RU2110792C1 - Acoustic emission piezoelectric transducer - Google Patents

Abstract

FIELD: nondestructive testing. SUBSTANCE: transducer has piezoelement, flexible member, waveguide and membrane enclosed in case. Membrane is coupled to case over external contour. Piezoelement is made as thin disc secured on one side of flexible member in the form of membrane rigidly coupled with its central part on, other side with the other end of waveguide. External contour of flexible member is coupled rigidly to case. Electrodes in the shape of concentric disc and ring are provided on nonoperating surface of piezoelement. Lateral size of waveguide is selected to be less than disc electrode diameter. Flexible member is fitted with ring arranged over its external contour on waveguide side. EFFECT: higher test results. 6 cl, 4 dwg

Description

 The invention relates to non-destructive testing of objects in extreme conditions of exposure to high temperature, γ-η - radiation, superheated steam, vibration, etc., namely piezoelectric transducers of acoustic emission, and can be used, in particular, to control the tightness of the first circuits of nuclear reactor installations power plants, during which surface waves with a ridge periodic structure are recorded (Rayleigh, Lamb waves and quasi-Rayleigh waves - hereinafter referred to as “Rayleigh waves”) in frequency range from 20 to 200 kHz, with a length comparable to the size of the converter.

 The primary converters of any diagnostic systems have very high requirements for technical characteristics that determine the resolution (electro-acoustic conversion coefficient, the width of the working frequency range, frequency response unevenness, etc.).

 Primary piezoelectric transducers of acoustic emission for monitoring the tightness of equipment of nuclear power plants are subject to additional requirements for ensuring special functional properties: the selectivity of the reception of Rayleigh waves among other modes of oscillation and the differential output of the electrical signal (Rayleigh waves carry information about the "sounding" defects of extended objects, while body (longitudinal) waves contain information about events occurring inside pipelines, for example vitations, and in this case are mechanical noise) [1a].

 The peculiarity of the process of controlling the leakage of pipelines of equipment of nuclear power plants by analyzing acoustic emission signals in the frequency range from 20 to 200 kHz is that a useful signal containing information about the condition of pipelines and the occurrence of defects in them is substantially drowned out by the mechanical noise of superheated water and steam. In addition, due to the limited access to the monitored object (including the inability to place amplification-conversion equipment near primary converters), long cable lines up to 200 m long are used, which sharply worsen the signal-to-noise ratio. To ensure noise immunity and reliability in such extreme conditions, the design of the converter should be as simple as possible, include a minimum number of parts, especially piezoelectric elements, have a differential output and, preferably, a large own electric capacitance compared to the capacity of the cable line.

 Known piezoelectric transducer for detecting Rayleigh waves, containing a set of piezoelectric plates located in one plane at a distance of half the wavelength of the detected mode for plates of opposite polarization [1b]. The choice of the sizes of the piezoelectric plates ensures the selectivity of Rayleigh wave reception and maximum sensitivity at the recording frequency. However, the use of this converter is limited due to the very narrow bandwidth, radiation pattern and an unacceptably large number of piezoelectric elements.

 The closest in technical essence is the piezoelectric transducer [2] for recording the acoustic emission of high-temperature objects. The transducer consists of one or more piezoelectric elements, previously preloaded by springs to the supporting support, connected to the housing by a thin membrane. The housing is equipped with three supports. The bearing support extends beyond the supports of the housing and is made in the form of a blunt waveguide, mainly in the form of an exponential funnel. Ultrasonic waves are received through the bearing support in contact with the surface of the controlled object. However, this transducer equally records both surface and longitudinal waves. Equal displacements of the surface of the controlled object at the point of contact of the bearing support of the transducer in the axial direction, regardless of the oscillation mode, will cause the appearance of equal electrical signals, since the piezoelectric elements are acoustically decoupled from the body (mechanical connection of the piezoelectric elements mounted on the bearing support and the body is carried out by means of a thin membrane).

 In addition, in this design, due to the phase shift (the path length of the acoustic signal to each piezoelectric element is different) and the different load of the piezoelectric elements (the lower piezoelectric element is loaded with the upper piezoelectric element and the inertial element, and the upper piezoelectric element is only the inertial element), the possibility of differential output of the electrical signal is limited.

 The problem solved by the invention is aimed at creating a piezoelectric transducer of acoustic emission sensitive to surface waves with a comb periodic structure, a length comparable to the size of the sensor.

 The technical result of the present invention is the selectivity of the reception of Rayleigh waves and increase the noise immunity of the piezoelectric transducer of acoustic emission.

The technical result is achieved by the fact that in the known piezoelectric transducer of acoustic emission comprising a housing, a piezoelectric element located in the housing, an elastic element, a waveguide and a membrane connected along the external circuit with the housing and in its central part with one end of the waveguide, the piezoelectric element is made in the form of a thin a disk mounted on one side of the elastic element in the form of a membrane. The central part of the elastic element, on the other hand, is rigidly connected to the other end of the waveguide, and the outer contour is connected to the body. On the non-working surface of the piezoelectric element, electrodes are made in the form of a concentric disk and a ring, while the transverse size of the waveguide is chosen smaller than the diameter of the disk electrode. The transverse size of the waveguide is less than half the minimum Rayleigh wavelength. The internal transverse dimension of the body is selected to be more than half the average Rayleigh wavelength. The elastic element is equipped with a ring located on the outer contour of the elastic element from the side of the waveguide, while the diameter of the piezoelectric element D _pe selected from the ratio:
D ₁ ≤D _pe <D ₂ ,
Where
D ₁ - the inner diameter of the ring of the elastic element;
D ₂ - the outer diameter of the ring of the elastic element.

 The ring of the elastic element is connected to the body by means of an adhesive layer with a thickness greater than 0.03 • λ, where λ is the longitudinal wavelength in the adhesive. The diameter of the disk electrode and the inner diameter of the ring electrode are selected within (0.55-0.8) of the inner diameter of the ring of the elastic element.

 The selectivity of receiving Rayleigh waves among all modes of ultrasonic vibrations of the controlled object and increasing the noise immunity of the piezoelectric transducer of acoustic emission is achieved by mechanical coupling of the piezoelectric element with the transducer elements that are in direct contact with the surface of the controlled object at a certain distance from each other, namely with the body through the elastic element and with the waveguide. The mechanical connection is formed due to the rigid connection of the piezoelectric element with the elastic element and the elastic element with the body (along the outer contour) and with the waveguide (along the inner contour). Displacements of the housing and the waveguide relative to each other cause deformation of the piezoelectric element and, accordingly, the appearance of an electrical signal. In this case, the execution of the elastic element in the form of a membrane and a piezoelectric element fixed in it in the form of a thin disk makes it possible to obtain high sensitivity to waves with a comb structure, since the bending stiffness of these elements is small and, therefore, the loss of wave energy as a result of their reflection is minimal.

 When Rayleigh waves propagate at the installation site of the transducer, the waveguide shifts relative to the housing and, when the piezoelectric element is bent, two annular deformation zones are formed, within which deformations opposite in direction are observed. At the same time, the execution of two electrodes on the inoperative surface of the piezoelectric element in the form of a concentric disk and ring and at the same time the choice of the transverse waveguide size smaller than the diameter of the disk electrode allows two channels to be received on one piezoelectric element and to remove electrical signals of different signs from the electrodes, which allows differential signal output.

 Under normal incidence of longitudinal waves from the depth of the pipeline material and simultaneous axial displacement of the waveguide and the body, the membrane of the elastic element and the piezoelectric element under the action of inertia are bent so that three ring deformation zones are formed within them, and the deformation of the inner ring zone is opposite to the extreme zones. Within each electrode, bending deformations opposite in the direction of deformation are observed simultaneously, mutually compensating for most of the electrical signal of mechanical interference. The uncompensated electric charge removed from the electrodes of the piezoelectric element is the same both in magnitude and in sign, and with the differential connection of the amplifier is subtracted. Thus, the placement of two electrodes in the form of a concentric disk and a ring on the non-working surface of the piezoelectric element, as well as limiting the transverse size of the waveguide, enable differential signal output, Rayleigh wave isolation, and signal suppression caused by the action of longitudinal waves.

The execution of the elastic element in the form of a membrane with a ring along the outer contour and the inequalities D ₁ ≤D _pe <D ₂ allow us to obtain the necessary distribution of the mechanical radial σ _r and tangential σ _t piezoelectric stresses, at which the signals recorded from both electrodes are equal. The lower boundary of the relationship (D ₁ ≤ D _pe ) simultaneously provides conditions for the absence of movement and the angle of rotation of the piezoelectric element on the outer contour of the membrane of the elastic element, since in this region the piezoelectric element does not experience bending deformations, and the overlap of the region of maximum bending stresses σ _r and σ _t near the outer contour . The upper boundary of the inequality (D _pe <D ₂ provides the absence of acoustic and electrical contacts of the forming surface of the piezoelectric element with the housing, and thereby eliminates the interference caused by the "ringing" of the housing, and short circuit with the ring electrode.

 The choice of the transverse size of the waveguide is less than half the minimum Rayleigh wavelength and the internal transverse size of the body is more than half the average Rayleigh length allows you to get the amplitude-frequency response of a bell-shaped converter with a maximum value in the middle of the working frequency range and a high slope of the frequency response outside it. Moreover, the limitation of the transverse size of the waveguide provides a significant suppression of high-frequency signals, and the limitation of the internal size of the housing of low-frequency signals.

 The connection of the housing with the ring of the elastic element by means of an adhesive layer with a thickness of more than 0.03 • λ (mainly, the thickness of the adhesive layer should be chosen closer to the minimum size: 0.03 • λ) allows you to reduce the effect of vibrations of the housing on the transducer readings. At the boundaries "body - glue" and "ring - glue" as a result of a significant difference in the wave impedances of the materials of the mating elements, interference is reflected. The sizes of the disk and ring electrodes are selected from the conditions for ensuring the maximum sensitivity of the converter during differential output of the electric signal and the maximum width of the insulating gap between the electrodes.

Optimizing the diameter of the disk electrode D _d and the inner diameter of the ring electrode D _k is a very time-consuming task, since the equations of bending of multilayer sensitive piezoelectric elements are quite complex and cumbersome. In the inventive Converter, the indicated diameters must be selected in the range (0.55-0.8) • D ₁ , the values of which are obtained by calculation. The annular zone, limited by diameters of 0.55 • D ₁ and 0.8 • D ₁ , represents the region of all possible combinations of diameters D _d and D _k and conditionally divides the piezoelectric element into two parts (two channels) with equal transform coefficients. The conversion coefficients were calculated based on the known dependences of the electric field strength of piezoelectric ceramic elements in the absence of an external electric field. The electric field strength and, accordingly, the conversion coefficient are functions of the sum of mechanical stresses σ _r and σ _t . Under the influence of Rayleigh waves near a diameter of 0.55 • D _{1, the} radial mechanical stresses σ _r change their sign, and near a diameter of 0.8 • D ₁ - tangential mechanical stresses σ _t . In the ring zones from d, to 0.55 • D ₁ and from 0.8 • D ₁ to D _{1, the} sum of the mechanical stresses of the piezoelectric element σ _r and σ _t , and, accordingly, the conversion coefficients are maximum. Inside the annular zone from 0.55 • D ₁ to 0.8 • D _{1, the} sum of the mechanical stresses of the piezoelectric element σ _r and σ _t has a minimum value, and in the diameter range (0.6-0.7) • D ₁ is close to to zero. Thus, the width of the insulating gap between the electrodes can be quite large and reach a value of 0.25 • D ₁ , since the influence of changes in the size of the electrodes D _d and D _k within their area of existence (0.55 - 0.8) • D ₁ on channel conversion coefficients and, accordingly, the difference in the readings of both channels is not significantly affected.

In addition to the above distinguishing features, it should also be noted that the shape of the piezoelectric element is that the diameter of the piezoelectric element D _pe is many times greater than its thickness. This ratio of the size of the piezoelectric element leads to high values of the converter’s own capacitance, reaching several dozens of nF and allowing the transmission of an electrical signal without amplification outside the environmentally and biologically hazardous rooms of a nuclear power plant with a very significant load capacity of the cable. In addition, this design is made on a single piezoelectric element, is quite simple and includes a minimum number of parts.

 Let us consider in more detail the features of the set of distinctive features of the claimed technical solution using the example of a piezoelectric acoustic emission transducer for equipment of nuclear power plants.

 The essence of the claimed technical solution will be clear from the attached explanations and illustrations. Figure 1 shows a structural diagram of a piezoelectric transducer of acoustic emission; figure 2 - dependence of radial and tangential mechanical stresses on the radius of the piezoelectric element during the propagation of Rayleigh waves; figure 3 - the same, with a normal incidence of longitudinal waves; figure 4 - dependence of the conversion coefficient of displacement into an electric charge on the radius of the disk electrode.

Figure 1 shows: 1 - housing; 2 - a piezoelectric element; 3 - elastic element; 4 - waveguide; 5 - base; 6 - disk electrode; 7 - ring electrode; 8 - ring; 9 - a layer of glue; 10 - cable; 11 - cable conductors; D _pe - the diameter of the piezoelectric element; D _d - the diameter of the disk electrode; D _to - the inner diameter of the ring electrode; D ₁ - the inner diameter of the ring of the elastic element; D ₂ - the outer diameter of the ring of the elastic element; d is the waveguide diameter. Figure 1 shows an embodiment of the design of the Converter, in which the connection of the outer contour of the elastic element 3 with the housing 1 is carried out by means of an adhesive layer 9.

Figure 2 shows: σ _r is the dependence of the radial mechanical stresses of the piezoelectric element on the radius, σ _t is the dependence of the tangential mechanical stresses on the radius.

In FIG. Figure 4 shows: ξ _q is the dependence of the conversion coefficient of displacement into an electric charge on the radius of the disk electrode.

The piezoelectric acoustic emission transducer comprises a housing 1, in which a piezoelectric element 2, an elastic element 3, a waveguide 4 and a membrane 5 are placed. The membrane 5 is connected along the external circuit to the housing 1, and along the internal circuit to the waveguide 4. The piezoelectric element 2 is made in the form of a thin disk and is rigidly fixed to the membrane of the elastic element 3. The central part of the membrane of the elastic element 3 is rigidly connected to the waveguide 4, and the outer circuit is connected to the housing 1. On the idle surface of the piezoelectric element 2, a disk electrode 6 and an annular elec od 7. The diameter of the waveguide 4 is selected smaller than the diameter of the electrode roller 6 and simultaneously less than half the minimum wavelength of the Rayleigh. The internal size of the housing 1 is selected to be greater than half the average Rayleigh wavelength. The elastic element 3 along the outer contour is provided with a ring 8. The diameter of the piezoelectric element 2 is selected from the relation: D ₁ ≤D _pe <D ₂ . Ring 8 is connected to the housing 1 by means of a layer of glue 9, with a thickness greater than 0.03 • λ. The diameter of the disk electrode 6 and the inner diameter of the ring electrode 7 are selected in the range (0.55-0.8) • D ₁ . In the upper part of the housing, a two-core shielded cable 10 is fixed, the cores 11 of which are connected to the electrodes 6 and 7.

Referring to FIG. 2 and 3, the dependences of the bending mechanical stresses σ _r and σ _t and the transformation coefficient ξ _q on a variable radius were obtained by calculation. Let us consider in more detail the order of this calculation by analyzing from variables: radius r; waveguide diameter d; the diameter of the disk electrode D _d ; the inner diameter of the ring D ₁ . For this, we consider the well-known equilibrium equation for a thin plate in cylindrical coordinates. It should be noted that for Rayleigh waves with a length comparable to the size of the transducer, waveguide 4 can be represented as a non-deformable rod transmitting vibrations of the surface of a controlled object to the membrane of elastic element 3 with almost no distortion. In this case, the total energy loss can be neglected. In addition, the hypothesis of direct normals was accepted in the calculation. All stress components having normal directions to the neutral plane are very small compared to other stresses and, therefore, they can be neglected.

 The rigidly connected piezoelectric element 2 and the elastic element 3 form a sensitive element of the transducer. In the calculation, a five-layer (upper electrode - piezoceramic material - lower electrode - adhesive layer - elastic element material) sensitive element is replaced by an equivalent homogeneous membrane with similar thickness and cylindrical stiffness.

The movement of the waveguide is equivalent to the action of the force F _r distributed along the inner contour of the membrane.

,
где
M_r - изгибающий момент в радиальном направлении; M_t - изгибающий момент в тангенциальном направлении; r - переменный радиус; K - жесткость чувствительного элемента; δ - перемещение волновода.

,
Where
M _r is the bending moment in the radial direction; M _t is the bending moment in the tangential direction; r is the variable radius; K is the stiffness of the sensing element; δ is the movement of the waveguide.

где
C - цилиндрическая жесткость эквивалентной мембраны; φ - угол поворота чувствительного элемента; r - переменный радиус; μ -коэффициент Пуассона эквивалентной мембраны.Bending moments of the sensing element:

Where
C is the cylindrical stiffness of the equivalent membrane; φ is the angle of rotation of the sensing element; r is the variable radius; μ is the Poisson's ratio of the equivalent membrane.

,
где
E_к - модуль упругости пьезокерамического материала; E_м - модуль упругости материала упругого элемента; h_к - толщина пьезоэлемента; h_м - толщина мембраны упругого элемента.Cylindrical stiffness of the sensing element:

,
Where
E _to - the elastic modulus of the piezoceramic material; E _m - the modulus of elasticity of the material of the elastic element; h _to - the thickness of the piezoelectric element; h _m - the thickness of the membrane of the elastic element.

,
Постоянные интегрирования находятся из граничных условий. На внутреннем (при r = d/2) и внешнем контуре (при r = D₁/2) чувствительного элемента угол поворота φ равен нулю. Граничные условия образуют систему уравнении, имеющих решение:

,
где
a = D₁/d.Angle of rotation φ:

,
Integration constants are found from the boundary conditions. On the internal (for r = d / 2) and external circuit (for r = D _1/2 ) of the sensing element, the rotation angle φ is equal to zero. Boundary conditions form a system of equations having a solution:

,
Where
a = D ₁ / d.

.After finding the integration constant, the stiffness of the sensitive element:

.

,

,
где
z - переменная по толщине чувствительного элемента.The mechanical stresses of the bend of the piezoelectric element in the radial direction σ _r and in the tangential direction σ _t and when exposed to Rayleigh waves are determined by the formulas:

,

,
Where
z is a variable over the thickness of the sensing element.

In FIG. Figure 2 shows the dependences of the mechanical radial σ _r and the tangential σ _t piezoelectric stresses.

Similarly, the bending stresses of the piezoelectric element are found under normal incidence of longitudinal waves with a difference in terms of setting boundary conditions (there is no movement of the waveguide relative to the body) and distributed inertia forces. Figure 3 shows the dependences of the stresses σ _r and σ _t acting in the normal incidence of longitudinal waves.

The electric field strength, and accordingly the conversion coefficient of displacement into an electric charge ξ _q, is a function of the sum of the mechanical stresses of the piezoelectric element σ _r and σ _t , the radius distribution of which depends on the mode of oscillations.

,
где
E_эл - напряженность электрического поля пьезокерамического элемента в отсутствии внешнего электрического поля; d₃₁ - пьезомодуль пьезокерамического материала; ε $\genfrac{}{}{0ex}{}{\text{t}}{\text{33}}$ - диэлектрическая проницаемость пьезокерамического материала; ε₀ - диэлектрическая проницаемость вакуума.

,
Where
E _el - the electric field strength of the piezoceramic element in the absence of an external electric field; d ₃₁ - piezoelectric module of a piezoceramic material; ε $\genfrac{}{}{0ex}{}{\text{t}}{\text{33}}$ - dielectric constant of a piezoceramic material; ε ₀ - dielectric constant of vacuum.

,
где
u - расстояние от нейтральной плоскости чувствительного элемента до клеевого слоя между пьезоэлементом и мембраной упругого элемента. Положение нейтральной плоскости чувствительного элемента находится из условия отсутствия деформаций нейтральной плоскости:

.The conversion coefficient of the channel formed by the part of the piezoelectric element bounded by the disk electrode ε $\genfrac{}{}{0ex}{}{\text{1}}{\text{q}}$ determined by the formula:

,
Where
u is the distance from the neutral plane of the sensing element to the adhesive layer between the piezoelectric element and the membrane of the elastic element. The position of the neutral plane of the sensing element is found from the condition of the absence of deformations of the neutral plane:

.

.The conversion coefficient of the channel formed by the part of the piezoelectric element bounded by a ring electrode ξ $\genfrac{}{}{0ex}{}{\text{2}}{\text{q}}$ calculated by the formula:

.

Figure 4 shows the dependence of the conversion coefficient of displacement into an electric charge ξ _q on the electrode radius D _d .

Analysis of dependences of transformation coefficients ξ $\genfrac{}{}{0ex}{}{\text{1}}{\text{q}}$ and ξ $\genfrac{}{}{0ex}{}{\text{2}}{\text{q}}$ (10, 12) shows that for any diameter of the disk electrode D _d the equality ξ $\genfrac{}{}{0ex}{}{\text{1}}{\text{q}}$ = -ξ $\genfrac{}{}{0ex}{}{\text{2}}{\text{q}}$ under the condition D _d ≈D _k (i.e., at the minimum possible width of the insulating gap between the electrodes). However, when choosing the diameters D _d and D _k in the range (0.55-0.8) • D _1, not only the conversion coefficients take the maximum values, but the diameters themselves can vary significantly, which is important to ensure manufacturability.

 Thus, the proposed piezoelectric transducer of acoustic emission provides selective reception of Rayleigh waves and improves noise immunity.

 The device operates as follows.

 During the propagation of Rayleigh waves on the surface of the controlled object, the waveguide shifts relative to the housing and bends the membrane of the elastic element and the piezoelectric element. Two annular zones with oppositely directed deformations are formed on the piezoelectric element. The electrical signals taken from the electrodes of the piezoelectric element are in antiphase and add up when the differential output. When longitudinal waves are incident, the electrical signals are in phase and substantially suppressed.

 The practical implementation of the inventive Converter was carried out as follows.

 Laboratory samples of acoustic emission piezoelectric transducers were made on the basis of a disk piezoelectric element from TsTS-26 piezoceramics with a diameter of 21 mm, with two electrodes on the upper plane: a disk electrode with a diameter of 13.5 mm and a ring electrode with an inner diameter of 14.5 mm and an outer diameter of 20 mm . The piezoelectric element was fixed using high-temperature adhesive on an elastic element in the form of a titanium alloy membrane with a ring along the outer contour and a central cylindrical rod playing the role of a waveguide. The inner diameter of the ring of the elastic element was 20 mm, and the outer diameter was 22 mm, the height of the ring of the elastic element was 6 mm. The ring of the elastic element is rigidly connected to the body through an adhesive layer 0.5 mm thick. A waveguide with a diameter of 4 mm is connected to the inner contour of the lower membrane, which plays the role of the base of the transducer and protects the internal cavity. On the outer contour, the lower membrane is connected to the housing. The inner diameter of the case is 25 mm. The electrical signal is removed from the electrodes of the piezoelectric element by means of a high-temperature cable of the type KNMS-2C, mounted on the housing cover.

Under the influence of Rayleigh waves, surface vibrations of a controlled object are transmitted to a piezoelectric element experiencing bending deformations. With the differential connection of the preamplifier, the signals of the received mode were added, and the interference signals were subtracted. The suppression of the signals of the longitudinal waves compared with the signals of the Rayleigh waves was 15 - 20 dB. The conversion coefficient of the displacement into the electric charge of laboratory samples was ≈4 • 10 ^-2 C / m. Relatively high values of electric capacitance (≈4 • 10 ^-9 F) made it possible to transmit the signal of the converters without amplification over distances of several tens of meters.

Claims (6)

 1. A piezoelectric acoustic emission transducer comprising a piezoelectric element, an elastic element, a waveguide and a base in the form of a membrane connected along the external circuit with the body and in its central part with a waveguide, characterized in that the piezoelectric element is made in the form of a thin disk fixed on an elastic element made in the form of a membrane, the central part of which is rigidly connected to the waveguide, and the outer contour - to the body, on the upper surface of the piezoelectric element electrodes are placed in the form of concentric disk and ring, and the transverse dimension of the waveguide is chosen less than the diameter of disk electrode.  2. The Converter according to claim 1, characterized in that the transverse dimension of the waveguide is selected to be less than half the minimum Rayleigh wavelength.  3. The Converter according to claim 1, characterized in that the inner transverse dimension of the housing is selected to be greater than half the average Rayleigh wavelength. 4. The Converter according to any one of paragraphs.1 to 3, characterized in that the elastic element is provided with an annular rib along the outer contour, while the diameter of the piezoelectric element D _pe selected from the ratio
D ₁ ≤ D _{p e} <D ₂ ,
where D ₁ is the inner diameter of the annular ribs of the elastic element;
D ₂ - the outer diameter of the annular ribs of the elastic element.  5. The Converter according to claim 4, characterized in that the annular rib of the elastic element is connected to the housing by means of an adhesive layer with a thickness greater than 0.03λ, where λ is the longitudinal wavelength in the adhesive.  6. The Converter according to claim 4 or 5, characterized in that the diameter of the disk electrode and the inner diameter of the ring electrode are selected within (0.55 - 0.8) of the inner diameter of the annular rib of the elastic element.

Images