Metal plate thickness measuring method and system based on electromagnetic eddy current

Abstract

The invention discloses a metal plate thickness measurement method and system based on electromagnetic eddy current, and the method comprises the steps: building an eddy current detection system, employing a coaxial excitation and reception eddy current sensor, and obtaining the mutual inductance signal change caused by a metal plate in a frequency sweep measurement mode through the eddy current sensor; calculating a characteristic frequency point with equal real part and imaginary part of the mutual inductance signal, and recording the characteristic point as a crossover frequency; respectively adjusting the lift-off distance for n times, and calculating the equal cross frequency of the real part and the imaginary part of the mutual inductance signal again; logarithmic processing is carried out on the n cross frequency points, and the relation between the logarithm of the cross frequency and the lift-off distance is obtained through linear fitting; a large amount of calculation finds that the slope of the fitting straight line is irrelevant to the conductivity, so that additional plate electromagnetic characteristic calibration is not needed, the slope of the fitting straight line is only determined by the thickness of the plate, and effective measurement of the plate thickness can be realized.

Description

Technical Field

[0001] The invention relates to the technical field of non-contact metal material dimension measurement using magnetic eddy current, and in particular to a metal plate thickness measurement method and system based on electromagnetic eddy current. Background Art

[0002] Metal sheets are often used to make supporting tower materials for transmission lines, such as steel and aluminum alloy. These materials need to have good mechanical strength and corrosion resistance. In addition, power fittings are metal parts used to support, fix, connect and protect wires and equipment in power systems, such as suspension clamps, tension clamps, connection fittings, etc. These fittings are usually made of high-strength aluminum alloy materials to ensure their reliability and durability when carrying power loads. Since these metal sheets are more likely to deform or break under abnormal loads during service, the reduction in thickness will further reduce the stability of metal components that need to withstand complex loads such as bending moments and shear forces. The thickness of the metal sheet is directly related to the bearing capacity and durability of the structure, and accurate thickness detection can ensure the safety of the structure. Eddy current testing is a non-destructive testing method based on the principle of electromagnetic induction. It evaluates certain properties or thickness information of conductive materials and their workpieces by detecting changes in induced eddy currents in the object being tested, which is of great significance for monitoring the quality of metal products. The present invention discloses a metal plate thickness measurement method based on electromagnetic eddy current, which adopts a coaxial excitation-receiving eddy current sensor, obtains the change of mutual inductance signal caused by the metal plate under different lifting distances through the eddy current sensor in the sweep frequency measurement mode, and calculates the crossover frequency at which the real part and the imaginary part of the mutual inductance signal are equal. The relationship between the logarithm of the crossover frequency and the lifting distance is obtained by linear fitting, and the slope of the fitting line has nothing to do with the conductivity and is only determined by the thickness of the plate, so that the effective measurement of the plate thickness can be achieved. Summary of the invention

[0003] The present invention aims to solve one of the technical problems in the related art at least to a certain extent.

[0004] To this end, the present invention proposes a metal sheet thickness measurement method based on electromagnetic eddy current, which utilizes the measurement results at different lifting distances to eliminate the influence of the metal electromagnetic properties on the thickness, and theoretically improves the accuracy of metal sheet thickness measurement.

[0005] Another object of the present invention is to provide a metal sheet thickness measurement system based on electromagnetic eddy current.

[0006] To achieve the above-mentioned purpose, the present invention provides, on one hand, a metal sheet thickness measurement imaging method based on electromagnetic eddy current, comprising:

[0007] Build an eddy current detection system and design a coaxial one-excitation-one-receiving cylindrical eddy current sensor;

[0008] Based on the structure of the sensor, an eddy current analytical theoretical model is established, and eddy current detection is performed on metal sheets;

[0009] The mutual inductance frequency sweep signal of the plate coil is collected based on the adjusted coil lifting distance, and the frequency point where the real part and the imaginary part of the mutual inductance frequency sweep signal are equal is extracted and recorded as the crossover frequency;

[0010] When it is determined that the required number of coil lift-off distance adjustment times is reached, different crossover frequencies at different lift-off distances are obtained;

[0011] The fitting slope is obtained by linearly fitting the relationship between the logarithm of different crossover frequencies and the lift-off distance;

[0012] The plate thickness measurement result is obtained based on the relationship between the fitting slope and the plate thickness.

[0013] The metal sheet thickness measurement method based on electromagnetic eddy current in the embodiment of the present invention may also have the following additional technical features:

[0014] In one embodiment of the present invention, an eddy current analytical theoretical model is established based on the structure of the sensor, including:

[0015]

[0016] In the formula

[0017]

[0018]

[0019]

[0020]

[0021] Among them, σ is the conductivity, d is the thickness of the metal plate, K is the coefficient of the sensor, and φ(α) is related to the electromagnetic properties and thickness of the metal component, which determines the phase of the eddy current response signal.

[0022] In one embodiment of the present invention, the fitting slope is determined by the thickness of the plate:

[0023] log(ω n+1 )=k1l n+1 +b1 (6)

[0024] Among them, k1 is the fitting slope, l n+1 For different lifting distances.

[0025] In one embodiment of the present invention, a linear fitting is used to characterize the relationship between the thickness of the plate and the fitting slope k1:

[0026] k1=k2d+b2(7).

[0027] In one embodiment of the present invention, when the coil probe is located above the jth metal plate to be tested, the crossover frequency of the mutual inductance signal at different lift-off distances is extracted, and the relationship between the lift-off distance, the crossover frequency and the logarithm is described using equation (6). The slope k j Substituting into formula (7), we can get the thickness of the jth metal sheet to be tested:

[0028]

[0029] To achieve the above object, the present invention provides a metal sheet thickness measurement system based on electromagnetic eddy current, comprising:

[0030] Sensor design module, used to build eddy current detection system and design coaxial-excitation-receiving cylindrical eddy current sensor;

[0031] Eddy current detection module, used to establish an eddy current analytical theoretical model based on the structure of the sensor and perform eddy current detection on metal plates;

[0032] The frequency point extraction module is used to collect the mutual inductance frequency sweep signal of the plate coil based on the adjusted coil lifting distance, and extract the frequency point where the real part and the imaginary part of the mutual inductance frequency sweep signal are equal and record it as the crossover frequency;

[0033] A different crossover frequency calculation module is used to obtain different crossover frequencies under different lift-off distances when it is determined that the required number of coil lift-off distance adjustment times has been reached;

[0034] A fitting slope calculation module, used to obtain a fitting slope by linearly fitting the relationship between the logarithm of different crossover frequencies and the lift-off distance;

[0035] The plate thickness measurement module is used to obtain the plate thickness measurement result according to the relationship between the fitting slope and the plate thickness.

[0036] The metal plate thickness measurement method and system based on electromagnetic eddy current in the embodiment of the present invention finds through a large number of calculations that the slope of the fitting straight line is independent of the electrical conductivity, so there is no need for additional calibration of the electromagnetic properties of the plate. The slope of the fitting straight line is only determined by the thickness of the plate, which can achieve effective measurement of the plate thickness.

[0037] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0039] Figure 1 is a flow chart of a metal sheet thickness measurement method based on electromagnetic eddy current according to an embodiment of the present invention;

[0040] Figure 2 is a schematic diagram of a frequency-sweep eddy current response signal according to an embodiment of the present invention;

[0041] Figure 3 is a relationship diagram between the crossover frequency logarithm and the lift-off distance of the eddy current response signal caused by samples with different electrical conductivities according to an embodiment of the present invention;

[0042] Figure 4 is a graph showing the relationship between the slope of the fitted straight line and the thickness of the metal sheet according to an embodiment of the present invention;

[0043] Figure 5 4 is a structural diagram of a metal sheet thickness measurement system based on electromagnetic eddy current according to an embodiment of the present invention. DETAILED DESCRIPTION

[0044] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0045] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0046] The following describes a metal sheet thickness measurement method and system based on electromagnetic eddy current according to an embodiment of the present invention with reference to the accompanying drawings.

[0047] Figure 1 is a flow chart of a method for measuring the thickness of a metal sheet based on electromagnetic eddy current according to an embodiment of the present invention. Figure 1 As shown, the method includes:

[0048] S1, build an eddy current detection system and design a coaxial one-excitation-one-receiving cylindrical eddy current sensor.

[0049] The embodiment of the present invention builds an eddy current detection system, designs a coaxial one-excitation one-receiving cylindrical eddy current sensor, and places it above the metal plate.

[0050] Specifically, the coaxial-excitation-receiving cylindrical eddy current sensor has an inner diameter and an outer diameter of the coil of 0.5 mm and 2 mm respectively, a spacing between the two coils of 1 mm, and a number of turns of each coil of 100.

[0051] S2, establish an eddy current analytical theoretical model based on the structure of the sensor, and perform eddy current detection on metal sheets.

[0052] An eddy current analysis theoretical model is established in the embodiment of the present invention, and eddy current detection is performed on the metal plate. The obtained plate coil mutual inductance sweep frequency signal M(ω) is stored in a computer. The computer extracts the frequency point where the real part and the imaginary part of the coil sweep frequency signal are equal, and the frequency point is recorded as the crossover frequency.

[0053] Specifically, the electromagnetic parameters of the sample in the analytical theoretical model are set, and the analytical theoretical model is established, including the conductivity σ and the thickness d of the metal plate. The analytical model is specifically:

[0054]

[0055] In the formula

[0056]

[0057]

[0058]

[0059] Where K is the coefficient of the sensor, which determines the amplitude of the signal. φ(α) is related to the electromagnetic properties and thickness of the metal component, and determines the phase of the eddy current response signal.

[0060] S3, based on the adjusted coil lifting distance, the mutual inductance frequency sweep signal of the plate coil is collected, and the frequency point where the real part and the imaginary part of the mutual inductance frequency sweep signal are equal is extracted and recorded as the crossover frequency.

[0061] The embodiment of the present invention adjusts the height of the sensor to change the lifting distance of the coil. Under the new lifting distance, the collected mutual inductance sweep frequency signal M(ω) of the plate coil is stored in the computer, and the computer still sets the crossover frequency of the real part and the imaginary part of the coil sweep frequency signal to be equal.

[0062] Specifically, the plate coil mutual inductance sweep signal M(ω) is obtained by sweeping eddy currents on the metal plate using an analytical model, and the excitation frequency range is 10 Hz to 1 MHz. The real part of the mutual inductance change mainly represents the reflected magnetic field energy, and the imaginary part of the mutual inductance change represents the heat loss of the eddy current. When the real part and the imaginary part are equal, the energy of the emitted magnetic field is equal to the energy of the heat loss. This characteristic point is closely related to the thickness of the metal. The crossover frequency point where the real part and the imaginary part of the coil sweep signal are equal is extracted, as shown in Figure 2 shown.

[0063] S4, when it is determined that the required number of coil lift-off distance adjustment times is reached, different crossover frequencies at different lift-off distances are obtained.

[0064] Specifically, step S3 is repeatedly performed until the maximum lift-off distance height required is reached, and the crossover frequency ω under different lift-off distances is obtained. n+1 .

[0065] S5, the fitting slope is obtained by linearly fitting the relationship between the logarithm of different crossover frequencies and the lift-off distance.

[0066] For different crossover frequencies ω n+1 Perform logarithmic processing and use linear fitting to obtain the crossover frequency logarithm and lift-off distance l n+1 The slope k1 of the fitting line has nothing to do with the conductivity σ and is only determined by the thickness d of the plate;

[0067] log(ω n+1 )=k1l n+1 +b1(6)

[0068] Specifically, the crossover frequency ω at different lift-off distances n+1 Perform logarithmic processing and use linear fitting to obtain the crossover frequency logarithm and lift-off distance l n+1 The slope k1 of the fitting line has nothing to do with the conductivity σ. The following steps are further included:

[0069] The analytical theoretical model is used to calculate l at different lift-off distances. n+1 The crossover frequency ω n+1 , the lift distance ranges from 1mm to 3mm, for the crossover frequency ω n+1 Logarithmic processing is performed and a linear fitting function is used to characterize the crossover frequency logarithm and the lift-off distance l n+1 relationship;

[0070] The conductivity σ of the metal sheet was changed to 18MS / m, 38MS / m and 58MS / m respectively, and the l under different lifting distances was calculated based on the analytical theoretical model. n+1 The crossover frequency ω n+1 , and then the crossover frequency ω n+1 Perform logarithmic processing to calculate the crossover frequency logarithm and lift-off distance l n+1 The slope of the linear fitting function of is found to be constant with the change of conductivity σ, as Figure 3 As shown;

[0071] The thickness d of the metal sheet is changed from 1mm to 5mm with an interval of 1mm. Then the analytical theoretical model is used to calculate the l under different lifting distances. n+1 The crossover frequency ω n+1 , for the crossover frequency ω n+1 Perform logarithmic processing to calculate the crossover frequency logarithm and lift-off distance ln+1 The slope of the linear fitting function is shown in Figure 2, and it is found that the fitting slope changes with the change of thickness d.

[0072] S6, obtaining the plate thickness measurement result according to the relationship between the fitting slope and the plate thickness.

[0073] Specifically, the slope k1 of the fitted straight line corresponding to the plates of different thicknesses is measured, and the linear fitting is still used to characterize the relationship between the thickness d and the slope k1 of the straight line, such as Figure 4 As shown;

[0074] k1=k2d+b2 (7)

[0075] Furthermore, in one embodiment of the present invention, the slope k1 of the fitted line corresponding to the plates of different thicknesses is measured, and the relationship between the thickness d and the slope k1 of the line is characterized by linear fitting; the characteristic is that the fitting slope k1 under different thicknesses is calculated, and it decreases monotonically with the increase of thickness, so the thickness information of the plate can be uniquely determined by the fitting slope k1. A linear fitting function is used to describe the relationship between the thickness d and the slope k1 of the line, and the correlation coefficient of the fitting can reach 0.999.

[0076] When the coil probe is located above the jth metal plate to be tested, the crossover frequency of the mutual inductance signal at different lifting distances is extracted. Using the linear equation log(ω n+1 ) = k j l n+1 +b j Describe the relationship between the lift-off distance and the crossover frequency and logarithm. j Substituting into formula (8), the thickness of the jth metal sheet to be tested can be obtained:

[0077]

[0078] According to the metal sheet thickness measurement method based on electromagnetic eddy current in the embodiment of the present invention, through a large number of calculations, it is found that the slope of the fitting line is independent of the conductivity, so there is no need to calibrate the electromagnetic properties of the plate. The slope of the fitting line is only determined by the thickness of the plate, and the thickness of the plate can be effectively measured. At the same time, the measurement results at different lifting distances are used to eliminate the influence of the electromagnetic properties of the metal on the thickness, and the accuracy of the thickness measurement of the metal plate is theoretically improved.

[0079] In order to implement the above embodiment, Figure 5 As shown, this embodiment also provides a metal sheet thickness measurement system 10 based on electromagnetic eddy current, including:

[0080] The sensor design module 100 is used to build an eddy current detection system and design a coaxial one-excitation-one-receiving cylindrical eddy current sensor;

[0081] The eddy current detection module 200 is used to establish an eddy current analytical theoretical model based on the structure of the sensor and perform eddy current detection on the metal plate;

[0082] The frequency point extraction module 300 is used to collect the mutual inductance frequency sweep signal of the plate coil based on the adjusted coil lifting distance, and extract the frequency point where the real part and the imaginary part of the mutual inductance frequency sweep signal are equal and record it as the crossover frequency;

[0083] A different crossover frequency calculation module 400 is used to obtain different crossover frequencies under different lift-off distances when it is determined that the required number of coil lift-off distance adjustment times has been reached;

[0084] A fitting slope calculation module 500 is used to obtain a fitting slope by linearly fitting the relationship between the logarithm of different crossover frequencies and the lift-off distance;

[0085] The plate thickness measurement module 600 is used to obtain a plate thickness measurement result according to the relationship between the fitting slope and the plate thickness.

[0086] Furthermore, an eddy current analytical theoretical model is established based on the structure of the sensor, including:

[0087]

[0088] In the formula

[0089]

[0090] Among them, σ is the conductivity, d is the thickness of the metal plate, K is the coefficient of the sensor, and φ(α) is related to the electromagnetic properties and thickness of the metal component, which determines the phase of the eddy current response signal.

[0091] Furthermore, the fitting slope is determined by the thickness of the plate:

[0092] log(ω n+1 )=k1l n+1 +b1 (6)

[0093] Among them, k1 is the fitting slope, l n+1 For different lifting distances.

[0094] Furthermore, linear fitting is used to characterize the relationship between the thickness of the plate and the fitting slope k1:

[0095] k1=k2d+b2 (7).

[0096] Furthermore, when the coil probe is located above the jth metal plate to be tested, the crossover frequency of the mutual inductance signal under different lifting distances is extracted, and the relationship between the lifting distance and the crossover frequency and logarithm is described using equation (6). The slope k j Substituting into formula (7), we can get the thickness of the jth metal sheet to be tested:

[0097]

[0098] According to the metal sheet thickness measurement system based on electromagnetic eddy current in the embodiment of the present invention, through a large number of calculations, it is found that the slope of the fitting line is independent of the conductivity, so there is no need to calibrate the electromagnetic properties of the sheet metal. The slope of the fitting line is only determined by the thickness of the sheet metal, and the thickness of the sheet metal can be effectively measured. At the same time, the measurement results at different lifting distances are used to eliminate the influence of the electromagnetic properties of the metal on the thickness, and the accuracy of the thickness measurement of the metal sheet metal is theoretically improved.

[0099] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0100] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

Claims

1. A method for measuring the thickness of a metal sheet based on electromagnetic eddy current, characterized in that: include: Build an eddy current detection system and design a coaxial one-excitation-one-receiving cylindrical eddy current sensor; Based on the structure of the sensor, an eddy current analytical theoretical model is established, and eddy current detection is performed on metal sheets; The mutual inductance frequency sweep signal of the plate coil is collected based on the adjusted coil lifting distance, and the frequency point where the real part and the imaginary part of the mutual inductance frequency sweep signal are equal is extracted and recorded as the crossover frequency; When it is determined that the required number of coil lift-off distance adjustment times is reached, different crossover frequencies at different lift-off distances are obtained; The fitting slope is obtained by linearly fitting the relationship between the logarithm of different crossover frequencies and the lift-off distance; The plate thickness measurement result is obtained based on the relationship between the fitting slope and the plate thickness.

2. The method according to claim 1, characterized in that The eddy current analytical theoretical model is established based on the structure of the sensor, including: In the formula Among them, σ is the conductivity, d is the thickness of the metal plate, K is the coefficient of the sensor, and φ(α) is related to the electromagnetic properties and thickness of the metal component, which determines the phase of the eddy current response signal.

3. The method according to claim 1, characterized in that The fitting slope is determined by the thickness of the sheet: log(ω n+1 )=k1l n+1 +b1(6) Among them, k1 is the fitting slope, l n+1 For different lifting distances.

4. The method according to claim 1, characterized in that: Linear fitting is used to characterize the relationship between the thickness of the plate and the fitting slope k1: k1=k2d+b2(7).

5. The method according to claim 1, characterized in that When the coil probe is located above the jth metal plate to be tested, the crossover frequency of the mutual inductance signal under different lifting distances is extracted. The relationship between the lifting distance and the crossover frequency and logarithm is described using equation (6). The slope k j Substituting into formula (7), we can get the thickness of the jth metal sheet to be tested:

6. A metal sheet thickness measurement system based on electromagnetic eddy current, characterized in that: include: Sensor design module, used to build eddy current detection system and design coaxial-excitation-receiving cylindrical eddy current sensor; Eddy current detection module, used to establish an eddy current analytical theoretical model based on the structure of the sensor and perform eddy current detection on metal plates; The frequency point extraction module is used to collect the mutual inductance frequency sweep signal of the plate coil based on the adjusted coil lifting distance, and extract the frequency point where the real part and the imaginary part of the mutual inductance frequency sweep signal are equal and record it as the crossover frequency; A different crossover frequency calculation module is used to obtain different crossover frequencies under different lift-off distances when it is determined that the required number of coil lift-off distance adjustment times has been reached; A fitting slope calculation module, used to obtain a fitting slope by linearly fitting the relationship between the logarithm of different crossover frequencies and the lift-off distance; The plate thickness measurement module is used to obtain the plate thickness measurement result according to the relationship between the fitting slope and the plate thickness.

7. The system according to claim 6, characterized in that The eddy current analytical theoretical model is established based on the structure of the sensor, including: In the formula Among them, σ is the conductivity, d is the thickness of the metal plate, K is the coefficient of the sensor, and φ(α) is related to the electromagnetic properties and thickness of the metal component, which determines the phase of the eddy current response signal.

8. The system according to claim 6, characterized in that The fitting slope is determined by the thickness of the sheet: log(ω n+1 )=k1l n+1 +b1(6) Among them, k1 is the fitting slope, l n+1 For different lifting distances.

9. The system according to claim 6, characterized in that Linear fitting is used to characterize the relationship between the thickness of the plate and the fitting slope k1: k1=k2d+b2(7).

10. The system according to claim 6, characterized in that When the coil probe is located above the jth metal plate to be tested, the crossover frequency of the mutual inductance signal under different lifting distances is extracted. The relationship between the lifting distance and the crossover frequency and logarithm is described using equation (6). The slope k j Substituting into formula (7), we can get the thickness of the jth metal sheet to be tested:

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