasing contact pressure after lubrication. The
maximum efficiency was increased from 28% in the dry
condition to 68% in the lubricated condition. In addition,
the motor efficiency reached 44% at 300 Vp-p applied voltage and 37.2 MPa contact pressure, which was the largest
applicable pressure of the spring we used. Thus, the motorwith lubrication operates well even under relatively highpressure, which shows that it can be utilized in applications for which high torque is required.
Figs. 15(a) and 15(b) depict the maximum torque andthe no-load speed as functions of contact pressure, both at
300 Vp-p. When the contact pressure was low, the maximum torque in the dry condition showed clear superiority
to that in the lubricated condition, and 0.3 N·m torque
was obtained at 10.1 MPa contact pressure. In contrast,

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IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control , vol. 60, no. 4, April 2013

Fig. 13. Diagram of experimental setup.

as the contact pressure increased, the maximum torqueincreased in the lubricated condition until the maximum
value of 0.36 N·m was achieved at 27.0 MPa contact pressure. When the contact pressure increased further, themaximum torque was still maintained at high values, cor-

Fig. 15. (a) Maximum torque and (b) no-load speed as functions of static
preload.

Fig. 14. Motor efficiency as a function of static preload in (a) dry and (b)lubricated condition. In the experiments, 5 MPa equals approximately
28.6 N.

responding to the high efficiency. There was a discrepancy between the simulation and the experimental results
for the no-load speed. The no-load speed with lubrication was low at low static preloads, in contrast with thehigh no-load speed in simulation. This is probably becauseof the power loss in the ball bearing used in the experiment, which was neglected in simulation. Another possible reason is that the fluid connects the rotor and thestator because of its surface tension even when there isno normal force acting on the contact surfaces, which was
also not simulated. The reverse torque was induced by
the fluid drag even the contact duration was small, which
seems to have caused the low no-load speed. Therefore,
if taken into account in simulation, these factors makethe no-load speed in the lubricated condition lower thanin the dry condition at low preloads, where the efficiency
and the maximum torque are even lower. As the contactpressure increased, the no-load speed reached the maximum, and began to decrease when the contact pressure
further increased. No-load speed as high as 14.95 rad/s
was achieved even at 37.2 MPa contact pressure, which isa distinctive difference between conditions with and without lubrication.

qiu et al.: efficiency improvement of hybrid transducer-type ultrasonic motor

V. Conclusions
We analytically and experimentally compared the
HTUSM characteristics in dry and lubricated conditions
and examined the effect of improving the motor efficiency
with lubrication at high static preloads. With lubrication,
the motor performance at low static preloads, including
the motor efficiency, the no-load speed, and the maximum
torque, was lower than that without lubrication. However,it was drastically improved at high static preloads, whichindicates that high pressure is required to keep sufficient
traction force if lubricant is applied. The transduction
efficiency of the motor was enhanced from 28% in thedry condition to 68% in the lubricated condition. Undermuch higher static preloads than in the dry condition,
the HTUSM was operated with relatively high efficiency
after being lubricated, which is desirable for high-torqueusages. In addition, because the contact surfaces were well
protected by the lubricant, the motor lifetime can be potentially prolonged. We believe that this study will leadto a breakthrough for broadening the application fields ofultrasonic motors.
Acknowledgments
The authors are indebted to Prof. T. Ishii at the University of Yamanashi, Japan, for his discussion and helping
us to polish the contact surfaces. They are also grateful tothe staff of the Technical Department, Tokyo Institute ofTechnology, Japan, for machining the metal componentsof the ultrasonic motors used in the experiments. In addition, they wish to thank an anonymous reviewer of an
earlier version of this paper for the reviewer’s constructive
and insightful comments and suggestions.
References[1]	S. Ueha and Y. Tomikawa, Ultrasonic Motors—Theory and Applications. New York, NY: Oxford University Press, 1993.
[2] K. Uchino, Piezoelectric Actuators and Ultrasonic Motors. New
York, NY: Kluwer Academic, 1997.
[3] K. Nakamura, M. Kurosawa, and S. Ueha, “Design of a hybridtransducer type ultrasonic motor,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 40, no. 4, pp. 395–401, 1993.
[4] T. Morita, M. Kurosawa, and T. Higuchi, “Design of a cylindricalultrasonic micromotor to obtain mechanical output,” Jpn. J. Appl.
Phys., vol. 35, no. 5B, pp. 3251–3254, 1996.
[5] M. Aoyagi and Y. Tomikawa, “Improvement in performance of longitudinal and torsional vibrator combination-type ultrasonic motor,” Jpn. J. Appl. Phys., vol. 38, no. 5B, pp. 3342–3346, 1999.
[6] J. Satonobu, D. Lee, K. Nakamura, and S. Ueha, “Improvement of
the longitudinal vibration system for the hybrid transducer ultrasonic motor,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol.
47, no. 1, pp. 216–221, 2000.
[7] K. Nakamura, M. Kurosawa, H. Kurebayashi, and S. Ueha, “Anestimation of load characteristics of an ultrasonic motor by measuring transient responses,” IEEE Trans. Ultrason. Ferroelectr. Freq.
Control, vol. 38, no. 5, pp. 481–485, 1991.
[8] K. Uchino, “Piezoelectric ultrasonic motors: Overview,” Smart Mater. Struct., vol. 7, no. 3, pp. 273–285, 1998.

793[9] T. Ishii, T. Shinkoda, S. Ueha, K. Nakamura, and M. Kurosawa,“Efficiency improvement of an ultrasonic motor driven with rectangular waveform,” Jpn. J. Appl. Phys., vol. 35, no. 5B, pp. 3281–3285,
1996.
[10] T. Ishii, H. Takahashi, K. Nakamura, and S. Ueha, “A low-wear
driving method of ultrasonic motors,” Jpn. J. Appl. Phys., vol. 38,
no. 5B, pp. 3338–3341, 1999.
[11] K. Nakamura, M. Maruyama, and S. Ueha, “A new ultrasonic motorusing electro-rheological fluid and torsional vibration,” Ultrasonics,
vol. 34, no. 2–5, pp. 261–264, 1996.
[12] F. W. Heilich and E. E. Shube, Traction Drive. New York, NY:
Marcel Dekker, 1983.
[13] H. Czichos, Tribology—A System Approach to the Science and Technology of Friction, Lubrication and Wear. New York, NY: Elsevier
Scientific, 1978.
[14] F. P. Bowden and D. Tabor, Friction—An Introduction to Tribology.
New York, NY: Krieger, 1982.
[15] K. Nakamura, M. Kurosawa, and S. Ueha, “Characteristics of a
hybrid transducer-type ultrasonic motor,” IEEE Trans. Ultrason.
Ferroelectr. Freq. Control, vol. 38, no. 3, pp. 188–193, 1991.

Wei Qiu was born in Jiangsu Province, China, on
June 27, 1987. He received the B.E. degree in electrical engineering from Nantong University, Nantong, China, in 2009; the M.Sc. degree in electricaland electronic engineering from The University ofHong Kong, Hong Kong, China, in 2010; and the
M.E. degree in information processing from theTokyo Institute of Technology, Yokohama, Japan,
in 2012. He is presently a doctoral candidate in
the Precision and Intelligence Laboratory, Tokyo
Institute of Technology, Kanagawa, Japan. His research interests include ultrasonic motors and actuators, with a particular interest on lubrication in ultrasonic motors, and chaotic mixing.
He received an award for the excellent student presentation at theannual meeting of the Acoustical Society of Japan (ASJ) in 2012. He isa student member of IEEE and ASJ.

Yosuke Mizuno was born in Hyogo, Japan, on
October 13, 1982. He received the B.E., M.E., andDr.Eng. degrees in electronic engineering from the
University of Tokyo, Japan, in 2005, 2007, and2010, respectively.
In 2004, he worked on optical information processing at the University of Tokyo. From 2005 to
2007, he was engaged in spintronics and studied
spin hot-carrier transistors for his master’s degree.
From 2007 to 2010, he was engaged in Brillouin
optical correlation-domain reflectometry at the
University of Tokyo for his Dr.Eng. degree. From 2007 to 2010, he wasa Research Fellow (DC1) of the Japan Society for the Promotion of Science (JSPS). From 2010 to 2012, as a Research Fellow (PD) of JSPS,
he worked on polymer optics at Tokyo Institute of Technology, Japan.
In 2011, he stayed at BAM Federal Institute for Materials Research andTesting, Germany, as a Visiting Research Associate. Since 2012, he has
been an Assistant Professor at the Precision and Intelligence Laboratory,
Tokyo Institute of Technology, where he is active in fiber-optic sensing,
polymer optics, and ultrasonics.
Dr. Mizuno is the winner of the Funai Research Award 2010, theAndo Incentive Prize for the Study of Electronics 2011, and the NF
Foundation R&D Encouragement Award 2012. He is a member of theJapanese Society of Applied Physics (JSAP), and the Institute of Electronics, Information, and Communication Engineers (IEICE) of Japan.

794IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control , vol. 60, no. 4, April 2013

Daisuke Koyama was born in Kyoto, Japan, in
September 1977. He received a Dr.Eng. degreefrom Doshisha University, Kyoto, Japan. He
joined the Precision and Intelligence Laboratory,
Tokyo Institute of Technology, as a research associate in 2005, and was promoted to assistant professor in 2007 and associate professor in 2011. He
is currently an associate professor in the Facultyof Science and Engineering, Doshisha University.
He received an award for the encouragement ofyoung scientists at the Symposium for UltrasonicElectronics and Engineering in 2003 and the Awaya Kiyoshi Award forencouragement of research from the Acoustical Society of Japan in 2008.
His main fields of research are in ultrasonic electronics, especially ultrasonic levitation in high-intensity ultrasound and applications of nonlinear microbubble vibration and destruction in medical ultrasonics. Dr.
Koyama is a member of the IEEE, the Acoustical Society of Japan, andthe Institute of Electrical Engineers of Japan.

Kentaro Nakamura was born in Tokyo, Japan,
on July 3, 1963. He received the B.E., M.E., andDr.Eng. degrees from the Tokyo Institute of Technology, Tokyo, Japan, in 1987, 1989, and 1992,
respectively. He has been a professor of the Precision and Intelligence Laboratory, Tokyo Instituteof Technology, since 2010. His research fields are
the application of ultrasonics and the measurement of vibration and sound using optical methods as well as fiber optic sensors.
He received the Awaya Kiyoshi Award for encouragement of research from the Acoustical Society of Japan in 1996.
He was awarded the best paper awards from the Institute of Electronics,
Information and Communication Engineers in 1998 and from UltrasonicElectronics in 2007 and 2010, respectively. He received the JJAP editorial contribution award from the Japan Society of Applied Physics in2007. Dr. Nakamura is a member of the IEEE, and served as the vicepresident of the Acoustical Society of Japan from 2010 to 2011.