A pulse eddy current decoupling method based on frequency domain amplitude centroid and time domain ratio characteristic quantity

Abstract

This invention relates to the field of nondestructive testing (NDT) technology, solving the technical problems of large measurement errors and insufficient detection accuracy in existing technologies under lift-off fluctuations caused by changes in coating thickness. Specifically, it relates to a pulsed eddy current lift-off and thickness decoupling method based on the characteristic quantity of the frequency domain amplitude centroid and the time domain ratio. Based on a pulsed eddy current analytical model, the pipe under test with a coating layer is equivalent to a three-layer flat plate structure, and the time-domain induced voltage signal of the pipe is obtained through a detection platform. A fast Fourier transform is performed on the time-domain induced voltage signal to obtain the corresponding frequency-domain induced voltage spectrum, and the frequency-domain amplitude centroid used to characterize the lift-off height is calculated based on the frequency-domain induced voltage spectrum. This invention uses the raw induced voltage signal as the analysis object, eliminating the need for complex preprocessing. The signal processing flow is simple and efficient, reducing computational complexity while ensuring detection accuracy, thus improving detection efficiency and engineering application feasibility.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing technology, and in particular to a pulse eddy current extraction and thickness decoupling method based on the characteristic quantity of frequency domain amplitude centroid and time domain ratio. Background Technology

[0002] Coated ferromagnetic pipes are widely used in the petrochemical industry. However, during long-term service, the pipe wall thickness is prone to thinning due to factors such as corrosion from the transported medium, environmental effects, and operating conditions, thus threatening equipment safety. Therefore, wall thickness testing of coated ferromagnetic pipes is of great significance. Traditional ultrasonic testing usually requires the removal of the external coating, which is not only inefficient but also fails to meet the requirements of online, non-contact testing.

[0003] Pulsed eddy current testing, a non-contact non-destructive testing method, uses a pulsed square wave with a certain duty cycle as the excitation signal. Because this excitation signal has a rich spectral composition and strong penetration capability, it can effectively detect deep defects or wall thickness changes, and therefore has been widely used in the corrosion thinning assessment of pressure pipelines with cladding layers.

[0004] Currently, in studies on the wall thickness detection of coated ferromagnetic pipes, parameters such as inflection point time, late signal attenuation rate, or late slope are commonly used as evaluation characteristics. However, these characteristic parameters are highly sensitive to changes in probe lift-off and fluctuations in coating thickness, and are easily affected by factors such as changes in the detection gap, leading to increased measurement errors and making it difficult to meet the practical needs of high-precision quantitative wall thickness detection.

[0005] To address the aforementioned issues and improve detection accuracy, effective decoupling of lift-off height and wall thickness parameters is necessary. However, pulsed eddy current (PECT) signals exhibit significant temporal diffusion characteristics, and the lift-off effect, wall thickness variations, and cladding parameters are interconnected, making feature extraction susceptible to noise interference and resulting in insufficient information mining and inadequate feature description capabilities. Therefore, investigating the influence of lift-off height on pulsed eddy current signals and establishing a decoupling method between lift-off height and wall thickness parameters is crucial for improving the accuracy of detection results. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a pulse eddy current lift-off and thickness decoupling method based on the characteristic quantity of frequency domain amplitude centroid and time domain ratio. This method solves the technical problems of large measurement errors and insufficient detection accuracy of existing technologies under lift-off fluctuation conditions caused by changes in coating thickness.

[0007] To address the aforementioned technical problems, this invention provides a method for pulse eddy current extraction and thickness decoupling based on the frequency domain amplitude centroid and time domain ratio characteristic. This method includes the following steps: S1. Based on the pulsed eddy current analytical model, the pipeline under test with a coating layer is equivalent to a three-layer flat plate structure, and the time-domain induced voltage signal of the pipeline under test is obtained through detection platform. ; S2, regarding the time-domain induced voltage signal A fast Fourier transform is performed to obtain the corresponding frequency domain induced voltage spectrum, and the frequency domain amplitude centroid used to characterize the lift-off height is calculated based on the frequency domain induced voltage spectrum. ; S3, convert the time-domain induced voltage signal Divided into including the early stage Mid-term stage and the later stage The time intervals are determined, and the signal energy corresponding to each time interval is calculated to construct a time-domain ratio feature quantity to characterize the wall thickness parameter variation information. ; S4. Establish the centroid for quantizing the frequency domain amplitude through calibration experiments. The linear relationship between lift-off height and time-domain ratio features. The target fitting curve of the linear relationship between the lift-off height and the wall thickness is obtained to achieve effective decoupling of the lift-off height and wall thickness parameters and obtain the wall thickness value of the pipe under test.

[0008] Furthermore, the three-layer flat plate structure consists of, from bottom to top, air inside the pipe, pipe wall, and cladding layer, with the pulsed eddy current probe located above the three-layer flat plate structure.

[0009] Furthermore, the detection platform includes: The system includes a detection module for generating pulse excitation signals and acquiring induced voltage signals, an analysis module for processing induced voltage signals and performing parameter inversion, and a pulse eddy current probe consisting of a hollow excitation coil and a receiving coil. The pulsed eddy current probe is connected to the detection module via a signal line, and the detection module and the analysis module exchange data via a signal transmission line.

[0010] Furthermore, the frequency domain induced voltage spectrum includes a frequency domain amplitude spectrum. and frequency domain phase spectrum The expressions are as follows: ; ; in, Time-domain induced voltage signal The frequency domain complex representation obtained after Fast Fourier Transform; This indicates the argument operation for a complex number, which returns the phase value of the complex number; Calculate the frequency domain amplitude spectrum respectively and frequency domain phase spectrum frequency domain amplitude centroid and frequency domain phase centroid The calculation formulas are as follows: ; ; in, The total number of frequency points participating in the calculation of the centroid of the frequency domain amplitude; For the first The frequency value corresponding to each frequency point.

[0011] Furthermore, in step S3, the specific process includes the following steps: S31, Regarding the time-domain induced voltage signal Discrete sampling is performed to obtain discrete time series signals. , , representing the time series sequence number corresponding to the sampling time; S32, The discrete time series signal Divided into early stages Mid-term stage and the later stage ; S33, Calculate the early stages separately. Mid-term stage and the later stage The signal energy is calculated using the following formula: ; ; ; in, Signal energy in the early stages; Signal energy in the intermediate stage; It is the signal energy in the late stage;

[0012] S34, Signal energy in the late stage Signal energy in the intermediate stage The ratio is defined as the time-domain ratio characteristic. .

[0013] Furthermore, in step S4, the specific process includes the following steps: S41. Obtain time-domain induced voltage signals under different lift-off height conditions through calibration experiments. And establish the centroid of frequency domain amplitude. The first-order fitting relationship between the lift-off height and the lift-off height is expressed as: ; in, The lift-off height is caused by the thickness of the coating layer; This is the proportionality coefficient; This is the bias constant; S42. The centroid f of the frequency domain amplitude corresponding to the pipe under test c,A Substitute the lifting height into the first-order fitting relationship to obtain the lifting height under the current detection state; S43. Using the pipe wall thickness parameter as the independent variable, the time-domain ratio characteristic quantity under different lift-off height conditions. Using the wall thickness as the dependent variable, a linear fit is performed to obtain the inversion fitting curve of the corresponding wall thickness parameter, forming a calibration database containing multiple sets of calibration lift-off height parameters; S44. Match the target fitting curve corresponding to the lift-off height in the calibration database; S45. The time-domain ratio characteristic quantity of the pipeline under test. Substitute the results into the target fitting curve to obtain the pipe wall thickness detection results.

[0014] Furthermore, when the lift-off height is between two adjacent calibrated lift-off height parameters, an interpolation method is used to determine the corresponding target fitting curve.

[0015] By employing the above technical solution, the present invention provides a method for pulse eddy current extraction and thickness decoupling based on the characteristic quantity of frequency domain amplitude centroid and time domain ratio, which has at least the following beneficial effects: 1. This invention uses frequency domain amplitude centroid and time domain ratio characteristic quantities as joint input parameters. It comprehensively considers the sensitivity of frequency domain information to lift-off changes and the response characteristics of time domain information to thickness changes, and constructs an analysis method for decoupling pulse eddy current lift-off and thickness, so as to achieve effective separation of lift-off effect and thickness information, thereby improving the accuracy and stability of detection results.

[0016] 2. This invention uses the original induced voltage signal as the analysis object, without the need for a complicated preprocessing process. The signal processing flow is simple and efficient, which can reduce the computational complexity while ensuring detection accuracy, and improve detection efficiency and engineering application feasibility. Attached Figure Description

[0017] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1This is a schematic diagram of the pulsed eddy current analytical model in this invention; Figure 2 This is a fitting curve of the frequency domain amplitude spectrum under different thicknesses in this invention; Figure 3 This is a fitting curve of the frequency domain phase spectrum under different thicknesses in this invention; Figure 4 This is a fitting curve of the centroid of the frequency domain amplitude versus the thickness under different thicknesses in this invention; Figure 5 This is a fitting curve of the frequency domain amplitude spectrum at different lift-off heights in this invention; Figure 6 This is a fitting curve of the frequency domain phase spectrum at different lift-off heights in this invention; Figure 7 This is a fitting curve of the centroid of frequency domain amplitude and thickness at different lift-off heights in this invention; Figure 8 This is a fitting curve of the centroid of the frequency domain amplitude versus the thickness in the calibration experiment of this invention; Figure 9 This is a fitting curve of the centroid of the frequency domain amplitude and the lift-off height in the calibration experiment of this invention; Figure 10 This is a schematic diagram of the time interval division in the calibration experiment of this invention; Figure 11 This is a fitting curve of the R1 feature quantity and thickness in this invention; Figure 12 This is a visualization diagram of the R1 feature quantity and thickness under different extraction conditions according to the present invention. Detailed Implementation

[0018] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. This will allow for a full understanding of how the present application uses technical means to solve technical problems and achieve technical effects, and to facilitate its implementation.

[0019] This embodiment proposes a pulse eddy current extraction and thickness decoupling method based on the frequency domain amplitude centroid and time domain ratio characteristics. It uses the original induced voltage signal as the analysis object, eliminating the need for complex preprocessing. The signal processing flow is simple and efficient, reducing computational complexity while ensuring detection accuracy, thus improving detection efficiency and engineering application feasibility. Figure 1 As shown, the method includes the following steps: S1. Based on the pulsed eddy current analytical model, the pipeline under test with a coating layer is equivalent to a three-layer flat plate structure, and the time-domain induced voltage signal of the pipeline under test is obtained through detection platform. In this embodiment, the pulsed eddy current analytical model refers to the theoretical model that derives the electromagnetic diffusion equation based on Maxwell's equations, applies boundary conditions to the multi-layered flat / cylindrical structure, obtains the analytical solution through Laplace transform (or Fourier transform), and then obtains the time-domain induced voltage signal through inverse transform.

[0020] Specifically, the pulsed eddy current analytical model consists of three layers: the first layer represents the air in the pipe, the second layer represents the pipe wall, and the third layer represents the coating layer. Large-area corrosion defects on the pipe wall surface are represented as uniformly thinned regions. A pulsed eddy current probe, composed of coaxially arranged hollow excitation and receiving coils, is positioned above the three-layer flat structure. Based on the model's axisymmetry, a cylindrical coordinate system is used. ,in =0 coincides with the upper surface of the 3rd layer The axis is aligned with the axis of symmetry of the excitation coil.

[0021] The detection platform includes a detection module for generating pulse excitation signals and acquiring induced voltage signals, an analysis module for processing induced voltage signals and performing parameter inversion, and a pulsed eddy current probe composed of a hollow excitation coil and a receiving coil. The pulsed eddy current probe is connected to the detection module via a signal line, and the detection module and the analysis module interact with each other via a signal transmission line.

[0022] The time-domain induced voltage signal of the pipeline under test is obtained through the detection platform. At this time, the pulse excitation signal is a current-type time-domain signal with a square wave waveform. Then, based on the established pulse eddy current detection analytical model, the detection platform detects the pipeline under test. The analysis module controls the excitation coil to apply a pulse excitation signal, inducing eddy currents in the pipeline. The receiving coil then collects the corresponding pulse eddy current response signal, thereby obtaining the time-domain induced voltage signal of the pipeline under test. This is used for subsequent feature extraction and wall thickness inversion analysis.

[0023] This embodiment decomposes the pulse excitation signal into multiple harmonics by performing a Fourier transform on it, where any one of the harmonics is denoted as . , Indicates the harmonic sequence number. Indicates time, and each harmonic Each harmonic corresponds to a unique number; therefore, each harmonic Induced voltage signal for: ; In the formula, The imaginary unit; For the first The angular frequency of the harmonic; The amplitude of the harmonic current; Permeability of free space; For characteristic quantity The number of elements is constant. For each feature quantity The corresponding number; The lift-off height between the pulsed eddy current probe and the pipe surface; The lift-off coefficient is used to characterize the effect of probe lift-off on the induced voltage; For the first Features The coil coefficient; This represents the generalized reflection coefficient from the third to the fourth layer.

[0024] Induced voltage signal for all harmonics Perform inverse discrete Fourier transform to obtain the time-domain induced voltage signal. The Fourier transform expression for the pulse excitation signal is as follows: ; In the formula, Indicates a square wave excitation signal; Indicates the amplitude of the excitation signal; This indicates the frequency of the excitation signal.

[0025] After inverse Fourier transform, the pulse excitation signal can be equivalently represented as a superposition of multiple sinusoidal harmonic components. Let... For harmonic sequence numbers, when The time corresponds to the fundamental frequency (first harmonic). The time corresponds to the third harmonic. The time corresponds to the fifth harmonic. The seventh harmonic corresponds to the first harmonic, and so on for the others. In actual calculations, the number of harmonics involved in the decomposition can be determined based on the required accuracy and computational cost. The more harmonics, the closer the reconstructed waveform is to the original square wave signal.

[0026] The time-domain induced voltage signal of the receiving coil for: ; In the formula, This represents the total number of harmonics. For the first The angular frequency of a harmonic.

[0027] Furthermore, the side of the two radial end faces of the coil closer to the cladding layer is called the proximal end face, and the side farther from the cladding layer is called the distal end face. The distance from the proximal end face of the hollow excitation coil to the outer surface of the cladding layer is kept consistent for both the excitation and receiving coils. Then the coil coefficient... The expression is: ; In the above formula, ; Indicates the coil height; and These represent the inner and outer radii of the coil, respectively. Number of coil turns; subscript and These represent the excitation coil and the receiving coil, respectively.

[0028] Among them, characteristic quantity This can be derived from the following expression: ; in It is a first-order Bessel function of the first kind; The height of the coil.

[0029] Furthermore, the generalized reflectance coefficient The expression for is obtained through the following formula: ; ; In the above formula, ; The number of layers in a three-layer flat panel structure; , These represent the pipe wall thickness and the lift-off height caused by the cladding layer, respectively. For the first Layer and first Reflection coefficient between layers; For the 1st to the 1st The generalized reflectance coefficient of the layer, ; and These are the first and second type Bessel functions, respectively; The longitudinal wave number of the electromagnetic wave; and The first The magnetic permeability and electrical conductivity of the layer.

[0030] S2, regarding the time-domain induced voltage signal A fast Fourier transform is performed to obtain the corresponding frequency domain induced voltage spectrum, and the frequency domain amplitude centroid used to characterize the lift-off height is calculated based on the frequency domain induced voltage spectrum. In this embodiment, the calculated frequency domain amplitude centroid... It is not sensitive to changes in pipe wall thickness, but is sensitive to changes in lift height, and therefore can be used to characterize lift height.

[0031] Specifically, the frequency domain induced voltage spectrum includes the frequency domain amplitude spectrum. and frequency domain phase spectrum The expressions are as follows: ; ; in, Time-domain induced voltage signal The frequency domain complex representation obtained after Fast Fourier Transform; This indicates the operation of taking the argument (phase angle) of a complex number, that is, returning the phase value of the complex number.

[0032] Calculate the frequency domain amplitude spectrum respectively and frequency domain phase spectrum frequency domain amplitude centroid and frequency domain phase centroid The calculation formulas are as follows: ; ; in, The total number of frequency points participating in the calculation of the centroid of the frequency domain amplitude; For the first The frequency value corresponding to each frequency point.

[0033] S3, convert the time-domain induced voltage signal Divided into including the early stage Mid-term stage and the later stage The time intervals are determined, and the signal energy corresponding to each time interval is calculated to construct a time-domain ratio feature quantity to characterize the wall thickness parameter variation information. In step S3, the specific process includes the following steps: S31, Regarding the time-domain induced voltage signal Discrete sampling is performed to obtain discrete time series signals. , , which represents the time series sequence number corresponding to the sampling time.

[0034] S32, The discrete time series signal Divided into early stages Mid-term stage and the later stage The three time intervals are divided based on the temporal evolution characteristics of the pulsed eddy current signal: the early stage corresponds to the period of high and drastic rise and peak amplitude of the signal after the excitation is applied, with a sampling point range of [missing information]. The intermediate stage corresponds to the transition range where the signal amplitude gradually decreases, and the sampling point range is... The late stage corresponds to the attenuation tail region where the signal amplitude is weak and mainly reflects the diffusion characteristics of the pipe wall thickness; the sampling point range is... N is the total number of discrete sampling points. The division nodes for the three stages. and Based on the excitation signal parameters and pipe material properties, these parameters are predetermined and kept fixed under the same detection conditions to ensure the consistency and repeatability of feature extraction.

[0035] S33, Calculate the early stages separately. Mid-term stage and the later stage The signal energy is calculated using the following formula: ; ; ; in, Signal energy in the early stages; Signal energy in the intermediate stage; It is the signal energy in the late stage; S34, Signal energy in the late stage Signal energy in the intermediate stage The ratio is defined as the time-domain ratio characteristic. The expression is: .

[0036] S4. Establish the centroid for quantizing the frequency domain amplitude through calibration experiments. The linear relationship between lift-off height and time-domain ratio features. The target fitting curve of the linear relationship between the lift-off height and the wall thickness is obtained to effectively decouple the lift-off height from the wall thickness parameter and obtain the wall thickness value of the pipe under test. In step S4, the specific process includes the following steps: S41. Obtain time-domain induced voltage signals under different lift-off height conditions through calibration experiments. And establish the centroid of frequency domain amplitude. The first-order fitting relationship between the lift-off height and the lift-off height is expressed as: ; in, The lift-off height is caused by the thickness of the coating layer; This is the proportionality coefficient; This is the bias constant; S42. Centroid of frequency domain amplitude corresponding to the pipe under test Substitute the lifting height into the first-order fitting relationship to obtain the lifting height under the current detection state; S43. Using the pipe wall thickness parameter as the independent variable, the time-domain ratio characteristic quantity under different lift-off height conditions. Using the wall thickness as the dependent variable, a linear fit is performed to obtain the inversion fitting curve of the corresponding wall thickness parameter, forming a calibration database containing multiple sets of calibration lift-off height parameters; S44. Match the target fitting curve corresponding to the lift-off height in the calibration database. When the lift-off height is between two adjacent calibration lift-off height parameters, use interpolation to determine the corresponding target fitting curve. S45. The time-domain ratio characteristic quantity of the pipeline under test. Substitute the results into the target fitting curve to obtain the pipe wall thickness detection results.

[0037] This embodiment is used to verify the high sensitivity of the frequency domain amplitude centroid to changes in lift-off height, verify the linear correspondence between the frequency domain amplitude centroid and lift-off height, and verify the high sensitivity of the time domain ratio characteristic quantity to changes in pipe thickness and its linear correspondence with wall thickness. Simultaneously, under the lift-off effect caused by changes in cladding thickness, the lift-off amount is characterized by combining the frequency domain amplitude centroid, and the pipe thickness is inverted using the time domain ratio characteristic quantity to verify the applicability of the proposed method under complex working conditions and the accuracy of the thickness inversion results.

[0038] In this verification example, the pipe wall with the cladding is mostly made of carbon steel with an electrical conductivity of 1.6 MS / m, a relative permeability of 600, and a dielectric constant of 1. The cladding layer has an electrical conductivity of 0 MS / m, a relative permeability of 1, and a dielectric constant of 1. The cladding layer thickness is set to decrease from 10 mm to 0 mm, with pipe wall thicknesses of 20.00 mm, 25.00 mm, 30.00 mm, 35.00 mm, and 40.00 mm, respectively. The pulse excitation signal is a square wave current with an amplitude of 4 A, a duty cycle of 50%, and a frequency of 10 Hz. The pulsed eddy current probe consists of an excitation coil and a receiving coil, with detailed parameters shown in Table 1.

[0039] Table 1 Parameters of Pulsed Eddy Current Probe

[0040] To verify the response relationship between the centroid characteristic quantity and the lift-off height and pipe thickness in this method, a pulsed eddy current analytical model was used to obtain time-domain induced voltage signals under different operating conditions, with pipe thicknesses set to 20.00 mm, 25.00 mm, 30.00 mm, 35.00 mm, and 40.00 mm. Fast Fourier Transform was performed on the time-domain induced voltage signals obtained under each thickness condition to obtain the corresponding frequency domain amplitude spectrum and frequency domain phase spectrum. For example... Figure 2 and Figure 3As shown, under different thickness conditions, the frequency domain amplitude spectrum and frequency domain phase spectrum of each signal show relatively small differences; however, under different lift-off height conditions, the frequency domain amplitude spectrum and frequency domain phase spectrum change more significantly, such as... Figure 5 and Figure 6 As shown. In contrast, the variation pattern of the frequency domain amplitude spectrum is more significant and regular. Therefore, the frequency domain amplitude spectrum is selected as the feature extraction object, and the frequency domain amplitude centroid is introduced to quantitatively characterize its distribution. The results show that the frequency domain amplitude centroid basically does not change with the pipe thickness, as shown. Figure 4 and Figure 8 As shown; however, the centroid of the frequency domain amplitude exhibits a good linear relationship with the increase of the lift-off height, as shown in the figure. Figure 7 and Figure 9 As shown, the frequency domain amplitude centroid is highly sensitive to lift-off height but insensitive to changes in pipe thickness, making it an effective characteristic parameter for lift-off height assessment. A linear equation is then fitted. Figure 7 The linear relationship shown is expressed by the following formula: ; In the formula, The lift-off height is caused by the thickness of the coating layer; It is the centroid of the frequency domain amplitude.

[0041] To evaluate the lift-off accuracy of the above fitted equation, a lift-off height of 2.00 mm was selected as the verification target. Frequency domain amplitude centroid. Substituting this into the above formula, the calculated predicted lift-off height is 1.93 mm, and its relative error is 3.5%. This error is within the acceptable range, indicating that the frequency domain amplitude centroid can achieve quantitative assessment of lift-off with high accuracy. The formula for calculating the relative error is: ; In the formula, This is relative error; This refers to the actual lift-off height.

[0042] By following the time-domain induced voltage signal as follows Figure 10 The time window shown is divided into early, middle, and late stages, and the signal amplitudes within each stage are summed. Furthermore, the summed amplitude values ​​of the late stage are compared with those of the middle stage to obtain the time-domain ratio characteristic. .like Figure 11 As shown, the time-domain ratio characteristic quantity The time-domain ratio exhibits a good linear relationship with pipe thickness variation, indicating its high sensitivity to wall thickness changes. Further thickness inversion verification was performed using a 40.00 mm thick specimen, and the results showed a relative error of 3.07%, which is within an acceptable range. Therefore, the time-domain ratio characteristic can effectively characterize pipe thickness variations and can serve as an effective characteristic parameter for pipe wall thickness assessment and inversion.

[0043] Meanwhile, to further verify the impact of the lift-off height variation caused by the coating layer on the characteristic quantities, experiments were conducted under coating layer thicknesses of 0 mm, 10 mm, 20 mm, 30 mm, and 40 mm. Detection signals under different coating layer thicknesses and different pipe thicknesses were extracted, and the corresponding time-domain induced voltage signals were obtained. Their time-domain ratio characteristic quantities were then calculated. The results are as follows: Figure 12 As shown. By Figure 12 It is evident that under different lift-off heights, the time-domain ratio characteristic quantity and the pipe thickness maintain a good linear relationship, with the slopes of the fitted lines remaining essentially constant, while the intercepts shift significantly with changes in lift-off height. This result indicates that the lift-off effect primarily influences the baseline position of the characteristic quantity, while having a relatively small impact on the thickness sensitivity law. Therefore, the lift-off sensitivity feature can be used to decouple the lift-off quantity first, and then a target fitted curve under the corresponding lift-off condition can be selected to accurately invert the actual pipe wall thickness, achieving quantitative assessment of pipe thickness under complex coating conditions.

[0044] Those skilled in the art will understand that all or part of the steps in the methods of the above embodiments can be implemented by a program instructing related hardware. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, this application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

Claims

1. A method for pulse eddy current extraction and thickness decoupling based on the frequency domain amplitude centroid and time domain ratio characteristic, characterized in that, The method includes the following steps: S1. Based on the pulsed eddy current analytical model, the pipeline under test with a coating layer is equivalent to a three-layer flat plate structure, and the time-domain induced voltage signal of the pipeline under test is obtained through detection platform. ; S2, regarding the time-domain induced voltage signal A fast Fourier transform is performed to obtain the corresponding frequency domain induced voltage spectrum, and the frequency domain amplitude centroid used to characterize the lift-off height is calculated based on the frequency domain induced voltage spectrum. ; S3, convert the time-domain induced voltage signal Divided into including the early stage Mid-term stage and the later stage The time intervals are determined, and the signal energy corresponding to each time interval is calculated to construct a time-domain ratio feature quantity to characterize the wall thickness parameter variation information. ; S4. Establish the centroid for quantizing the frequency domain amplitude through calibration experiments. The linear relationship between lift-off height and time-domain ratio features. The target fitting curve of the linear relationship between the lift-off height and the wall thickness is obtained to achieve effective decoupling of the lift-off height and wall thickness parameters and obtain the wall thickness value of the pipe under test.

2. The pulsed eddy current lifting and thickness decoupling method according to claim 1, characterized in that, The three-layer flat plate structure consists of, from bottom to top, the air inside the pipe, the pipe wall, and the cladding layer, with the pulsed eddy current probe located above the three-layer flat plate structure.

3. The pulsed eddy current lifting and thickness decoupling method according to claim 1, characterized in that, The detection platform includes: The system includes a detection module for generating pulse excitation signals and acquiring induced voltage signals, an analysis module for processing induced voltage signals and performing parameter inversion, and a pulse eddy current probe consisting of a hollow excitation coil and a receiving coil. The pulsed eddy current probe is connected to the detection module via a signal line, and the detection module and the analysis module exchange data via a signal transmission line.

4. The pulsed eddy current lifting and thickness decoupling method according to claim 1, characterized in that, The frequency domain induced voltage spectrum includes the frequency domain amplitude spectrum. and frequency domain phase spectrum The expressions are as follows: ； ； in, Time-domain induced voltage signal The frequency domain complex representation obtained after Fast Fourier Transform; This indicates the argument operation for a complex number, which returns the phase value of the complex number; Calculate the frequency domain amplitude spectrum respectively and frequency domain phase spectrum frequency domain amplitude centroid and frequency domain phase centroid The calculation formulas are as follows: ； ； in, The total number of frequency points participating in the calculation of the centroid of the frequency domain amplitude; For the first The frequency value corresponding to each frequency point.

5. The pulsed eddy current lifting and thickness decoupling method according to claim 1, characterized in that, In step S3, the specific process includes the following steps: S31, Regarding the time-domain induced voltage signal Discrete sampling is performed to obtain discrete time series signals. , , representing the time series sequence number corresponding to the sampling time; S32, The discrete time series signal Divided into early stages Mid-term stage and the later stage ; S33, Calculate the early stages separately. Mid-term stage and the later stage The signal energy is calculated using the following formula: ； ； ； in, Signal energy in the early stages; Signal energy in the intermediate stage; It is the signal energy in the late stage; S34, Signal energy in the late stage Signal energy in the intermediate stage The ratio is defined as the time-domain ratio characteristic. .

6. The pulsed eddy current lifting and thickness decoupling method according to claim 1, characterized in that, In step S4, the specific process includes the following steps: S41. Obtain time-domain induced voltage signals under different lift-off height conditions through calibration experiments. And establish the centroid of frequency domain amplitude. The first-order fitting relationship between the lift-off height and the lift-off height is expressed as: ； in, The lift-off height is caused by the thickness of the coating layer; This is the proportionality coefficient; This is the bias constant; S42. Centroid of frequency domain amplitude corresponding to the pipe under test Substitute the lifting height into the first-order fitting relationship to obtain the lifting height under the current detection state; S43. Using the pipe wall thickness parameter as the independent variable, the time-domain ratio characteristic quantity under different lift-off height conditions. Using the wall thickness as the dependent variable, a linear fit is performed to obtain the inversion fitting curve of the corresponding wall thickness parameter, forming a calibration database containing multiple sets of calibration lift-off height parameters; S44. Match the target fitting curve corresponding to the lift-off height in the calibration database; S45. The time-domain ratio characteristic quantity of the pipeline under test. Substitute the results into the target fitting curve to obtain the pipe wall thickness detection results.

7. The pulsed eddy current lifting and thickness decoupling method according to claim 6, characterized in that, When the lift-off height is between two adjacent calibrated lift-off height parameters, the corresponding target fitting curve is determined by interpolation.

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