阅读说明：本技术 一种变模态磁集中器式兰姆波电磁声换能器 (Variable-mode magnetic concentrator type lamb wave electromagnetic acoustic transducer ) 是由 刘增华 邓黎明 张永琛 何存富 吴斌 于 2019-05-29 设计创作，主要内容包括：本发明公开了一种变模态式磁集中器式兰姆波电磁声换能器,该换能器包括钕铁硼磁铁,A类磁集中器,B类磁集中器,柔性电路板中的双层变间距回折线圈。将线圈放置在铝板表面,磁集中器与的上表面与钕铁硼磁铁吸合后固定在线圈正上方,当选用A类磁集中器时,控制磁集中器下表面放置在线圈中心距为d<Sub>1</Sub>的线圈上,则可激励主模态为A<Sub>0</Sub>模态的导波；当选用B类磁集中器时,控制磁集中器下表面放置在线圈中心距为d<Sub>2</Sub>的线圈上,则可激励主模态为S<Sub>0</Sub>模态的导波。通过频率特性测试验证了所研制的变模态磁集中器式兰姆波电磁声换能器具有较好的频率响应特性,同时验证了换能器设计的合理性,实现对板结构的结构健康监测。(The invention discloses a variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer, which comprises a neodymium iron boron magnet, a class A magnetic concentrator, a class B magnetic concentrator and a double-layer variable-pitch inflection coil in a flexible circuit board, wherein the coil is placed on the surface of an aluminum plate, the upper surface of the magnetic concentrator and the neodymium iron boron magnet are fixed right above the coil after being attracted, when the class A magnetic concentrator is selected, the lower surface of the magnetic concentrator is controlled to be placed on the coil with the coil center distance of d 1 , so that guided waves with the main mode of A 0 can be excited, when the class B magnetic concentrator is selected, the lower surface of the magnetic concentrator is controlled to be placed on the coil with the coil center distance of d 2 , so that guided waves with the main mode of S 0 can be excited, the developed variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer has good frequency response characteristics through frequency characteristic tests, meanwhile, the rationality of the design of the transducer is verified, and the structural health monitoring of a plate structure is realized.)

1. A variable-mode magnetic concentrator type lamb wave electromagnetic acoustic transducer comprises a rubidium-iron-boron magnet (1), a magnetic concentrator (2) and a multi-cluster circular inflection coil (3) in a flexible circuit board; the method is characterized in that: the neodymium iron boron magnet (1) is a cylindrical permanent magnet; the magnetic concentrator (2) is composed of an upper cylindrical body and a lower four circular columns; the cylindrical top surface of the magnetic concentrator (2) is attracted with the neodymium iron boron magnet (1), and the contact surfaces are superposed; a plurality of clusters of circular folding coils (3) in the flexible circuit board are arranged at the lower end of the magnetic concentrator (2), and the bottom surface of the magnetic concentrator (2) is superposed with the plurality of clusters of circular folding coils (3) in the flexible circuit board; a plurality of clusters of circular folding coils (3) in the flexible circuit board are placed on an aluminum plate, and the magnetic concentrator (2) and the neodymium iron boron magnet (1) are fixed right above the folding coils (3).

2. A variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer according to claim 1 in which: in the variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer, a rectangular permanent magnet provides a static magnetic field, the magnetic field is guided and concentrated by a magnetic concentrator, coils generate eddy currents in a tested piece, the eddy currents and the static magnetic field act together to generate Lorentz force, and the center distance of the magnetic field in contact with the coils is changed by different magnetic concentrators, so that the wavelength of a generated signal is changed, and the mode of excited lamb waves is changed.

3. A variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer according to claim 1, characterized in that the coil spacing d ₁ of the double-layer variable-spacing folded coil (4) in the flexible circuit board is half of the wavelength of A ₀ mode satisfying the constructive interference at 272kHz, namely, the A ₀ mode satisfying the constructive interference can be excited by applying a static magnetic field perpendicular to the eddy current direction generated by the wires to the two wires with the d ₁ as the center distance.

4. The variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer according to claim 1, wherein the double-layer variable-pitch meander coil (4) in the flexible circuit board has a coil pitch d ₂ of half the wavelength of the S ₀ mode satisfying the constructive interference at 272kHz, that is, the S ₀ mode satisfying the constructive interference can be excited by applying a static magnetic field perpendicular to the direction of the eddy current generated by the two wires with the center distance d ₂.

5. A variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer according to claim 1 in which: the double-layer variable-pitch inflection coil (4) in the flexible circuit board adopts a double-layer wiring mode, so that the amplitude of a signal excited by the transducer is improved.

6. a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer according to claim 1 in which: the coil pitch of the double-layer variable-pitch inflection coil (4) in the flexible circuit board is half of the excited wavelength, and the transducer meets the principle of constructive interference, so that the amplitude of an excited signal of the transducer is improved.

7. A variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer according to claim 1 in which: the transducer has acoustic field directivity and frequency response characteristics.

Technical Field

The invention relates to a variable-mode magnetic concentrator type lamb wave electromagnetic acoustic transducer, which belongs to the field of ultrasonic nondestructive testing, and can excite different-mode guided waves in a plate structure under the condition that a series of parameters such as a coil, a magnet, excitation frequency and the like are not changed and only different magnetic concentrators are changed.

Background

Sheet metal structures are widely used in aerospace, civil engineering, machinery and other fields. In the production and use processes, the metal plate structure can be inevitably damaged, and the service life of the material is greatly reduced. In order to ensure the safety of the use of the metal plate structure, it is necessary to perform nondestructive inspection on the metal plate. In the traditional ultrasonic detection technology, the piezoelectric ultrasonic technology is an important component of the traditional ultrasonic detection technology and is widely applied in the early development stage of the ultrasonic detection technology, but the piezoelectric ultrasonic technology has the defects of high requirements on test pieces, complex required pretreatment technology, need of a coupling agent and the like, so that the detection cost, complexity and limitation are greatly increased. As a novel nondestructive testing technology, when an Electromagnetic Acoustic Transducer (EMAT) is used for nondestructive testing, a coupling agent is not needed, the application range is wide, and non-contact testing and online monitoring can be performed on a test piece.

At present, EMAT is mainly adopted to excite the lowest-order Lamb waves and SH waves, including A ₀, S ₀ and SH ₀ modes, a plate structure is detected and imaged, the low-frequency band A ₀ mode of the A ₀ mode EMAT mainly takes out off-plane displacement, and the magnetostrictive effect is difficult to generate magnetostrictive force perpendicular to the surface of a plate, so that the currently developed A ₀ mode EMAT is based on a Lorentz force mechanism, Guo and the like utilize a retrace coil and a permanent magnet providing a vertical magnetic field to excite a single A ₀ mode with directivity in an aluminum plate and realize the detection of groove defects in the aluminum plate.

Disclosure of Invention

the invention aims to design a variable-mode magnetic concentrator type lamb wave electromagnetic acoustic transducer, namely, a variable-mode magnetic concentrator is added on the basis of the traditional electromagnetic acoustic transducer, so that the center distance of a static magnetic field at a part in contact with a coil is changed, the center distance of generated Lorentz force is changed, the wavelength of a generated signal is changed, and the proportion of modes in the excited signal is changed. By utilizing the variable-mode magnetic concentrator type lamb wave electromagnetic acoustic transducer, under the condition that a series of parameters such as a coil, a magnet, excitation frequency and the like are not changed and different magnetic concentrators are only changed, the variable-mode magnetic concentrator type lamb wave electromagnetic acoustic transducer changes the condition that different modes of guided waves are excited in a plate structure and can be used for realizing the structural health monitoring of the plate structure.

In order to achieve the purpose, the invention adopts the following design scheme:

A variable-mode magnetic concentrator type lamb wave electromagnetic acoustic transducer comprises a neodymium iron boron magnet 1, a class A magnetic concentrator 2, a class B magnetic concentrator 3 and a double-layer variable-interval inflection coil 4 in a flexible circuit board, and is characterized in that the neodymium iron boron magnet 1 is a cuboid magnet, the lower surface of the magnet is attracted with the upper surface of the magnetic concentrator, the material of the class A magnetic concentrator 2 is defined as ferrite, the shapes of the two magnetic concentrators are basically consistent, the upper part of the class A magnetic concentrator is cuboid, the lower part of the class A magnetic concentrator is composed of two cuboids, the difference is that the distance between the two cuboids on the lower part of the class A magnetic concentrator is different, the double-layer variable-interval inflection coil 4 in the flexible circuit board is placed on the surface of an aluminum plate, the class A magnetic concentrator 2 and the neodymium iron boron magnet are fixed right above the coil 4, the relative position of the bottom surface of the class A magnetic concentrator 2 and the coil is adjusted, guided waves of the A mode ₀ can be excited, the double-layer variable-interval inflection coil 4 in the flexible circuit board is placed on the surface of the aluminum plate, the class B magnetic concentrator 3 and the B magnetic concentrator 4 are adjusted and the relative position of the B magnetic concentrator and the B magnetic concentrator is opposite to the guided waves.

The variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer is characterized in that: the ndfeb magnet 1 is a rectangular parallelepiped, and is polarized in the direction of the height of the magnet, so that the magnetic field lines are transmitted in the direction of the height of the magnet (clockwise or counterclockwise).

The variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer is characterized in that a class A magnetic concentrator 3 is made of ferrite, the length and the width of a cuboid at the upper part are respectively equal to those of a cuboid neodymium iron boron magnet, the center distance between two cuboids at the lower part is equal to a coil interval l ₄ in a coil 4, the top surface of the magnetic concentrator 3 is attracted with the bottom surface of the neodymium iron boron magnet 1, and after the magnetic concentrator is added, a magnetic field is guided and concentrated by the magnetic concentrator, so that guided waves with a main mode of A ₀ can be excited.

The variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer is characterized in that a B-type magnetic concentrator 4 is made of ferrite, the length and the width of a cuboid at the upper part are respectively equal to those of a cuboid neodymium-iron-boron magnet, the center distance between two cuboids at the lower part is equal to a coil interval l ₇ in a coil 4, the top surface of a magnetic concentrator 3 is attracted with the bottom surface of a neodymium-iron-boron magnet 1, and after the magnetic concentrator is added, a magnetic field is guided and concentrated by the magnetic concentrator, so that guided waves with a main mode of S ₀ can be excited.

The variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer is characterized in that: the double-layer variable-pitch inflection coil 4 in the flexible circuit board adopts a double-layer wiring mode, and changes the center distance of a magnetic field of a part in contact with the coil through different magnetic concentrators, so that the wavelength of a generated signal is changed, and the mode of excited lamb waves is changed.

The invention can obtain the following beneficial effects:

1. In a variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer, a rectangular permanent magnet provides a static magnetic field, the magnetic field is guided and concentrated through a magnetic concentrator, coils generate eddy currents in a tested piece, the eddy currents and the static magnetic field act together to generate Lorentz force, and the center distance of the magnetic field in contact with the coils is changed through different magnetic concentrators, so that the wavelength of a generated signal is changed, and the mode of excited lamb waves is changed;

2. the wavelength of A ₀ mode meeting constructive interference is half of that of a double-layer variable-pitch inflection coil 4 in the flexible circuit board when the coil pitch d ₁ is 272kHz, namely a static magnetic field perpendicular to the eddy current direction generated by two wires taking d ₁ as the center distance is applied to the two wires, so that the A ₀ mode meeting the constructive interference can be excited;

3. the coil spacing d ₂ of the double-layer variable-spacing folded coil 4 in the flexible circuit board is half of the wavelength of the S ₀ mode which meets the constructive interference under 272kHz, namely a static magnetic field which is vertical to the eddy current direction generated by the leads is applied to the two leads which take d ₂ as the center distance, so that the S ₀ mode which meets the constructive interference can be excited;

4. The double-layer variable-pitch inflection coil 4 in the flexible circuit board adopts a double-layer wiring mode to improve the amplitude of a signal excited by the transducer;

5. The coil pitch of the double-layer variable pitch inflection coil 4 in the flexible circuit board is half of the excited wavelength, and the transducer meets the principle of constructive interference, so that the amplitude of an excited signal of the transducer is improved;

6. the sound field directivity and the frequency response characteristic of the transducer are good.

Drawings

Fig. 1 is an exploded schematic view of a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer, a) a transducer structure diagram when an excitation mode is a ₀, and b) a transducer structure diagram when the excitation mode is a ₀;

FIG. 2 is a schematic view of a rubidium-iron-boron magnet;

FIG. 3 is a schematic view of a class A magnetic concentrator: a) a front view of a class A magnetic concentrator, b) a top view of a class A magnetic concentrator;

FIG. 4 is a schematic view of a class B magnetic concentrator: a) a front view of a class B magnetic concentrator, B) a top view of a class B magnetic concentrator;

FIG. 5 is a schematic diagram of a dual layer variable pitch meander coil in a flexible circuit board;

in FIG. 6, a) and b) are ultrasonic guided wave group velocity and phase velocity dispersion curves of an aluminum plate with the thickness of 1mm respectively;

FIG. 7 is a schematic diagram of an experimental system for a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer;

FIG. 8 is a schematic diagram of an experimental system for an electromagnetic acoustic transducer;

FIG. 9 illustrates a received signal of a conventional electromagnetic acoustic transducer at an excitation frequency of 272 kHz;

FIG. 10 is a schematic of a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer using a received signal from a class A magnetic concentrator at an excitation frequency of 272 kHz;

FIG. 11 is a schematic of a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer using the received signal of a class B magnetic concentrator at an excitation frequency of 272 kHz;

FIG. 12 is a graph of the rate response characteristics of a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer using a class A magnetic concentrator;

FIG. 13 is a graph of the rate response characteristics of a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer using a class B magnetic concentrator;

FIG. 14 is a schematic diagram of a transducer arrangement for a directivity experiment of a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer;

FIG. 15 is a graph of acoustic field directivity for a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer using a class A magnetic concentrator;

FIG. 16 is a graph of acoustic field directivity for a variable mode magnetic concentrator lamb wave electromagnetic acoustic transducer using a class B magnetic concentrator;

In the figure: 1. neodymium iron boron magnet, 2, A type magnetic concentrator, 3, B type magnetic concentrator, 4, double-layer variable-pitch inflection coil in the flexible circuit board, 5, high-energy pulse excitation receiving device RPR4000, 6, digital oscilloscope, 7, excitation end impedance matching module, 8, receiving end impedance matching module, 8, variable-mode magnetic concentrator type lamb wave electromagnetic sound excitation transducer, 9, variable-mode magnetic concentrator type lamb wave electromagnetic sound receiving transducer, 10, aluminum plate, 11, traditional electromagnetic sound excitation transducer, 12, traditional electromagnetic sound receiving transducer.

Detailed Description

The invention is further illustrated by the following figures and examples.

Based on the Lorentz force effect, a variable-mode magnetic concentrator type electromagnetic acoustic transducer is designed, and the transducer is utilized to excite single-mode A ₀ mode guided wave S ₀ mode guided wave on an aluminum plate.

a structural schematic diagram of a variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer is shown in fig. 1, wherein a) a transducer structure schematic diagram when an excitation mode is a ₀, B) a transducer structure schematic diagram when an excitation mode is a ₀, the variable-mode magnetic concentrator electromagnetic acoustic transducer comprises a neodymium-iron-boron magnet 1, a class-a magnetic concentrator 2, a class-B magnetic concentrator 3 and a double-layer variable-pitch inflection coil 4 in a flexible circuit board, a detection object is an aluminum plate with a specification of 1000 × 1000 × 1 (unit: mm), fig. 6 is an ultrasonic guided wave group velocity and phase velocity frequency dispersion curve of the aluminum plate, a coil center distance of 10mm, namely a signal wavelength of 20mm is selected according to the principle, in fig. 6B), the slope is the wavelength, and therefore a straight line with a slope of 20 is drawn to observe an intersection point of modes 865 and S ₀, the horizontal coordinate of the point is S ₀ mode satisfying phase length interference, namely the theoretical center frequency of the mode of S ₀ of an excitation signal, the theoretical center frequency v _g of the theoretical group mode of the mode is observed in 6a) of the mode through the theoretical center frequency, so that the theoretical center frequency of the mode is observed in a) for flight, the calculation of the variable-mode, the variable-frequency calculation of the double-mode electromagnetic acoustic transducer is used for calculating the calculation of the transition of the theoretical center frequency of the double-layer electromagnetic wave electromagnetic acoustic transducer, the transition of the excitation signal, the theoretical center distance from the theoretical center of the excitation mode under the theoretical center of the transition of.

as shown in fig. 2, the length l ₁, l ₂ and h ₁ of the rectangular parallelepiped magnet are 25mm, 25mm and 15mm, respectively.

the class-a magnetic concentrator is shown in fig. 3, where a) is a front view of the magnetic concentrator, b) is a top view, the upper portion of the magnetic concentrator is a rectangular parallelepiped with equal length and width, l ₃ ═ ₆ ═ 25mm, h ₂ ═ 4.5mm in height, the lower portion is two rectangular parallelepipeds with equal length, width and height, l ₅ ═ 3mm, h ₃ ═ 5mm in height, and the center distance l ₄ of the two rectangular parallelepipeds is 4.5 mm.

The class-B magnetic concentrator is shown in fig. 3, wherein a) is a front view of the magnetic concentrator, B) is a top view, the sizes of the class-A magnetic concentrator and the class-B magnetic concentrator are completely consistent except that the center distances of two cuboids at the lower parts of the class-A magnetic concentrator and the class-B magnetic concentrator are different, and the center distance l ₇ of the two cuboids at the lower part of the class-B magnetic concentrator is 10 mm.

as shown in fig. 5, the double-layer variable-pitch folded coil 4 in the flexible circuit board has a wire width of 0.2mm, and adopts a rectangular folded wiring mode, and double-layer wiring, wherein the coil center distance d ₁ is 4.5mm, and the coil center distance d ₂ is 10 mm.

the experimental system is shown in fig. 6 and comprises a high-energy pulse excitation receiving device RPR40004, a digital oscilloscope 5, an excitation end impedance matching module 6, a receiving end impedance matching module 7, an excitation transducer 8, a receiving transducer 9 and an aluminum plate 10. The RPR40004 can generate a high-energy excitation signal, and the digital oscilloscope 5 is used for observing and storing the signal; the impedance matching modules 6 and 7 have the function of enabling the transducer coil to obtain maximum energy and improving the transduction efficiency of the transducer.

the magnetic concentrator is characterized in that a neodymium iron boron magnet 1 is a cuboid magnet, the lower surface of the magnet is attracted with the upper surface of a magnetic concentrator, a class A magnetic concentrator 2 and a class B magnetic concentrator 3 are made of ferrite, the shapes of the two magnetic concentrators are basically consistent, the upper part of the class A magnetic concentrator is a cuboid, the lower part of the class A magnetic concentrator is composed of two cuboids, the distance between the two cuboids is different, a double-layer variable-pitch inflection coil 4 in a flexible circuit board is placed on the surface of an aluminum plate, the class A magnetic concentrator 2 and a neodymium iron boron magnet are fixed right above the coil 4, the relative position of the bottom surface of the class A magnetic concentrator 2 and the coil is adjusted, guided waves of an A ₀ mode can be excited, the double-layer variable-pitch inflection coil 4 in the flexible circuit board is placed on the surface of the aluminum plate, the class B magnetic concentrator 3 and the neodymium iron boron magnet are fixed right above the coil 4, the relative position of the bottom surface of the class B magnetic concentrator 2 and the coil is adjusted, and guided waves of an S35.

The experimental system is shown in fig. 7 and comprises a high-energy pulse excitation receiving device RPR40005, a digital oscilloscope 6, an excitation end impedance matching module 7, a receiving end impedance matching module 8, a variable-mode magnetic concentrator type electromagnetic sound excitation transducer 9, a variable-mode magnetic concentrator type electromagnetic sound receiving transducer 10 and an aluminum plate 11. The RPR40005 can generate high-energy excitation signals, and the digital oscillograph 6 is used for observing and storing the signals; the impedance matching modules 7 and 8 are used for enabling the transducer coil to obtain maximum energy and improving the transduction efficiency of the transducer.

1) Modal testing

_{0 0 0 0 0 0 0 0 0 0 0 0 0}in order to verify the feasibility of the variable-mode magnetic concentrator electromagnetic acoustic transducer, a mode test experiment is performed on the variable-mode magnetic concentrator electromagnetic acoustic transducer, the schematic diagram of the experimental system is shown in fig. 7 and 8, wherein fig. 7 is a schematic diagram of the experimental system of the variable-mode magnetic concentrator electromagnetic acoustic transducer, fig. 8 is a schematic diagram of the experimental system of the conventional electromagnetic acoustic transducer, except for the difference between the excitation and receiving transducers, the other conditions are the same, the excitation transducer is 325mm away from the left end face of the aluminum plate, the distance between the excitation transducer and the receiving transducer is 200mm, the receiving transducer is 475mm away from the right end face of the aluminum plate, the excitation transducer and the receiving transducer are on the same horizontal line and 300mm away from the rear end face of the aluminum plate, the excitation signal is a five-cycle sine wave modulated by a hanning window in the experiment, the excitation frequency is 272kHz, an excitation frequency is adopted in an excitation-receiving experiment mode, the amplitude of the conventional electromagnetic acoustic transducer, the variable-mode a type magnetic concentrator blue-wave electromagnetic acoustic transducer and the variable-type magnetic concentrator blue-type electromagnetic acoustic wave electromagnetic transducer is used for collecting the direct wave signals of the conventional electromagnetic acoustic signals of the variable-mode packet, the electromagnetic acoustic transducer of the variable-type concentrator blue-type electromagnetic acoustic-type acoustic transducer, and acoustic-type acoustic transducers, and acoustic-type acoustic-.

2) Frequency response characteristic test

Carry out the frequency scanning experiment respectively to above-mentioned two kinds of transducers, the experimental system schematic diagram is shown in fig. 7, and wherein the excitation transducer is apart from aluminum plate left end face 325mm, and the interval of excitation transducer and receiving transducer is 200mm, and the receiving transducer is apart from aluminum plate right-hand member face 475mm, and excitation transducer and receiving transducer are on same water flat line and apart from aluminum plate rear end face 300 mm. In the experiment, the excitation signal is a five-period sine wave modulated by a Hanning window, the initial frequency of the frequency scanning experiment in the experiment is 172kHz, the frequency is increased to 372kHz by taking 10kHz as a step length, the signal amplitude is observed and normalized in an experiment mode of one excitation and one receiving, and the frequency response characteristic of the transducer is shown in figures 12 and 13.

The frequency response characteristic of the variable-mode class-a magnetic concentrator lamb wave electromagnetic acoustic transducer is shown in fig. 12, a circle is a normalized amplitude of a direct wave signal at each frequency acquired through experiments, a curve is first-order gaussian fitting of a series of normalized amplitudes, the curve can be obtained through the first-order gaussian fitting, the actual central frequency of the variable-mode class-a magnetic concentrator lamb wave electromagnetic acoustic transducer is 270kHz, the actual central frequency is quite close to the theoretical central frequency of 272kHz, and the relative error is 0.7%. Similarly, in fig. 13, the actual center frequency of the variable mode B type magnetic concentrator lamb wave electromagnetic acoustic transducer is 268kHz, which is very close to the theoretical center frequency of 272kHz, with a relative error of 1%. Therefore, the developed variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer has better frequency response characteristics, and simultaneously, the rationality of the design of the transducer is verified.

3) Sound field directivity test

In order to verify whether the sound field directivity of the variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer is good or not, a sound field directivity experiment is performed on the variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer, the arrangement diagram of transducers of the sound field directivity experiment is shown in fig. 14, receiving transducers are uniformly placed on a semicircle with the excitation transducer as the center of a circle and the radius of 200mm, and the placing rules of the receiving transducers are as follows: the method comprises the steps of uniformly placing the transducers from 0 DEG to 180 DEG in a 5-degree step length mode, namely placing the receiving transducers once every 5 DEG, wherein in an experiment, an excitation signal is a five-period sine wave modulated by a Hanning window, the excitation frequency is 272kHz, extracting signal amplitudes received by the receiving transducers at all angles by adopting an experiment mode of one excitation and one receiving, and carrying out normalization processing so as to observe the sound field directivity of the variable-mode magnetic concentrator type lamb wave electromagnetic acoustic transducer, wherein an A-type magnetic concentrator and a B-type magnetic concentrator are respectively shown in figures 15 and 16, a circle is the normalized amplitude of the signal collected in the experiment at all angles, and black circles are connected to form a blue broken line. As can be seen from fig. 15 and 16, the variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer has a high normalized amplitude in the range of 75 ° to 105 °, so that the directivity of the variable-mode magnetic concentrator lamb wave electromagnetic acoustic transducer is good.