EP0142178A1 - Ultrasonic transducer - Google Patents

Ultrasonic transducer Download PDF

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Publication number
EP0142178A1
EP0142178A1 EP84201200A EP84201200A EP0142178A1 EP 0142178 A1 EP0142178 A1 EP 0142178A1 EP 84201200 A EP84201200 A EP 84201200A EP 84201200 A EP84201200 A EP 84201200A EP 0142178 A1 EP0142178 A1 EP 0142178A1
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Prior art keywords
piezoelectric material
layer
medium
acoustic impedance
layers
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German (de)
French (fr)
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EP0142178B2 (en
EP0142178B1 (en
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Claude Robert Mequio
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Laboratoires dElectronique Philips SAS
Koninklijke Philips NV
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Laboratoires dElectronique Philips SAS
Laboratoires dElectronique et de Physique Appliquee
Philips Gloeilampenfabrieken NV
Koninklijke Philips Electronics NV
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    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/02—Mechanical acoustic impedances; Impedance matching, e.g. by horns; Acoustic resonators

Definitions

  • the present invention relates to an ultrasonic transducer comprising a substrate constituting a rear medium, a layer of piezoelectric material and one or more matching layers whose acoustic impedance has a value between that of the piezoelectric material and that of a medium before spread.
  • An ultrasonic transducer essentially consists, in a conventional manner, of a substrate constituting a rear absorption or reflection medium, a layer of piezoelectric material equipped with electrodes on its front and rear faces and at least one layer acoustic impedance matching, placed in front of the piezoelectric material, between it and the propagation medium.
  • Transducers of this type are described in particular in the article "The effects of backing and matching on the performance of piezoelectric ceramic transducers" by G. Kossoff, published in the journal IEEE Transactions on sonics and ultrasonics, volume SU-13, March 1966 , pages 20 to 30.
  • the implementation of one or more of these adaptation layers has the main effect of improving the sensitivity of the transducers and also contributes to increasing their bandwidth.
  • the ultrasonic transducers used in ultrasound must combine two main qualities at the level of transduction: not only good sensitivity (because the increase in signal-to-noise ratio facilitates the processing of received signals) but also sufficient damping (because the brevity of the impulse response conditions the axial resolution).
  • the object of the invention is to propose an ultrasonic transducer which easily reconciles the requirements of sensitivity and damping.
  • a first embodiment of the ultrasonic transducer according to the invention is characterized in that the adaptation layer or layers are placed between the piezoelectric material and the front medium of propagation, in that the rear medium has a sufficiently high acoustic impedance value compared to that of the piezoelectric material to be considered as rigid, and in that the thickness of the layer of piezoelectric material is equal a quarter of the wavelength associated with the resonant frequency of the transducer.
  • a second embodiment of the ultrasonic transducer according to the invention is characterized in that the adaptation layers are placed in identical number on either side of the piezoelectric material, the layers located symmetrically two by two having the same impedance value acoustic and the same thickness, in that the rear medium has an acoustic impedance value substantially equal to that of the medium before propagation, and in that the thickness of the layer of piezoelectric material is equal to half the length wave associated with the resonant frequency of the transducer, so that the structure is symmetrical with respect to the median plane of the layer of piezoelectric material.
  • the first embodiment represented in FIG. 1, consists of an ultrasonic transducer with vibration in thickness mode, composed of a substrate 10 constituting the rear middle of the transducer, a layer 20 of piezoelectric material covered on its front faces and back of metal sheets 21 and 22 constituting first and second electrodes (connected in a known manner to a polarization circuit not shown which provides the excitation potential), and, between this layer 20 and the medium before propagation 50, of two acoustic impedance matching layers 30 and 40 (called quarter-wave interference layers).
  • the substrate 10 has, with respect to the layer 20 of piezoelectric material, a value of acoustic impedance very much higher, and sufficiently high in any case so that this substrate can be considered as rigid relative to the material piezoelectric, i.e. as a rear center at zero deformation.
  • the thickness of the layer 20 is equal to a quarter of the wavelength associated with the resonant frequency of the transducer.
  • the values of the impedances of this layer 20, of the adaptation layers 30 and 40 and of the propagation medium form, considered in this order, a decreasing sequence, for example and without limitation an arithmetic or geometric sequence.
  • a second ultrasonic transducer completely symmetrical, comprising a substrate 10 serving as rear medium, a layer 20 of piezoelectric material of thickness equal to half the wavelength associated with the resonant frequency of the transducer, and two acoustic impedance matching layers 30,40 on the one hand between the rear medium and the piezoelectric material and on the other hand between this material and the medium before propagation 50.
  • the values of the acoustic impedances likewise form a decreasing sequence from that of the piezoelectric material and these values as well as the thicknesses of the layers 30, 40 are symmetrical on both sides of this material.
  • the tests and simulations carried out with a structure thus constituted show that the spectrum (or module of the Fourier transform) of the electrical response in ultrasound mode to an electrical excitation of impulse type and of effective duration equal to the time of flight in the piezoelectric material (the flight time is the duration of the path of the ultrasonic waves from one face to the other of the vibrating piezoelectric material according to its thickness equal to half of the ultrasonic wavelength at the emission frequency of the transducer is of shape Gaussian; consequently, the envelope of the electrical response is also Gaussian and this response is quickly absorbed.
  • Such a medium is fairly easily achievable if the chosen piezoelectric material does not have too high an acoustic impedance: hence the proposal of the first structure, called virtual symmetry and therefore comprising a rigid rear medium, a piezoelectric layer having an equal thickness at a quarter of said wavelength, and the acoustic impedance matching layers, this structure having the same damping characteristics as the second completely symmetrical structure but improved sensitivity.
  • the sensitivity is characterized by a sensitivity index whose expression in dB is of the type 20 lo g V S / U REF where V REF is for an internal impedance generator adapted to its load the voltage allowing l emission of a rectangular resonant pulse and where V s is the peak-to-peak voltage of the response, and that the damping is generally characterized by the relative bandwidth at -6 dB dF / F of the fundamental spectrum, expressed in% and in which dF is the difference between the points where the electrical amplitude is at -6 dB below the maximum and F the central frequency corresponding to said maximum.
  • the essential characteristic of the structure with total symmetry is very good damping.
  • the advantages of the structure with virtual symmetry are the following: gain of 6 dB (maximum) on the sensitivity index of the structure with total symmetry, thanks to the effect of "acoustic mirror" of the rigid rear medium which reflects all the acoustic energy towards the front, maintaining the same damping as that, very satisfactory, of the structure with total symmetry, thickness of the piezoelectric material twice less, for a given working frequency, than 'with conventional transducers with a piezoelectric layer in ⁇ / 2 (this last characteristic is important for piezoelectric polymers such as the polyvinylidene fluoride mentioned above, which are difficult to obtain in high thicknesses).

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Multimedia (AREA)
  • Transducers For Ultrasonic Waves (AREA)
  • Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)

Abstract

Transducteur ultrasonore comprenant un substrat (10) constituant un milieu arrière, une couche de matériau piézoélectrique et une ou plusieurs couches d'adaptation (30, 40) dont l'impédance acoustique a une valeur comprise entre celle du matériau piézoélectrique et celle du milieu avant de propagation (50). La ou les couches d'adaptation sont placées uniquement entre le matériau piézoélectrique et le milieu avant de propagation, le milieu arrière (10) a une valeur d'impédance acoustique suffisamment élevée par rapport à celle du matériau piézoélectrique pour pouvoir être considéré comme rigide, et l'épaisseur de la couche de matériau piézoélectrique est égale au quart de la longueur d'onde associée à la fréquence de résonance du transducteur.Ultrasonic transducer comprising a substrate (10) constituting a rear medium, a layer of piezoelectric material and one or more adaptation layers (30, 40) whose acoustic impedance has a value between that of the piezoelectric material and that of the front medium propagation (50). The adaptation layer or layers are placed only between the piezoelectric material and the medium before propagation, the rear medium (10) has a sufficiently high acoustic impedance value compared to that of the piezoelectric material to be able to be considered as rigid, and the thickness of the layer of piezoelectric material is equal to a quarter of the wavelength associated with the resonant frequency of the transducer.

Description

La présente invention concerne un transducteur ultrasonore comprenant un substrat constituant un milieu arrière, une couche de matériau piézoélectrique et une ou plusieurs couches d'adaptation dont l'impédance acoustique a une valeur comprise entre celle du matériau piézoélectrique et celle d'un milieu avant de propagation.The present invention relates to an ultrasonic transducer comprising a substrate constituting a rear medium, a layer of piezoelectric material and one or more matching layers whose acoustic impedance has a value between that of the piezoelectric material and that of a medium before spread.

Un transducteur ultrasonore est constitué essentiellement, de façon classique, d'un substrat constituant un milieu arrière d'absorption ou de réflexion, d'une couche de matériau piézoélectrique équipée d'électrodes sur ses faces avant et arrière et d'au moins une couche d'adaptation d'impédance acoustique, placée devant le matériau piézoélectrique, entre celui-ci et le milieu de propagation. Des transducteurs de ce type sont notamment décrits dans l'article "The effects of backing and matching on the performance of piezoelectric ceramic transducers" de G. Kossoff, paru dans la revue IEEE Transactions on sonics and ultrasonics, volume SU-13, mars 1966, pages 20 à 30. La mise en place d'une ou de plusieurs de ces couches d'adaptation a pour effet principal d'améliorer la sensibilité des transducteurs et contribue également à augmenter leur largeur de bande.An ultrasonic transducer essentially consists, in a conventional manner, of a substrate constituting a rear absorption or reflection medium, a layer of piezoelectric material equipped with electrodes on its front and rear faces and at least one layer acoustic impedance matching, placed in front of the piezoelectric material, between it and the propagation medium. Transducers of this type are described in particular in the article "The effects of backing and matching on the performance of piezoelectric ceramic transducers" by G. Kossoff, published in the journal IEEE Transactions on sonics and ultrasonics, volume SU-13, March 1966 , pages 20 to 30. The implementation of one or more of these adaptation layers has the main effect of improving the sensitivity of the transducers and also contributes to increasing their bandwidth.

On rappellera ici que les transducteurs ultrasonores utilisés en échographie doivent réunir deux qualités principales au niveau de la transduction : non seulement une bonne sensibilité (car l'augmentation du rapport signal-sur-bruit facilite le traitement des signaux reçus) mais aussi un amortissement suffisant (car la brièveté de la réponse impulsionnelle conditionne la résolution axiale).It will be recalled here that the ultrasonic transducers used in ultrasound must combine two main qualities at the level of transduction: not only good sensitivity (because the increase in signal-to-noise ratio facilitates the processing of received signals) but also sufficient damping (because the brevity of the impulse response conditions the axial resolution).

Le but de l'invention est de proposer un transducteur ultrasonore conciliant de façon simple les exigences de sensibilité et d'amortissement.The object of the invention is to propose an ultrasonic transducer which easily reconciles the requirements of sensitivity and damping.

A cet effet une première réalisation du transducteur ultrasonore conforme à l'invention est caractérisée en ce que la ou les couches d'adaptation sont placées entre le matériau piézoélectrique et le milieu avant de propagation, en ce que le milieu arrière a une valeur d'impédance acoustique suffisamment élevée par rapport à celle du matériau piézoélectrique pour pouvoir être considéré comme rigide, et en ce que l'épaisseur de la couche de matériau piézoélectrique est égale au quart de la longueur d'onde associée à la fréquence de résonance du transducteur.To this end, a first embodiment of the ultrasonic transducer according to the invention is characterized in that the adaptation layer or layers are placed between the piezoelectric material and the front medium of propagation, in that the rear medium has a sufficiently high acoustic impedance value compared to that of the piezoelectric material to be considered as rigid, and in that the thickness of the layer of piezoelectric material is equal a quarter of the wavelength associated with the resonant frequency of the transducer.

Une deuxième réalisation du transducteur ultrasonore conforme à l'invention est caractérisée en ce que les couches d'adaptation sont placées en nombre identique de part et d'autre du matériau piézoélectrique, les couches situées symétriquement deux à deux ayant la même valeur d'impédance acoustique et la même épaisseur, en ce que le milieu arrière a une valeur d'impédance acoustique sensiblement égale à celle du milieu avant de propagation, et en ce que l'épaisseur de la couche de matériau piézoélectrique est égale à la moitié de la longueur d'onde associée à la fréquence de résonance du transducteur, de façon que la structure soit symétrique par rapport au plan médian de la couche de matériau piézoélectrique.A second embodiment of the ultrasonic transducer according to the invention is characterized in that the adaptation layers are placed in identical number on either side of the piezoelectric material, the layers located symmetrically two by two having the same impedance value acoustic and the same thickness, in that the rear medium has an acoustic impedance value substantially equal to that of the medium before propagation, and in that the thickness of the layer of piezoelectric material is equal to half the length wave associated with the resonant frequency of the transducer, so that the structure is symmetrical with respect to the median plane of the layer of piezoelectric material.

Les particularités et avantages de l'invention vont être maintenant décrits ci-dessous plus en détail en se référant aux figures 1 et 2, données à titre d'exemples non limitatifs et qui montrent deux réalisations de transducteurs conformes à l'invention.The features and advantages of the invention will now be described below in more detail with reference to Figures 1 and 2, given by way of non-limiting examples and which show two embodiments of transducers according to the invention.

La première réalisation, représentée sur la figure 1, consiste en un transducteur ultrasonore à vibration en mode d'épaisseur, composé d'un substrat 10 constituant le milieu arrière de transducteur, d'une couche 20 de matériau piézoélectrique recouverte sur ses faces avant et arrière de feuilles métalliques 21 et 22 constituant des première et deuxième électrodes (reliées de façon connue à un circuit de polarisation non représenté qui fournit le potentiel d'excitation), et, entre cette couche 20 et le milieu avant de propagation 50, de deux couches 30 et 40 d'adaptation d'impédance acoustique (dites couches interférentielles quart d'onde).The first embodiment, represented in FIG. 1, consists of an ultrasonic transducer with vibration in thickness mode, composed of a substrate 10 constituting the rear middle of the transducer, a layer 20 of piezoelectric material covered on its front faces and back of metal sheets 21 and 22 constituting first and second electrodes (connected in a known manner to a polarization circuit not shown which provides the excitation potential), and, between this layer 20 and the medium before propagation 50, of two acoustic impedance matching layers 30 and 40 (called quarter-wave interference layers).

Dans cette première structure selon l'invention, le substrat 10 présente par rapport à la couche 20 de matériau piézoélectrique une valeur d'impédance acoustique très nettement supérieure, et suffisamment élevée en tout cas pour que ce substrat puisse être considéré comme rigide relativement au matériau piézoélectrique, c'est-à-dire comme un milieu arrière à déformation nulle. En outre, l'épaisseur de la couche 20 est égale au quart de la longueur d'onde associée à la fréquence de résonance du transducteur. Enfin, si l'on veut optimiser le transfert d'énergie de la couche 20 de matériau piézoélectrique vers le milieu avant de propagation 50, les valeurs des impédances de cette couche 20, des couches d'adaptation 30 et 40 et du milieu de propagation forment, considérées dans cet ordre, une suite décroissante, par exemple et de façon non limitative une suite arithmétique ou géométrique.In this first structure according to the invention, the substrate 10 has, with respect to the layer 20 of piezoelectric material, a value of acoustic impedance very much higher, and sufficiently high in any case so that this substrate can be considered as rigid relative to the material piezoelectric, i.e. as a rear center at zero deformation. In addition, the thickness of the layer 20 is equal to a quarter of the wavelength associated with the resonant frequency of the transducer. Finally, if we want to optimize the energy transfer from the layer 20 of piezoelectric material to the medium before propagation 50, the values of the impedances of this layer 20, of the adaptation layers 30 and 40 and of the propagation medium form, considered in this order, a decreasing sequence, for example and without limitation an arithmetic or geometric sequence.

Pour comprendre maintenant comment la première structure ainsi décrite présente à la fois une bonne sensibilité et un excellent amortissement, on imagine (voir la figure 2) un deuxième transducteur ultrasonore, totalement symétrique, comprenant un substrat 10 servant de milieu arrière, une couche 20 de matériau piézoélectrique d'épaisseur égale à la moitié de la longueur d'onde associée à la fréquence de résonance du transducteur, et deux couches 30,40 d'adaptation d'impédance acoustique d'une part entre le milieu arrière et le matériau piézoélectrique et d'autre part entre ce matériau et le milieu avant de propagation 50. Dans cette deuxième structure, les valeurs des impédances acoustiques forment de même une suite décroissante à partir de celle du matériau piézoélectrique et ces valeurs ainsi que les épaisseurs des couches 30, 40 d'adaptation sont symétriques de part et d'autre de ce matériau. Les tests et simulations effectués avec une structure ainsi constituée montrent que le spectre (ou module de la transformée de Fourier) de la réponse électrique en mode échographique à une excitation électrique de type impulsionnel et de durée effective égale au temps de vol dans le matériau piézoélectrique (le temps de vol est la durée du parcours des ondes ultrasonores d'une face à l'autre du matériau piézoélectrique vibrant suivant son épaisseur égale à la moitié de la longueur d'onde ultrasonore à la fréquence d'émission du transducteur est de forme gaussienne ; par suite, l'enveloppe de la réponse électrique est également gaussienne et cette réponse s'amortit rapidement. Par ailleurs, de la symétrie de la structure, il résulte que les déformations sur les deux faces du matériau piézoélectrique sont identiques (puisque ces deux faces sont, acoustiquement, chargées de façon identique) et que, par suite, la déformation est nulle dans le plan médian de ce matériau. La partie de la deuxième structure qui se trouve située d'un seul côté de ce plan médian est donc équivalente à un milieu arrière infiniment rigide, c'est-à-dire à déformation nulle. Un tel milieu est assez facilement réalisable si le matériau piézoélectrique choisi ne possède pas une impédance acoustique trop élevée : d'où la proposition de la première structure, dite à symétrie virtuelle et comprenant donc un milieu arrière rigide, une couche piézoélectrique ayant une épaisseur égale à un quart de ladite longueur d'onde, et les couches d'adaptation d'impédance acoustique, cette structure présentant les mêmes caractéristiques d'amortissement que la deuxième structure totalement symétrique mais une sensibilité améliorée.To understand now how the first structure thus described has both good sensitivity and excellent damping, we imagine (see Figure 2) a second ultrasonic transducer, completely symmetrical, comprising a substrate 10 serving as rear medium, a layer 20 of piezoelectric material of thickness equal to half the wavelength associated with the resonant frequency of the transducer, and two acoustic impedance matching layers 30,40 on the one hand between the rear medium and the piezoelectric material and on the other hand between this material and the medium before propagation 50. In this second structure, the values of the acoustic impedances likewise form a decreasing sequence from that of the piezoelectric material and these values as well as the thicknesses of the layers 30, 40 are symmetrical on both sides of this material. The tests and simulations carried out with a structure thus constituted show that the spectrum (or module of the Fourier transform) of the electrical response in ultrasound mode to an electrical excitation of impulse type and of effective duration equal to the time of flight in the piezoelectric material (the flight time is the duration of the path of the ultrasonic waves from one face to the other of the vibrating piezoelectric material according to its thickness equal to half of the ultrasonic wavelength at the emission frequency of the transducer is of shape Gaussian; consequently, the envelope of the electrical response is also Gaussian and this response is quickly absorbed. Furthermore, from the symmetry of the structure, it follows that the deformations on both sides of the piezoelectric material are identical (since these two faces are acoustically loaded identically) and that, consequently, the deformation is zero in the median plane of this material. tenth structure which is located on one side of this median plane is therefore equivalent to an infinitely rigid rear medium, that is to say with zero deformation. Such a medium is fairly easily achievable if the chosen piezoelectric material does not have too high an acoustic impedance: hence the proposal of the first structure, called virtual symmetry and therefore comprising a rigid rear medium, a piezoelectric layer having an equal thickness at a quarter of said wavelength, and the acoustic impedance matching layers, this structure having the same damping characteristics as the second completely symmetrical structure but improved sensitivity.

Les essais et simulations réalisés (dans d'égales conditions électriques d'émission et de réception) ont montré la possibilité d'obtenir effectivement diverses structures répondant aux objectifs de l'invention (sensibilité et amortissement simultanément satisfaisants). Dans le cas où le matériau piézoélectrique est une céramique ferroélectrique de type PZT-5 (matériau piézoélectrique à base de zirconate titanate de plomb : voir l'ouvrage "Physical Acoustics, Principles and Methods", de Warren P. Mason, Vol.l, partie A, page 202), on peut citer les exemples suivants (exemples à deux couches d'adaptation d'impédance acoustique) :

  • (1) première structure (à symétrie virtuelle) :
    • (a) impédances (en kg/cm2.sec x 10 6) :
      • - milieu arrière : 1000 (simulation)
      • - matériau piézoélectrique : 30
      • - première couche d'adaptation : 4
      • - deuxième couche d'adaptation : 1,8
      • - milieu avant de propagation : 1,5
    • (b) résultats obtenus :
      • - indice de sensibilité = -10,03 dB
      • - largeur de bande relative à -6 dB = 55
      • - durée de réponse à -20 dB = 7,6 T
      • - durée de réponse à -40 dB = 8,9 T
The tests and simulations carried out (under equal electrical conditions of emission and reception) showed the possibility of effectively obtaining various structures meeting the objectives of the invention (sensitivity and damping simultaneously satisfactory). In the case where the piezoelectric material is a ferroelectric ceramic of the PZT-5 type (piezoelectric material based on lead zirconate titanate: see the work "Physical Acoustics, Principles and Methods", by Warren P. Mason, Vol.l, part A, page 202), the following examples can be cited (examples with two layers of acoustic impedance adaptation):
  • (1) first structure (with virtual symmetry):
    • (a) impedances (in kg / cm 2 .sec x 10 6 ) :
      • - rear center: 1000 (simulation)
      • - piezoelectric material: 30
      • - first adaptation layer: 4
      • - second adaptation layer: 1.8
      • - medium before propagation: 1.5
    • (b) results obtained:
      • - sensitivity index = -10.03 dB
      • - relative bandwidth at -6 dB = 55
      • - response time at -20 dB = 7.6 T
      • - response time at -40 dB = 8.9 T

(On rappellera ici que la sensibilité est caractérisée par un indice de sensibilité dont l'expression en dB est du type 20 log VS/UREF où VREF est pour un générateur d'impédance interne adaptée à sa charge la tension permettant l'émission d'une impulsion résonnante rectangulaire et où Vs est la tension crête-à-crête de la réponse, et que l'amortissement est généralement caractérisé par la largeur de bande relative à -6 dB dF/F du spectre fondamental, exprimée en % et dans laquelle dF est l'écart entre les points où l'amplitude électrique est à -6 dB sous le maximum et F la fréquence centrale correspondant audit maximum. Cependant, cette dernière information est insuffisante pour caractériser complètement l'amortissement puisqu'elle ne tient compte ni de la forme, qui peut être irrégulière, du spectre fondamental ni de la présence d'harmoniques supérieurs qui perturbent la fin des échos, et elle est complétée par deux autres indicateurs temporels qui sont les durées de la réponse électrique à -20 dB et à -40 dB (ces points à -20 et -40 dB étant définis par les instants auxquels l'amplitude crête-à-crête est devenue inférieure respectivement au dixième et au centième de sa valeur initiale) à une impulsion résonnante rectangulaire de durée T, ces durées étant normées c'est-à-dire exprimées par référence audit temps de vol T.)(It will be recalled here that the sensitivity is characterized by a sensitivity index whose expression in dB is of the type 20 lo g V S / U REF where V REF is for an internal impedance generator adapted to its load the voltage allowing l emission of a rectangular resonant pulse and where V s is the peak-to-peak voltage of the response, and that the damping is generally characterized by the relative bandwidth at -6 dB dF / F of the fundamental spectrum, expressed in% and in which dF is the difference between the points where the electrical amplitude is at -6 dB below the maximum and F the central frequency corresponding to said maximum. However, this latter information is insufficient to fully characterize the damping since it takes into account neither the shape, which can be irregular, the fundamental spectrum nor the presence of higher harmonics which disturb the end of the echoes, and it is supplemented by two other time indicators which are the durations of the electrical response at -20 dB and at -40 dB (these points at -20 and -40 dB being defined by the instants at which the peak-to-peak amplitude became less respectively to the tenth and to the hundredth of its initial value) to a rectangular resonant pulse of duration T , these durations being normalized, that is to say expressed by reference to said flight time T. )

(2) deuxième structure, à symétrie totale, duale de la précédente :

  • (a) impédances :
    • - milieu arrière : 1,5
    • - couches d'adaptation : 1,8 et 4
    • - matériau-piézoélectrique : 30
    • - couche d'adaptation : 4 et 1,8
    • - milieu avant de propagation : 1,5
  • (b) résultats obtenus :
    • - indice de sensibilité : -13 dB
    • - largeur de bande relative à -6 dB = 53 %
    • - durée de réponse à -20 dB = 7,79 T
    • - durée de réponse à -40 dB = 9,8 T
(2) second structure, with total symmetry, dual from the previous one:
  • (a) impedances:
    • - rear center: 1.5
    • - adaptation layers: 1.8 and 4
    • - piezoelectric material: 30
    • - adaptation layer: 4 and 1.8
    • - medium before propagation: 1.5
  • (b) results obtained:
    • - sensitivity index: -13 dB
    • - relative bandwidth at -6 dB = 53%
    • - response time at -20 dB = 7.79 T
    • - response time at -40 dB = 9.8 T

Dans le cas où le matériau piézoélectrique est du polyfluorure de vinylidène, on peut citer de même les exemples suivants (exemples à une couche d'adaptation d'impédance acoustique) :

  • (3) première structure (à symétrie virtuelle) :
    • (a) impédances :
      • - milieu arrière : 46
      • - matériau piézoélectrique : 4,6
      • - couche d'adaptation : 1,8
      • - milieu avant de propagation : 1,5
    • (b) résultats obtenus :
      • - indice de sensibilité = -19,66 dB
      • - largeur de bande relative à -6 dB = 82 %
      • - durée de réponse à -20 dB = 5,4 T
      • - durée de réponse à -40 dB = 7,8 T
  • (4)deuxième structure, à symétrie totale, duale de la précédente :
    • (a) impédances :
      • - milieux arrière et avant : 1,5
      • - couches d'adaptation arrière et avant : 1,8
      • - matériau piézoélectrique : 4,6
    • (b) résultats obtenus :
      • - indice de sensibilité = -23,8 dB
      • - largeur de bande relative à -6 dB = 75 %
      • - durée de réponse à -20 dB = 5,63 T
      • - durée de réponse à -40 dB = 8 T
In the case where the piezoelectric material is polyvinylidene fluoride, the following examples can also be cited (examples with an acoustic impedance matching layer):
  • (3) first structure (with virtual symmetry):
    • (a) impedances:
      • - rear center: 46
      • - piezoelectric material: 4.6
      • - adaptation layer: 1.8
      • - medium before propagation: 1.5
    • (b) results obtained:
      • - sensitivity index = -19.66 dB
      • - relative bandwidth at -6 dB = 82%
      • - response time at -20 dB = 5.4 T
      • - response time at -40 dB = 7.8 T
  • (4) second structure, with total symmetry, dual from the previous one:
    • (a) impedances:
      • - rear and front midpoints: 1.5
      • - rear and front adaptation layers: 1.8
      • - piezoelectric material: 4.6
    • (b) results obtained:
      • - sensitivity index = -23.8 dB
      • - relative bandwidth at -6 dB = 75%
      • - response time at -20 dB = 5.63 T
      • - response time at -40 dB = 8 T

La caractéristique essentielle de la structure à symétrie totale(figure 2) est un très bon amortissement. Les avantages de la structure à symétrie virtuelle (figure 1) sont, eux, les suivants : gain de 6 dB (au maximum) sur l'indice de sensibilité de la structure à symétrie totale, grâce à l'effet de "miroir acoustique" du milieu arrière rigide qui réfléchit toute l'énergie acoustique vers l'avant, maintien du même amortissement que celui, très satisfaisant, de la structure à symétrie totale, épaisseur du matériau piézoélectrique deux fois plus faible, pour une fréquence de travail donnée, qu'avec les transducteurs classiques à couche piézoélectrique en λ/2 (cette dernière caractéristique est importante pour des polymères piézoélectriques tels que le polyfluorure de vinylidène cité plus haut, qui sont difficiles à obtenir en fortes épaisseurs).The essential characteristic of the structure with total symmetry (Figure 2) is very good damping. The advantages of the structure with virtual symmetry (figure 1) are the following: gain of 6 dB (maximum) on the sensitivity index of the structure with total symmetry, thanks to the effect of "acoustic mirror" of the rigid rear medium which reflects all the acoustic energy towards the front, maintaining the same damping as that, very satisfactory, of the structure with total symmetry, thickness of the piezoelectric material twice less, for a given working frequency, than 'with conventional transducers with a piezoelectric layer in λ / 2 (this last characteristic is important for piezoelectric polymers such as the polyvinylidene fluoride mentioned above, which are difficult to obtain in high thicknesses).

Bien entendu la présente invention n'est pas limitée aux exemples de réalisation décrits et représentés, à partir desquels des variantes peuvent être proposées sans pour cela sortir du cadre de l'invention, en particulier celles dans lesquelles on aurait choisi un nombre différent de couches d'adaptation d'impédance acoustique entre le matériau piézoélectrique et les milieux extrêmes.Of course, the present invention is not limited to the embodiments described and shown, from which variants can be proposed without departing from the scope of the invention, in particular those in which a different number of layers would have been chosen. adaptation of acoustic impedance between the piezoelectric material and extreme environments.

Claims (3)

1. Transducteur ultrasonore comprenant un substrat (10) constituant un milieu arrière, une couche de matériau piézoélectrique et une ou plusieurs couches d'adaptation (30,40) dont l'impédance acoustique a une valeur comprise entre celle du matériau piézoélectrique et celle d'un milieu avant de propagation (50), caractérisé en ce que la ou les couches d'adaptation sont placées entre le matériau piézoélectrique et le milieu avant de propagation, en ce que le milieu arrière (10) a une valeur d'impédance acoustique suffisamment élevée par rapport à celle du matériau piézoélectrique pour pouvoir être considéré comme rigide, et en ce que l'épaisseur de la couche de matériau piézoélectrique est égale au quart de la longueur d'onde associée à la fréquence de résonance du transducteur.1. Ultrasonic transducer comprising a substrate (10) constituting a rear medium, a layer of piezoelectric material and one or more adaptation layers (30.40) whose acoustic impedance has a value between that of the piezoelectric material and that of '' a front propagation medium (50), characterized in that the adaptation layer or layers are placed between the piezoelectric material and the front propagation medium, in that the rear medium (10) has an acoustic impedance value sufficiently high compared to that of the piezoelectric material to be considered as rigid, and in that the thickness of the layer of piezoelectric material is equal to a quarter of the wavelength associated with the resonant frequency of the transducer. 2. Transducteur ultrasonore comprenant un substrat (10) constituant un milieu arrière, une couche de matériau piézoélectrique et une ou plusieurs couches d'adaptation (30,40) dont l'impédance acoustique a une valeur comprise entre celle du matériau piézoélectrique et celle d'un milieu avant de propagation (50), caractérisé en ce que les couches d'adaptation sont placées en nombre identique de part et d'autre du matériau piézoélectrique, les couches situées symétriquement deux à deux ayant la même valeur d'impédance acoustique et la même épaisseur, en ce que le milieu arrière (10) a une valeur d'impédance acoustique sensiblement égale à celle du milieu avant de propagation, et en ce que l'épaisseur de la couche de matériau piézoélectrique est égale à la moitié de la longueur d'onde associée à la fréquence de résonance du transducteur, de façon que la structure soit symétrique par rapport au plan médian de la couche de matériau piézoélectrique.2. Ultrasonic transducer comprising a substrate (10) constituting a rear medium, a layer of piezoelectric material and one or more adaptation layers (30.40) whose acoustic impedance has a value between that of the piezoelectric material and that of '' a propagation medium (50), characterized in that the matching layers are placed in identical number on either side of the piezoelectric material, the layers located symmetrically two by two having the same value of acoustic impedance and the same thickness, in that the rear medium (10) has an acoustic impedance value substantially equal to that of the medium before propagation, and in that the thickness of the layer of piezoelectric material is equal to half the wavelength associated with the resonant frequency of the transducer, so that the structure is symmetrical with respect to the median plane of the layer of piezoelectric material. 3. Transducteur selon l'une des revendications 1 et 2, caractérisé en ce que les valeurs des impédances de la couche de matériau piézoélectrique, des couches d'adaptation d'impédance acoustique et du milieu avant de propagation forment, considérées dans cet ordre, une suite décroissante.3. Transducer according to one of claims 1 and 2, characterized in that the values of the impedances of the layer of piezoelectric material, of the acoustic impedance adaptation layers and of the medium before propagation form, considered in this order, a decreasing sequence.
EP84201200A 1983-08-31 1984-08-20 Ultrasonic transducer Expired - Lifetime EP0142178B2 (en)

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FR8313986 1983-08-31
FR8313986A FR2551611B1 (en) 1983-08-31 1983-08-31 NOVEL ULTRASONIC TRANSDUCER STRUCTURE AND ULTRASONIC ECHOGRAPHY MEDIA EXAMINATION APPARATUS COMPRISING SUCH A STRUCTURE

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EP0142178A1 true EP0142178A1 (en) 1985-05-22
EP0142178B1 EP0142178B1 (en) 1990-01-03
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EP2775730A1 (en) 2013-03-05 2014-09-10 British Telecommunications public limited company Video data provision
EP2775731A1 (en) 2013-03-05 2014-09-10 British Telecommunications public limited company Provision of video data
WO2014135826A1 (en) 2013-03-05 2014-09-12 British Telecommunications Public Limited Company Video data provision
WO2014135827A1 (en) 2013-03-05 2014-09-12 British Telecommunications Public Limited Company Provision of video data
EP2819418A1 (en) 2013-06-27 2014-12-31 British Telecommunications public limited company Provision of video data

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JPS6084099A (en) 1985-05-13
US4771205A (en) 1988-09-13
EP0142178B2 (en) 1994-01-12
FR2551611A1 (en) 1985-03-08
EP0142178B1 (en) 1990-01-03
IL72791A0 (en) 1984-11-30
CA1260603A (en) 1989-09-26
JPH0640676B2 (en) 1994-05-25
DE3480968D1 (en) 1990-02-08
IL72791A (en) 1988-08-31
FR2551611B1 (en) 1986-10-24

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