Sensors and Actuators A 217 (2014) 124–128

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Sensors and Actuators A: Physicaljournal homepage: www.elsevier.com/locate/sna

Non-contact piezoelectric rotary motor modulated by giant
electrorheological ﬂuid
Wei Qiu a,∗ , Yaying Hong b , Yosuke Mizuno a , Weijia Wen b , Kentaro Nakamura aabPrecision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan
Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, Chinaa r t i c l e

i n f oArticle history:
Received 3 February 2014
Received in revised form 21 April 2014
Accepted 18 May 2014
Available online 27 May 2014
Keywords:
Non-contact ultrasonic motor
Giant electrorheological ﬂuids
Torsional vibratora b s t r a c t
A bidirectional non-contact rotary motor using a piezoelectric torsional vibrator and the giant electrorheological (GER) ﬂuid is described in this paper. By applying the dynamic electric signal with a square
waveform to the GER ﬂuid, which is in phase with the vibration velocity of the torsional vibrator, bidirectional rotation at an excitation frequency of 118 Hz is achieved. This motor generates 1.04 mN m torquewhen the electric ﬁeld strength of 2 kV/mm with 30% duty cycle is applied to the GER ﬂuid, and therotational speed of up to 7.14 rad/s is achieved if the electric ﬁeld strength is increased to 2.5 kV/mm.
Similarities and differences of the motor characteristics between this motor and the conventional standing wave ultrasonic motors are discussed. The motor performance is not ideal under high electric ﬁeld
strength, indicating that the response time of the GER ﬂuid is dependent on the electric ﬁeld strength.
© 2014 Elsevier B.V. All rights reserved.

1. Introduction
Non-contact ultrasonic motors, which are usually driven byradiation pressure or acoustic streaming, have been proposed as
an alternative to the conventional contact-type ultrasonic motors[1–3]. Unlike the conventional ultrasonic motors, such kinds ofmotors do not suffer from the wear between the stator androtor/slide due to their non-contact nature, which greatly prolongs the motor lifetime. However, non-contact ultrasonic motors
are lacking in the most attractive feature of ultrasonic motors, i.e.
high torque at low speed, and the output force of those motors
is extremely low, which signiﬁcantly limits their applications. To
overcome this problem, electrorheological (ER) ﬂuids, the rheological characteristics of which can be varied by applying electric
ﬁeld, are employed to non-contact piezoelectric motors. This idea
is inspired from the desirable driving mechanism of contact-typeultrasonic motors, where the modulation of the positive and thenegative friction forces should be efﬁcient [4,5]. This type of noncontact piezoelectric motor was initially studied by Nakamura et al.
[6], and the principle was veriﬁed. Meanwhile, another type of
linear stepping piezoelectric motor using ER ﬂuid was also developed [7,8]. However, previous studies have proven that the motor
performance is highly dependent on the characteristics of the ER
ﬂuids, including the response time, yield stress, and viscosity atzero ﬁeld. The microscopic motion opposite to the macroscopic
motional direction was also observed in the existing motors [9],originating from the undesirable properties of the ER ﬂuids. The
recent breakthrough in ER ﬂuids has been the discovery of giant
electrorheological (GER) effect [10,11], which is different fromthe conventional dielectric ER ﬂuids on the point that nano-sizedmolecular dipoles are utilized [12–15]. This effect provides the ER
ﬂuids with not only shorter response time (∼1 ms) but also muchhigher yield stress (>250 kPa), which exceeds the theoretical upper
limit of the conventional dielectric ER ﬂuids by one to two ordersof magnitude, leading to the potential applications in non-contact
piezoelectric motors.
In this paper, we report on a non-contact rotary motor actuatedby a piezoelectric torsional vibrator the vibrational force of which
is modulated by the GER ﬂuid. After introducing the GER effect,
the motor conﬁguration and operating principle are described.
The motor mechanical characteristics are measured and as highas 1.04 mN m torque is achieved at the rotational speed of up to
6.98 rad/s, showing signiﬁcant improvement compared with the
motor using the conventional dielectric ER ﬂuid. The effect of the
GER ﬂuid under various duty cycles on torsional vibration velocity
is clariﬁed and the similarities and differences between this type of
motor and standing wave ultrasonic motors are also discussed.

2. The GER effect∗ Corresponding author. Tel.: +81 45 924 5052; fax: +81 45 924 5091.
E-mail address: qiu@sonic.pi.titech.ac.jp (W. Qiu).
http://dx.doi.org/10.1016/j.sna.2014.05.019
0924-4247/© 2014 Elsevier B.V. All rights reserved.

The discovery of the GER effect was motivated by the insufﬁcient yield stress of traditional dielectric ER ﬂuids, which largely

W. Qiu et al. / Sensors and Actuators A 217 (2014) 124–128

limits their practical applications [13]. The GER effect is reported
to be attributed to the saturation polarization of molecular dipoles,
instead of the induced polarization mechanism in dielectric ER ﬂuids [10,13,15]. This mechanism is achieved by dispersing barium
titanyl oxalate (BaTiO(C2 O4 )2 ) nanoparticles (∼50 nm in diameter) with urea coating (2–5 nm in thickness) in silicone oil. The
generated suspension exhibits strong ER behavior, resulting ina signiﬁcantly improved yield stress of ER ﬂuids [10,11,13]. In
addition, the GER ﬂuid also reveals several excellent features,
including the low current density, the near-linear dependenceof the yield stress on electric ﬁeld strength, and the short
response time (∼1 ms), which cannot be observed in dielectric ER
ﬂuids.
The GER mechanism has been extensively discussed from bothphenomenological [10] and microscopic explanations [14]. The
former has proven that the surface saturation polarization in thecontact zone of the neighboring particles directly contributes to the
GER effect. Meanwhile, the latter has predicted that the formationof aligned (urea) molecular dipolar ﬁlaments induces the significant interaction between the two boundaries of the nanoscaleconﬁnement. In particular, the short-range (other than dipolar)interactions between the coating molecules and the core (barium
titanyl oxalate) particle, and/or the oil, must be such that the molecular dipoles are unlocked from antiparallel pairings (which would
imply insensitivity to the external ﬁeld and hence a small dielectric
response).
Under an applied electric ﬁeld, induced polarization in the particles causes their aggregation into columns aligned along the ﬁeld
direction. These columns are responsible for the solid yield stresswhen the GER ﬂuid is sheared perpendicular to the columns. Owing
to the unideal thermodynamic nature of the urea-water solution, it
is known that urea molecules tend to predominantly aggregate at
the surface of hydrophilic solid particles, forming a (nonuniform)
liquid-like coating. Formation of water ﬁlaments under constrained
conditions may give rise to forces far stronger and longer in range
than the van der Waals interaction, resulting in high yield stress.
The ﬁnding of the GER effect has thus widened the potential applications in the mechanical devices, where the active control of force
transmission is required.

125

Fig. 1. (a) Conﬁguration of the prototype piezoelectric motor using torsional vibrator and the GER ﬂuid; (b) A completed prototype motor.

3. Materials and methods
The key difﬁculty in designing a torsional vibrator for this motor
is to lower the resonance frequency in order to compensate for the
relatively long response time of the GER ﬂuid compared with piezoelectric materials. The unique solution here is to press/stretch thetwo ends of an S-shaped spring with two multilayer PZT actuators
driven by one signal, as shown in Fig. 1. The multilayer PZT actuators(NEC-Tokin Inc., Tokyo, Japan) with a dimension of 5 × 5 × 18 mm
were bonded to the two ends of the spring with epoxy glue. The
spring composed of phosphor bronze was embedded in a 6-mmdiameter shaft with an angle of 60◦ , which was connected witha 36-mm-diameter cylinder. The resonance frequency of the torsional vibrator is determined by the thickness of the spring, anda 0.5-mm-thick spring was employed in this experiment, resulting in a resonance frequency of the torsional vibrator at 118 Hz
which is approximately 8 times longer than the response time of
the GER ﬂuid. The rotor with 42 mm diameter, acting like a reservoir of the GER ﬂuid, was mounted on the base with a ball bearing,leaving a 0.4-mm gap from the bottom of the torsional vibrator.
The weight ratio of the solid particles in the GER ﬂuid used in thisexperiment was 44%, resulting in the viscosity of <100 Pa s without
external electric ﬁeld and 27.54 kPa s under 3 kV/mm electric ﬁeld
strength.

Fig. 2. Diagram of experimental setup.

4. Operating principle
The schematic diagram of the experimental setup is depicted
in Fig. 2. A function generator with a phase shifter was employed
to drive the proposed motor. The driving power to the GER ﬂuid
was ampliﬁed with a high-voltage power ampliﬁer (Matsusada
Precision Inc., Kusatsu, Japan). The square-wave electric pulses synchronized with the vibration velocity of the torsional vibrator wereapplied to the GER ﬂuid to drive the motor. When the torsional
vibration velocity was high, the external electric ﬁeld was applied to126

W. Qiu et al. / Sensors and Actuators A 217 (2014) 124–128

Fig. 4. Dependence of the rotational speed on the phase difference between theelectric ﬁeld applied to the GER ﬂuid and the torsional vibration velocity.

Fig. 3. (a) and (b) Illustration of the operating principle of the motor.

the GER ﬂuid and hence the viscosity became high and large torque
was transmitted, as illustrated in Fig. 3(a). In contrast, the electricﬁeld applied to the GER ﬂuid was switched off when the torsional
vibration velocity was lower or opposite to the rotational velocity of the rotor, as shown in Fig. 3(b); the negative torque became
small owing to the low viscosity of the GER ﬂuid. As a result, theoutput torque in one vibration cycle, regarded as the summation of
the positive and the negative torques, was always positive, and the
motor was rotated in one direction. If the phase difference between
the applied electric ﬁeld to the GER ﬂuid and the torsional vibration
velocity was shifted by 180◦ , the rotational direction was reversed.
The operating principle of this motor is analogous to that ofstanding wave ultrasonic motors, in particular, hybrid transducertype ultrasonic motors (HTUSMs), where a torsional vibrator isalso utilized [16]. Their difference lies in that the traction force
is actively modulated by the GER ﬂuid in the non-contact piezoelectric motor developed in this paper, instead of the modulationof the dynamic preload by the longitudinal vibration in HTUSMs,
where friction loss and wear often occurs owing to the frictiondrive mechanism and the poor modulation of the friction force(note that strong longitudinal vibration can hardly be achieved ifhigh static preload is applied to obtain sufﬁcient torque) [5,17].
The active modulation of the traction force is ideal for piezoelectricmotors since they convert the alternating vibration generated by
the piezoelectric vibrator to one-direction movement. Such kind offorce modulation in piezoelectric motors requires considerable difference between the positive and the negative forces as well as thefast response of a conversion material. The desirable characteristicsof the GER ﬂuid, i.e. the high viscosity (or yield stress) change ratio
and short response time due to the GER effect, enable its potential
utilization in non-contact piezoelectric motors.
5. Results and discussion
The dependence of the motor rotational speed on the phase difference between the applied electric ﬁeld to the GER ﬂuid and thetorsional vibration velocity is shown in Fig. 4. The electric ﬁeld

Fig. 5. Dependence of the torsional vibration velocity on duty cycle.

strength applied to the GER ﬂuid was 2 kV/mm with a duty cycleof 30% and the peak-to-peak voltage applied to the multilayer PZT
actuators was ﬁxed at 80 V. The motor rotational speed was measured by a high-speed digital camera (Integrated Design Tools Inc.,
Tallahassee, FL). The highest speeds in two rotational directions
were obtained with 0◦ and 180◦ phase differences, which not only
validates our original expectation but indicates that the responsetime of the GER ﬂuid is sufﬁciently short at the frequency of 118 Hz.
The motor performance using one kind of the conventional dielectric ER ﬂuid (ER tec, Minoh, Japan) was also compared under thesame electric ﬁeld strength with a duty cycle of 10%. Without the
GER effect, this conventional ER ﬂuid possessed the response timeof approximately 4 ms and the viscosity of 0.3 Pa s at zero ﬁeld and
3.7 Pa s under 2 kV/mm electric ﬁeld strength. The highest rotational speed was obtained with 20◦ phase difference and the motor
was not drivable under the duty cycle of 30%, indicating that the
response time of this type of ER ﬂuid was too long at the frequency of 118 Hz. In addition, the rotational speed was much lower
than that of the motor using the GER ﬂuid, attributing to the muchsmaller change in viscosity and also the longer response time.
Fig. 5 illustrates the effect of the duty cycle on the torsional
vibration velocity when the phase difference was 0◦ . Same electric ﬁeld strength and driving voltage were applied to the GER ﬂuid

W. Qiu et al. / Sensors and Actuators A 217 (2014) 124–128

127

Fig. 6. Rotational speed and maximum torque as functions of duty cycle.

and the multilayer PZT actuators. The torsional vibration velocity
was measured by a 2D laser Doppler vibrometer (Polytec Gmbh,
Waldbronn, Germany). The torsional vibration velocity decreased
with the increasing duty cycle, which is analogical to the effect of
the contact duration on the vibration velocity in driving direction
in standing wave ultrasonic motors, where the vibration velocity
decreases with the contact duration increasing. This is becausethat long contact duration increases the load to the vibrator andhence the vibration velocity is lowered. For the non-contact motor
described here, high duty cycle increased the average viscosity of
the GER ﬂuid in one vibration cycle, and hence increased the resistance to the torsional vibrator when the torsional vibration velocity
was lower or opposite to the rotational velocity of the rotor, resulting in the reduction of the torsional vibration velocity.
Then, the dependences of the motor rotational speed and the
maximum torque on duty cycle were investigated with the same
driving conditions mentioned above, as shown in Fig. 6. Different
weights were pulled to measure the torque, and the maximum
torque was calculated from the weight that just stopped the motor.
Desirable motor performance was obtained at 30% duty cycle,
where the rotational speed of as high as 6.98 rad/s and the maximum torque of as large as 1.04 mN m were achieved. The motor
mechanical performance became poor as the duty cycle was too
low or too high, and the motor was not driven if the duty cycle washigher than 80%. When the duty cycle was too low, though the torsional vibration velocity was high, the driving force was drasticallyreduced and the reserve torque induced by the ﬂuid drag at zeroﬁeld as well as the power loss in the ball bearing became dominant.
On the other hand, if too high duty cycle was employed, the reverse
torque was increased due to the long active time of the GER ﬂuid
in one vibration cycle, stopping the motor.
The relationship between the motor characteristics and the electric ﬁeld strength applied to the GER ﬂuid was also investigatedat the duty cycle of 30%, as shown in Fig. 7. Low rotational speed
and small maximum torque were obtained when the electric ﬁeld
strength was low, due to the small change in the viscosity of the
GER ﬂuid. With the increase of the electric ﬁeld strength, the rotational speed became closer to the torsional vibration velocity owing
to the increase of the viscosity change ratio, which was describedin Ref. [6]. However, the maximum torque and the rotational speed
slightly decreased when the electric ﬁeld strength was higher than
2 kV/mm and 2.5 kV/mm, respectively, which was different fromour expectation. Under higher electric ﬁeld strength, the change inthe viscosity of the GER ﬂuid with and without electric ﬁeld is more

Fig. 7. Rotational speed and maximum torque as functions of electric ﬁeld strength
applied to the GER ﬂuid.

considerable if the response time keeps the same, which should bedesirable for this type of motor. Nevertheless, it was found that thehighest rotational speeds under 2.5 kV/mm and 3 kV/mm electricﬁeld strength were obtained with 10◦ and 20◦ phase difference,
respectively. This phenomenon indicates that the response timeof the GER ﬂuid is prolonged after applying higher electric ﬁeld
strength. Therefore, the reverse torque was increased due to thelonger response time of the GER ﬂuid, resulting in the decrease of
the maximum torque and the rotational speed, though same duty
cycle was applied.

6. Conclusions
In conclusion, we developed a non-contact rotary motor using
a piezoelectric torsional vibrator and the GER ﬂuid. By comparing
with the motor using the conventional dielectric ER ﬂuid, drastic
improvement in the motor characteristics was observed becauseof the short response time and high yield stress of the GER ﬂuid.
Ideal motor performance was obtained under 2 kV/mm electricﬁeld strength with 30% duty cycle, and 1.04 mN m torque at therotational speed of up to 6.98 rad/s was achieved, offering force
at least two orders of magnitude larger than that of conventional
non-contact ultrasonic motors. Several similarities and differences
between this motor and contact-type standing wave ultrasonicmotors were discussed, verifying the operating principle of thistype of non-contact motor. The response time of the GER ﬂuid
was affected by the applied electric ﬁeld, which limited the motor
performance under high electric ﬁeld strength.

Acknowledgements
The authors are indebted to Prof. K. Yoshida at Tokyo Institute of Technology, Japan, for his insightful discussion and lending
us a high-voltage power ampliﬁer. They are also grateful to thestaffs of the Technical Department, Tokyo Institute of Technology,
Japan, for machining the metal components of the motor used in
the experiment. W. Qiu is grateful to the Research Fellowships for
Young Scientists from the Japan Society for the Promotion of Science (JSPS).

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Biographies
Wei Qiu received the B.E. degree in electrical engineering from Nantong University, China, in 2009, M.Sc. degree in electrical and electronic engineering fromThe University of Hong Kong, Hong Kong, in 2010, and M.E. degree in information
processing from Tokyo Institute of Technology, Japan, in 2012. Since 2012, he has
been a doctoral candidate in the Precision and Intelligence Laboratory, Tokyo Institute of Technology, Japan. In 2014, he became a Research Fellow (DC2) of the Japan
Society for the Promotion of Science (JSPS). His research interests include ultrasonicmotors and actuators with a particular interest on lubrication in ultrasonic motors,
and chaotic mixing.
Yaying Hong received the M.Eng. degree (ﬁrst-class honors) in chemical engineering from University of Birmingham, UK, in 2010. Since 2012, she has been a Ph.D.
candidate in department of physics, Hong Kong University of Science and Technology (HKUST), Hong Kong. Her research interests focus on electrorheological ﬂuid,
microﬂuidic chips, and soft matter.
Yosuke Mizuno received the B.E., M.E., and Dr.Eng. degrees in electronic engineeringfrom the University of Tokyo, Japan, in 2005, 2007, and 2010, respectively. From2007 to 2010, he was engaged in Brillouin optical correlation-domain reﬂectometry
for his Dr.Eng. degree at the University of Tokyo. From 2007 to 2010, he was a
Research Fellow (DC1) of the Japan Society for the Promotion of Science (JSPS). From2010 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 Institutefor Materials Research and Testing, 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.
Weijia Wen earned his B.S. (1982) and M.S. (1988) degrees at Chongqing University, China. He completed his Ph.D. (1995) degree in Institute of Physics, Chinese
Academy of Science, Beijing, China. He was a postdoctoral fellow at Hong Kong
University of Science and Technology (HKUST) from 1995 to 1997 and Universityof California, Los Angeles (UCLA) from 1997 to 1999. He joined HKUST in 1999
and currently is a professor in the department of Physics, HKUST. Professor Wen’s
main research interests include soft condensed matter physics, electrorheological (ER) and magnetorheological (MR) ﬂuids, ﬁeld-induced pattern and structuretransitions, micro- and nano-ﬂuidic controlling, microsphere and nanoparticle fabrications, thin ﬁlm physics, band gap materials, metamaterials and nonlinear opticalmaterials.
Kentaro Nakamura received the B.E., M.E., and Dr.Eng. degrees from Tokyo Instituteof Technology, Japan, in 1987, 1989, and 1992, respectively. He served as an associate
professor in the Precision and Intelligence Laboratory, Tokyo Institute of Technology
from 1996 to 2010, and became a professor in 2010. Professor Nakamura’s researchinterests focus on the various applications of power ultrasonics, including ultrasoniclevitation, non-contact transportation, and the development of ultrasonic motors
and actuators. He is also working on the measurement of vibration and sound using
optical methods as well as ﬁber-optic sensors.