EP0162515B1 - Ultrasonic transducer devices using an array of piezoelectric transducer elements - Google Patents

Description

The present invention relates to an ultrasonic transduction device comprising a linear array of parallel piezoelectric transducer elements. The transducer elements have in such a device a length L large compared to the other dimensions (the width W and the thickness T). This device can be used for example in the field of non-destructive testing of materials or in that of the exploration of biological tissues.

United States Patent No. 4101795 issued July 18, 1978 in the name of the transferee Matsushita Electric Industrial Company describes an ultrasonic transducer device whose piezoelectric transducer elements (see Figures 1 to 3 of this patent) can , thanks to particular geometrical arrangements, to vibrate in pure thickness mode, that is to say in the ideal way in which a piston moves, without undesirable coupling with disturbing vibratory modes.

Knowledge of the vibration modes of thin piezoelectric elements is important for the design of linear arrays of transducers. Such knowledge can be brought in experimentally (or theoretically by means of a two- or three-dimensional modeling exploited for example by a finite element method) by carrying out the operation called characterization of a determined piezoelectric material. , which consists in establishing as completely as possible the relationships between the parameters on which the operation of the transduction device made with this material depends. These relationships can be viewed in the form of various curves, and in particular in the form of so-called Fabian-Sato diagrams which represent the dispersion curves of the resonance frequencies of the material concerned (see EL Fabian, studies presented in MASON, "Physical Acoustics ", volume 1, part A, chapter 6, pages 456 and 457, Edition Academic Press, 1964; see also the patent cited above, of which Mr. Sato is co-applicant). These curves show for the different modes of vibration of the material (fundamental and harmonics) the relationship between the W / T ratio and the product FT of the resonant frequency by the thickness of the piezoelectric elements: Figure 4 of the cited document shows an example of such a network of curves.

As can be seen from the examination of this network, the unimodal operation of the device described in the cited patent is obtained by imposing on the W / T ratio an upper limit of the order of 0.8, a value below which, in addition, the effective electromechanical coupling coefficient takes a higher value (a variation curve of the electromechanical coupling coefficient, such as that of FIG. 9 of the cited patent, provides information on the relative amplitude of the vibrations obtained in the vibration mode considered, depending on the choice of W / T). However, the inherent constraint in the choice of such values of W / T is a greater complexity of production, the grooving between successive piezoelectric elements of the strip being all the more difficult to achieve the narrower the width of these elements.

The object of the invention is to propose a new bar structure which is no longer subject to this constraint relating to the W / T ratio and which is therefore simpler to produce while remaining efficient.

The invention relates to this effect an ultrasonic transduction device comprising a linear array of parallel piezoelectric transducer elements of width W, characterized in that the thickness T of said transducer elements is equal to half the wavelength corresponding to a frequency F equal to the average of two successive piezoelectric resonance frequencies of the piezoelectric material concerned, for which the products of this thickness by said resonance frequencies frame, on the two-dimensional diagram of the curves FT = f (W _/ T) of dispersion of the resonance frequencies relating to the piezoelectric material concerned, so-called coupling zones of the two successive vibration modes corresponding to said two frequencies, said coupling zones being defined by the fact that, in these zones, the resonance frequencies and the electromechanical coupling efficiencies of these two modes are respectively neighbors.

The invention also relates to an ultrasonic transduction device comprising several parallel linear bars of piezoelectric parallelepipedal transducer elements of length L and width W, characterized in that the thickness T of said transducer elements is equal to half the length of wave corresponding to a frequency F equal to the average of two successive piezoelectric resonance frequencies of the piezoelectric material concerned, for which the products of this thickness by said resonance frequencies frame, on the three-dimensional diagram of the curves FT = f (W / T, LfT) of dispersion of the resonance frequencies relative to the piezoelectric material concerned, so-called coupling zones of the two successive vibrational modes corresponding to said two frequencies, said coupling zones being defined by the fact that, in these zones, the resonance frequencies and the coupla efficiencies electromechanical ge of these two modes are respectively close.

In the structures thus proposed, the originality rests on the way of exploiting vibrational modes coexisting in the so-called coupling zones of the diagram of dispersion of the resonance frequencies of the piezoelectric material used. This operation is carried out by a judicious choice of the geometrical characteristics of the piezoelectric elements, and in particular of their thickness, and by placing themselves voluntarily in areas of operation of the transduction device where this operation is not unimodal. The transduction sensitivity is thus increased due to the exploitation of several resonance modes having high electromechanical couplings and, simultaneously, due to the good damping of the residual and harmonic modes.

• les figures 1 et 2 donnent des exemples de diagrammes de Fabian-Sato montrant respectivement les courbes de dispersion des fréquences de résonance piézoélectrique et de résonance élastique rigidifiée, ou antirésonance, du dispositif de transduction selon son épaisseur et selon sa largeur;
• la figure 3 montre la courbe de variation du module IIEI de l'impédance électrique en fonction de la fréquence dans le cas de la zone de couplage correspondant à l'encadré C de la figure 2;
• les figures 4 et 5 montrent les courbes de variation de la fonction de transfert unidimensionnelle RVE (rapport vitesse vibratoire/excitation électrique) associées à la figure 3 dans le cas des zones de couplage correspondant respectivement aux encadrés B et C de la figure 2;
• les figures 6 et 8 montrent l'évolution de la courbe de la figure 5 d'une part lorsque seules les pertes internes du matériau sont prises en compte par rapport à cette figure 5 et d'autre part lorsque le dispositif de transduction a été adapté à l'aide d'une structure interférentielle de fonction de transfert TFE donnée par la figure 7;
• la figure 9 montre un exemple de diagramme tridimensionnel de Fabian-Sato;

The features and advantages of the invention will now appear more precisely in the description which follows and which refers to the appended figures, in which:

• FIGS. 1 and 2 give examples of Fabian-Sato diagrams respectively showing the dispersion curves of the frequencies of piezoelectric resonance and of stiffened elastic resonance, or antiresonance, of the transduction device according to its thickness and according to its width;
• FIG. 3 shows the variation curve of the IIEI module of the electrical impedance as a function of the frequency in the case of the coupling zone corresponding to box C of FIG. 2;
• FIGS. 4 and 5 show the variation curves of the one-dimensional transfer function RVE (vibrational speed / electrical excitation ratio) associated with FIG. 3 in the case of the coupling zones corresponding respectively to boxes B and C of FIG. 2;
• FIGS. 6 and 8 show the evolution of the curve of FIG. 5 on the one hand when only the internal losses of the material are taken into account with respect to this FIG. 5 and on the other hand when the transduction device has been adapted using an interference structure of the TFE transfer function given by FIG. 7;
• FIG. 9 shows an example of a three-dimensional Fabian-Sato diagram;

où n est entier positif ou nul, et v_T la vitesse de propagation des ondes ultrasonores suivant T (supposée indépendante du rapport W/T). En conséquence, le produit F.T (qui est la grandeur représentée en ordonnée sur les diagrammes de Fabian-Sato) est donné par l'expression:

à laquelle correspond un réseau de droites parallèles à l'axe des abscisses (voir la figure 1 ci- jointe).If we consider a simple parallelepiped bar supposed to be elastic, the vibratory state of the resonant cavity that it constitutes is said to be decoupled when the elastic vibrations along the thickness T are independent of those along the width W (and vice versa). The resonance frequencies according to the thickness T of the cavity are then given by the expression:

where n is a positive or zero integer, and v _T the propagation speed of the ultrasonic waves according to T (assumed to be independent of the W / T ratio). Consequently, the product FT (which is the quantity represented on the ordinate on the Fabian-Sato diagrams) is given by the expression:

which corresponds to a network of lines parallel to the abscissa axis (see Figure 1 attached).

où v_w est la vitesse de propagation suivant W (supposée aussi indépendante du rapport W/T), et le produit F.T par l'expression:

à laquelle correspond un réseau d'hyperboles également représenté sur la figure 1.Similarly, the resonance frequencies of the cavity along the width W are given by the expression:

where v _w is the propagation speed according to W (also assumed to be independent of the W / T ratio), and the product FT by the expression:

to which corresponds a network of hyperbolas also represented in FIG. 1.

This network of lines and this network of hyperbolas are ideal networks of asymptotes which are the limits obtained in the case of a decoupled bar, asymptotes of the dispersion curves observed in the case of a piezoelectric bar whose states vibratory according to the thickness and the width are coupled. In the latter case, the frequency dispersion diagram takes the form of that shown in FIG. 2. The observation of the curves of this diagram shows for example that, in the vicinity of W / T = 0.5 (see the box A of this figure 2), the fundamental resonance of thickness RFE (first "horizontal" asymptote) corresponds approximately to half of the fundamental resonance of width RFL (first hyperbolic asymptote) or, which is equivalent, that the fundamental resonance of width RFL corresponds approximately to harmonic 2 of the fundamental resonance of thickness RFE. The piezoelectric point of view, the excitation of the resonance of thickness implies a small excitation of the resonance width, which also results in an increase in the vicinity of W / T = _{0, 5,} the effective electromechanical coupling coefficient associated with the thickness resonance. It is the obtaining of this unimodal resonance which is exploited in the aforementioned patent, where one therefore gets rid of disturbing vibratory modes in favor of a single vibratory mode.

In the case of the invention, paradoxically the reverse approach is carried out, namely that one selects on the Fabian-Sato diagram corresponding to a determined piezoelectric material of the resonance coupling zones. This selection is made by choosing values of the W / T ratio corresponding to the intersections of the asymptotes of the lateral resonance and thickness characteristics (examples of such intersections are indicated in boxes B and C of FIG. 2). In fact, in the zones surrounding these intersections, the simultaneous presence of two resonance modes is observed, the frequencies and the electromechanical coupling efficiencies of which are close. Compared to these so-called twin modes, the other modes are, as shown in FIG. 2, much more distant in frequency (or are much lower in electromechanical coupling efficiency).

When characterizing a piezoelectric material, it is interesting to establish a different type of relation than the diagrams already mentioned, namely that which links the module of the electrical impedance IE of the material and the working frequency of the device. ultrasonic transduction performed with this material. A curve translating this relation is represented on figure 3. The reading of this curve makes it possible to know the values of the piezoelectric resonance frequencies of the material (these are the values of frequency for which, the impedance having a relative minimum, the conversion d energy operated by the transducer is maximum) as well as the values of its anti-resonance frequencies, said frequencies of stiffened elastic resonance and to which, on the contrary, correspond maximums of the value of the electrical impedance.

The ultrasonic transduction device described here preferably comprises the following structure, namely a network of piezoelectric transducer elements in the form of rectangular plates of piezoelectric material (generally produced from a single plate which has been cut out) , these plates of length L, of width W and of thickness T having their front and rear faces equipped with electrodes and being arranged parallel to each other and at regular intervals with their faces of dimensions L and T facing each other. The structure according to the invention is then characteristic, in the sense that the thickness of the piezoelectric elements is chosen to be equal to half the wavelength corresponding to a frequency substantially equal to the average of two successive resonant frequencies of the piezoelectric material concerned.

The impedance curve of FIG. 3 corresponds to a curve of the associated one-dimensional transfer function (examples corresponding to the paired modes of the boxed zones B and C of FIG. 2 are given in FIGS. 4 and 5 respectively), which translates the variation of the! RVEI module of the vibratory speed / electrical excitation ratio at the terminals as a function of the frequency. If such a transfer function takes into account the internal losses of the piezoelectric material, the resonances presented by this transfer function are damped (see FIG. 6, corresponding to the area C of FIG. 2).

The study carried out so far considered the case of an ultrasonic transduction device without adaptation layers, with simply two propagation media of semi-infinite type on the front and rear electrode surfaces. The device can be equipped with an interference transmittance structure resonating on the frequency F _A , this structure comprising one or more adaptation layers at the front, or at the rear, or at the front and at the rear. piezoelectric material; F _A is the average frequency, in the example of FIG. 6, of the frequencies F _R2 and F _R3 corresponding to the maximums of the transfer function, these maximums corresponding themselves, as we have seen, to the minima of the curve associated electrical impedance. The adaptation is carried out for example with a single interference layer known as a quarter wave tuned to the frequency F _A. The difference l: 1F visible in FIG. 7 shows the transfer function corresponding to this adaptation structure, and is more precisely the width at half height of the transmittance of the quarter wave layer tuned to F _A with tap taking into account the acoustic impedances of the adjacent media. If the adaptation thus carried out is such that the extent AF / F _A is greater than the relative difference between the paired modes concerned, (FR3-FR2) IFp in the case of modes 2 and 3 concerned by zone C of the figure 2, then the transfer function which, in figure 6, still let appear despite the damping due to the losses the maximums due to the coexistence of two modes, now presents the form appearing in figure 8. More precisely, one is is then reduced to the case of quasi-Guassian unimodality, the advantages of which are known and which makes it possible to obtain an impulse response of quasi-Guassian envelope, the absence or the presence of higher harmonics being able to be further controlled by the bias of the electrical charge conditions of the transducer in transmission and reception.

These load conditions can also be used to improve, via electrical adaptation, the Guassian aspect of the module of the spectrum of the impulse response. For example, in the case of the paired modes corresponding to the boxed area B of FIG. 2, the relative difference of the coupled modes 1 and 2 is such that it is then necessary to associate with the transduction device not only a structure d broadband adaptation-several layers of quarter-wave type, with possibly offset chords-but also an electrical adaptation network, for example simply consisting of a resistor in series and an inductor in parallel.

By the way, throughout the description, it is necessary to hear, by average, any simple, arithmetic or geometric average, or an average of a more complex nature, such as a quadratic average, or a weighted average, the weighting of each frequency can then for example be effected by the electromechanical coupling coefficient associated with each of them in the vibration mode concerned.

Finally, it can be specified that the invention is applicable in a rigorously similar manner to the case of three-dimensional vibrational states, when the ultrasonic transduction device is a grooved two-dimensional strip with a network of piezoelectric parallelepipedal transducer elements. For this, it suffices to consider a three-dimensional generalization of the Fabian-Sato digrams, the FT product this time being expressed not as a function only of the W / T ratio but of the two geometrical configuration reports. W / T and L / T. It is moreover evident that a two-dimensional Fabian-Sato diagram such as that of FIG. 2 is the limit, when L and therefore L / T become large, of a three-dimensional Fabian-Sato diagram. The plane coupling zones observed on the three-dimensional diagrams become, in this case of the three-dimensional generalization, three-dimensional coupling zones, tubular regions, such as for example the region R indicated by an arrow in FIG. 9 which shows the look of a three-dimensional Fabian-Sato diagram. It will also be noted that, given the reversibility between the dimensions L and W depending on whether one or the other is greater than the other, this three-dimensional diagram and the particular coupling zones which are observed there have symmetry with respect to the bisector plane of the axes (0, LIT), (0, W / T).

Claims (2)

1. An ultrasonic transducer device, comprising a linear array of parallel piezoelectric transducer elements having a width W, characterized in that the thickness T of said transducer elements is equal to half the wavelength corresponding to a frequency F equal to the mean value of two successive piezoelectric resonance frequencies of the relevant piezoelectric material for which the products of this thickness and said resonance frequencies frame, in the two-dimensional diagram of the dispersion curves F.T=f(W/T) of the resonance frequencies of the relevant piezoelectric material, coupling zones of the two successive vibratory modes corresponding to said two frequencies, said coupling zones being defined by the fact that in said zones the resonance frequencies and the electromechanical coupling efficiencies of these two modes approximate one another.

2. An ultrasonic transducer device as claimed in Claim 1, comprising several parallel arrays of parallelepiped piezoelectric transducer elements having a length L and a width W, characterized in that the thickness T of said transducer elements is equal to half the wavelength correspondnig to a frequency F equal to the mean value of two successive piezoelectric resonance frequencies of the relevant piezoelectric material for which the products of this thickness and said resonance frequencies frame, in the three-dimensional diagram of the dispersion curves F.T=f(W/T, L/T) of the resonance frequencies of the relevant piezoelectric material, coupling zones of the two successive vibratory modes corresponding to said two frequencies, said coupling zones being defined by the fact that in said zones the resonance frequencies and the electromechanical coupling efficiencies of these two modes approximate one another.

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