Non-magnetic pipeline conductivity measurement method based on double-frequency eddy current detection

Abstract

The invention provides a non-magnetic pipeline conductivity measurement method based on double-frequency eddy current detection, and the method comprises the steps: carrying out the conductivity estimation of a non-magnetic pipeline based on the double-frequency eddy current detection, and determining a coil lift-off distance through selecting a coil complex inductance variable quantity real part corresponding to a high-frequency-band single frequency; after the lift-off distance is obtained, a pipeline asymmetric model analytic solution is utilized, a coil phase corresponding to a single frequency in a low-frequency band is selected as a target function value, and the conductivity of the pipeline is obtained through iterative solution of an improved Newton-Raphson method; according to the invention, the conductivity of the pipeline can be obtained only through double-frequency measurement under the condition that the insulation of the pipeline is unknown, so that the accuracy and the measurement efficiency of conductivity measurement are improved.

Description

Technical Field

[0001] The invention relates to the field of measurement technology, in particular to a non-magnetic pipeline conductivity measurement method based on dual-frequency eddy current detection. Background Art

[0002] With the rapid development of the aviation industry, the requirements for aircraft performance are increasing, especially in terms of safety and reliability. As an important part of the aircraft structure, metal pipes undertake the important task of transporting multiple media such as fuel, hydraulic oil, and air. The material selection and performance of these pipes directly affect the overall performance and flight safety of the aircraft. As a basic physical parameter of metal materials, conductivity can not only be combined with hardness measurement to evaluate the internal quality of the pipe, but also can be used to identify and classify different materials. In addition, corrosion can affect the conductivity and thickness of the specimen. Regular inspection can timely identify potential risks and avoid serious accidents such as leakage and explosion caused by material failure. Therefore, the development of an accurate, efficient, and non-destructive conductivity measurement technology for metal pipes has important practical application value for extending the service life of pipes, reducing maintenance costs, and improving industrial safety levels.

[0003] At present, the main methods for measuring metal conductivity are four-point probe test method and eddy current detection method. The four-point probe test method is a traditional conductivity measurement technology, but it requires direct contact with the sample surface, which may cause damage to the sample surface, and its application is limited to bulk solids with uniform conductivity. Compared with the four-point probe test method, the eddy current detection method based on the principle of electromagnetic induction has the advantages of non-contact, fast and efficient. In recent years, scholars have done a lot of research on the use of eddy current detection method to achieve conductivity measurement, but they are mainly aimed at metal flat plates, and rarely discuss pipeline models. Summary of the invention

[0004] The present invention proposes a non-magnetic pipeline conductivity measurement method based on dual-frequency eddy current detection. When the pipeline insulation is unknown, its conductivity can be obtained only through dual-frequency measurement, thereby improving the accuracy and efficiency of conductivity measurement.

[0005] The present invention adopts the following technical solutions.

[0006] A method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection. The method estimates the electrical conductivity of the non-magnetic pipeline based on dual-frequency eddy current detection, and determines the coil lift-off distance by selecting the real part of the change in the complex inductance of the coil corresponding to a single frequency in a high-frequency band; after obtaining the lift-off distance, an analytical solution of an asymmetric model of the pipeline is used, and the coil phase corresponding to a single frequency in a low-frequency band is selected as the objective function value, and the pipeline conductivity is obtained by iteratively solving the improved Newton-Raphson method.

[0007] When the insulation value of the non-magnetic pipeline is unknown, the measurement method is used to obtain the electrical conductivity of the pipeline only through dual-frequency measurement.

[0008] The measuring method comprises the following steps:

[0009] Step 1: Use a sensor to measure the coil impedance change ΔZ in the high frequency band and the low frequency band respectively;

[0010] Step 2: Determine the lifting distance;

[0011] Step 3: Calculate the conductivity.

[0012] In step 1, ΔZ=ZZ air , where Z is the coil impedance measured when containing the sample to be tested, Z air It is the coil impedance measured without the sample to be tested.

[0013] In step 2, the real part of the complex inductance variation of the non-magnetic pipeline in the high frequency band tends to a constant value C, which is independent of the pipeline thickness and conductivity and only related to the lift-off, such as Figure 2 As shown; and when the lift-off distance is within a preset range, the constant and the natural logarithm of the lift-off satisfy a quadratic function relationship, that is, C = a × ln 2 (l)+b×ln(l)+c, where a, b, c are coefficients of the expression;

[0014] In step 2, the lift-off distance l is determined by the measured constant value C.

[0015] In step 2, the change in complex inductance ΔL in the high frequency band is obtained from the change in coil impedance ΔZ. The lifting distance is then determined by the real part of the change in complex inductance in the high frequency band.

[0016] In step 3, according to the phase at low frequency As well as the established analytical solution, the conductivity estimation value is obtained by using the improved Newton-Raphson iteration method, which is:

[0017] Select the initial iteration value of conductivity, substitute the lift-off distance l into the forward problem to obtain the calculated Re(ΔL) and Im(ΔL), and then obtain the phase The modified Newton-Raphson method is used to determine the conductivity value that minimizes the square of the Euclidean distance between the calculated phase and the measured phase, which will be used as the final conductivity estimate; phase The specific calculation formula is:

[0018]

[0019]

[0020] The coil coefficient C s (α,m) is calculated using the magnetic flux density generated by the coil in free space.

[0021] In the non-magnetic pipeline, the top view of the pipeline and the conical coil is as follows: Figure 4 As shown, the axis of the tube is defined as the z-axis of the cylindrical coordinate system, and all subsequent calculations will be performed in this coordinate system;

[0022] First, in the simulation software, the coil in free space is extracted at the surface of the pipe r = r o Where Then the following expression is used to obtain:

[0023]

[0024] is r=r o Two-dimensional Fourier transform results of the z component of the free space magnetic flux density on the surface;

[0025] The second-order vector potential method is used to calculate the pipeline coefficient R(α, m), and the final calculation formula for R(α, m) is:

[0026]

[0027] in,

[0028] k 2 =-jωμ0σ

[0029]

[0030] I pq =I m (α p r q ),K pq =K m (α p r q )

[0031]

[0032]

[0033] I m (x) is the modified Bessel function of the first kind of order m, K m (x) is the modified Bessel function of the second kind of order m.

[0034] ω——Excitation current angular frequency

[0035] μ0——vacuum magnetic permeability

[0036] I——excitation current

[0037] σ——Pipeline conductivity

[0038] r i ——Inner diameter of pipe

[0039] r o ——Outer diameter of the pipe.

[0041] In step 3, the phase After calculating the formula, the actual coil phase measured in the low frequency band is As the objective function value of the improved Newton-Raphson method, and the final conductivity estimation value is obtained by the improved Newton-Raphson method;

[0042] The pipeline conductivity estimation algorithm is as follows:

[0043] Input: Initial guess of conductivity σ 1 , tolerance ε, maximum number of iterations n i , the objective function value Output: conductivity σ;

[0044] Initialization: Set Let k = 1, 2, ..., n i And use it as the count to iterate in the loop. In each iteration,

[0045]

[0046] when Stop iteration and output the calculated value.

[0047] The coil is a conical coil or a frustum-shaped coil, and the change in complex inductance at high frequency tends to a constant value C. When the lifting distance is within the range of 1 cm, the constant and the natural logarithm of the lifting distance satisfy a quadratic function relationship.

[0048] The present invention proposes a novel method for measuring the electrical conductivity of non-magnetic metal pipelines. The electrical conductivity of non-magnetic pipelines is estimated based on dual-frequency eddy current detection. In view of the influence of lift-off fluctuations on eddy current detection results, the present invention proposes a method for determining the coil lift-off distance by selecting the real part of the change in the complex inductance of the coil corresponding to a single frequency in the high-frequency band. After obtaining the lift-off distance, the analytical solution of the pipeline asymmetric model is used, and the coil phase corresponding to a single frequency in the low-frequency band is selected as the objective function value. The electrical conductivity of the pipeline is obtained by iteratively solving the improved Newton-Raphson method. This method can obtain the electrical conductivity of the pipeline only through dual-frequency measurement when the pipeline insulation is unknown, thereby improving the accuracy and efficiency of the conductivity measurement.

[0049] The invention belongs to an accurate, efficient and non-destructive metal pipeline conductivity measurement technology, which has important practical application value for extending pipeline service life, reducing maintenance costs and improving industrial safety levels.

[0050] In view of the existing methods used in the field of metal conductivity detection, the present invention proposes a method for measuring the conductivity of non-magnetic metal pipelines based on dual-frequency eddy current measurement. In view of the problem that lift-off fluctuations in eddy current detection may cause measurement errors, it is proposed to obtain the accurate value of lift-off based on the relationship between the real part of the complex inductance change at high frequency and the lift-off, thereby avoiding the influence of lift-off fluctuations on the conductivity inversion process. Then, the coil phase measured at low frequency is used as the objective function value of the improved Newton-Raphson method, and the coil coefficient flux indirect method is used to analytically solve the estimated value of the conductivity of the non-magnetic metal pipeline. The present invention reduces the complex integral calculations in the forward model and improves the efficiency of conductivity estimation. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0052] Attached Figure 1 It is a schematic diagram of the flow of conductivity estimation measurement of the present invention;

[0053] Attached Figure 2 It is a schematic diagram of the curve of the real part of the complex inductance variation with frequency;

[0054] Attached Figure 3 It is a schematic diagram of the degree of fit between the fitting function and the simulation and experimental results in the experiment (the experiment is used to verify that the constant value C and the natural logarithm of the lift-off satisfy the quadratic function relationship);

[0055] Attached Figure 4 Schematic diagram of the top view of the pipe and conical coil at any lift-off distance l. DETAILED DESCRIPTION

[0056] As shown in the figure, a method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection is described. The method estimates the electrical conductivity of the non-magnetic pipeline based on dual-frequency eddy current detection, and determines the coil lift-off distance by selecting the real part of the change in the complex inductance of the coil corresponding to a single frequency in the high-frequency band; after obtaining the lift-off distance, the analytical solution of the pipeline asymmetric model is used, and the coil phase corresponding to a single frequency in the low-frequency band is selected as the objective function value, and the pipeline conductivity is obtained by iteratively solving the improved Newton-Raphson method.

[0057] When the insulation value of the non-magnetic pipeline is unknown, the measurement method is used to obtain the electrical conductivity of the pipeline only through dual-frequency measurement.

[0058] The measuring method comprises the following steps:

[0059] Step 1: Use a sensor to measure the coil impedance change ΔZ in the high frequency band and the low frequency band respectively;

[0060] Step 2: Determine the lifting distance;

[0061] Step 3: Calculate the conductivity.

[0062] In step 1, ΔZ=ZZ air , where Z is the coil impedance measured when containing the sample to be tested, Z air It is the coil impedance measured without the sample to be tested.

[0063] In step 2, the real part of the complex inductance variation of the non-magnetic pipeline in the high frequency band tends to a constant value C, which is independent of the pipeline thickness and conductivity and only related to the lift-off, such as Figure 2 As shown; and when the lift-off distance is within a preset range, the constant and the natural logarithm of the lift-off satisfy a quadratic function relationship, that is, C = a × ln 2 (l)+b×ln(l)+c, where a, b, c are coefficients of the expression; Figure 3 The simulation and experimental results are shown in the data curves.

[0064] In step 2, the lift-off distance l is determined by the measured constant value C.

[0065] In step 2, the change in complex inductance ΔL in the high frequency band is obtained from the change in coil impedance ΔZ. The lifting distance is then determined by the real part of the change in complex inductance in the high frequency band.

[0066] In step 3, according to the phase at low frequency As well as the established analytical solution, the conductivity estimation value is obtained by using the improved Newton-Raphson iteration method, which is:

[0067] Select the initial iteration value of conductivity, substitute the lift-off distance l into the forward problem to obtain the calculated Re(ΔL) and Im(ΔL), and then obtain the phase The modified Newton-Raphson method is used to determine the conductivity value that minimizes the square of the Euclidean distance between the calculated phase and the measured phase, which will be used as the final conductivity estimate; phase The specific calculation formula is:

[0068]

[0069] The coil coefficient C s (α,m) is calculated using the magnetic flux density generated by the coil in free space.

[0070] In the non-magnetic pipeline, the top view of the pipeline and the conical coil is as follows: Figure 4 As shown, the axis of the tube is defined as the z-axis of the cylindrical coordinate system, and all subsequent calculations will be performed in this coordinate system;

[0071] First, in the simulation software, the coil in free space is extracted at the surface of the pipe r = r o Where Then the following expression is used to obtain:

[0072]

[0073] is r = r o Two-dimensional Fourier transform results of the z component of the free space magnetic flux density on the surface;

[0074] The second-order vector potential method is used to calculate the pipeline coefficient R(α, m), and the final calculation formula for R(α, m) is:

[0075]

[0076] in,

[0077] k 2 =-jωμ0σ

[0078]

[0079] I pq =I m (α p r q ),K pq =K m (α p r q )

[0080]

[0081]

[0082] I m (x) is the modified Bessel function of the first kind of order m, K m (x) is the modified Bessel function of the second kind of order m.

[0083] ω——Excitation current angular frequency

[0084] μ0——vacuum magnetic permeability

[0085] I——excitation current

[0086] σ——Pipeline conductivity

[0087] r i ——Inner diameter of pipe

[0088] ro ——Outer diameter of the pipe.

[0090] In step 3, the phase After calculating the formula, the actual coil phase measured in the low frequency band is As the objective function value of the improved Newton-Raphson method, and the final conductivity estimation value is obtained by the improved Newton-Raphson method;

[0091] The pipeline conductivity estimation algorithm is as follows:

[0092] Input: Initial guess of conductivity σ 1 , tolerance ε, maximum number of iterations n i , the objective function value Output: conductivity σ;

[0093] Initialization: Set Let k = 1, 2, ..., n i And use it as the count to iterate in the loop. In each iteration,

[0094]

[0095] when Stop iteration and output the calculated value.

[0096] The process of obtaining the final conductivity estimate by the improved Newton-Raphson method is shown in the pseudo code in Table 1.

[0097] Table 1 Newton-Raphson method process

[0098]

[0099] The coil is a conical coil or a frustum-shaped coil, and the change in complex inductance at high frequency tends to a constant value C. When the lifting distance is within the range of 1 cm, the constant and the natural logarithm of the lifting distance satisfy a quadratic function relationship.

[0100] In this example, coils are used to generate alternating magnetic fields of different frequencies. The magnetic field propagates in the space around the pipeline, thereby inducing eddy currents in the pipeline. The conductivity of the pipeline is measured by detecting the changes in the eddy currents, and the lift-off height is adjusted by changing the distance between the coil and the pipeline.

Claims

1. A method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection, characterized in that: The measurement method estimates the conductivity of the non-magnetic pipeline based on dual-frequency eddy current detection, and determines the coil lift-off distance by selecting the real part of the coil complex inductance change corresponding to a single frequency in the high-frequency band; after obtaining the lift-off distance, the analytical solution of the pipeline asymmetric model is used, and the coil phase corresponding to a single frequency in the low-frequency band is selected as the objective function value, and the pipeline conductivity is obtained by iteratively solving the improved Newton-Raphson method.

2. According to claim 1, a method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection is characterized in that: When the insulation value of the non-magnetic pipeline is unknown, the measurement method is used to obtain the electrical conductivity of the pipeline only through dual-frequency measurement.

3. The method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection according to claim 1 is characterized in that: The measuring method comprises the following steps: Step 1: Use a sensor to measure the coil impedance change ΔZ in the high frequency band and the low frequency band respectively; Step 2: Determine the lifting distance; Step 3: Calculate the conductivity.

4. The method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection according to claim 3 is characterized in that: In step 1, ΔZ=ZZ air , where Z is the coil impedance measured when containing the sample to be tested, Z air It is the coil impedance measured without the sample to be tested.

5. The method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection according to claim 4 is characterized in that: In step 2, the real part of the change in complex inductance of the non-magnetic pipe in the high frequency band tends to a constant value C, which is only related to the lift-off distance; and when the lift-off distance is within a preset range, the constant and the natural logarithm of the lift-off satisfy a quadratic function relationship, that is, C = a × ln 2 (l)+b×ln(l)+c, where a, b, c are coefficients of the expression; In step 2, the lift-off distance l is determined by the measured constant value C.

6. The method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection according to claim 5 is characterized in that: In step 2, the change in complex inductance ΔL in the high frequency band is obtained from the change in coil impedance ΔZ. The lifting distance is then determined by the real part of the change in complex inductance in the high frequency band.

7. The method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection according to claim 6 is characterized in that: In step 3, according to the phase at low frequency As well as the established analytical solution, the conductivity estimation value is obtained by using the improved Newton-Raphson iteration method, which is: Select the initial iteration value of conductivity, substitute the lift-off distance l into the forward problem to obtain the calculated Re(ΔL) and Im(ΔL), and then obtain the phase The modified Newton-Raphson method is used to determine the conductivity value that minimizes the square of the Euclidean distance between the calculated phase and the measured phase, which will be used as the final conductivity estimate; phase The specific calculation formula is: The coil coefficient C s (α,m) is calculated using the magnetic flux density generated by the coil in free space.

8. The method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection according to claim 7 is characterized in that: In the non-magnetic pipeline, the axis of the pipeline is defined as the z-axis of the cylindrical coordinate system, and subsequent calculations are performed in this coordinate system; First, in the simulation software, the coil in free space is extracted at the surface of the pipe r = r o Where Then the following expression is used to obtain: is r=r o Two-dimensional Fourier transform results of the z component of the free space magnetic flux density on the surface; The second-order vector potential method is used to calculate the pipeline coefficient R(α, m), and the final calculation formula for R(α, m) is: in, k 2 =-jωμ0σ I pq =I m (a p r q ),K pq =K m (a p r q ) I m (x) is the modified Bessel function of the first kind of order m, K m (x) is the modified Bessel function of the second kind of order m. ω——Excitation current angular frequency μ0——vacuum magnetic permeability I——excitation current σ——Pipeline conductivity r i ——Inner diameter of pipe r o ——Outer diameter of the pipe.

9. The method for measuring the electrical conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection according to claim 8, characterized in that: In step 3, the phase After calculating the formula, the actual coil phase measured in the low frequency band is As the objective function value of the improved Newton-Raphson method, and the final conductivity estimation value is obtained by the improved Newton-Raphson method; The pipeline conductivity estimation algorithm is as follows: Input: Initial guess of conductivity σ 1 , tolerance ε, maximum number of iterations n i , the objective function value Output: conductivity σ; Initialization: Set Let k = 1, 2, ..., n i And use it as the count to iterate in the loop. In each iteration, when Stop iteration and output the calculated value.

10. The method for measuring the conductivity of a non-magnetic pipeline based on dual-frequency eddy current detection according to claim 8, characterized in that: The coil is a conical coil or a frustum-shaped coil, and the change in complex inductance at high frequency tends to a constant value C. When the lifting distance is within the range of 1 cm, the constant and the natural logarithm of the lifting distance satisfy a quadratic function relationship.

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