US3187207A - Transducers - Google Patents

Description

June 1, 1965 s. R. ToMES 3,187,207

TRANSDUCERS Filed Aug. 8. 1960 INV ENT OR 5/0 A. 73mm,

ATTORNEYS United States Patent 3,187,207 TRANSDUCERS Sidney R. Tomes, Whitestone, N.Y., assignor, by mesne assignments, to Giannini Controls Corporation, Duarte,

Calif.

Filed Aug. 8, 1960, Ser. No. 48,275 8 Claims. (Cl. SIG-8.7)

This invention relates to transducers, and more particularly, to a composite transducer unit to be used with apparatus for generating and controlling compression waves of ultrasonic frequencies in a variety of devices.

It is well known that elements made of electrostrictive material exhibit the characteristic of expanding or contracting when subjected to an electric field, and corrversely, develop an electric field when they are physically stressed. Use of this characteristic ofelectrostrictive elements has been made in a variety of devices. Such elements or a group of such elements are commonly referred to as transducers.

In the field of ultrasonic cleaning, use has been made of the expanding and contracting characteristic of transducers in order to create compressional wave energy. In the conventional type of ultrasonic cleaning device, an electromechanical transducer is placed in contact with a tank or container which is filled with a liquid, usually a cleaning fluid. An electronic frequency generator or oscillator is arranged to generate a wave at ultrasonic frequencies which is applied to excite the transducer. In turn, the transducer by expanding and contracting develops compression waves at ultrasonic frequencies in the cleaning fluid. These waves are in the form of a conical, divergent beam directed away from the transducer. An object may be cleaned by being placed in the cleaning fluid within the zone of the beam.

It is the usual practice to place the transducer at the bottom of the tank so that the beam will be directed upwardly, and the objects to be cleaned are suspended, as by a tray or basket, above the transducer and within the zone of the beam. Sometimes several electromechanical transducers are used.

The cleaning action is caused by the low intensity standing compression waves produced within the cleaning fluid by the transducer. The ditferential pressures between the nodes and anti-nodes of the waves produce a violent circulation of the cleaning fluid. It is well known that in any given frequency range, maximum activity of the cleaning fluid will occur when the operating frequency of the transducer is such that the height of the liquid in the tank is an exact number of one-half wave lengths produced by the transducer. It is desirable, therefore, that the transducer be tuned to vibrate at the frequency which will produce an exact number of one-half wave lengths in the fluid, and at the same time, be in the region of the transducer resonance.

A major disadvantage of conventional ultrasonic frequency devices is that the frequency generator systems must be adjusted by hand in order to tune the transducers to the desired frequency so that maximum ampiitude of the compression waves will be produced. Several variable factors determine the most efficient frequency, and all must be considered simultaneously, otherwise the transducer will operate at an incorrect frequency and low efl'iciency will result. Certain of the factors which affect the frequency which should be applied to the transducer are, for example, the level of the liquid in the tank, the load placed in the tank, the heating of the transducer, etc.

In previous types of ultrasonic frequency devices, it is necessary to continuously adjust the frequency by hand in order to compensate for such variable factors so as to maintain the transducer vibrating at the frequency which "ice will give optimum results. The manual procedure required to keep the transducer vibrating at the desired frequency is not only cumbersome and inefiicient, but it is impossible to keep the transducer tuned to exactly the correct frequency due to the fact that the variable factors may be changing continuously. Manual adjustment of the transducer will usually lag the frequency at which the transducer should be driven for optimum results.

Reference is made to ultrasonic cleaning devices for the purpose of example only. Other useful applications of ultrasonic wave energy have been made such as in ultrasonic drilling, ultrasonic heating of materials, etc. Certain of these other applications of ultrasonic wave energy employ electromechanical transducers which must develop high temperatures by producing compression waves of high amplitude. Transducers of this type will give the maximum amplitude, and, consequently, the highest efficiency when they are driven at a frequency very .close to their natural resonance.

All of these conventional types of devices, however, employ only electromechanical transducers, i.e., transducers that expand or contract when a voltage is passed across them. Consequently, the conventional system must be adjusted or tuned by hand in order that the correct frequency may be impressed across the electromechanical transducer so that it will produce compression waves of maximum energy.

This application is a continuation-in-part of my copending application, Serial No. 837,983, filed September 3, 1959, to which reference may be made for an understanding of a self-monitoring ultrasonic frequency vibrating system wherein the frequencies, impressed upon an electromechanical transducer are automatically controlled so as to maintain oscillation of the transducer unit at the frequency which will set up the most efficient standing waves in a load device.

Manual adjustment of the frequency generator is eliminated by feeding back to the generator :1 signal which corresponds to the compression waves produced in the load device. If these compression waves are not at the optimum frequency then the phase and amplitude of the feedback will be such as to cause the oscillator to seek a new frequency which will give maximum compression wave energy in the load device. The generator will then adjust automatically to correct the frequency output impressed upon the transducer unit and also the load device, so that the new frequency will produce maximum compression wave energy in the load device.

One object of this invention is to provide a transducer unit adapted for use with a frequency generator system.

Another object of this invention is to utilize, in a composite transducer unit, the characteristics of electrostrictive material of expanding or contracting when subject to an electric field and the characteristic of developing an electric field when stressed.

Another object of this invention is to provide a transducer unit for creating compression wave energy from electrical energy and for developing electrical energy for controlling the frequency of such compression wave energy.

Another object of this invention is to provide a composite transducer unit for applying compression wave energy at a certain frequency to a load device and for sensing the frequency of the compression waves in the load device.

Another object of this invention is to provide a new use of transducers.

These objects, as well as others which will become apparent to those skilled in the art, may be accomplished, according to one preferred embodiment of the invention, by providing a composite half-wave transducer unit made of a series of disc-shaped, active, electrostrictive elements and inactive metal elements. The active and inactive elements are arranged in pairs and a unit includes at least two of these pairs, one of which serves as a driving section and the other as a driven section. The various elements are stacked togetherand held by a suitably strong adhesive, which for example, may be an epoxy type cement. -The faces of theactive, electrostrie'tive elements are si-l'veredin order to-present maximum electrical con: tact, and means are-provided whereby damage stresses in the unit are relieved.

The embodiment of the invention explained above is illustrated inthe accompanying drawings in-which:

FIG. 1 is an exploded view of the composite transducer unit of this invention; and

FIG. 2 is a cross sectional view of the transducer unit with a load device to which the unit may be attached shown by broken lines.

The transducer unit of this invention isdesignate'd gen orally by reference numeral 10. This unit i comprised of electrostrictive elements and 14, which, for example, may be of barium tita'r'late in a ceramic state. Magnetost'ric'tive or piezoelectric elements may be used also. The elements 12 and 14 are of disc sliape'as shown in FIG. 1 and have both of their flat surfaces silverediri a manner that is well known; in the art;

Inactive or passive 'eTinefitS 16 iin1d 1d areat opposite ends of the transducer unit 10 andaligned with the active elements 12'and 14'. The'passive elements 16 and 18 may be made up of steel, aluminum or ceramic, for example, or any material with low acoustic loss.

In" fac-tb face contact with; each of the flat surfaces of the active elements 12 and 14, there are relatively thin washers or discs '20, 22, 24' and 26. The disc 26 is be; tween the active element 12 and the passive element 16, andthe disc 26 is between the active element 14 and the passive elementlB. Between the discs 22 and 24 and in face-to-face contact with these discs there is a conductor plate 28 which may be made of aluminum", for example. The plate 28 serves as a common terminal for the active elements 12an'd-14', and has a holethrough a portion which extends to one side of the unit 10; The hole 30 serves as a convenient connection point for a lead (not illustrated). A terminal 32 is provided for the passive element 16 to which a lead 34 may be connected. Similarly, a terminal 36 is provided for the passive element 18 for connecting a lead 38".

Both active elements 12 and 14, the disc 20', 22, 24 and 26 and the plate 28 have a hole extending through the center portion thereof as indicated at 40. The passive element 16 has a bore 42 connecting with the hole 40, and the passive element 18' has a bore 44 also connecting with the hole 40.

In the assembled composite unit 10, a locating pin 46 extends through the several holes in the active and passive elements, the discs and the plate, and into the bores 42 and 44, thereby serving to center the elements and to aid in retaining them in aligned; position. The pin 46 is made from any suitable high dielectric strength insulating material. In assembling the transducer unit 10, a strong adhesive such as an epoxy cement is applied to all of the face-to-face contact surfaces between the disc-shaped elements.

During the use of the unit 10 it is subject to high temperatures created by the active elements 12 and 14. The active and the passive members are usually made of a ditferent material, therefore, there exists an unequal coefficient of expansion between these members. I have found that it is this unequal expansion of the various members of conventional type transducers that often causes them to breakdown. Severe stresses are set up in the ceramic active elements, and these stresses may crack the ceramic. Also, the stresses may cause a parting of the adhesive bonds.

I have found that this ditliculty may be completely solved by the use of the discs 26, 22, 24 and 26. These discs may be made of lead or other such ductile material as tin, for example. Very satisfactory results may be obtained if the'discs are approximately .008 of an inch thick, although this dimension may be varied. The discs are soft and ductile enough to sutficiently reduce the stresses brought about by the unequal expansion of the various elements, and although the discs are poor acoustic conductors, their relatively short length will not introduce measurable losses.

In use, the transducer unit 10 is attached to a body or load device L, represented in FIG. 2 by broken lines. The load device may be, for example, a tank containing cleaningiluid, or a drill piece of an ultrasonic drilling apparatus, or the contact end of an ultrasonic heating apparatus. The connection may be made by bolts threaded into tapped holes provided in the outside face of the passive section 16. Instead, a strong adhesive, such as an epoxy cement may be used, or both bolts and an adhesive maybe used.

The active element 12 and the passive element 16 together form a driving section A which is connected to an ultrasonic frequency generator as by lead 34. The recur ring pulses which are maintained at the frequency produced by the generator are impressed across the active element 12 so as 0t cause it to create compression waves at a corresponding frequency. These compression waves are then transmitted to the load device L. If the load device is a tank of cleaning fluid, for example, a zone of standing compression waves is created in the fluid. The high frequency of the waves results in violent agitation of the liquid. If the load device is a heating apparatus, high temperatures are created by the compression waves which, in this case, usually are of large amplitude.

. The active element 14 and the passive element 18 together form a driven section B. The compression waves from the load device L are reflected to the driven section B and stress the active element 18, causing it to develop pulsating voltages corresponding to the compression waves received by it. These pulsating voltages are then transmitted, as by the lead 38, to the ultrasonic frequency generator to control the frequency of the pulses put out by the generator to' the driving section A. The driving section A and the driven section B are acoustically coupled by the bonds between the pairs of active and passive elements.

Thus, the driving section A of the transducer unit 10 receives electric pulses at a certain frequency from the generator and converts these electric pulses to mechanical pulses or waves which are then transmitted to the load device L during which the entire unit 1% acts as a halfwave resonator. The driven section B of the transducer unit 10 senses or detects these mechanical pulses and converts them to electrical pulses at a corresponding frequency and feeds these electrical pulses back to the generator. The generator then uses the feed back pulses to control the frequency at which the electrical pulses will be impressed upon the driving section A of the transducer unit 10.

Reference may be made to my co-pending application Serial No. 837,983, filed September 3, 1959, which describes a generator system by which the output frequency of a generator is controlled by the feed back frequency.

It is well known that an electromechanical transducer element will give maximum efficiency when it is driven at or very close to its natural resonant frequency, or some exact multiple of the resonant frequency. Conventional transducer units used in such applications as ultrasonic cleaning, ultrasonic drilling, and ultrasonic heating employ only electromechanical transducer elements, that is, transducers that create compressional wave energy from electrical pulses. The resonant frequency at which these transducers should be driven for maximum efficiency often changes during an operation, due to changes in ambient temperature, changes in the work load conditions, or temperature rises due to losses in the transducers, for example. Therefore, the frequency 7 generator which drives the transducers must be of the type which may be adjusted so that the transducers may always be driven at the proper frequency. Of course, such adjustment must be by hand.

The transducer unit 10 of this invention enables this disadvantage in conventional transducer units to be eliminated by providing for the automatic adjustment of the frequency at which the driving section A is driven in response to the varying operating conditions. In the transducer unit 10, the driving section will always oscillate at a frequency which will create maximum compression wave energy. The maximum wave energy put out by the driving section A will result in maximum vibration energy being reflected upon the driven section B, and maximum voltage feedback pressed upon the generator. Any wave energy less than maximum will result in less than maximum feedback voltage. If the generator receives less than maximum voltage feedback it is forced to oscillate at a changed frequency which will produce maximum wave energy, and this changed frequency is the frequency at which the driving section A subsequently will be driven.

The passive elements 16 and 18 serve to reduce the amount of electrostrictive material required for the driving and driven sections A and B. It is necessary that the resonant frequency of the driving and driven sections be predetermined. The velocity constant of the metals, steel or aluminum for example, used for the passive elements 16 and 18 is known and does not vary appreciably from sample to sample, however, the material used for the active elements 12 and 14, which often is barium titanate in a ceramic state, may vary considerably. Consequently, the greater the ratio of the size of the passive elements 16 and 18 to the active elements 12 and 14, the easier it is to determine, maintain and control the resonant frequency of the driving and driven sections.

Different electrost'rictive materials offer different vibrational characteristics, and the active elements 12 and 14 may be made of the type of electrostrictive material which will give the particular results desirable. The selection of the size and material of the active and passive elements 12 and 14 provides a means of obtaining optimum impedance matching between the transducer unit and the load. The ratios of the active and passive materials in the driving and driven sections A and B will control optimum impedance matching, the desired output impedance and voltage of the driven section, and also the voltage gradients in the active element 12. The selection may be made according to the characteristics desired of the transducer unit.

In certain applications of the transducer unit 10, a very high output of compression wave energy may be desirable. In this event, several driving sections may be used with a single driven section. If, for example, 50 driving sections are used with a single driven section, a low power amplifier may be added between the driven section and the generator. While this requires the added expense of an amplifier, such expense may be offset by the reduced number of driven sections used. This illustration is given as an example only to show the versatility of the transducer unit. When multiple transducer elements are used they are connected in parallel.

Other modifications may be made to change the characteristics of the unit 10. For example, the active elements 12 and 14 may be separated by an additional passive element. In the embodiment described, the driving section A is placed in contact with the load device L, but equally satisfactory results may be obtained if the positions of the driven section A and the driven section B are reversed. The unit may be used with sonic as well as ultrasonic frequencies.

Although a certain embodiment of this invention has been described in detail, it will be apparent to persons skilled in the art that modifications may be made. Consequently, it is intended that the foregoing description should be considered as exemplary only and that the scope of the invention should be determined from the following claims.

I claim:

1. A compressional wave transducer comprising a pair of passive terminal elements, both electrically conductive, a pair of non-metallic electrically active elements between the terminal elements, a metallic conductive plate element between the active elements, a ductile element of conductive material between each pair of said adjacent elements, each ductile element being bonded with cement to the adjacent elements whereby stresses between the adjacent elements are accommodated by the ductile element.

2. The transducer of claim 1 wherein the f aces of the active elements are metallized.

3. The transducer of claim 1 wherein the elements are disc shaped.

4. The transducer of claim 1 wherein cement constitutes the sole attachment between adjacent elements.

5. The transducer of claim 1 wherein the terminal elements constitute individual electrodes separately connected to the active elements.

6. The transducer of claim 5 wherein a rod is positioned centrally of said elements.

7. In a compressional wave transducer, a non-metallic electrically active element, a metallic passive element, both stacked against a stress relieving element of ductile material, said elements each being bonded with cement to the adjacent element whereby stresses between the first two elements are accommodated by the ductile element.

8. A compressional wave transducer comprising a nonmetallic electrically active element having a metallized face, a metallic passive element having a congruent face, said elements being stacked with their congruent faces adjacent, a stress relieving element of ductile material between said congruent faces, said elements each being bonded with cement to the adjacent element whereby stresses between the first two elements are accommodated by the ductile element.

References Cited by the Examiner UNITED STATES PATENTS 1,912,213 5/33 Nicolson 310-81 2,137,852 I l/=38 Nicolson BIO-8.8 2,625,035 1/53 Firestone 3 l0-8.6 2,947,889 8/60 Rich 3-108.7 3,066,232 11/62 Brason 3 l09.8 3,094,314 6/63 Kearney et al. 3108.7

MILTON O. HIRSHFIELD, Primary Examiner.

STEPHEN W. CAPELLI, Examiner.

Claims (1)

1. A COMPRESSIONAL WAVE TRANSDUCER COMPRISING A PAIR OF PASSIVE TERMINAL ELEMENTS, BOTH ELECTRICALLY CONDUCTIVE, A PAIR OF NON-METALLIC ELECTRICALLY ACTIVE ELEMENTS BETWEEN THE TERMINAL ELEMENTS A METALLIC CONDUCTIVE PLATE ELEMENT BETWEEN THE ACTIVE ELEMENTS, A DUCTILE ELEMENT OF CONDUCTIVE MATERIAL BETWEEN EACH PAIR OF SAID ADJACENT ELEMENT, EACH DUCTILE ELEMENT BEING BONDED WITH CEMENT TO THE ADJACENT ELEMENTS WHEREBY STRESSES BETWEEN THE ADJACENT ELEMENTS ARE ACCOMMODATED BY THE DUCTILE ELEMENT.

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