Ultra-sonic wave transducers

2,779,880

G· MALHERBE

ULTRA-SONIC WAVE TRANSDUCERS

Jan. 29, 1957

Filed March 18. 1952

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INVENTOR

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ATTORNEY

Jan. 29, 1957

2,779,880

G. MALHERBE

ULTRA-SONIC WAVE TRANSDUCERS

Filed March 18, 1952

Sheets-Sheet 2

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ATTORNEY

United States Patent Office

2,779,880

Patented Jan. 29, 1957

2,779,880

ULTRA-SONIC WAVE TRANSDUCERS 5

Georges Malherbe, Montigny-le-TiHeuI, Belgium, assignor to Ateliers de Constructions Electriques de Charleroi, Brussels, Belgium, a company of Belgium

Application March 18,1952, Serial No. 277,330

Claims. (Cl. 310—8.3)

The present invention relates to devices intended to transmit elastic waves, arid particularly ultra-sonic waves, from an emitter device to a body to be examined or treated with these waves.

These devices are commonly called ultra-sonic wave “transducers” and the invention relates more particularly to ultra-sonic transducers comprising a . container filled with a wave-conducting liquid which is in contact both with the wave-emitting device, for example a piezo-electric crystal, and with a flexible wall of the container adapted to engage the body to be examined of treated.

One example of these ultra-sonic transducers has been described in prior patent application, Serial No. 684,614, now Patent No, 2,532,507 issued December 5, 1950 relating to “turgescent” feeler apparatus. Ultra-sonic wave transducers produced in the form of turgescent feelers generally work well and are in current use.

There are, however, cases of practical application where the turgescent feelers such as described in the said patent application are difficult, if not impossible, to use because of their large dimensions.

_ These ultra-sonic wave transducers, as now constructed in the form Of turgescent feelers, comprise a hollow body of cylindrical . general shape, filled with a waveconducting liquid, the. two ends being respectively closed by a piezo-electric quartz crystal element capable of emitting ultra-sonic waves and by a flexible wall capable of bulging under the pressure of the filling liquid and of engaging snugly a portion of the surface of a body to be examined or treated, without any appreciable air gap between the two surfaces in contact. For reasons of directivity, i. e. concentration of the ultra-sonic waves into a relatively narrow beam in a given direction, and taking account of the wave-length of the emission, it has been found desirable to use an emitting device having a relatively large surface, which entails a cylindrical container of correspondingly: large diameter.. This results in a transducer of. considerable overall size and having a flexible wall of fairly large area at the outlet end. For example, in an ultra-sonic wave transducer which is currently manufactured, the diameter of the emitter device is about 20 mm. and the external diameter of the cylindrical container is about 40 mm. If account be taken of the. area lost in the mounting of the crystal, there is thus obtained a directivity along the axis of the container equal to the theoretical directivity of a vibrating surface about 15 mm. in diameter.

In these conditions, it is easily understood that an ultra-sonic wave transducer having such dimensions cannot be employed in all cases of practical application. It is particularly unsuitable for exploring or treating a body having a surface that is difficult of access, as, for example, a body having rectangular or trapezoidal grooves or recesses with a narrow entry area, less than 40 mm. in the case considered. .

-.. A solution which comes immediately to mind in this case is to use a transducer of smaller size, which can be introduced into the recesses so as to secure proper contact between the bottom of a recess and the flexible wall of the transducer. That would involve the use as waveemitter of a piezo-electric crystal of smaller diameter, corresponding to a very small effective vibrating area, from which it follows that, for convenient excitation voltages and powers, the intensity of the radiations emitted ^wQuld be very low. Moreover, all other things being 10 equai, any reduction of the diameter of the vibrating surface leads to a great reduction of the directivity.

Another solution which might be considered consists in using, with a quartz crystal wave-emitter of normal dimension, for example 20 mm. diameter, a hollow sup15 porting member of frusto-conical shape instead of the cylindrical member usually employed. The axis of the cone frustum being coincident with that of the crystal, the small base of the cone, opposite to the quartz, could have a sufficiently small diameter for the transducer thus 20 constructed to be introduced into places difficult of access, as, for example, into a recess with a narrow entry area. There would thus be obtained a radiation of suitable intensity with an exciting voltage and power which are admissible.

However, tests carried out upon ultra-sonic wave transducers constructed in this way show that, although the . waves emitted by the crystal may be concentrated upon a small area at the opposite end of the cone frustum, the energy emitted diverges widely at the outlet from 30 the cone. In a plane parallel to the emitter crystal, around a central zone of high transmitted energy, there is a zone of lower intensity surrounded by an annular area of high energy, the position and intensity of which differ from those corresponding to the first diffraction 35 ring provided by Huygens’theory.

This arises from the fact that the part of the ultrasonic waves coming from the periphery of the piezoelectric crystal is reflected by the walls of the cone and emerges from the latter at an angle to its axis. This 40 angle can be calculated by the general laws of reflection in geometrical optics.

The present invention provides a wave transducer of small dimensions at the place of contact with the body, to be examined or treated and possessing a small outlet ^J area, while not having the drawbacks pointed out above of the transducer with small emitting element or the transducer in the form of a convergent cone..

More, particularly the invention relates to an improvement in ultra-sonic wave transducers with liquid conducting medium, like the turgescent feelers described in the. aforesaid patent application, and consisting in the fact that the container enclosing the liquid conducting the ultra-sonic waves from the wave-emitter device to a ₅₅ body to be examined or treated with the said waves comprises two hollow frusto-conical portions arranged coaxially and joined at their small bases, the two ends of the convergent-divergent double cone frustum thus constituted being of different cross-sectional areas, the wave60 emitting device being disposed normally to the axis of the double cone frustum and at its larger end, and the smaller, end of the double cone frustum, closed by a flexible wall, constituting the outlet for the ultra-sonic waves.

The invention is hereinafter described with reference 65 to the accompanying drawings, in which:

Fig. 1 represents diagrammatically an ultra-sonic wave transducer such as described in the aforesaid patent application.

Fig. la is a graph of directivity of the transducer rep' resented in Fig. 1, as explained below.

Figs. 2 and 2α represent an ultra-sonic wave transducer

9,779,880 similar to that of Fig. 1 but of considerably smaller outlet area, and its directivity graph.

Figs. 3 and 3a represent an ultra-sonic wave transducer in the form of a convergent cone, and its directivity graph. .

- Figs. 4 and 4a represent an ultra-sonic wave transducer according to the invention, having an interior profile in the form of a convergent-divergent double cone frustum, and its directivity granh.

Fig. 5 represents an example of the use of the ultrasonic wave^: transducer according to the invention.

Fig. 6 represents another embodiment of the invention.

In Figs. 1 to 4, the character G designates wave-emitting means, e. g. a quartz crystal emitting ultra-sonic waves, E is the container forming the body or housing of the transducer and containing a liquid for conducting the ultra-sonic waves, P indicates a flexible wall, made, for example, of rubber, closing the end of the housing opposite the crystal C and intended to establish proper contact between the transducer and the body to be examined or treated with the ultra-sonic waves. ?

In the embodiment of the invention illustrated in Fig. 4, the housing E of the transducer comprises two hollow frusto-conical portions Ei and Ea which have a common axis X. The small ends of the frusto-conical portions are of equal cross-section and are joined in a neck portion N. The large ends of the frusto-conical portions Ei and Ea are of unequal size, the large end ei of portion Ei being larger than the large end ea of portion Ea. The cross-section of the neck N is smaller than either of the ends ei or ez of the housing. Thus, proceeding from the large end ei at which the emitting means C is located, the walls of the housing E first converge to the neck N and then diverge to the opposite end ez of the housing which is closed by the flexible wall P. However, the total amount of divergence is less than the total convergence so that the cross-sectional area of the end ea of the housing closed by the wall P is less than that of the end ei where the crystal C is located. This is achieved by having the housing portion Ea shorter than the portion Ei in an axial direction or by having the angle of the truncated cone forming the portion Ea smaller than that forming the portion Ei or by a combination of both of these factors. It has been found that good results are obtained when the ratio of the cone angle of portion Ei to that of portion Ea has a value between 1:1 and 2:1.

The directivity graphs of the transducers represented in. Figs. 1 to 4 are shown in Figs, la to 4a, respectively, and have been obtained in the following manner: The transducer and a receiver of ultra-sonic waves were arranged in a medium conductive to the ultra-sonic waves, for example water, at a convenient distance from one another. The receiver, which was centered beforehand exactly on the axis of the emitter, was displaced along a straight line lying in a plane perpendicular to that axis. . At each position upon that straight line, the receiver was orientated accurately towards the emitter. There were then plotted as ordinates upon the diagrams of Figs, la to 4a, the deflections of an apparatus measuring the energy transmitted to the receiver, as a function of the distance from the receiver to the axis of the emitter, this distance being measured in.the plane normal to the, axis, and being plotted as abscissa.

: 'The scales used were the same, both for abscissae and for ordinates, for all of the graphs shown in Figs, la to 4a, and all the measurements were effected with the same voltage, for excitation of the emitter device.

Lastly, the sectional area of the emitter device was the same for the transducers of Figs. 1, 3 and 4, while the outlet area for the ultra-sonic waves from the transducer was the same for the transducers of Figs. 2, 3 and 4.

Consideration of these figures shows clearly the advantage of the transducer represented in Fig. 4, both as regards directivity and as regards intensity of the. radiation emitted, over the transducers represented in Figs.

and 3. In the case of Fig·. 2, the small area of the emitter device results in a lower sensitivity of the crystal and in spacing of the first and second lateral maxima, under Huygens’ theory, farther from the axis of the transducer. The inferior directivity of the transducer represented in Fig. 3 is due to the, path followed by the portion of the wave emitted by the peripheral portion of the crystal. The paths followed by the different portions of the emitted wave in the case of Figs. 3 and 4 are shown in broken lines and clearly illustrate the superior directivity of the wave transducer; according to the invention.

When the cone angle of the housing portion E2 of the embodiment shown in Fig. 4 is equal to that of the housing portion Ei, the paths of the waves reflected first by ;

the. side walls of the portion Ei and then by the side walls of the portion E2 are parallel to one another. If the cone angle of the housing portion E2 is less than that of the housing portion Ei, the paths of the reflected waves in the peripheral portion of the housing are caused to converge and are thereby focused.

Fig. 5 shows by way of example a practical application of the improved transducer, where the transducer represented in Fig. 1 is not capable of being used. T designates a metallic body to be examined with the ultrasonic waves, this body being provided with a recess or notch L the width of which is less than the diameter of the transducer represented in Fig. 1, but of such size that it is possible to introduce the improved transducer into the recess. It is clear that in this case the improved transducer can be used to secure perfect contact between the bottom of the notch and the flexible wall of the transducer while the transducer shown in Fig. 1 cannot be used.

The foregoing description covers one embodiment of the improved wave transducer, having a small surface for contact with the bodies to be. examined or treated with ultra-sonic waves, and its advantages over other types of transducers have been explained, these advantages consisting particularly in good directivity of the waves leaving the transducer and high intensity of radiation emitted in normal conditions of use. It is possible to obtain substantially the same results by using an ultrasonic wave transducer of modified shape and particularly a transducer of biparaboloidal internal profile according to a second embodiment of the invention.

In this embodiment of the invention, the ultra-sonic wave transducer, with liquid conducting medium, comprises two hollow paraboloids, aligned on the same axis, having the same focus and delimited by end sections perpendicular to said axis, the principal parameters and the axial lengths of the two paraboloids being such that said end sections are of different sizes, the two paraboloids being united apically by an annular neck portion, the wave-emitting device being arranged normally to said axis at the larger of the delimiting end sections, and the smaller of these end sections being closed by a flexible wall constituting the outlet for the ultra-sonic waves.

Fig. 6 represents diagrammatically an ultra-sonic wave transducer constructed, according to the invention, in the form of two coaxial paraboloids having the same focus, the transducer being shown schematically in section on a plane containing the common axis. The transducer comprises a piezo-electric crystal C emitting ultra-sonic waves, a container E intended to enclose the liquid conductive to these waves and comprising two paraboloids Pi and P2 and a flexible wall P for pressing upon the surface of a body to be examined or treated with the said waves. There is also, indicated at M a sleeve or neck portion uniting the two paraboloids Pi and P2 constituting the container or enclosure E.

It will be easily understood that it is possible to obtain by means of a wave transducer constructed in this way an optimum directivity of the emitted waves along the axis of the transducer, while having very small outlet area and overall size. In fact, ail the ultra-sonic

2,779,880 wayes emitted normally to the crystal C, except a small central portion corresponding in area to the cross-section of the neck M, are concentrated at the focus of the first paraboloid. As this focus is also that of the second paraboloid, the ultra-sonic waves will be reflected from the walls of the latter paraboloid and will finally leave the transducer parallel to the axis of this latter. The waves from the aforementioned small central area of the crystal pass straight through the neck portion M. It is possible to select the outlet area of the transducer as small as may be desired by adopting suitable values for the principal parameters and the axial lengths of the two paraboloids, or of the two parabolae obtained by taking a section of the paraboloids on a plane passing through their common axis.

The equation of a parabola relative to its axis and to its tangent at the apex being y²=2px, where p represents the principal parameter, it is possible, for example, to obtain an outlet area of a diameter equal to half the effective diameter of the emitter element by adopting principal parameters pi, pa and axial lengths Li, La satisfying the relation:

2hii₌₄P2L2 wherein the suffix 1 relates to the larger parabola Pi and the suffix 2 to the smaller parabola P2. For example, the relations adopted might be:

F₂=—, and Δ Δ

It is possible in this way to reduce the overall size of the transducer so as to permit its use in the same applications as described above for the transducer in the form of a double cone.

As will be seen from the foregoing description, the ideal form of realization of the transducer according to the invention is formed by two cofocal paraboloids, whether or not joined by a cylindrical connection portion. However, in many cases, it is satisfactory to approximate this ideal form by simpler surfaces, as, for example, by one or more rectilinear surfaces of revolution inscribed on each of the ideal cofocal paraboloids. Thus, the embodiments shown in Figs. 4 and 5 having two frusto-conical portions are approximations, of the ideal biparaboloidal profile such as that shown in Fig. 6 and have been found to produce good results while being easier to manufacture.

While several forms of the invention have been illustrated and described, it will be understood that the invention is thus not limited to the specific embodiments shown by way of example in the drawings.