Electromagnetic ultrasonic transducer with annular coil structure and design method thereof

Abstract

The invention relates to the technical field of electromagnetic ultrasonic detection, in particular to an electromagnetic ultrasonic transducer of an annular coil structure and a design method of the electromagnetic ultrasonic transducer of the annular coil structure. The electromagnetic ultrasonic transducer of the annular coil structure comprises a cylindrical permanent magnet and a PCB annular array coil, circumferential and axial static bias magnetic fields are provided; the PCB annular array coil is composed of a first coil and a second coil which are located on the same axial plane, the first coil and the second coil are annular coils, the first coil and the second coil are connected in an end-to-end mode and are wound in a double-layer mode, the head end and the tail end of each coil are connected with an external wire respectively, and the wires guide external excitation current. The electromagnetic ultrasonic transducer of the annular coil structure can control the phase length and destructive interference of symmetric and antisymmetric modes in Lamb waves according to the mode of controlling the width of the coil, namely the number of turns and the like, and excitation of the omnidirectional single-mode Lamb waves is achieved.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic ultrasonic testing technology, and in particular to an electromagnetic ultrasonic transducer with a ring coil structure and its design method. Background Technology

[0002] For large-area structures such as aluminum plates, the surface is often coated with dust or water, making traditional piezoelectric ultrasonic testing difficult to adapt to on-site inspection due to the need for coupling agents and surface treatments. Electromagnetic Acoustic Transducers (EMATs), as a newly developed ultrasonic testing device, directly generate ultrasonic waves inside a metal conductor through electromagnetic coupling, offering advantages such as non-contact measurement and no need for surface treatment. Using EMAT arrays not only enables efficient and rapid inspection of large plate structures but also allows for the visualization of defects in the structure through various imaging algorithms.

[0003] However, due to the multimodal characteristics of guided waves, multiple modes are usually generated simultaneously when Lamb waves are excited, which brings difficulties to defect signal extraction and imaging. Electromagnetic ultrasonic guided wave transducers are the front end of guided wave nondestructive testing systems, and their performance is crucial to the efficiency and quality of guided wave testing. Therefore, researching electromagnetic ultrasonic transducers capable of exciting a single guided wave mode is of great significance. Summary of the Invention

[0004] The purpose of this invention is to provide an electromagnetic ultrasonic transducer with a ring coil structure and its design method, which solves the problem that existing electromagnetic ultrasonic transducers are difficult to efficiently excite and receive omnidirectional single-mode Lamb waves.

[0005] To achieve the above objectives, the present invention provides an electromagnetic ultrasonic transducer with a ring coil structure. The electromagnetic ultrasonic transducer with a ring coil structure includes a cylindrical permanent magnet and a PCB ring array coil. The PCB ring array coil is disposed below the cylindrical permanent magnet. The cylindrical permanent magnet is a high-permeability neodymium iron boron magnet used to provide circumferential and axial static bias magnetic fields. The PCB ring array coil is composed of a first coil and a second coil in the same plane. Both the first coil and the second coil are ring coils. The first coil and the second coil are connected end to end and are double-wound. The beginning and end ends of the PCB ring array coil are respectively connected to external wires, which guide external excitation current.

[0006] The cylindrical permanent magnet is magnetized in the axial direction, and the static bias magnetic field it generates includes an axial magnetic field perpendicular to the surface of the conductive specimen and a circumferential magnetic field along the surface of the conductive specimen.

[0007] The coils in the PCB ring array are wound with wires according to the diameter of each coil. The first coil and the second coil are wound in opposite directions, and the number of coil turns is evenly distributed on the top and bottom layers. The top and bottom layers are connected by vias. Specifically:

[0008] The top layer coil is wound from the outside in. The outer coil is wound clockwise / counterclockwise, and the inner coil is wound counterclockwise / clockwise. The inner coil is connected to the bottom layer by a via.

[0009] The bottom coil is wound from the inside out, and the two coils are wound in the same direction as the top coil.

[0010] When alternating current is passed through the coil, the current first flows in from the outside to the inside through the top layer, and then flows out from the inside to the outside through the bottom layer.

[0011] The innermost coil of the PCB ring array has a diameter of half a wavelength, and the spacing between adjacent coils is also half a wavelength.

[0012] The PCB ring array coil has pads at both ends for connecting to external wires.

[0013] In the electromagnetic ultrasonic transducer with the ring coil structure, after an alternating excitation current is applied, the PCB ring array coils, under the action of a static bias magnetic field, generate corresponding S0 / A0 mode Lamb waves with the same amplitude but opposite phase. The two waves are superimposed to form destructive interference, so that the amplitude of one mode wave is reduced to 0; the other mode wave forms constructive interference, and the amplitudes are added to achieve the effect of enhancing the amplitude, so that there is only a single mode wave.

[0014] The electromagnetic ultrasonic transducer with the ring coil structure generates Lorentz force in a non-ferromagnetic conductive test specimen in a non-contact manner, and generates omnidirectional Lamb waves through the Lorentz force.

[0015] The electromagnetic ultrasonic transducer with the ring coil structure generates Lorentz force and magnetostrictive force in the ferromagnetic conductive test specimen in a non-contact manner, and generates omnidirectional Lamb waves through the combined action of the two.

[0016] This invention also provides a design method for an electromagnetic ultrasonic transducer with a ring coil structure, applicable to the electromagnetic ultrasonic transducer with a ring coil structure as described above, comprising the following steps:

[0017] The dispersion curve of the Lamb wave is calculated based on the material being tested. The center frequency f of the desired Lamb wave is determined, and the phase velocity C corresponding to the frequency-thickness product is found on the dispersion curve. p ;

[0018] The wavelength of the sound wave is calculated based on the phase velocity and center frequency, C. p / f= The diameters of the first and second coils are determined based on the wavelength: D1 = D2-D1= ;

[0019] Based on the magnetic field distribution of the cylindrical permanent magnet, the magnetic field component B along the coil diameter is obtained. r B z ;

[0020] Under the above parameters, calculate the frequency-displacement curves of A0 and S0 modes. By examining the frequency-displacement curves, observe the displacement amplitude ratio of A0 to S0 under these parameters. Adjust the coil wire width to minimize the amplitude of the S0 / A0 mode at the center frequency and maximize the amplitude of the A0 / S0 mode.

[0021] Based on the obtained parameters, the PCB coil is drawn, and the specific structure design of the PCB ring array coil of the electromagnetic ultrasonic transducer with the ring coil structure is completed.

[0022] The design method of the electromagnetic ultrasonic transducer with the ring coil structure also includes the following preliminary steps: assuming that the eddy current distribution induced by the tightly wound helical coil and the ring coil is the same, and that the coil width is the product of the number of coil turns and the wire width; the width of the Lorentz force is the same as the coil width, and since the current flowing through the coil is the same, the Lorentz force is proportional to the bias magnetic field strength at the location of the coil; the magnitude and direction of the average bias magnetic field at each point within the coil width range are the same as the magnitude and direction of the bias magnetic field of the coil diameter.

[0023] The PCB ring array coil can be expanded from two coils to multiple coils to improve the excitation efficiency of the transducer.

[0024] This invention discloses an electromagnetic ultrasonic transducer with a ring coil structure and its design method. The ring coil structure electromagnetic ultrasonic transducer includes a cylindrical permanent magnet and a PCB ring array coil. The cylindrical permanent magnet is a high-permeability neodymium iron boron magnet, providing circumferential and axial static bias magnetic fields. The PCB ring array coil consists of a first coil and a second coil in the same plane. Both the first coil and the second coil are ring coils, connected end-to-end and double-wound. The beginning and end ends of the coils are connected to external wires, which guide external excitation current. The ring coil structure electromagnetic ultrasonic transducer can control the constructive and destructive interference of symmetrical and antisymmetric modes in the Lamb wave by controlling the coil width (i.e., the number of turns), thereby achieving excitation of an omnidirectional single-mode Lamb wave. Furthermore, since this design method is not specific to any particular structure, a corresponding single-mode Lamb wave EMAT can be designed at any desired excitation center frequency, providing a new design approach for achieving efficient excitation of omnidirectional single-mode Lamb waves. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram illustrating the structural principle of the electromagnetic ultrasonic transducer with a ring coil structure provided by the present invention.

[0027] Figure 2 This is a diagram showing the bias magnetic field distribution of a cylindrical permanent magnet provided by the present invention.

[0028] Figure 3 This is a displacement-frequency curve of the electromagnetic ultrasonic transducer with a ring coil structure provided by the present invention.

[0029] Figure 4 This is a comparison chart of displacement amplitudes excited by EMAT provided by the present invention.

[0030] Figure 5 This is a structural diagram of the electromagnetic ultrasonic transducer with a ring coil structure (a PCB ring array coil has three coil structures) provided by the present invention.

[0031] Figure 6 This is a displacement-frequency curve of the electromagnetic ultrasonic transducer with a ring coil structure (a PCB ring array coil has three coil structures) provided by the present invention.

[0032] Figure 7 This is a displacement waveform diagram excited by the electromagnetic ultrasonic transducer with a ring coil structure (the PCB ring array coil has three coil structures) provided by the present invention.

[0033] Figure 8 This is the PCB layout of the PCB ring array coil (with three coil structures) provided by the present invention.

[0034] Figure 9 This is a signal diagram excited by the electromagnetic ultrasonic transducer with a ring coil structure (the PCB ring array coil has three coil structures) provided by the present invention.

[0035] Figure 10 This is a flowchart illustrating the design steps of the electromagnetic ultrasonic transducer with a ring coil structure provided by the present invention.

[0036] 1-Cylindrical permanent magnet, 2-First coil, 3-Conductive test piece, 4-Second coil, 5-Third coil. Detailed Implementation

[0037] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0038] Please see Figure 1 This invention provides an electromagnetic ultrasonic transducer with a ring coil structure. The electromagnetic ultrasonic transducer with a ring coil structure includes a cylindrical permanent magnet 1 and a PCB ring array coil. The PCB ring array coil is disposed below the cylindrical permanent magnet 1. The cylindrical permanent magnet 1 is a high-permeability neodymium iron boron strong magnet used to provide circumferential and axial static bias magnetic fields. The PCB ring array coil is composed of a first coil 2 and a second coil 4 in the same plane. The first coil 2 and the second coil 4 are both ring coils. The first coil 2 and the second coil 4 are connected end to end and are double-wound. The beginning and end ends of the PCB ring array coil are respectively connected to external wires, and the external wires guide external excitation current.

[0039] The cylindrical permanent magnet 1 is magnetized in the axial direction, and the static bias magnetic field generated includes an axial magnetic field perpendicular to the surface of the conductive test piece 3 and a circumferential magnetic field along the surface of the conductive test piece 3.

[0040] The coils in the PCB ring array are wound with wires according to the diameter of each coil. The first coil 2 and the second coil 4 are wound in opposite directions. The number of coil turns is evenly distributed on the top and bottom layers, which are connected by vias. Specifically:

[0041] The top layer coil is wound from the outside in. The outer coil is wound clockwise / counterclockwise, and the inner coil is wound counterclockwise / clockwise. The inner coil is connected to the bottom layer by a via.

[0042] The bottom coil is wound from the inside out, and the two coils are wound in the same direction as the top coil.

[0043] When alternating current is passed through the coil, the current first flows in from the outside to the inside through the top layer, and then flows out from the inside to the outside through the bottom layer.

[0044] In this embodiment, the electromagnetic ultrasonic transducer with the ring coil structure includes the cylindrical permanent magnet 1 and the PCB ring array coil. The cylindrical permanent magnet 1 is a high-permeability neodymium iron boron magnet that provides circumferential and axial static bias magnetic fields. The PCB ring array coil consists of a first coil 2 and a second coil 4 in the same plane. Both the first coil 2 and the second coil 4 are ring coils. The first coil 2 and the second coil 4 are connected end to end and are double-wound. The beginning and end ends of the coils are connected to external wires, which guide external excitation current. The electromagnetic ultrasonic transducer with the ring coil structure can control the constructive and destructive interference of symmetrical and antisymmetrical modes in the Lamb wave by controlling the coil width, i.e., the number of turns, etc., to achieve excitation of an omnidirectional single-mode Lamb wave.

[0045] Furthermore, for a single PCB ring array coil, let the coil's mid-diameter be r. c Any position on the aluminum plate below the coil (the conductive specimen 3 under test) Lorentz force generated at ) It can be obtained from equation (1):

[0046] Equation (1);

[0047] In the formula, t is time. The eddy current density generated by the coil on the aluminum plate surface. and These represent the axial and circumferential magnetic field strengths of the coil below the magnet, respectively, for any given angle. ,exist place In the plane, across the entire region Integrating, we can obtain the total Lorentz force acting on the toroidal region:

[0048] Equation (2);

[0049] exist Under the action of [the mechanism], the boundary conditions on the upper and lower surfaces of the aluminum plate can be decomposed into symmetric and antisymmetric modes;

[0050] Top surface:

[0051] Equation (3);

[0052] Bottom surface:

[0053] Equation (4);

[0054] In the formula, and These represent the shear stress and normal stress acting on the surface of the aluminum plate, respectively. For aluminum plate thickness, The upper surface of the aluminum plate, For the lower surface of the aluminum plate, the first half of the shear stress on the decomposed upper and lower surfaces constitutes an antisymmetric mode, while the second half constitutes a symmetric mode. Substituting the antisymmetric mode parts in equations (3) and (4) into the analytical expression for the Lamb wave stress of the antisymmetric mode of the circular wavefront, omitting the exponential term, and combining it with the excitation source position r = r c The analytical expression for the displacement of the antisymmetric mode Lamb wave on the circular wavefront can be obtained:

[0055] Equation (5);

[0056] Equation (6);

[0057] in , Depend on and Decide, and These are the longitudinal wave velocity and transverse wave velocity of the material, respectively. , Let r and z be the displacements in the r and z directions, respectively. Lamé constant, where k is the wave number. Angular frequency, It is a first-order Bessel function. It is the zeroth order Bessel function.

[0058] Similarly, the analytical expression for the displacement of the Lamb wavefront of the symmetric mode circular wavefront can be obtained.

[0059] According to the principle of Lorentz force, a radial Lorentz force is generated when a normal bias magnetic field interacts with a circumferential eddy current, while an out-of-plane Lorentz force is generated when the radial bias magnetic field interacts with a circumferential eddy current. Both directions of Lorentz force contribute to the excitation of Lamb waves in the S0 and A0 modes. The radial Lorentz force contributes more to the excitation of the S0 mode, while the normal Lorentz force contributes more to the excitation of the A0 mode. Therefore, simplifying the effect of the bias magnetic field in either direction during design cannot accurately reflect the true state of the excitation load, resulting in unsatisfactory excitation effects. When both bias magnetic fields are considered, due to the existence of excitation loads in two directions, the stress boundary condition for the excitation of a single coil on the plate surface becomes:

[0060] and Equation (7);

[0061] Similarly, the boundary load can be decomposed into symmetric and antisymmetric modes, since the displacement excited by the load in any direction can be expressed as an axial component. and radial component The sum of the resulting displacements:

[0062] Equation (8);

[0063] Furthermore, the displacements excited by the Lorentz force in each direction include both symmetric and antisymmetric modes:

[0064] Equation (9);

[0065] Equation (10);

[0066] Based on the relationship between the displacement excited by a single coil and the Lorentz force expressed in equation (5), the relationship between the displacement excited by two coil arrays and the Lorentz force can be obtained:

[0067] Equation (11);

[0068] In the formula:

[0069]

[0070]

[0071]

[0072]

[0073]

[0074]

[0075] in, Let be the width of the Lorentz force acting on the plate surface. and The radial and axial Lorentz forces generated by the first and second coils, respectively. Let be the out-of-plane displacement component of the plate. These are the diameters of the first and second coils, respectively. These are the widths of the first and second coils, respectively. These are the zeroth and first order Hankel functions of the second kind, respectively. These are the wavenumbers corresponding to the 0th order antisymmetric mode (A0) and the symmetric mode (S0), respectively. The expressions are similar and will not be given separately in this technical solution.

[0076] Equation (11) above gives the relationship between the displacement of the array coil and the Lorentz force. Therefore, it is only necessary to make the design such that... or When the value is 0, destructive interference of the modes can be achieved, and correspondingly, constructive interference of the other mode will occur, resulting in only a single mode wave.

[0077] Please see Figures 2 to 10 The present invention also provides a design method for an electromagnetic ultrasonic transducer with a ring coil structure, applicable to the electromagnetic ultrasonic transducer with a ring coil structure as described above, comprising the following steps:

[0078] Step 1: Assume that the eddy current distribution induced in the tightly wound helical coil and the toroidal coil is the same, and the coil width is the product of the number of coil turns and the wire width; the Lorentz force has the same width as the coil. Since the current flowing through the coil is the same, the Lorentz force is proportional to the bias magnetic field strength at the location of the coil; the magnitude and direction of the average bias magnetic field at each point within the coil width are the same as the magnitude and direction of the bias magnetic field at the coil diameter.

[0079] Step 2: Calculate the dispersion curve of the Lamb wave based on the material being tested, determine the center frequency f of the desired Lamb wave, and find the corresponding phase velocity C on the dispersion curve. p ;

[0080] Step 3: Calculate the wavelength of the sound wave, C, based on the phase velocity and center frequency. p / f= The diameters of the first coil 2 and the second coil 4 are determined based on the wavelength: D1 = D2-D1= ;

[0081] Step 4: Based on the magnetic field distribution of the cylindrical permanent magnet 1, obtain the magnetic field component B along the diameter of the coil.r B z ;

[0082] Step 5: Under the above parameters, calculate the frequency-displacement curves of A0 and S0 modes. By looking at the frequency-displacement curves, check the displacement amplitude ratio of A0 and S0 under the parameter conditions. Adjust the width of the coil wire so that the amplitude of S0 / A0 mode wave at the center frequency is minimized while the amplitude of A0 / S0 mode wave is maximized.

[0083] Step Six: Draw the PCB coil based on the parameters obtained in Step Five, and complete the specific structural design of the PCB ring array coil of the electromagnetic ultrasonic transducer with the ring coil structure.

[0084] In this embodiment, based on the magnetic field distribution of the cylindrical permanent magnet 1 (e.g., Figure 2 As shown, since the magnet has an axially symmetric structure, only one side of the magnetic field distribution is shown. The radial component B of the magnetic field along the diameter of the coil is obtained. r Axial component B z ;

[0085] The parameters determined in steps two through four above are substituted into equation (11) to calculate the frequency-displacement curves of modes A0 and S0, as follows: Figure 3 As shown, at a given center frequency, the S0 mode wave is eliminated, and the required A0 mode wave amplitude reaches its maximum.

[0086] Furthermore, finite element simulation was performed on EMAT to obtain the displacement waveform excited at the center frequency, as shown below. Figure 4 As shown in the figure, C1 and C2 represent the displacements excited by the first coil 2 and the second coil 4 individually, C1+C2 represents the sum of the displacements excited by the first coil 2 and the second coil 4 individually, and "array" represents the displacement excited simultaneously by an array structure composed of the first coil 2 and the second coil 4. It can be seen from the figure that the result of adding C1 and C2 is the same as the result of exciting using the coil array structure; both suppress the generation of the S0 mode wave, and the displacement amplitude of the A0 mode wave is enhanced after superposition.

[0087] The PCB ring array coil can be expanded from two coils to multiple coils to improve the excitation efficiency of the transducer, such as... Figure 5 As shown, an ultrasonic transducer with a PCB ring three-coil array is constructed. A third coil 5 is arranged around the second coil 4, and the distance between the third coil 5 and the second coil 4 is also half a wavelength. The current directions of the first coil 2 and the second coil 4 are opposite, while the current directions of the first coil 2 and the third coil 5 are the same, so that the Lorentz forces excited by the adjacent coils are opposite. Similarly, equation (11) is further extended to a displacement expression excited by the three-coil array. The result is obtained by adjusting the parameters of the three coils. Figure 6 The displacement-frequency curve shown in the figure indicates that at the specified center frequency, the displacement amplitude of the S0 mode wave is 0, while the displacement amplitude of the A0 mode wave reaches its maximum. Similarly, simulation of EMAT yields the displacement waveform excited at the center frequency as shown below. Figure 7 As shown, similar to the dual-coil array structure, the displacements of C1+C2+C3 and the array excitation are the same, and the three-coil array structure EMAT can achieve better suppression effect.

[0088] Based on the simulation and the results obtained in step five, draw the PCB layout of the three-coil array as follows: Figure 8 As shown, the three coils are connected in series, with the second coil 4 wound in the opposite direction to the first coil 2 and the third coil 5. The coil turns are evenly distributed on the top and bottom layers, which are connected by vias.

[0089] The PCB ring-shaped three-coil array was combined with the cylindrical permanent magnet 1 to form an EMAT for experimentation. The acquired signal is as follows: Figure 9 As shown, no S0 mode wave signal was observed in the received signal. The experimental results demonstrate that the proposed EMAT ring coil array structure can effectively suppress the S0 mode Lamb wave and selectively excite the desired single A0 mode wave. Due to the symmetry of the structure, the excited Lamb wave is omnidirectional.

[0090] Simulation and experimental results both verify that the transducer can excite a pure omnidirectional single A0 mode Lamb wave. Furthermore, based on this method, an omnidirectional single S0 mode Lamb wave can be designed. The designed EMAT has the characteristics of simple structure and small size, providing a new design idea for achieving efficient excitation of omnidirectional single mode Lamb waves.

[0091] In summary, the electromagnetic ultrasonic transducer with a ring coil structure and its design method provided in this technical solution can excite and receive omnidirectional single-mode Lamb waves. The electromagnetic ultrasonic transducer with a ring coil structure includes a cylindrical permanent magnet 1 and a PCB ring array coil. The cylindrical permanent magnet 1 is a high-permeability neodymium iron boron magnet, providing circumferential and axial static bias magnetic fields. The PCB ring array coil consists of two ring coils in the same plane, connected end-to-end and double-wound. The beginning and end of the coils are connected to external wires, which guide the external excitation current. The electromagnetic ultrasonic transducer with a ring coil structure can control the constructive and destructive interference of symmetrical and antisymmetric modes in the Lamb wave by controlling the coil width (i.e., the number of turns), thereby achieving the excitation of omnidirectional single-mode Lamb waves. Since this design method is not specific to any particular structure, a corresponding single-mode Lamb wave (EMAT) can be designed according to this method at any desired excitation center frequency, providing a new design approach for the efficient excitation of omnidirectional single-mode Lamb waves.

[0092] The above description discloses only one preferred embodiment of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. An electromagnetic ultrasonic transducer with a ring coil structure, characterized in that, The device includes a cylindrical permanent magnet and a PCB ring array coil. The PCB ring array coil is located below the cylindrical permanent magnet. The cylindrical permanent magnet is a high-permeability neodymium iron boron magnet used to provide circumferential and axial static bias magnetic fields. The PCB ring array coil consists of a first coil and a second coil in the same plane. Both the first coil and the second coil are ring coils. The first coil and the second coil are connected end to end and are double-wound. The beginning and end of the PCB ring array coil are respectively connected to external wires, which guide external excitation current. The cylindrical permanent magnet is magnetized in the axial direction, and the static bias magnetic field it generates includes an axial magnetic field perpendicular to the surface of the conductive specimen and a circumferential magnetic field along the surface of the conductive specimen. The coils in the PCB ring array are wound with wires according to the diameter of each coil. The first coil and the second coil are wound in opposite directions, and the number of coil turns is evenly distributed on the top and bottom layers. The top and bottom layers are connected by vias. Specifically: The top layer coil is wound from the outside in. The outer coil is wound clockwise / counterclockwise, and the inner coil is wound counterclockwise / clockwise. The inner coil is connected to the bottom layer by a via. The bottom coil is wound from the inside out, and the two coils are wound in the same direction as the top coil. When alternating current is passed through the coil, the current first flows in from the outside to the inside through the top layer, and then flows out from the inside to the outside through the bottom layer.

2. The electromagnetic ultrasonic transducer with a ring coil structure as described in claim 1, characterized in that, The innermost coil of the PCB ring array has a diameter of half a wavelength, and the spacing between adjacent coils is also half a wavelength.

3. The electromagnetic ultrasonic transducer with a ring coil structure as described in claim 1, characterized in that, The PCB ring array coil has pads at both ends for connecting to external wires.

4. The electromagnetic ultrasonic transducer with a ring coil structure as described in claim 1, characterized in that, When an alternating excitation current is applied to the electromagnetic ultrasonic transducer with the ring coil structure, the PCB ring array coils, under the action of a static bias magnetic field, generate corresponding S0 / A0 mode Lamb waves with the same amplitude but opposite phase. The two waves are superimposed to form destructive interference, causing the amplitude of one mode wave to be reduced to 0; the other mode wave forms constructive interference, and the amplitudes are added together, achieving the effect of enhancing the amplitude, thus resulting in only a single mode wave.

5. The electromagnetic ultrasonic transducer with a ring coil structure as described in claim 1, characterized in that, The electromagnetic ultrasonic transducer with the ring coil structure generates Lorentz force in a non-ferromagnetic conductive test specimen in a non-contact manner, and generates omnidirectional Lamb waves through the Lorentz force.

6. The electromagnetic ultrasonic transducer with a ring coil structure as described in claim 1, characterized in that, The electromagnetic ultrasonic transducer with the ring coil structure generates Lorentz force and magnetostrictive force in the ferromagnetic conductive test specimen in a non-contact manner, and generates omnidirectional Lamb waves through the combined action of the two.

7. A design method for an electromagnetic ultrasonic transducer with a ring coil structure, applied to the electromagnetic ultrasonic transducer with a ring coil structure as described in claim 1, characterized in that, Includes the following steps: The dispersion curve of the Lamb wave is calculated based on the material being tested. The center frequency f of the desired Lamb wave is determined, and the phase velocity C corresponding to the frequency-thickness product is found on the dispersion curve. p ; The wavelength of the sound wave is calculated based on the phase velocity and center frequency, C. p / f= The diameters of the first and second coils are determined based on the wavelength: D1 = D2-D1= ; Based on the magnetic field distribution of the cylindrical permanent magnet, the magnetic field component B along the coil diameter is obtained. r B z ; Under the above parameters, calculate the frequency-displacement curves of A0 and S0 modes. By examining the frequency-displacement curves, observe the displacement amplitude ratio of A0 to S0 under these parameters. Adjust the coil wire width to minimize the amplitude of the S0 / A0 mode at the center frequency and maximize the amplitude of the A0 / S0 mode. Based on the obtained parameters, the PCB coil is drawn, and the specific structure design of the PCB ring array coil of the electromagnetic ultrasonic transducer with the ring coil structure is completed.

8. The design method of the electromagnetic ultrasonic transducer with a ring coil structure as described in claim 7, characterized in that: The design method of the electromagnetic ultrasonic transducer with the ring coil structure also includes the following preliminary steps: assuming that the eddy current distribution induced by the tightly wound helical coil and the ring coil is the same, and that the coil width is the product of the number of coil turns and the wire width; the width of the Lorentz force is the same as the coil width, and since the current flowing through the coil is the same, the Lorentz force is proportional to the bias magnetic field strength at the location of the coil; the magnitude and direction of the average bias magnetic field at each point within the coil width range are the same as the magnitude and direction of the bias magnetic field of the coil diameter.

9. The design method of the electromagnetic ultrasonic transducer with a ring coil structure as described in claim 7, characterized in that: The PCB ring array coil can be expanded from two coils to multiple coils to improve the excitation efficiency of the transducer.

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