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Publication numberUS3360665 A
Publication typeGrant
Publication dateDec 26, 1967
Filing dateApr 15, 1965
Priority dateApr 15, 1965
Publication numberUS 3360665 A, US 3360665A, US-A-3360665, US3360665 A, US3360665A
InventorsBoswell Vance F
Original AssigneeClevite Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Prestressed piezoelectric transducer
US 3360665 A
Abstract  available in
Previous page
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Claims  available in
Description  (OCR text may contain errors)

Dec. 26, 1967 v. F. BOSWELL 3,360,665

PRESTRESSED PIEZOELECTRIC TRANSDUCER Filed April 15, 1965 I IIIIIIIIII/III L\'\ ENTOR VANCE F. BOSWELL WMQW ATTORNEY United States Patent Otifice 3,360,665 Patented Dec. 26, 1967 ABSTRACT OF THE DISCLOSURE A longitudinally elongated piezoelectrically responsive transducer encapsulated, except for the end faces thereof, by a layer of plastic material having a controlled shrinkage. The axial ends of the layer are anchored either to the piezoelectric element directly or to metal end pieces.

The resent invention relates to a transducer for generating or receiving energy. More particularly, the invention concerns a prestressed piezoelectrically responsive transducer for converting between electrical and mechanical forms of energy, and which may be adapted to be employed as a voltage generator, acoustic wave generator, or like device such as are commonly used for instance in sonar systems, accelerometers, impact fuses and piezoelectric ignition systems.

The general category of transducers herein under consideration is known in the art and reference may be had to Us. Patents Nos. 2,930,912 to H. B. Miller, 3,110,825 to H. B. Miller and 3,082,333 to Huilerd et a1. These patents are assigned to the same assignee as the instant application.

In the prior art, as is illustrated in the aforementioned patents, transducers are prestressed by externally arranged mechanical devices, such as bolts, springs and tension screws. The difiiculty of maintaining equal external pre-loading pressure upon the piezoelectric element over an extended period of time without affecting mechanical changes in the external equipment is well known. Alignment of the piezoelectric elements comprising the transducer is accomplished by forces which are exerted by externally arranged mechanical structures. The transducers, and more particularly the individual components thereof, are prefabricated and subsequently put together, resulting in a structure having no inherent support feature of the character herein under consideration. As a result, during the operation of the piezoelectric transducer, particularly as applied to high voltage generators, the position of the piezoelectric elements is caused to shift due to a lack of rigidity of the structural system. The consequent result is a drop in output and efficiency and in severe cases a fracture of the element.

It is therefore the primary object of this invention to provide a monolithic transducer structure which is prestressed to the desired degree without the use of externally arranged mechanical prestressing means.

It is a further object of this invention to provide a piezoelectric transducer which is capable to maintain physical orientation of the main components thereof over a substantially long period of time for the protection of frangible materials against mechanical damage without additional external support or stiffening.

It is a still further object of this invention to provide a piezoelectric transducer which is provided with environmental protection against deleterious substances and to revent piezoelectric crystal arc-over and unwanted arcing to external structures.

An aspect of the present invention resides in the provision of a prestressed piezoelectric transducer for generating or receiving energy. The transducer includes a longitudinally elongated element which is piezoelectrically responsive in compression and a compression retainer associated with the element at each outer end thereof. A relatively thick layer composed of plastic material having controlled shrinkage encapsulates the periphery of the element and the retainer, that is, all surfaces except the outer ends, and is effective to permanently preload the element in the longitudinal direction between the location of each retainer.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawing:

FIGURE 1 is a longitudinal cross sectional view of a transducer in accordance with this invention;

FIGURE 2 is a view similar to FIGURE 1 showing a modified transducer comprising two piezoelectrically active elements;

FIGURE 3 is a view trating an active and a and FIGURE 4 is a view also similar to FIGURE 1 illuspiezoelectrically inactive element;

similar to FIGURE 1 showing a still further modification of this invention.

The transducer in accordance with this invention is hereinafter generally described in terms of a voltage generator. The applicability of the invention to wave generators and the like and the equivalent terms therefor will be immediately apparent to those skilled in the art and the claims appended to this specification shall be interpreted accordingly.

Turning now to FIGURE 1, there is shown an elongated element 10 formed of electromechanically sensitive material of the piezoelectric type. Preferably, the element is composed of a polycrystalline ceramic material such as barium titanate, lead titanate-zirconate or the like.

The piezoelectric element 10 is polarized parallel to the longitudinal axis and is suitably electroded on the peripheral surfaces thereof to provide a piezoelectric response in the compression mode when the element is squeezed longitudinally. The field of polarization is such that one end of the ceramic element is normally positive while the opposite end has normally a negative voltage potential.

Axially aligned with the piezoelectric member 10 are a couple of compression retainer blocks, or end pieces, 12 and 14, one thereof being disposed at each end of the elongated ceramic member 10. In the preferred embodiment the end pieces 12 and 14 are fabricated of a suitable metallic material.

The end pieces are provided with a circumferentially extending radial groove 16 for reasons which hereinafter will become more apparent.

The ceramic element 10 is held together with the end pieces 12 and 14 by a sleevelike layer 18 composed of a thermoplastic material which encapsulates the periphery of the piezoelectric ceramic and the compression retainer end pieces 12 and 14.

Preferably the thermoplastic material is composed of polypropylene although some other materials, or mixed systems, have shown some promise. Into the latter category fall thermosetting plastics of the type which provide a sufficient degree of shrinkage during the curing process.

The polypropylene encapsulant, as used in the preferred embodiment of this invention, is molded onto the aforedescribed element ultizing conventional molding methods. During the cooling process following the casting of the material, the encapsulant shrinks both longitudinally and circumferentially. In the device as depicted in FIGURE 1, in view of the ratio of length to diameter, shrinkage in the longitudinal direction causes unit loading on the end pieces 12 and 14 in excess of the unit load exerted upon the periphery of the element. Selection of suitable coating thickness and overall length and the shrinkage coefiicient of the plastic will establish the degree of compression. The anchorage provided by the groove 16 of the end pieces 12 and 14 causes the sleeve to apply pressure through the metal end pieces to the piezoelectric elements during and permanently after this shrinkage occurs.

A conductive plastic material may be superimposed on the basic encapsulating sleeve to serve both as a radio frequency shield and to establish a low conductivity path between the end pieces 12 and 14 to completely surround the device with a radio frequency leak-proof coating.

The heat required to cause the thermoplastic material to melt or the exothermic reaction involved in thermosetting plastic, may be used to cause direct adhesion of the encapsulating material to the piezoelectric element or, alternatively, may serve to assist in shrinking an intermediate layer of heat resistive material into intimate contact with the ceramic element. As noted above, this intimacy is accentuated by the controlled circumferential shrinking of the plastic encapsulant. The degree of shrinkage obtainable by the materials herein under consideration are well known in the art. For example, reference may be had to Machine Design, Plastics Book Issue, dated Sept. 20, 1962. The mold shrinkage for polypropylene as indicated therein may be selected from the range of 0.010 to 0.025 inch per inch. A shrinkage of this magnitude, when used in conjunction with compression retainer end pieces 12 and 14, establishes a substantial preload upon the ceramic element and the resulting device exhibits and maintains an excellent symmetry about the longitudinal axis. This improvement is also of significant importance in devices such as shown in FIG- URE 2 which utilize a plurality of elements. The elements are held together in intimate contact and can be caused to vibrate as a unit.

More particularly with respect to FIGURE 2, it should be noted that the elements in this embodiment are arranged mechanically in series and electrically in parallel. A hardened conductive centerpiece 20 is interposed between elements and serves as an electrode from which energy may be delivered to an external load. The centerpiece is ground to a sufiicient degree of flatness to assure intimate contact with the equally flat ceramic elements and to maintain over the Whole area suitable contiguity therebetween.

By virtue of a controlled shrinkage in the polypropylene material composing sleeve 18, the members 10, 12 and 14 are preloaded longitudinally to the extent that intimate contact is again established between each of these components by the dimensional shrinkage in the longitudinal direction of the plastic material. The degree of contact and intimate relationship between the components is manifested by the fact that the device as a whole will resonate at a frequency determined by the frequency constants of the ceramic materials and the loading provided by the intimately coupled end pieces. For example, assuming that the ceramic elements are composed of PZT4, a trademark of Clevite Corporation, and have a cylindrical configuration of about .6 in diameter, an overall resonance of the device is established in the vicinity of 23 kilocycles per second while the ceramic elements alone would resonate at 60 kilocycles per second. Thus, the sleeve 18 formed by molding under suitable conditions of temperature and pressure serves the dual function of affecting electrical and mechanical characteristics.

A further modification of the invention is shown in FIGURE 3 wherein a single element 10 of polycrystalline ceramic is disposed end to end and longitudinally aligned with a piezoelectrically inactive element 22 for generating acoustic waves in response to an electric signal applied to the active element 10, or to receive such acoustic wave energy and causing an electric signal to be generated. The element 22 is referred to as inactive inasmuch as this element is incapable of a piezoelectric response. In the preferred embodiment such inactive element 22 is composed of a suitable metal.

In accordance with this invention both, the active element 10 and the inactive element 22, are provided with compression retainer grooves 16 as hereinabove described.

The FIGURE 4 illustrates a further modification of the invention and more particularly of the embodiment shown in FIGURE 2. In this modification the separate end pieces 12 and 14 have been eliminated and the provision for anchoring the layer of plastic material is directly incorporated into each piezoelectric element 24 and forms an integral part thereof.

While there have been described what are at present considered to be the preferred embodiments 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 aimed, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A unitary prestressed piezoelectric transducer for generating or receiving energy, comprising: piezoelectric element means, and a layer of shrunk in situ plastic material symmetrically encapsulating said element means between the axial end faces and being anchored thereto to permanently place the element means under compression.

2. A prestressed piezoelectric transducer according to claim 1, and a metallic member at each axial end of said element means adapted to anchoringly receive the layer of plastic material.

3. A prestressed piezoelectric transducer according to claim 1, wherein said layer of plastic material is directly anchored to the remote ends of the element means.

4. A prestressed piezoelectric transducer according to claim 1, wherein said element means comprises a plurality of members stacked end to end and at least one thereof is piezoelectrically responsive in the compression mode.

5. A prestressed piezoelectric transducer according to claim 4, wherein one of said members is piezoelectricnlly inactive.

6. A prestressed piezoelectric transducer according to claim 4 wherein at least two of said members are piezoelectrically responsive and disposed mechanically in series and electrically in parallel; and an electrically conducting rigid member interposed between the piezoelectric elements.

7. A prestressed piezoelectric transducer according to claim 1, wherein said material is composed of polypropylene.

References Cited UNITED STATES PATENTS 2,872,600 2/1959 Peck 3l0-S.7 2,871,787 2/1959 Rizer 310-8.7 3,031,591 4/1962 Cary 310-8.7 3,213,666 10/1965 Pudnick 310-8] 3,230,402 1/1966 Nightingale 3l0-8.7

MILTON O. HIRSHFIELD, Primary Examiner.

I. D. MILLER, Assislant Examiner.

Patent Citations
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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3396285 *Aug 10, 1966Aug 6, 1968Trustees Of The Ohio State UniElectromechanical transducer
US3466473 *Dec 30, 1966Sep 9, 1969Univ OhioHigh voltage sonic pulse generator
US3509389 *Mar 5, 1969Apr 28, 1970Us ArmyPiezo-electric crystal construction
US3585417 *Mar 6, 1969Jun 15, 1971Mallory & Co Inc P RPiezoelectric device having a resistor and a plastic insulating casing
US3716828 *Feb 2, 1970Feb 13, 1973Dynamics Corp Massa DivElectroacoustic transducer with improved shock resistance
US3725986 *Nov 8, 1971Apr 10, 1973Mechanical Tech IncMethod of making power transducers
US3749948 *Jun 21, 1971Jul 31, 1973Seismic LogsPressure transducer
US3992640 *Nov 29, 1974Nov 16, 1976Eastman Kodak CompanyPiezo crystal housing and mount
US4193009 *Nov 7, 1977Mar 11, 1980Durley Benton A IiiUltrasonic piezoelectric transducer using a rubber mounting
US4511598 *Oct 4, 1982Apr 16, 1985Xerox CorporationElectromechanical transducer protecting
US4545625 *Feb 28, 1983Oct 8, 1985International Business Machines CorporationPrestressed cylindrical squeeze bearing member
US4577131 *Dec 29, 1983Mar 18, 1986Zygo CorporationPiezoelectric micromotion actuator
US4823802 *Apr 3, 1987Apr 25, 1989Vsesojuzny Nauchno-Issledovatelsky I Ispytatelny Institut Meditsinskoi TekhnikiDevice for measurement of arterial blood pressure
US4862893 *Feb 2, 1988Sep 5, 1989Intra-Sonix, Inc.Ultrasonic transducer
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US7990023 *May 22, 2007Aug 2, 2011Robert Bosch GmbhArrangement with a coated piezoelectric actuator
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US20090199379 *May 22, 2007Aug 13, 2009Albano De PaoliArrangement with a coated piezoelectric actuator
US20110025169 *Apr 29, 2008Feb 3, 2011Robert Bosch GmbhPiezoelectric drive unit
US20130074602 *Sep 23, 2011Mar 28, 2013Ascent Ventures, LlcUltrasonic transducer wear cap
US20140209599 *Jan 25, 2013Jul 31, 2014Energyield, LlcEnergy harvesting container
EP1588782A1 *Apr 15, 2005Oct 26, 2005Elliptec Resonant Actuator AGMolded piezoelectric apparatus
EP1939950A3 *Nov 9, 2007Nov 2, 2011Robert Bosch GmbhPiezo-electric actuator
WO1994007615A1 *Sep 24, 1993Apr 14, 1994Endress U. Hauser Gmbh U. Co.Sonic or ultrasonic transducer
WO2008135457A1 *Apr 29, 2008Nov 13, 2008Robert Bosch GmbhPiezoelectric drive unit
U.S. Classification310/328, 310/339, 310/340
International ClassificationB06B1/06, H01L41/053, H01L41/00
Cooperative ClassificationH01L41/053, B06B1/0644, H01L41/0536
European ClassificationH01L41/053P, B06B1/06E, H01L41/053