Sensors and Actuators A 149 (2009) 113–119

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Sensors and Actuators A: Physical

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A multilayer TWILA ultrasonic motor

Lionel Petit ∗, Paul GonnardINSA Lyon-LGEF, 8 rue de la physique, 69621 Villeurbanne Cedex, France

a r t i c l e i n f o

Article history:Received 14 May 2008Received in revised form30 September 2008Accepted 30 September 2008Available online 17 October 2008

Keywords:Ultrasonic motorPiezoelectric multilayers

a b s t r a c t

A fully operational ultrasonic motor using bulk piezoelectric longitudinal actuators has been alreadyintroduced in our previous works. But despite of its interesting characteristics and its easily implementa-tion, its high supply voltage (from 200 Vrms) could be a technological lock-in by imposing supplementaryconstraints in order to satisfy the electrical safety standards.

To overcome this drawback, we have developed a new stator version which integrated multilayerlongitudinal actuators, based on the bulk TWILA ultrasonic motor.

If the global motor structure stays relatively closed to the bulk one, the new longitudinal actuatortopology has been optimised with the help of a Finite Element Modeling.

The higher capacitance value of the multilayer actuators requires the use of inductance compensationto optimise the motor drive.

Finite Element Model

Longitudinal actuators Finally, The mechanical performances are not only preserved but are now obtained under a main supplyvoltage (12 Vrms) 17 times lower than the previous bulk version. Moreover, the global efficiency is largely

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improved by reaching 20%

. Introduction

Our previous works [1–3] on ultrasonic motors led to a func-ional traveling wave induced by longitudinal actuators (TWILAs)

otor which combines good mechanical performances and lowroduction cost. This motor belongs to the traveling wave ultra-onic motor (TWUM) family because it uses the propagation of aexional traveling wave inside a ring-shaped stator. Each point ofhe surface then describes an elliptical movement by rotation ofhe ring cross-section. This traveling wave results from the super-osition of two stationary waves spatially dephased of a quarterf wavelength in space and of a pi/2 factor in time. The flexionalibrations of the stator are generated by longitudinal vibrations ofomposite piezoelectric actuators (Langevin type) perpendicularlyisposed to the stator and distributed along the circumference. Theods are divided into two electrical channels, each one creating aexion stationary wave (Fig. 1a).

The key point of the TWILA design relies on a particular rodsrrangement “in three quarter of wavelength” that not only allows

reduction of the rods number but also an increase of their diame-

er. This last point, associated with the use of the conical couplers,nhibits the parasitic flexion of the longitudinal actuators, so harm-ul for the motor efficiency.

∗ Corresponding author.E-mail address: lionel.petit@insa-lyon.fr (L. Petit).

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924-4247/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2008.09.020

© 2008 Elsevier B.V. All rights reserved.

A maximal mechanical output power of 1.3 W is then obtained in63 cm3 (20 mm × 52 mm) and 40 g weight tubular motor (Fig. 1b).

The longitudinal actuators integrate bulk hard PZT materialhich needs a high supply voltage (200 Vrms for the standard ver-

ion). This point could be industrially penalizing by imposing someupplementary constraints in order to satisfy the electrical safetytandards (shielding, twin wiring. . .). Moreover, the power sup-ly design has to take into account the inter-phases mechanicaloupling [4] because the motor is obligatory driven near of its oper-tional resonance frequency.

To overcome theses limitations, a new version of the TWILAntegrated MultiLayer actuators (so-called ML TWILA) has beenesigned with two purposes:

to decrease the supply voltageto open new perspectives of easy-strategy command, such asdeveloped in [5].

Multilayer piezoelectric actuators are now commonly used inome industrial applications. The more significative is certainly theiezoelectric Common Rail Injectors for diesel engine such as theiemens AG [6] or Bosch [7] solutions.

But it should be noted that these applications use a staticr quasistatic working of the multilayer elements whereas ultra-onic motors need a resonant state condition in order to increasehe mechanical output. For that reason integration of mutlilayerlements in ultrasonic motors is now a technological key to over-

114 L. Petit, P. Gonnard / Sensors and Actuators A 149 (2009) 113–119

figur

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uTt

2u

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b

Fig. 1. Principle (a) and general con

ome. Funakubo and Tomikawa [8] have presented a linear motorsing multilayer actuators in order to lower the operating volt-ge. Kui et al. [9] have developed a longitudinal-bending modeltrasonic motor using stacked multilayer piezoelectric actuators,hich were self-developed from hard lead zirconate titanate (PZT)

eramic.Doshida et al. [10] have investigated a miniature cantilever-type

ltrasonic motor using Pb-free multilayer piezoelectric ceramics.hey showed that the motor needed 10 times as high a voltage ashe PZT motor in electrical driving.

This paper is organized as follows:

In paragraph 2, the opportunity of using “hard material” multi-layer actuators in ultrasonic motor is firstly established. It is shownthat the dynamical behavior of these actuators meets the motorspecifications criterions.

The different steps of the stator design are then described in para-graph 3. A Finite Element Model permits stator optimisation.Finally in paragraph 4, the performances of this new version aregiven and compared with the previous version which using bulkceramics.

Fig. 2. Characteristics of the NO

ation (b) of the bulk TWILA motor.

. On the possibility of a MutiLayer integration inltrasonic motors

Because an ultrasonic motor stator is a high power device, theecommended piezoelectric material should be a “Hard” type com-osition [11]. Mechanical and electrical losses are then limited inhat case. At the begin of the present work, only the NOLIAC [12]ompany provided a standard hard multilayer: the actuator CMAP11Fig. 2) whose main characteristics are summarized in the right-able.

The first step of this work had consists of validate the dynamicalehavior of the multilayer actuators under the motor specifications.

The chosen criterions are

(a) good stability of the dynamic characteristics under resonant state

condition.

As previously said, a high output power of the motor isobtained when the vibration amplitudes of the stator are large.This can naturally obtain by supplying the active elements neartheir resonant frequency. But because internal stresses become

LIAC CMAP11 MultiLayer.

L. Petit, P. Gonnard / Sensors and Act

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ig. 3. Evolution of the loss angle tan ı as a function of the applied electric field (forCMAP11 stack) at 1 kHz.

large under resonant state, it is necessary to verify the goodmechanical resistance of the specific mutilayer structures.

b) a steady behavior under mechanical static preload.The maximal acceptable mechanical stress before failure is

higher in compression than in extension for a piezoelectricmaterial. Moreover, a risk of delamination exists in multilayerin this last case. For this reason, it is necessary to apply a staticpreload on the mutilayer in order to shift the working point inthe compression area.

.1. Electrical and mechanical characteristics under highlectrical field

A high electrical field solicitation must not induce large electri-al and/or mechanical losses on pain seriously limit the availableechanical power.The electrical losses are determined by the measurement of the

oss angle tan ı (Fig. 3). The observed behavior is similar to bulk PZTeramics one, leading to a loss angle included in the bulk ceramicsange.

The total absorbing current I of a piezoelectric transducer underarmonic range is the sum of (i) the derived current IC0 in its block-

ng capacitance C0 and (ii) the motional current Im:

= IC0 + Im = jC0ωV + ˛u̇z (1)

here ˛ is the force factor of the transducer, u̇z is the vibratingelocity and V is the applied voltage.

The motional current Im modulus and the electric power P mod-lus are calculated from the absorbed current I measurement at the

esonance frequency by

Im =√

I2 − I2C0

P = VI(2)

ip4tA

able 1easured and calculated values of the CMAP11-stack characteristics.

(kV/mm) Measured u̇z (m/s) Measured I (A) Calculated

0 0 0 00.463 0.0035 0.037 0.0091.67 0.0085 0.104 0.0353.03 0.0175 0.220 0.0636.58 0.032 0.410 0.1388.67 0.054 0.691 0.182

13.77 0.075 1.067 0.289

uators A 149 (2009) 113–119 115

here the derived current IC0 modulus (=C0ωV) is estimated inssuming that the blocking capacitance C0 is constant.

The force factor ˛ is deduced from the vibrating velocity u̇z mea-urement by

= Im

u̇z=

√I2 − I2

C0

u̇z(3)

Finally, the mechanical losses � are estimated by

= V

Im(4)

hese kind of measurements (Table 1 and Fig. 4) have been doneith a stack composed by 3 CMAP11. The electric field is voluntary

imited to 14 kV/m with respect to the current limit (1 A) of the setp measurement. No mechanical losses runaway is observed andalues stay in the typical range of bulk PZT ceramics.

.2. Behavior under static preload

Our previous works showed that the application of a longitudi-al static preload is absolutely necessary to limit the risk of materialrack under dynamic stresses. In this order, piezoelectric proper-ies (especially the d33 coefficient) have to be preserved. In theresent case (Fig. 5), no degradation due to the preload application

s observed. It should be noted that the increase of the coefficient33 during the 1st preload application is typical of hard PZT mate-ial.

Finally, the multilayers actuators can be considered as also per-orming than bulk ones in term of electromechanical behavior. Theyave a satisfactory dynamical performance under high electricalolicitation and endure static preload without degradations. Theirntegration in ultrasonic motors is then feasible, a priori.

. Design of the stator

.1. Finite Element Modeling

The same approach used in our previous design has been fol-owing here. We do not develop again the main steps of the statoresign which stay close than those developed in [3]. For practicaleason, the maximum of motor components have to be commonith the bulk TWILA version. This imposed that the total length

f the longitudinal actuator la (including ML actuators and conicalouplers) has to be near than 30 mm such as for the bulk version. Areliminary design phase based on a Finite Element Model permitso choose the optimal number of CMAP-stack. The aim is to preservegood coupling coefficient of the longitudinal actuators because

he global coupling coefficient of the stator directly depends from

t. The simulations of the complete longitudinal actuator (Fig. 6) hadermitted to deduce the coupling coefficient evolution for 1, 2, 3 orCMAP stacks compared to the bulk reference values. As expected,

he coupling coefficient directly depends on the stack length ls.compromise between CMAP number (by this way, reduction of

IC0(A) Calculated Im from (2) (A) Calculated P from (2) (W)

0 00.036 0.0010.098 0.0110.211 0.0450.386 0.1800.667 0.4011.027 0.985

116 L. Petit, P. Gonnard / Sensors and Actuators A 149 (2009) 113–119

Fig. 4. Evolution of the vibration velocity u̇z , the electric power P, the mechanical lossesCMAP11 stack) at the resonance frequency.

Ft

cCl

nc

Ft

tntfisttna

3

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ig. 5. Evolution of d33 coefficient for a 3 CMAP11 stack as a function of the longi-udinal static preload.

ost!) and high coupling coefficient has to be done. Finally, a 3MAP-stack has been chosen in the wanted range of total length

a (grey area in Fig. 6).The second design step concerns with the whole stator where a

ew definition of the conical coupling (Fig. 7) optimise the couplingoefficient while minimizing the parasitic flexion of the actuators. If

ig. 6. Evolution of the coupling coefficient k for different CMAP11 stacks as a func-ion of the actuator length la .

itt

4

m

P

w

� (from (4)) and the force factor ˛ (from (3)) versus the applied electric field (for 3

he previous TWILA version used cylinder-Langevin actuators, thisew version integrates quadrate cross-section actuators becausehe industrial ML actuators are only available in this geometric con-guration. This point does not affect the dynamic behavior of thetator and preserve the inhibition of the parasitic flexion. The sta-or ring has also undergone new evolutions, especially in term ofooth’s geometry. The tooth’s flexion in the tangential direction isow favoured in order to increase the tangential vibrating velocityt its extremity, and so on, increase the motor rotational speed.

.2. Prototyping

A new assembly tool set has been developed (Fig. 8). A partialssembly is firstly pressed in order to crimp the couplers in thend-facings machined in the lower side of the stator ring (Fig. 8b).second tool (Fig. 8c) permits the centering and alignment of theL actuators with the conical couplers and with the statoric ring.The components of a ML TWILA stator and an assembly stator

re shown in Fig. 9. The complete motor is described in Fig. 10. Asn the case of the bulk version, the static preload applied on the MLrough the center prestress axe is controlled by the screwing downorque during the assembly process.

. Performances

We show in our previous works [3] that the maximal outputechanical power of the motor could be written as

nom = Acm˛kdQ

√CLF

mV2 (5)

here

Acm is a performance variable that depends on contact mechanismparameters (rotor geometry, friction coefficient. . .),CLF = C0 + (˛2/K) is the stator low frequency capacitance with C0 is

the blocking capacitance,Q is the stator quality factor,m is the stator dynamic mass,V is the applied voltage,kd is the electromechanical coupling coefficient:

L. Petit, P. Gonnard / Sensors and Actuators A 149 (2009) 113–119 117

Fig. 7. FEM analysis of the stator for the operational flexional mode (rank 3).

Fig. 8. (a) Specific tools for ML TWILA stator assembly: (b) tool for the conical couplers crimp on the stator and (c) tool for the parts centering operation.

Fig. 9. Stator components (a) and assembly (b).

118 L. Petit, P. Gonnard / Sensors and Actuators A 149 (2009) 113–119

Fv

k

wt

4

e

cenf

L

h

o

Fig. 11. Admittance modulus |Y| of the free stator with and without compensationinductor.

Fig. 12. Mechanical characteristics of the ML TWILA versus Bulk TWILA: speed-tri

taAtio

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ig. 10. Complete ML TWILA motor: (a) partial exploded view and (b) longitudinaliew (size in mm).

2d = ˛2

KCLF= 1

1 + ((KC0)/˛2)(6)

here ˛ and K are respectively the force factor and the stiffness ofhe stator.

.1. Electromechanical characteristics

The high blocking capacitance C0 of the ML elements limits thelectromachanical coupling coefficient k, as shown in relation (6).

However, this limitation can be easily bypassed by placing aompensation inductor Lc in parallel on the blocking capacitance ofach stator phase which induces a parallel electrical resonance phe-omenon. The inductor value is deduced near the stator resonance

requency fs by

c = 1

4�2C0f 2s

(7)

Practically, the capacitance effect is never totally cancel butighly decreased as shown in Fig. 11.

Electromechanical characteristics for a free stator phase (with-ut rotor) are summarized in Table 2. The compensation permits

pivd

able 2lectrical characteristics without and with compensation inductance (Lc = 7 �H) compare

Resonance freq., fs(Hz)

Blockingcapacitance, C0 (nF)

Stiffness, K(MPa)

Dynmas

ithout comp. 81,700 535 2800 11.5ith comp. 81,780 72 2700 11.3

ulk version 74,150 0.18 2500 12

orque and mechanical power-torque characteristics for the ML version areespectively nML and PML . Speed-torque and mechanical power-torque character-stics for the Bulk version are respectively nbulk and Pbulk .

o divide by 7 the effective value of the blocking capacitance C0nd then leads to a great increase of the coupling coefficient kd.ssuming that the performance variable Acm is the same for the

wo versions, the theoretical value of maximal mechanical powers derived from (5) and then is given in the last column as a functionf the main voltage V.

This clearly shows the main advantage in using ML actuatorsecause the same theoretical mechanical power can be obtainedith a main voltage 17 times lower (with compensation).

.2. Mechanical characteristics

The mechanical performances are shown in Fig. 12 and are com-

ared to Bulk version’s ones. A maximal output mechanical power

n the order of magnitude of 1.5 W is obtained for the two motorersions even if the rated working points (RML and Rbulk) are quiteifferent.

d with bulk version characteristics.

amics, m (g)

Coupling coef.,kd (%)

Qualityfactor, Q

Force factor, ˛(N/V)

TheoreticalPnom from (5)

4.5 500 1.75 0.270AcmV2

12 740 1.8 0.405AcmV2

15 30 0.1 0.00135AcmV2

L. Petit, P. Gonnard / Sensors and Act

Table 3Mechanical performances of the ML TWILA versus Bulk TWILA.

Bulk version ML version

n0 (rpm) 250 300� max (mN m) 110 95Pmax (W) 1.64 1.54�n�V

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nom (mN m) 85 75nom (rpm) 185 200max (%) 10 20(Vrms) 200 12

But the more significant difference concern with the main volt-ge: 12 Vrms for the ML version instead of 200 Vrms for the Bulk’sne. This clearly confirms theoretical power values of Table 2 andhe theoretical decrease ratio of 17.

Finally, the main mechanical characteristics are given in Table 3or these two versions.

.3. Conclusion

Using Hard material multilayer actuators in a resonant state isow a feasible challenge. We had shown that electromechanicalharacteristics are not degraded compared to the equivalent bulkaterial. Integrated in the TWILA ultrasonic motor concept, they

ead to a severe decrease of the supply voltage.A Finite Element Modeling was useful to design these new lon-

itudinal actuators and to valid the working of this new motoronfiguration.

New assembly tool permits to build several motor prototypesith good characteristics repeatability. Finally a 1.5 W outputower motor with a 12 V supply voltage is now operational. Thisepresents a 17 times decrease of the supply voltage. In addition,he efficiency is largely improved by reaching now 20%.

eferences

[1] L. Petit, et al., A piezomotor using longitudinal actuators, IEEE Trans. Ultrason.Ferroelectr. Freq. Control 45 (2) (1998) 277–284.

[2] L. Petit, P. Gonnard, B. Grehant, A piezomotor for the small actuator market.Applications of Ferroelectrics, ISAF 2000, 2000.

atsn1

uators A 149 (2009) 113–119 119

[3] L. Petit, P. Gonnard, Industrial design of a centimetric “TWILA” ultrasonic motor,Sens. Actuators A: Phys. 120 (1) (2005) 211–224.

[4] L. Petit, P. Gonnard, Inter-phases mechanical coupling in ultrasonic motors,Sens. Actuators A: Phys. 116 (3) (2004) 492–500.

[5] E. Bekiroglu, Microcontroller-based full control of ultrasonic motor with fre-quency and voltage adjusting, Sensors and Actuators A: Phys. 141 (1) (2008)151–159.

[6] Siemens AG, Piezoelectrics Injector available at https://www.ct.siemens.com/en/technologies/ps/beispiele/piezo.html, 2007.

[7] The 3rd generation of Common Rail from Bosch, available at http://www.boschautoparts.co.uk, 2007.

[8] T. Funakubo, Y. Tomikawa, Characteristics of multilayer piezoelectric actuatormade of high Q material for application to ultrasonic linear motor, Jpn. J. Appl.Phys. 41 (2002) 7144–7148.

[9] Y. Kui, B. Koc, K. Uchino, Longitudinal-bending mode micromotor using multi-layer piezoelectric actuator, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 48(4) (2001) 1066–1071.

10] Y. Doshida, et al., Miniature cantilever-type ultrasonic motor using Pb-free mul-tilayer piezoelectric ceramics Japanese, J. Appl. Phys. 46 (7B) (2007) 4921–4925.

11] P. Gonnard, L. Petit, Non linear effects in piezoelectric materials, Boletin De LaSociedad Espanola De Ceramica Y Vidrio 41 (1) (2002) 107–114.

12] B. Andersen, et al., Performance of piezoelectric ceramic multilayer componentsbased on hard and soft PZT in Actuators 2000, Germany, 2000.

iographies

ionel Petit received the M.S. degree in 1992 from the University of Claude Bernard,yon, France and the PhD in acoustics in 1996 from the Institut National des Sciencesppliqués (INSA), Lyon for his work on piezoelectric motors. He joined the Labora-

oire de Génie Electrique et Ferroélectricité (LGEF) where he is Associate Professorince 1998. He received the diploma of “Habilitation à Diriger des Recherches” in006 for his work on the multiphysic couplings applied on the piezoelectric motorsnd on the vibration control and the energy harvesting. His current research inter-sts include development and characterization of electroactive smart materials andystems, energy harvesting and smart self-powered systems. He holds five patentsoncerning ultrasonic motors.

aul Gonnard received the engineer degree in physical engineering from the Insti-ut National des Sciences Appliquées, Lyon, France, in 1964 and the PhD in 1968 fromyon University France. From 1964 to 1969 he worked at the Commissariat à l’Energietomique and then he joined the Laboratoire de Génie Electrique et Ferroélectricité

nd 2000. He has developed more particularly the characterization of piezoelec-ric materials in high power applications, including a participation to an Europeantandardization work and he has managed a joint research involving 10 French orga-izations in the study of ultrasonic motors. He holds six patents and has published10 articles.