EP0958649A2

EP0958649A2 - High torque ultrasonic motor system

Info

Publication number: EP0958649A2
Application number: EP97923449A
Authority: EP
Inventors: Yoseph Bar-Cohen; Shyh-Shiuh Lih; Willem A. Grandia
Original assignee: California Institute of Technology CalTech
Current assignee: California Institute of Technology CalTech
Priority date: 1996-04-17
Filing date: 1997-04-17
Publication date: 1999-11-24
Also published as: WO1997039520A3; WO1997039520A2; AU2925097A

Abstract

The present invention is a compact, low mass, and low power consumption piezoelectric ultrasonic motor with high torque. The piezoelectric ultrasonic motor (54) of the present invention includes a polarized piezoelectric drive ring wafer (34) comprised of a plurality of arc piece piezoelectric elements (32). The piezoelectric ultrasonic motor (54) can utilize one or a plurality of piezoelectric wafers (34) stacked as layers (34) to increase the torque of the motor. Also, a series of the piezoelectric motors (34) can be stacked and operated synchronously, as one motor (52), to maximize the torque within the volume, mass and power specification limitations of a specific application.

Description

HIGH TORQUE ULTRASONIC MOTOR SYSTEM BACKGROUND OF THE INVENTION Qriσin of the Invention: The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the contractor has elected to retain title.

Field of the Invention

The present invention relates to compact low mass motors, and in particular to piezoelectric ultrasonic motors with high torque.

Related Art

Ultrasonic motors are a new actuation technology that has emerged in the last few years in commercial products such as cameras and watches. However, ultrasonic motors, utilized in such commercial applications as cameras and watches, have been limited in torque capability and are complex to manufacture. For instance, many of these commercial motors are limited to 6 in-lb of torque. Figure 1 is a typical ultrasonic motor of the prior art. The ultrasonic motor 10 includes standard components, such as a cross roller link 12, an encoder 14, a flat spring 16, a rotor 18, a stator 20, and a piezoelectric element 22. The piezoelectric element 22 is made of piezoelectric material and is shaped as a flat, annular, continuous ring or wafer.

Figure 2 is the ultrasonic motor cf Figure 1 illustrating the detail cf the continuous piezoelectric wafer. The piezoelectric wafer 22 has sections with either positive 24 and negative 26 poling directions. For actuation, an outside source (not shown) provides the piezoelectric wafer 22 with an electrical potential. The electrical potential causes the piezoelectric wafer 22 to undergo mechanical deformation and induce flexural ultrasonic traveling waves in a direction as indicated by arrow 28.

The traveling waves are transmitted to the stator 20 and provide the stator 20 with an orbital motion at the point the stator 20 and the rotor 18 are in contact. This motion is achieved through control of the piezoelectric wafer 22. The interface between the rotor 18 and stator 20 is frictional, and thus, rectifies the micro-motion of the stator 20 to produce macro-motion of the rotor 18. Therefore, the interconnection between the rotor 18 and the stator 20, rotates the rotor 18 in the appropriate direction as indicated by arrow 30.

To induce a flexural traveling wave in the direction indicated by arrow 28, the piezoelectric wafer 22 is divided into sections or zones with positive or negative poling directions that are repolarized. The repolarized zones are produced in a sequence that is defined by the desired number of drive wavelength (commonly in the range of 6 to 10) . However, the process of repolmg is time consuming and adds a step to the entire process and causes the repoled zones to induce a less effective and non-uniform piezoelectric effect. When compared to the level prior tc repoling, the loss of efficiency is expressed m terms of a lower piezoelectric coefficients d₃₁ and d₃₃. Generally, a single layer of piezoelectric material induces a relative small displacement at the level of several fractions of a micron. However, the process of repolmg does not allow the αse of more than one layer of a piezoelectric wafer. As a result, the motor has limited torque, and is complex to manufacture.

Therefore, what is needed is a compact low mass motor with low power consumption. What is also needed is a piezoelectric ultrasonic motor with relatively high torque. What is additionally needed is an ultrasonic motor that can utilize more than one piezoelectric wafer layer. What is further needed is a piezoelectric ultrasonic motor that is relatively easy to manufacture.

Whatever the merits of the above mentioned systems and methods, they do not achieve the benefits of the present invention.

SUMMARY OF THE INVENTION To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention is a compact, low mass, and low power consumption piezoelectric ultrasonic motor with high torque .

The piezoelectric ultrasonic motor of the present invention includes a stator with teeth, a rotor, and a pre- polarized piezoelectric drive ring wafer. The piezoelectric drive ring is comprised of a plurality of arc piece piezoelectric elements, each with a predetermined pole direction (+ or -) . The piezoelectric arc pieces are assembled sequentially to make up the full drive ring. This sequential assembly corresponds to the overall desired pole position sequence of the arc pieces as required to excite traveling flexural waves.

In addition, electrodes interconnected between the piezoelectric drive ring wafer and an outside electrical source provide an electrical potential to the drive ring wafer and electrically interconnect the arc pieces. The stator is positioned between the piezoelectric drive ring and the rotor and receives propagation from the traveling waves. The stator harnesses the action of the traveling wave and propels - the rotor through the frictional contact between the rotor and the stator' s teeth. The propelling of the rotor creates mechanical motion. Thus, the motor utilizes the amplification and repetition of the micro-deformations of the piezoelectric drive ring to generate intensified mechanical motion.

The piezoelectric ultrasonic motor can utilize one or a /US97/06871

plurality of piezoelectric wafers stacked as layers to increase the torque of the motor. Also, a series of the piezoelectric motors can be stacked and operate synchronously, as one motor, to maximize the torque withm the volume, mass ana power specification limitations of a specific application.

A feature of the piezoelectric ultrasonic motor of the present invention is to have a piezoelectric wafer with arc pieces having predetermined pole directions. Another feature of the ultrasonic motor of the present invention is the stacking of piezoelectric elements for the utilization of more than one piezoelectric wafer layer per motor. Another feature of the present invention is the stacking of ultrasonic motors.

Yet another feature of the present invention is to have torque and rotary actuation delivered in 90 degrees to the driving stator.

An advantage of the piezoelectric ultrasonic motor of the present invention is the ability to produce relatively high torque. Another advantage of the present invention is that the piezoelectric wafer does not need to be continuously repoled. Another advantage of the piezoelectric ultrasonic motor of the present invention is that it is relatively easy to manufacture. Yet another advantage of the piezoelectric ultrasonic motor system of the present invention is that it can be mass produced at low cost. The foregoing and still further features and advantages of the present invention as well as a more complete understanding thereof will be made apparent from a study of the following detailed description of the invention in connection with the accompanying drawings and appendeα claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers represent corresponding parts throughout :

Figure 1 is an ultrasonic motor assembly of the prior art; Figure 2 is the ultrasonic motor of Figure 1 illustrating the detail of the continuous piezoelectric repoled wafer;

Figure 3 is an individual piezoelectric crystal of the present invention;

Figure 4 illustrates a piezoelectric drive ring wafer of the present invention comprised of numerous piezoelectric crystals of Figure 3;

Figure 5 illustrates a stacked piezoelectric element of the present invention comprised of numerous piezoelectric wafers of Figure 4; Figure 6 is an exploded view of an ultrasonic motor of the present invention with a stator, rotor, and shaft;

Figure 7 is a side sectional view of the stator and rotor of Figure 6;

Figure 8 illustrates a series of ultrasonic motors connected along a single shaft and comprised of numerous ultrasonic piezoelectric motors of the present invention; Figure 9 illustrates a side view of a the ultrasonic piezoelectric motor with bearings; and

Figure 10 illustrates the ultrasonic motor of Figure 7 with a rotor normal to the stator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. Figure 3 is an individual piezoelectric crystal of the present invention. Existing motors utilize a single continuous piezoelectric wafer (as shown in Figure 2) as a driving ring to urge the stator, which rotates the rotor. In contrast, the present invention utilizes a series of arc-shaped piezoelectric elements 32. Each piezoelectric arc piece 32 is preferably made of a piezoelectric material with high d₃₁ and d₃₃ coefficients, such as a commercially available crystographic oxide material or piezoceramic crystal based on Navy Code PZT-4 (PbZnTn i.e., Plumbum, Zirconium, Titanium oxide) . The piezoelectric arc piece can have either a positive (+) or negative poling direction (-) . Figure 4 illustrates a piezoelectric drive ring wafer of the present invention comprised of numerous piezoelectric crystals of Figure 3. A piezoelectric drive ring wafer 34 preferably comprises a first section 36 and a second section 38. Each section contains the arc-shaped piezoelectric elements 32 of Figure 3 assembled sequentially to make up the full drive ring 34.

The arc pieces 32 are preferably sequentially assembled to one another with an epoxy, such as silver epoxy. Also, the arc pieces 32, with predetermined pole directions (+,-) , are sequentially assembled to one another to form the drive ring. The sequential arrangement corresponds to the overall desired pole position sequence of the arc pieces 32 as required to excite traveling flexural waves (discussed in detail below in Figure 7) . Such a sequence could include alternating positive and negative poling directions of the arc pieces 32, as shown in Figure 4.

In addition, each section 36, 38 contains an inside edge 40, 42, respectively, an outside edge 44, 46, respectively, and first and second ground elements 48, 50, respectively. Preferably, the inside edge 40 of the first section 36 is a first orthogonal mode with a sinwt wire electrode and the inside edge 42 of the second section 38 is a second orthogonal mode with a coswt wire electrode . The two orthogonal modes are needed to generate the traveling wave discussed in detail below in Figure 7. The arrangement of the first and second on the results that are desired.

The electrodes are connected to an cutside electrical source (net shownj and provide an electrical potential to the drive rmg wafer 34. Also, the electrodes electrically interconnect the arc pieces 32 together. The first and second ground arc elements 48, 50 are a common ground and connect the two sections 36, 38 to each other. Further, the outside edges 44, 46 of the piezoelectric arc pieces 32 could be scarfed to prevent shorting of the inside edge electrodes 40, 42.

This drive ring arrangement allows the ultrasonic motor system of the present invention to be easily manufactured. Since the piezoelectric wafer of the present invention is comprised of plural piezoelectric arc pieces 32 placed with their pole in a desired direction, the complex current process of repoling zones in a continuous drive wafer as Figures 1 and 2 is avoided.

Figure 5 illustrates a stacked piezoelectric element of the present invention comprised of numerous piezoelectric drive ring wafers 34 of Figure 4. Since the drive ring 34 of Figure 4 is made of the individual piezoelectric arc pieces 32 of Figure 3, each piezoelectric arc piece 32 can either be made of a single piezoelectric drive ring layer 34 or a stack of piezoelectric drive rmg layers 52 (stack actuators) to enhance the level of actuation.

A stack 52 of piezoelectric drive rings 34 utilizes the „_™„„,

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displacement induced by each piezoelectric drive rings 52 to maximize the overall displacement. The use of stacked drive ring wafers 52 instead of the use of a single drive ring wafer 34 allows the inducement of nigh electric fields wnile asing relatively low voltage levels (below 100 Volts) .

For example, when a single piezoelectric drive ring wafer 34 is subjected to the electric field, a displacement at a level of several fractions of a micron is produced. Consequently, when a stack 52 of drive rmg wafers 34 is used, such as a stack of thirty or more piezoelectric drive rmg wafers 34, a ten to twenty micron displacement can be obtained. The displacement depends on the type of electroactive material and the number and thickness of the drive rmg wafers 34 contained m the stack 52. This increased displacement significantly enhances the motor output torque.

Figure 6 is an exploded view of an ultrasonic motor of the present invention. Figure 7 is a detailed view of the stator and rotor of Figure 6. The ultrasonic motor 54 includes a stator driver 56, preferably with teeth 58, a rotor

60, and a shaft 62. The piezoelectric drive rmg wafer 34 excites a traveling bending wave 65 withm the stator 56 when the piezoelectric drive rmg 34 is SUDJected to the electric field. The wave 65, not the stator 56, propagates at a rotational speed Ω. As the traveling wave 65 passes along a centerline 67 of the stator 56, vertical motion is created 6871

with amplitude and frequency equal to that of the traveling wave 65.

Nevertheless, bending movement of the stator 56 causes points that are off the centerline 67 to move back and forth horizontally as well as vertically. The resulting trajectory is an elliptical orbital motion 69. In other words, the traveling waves 65 are transmitted to the stator 56 and provide the stator 56 with the elliptical orbital motion 69 at a contact point 64 between the stator 56 and the rotor 60. As a result, moving farther off the centerline 67 creates greater horizontal velocities, thus widening the elliptical orbital motion 69. An instantaneous velocity within the teeth 58 is indicated by arrows 71. At the peak of the orbital motion 69, vertical velocity is zero and the horizontal velocity is in the opposite direction of the traveling wave 65.

A frictional force is created between the rotor 60 and the stator 56 by pressing the rotor 60 on top of the stator 56. The frictional force between the surface of the horizontally moving stator 56 and the rotor 60 causes the rotor 60 to spin. It is important to note that the rotational speed of the rotor 60 is not equal to the frequency of the traveling wave 65. As a result, the motor 54 is operated by friction forces transferring the traveling wave 65 action from the teeth 58 of the stator 56 to the rotor 60, converting micron motion to a macro level movement of the rotor 60.

The orbital motion 69 within the stator 56 can be harnessed, for example, to provide a macnmmg action. Thus, the ultrasonic motor 54 can be used for micro-machinmg applications on controlled surfaces. One example of an application is the surface preparation of rotors and stators to allow meeting their smoothness and surface parallelism specifications.

Figure 8 illustrates a series of ultrasonic motors of the present invention connected along a single shaft. A system 68 having a series of motors 54 can be connected along a common shaft 70. The series of motors operate synchronously so that they operate as one motor. This arrangement further increases the total torque capability of the system 68. As a result, the system 68 of a series of motors 54 is highly reliable and maximizes torque within volume, mass, and power specifications bounds of the desired application.

The system 68 of a series of motors 54 has one shaft 70 common to each motor 54 and a front bearing 72 and a rear bearing 74. Also, each motor 54 of the system 68 of a series of motors 54 includes a case (not shown) , a stator 56, which preferably has teeth 58, a rotor 60, and a piezoelectric drive ring wafer 34, which is preferably a stack 52 of drive rings 34 as described in detail above m Figure 5. The rotor 60 rotates the shaft 70 in the direction indicated by arrow 78. All of the stators 56 and cases of the individual motors are connected to each other, whereas each rotor 60 is connected to the center shaft 70. Also, the system 68 of a series of motors 54 is capable of economical mass production and can be made with enhanced driving piezoceramic actuators which can generate even higher torque.

Figure 9 illustrates a side view of the ultrasonic piezoelectric motor with bearings. The mass of the series of ultrasonic motors 68 of Figure 8 is smaller than the mass of the same number of motors 68 as single units. This is because some of the components of a single motor are not needed when making the series of motors as shown in Figure 8. In other words, system mass is reduced because the duplication of components present when separate motors are connected to operate individually is eliminated when the motors are mounted on a single shaft.

For example, as shown in Figure 9, two single motors 80, 82 each use two bearings 84, 86 to hold their respective shafts steady and assure their stability during rotation. However, when connecting the two motors, only two bearings are necessary if they are placed at the front and rear of the connected motors. These front and rear bearings function m the same manner as each respective bearing on each single motor.

Specifically, referring back to Figure 8, the system 68 of a series of motors 54, as a whole, have only two bearings that are shared, namely the front and rear bearings 72, 74. Thus, two bearings are not required for each individual motor. This approach eliminates extra bearings each time another _n™,„,_o„~

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motor is connected. This arrangement significantly reduces the total mass of tne system 68 of a series of motors 54. Furtner, only one set of other motor components are required if tne component is common to the series of motors, such as a case enclosing and a set of wires.

Figure 10 illustrates the ultrasonic motor of Figure 8 with a rotor normal to a stator The rotor 60 can be rotate parallel to the surface of the stator 58 as shown in Figure 8, but also a rotor 90 can rotate normal (90 degrees) to the surface of a stator 92 with teeth 94. This is accomplished by placmg the rotor 90 normal (or in the desired direction) to the stator 92. This arrangement offers unique drive capabilities that do not require gears or other mechanisms to transfer motion from the piezoelectric drive rmg 34, to the stator 92, to the rotor 90, and to a shaft 96, m a perpendicular direction to the drive mechanism. The arrangement of Figure 10 can be used with either a single motor system or a series of motors, similar to the system 68 of Figure 8. The ultrasonic motor of the present invention has high self-holding force assuring control with no need for power durmg idle stage, simple construction, provide direct drive, and has quiet operation, and a quick response. Applications The ultrasonic motor system of the present mvention has many unique characteristics that are enaolmg technologies for a wide variety of applications, including space missions. Further, the ultrasonic motor system of the present invention can be applied to drive a crawler for inspection of aircraft structures as well as actuate medical instrumentation, particularly operation tools.

The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A piezoelectric ultrasonic motor having a stator and a rotor coupled to said stator, said motor further comprising: a drive wafer coupled to said stator and comprising a first section interconnected to a second section by grounded arc pieces; wherein said first and second sections comprise a plurality of separate individual arc pieces sequentially coupled to one another in an order with alternating poling directions; a first electrode interconnecting together each one of said arc pieces of said first section with another one of said arc pieces; a second electrode interconnecting together each one of said arc pieces of said second section with another one of said arc pieces; and an outside source coupled to said first and second electrodes for exciting traveling flexural waves within said stator to deform said stator, wherein said rotor coupled to stator receives said deformation to thereby move said rotor.

2. The invention as set forth in claim 1, wherein each arc piece is made of a piezoelectric material with high d₃₁ and d₃₃ coefficients.

3. The invention as set forth m claim 2, wherein each arc piece is made of piezoelectric crystal.

4. The mvention as set forth m claim 1, wherem each one of said arc pieces is coupled to another one of said arc pieces by an adhesive bond.

5. The invention as set forth in claim 1, wherem said plurality of arc pieces of said first and second sections are sequentially coupled to one another in an order with alternating poling directions.

6. The invention as set forth in claim 1, wherem said first electrode is driven by a reference cyclic signal and said second electrode is driven by an orthogonal cyclic signal with 90 degrees phase difference.

7. The invention as set forth in claim 1, wherem said stator further comprises teeth located between said stator and said rotor.

8. The invention as set forth in claim 1, wherein each one of said arc pieces has a scarfed outside edge for preventing shorting of said first and second electrodes.

9. The mvention as set forth in claim 1, wherein said stator further comprises a top surface with teeth and said rotor further comprises an elongated shaft extending normal from a top and a bottom of said rotor, and wherem said rotor is coupled to said teeth above said top surface of said stator so that said shaft extends normal to the plane of said rotor.

10. The invention as set forth in claim 9, wherein said rotor is coupled to said teeth above said top surface of said stator so that said shaft extends parallel to the plane of said rotor.

11. A compact, low mass, and low power consumption piezoelectric ultrasonic motor with high torque having a stator and a rotor coupled to said stator, said motor further comprising: a plurality of stacked drive wafers coupled to said stator, each of said stacked drive wafer comprising a first section interconnected to a second section by grounded arc pieces, wherein said first and second sections comprise a plurality of separate individual arc pieces sequentially coupled to one another in an order with alternating poling directions; a first electrode interconnecting said arc pieces of said first sections together and a second electrode interconnecting said arc pieces of second sections together; and an outside source coupled to said first and second electrodes for exciting traveling flexural waves within said stator to deform said stator, wherein said rotor receives said deformation from said stator to move said rotor.

12. The invention as set forth in claim 11, wherein each one of said arc pieces is coupled to another one of said arc pieces by an adhesive bond.

13. The invention as set forth in claim 11, wherein said first electrode is driven by a reference cyclic signal and said second electrode is driven by an orthogonal cyclic signal with 90 degrees phase difference.

14. The invention as set forth in claim 11, wherem each one of said arc pieces has a scarfed outside edge for preventing shorting of said first and second electrodes.

15. The invention as set forth in claim 11, wherein said stator further comprises a top surface with teeth and said rotor further comprises an elongated shaft extending normal from a top and a bottom of said rotor, and wherein said rotor is coupled to said teeth above said top surface of said stator so that said shaft extends normal to the plane of said rotor.

16. The invention as set forth in claim 11, wherein said rotor is coupled to said teeth above said top surface of said stator so that said shaft extends parallel to the plane of said rotor.

17. A compact, low mass, and low power consumption piezoelectric ultrasonic motor with high torque having a plurality of stators, a first bearing connected to a shaft at a top end of the motor, a second bearing connected to the shaft at a bottom end of the motor, and a plurality cf rotors each coupled to one of the stators and each connected to the shaft, said motor further comprising: a plurality of synchronized stacked drive wafers each coupled to each one of said stators, each one of said stacked drive wafers comprising a first section interconnected to a second section by grounded arc pieces, wherein said first and second sections comprise a plurality of separate individual arc pieces sequentially coupled to one another in an order with alternating poling directions; wherein each stacked drive wafer further includes a first electrode interconnecting each one of said arc pieces of each one of said first sections together and interconnecting all of said first sections together, and a second electrode interconnecting each one of said arc pieces of each one of said second sections together and interconnecting all of said second sections together; an outside source coupled to each electrode for exciting traveling flexural waves within each stator to deform each one of said stators, wherein each one of said rotors synchronously receives said deformation from each one of said stators to move said shaft and said bearings synchronously.

18. The invention as set forth in claim 17, wherein each one of said arc pieces is coupled to another one of said arc pieces by an adhesive bond.

19. The invention as set forth in claim 17, wherein said first electrode is driven by a reference cyclic signal and said second electrode is driven by an orthogonal cyclic signal with 90 degrees phase difference.