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Sensors and Actuators A 235 (2015) 240–255

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

j ourna l ho me page: www.elsev ier .com/ locate /sna

eview

review of long range piezoelectric motors using frequencyeveraged method

uxin Penga, Yulong Pengb, Xiaoyi Gua, Jian Wangc, Haoyong Yua,∗

Department of Biomedical Engineering, National University of Singapore, Singapore 117575, SingaporeSchool of Mechanical & Electrical Engineering, Zhongyuan University of Technology, Zhengzhou 450007, ChinaDepartment of Exercise and Sports Science, Zhejiang University, Hangzhou 310028, China

r t i c l e i n f o

rticle history:eceived 30 March 2015eceived in revised form 25 August 2015ccepted 5 October 2015vailable online 14 October 2015

eywords:ong range

a b s t r a c t

This paper provides a comprehensive review of the literature regarding precision piezoelectric motorsover long ranges based on the principle of repeating a series of small periodic step motions, named “fre-quency leveraged motors” in this paper. A summary of recent research into frequency leveraged motorsis presented. Work is classified into three categories by different frequency driving methods, includingultrasonic motors, quasi-static motors (non-resonant motors), and motors combined resonant and quasi-static operations. Pros and cons of each motor type are discussed in term of their principle, structure,and performance. In addition, future perspectives and improvements of the frequency leveraged motor

iezoelectric motorrequency leveragedltrasonic motor

nchworm mechanismalking drive

are also provided. It is summarized in such a way can provide a better understanding of the core charac-teristics of each type of long range piezoelectric motor. Moreover, it also aids in determining successfuldesigns, suitability for applications and further research areas.

© 2015 Elsevier B.V. All rights reserved.

nertia motormpact friction drive

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2412. Piezoelectric ultrasonic motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

2.1. Principle of the piezoelectric ultrasonic motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2422.2. Standing wave type motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

2.2.1. Different vibration modes of standing wave type motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2422.2.2. Standing wave type motors of different composite vibration modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2432.2.3. Multi-DOF standing wave type motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

2.3. Traveling wave type motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2452.4. Performance analysis and perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

3. Quasi-static motors (non-resonant motors) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2463.1. Clamping and feeding mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

3.1.1. Inchworm mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2463.1.2. Seal mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2483.1.3. Walking drive mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

3.2. Inertia drive mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2493.2.1. Impact drive mechanisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2493.2.2. Smooth impact drive mechanisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .250

3.3. Performance analysis of inertial motors . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Fax: + 65 6872 3069.E-mail address: bieyhy@nus.edu.sg (H. Yu).

ttp://dx.doi.org/10.1016/j.sna.2015.10.015924-4247/© 2015 Elsevier B.V. All rights reserved.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252

Y. Peng et al. / Sensors and Actuators A 235 (2015) 240–255 241

4. Motors combined resonant and quasi-static operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2535. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253Biography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

1

mo[tvim

arcfieeofmaaumddo[(oectma[ttfsms

. Introduction

A piezoelectric motor is a device that creates a linear or rotaryotion by means of converse piezoelectric effect. It aims to move an

bject over a certain distance with a high resolution and accuracy1–6]. Piezoelectric motors with long motion range, high resolu-ion, compact size, high speed and high bandwidth are desired inarious applications, such as the manipulation of biological spec-mens, semiconductor equipment, optical element alignment and

easuring instruments.The piezoelectric motor usually works with the piezoelectric

ctuator to achieve long motion range [6]. Piezoelectric actuatorsefer to actuators made of piezoelectric materials such as lead zir-onate titanate (PZT), which can provide motion with extremelyne resolution (sub-nanometer) but with a small travel range (sev-ral micrometers) [6]. To overcome this drawback, a number offforts have been made for magnifying the output displacementf PZTs. Basically, there are three different displacement ampli-ying methods: internally leveraged method, externally leveraged

ethod, and frequency leveraged method [2]. The internally lever-ged method uses the internal structure of the PZT to generatemplified displacement, while the externally leveraged methodses the external mechanical component to amplify the displace-ent of the PZT. The frequency leveraged motor amplifies the

isplacement by using the frequency performance of the PZT torive the motor in a series of small steps. A typical applicationf the internally leveraged method is the stacked multilayer PZT3–10]. However, the travel range of stacked PZTs is still limitedseveral tens of micrometers order). Thus, to improve displacementf the PZT, externally leveraged mechanisms have been widelymployed using amplification mechanisms. The typical type is theompliant mechanisms, which are flexible monolithic structureshat transmit motion and/or force through elastic body deformation

echanisms. Flexure hinges are usually utilized as a displacementmplifier in these mechanisms to enlarge the stroke of the PZT11–14]. Even though the flexible mechanism can realize nanome-er and relatively large motion ranges, the amplification factor ofhe mechanism stroke is limited, resulting in difficulty achievingurther larger motion ranges (such as millimeter level). In addition,

uch a mechanism for long travel range not only needs a large size ofagnification mechanism, but also requires long PZTs with neces-

ary strokes [15–17]. On the other hand, the actuating technique

Frequency le

Ultrasonic motors Q

Standing wave type Travell ing wave type Clampin

Inchworm mec hani sms

Sea l mec hani sms

Walking mec han

Fig. 1. Classification of freque

of a frequency leveraged motor is to drive the moving elementby repeating the step motion of the PZT itself. By accumulatingthe displacement over many periods of the driving voltage appliedto the PZT, it can generate long range movement through repeti-tion and accumulation of micro-deformations of the PZT. Comparedwith the internally leveraged and externally leveraged methods,frequency leveraged motors can provide longer travel ranges (mil-limeters or more) without increasing the internal structure of thePZT or using external amplification mechanisms. Therefore, fre-quency leveraged motors have great potential to be designed witha simple structure and a compact size, which might be attractivefor long travel range applications in micro-surgery, insect scaledrobots and micro-positioning stages [18].

In this paper, frequency leveraged motors are classified intothree groups by their frequencies of operation: ultrasonic motors,quasi-static motors (also called non-resonant motors), and motorscombined resonant and quasi-static operations. Usually, an ultra-sonic motor utilizes the mechanical resonance at high frequencyto increase output motion, whereas quasi-static motors are oper-ated at low frequency with a stable response [19,20]. The motorcombined resonant and quasi-static operations is an emergingmotor which utilizes the advantages of both operations by mergingthem into a single system. The whole classification system of fre-quency leveraged motors is shown in Fig. 1. Based on different wavepropagation methods, the ultrasonic motors can be classified intostanding wave type motors and traveling wave type motors. On theother hand, quasi-static motors are based on either the clampingdrive principle or the inertia drive principle [19,20]. In the clamp-ing drive principle, the motor is driven by clamping and feedingPZTs step by step. The typical motors of this type include inchwormmechanisms, seal mechanisms, and walking drive mechanisms. Inthe inertia drive principle, inertia and friction force are utilized todrive the motor over a long motion range. Two typical types of iner-tia motors are impact drive mechanisms and smooth impact drivemechanisms.

Correspondingly, this paper is organized as follows. In Section2, piezoelectric ultrasonic motors are classified into standing wavetype motors and traveling wave type motors. In Section 3, quasi-static motors are categorized into two main types. One is clamping

and feeding mechanisms, which include inchworm mechanisms,seal mechanisms, and walking drive mechanisms. The other is iner-tia drive mechanisms, which include impact drive mechanisms and

verag ed motor s

uasi-static motors

g drive Inertial drive

Impact drive mec hani sms

Smooth impact drive mec hanisms

drive isms

Motors combined resonant and qu asi-

static operations

ncy leveraged motors.

242 Y. Peng et al. / Sensors and Actuators A 235 (2015) 240–255

A

BVibratory piece PZT

Slider/rotor

(a)

Sli der/rotor

Stator

Propagation direc tion

Moving direc tion

(b)

Stator

Fw

seaCm

2

2

tdtsco

tPtwmmspbitsar

UgestGs

ig. 2. Principle of piezoelectric ultrasonic motors: (a) principle of the standingave type motor(b) principle of the traveling wave type motor.

mooth impact drive mechanisms. In Section 4, we introduce sev-ral motors combined resonant and quasi-static operations whichttempt to utilize their advantages but mitigate their weaknesses.onclusions and future research directions of frequency leveragedotors are given in Section 5.

. Piezoelectric ultrasonic motors

.1. Principle of the piezoelectric ultrasonic motor

An ultrasonic motor is a motor powered by the ultrasonic vibra-ion excited at resonance. This vibration energy can produce ariving force to a stator. A rotor or slider, placed against the sta-or, can be then driven by using the frictional contact between thetator and the rotor/slider. The displacement of an ultrasonic motoran be accumulated by converting the vibrations into the motionf the rotor/slider.

A piezoelectric ultrasonic motor obtains a standing wave orraveling wave (surfing wave) produced by resonantly excitedZTs. Hence, piezoelectric ultrasonic motors can be divided intowo groups according to wave propagation methods, standingave type motors and traveling wave type motors. To induce theechanical movement for rotating or sliding motion, an ellipticotion of the stator should be formed by mechanical waves. As

hown in Fig. 2(a), the stator of a standing wave type motor is com-osed of a PZT and a vibratory piece. The vibratory piece generatesending and the PZT is excited with alternating voltage, resulting

n elliptical trajectories at the tip of the stator. From A to B, the sta-or contacts to the slider/rotor and thus pushes the slider/rotor forlight movement. From B to A, the stator is released from the rotor,nd no movement is transmitted to the slider/rotor. Therefore, longange traveling can be achieved by repeating the elliptic motion.

Fig. 2(b) shows the principle of a traveling wave type motor.nlike the standing type motor which utilizes one elliptical motionenerated in the stator, every point on the stator face of the trav-ling wave type motor follows an elliptical trajectory. Therefore, a

urface particle of the contact surface of the stator can draw an ellip-ical locus and cause intermittent movement of the slider/rotor.enerally, a traveling wave can be generated by combining of twotanding waves with 90◦ phase difference. Traveling wave in the

Fig. 3. Different vibration modes in ultrasonic motors: (a) circumferential bendingvibration mode, (b) bending vibration mode, (c) radial vibration mode, (d) longitu-dinal/axial vibration mode, (e) torsional vibration mode.

opposite direction can be generated by changing the phase differ-ence to −90◦.

2.2. Standing wave type motors

2.2.1. Different vibration modes of standing wave type motorsSince different resonant vibration modes are the basis of design

for standing wave type motors, the classification according to thevibration modes is thought to essentially reflect the characteristicsof these motors. In general, typical vibration modes used in stand-ing wave type motors can be classified into five major categories asfollows.

(1) Circumferential bending vibration mode

Fig. 3(a) shows the circumferential bending vibration mode of amembrane-like stator. The stator is usually coated with an excitingPZT. When the PZT is excited at the resonant mode, the axial dis-placement of the stator vibrates as a function of the circumferential.Motors of Sashida’s type belong to this category.

(2) Bending vibration mode

Bending vibration mode is characterized by bending deforma-tions perpendicular to a longitudinal axis of the stator (see Fig. 3(b)).

(3) Radial vibration mode

As shown in Fig. 3(c), in the radial vibration mode, the displace-ment of the stator is in the radial direction. Generally, to achieveelliptic motion, radial modes are always combined with bendingmodes.

(4) Longitudinal/axial vibration mode

The displacement of the longitudinal vibration mode is in theaxial direction (see Fig. 3(d)). Longitudinal vibration modes areusually combined with torsional or bending modes.

Y. Peng et al. / Sensors and Actuators A 235 (2015) 240–255 243

(

vtw

2m

utt

(

ceisTXp

bbTapom

atifbdFtstoT

Fig. 4. Composite orthogonal bending vibration modes.

5) Torsional vibration mode

Fig. 3(e) shows the mode of the torsional vibration. It is angularibration excited by shear deformation of the upper part of the sta-or. Torsional vibration modes are commonly used in combinationith other orthogonal modes.

.2.2. Standing wave type motors of different composite vibrationodes

In general, the elliptical motion of standing wave type motors issually created by coupling at least two vibration modes. Therefore,he standing wave type motors in this paper are classified based onhe way of combination of different vibration modes.

1) Motors using orthogonal bending vibration modes

As shown in Fig. 4, two orthogonal bending modes of the beaman be isolated by driving the PZT at the correct frequencies. Forxample, a single PZT with multiple electrodes can be employedn this structure. When the electrodes are driven by two drivingignals simultaneously, two orthogonal bending modes are created.herefore, the tip of the beam rotates with an elliptical path in the-Y plane. The rotation direction can be reversed by reversing thehase of the electrical sources.

The first bending vibration type ultrasonic motor was developedy Kurosawa et al. [21]. In their research, the motor was constructedy using bending vibrations of a short cylinder with free–free ends.he motor was designed to have a size of 20 mm in diameter andbout 75 mm in length. It could also realize a high mechanical out-ut of more than 1 W. Its revolution speed without load was 200r 300 r/min. This motor had been widely used in the auto focusingodel of Canon’s “EOS” camera series.Subsequently, Morita et al. used a thin-film PZT deposited by

hydrothermal method onto the surface of a titanium tube andhen successfully developed a cylindrical ultrasonic motor (2.4 mmn diameter and 10 mm in length) [22]. Electrodes were formed inour places on the PZT layer in a circumferential direction to exciteending vibration modes. The rotor on the stator could achieve bi-irectional movement with maximum revolution speed of 295 rpm.urthermore, Morita et al. improved this design in 2000 [23]. Thehin-film PZT was deposited by the hydrothermal method on a

mall tube as before. By using an “improved nucleation process”,he dimensions of the stator were miniaturized to be 1.4 mm inuter diameter, 1.2 mm in inner diameter, and 5.0 mm in length.herefore, with a 17% volume of the previous motor, the smaller

Fig. 5. Longitudinal-bending composite vibration modes.

motor can achieve a maximum velocity of 680 rpm and a maximumtorque of 0.67 �Nm.

In collaboration with Samsung Electromechanics, Korea, Kocet al. [24] proposed an alternative bending mode motor by usingtwo orthogonal bending modes of a hollow cylinder. The outsidesurface of the cylinder is flattened on two sides at 90◦ to each other.Two rectangular piezoelectric plates were bonded on each surface,respectively. Since the cylinder combined the circular and squarecross sections, the stator had two degenerated bending modes ofslightly different resonance frequencies. Therefore, both modescould be excited when only one PZT plate was excited at a frequencybetween the above two bending modes. Then, an elliptical motioncould be generated at the stator tip, and the rotation direction couldbe reversed by exciting the other PZT element. Another commercialproduct is the SQUIGGLE motor developed by New Scale Technolo-gies [25]. Four PZTs of the motor were attached to a nut (stator),with a mating screw (shaft) inside. By using orthogonal bendingmodes driven by the PZTs, a wobbling motion could be created inthe nut, which could be converted to a linear displacement throughthe screw. According to Newscale’s reports, the SQUIGGLE motorhas found applications in medical devices, lab-on-chip microfluidicpumps, and micro optical instruments.

(2) Motors using longitudinal-bending composite vibration modes

The working principle of a longitudinal-bending ultrasonicmotor is shown in Fig. 5. By combining a longitudinal and a bend-ing vibration mode, a desired elliptic motion can be produced.Tomikawa et al. proposed a linearly self-moving longitudinal-bending ultrasonic motor in 1989 [26]. The first longitudinal andfourth bending modes were utilized to form a multi-mode vibrator.Therefore, elliptic motion in the same direction could be generatedat both ends of the vibrator. In 1990, Ohnishi et al. developed a�-shaped linear ultrasonic motor [27]. Two multilayer PZTs werefixed at the two leg parts of the �-shaped frame. When the twoPZTs were driven with a 90◦ phase difference, longitudinal and lat-eral bending modes of the leg parts could be stimulated. As a result,an elliptical motion was synthesized to drive the guide rails of themotor.

To realize miniaturization of ultrasonic motors, Suzuki et al. [28]developed a new type piezoelectric micro motor which employeda flat spring to adjust contact conditions. The structure had minia-

turized the motor to as small as 2 mm in diameter and 0.3 mm inheight. The stator with three cantilever oscillators glued to threePZTs had a flat configuration. When a PZT stimulated vibration andflexion of the cantilever oscillator, elliptical movements at the free

244 Y. Peng et al. / Sensors and Actuat

et11

mcPodt8

(

utsoewtoftfUm

cdwdov

tst

Fig. 6. Longitudinal-torsional composite vibration modes.

nd of the cantilever oscillator could be created and transferredo the rotor. The micro motor could be operated at approximately800 rpm at 14 Vp-p when the contact pressure was set to be.6 MPa.

A different use of a longitudinal-bending coupling vibrationode was proposed by Aoyagi et al. [29]. In their design, the stator

onsisted of a stainless steel vibrator sandwiched between two thinZT plates. As the PZTs excited the fundamental axial and secondrder bending modes in the vibrator, an elliptical motion could beelivered at the contact point with the rotor. With a stator vibra-or 0.55 mm thick, a shaft 1.5 mm in diameter could be driven over000 rpm.

3) Motors using longitudinal-torsional composite vibrationmodes

Fig. 6 shows the operation principle of a longitudinal-torsionalltrasonic motor. Longitudinal vibration in the axial direction andorsional vibration in the circumferential direction are excitedimultaneously. As a result, an elliptic motion is generated at the tipf the stator. An example of this class was developed by Kurosawat al. in 1991 [30]. The stator consisted of two kinds of PZTs whichere driven by separate electric signals. One PZT was operated in

he torsional vibration mode to drive the rotor, and the other wasperated in the longitudinal vibration mode to control the frictionalorce. The vibration displacement of the latter is perpendicular tohat of the former, and they were excited simultaneously. There-ore, an elliptical motion was produced at the end of the stator.nloaded speed of the motor was measured to be 100 rpm, and theaximum output torque was 7 kg cm.The design by Watson et al. [31] is a recent example of this

lass of motors. The motor used coupled axial-torsional vibrationerived from a helically cut stator. The stator was a metallic tubeith diameter of 0.25 mm and length of 1 mm. The motor could beriven at a maximum angular velocity of 1295 rpm with a torquef 13 nNm. The proposed micro motor could be used in areas of “inivo” surgery and micro-robotics.

Compared with other kinds of ultrasonic motors, a longitudinal-orsional hybrid ultrasonic motor has merits of larger torque andtability at lower speeds. It has good controllability and driveshe motor precisely, so it is suitable for use in some potential

Fig. 7. Radial-bending comp

ors A 235 (2015) 240–255

fields, such as aeronautics and astronautics. However, the coupledaxial/torsional hybrid design is complex and difficult to fabricate,making them unsuitable for use as a micro/milli-scale motor.

(4) Motors using radial-bending composite vibration modes

Compared with the longitudinal-bending ultrasonic motors, aradial mode of a radial-bending ultrasonic motor is used instead ofthe longitudinal mode. As shown in Fig. 7, combined with a bendingvibration mode, a radial-bending ultrasonic motor usually employsa ring-like structure to vibrate in the radial direction. Therefore,based on the similar principle as a longitudinal-bending motor,the elliptic motion is generated in a specified direction which isdependent on the bending vibration mode.

An example of a radial-bending spherical motor was proposedby Aoyagi et al. [32]. In their research, the motor was designed witha sandwich structure in which a spherical rotor was held by two sta-tors. A disk shaped vibrator could excite a radial vibration mode andtwo bending vibration modes. The three vibration modes were per-pendicular to each other. By combining the radial vibration modeand one bending mode, the spherical rotor could rotate around X-or Y-axis. In addition, Z-axis rotation could be generated by com-bining the two bending vibration modes. The spherical motor wassuitable for use in multi-degree-of-freedom (multi-DOF) joints ofrobots or manipulators.

In 2011, Lu et al. put forward a rotor-embedded-type sphericalmotor [33]. Unlike the former sandwich structure, a single annularstator vibrator was fabricated with a spherical inner surface in theirnovel design. The rotor is embedded into the center of stator torotate around three axes. With a monolithic construction, the motorused fewest components to realize a simple, lightweight, compactstructure. Compared to the sandwich-type motor, this novel onecould eliminate harmful vibration without any support and preloadcomponents for the stator.

In addition, there are standing wave type motors based onsome other composite vibration modes such as torsional-bendingvibration mode [34], longitudinal-shear vibration mode [35] andradial-torsional vibration mode [36]. Therefore, compared with thetraveling wave type motors, standing wave type motors are supe-rior in their various design and construction. The structure of themis less complicated and more suitable for miniaturization. It shouldbe noted that the comparatively large outputs of this type areobtained by the piezoelectric strain amplified at resonance, whichvaries with different vibration modes. The speed of operation is alsodetermined by the resonant conditions. The service life operated atresonance of the motors should also be considered during design.

2.2.3. Multi-DOF standing wave type motorsAs elaborated above, a single-DOF motion is excited by a cou-

pling vibration mode, which usually needs two resonance vibrationmodes excited simultaneously. On the other hand, a multi-DOF

ultrasonic motor is driven by a single stator with several cou-pling vibration modes. It is required multiple resonance vibrationmodes (three or more) to generate several different couplingvibration modes (two or more), resulting in multi-DOF motion

osite vibration modes.

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Y. Peng et al. / Sensors and

orrespondingly. Therefore, compared with the multi-DOF motiony superposing several single-DOF motors, a multi-DOF ultrasonicotor has a simple and compact structure, which is desired in manyicro-mechatronic systems.Considerable work has been done in an attempt to realize

ulti-DOF ultrasonic motors. The most common multi-DOF ultra-onic motors are planar, cylindrical and spherical types. Planarype ultrasonic motors, also known as surface motors, can gen-rate two (XY motion) or three (XY� motion) DOFs in a flat andamping free surface. They are preferred in the applications oflanar micro-positioning. Many researchers used only composite

ongitudinal-bending vibration modes to move the motor straightn any direction on the XY plane [37,38]. A cylindrical type ultra-onic motor is a 2-DOF motor which can move along and around the-axis. A lot of research has been done to realize linear and rotaryotion. One example of this was proposed by Iwatsuki et al. [39].

he stator reported in their research could move a shaft along andround the Z-axis by combining different longitudinal and bend-ng modes of the corresponding PZTs. However, a pair of identicalltrasonic motors was employed in the design and might increasehe total volume. In 2009, a compact ultrasonic motor with a cubictator with a through-hole was proposed by Mashimo et al. [40].he stator consists of a metallic cube with four PZTs adhered to itsides. The rotary motion was generated by a vibration mode whichxcited three waves along the circumference of the through-hole.he linear motion was generated by combining the first longitu-inal mode and the second longitudinal mode. Later, the designas also miniaturized by Mashimo et al. in 2014 [41]. The volume

f stator was design to be approximately 1 mm3, which was one ofhe smallest among currently existing motors. Spherical type ultra-onic motors are widely used in spatial positioning applications. Aall is used in this type as the rotor. There are several methodso construct a spherical type motor. Takemura et al. developed aar-shaped ultrasonic motor capable of generating rotations of apherical rotor around X-, Y- and Z-axis [42]. Rotation around the X-r Y-axis was excited by combining a longitudinal vibration modend a bending vibration mode, while the rotation around Z-axisould be produced when the two bending modes (one in Z-X planend one in Y-Z plane) were combined. In Aoyagi et al. research43], three bending vibrations were applied to rotate a sphericalotor. Rotations in three orthogonal directions could be realized byombining two of the three bending vibrations. Aoyagi et al. alsoodified this design by using two composite radial-bending vibra-

ion modes to rotate the spherical rotor around the X- or Y-axis, andy combining two bending vibration modes to generate Z-axis rota-ion [32]. They also further advanced the motor to be an all-in-onetructure in 2011 [33].

Tables 1 and 2 summarize the performance data of the reviewedtanding wave type motors. Classification in such a way can pro-ide a good understanding of the operating principle, construction,pplications and performance of standing wave type motors.

.3. Traveling wave type motors

The most famous type of the traveling wave type motors arenown as “surfing” motors. In the influential work of Sashida [44],he travelling wave was induced by a thin piezoelectric ring. A ring-haped elastic body was bonded to the piezoelectric ring. A ring-haped rotor in contact with the elastic body could be driven in bothirections by exchanging the sine and cosine voltage inputs. Theotor could be driven continuously by the traveling wave and thebrasion on the contact surface could be decreased. Compared with

onventional electromagnetic motors, this motor could be silentlyriven with a propagating wave frequency of 44 kHz without anyeduction mechanism such as gears. In addition, the thin design ofhe motor made it suitable for integrating into a camera as an auto-

ors A 235 (2015) 240–255 245

focusing mechanism. In 1985, Canon installed the motor compactlyin its EF 300 mm f/2.8 L lens. Most of Canon’s “EOS” camera serieshave already been replaced by the ultrasonic motor of Sashida’stype [45].

The “surfing” traveling wave type motors can provide highspeed and high output force operation due to the small amplitudeand high frequency waves. However, the complex design limitstheir miniaturization to a scale of a few millimetres. For example,without a sufficient buffer gap between the adjacent electrodes,electrical poling process easily initiates cracks on the electrode gapdue to the residual stress concentration.

Another type of traveling wave type motor is the surface acous-tic wave (SAW) motor [46–48]. A SAW, also known as a Rayleighwave, is an acoustic wave traveling along the surface of an elas-tic material. It is a coupled wave of the longitudinal wave andthe shear wave which has normal displacement component to aboundary. As it moves across the surface, each surface point in theelastic medium moves along an elliptical locus. The first SAW motorwas reported by Kurosawa et al. in 1996 [46]. A 127.8◦ Y-rotatedX-propagating piezoelectric substrate (LiNbO3) with two pairs ofinterdigital transducers (IDT’s) was used for the SAW propagation.By exciting SAW in X- or Y-axis direction, a slider can be drivento move two-dimensionally in the plane. The driving frequencyof the planner motor was around 10 MHz, which could producea maximum transfer speed of 200 mm/s. The miniaturization of theSAW motor was investigated by Shigematsu and Kurosawa in 2006[47]. In their study, the stator of the motor was miniaturized to be3 mm × 12.5 mm × 0.5 mm using a 100 MHz driving frequency. Anarbitrary axis rotating SAW micro motor was reported by Tjeunget al. [48]. To achieve arbitrary rotating motion, the substrate mate-rial of the motor was PZT rather than LiNbO3 due to the anisotropicnature of the material. A micro metal sphere as the rotor was placedonto a blind hole drilled on the PZT surface. It was verified that themaximum rotation speed and torque achieved were 1000 rpm and14 �Nmm respectively.

SAW motors are suitable for precision positioning (nanometer)due to their very high-frequency SAW driving mechanisms. Theyalso have great potential for high speed, high output force and quickresponse operation in a wide range of applications. Operating fre-quency of SAW motors is far higher than the common ultrasonicmotors. It means that the operating wavelength of a SAW motoris far shorter and more convenient for miniaturization. However,further miniaturization is also restricted due to the necessity tofabricate complex IDT’s. In addition, the efficiency of SAW motorsis still not so high, which is less than 10% at present. High wearrate induced by high-frequency operation is another hurdle to theircommercialization.

2.4. Performance analysis and perspective

Compared with other types of motors, especially conven-tional electromagnetic motors, piezoelectric ultrasonic motorshave many merits, including lightweight, compact size, high rev-olution (rotational types) or speed (linear types), self-brakingwithout power, no noise, and electromagnetic immunity. Due tohigh driving torque at low speed, ultrasonic motors need no decel-eration mechanism. Therefore, by using piezoelectric ultrasonicmotors, micro mechanical systems can be realized for precise posi-tioning over long strokes. The demerits of ultrasonic motors includenecessity for a high frequency power supply, drooping torque-speed characteristics, and control complexity due to multiple input

signals. It is required that the motor should have special frequency,amplitude and phase of driving signals. The driving signals shouldbe adjusted appropriately to keep a stable output when the oper-ational environments change, such as temperature, humidity and

246 Y. Peng et al. / Sensors and Actuators A 235 (2015) 240–255

Table 1Performance of standing wave type motors.

Reference Year Vibration mode Stator type Stator size (mm) Velocity Output

Kurosawa et al. [21] 1989 Orthogonal bending Sandwich type ˚20 × 16 300 rpm 150 mNmMorita et al. [22] 1995 Orthogonal bending Thin-film PZT on tube ˚2.4 × 10 295 rpm N/AMorita et al. [23] 2000 Orthogonal bending Thin-film PZT on tube ˚1.4 × 5 680 rpm 670 nNmKoc et al. [24] 2002 Orthogonal bending Tube with two flattened sides ˚2.4 × 10 360 rpm 1.8 mNmTomikawa et al. [26] 1989 Longitudinal-bending Flat rectangular type 50 × 10 × 3.6 240 mm/s 25 mNmOhnishi et al. [27] 1990 Longitudinal-bending �-shaped ≈36 × 15 × 5 300 mm/s 10 NSuzuki et al. [28] 2000 Longitudinal-bending Three cantilevers glued to PZTs ˚2.4 × 10 1800 rpm 3.2 mNmAoyagi et al. [29] 2004 Longitudinal-bending Thin plate with a vibrating piece 16.2 × 2.5 × 0.55 8000 rpm 0.06 mNmKurosawa et al. [30] 1991 Longitudinal-torsional Sandwich type ˚50 × 82 100 rpm 686 mNmWatson et al. [31] 2009 Longitudinal-torsional Helically cut tube ˚0.24 × 0.88 1295 rpm 13 nNmAoyagi et al. [32] 2004 Radial-bending Disk type ˚60 × 1 N/A 91 mNmLu et al. [33] 2011 Radial-bending Single annular stator ˚58 × 14 N/A 15 mNm

Table 2Performance of multi-DOF standing wave type motors.

Reference Year DOF Vibration mode Size (mm) Velocity Output

Shi et al. [37] 2009 Planar Longitudinal mode, bending mode 210 × 210 × 30 960 mm/s 103 NYan et al. [38] 2012 Planar Longitudinal mode, bending mode ˚30 × 50 N/A N/AIwatsuki et al. [39] 1996 Linear-rotary Longitudinal mode, bending mode 350 × 300 × 36 2700 rpm, 160 mm/s N/AMashimo et al. [40] 2008 Linear-rotary Three-wave mode, longitudinal mode 14 × 14 × 14 160 rpm, 63 mm/s N/A

ngituendin

pt

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3

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Mashimo et al. [41] 2014 Linear-rotary Three-wave mode, loTakemura et al. [42] 2000 Spherical Longitudinal mode, bAoyagi et al. [43] 2002 Spherical Bending mode

re-load. Therefore, it is difficult to tune the resonator and adjusthe pre-load of an ultrasonic motor [49,50].

One of the greatest potential directions in future work is theiniaturization and integration of ultrasonic motors. A miniature

ltrasonic motor is demanded in many micro areas, such as ininiature zoom lens, micro-robotics and micro surgery. There-

ore, the driving mechanisms and stator design should continue toe reduced in scale, and possible lightweight materials should be

nvestigated to obtain a large torque-to-weight ratio. To get a higherutput and new motor designs, improving and developing manu-acturing processes will continue be an active area of research. Onhe other hand, further research into PZT fabrication and PZT mate-ials cannot only generate larger vibration amplitudes and higherutputs, but also construct new types of micro motors. Since theovement of an ultrasonic motor is generated though the frictional

oupling between the stator and the slider/rotor, wear and fatiguen the contact surface is an inevitable problem. Therefore, it is nec-ssary to investigate low wear frictional materials for improvinghe durability of ultrasonic motors. In addition, the improvementf adhesive materials and adhesive bonding techniques can alsomprove the motor’s adaptability to environmental conditions andhe stability of its performance. Additionally, the improved PZT

aterials and bonding techniques could potentially increase thefficiency of ultrasonic motors.

. Quasi-static motors (non-resonant motors)

An ultrasonic motor is driven by using the resonance of the sta-or, which has difficulties of tuning the resonator and adjusting there-load. Therefore, to drive the moving element within a largerandwidth of the driving frequency, some quasi-static motorsnon-resonant motors) have been proposed in recent decades. Gen-rally, there are two positioning modes of these motors. The first is

long stroke mode. At high frequencies, the PZT is actuated rapidlynd repeatedly in a series of small steps. The motor can thereforechieve a theoretically unlimited travel range by accumulating each

troke of the step motion. The second positioning mode is a fineositioning mode. When a slowly changing DC voltage is appliedo the PZT, nanometer scale positioning can be achieved withinhe stroke of the PZT. Therefore, without any external mechanical

dinal mode 1 × 1 × 1 2500 rpm 20 nNmg mode ˚10 × 32 183 rpm 5 mNm,

20 × 20 × 3.25 1200 rpm N/A

magnification components, a quasi-static motor is capable of coarsepositioning as well as fine positioning with simple construction anda compact size.

There are several ways for a quasi-static motor to achieve amacroscopic motion from a very small strain generated by the PZTitself. Two main groups are classified based on different operationprinciples: clamping and feeding mechanisms and inertia drivemechanisms. In a clamping and feeding mechanism, long travelrange is achieved by repeating sets of clamping and feeding motionsof a number of PZTs. On the other hand, the motion range of an iner-tia drive mechanism is extended by utilizing inertia and frictionforce.

3.1. Clamping and feeding mechanisms

3.1.1. Inchworm mechanismsAn “inchworm” mechanism is a type of clamping and feed-

ing mechanism which imitates the movement of the inchworm innature. It can deliver nanometer-precision positioning over a longmotion range. Piezoelectric inchworm motors presented in the lit-erature can be categorized into three groups. In the first group, thebody of the actuation mechanism can move through a fixed guideway, which is known as a “walker” [51,52]. The “pusher” is the sec-ond configuration. Here, the shaft moves through a fixed actuationmechanism body [53,54]. The third technique can be referred to asthe hybrid “walker-pusher”, which mixes the actuation methods ofthe two previous groups [55,56].

An inchworm mechanism usually consists of three PZTs. Thecentral one is used as a feeding mechanism to produce displace-ment along the motor shaft, while the other two serve as clamps.As shown in Fig. 8, the actuation sequence of a “walker” inchwormmechanism is similar to that of an inchworm in nature. On the otherhand, the principle of a “pusher” is shown in Fig. 9. The motion ofthe shaft can be achieved by coordinating sequential activation ofthe feeding and clamping PZTs. One complete cycle is as follows: (i)PZT 3 releases its grip on the shaft; (ii) PZT 2 expands to move the

shaft to the left; (iii) PZT 3 clamps the shaft; (iv) PZT 1 relaxes itsclamp of the shaft; (v) PZT 2 contracts to feed the shaft to the leftagain; (vi) PZT 1 clamps the shaft and the cycle begins again. Fig. 10shows the principle of a “walker-pusher” inchworm mechanism.

Y. Peng et al. / Sensors and Actuators A 235 (2015) 240–255 247

Tmtta

totdlo

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Fig. 8. Working principle of a “walker” inchworm mechanism.

he clamping PZTs are fixed on the base as that in the “pusher”echanism, while the feeding PZT serves as the moving shaft as in

he “walker” mechanism. The feeding PZT can expand and contracto moving forward when the clamping PZTs alternately clamp it in

well-defined order.Therefore, by repeating sets of clamping and feeding motions,

he shaft can be driven continuously over a long range. The directionf the shaft can be reversed by exchanging the clamping sequence ofhe two clamps. The stroke of each cycle is limited by the maximumisplacement of the central PZT. Theoretically, there is no range

imitation of an inchworm motor, but it still depends on the lengthf the guide way.

Many investigations have been conducted to construct variousnchworm motors. The first example of a “walker” inchworm motor

as developed by Brisbane et al. in 1968 [51]. This motor usedhree cylindrical PZTs, two for clamping and one for feeding. The

otor could produce an increment step size down to 5 �m with alaimed actuation speed of 50 mm/s. The first “pusher” inchwormotor was patented by Burleigh Instruments Inc. in 1975 [53]. It

ould provide a nanometer resolution and 200 mm motion range.n output force of 15 N is achieved at a speed of 2 mm/s. Hsu et al.

ntroduced the first patented “walker-pusher” inchworm motor in966 [55]. A piezoelectric tube was utilized to create the forwardotion, while the clamping mechanisms used two annular wedge

urfaces inclined to the shaft’s axis such that the wedged mem-

ers could prevent motion in either direction. It could produce annloaded velocity of 6.3 mm/s at 60 Hz and approximately 38 mm/st 400 Hz. Another famous “walker-pusher” inchworm motor waseveloped by Locher et al. in 1967 [56]. The motor used clamp-

Fig. 9. Working principle of a “pusher” inchworm mechanism.

ing elements to engage a “clam-claw” mechanism resulting in anindividual step as small as 13 �m.

Since the performance of the clamping mechanisms decides thefeasibility of inchworm motors, design of better clamping mech-anisms is part of the focus in inchworm motor development. Toguarantee enough clamping force, many researchers used flexurestructures or compliant mechanisms to enlarge the PZT elongation.In 1999, Roberts et al. designed a linear inchworm motor oper-ating inside a metal tube [57]. The clamping mechanism of themotor could expand to push a piston outwards and allow four quad-rants to splay apart thus gripping against the inside surface of themetal tube. In the design of Li et al. [58], each clamping mechanismconsisted of a tubular PZT, a compliant mechanism with three iden-tical flexural arms and a pre-load mechanism. Ma et al. proposed akind of inchworm motor with a symmetry leveraged flexible hingedisplacement amplification mechanism [59]. Experimental resultsshowed that the clamping force of the mechanism was 17 N, andthe bearing capacity was 11 N.

In addition to linear inchworm motors, rotary inchworm motorswere also proposed by some researchers. Ohnishi et al. developeda rotary inchworm motor in 1990 [60]. Three pairs of longitudinalPZTs were utilized as the clamping mechanisms, while a torsionalPZT served as the feeding mechanism. The rotor could rotate basedon a similar actuation sequence to a linear inchworm motor. In1995 [61], Duong et al. proposed a rotary inchworm motor with notorsional PZT used. The motor consisted of two clamping mecha-nisms to provide the holding force and a swinger to provide angular

motion. The swinger consisted of one PZT and a flexure mechanismwhich generated rotation about the center of the motor, while theclamping mechanisms alternately griped the rotor. Li et al. pro-posed a rotary motor with nine PZTs [62]. The stator included two

248 Y. Peng et al. / Sensors and Actuators A 235 (2015) 240–255

ltuus[tio

mrOlcsItrodct

lts

Fig. 10. Working principle of a “walker-pusher” inchworm mechanism.

ayers connected by flexure hinges. Each layer employed three PZTso push three clamping units respectively. Three other PZTs weretilized to produce the torque between the upper layer and thender layer of the stator. A rotary inchworm motor utilizing a Y-haped stator and ring rotor was proposed by Zhou et al. in 201463]. One PZT was employed and the Y-shaped stator could bereated as three beams to clamp the rotor alternately. It was ver-fied that the motor could achieve a maximum rotational velocityf 26.3 rpm and a stall torque of 96 �Nm.

On the other hand, development has been done to constructulti-DOF inchworm motors. Yan et al. developed a 3-DOF mobile

obot utilizing a rhombic flexure hinge mechanism in 2006 [64].ne PZT was used to drive the robot, while four electromagnetic

egs were utilized to clamp the robot. By controlling the legs tolamp and release appropriately, the robot could achieve largetroke translation and rotation with high resolution on a platform.n 2011, Torii et al. proposed a tripedal robot with three PZTs andhree electromagnets [65]. The deformation of the PZTs moved theobot, while the adhesion of the electromagnets held the robotn a magnetic floor. Another 3-DOF inchworm mechanism waseveloped by Fuchiwaki et al. in 2010 [66]. The mechanism wasomposed of four PZTs as feeding mechanisms and a pair of elec-romagnets as clamping mechanisms.

Inchworm mechanisms have many advantages, e.g., high reso-ution positioning, compact size, no backlash, etc. Strictly speaking,he inchworm motor is also a friction-type motor, which pushes thehaft through friction at the interface of the clamp and the shaft.

Fig. 11. Working principle of a “seal” mechanism.

The load capacity of the inchworm motor depends on the staticfriction between the clamp and the shaft. Compared to other fric-tion drive mechanisms, the clamping mechanism of an inchwormmotor can generate a higher driving force and a quasi-static opera-tion. It also reduces wear due to the clamping operation when theshaft is stationary. However, in the meantime, the clamping opera-tion and relatively large volume reduce the PZT driving frequency.An inchworm mechanism needs at least three phases, causing itsoperation to be complex. The discontinuous clamping operationmay also cause vibration to the shaft. In addition, the mechanisms,especially the chucks, seem to be hard to miniaturize. High preci-sion in manufacturing is also a challenge so that the clamps workreliably.

3.1.2. Seal mechanismsA seal mechanism is another clamping and feeding mechanism,

which is very similar to the inchworm mechanism but with onlyone clamping mechanism. As shown in Fig. 11, a seal mechanismis composed of one PZT and two friction elements (1 and 2). Fric-tion element 1 is a passive device and applies a constant frictionalforce. Only friction element 2 is controlled by an on-off action, caus-ing a clamping-releasing operation at the base. The frictional forceshould be designed as:

F2off < F1 < F2on

where F1 is a constant frictional force between friction element 1and the base, while F2on and F2off are frictional forces in the cases ofclamping and releasing friction element 2, respectively. The actua-tion sequence of the seal mechanism is as follows: (i) The frictionelement 2 is off; (ii) Due to F2off < F1, the PZT expands to move thefriction element 2 to the right when the friction element 1 keepsstationary; (iii) The friction element 2 is turned on; (iv) The PZTcontracts to move the friction element 1 to the right because F2on isbigger than F1. The moving direction can be reversed by exchangingthe extension and contraction of the PZT in the sequence above.

Furutani et al. developed an l-shaped seal mechanism with 3-DOFs in 2002 [67]. This mechanism consisted of two controlledelectromagnets connected by two PZTs. An electromagnet withconstant friction was used to connect the two PZTs at a right angle.By controlling the two controlled electromagnets with an on-offaction, the mechanism could move with micrometer order steps inthe x-, y- and �-directions.

Compared with the inchworm mechanism, a seal mechanism

using passive devices decreases the number of controlled actu-ators but still retaining the same performance as the inchwormmechanism [67,68].

Y. Peng et al. / Sensors and Actuators A 235 (2015) 240–255 249

(i)

(ii)

(iii)

(iv)

Shaft

Feeding PZT Clamping PZT

3

dssamt

iafitcmpts

dafiaciw

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PZT Inertial massMain body

Slow extension

Rapid contraction

Fig. 12. Working principle of a walking drive mechanism.

.1.3. Walking drive mechanismsAnother type of clamping and feeding mechanism is a “walking

rive mechanism”. It simulates the walking motions of a biologicalpecies. A walking drive mechanism usually uses a series of theame driving elements (at least two “legs”), each of which can clampnd feed the moving element separately. During the long walkingotion, the mechanism can be alternately supported and driven by

he legs step by step.The working principle of a walking drive mechanism is shown

n Fig. 12. The motion of one leg can clamp and feed the mech-nism independently. When or before the feeding of one step isnished, the clamping of the next step is initiated. This means thathe clamping and feeding of the next step can also be carried outoncertedly before the previous step finishes. Therefore, two orore legs should be used to accomplish an overlap of the motion

atterns. Similar to walking motions in animals or human beings,his mechanism can by driven in a continuous motion by repeatingets of clamping and feeding operations of the legs.

In 1997, Shamoto et al. proposed a walking drive mechanismriven by three “legs” [69]. Each leg consisted of a clamping PZT and

feeding PZT. By shifting the phases of the legs, the clamping andeed motions were repeated alternatively in a way similar to walk-ng motions in animals. At least two clamping mechanisms clampednd guided the shaft rigidly at any time. Therefore, the mechanismould be driven continuously over a long stroke. Based on the walk-ng drive principle, Shamoto et al. also proposed an XY� and 6-axis

alking drive mechanisms in 1997 and 2000, respectively [70,71].Pan et al. developed a motor for translating a prism using six

hear-mode PZTs [72]. Each PZT consecutively sheared and slidackward along the prism, while all the six PZTs simultaneouslyeturned to their original position and drove the prism forward. Onhe other hand, a “piezolegs motor” was developed by Piezomo-or Uppsala AB [73]. It consisted of four PZT legs and a drive rodressed against the legs by a pre-load. Based on a bimorph work-

ng principle, the legs could elongate or bend in a two-dimensionallane. Therefore, the motor could perform a walking movement byynchronizing the movement of each pair of the four legs. Sincehe motor is non-resonant, it is easy to scale up and down in size.

any “piezolegs motors” have been commercialized for various

pplications, such as tuning and aligning lenses, high-precisionuto-focusing, precision positioning in lab-on-a-chip or semicon-uctor industries, and manipulating samples in a Scanning Electronicroscope (SEM) or Transmission Electron Microscope (TEM) [73].

Fig. 13. Working principle of an impact drive mechanism.

In general, an inchworm mechanism has a simpler structure.However, the motion is intermittent due to only one feeding actu-ator. The feeding actuator must stop when changing the clampingactuators to hold the shaft. Therefore, the clamping and feedingmotion is not simultaneous but with a sequential alternation. Com-pared with the inchworm mechanism, the clamping motion of oneleg of the walking drive mechanism can be carried out during itsfeeding phase. Hence, the leg can cooperate with the movementof the shaft fed by other legs. In addition, the leg can clamp orrelease the shaft at any time when other legs feed the shaft. Itmeans that the shaft is fed by several legs or at least one leg duringthe entire movement. Therefore, the walking drive mechanism canmove continuously and smoothly with high rigidity.

All the typical types of the aforementioned mechanisms arebased on the sequenced operation of clamping and feeding. Aclamping and feeding mechanism is capable of providing a largeforce with high efficiency, but it is characterized by low speed dueto the low working frequency in the quasi-static state. The mecha-nism needs at least one clamping mechanism as well as one feedingmechanism to operate, which is too complicated in both structureand control. In addition, the reliability and applications of the mech-anism in small space and weak signal measurements all becomesevere issues [74].

3.2. Inertia drive mechanisms

3.2.1. Impact drive mechanismsAn impact drive mechanism (IDM) is a method utilizing static

friction and the impulsive inertial force caused by the rapid defor-mations of a PZT. As shown in Fig. 13, this mechanism is composedof a main body, a PZT and an inertial mass. The main body is placedon a guide surface, while the inertial mass is not in contact withthe guide surface. When the PZT expands slowly, the mass movesforward while the main body keeps stationary due to the staticfriction on the guide surface. Then, the PZT shrinks quickly to gen-erate an impulsive force and the main body gets the momentumof the mass. At this time, the inertial force generated by the massexceeds the maximum static force of the main body. Consequently,the main body moves forward against static friction. By repeatingthese steps, the main body can move forward in infinite distancecontinuously. The backward motion can be obtained by reversingthe sequence of extension and contraction of the PZT.

The impact drive mechanism was first proposed by Higuchi et al.in 1990 [75,76]. In his patent [75], several impact drive mechanismswith multi-DOFs were proposed, including XY and XY� stages. Inthe paper of Higuchi et al. [76], one rotating joint was constructedby an arm with a shaft, a spring and a stand. A pair of PZTs with iner-

tial masses was connected to the arm. Based on an impact frictiondrive, the PZTs could rotate the joint with a velocity of 0.048 rad/secwhen the pulse rate was 1.2 kHz at 80 V. A three-DOF joint was alsoproposed in the paper. The arm was supported by a ball-joint mech-

2 Actuators A 235 (2015) 240–255

atofaamiio[efCsd[cttp�iaYdpautitiHd

otfm“otststt

3

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PZT

Friction element(Inertial mass )Moving body

Slow extension

Rap id contraction

Fig. 14. Working principle of an initial smooth impact drive mechanism.

Base

PZT Friction element

Moving body

Slow expansion

Rapid contrac tion

50 Y. Peng et al. / Sensors and

nism. The cross section of the arm was an equilateral triangle, andhe sides were attached with six PZTs and six inertial masses. Basedn an impact friction drive, the PZTs could generate a couple oforces around the rotating axes of X, Y, and Z. Then, Higuchi et al.lso proposed a four-DOF micro robot arm based on the two jointsbove. The IDM developed by Higuchi et al. have already been com-ercialized and being used in many institutes, such as for smooth

nsertion of a micro pipette into the cytoplasm in case of spermnjection or DNA transplantation [77]. Cedrat technologies devel-ped a Stepping Piezo Actuator (SPA) based on the principle of IDM78]. The displacement of the PZT was magnified by an externallliptical shell, and a clamping mechanism was used to generateriction force. Several customized SPAs have been developed byedrat Technologies to meet various environments (medical MRI,pace. . .) and customers’ needs. Yamagata and Higuchi et al. alsoeveloped an XY� positioning table for use in an ultrahigh vacuum79]. The table has a hexagonal shape with six PZTs and massesonnected to the inside wall of the body. By utilizing friction andhe inertial forces caused by rapid deformations of the PZTs, theable could be driven in the directions of X, Y, and �. Nomura et al.roposed an impact drive mechanism that could provide X, Y, and

motions [80]. Four PZTs were connected to the main body and thenertial mass. By alternating the phase shift of the PZTs, this mech-nism could generate not only translational displacement in X and

directions but also a rotational one, �. To construct an impactrive rotary precision actuator, Zhang et al. used one end-loadediezoelectric cantilever bimorph and one inertial mass as an actu-ting unit [81]. Two actuating units with a symmetrical layout weresed to rotate the actuator. By slow bending and rapid restoration,he piezoelectric bimorph could rotate the actuator with a theoret-cally unlimited working range. As the deformation magnitude athe free-end was more than 10 times that of a stacked PZT, a heav-er end-mass could be easily driven by the piezoelectric bimorph.ence, the actuator was thought to be able to possess heavy loadriving ability.

Unlike other friction-type motors, the impact drive mechanismnly moves in the “slip” period and stands through the static fric-ion period. The inertial force is used to overcome the frictionalorce to actuate the main body. Since the PZT and the inertial mass

ove with the movement of the main body, the whole system is aself-moving” mechanism. It should be noted that the inertial massf the IDM must be relatively large for effective transmission tohe moving body, resulting in reduced resonance frequency of thetage. Besides, it is impossible to utilize the stroke of the PZT itselfo move the moving body. This is because the static friction forcehould be overcome to move the main body. It is required rela-ively large voltage to generate enough impact force, which limitshe minimum positioning displacement of the main body.

.2.2. Smooth impact drive mechanismsThe initial smooth impact drive mechanism (SIDM) was mod-

fied from the construction of an impact drive mechanism byoshida et al. in 1999 [82]. As shown in Fig. 14, the inertial mass ofhe IDM is modified to become a friction element, which contactsith the guide surface. The frictional force on the friction element

s much larger than that on the moving body. Therefore, when theZT expands slowly, the friction element keeps at the original posi-ion and the moving body moves forward. When the PZT contractsapidly, the moving body and the friction element are draggedimultaneously. The moving body can move a small displacementith respect to its original position. Hence, a long forward stroke is

btained by repeating the above operations. The whole mechanism

f the initial SIDM is self-propelling and can be driven using a fineositioning mode by using the displacement of the PZT itself. Bytilizing the principle of the mechanism, Morita et al. developed ahree-DOF parallel link mechanism in 2002 [83]. The mechanism

d

Fig. 15. Working principle of a smooth impact drive mechanism.

employed three SIDMs for positioning in X, Y, and � directions overa long stroke with fine positioning resolution. In previous researchby the authors [84], a pair of PZTs and friction elements with a sym-metrical layout was employed to support and drive a moving body.Therefore, the moving body didn’t need to contact with the guidesurface to obtain a relatively smooth movement.

Subsequently, Yoshida et al. also improved the SIDM by using afriction element to transmit the movement of the PZT to a movingbody [85]. As shown in Fig. 15, one end of the PZT is mounted on abase and the other is attached to the friction element. The movingbody is placed on the friction element by a pre-load mechanism. Inthe long stroke mode, the PZT is driven by a saw-tooth wave volt-age of slow increase and rapid decrease. When the PZT is drivenslowly, the moving body can be moved by the frictional force. Then,the applied voltage rapidly decreases and the PZT shrinks very fast.The moving body cannot follow the fast motion of the friction ele-ment and remains in place due to its inertia. Therefore, the movingbody can obtain an unlimited displacement by continuously repeat-ing these operations. On the other hand, in the fine positioningmode, a slowly changed DC voltage is applied to the PZT. Due to thestatic friction between the moving body and the friction element,the moving body can move with the movement of the PZT itselfwithout slippage. Therefore, the moving body can demonstrate thesame positioning faculty of the PZT itself, which is at nanometerlevel. The SIDM developed by Yoshida et al. was much smaller andcould be installed in many mobile devices. Konica Minolta Opto hasalready mass-produced SIDMs in several applications such as auto-

focus actuators for mobile camera lens and image stabilizing unitof cameras called “Anti-shake” [86]. A number of SIDM-based com-mercial motors have also been developed by Physik Instrumentefor precision positioning [17].

Y. Peng et al. / Sensors and Actuat

Slow expa nsion

Rap id contraction

PZT

Friction element

Moving body

F

l[gfdrTsdfamtwmcssnaL

c“TamfawmtlsmfDaTitmP

ig. 16. Working principle of a shear-mode smooth impact drive mechanism.

To construct a multi-DOF SIDM, Zhang et al. developed a rotary-inear motor by superimposing a linear SIDM and a rotary SIDM87]. In the rotary SIDM, two PZTs were situated along the tan-ential direction of the motor system, and two correspondingriction pieces were placed against the perimeter of the rotary cylin-er. Therefore, based on the same principle of a linear SIDM, theotary SIDM could move with an unlimited angular displacement.o avoid the large size of the stacked structure, Gao et al. con-tructed a compact linear-rotary stage by utilizing two L-shapedriving units [49]. Each driving unit consisted of two PZTs and ariction element made by permanent magnet. The two PZTs wereligned along the axial direction and tangential direction of theoving body, respectively. Hence, based on the principle of SIDM,

he stage could move along the axial and tangential directionsith large motion ranges. It should be noted that the permanentagnet employed as the friction element could generate constant

ontact force without any pre-load mechanisms, which greatlyimplified the SIDM-based stage configuration. Subsequently, aecond-generation linear-rotary micro-stage whose volume waso more than 1 cm3 was constructed in Gao’s lab [50]. In 2013,

compact XY micro-stage was also proposed by using the same-shaped driving unit by Gao’s lab [88].

Some other deformation modes of the PZT were also used toonstruct a SIDM-based motor. One typical type is referred to asstick-slip” motor utilizing shear deformation of the PZT [89–93].he principle is shown in Fig. 16. A moving body is supportednd actuated by a deformable PZT leg. During the slow defor-ation of the leg, the moving body follows the leg owing to

rictional force, whereas it slips due to its inertia when the PZT legbruptly shrinks backwards. Compared with the “piezolegs motor”,hich needs at least two legs actuated alternately, the “stick-slip”otor can be actuated by only one leg because of a smooth fric-

ional drive [89]. In the research of Breguet et al. [90], two PZTegs in shearing mode were employed to drive the moving bodyimultaneously. They proposed and fabricated several stick-slipicro-manipulators including a 6-DOF platform, a sample holder

or AFM, two 3-DOF mobile micro-robots, a 4-DOF micro-Electricischarge Machining (EDM) machine, a micro-assembly systemnd a micro-telemanipulation system for biological specimens.hese manipulators had several DOFs and were easy to integrate

nto complex mechanical systems. Zesch et al. proposed a rota-ional motor relying on the stick-slip effect [91]. The rotor of the

otor was supported by 5 ruby hemispheres glued on top of 3 shearZTs. The PZTs could move the ruby hemispheres tangentially to

ors A 235 (2015) 240–255 251

the rotor’s circumference, thereby causing rotation of the axle. Aspherical motor which could be rotated about two or three orthog-onal axes was proposed by Howald et al. [92]. Three PZT tubes orshear PZTs were aligned perpendicular to each other. Therefore, thespherical motor could be rotated about any desired axis throughits center. Zou et al. designed a rotary motor by utilizing three PZTbimorph actuators mounted at 120◦ angles from each other [93]. Asteel ball was attached to the top of each actuator and a glass diskwas placed on the steel balls. Therefore, the disk could be rotatedover unlimited angular range when the three PZT bimorph actua-tors were actuated in the stick-slip operation. Morita et al. reporteda rotational motor using a hollow-cylinder torsional actuator [94].The torsional actuator was made of multi-layer PZTs. To excite tor-sional displacement, the poling directions of the PZTs were alignedin a circumferential direction. Therefore, the rotor on the torsionalactuator could be driven continuously when a saw-tooth wave volt-age was applied to the actuator. The research of Han et al. showedthe feasibility of making an impact rotary motor based on a tubularpiezoelectric fiber actuator with helical electrodes [95]. In their fur-ther study [96], a cylindrical torsional actuator with grooved helicalelectrodes planted on its side face was developed. The motor couldrotate at a speed of 22.5 r/min with a braking torque of 0.1 mNm,and the stall torque could reach up to 1.6 mNm.

In addition, some researchers have also used disk-type PZTs inthe SIDM. In 2006, Kang et al. invented a tiny linear motor using thevibration of a PZT transducer [97]. The transducer consisted of twoPZT disks with a metal disk between them. A shaft was mounted tothe top PZT disk and a mobile element was compressed to the shaft.Long range linear motion of the mobile element could therefore beachieved by successive smooth friction driving. A similar mecha-nism developed for braille displays was also found in Ref. [98]. Alinear motor using a unimorph structure PZT disk was developedby Jun et al. [99]. The disk was fabricated as a ring shape and a shaftwas positioned in the hole of the disk. A saw-tooth wave voltagewas applied to the disk by utilizing the principle of SIDM. As a result,the shaft moved smoothly with a long travel range.

Both the IDM and SIDM utilize an inertial force and a frictionalforce to drive the moving element over a long motion range, andmany publications called them “inertia motors”. Compared withthe IDM, the SIDM might be superior in respect of its positioningresolution. An SIDM can demonstrate the same positioning resolu-tion of the PZT itself due to the driving static force between the PZTand the friction element. With the IDM, the moving body is drivenusing impact step motions. In other words, the frictional force playsa positive role for the actuation movement in the SIDM, whereasthe inertia force plays a positive role in the IDM. In addition, a typ-ical SIDM usually consists of two separate moving systems. Oneis the PZT and the friction element, the other is the moving body.Each of these can move independently. This allows the resonantfrequency in the first system to be designed separately as high aspossible. Therefore, the SIDM usually has a wider bandwidth withless vibration. However, unlike the unlimited motion range of theIDM because of its self-moving characteristic, the travel range ofthe SIDM is always limited by the length of the friction couplingmechanisms.

An inertia motor is rather simple made superior by its compactstructure, simple operation, and accurate step capability comparedwith other types of motors, but it is not very rigid and prone tovibration [74]. In addition, its speed, output pushing force, and effi-ciency are fairly low because of its quasi-static operation, frictioncoupling mechanism, and sliding friction dissipation. For example,compared to the ultrasonic motors, the inertia motor is simpler, but

the holding force is 1/10 smaller. In Table 3 we summarize the per-formance data of typical quasi-static motors reviewed according toclassification to provide an easy reference for review.

252 Y. Peng et al. / Sensors and Actuators A 235 (2015) 240–255

Table 3Performance of quasi-static motors (non-resonant motors).

Reference Year Principle DOF Size (mm) Velocity Resolution Output

Burleigh design [53] 1973 Inchworm Linear 44 × 33 × 28 2 mm/s N/A 15 NRoberts et al. [57] 1999 Inchworm Linear ˚28 × 290 0.073 mm/s N/A 22 NMa et al [59] 2014 Inchworm Linear N/A 1.259 mm/s 60 nm 17 NDuong et al. [61] 1995 Inchworm Rotary ≈˚100 × 50 2 rpm N/A N/ALi et al. [62] 2013 Inchworm Rotary ˚80 × 25 0.06 rpm 4.95 �rad 93 mNmZhou et al. [63] 2014 Inchworm Rotary N/A 26.3 rpm N/A 0.096 mNmYan et al. [64] 2006 Inchworm XY� 55 × 35 × 20 N/A N/A N/ATorii et al. [65] 2011 Inchworm XY� ≈65 × 60 × H N/A N/A N/AFuchiwaki et al. [66] 2010 Inchworm XY� 50 × 50 × 23 18.3 mm/s 10 nm N/AFurutani et al. [67,68] 2002 Seal XY� 63 × 63 × 16 0.65 mm/s, 0.066 rpm 400 nm, 10 �rad 1.4 NShamoto et al. [70] 1997 Walking drive XY� 370 × 160 × 42 6 mm/s 10 nm N/AHiguchi et al. [79] 1990 IDM XY� ˚120 × 33 N/A 700 nm N/ANomura et al. [80] 2007 IDM XY� 28 × 28 × 16.5 1 mm/s 17 nm N/AZhang et al. [81] 2008 IDM Rotary 171 × 20 × 0.4 0.032 rpm 1 �rad 20 mNmMorita et al. [83] 2002 SIDM XY� ˚6 × 70 20 mm/s N/A 0.68 NPeng et al. [84] 2011 SIDM Linear 20 × 10 × 3 5.4 mm/s 6 nm N/AZhang et al. [87] 2006 SIDM Linear-rotary ˚10 × 50 7.3 mm/s, 2 rpm 26 nm, 0.019◦ 2.09 N, 12.2 mNmGao et al. [49] 2010 SIDM Linear-rotary 45 × 45 × 35 16 mm/s, 0.5 rpm 10 nm, 0.2′′ N/APeng et al. [50] 2013 SIDM Linear-rotary 11 × 11 × 5.7 5.7 mm/s 25 nm N/AShimizu et al. [88] 2013 SIDM XY 24 × 24 × 5 11 mm/s 10 nm 0.06 NZesch et al. [91] 1995 SIDM Rotary 10 × 8 × 8 1 rpm 0.1 �rad 0.37 mNmMorita et al. [94] 1999 SIDM Rotary ˚15×11 27 rpm N/A 5.5 mNm

15

10

× 10

3

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Han et al. [95] 2009 SIDM Rotary ˚1 ×Zhang et al. [96] 2012 SIDM Rotary ˚8 ×Kang et al. [97] 2006 SIDM Linear ˚3.5

.3. Performance analysis of inertial motors

Generally, it is assumed that the inertial motor is actuated instick-slip” mode and is also called a “stick-slip” motor in someiterature [89–93]. The moving body of the motor is classicallyegarded as being driven in small steps which are composed of atick phase and a slip phase, between the friction partners. How-ver, strictly speaking, the “stick-slip” mode involving friction andliding only occurs at a specific frequency. Actually, the inertiaotor can also operate in “slip–slip” mode without any static fric-

ion phase [82,100]. This means that the movement of an inertiaotor not only occurs at a low specific frequency, but can be driven

sing a high bandwidth under resonance. In a “stick-slip” mode, ithould be operated at a specific frequency exactly to obtain staticriction in the “stick” phase and dynamic friction in the “slip” phaseimultaneously. When the driving frequency becomes larger, theoving element is driven by dynamic friction force instead of static

riction force, even in the previous “stick” phase. Therefore, slip-age also occurs not only in the “slip” phase, but also in the previousstick” phase. This phenomenon is called a “slip–slip” mode becauselippage occurs in both phases. In this mode, the amount of the dis-lacement of each step is determined by the difference between theorward slippage and the backward slippage. Although backwardlippage occurs throughout the motion, the moving body movesorward continuously because the duration of its forward force isonger than that of its backwards force. The maximum velocityeachable in “stick-slip” mode is limited, while “slip–slip” opera-ion allows inertia motors to be driven at higher frequencies forigher velocities.

Although the velocity of the inertia motor could be improveduring the “slip–slip” mode, this movement might be not “smooth”t a low driving frequency. This is because only partial slippageccurs in the backward “slip” phase, and the moving body partiallyollows the backward motion of the PZT. Therefore, the movingody moves forward with vibration due to its partial backwardotion. When the driving frequency becomes high enough, the

oving body moves forward so fast that it cannot follow the back-ard movement of the PZT. Therefore, no vibration occurs and theoving body moves forwards smoothly and fast, which is desired

or inertia motors. In Zhang et al. research [87], five cases were

90 rpm N/A 0.08 mNm215 rpm N/A 1.6 mNm6.5 mm/s N/A 0.18 N

described for identifying the relationship between the output dis-placement of the moving body and the PZT according to stick andslip dynamic conditions.

On the other hand, some researchers have used a saw-tooth volt-age to drive the PZT in order to obtain slow expansion and rapidcontraction of the PZT. However, the output displacement of thePZT is distorted by the non-flat transfer function of the apparatussuch as the PZT, the electronic circuits and the mechanical compo-nents. The distorted output displacement of the PZT is not efficientfor the impact friction motion because it may reduce the velocityand cause undesirable motion of the moving body. To enhance thevelocity of the moving body, the waveform of the voltage shouldbe optimized to generate a quasi-sawtooth output displacement.In Yoshida et al. research [82], a rectangular voltage was reportedfor driving the SIDM. In the authors’ research [50], the desired volt-age waveform at each frequency was calculated by inverse Laplacetransformations.

Some other performances of inertia motors were also investi-gated in numerous publications. The movement performance ofthe IDM on fluid lubricated surfaces was investigated by Furutaniet al. [101]. It was verified that the movement of IDM on a fluidlubricated surface was almost the same as that on a dry surfacewhen the fluid lubricant was thin and the device moved slowly,and appropriate fluid lubricants could make the movement stable.In [102], it was found that the performance was not affected byexposure to dust and humidity. However, slight lubrication withmachine oil resulted in a significant reduction in driving speed andmaximum load. The effect of frictional force on driving speed is alsofound in literature. In [82], the driving speed changed little with theincreasing of the frictional force, and began to decline when the fric-tional force reached a certain value. The thermal effect on inertiamotors was investigated by Li et al. [103]. Their experiments fur-ther showed that a temperature rise reduced the displacement ofthe motor at low voltage, while there was no significant change athigh voltage.

4. Motors combined resonant and quasi-static operations

Traditionally, ultrasonic motors are operated at resonancethereby achieving high speed, while quasi-static motors are driven

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Y. Peng et al. / Sensors and

t off-resonance and may generate higher force/torque and resolu-ion but cannot generate significantly high speed. To take advantagef the merits of both approaches, the recent trend is to combine theesonant and quasi-static operations together.

In Mangeot’s research [104], a piezoelectric motor was con-tructed by merging quasi-static and resonant operations into aingle system. The motor could be operated over the whole fre-uency from 0 Hz up to 10% above resonance. Therefore, the motorould demonstrate high torque in quasi-static operation and highotation speed at resonance. To combine the fast positioning capa-ility of resonant motors with a stepping function enabling fineositioning in the nanometer range, Devos et al. designed anllipse-shaped linear drive unit by means of only two PZTs [105]. Inhe resonant mode, the horizontal and the vertical vibration modesere used to create an elliptical motion. In the stepping position-

ng mode, the motor operated as a walking drive based on thenchworm principle. Therefore, by combining the resonant and thetepping positioning modes, the motor could achieve a high speedf more than 100 mm/s and position accurately and smoothly atery low speeds. Based on the same driving concept, Devos et al.lso built a planar piezoelectric drive by employing four PZTs for aesired planar motion [106].

Because of the low mechanical quality factor and heat gen-ration of multi-layered PZTs, the traditional SIDM operated atff-resonant frequency is not suitable for sufficient high-speedovement. Several attempts were made to increase the driving fre-

uency of the SIDM up to its resonance, which could improve theriving speed and thrust significantly. Since a saw-tooth voltageannot generate a saw-tooth shaped displacement in the vicinityf resonance [107], some researchers used an uneven rectangu-ar voltage wave to drive the SIDM at the resonant frequency.nother method is to combine two resonant frequency modes

108]. Tuncdemir et al. designed and manufactured a translational-otary motor by using a single actuator [107]. Translational andotary motions were obtained at two distinct resonant frequen-ies by means of SIDM applied on slanted PZTs. To generate aaw-tooth shaped displacement profile, the SIDM was driven byn asymmetric square wave voltage at the resonant frequency.ompared with the SIDM driven at off-resonance, the SIDM drivent resonance not only reduced the power requirement but alsonhanced the efficiency. In 2012, Morita et al. proposed a resonant-ype SIDM to introduce the high speed and high thrust operation108]. A quasi-sawtooth movement was generated by combininghe first and third longitudinal resonant vibration modes. The ratiof the first and third vibration resonant frequencies was adjustedo be 1:2 by a bolt-clamped Langevin transducer. Therefore, the

otor could be driven with a high speed of 280 mm/s. In 2014, resonant-type inertia linear motor based on waveform synthe-is was proposed by Pan et al. [109]. The motor’s stator driveny PZT plates could produce a quasi-sawtooth shaped motion byombining the second and the first resonant modes. In addition,he driving speed as well as the output power could be poten-ially enhanced with optimized design. Their lab also proposed aiezoelectric motor combined the resonant actuation of ultrasonicotors with the control mechanism of inchworm motors to uti-

ize their advantages but mitigate their weaknesses [110]. Based onhe synchronized switching-mode operation of harmonic vibration,he prototype demonstrated higher output force than ultrasonic

otors, and higher efficiency than the motors with sliding frictionoupling mechanism.

. Conclusions

In this paper, a critical review of long range piezoelectricotors using frequency leveraged method is presented. The work is

ors A 235 (2015) 240–255 253

classified into three groups by actuation frequency, including ultra-sonic motors (resonant motors), quasi-static motors (non-resonantmotors) and motors combining resonant and quasi-static opera-tions. According to different wave propagation methods, ultrasonicmotors have been classified into standing wave type motors andtraveling wave type motors. The standing wave type motors havebeen discussed according to different combinations of vibrationmodes. Also, two typical types of traveling wave type motors havebeen introduced. Quasi-static motors have been classified to beclamping and feeding mechanisms and inertia drive mechanisms.Various clamping and feeding mechanisms have been introducedaccording to their basis of operation, including inchworm mech-anisms, seal mechanisms, and walking drive mechanisms. Impactdrive mechanisms and smooth impact drive mechanisms, whichare two main typical types of inertia drive mechanisms, have beenalso discussed. Some issues of movement performance and drivingmethods for inertia motors have been clarified. In addition, prosand cons for each type of motor have been discussed in this paper.Finally, recent attempts at constructing a motor by combining theresonant and quasi-static operations are also reviewed. The currentstate of frequency leveraged motors summarized in such a way canhelp researchers and engineers to design long range piezoelectricmotors with improved performance.

Future research directions are suggested as follows:

(1) Since almost all the frequency leveraged motors utilize fric-tional force to repeat an intermittent movement to obtain a longmotion range, frictional behavior varies depending on pressureand contacting surfaces. Applications are limited to light duty,and open-loop repeatability is limited. Therefore, fabricationand materials at the contact surface should be further inves-tigated. The optimized friction coupling can not only improvethe wear and fatigue of the contact surface but also enhance thedriving speed, repeatability and load capacity.

(2) Another significant potential in the future is the miniaturizationof the frequency leveraged motors. Therefore, optimization ofstructural design, lightweight material and novel manufactur-ing processes will continue to be active areas for the future.

(3) Design of new driving mechanisms also plays an important rolein the future. It is expected that more and more novel struc-tures will be developed based on the principles of frequencyleveraged motors.

(4) Innovation of new driving principles and methods should con-tinue to be explored.

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Biographies

Yuxin Peng received his Bachelor and Master degrees from Chongqing University,China, in 2007 and 2010. He received his PhD from Department of Nanomechanics,Tohoku University, Japan. He is currently a Research Fellow at the Department ofBiomedical Engineering, National University of Singapore, Singapore. His researchfocuses on piezoelectric motors, sensor calibration and robotics.

Yulong Peng received his Bachelor of Mechanical Engineering degree from Collegeof Information & Business, Zhongyuan University of Technology, China. He is now aMaster course student in Mechanical & Electrical Engineering, Zhongyuan Universityof Technology, China.

Xiaoyi Gu received his Bachelor degree from Department of Mechanical Engineeringand Automation, Shanghai Jiaotong University, China, in 2013. He is now a Master ofEngineering student in Department of Biomedical Engineering, National universityof Singapore.

Jian Wang received his Bachelor degree from the Department of Physical Education,Shanxi University, China in 1982. He received his Master and PhD degrees fromthe Department of Biology and Department of Engineering Psychology, HangzhouUniversity, China in 1987 and 1996. He is currently a professor and director of theDepartment of Exercise and Sports Science, Zhejiang University. His interests includesmart actuators, rehabilitation robots, exercise physiology and ergonomics. He is amember of Chinese Human Ergonomics Society and Chinese Physiology Society.

Haoyong Yu received his PhD in Mechanical Engineering from the MassachusettsInstitute of Technology (MIT), Cambridge, in 2002. He is an Assistant Professor of

Organization (DSO), National Laboratories of Singapore as a Principal Member ofTechnical Staff. His areas of research include smart actuators, medical robotics,rehabilitation engineering and assistive technologies. He is a member of IEEE.