A pulsed eddy current detection method and device for steel bar diameter based on parameter inversion

Abstract

The present invention discloses a method and device for detecting the diameter of a steel bar based on parameter inversion. Based on the time domain analytical solution of the steel bar pulse eddy current detection model, the induced voltage measurement curve is used to establish a least squares problem between the induced voltage time domain signal measurement value and the theoretical calculation value to invert the diameter, relative magnetic permeability and electrical conductivity of the inspected steel bar. When the relative error sum between the induced voltage measurement curve and the theoretical calculation curve is the minimum, the diameter, relative magnetic permeability and electrical conductivity of the inspected steel bar can be obtained, wherein the inverted steel bar diameter is the detection result, thereby obtaining the size specification of the inspected steel bar. The method and device of the present invention do not require standard test blocks for calibration, can implement more accurate non-destructive detection of the parameters of the inspected steel bar, and eliminate the influence of the electromagnetic parameters of the steel bar on the detection results.

Description

Technical Field

[0001] The present invention belongs to the technical field of electromagnetic nondestructive testing, and relates to a method and device for detecting steel bar diameter by pulsed eddy current based on parameter inversion. Background Art

[0002] In the construction field, reinforced concrete is widely used due to its excellent compressive strength and tensile strength. Parameters such as the nominal diameter, position and protective layer thickness of the steel bars in concrete have an important influence on the structural stability and mechanical properties of the building. Therefore, the detection and evaluation of the parameters of reinforced concrete are very important in the fields of project acceptance, maintenance of old buildings, etc.

[0003] At present, in the construction field, electromagnetic induction method is often used to detect the parameters of steel bars in concrete. Electromagnetic induction method can use DC, AC and pulse excitation methods to generate a magnetic field in the target area to magnetize the steel bars, thereby generating eddy currents inside the steel bars, and then generating a secondary magnetic field. The disturbance of the magnetic field is detected by the sensor to estimate the parameters of the steel bars. At present, common detection equipment can already measure the position of the steel bars and the thickness of the protective layer, but due to the lack of direct electromagnetic field theory research, it still cannot accurately detect the diameter of the steel bars.

[0004] The pulsed eddy current method is a non-contact electromagnetic non-destructive testing method. A pulsed excitation current is passed through the excitation coil to generate a pulsed strong magnetic field. The changing magnetic field induces a transient eddy current field in the steel bar, and this eddy current field induces a voltage signal at both ends of the detection coil. By measuring the attenuation process of the induced voltage, the position of the steel bar is determined and the diameter of the steel bar is detected. The pulsed eddy current method applies a pulse excitation, which can generate a transient strong magnetic field. Its biggest advantage is its strong penetration ability and higher sensitivity to changes in parameters such as the diameter of the steel bar and the thickness of the concrete outside the steel bar. In addition, concrete is a non-conductive and non-magnetic material, which will not interfere with the distribution and detection signal of the pulsed eddy current field. Summary of the invention

[0005] In view of the above problems, the present invention proposes a pulse eddy current detection method and device for steel bar diameter based on parameter inversion. Firstly, the actual position of the steel bar is determined by the designed device using pulse eddy current continuous scanning to detect the time domain induced voltage signal, and then the induced voltage time domain signal directly above the steel bar is used for parameter inversion to obtain the diameter of the inspected steel bar.

[0006] A pulse eddy current detection method for steel bar diameter based on parameter inversion, characterized in that the steps are designed as follows:

[0007] Step 1: Determine the location of buried steel bars

[0008] Step 2: Use the induced voltage time domain signal directly above the steel bar to perform parameter inversion to obtain the diameter of the inspected steel bar, specifically:

[0009] First, the diameter D and relative magnetic permeability μ of the steel bar to be tested are r , conductivity σ is set as an unknown parameter, and the parameter vector x to be inverted is (D, μ r ,σ) T ; The induced voltage measurement data collected above the inspected steel bar are (t1,u1),(t2,u2),…,(t m ,u m ). Where t and u are the time domain signal curves of the induced voltage u m The time point on (t) and the induced voltage corresponding to the time point are compared with the theoretical value of the induced voltage u(x,t). The parameter x is inverted by minimizing the sum of squares of the relative errors between the measured value of the induced voltage signal and the theoretical calculated value obtained by the time domain expression of the induced voltage, and the least squares problem is established:

[0010]

[0011] The residual percentage function is:

[0012]

[0013] And let the residual function vector be:

[0014] r(x)=(r1(x),r2(x),...,r m (x) T .

[0015] Then, the iterative algorithm is used to solve the optimal solution x of the least squares problem. * , the calculation steps of the iterative algorithm are:

[0016] (1) Given an initial point (Where D (1) =5~40mm, σ (1) =1~10MS / m), allowable error ε>0 (generally ε=10 -3 ), set k=1.

[0017] (2) The parameter vector of step k Substituting into equation (1), we can calculate the i The theoretical value of the induced voltage u(x (k) ,t i ), and then with the measured value of the induced voltage u i Subtract and calculate the residual percentage function value

[0018]

[0019] And get the residual function vector r (k); Then, the first-order partial derivative of the induced voltage theoretical curve with respect to the diameter D is further calculated by formula (1):

[0020]

[0021] And the induced voltage theoretical curve versus relative magnetic permeability μ r The first partial derivative of

[0022]

[0023] And the first-order partial derivative of the induced voltage theoretical curve with respect to conductivity σ

[0024]

[0025] Get the m×3 matrix A k =(a ij ) m×3 , j=1,2,3.

[0026] (3) Solving the equations

[0027]

[0028] Find the direction vector b (k) ;

[0029] (4) From the parameter vector x (k) Departure along b (k) Perform a one-dimensional search and find the step length α k , so that

[0030]

[0031] And order

[0032] x (k+1) =x (k) +α k b (k) ;

[0033] (5) If ||x (k+1) -x (k) ||≤ε, then stop the calculation and get the optimal solution of the least squares problem Otherwise, set k:=k+1 and return to step (2).

[0034] The optimal solution of the least squares problem is solved by the above iterative algorithm After that, the inversion result of the diameter of the inspected steel bar is obtained. Inversion results of relative magnetic permeability Inversion results of conductivity

[0035] The advantages of the present invention are:

[0036] 1. The pulse eddy current detection method and device for steel bar diameter based on parameter inversion of the present invention can implement more accurate non-destructive detection of the parameters of the inspected steel bars.

[0037] The present invention uses the time domain analytical solution of the induced voltage of the steel bar pulse eddy current detection model and the induced voltage measurement curve to establish the least square problem between the measured value and the calculated value of the induced voltage signal, so as to minimize the sum of the squares of the relative errors between the two to invert the parameters of the inspected steel bar. Compared with the previous electromagnetic detection method of extracting detection feature quantities from the detection signal, this method not only extracts a few special points on the signal curve, but makes full use of the information on the entire induced voltage signal curve, and can more effectively evaluate the entire attenuation process of the pulse eddy current electromagnetic field, so that more accurate eddy current detection can be implemented on the parameters of the inspected steel bar.

[0038] 2. The pulse eddy current detection method and device for steel bar diameter based on parameter inversion of the present invention eliminates the influence of the electromagnetic parameters of the steel bar on the detection results.

[0039] Electromagnetic parameters such as the magnetic permeability and electrical conductivity of steel bars will change due to the influence of production processes and environmental factors such as temperature and stress. Therefore, when electromagnetic testing is performed on steel bars, the magnetic permeability and electrical conductivity of the steel bars cannot be accurately determined, thereby affecting the measurement results of the steel bar diameter. In the present invention, the magnetic permeability and electrical conductivity of the steel bars to be tested are set as unknown parameters, and the parameter inversion method of the present invention is used to determine the values ​​of the magnetic permeability and electrical conductivity of each steel bar and each time it is tested, thereby eliminating the influence of electromagnetic parameter changes on the test results and improving the detection accuracy and reliability of the steel bar diameter.

[0040] 3. The pulse eddy current detection method and device for steel bar diameter based on parameter inversion of the present invention do not require calibration with standard test blocks.

[0041] Old buildings often lack relevant information such as steel bar grades and sizes, and their electromagnetic parameters cannot be calibrated by standard steel bars. In the absence of prior information about steel bars, it is difficult to accurately detect the diameter of steel bars. The method of the present invention does not require calibration of steel bars. The diameter and electromagnetic parameters of the steel bars under test can be determined simultaneously by parameter inversion of the induced voltage signal at both ends of the detection coil, thus realizing non-destructive detection of the diameter of steel bars buried in concrete, and making the method adaptable to more application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is the structural diagram of the pulsed eddy current electromagnetic nondestructive testing system for buried steel bars.

[0043] Figure 2 This is a cross-sectional diagram of the cylindrical coil probe.

[0044] Figure 3It is the flow chart of pulse eddy current detection of steel bar diameter.

[0045] Figure 4 This is a comparison chart between the experimental measurement results of induced voltage and the theoretical calculation results.

[0046] Figure 5 It is the difference curve of induced voltage.

[0047] Figure 6 It is a pulse eddy current detection signal curve at different probe eccentricity distances.

[0048] Figure 7 It is a comparison chart between the experimental measurement curve of induced voltage collected from steel bars of different diameters and the theoretical calculation curve.

[0049] In the figure:

[0050] 1- coil probe 2- computer 3- DA digital-to-analog converter 4- power amplifier circuit

[0051] 5-Data acquisition card 101-Coil skeleton 102-Excitation coil 103-Detection coil DETAILED DESCRIPTION

[0052] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0053] The present invention uses a pulsed eddy current detection method for steel bar diameter based on parameter inversion. First, the actual position of the steel bar needs to be determined by continuous pulsed eddy current scanning detection signals. Then, the induced voltage time domain signal directly above the steel bar is used for parameter inversion to obtain the diameter of the steel bar to be detected. The steel bar to be detected can be a common round steel bar or a threaded steel bar buried in concrete, such as Figure 3 As shown, the specific detection steps are:

[0054] Step 1: Continuously scan the buried steel bars using a pulsed eddy current electromagnetic nondestructive testing device.

[0055] The pulsed eddy current electromagnetic nondestructive testing device is as follows: Figure 1 As shown, it includes a coil probe 1, a computer 2, a DA digital-to-analog converter 3, a power amplifier circuit 4 and a data acquisition card 5. The cylindrical coil probe 1 is used to obtain the induced voltage time domain signal during continuous scanning and send it to the computer 2 for storage. Figure 2As shown, the coil probe 1 is composed of a coil skeleton 101, an excitation coil 102 and a detection coil 103. The coil skeleton 101 is arranged at the center of the excitation coil 102, and the outside of the excitation coil 102 is the detection coil 103; or the coil skeleton 101 is arranged at the center of the detection coil 103, and the outside of the detection coil 103 is the excitation coil 102. The computer 2 is used to realize functions such as signal acquisition, processing, result display, and data storage, and processes the received discrete signal data to obtain information such as the position and diameter of the steel bars in the concrete. The power amplifier is used to realize signal amplification. The data acquisition card 5 is used to collect the induced voltage time domain signal at both ends of the detection coil 103. The DA digital-to-analog converter 3 is used to convert the excitation digital signal output by the computer 2 into an analog signal.

[0056] With the above detection device, the coil probe 1 is placed close to the outer layer of concrete, and a continuous scan is performed in one direction along the vertical direction of the buried steel bars, with the radius r of the circular coil probe 1 as the step length.

[0057] Step 2: Perform a pulse eddy current detection signal acquisition for each detection point to obtain the induced voltage time domain signal of each detection point.

[0058] The acquisition of induced voltage time domain signals using a pulsed eddy current testing system is called the pulsed eddy current testing signal acquisition procedure, Signal Acquisition Procedure, SAP. The specific steps are as follows:

[0059] SAP-1: Connect the two ends of the exciting coil 102 in the coil probe 1 to the output ends of the power amplifier circuit, and connect the two ends of the detecting coil 103 to the input ends of the AD analog-to-digital converter in the data acquisition card 5 .

[0060] SAP-2: The computer 2 outputs an excitation digital signal with a continuous pulse width of 10 to 1000ms and an amplitude of 0.1 to 1V; after passing through the DA digital-to-analog converter 3, it becomes an excitation analog signal with a continuous pulse width of 10 to 1000ms and an amplitude of 0.1 to 1V, and is output to the power amplifier circuit 4; after amplifying the power of the power amplifier circuit 4, a pulse excitation current with a continuous pulse width of 10 to 1000ms and an amplitude of 0.1 to 10A is output to the excitation coil 102;

[0061] SAP-3: The induced voltage time domain signal u(t) at both ends of the detection coil 103 is collected through the data acquisition card 5, in V, and the collected induced voltage time domain signal u(t) is stored in the computer.

[0062] In the above process, in the steel bar diameter pulse eddy current detection model, when the pulse excitation current i(t) is passed through the excitation coil 102, the time domain expression of the induced voltage at both ends of the detection coil 103 is:

[0063]

[0064] In formula (1), π is 3.14; e is the base of natural logarithm, which is 2.72; σ is the conductivity of the steel bar under test, in S / m; D is the diameter of the steel bar under test, in m; μ0 is the magnetic permeability of vacuum, which is 4π×10 -7 H / m; μ r is the relative magnetic permeability of the steel bar under inspection; i(t) is the pulse excitation current, in A; i′(t) represents the derivative of the pulse excitation current with respect to time; “*” represents the convolution operation with respect to time; I m (x) and K m (x) are the first and second m-order modified Bessel functions respectively; λ is the integral variable.

[0065] The coefficients in formula (1) are:

[0066]

[0067]

[0068]

[0069] F(ξ,m)=mJ m (ξ)-ξJ m+1 (ξ);

[0070] Among them, ξ t is the denominator expression Y m (ξ)=0's tth positive real root, in formula (1), Y' m (ξ) represents the denominator Y m (ξ) is the derivative of the variable ξ; C p (λ,m) is the coil coefficient of the detection coil, and its expression is:

[0071]

[0072] Where h is the thickness of the concrete layer on the steel bar surface; s is the variable after Laplace transformation; h p is the height of the detection coil; r pi and r po are the inner and outer radii of the detection coil; r is the integral variable, and its integral interval is the inner and outer radii of the detection coil; l0 is the eccentricity distance of the probe (the distance from the center axis of the coil probe to the axis of the steel bar); N p is the number of turns of the detection coil; C d (λ,m) is the coil coefficient of the excitation coil, and its expression is the same as C p(λ,m) are the same, and the variables with the subscript p are the relevant parameters of the detection coil, which are related to the parameters of the coil itself. It is only necessary to replace these parameters with the relevant parameters of the excitation coil.

[0073] Step 3: Judge the induced voltage time domain signals of three adjacent detection points to determine the position of the steel bar.

[0074] First, suppose that the induced voltage time domain signal collected at the middle detection point among the three adjacent detection points in the time period t is u m (t), the time domain signal of the induced voltage at the previous detection point is u m-1 (t), the time domain signal of the induced voltage at the next detection point is u m+1 (t). Draw the difference curve u of the induced voltage m (t)-u m-1 (t), and determine the peak time t0 of the difference curve, such as Figure 5 As shown; then the voltage amplitude of the induced voltage time domain signal at the time t0 of the three adjacent detection points is extracted respectively. m-1 、V m and V m+1 , stored in the host as the corresponding signal feature quantity, such as Figure 6 As shown. If the feature quantity satisfies V m >V m-1 and V m >V m+1 , it means that the middle detection point is located directly above the steel bar to be inspected, and the induced voltage signal u m (t) and the position information of the current point are stored in the computer, and the detection point directly above the inspected steel bar is recorded as Q j , then implement step 4 to detect the steel bar diameter; otherwise, return to step 1 and repeat the process to perform continuous pulse eddy current scanning detection at other locations outside the concrete.

[0075] Step 4: Inverse the parameters of the inspected steel bar to determine the diameter of the inspected steel bar.

[0076] After obtaining the induced voltage time domain signal at both ends of the detection coil at the detection point directly above the inspected steel bar in step 3, how to invert the diameter of the inspected steel bar is the key to signal processing in the pulse eddy current detection of steel bar diameter. Therefore, in the present invention, the induced voltage time domain signal at both ends of the detection coil at the detection point directly above the inspected steel bar is obtained in step 3. After that, a least squares problem is established to invert the diameter, relative magnetic permeability and electrical conductivity of the inspected steel bar at the detection point. This process is called the parameter inversion procedure, Parameters Inversion Procedure, PIP, and the specific steps are as follows:

[0077] Step PIP-1: The diameter D and relative magnetic permeability μ of the steel bar to be tested r, the conductivity σ is set as an unknown parameter, that is, the parameter vector to be inverted is x = (D, μ r ,σ) T ;

[0078] Step PIP-2: According to the SAP step, the time domain induced voltage measurement data at both ends of the detection coil 3 collected by the data acquisition card 11 at the position above the inspected steel bar is (t1, u1), (t2, u2), ..., (t m ,u m ), where t and u are the time domain signal curves of the induced voltage u m The time point on (t) and the induced voltage corresponding to the time point, such as Figure 4 As shown by the solid line in the middle, it is compared with the theoretical value of the induced voltage u(x, t) calculated by equation (1), and the parameter x is inverted by minimizing the sum of squares of the relative errors between the measured value of the induced voltage signal and the theoretical calculated value, that is, establishing a least squares problem:

[0079]

[0080] Where R is the real number field;

[0081] The residual percentage function is:

[0082]

[0083] And let the residual function vector be:

[0084] r(x)=(r1(x),r2(x),...,r m (x) T .

[0085] Step PIP-3: In computer 2, use an iterative algorithm to solve the optimal solution x of the least squares problem (Equation (2)) * , the calculation steps of the iterative algorithm are:

[0086] (1) Given an initial point (Where D (1) =5~40mm, σ (1) =1~10MS / m), allowable error ε>0 (generally ε=10 -3 ), set k = 1;

[0087] (2) The parameter vector of step k Substituting into equation (1), we can calculate the i The theoretical value of the induced voltage u(x (k) ,t i ), and then with the measured value of the induced voltage u i Subtract and calculate the residual percentage function value

[0088]

[0089] And get the residual function vector r (k) ; Then, the first-order partial derivative of the induced voltage theoretical curve with respect to the diameter D is further calculated by formula (1):

[0090]

[0091] And the induced voltage theoretical curve versus relative magnetic permeability μ r The first partial derivative of

[0092]

[0093] And the first-order partial derivative of the induced voltage theoretical curve with respect to conductivity σ

[0094]

[0095] Get the m×3 matrix A k =(a ij ) m×3 , j=1,2,3.

[0096] (3) Solving the equations

[0097]

[0098] Find the direction vector b (k) ;

[0099] (4) From the parameter vector x (k) Departure along b (k) Perform a one-dimensional search and find the step length α k , so that

[0100]

[0101] And order

[0102] x (k+1) =x (k) +α k b (k) ;

[0103] (5) If ||x (k+1) -x (k) ||≤ε, then stop the calculation and get the optimal solution of the least squares problem (2) Otherwise, set k:=k+1 and return to step (2).

[0104] The optimal solution of the least squares problem (2) is obtained by the above iterative algorithm After that, the inversion result of the diameter of the inspected steel bar is obtained. Inversion results of relative magnetic permeability Inversion results of conductivity Optimize the parameters Substituting into equation (1), the theoretical calculation result of induced voltage is calculated as follows: Figure 4 Finally, the inversion results are matched with the position information of the inspected steel bars and saved in the computer.

[0105] Example 1

[0106] An example of pulse eddy current testing of steel bar diameter using parameter inversion in the present invention is given below.

[0107] There are four threaded steel bars with inner diameters of 24.4mm, 21.7mm, 19.4mm and 17.4mm respectively. The detection points directly above the four steel bars are marked as Q0, Q1, Q2 and Q3 respectively. The lifting distance between the lower edge of the coil probe and the surface of the steel bar to be detected is 25.0mm, and the eccentric distance of the probe is 0.

[0108] According to the SAP steps proposed in the present invention, pulse eddy current detection is performed on four steel bars to obtain the induced voltage signal detection signal as follows: Figure 7 The solid line in the middle shows that the sampling rate is 50 kS / s. Then, according to the PIP steps, the parameters of the induced voltage time domain signals at the four detection points are inverted, and finally the parameter inversion results of the four steel bars are obtained as shown in Table 1. Substituting the parameter inversion results into formula (1), the theoretical calculation results of the induced voltage corresponding to steel bars of different diameters are calculated as follows: Figure 7 As shown in the solid point, the experimental measurement results of the induced voltage are in good agreement with the theoretical calculation results. By comparing the diameter inversion results of the four steel bars with the actual diameter values, it is calculated that the relative error of the steel bar diameter detection result is no more than 3%, which meets the accuracy requirements in actual engineering and verifies the feasibility and reliability of the pulse eddy current method in the present invention to detect the steel bar diameter.

[0109] Table 1 Parameter inversion results of different steel bars

[0110]

Claims

1. A pulsed eddy current detection method for steel bar diameter based on parameter inversion, characterized in that: The steps are designed as follows: Step 1: Use pulsed eddy current continuous scanning detection signals to determine the actual location of buried steel bars; a. Place the circular coil probe close to the outer layer of concrete, and use the radius r of the coil probe as the step length to continuously scan in one direction along the vertical direction of the buried steel bars; b. Collect pulse eddy current detection signals at each detection point to obtain the induced voltage time domain signals at each detection point; c. Judge the induced voltage time domain signals of three adjacent detection points to determine the position of the steel bar; First, suppose that the induced voltage time domain signal collected at the middle detection point among the three adjacent detection points in the time period t is u m (t), the time domain signal of the induced voltage at the previous detection point is u m-1 (t), the time domain signal of the induced voltage at the next detection point is u m+1 (t); draw the difference curve u of the induced voltage m (t)-u m-1 (t), and determine the peak time t0 of the difference curve; then extract the voltage amplitude of the induced voltage time domain signal of the three adjacent detection points at time t0 as V m-1 、V m and V m+1 , as the corresponding signal feature; if the feature satisfies V m >V m-1 and V m >V m+1 , it means that the middle detection point is located directly above the inspected steel bar; Otherwise, return to step a and repeat the process to perform continuous pulsed eddy current scanning inspection at other locations outside the concrete; Step 2: Use the induced voltage time domain signal directly above the steel bar to perform parameter inversion to obtain the diameter of the inspected steel bar, specifically: First, the diameter D and relative magnetic permeability μ of the steel bar to be tested are r , conductivity σ is set as an unknown parameter, and the parameter vector x to be inverted is (D, μ r ,σ) T ; The induced voltage measurement data collected above the inspected steel bar are (t1,u1),(t2,u2),…,(t m ,u m ), where t and u are the time domain signal curves of the induced voltage u m The time point on (t) and the induced voltage corresponding to the time point are compared with the theoretical value of the induced voltage u(x,t). The parameter x is inverted by minimizing the sum of squares of the relative errors between the measured value of the induced voltage signal and the theoretical calculated value obtained by the time domain expression of the induced voltage, and the least squares problem is established: The residual percentage function is: And let the residual function vector be: r(x)=(r1(x),r2(x),...,r m (x)) T ; Then, the iterative algorithm is used to solve the optimal solution x of the least squares problem. * , the calculation steps of the iterative algorithm are: (1) Given an initial point Where D (1) =5~40mm, σ (1) =1~10MS / m, allowable error ε>0, take ε=10 -3 , set k = 1; (2) The parameter vector of step k Substituting into equation (1), we can calculate the i The theoretical value of the induced voltage u(x (k) ,t i ), and then with the measured value of the induced voltage u i Subtract and calculate the residual percentage function value And get the residual function vector r (k) ; Then, the first-order partial derivative of the induced voltage theoretical curve with respect to the diameter D is further calculated by formula (1): And the induced voltage theoretical curve versus relative magnetic permeability μ r The first partial derivative of And the first-order partial derivative of the induced voltage theoretical curve with respect to conductivity σ Get the m×3 matrix A k =(a ij ) m×3 , j = 1, 2, 3; (3) Solving the equations Find the direction vector b (k) ; (4) From the parameter vector x (k) Departure along b (k) Perform a one-dimensional search and find the step length α k , so that And order x (k+1) =x (k) +a k b (k) ; (5) If ||x (k+1) -x (k) ||≤ε, then stop the calculation and get the optimal solution of the least squares problem Otherwise, set k:=k+1 and return to step (2); The optimal solution of the least squares problem is solved by the above iterative algorithm After that, the inversion result of the diameter of the inspected steel bar is obtained. Inversion results of relative magnetic permeability Inversion results of conductivity 2. A method for detecting steel bar diameter by pulsed eddy current based on parameter inversion as claimed in claim 1, characterized in that: The pulsed eddy current detection signal acquisition method is: 1) Connect the two ends of the excitation coil in the coil probe to the output end of the power amplifier circuit, and connect the two ends of the detection coil to the input end of the AD analog-to-digital converter in the data acquisition card; 2) The computer outputs an excitation digital signal with a continuous pulse width of 10 to 1000ms and an amplitude of 0.1 to 1V; after passing through a DA digital-to-analog converter, it becomes an excitation analog signal with a continuous pulse width of 10 to 1000ms and an amplitude of 0.1 to 1V, and outputs it to a power amplifier circuit; after amplifying the power of the power amplifier circuit, it outputs a pulse excitation current with a continuous pulse width of 10 to 1000ms and an amplitude of 0.1 to 10A to the excitation coil; 3) The induced voltage time domain signal u(t) at both ends of the detection coil is collected by a data acquisition card, and the collected induced voltage time domain signal u(t) is stored in a computer.

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