BRIEF NOTEJapanese Journal of Applied Physics 55, 018001 (2016)
http://doi.org/10.7567/JJAP.55.018001

Traveling wave ultrasonic motor using polymer-based vibrator
Jiang Wu*, Yosuke Mizuno, Marie Tabaru, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan*E-mail: wujiang@sonic.pi.titech.ac.jp
Received September 25, 2015; accepted October 19, 2015; published online November 25, 2015
With the characteristics of low density, low elastic modulus, and low mechanical loss, poly(phenylene sulﬁde) (PPS) is a promising material for
fabricating lightweight ultrasonic motors (USMs). For the ﬁrst time, we used PPS to fabricate an annular elastomer with teeth and glued a piece ofpiezoelectric-ceramic annular disk to the bottom of the elastomer to form a vibrator. To explore for a material suitable for the rotor surface coming incontact with the PPS-based vibrator, several disk-shaped rotors made of different materials were fabricated to form traveling wave USMs. The
polymer-based USM rotates successfully as the conventional metal-based USMs. The experimental results show that the USM with the aluminumrotor has the largest torque, which indicates that aluminum is the most suitable for the rotor surface among the tested materials.
© 2016 The Japan Society of Applied Physics

Having the characteristics of low speed, high torque, andquick response, ultrasonic motors (USMs) have been applied
to robots and optical instruments.1–15) A lightweight andpowerful actuator is highly required to decrease the weightof the mechanical system and increase operability. Using
polymers as the main body of USMs is a potentially feasible
method to obtain a lightweight USM because some newly
invented functional polymers have low densities and excellent workability.16–19) However, to the best of our knowledge,polymers have not been used as the elastomer in USMs
because polymers usually have large attenuations for ultrasounds. In our recent report, through the mechanical-loss
evaluation of some functional polymers, we concluded thatthe mechanical quality factor of poly(phenylene sulﬁde)(PPS) reaches 450,20) which indicates that PPS is potentiallyapplicable to USMs. In this study, we used PPS as theelastomer to fabricate an annular vibrator and glued a pieceof ring-shaped piezoelectric ceramic element to the bottomof the PPS-based elastomer to form a vibrator. When twovoltage sources with a 90° phase diﬀerence are applied,a traveling wave is generated on the vibrator, and thus, therotor pressed on the top of the vibrator is driven to rotate.
Because the surface of the rotor coming in contact with thePPS-based vibrator also has inﬂuence on the motor performance, we tried using some typical metals and functional
polymers for rotors and experimentally evaluated the mechanical characteristics of the USMs in order to ﬁnd thematerial suitable for the rotor surface in contact to the PPSbased vibrator.
Figures 1(a) and 1(b) show the structure of the USM andthe dimensions of the vibrator. The elastomer of the vibrator
is made of PPS, whose mechanical parameters are listed inTable I. One piece of the ring-shaped piezoelectric ceramicelement (Fuji Ceramics C213) with a thickness of 1 mm and adiameter of 28 mm, is glued to the bottom of the elastomerusing epoxy. The outer diameter of the vibrator is 28 mm andthe width of the contacting part is 3 mm. 36 teeth with 2 mm
depth are formed for the contacting part. The method of
exciting traveling waves and the working principle of themotion are the same as those of the conventional metal-based
USM. As Fig. 1(c) shows, the electrodes are divided intoten parts along the circumference, which is as long as ﬁve
wavelengths. The polarization directions alternate every half
wavelength and two parts with 1=4 and 3=4 wavelengths areset without polarization. Being separated by the 1=4- andFig. 1. (Color online) (a) Structure of the traveling wave rotary USM,(b) dimensions of the PPS-based ring-shaped vibrator with teeth,(c) polarization of the piezoelectric ceramic element, and (d) 4th-order
bending mode of the vibrator.

Table I. Mechanical parameters of the tested PPS.
Density (103 kg=m3)

1.35

Elastic modulus (GPa)

3.45

Poisson’s ratio

0.36

3=4-wavelength slots, the electrodes are divided into twogroups. With two channels of sinusoidal driving voltages
applied to the two groups, two standing waves with a90° phase diﬀerence in space are generated. Figure 1(d)shows the 4th-order bending mode, which is generated on
the vibrator in the experiments stated later. When thephase diﬀerence between the voltages is set to 90°, travelingwaves are generated. Thus, a rotor pressed on the top of
the vibrator is driven to rotate by the traveling waves. To
select the material suitable for the rotor is also an objectiveof this study. The disk-shaped rotors were fabricated using
poly(ether ether ketone) (PEEK), polyacetal (POM), PPS,aluminum, and stainless steel, whose structures are shown in
Fig. 1(a).

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J. Wu et al.

(a)Fig. 2. Vibration velocity at one point at external edge of the vibrator as afunction of driving frequency.

(b)
Fig. 4. (Color online) (a) Position of vertical-vibration-velocity
measurement and (b) vibration velocity distribution on external
circumference.
Fig. 3. Angular velocity as a function of phase diﬀerence between twochannels of voltages.

The vibration characteristics of the polymer-based vibrator
were measured. Using a laser Doppler velocimeter (PolytecNLV1232), the vertical vibration velocities were measured
at one point of the outer edge by sweeping the frequencies
from 28.70 to 28.95 kHz. Figure 2 shows the dependence ofvertical vibration velocity on driving frequency at a zero-topeak voltage of 180 V. The vibration velocity reaches its peakat a driving frequency of 28.82 kHz. The mechanical quality
factor Q of the vibrator is calculated to be 262 using
Q¼f0;
f

ð1Þ

where f0 and Δ f represent the resonance frequency andbandwidth for 1=√2 of the amplitude, respectively.
Next, we placed a PPS-based rotor on the vibrator to determine whether a traveling wave is generated on the vibrator.
When the phase between the two voltages was varied, therotation velocity was measured using a high-speed camera(Integrated Designed Tools M5) at a driving frequency of
28.82 kHz and a zero-to-peak voltage of 180 V. Similarly tothe conventional USM with the metal-based vibrator, theprototype USM rotated clockwise and counterclockwise as
the phase diﬀerence changes. Figure 3 shows that the angularvelocity reaches maximum in clockwise and counterclockwise directions at the phases of 70 and 260°, respectively.
The maximum speed in the counterclockwise direction isapproximately half of that in the clockwise direction becauseof some irregularities in assembly. At the driving frequencyof 28.82 kHz and the phase of 70°, the vibration velocity
distribution on the circumference of the vibrator was measured. The amplitude and phase were measured using the laser
Dropper velocimeter and a lock-in voltmeter (NF ElectronicInstruments 5560), respectively. The driving voltage forone channel was used as a reference signal to the lock-involtmeter. Figure 4 shows that the phase varies from −180 to
180° almost linearly ﬁve times, which indicates that a traveling wave of ﬁve wavelengths is generated on the vibrator.
On the other hand, the maximum and minimum velocities(vmax and vmin) are 287 and 212 mm=s, respectively. Thus, thestanding wave ratio is 1.35, which indicates that the existenceof a standing wave is satisfactorily small.
To discuss the motor performance, we summarized themechanical characteristics for the USMs with the rotors madeof diﬀerent materials under approximately the same preloads.
The driving frequency, zero-to-peak voltage, and phase difference were set to be 28.82 kHz, 180 V, and 70°, respectively. Figure 5 shows the mechanical characteristics of thetested USMs. The no-load angular velocity of the USM with
the PPS-based rotor reaches 18 rad=s, which is higher thanthose of the other USMs. The angular velocities of the USMs
with the PPS-, POM-, and PEEK-based rotors decrease
sharply as the preload increases and fell to zero when thetorque reaches approximately 200 µNm. The maximum
torques of the USMs with the aluminum- and stainlesssteel-based rotor are 670 and 500 µNm, which are larger thanthose of the USMs with polymer-based rotors. As the USM
with aluminum-based rotor has a larger torque than the other018001-2© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 018001 (2016)

J. Wu et al.

diameter. The vibration velocity of the polymer-based
vibrator decreases sharply as the preload increases and onlya small preload can be applied to this vibrator. This unique
phenomenon for the polymer-based vibrator needs to beinvestigated in the future. Meanwhile, some low-density
piezoelectric materials should be used to reduce the weightof USMs.
Acknowledgment The authors thank Daicel Corporation for providing
some polymer samples tested in this study.

Fig. 5. (Color online) Mechanical characteristics of USMs with the PPS-,POM-, PEEK-, aluminum-, and stainless-steel-based rotors.

tested USMs, aluminum is considered to be the material
suitable for forming the surface of a rotor coming in contact
with the PPS-based vibrator among the tested materials. Themaximum torque is determined by preload, friction coeﬃcient, and arm length (the distance between the action
point and the axis).1) As the arms are the same in length inall experiments and the preload is set to be approximately
the same, the maximum torque should be mainly related tothe eﬀective friction coeﬃcient on the contacting surface
between the vibrator and the rotor. The eﬀective friction
coeﬃcient between the PPS-based stator and the metal-based
rotor is larger than that between the PPS-based stator and thepolymer-based rotor.
In practice, to reduce the weight of the USM, polymers
are suitable choices as the main body of the rotor because of
their low densities. On the other hand, an aluminum annularthin sheet as the friction material should be attached to thebottom of the polymer-based rotor to increase the friction.
In this study, PPS is used as the elastomer in USM andthe weight of the elastic body was largely reduced. However,
the torque of the prototype USM is 0.05 times smaller thanthat of the commercial metal-based USMs with the same

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