Sensors and Actuators A 282 (2018) 183–191

Contents lists available at ScienceDirect

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

Ultrasonic motor performance inﬂuenced by lubricant properties
Wei Qiu a,∗ , Yosuke Mizuno b , Koshi Adachi c , Kentaro Nakamura b,∗a

Department of Physics, Technical University of Denmark, DTU Physics Building 309, DK-2800 Kongens Lyngby, Denmark
Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japanc
Department of Mechanical Systems Engineering, Graduate School of Engineering, Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579,
Japanba r t i c l e

i n f oArticle history:
Received 16 July 2018
Received in revised form 16 August 2018
Accepted 8 September 2018
Available online 10 September 2018
Keywords:
Ultrasonic motors
Efﬁciency
TorqueLubricantViscosity
Lubrication mechanisms

a b s t r a c t
Functional lubrication has been shown to be a promising method for improving the performance ofultrasonic motors (USMs). However, the complex lubrication mechanisms have not been studied and theeffect of lubricant properties on motor performance remains unknown. In this paper, both low-traction
ﬂuids and high-traction ﬂuids with a wide viscosity range are applied, and the lubricating effects areinvestigated in hybrid transducer-type ultrasonic motors. The results clearly show that high-traction
ﬂuids have signiﬁcant superiority to low-traction ﬂuids in terms of motor efﬁciency and reduction offriction loss at the entire range of the applied preload. A high friction coefﬁcient in boundary lubrication
regime and efﬁcient modulation in friction are conﬁrmed to be essential for achieving high-performance
lubricated USMs. It is also found that the motor performance is independent of lubricant viscosities inthe tested range, irrespective of lubricant types. The reasons for the distinct lubricating effects with thetwo types of lubricants are analyzed in detail, which leads to the criteria of selecting lubricants and more
understanding in the lubrication mechanisms in USMs.
© 2018 Elsevier B.V. All rights reserved.

1. Introduction
Ultrasonic motors (USMs) possess several unique features, e.g.
high torque at low speed, high power density, precise positioning
capability without using gear systems, etc., that electromagnetic
motors generally do not have [1,2]. Owing to these attractive features, USMs have been expected to have wide markets since theirinfancy. However, in spite of the successful application in cameras, USMs have not been considerably utilized as the actuatorsin other mechanical systems. Their inherently low efﬁciency andshort lifetime are always the major obstacles, and contemporarycommercial USMs are solely used for the actuation tasks where
neither high-efﬁciency nor long-time operation is required.
The inherent problems of USMs are mainly caused by theirfriction drive mechanism. In most of the USMs, the alternating
vibrations in both tangential (driving) and vertical (loading) directions exist. The vertical vibration varies the loading force and hence
the friction force between the rotor and the stator is instantaneously changed, corresponding to the sinusoidal tangential force.
Consequently, both positive (when the velocity of the stator is∗ Corresponding authors.
E-mail addresses: weiqiu@fysik.dtu.dk (W. Qiu), knakamur@sonic.pi.titech.ac.jp(K. Nakamura).
https://doi.org/10.1016/j.sna.2018.09.022
0924-4247/© 2018 Elsevier B.V. All rights reserved.

higher than that of the rotor) and negative friction forces exist during the motor operation, and the positive friction force in one periodof vibration should always be larger than the negative friction force.
Friction loss is generated by the slip between the rotor and the stator, and large slip appears when negative friction force is present,
which should ideally be minimized in order to achieve high motor
efﬁciency. Though numerous attempts have been made to enhance
the motor efﬁciency by designing ideal contact and vibration conditions [3–10], by applying the friction materials with desirable
tribological performance [11,12], or by optimizing drive and control schemes [13–15], these methods are still based on friction drive
mechanism and cannot effectively reduce the friction loss without
sacriﬁcing output torque. Several types of non-contact USMs, ofwhich the rotor does not contact with the stator, have been developed to eliminate the friction loss [16–20]. However, the output
torque of non-contact USMs is greatly lower than that of contacttype USMs, signiﬁcantly limiting their applications.
In order to tackle the inherent problems in USMs, the authors
proposed a novel driving mechanism, i.e. using lubrication at thecontact interface, which had not been considered to be applicableto friction drive USMs. Lubricants can work in different lubrication regimes according to the contact conditions, which was
described by the well-known Stribeck curve [21]. The presence of
low lubricant viscosity, low relative velocity between the two contact surfaces, or high load leads to boundary lubrication regime,

184

W. Qiu et al. / Sensors and Actuators A 282 (2018) 183–191

Fig. 1. (a) Sketch and dimension of the HTUSM used in experiments. (b) Illustration of boundary lubrication regime in HTUSM where low relative velocity between the rotor
and the stator and high loading force are present. (c) Illustration of mixed (or hydrodynamic) lubrication regime in HTUSM where high relative velocity between the rotor
and the stator and low loading force are present.

which usually provides a high friction coefﬁcient. In contrast, thefriction coefﬁcient can be signiﬁcantly reduced in hydrodynamic
lubrication regime, which is achieved by high lubricant viscosity,
high relative velocity, or low load. Mixed lubrication regime falls in
between boundary and hydrodynamic lubrication regimes, whichis also able to reduce the friction coefﬁcient to some extent. In
USMs, both low relative velocity between the rotor and the statorand high load can be obtained in the driving stage (the velocity ofthe stator is higher than that of the rotor), corresponding to boundary lubrication regime; while mixed or hydrodynamic lubrication
regime can be achieved when the rotor moves in an opposite direction to the stator, where high relative velocity and low load arepresent. Based on this lubrication mechanism, friction coefﬁcient
can be varied to reduce the friction loss without losing the driving force. This novel driving mechanism has been proven to beeffective in our early studies in hybrid transducer-type ultrasonicmotors (HTUSMs), where both motor efﬁciency and torque weredrastically improved with lubrication [22,23].
Lubricant is always the key element in the machineries that
require lubrication. In lubricated USMs, the ideal lubricant shouldbe able to vary lubrication regimes corresponding to the alternating
vibrations of the motor, in order to reduce friction loss and obtainhigh output force. Though drastic improvement in motor performance has been observed in the previous studies, the knowledge
regarding the role of lubricant properties is still lacking due to thepoor understanding of the complex lubrication mechanisms, which
is an obstacle to establish criteria for the selection of lubricants for
USMs.
In this paper, we investigate the lubricating effects of both
low-traction and high-traction ﬂuids with various viscosities inan HTUSM. After presenting a method to calculate the friction
loss in HTUSM using an equivalent circuit, we experimentallydemonstrate how the friction loss and motor efﬁciency vary using
low-traction and high-traction ﬂuids under different preloads.
Then, the dependence of the mechanical characteristics of HTUSM,
i.e. efﬁciency, torque, and no-load speed, on lubricant viscosity
is studied. Finally, the signiﬁcance of high friction coefﬁcient inboundary lubrication regime and efﬁcient modulation of friction
according to the Stribeck curve is conﬁrmed, and the criteria for
selecting lubricants for USMs are established.

2. Materials and methods
2.1. Motor materials and structure
A Langevin transducer (diameter  = 25 mm, height h = 40 mm)
comprising of both torsional lead zirconate titanate (PZT) disks(outer diameter o = 25 mm, inner diameter i = 12 mm, thick-ness d = 4 mm) and longitudinal PZT disks (o = 25 mm, i = 12 mm,
d = 1 mm) was used as the stator of HTUSM, as shown in Fig. 1. A
rotor ( = 29 mm) was pressed to the stator by a coil spring, and thepreload was adjusted by varying the pressing force applied to thespring. The rotors with different heights were fabricated in orderto match the resonance frequencies of longitudinal and torsional
vibrations under different preloads. Lubricant was placed on top of
the stator before applying preload, and the surfaces of the friction
materials were completely wetted by the lubricant.
Alumina (Al2 O3 , 99.5%) and zirconia (ZrO2 , 94.0%) were chosen
as the friction materials and bonded on the stator and the rotorsides, owing to their excellent tribological performance in lubricated USMs [24]. The average roughness of Al2 O3 and ZrO2 surfaces
were 0.12 and 0.27 ␮m, respectively. According to our previous
study, the surface roughness and morphology of both materials are
not signiﬁcantly changed during the operation of lubricated USMs[24], which is important to conduct a fair comparison using different lubricants. The dimensions of the friction materials followed
our previous design described in Ref. [22].

2.2. Lubricants
Both low-traction and high-traction ﬂuids with various viscosities were prepared for this study. All the properties of lubricants
are shown in Table 1. High-traction ﬂuid is a category of lubricantsthat transmits shear force in elastohydrodynamic lubrication (EHL)
regime. Therefore, they typically possess high traction coefﬁcients
which enable efﬁcient power transmission in traction drive systems(e.g. continuously variable transmission). In contrast, low-traction
ﬂuids cannot efﬁciently transmit shear force in EHL regime due totheir low traction coefﬁcients, and hence are not used for traction
drive. Though EHL regime is unlikely to occur in USMs due to theconformal contact and relatively low contact pressure (<0.1 GPa),these two types of lubricants were selected because of their distinct
properties.
For each type of lubricant, three viscosity grades (VG) wereselected, i.e. VG 100, VG 220, and VG 460. According to the viscosities at 40 ◦ C and 100 ◦ C as well as the viscosity index, the viscosities
at 20 ◦ C (the temperature at which the experiments were conducted) were calculated, and both types of lubricants cover a wide
viscosity range. Zinc dialkyldithiophosphate (ZDDP) was the only
additive used in the lubricants, since the lubricating effect might
be very complex if more additives were applied. ZDDP is a common antiwear additive which reduces wear at moderate loads andtemperatures. Though the working mechanisms of ZDDP in metal
contact and ceramic contact are different, a reduction in friction andwear in ceramic contact (including Al2 O3 and ZrO2 ) has been found[25]. In this experiment, preventing wear is essential because theW. Qiu et al. / Sensors and Actuators A 282 (2018) 183–191

185

Table 1
Properties of lubricants.
Low-traction ﬂuid
(VG 100)

Low-traction ﬂuid
(VG 220)

Low-traction ﬂuid
(VG 460)

High-traction ﬂuid
(VG 100)

High-traction ﬂuid
(VG 220)High-traction ﬂuid
(VG 460)

Base oil

Polyalphaoleﬁn

Polyalphaoleﬁn

Polyalphaoleﬁn

Additives
Viscosity at 40 ◦ C(cSt)Viscosity at 100 ◦ C(cSt)Viscosity at 20 ◦ C(cSt)Viscosity index
Density at 20 ◦ C(kg/m3 )

ZDDP
104.0

ZDDP
214.5

ZDDP
474.0

Synthetic
naphthene
ZDDP
100.6

Synthetic
naphthene
ZDDP
229.3

Synthetic
naphthene
ZDDP
421.5

15.6

25.7

46.5

8.3

11.8

15.1

279.5

646.7

1561.4446.7

1421.73358.1

160
833.8

152
841.1

155
847.3

16
974.5

−37
979.4

−81
980.7

Table 2
Parameters used for calculating friction loss.
Torque factor A
Free admittance Ym0
Torsional resonance frequency fT0
Clamped capacitance CdDielectric loss Rd
Contact radius r

Fig. 2. Equivalent circuit of HTUSM for calculating friction loss.

effect of lubricants would become unclear, if the contamination of
the lubricants occurs due to the formation of wear debris. In addition, from the perspective of motor lifetime, the lubricants whichcannot prevent the wear of friction materials should not be selected
for USMs. Therefore, ZDDP was added to the lubricants, since theeffectiveness of ZDDP in preventing wear in lubricated USMs has
been conﬁrmed [24].
2.3. Equivalent circuit model for calculating friction loss
USMs typically consist of three types of energy loss, i.e. dielectric loss, vibration loss, and friction loss. The purpose of applying
lubricant to USMs is to reduce the friction loss. It is therefore important to compare not only the motor characteristics but also thefriction loss when different lubricants are present. We employ anequivalent circuit model which represents the torque transmission mechanism of the torsional vibrator, as shown in Fig. 2. The
left arm represents the electrical terminal of the torsional vibrator,
where Rd and Cd are the dielectric loss and clamped capacitance of
torsional PZT disks while im is the motional current. The applied
voltage to the torsional vibrator UT = U0 cosωt, where U0 and ω indicate the voltage amplitude and the angular frequency, respectively.
The right arm represents the mechanical terminal of the torsional
vibrator, where lm , cm , and rm are equivalent to the moment ofinertia, the reciprocal of the torsion coefﬁcient, and the rotational
friction (vibration loss) of the torsional vibrator, while ˝T , ˝R , and 0 are the angular vibration velocity, the motor rotational speed,
and the open-circuited torque, respectively. The electrical and themechanical terminals are connected by the torque factor A :

 im = A ˝T ,
0 = A UT .

(1)

The output torque  is transmitted by friction force, and hence thefriction loss can be calculated by subtracting the dielectric loss, thevibration loss, and the output power from the input power to thetorsional transducer. It should be noted that the friction characteristics are represented by a question mark box. In dry contact,

0.00445 Nm/V
2.75 mS
19.7 kHz2.28 nF
697 k
10.75 mmthe friction coefﬁcient  is typically assumed to be a constantin numerical simulation, hence the friction characteristics can be
represented by a twin Zener diode, as described in Refs. [26,27].
However, when lubricant is present,  is no longer a constantbut varies based on the Stribeck curve. In this case, an equivalent
electronic component which can represent the complex friction
characteristics is difﬁcult to ﬁnd, and a question mark box is therefore used in the equivalent circuit. It also indicates that an accurate
model for lubricated USMs remains challenging [22], and thus only
experimental investigations are performed here. Since in this study
we mainly discuss the friction loss, the efﬁciency deﬁned in thispaper is the transduction efﬁciency of the HTUSM, which is theratio of the motor output power to the input electrical power of
the torsional vibrator (longitudinal vibration does not contributeto the torque transmission). All the parameters used for calculating
friction loss are summarized in Table 2.
2.4. Experimental setup and methods
Fig. 3 illustrates the schematic diagram of the experimental
setup. The signal to drive the torsional vibrator was generated froma function generator and subsequently ampliﬁed using an ampliﬁer
and a transformer. The longitudinal vibrator was driven by the signal from the same function generator, but a phase difference was
set using a phase shifter before the signal was ampliﬁed. ˝T was
measured using an in-plane laser Doppler vibrometer (IPV-100,
Polytec Gmbh, Waldbronn, Germany), while ˝R was measured byboth a high-speed digital camera (M3, Integrated Design Tools, Inc.,
Tallahassee, USA) and a rotary encoder (Canon Inc., Tokyo, Japan).
Different weights were employed to obtain the load characteristics of the motor. The electrical power to the torsional vibrator was
calculated from the deﬁnition of the effective power with the voltage and current waveforms, and also veriﬁed using a digital power
meter (Power HiTESTER 3332, Hioki, Ueda, Japan).
The applied voltage to the longitudinal PZT disks was ﬁxed
at 61 Vrms for all the measurements, in order to avoid the effectof longitudinal vibration on motor transduction efﬁciency. Under
different preloads, the maximum motor efﬁciency is achievedat different U0 . In general, the maximum efﬁciency under low
preloads is obtained at low U0 , while high U0 is required to achieve
maximum efﬁciency under high preloads. The details regarding this186

W. Qiu et al. / Sensors and Actuators A 282 (2018) 183–191

Fig. 3. Schematic diagram of the experimental setup.

Fig. 4. The percentage of different contributions of power consumption in lubricated HTUSMs using (a) low-traction ﬂuid (VG 100) and (b) high-traction ﬂuid (VG 100) under
different preloads.

phenomenon have been discussed in our previous studies [22,26].
In the present work, U0 was tuned to ﬁnd the maximum efﬁciency
under each preload (the optimal U0 values for the same type of
lubricant with different viscosities under the same preload had
little variation (maximum 3.2%), but the optimal U0 values for hightraction ﬂuids were higher (from 27.4% to 73.7% under different
preloads) than that for low-traction ﬂuids). The U0 values usedin the experiments ranged from 13.44 to 147.81 Vrms , corresponding to a preload range from 171 to 682 N. The no-load speed andmaximum torque under each preload were measured with U0 , atwhich maximum efﬁciency can be achieved. The maximum torqueunder different conditions was calculated from the weights thatjust stopped the motor. All the experiments were conducted atroom temperature (20 ◦ C); for each measurement, the motor was
operated for a short time (less than 10 s) to avoid the temperaturerise. Thus, the viscosities of the lubricants were not signiﬁcantly
varied during each measurement. Prior to changing the lubricant,the surfaces of the friction materials were cleaned with toluene
for 15 min, followed by another 15-min cleaning with isopropanol,
both in an ultrasonic cleaner, in order to avoid any residuals of thepreviously tested lubricants. All the measurements were performed
ﬁve times under each condition in order to improve the statistics.
3. Results
Different contributions of power consumption in lubricated
HTUSMs under various preloads using both low-traction and hightraction ﬂuids with a viscosity grade of 100 are illustrated in Fig. 4.
The dielectric loss of the torsional PZT disks only consumed very
limited amount of the input power (≈1%) under all preloads forboth types of lubricants. The percentage of vibration loss was rel-

atively high under low preloads, but decreased with the preload
increasing and became saturated under high preloads. When lowtraction ﬂuid was applied, the percentage of vibration loss washigher than that using high-traction ﬂuid, and the difference under
low preloads was rather distinct. More signiﬁcant difference wasfound for the two types of lubricants in the percentage of friction
loss and motor efﬁciency. It is clearly shown that friction loss is thedominant loss in HTUSMs, particularly under high preloads. The
percentage of friction loss using low-traction ﬂuid was higher thanthat lubricated by high-traction ﬂuid at the whole preload range,
resulting in lower motor efﬁciency.
The dependencies of the motor efﬁciency and the percentage
of friction loss on static preload with all lubricant viscosities areshown in Figs. 5 and 6 . Surprisingly, the motor efﬁciency andthe percentage of friction loss did not show any viscosity dependence, resulting in similar efﬁciency and percentage of friction loss
under all applied preloads. With low-traction ﬂuids, the motor
efﬁciency initially increased with the preload increasing, then saturated under moderate preloads but ﬁnally decreased under highpreloads. Correspondingly, the percentage of friction loss showed ageneral trend of increasing, while it increased more rapidly underhigh preloads than that under low preloads. In contrast, when hightraction ﬂuids were applied, the motor efﬁciency increased with thepreload increasing, and was maintained at high values (>53%). In
terms of the percentage of friction loss, it initially decreased until
it reached the lowest value, and then was kept at relatively low
values (<37%) under high preloads.
The maximum torque and no-load speed using different lubricants were also examined, as illustrated in Figs. 7 and 8 . It should be
noted that the maximum torque and no-load speed were measured
with the applied voltages to the torsional vibrator U0 , at which the

W. Qiu et al. / Sensors and Actuators A 282 (2018) 183–191

187

Fig. 5. Motor efﬁciency as a function of static preload using (a) low-traction ﬂuids and (b) high-traction ﬂuids with different viscosities. The upper ﬁgures show the dataplots in a large scale for y-axis (from 0 to 100%), while the lower ﬁgures plot the same data as the upper ones, but in a small scale for y-axis in order to show the standard
errors clearly.

Fig. 6. The percentage of friction loss as a function of static preload using (a) low-traction ﬂuids and (b) high-traction ﬂuids with different viscosities. The upper ﬁgures show
the data plots in a large scale for y-axis (from 0 to 100%), while the lower ﬁgures plot the same data as the upper ones, but in a small scale for y-axis in order to show thestandard errors clearly.

highest motor efﬁciency was obtained under different preloads.
Therefore, the results in Fig. 7 are not the absolute maximum torqueof motor under each preload, which should be even larger with ahigher U0 value [23,26]. Since the U0 values for the same type of
lubricant with different viscosities were rather close but varied for
different types of lubricants, the comparison was only conducted
for the same type of lubricant. The maximum torque using bothtypes of lubricants showed an increasing trend at the entire preload
range. The no-load speed with high-traction ﬂuids increased withthe preload increasing, while with low-traction ﬂuids, the no-load
speed initially increased until it became saturated when the preload
was higher than 500 N. Again, the maximum torque and no-load
speed did not show any viscosity dependence under all preloads,

which is the same with motor efﬁciency and percentage of friction
loss.
4. Discussion
High-traction ﬂuids showed signiﬁcant superiority to lowtraction ﬂuids in terms of motor efﬁciency and reduction in friction
loss. In order to understand the distinct lubricating effects, thefriction coefﬁcients under two different conditions were measured. Owing to the difﬁculties in measuring instantaneous friction
coefﬁcients during motor operation, the friction coefﬁcient wasestimated from the falling curve right after switching off the input
power to the HTUSM, which shows the friction between the rotor
and the stator without ultrasonic vibration [11,24]. The retarding

188

W. Qiu et al. / Sensors and Actuators A 282 (2018) 183–191

Fig. 7. Maximum torque as a function of static preload using (a) low-traction ﬂuids and (b) high-traction ﬂuids with different viscosities.

Fig. 8. No-load speed as a function of static preload using (a) low-traction ﬂuids and (b) high-traction ﬂuids with different viscosities.

torque of USMs TR (the torque without ultrasonic vibration) can be
expressed as
TR = Jd˝R
˝R1 − ˝R2
=J
.
tf
dt

(2)

Here, J and tf are the moment of inertia of the rotor and the falltime. ˝R1 and ˝R2 represent the highest and the lowest rotational
speeds within tf . TR can also be obtained from
TR = Fr,(3)

where F, and r represent the preload and the contact radius, respectively. Combining Eqs. (2) and (3),  can be calculated as=J(˝R1 − ˝R2 )
.
tf Fr

(4)

In lubricated condition,  is dependent on the relative velocity ofthe sliding surfaces. Therefore, the obtained  value is the averagevalue within tf . The  values measured under condition (a) in Table
III was chosen to indicate the friction coefﬁcients in the stage where
the reverse torque occurs (˝T < ˝R ), while the values measured
under condition (b) indicates the driving stage (˝T  ˝R ). Here, P
represents the apparent contact pressure under F (P = F/A, where A

is the contact area), and V̄ is the average sliding velocity within tf ,which is calculated as
V̄ =

1
tf


˝R r dt.

(5)

tf

Under condition (a), the motor stopped from ˝1 = 18.6 rad/s to˝2 = 13.95 rad/s, resulting in a V̄ value of approximately 175 mm/s;
under condition (b), the motor stopped from ˝1 = 4.65 rad/s to˝2 = 0, resulting in a V̄ value of approximately 25 mm/s. Owing to
low P and high V̄ , condition (a) should be able to represent mixed
lubrication regime; while boundary lubrication regime is represented by condition (b) because of high P and low V̄ . Hydrodynamic
lubrication is extremely difﬁcult to achieve in HTUSMs due to theparallel contact surfaces (the lifting pressure is hard to build without a converging entry for the lubricant entrainment), unless therotor and the stator are detached due to the longitudinal vibration.
Under high F, the contact duration is 2 so that the rotor and the stator are always in contact; under low F, the contact duration might
be smaller than 2, but the period for detachment in one periodof vibration should be very limited due to the weak longitudinal
vibration. All the measurements were performed ﬁve times undereach condition, and individual measurement consumed less than 5s, which prevented the temperature rise. The average values of and their standard errors are summarized in Table 3.

W. Qiu et al. / Sensors and Actuators A 282 (2018) 183–191

189

Table 3
Friction coefﬁcients using different lubricants measured under conditions (a) and (b).
Low-traction ﬂuid
(VG 100)

Low-traction ﬂuid
(VG 220)

Low-traction ﬂuid
(VG 460)

High-traction ﬂuid
(VG 100)

High-traction ﬂuid
(VG 220)High-traction ﬂuid
(VG 460)

0.014 ± 0.001

0.017 ± 0.002

0.026 ± 0.002

0.043 ± 0.003

0.062 ± 0.003

0.087 ± 0.003(b) F = 682 N, P = 76 MPa, V̄ = 25 mm/s 0.067 ± 0.003

0.079 ± 0.003

0.082 ± 0.004

0.103 ± 0.004

0.121 ± 0.005

0.144 ± 0.005(a) F = 57 N, P = 6 MPa, V̄ = 175 mm/s

For both types of lubricants,  showed viscosity dependence
at each condition, giving higher  when the viscosity was higher.
The big difference in  between the two conditions at each viscosity proved that  is indeed able to be greatly varied during
motor operation owing to the presence of lubricant. The  values using high-traction ﬂuids were considerably higher than thoseusing low-traction ﬂuids in condition (b) at the entire range oftested viscosities, though they were also higher in condition (a).
The high  values of high-traction ﬂuids in boundary lubrication
regime is believed to be the most signiﬁcant reason for the higher
motor efﬁciency than that using low-traction ﬂuids. As expected,
it is essential to possess high  in the driving stage of lubricatedUSMs through boundary lubrication; otherwise the driving force
would be signiﬁcantly lowered, leading to low motor efﬁciency,
which is the case using low-traction ﬂuids. Though it is also important to achieve low  in mixed lubrication regime in order to reduce
the reverse torque, the higher efﬁciency with high-traction ﬂuids
shows that a high  value in boundary lubrication regime is a more
dominant factor for achieving high-performance lubricated USMs,
if the range of variation in  is comparable among different typesof lubricants. The  values in boundary lubrication regime using
high-traction ﬂuids are even comparable with the  values in USMs
using some of the pairs of friction materials in dry contact [11].
However, using high-traction ﬂuids, the  values in mixed lubrication regime (when the velocity of the stator is opposite to thatof the rotor) are considerably lower than those in boundary lubrication regime, which is not possible in dry contact. This fact leads
to efﬁcient modulation of friction force in lubricated USMs, sinceboth  and F are lowered when negative friction force is present,
while typically only F is reduced in dry contact. In addition, the highaverage  values in one period of vibration using high-traction ﬂuids also lower ˝T [20,23], which decreases the vibration loss (seeFig. 4).
We take the results obtained from low-traction and hightraction ﬂuids with a viscosity grade of 100 as an example toillustrate how the friction loss varied under different conditions.
The friction loss PF can be calculated as [3,28]
PF =

1
t

 t

f (t)[vs (t) − vr (t)] dt.

(6)0Here, f, vs , and vr (all time-dependent) are the tangential force
between the rotor and the stator and the tangential velocities ofthe stator and the rotor, respectively, where f is mainly determinedby the friction force. The slip velocity v in one period of vibration
T is calculated as
v =

1
T

 T

|vs (t) − vr (t)| dt.

(7)0

The measured input power Pin , PF , and v using low-traction
and high-traction ﬂuids (VG 100) under different F are shown in
Fig. 9. All the three parameters exhibited increasing trends when
F increased, irrespective of lubricant types, resulting from theincreasing U0 values when F became higher. Under most of theapplied F, the Pin values using high-traction ﬂuid were higher thanthat using low-traction ﬂuid. Despite low Pin , using low-traction
ﬂuid, PF was always higher than that using high-traction ﬂuid
except under very low F, corresponding to larger v and lower
efﬁciency (see Fig. 5). The low efﬁciency using low-traction ﬂuid ismainly caused by high PF and low output power. Large v is one
important reason to induce high PF , as analyzed in Ref. [13], while
low output power is attributed to the low  values in boundary
lubrication regime. The smallerv using high-traction ﬂuid thanthat using low-traction ﬂuid was mainly due to its higher average
, resulting in less PF especially under high F. Less PF and vibration
loss with high-traction ﬂuids led to much higher efﬁciency thanthat using low-traction ﬂuids. Again, the large output power andhigh efﬁciency using high-traction ﬂuids under high F is a strong
evidence that the friction modulation is efﬁcient with this type of
lubricant, which is not possible in dry contact owing to the weaklongitudinal vibration under high F and similar  when postive andnegative friction forces are present.

Fig. 9. (a) Input power Pin , (b) friction loss PF , and (c) slip velocity v in one period of vibration T as functions of static preload using low-traction and high-traction ﬂuids(VG 100). All the values were measured when the highest efﬁciency was achieved under each preload.

190

W. Qiu et al. / Sensors and Actuators A 282 (2018) 183–191

Somewhat counter intuitively, all the mechanical characteristics
of motor were very close when different viscosities of lubricants
were applied (see Figs. 5–8), though the  values in boundary andmixed lubrication regimes were different. It indicates that hydrodynamic lubrication did not play a signiﬁcant role even under low F;
otherwise the motor characteristics should be quite distinct when
the lubricants with different viscosities were used (the drag force
in hydrodynamic lubrication regime is larger with high-viscosity
lubricant than that using low-viscosity lubricant, which would
induce higher PF ). It also indicates that the average friction force
and slip in one T are fairly close with different viscosities. To investigate this, the measurements of the instantaneous  values during
motor operation have to be performed, which is extremely difﬁcult with the existing techniques. At the present time, however, we
think it is a good hypothesis that the variation of  in one T using
the two types of lubricants tested here with different viscosities
leads to similar average friction force and slip. The independence ofthe motor performance on lubricant viscosity also illustrates thatthe viscosity variation of the lubricants at a relatively wide temperature range in continuous motor operation will not affect thecharacteristics of lubricated USMs.

5. Conclusion
In this paper, we experimentally examined the lubricating
effects of both low-traction and high-traction ﬂuids with a wide
range of viscosity in HTUSM. The results clearly show that hightraction ﬂuids have signiﬁcant superiority to low-traction ﬂuids
in terms of motor efﬁciency and reduction of friction loss at theentire range of the applied preload. It is also found that the motor
performance is independent of lubricant viscosities in the tested
range, irrespective of lubricant types. The experimental results
were qualitatively analyzed in detail, indicating that hydrodynamic
lubrication can hardly be achieved in lubricated USMs, and the
motor mainly operated in boundary and mixed lubrication regimes.
Low-traction ﬂuids showed low friction coefﬁcient  in boundary
lubrication regime and large slip, resulting in high friction loss andlow motor efﬁciency. In contrast, using high-traction ﬂuids, thefriction modulation remained efﬁcient even under high preloads
in addition to high  in boundary lubrication regime and the slip
between the rotor and the stator was small, leading to low friction loss and high efﬁciency. This study conﬁrmed that high  inboundary lubrication regime and the efﬁcient modulation of thefriction force are essential for achieving high-performance lubricated USMs, which should be the criteria for selecting lubricantsfor USMs. Since it may be difﬁcult to increase  in boundary lubrication regime for high-traction lubricants due to the potential
severe wear, further improvement in motor performance can still
be expected if the  value in mixed lubrication regime is reduced.
The results obtained in this work also provide a step forward inunderstanding the complex lubrication mechanisms in USMs.

Acknowledgements
This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 15K13896 and a
Grant-in-Aid for JSPS Research Fellows (No. 26011842). The authors
are indebted to Dr. Yoji Sunagawa (Drive Train Lubricants Group,
Lubricants Research Laboratory, Idemitsu Kosan Co., Ltd.) for theinsightful discussions and providing all the lubricants. They also
appreciate the staff members of the Precision and Manufacturing
Center (Technical Department, Tokyo Institute of Technology) for
machining the metal components of the motors used in the experiment.

References[1] S. Ueha, Y. Tomikawa, Ultrasonic Motors – Theory and Applications, Oxford
Science Publications, New York, 1993.
[2] K. Uchino, Piezoelectric Actuators and Ultrasonic Motors, Kluwer Academic
Publisher, New York, 1996.
[3] T. Maeno, T. Tsukimoto, A. Miyake, Finite-element analysis of the rotor/stator
contact in a ring-type ultrasonic motor, IEEE Trans. Ultrason. Ferroelectr. Freq.
Control 39 (1992) 668–674.
[4] T. Maeno, I. Okumura, T. Tsukimoto, T. Mori, How to make high-efﬁciency
ultrasonic motors, Proc. Intl. Conf. Acoustics (2004) I409–I412.
[5] T. Morita, M. Kurosawa, T. Higuchi, Design of a cylindrical ultrasonicmicromotor to obtain mechanical output, Jpn. J. Appl. Phys. 35 (1996)
3251–3254.
[6] M.K. Kurosawa, O. Kodaira, Y. Tsuchitoi, T. Higuchi, Transducer for high speed
and large thrust ultrasonic linear motor using two sandwich-type vibrators,
IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45 (1998) 1188–1195.
[7] M. Aoyagi, Y. Tomikawa, Improvement in performance of longitudinal andtorsional vibrator combination-type ultrasonic motor, Jpn. J. Appl. Phys. 38(1999) 3342–3346.
[8] J. Satonobu, D. Lee, K. Nakamura, S. Ueha, Improvement of the longitudinal
vibration system for the hybrid transducer ultrasonic motor, IEEE Trans.
Ultrason. Ferroelectr. Freq. Control 47 (2000) 216–221.
[9] M. Takano, M. Takimoto, K. Nakamura, Electrode design of multilayered
piezoelectric transducers for longitudinal-bending ultrasonic actuators,
Acoust. Sci. Technol. 32 (2011) 100–108.
[10] F.R.M. Romlay, W.A.W. Yusoff, K.A.M. Piah, Increasing the efﬁciency oftraveling wave ultrasonic motor by modifying the stator geometry,
Ultrasonics 64 (2016) 177–185.
[11] K. Nakamura, M. Kurosawa, H. Kurebayashi, S. Ueha, An estimation of load
characteristics of an ultrasonic motor by measuring transient responses, IEEE
Trans. Ultrason. Ferroelectr. Freq. Control 38 (1991) 481–485.
[12] K. Uchino, Piezoelectric ultrasonic motors: overview, Smart Mater. Struct. 7(1998) 273–285.
[13] T. Ishii, T. Shinkoda, S. Ueha, K. Nakamura, M. Kurosawa, Efﬁciency
improvement of an ultrasonic motor driven with rectangular waveform, Jpn.
J. Appl. Phys. 35 (1996) 3281–3285.
[14] T. Ishii, H. Takahashi, S. Ueha, et al., A low-wear driving method of ultrasonicmotors, Jpn. J. Appl. Phys. 38 (1999) 3338–3341.
[15] A. Glazounov, S. Wang, Q. Zhang, C. Kim, High-efﬁciency piezoelectric motor
combining continuous rotation with precise control over angular positioning,Appl. Phys. Lett. 75 (1999) 862–864.
[16] K. Nakamura, T. Ito, M. Kurosawa, S. Ueha, A trial construction of an ultrasonicmotor with ﬂuid coupling, Jpn. J. Appl. Phys. 29 (1990) L160–L161.
[17] Y. Yamayoshi, Ultrasonic motor not using mechanical friction force, Int. J.
Appl. Eletromagn. Mater 3 (1992) 179–182.
[18] T. Yamazaki, J. Hu, K. Nakamura, S. Ueha, Trial construction of a noncontact
ultrasonic motor with an ultrasonically levitated rotor, Jpn. J. Appl. Phys. 35(1996) 3286–3288.
[19] K. Nakamura, M. Maruyama, S. Ueha, A new ultrasonic motor using
electro-rheological ﬂuid and torsional vibration, Ultrasonics 34 (1996)
261–264.
[20] W. Qiu, Y. Hong, Y. Mizuno, W. Wen, K. Nakamura, Non-contact piezoelectricrotary motor modulated by giant electrorheological ﬂuid, Sens. Actuators A –Phys. 217 (2014) 124–128.
[21] R. Stribeck, Die wesentlichen eigenschaften der gleit-und rollenlager, Z. Ver.
Deutsch. Ing. 46 (1902) 1341–1348, 1432–1438, 1463–1470.
[22] W. Qiu, Y. Mizuno, D. Koyama, K. Nakamura, Efﬁciency improvement ofhybrid transducer-type ultrasonic motor using lubricant, IEEE Trans. Ultrason.
Ferroelectr. Freq. Control 60 (2013) 786–794.
[23] W. Qiu, Y. Mizuno, M. Tabaru, K. Nakamura, Can lubricant enhance the torqueof ultrasonic motors? An experimental investigation, Appl. Phys. Lett. 105(2014) 224102, 1–4.
[24] W. Qiu, Y. Mizuno, K. Nakamura, Tribological performance of ceramics inlubricated ultrasonic motors, Wear 352 (2016) 188–195.
[25] G. Stachowiak, A.W. Batchelor, Engineering Tribology, fourth ed.,Butterworth-Heinemann, Oxford, 2013.
[26] K. Nakamura, M. Kurosawa, S. Ueha, Characteristics of a hybridtransducer-type ultrasonic motor, IEEE Trans. Ultrason. Ferroelectr. Freq.
Control 38 (1991) 188–193.
[27] K. Nakamura, M. Kurosawa, S. Ueha, Design of a hybrid transducer typeultrasonic motor, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 40 (4) (1993)
395–401.
[28] H. Hirata, S. Ueha, Characteristics estimation of a traveling wave typeultrasonic motor, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 40 (1993)
402–406.

Biographies
Wei Qiu received the B.E. degree in electrical engineering from Nantong University, Nantong, China, in 2009, M.Sc degree in electrical and electronic engineeringfrom The University of Hong Kong, Hong Kong, China, in 2010, M.E. and Dr.Eng.
degrees in information processing (specialized in ultrasonics) from Tokyo Instituteof Technology, Yokohama, Japan, in 2012 and 2015, respectively. He stayed at Tri-

W. Qiu et al. / Sensors and Actuators A 282 (2018) 183–191
bology Group, Department of Mechanical Engineering, Imperial College London, UK,
as a visiting student in 2014. From 2014 to 2015, he was a Research Fellow (DC2)of the Japan Society for the Promotion of Science (JSPS). From 2015 to 2016, heworked at the Precision and Intelligence Laboratory (presently, Institute of Innovative Research), Tokyo Institute of Technology, as a Research Fellow (PD) of JSPS.
Since 2016, he has become an H.C. Ørsted COFUND Postdoc Fellow (co-funded withMarie Curie Actions) at Department of Physics, Technical University of Denmark. His
research interests include microscale acoustoﬂuidics as well as ultrasonic motors
and actuators. Dr. Qiu is the recipient of the Young Scientist Award at the Symposium on Ultrasonic Electronics in 2013, the RWB Stephens Prize at 2015 International
Congress on Ultrasonics, the Seiichi Tejima Doctoral Dissertation Award in 2016, andthe Terrence Coakley Poster Award at Acoustoﬂuidics 2018.
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 involved 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 in the Precision and Intelligence Laboratory(presently, Institute of Innovative Research), Tokyo Institute of Technology, where
he is active in ﬁber-optic sensing, polymer optics, and ultrasonics. Dr. Mizuno is
the winner of the Ando Incentive Prize for the Study of Electronics 2011, the Konica
Minolta Imaging Science Award 2014, the Japanese Society of Applied Physics (JSAP)
Young Scientist Presentation Award 2015, and the Optical Society of Japan (OSJ)
Optics Paper Award 2017, the Optical Design Award 2018. He is a Senior Member of
the IEEE Photonics Society and a member of the JSAP, the OSJ, and the Institute ofElectronics, Information, and Communication Engineers of Japan.
Koshi Adachi graduated in Mechanical Engineering from Tohoku University in 1988
and obtained his Ph.D. degree for research in Tribology from Tohoku University,

191

Japan, in 1998. He started to work as a research associate in 1990, and he is currently a full professor at the Faculty of Engineering, Tohoku University. He is the headof Laboratory of Tribology and Nanointerface Engineering and the director of Center for Tribologically-based Machine Design, in Division of Mechanical Engineering,Tohoku University. His research interests span a wide range of tribology, including
fundamentals and applications of tribology, with a particular interest in friction andwear mechanisms of advanced materials, technologies for super-low friction suchas surface texturing and new coatings, science and technology for running-in control and functional interface (nanointerface) formed during friction automatically,
environmentally friendly mechanical systems with water and gas lubrication, friction drive with ultrasonic and surface acoustic wave for precise positioning systems
as well as control of tribo-chemical reaction. Prof. Adachi has won eighteen awards
covering different aspects of his research, including the Best Paper Awards fromThe Japan Society of Mechanical Engineers in 1999 and from Japanese Society of Tribologists in 2003, 2014, and 2018. He serves as the editorial board member for six
international journals, such as International Journal of WEAR, Lubrication Science,and so on.
Kentaro Nakamura received the B.E., M.E., and Dr.Eng. degrees from Tokyo Instituteof Technology, Japan, in 1987, 1989, and 1992, respectively. Since 2010, he has beena Professor in the Precision and Intelligence Laboratory (presently, Institute of Innovative Research), Tokyo Institute of Technology. His research ﬁeld is the applicationsof ultrasonic waves, measurement of vibration and sound using optical methods, andﬁber-optic sensing. Prof. Nakamura is the winner of the Awaya Kiyoshi Award for
Encouragement of Research from the Acoustical Society of Japan (ASJ) in 1996, andthe Best Paper Awards from the Institute of Electronics, Information and Communication Engineers (IEICE) in 1998 and from the Symposium on Ultrasonic Electronics
in 2007 and 2011. He also received the Japanese Journal of Applied Physics Editorial
Contribution Award from the Japan Society of Applied Physics (JSAP) in 2007, theOptical Design Award 2018. He is a member of the IEEE, the ASJ, the JSAP, the IEICE,
and the Institute of Electrical Engineers of Japan. He served as the president of ASJ
from 2015 to 2017.