Method Of Making Power Transducers

Abstract

A method of making a power transducer device by thermally shrink-fitting a transducer element and a metallic transducer element support member within an opening in a metallic supporting body member so that the body member, the transducer element and the transducer element support member are disposed in a mechanical interference-fit stress producing relationship with each other to subject the transducer element to, and maintain it at, a desired compressive loading.

Parent Case Text

This application is a division of copending application Ser. No. 26,950 filed Apr. 9, 1970 now Pat. No. 3,657,581, issued Apr. 18, 1972 and entitled,"Power Transducers."

Description

The invention described herein was made in the performance of work under NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).

This invention relates generally to power transducers and more particularly to a method of making such transducers. The invention has a wide range of applications, only some of which, wherein the invention is especially adapted and useful, are described in detail herein.

In recent years ultrasonic energy systems have found wide use in many commercial and industrial applications such as in cleaning, soldering and in bearings. Briefly, in such systems a transducer element of the magnetostrictive or electrostrictive (piezoelectric) type is driven from a suitable source of oscillatory energy to produce the desired vibrations in the transducer element. As is well known, magnetostrictive transducer elements have the property of changing physical dimensions when subjected to an applied magnetic field while electrostrictive transducer elements have the property of changing physical dimensions when subjected to an applied electric field. Conversely when such elements are subjected to an applied force they have the property of modifying the applied field.

Thus, for example, when transducer elements are used as mechanical drivers they convert electrical energy to mechanical energy. The electrical energy may be supplied, for example, in the form of an oscillating electrical current. The resulting mechanical output is in the form of repetitive expansions and contractions of the transducer. The frequency of the oscillatory response (forced vibration) of the transducer corresponds to the frequency of the driving electrical output.

When the transducer elements are used as force sensors they convert mechanical force impulses to corresponding electrical impulses which can be measured by conventional means and a measure of the mechanical force imposed on the transducer is thus obtained. Calibration of such a device to obtain the mechanical force to electrical signal correspondence may be accomplished in any suitable manner known in the art.

The power generated by power transducers is transmitted to the working area by their supporting structure, the design of which will usually be determined by the transducer application. Efficient transmission of the power from the transducer element to the supporting body member, or other supporting structure, requires that hard and uniform coupling be provided between their interfaces. This is especially true when ultrasonic energy is involved. In addition, many very desirable power transducer elements are made of crystalline ceramic material, which materials have high compressive strength but low tensile strength. Accordingly, at higher power levels the transducer element must be preloaded and adequately supported to prevent failure due to internal tensile stresses. Many attempts have been made in the prior art to achieve the required preloading and hard and uniform coupling but none have been entirely satisfactory. For example, attempts have been made to preload the elements by clamping arrangements employing external bolts or other fastening means. Such an arrangement has the disadvantage that considerable power dissipation takes place in the bolted joints with severe local heating. The efficiency of power transfer is thereby much reduced and the structural failure rate is high.

It is a primary object of the present invention to provide a new and improved transducer arrangement which overcomes one or more of the foregoing prior art problems and in addition offers a number of distinct advantages in operation, ease of manufacture and reliability.

It is another object of the invention to provide a transducer arrangement exhibiting superior transducer element support and giving high transmission efficiency between transducer element and supporting structure.

Another object of the invention is to provide a transducer arrangement exhibiting high reliability and long operating life.

Still another object of this invention is to provide a transducer arrangement wherein the energy density of transmission is high thereby making possible the use of smaller ceramic crystals and operation at lower voltages.

Yet another object of the invention is a transducer arrangement which readily lends itself to the use of mass production techniques with consequent cost savings.

A still further object of the invention is to provide a new and improved transducer arrangement having high dimensional stability permitting the device to be used as a mechanical reference, for example, in bearing metrology.

Briefly stated, in accordance with one aspect of the invention, there is provided a novel method which achieves both preloading and supporting of the transducer element or elements in the supporting structure, or body, as well as hard and uniform coupling thereto by shrink fitting a transducer element and a transducer element support member with a supporting body member. The transducer element may be constructed of a magnetostrictive material, or an electrostrictive material. Conveniently, this may be accomplished by machining corresponding internal and external dimensions of the supporting body member, transducer elements, and mating transducer elements support means to tolerances of mechanical interference fit and shrink fitting the transducer element support means and the supporting body member together into a unitary assembly. If desired the transducer element may be shrink fitted to the body member without utilizing any separate support means. The use of such a support means however allows for a very convenient means of applying an electric field across the element.

The novel features believed characteristic of this invention are set forth with particularity in the appended claims. The invention itself, together with its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is the schematic plan view of a transducer device constructed in accordance with one embodiment of the invention;

FIG. 2 is the schematic cross-section of the arrangement shown in FIG. 1 taken along the line 2--2 and showing, in addition, attachment of suitable electrical connections;

FIG. 3 is a section view showing another embodiment of the invention;

FIG. 4 is a section view showing yet another embodiment of the invention;

FIG. 4a is a perspective view of the transducer element support member disposed between the rectangular transducer elements in FIGS. 3 and 4.

Referring now more particularly to the drawing, there is shown in FIGS. 1 and 2 one embodiment of a transducer constructed in accordance with this invention. As shown, the transducer device comprises a transducer element 11, which may be of a suitable magnetostrictive or electrostrictive material. As illustrated, element 11 is of hollow cylindrical configuration. To achieve maximum amplitude vibrations when subjected to an applied field in the radial direction, the material of transducer element 11 is polarized in the radial direction.

In accordance with this invention a lengthwise slit 22 is provided in transducer element 11 to prevent the introduction of hoop stresses in the element during assembly or operation. Hoop stresses do not contribute to the generation of power by the transducer element but rather tend to cause damage to it. Transducer element 11 is shrink fitted into a supporting body 12 of any desired shape. Element 11 has a transducer element support means, shown as a concentric pin 13 shrink fitted into it, as shown. For example, transducer element 11 and pin 13 form a transducer element assembly which is disposed in shrink-fit relationship with the supporting body 12. The supporting body 12 and the internal pin 13 may be conveniently used as terminals as well as supports for the transducer device 10.

As illustrated in FIG. 2, pin 13 may be provided with suitable flexible members which make it possible to attach electrical leads at locations where they are not exposed to the adverse effects of high frequency vibrations. These members can also be used as points of attachment of external supports or suspensions for the transducer device. To this end, annular diaphragms 14 and 15 are provided which terminate in ring sections 16 and 17, respectively. Diaphragms 14 and 15 are formed integral with the pin 13 and the supporting body 12, respectively. Such members are operative to assure that vibrations of the pin 13 or body 12 are not transmitted to the face portions 18 and 19 of the ring sections 16 and 17, respectively. Power leads 20 and 21 may be attached to the face portions 18 and 19, as shown in FIG. 2. Oscillatory current may thus be conveniently supplied from a suitable power source (not shown) to the transducer element 11 through leads 20 and 21, annular diaphragms 14 and 15, the internal pin 13, and the supporting body 12. When oscillatory current is so supplied, the transducer element 11 is set into a radial or thickness mode of oscillation. The hard and uniform coupling provided by the novel mounting arrangement of this invention assures that the mechanical energy so produced is transmitted to the desired working area of the transducer device.

Any suitable electrostrictive or magnetostrictive material may be used for the transducer elements 11. Some examples of materials known to exhibit highly magnetostrictive characteristics are permanickel, nickel and permendur. Especially desirable electrostrictive materials are the piezoelectric ceramic materials such as lead titanite-lead zirconite. One especially suitable electrostrictive material of this type is a ceramic material manufactured and sold under the designation PZT4 by the Clevite Corporation. Transducer elements 11 of such material can be readily obtained in finish machined form. For some hollow cylindrical transducer elements it is desirable, to assure uniform internal stresses during operation and allow for operation at low voltages, that the thickness of the transducer elements be made smaller than the radius thereof. For example, in one particular transducer arrangement the transducer thickness was made less than about one-eighth inch. Operation at low power input has the added advantage that operating temperatures are lower and any thermally caused frequency drift is much reduced.

The material of the supporting body 12 is not especially critical, although appropriate physical properties of the transducer element and supporting body should be properly matched. For example, the materials for element 11, pin 13 and body 12 should be selected so that their moduli of elasticity are approximately the same. If a transducer element of PZT4 ceramic material is used, a suitable material for supporting body 12 and pin 13 would be aluminum or titanium.

The geometric configuration of the supporting body 12 will usually be determined by the type and shape of transducer device desired, and the purpose for which the device is intended. The openings into which the transducer elements 11 are to be fitted can be machined accurately by conventional means, for example, by boring, broaching or any other suitable technique. The material of the internal pin 13 may be the same as that of the supporting body 12. The desired outside finish of pin 13 can be obtained in any suitable manner such as, for example, by grinding. The machining tolerances of all mating surface dimensions are such as to provide for a mechanical interference fit. Control of the degree of shrink fit is important as this determines the power density which can be transmitted from the transducer element to the supporting body. Moreover, uneven or excessive loading of the element 11 may damage or depolarize it.

The method of making a power transducer in accordance with this invention can best be explained by reference to FIGS. 1 and 2. Selecting transducer element 11, for example, of Clevite PZT4 electrostrictive ceramic material and the supporting body 12 and internal pin 13 of aluminum, the desired compressive preload is achieved with an interference fit of 0.0006 to 0.003 inches per linear inch of corresponding component dimension. This is conveniently provided by heating the body 12 to a temperature in the range of about 250.degree. to 280.degree. C. The ceramic transducer element 11 is slit axially to remove and prevent hoop stresses from being developed and the pin 13 is inserted in the central opening thereof. The hollow cylindrical transducer element 11 with the pin 13 therein is cooled to about -20.degree. C and disposed in the opening in the heated body 12. When body 12 is returned to room temperature the transducer element assembly is supported and mounted in body member 12 and the transducer element 11 is in shrink-fit relationship with such body member and subjected to a preselected compressive loading. In a transducer device constructed as just described, the preloading of the transducer element 11 may be of the order of 2,000 to 10,000 psi. The critical maximum temperature to which the transducer element may be exposed is the Curie point of the material at which temperature the element depolarizes. The Curie point of Clevite PZT4 ceramic material, for example, is about 325.degree. C and the highest compressive load to which it should be subjected is about 10,000 psi.

One criterion by which suitable design and machining tolerances of components and the correct assembly procedure can be assessed is the internal power loss under normal operating conditions which can be tolerated. This can be expressed conveniently as Q, the ratio of (energy stored in the transducer element at zero velocity/energy dissipated per cycle). The larger Q, the better the design and the higher the conversion efficiency of the device. Thus, for example, a prior art device using separate flanged flexures, large ceramic elements and clamping bolts was considered excellent with Q equal to about 300. On the other hand, the Q of a device in accordance with FIG. 5 of this invention is about 2,500.

Other embodiments of the invention are illustrated in FIGS. 3 and 4. The basic concept involved in the arrangements illustrated in FIGS. 3 and 4 is the same as that already described. That is, the arrangement comprises a transducer element shrink fitted to the body member. In the particular embodiments illustrated in FIGS. 3 and 4 the arrangement comprises a transducer element assembly including a transducer element and a transducer element support means. In these embodiments, however, the transducer element assembly comprises a pair of rectangular transducer element members with a sheet material member disposed therebetween. This assembly is then suitably shrink fitted in accordance with the method of this invention in a suitable cylindrical opening provided in the supporting body member. Since the transducer element assembly is of a rectangular configuration, a suitable insert means is provided to achieve a cylindrical surface which is convenient and effective in obtaining the required shrink-fit relationship. The insert also assures a pressure uniformity which otherwise may be difficult to obtain.

In the embodiments shown in FIGS. 3 and 4, therefore, transducer elements 31 and 32 are shaped in the form of short rectangular parallelpipeds. The transducer element support means is in the form of a metallic sheet member 33 located between elements 31 and 32. Thus, sheet member 33 provides an internal support, an external suspension point if needed, and serves also as one of the electrodes. An insert means is provided to achieve the desired shrink-fit relationship. As shown in FIG. 3, the arrangement includes an insert means 34 of U-shaped cross section. In the arrangement of FIG. 4, on the other hand, the insert means includes two cylindrical segments 35 and 36. The assemblies of transducer elements, sheet support members, and insert means are shrink fitted into the body 37 of the device in the manner previously described in detail in connection with the embodiment of FIG. 1. Although not shown in FIGS. 3 and 4, electrical connections may also be provided in the manner already described.

While there has been described what are considered to be the preferred embodiments of the method of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

What we claim as new and desire to secure by United States Letters Patent is:

1. The method of providing a hard and uniform coupling between a transducer element means and a metallic supporting body member comprising the steps of:

a. Providing a transducer element means constructed of a material selected from the group consisting of electrostrictive and magnetostrictive materials;

b. Providing a metallic transducer element support means shaped to conform to a surface of said transducer element means;

c. Fitting said transducer element means and said transducer element support means together to provide a transducer element assembly;

d. Providing a metallic supporting body member;

e. Forming at least one opening in said metallic supporting body member, said opening being dimensioned smaller than said transducer element assembly to initially prevent insertion of said transducer element assembly into said opening;

f. Heating said supporting body member to a temperature sufficient to cause a desired expansion thereof and enlargement of the opening therein but below the Curie point of the transducer element material;

g. Cooling said transducer element assembly to a predetermined low temperature;

h. Disposing said cooled transducer element assembly into the thermally enlarged opening of said heated supporting body member; and

i. Returning said supporting body member to its normal operating temperature whereby said transducer element assembly is mounted in said opening in a mechanical interference-fit relationship with said supporting body member to effect and maintain a hard and uniform coupling therebetween and said transducer element is subjected to a desired preselected compressive loading.

2. The method recited in claim 1 wherein said transducer element is constructed of an electrostrictive ceramic material and said supporting body member is heated to a temperature in the range of about 250.degree. to 280.degree. C.

3. The method recited in claim 2 wherein said transducer element assembly is cooled to a temperature of about -20.degree. C.

4. The method of providing a hard and uniform coupling between a transducer element means and a metallic supporting body member comprising the steps of:

a. Providing a hollow cylindrical transducer element means constructed of a material selected from the group consisting of electrostrictive and magnetostrictive materials;

b. Slitting said hollow cylindrical transducer element means axially to prevent development of hoop stress therein;

c. Shaping a metal cylindrical transducer element support member and fitting said member within the central opening of said transducer element to form an assembly;

d. Providing a metal supporting body member and forming at least one opening therein dimensioned smaller than the outside diameter of said transducer element assembly to initially prevent insertion thereof into said opening but effective to receive said hollow cylindrical transducer element assembly in mechanical interference-fit relationship;

e. Bringing said supporting body member to a preselected elevated temperature sufficient to cause thermal expansion thereof and allow said transducer element assembly to be freely disposed in said opening therein but below the curie point of the transducer element material;

f. Disposing said transducer element assembly within the thermally enlarged opening of said higher temperature supporting body member; and

g. Returning said supporting body member to its normal operating temperature whereby said hollow cylindrical transducer element means is mounted and supported in the opening in said supporting body member mechanical interference-fit relationship to effect and maintain a hard and uniform coupling between the transducer element and said supporting body and said transducer element is subjected to a preselected compressive loading.

5. The method recited in claim 4 wherein said transducer element is constructed of an electrostrictive ceramic material and said supporting body member is brought to a temperature in the range of about 250.degree. to 280.degree. C.

6. The method recited in claim 4 wherein said assembly is cooled to a preselected low temperature prior to being disposed within the opening of said supporting body member.

7. The method recited in claim 6 wherein said assembly is cooled to a temperature of about -20.degree. C.

8. The method recited in claim 6 wherein said transducer element is constructed of an electrostrictive ceramic material having a curie point of about 325.degree. C and said supporting body member is brought to a temperature in the range of about 250.degree. C to 280.degree. C.

9. The method recited in claim 8 wherein said assembly is cooled to a temperature of about -20.degree.C.

10. The method recited in claim 1 wherein said interference-fit is in the range of about 0.0006 to 0.003 inches per linear inch.

11. The method recited in claim 4 wherein said interference-fit is in the range of about 0.0006 to 0.003 inches per linear inch.