Compound ultrasonic transducer and mounting means therefor

310-322

SR

XR

3»198»489

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SEARCH Ι00Β

Aug. 3, 1965

Η. T. FINCH

3,198,489

COMPOUND ULTRASONIC TRANSDUCER AND MOUNTING MEANS THEREFOR

Filed Feb. 16, 1962

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ATTORNEY

United States Patent Office

3,198,489

Patented Aug. 3, 1965

3,198,489

COMPOUND ULTRASONIC TRANSDUCER AND MOUNTING MEANS THEREFOR

Horace T. Finch, Arcadia, Calif., assignor to The Birtcher Corporation, Los Angeles, Calif., a corporation of California

Filed Feb. 16,1962, Ser. No. 173,640 15 Claims. (CI. 259—1)

This invention relates to an ultrasonic transducer and supporting means therefor and more particularly to flangemounted compound electroacoustic transducer of the type used in connection with the generation of ultrasonic energy in tanks, containers, or similar devices.

Holes (gas bubble cavities) can be created in a liquid by high intensity sound waves. When such a cavity collapses, extremely high pressures are produced. This process, called cavitation, is the original of a number of mechanical, chemical and biological effects. For example, cavitation effects can be used to disperse metals and sulphur in solutions to produce extra fine grain photographic emulsions, and to achieve a smaller size and more uniform alloying of molten metals. In chemistry, cavitation can be used to break long-chain polymers into shorter chains, affording a polymer of more uniform chain length than is possible with other depolymerizing methods. Cavitation forces also can be used to sterilize milk.

Ultrasonic energy is used widely in the cleaning of metal parts. The large acoustic forces generated actually break off particles and contamination from metal surfaces.

Any cleaning advantage obtained by an ultrasonic cleaning system must be the result of power developed by the compressive mode of operation of the active element. Heretofore, the transducer has been cemented or otherwise attached to the outer face of the tank. This method in- 35 variably results in predominate shear mode coupling from the driver element. Inasmuch as shear mode vibration cannot be supported by a liquid, this mode of ultrasonic energy generation represents a loss of power. The present invention employs a novel and improved transducer and 40 mounting means therefor which minimize shear mode transfer to the container.

The transducer of the present invention comprises a half-wave compound oscillator which is flange mounted at its quarter-wave point, thus allowing a portion of one- 45 quarter wave length of active resonator to extend within the tank and the remaining quarter wave length to extend outside the tank. This configuration also provides for coupling at the zero motional mode of the compound system. Coupling at the zero motional point of the half- 50 wave resonator system, results in minimum amount of shear wave energy being transmitted at the coupling point to the tank. As a result, the half-wave system is free to vibrate at the maximum “Q” point of the fundamental half-wave frequency.

Accordingly, it is a principal object of the present invention to provide a novel and improved compound oscillator for generating ultrasonic energy in a vessel.

It is a further object of the invention to provide novel and improved mounting means for piezoelectric and similar types of transducers whereby the mechanical ruggedness is improved without loss in acoustic efficiency.

Another object of the invention is to provide novel and improved ultrasonic transducer apparatus for efficiently coupling ultrasonic energy to fluid within a vessel, with a minimum of shear-mode power.

Yet another object of the invention is to provide a novel and improved ultrasonic compound oscillator adapted to directly and efficiently couple ultrasonic energy to γθ liquids within a container yet mounting the activated element of the oscillator outside the container.

Still another object of the invention is to provide novel and improved flange mounting means for ultrasonic transducers of the type employed in conjunction with liquid containers, in which minimum shear mode coupling exists 5 between the excited element and the liquid container.

A general object of this invention is to provide new and improved transducing apparatus which overcomes disadvantages of previous means and methods heretofore intended to accomplish generally similar purposes.

These and other objects of the invention will be understood more completely from the following detailed description, taken in conjunction with the drawings, in which:

FIGURE 1 is a sectional view of a preferred embodiment of the invention showing the manner in which the j 5 transducer is attached to the liquid container. This section is taken along line 1—1 of FIGURE 2.

FIGURE 2 is a perspective view, partially broken away, illustrating the transducer of FIGURE 1.

FIGURE 3 is a bottom plan view of the apparatus of ₂₀ FIGURE 4.

FIGURE 4 is a sectional view taken along line 4—4 of FIGURE 3.

Looking now at FIGURE 1 there is shown a first embodiment of the invention as utilized in connection with 25 the generation of ultrasonic sound waves in a liquid container. This application is a typical ultrasonic cleaning arrangement. Container 1 may be of any suitable configuration adapted to hold a solvent, cleaning fluid, water, etc. The bottom of the container is provided with an aperture 39 for receiving the transducer, and through which a vibratile face of the transducer may extend. The transducer comprises a compound oscillator having an electromechanical element 2 which is bonded to a resonator element 3. Element 2 is a circular plate of piezoelectric material such as crystalline quartz or barium titanate, or may be a magnetostrictive material such as a ferrite. A resilient gasket 4 of neoprene or other suitable material seals the junction between the transducer and container 1. An alternatingcurrent electrical excitation voltage is supplied to element 2 by means of foil conductors or electrodes 5-8.

Electrodes 5-8 may comprise copper foil strips. The two major faces of the piezoelectric element are preferably silverplated, after which the foil electrodes 5-8 are soldered thereto. Alternatively, the faces of element 2 may have an adherent metallic paint coating to which the foil or wire leads may be attached. In a preferred embodiment, element 2 comprises a barium titanate disc which is excited to compressional vibration (thickness mode) as the element is excited by an alternating voltage of suitable frequency. In a typical application, one terminal of the excitation source 30 would be connected to conductors 5 and 7, which are in electrical contact with the upper surface of element 2, and the remaining terminal of the excitation source is connected to conductors 6 and 8 which 55 are in electrical contact with the bottom surface of element 2. Barium-titanate element 2 is bonded to resonator 3 by means of an epoxy resin or other suitable adhesive applied to the interface 9.

Gasket 4 is placed between the flange 3A and the botθθ tom of the tank 1 in order to seal the opening in the tank against leakage. The opening in the tank is slightly larger in diameter than the diameter of the resonator 3 so that shear mode motion will not be directly transmitted to the bottom of the tank.

Gasket 4 may be sealed to the peripheral flange portion 3A of resonator 3 by means of a suitable adhesive. Similarly, the adjoining surfaces of gasket 4 and container 1 may be sealed by means of any suitable adhesive.

Resonator 3 is fabricated from a metal which is compatible with the liquid to be contained in tank 1. In a typical construction, resonator 3 may be fabricated from a bronze alloy such as Duronze III.

3,198,489

Since the transducer employs a driven element 2 which imparts an oscillatory motion to a tuned resonator 3 coupled therewith, the device may be described as a compound oscillator. The combined structure is designed to operate as a modified edge-clamped disc.

To provide a means for mounting the device, the resonator portion of the transducer has a thin flange 3A around its periphery by means of which it may be held and supported. This flange is located at a nodal point along the longitudinal axis of the transducer.

In this system the mounting flange 3A provides minimal motional coupling to the tank 1 at, or near, one-quarter wavelength of the compound transducer. The metallic portion of the system is flange-mounted at the quarterwave length point (zero motional impedance point) of the cleaning tank, to the end result that minimum shear mode coupling exists between the oscillator and the tank proper. Since the transducer is mounted at the nodal point, the reduction of its activity is minimized as compared with conventionally clamped disc generators. External mounting of the transducer element, as employed heretofore, results in the generation of considerable shear mode power, since the part is clamped at its active surface. Since shear mode vibration cannot be supported by the liquid, this method of coupling represents a substantial loss of power.

The effective radiating surface of the resonator portion of the transducer (the upper surface of resonator 3) is located within the tank 1 in direct contact with the liquid load. Thus, the mounting favors compressional mode (X axis) vibration and the shear mode is mechanically removed by the gasket 4. The axis of the transducer is substantially coplanar with that of the mounting assembly; this arrangement will suppress transmission of the shear mode into the mounting and permit full play of the compressional mode.

In operation, the transducer is driven in thickness mode (parallel to the longitudinal axis) by an ultrasonic signal generating circuit (not shown) so that it elongates and compresses at the excitation frequency.

Since the effective radiating surface presented to the load is considerably greater for thickness vibration than for other modes of vibration, the electro-acoustic efficiency of this mode of vibration is relatively high. Furthermore, the dimensions of the radiating surface are large in comparison with the wavelength radiated. For this reason, the transducer of the present invention will produce ultrasonic beams of small angular divergence.

It is generally believed, by those versed in the art, that ultrasonic cleaning is best achieved at frequencies between 25 kilocycles and 100 kilocycles. These frequencies are preferred since higher frequencies produce gross shadowing effects. Also, most metallic parts which are to be cleaned will be of a size which is a fraction of the fundamental wavelength in this frequency range.

For the purpose of describing a typical construction, assume that the fundamental frequency of the transducer is to be 90 kilocycles.

At a fundamental frequency of 90 kilocycles (kc.), a barium-titanate element will be found to be aplevels of the order of 5 watts/cm.², an applied voltage stress of ten volts/mil is required. Therefore the stress required for a one inch element, for this power level, is 10,000 volts.

Since the average impedance of a half wavelength 90 kc. element is of the order of 3,500 ohms at a fundamental frequency, stress in mils times volts presents an impossible matching condition for any practical oscillator system.

In a half-wave compound oscillator according to the invention, utilizing barium-titanate as a piezoelectric element which is one-quarter of the wavelength thick (one half of the thickness of the compound oscillator), ten volts per mil must be divided by two for the same stress per mil at any frequency. In addition to this, proper choice of the material which is to comprise the remaining quarter-wave thickness of the compound oscillator has been found to considerably increase the “Q” of the overall system. As a result, the normal stress in volts per mil, applied to the barium-titanate element, will produce a higher motional impedance at the interface of the metallic portion of the system to the interface of the liquid.

The following formulas may be employed to design a compound oscillator according to the invention at any frequency. These formulas are predicated on the utilization of barium titanate elements which are less than onehalf wave length of the frequency of the final compound oscillator system in thickness. The thickness of the barium titanate element in a compound oscillator is a matter of choice.

λ=ν// λ=οηβ wave length of the frequency of the final compound oscillator (employing barium titanate).

v=velocity of sound in the compound oscillator. f=frequency of the final compound oscillator.

λ/2=ν/2/

Τ=τ%λ/2 where

T=thickness of barium titanate.

T=percent of one-half wave length composed of barium titanate.

X72=v72/

X'=one wave length of the frequency of the final compound oscillator in the metallic portion.

v'=velocity of sound in the metallic portion.

Γ=(100-τ)%λ72/ where

T'=thickness of the metallic portion of the compound oscillator.

The flange is located at or near X/4 from the exposed or upper surface of the metallic part.

The barium titanate element is a preferred type of material; however, this element is a member of a class called ferroelectric ceramics which aer selected materials that require prepolarization. It should be understood that natural piezoelectric materials are also satisfactory for this application. Other types of piezoelectric materials include ammonium dihydrogen phosphate, crystalline quartz, ethylene diamine tartrate, and dipotassium tartrate.

There is shown in FIGURES 3 and 4 an alternative embodiment of the device in which the transducer is attached to the liquid vessel by means of a plurality of individual fasteners rather than by adhesive bonding. With reference to FIGURE 3, which is a bottom plan view of the device, the flanged portion of the resonator 10 is provided with a plurality of spaced bolt holes 12-19. Assuming that eight holes are used, they may be spaced at 45° intervals. Gasket 20 is provided with a corresponding number of holes which are aligned with the holes in the flange potrion of the resonator 10.

Gasket 20 is fabricated from neoprene or similar material and provides a fluid-tight seal between the flange and the bottom of the vessel 23. A plurality of bolts, two of which are shown at 21 and 22 in FIGURE 4, extend through corresponding ones of the bolt holes in the base of vessel 23. The fastening bolts are provided with mating nuts, two of which are shown at 24 and 25.

Element 11 is bonded to resonator 10 in a manner similar to that described in connection with the embodiment of FIGURE 1. Also, element 11 is supplied with electrodes 26-29, by means of which the excitation voltages may be applied to the device.

As will be obvious to those skilled in the art various other types of fastening means may be employed in lieu of bolts 21 and 22, or the adhesive bonding means employed in the apparatus of FIGURES 1 and 2.

Other modifications will be apparent to those skilled

3,198,489 in the art. For example, a single pair of conductor electrodes may be employed rather than the two pairs of conductors (26-27, 28-29) shown; such modification being determined by the power requirements of a particular application and/or other electrical charatceristics of the electromechanical element 10.

In the description of both of the embodiments shown, there is the implication that the container or tank is provided with a single transducer. It should be understood, however, that any number of transducers may be employed in a single container and such transducers may be arranged in rows or circular patterns as dictated by application requirements.

Since certain changes may be made in the above described apparatus, without departing from the scope of the invention herein involved, it is intended that all material contained in the above description, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

In summary, the flange-mounting technique employed by the compound oscillator of the present invention permits the generation of relatively high power levels in the liquid load of an ultrasonic cleaning tank or other vessel with greater efficiency than obtainable with prior devices.

Heretofore, the liquid-containing tank has employed a piezoelectric element having its upper face cemented directly to the bottom of the container. Since the lower end of the element is not anchored, movement of the piezoelectric element is primarily in the direction of the lower end and only a small portion of the energy can be transmitted via the upper end to the liquid load in the container. Further, when the upper face of the element is cemented to the bottom of the container, the Y-axis shear mode of oscillator motion is transmitted to the bottom of the container thereby resulting in substantial losses in useful energy and undesired heating of the tank. The transducer of the present invention, being supported by an integral flange, peripherally disposed about its central plane, and mounted at the minimum energy point permits optimum transfer of acoustic energy to the liquid load.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to preferred embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated and in their operation may be made by those skilled in the art, without departing from the spirit of the invention; therefore, it is intended that the invention be limited only as indicated by the scope of the following claims.