WO2007129987A1 - Piezoelectric micromotor construction and method of driving the same - Google Patents

Abstract

A method for driving an ultrasonic motor uses a high dynamic clamping load, which allows a high optimum preload for the operation of the motor under non-slippage conditions, resulting in a high frictional force between the rotor and the stator, and giving rise to a high output torque. The high dynamic clamping load may be achieved, for example, via relaxor-based single-crystal driving elements, and a stator comprising a metal cylinder of high specific modulus. A pulsed input voltage allows use of the micromotor as a stepper motor having accurate angular positioning. The micromotor may be several mm or tens of mm in diameter, and achieve ratios of output torque to input voltage greater than 10 μNm/Vpp (eg, an output torque of 0.56 mNm at 40 Vpp).

Description

Piezoelectric micromotor construction and method of driving the same

TECHNICAL FIELD

This invention relates to the construction or configuration of a motor having a small form factor, also known as micromotor or ultrasonic motor, particularly those having at least a drive element made from piezoelectric materials. It also discloses a method for driving the micromotor, including providing stepping function and rotation speed control.

BACKGROUND ART The terms "ultrasonic motors" or "micromotors" are well-known in the art as motors having small dimensions or form factor, generally in the range of a few millimetres to centimetres, which have high output torque and low input power. They are light, compact, operate quietly and produce little or no magnetic interference, and have therefore found good use in appliances such as cameras, watches, precision movement devices. Cylindrical shape micromotors of a few millimetres in diameter have been reportedly used in miniature medical and industrial devices such as catheters and microrobots as mentioned in references nos. 21 - 26 of Other References & Prior Art.

Another desirable feature of micromotors is their low rotation speeds, even for larger size micromotors, so that reduction gear-chain mechanisms need not be provided in the appliance design, as is required if low-speed induction motors were employed.

Micromotors generally employ piezoelectric elements as drive parts.

The operating principle generally involves having a stator made of a metal tube and driven by piezoelectric materials such as lead-zirconate-titanate (PZT) thin films [ref. nos. 21 , 22, 24], a bulk PZT tube (i.e. hollow pith) [ref. nos. 23, 25], or bulk PZT rod (i.e. solid pith) [ref. nos. 26] in a generally cylindrical shaped micromotor as shown in FIGURE 1. It may be driven by coupling two fundamental bending modes excited electrically with a 90° phase difference, as shown in FIG. 1(b), to produce a travelling wave motion at one of the end face or at both end faces of the stator. This produces frictional forces to drive the rotors.

Preloading of piezoelectric motors is known over 20 years ago, such as that mentioned in U.S. Patent No. 4,460,842 (U.S. Philips Corporation). As late as 2005, such as U.S. Patent No. 6,870,304 (Elliptec Resonant Actuators AG), preloading is described with clamping of piezoelectric motors while clamping is mentioned as the physical clamping of drive elements rather than as a result of over-preloading. Despite the general knowledge that preload is required to operate micromotors, the importance of preload has so far not been examined. On the other hand, when the preload is too high, it would effectively clamps the rotors onto the stator and thus prevented the micromotor from functioning. This is due to the fact that, when the longitudinal strain amplitude (which in operation determines velocity) produced by the piezoelectric driving elements is insufficient to counter that produced by the preload as in the case of micromotors driven by PZT ceramic driving elements at moderately high preload. Accordingly, this leads to the limited output torque produced by prior art micromotors.

Another object of our research concerns maintaining the rotation of micromotors at low speeds, e.g. typically at < 500 rpm, for a micromotor having a diameter of a size in the centimetre range. Generally, the rotation speed of micromotors increases with decreasing diameter of the motor. A conventional way of maintaining the low rotation speeds is to keep the applied voltage low. However, since both the rotation speed and output torque of micromotors decrease with applied voltage, decreasing voltage adversely affect the output torque of the motor.

OBJECTS OF THE INVENTION

One of the objects of our research is to look into the effect of varying preload to the output torque of the micromotor. Another object is to investigate if the output torque derivable from a micromotor may be determined by the magnitude of the preload applied. Our research also looked into the design and fabrication of cylindrical or rod-shape ultrasonic micromotors of high output torque using suitable driving materials and having the following new features:

(a) use of relaxor-based piezo single crystals as the driving elements, such as lead-zinc-niobate-lead-titanate [Pb(Zni_/3Nb_2/3)O₃-PbTiO₃ or PZN-PT] and lead-magnesium-niobate-lead-titanate [Pb(Mgi_/3Nb_2/3)O₃-PbTiO₃ or PMN- PT] solid solution single crystals, of appropriate orientation cuts to raise the dynamic clamping load of the rotor-stator assembly;

(b) use of multilayer single crystal driving elements to lower the driving voltage of the micromotors;

(c) use of a hollow rotor shaft for guide wire insertion and other appropriate functions, and

(d) use of a pulsed drive scheme to enable the ultrasonic micromotor to operate as a stepper motor and/to run at very low rotation speeds while maintaining its high output torque. SUMMARY OF DISCLOSURE

As a general embodiment of our invention, a method is provided for driving an ultrasonic micromotor with the concept of utilizing a rotor-stator assembly of high dynamic clamping load. "High dynamic clamping load" is defined as the critical preload to cause the rotor-stator assembly to vibrate as a single body and the motor to cease to function and/or rotate. This concept may preferably be used for the design and fabrication of ultrasonic micromotors of high output torques and of a few to low tens mm in diameter. The high dynamic clamping load entails a high optimum preload for the operation of the motor under non-slippage conditions, which results in a high frictional force between the rotor and the stator thereby giving rise to the high output torque of the micromotor.

In a specific embodiment, the clamping load at a high and dynamic clamping load value is achieved with piezoelectric active elements having high ratio of d_3ϊ

wherein d_3l is the transverse mode piezoelectric strain constant; and

_SE is the elastic compliance constant along the length direction of the d₃i mode actuator determined under constant electric field condition.

In a preferred embodiment, the high dynamic clamping load is achieved with a bending vibrator in the form of a thin-walled metal tube having 29

a length-to-diameter ratio of > 2.0 and a high specific modulus material. Preferably, the high dynamic clamping load is achieved with at least a rotor having high inertia, made from high density material and/or diameter larger than that of stator, and suitable dimension according to design of the micromotor.

In another preferred embodiment, the high dynamic clamping load is achieved with a relaxor-based single crystal as driving element wherein a high preload may be applied to the micromotor to attain ouput torque of >2.5μNm/Vp_P, micromotor stator and/or rotor is within a few millimeters in diameter and the output torque is achievable to > 2.5 μNm/V_Pp, more preferably > 5 μNmΛ/_pp. This dynamic clamping load of the stator-rotor assembly may be achieved with the use of relaxor-based ferroelectric single crystal driving elements at low applied voltages.

The frictional coefficient between the rotor and the stator may preferably be increased via conventional means to increase the frictional force and hence output torque of the micromotor.

In another aspect of our invention, a micromotor is disclosed having at least a means for preloading at least one drive element of said micromotor over its piezoelectric operating range, wherein said preloading is user- adjustable within a range optimal to torque output of said micromotor but below that of clamping load. In yet another aspect, a micromotor is disclosed comprising a piezoelectric rotor-stator assembly wherein at least one element of said rotor- stator assembly may be dynamically preloaded at a high optimum preload, resulting in a high frictional force between rotor and stator of the rotor-stator assembly, thereby giving rise to a high output torque. Preferably, the micromotor is of high output torque and dimension ranging from a few to low tens of millimetres in diameter.

In a preferred embodiment, the high dynamic clamping load value is achieved with piezoelectric active elements having high ratio of

4 wherein d_n is the transverse mode piezoelectric strain constant; and _sε is the elastic compliance constant along the length direction of the c/₃i mode actuator determined under constant electric field condition.

Preferably, the high dynamic clamping load is achieved with a bending vibrator in the form of a thin-walled metal tube having a length-to-diameter

ratio of > 2.0 and a high specific modulus material. More preferably, the high

dynamic clamping load is achieved with at least a rotor having high inertta, made from high density material and/or diameter larger than that of stator, and suitable dimension according to design of the micromotor. Most preferably, the high dynamic clamping load is achieved with a relaxor-based single crystal as driving element wherein a high preload may be applied to the micromotor to attain ouput torque of >2.5μNm/V_pp,

In a specific embodiment of the micromotor, the drive element may include at least one piezoelectric element comprising a relaxor-based single- crystal element which is excitable to clamping load at a high and dynamic load value. Preferably, the diameter of rotor and/or stator is within a few millimetres, in which high dynamic clamping load provides an operating range below said clamping load and optimally drives the micromotor at an output torque of > 2.5 μNm/V_pp. The single crystal element may preferably comprises of at least one or combination of the following solid solutions of suitable compositions: - lead-zinc-nobiate-lead-titanate [Pb(Zni_/3Nb2/3)O₃-xPbTiθ3 or PZN-yPT] and - lead-magnesium-nobiate-lead titanate [Pb(Mgi_/3Nb2_/3)θ₃-yPbTiO₃ or PMN-

yPT] wherein 0.045 < x < 0.09, 0.27 < y < 0.34 wherein x and y are in mole fraction and including their doped derivatives.

In another specific embodiment of the micromotor, the single crystal element may comprise of individual plates, and/or stacks of at least two plates in appropriate poling combinations to provide the optimal bending vibration to

drive the micromotor, wherein poled single crystal c/_3rplates of high -Q₃₁ or

I d_3ϊ

I > 10 CIm², and of a reasonably high electromechanical coupling factor of /C3₁ >

0.50, are used as the driving elements to drive the micromotor. Preferably,

-c/₃₁ is > 1200 pC/N and I J₃₁ As₁* I > 15 C/m² and of a reasonably high

electromechanical coupling factor of /C₃₁ > 0.50, are used as the driving

elements to drive the micromotor.

In a preferred embodiment of the micromotor, each active element comprises of a single or a plurality of d₃i-mode single crystal plate. In another preferred embodiment of the micromotor, the single crystal active element(s) is(are) in the form of suitably poled tube(s) of appropriate orientation cut, either with or without the use of an internal or external metallic tube. Alternatively, the single crystal active element(s) is(are) in the form of suitably poled rod(s) of appropriate orientation cut, with or without the use of an external metallic tube.

In one aspect of the micromotor, a solid rotor shaft is used. A hollow rotor may also be used, instead of a solid shaft, for the insertion of guide wire or other functions.

In another aspect of the micromotor, the stator may contain appropriate end caps, either at one or both ends.

In another aspect of our invention, the concept of ultrasonic micromotor design is based on the frictional coefficient between the rotor and the stator being increased via established or conventional means to increase the frictional force and hence output torque of the micromotor. Preferably, user- adjustable means are provided which may include any one or combination of mechanical, electromechanical, magnetic and electromagnetic means for varying the output torque range of said micromotor. Preferably still, one end of the stator is actuated to drive the rotor. More preferably, a plurality of stators is used to drive the rotor of said micromotor collectively.

The motor or micromotor of our invention may preferably be driven by

one electric source with a 90° phase shifter or by more than one electric source with or without appropriate phase shifters depending on the poling configuration of the driving elements, and wherein the electrical circuits may be appropriately biased when so desired. Preferably, the micromotor has multilayer driving elements each with electrical circuit for selective and/or collective actuation over user-selectable range of output torque.

In another aspect of our micromotor, a pulsed drive scheme may preferably be provided to enable the micromotor to operate at very low rotation speeds, including speeds from fractions of a revolution per minute (rpm) up to its maximum speed while maintaining its high output torque throughout the speed range. The stators are driven by pulsed drive scheme comprising an active phase containing input signals ranging from one to a few tens cycles of the design resonant frequency of the micromotor and at sufficiently high voltage amplitude to set the motor in brief or intermittent bending vibration motion in driving the micromotor, and optionally biasing the electrical circuit appropriately to avoid depoling of the driving elements as required.

Preferably, the input voltage signals within the pulsed input has at least one input signal having an amplitude large enough to set the micromotor in brief or intermittent bending motion. With such accurate angular displacement positioning control and actuation, the micromotor may thus function as a stepper motor, particularly with the pulse-driven scheme.

In a preferred embodiment, the stators are driven by pulsed-driven scheme comprising an active phase containing input signals ranging from one to a few tens cycles of the design resonant frequency of the micromotor and at sufficiently high voltage amplitude to set the motor in brief or intermittent bending vibration motion, followed by or interspersed with at least an idle phase of suitable duration. The brief or intermittent bending vibration motion is preferably enabled with input voltage signals within the active phase having, at least a fraction of said signals, voltage of sufficient amplitude to set off said bending vibration motion. Preferably still, the signals within the idle phase of the micromotor drive comprises any one of :

- nil signal input;

- the signal in DC or AC which frequency lies outside the resonant frequency range of the motor; or - the signal in DC or AC which frequency lies within the resonant frequency range of the motor but is of insufficient voltage amplitude to drive the motor.

In yet another preferred embodiment of the micromotor, the rotation speed of the micromotor drive is controlled, including any one or combination of by fixing the micromotor's rotation speed from a fraction of rpm to low hundreds of rpm or varying the rotation speed from a fraction of rpm to its maximum rotational speed as attainable with continuous drive means.

Another preferred embodiment provides the driving circuit to incorporate control means for amplitude of the applied voltage, driving signal waveform, working frequency, voltage and/or current limiter, preload level, and other conventional parameters in controlling the rotation speed and angular displacement of the micromotor. Preferably, the driving circuit includes means for feedback control of any one or combination of rotation speed, output torque and/or output power of the micromotor, resonance frequency changes due to heat generated during continuous operation or changes in ambient temperature.

The various embodiments of the micromotor described above may preferably be packaged into an appropriate housing incorporating securing and bearing means, or incorporated in a device or appliance. LIST OF ACCOMPANYING DRAWINGS

The present invention will now be described in detail with reference to the accompanying drawings that follows, wherein specific embodiments are described as non-limiting examples or illustrations of the workings of the invention, in which:

FIGURE 1 (Prior Art) shows conventional micromotor operating principles wherein Fig. 1(a) illustrates the operating cycle of cylindrical ultrasonic micromotors and Fig. 1(b) schematically shows a typical driving circuit;

FIGURE 2 shows 2 graphs illustrating the relationship between dynamic clamping effect between the rotor and the stator pair with the operation characteristics of ultrasonic micromotors wherein graph (a) shows the effects of low dynamic clamping load and graph (b) shows the effects of high dynamic clamping load;

FIGURE 3 shows a graph illustrating the relationship between the output torque range of PZT-ceramic-driven ultrasonic micromotors and predicted ranges of relaxor-based single crystal driving elements of single- layer (n=1) and dual-layer (n=2) structures being compared;

FIGURE 4 shows cross-sectional views of the construction of the single crystal ultrasonic micromotor prototype used in the demonstration of our invention;

FIGURE 5 shows a schematic cross sectional views of our proposed micromotors showing the polarization orientations of adjacent single crystal plates. Each stacked driving element contains an even number of single crystal plates in this case, (a) and (b) show two different driving circuits for the orientations shown;

FIGURE 6 shows graphs illustrating the performance of a 2.4 mm stator diameter single crystal micromotor (with n=2) prototype according to our invention;

FIGURE 7 (Prior Art) shows a graphical representation of conventional electrical input signals for driving micromotors of our invention;

FIGURE 8 shows a graphical representation of a pulsed electrical signal for accurate angular displacement control of micromotors according to our invention;

FIGURE 9 shows a graphical representation of another possible pulsed electrical signal for accurate angular displacement control of micromotors according to our invention;

FIGURE 10 shows a graphical representation of an "active-idle" cycle for speed control of micromotors of our invention;

FIGURE 11 shows schematic representations of examples of variants of the present invention which include the use of individual single crystal plates or stacks containing odd numbers of single crystal plates as the driving elements wherein Fig. 11 (a) and Fig. 11(b) show two possible driving circuits for the orientations shown;

FIGURE 12 shows an alternative design of the single crystal driven micromotor of our invention;

FIGURE 13 shows an example of another design variant of our proposed micromotor construction;

FIGURE 14 shows an example of micromotor designed with a plurality of short stators for greatly improved output torque; and

FIGURE 15 shows a graphical representation of a variant of the pulsed drive scheme according to the present invention and the method of determining n_a and f_p values. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Concept of Dynamic Clamping Load and Output Torque

Although extensive studies have been performed on cylindrical ultrasonic micromotors in prior art, aspects concerning their maximum output torque have not been adequately addressed. For instance, Morita ef a/, [ref. nos. 22, 24] have used the equivalent circuit approach to extract the force factor of their micromotor. This technique, however, applies to the condition of negligible external load. The effect of preload on the maximum output torque of the micromotors has also not been addressed thus far. This invention thus evaluates for the first time the dynamic clamping load for the rotor-stator assembly and its relation to the output torque of cylindrical micromotors.

Micromotors are driven by the frictional forces between the stator and the rotor. The stator transducer is often made in cylindrical or rod-shape. Assuming a hypothetical composite stator made of a metal tube of outer radius r_o and powered by 4 piezoelectric driving elements with their lengths lying along the axis of the stator tube, i.e. in longitudinal direction, Under free-

to-free end bending vibration condition, the natural vibration frequencies ω_f

and corresponding mode shapes W_t(z) of the stator transducer can be

estimated using the Timoshenko beam theory [which are described ϊn ret nos. 22, 28] based on the following equation 1.

and W_t(z) = C,.[sin(α. /2)cosh(a_tz/L) -sinh(α. /2)cos(α,.z/£)] (2)

where α, is the frequency parameter (α, = 4.7 for the fundamental bending

mode, i.e. /=1 );

E₅ is Young's modulus;

G₅ is the shear modulur;

p_s is the density of the stator material;

L is the length;

A₅ is the cross section area,

I₅ is the polar moment of inertia of the stator transducer; and

Cj and η are adjustment coefficients.

The triangular brackets in equation (2) denote functions of the argument shown and similarly thereafter.

For cylindrical micromotors, the fundamental bending mode is employed, i.e. / = 1 in equations (1) and (2). In this case, the longitudinal

displacement U₁ (z) at both end faces of the stator transducer, i.e., at z = i/2,

is given by /2) (3)

where

-Ce₁ sin(α_j /2)sinh(α, ll)

A₁ =

Sm(CC₁ / 2) COSh(CK₁ / 2) - sinli^ / 2) cos(α_j / 2)

Alternatively, M₁(I, /2) may be described as the static displacement in

the longitudinal direction, S₁(LH) , magnified by a quality factor, Q, as

Parameters. S₁(LIl) can be estimated from a static analysis of

piezoelectric plate induced displacement as follows. Consider a stator tube, of length L, outer radius r₀, and inner radius η, excited by a pair of

piezoelectric plates, or length L, width b and thickness h. Let S₁^ be the elastic

compliance in the length direction and dβi be the transverse piezoelectric strain coefficient of the piezoelectric material. Then, for a piezoelectric plate of length much larger than the other dimensions, the constitutive equation can be simplified into a one-dimensional equation as,

^ =^ +^₃ (5) where £₃ is the electric field applied across the thickness direction of the

piezoelectric plates;

T_x is the stress in the length direction developed in the piezoelectric

plates due to both the externally applied load and induced stresses due to the constraint effect of the stator tube; and

S₁ is the resultant strain in the length direction at the interface between

the piezoelectric plate and the stator.

Under the applied electric field, the moment M₅ induced by a pair of

facing piezoelectric plates causes the stator tube to bend with curvature, κ₅ ,

and is given by

M_s =-2T,bhr_o =E_sI_sK_s (6)

where E₅ is the Young's modulus;

I₅ is the bending inertia of the stator tube; I₅ = π(r* -r*)/4 for thin

cylinder. The curvature κ₅ is related to the strain S₁ at the surface layer of the

stator tube by K₅ =S₁ Ir₀.

From equations (5) and (6), the strain can be obtained as

S_λ =d_mξ₃ /φ (7) and δ_ι(L/2) = S_i - = (d₃₁ξ₃L/2)/φ (8)

_{where (9)}

Ef₁ (= M Su) is the Young's modulus of the piezoelectric material along

the longitudinal direction and A_s (=π{r* -r,²}) and (4bh) are, respectively, the

effective cross-sectional area of the non-piezoelectric part and that of the piezoelectric part of the stator transducer. The term within the first brackets in equation (9) gives the stiffness ratio of the non-piezoelectric and piezoelectric of the stator transducer.

FIGURE 1 (Prior Art) shows a conventional micromotor operating principles wherein Fig. 1(a) illustrates the operating cycle of cylindrical micromotors' motion and Fig. 1(b) schematically shows a typical conventional circuit for driving a micromotor, wherein V_pp and f_r are the peak-to-peak driving voltage and the resonant frequency of the micromotor, respectively.

Since the rotor is driven by a travelling wave formed at one or both ends of the stator, at any instant of time, only one point (or a very small area) of the stator end, which displays the maximum longitudinal displacement, should be in contact with the rotor (Figure 1a). At this contact point, the particle tangential velocity is at the maximum while the particle longitudinal velocity is zero. To maintain the non-slip condition at the contact point, it is imperative that the longitudinal velocity of the rotor is restored to that of the stator by the preload almost immediately after the particle longitudinal of the stator velocity at the contact point has changed sign. The use of a spring of appropriate compliance plays a key role in this respect. It enables the preload to adjust itself during the operation of the micromotor.

FIGURE 2 shows 2 graphs illustrating the relationship between dynamic clamping effect between the rotor and the stator pair with the operation characteristics of ultrasonic micromotors. "Dynamic clamping" here refers to the condition under which the rotors are dynamically clamped onto the stator by the preload and become a part of the stator in the bending vibration. The graph in Fig. 2(a) pertains to that of low dynamic clamping load and graph (b) pertains to that of high dynamic clamping load. For high output torque, the micromotor should be operated in the non-slip slightly-to-partially- clamped region marked in the figure. Increasing both the dynamic clamping load of the stator-rotor assembly and the preload level are essential in designing ultrasonic micromotors of high output torques.

Figure 2 shows schematically the effect of preloads on the rotation speed of an ultrasonic micromotor. At low preloads, slip occurs readily between the rotor and the stator. This phenomenon can be readily understood especially when the preload is negligible. In this case, the force generated by the bending vibration is sufficient to set the rotor to continue to move upwards as a result of the inertia effect even when the particle longitudinal velocity of the stator at the contact point has changed sign. The application of a sufficiently high preload to counter this inertia effect is thus essential in order to ensure that the rotor remains in contact with the stator at the contact point at all times. This occurs at the optimum preload level at which the rotation velocity is also at its maximum as shown in Figure 2.

Figure 2 further shows that with further increase in the preload level, although non-slip condition is realized, the rotation speed drops, gradually first and then more abruptly. The micromotor eventually ceases to rotate at sufficiently high preloads. The sudden decrease in the rotation speed with increasing preload in this region can be attributed to the increased "clamping" effect produced by the preload such that the rotor is "clamped" onto the stator and vibrate together with the stator as though they are one single body.

Since the frictional force which drives the rotor is originated from the preload applied, the higher the preload, the higher will be the frictional force attainable and hence the output torque. However, as shown schematically in Figure 2, the maximum preload allowed is that corresponds to the dynamic clamping load of the rotor-stator assembly. As will be shown below, the dynamic clamping load is directly related to the longitudinal displacement amplitude of the stator and hence the type of piezoelectric driving material used. Consider the instance when the rotor is in contact with the stator at the highest displacement point and that the stator starts to retract in the opposite direction. Assuming that the rotor now acts as a free body. Under the action

of the preload F_p , the rotor will move in the negative longitudinal direction

with an acceleration a_R given by

F_p = m_Ra_R (10)

where m_R is the mass of the rotor. In the absence of the stator, the downward

velocity of the rotor will increase according to equation (10). Clamping of rotor occurs when at all times the velocity produced by the preload is equal or larger than the maximum longitudinal velocity of the stator at the end face. For a point on the end face of the stator in bending vibration, the maximum longitudinal velocity occurs when its tangential velocity is nil, and this occur after the stator and hence the rotor have spent a quarter of the period in motion from its maximum longitudinal displacement point. The particle

longitudinal velocity of the stator v_{S max} and that of the rotor v_R at his instance

are given by

(H)

and v_R=F_p(T/4)/m_R=F_p/4f_rm_R (12)

where T and ω_r (and f_r , where ω_r = 2πf_r) are the period and resonant

frequency describing the bending vibration of the stator transducer, i.e. T =

1//_r ; φ is given by equation (9). The condition for clamping corresponds to

whenv_Λ>v_Sjmax,or

4πm_RQf_r ²Ld₃₁ξ₃

K = (13) φ

For a thin-wall cylinder, the shear effect on the resonant frequency in

equation (1) may be ignored, i.e., f_r «— 1 I C —a₁W I I ~E^s ₈ -I ^s, Ϋ² , giving 2π\L) {p_sA_s^

where I_s =π(r*-r_t )/4 and A_s=π(r;-r?) are invoked in equation (14).

4

CΛ-— — ) is a constant. V_pp in equation (14) is the applied peak-to-peak

4π

voltage across the thickness h of the piezoelectric driving element, i.e. ξ₃ =

V_ppf2h.

The maximum output torque, 7_max, is thus given by G2006/000129

25

_τ _ _{?( f F} v _ _{?r f m} ^Es ^rlV + fo Ir₀?] d_3λ Q V_pp

Ps L S_n φ h

where f_c is the coefficient of friction between the stator and the rotor. In equation (15) above, it is assumed that the rotors are driven by both the top and bottom faces of the stator, as in our example demonstration below.

Under this condition, the output torque is not (f_cF_c)r₀ but 2(f_cF_c)r₀. Equation

(15) can be rewritten as

T_^ /V_pp = 2CJ_cf_Rf_Smf_Sdf_Pm f_s__pf_Pd (16)

where f_c (=coefficient of friction) is a parameter related to the properties of the

rotor-stator pair, f_R(= m_R) to the property of the rotor,

fsm (=E_S / p_s , or specific modulus) to the property of the stator material,

f_Sd to the size and design of the stator transducer,

fp_m (⁼^₃i l^su) ^{to tne} properties of the piezoelectric driving element;

fs-p (-Ql φ) to ^tne interactive effect of both the piezoelectric and non-

piezoelectric parts of the stator transducer; and

f_Pd (= Mh) to the design of the piezoelectric driving elements of the

stator transducer (i.e. its thickness, number of layers, etc.). Note from equation (16) that given the rotor and stator materials (via f_c, f_R, and fsm), the voltage-normalized maximum output torque (i.e. T_may/V_Pp) depends only on the design and construction of the stator transducer (i.e., via fs_d and f_s.p) and the properties (i.e. via fp_m) and the design and construction

(via fpβ) of the driving piezoelectric elements.

It should be noted that at preload levels equal to and above the dynamic clamping load, the rotor is "effectively clamped" onto the stator, i.e., both the rotor and the stator are in full contact and vibrate as though they are a single body. As the preload is decreased gradually from this critical value, the rotor will gradually free itself from the full contact situation, i.e., the overall contact area will decrease, or at least intermittently, and the rotation speed of the rotor will increase accordingly. At the point when the preload is decreased to the extent such that only a very small area of contact is realized between the rotor and stator, the rotation speed is the maximum. This corresponds to the condition of optimum preload. Note that for the preload range from the optimum preload to the dynamic clamping load, there is no slip between the rotor and the stator. Below the optimum preload (for a given applied voltage), slip occurs between the rotor and the stator and the rotation speed of the rotor decreases accordingly.

A non-slip condition for the operation of cylindrical ultrasonic micromotors is desirable because slip results in wear, noise, heat, etc., and hence degrades the performance and life time of the micromotors. The operating range of an ultrasonic micromotor of high output torque is thus that corresponds to when the applied preload is slightly above the optimum value but below the dynamic clamping load value, i.e., in the "non-slip slightly-to- partially-clamped" region marked in Figure 2. Since this operating region scales with the magnitude of dynamic clamping load of the system, the adoption of rotor-stator assembly of a high dynamic clamping load is essential for the making of ultrasonic micromotors of high output torque. As shown in the present invention, the dynamic clamping load is determined by the stator design, the electromechanical properties of the driving elements used and other factors through Equation (16).

FIGURE 3 shows a graph illustrating the relationship between the output torque range of PZT-ceramic-d riven ultrasonic micromotors of 1.0-2.5 mm stator-diameter according to prior art, the low output torque being a result of their resultant low dynamic clamping loads. The predicted ranges of output torque of micromotors driven by relaxor-based single crystal driving elements of single-layer (n=1) and dual-layer (n=2) structures are included for comparison. The open symbols are the measured maximum output torques obtained from a prototype single-crystal driven micromotor of 2.4 mm stator- diameter constructed according to the preferred embodiment of the present Invention (with n=2). (See also Figure 6 and its description). Given in Figure 3 is the estimated maximum output torque derivable from micromotors of 1.0 to 2.5 mm in stator-diameter driven by state-of-the-art

lead zirconate titanate (PZT) ceramics (d₃₁ « 100-250 pC/N; sf_x « 20 pm²/N).

It is evident that the maximum output torque remains marginal in this case,

being < 100 μNm even when driven at 50 V_pp (or 17 V_rm_S), or < 2.0 μNm/V_pp

typically. Also given in Figure 3 are the expected maximum output torques for micromotors driven by relaxor-based single crystal driving elements, which

are of superior piezoelectric properties {d₃ή « 1000-4000 pC/N; s^ « 40-200

pm²/N). Compared with PZT ceramics, the output torques of single crystal driven micromotors are several times higher at the same applied voltage, i.e. 2.5-10 μNm/V_Pp (for n = 1). Thus, we may conclude that relaxor single crystal driving elements are essential for a high dynamic clamping load and hence output torque of ultrasonic micromotors as opposed to that available from micromotors of prior art.

Note that equation (16) is the output torque for ultrasonic micromotors driven by piezoelectric plate of thickness h of single layer construction, such

that ξ₃ = V_Pp/2h. One way to maintain the same torque but at lower applied

voltage is to use multilayer driving elements of identical overall dimensions. Let n be the number of layers within each plate of driving element and t the

thickness of each layer, i.e. t = bin, using ξ₃ = V_ppl2t, equation (16) becomes

GT-, / y_» ) "-layer = - n{2CJJ_Rf_Sm f_sd /_Λ f_s__pf_M ) (17) Figure 4 shows a cross-sectional view of the construction of the single crystal ultrasonic micromotor prototype used in the demonstration of our invention. Each of the four driving elements 1 consists of two single-crystal darplates (i.e. n=2), of opposite polarization, bonded back-to-back using a conductive epoxy. The driving elements were bonded onto the four flattened surfaces of the metal stator tube 2. The stator is assembled with an upper rotor 3a, a spring 4, a lock nut 5 and a hollow rotor shaft 6 comprising the lower rotor 3b into the motor. The spring is adjusted to obtain the desired preload level, which determines the maximum output torque of the motor. A solid rotor shaft may also be used instead when appropriate.

A prototype single crystal micromotor was fabricated to attest the present invention. Figure 4 gives a schematic explaining its construction. The main part of the stator is a metallic tube 2 (which is an aluminum tube in the demonstration used although other metal tubes of reasonably high

specific modulus, E_sl ρ_s , also work well), of 2.4 mm outer diameter, 1.2 mm

inner diameter, and 13 mm length. The driving elements 1 were made of single crystal stacks of 0.4 mm thick, formed by bonding two [001]-poled PZN- 7%PT single crystal plates, of 12.5^L*1.2^wχ0.2^τ mm³ in dimensions each, back-to-back using conductive epoxy. The length direction of the single crystal plates is along the [110] pseudo cubic crystal direction. For PZN-7%PT single crystal plates, this poling-direction-length-orientation combination gives

relatively high d₃₁ but low ^f₁ values (of around -1300 pC/N and 40*10¹² m²/N, respectively, or a ratio of d_3X lsf_x >30 C/m²) arid high electro-mechanical

coupling factor (i.e., Zc₃₁ « 0.85). The poling directions of adjacent single

crystal plates within each stack are given in Figure 5. To ensure more efficient power transmission, the contact areas of the end faces of the stator

were tapered inward at a suitable angle, being 10-45° typically (a taper angle

of 18° was used in the demonstration which was determined based on the

dimensions of the stator tube).

FIGURE 5 shows a schematic cross sectional views of our proposed micromotors showing the polarization orientations of the two single crystal plates within each of the four stacked driving elements used in the demonstration unit. When driven by (a) single electric source (plus a 90°

phase shifter) and (b) two electric sources of 90° phase difference (or equivalent circuits). The driving circuits may be appropriately biased when so desired. The figures on the right show typical polarization arrangements when stacked driving elements containing even numbers (i.e., 4, 6, 8, etc.) of single crystal plates are used. V_pp and f_r are the peak-to-peak driving voltage and the resonant frequency of the micromotor, respectively.

In the demonstration used, the facing stacks of single crystal plates were bonded using conductive epoxy onto the four flattened surfaces of a metallic stator tube in such a manner that they produce a tensile and compressive pair when connected to the same voltage input (Figure 5a). In this way, an electrical source plus a 90° phase shifter, corresponding to two

electrical sources with 90° phase difference, suffice to excite the motor with

four driving elements. The resonance frequency of the assembled motor with preload covers from 37 to 44 kHz, which is slightly lower than the value of 45 kHz obtained for the stator transducer alone. The driving circuits used are

shown in Figure 5. On changing the phase difference to -90°, the motor runs

in the reverse direction.

FIGURE 6 shows graphs illustrating the performance of a 2.4 mm stator diameter single crystal micromotor (with n=2) prototype according to our invention used in the demonstration. The operating preload ranges according to the present invention are shaded for easy reference. The effects of preload on the rotation speed and the maximum output torque of the micromotor under an applied voltage of 20 V_pp (or 7

respectively, in continuous drive mode are shown in Figure 6.

The rotation speed was determined by means of the laser vibrometer and the maximum output torque by the weight-lifting technique. The latter was taken as the applied torque (determined by the added weights being lifted) under which the motor ceases to rotate. The non-slip slightly-to-partially- clamped region has been utilized for the operation of the prototype micromotor, as indicated in the figure. The registered maximum output

torques are about 280 μNm at 20 V_pp and 560 μNm at 40 V_PPi respectively.

This is equivalent to 14 μNm/V_pp, which is at least an order of magnitude higher than the highest value of output torque for micromotors of similar dimensions reported in prior art. It should also be noted that the maximum output torque of the micromotor used in the demonstration can be further improved by increasing both the preload and applied voltage accordingly.

Pulsed Drive Scheme for Stepping Function and Rotation Speed Control

FIGURE 7 (Prior Art) shows a graphical representation of conventional electrical input signals for driving ultrasonic motors or micromotors. The signal has a frequency in the ultrasonic range, which is also the resonant frequency (f_r) of the motor. V_pp is the peak-to-peak driving voltage (Prior art).

Depending on the actual design, ultrasonic motors or micromotors reported in prior art [1-27] can be divided into either standing or travelling wave type. While standing-wave ultrasonic motors often need a single electric source to operate, the traveling wave type requires two or more electric sources of appropriate phase difference to drive. Both types of ultrasonic motors are excited by continuous electrical inputs at ultrasonic frequencies, which are also the resonant frequency of the stator-rotor assembly of the motor concerned. A schematic of a typical sinusoidal input signal for operating ultrasonic motors is given in Figure 7. The period of the electrical signal is 1/f_r, where f_r is the resonant frequency of the stator-rotor assembly. In a conventional drive system, the amplitude of the applied voltage is sufficiently high to set the stator in vibration of the intended resonance mode, which in turn causes the rotor to rotate via friction.

Typical ultrasonic motors or micromotors run at resonant frequencies in the range of 20-200 kHz. The period of the driving electrical signal is thus

about 5-50 μsec. This small period provides an opportunity for redesigning the

driving input signals to enable more accurate control of the angular displacement of the motor and its rotation speed.

FIGURE 8 shows a graphical representation of a pulsed electrical signal for accurate angular displacement control of ultrasonic motors or micromotors according to our invention. Each pulse consists of a few to few tens cycles of 1/f_rperiod signals of sufficiently high voltage amplitude to set the motor in motion, f_r being the resonant frequency of the motor. The pulsed input signal may be appropriately biased when so desired.

The pulse driven scheme has an active phase, of pulse-like input signals containing a few to few tens cycles of driving signals of resonant frequency (f_r) and sufficiently high voltage amplitude, followed by an idle phase of durations depending on the intended application.

We have investigated the effects of the number of cycles of the Vf_r period signals of sufficiently high voltage amplitude, n_a, hereafter referred to as "the pulse content", on the operational smoothness of the ultrasonic micromotor. The results showed that our experimented motor operated in a

controlled manner when n_a >5 but is a less controllable when n_a< 5.

Assuming that the motor takes about one full cycle to set into and subside from resonance, respectively, the actual number of 1/f_rperiod cycles during which the motor is activated in resonance by a n_a=5 pulse is thus (5-2)

or 3. This corresponds to a response time of 15-150 μsec. For an ultrasonic

micromotor with an original rotation speed of, say, 900 rpm (or 15 rev/s) when excited with the continuous input signal (of f_r), this corresponds to about

0.0014 rad (or about 0.08°) in angular displacement for f_r = 200 kHz and to

about 0.014 rad (or about 0.8°) for f_r = 20 kHz. In other words, with the pulsed input shown in Figure 8, ultrasonic motors or micromotors can function as stepper-motors with an angular positioning accuracy in the range of 0.001- 0.01 rad (or about 0.06-0.6°) depending on the operating frequency.

FIGURE 9 shows a graphical representation of another possible pulsed electrical signal for accurate angular displacement control of ultrasonic motors or micromotors of our invention. Each pulse consists of a few to few tens cycles of 1/frperiod signals, f_r being the resonant frequency of the micromotor. Except one as shown, or a few in other cases depending on applications, the voltage amplitudes of the 1/f_rperiod signals within the pulse are not high enough to set the motor in motion. The pulsed input signal may be appropriately biased when so desired. To further increase the angular positioning accuracy of the motor, the pulse content shown in Figure 9 or of equivalent nature can be used. In this case, other than the large-amplitude 1/f_rperiod signal(s) which generates sufficient displacement via the piezoelectric driving elements to set the motor in motion, the remaining 1/f_rperiod signals are too weak to cause the motor to rotate. In other words, n_a ~1 in this case and the accuracy in angular

positioning is improved to better than 0.0005 rad (or < 0.05°) for an ultrasonic

motor operated above 100 kHz. The positioning accuracy can be improved further should ultrasonic motors of larger diameters be used, which yield lower rotation speeds when driven with the continuous drive scheme.

Figure 10 shows a graphical representation of an "active-idle" cycle for speed control of micromotors of our invention. In a pulsed drive system, each input pulse (of n_a numbers of 1/f_rperiod signals for the case of Figure 8 or n_a=1 for the case of Figure 9), which forms the active part of the input signal, is followed by an idle phase. Figure 10 gives an example of such an "active- idle" pulsed drive scheme. The motor is set into rotation during the "active" phase but stops to rotate during the "idle" phase of the input signal. Let f_p be the pulse frequency, i.e. that of a complete "active-idle" cycle (Figure 8), by engineering the values of n_a and f_p, the proposed pulsed drive scheme can be used to control the rotation speed of the ultrasonic motor or micromotor.

The "active" phase consists of a pulse which, in turn, is made up of a few to few hundreds cycles (say, n_a cycles) of Mf _r period signals of sufficiently high voltage amplitude to set the motor in motion, f_r being the resonant frequency of the motor. The motor remains at rest in the subsequent "idle" phase. f_p is the pulse frequency, i.e. that of a complete "active-idle" cycle. With suitable combinations of the pulse content (n_a) and pulse frequency (f_p) values, the motor can be made to have a wide range of rotation speed to suit various applications, from fractions of a rpm to the full rotation speed without the idle phase. The input signal may be appropriately biased when so desired.

For instance, let VR be the rotation speed (in rpm) obtained with the pulsed input signal (of n_a and f_p as in Figure 10) and V₀ be the rotation speed

(in rpm) obtained with the conventional continuous input signal without the idle phase. The rotation speed of an ultrasonic motor driven by the proposed pulsed drive scheme is thus given by,

F_Λ « F₀ (^^) when n_a * fp < f_r. (18a)

and V_R ~V₀ when n_a ^χ f_p ≥ f_r (18b)

Taking f_r =50 kHz and assuming a maximum rotation speed (V₀) of 900 rpm when driven with the continuous signal, the rotation speeds of the motor

(VR) with different combinations of the pulse content (n_a) and pulse frequency

(fp) values can be estimated using equation (18). The results are given in

Table 1. This table shows that with proper selection of the values of the pulse content (n_a) and pulse frequency (f_p), an ultrasonic motor or micromotor can be tailored to operate at very low rotation speeds, being a fraction of rpm. It can also function as a variable speed motor if ways and means are provided to alter the pulse content (n_a) and/or the pulse frequency (f_p) before or after the motor is set in motion. With such a pulsed drive scheme, the rotation speed of a ultrasonic motor can be varied over a wide range, from fractions of a rpm to its full speed with continuous drive, or a fraction of the above range.

Table 1 - Overall rotation speed VR (in rpm) for different values of n_a and f_p for an ultrasonic motor with f_r = 50 kHz and V₀ =900 rpm

^Equivalent to continuous drive at resonance frequency.

FIGURE 11 shows schematic representations of examples of variants of the present invention which include the use of individual single crystal plates or stacks containing odd numbers of single crystal plates as the driving elements wherein Fig. 11 (a) and Fig. 11(b) show two possible polarization orientations and the associated driving circuits when individual single crystal plates are used. The driving circuits may be appropriately biased when so desired. The figures on the right show typical polarization arrangements when stacked driving elements containing odd numbers (i.e., 3, 5, 7, etc.) of single crystal plates are used. V_pp and f_r are the peak-to-peak driving voltage and the resonant frequency of the micromotor, respectively.

Variants of the present invention include ultrasonic micromotors activated by single crystal driving elements of different forms, such as individual plates, single crystal stacks of two or more layers (of either even or odd number), and in the form of a suitably poled tube or solid rod, to provide the bending vibration mode to drive the micromotors, either with or without the use of an internal and/or external metallic tube when applicable. Schematics of such variants in which individual single crystal plates or stacks containing different numbers of single crystal plates are used as the driving elements can be found in Figures 5 and 11. Also shown in these figures are possible poling orientations and associated driving circuits. The latter may include an appropriate d-c bias when driving at large voltage amplitudes, which serves to prevent depoling of the single crystal driving elements.

Although PZN-7%PT single crystal c/₃rdriving elements of [110]-length cut and poled in the [001] crystal direction were used in the demonstration, other single crystal o^-mode driving elements, preferably of but not limited to PZN-PT and PMN-PT compositions, of suitable orientation cuts with reasonably high c^ and Zc_3? values but low ^ values, i.e., with /C_3? > 0.5 and

ratio of d_3l /s*_x > 14 C/m², and/or other suitable shapes, such as in tube or rod

form, may also be used.

The stator used in our demonstration is one without the end caps. The use of end caps of appropriate materials and designs helps smoothen the bending vibration motion at the end faces of the stator and is highly desirable when the entire stator is made of a bulk single crystal tube or rod which are typically brittle. Preferably, the end caps should be bonded firmly onto the stator tube or rod with an appropriate means such as an epoxy. An example of such can be found in Figure 12.

FIGURE 12 shows an alternative design of the single crystal driven micromotor of our invention which has a stator 8 with two end caps 9a and 9b and two rotor disks 10a and 10b, one at each side of the stator, a hollow and grooved rotor shaft 11, a disk spring 12 and a C-clip 13. A solid rotor shaft may be used when so desired. The rotor shaft assembly may be of other appropriate multi-piece designs to facilitate their fabrication, assembling and maintenance. For instance, instead of a threaded shaft, other reliable means may be used for precise preloading purposes. Figure 12 shows, for example, a grooved rotor shaft 11 design for use with a disk spring 12 and C-dϊp 13. Other alternatives include the use of epoxy for fixing the preloading nut and/or ring. Appropriate friction materials or coatings may be incorporated at the contact surface between the stator and the rotor, which serves to increase the friction between the pair and hence to provide improved output torque of the ultrasonic motor or micromotor.

In the case when a much wider range of adjustable output torque is desired, features allowing for the adjustment of preloads, be it mechanical, electromechanical, magnetic, electromagnetic, etc., may be incorporated in the motor design. In addition, the driving circuit may also include features which enable different layers of the multilayer driving elements to be activated selectively or collectively.

When the overall length of the micromotor must be keep low, the use of other appropriate forms of springs in place of the coil spring may be employed. Similarly, a retaining ring instead of a standard nut may be used.

Figure 12 shows an example of such in which a disk spring 12 and a C-ring

13 are used instead.

FIGURE 13 shows an example of another design variant of our proposed micromotor construction in which only one end of the stator 14 is used to drive the hollow rotor shaft 15 while the other end in a clamped position. A bearing 16 may be used to facilitate the smooth operation of the motor or micromotor. A longer hollow rotor shaft may be used which may 00129

41

protrude through the bottom end for insertion of guide wire or other suitable functions. A solid rotor shaft may also be used instead when so desired.

When desired, only one end of the stator may be used to drive the rotor while keeping the other end in a clamped state. An example of such is given in Figure 13. The use of a ball bearing 14 or other suitable type of bearing is advantageous in this case. Note, however, that with this design, the ensuring output torque may be significantly lower compared with the dual-end drive design.

FIGURE 14 shows an example of micromotor designed with a plurality of short stators for greatly improved output torque. Note from Eq. (15) that the maximum output torque is proportional to the cube of the radius-to-length ratio of the stator. For a given stator radius, its length should be kept short whenever possibly to attain high output torque. An alterative design is to use plurality of short stators of identical or similar designs. An example of such is given in Figure 14. In such designs, the output torque will be greatly enhanced when individual stators have nearly identical resonant frequency. For instance, the output torque will be doubled for a dual-stator design, tripled for a three-stator design, and so on. It should be stressed that although single crystal driven stators are preferred in the present invention, this multi-stator design is also valid for stators made of a metal tube driven by piezoceramic driving element or a bulk piezoceramic tube, which are suitably poled and electrically powered into bending vibration. G2006/000129

42

Although the preferred embodiment of the present invention and its variants concern motors or micromotors with a hollow rotor shaft, they also apply to motors or micromotors with a solid rotor shaft. And, instead of a hollow stator made of a metal tube driven by single crystal piezoelectric element or single crystal tube, appropriately poled single crystal rod may also be used as the stator when so desired.

The proposed active-idle cycle may be executed in any suitable form. In general, any part of a driving signal of resonant frequency and sufficiently high voltage amplitude to set the ultrasonic motor or micromotor in intermittent rotation would constitute the active phase, while any part of the input signal over which the motor ceases to rotate would constitute the idle phase. An example of such is given in Figure 14. As shown in this figure, the active phase of the signal needs not be of a constant voltage. The pulse content n_a (i.e. the number of cycles of 1/fr-period signals of sufficiently high voltage amplitude to drive the motor in the pulse input signal) also needs not be fixed.

Similarly, the idle phase of the input signal needs not be of nil signals and may include signals of resonant frequency but of sufficiently low voltage amplitude, any d-c signals, or signals of frequency lying outside the resonant frequency of the ultrasonic micromotor. The pulse frequency f_p also needs not be a constant although a fixed f_p value would facilitate the control aspect of the motor. An appropriate d-c bias may also be used to prevent depoling of the single crystal driving elements when driven at large applied voltages.

FIGURE 15 shows a graphical representation of a variant of the pulsed drive scheme according to the present invention and the method of determining n_a and f_p according to the embodiment of the present invention. The input signal may be appropriately biased when so desired.

Other variants of the present invention include the use of the proposed pulsed drive concept together with other standard control schemes, such as via the amplitude of the applied voltage, driving signal waveform, working frequency, voltage and/or current limiter, preload level, and etc., for controlling the rotation speed and angular displacement of the single crystal driven ultrasonic motor or micromotor. The use of appropriate feedback circuit, such as with an encoders and/or other suitable means, may also be incorporated when so desired. The driving circuit may also include a temperature compensation unit which addresses the possible temperature effect on the resonance frequency of the motor or micromotors, either as a result of heating during continuous operation or change in environment temperature.

The housing of the micromotor may be made of any suitable material. Preferably, the stator is to be secured in the housing at its nodal points such that the bending vibration of the stator is not affected by the clamping effect of the housing. For free-free end vibration, the nodal positions are about 0.22L from each end of the stator, L being the length of the stator. For free-clamped end design, the nodal point is at the clamped end.

The housing, or motor packaging, may also incorporate appropriate bearings or bearing material to limit the extent of wobbling of the rotor shaft. The use of single-piece plastic or powder-metallurgy bearings suffices under low-rotation speed and/or low-torque application. The use of ball bearings is preferred for motors or micromotors for high rotation speed and output torque applications.

Securing features may be incorporated in the housing design for ease of motor installation purposes. Alternatively, the motor may be installed onto the base structure either by mechanically means provided with the base structure, or bonded onto the base structure with an appropriate epoxy.

Other References and Prior Art

1. M. Katsuma, H. Murakami and A. Hiramatsu, Vibration wave motor. U.S. Patent No. 4,513,219 (April 23 1985).

2. T. Sashida, Motor device utilizing ultrasonic oscillation. U.S. Patent No. 4,562,374 (December 31, 1985). 3. I.Okumura and K. Izukawa, Vibration wave motor with plural projection vibrator. U.S. Patent No. 4,580,073 (April 1, 1986).

4. Y. Imasaka, H. Yoneno, M. Sumibara and A. Tokushima, Ultrasonic motor. U.S. Patent No. 4,739,212 (April 19, 1988).

5. O. Kawasaki, R. Inaba, A. Tokushima and K. Takeda, Ultrasonic motor with stator projections and at least two concentric rings of electrodes. U.S. Patent No. 4,829,209 (May 9, 1989). 6. T. lijima, Method for converting standing wave vibrations into motion and standing wave motor therefore. U.S. Patent No. 4,882,500 (21 Nov 1989).

7. N. Fujie and Y. Kuwabara, Vibration wave motor. U.S. Patent No. 4,937,488 (June 26, 1990).

8. K. Kataoka, Vibration wave motor. U.S. Patent No. 5,001,404 (19 Mar 1991).

9. M. Kasuga, T. Mori and N. Tsukada, Traveling-wave motor. U.S. Patent No.5,006,746 (April 9, 1991 ).

10. M. Kasuga, N. Tsukada and H. Kitamura, Standing-wave type ultrasonic motor and timepiece. U.S. Patent No. 5,079,470 (January 7, 1992). 11. T. Fujimura, Ultrasonic oscillator and ultrasonic motor using the same. U.S. Patent No. 5,162,692 (November 10, 1992).

12. Y. Kawai, A. Takeuchi and K. Asai, Ultrasonic motor. U.S. Patent No. 5,172,023 (Dec. 15, 1992).

13. O. Miyazawa and K. Takedo, Ultrasonic motor. US. Patent No. 5,247,220 (Sep. 21, 1993).

14. M. Yano, Y. Takemura and T. Suzuki, Stator of ultrasonic motor and Method for manufacturing the same.. U.S. Patent No. 5,363,006 (8 Nov

1994). T/SG2006/000129

46

15. O. Miyazawa, T. Furukawa and J. Kitahara, Ultrasonic step motor. U.S. Patent No. 5,343,108 (August 30, 1994).

16. H. lmabayashi and T. Funakubo, Ultrasonic motor. U.S. Patent No. 5,376,858 (December 27, 1994).

17. R. Suzuki, Vibration driven motor, U.S. Patent No. 5,428,260 (January 27, 1995). 18. M. Kasuga, K. Suzuki, M. Suzuki and M. Suzuki, Ultrasonic motor and electronic apparatus equipped with ultrasonic motor. US. Patent No. 5,592,041 (January 7, 1997).

19. T. Takagi and T. Ashizawa, Ultrasonic motor. U.S. Patent No. 5,821 ,667 (October 13, 1998).

20. M. Kurosawa, K. Nakamura, T. Okamoto and S. Ueha, An ultrasonic motor using bending vibration of a short cylinder. IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 36 (1989), 517.

21. T. Morita, M. Kurosawa and T. Higuchi, An ultrasonic micromotor using a bending cylindrical transducer based on PZT thin film. Sensor and Actuators A, 50_.(1995), 75. 22. T. Morita, M. Kurosawa and T. Higuchi, A cylindrical micro ultrasonic motor using PZT thin film deposited by single process hydrothermal method (φ2.4 mm, L=10 mm stator transducer). IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 45 (1998), 1178. 23. T. Morita, M. K. Kurosawa and T. Higuchi, Cylindrical micro ultrasonic motor utilizing bulk lead zirconate titanate (PZT). Japanese Journal of Applied Physics, 38 (1999), 3347.

24. T. Morita, M. K. Kurosawa and T. Higuchi, A cylindrical shaped micro ultrasonic utilizing PZT thin film (1.4 mm in diameter and 5.0 mm long stator transducer). Sensor and Actuators A, 83 (2000), 225.

25. S. Dong, S.P. Lim, K.H. Lee, J. Zhang, LC. Lim and K. Uchino, Piezoelectric ultrasonic micromotor with 1.5 mm diameter. IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 50 (2003), 361.

26. X. Chu, L. Yan and L. Li₁ Characteristic analysis of an ultrasonic micromotor using a 3 mm diameter piezoelectric rod. Journal of Smart Materials and Structures, 13 (2004), N17.

27. P. Lu, K.H. Lee, S.P. Lim and W.Z. Lin, A kinematic analysis of cylindrical ultrasonic micromotors. Sensors and Actuators A, 87_(2001), 194.

Claims

1. A method of driving an ultrasonic micromotor with the concept of utilizing a rotor-stator assembly of high dynamic clamping load,

- defined as the critical preload to cause the rotor-stator assembly to vibrate as a single body and the motor to cease to function/rotate, for the design and fabrication of ultrasonic micromotors of high output torques and of a few to low tens mm in diameter wherein the high dynamic clamping load entails a high optimum preload for the operation of the motor under non-slippage conditions, which results in a high frictional force between the rotor and the stator thereby giving rise to the high output torque of the micromotor.

2. A method according to Claim 1 wherein the micromotor is of high output torque and dimension ranging from a few to low tens of millimetres in diameter.

3. A method according to Claim 1 Wherein the clamping load is at a high and dynamic clamping load value is achieved with piezoelectric active elements having high ratio of ,* wherein J₃₁ is the transverse mode piezoelectric strain constant; and

¹H is the elastic compliance constant along the length direction of the cfei mode actuator determined under constant electric field condition.

4. A method according to Claim 1 wherein the high dynamic clamping load is achieved with a bendiηg vibrator in the form of a thin-walled metal tube having a length-to-diameter ratio of > 2.0 and a high specific modulus material.

5. A method according to Claim 1 wherein the high dynamic clamping load is achieved with at least a rotor having high inertia; - made from high density material and/or diameter > that of stator, and suitable dimension according to design of the micromotor.

6. A method according to any one of Claims 1 - 3 wherein the high dynamic clamping load is achieved with a relaxor-based single crystal as driving element wherein a high preload may be applied to the micromotor to attain ouput torque of >2.5μNm/V_pp, micromotor stator and/or rotor is within a few millimeters in diameter and the output torque is achievable to > 2.5

7. A method according to Claim 6 wherein the output torque is > 5

8. A method according to Claim 1 wherein frictional coefficient between the rotor and the stator is increased via conventional means to increase the frictional force and hence output torque of the micromotor.

9. A micromotor according to Claims 1 to 8 having at least a means for preloading at least one drive element of said micromotor over its piezoelectric operating range, wherein said preloading is user-adjustable within a range optimal to torque output of said micromotor but below that of clamping load.

10. A micromotor according to Claims 1 to 8 comprising a piezoelectric rotor-stator assembly wherein at least one element of said rotor-stator assembly may be dynamically preloaded at a high optimum preload, resulting in a high frictional force between rotor and stator of the rotor-stator assembly, thereby giving rise to a high output torque.

11. A micromotor according to Claim 10 wherein the micromotor is of high output torque and dimension ranging from a few to low tens of millimetres in diameter.

12. A micromotor according to Claim 10 wherein the high dynamic clamping load value is achieved with piezoelectric active elements having high ratio of ^si_

4

wherein d_n is the transverse mode piezoelectric strain constant; and _S ^E is the elastic compliance constant along the length direction of the cfei mode actuator determined under constant electric field condition.

13. A micromotor according to Claim 10 wherein the high dynamic clamping load is achieved with a bending vibrator in the form of a thin-walled metal tube having a length-to-diameter ratio of > 2.0 and a high specific modulus material.

14. A micromotor according to Claim 10 wherein the high dynamic clamping load is achieved with at least a rotor having high inertia; made from high density material and/or diameter > that of stator, and suitable dimension according to design of the micromotor.

15. A micromotor according to any one of Claims 10 - 14 wherein the high dynamic clamping load is achieved with a relaxor-based single crystal as driving element wherein a high preload may be applied to the micromotor to attain ouput torque of >2.5μNm/V_pp,

16. A micromotor according to Claim 9 wherein the drive element includes at least one piezoelectric element comprising a relaxor-based single-crystal element which is excitable to clamping load at a high and dynamic load value.

17. A micromotor according to Claim 16 wherein the diameter of the rotor and/or stator is within a few millimeters, in which high dynamic clamping load provides an operating range below said clamping load and optimally drives the micromotor at an output torque of > 2.5 μNm/V_pp.

18. A micromotor according to Claim 15 wherein the single crystal element comprises of at least one or combination of the following solid solutions of suitable compositions: lead-zinc-nobiate-lead-titanate [Pb(Zn₁Z₃Nb₂Z₃)O₃-XPbTiO₃ or PZN-yPT] and lead-magnesium-nobiate-lead titanate [Pb(Mgi_/3Nb₂z₃)O₃-yPbTiO₃ or PMN-yPT] wherein 0.045 < x < 0.09 and 0.27< y < 0.34, wherein x and y are in mole fraction and including their doped derivatives.

19. A micromotor according to Claim 15 - 16 wherein the single crystal element comprises of individual plates, and/or stacks of at least two plates in appropriate poling combinations to provide the optimal bending vibration to

drive the micromotor, wherein poled single crystal cferplates of high -d₃i or

I d₃₁ /s*| value, preferably including of -cf₃₁ > 800 pC/N and I d_3l /j* I > 10

CIm², and of a reasonably high electromechanical coupling factor of k_& ≥

0.50, are used as the driving elements to drive the micromotor.

20. A micromotor according to Claim 19 wherein

more preferably, -c/₃i > 1200 pC/N and I d_3l /s* I > 15 C/m². and of a

reasonably high electromechanical coupling factor of kz<_\ > 0.50, are used as the driving elements to drive the micromotor.

21. A micromotor according to Claim 15 and 19 wherein each active element comprises of a single or a plurality of d₃i-mode single crystal plate.

22. A micromotor according to Claims 15, 19 and 21 wherein single crystal active element(s) is(are) in the form of suitably poled tube(s) of appropriate orientation cut, either with or without the use of an internal or external metallic tube.

23. A micromotor according to Claims 15 and 19 - 22 wherein the single crystal active element(s) is(are) in the form of suitably poled rod(s) of appropriate orientation cut, with or without the use of an external metallic tube.

24. A micromotor according to any one of Claims 9 - 23 wherein a solid rotor shaft is used.

25. A micromotor according to any one of Claims 9 - 24 wherein a hollow rotor shaft is used, instead of a solid shaft, for the insertion of guide wire or other functions.

26. A micromotor according to any one of Claims 9 - 25 wherein the stator contains appropriate end caps, either at one or both ends.

27. The concept of ultrasonic micromotor design as in Claims 1 to 26 whereby the frictional coefficient between the rotor and the stator is increased via established or conventional means to increase the frictional force and hence output torque of the micromotor.

28. A micromotor according to any one of Claims 9 - 26 wherein user- adjustable means are provided, including any one or combination of mechanical, electromechanical, magnetic and electromagnetic means for varying the output torque range of said micromotor.

29. A micromotor according to any one of Claims 9 - 28 wherein only one end of the stator is actuated to drive the rotor.

30. A micromotor according to any one of Claims 9 - 29 wherein a plurality of stators are used to drive the rotor of said micromotor collectively.

31. A micromotor according to any one of Claims 9 - 30 wherein the motor

or micromotor may be driven by one electric source with a 90° phase shifter or

by more than one electric source with or without appropriate phase shifters depending on the poling configuration of the driving elements, and wherein the electrical circuits may be appropriately biased when so desired.

32. A micromotor according to any one of Claims 9 - 31 wherein having multilayer driving elements each with electrical circuit for selective and/or collective actuation over user-selectable range of output torque.

33. A micromotor according to any one of Claims 9 - 32 wherein the stators are driven by pulsed drive scheme comprising an active phase containing input signals ranging from one to a few tens cycles of the design resonant frequency of the micromotor and at sufficiently high voltage amplitude to set the motor in brief or intermittent bending vibration motion in driving the micromotor, and optionally biasing the electrical circuit appropriately to avoid depoling of the driving elements as required.

34. A micromotor according to Claim 33 wherein the input voltage signals within the pulsed input has at least one input signal having an amplitude large enough to set the micromotor in brief or intermittent bending motion.

35. A micromotor according to any one of Claims 33 and 34 wherein accurate angular displacement control of the micromotor is achieved as in actuation of a stepper motor.

36. A micromotor according to any one of Claims 9 and 33 wherein the stators are driven by pulsed-driven scheme comprising an active phase containing input signals ranging from one to a few tens cycles of the design resonant frequency of the micromotor and at sufficiently high voltage amplitude to set the motor in brief or intermittent bending vibration motion, followed by or interspersed with at least an idle phase of suitable duration.

37. A micromotor according to Claim 36 wherein the brief or intermittent bending vibration motion is enabled with input voltage signals within the active phase having, at least a fraction of said signals, voltage of sufficient amplitude to set off said bending vibration motion.

38. A micromotor according to any one of Claims 33 - 37 wherein the signals within the idle phase of the micromotor drive comprises any one of

- nil signal input;

- the signal in DC or AC which frequency lies outside the resonant frequency range of the motor; or

- the signal in DC or AC which frequency lies within the resonant frequency range of the motor but is of insufficient voltage amplitude to drive the motor.

39. A micromotor according to any one of Claims 36 - 38 wherein the rotation speed of the micromotor drive is controlled, including any one or combination of:

- fixing the micromotor's rotation speed from a fraction of rpm to tow hundreds of rpm; - varying the rotation speed from a fraction of rpm to its maximum rotational speed as attainable with continuous drive means.

40. A micromotor according to any one of Claims 36 - 39 wherein the driving circuit incorporates control means for amplitude of the applied voltage, driving signal waveform, working frequency, voltage and/or current limiter, preload level, and other conventional parameters in controlling the rotation speed and angular displacement of the micromotor.

41. A micromotor according to Claim 40 wherein the driving circuit includes means for feedback control of any one or combination of rotation speed, output torque and/or output power of the micromotor, resonance frequency changes due to heat generated during continuous operation or changes in ambient temperature.

42. A micromotor according to any of the preceding claims packaged into an appropriate housing incorporating securing and bearing means.

43. A device incorporating a micromotor according to any one of the preceding claims.

44. An appliance incorporating a micromotor according to any one of the preceding claims.