US2875354A - Piezoelectric transducer - Google Patents

Description

United States Patent O 2,875,354 PIEZOELECTRIC TRANSDUCER Robert V. Harris, Darien, Conn., l:assignor to Branson Instruments, Inc., Stamford, Conn., a corporation of 'Connecticut Application January 29, 1954, Serial No. 407,020

Claims. (Cl. S10-8.2)

This invention relates to transducers and more particularly to an ultrasonic crystal transducer for use with inspection, measuring and testing equipment or the like.

Ultrasonic inspection equipment, such as is described in Patent No. 2,280,226 to F. A. Firestone; Patent No. 2,431,233 to W. S. Erwin; and in Patent No. 2,522,924 to N. G. Branson, is usually designed to operate with quartz crystal transducers having an impedance of 100,- 000 ohms or higher. However, the output impedance of the equipment is limited, so that at lower frequencies, and limited transducer surface dimensions, the transducer impedance becomes considerably higher than the output impedance of the generating equipment, resulting in a poor transfer of energy between the equipment and the transducer.

It is the principal object of this invention to provide a transducer which will have the desired impedance characteristic at the desired resonant frequency and having the desired surface dimensions.

There are in general use in ultrasonic inspection equipment two types of piezoelectric materials having opposite impedance characteristics, materials such as quartz having impedances of 100,000 ohms or higher and ceramic materials such .as polarized barium titanate having impedances of 100 ohms or lower.

It is another object of this invention to provide a transducer having an intermediate impedance, which can be used interchangeably with conventional quartz transducers in existing equipment.

At the lower frequencies used for metal inspection (below 1.5 me), operation with practical sized quartz transducers becomes increasingly difficult, due to the fact that the impedance of the quartz transducer becomes considerably higher than the plate circuit of the oscillator which drives it. Transducers constructed in accordance with this invention permit improved operation of the equipment with smaller transducers at such lower frequencies.

In accordance with the invention a compound transducer is formed comprising a sandwich of two piezoelectric materials having dierent impedance characteristics secured together by a suitable cement and so arranged, relative to their polarities, that both elements expand and contract together with the same polarity of applied voltage.

In the drawings, Fig. 1 is a greatly enlarged schematic cross-section through a compound transducer; Fig. 2 is a similar view of a preferred practical design of complete transducer unit including a support or holder.

Referring to Fig. 1, which illustrates the basic construction of a transducer according to the invention, a slab 4 of piezoelectric material such as X-cut quartz, which has a relatively high impedance, is secured to a second slab 5 of piezoelectric material having a relatively low impedance such as polarized barium titanate, by means of a layer of adhesive 6. The polarity of the two slabs 4 and 5 is selected so that they both contract and exg 2,875,354 Patented Feb. 24, 1959 pand together and, being secured to one another, they react as a unit. In this embodiment, electrical contact with the transducer is achieved by means of a conductive coating 8 such as silver plating, and by contact of the element 5 with the work 9 which is assumed to be a metal plate or other conductive body being examined. Suitable generating equipment of ultrasonic frequencies is indicated by the radio frequency oscillator unit 10, the output of which is connected through wire 11 to conductive coating 8 and through wire 12 with the conductive work surface 9.

The fundamental resonant frequency of the transducer unit as a whole is determined by the total thickness T and the sound velocity constants of the materials used in its construction. The impedance of the transducer unit as a whole is determined by the proportionate thicknesses of the two piezoelectric elements and the thickness of the cement 6 and the surface area. The impedance of the transducer will approach that of material 4 as its thickness t, approaches of the total thickness T and the impedance of the unit as a whole will approach that of material 5 as its thickness t3 approaches 100% of the total thickness T.

In actual practice, a transducer element is preferably made as shown in Fig. 2, the piezoelectric elements being provided with a pair of conductive surfaces and protected by a wear plate which is applied to the work, the transducer being attached to a suitable holder. By way of example, a rigid plastic disc 14 having good dielectric properties is mounted in a housing 15 having an opening 16 for a coaxial cable 18 with inner and outer conductors 19 and 20. Secured to the disc 14 is a compound transducer element made up of a low impedance piezoelectric disc 21 secured by cement 22 to a high impedance piezoelectric disc 24. The upper disc 21 is provided with a conductive coating 25 of silver or the like and a similar coating 26 is provided on the lower surface of disc 24, the latter coating being protected by a wear plate 28 secured to the coating 26 by cement 29. .The wear plate may be a thin wafer of random-cut quartz, glass or similar material having no piezoelectric properties. The transducer element is secured to disc 14 by a layer of cement 30 and the disc 14 is perforated at 31 and 32 to permit passage of the connecting wires 33 and 34 which are electrically bonded to the conductive coatings 25 and 26 respectively. After the transducer element has been secured to dise 14, its exposed edges are enclosed in a thick coating of moisture-proof cement 35 in which the connecting wire 34 is imbedded.

By way of specific example and as illustrative of the advantages of the compound transducer of the invention, a transducer made up of round discs 3A in diameter of polarized barium titanate .028" thick, X-cut quartz .009" thick and a random-cut quartz wear plate .009" thick, having an overall thickness including the cement of .047", has a natural resonant frequency of 2.3 mc. at which the impedance is 50,000 ohms. As the frequency decreases the impedance becomes 200,000 ohms at 2 mc., 400,000 ohms at 1.5 me. and 700,000 ohms at l mc. By contrast, a single piece of X-cut quartz provided with a wear plate and of comparable overall dimensions, resonates also at 2.3 mc. but has an impedance of 800,000 ohms at that frequency which increases to 1 megohm at 2 mc., 2 megohms at 1.5 mc. and 4 megohms at 1 mc.

It will be appreciated that to obtain the desired lower impedance utilizing only quartz would necessitate greatly increasing the size of the crystal element of the transducer. For example, a compound transducer made in accordance with the invention and only l" in diameter will have approximately the same sensitivity as a 1% diameter X-cut quartz crystal.` For the purposes of testing, thick f'ice I ness measurement, etc., for which ultrasonic transducer elements are used, it is highly desirable that the transdueer dimensions be kept small while at the same time the transducers be capable of operating with a good transfer of energy when using existing equipment designed for use with transducers having an impedance in the vicinity of 100,000 ohms.

The above described specific embodiment is merely illustrative of the invention which is not limited to the details thereof but is to be construed in accordance with the appended claims.

What is claimed is:

1. In apparatus for generating vibrations in an object, the combination of a composite piezoelectric transducer unit, comprising a sandwich of two different piezoelectric materials having widely dilerent impedance characteristics held together in face to face relationship, an alternating current generator, and means for applying the output of said generator across said transducer.

2. The combination defined in claim l in which said materials are arranged to expand and contract together under the applied alternating potential.

3. The combination dened in claim 1 in which one of said materials is X-cut quartz and the other polarized barium titanate.

4. The combination delned in claim 1 in which said materials are joined together by a non-conducting bonding material.

5. In apparatus adapted to set up mechanical vibrations in a medium, the combination of two piezo-electric elements formed from materials having widely different impedance characteristics and held together in face to face relationship, conductive layers on the opposite external faces of both of said elements, and means for applying a varying electric potential to both of said conductive layers to cause said elements to vibrate together, whereby said apparatus has an impedance characteristic intermediate those of said elements.

References Cited in the lc of this patent UNITED STATES PATENTS 2,321,285 Ehret June 8, 1943 2,410,825 Lane Nov. 12, 1946 2,484,626 Keller Oct. 11, 1949 2,484,636 Mason Oct. 11, 1949 2,666,862 Branson Jan. 19, 1954 FOREIGN PATENTS 856,314 Germany Nov. 20, 1952 OTHER REFERENCES Piezotronic Technical Data, published 1953 by the Brush Electronics Co., pages 2 and 17.

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