A piezoelectric acoustic emission sensor

Abstract

The present invention provides a piezoelectric acoustic emission sensor. It includes: a matching layer, a piezoelectric element, a backing layer, and an internal circuit that are connected in sequence; the matching layer is used to achieve acoustic impedance matching between the piezoelectric element and the working medium, and the thickness of the matching layer is designed according to the loss of signal energy during transmission; the piezoelectric element is used to capture acoustic emission signals from the working medium through the matching layer, convert the acoustic emission signals into electrical signals, and transmit the electrical signals to the backing layer; the backing layer is used to support the piezoelectric acoustic emission sensor, absorb the energy of the acoustic emission signals transmitted by the piezoelectric element, and transmit the acoustic emission signals to the internal circuit, and the internal circuit is used to perform band-pass filtering and amplification processing on the acoustic emission electrical signals sensed by the piezoelectric ceramics. The piezoelectric acoustic emission sensor of the present invention solves the problem of sensing early weak fault signals, can realize early fault diagnosis of the measured equipment, and provides a mathematical model and theoretical support for the design of acoustic emission sensors.

Description

Technical Field

[0001] The present invention relates to the technical field of acoustic emission sensors, and particularly to a piezoelectric acoustic emission sensor. Background Art

[0002] In the early stage of fault occurrence, its characteristics are relatively mild, the impact on the monitored system is relatively small, and the maintenance cost is also relatively low. If the fault is not diagnosed or not taken seriously enough in the early stage, when it develops and accumulates to a certain extent, it will trigger major or even catastrophic accidents. Therefore, a signal sensing technology with high sensitivity and filtering characteristics is of great significance for the safe operation of the measured structure.

[0003] At present, fault diagnosis technologies mainly focus on temperature detection methods, vibration detection methods, and oil fluid detection methods. Since temperature detection methods and oil fluid detection methods have low sensitivity to early faults and are not suitable for online detection; vibration detection methods are easily interfered by the noise of the mechanical structure body and have low sensitivity and are not suitable for early state diagnosis. Acoustic emission technology has high-frequency and high-sensitivity characteristics compared with the above technologies, can directly respond to the signals of the faults themselves, and is suitable for early fault diagnosis.

[0004] An acoustic emission sensor is the core and front-end unit for sensing acoustic emission signals, and mainly completes functions such as electromechanical coupling, signal amplification, and filtering of acoustic emission signals. At present, there is no effective design method for the mathematical model of piezoelectric acoustic emission sensors in the prior art. Summary of the Invention

[0005] Embodiments of the present invention provide a piezoelectric acoustic emission sensor to realize the design of acoustic emission sensors with different center frequencies.

[0006] To achieve the above object, the present invention adopts the following technical solutions.

[0007] A piezoelectric acoustic emission sensor, comprising: a matching layer, a piezoelectric element, a backing layer, and an internal circuit connected in sequence;

[0008] The matching layer is used to achieve acoustic impedance matching between the piezoelectric element and the working medium, and design the thickness of the matching layer according to the loss of signal energy during transmission;

[0009] The piezoelectric element is used to capture acoustic emission signals from the working medium through the matching layer, convert the acoustic emission signals into electrical signals, and transmit the electrical signals to the backing layer;

[0010] The backing layer is used to support the piezoelectric acoustic emission sensor, absorb the energy of the acoustic emission signals transmitted by the piezoelectric element, and transmit the acoustic emission signals to the internal circuit.

[0011] The built-in circuit is used to perform band-pass filtering and amplification processing on the acoustic emission electrical signals sensed by the piezoelectric ceramic.

[0012] Preferably, the piezoelectric element includes a piezoelectric ceramic, the resonant frequency of the piezoelectric ceramic is selected as 150 kHz, and the piezoelectric ceramic operates near the resonant frequency.

[0013] Preferably, the backing layer includes epoxy resin, and tungsten powder is added to the epoxy resin matrix.

[0014] Preferably, the built-in circuit includes a receiving circuit and a transmitting circuit. The receiving circuit completes the band-pass filtering and amplification functions of the piezoelectric signal, and the transmitting circuit realizes the functions of acoustic wave emission and sensor self-check.

[0015] Preferably, the receiving circuit is composed of an LTC6230. The operational amplifier uses a differential connection method to amplify the differential mode signal. The transmitting circuit is based on a timer to form a multivibrator and emits square wave pulses with a fixed duty cycle.

[0016] It can be seen from the technical solutions provided by the embodiments of the present invention above that the mathematical model and design method of the piezoelectric acoustic emission sensor in the embodiments of the present invention: solve the problem of sensing early weak fault signals, can realize the early fault diagnosis of the equipment to be measured, provide a mathematical model and theoretical support for the design of acoustic emission sensors, especially for the design of the matching layer thickness, for the first time considers the attenuation of sound waves, explains the problem of matching layer thickness design from a quantitative perspective, and can realize the optimal matching layer thickness corresponding to different materials. The mathematical model and design method proposed by the present invention have high accuracy and can meet the requirements of high efficiency and accuracy in the design of acoustic emission sensors.

[0017] Additional aspects and advantages of the present invention will be given in part in the following description, and these will become apparent from the following description, or can be understood through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0019] Figure 1 It is a schematic structural diagram of an acoustic emission sensor provided by an embodiment of the present invention;

[0020] Figure 2 It is a relationship diagram between the resonant frequency and thickness of a piezoelectric ceramic provided by an embodiment of the present invention;

[0021] Figure 3The sound intensity transmission coefficient t provided by the embodiments of the present invention I Schematic diagram of the model of the matching layer thickness and frequency

[0022] Figure 4 Schematic diagram of the optimal matching layer thickness corresponding to different materials (different attenuation coefficients) provided by the embodiments of the present invention

[0023] Figure 5 Schematic diagram of the influence of damping on the vibration response of the sensor provided by the embodiments of the present invention

[0024] Figure 6 Schematic diagram of the sound absorption effect of the backing layer under different tungsten powder contents provided by the embodiments of the present invention Detailed implementation manners

[0025] The following details the implementation manners of the present invention. The examples of the implementation manners are shown in the drawings, where the same or similar reference numerals represent the same or similar elements or elements with the same or similar functions throughout. The implementation manners described below by referring to the drawings are exemplary and are only used to explain the present invention, and cannot be construed as a limitation to the present invention.

[0026] Those skilled in the art of the present technology can understand that unless specifically stated, the singular forms "a", "an", "the" and "said" used herein may also include the plural forms. It should be further understood that the term "including" used in the description of the present invention means the presence of the described features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or their groups. It should be understood that when we say an element is "connected" or "coupled" to another element, it can be directly connected or coupled to other elements, or there may also be intermediate elements. In addition, the "connection" or "coupling" used herein may include wireless connection or coupling. The phrase "and / or" used herein includes any and all combinations of one or more of the associated listed items.

[0027] Those skilled in the art of the present technology can understand that unless otherwise defined, all terms (including technical terms and scientific terms) used herein have the same meaning as the general understanding of those of ordinary skill in the technical field to which the present invention belongs. It should also be understood that terms such as those defined in a general dictionary should be understood to have a meaning consistent with the meaning in the context of the prior art, and will not be interpreted with an idealized or overly formal meaning unless defined as such here.

[0028] For the convenience of understanding the embodiments of the present invention, the following will further explain with several specific embodiments by referring to the drawings, and each embodiment does not constitute a limitation to the embodiments of the present invention.

[0029] As shown in the schematic diagram of the structure of an acoustic emission sensor provided by an embodiment of the present invention, Figure 1 it includes a piezoelectric element, a matching layer, a backing layer, and an internal circuit. The acoustic emission sensor mentioned in the present invention is designed in various aspects based on the d33 mode of the PZT-5A piezoelectric element.

[0030] The piezoelectric element is the core part of the acoustic emission sensor, which plays the role of accurately and reliably capturing the acoustic emission signal and performing acoustic-electric signal conversion. The piezoelectric element can be a piezoelectric ceramic. By studying the resonance frequency and vibration mode of the cylindrical piezoelectric ceramic, the relationship between the thickness t of the piezoelectric ceramic and the resonance frequency f s can be determined. Since the main frequency band of the acoustic emission signal of the metal material is between 20 kHz and 400 kHz, the resonance frequency of the piezoelectric ceramic is selected as 150 kHz. When the piezoelectric ceramic works near the resonance frequency, more charges are generated and the sensitivity is greater.

[0031] f s = 310.1 - 21.68t (1)

[0032] f s is the resonance frequency of the piezoelectric ceramic (also called the sensor center frequency in the present invention), and t is the thickness of the piezoelectric ceramic.

[0033] Figure 2 As shown in the relationship diagram between the resonance frequency and thickness of a piezoelectric ceramic provided by an embodiment of the present invention, Figure 2 within 5 - 10 mm, the thickness of the piezoelectric ceramic and the resonance frequency are linearly correlated, providing a good theoretical basis for the size selection of the piezoelectric element.

[0034] The matching layer can not only achieve acoustic impedance matching between the piezoelectric element and the working medium, but also effectively protect the piezoelectric element and increase the reliability of use. In the present invention, a single-layer matching layer structure is selected for sensor design. When designing the thickness of the matching layer, first obtain the acoustic intensity transmission coefficient t I without attenuation, and its model with the thickness and frequency of the matching layer is as Figure 3 shown:

[0035]

[0036] t I represents the acoustic intensity transmission coefficient of the matching layer; I t is the acoustic intensity of the transmitted wave; I i is the ratio of the acoustic intensity of the incident wave; p ta is the acoustic pressure of the transmitted wave; p ia is the acoustic pressure of the incident wave; ρ p is the density of the outgoing material; ρ s is the density of the incident material; c pis the sound velocity of the outgoing material; c s is the sound velocity of the outgoing material; Z s is the acoustic impedance of the structure under test; Z m is the acoustic impedance of the matching layer; Z p is the acoustic impedance of the piezoelectric ceramic; k2 is the wave number; D is the acoustic wave propagation distance. Considering that the energy loss during transmission is mainly caused by scattering attenuation and absorption attenuation, and shows an exponential attenuation form, as shown in Equation (3).

[0037] v = Ae -βt (3)

[0038] v is the vibration amplitude of the mass point per unit area; A is the maximum amplitude value; β is the attenuation coefficient; t is the thickness of the matching layer.

[0039] By considering the acoustic wave attenuation law in the matching layer into the matching layer thickness design model, a mathematical model of the sound intensity transmission coefficient with respect to the matching layer thickness and attenuation coefficient can be obtained. Furthermore, the optimal matching layer thickness corresponding to different materials (different attenuation coefficients) can be determined, as Figure 4 shown. From Figure 4 it can be seen that as the material attenuation coefficient continuously increases, the optimal matching layer thickness shows a gradually decreasing trend.

[0040] Finally, when performing vibration analysis on the entire sensor structure, the piezoelectric structure composed of the matching layer and the piezoelectric ceramic is regarded as an ideal elastic body. Under the action of axial force, it undergoes a very small thickness expansion and contraction deformation. The backing layer is pasted and cured on the back of the piezoelectric ceramic, mainly playing a damping role in the vibration of the piezoelectric ceramic. Figure 5 is a schematic diagram of the influence of damping on the vibration response of the sensor provided by an embodiment of the present invention.

[0041] From Figure 3 it can be known that the sound intensity transmission coefficient t I has a certain relationship with the frequency of the acoustic wave and the thickness of the matching layer, where t I is mainly related to the thickness of the matching layer. On this basis, combined with the acoustic wave attenuation law proposed by the present invention, the improved law of the sound intensity transmission coefficient t I and the thickness of the matching layer is obtained. Finally, the relationship between the optimal matching layer thickness and the sound intensity transmission coefficient can be obtained.

[0042] The optimal matching layer thickness is related to the center frequency of the acoustic emission sensor, but in the present invention, the influence is small and can be ignored, mainly related to the inherent characteristics of the material.

[0043] Different center frequencies f sWhen designing an acoustic emission sensor, it is first necessary to select a piezoelectric ceramic material with excellent piezoelectric properties, and then determine a diameter. According to the center frequency, the thickness of the piezoelectric ceramic can be determined. Subsequently, based on the matching layer design theory and matching layer materials, the thickness of the matching layer is determined, and the diameter is slightly larger than that of the piezoelectric ceramic. The backing layer can be determined according to the sound absorption effect of the sensor, which is generally a composite material of tungsten powder and epoxy resin, with a tungsten powder volume fraction of 9%. Finally, the acoustic emission sensor is encapsulated.

[0044] The backing layer not only needs to support the piezoelectric ceramic, but more importantly, it needs to absorb the energy of high-frequency acoustic emission signals to prevent reflection and interference with the electrical signal. When the sound 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 decrease. Therefore, epoxy resin with good sound attenuation performance is selected as the backing layer, and adding tungsten powder to the epoxy resin matrix can effectively increase the acoustic impedance of the backing layer and improve the sound absorption ability of the backing layer, thereby reducing the sound wave reflection between the piezoelectric ceramic and the backing layer. The backing layer can be regarded as damping, and its influence on the frequency and displacement response of the sensor is as Figure 5 shown.

[0045] In determining the backing layer material, the ratio of epoxy resin and tungsten powder is crucial. In this invention, the sound absorption effect of the backing layer under different tungsten powder contents is studied, and the results are as Figure 6 shown. It is found that when the tungsten powder volume fraction is about 9%, the sound attenuation of the backing layer is relatively large, indicating that its sound absorption effect is better.

[0046] Internal circuit design: The electrical signal output by the piezoelectric ceramic is very weak, so it must be amplified. The design of the amplification circuit needs to consider the influence of noise. The acoustic emission sensor contains an internal integrated circuit, which is divided into a receiving circuit and a transmitting circuit. The receiving circuit mainly completes the band-pass filtering and amplification functions of the piezoelectric signal, and the transmitting circuit mainly realizes the functions of sound wave emission and sensor self-test.

[0047] The receiving circuit is mainly composed of LTC6230. The operational amplifier uses a differential connection method, which can effectively suppress the common-mode signal and amplify the differential-mode signal. The transmitting circuit is based on a 555 timer to form a multivibrator, generating a square wave pulse with a fixed duty cycle.

[0048] The above research did not consider strength. Finally, an impact simulation of the sensor is carried out. On the premise of meeting the structural strength requirements, the components of each part are prepared, the technological process is determined, the sensor encapsulation process is determined, and finally the acoustic emission sensor samples are tested.

[0049] During the operation of the device under test, if early faults occur, according to the nature of acoustic emission, the acoustic emission signals generated by early weak faults belong to weak and high-frequency signals. To accurately sense this signal, it means that the acoustic emission sensor must have the characteristics of high frequency and high sensitivity. The acoustic emission sensor mainly includes: piezoelectric ceramics, matching layer and backing layer. Piezoelectric ceramics mainly complete the electromechanical conversion of the weak and high-frequency signals. In order to amplify the weak signal as much as possible, the center frequency (resonant frequency) of the piezoelectric ceramics in design should be consistent with the frequency of the signal, so as to utilize the resonance phenomenon to amplify the signal. For the matching layer, due to the acoustic impedance difference between the structure under test and the piezoelectric ceramics directly, this will reduce the transmission efficiency of sound waves between the interfaces (acoustic intensity transmission coefficient), and by designing a reasonable matching layer, the transmission rate of sound waves can be improved. The backing layer is mainly responsible for absorbing sound waves to prevent reflection and contaminating the original signal.

[0050] The electrical signals generated by the piezoelectric ceramics are amplified and filtered by the built-in circuit and then collected. At this time, the acquired signals are analyzed on the upper computer, and through the analysis of the time-domain parameters and frequency-domain parameters of the signals, the fault discrimination and the assessment of the equipment damage degree are completed.

[0051] To sum up, the acoustic emission method adopted in the embodiment of the present invention is a dynamic detection method. The frequency range of its sensor is generally above 100 kHz, which is much larger than the audio noise and vibration noise generated by the equipment operation. And the detected signals come from the defects of the object under test itself, which can provide real-time information on the changes of early fault defects with load, time, temperature, etc. This technology can detect elastic waves with an amplitude of 10 -14 m, and the detection sensitivity is very high.

[0052] The present invention establishes a mathematical model for each part structure of the acoustic emission sensor, and proposes a design method, which can realize the design of acoustic emission sensors with different center frequencies, and has strong theoretical guiding significance.

[0053] When designing the matching layer of the acoustic emission sensor, considering the actual scenario, the attenuation factor of sound waves is considered. Given the material, the calculation of the optimal matching layer can be realized, and the experimental results show that the model has high accuracy.

[0054] The design process of this method is simple, and the calculation results are accurate, which is very suitable for the design and research and development of acoustic emission sensors.

[0055] Those of ordinary skill in the art can understand that the drawings are only schematic diagrams of an embodiment, and the modules or processes in the drawings are not necessarily essential for implementing the present invention.

[0056] Each embodiment in this specification is described in a progressive manner. For the same or similar parts among the embodiments, reference can be made to each other. Each embodiment focuses on the differences from other embodiments. In particular, for the device or system embodiments, since they are basically similar to the method embodiments, they are described relatively simply. For the relevant parts, reference can be made to the partial description of the method embodiments. The device and system embodiments described above are only illustrative. The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed to multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this embodiment. A person of ordinary skill in the art can understand and implement it without creative efforts.

[0057] As mentioned above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A piezoelectric acoustic emission sensor, characterized in that, Comprising: A matching layer, a piezoelectric element, a backing layer, and an internal circuit connected in sequence; The said matching layer is used to achieve acoustic impedance matching between the piezoelectric element and the working medium, and the thickness of the matching layer is designed according to the loss of signal energy during transmission; The said piezoelectric element is used to capture acoustic emission signals from the working medium through the matching layer, convert the acoustic emission signals into electrical signals, and transmit the electrical signals to the backing layer; The said backing layer is used to support the piezoelectric acoustic emission sensor, absorb the energy of the acoustic emission signals transmitted by the piezoelectric element, and transmit the acoustic emission signals to the internal circuit, The said internal circuit is used to perform band-pass filtering and amplification processing on the acoustic emission electrical signals sensed by the piezoelectric ceramics; A single-layer matching layer structure is selected for the sensor design. When designing the thickness of the matching layer, the sound intensity transmission coefficient t without attenuation is first obtained I : t I represents the sound intensity transmission coefficient of the matching layer; I t is the sound intensity of the transmitted wave; I i is the ratio of the sound intensity of the transmitted wave to that of the incident wave; p ta is the sound pressure of the transmitted wave; p ia is the sound pressure of the incident wave; ρ p is the density of the outgoing material; ρ s is the density of the incident material; c p is the sound velocity of the outgoing material; c s is the sound velocity of the outgoing material; Z s is the acoustic impedance of the structure under test; Z m is the acoustic impedance of the matching layer; Z p is the acoustic impedance of the piezoelectric ceramic; k2 is the wave number; D is the acoustic wave propagation distance, and the loss of energy during transmission is caused by scattering attenuation and absorption attenuation, and shows an exponential attenuation form; Taking the acoustic wave attenuation law in the matching layer into account in the matching layer thickness design model, a mathematical model of the sound intensity transmission coefficient with respect to the matching layer thickness and attenuation coefficient is obtained, as shown in Equation (3); v = Ae -βt (3) v is the vibration amplitude of the mass point per unit area, A is the maximum amplitude value, β is the attenuation coefficient, t is the thickness of the matching layer, and then the optimal matching layer thickness corresponding to different materials is determined; The said internal circuit includes a receiving circuit and a transmitting circuit. The receiving circuit completes the band-pass filtering and amplification functions of the piezoelectric signal, and the transmitting circuit realizes the functions of acoustic wave emission and sensor self-check. The said receiving circuit consists of LTC6230. The operational amplifier uses a differential connection method to amplify the differential-mode signal. The transmitting circuit is based on a timer to form a multivibrator to emit square wave pulses with a fixed duty cycle; The piezoelectric element described above includes piezoelectric ceramics. The relationship between the thickness t of the piezoelectric ceramics and the resonance frequency f s is as follows: f s = 310.1 - 21.68t (1) f s is the resonant frequency of the piezoelectric ceramic, t is the thickness of the piezoelectric ceramic. The resonant frequency of the piezoelectric ceramic is selected as 150 kHz, and the piezoelectric ceramic operates near the resonant frequency during operation; The said backing layer includes epoxy resin, and tungsten powder is added to the epoxy resin matrix. The volume fraction of the tungsten powder is 9%.

Images