A method and system for assessing damage to metal substrates under coatings based on magnetic field detection technology

Abstract

The present invention provides a method and system for assessing damage to a metal substrate under a coating based on magnetic field detection technology. The method utilizes eddy current excitation and a magnetic field detection system to obtain a magnetic field intensity distribution image of the metal substrate under the coating, determines the degree of damage to the metal substrate under the coating based on the characteristics of the obtained magnetic field intensity distribution image, assesses the degree of damage to the metal substrate under the coating and performs grading, and provides an assessment result. The eddy current excitation magnetic field system includes a multi-frequency eddy current excitation signal generator, an excitation coil, a magnetic field data collector, a linear array magnetic field detection sensor, a data processing module, and a display. The linear array magnetic field detection sensor is fixedly connected to the excitation coil and is located inside the excitation coil. The scheme of the present invention can not only quantitatively provide the degree of damage to the metal substrate under the coating, but also provide a damage rating for each depth interval of the metal substrate under the coating, thereby improving the accuracy of damage assessment of the metal substrate under the coating and having the advantages of being non-destructive, efficient, and convenient.

Description

Technical Field

[0001] The present invention belongs to the technical field of non-destructive testing, and in particular relates to a method and system for assessing damage to a metal substrate under a coating based on magnetic field detection technology. Background Art

[0002] Eddy current testing (using coil excitation to obtain inductive reactance data) is a commonly used nondestructive testing technique, primarily used to detect defects on or near the surface of a workpiece. Alternating current can generate eddy currents on the surface of metal materials. When the metal surface is damaged by cracks, corrosion, or other defects, the magnetic field generated by the eddy currents changes, and the detected magnetic field data can reflect the damage to the metal surface. The existing document "Pulsed Eddy Current Magnetic Field Measurement Technology for Defect Detection in Multilayer Structures" discloses the use of pulsed eddy current magnetic fields to identify defects in multilayer structures; the existing document "Three-Dimensional Magnetic Field Measurement and Defect Quantitative Assessment of Rectangular Pulsed Eddy Current Sensors" discloses a pulsed eddy current testing system designed based on the pulsed eddy current testing principle, which measures defect length through curve characteristics. However, these existing nondestructive testing techniques focus on the presence or absence of defects and do not quantitatively determine the extent of damage to the metal substrate beneath the coating (the coating on the metal substrate beneath the coating), resulting in inaccurate damage assessments. Summary of the Invention

[0003] The purpose of the present invention is to provide a method and system for assessing damage to a metal substrate under a coating based on magnetic field detection technology, which can at least solve the technical problem in the prior art of "not quantitatively giving the degree of damage to the metal substrate under the coating, resulting in insufficient accuracy in damage assessment."

[0004] The technical solutions adopted in the present invention are as follows.

[0005] A method for assessing damage to a metal substrate under a coating based on magnetic field detection technology comprises the following steps:

[0006] Step 1: using an eddy current excitation magnetic field system to obtain a magnetic field intensity distribution image of the metal substrate under the coating, and determining the degree of damage to the metal substrate under the coating based on the characteristics of the obtained magnetic field intensity distribution image;

[0007] Step 2: Using an eddy current excitation magnetic field system to obtain a transverse damage image of the metal substrate under the coating, and determining the coordinates of the damaged area and the transverse damage morphology based on the contour features on the obtained transverse damage image;

[0008] Step 3: Determine the damage type and corresponding quantity based on the obtained damage depth interval and lateral damage morphology. The damage types include cracks, deep corrosion pits, and shallow corrosion pits.

[0009] Step 4: Evaluate the damage degree of the metal substrate under the coating according to formula (I), grade it according to the obtained value S, and give the evaluation results, which are divided into four levels: general (S<2), more serious (2≤S<4), serious (4≤S<6), and very serious (S≥6);

[0010] S=0.5m+0.35n+0.15p (Ⅰ)

[0011] Where m represents the number of cracks, n represents the number of deep corrosion pits, and p represents the number of shallow corrosion pits.

[0012] As a preferred solution, in step 1, by changing the excitation frequency of the eddy current excitation magnetic field system, the magnetic field intensity distribution images of different depth areas of the metal substrate under the coating are obtained, and the damage depth range of the metal substrate under the coating is determined by combining the magnetic field intensity distribution characteristics of different depth areas.

[0013] As a preferred solution, the number of layers required to be scanned in the thickness direction of the metal substrate under the coating is determined; when the magnetic field intensity distribution image obtained by scanning the Kth layer has a steep change feature in a certain area, and the magnetic field intensity distribution images obtained by scanning the 1st to K-1th layers have no steep change feature, then the damage depth interval is determined to be the thickness area from the K-1th layer to the Kth layer.

[0014] In the present invention, the eddy current excitation magnetic field system includes a multi-frequency eddy current excitation signal generator, an excitation coil, a magnetic field data collector, a linear array magnetic field detection sensor and a data processing module and a display, wherein the linear array magnetic field detection sensor is fixedly connected to the excitation coil and is located inside the excitation coil; wherein the multi-frequency eddy current excitation signal generator is used to generate a multi-frequency excitation signal and form a magnetic field at the measured part, and synchronously generate a frequency mark signal, and synchronously transmit the mark signal to the magnetic field data collector; the linear array magnetic field detection sensor is used to detect the magnetic field data under different excitation frequencies, and correspondingly record and extract the magnetic field data under different excitation frequencies, and transmit the magnetic field data to the magnetic field data collector; the magnetic field data collector is used to obtain the magnetic field data and the frequency mark signal, match the magnetic field data with the frequency mark signal, store it, and feed it back to the data processing module; the data processing module is used to generate a magnetic field data graph from the read magnetic field data and output it. Among them, the multi-frequency eddy current excitation signal generator can continuously generate multiple different frequencies within a second period. A programmable signal generator can be used. These required frequency levels can be set by those skilled in the art through programming. For example, the multi-frequency excitation signal includes a first frequency level emitted in the first millisecond period T1, a second frequency level emitted in the second millisecond period T2, and so on. In the Kth millisecond period T KThe total duration of all millisecond-level periods is controlled within one second. The detection thickness range corresponding to the first frequency is 0-h1 mm, the detection thickness range corresponding to the second frequency is 0-h2 mm, and the detection thickness range corresponding to the Kth frequency is 0-h K mm, the position of 0 mm indicates the detection reference surface, and the position of h1 mm indicates the depth h below the detection reference surface. K mm position, h1<h2<h K .

[0015] The present invention also provides a system for assessing damage to a metal substrate under a coating based on magnetic field detection technology, comprising a computer connected to an eddy current excitation device, the computer comprising a memory, a processor, and a program stored in the memory and executable on the processor, wherein the processor implements the following steps when executing the program:

[0016] S1, real-time reading of magnetic field intensity distribution image data fed back by the eddy current excitation magnetic field system; real-time reading of lateral damage image fed back by the eddy current excitation magnetic field system;

[0017] S2, under the set excitation frequency, after each scan of the metal substrate under the coating, obtain the abrupt change feature and its coordinate area in the obtained magnetic field intensity distribution image, and define the coordinate area of ​​the abrupt change feature in the magnetic field intensity distribution image obtained by scanning the Nth layer as P N (X N , Y N 、Z N ), N=1, 2, 3…;

[0018] S3, extract the same two-dimensional coordinates P N (X N , Y N ) The height data Z of the region has a steep change feature N , and according to the height data Z N The location determines the damage depth range of the metal substrate under the coating;

[0019] S4, identifying the contour features on the obtained transverse damage image. When the contour feature is linear and its ends do not overlap, the defect corresponding to the contour feature is determined to be a crack; when the contour feature is flaky and its corresponding maximum damage depth interval exceeds a preset threshold, the defect corresponding to the contour feature is determined to be a deep corrosion pit; when the contour feature is flaky and its corresponding maximum damage depth interval does not exceed a preset threshold, the defect corresponding to the contour feature is determined to be a shallow corrosion pit;

[0020] S5: Count the number of damage types, evaluate the damage degree of the metal substrate under the coating according to formula (I), and classify it according to the obtained value S into four levels: general damage (S<2), relatively serious damage (2≤S<4), serious damage (4≤S<6), and very serious damage (S≥6), and output the corresponding evaluation results;

[0021] S=0.5m+0.35n+0.15p (Ⅰ)

[0022] Where m represents the number of cracks, n represents the number of deep corrosion pits, and p represents the number of shallow corrosion pits.

[0023] Beneficial effects: By adopting the solution of the present invention, without removing the surface coating of the metal substrate under the coating, not only the damage degree of the metal substrate under the coating can be quantitatively given, but also the damage rating of each depth interval of the metal substrate under the coating can be given, thereby improving the accuracy of the damage assessment of the metal substrate under the coating, and having the advantages of being non-destructive, efficient and convenient; the present invention determines the damage type by combining the damage depth and the lateral damage morphology, greatly simplifying the damage type determination model and process, which is conducive to improving the efficiency of the damage degree assessment of the metal substrate under the coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of lateral damage images obtained by scanning different depth areas of the metal substrate under the coating in the embodiment;

[0025] Figure 2 Schematic diagram of the magnetic field data of general damage to the metal substrate under the coating obtained in the embodiment;

[0026] Figure 3 This is a schematic diagram of the magnetic field data showing severe damage to the metal substrate under the coating obtained in the embodiment;

[0027] Figure 4 Schematic diagram of the magnetic field data showing severe damage to the metal substrate under the coating obtained in the embodiment;

[0028] Figure 5 This is a schematic diagram of the magnetic field data showing that the metal substrate under the coating obtained in the embodiment is severely damaged. DETAILED DESCRIPTION

[0029] The present invention will be further described below with reference to the embodiments and accompanying drawings. Example 1

[0030] A method for assessing damage to a metal substrate under a coating based on magnetic field detection technology comprises the following steps:

[0031] Step 1: Using an eddy current excitation magnetic field system to obtain magnetic field intensity distribution images at different depths of the metal substrate under the coating, and determining the damage depth range of the metal substrate under the coating based on the characteristics of the obtained magnetic field intensity distribution images; in this step, specifically, by changing the excitation frequency of the eddy current excitation magnetic field system, magnetic field intensity distribution images of different depth regions of the metal substrate under the coating are obtained, and the damage depth range of the metal substrate under the coating is determined based on the magnetic field intensity distribution characteristics of the different depth regions;

[0032] Step 2: Using an eddy current excitation device to obtain a transverse damage image of the metal substrate under the coating, and determining the coordinates of the damaged area and the transverse damage morphology based on the contour features on the obtained transverse damage image;

[0033] Step 3: Determine the damage type and corresponding quantity based on the obtained damage depth and lateral damage morphology. The damage types include cracks, deep corrosion pits, and shallow corrosion pits.

[0034] Step 4: Evaluate the damage degree of the metal substrate under the coating according to formula (I), and classify it according to the obtained value S into four levels: general damage (S<2), relatively serious damage (2≤S<4), serious damage (4≤S<6), and very serious damage (S≥6);

[0035] S=0.5m+0.35n+0.15p (Ⅰ)

[0036] Where m represents the number of cracks, n represents the number of deep corrosion pits, and p represents the number of shallow corrosion pits.

[0037] Among them, the eddy current excitation magnetic field system includes a multi-frequency eddy current excitation signal generator, an excitation coil, a magnetic field data collector, a linear array magnetic field detection sensor and a data processing module and a display. The linear array magnetic field detection sensor is fixedly connected to the excitation coil and is located inside the excitation coil; wherein the multi-frequency eddy current excitation signal generator is used to generate a multi-frequency excitation signal and form a magnetic field at the measured part, and synchronously generate a frequency mark signal, and synchronously transmit the mark signal to the magnetic field data collector; the linear array magnetic field detection sensor is used to detect the magnetic field data under different excitation frequencies, and correspondingly record and extract the magnetic field data under different excitation frequencies, and transmit the magnetic field data to the magnetic field data collector; the magnetic field data collector is used to obtain magnetic field data and frequency mark signals, match the magnetic field data with the frequency mark signals, store them, and feed them back to the data processing module; the data processing module is used to generate a magnetic field data graph from the read magnetic field data and output it; the multi-frequency excitation signal includes a first frequency emitted in the first millisecond period T1, a second frequency emitted in the second millisecond period T2... in the Kth millisecond period T KThe total duration of all millisecond-level periods is controlled within one second. The detection thickness range corresponding to the first frequency is 0-h1 mm, the detection thickness range corresponding to the second frequency is 0-h2 mm, and the detection thickness range corresponding to the Kth frequency is 0-h K mm, the position of 0 mm indicates the detection reference surface, and the position of h1 mm indicates the depth h below the detection reference surface. K mm position, h1<h2<h K .

[0038] During use, first determine the number of layers of the metal substrate under the coating that need to be scanned in the thickness direction; when the magnetic field intensity distribution image obtained by scanning the Kth layer has a steep change feature in a certain area, and the magnetic field intensity distribution images obtained by scanning the 1st to K-1th layers have no steep change feature, then the damage depth interval is determined to be the thickness area from the K-1th layer to the Kth layer. Example 2

[0039] A system for assessing damage to a metal substrate under a coating based on magnetic field detection technology includes a computer connected to an eddy current excitation device. The computer includes a memory, a processor, and a program stored in the memory and executable on the processor. When the processor executes the program, the following steps are performed:

[0040] S1, real-time reading of magnetic field intensity distribution image data fed back by the eddy current excitation magnetic field system; real-time reading of lateral damage image fed back by the eddy current excitation magnetic field system;

[0041] S2, under the set excitation frequency, after each scan of the metal substrate under the coating, obtain the abrupt change feature and its coordinate area in the obtained magnetic field intensity distribution image, and define the coordinate area of ​​the abrupt change feature in the magnetic field intensity distribution image obtained after scanning the Nth layer as P N (X N , Y N 、Z N ), N=1, 2, 3…;

[0042] S3, extract the same two-dimensional coordinates P N (X N , Y N ) The height data Z of the region has a steep change feature N , and according to the height data Z N The location determines the damage depth range of the metal substrate under the coating;

[0043] S4, identifying the contour features on the obtained transverse damage image. When the contour feature is linear and its ends do not overlap, the damage corresponding to the contour feature is determined to be a crack; when the contour feature is flaky and the corresponding maximum damage depth interval exceeds a preset threshold, the damage corresponding to the contour feature is determined to be a deep corrosion pit; when the contour feature is flaky and the corresponding maximum damage depth interval does not exceed a preset threshold, the damage corresponding to the contour feature is determined to be a shallow corrosion pit;

[0044] S5: Count the number of damage types, evaluate the damage degree of the metal substrate under the coating according to formula (I), and classify it according to the obtained value S into four levels: general damage (S<2), relatively serious damage (2≤S<4), serious damage (4≤S<6), and very serious damage (S≥6), and output the corresponding evaluation results;

[0045] S=0.5m+0.35n+0.15p (Ⅰ)

[0046] Where m represents the number of cracks, n represents the number of deep corrosion pits, and p represents the number of shallow corrosion pits.

[0047] To facilitate a better understanding of the solution in the embodiment, further explanation is given: the thickness of a certain aluminum alloy sample is 1.6 mm, and the eddy current excitation equipment used has multiple excitation frequencies, and four excitation frequencies can be selected, including a first frequency (40 kHz) emitted in the first millisecond period T1 (0-100 ms), a second frequency (30 kHz) emitted in the second millisecond period T2 (100-200 ms)… and a fourth frequency (10 kHz) emitted in the fourth millisecond period T4 (300-400 ms). The total duration of all millisecond periods is 400 ms; the detection thickness range corresponding to the first frequency is 0-h1 mm (h1=0.4 mm), the detection thickness range corresponding to the second frequency is 0-h2 mm (h2=0.8 mm), and the detection thickness range corresponding to the fourth frequency is 0-h K mm (h4=1.6mm), the position of 0 mm indicates the detection reference surface, and the position of h1 mm indicates the depth h below the detection reference surface. K mm position, h1<h2<h K During operation, first select the first to fourth frequency, move the excitation coil and the linear array magnetic field detection sensor at a constant speed along the detection path, and complete the scanning of the area with a thickness (i.e. depth) of 0-1.6mm at one time. After the completion of this scan, the magnetic field data map corresponding to the first frequency is obtained after data processing (the transverse damage image is as follows Figure 1 The left picture in the figure), the magnetic field data corresponding to the second frequency (its transverse damage image is as follows Figure 1 The middle figure in the figure) ... the magnetic field data corresponding to the fourth frequency (its transverse damage image is as follows Figure 1 Analyze the magnetic field data plots for the first through fourth frequency ranges. If the magnetic field data plots for the first through third frequency ranges show no damage signatures, but the fourth frequency range shows damage signatures, this indicates damage in the 1.2-1.6 mm thickness range.

[0048] Test samples with different damages and analyze the magnetic field data to obtain the damage types such as cracks, corrosion pits, and superficial corrosion. Count the number and calculate the damage coefficient S:

[0049] The lateral damage morphology of sample 1 is obtained using eddy current excitation equipment, such as Figure 2 As shown in the figure, through identification and analysis, it is found that the metal substrate under the coating of this sample has 1 crack and 4 corrosion damages, of which 3 corrosion damages are shallow corrosion pits and 1 corrosion damage is a deep corrosion pit. According to the formula S=0.5m+0.35n+0.15p, S=1.3 is calculated, which is a general damage;

[0050] The lateral damage morphology of sample 2 is obtained using eddy current excitation equipment, such as Figure 3 As shown in the figure, through identification and analysis, it was found that the metal substrate under the coating of this sample had 4 cracks and 4 corrosion damages, of which 3 corrosion damages were shallow corrosion pits and 1 corrosion damage was a deep corrosion pit. According to the formula S=0.5m+0.35n+0.15p, S=2.8 was calculated, which is a relatively serious damage.

[0051] The lateral damage morphology of sample 3 was obtained using eddy current excitation equipment, such as Figure 4 As shown in the figure, through identification and analysis, it was found that there were 6 cracks and 4 corrosion damages under the coating of the sample, of which 3 corrosion damages were shallow corrosion pits and 1 corrosion damage was a deep corrosion pit. According to the formula S=0.5m+0.35n+0.15p, S=4.2 was calculated, which was a serious damage.

[0052] The lateral damage morphology of sample 4 was obtained using eddy current excitation equipment, such as Figure 5 As shown in the figure, through identification and analysis, it was found that there were 10 cracks and 5 corrosion damages under the coating of the sample, of which 3 corrosion damages were shallow corrosion pits and 2 corrosion damages were deep corrosion pits. According to the formula S=0.5m+0.35n+0.15p, S=6.15 was calculated, which is a very serious damage;

[0053] The lateral damage morphology of the sample is obtained using eddy current excitation equipment, such as Figure 1 As shown in the figure, through identification and analysis, it is found that there are cracks and corrosion damage in each scanning depth interval under the coating of this sample. Combined with the formula S=0.5m+0.35n+0.15p, it is calculated that S=1, and each scanning depth interval belongs to general damage.

Claims

1. A method for assessing damage to a metal substrate under a coating based on magnetic field detection technology, characterized in that the steps include: Step 1: using an eddy current excitation magnetic field system to obtain magnetic field intensity distribution images at different depths of the metal substrate under the coating, and determining the degree of damage to the metal substrate under the coating based on the characteristics of the obtained magnetic field intensity distribution images; Step 2: Using an eddy current excitation magnetic field system to obtain a transverse damage image of the metal substrate under the coating, and determining the coordinates of the damaged area and the transverse damage morphology based on the contour features on the obtained transverse damage image; Step 3: Determine the damage type and corresponding quantity based on the obtained damage depth interval and lateral damage morphology. The damage types include cracks, deep corrosion pits, and shallow corrosion pits. Step 4: Evaluate the damage degree of the metal substrate under the coating according to formula (I), classify it according to the obtained value S, and give the evaluation results, which are divided into four levels: general damage S<2, more serious 2≤S<4, serious 4≤S<6, and very serious S≥6; S=0.5m+0.35n+0.15p (Ⅰ) Where m represents the number of cracks, n represents the number of deep corrosion pits, and p represents the number of shallow corrosion pits; The eddy current excitation magnetic field system includes a multi-frequency eddy current excitation signal generator, an excitation coil, a magnetic field data collector, a linear array magnetic field detection sensor and a data processing module and a display. The linear array magnetic field detection sensor is fixedly connected to the excitation coil and is located inside the excitation coil; wherein the multi-frequency eddy current excitation signal generator is used to generate a multi-frequency excitation signal and form a magnetic field at the measured part, and synchronously generate a frequency mark signal, and synchronously transmit the mark signal to the magnetic field data collector; the linear array magnetic field detection sensor is used to detect magnetic field data under different excitation frequencies, and correspondingly record and extract magnetic field data under different excitation frequencies, and transmit the magnetic field data to the magnetic field data collector; the magnetic field data collector is used to obtain magnetic field data and frequency mark signals, match and store the magnetic field data with the frequency mark signals, and feed them back to the data processing module; the data processing module is used to generate a magnetic field data graph from the read magnetic field data and output it; the multi-frequency excitation signal includes a first frequency emitted in the first millisecond period T1, a second frequency emitted in the second millisecond period T2... in the Kth millisecond period T K The total duration of all millisecond-level periods is controlled within one second; the detection thickness range corresponding to the first frequency is 0-h1 mm, the detection thickness range corresponding to the second frequency is 0-h2 mm, and the detection thickness range corresponding to the Kth frequency is 0-h K mm, the position of 0 mm indicates the detection reference surface, and the position of h1 mm indicates the depth h below the detection reference surface. K mm position, h1<h2<h K .

2. The method for assessing damage to a metal substrate under a coating based on magnetic field detection technology according to claim 1, wherein: In step 1, by changing the excitation frequency of the eddy current excitation magnetic field system, the magnetic field intensity distribution images of different depth areas of the metal substrate under the coating are obtained, and the damage depth range of the metal substrate under the coating is determined by combining the magnetic field intensity distribution characteristics of different depth areas.

3. The method for assessing damage to a metal substrate under a coating based on magnetic field detection technology according to claim 2, wherein: Determine the number of layers that need to be scanned in the thickness direction of the metal substrate under the coating; when the magnetic field intensity distribution image obtained by scanning the Kth layer has a steep change feature in a certain area, and the magnetic field intensity distribution images obtained by scanning the 1st to K-1th layers have no steep change feature, then the damage depth interval is determined to be the thickness area from the K-1th layer to the Kth layer.

4. A system for assessing damage to metal substrates under coatings based on magnetic field detection technology, characterized in that: The invention comprises a computer connected to the eddy current excitation magnetic field system, the computer comprising a memory, a processor and a program stored in the memory and executable on the processor, wherein the processor implements the following steps when executing the program: S1, real-time reading of magnetic field intensity distribution image data fed back by the eddy current excitation magnetic field system; real-time reading of lateral damage image fed back by the eddy current excitation magnetic field system; S2, under the set excitation frequency, after each scan of the metal substrate under the coating, obtain the abrupt change feature and its coordinate area in the obtained magnetic field intensity distribution image, and define the coordinate area of ​​the abrupt change feature in the magnetic field intensity distribution image obtained by scanning the Nth layer as P N (X N , Y N , Z N ), N=1, 2, 3…; S3, extract the same two-dimensional coordinates P N (X N , Y N ) The height data Z of the region has a steep change feature N , and according to the height data Z N The location determines the damage depth range of the metal substrate under the coating; S4, identifying the contour features on the obtained transverse damage image. When the contour feature is linear and its ends do not overlap, the defect corresponding to the contour feature is determined to be a crack; when the contour feature is flaky and its corresponding maximum damage depth interval exceeds a preset threshold, the defect corresponding to the contour feature is determined to be a deep corrosion pit; when the contour feature is flaky and its corresponding maximum damage depth interval does not exceed a preset threshold, the defect corresponding to the contour feature is determined to be a shallow corrosion pit; S5, count the number of each damage type, evaluate the damage degree of the metal substrate under the coating according to formula (I), and classify it according to the obtained value S into four levels: general damage S<2, more serious 2≤S<4, serious 4≤S<6, and very serious S≥6, and output the corresponding evaluation results; S=0.5m+0.35n+0.15p (Ⅰ) Where m represents the number of cracks, n represents the number of deep corrosion pits, and p represents the number of shallow corrosion pits; The eddy current excitation magnetic field system includes a multi-frequency eddy current excitation signal generator, an excitation coil, a magnetic field data collector, a linear array magnetic field detection sensor and a data processing module and a display. The linear array magnetic field detection sensor is fixedly connected to the excitation coil and is located inside the excitation coil; wherein the multi-frequency eddy current excitation signal generator is used to generate a multi-frequency excitation signal and form a magnetic field at the measured part, and synchronously generate a frequency mark signal, and synchronously transmit the mark signal to the magnetic field data collector; the linear array magnetic field detection sensor is used to detect magnetic field data under different excitation frequencies, and correspondingly record and extract magnetic field data under different excitation frequencies, and transmit the magnetic field data to the magnetic field data collector; the magnetic field data collector is used to obtain magnetic field data and frequency mark signals, match and store the magnetic field data with the frequency mark signals, and feed them back to the data processing module; the data processing module is used to generate a magnetic field data graph from the read magnetic field data and output it; the multi-frequency excitation signal includes a first frequency emitted in the first millisecond period T1, a second frequency emitted in the second millisecond period T2... in the Kth millisecond period T K The total duration of all millisecond-level periods is controlled within one second; the detection thickness range corresponding to the first frequency is 0-h1 mm, the detection thickness range corresponding to the second frequency is 0-h2 mm, and the detection thickness range corresponding to the Kth frequency is 0-h K mm, the position of 0 mm indicates the detection reference surface, and the position of h1 mm indicates the depth h below the detection reference surface. K mm position, h1<h2<h K .

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