A piezoelectric ultrasonic transducer for industrial rotary machine condition monitoring

Abstract

The application discloses a piezoelectric ultrasonic transducer for industrial rotary machine state monitoring, and belongs to the technical field of ultrasonic transducers, which comprises an insulating base, a waveguide column and a piezoelectric element which are sequentially connected from bottom to top and are arranged in a metal shell, the waveguide column and the piezoelectric element are completely wrapped by a backing layer, and a connecting head for connecting with a cable is arranged at the top of the metal shell; two electrodes are led out on the piezoelectric element for contacting with the cable, and the piezoelectric ultrasonic transducer is connected with a measured rotary machine structure through the insulating base. The piezoelectric ultrasonic transducer solves the problem of early weak fault signal sensing, and provides a mathematical model and theoretical support for the structural design of a bending vibration mode ultrasonic transducer.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ultrasonic transducers, and particularly relates to a piezoelectric ultrasonic transducer for industrial rotating machinery state monitoring. BACKGROUND

[0002] Vibration analysis, as a mature mechanical fault diagnosis technology, is widely used in industrial sites. By monitoring the vibration state of rotating machinery through vibration transducers, potential problems can be found in time. However, due to the low range of monitoring frequency signals, which is basically below 10 kHz, there is an overlap with the environmental noise band, which makes the collected signals susceptible to interference, affecting the accuracy of fault diagnosis.

[0003] When early damage occurs in rotating machinery, the ultrasonic signals generated by the change of frictional stress at the damage site provide a new monitoring means for faults. The frequency of such ultrasonic signals is above 20 kHz, much higher than environmental noise, and it has a higher signal-to-noise ratio and is more sensitive to early fault detection. If such ultrasonic signals can be effectively monitored, the damage to rotating machinery can be detected earlier, and timely measures can be taken to avoid potential safety hazards.

[0004] Currently, although there are some ultrasonic transducers for rotating machinery fault monitoring, they still have certain limitations in design and application. Most ultrasonic transducers are designed based on the radial polarization mode of piezoelectric ceramics, which results in a larger transducer size and a single installation method, which can only rely on adhesive to fix on the surface of the machine. This method not only limits the application range of the transducer, but also may introduce additional installation errors and inconvenience.

[0005] Therefore, it is particularly important to develop an ultrasonic transducer based on the bending vibration mode of piezoelectric ceramics. This new type of transducer is expected to solve the problems of existing ultrasonic transducers in size, installation method and application flexibility. However, there is a lack of analysis of the theoretical model of piezoelectric ceramic bending vibration, and there is a lack of an effective mathematical model of ultrasonic transducers. Through in-depth research on the transducer, a more compact and flexible ultrasonic transducer can be designed to meet the monitoring needs of various industrial rotating machinery. SUMMARY

[0006] The application provides an ultrasonic transducer based on the bending vibration mode of piezoelectric ceramics, and provides the necessary mathematical model and theoretical support for its design.

[0007] In order to achieve the above purpose, the application adopts the following technical solutions.

[0008] A piezoelectric ultrasonic transducer for industrial rotary machinery state monitoring, comprising an insulating base, a waveguide column and a piezoelectric element connected in sequence from bottom to top, which are arranged inside a metal shell, the waveguide column and the piezoelectric element are completely wrapped by a backing layer, and a connecting head for connecting with a cable is arranged at the top of the metal shell; two electrodes are led out from the piezoelectric element for contacting with the cable, and the piezoelectric ultrasonic transducer is connected with a rotary machinery structure to be measured through the insulating base.

[0009] Preferably, the insulating base comprises a connecting base arranged at the lower layer and an annular layer arranged on the connecting base, a threaded hole is arranged in the inner core of the bottom of the connecting base, and a column groove is arranged in the annular layer at the top of the connecting base.

[0010] Preferably, the waveguide column is installed in the column groove, the piezoelectric element is arranged at the top end of the waveguide column, and the backing layer is arranged in the annular layer.

[0011] Preferably, a stepped column cavity that is matched with the insulating base is arranged inside the metal shell, a flange is arranged at the bottom, and the insulating base is clamped in the metal shell.

[0012] Preferably, the piezoelectric ultrasonic transducer is connected with the rotary machinery structure to be measured through the threaded column of the connecting base of the insulating base, or is installed on the rotary machinery structure to be measured through a magnetic attraction base and a glue joint base.

[0013] Preferably, the piezoelectric element comprises a piezoelectric ceramic, a rectangular piezoelectric bimorph is selected, the resonant frequency of which is 40 kHz, the insulating base adopts alumina, the waveguide column adopts a brass column, and the metal shell adopts a stainless steel shell.

[0014] Preferably, the backing layer adopts silicone rubber, which forms a covering and wrapping for the piezoelectric element.

[0015] Preferably, the piezoelectric element adopts a rectangular piezoelectric bimorph, under the same parameters, the output performance of the piezoelectric bimorph is better than the sum of two monocrystals, that is, the piezoelectric bimorph can perceive early and more weak fault signals of the rotary machinery. The ultrasonic signal frequency of the rotary machinery wear is about 40 kHz, therefore, the center frequency of the piezoelectric bimorph should also be 40 kHz when it is designed, so that the ultrasonic transducer also works near the resonant frequency, and the output charge of the transducer is more and the sensitivity is greater.

[0016] At present, there is no complete theoretical model for solving the resonant frequency of the rectangular piezoelectric bimorph, the present application combines the bending vibration theory of rectangular thin plates of the whole element method (WEM) to study the resonant frequency of the rectangular piezoelectric bimorph, and the vibration differential equation of the rectangular piezoelectric bimorph with free bending vibration on four sides can be expressed as:

[0017]

[0018] wherein a = a / b, a and b represent the length and width of the rectangular piezoelectric bimorph respectively, w is the analytical solution of the piezoelectric bimorph differential equation, and Ω is the natural frequency of the piezoelectric bimorph.

[0019] When w satisfies equation (1) and satisfies the free boundary condition, the expression of Ω is:

[0020]

[0021] wherein ω represents the angular frequency, D is the bending stiffness of the piezoelectric bimorph, ρ represents the density of the piezoelectric bimorph, and h represents the thickness of the piezoelectric bimorph.

[0022] The piezoelectric bimorph resonant frequency is generally in the third order vibration mode of the rectangular thin plate, and in combination with the basic properties of the piezoelectric ceramic, for a square piezoelectric ceramic sheet, the resonant frequency f s is

[0023]

[0024] wherein is the elastic compliance constant of the rectangular piezoelectric bimorph.

[0025] The relationship between the side length and thickness of the piezoelectric bimorph and the resonant frequency is shown in equation (3), and the size parameters of the piezoelectric bimorph can be designed according to equation (3).

[0026] The insulating base not only realizes the acoustic impedance matching of the piezoelectric element and the measured machine, but also realizes the electrical insulation between the transducer and the rotating machine shell, protects the piezoelectric element, and increases the use reliability. The waveguide column plays a supporting role for the piezoelectric element, and conducts the ultrasonic signal captured by the insulating base from the measured rotating machine structure to the center of the piezoelectric element, triggering the free boundary bending vibration of the piezoelectric element. The insulating base and the waveguide column form a double-layer matching layer structure, and for the double-layer matching layer structure, the acoustic energy transmission coefficient t is

[0027]

[0028] wherein P 1ta is the amplitude of the transmitted sound pressure of the measured component, P 4ta is the amplitude of the sound pressure transmitted to the inside of the piezoelectric ceramic, Z1 and Z4 are the characteristic acoustic impedances of the measured component and the piezoelectric ceramic respectively.

[0029] When the attenuation of the sound wave in the medium is not considered,

[0030]

[0031] Wherein, Z2, Z3 are the characteristic acoustic impedance of the insulating base and the waveguide column respectively, k2, k3 are the wave number in the insulating base and the waveguide column respectively, d1, d2 are the thickness of the insulating base and the waveguide column respectively.

[0032] Considering that the loss of the sound wave in the medium during the propagation is mainly caused by scattering attenuation and absorption attenuation, and presents an exponential decay form, as shown in formula (6)

[0033] V=Ae -βd (6)

[0034] Wherein, V is the vibration amplitude of the unit area particle, A is the maximum amplitude, β is the attenuation coefficient, and d is the medium thickness.

[0035] Considering the attenuation law of the sound wave in the medium into the matching layer thickness design model, the mathematical model of the sound intensity transmission coefficient and the matching layer thickness and the attenuation coefficient can be obtained, and the best matching layer thickness corresponding to different materials (different attenuation coefficients) is determined by taking the maximum sound intensity transmission coefficient as the solving target. When considering the attenuation of the sound wave in the medium:

[0036]

[0037] Wherein, β1, β2 are the acoustic attenuation coefficients of the insulating base and the waveguide column respectively.

[0038] The relationship between the thickness of the insulating base and the waveguide column and the sound intensity transmission coefficient is shown in formula (4) and formula (7), and the thickness parameters of the insulating base and the waveguide column can be selected according to formula (4) and formula (7) by taking the maximum sound intensity transmission coefficient as the solving target. The higher the sound intensity transmission coefficient, the better the performance of the ultrasonic transducer.

[0039] Preferably, the insulating base adopts alumina, which has excellent electrical insulation, high hardness, excellent high temperature resistance, can protect the piezoelectric ceramic from corrosion by chemical reaction pollutants, and its acoustic attenuation coefficient is 0.05396.

[0040] Preferably, the waveguide column adopts brass column, and its acoustic impedance is between the insulating base and the piezoelectric element, which can make more ultrasonic signals transmit to the piezoelectric element and improve the transmission rate of ultrasonic energy, and its acoustic attenuation coefficient is 0.067.

[0041] The backing layer forms a covering package for the piezoelectric element, absorbs the transmitted ultrasonic signal energy, and prevents reflection to interfere with the electric signal. When the ultrasonic wave enters the backing layer through the piezoelectric element, if the acoustic impedance difference between the two is large, the transmission rate of the sound wave will be reduced. In the study of the transmission of the sound wave to the backing layer, according to the sound wave propagation theory, the sound intensity transmission coefficient t of the backing layer can be obtained s The expression is shown in formula (8):

[0042]

[0043] In the formula, Z5 is the acoustic impedance of the backing layer.

[0044] Different backing materials have different acoustic impedance, and the selection of the backing material can be selected according to formula (8). At present, the backing layer research is mainly based on air backing or rigid backing, and rarely involves the backing layer structure with covering properties. When Z4>>Z5 and Z4<<Z5, the sound intensity transmission coefficient t s ≈0, that is, very few sound waves can be transmitted, and almost total reflection occurs. Taking air backing as an example, the acoustic impedance of piezoelectric ceramic is 35 MRayl, and the acoustic impedance of air is only 438.6 Rayl, at this time the sound intensity transmission coefficient is only about 0.005%. That is, the impedance of the backing layer is too large or too small, which cannot improve the transmission ability of the sound wave, so the impedance and sound absorption effect of the backing material is extremely important in the design of the backing layer, not only the sound attenuation ability of the material itself to the sound wave needs to be considered, but also the transmission ability of the sound wave needs to be considered.

[0045] Preferably, the backing layer adopts silicone rubber, the acoustic impedance of the silicone rubber is 1.2 MRayl, and the sound attenuation coefficient reaches 0.91. It can not only reduce the interference of echo, but also will not inhibit the harmonic response vibration of the piezoelectric ceramic, so that the ultrasonic transducer can maintain a high receiving sensitivity.

[0046] Preferably, the metal shell adopts a stainless steel shell, which has superior structural strength and high acoustic impedance characteristics, can effectively protect the internal components from external damage, and can significantly shield electromagnetic interference and noise interference, and ensure stable operation of the transducer.

[0047] Thanks to the above technical solutions, the technical progress achieved by the present application is:

[0048] The application solves the problem of early weak fault signal sensing of rotating machinery, designs a new type of bending vibration mode ultrasonic transducer structure, and provides a mathematical model and theoretical support for the design, and for the rectangular piezoelectric bimorph which is the core element of the transducer, in view of the current insufficient theoretical analysis of the resonant frequency of the piezoelectric bimorph under the free boundary condition, the resonant frequency of the rectangular piezoelectric bimorph is derived by combining the rectangular thin plate bending vibration theory of the whole element method (WEM) and the basic properties of piezoelectric ceramics, and the ultrasonic transducer with different resonant frequencies can be designed according to the formula. In view of the thickness design of the insulating base and the waveguide column, combined with the multi-matching layer theoretical model, the thickness problem of the insulating base and the waveguide column is explained from the quantitative angle on the basis of improvement, considering the attenuation of sound waves in the medium, the best thickness design of different materials can be realized, and with the gradual increase of the attenuation coefficient, the sound intensity transmission coefficient gradually decreases. When the attenuation coefficient is greater than 0.005, the thickness of the theoretical best insulating base and waveguide column should be thinner. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 is a structure schematic diagram of an ultrasonic transducer provided by an embodiment of the application;

[0050] Figure 2 is a resonant frequency and size relationship diagram of a piezoelectric bimorph provided by an embodiment of the application;

[0051] Figure 3 is a vibration mode analysis diagram of the piezoelectric bimorph when the waveguide column center supports the piezoelectric bimorph provided by an embodiment of the application;

[0052] Figure 4 is a relationship diagram of the sound intensity transmission coefficient and the thickness of the insulating base and the waveguide column provided by an embodiment of the application;

[0053] Figure 5 is a relationship diagram of the sound intensity transmission coefficient and the frequency provided by an embodiment of the application;

[0054] Figure 6 is an output signal waveform diagram of the transducer when the air backer is excited provided by an embodiment of the application;

[0055] Figure 7 is an output signal waveform diagram of the transducer when the soft silica gel backer is excited provided by an embodiment of the application;

[0056] Figure 8 is a schematic diagram of the insulating base;

[0057] Figure 9 is a schematic diagram of the metal shell;

[0058] Wherein, 1, the insulating base, 1-1, the connecting base, 1-2, the annular layer, 2, the waveguide column, 3, the piezoelectric element, 4, the backer layer, 5, the connecting head, 6, the metal shell. DETAILED DESCRIPTION

[0059] The technical solutions of the present application will be described clearly and completely below in combination with the embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work are within the protection scope of the present application.

[0060] A structural schematic diagram of the ultrasonic transducer provided by the embodiment of the present application is shown in Figure 1 and Figure 8 、 9 The ultrasonic transducer includes an insulating base 1, a waveguide column 2 and a piezoelectric element 3 which are sequentially connected from bottom to top and are arranged inside a metal shell 6, the waveguide column 2 and the piezoelectric element 3 are completely wrapped by a backing layer 4, and the metal shell 6 is provided with a connecting head 5 at the top for connecting with a cable; two electrodes are led out on the piezoelectric element for contacting with the cable, and the piezoelectric ultrasonic transducer is connected with a measured rotating mechanical structure through the insulating base 1.

[0061] The insulating base 1 includes a connecting base 1-1 arranged at the lower layer and an annular layer 1-2 arranged on the connecting base 1-1, a threaded hole is arranged in the inner core of the bottom of the connecting base 1-1, and a column groove is arranged in the annular layer 1-2 at the top of the connecting base 1-1.

[0062] The insulating base 1 is used for realizing the acoustic impedance matching between the piezoelectric element 3 and the measured rotating machine and realizing the electrical insulation with the rotating machine shell. The piezoelectric ultrasonic transducer is connected with the measured rotating mechanical structure through the threaded column of the connecting base 1-1 of the insulating base 1, and the detection rod, the magnetic attraction base, the glue joint base and other accessories can be replaced, and various installation modes are supported to cope with different monitoring scenes in the industrial field.

[0063] The waveguide column is inserted into the column groove of the insulating base 1 at one end and is fixedly bonded to the piezoelectric element 3 at the other end through epoxy resin glue, so as to conduct the ultrasonic signal captured by the insulating base from the measured rotating mechanical structure to the center of the piezoelectric element 3, thereby realizing the bending vibration excitation of the piezoelectric element. This design can make the piezoelectric element in the free boundary bending vibration, so that the ultrasonic transducer has higher sensitivity and response speed, and can perceive the ultrasonic signal generated by the rotation friction of the rotating machine. Under the free boundary condition, the resonance frequency response energy conversion efficiency of the piezoelectric ceramic is higher, and the running state of the rotating machine can be more accurately reflected.

[0064] The piezoelectric element 3 is used for converting the ultrasonic signal conducted by the waveguide column 2 into an electric signal and transmitting the electric signal to a later-stage circuit for processing, so as to realize the running state perception of the rotating machine.

[0065] The backing layer 4 forms a complete wrapping state to the piezoelectric element 3, on the one hand, absorbs the transmitted sound waves from the piezoelectric ceramic (piezoelectric element), reduces the interference of echo, and improves the resolution of the ultrasonic transducer; on the other hand, plays a damping role, affects the actual working bandwidth of the transducer. In addition, the backing layer also plays a role of shock absorption and buffering, avoiding damage to the piezoelectric element due to mechanical impact. After the waveguide column 2 and the piezoelectric element 3 are installed, soft silicone is injected into the annular layer 1-2 to solidify and form the backing layer.

[0066] The metal shell 6, as an element packaging shell, has superior structural strength and high acoustic impedance characteristics, which not only effectively protects the internal elements from external damage, but also significantly shields electromagnetic interference and noise interference, ensuring stable operation of the transducer.

[0067] The connecting head 5 is used for connecting the cable to transmit the voltage signal output by the ultrasonic transducer under excitation and induction, facilitating signal processing of the subsequent circuit.

[0068] The ultrasonic transducer mentioned in the application is designed based on the bending vibration mode of the PZT-5A piezoelectric bimorph.

[0069] The piezoelectric element is the core component of the ultrasonic transducer, which can accurately and reliably capture the ultrasonic signals emitted by the rotating machinery, and can convert the ultrasonic signals into electrical signals through the piezoelectric effect, facilitating processing by the subsequent circuit. The piezoelectric element can be piezoelectric ceramic. The relationship between the size of the piezoelectric bimorph and the resonance frequency f s of the piezoelectric bimorph is determined by studying the resonance frequency of the rectangular piezoelectric bimorph. The center frequency of the ultrasonic signal generated by the wear of the rotating machinery is about 40 kHz, so the center frequency of the piezoelectric bimorph is selected as 40 kHz. When the piezoelectric ceramic is working, it is near the resonance frequency, and more electric charges are generated, and the sensitivity is greater.

[0070] For a rectangular piezoelectric bimorph, the resonance frequency of the rectangular piezoelectric bimorph is studied by combining the full element method (WEM) rectangular thin plate bending vibration theory. The vibration differential equation of the rectangular piezoelectric bimorph with free edges can be expressed as:

[0071]

[0072] In the formula, alpha = a / b, a and b represent the length and width of the rectangular piezoelectric bimorph respectively, w is the analytical solution of the piezoelectric bimorph differential equation, and Omega is the natural frequency of the piezoelectric bimorph.

[0073] When w satisfies equation (1) and also satisfies the free boundary condition, the expression of Omega is:

[0074]

[0075] In the formula, ω represents the angular frequency, D is the bending stiffness of the piezoelectric bicrystalline wafer, ρ represents the density of the piezoelectric bicrystalline wafer, and h represents the thickness of the piezoelectric bicrystalline wafer.

[0076] The resonant frequency of a piezoelectric bicrystalline wafer is generally located in the third vibration mode of a rectangular thin plate. Considering the fundamental properties of piezoelectric ceramics, for a square piezoelectric ceramic sheet, its resonant frequency f... s for

[0077]

[0078] In the formula, Let be the elastic compliance constant of the rectangular piezoelectric bicrystalline wafer.

[0079] The relationship between the side length and thickness of the piezoelectric bicrystalline wafer and the resonant frequency is shown in formula (3). The size parameters of the piezoelectric bicrystalline wafer can be designed according to formula (3).

[0080] For the PZT-5A, ρ=7450kg / m 3 , Figure 2 The diagram showing the resonant frequency versus size relationship of the PZT-5A piezoelectric bicrystalline wafer provided in this embodiment of the invention is shown below. Figure 2 As shown, the resonant frequency of the piezoelectric bicrystalline wafer gradually decreases with increasing side length, while the resonant frequency gradually increases with increasing thickness. This piezoelectric model provides a good theoretical basis for the selection of piezoelectric bicrystalline wafer dimensions. To achieve a center frequency of 40kHz, the designed dimensions of the piezoelectric bicrystalline wafer are 8mm × 8mm × 0.5mm.

[0081] One end of the waveguide post is inserted into a groove on the upper surface of the insulating base, and the other end is bonded to the piezoelectric element from the center using epoxy resin. This transmits the ultrasonic signal captured by the insulating base from the rotating machinery under test to the center of the piezoelectric element, thereby exciting the piezoelectric element's bending vibration. This design allows the piezoelectric element to undergo free-boundary bending vibration, giving the ultrasonic transducer higher sensitivity and response speed, enabling it to detect even weaker ultrasonic signals generated by the friction of rotating machinery. Under free-boundary conditions, the piezoelectric ceramic exhibits higher energy conversion efficiency at its resonant frequency response, more accurately reflecting the operating state of the rotating machinery. Figure 3 Vibration mode analysis of the piezoelectric bicrystalline wafer supported by the waveguide post shows that the piezoelectric bicrystalline wafer exhibits free boundary vibration. When operating in bending vibration mode, the vibration frequency is 37.674 kHz.

[0082] The insulation base can not only realize the acoustic impedance matching of the piezoelectric element and the measured machine, but also realize the electrical insulation between the transducer and the rotating machine shell, protect the piezoelectric element and increase the use reliability. The upper surface of the insulation base is provided with a groove for placing and fixing the waveguide column. The insulation base and the waveguide column form a double-layer matching layer structure, and the transducer is designed by selecting a multi-layer matching layer structure. For the double-layer matching layer structure, the sound energy transmission coefficient t is

[0083]

[0084] In the formula, P 1ta is the amplitude of the emitted sound pressure of the measured component, P 4ta is the amplitude of the sound pressure transmitted into the piezoelectric ceramic, Z1 and Z4 are the characteristic acoustic impedances of the measured component and the piezoelectric ceramic respectively.

[0085] When the attenuation of the sound wave in the medium is not considered,

[0086]

[0087] In the formula, Z2 and Z3 are the characteristic acoustic impedances of the insulation base and the waveguide column respectively, k2 and k3 are the wave numbers in the insulation base and the waveguide column respectively, and d1 and d2 are the thicknesses of the insulation base and the waveguide column respectively.

[0088] It is considered that the loss of the sound wave in the medium during propagation is mainly caused by scattering attenuation and absorption attenuation, and presents an exponential attenuation form, as shown in formula (6)

[0089] V=Ae -βd (6)

[0090] In the formula, V is the vibration amplitude of the unit area particle, A is the maximum amplitude, β is the attenuation coefficient, and d is the thickness of the medium.

[0091] The attenuation law of the sound wave in the medium is considered in the matching layer thickness design model, the mathematical model of the sound intensity transmission coefficient and the matching layer thickness and the attenuation coefficient can be obtained, and the optimal matching layer thickness corresponding to different materials (different attenuation coefficients) can be determined. When the attenuation of the sound wave in the medium is considered:

[0092]

[0093] In the formula, β2 and β3 are the sound attenuation coefficients of the insulation base and the waveguide column respectively.

[0094] The relationship between the thicknesses of the insulation base and the waveguide column and the sound intensity transmission coefficient is shown in formula (4) and formula (7). The thickness parameters of the insulation base and the waveguide column can be selected according to formula (4) and formula (7) with the maximum sound intensity transmission coefficient as the solving target. The higher the sound intensity transmission coefficient is, the better the performance of the ultrasonic transducer is.

[0095] Taking alumina as the insulating base material and brass as the waveguide post material as an example, the acoustic attenuation coefficient of alumina is 0.05396, and that of brass is 0.067. The acoustic transmission coefficient varies with the thickness of the alumina and brass layers as shown in the curves. Figure 4 As shown, from Figure 4 As can be seen, the acoustic transmission coefficient decreases sharply with the increase of material thickness. Therefore, when designing an ultrasonic transducer, the thickness of the insulating base and waveguide post should be as thin as possible. In this design, the thickness of the insulating base is 1 mm and the thickness of the waveguide post is 2.5 mm. The calculated acoustic transmission coefficient is 0.6118.

[0096] Figure 5 The changes in the acoustic transmission coefficient at different frequencies are shown. It can be seen that the frequency has little effect on the acoustic transmission coefficient, and the curve shows a flat trend. This indicates that when designing the thickness of the insulating base and waveguide post, the influence of frequency can be ignored, and only the material properties need to be considered.

[0097] The backing layer covers and encapsulates the piezoelectric element, absorbing the energy of the propagating ultrasonic signal and preventing reflection that could interfere with the electrical signal. When ultrasound passes through the piezoelectric element and enters the backing layer, if the acoustic impedances of the two differ significantly, the transmittance of the sound wave will decrease. When studying the transmission of sound waves to the backing layer, based on sound wave propagation theory, the acoustic intensity transmission coefficient t of the backing layer can be obtained. s The expression is shown in equation (8):

[0098]

[0099] In the formula, Z5 is the acoustic impedance of the backing layer.

[0100] Different backing materials have different acoustic impedances, and the selection of backing materials can be based on formula (8). Currently, most backing layer research focuses on air backing or rigid backing, with very few studies involving backing layer structures with covering properties. When Z4 >> Z5 and Z4 << Z5, the sound intensity transmission coefficient t... s≈0, that is, very little sound wave can be transmitted, almost total reflection occurs. Taking air backing as an example, the piezoelectric ceramic acoustic impedance is 35 MRayl, and the air acoustic impedance is only 438.6 Rayl, at this time the sound intensity transmission coefficient is only about 0.005%. That is, the impedance of the backing is too large or too small, which cannot improve the transmission ability of the sound wave, therefore, the impedance and sound absorption effect of the backing material are extremely important in the design of the backing layer, not only the sound attenuation ability of the material itself to the sound wave needs to be considered, but also the transmission ability of the sound wave needs to be considered. Therefore, in the experimental example, soft silicone with good sound attenuation performance is selected as the backing layer, the acoustic impedance of the soft silicone is 1.2 MRayl, and the sound attenuation coefficient reaches 0.91. It can not only reduce the interference of the echo, but also will not inhibit the harmonic response vibration of the piezoelectric ceramic, so that the ultrasonic transducer can maintain higher receiving sensitivity. Figure 6 The change of the output signal of the ultrasonic transducer after a 40 kHz three-period sine wave pulse signal modulated by a Hanning window is given to the ultrasonic transducer with air backing, the tail signal oscillation of the output signal of the air backing decays, and serious tailing phenomenon occurs; Figure 7 The change of the output signal of the ultrasonic transducer when the same excitation signal as the air backing is used when the soft silicone backing is used, the output signal has high consistency with the excitation signal, which reflects the absorption effect of the backing layer on the ultrasonic energy, and it is explained that the soft silicone backing layer can greatly reduce the tailing time of the received signal and improve the resolution of the ultrasonic transducer.

[0101] The metal shell adopts a stainless steel shell, which has superior structural strength and high acoustic impedance characteristics, can effectively protect the internal components from external damage, and can significantly shield electromagnetic interference and noise interference, and ensure stable operation of the transducer.

[0102] The connecting head is used for connecting the cable, and transmits the voltage signal output by the ultrasonic transducer under excitation and induction, which is convenient for signal processing of the subsequent circuit.

[0103] The mathematical model of each part of the ultrasonic transducer is established, and the design method is proposed, so that the ultrasonic transducer design based on the bending vibration mode of the piezoelectric ceramic with different center frequencies can be realized, which has strong theoretical guiding significance.

Claims

1. A piezoelectric ultrasonic transducer for monitoring the condition of industrial rotating machinery, characterized in that: The device includes an insulating base (1), a waveguide post (2), and a piezoelectric element (3) arranged sequentially from bottom to top inside a metal casing (6). The waveguide post (2) and the piezoelectric element (3) are completely enclosed by a backing layer (4). A connector (5) for connecting to a cable is provided on the top of the metal casing (6). Two electrodes are led out from the piezoelectric element for contact with the cable. The piezoelectric ultrasonic transducer is connected to the rotating mechanical structure under test through the insulating base (1). The piezoelectric element (3) includes piezoelectric ceramic, which is a rectangular piezoelectric bicrystalline wafer with a resonant frequency of 40 kHz. The insulating base (1) is made of alumina, the waveguide post (2) is made of brass, and the metal shell (6) is made of stainless steel. The relationship between the side length and thickness of the rectangular piezoelectric bicrystalline wafer and the resonant frequency is shown in formula (3). The resonant frequency of the rectangular piezoelectric bicrystalline wafer is derived by combining the whole-element method of rectangular thin plate bending vibration theory and the basic properties of piezoelectric ceramics, as follows: The differential equation for the vibration of a rectangular piezoelectric bicrystalline wafer under free bending vibration on four sides is expressed as follows: (1) In the formula, α = a / b, where a and b represent the length and width of the rectangular piezoelectric bicrystalline wafer, respectively. This is the analytical solution to the differential equation of the piezoelectric bicrystalline wafer. This is the inherent frequency of the piezoelectric bicrystalline wafer; when When equation (1) and the free boundary conditions are satisfied, The expression is: (2) In the formula, This represents the angular frequency, and D is the bending stiffness of the piezoelectric bicrystalline wafer. The density of the piezoelectric bicrystalline wafer is represented by h, and the thickness of the piezoelectric bicrystalline wafer is represented by h. The resonant frequency of a piezoelectric bicrystalline wafer is generally located in the third vibration mode of a rectangular thin plate. Considering the fundamental properties of piezoelectric ceramics, for a square piezoelectric ceramic sheet, its resonant frequency f... s for: (3) In the formula, , Let be the elastic compliance constant of the rectangular piezoelectric bicrystalline wafer.

2. The piezoelectric ultrasonic transducer for monitoring the condition of industrial rotating machinery according to claim 1, characterized in that: The insulating base (1) includes a connecting base (1-1) disposed on the lower layer and an annular layer (1-2) disposed on the connecting base (1-1). The bottom inner core of the connecting base (1-1) is provided with a threaded hole, and the top of the connecting base (1-1) is provided with a column groove in the annular layer (1-2).

3. A piezoelectric ultrasonic transducer for monitoring the condition of industrial rotating machinery according to claim 2, characterized in that: Waveguide post (2) is installed in the post groove, piezoelectric element (3) is set at the top of waveguide post (2), and backing layer (4) is set in the annular layer (1-2).

4. A piezoelectric ultrasonic transducer for monitoring the condition of industrial rotating machinery according to claim 2, characterized in that: The metal shell (6) has a stepped cylindrical cavity inside that is compatible with the insulating base (1), and a flange is provided at the bottom. The insulating base (1) is fitted inside the metal shell (6).

5. A piezoelectric ultrasonic transducer for monitoring the condition of industrial rotating machinery according to claim 2, characterized in that: The piezoelectric ultrasonic transducer is connected to the rotating mechanical structure under test via a threaded post through the connecting base (1-1) of the insulating base (1), or by means of a magnetic base and an adhesive base.

6. A piezoelectric ultrasonic transducer for monitoring the condition of industrial rotating machinery according to claim 1, characterized in that: The backing layer (4) is made of silicone rubber, which forms a covering and wrapping around the piezoelectric element.

7. A piezoelectric ultrasonic transducer for monitoring the condition of industrial rotating machinery according to claim 1, characterized in that: The relationship between the thickness of the insulating base and the waveguide post and the acoustic intensity transmission coefficient is shown in formulas (4) and (7). The thickness parameters of the insulating base and the waveguide post can be selected according to formulas (4) and (7) with the maximum acoustic intensity transmission coefficient as the solution objective. The higher the acoustic intensity transmission coefficient, the better the performance of the ultrasonic transducer. The insulating base (1) and the waveguide post (2) constitute a double-layer matching layer structure. For the double-layer matching layer structure, its acoustic energy transmission coefficient t is: (4) In the formula, The amplitude of the emitted sound pressure of the component being measured. The sound pressure amplitude transmitted into the piezoelectric ceramic is the value of the sound pressure level. , The characteristic acoustic impedances of the tested component and the piezoelectric ceramic are respectively. By incorporating the attenuation law of sound waves in the medium into the matching layer thickness design model, a mathematical model of sound intensity transmission coefficient, matching layer thickness, and attenuation coefficient is obtained. Using the maximum sound intensity transmission coefficient as the solution objective, the optimal matching layer thickness for different materials is determined. This is done while considering the attenuation of sound waves in the medium. (7) In the formula, , The characteristic acoustic impedances of the insulating base and the waveguide post are respectively. , These represent the wavenumbers in the insulating base and the waveguide post, respectively. , The thicknesses of the insulating base and the waveguide post are respectively. , These are the acoustic attenuation coefficients of the insulating base and the waveguide post, respectively.

8. A piezoelectric ultrasonic transducer for monitoring the condition of industrial rotating machinery according to claim 1, characterized in that: Different backing materials have different acoustic impedances, and the material of the backing layer can be selected according to formula (8). (8) In the formula, t s The sound intensity transmission coefficient of the backing layer. The acoustic impedance of the backing layer.

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