Design method of low-distortion narrow magnet electromagnetic ultrasonic transducer and ultrasonic transducer

Abstract

The present invention relates to a design method of a low-distortion narrow magnet electromagnetic ultrasonic transducer and an ultrasonic transducer, the method comprising the following steps: constructing a zero-phase reference model, fitting and calculating the phase offset component affected by the magnetic field according to the magnetic field distribution; obtaining the number of coil wire distribution of the ultrasonic transducer to be designed, and constructing the phase offset expression of the Rayleigh wave excited by each wire in the electromagnetic ultrasonic transducer based on the phase offset component affected by the magnetic field; selecting two wires from all the wires as a typical characteristic pair representing the phase difference of the entire coil, and calculating the fold line spacing that makes the phase deviation between the two wires zero; and optimizing the arrangement of multiple wires of the coil with the fold line spacing. Compared with the prior art, the present invention has the advantages of effectively eliminating waveform distortion and low distortion.

Description

Technical Field

[0001] The invention belongs to the technical field of nondestructive testing, and in particular relates to a design method of a low-distortion narrow magnet electromagnetic ultrasonic transducer and an ultrasonic transducer. Background Art

[0002] Electromagnetic ultrasonic transducers (EMAT) mainly include magnets (electromagnets and permanent magnets) and coil structures. When a high-frequency current is passed through the coil, since the energy conversion occurs in the skin layer of the material being tested, no coupling agent is required, and non-contact measurement can be achieved, which can be detected in complex environments, making it particularly attractive in some non-destructive testing applications. EMAT does not require coupling or contact with the sample and can generate and detect multiple wave modes. However, the ultrasonic amplitude generated by EMAT is usually much lower than that obtained using more efficient piezoelectric transducers. Researchers have found that compared with traditional wide magnet EMAT, the Rayleigh wave signal excited by narrow magnet EMAT is stronger. At the same time, the narrow magnet EMAT design amplifies the problems that already exist in conventional EMAT. The presence of an unbalanced magnetic field causes the center frequency of the Rayleigh wave to shift and the frequency sidelobe phenomenon, which manifests itself as severe waveform distortion in the time domain, that is, waveform distortion. Some researchers have proposed a method to solve waveform distortion: by improving the EMAT coil structure by connecting parallel split conductors, the adjacent waveforms in the waveform superposition process can be smoothly transitioned, thereby reducing the waveform distortion problem. This technology of parallel splitting the wires weakens the effect of the horizontal magnetic field, so the signal enhancement effect brought by the narrow magnet EMAT design itself is weakened. At the same time, the iterative process of this method in the simulation uses the final excited waveform as the selection criterion, lacks necessary constraints, and the design criteria are relatively vague. Once the relevant test conditions change, the modification process of the wire splitting parallel scheme is relatively complicated and difficult.

[0003] Therefore, it is necessary to develop a new technology that can effectively eliminate the waveform distortion of narrow magnet electromagnetic ultrasonic transducers. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a design method and ultrasonic transducer of a narrow magnet electromagnetic ultrasonic transducer which can effectively eliminate waveform distortion and have low distortion.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A method for designing a low-distortion narrow magnet electromagnetic ultrasonic transducer comprises the following steps:

[0007] A zero-phase reference model is constructed, and the phase offset component affected by the magnetic field is obtained by fitting calculation according to the magnetic field distribution;

[0008] Obtaining the number of coil wires of the ultrasonic transducer to be designed, and constructing a phase offset expression of the Rayleigh wave excited by each wire in the electromagnetic ultrasonic transducer based on the phase offset component affected by the magnetic field;

[0009] Select two conductors from all the conductors as a typical characteristic pair representing the phase difference of the entire coil, and calculate the fold line spacing that makes the phase deviation between the two conductors zero;

[0010] The multiple wires of the coil are optimally arranged according to the fold line spacing.

[0011] Furthermore, the zero-phase reference model includes a permanent magnet and a reference wire, and when determining the phase offset component affected by the magnetic field, the reference wire is located at the origin.

[0012] Furthermore, when determining the phase offset component affected by the magnetic field, the magnetic field flux density distribution data of the surface of the test piece under the action of the zero-phase reference model is collected to obtain the relationship between the phase offset component and the position of the conductor, and the approximate analytical expression of the phase offset component is obtained by fitting.

[0013] Furthermore, a Fourier fitting method is used to obtain an approximate analytical expression of the phase offset component.

[0014] Furthermore, the approximate analytical expression is a piecewise expression.

[0015] Furthermore, the phase shift expression of the Rayleigh wave excited by each wire is:

[0016]

[0017] in, is the phase offset of conductor n, λ is the wavelength, L is the line spacing, is the phase shift component A function of the relationship between the wire position x and the wire position x.

[0018] Furthermore, the edge conductor and the center conductor are used as the typical characteristic pair representing the phase difference of the entire coil.

[0019] Furthermore, the wire is a double-turn wire.

[0020] The present invention also provides a narrow magnet electromagnetic ultrasonic transducer, which is designed using the design method described above.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The present invention corrects the waveform distortion caused by the unbalanced magnetic field, improves the distortion problem of the narrow magnet EMAT signal, accurately controls the output signal frequency, and effectively enhances the signal excitation amplitude.

[0023] 2. Starting from the phase shift components of a single wire, the fold spacing L of the zigzag coil is uniformly adjusted based on the selected typical characteristic pair representing the phase difference of the entire coil. There is no need to split the wire or arrange the wire unevenly. Only a simple wire arrangement is needed to improve the waveform distortion, which greatly reduces the complexity of the design and use of the narrow magnet EMAT.

[0024] 3. The technical solution provided by the present invention can help researchers obtain Rayleigh waveforms with higher peak values, purer spectral components and no obvious distortion.

[0025] 4. The present invention is conducive to subsequent accurate signal analysis and equipment status evaluation. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the process of the present invention;

[0027] Figure 2 In the embodiment of the present invention, Benchmark reference wire model;

[0028] Figure 3 Detailed configuration diagram of the narrow magnet EMAT in an embodiment of the present invention;

[0029] Figure 4 Schematic diagram comparing the Rayleigh wave signals excited by the narrow magnet EMAT with the existing design and the modified fold line spacing in the embodiment, wherein (a) is the time domain signal and (b) is the frequency domain signal. DETAILED DESCRIPTION

[0030] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0031] According to the research on the design of narrow magnet EMAT, it is found that narrow magnet EMAT achieves higher Rayleigh wave excitation than conventional wide magnet EMAT, but it is also found that the narrow magnet EMAT design will cause serious waveform distortion, and the center frequency of the signal will shift and will no longer be pure. In order to solve the above technical problems, the present invention designs and analyzes the Rayleigh wave phase shift phenomenon caused by the bias magnetic field, determines the calculation formula of the phase shift, corrects and changes the folding spacing L of the coil, determines a new wire arrangement method, and completes the solution to the waveform distortion problem. The present invention clarifies the cause of waveform distortion during the Rayleigh wave excitation process of the narrow magnet electromagnetic ultrasonic transducer, which is not clarified in the original technical solution, and provides a reliable basis for the subsequent Rayleigh waveform collection, signal analysis and equipment status evaluation.

[0032] Example 1

[0033] This embodiment provides a design method for a low-distortion narrow magnet electromagnetic ultrasonic transducer. Figure 1 As shown, the following steps are included:

[0034] Step S101: construct a zero-phase reference model, and obtain a phase offset component affected by the magnetic field by fitting calculation according to the magnetic field distribution.

[0035] The zero-phase reference model used in this embodiment is as follows: Figure 2 As shown, it specifically includes a square permanent magnet and a double-turn wire, the double-turn wire is used as a reference wire, and a 30mm thick 6061 aluminum alloy plate is set. The reference wire is located at the origin O, and the lifting distance is kept constant at 0.2mm. At the same time, the current direction in the wire is vertically outward, as shown in Figure 2 As shown in red in the figure. Using a 500kHz 10-cycle tone-burst signal as the stimulus, at the receiving point x r The off-surface displacement signal is received at the position of =-40mm.

[0036] According to the magnetic field flux density distribution data collected near the induced eddy current area on the surface of the aluminum plate, the phase offset component is obtained. The relationship between the position x of the wire. Contains the reference value of the phase shift component caused by the magnetic field and the phase offset relative to the reference conductor In this embodiment, the Fourier fitting method can be used to obtain The approximate analytical expression of the curve is divided into three parts according to the x coordinate: -20mm to -10mm, -10mm to 10mm and 10mm to 20mm. The fitting is divided into the following three parts:

[0037]

[0038] Wherein, a0, a1, b1, c, a2, b2 and p are seven coefficients obtained by fitting. In this embodiment, their values ​​are 1.7231, 0.4766, -0.2208, 0.3556, -0.1348, 0.004718 and -0.1070 respectively.

[0039] Step S102: obtaining the number of coil wires of the ultrasonic transducer to be designed, and constructing a phase offset expression of the Rayleigh wave excited by each wire in the electromagnetic ultrasonic transducer based on the phase offset component affected by the magnetic field.

[0040] In this embodiment, after obtaining the number of coil wires of the ultrasonic transducer to be designed, based on Figure 2 The zero-phase reference model shown in Figure 2 can be obtained as Figure 3 The detailed configuration of the narrow magnet EMAT is shown, where the parameters of the permanent magnet and the aluminum plate are the same as Figure 2 The wire distribution of the coil is as follows Figure 3 As shown in the red part, the red middle part indicates the direction of current, and each wire is numbered in sequence from left to right.

[0041] In this embodiment, the phase shift of the Rayleigh wave signal excited by the nth wire compared with the reference wire is for:

[0042]

[0043] in, is the phase offset of conductor n, λ is the wavelength, L is the line spacing, is the phase shift component A function of the relationship between the wire position x and the wire position x.

[0044] Step S103: Select two wires from all the wires as a typical characteristic pair representing the phase difference of the entire coil.

[0045] In a preferred embodiment, an edge conductor and a center conductor are selected as the typical characteristic pair representing the phase difference of the entire coil to improve the design accuracy.

[0046] refer to Figure 3 As shown, in this embodiment, conductor No. 1 is located at the outermost edge of the magnet, and conductor No. 6 is closest to the center of the magnet. The magnetic fields around these two conductors are most different from those around other conductors. Conductors No. 1 and No. 6 are selected as a typical characteristic pair representing the phase difference of the entire coil.

[0047] Step S104, calculating the fold line spacing that makes the phase deviation between the two selected conductors zero, thereby determining a suitable fold line spacing.

[0048] In this embodiment, the expression for making the theoretical phase difference between the No. 1 and No. 6 wires zero is:

[0049]

[0050] By calculating the above equation, we can get the appropriate folding distance L0. When the phase difference between the excitation signals of the central wire and the edge wire is the smallest, the phase difference of the entire system also tends to zero, and the waveform distortion can be corrected.

[0051] Step S105, determining a suitable coil arrangement method. After obtaining a suitable fold line spacing, the multiple conductors of the coil can be optimally arranged.

[0052] In this embodiment, when the excitation frequency f remains unchanged at 500kHz, the waveform distortion is corrected by changing the value of the folding spacing L, which is adjusted from the original L = λ / 2 = 2.94mm to the corrected folding spacing L0 = 2.6265mm. After correction, the phase difference between the Rayleigh waves excited by the central conductor and the edge conductor can be reduced from 1.8125rad to 0.00014rad, and the phase offset value The standard deviation σ II is 0.1239 rad, which is different from the standard deviation σ of the phase shift of each wire excitation signal in the existing design. I =1.2549rad, the standard deviation has been greatly improved, and it can be basically considered that there is no phase difference between the two sets of signals. Table 1 lists in detail the phase shift of the Rayleigh wave signal excited by all wires individually after the fold spacing L is adjusted to 2.6265mm size.

[0053] Table 1 Phase shift of different conductors when L = 2.6265 mm

[0054]

[0055] like Figure 4 As shown, the center frequency of the Rayleigh wave signal excited by the present invention returns to the desired position of 500kHz, and the excited waveform has a peak only at the frequency of 500.02kHz, that is, the excitation frequency deviation is only 0.004%, while the existing design has frequency side lobes, and the peak value of the side lobes is relatively high.

[0056] Example 2

[0057] This embodiment provides a narrow magnet electromagnetic ultrasonic transducer, which is designed using the design method described in Example 1.

[0058] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A method for designing a low-distortion narrow magnet electromagnetic ultrasonic transducer, characterized in that: The following steps are involved: A zero-phase reference model is constructed, and the phase offset component affected by the magnetic field is obtained by fitting calculation according to the magnetic field distribution; Obtaining the number of coil wires of the ultrasonic transducer to be designed, and constructing a phase offset expression of the Rayleigh wave excited by each wire in the electromagnetic ultrasonic transducer based on the phase offset component affected by the magnetic field; Select two conductors from all the conductors as a typical characteristic pair representing the phase difference of the entire coil, and calculate the fold line spacing that makes the phase deviation between the two conductors zero; Optimizing the arrangement of the multiple wires of the coil according to the fold line spacing; The zero-phase reference model includes a permanent magnet and a reference wire, and when determining the phase offset component affected by the magnetic field, the reference wire is located at the origin; The phase shift expression of the Rayleigh wave excited by each wire is: ， in, for n The phase offset of the signal conductor, is the wavelength, is the line spacing, is the phase shift component The position of the wire The relationship function between them.

2. The method for designing a low-distortion narrow magnet electromagnetic ultrasonic transducer according to claim 1, characterized in that: When determining the phase offset component affected by the magnetic field, the magnetic field flux density distribution data of the surface of the test piece under the action of the zero-phase reference model is collected to obtain the relationship between the phase offset component and the position of the conductor, and the approximate analytical expression of the phase offset component is obtained by fitting.

3. The method for designing a low-distortion narrow magnet electromagnetic ultrasonic transducer according to claim 2, characterized in that: The approximate analytical expression of the phase shift component is obtained by Fourier fitting.

4. The method for designing a low-distortion narrow magnet electromagnetic ultrasonic transducer according to claim 2, characterized in that: The approximate analytical expression is a piecewise expression.

5. The method for designing a low-distortion narrow magnet electromagnetic ultrasonic transducer according to claim 1, characterized in that: The edge conductor and the center conductor are used as the typical characteristic pair representing the phase difference of the entire coil.

6. The method for designing a low-distortion narrow magnet electromagnetic ultrasonic transducer according to claim 1, characterized in that: The wire is a double-turn wire.

7. A narrow magnet electromagnetic ultrasonic transducer, characterized in that: The method is designed according to any one of claims 1 to 6.

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