Coating layer wall thickness pulsed eddy current detection method and system based on multiple eddy current rings

Abstract

The embodiment of the invention provides a coating layer wall thickness pulse eddy current detection method and system based on multiple eddy current rings, and the method comprises the steps: applying a square wave excitation signal to an excitation coil, and carrying out the induction in a structural member with a coating layer to generate eddy current; collecting a voltage signal generated by induction of an attenuation magnetic field generated by the eddy current by using a receiving coil, wherein the attenuation process of the voltage signal comprises three stages; identifying a power function interval of the second stage to determine the starting time of the third stage; taking the starting time of the third stage as a reference, and selecting a preset thickness quantitative time interval in the third stage; calculating a first-order derivative of the voltage signal to time under a single logarithmic coordinate system in the thickness quantitative time interval to obtain a slope; according to the quantitative relation between the slope and the wall thickness of the structural part with the coating layer, the wall thickness value of the structural part with the coating layer is obtained through calculation.

Description

Technical Field

[0001] This application relates to the technical field of measuring electromagnetic variables, specifically to a pulsed eddy current detection method and system for coating wall thickness based on multiple eddy current rings. Background Technology

[0002] Clad metal structural components are widely used in pressure vessels, pressure pipelines, and boilers in industries such as petroleum, chemical, and power. These devices typically operate under harsh conditions and are prone to various defects that seriously jeopardize their normal operation; therefore, effective non-destructive testing is crucial.

[0003] Pulsed eddy current (PECT) technology has become an important method for inspecting clad metal structures due to its rich excitation signal spectrum and non-contact detection. Unlike traditional eddy current testing which uses sinusoidal excitation, PECT typically uses square wave or bipolar square wave excitation with a specific duty cycle as the signal source. Since the square wave excitation signal contains a series of spectral components from the fundamental frequency to higher harmonics, low-frequency signals can penetrate deeper into the test piece, while high-frequency signals are more sensitive to surface defects. Therefore, PECT can acquire information about different depths of the test piece with a single excitation, offering significant technical advantages. (See attached diagram) Figure 2 The diagram illustrates the basic principle of pulsed eddy current testing on coated pipe specimens. It includes a signal generator capable of exciting a square wave signal, a data acquisition unit capable of acquiring the voltage of the receiving coil, and a computer capable of processing the data. The basic principle of pulsed eddy current testing is as follows: a square wave voltage signal is applied to the excitation coil, generating a constant primary magnetic field during the continuous phase of the square wave. When the square wave voltage is turned off, the primary magnetic field rapidly decays, inducing eddy currents in the test specimen. These eddy currents then generate a secondary magnetic field. At this point, the receiving coil begins to acquire the decaying signal of the secondary magnetic field. (See attached diagram.) Figure 3 The diagram illustrates the working principle of the excitation and reception signals. Measuring wall thickness based on the attenuation rate of the received signal's magnetic field is one of the fundamental principles of pulsed eddy current thickness measurement. After the excitation is turned off, the attenuation rate of the eddy current magnetic field varies for specimens with different wall thicknesses; the larger the wall thickness, the flatter the attenuation curve becomes in the later stages. By analyzing the slope of the attenuation curve, the wall thickness of the specimen can be deduced. (Refer to Appendix) Figure 4 As shown, the magnetic field attenuation curves for different wall thicknesses (9.9 mm, 5.5 mm) are illustrated. In practical signal analysis, due to the rapid attenuation of the magnetic field caused by pulsed eddy currents, a semi-logarithmic coordinate system is typically used for analysis. The induced voltage signal of the receiving coil can be divided into three characteristic stages. Refer to the appendix. Figure 5 As shown, the first stage Phase Two The signal is mainly affected by the probe characteristics, originating from the strong induced voltage generated after the excitation coil voltage is instantaneously turned off. It changes rapidly within a very short time and is less affected by the parameters of the test piece, making it difficult to effectively reflect the test piece information. Third stage ( The first stage is the long-term decay stage, during which the range of induced eddy currents on the test piece spreads outwards and the eddy current density gradually decreases. The signal in this stage contains the effective information of the test piece.

[0004] Existing techniques typically extract the slope of the third-stage signal in a logarithmic coordinate system to establish a quantitative relationship with wall thickness. Classical theory states that the square of the wall thickness is proportional to the characteristic quantity (i.e., the reciprocal of the absolute value of the slope of the logarithmic curve). However, the slope of the third-stage signal is not a fixed value; its absolute value gradually decreases over time. Existing techniques generally select signals within a fixed voltage range or a fixed time period for calculation. However, when detection conditions such as lift-off height and specimen thickness change, this fixed-range selection method cannot accurately characterize the specimen thickness, exhibiting significant limitations.

[0005] Therefore, there is an urgent need for a new pulsed eddy current thickness measurement scheme that can more accurately and adaptively select the characteristic analysis period to improve detection accuracy and adaptability. Summary of the Invention

[0006] This application proposes a pulsed eddy current detection method and system for coating wall thickness based on multiple eddy rings, which is used to overcome the deficiencies of the prior art.

[0007] According to a first aspect of the embodiments of this application, a pulsed eddy current detection method for cladding wall thickness based on multiple eddy rings is provided, comprising: A square wave excitation signal is applied to the excitation coil, and eddy currents are induced in the clad structure. The voltage signal generated by the attenuating magnetic field generated by the eddy current is acquired by the receiving coil. The attenuation process of the voltage signal includes three stages: the first stage is the instantaneous response segment after the excitation magnetic field is turned off; the second stage is the segment in which the eddy current penetrates into the center region of the wall thickness; and the third stage is the segment in which the eddy current diffuses and attenuates radially. Identify the power function interval of the second stage to determine the start time of the third stage; Based on the equivalent model of time-sharing excitation of multi-vortex rings, a preset thickness quantitative time interval is selected within the third stage, with the start time of the third stage as the benchmark. Calculate the first derivative of the voltage signal with respect to time in a logarithmic coordinate system within the time interval for the thickness measurement, and use it to obtain the slope; The wall thickness of the clad structure is calculated based on the quantitative relationship between the slope and the wall thickness of the clad structure.

[0008] In some implementations, identifying the power function interval of the second stage includes: Take the logarithm of the voltage signal value and the corresponding time value respectively to obtain the voltage logarithmic vector and the time logarithmic vector; Data points are selected at fixed intervals on the time logarithm vector to perform linear fitting, and the linear distance from all data points to the fitted line is calculated. The interior points of the straight-line distance are filtered according to a preset distance threshold, and an interior point dataset is formed to find the interior point dataset corresponding to the maximum time difference. The least squares method was used to fit a straight line to the inlier dataset. The linear intervals in the double logarithmic coordinate system corresponding to the parameters of the fitted straight line are identified and defined as the power function intervals of the second stage.

[0009] In some embodiments, the thickness measurement time interval is the time interval during which the voltage signal decays to the point where the amplitude of the voltage signal is equal to a power function of α1 to α2, wherein the combinations of values ​​for α1 and α2 include: α1=0.7 and α2=0.5; or α1=0.6 and α2=0.4; or α1=0.5 and α2=0.3.

[0010] In some embodiments, applying a square wave excitation signal to the excitation coil includes: Based on the conditions of a duty cycle of 50% and an excitation frequency within 100Hz, the square wave excitation signal is applied to the excitation coil.

[0011] In some implementations, the slope is achieved through the following mathematical relationship: ; in, The slope is represented by the first derivative of the voltage signal with respect to time in a logarithmic coordinate system. This indicates taking the derivative with respect to time; This indicates that the voltage signal acquired by the receiving coil changes with time. A changing function.

[0012] In some embodiments, the mathematical relationship between the slope and the wall thickness of the cladding structure is as follows: ; ; in, Characteristic quantities used for thickness measurement; Indicates the slope; Indicates the wall thickness value of the structure with a cladding layer; Indicates proportional to; Indicates time The characteristic quantity of change; This indicates the start time of the third stage.

[0013] In some embodiments, before calculating the first derivative of the voltage signal with respect to time in a logarithmic coordinate system over the time interval of the thickness measurement, the following steps are included: The voltage signal is subjected to nonlinear compression and smoothing filtering to suppress noise.

[0014] In some embodiments, calculating the wall thickness value of the clad structure based on the quantitative relationship between the slope and the wall thickness of the clad structure includes: The feature quantity is compared with the preset thickness and feature quantity calibration curve to obtain the wall thickness value through inversion. The preset thickness and characteristic quantity calibration curve is obtained by measuring the characteristic quantities of multiple standard specimens with known thicknesses under the same testing conditions and fitting the relationship between the characteristic quantities and the wall thickness value.

[0015] According to a second aspect of this application, a pulsed eddy current detection system for cladding layer wall thickness based on a multi-vortex ring is provided, for implementing the pulsed eddy current detection method for cladding layer wall thickness based on a multi-vortex ring as described above, characterized in that it includes: A signal generator is used to generate a square wave excitation signal; Excitation coils are used to induce eddy currents in the workpiece being tested; A receiving coil is used to acquire the attenuated signal of the eddy current magnetic field. A data acquisition unit is used to acquire the voltage signal from the receiving coil. The processing unit is configured to perform quantitative wall thickness calculation, including identifying the power function interval of the received signal, determining the quantitative thickness time interval, calculating the slope of the voltage signal in a logarithmic coordinate system, and performing quantitative wall thickness calculation based on a preset slope-wall thickness relationship.

[0016] In some embodiments, the system further includes a probe, which is a coaxial cylindrical probe or a semi-circular focusing probe.

[0017] The beneficial effects of the pulsed eddy current detection method and system for coating wall thickness based on multi-eddy rings in this application include at least the following: This application first applies a square wave signal to the excitation coil, utilizing its rich spectral characteristics to induce eddy current fields with varying penetration depths in the test specimen, laying the physical foundation for obtaining comprehensive specimen thickness information. The receiving coil collects the voltage signal induced by the eddy current attenuation magnetic field, and by clearly defining its three attenuation stages, effectively distinguishes between invalid signal segments dominated by probe characteristics and valid signal segments containing specimen thickness information, thus pinpointing the target interval for subsequent precise analysis. Secondly, by identifying the power function interval of the second stage, the start time of the third stage is determined. This method uses the linear characteristics of the second stage in a double logarithmic coordinate system as a precise time scale, thereby objectively and repeatedly locating the effective starting point for thickness information analysis, avoiding the subjective arbitrariness of human selection. Based on the equivalent model of multi-eddy current ring time-division excitation, the thickness quantification time interval is selected based on this starting time, ensuring that the selection of the time interval is grounded in a profound physical model. This guarantees that the analyzed signal corresponds to the eddy current diffusion state under the same physical mechanism, significantly improving the accuracy and consistency of thickness quantification. Finally, by calculating the slope of the voltage signal in a single logarithmic coordinate system within the quantitative time interval of thickness, the complex signal attenuation pattern is transformed into a single, clear feature quantity, which greatly simplifies the process of extracting thickness information. By establishing a quantitative relationship between the slope and the wall thickness for the final calculation, the feature quantity extracted in the previous steps is directly correlated with the final physical quantity (wall thickness), realizing a reliable inversion from signal to thickness and achieving the final goal of nondestructive testing. Attached Figure Description

[0018] Figure 1 This is a schematic flowchart of the pulsed eddy current detection method for coating wall thickness based on multiple eddy rings according to an embodiment of this application. Figure 2 This is a schematic diagram illustrating the principle of pulsed eddy current detection for coated pipelines according to an embodiment of this application. Figure 3 This is a schematic diagram illustrating the working principle of pulsed eddy current detection excitation and signal reception in a pipe with a cladding layer according to an embodiment of this application. Figure 4 The graph shows the magnetic field attenuation curves for different wall thicknesses (9.9 mm, 5.5 mm) in embodiments of this application. Figure 5 This is a schematic diagram of the PECT response signal according to an embodiment of this application; Figure 6 This is a diagram illustrating the eddy current distribution and the location of the maximum eddy current density at different times in an embodiment of this application. Figure 7-8 These are curves of the pulsed eddy current detection signal at different stages, representing embodiments of this application. Figure 9-10The figures are graphs showing the changes in the voltage of the test receiving coil and the slope in a logarithmic coordinate system, respectively, according to embodiments of this application. Figure 11 These are different α-power function curves from embodiments of this application; Figure 12-14 The figures shown are the received voltage signal curves of simulated specimens of different thicknesses according to embodiments of this application. Figure 15 The characteristic quantities of different thickness quantitative ranges in the simulation of embodiments of this application With thickness Relationship curve diagram; Figure 16 This is a graph of the original noisy signal in an embodiment of this application; Figure 17 This is a graph of the test signal after nonlinear compression and smoothing filtering according to an embodiment of this application. Figure 18-22 The following are signal reception curves for Q235 steel plates of different thicknesses according to embodiments of this application; Figure 23-27 These are characteristic quantities of specimens of different thicknesses according to embodiments of this application. With thickness The relationship curve. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the following description of the pulsed eddy current detection method and system for cladding wall thickness based on multi-vortex rings, in conjunction with the accompanying drawings of the embodiments of this application, will clearly and completely describe the technical solutions of the embodiments of this application. Obviously, the described embodiments are only some embodiments of the embodiments of this application, and not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0020] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the claimed embodiments of the present application, but merely to illustrate selected embodiments of the present application. Other embodiments obtained by those skilled in the art based on the embodiments of the present application without inventive effort are all within the scope of protection of the embodiments of the present application.

[0021] It can be noted that similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it will not be further defined and explained in subsequent figures according to the embodiments of this application.

[0022] Related technologies have provided a simplified relationship between the slope and thickness of the signal response, omitting the coefficients of the accumulation terms which have a relatively small impact. The principle is roughly as follows: First, the voltage value... Taking the logarithm (the logarithm processing in this application is uniformly illustrated using base 10), the relationship is shown below: Equation (1-1); in, The coefficients are the fitting coefficients, and the subscripts are... Representing different terms in the fit, For all All are true; The magnetic permeability of the specimen; Electrical conductivity; This is the thickness value. It is a natural constant.

[0023] When the signal is in the third stage, that is In this case, the cumulative term can be omitted, and at this time: Equation (1-2); Differentiate with respect to time: Equation (1-3); Let an expression It equals the reciprocal of equation (1-3): Equation (1-4); For different parts of the same test piece, it can be considered that If it is an unchanging constant, then: Equation (1-5); From the above inference, we can conclude that With signal characteristics They are directly proportional, among which, The meaning is the absolute form of the derivative of the slope of a logarithmic curve.

[0024] See attached document Figure 5As shown in the embodiments of this application, it was found during the experiment that the slope of the later signal is not a fixed value, but changes with time, and this change cannot be ignored if the time is long. Regarding the specific measures for slope, for pulse eddy current thickness measurement of ferromagnetic components, related technologies, based on the classical eddy current ring equivalent theory and system theory, proposed a multi-eddy current ring coupled pulse eddy current thickness measurement model, equating each eddy current ring to a coil, and combining the circuit model to equating the pulse eddy current signal to the response of a high-order system under step current excitation, and modeling the received voltage time-domain signal during pulse eddy current detection. Based on this, the time-domain signal was simplified, and the slope of the later voltage signal in the single logarithmic coordinate domain was used to quantify the thickness of the specimen. When using the slope of the later voltage signal to quantify the specimen thickness, there is a problem of selecting a signal interval. Related technologies generally select a fixed voltage value interval or a fixed time period of signal in the later stage of the received signal. However, in a logarithmic coordinate system, the slope of the voltage received signal for the same pulse eddy current detection is not fixed in the later stages, but rather the absolute value of the slope gradually decreases over time. Furthermore, when detection conditions such as lift-off height and specimen thickness change, selecting a fixed interval of signal makes it difficult to accurately characterize the corresponding specimen thickness.

[0025] To accurately quantify pipe wall thickness, it is necessary to determine which time period to use for slope measurement to achieve a better quantitative result. Therefore, this application proposes a more refined quantitative method based on a multi-vortex ring time-sharing excitation equivalent model. This method is based on the equivalent vortex ring, rather than measuring the magnetic field attenuation rate in the same time period.

[0026] See attached document Figure 1 As shown in the embodiment of this application, a pulsed eddy current detection method for cladding layer wall thickness based on a multi-vortex ring is disclosed. This method is executed based on a pulsed eddy current detection system for cladding layer wall thickness based on a multi-vortex ring. The purpose is to obtain the subsequent signal expression by establishing an equivalent circuit model and to propose characteristic quantities for measuring the cladding layer wall thickness. The method includes steps 110-150.

[0027] Step 110: Apply a square wave excitation signal to the excitation coil and induce eddy currents in the clad structure.

[0028] In some implementations, applying the square wave excitation signal to the excitation coil includes applying the square wave excitation signal to the excitation coil based on a duty cycle of 50% and an excitation frequency within 100Hz.

[0029] For example, simulation results of the device (e.g., a flat plate) in this embodiment of the application show that eddy currents are induced on the surface of the flat plate after the excitation current is turned off during pulsed eddy current detection. Specifically, the eddy currents penetrate from the surface to the central region of the pipe wall along the depth direction of the wall thickness. When the maximum value of the eddy current reaches the central region of the pipe wall, the position of the maximum value of the eddy current no longer penetrates deeper into the wall thickness, but diffuses and decays in all directions over time, and the position of the maximum value always remains in the central region of the wall thickness. Based on this, a gradually expanding eddy current ring is formed.

[0030] Step 120: Use a receiving coil to collect the voltage signal induced by the attenuated magnetic field generated by the eddy current.

[0031] The attenuation process of the voltage signal includes three stages: the first stage is the instantaneous response stage after the excitation magnetic field is turned off, the second stage is the stage of eddy current penetrating into the center region of the wall thickness, and the third stage is the stage of eddy current attenuation along the radial diffusion.

[0032] Step 130: Identify the power function interval of the second stage to determine the start time of the third stage.

[0033] In some implementations, identifying the power function interval of the second stage includes: taking the logarithm of the voltage signal value and the corresponding time value to obtain a voltage logarithmic vector and a time logarithmic vector; selecting data points at fixed intervals on the time logarithmic vector for linear fitting, and calculating the linear distance from all data points to the fitted line; filtering the interior points of the line distance according to a preset distance threshold to form an interior point dataset, which is used to find the interior point dataset corresponding to the maximum time difference; performing linear fitting on the interior point dataset using the least squares method; and identifying and defining the linear interval in the double logarithmic coordinate system corresponding to the parameters of the fitted line as the power function interval of the second stage.

[0034] For example, Figure 5 The figure shows the eddy current distribution in an 8mm thick steel plate at different times after the excitation current is turned off. The figure also shows the location of the maximum eddy current density and the maximum modulus of the eddy current density at different times. From the received voltage, after the excitation current is turned off, the voltage of the receiving coil first rapidly reaches its maximum value, then rapidly decays, and finally slowly decays at the end of the signal duration. Figure 7-8 As shown, the received voltage signal can be divided into three stages.

[0035] The first stage: the excitation magnetic field is turned off, and the voltage signal of the receiving coil is the result of the combined effect of the excitation magnetic field and the eddy current field on the specimen. In this stage, the portion of the received signal after its maximum value appears as a straight line in a double logarithmic coordinate system. ,in , For constant terms, (For time), when converted to Cartesian coordinates, this part conforms to an inverse power-law function, where the constant term... Closely related to the structure of the probe, different probes have different constant terms in actual experiments. The signal coupled to the excitation coil in this stage is closely related to the probe's structure. If the coupling inductance between the receiver and the excitation is large, the component of the coupling between the excitation and receiver in the received signal will account for a large proportion, making it difficult to extract effective information about the specimen from the signal in this stage. For example, refer to the attached diagram. Figure 6 The first figure starting from the top (corresponding to the appendix) Figure 6 The image corresponding to a time of 0.0001s roughly illustrates this.

[0036] The second stage: After the excitation magnetic field is completely disconnected, the eddy currents on the surface of the conductor specimen continue to penetrate towards the center of the wall thickness along the wall thickness direction until the location of the maximum eddy current density reaches the center of the conductor specimen. During the penetration process, the eddy current density gradually decreases with increasing penetration depth. The location of the maximum eddy current density continues to move towards the center of the wall thickness, as exemplarily shown in the attached diagram. Figure 6 Figures 2 and 3 in the appendix (corresponding to the attached figures) Figure 6 The images corresponding to times of 0.0012s and 0.0073s). The duration of this process is closely related to the wall thickness. For the same conductor material, the thicker the wall, the longer the process lasts. During this stage, the signal from the receiving coil appears as a straight line in a double logarithmic domain coordinate system ( ,in , For constant terms, (where the time factor is 1), converting this segment to a Cartesian coordinate system, this segment corresponds to an inverse power-law function. The parameters in the inverse power-law at this stage... The absolute value is less than the exponential term of the inverse power law in the first stage. The absolute value of the value indicates that the signal attenuation rate in this stage is less than that in the first stage. For the same probe and the same material test piece, the end time of the inverse power law function is related to the material thickness; the thicker the material, the later the end time of the inverse power law function. If the influence of noise is removed, the receiving coil in this stage only receives the signal of the eddy current field, and this signal contains information about the test piece. In fact, the end time of the power function can be used to evaluate and quantify the thickness of the test piece, but in actual experiments, due to the presence of noise, the end time of the power function is difficult to find accurately, making it difficult to quantify the thickness of the test piece using this time.

[0037] Phase Three: After the maximum eddy current density reaches the center region of the test specimen's wall thickness, the location of the maximum eddy current density no longer penetrates deeper into the wall thickness. The eddy current diffuses radially along the eddy current ring until the eddy current density reaches zero. After the second phase (the initial stage of the third phase), there is a brief transition period where the trajectory of the maximum eddy current density location gradually changes from moving along the thickness depth direction to moving radially along the steel plate specimen. After this brief transition period, the location of the maximum eddy current density location consistently moves radially along the center region of the wall thickness, the eddy current diffuses outwards, and the maximum eddy current density gradually decreases. In this phase, the eddy currents are symmetrically distributed along the wall thickness depth direction, with the plane of symmetry being the 1 / 2 wall thickness plane. (See attached diagram). Figure 6 As shown in the 4th, 5th, and 6th images (the 4th, 5th, and 6th images correspond to the attached images respectively), Figure 6 (Images corresponding to times of 0.0182s, 0.0271s, and 0.0315s). See attached image. Figure 8-9 As shown, the received signal of the coil in the third stage above conforms to an exponential function with multiple exponential terms. (in , (where is a constant term), where, in the logarithmic domain, the attenuation curve of the received signal in this stage appears as a straight line. In reality, the slope of the voltage attenuation curve in the logarithmic coordinate system changes slowly over time, with the absolute value of the slope gradually decreasing over time. (See Appendix...) Figure 9-10 The voltage decay curve and slope change in the logarithmic coordinate system are shown. The received signal at this stage contains the wall thickness information of the specimen. In this embodiment, the received signal at this stage is processed to extract thickness features for more accurate thickness quantification.

[0038] In one exemplary embodiment, this application identifies the power function interval of the second stage, and uses a Sequence Sample Consensus (SEQSAC) algorithm to determine the start time of the third stage, thereby finding the power function interval segment of the received signal. In a log-log coordinate system, the least squares method is used to perform linear fitting on the data of the power function segment to obtain the parameters of the power function.

[0039] The specific implementation process of the Sequential Sample Consensus Algorithm (SEQSAC) includes: (1) taking the logarithm of both the received voltage signal value and the corresponding time value to obtain the voltage vector V=[log 10 (v1), log 10 (v2), log 10 (v3), ..., log 10 (v N )] and the time vector T=[log 10(t1), log 10 (t2), log 10 (t3), ..., log 10 (t N (2) Starting from the first voltage data point at the beginning of the data, take two data points at intervals of n. Draw a straight line through these two points and calculate the distance l from all data points to the line. Set a distance threshold ρ, find all data points whose distance to the line is less than ρ, and call these points interior points. All interior points are arranged in ascending order of time to form the interior point dataset I1. (3) Calculate the logarithmic time corresponding to the two endpoints of the interior point dataset I1 as lgT1 and lgT2, respectively, and calculate the logarithmic time difference Δ1 = lgT2 - lgT1 corresponding to the two endpoints. (4) Starting from the second voltage data point at the beginning of the data segment, take two data points at intervals of n. Repeat step (2) to obtain the interior point dataset I2. Repeat step (3) to calculate the logarithmic time difference Δ2 corresponding to the two endpoints of the current interior point dataset I2. (5) Repeat step (4) until the N-n+1th data point of the voltage vector V is reached. The interior point dataset I = [I1, I2, I3, ..., I...] is obtained. N–n+1 ] and the endpoint logarithmic time difference dataset of the interior point dataset △=[△1, △2, △3, ..., △ N–n+1 (6) Find the maximum value Δ from the dataset of time differences between the inner endpoints. i And find △ i Corresponding interior point dataset I i Using the least squares method on the interior point dataset I i Fit a straight line to all interior points and find the equation of the line y = . ·t+b, where is the slope of the line, and b is the constant term. (7) Finally, using the conversion formula of power functions in Cartesian coordinates and double logarithmic domain, V= T+b converted to a power function in Cartesian coordinates Among them, △ i The time value at the right endpoint corresponds to the end time of the power function segment, which is also the end time of the second stage of receiving the voltage signal and the start time of the third stage.

[0040] In practical applications, the fixed interval n can be selected according to the signal sampling density, usually 5 to 10 data points; the distance threshold ρ can be calibrated through pre-experimentation, and is generally set to 1.5 to 2 times the average distance of the data points.

[0041] Step 140: Based on the equivalent model of time-sharing excitation of multi-vortex rings, a preset thickness quantitative time interval is selected within the third stage, taking the start time of the third stage as the reference.

[0042] In some embodiments, the thickness measurement time interval is the time interval from when the voltage signal decays to when the amplitude of the voltage signal is equal to a power function of α1 to α2, wherein the combinations of values ​​for α1 and α2 include: α1=0.7 and α2=0.5; or α1=0.6 and α2=0.4; or α1=0.5 and α2=0.3.

[0043] In one exemplary embodiment, For example, after the transition section of the third stage, the eddy current distribution in the cross-section of the conductor specimen at 1 / 4 of its cross-section is as follows: Figure 6 As shown in images 4, 5, and 6. At a given moment, using the principle of current equivalence, the eddy currents within the conductor specimen can be equivalently represented as the current flowing through a hypothetical hollow coil with a finite cross-section. Within a small time range, the amplitude of the eddy current density in the conductor specimen continuously decays. In a short time, the size and position of the equivalent hypothetical eddy current loop remain approximately constant; therefore, in a short time, the hypothetical equivalent eddy current loop (the hollow coil with a finite cross-section) can be considered as... - First-order circuit.

[0044] The differential equation for the first-order circuit of the eddy current ring is: Equation (2-1); Solving the equation yields: Equation (2-2); in, Equation (2-3); in, The equivalent inductance of the hypothetical eddy current ring is expressed in H / m. The equivalent resistance of the hypothetical eddy current ring is expressed in Ω. is the time constant of a first-order circuit, measured in seconds (s). The thickness of the hypothetical eddy ring is given in meters (m); the "~" in the formula indicates a direct proportional relationship. The resistance value of the eddy ring can be determined from the formula for calculating resistance. The value is inversely proportional to the cross-sectional area (W*D) of the eddy current ring, which is also inversely proportional to the axial dimension D of the eddy current ring's thickness. D, in turn, is related to the conductor's thickness. Proportional, therefore the resistance of the eddy ring and Proportional. For the inductance of an eddy current ring... When the thickness D of the eddy current ring doubles, its corresponding inductance also doubles. Therefore, the inductance of the eddy current ring... With the thickness of the conductor Proportional. In summary, the time constant of a first-order eddy current ring circuit... With the square of the conductor's thickness Proportional. For specimens of uniform material and identical dimensions, if the relative positions of the eddy current rings in the specimen are the same, then the first-order circuit time constant τ of the eddy current rings is proportional. 环 The change only represents a change in thickness.

[0045] For example, selecting the quantitative time interval for measuring curve thickness using the power function of the receiving function includes: for a measured descending curve, when calculating thickness using the slope, the time interval for calculating the slope should be between the intersection points of the α1-fold and α2-fold power functions of the voltage received signal and the descending curve, where 0.7 ≥ α1 > α2 ≥ 0.3 and α1 - α2 = 0.2. (See Appendix) Figure 12-14 As shown, taking conductor specimens with thicknesses ranging from 2.4 mm to 20 mm as examples, and using simulation calculation results as examples, the received signals corresponding to conductor specimens of different thicknesses are obtained in a single logarithmic coordinate system.

[0046] Step 150: Calculate the first derivative of the voltage signal with respect to time in a logarithmic coordinate system within the time interval of the thickness measurement, and use it to obtain the slope.

[0047] In some implementations, the slope is achieved through the following mathematical relationship: Equation (4-1); in, This slope represents the first derivative of the voltage signal with respect to time in a logarithmic coordinate system. This indicates taking the derivative with respect to time; This indicates that the voltage signal collected by the receiving coil changes with time. A changing function.

[0048] In some implementations, before calculating the first derivative of the voltage signal with respect to time in a logarithmic coordinate system over the time interval of the thickness, the method includes performing nonlinear compression smoothing filtering on the voltage signal to suppress noise.

[0049] In the experiment, the pulsed eddy current excitation current was set to 2A, the square wave duty cycle to 50%, and the excitation frequency to 1Hz. (Refer to the appendix.) Figure 16 As shown, noise exists in the received signal of the coil during pulsed eddy current detection. After removing the tail noise segment and filtering the remaining part, the result is shown in the attached figure. Figure 17 As shown, the filtered data is smoother.

[0050] Step 160: Calculate the wall thickness of the clad structure based on the quantitative relationship between the slope and the wall thickness of the clad structure.

[0051] In some embodiments, the mathematical relationship between the slope and the wall thickness of the cladding structure is as follows: Equation (4-2); Equation (4-3); in, Characteristic quantities used for thickness measurement; This indicates the slope; Indicates the wall thickness value of the structure with a cladding layer; Indicates proportional to; Indicates time The characteristic quantity of change; This indicates the start time of the third phase.

[0052] In some embodiments, calculating the wall thickness value of the clad structure based on the quantitative relationship between the slope and the wall thickness of the clad structure includes: comparing the characteristic quantity with a preset thickness and characteristic quantity calibration curve to invert and obtain the wall thickness value.

[0053] The preset thickness and characteristic quantity calibration curve is obtained by measuring the characteristic quantity of multiple standard specimens with known thicknesses under the same testing conditions and fitting the relationship between the characteristic quantity and the wall thickness value.

[0054] For example, based on the numerical simulation results, from the third stage of the received voltage signal, select the following thickness measurement intervals: 0.7 to 0.5 times the power function, 0.6 to 0.4 times the power function, and 0.5 to 0.3 times the power function. Calculate the slope values ​​of the received voltage signal for different thicknesses on a logarithmic coordinate system. Thickness characteristic quantity. With thickness It exhibits a good linear relationship, see attached figure. Figure 15 As shown, the relationship between the slope of the received signal and the thickness of the conductor specimen is calculated by selecting different thickness quantitative intervals. From Figure 15 As can be seen from the above, the characteristic quantities of the three different thickness quantitative intervals selected above are... With thickness All of them present as Figure 15 The diagram shows a relatively good linear relationship. Regarding the characteristic quantities in the figure... With thickness The least squares method was used to fit the line, and the fitting results are shown in Table 1 below: Table 1: Characteristic quantities of different quantitative intervals With thickness Fitted straight line

[0055] As shown in Table 1, for steel plates with thicknesses within this range, selecting any of the three thickness quantitative intervals does not change the slope of the linear fitting curve between the thickness characteristic quantity and the thickness; only the intercept of the line changes. The larger the α value of the selected thickness quantitative interval, the earlier the thickness quantitative interval appears, and the higher the thickness value... With characteristic quantity The smaller the intercept of the fitted straight line, the better. If the thickness value... With characteristic quantity If the intercept of the fitted line is zero or close to zero, then the fitted line is a straight line passing through the origin. This can be achieved using only the thickness value of the reference specimen and the characteristic quantity of the received voltage. That will allow you to determine the thickness. With characteristic quantity The fitted straight line will bring great convenience in practical engineering applications. However, in reality, the thickness value... With characteristic quantity The relationship line does not pass through the origin, and it cannot be determined solely by the slope of a single reference specimen thickness and the received voltage signal. and The relationship curve. If a curve passing through the origin is drawn using a single point, it will introduce significant errors. In actual testing, during the third stage of voltage signal reception, the eddy currents continuously attenuate over time, their amplitude decreases, and they are easily affected by noise. To ensure the quality of the signal within the selected thickness quantitative range, the chosen range should be as early as possible.

[0056] This embodiment uses Q235 steel plate and a focusing probe as an example for illustration, but this method is also applicable to other metal materials (such as low-carbon steel and alloy steel) and various probe configurations (such as coaxial cylindrical probes). By adjusting the calibration curve, it can be adapted to different detection scenarios. In some embodiments, the method further includes: uniformly thinned thickness quantification, which can be achieved, for example, by performing a thickness quantification test on a Q235 disc-shaped steel plate of uniform thickness. The diameter of the steel plate used in the test is 400 mm, and the thickness of the steel plate is shown in Table 2. Due to processing errors, the thickness of the specimen at 5 different locations is measured using an ultrasonic thickness gauge, and the median is taken. Using a focusing probe, tests are conducted on Q235 disc-shaped steel plates of different thicknesses at different lift-off heights. To maintain consistency in the test, the probe is placed in the center of the steel plate during each test. In the test, the specimen of each thickness is measured 3 times at each lift-off height.

[0057] Table 2: Thickness of iron disc specimens (unit: mm)

[0058] Among them, the test received signals for steel plates of different thicknesses at lifting distances of 5 mm, 10 mm, 20 mm, 40 mm, and 60 mm are shown in the attached figure. Figure 18-22 As shown.

[0059] In some implementations, embodiments of this application can define the time interval between 0.7 and 0.5 times the power of each received voltage signal as the thickness measurement time interval. This allows for the determination of thickness characteristics at different lift-off heights. With thickness Fitted straight line reference appendix Figure 23-27 The experimental data shown are illustrated in Table 3, where the parameters of the fitted straight line are also shown. (From the appendix...) Figure 23-27 Experimental data show that the thickness characteristic quantity With thickness It exhibits a good linear relationship, and at a lift height of 60mm, the thickness characteristic value... With thickness It still maintains a good linear relationship. Therefore, the method proposed in this application not only has good linearity but also good lift-off suppression characteristics.

[0060] Table 3: Characteristic quantities of semi-circular focusing probes with different lift-offs With thickness Fitted line parameters

[0061] This application first applies a square wave signal to the excitation coil, utilizing its rich spectral characteristics to induce eddy current fields with varying penetration depths in the test specimen, laying the physical foundation for obtaining comprehensive specimen thickness information. The receiving coil collects the voltage signal induced by the eddy current attenuation magnetic field, and by clearly defining its three attenuation stages, effectively distinguishes between invalid signal segments dominated by probe characteristics and valid signal segments containing specimen thickness information, thus pinpointing the target interval for subsequent precise analysis. Secondly, by identifying the power function interval of the second stage, the start time of the third stage is determined. This method uses the linear characteristics of the second stage in a double logarithmic coordinate system as a precise time scale, thereby objectively and repeatedly locating the effective starting point for thickness information analysis, avoiding the subjective arbitrariness of human selection. Based on the equivalent model of multi-eddy current ring time-division excitation, the thickness quantification time interval is selected based on this starting time, ensuring that the selection of the time interval is grounded in a profound physical model. This guarantees that the analyzed signal corresponds to the eddy current diffusion state under the same physical mechanism, significantly improving the accuracy and consistency of thickness quantification. Finally, by calculating the slope of the voltage signal in a single logarithmic coordinate system within the quantitative time interval of thickness, the complex signal attenuation pattern is transformed into a single, clear feature quantity, which greatly simplifies the process of extracting thickness information. By establishing a quantitative relationship between the slope and the wall thickness for the final calculation, the feature quantity extracted in the previous steps is directly correlated with the final physical quantity (wall thickness), realizing a reliable inversion from signal to thickness and achieving the final goal of nondestructive testing.

[0062] Compared with traditional fixed interval selection methods, this invention adaptively determines the thickness quantitative time interval based on a multi-vortex ring model, significantly reducing the impact of lift-off height and thickness variations on measurement accuracy. As shown in Table 3, even at a lift of 60mm, and It still maintains good linearity (the slope of the fitted line changes by <0.005), while traditional methods typically have errors exceeding 10% under these conditions. According to a specific implementation of this application, the eddy current diffusion time constant, used to characterize the eddy current diffusion and decay timescales in conductive materials... The expression is shown as: Equation (2-4); Among them, eddy diffusion time This is the time characteristic of the eddy current from excitation to decay, and its value is 1 / 3 of the eddy current intensity decaying to its initial value. The required time represents a time scale. The eddy current diffusion time difference constant is not only related to the physical properties of the conductor material, but also directly related to the thickness of the conductor material. The thickness is... The conductor specimen has an eddy current diffusion time of . Thickness is The conductor specimen has an eddy current diffusion time of . The relationship between the eddy current diffusion time constants for two specimens of the same material is shown as follows: Equation (2-5); In the third stage of eddy current diffusion, the eddy current intensity in the conductor continuously decreases, and the time constant of the first-order circuit of the hypothetical eddy current ring... With eddy diffusion time constant Both are proportional to the square of the conductor thickness, and both parameters are time scales related to the material's permeability, conductivity, and thickness. Therefore, the time constant of a first-order circuit... With respect to the eddy current diffusion time constant of the conductor material Direct correspondence: Equation (2-6); therefore, Equation (2-7); Where "~" indicates a proportional relationship, and the time constant of the eddy current ring equivalent first-order circuit is... With respect to the eddy current diffusion time constant of the conductor material They are directly proportional. And the eddy current diffusion time constant... With the thickness of the material Directly related. Therefore, the time constant of a first-order circuit can be used. The relationship between the material thickness and the thickness is expressed as follows: Equation (2-8); During a short period of time in the third stage, the voltage of the receiving coil will be... Expressed in a logarithmic coordinate system, it is calculated using the following expression: Equation (2-9); The first derivative of the voltage decay curve with respect to time in a logarithmic coordinate system is: Equation (2-10); From this formula, we can see that during a short period of time in the third stage, the slope of the voltage signal of the receiving coil in a logarithmic coordinate system is... Equivalent first-order circuit constant of eddy current ring Related. The time constant of the first-order circuit equivalent to the eddy current ring. The specific value is difficult to calculate directly in both simulation and actual experiments, but the slope can be easily obtained by performing a first-order fit on the received voltage signal. Value, and Less than zero. Due to the slope It can be obtained , Equation (2-11); Among them, the characteristic quantities of the received signal for, Equation (2-12); If the slope of the standard thickness specimen is known and The value is obtained by calculating the slope of the test piece. Using the above formula and The relationship between the conductor specimen thickness and the thickness can be obtained. The changes.

[0063] By combining the above formulas, we can obtain... and Relationship, Equation (2-13); That is, Equation (2-14); In this formula, the slope of the received voltage signal of the standard reference conductor specimen is... , characteristic quantity is Thickness is The slope of the received voltage signal of the tested device is , characteristic quantity is Thickness is .

[0064] The extraction of conductor thickness characteristics includes the following: The application of formula (2-14) assumes that the thickness to be measured and the thickness of the standard part are within the same eddy current loop. However, when the decay time span of the entire eddy current magnetic field is large, the equivalent eddy current loop corresponding to the third segment of the received voltage signal will undergo significant changes. Therefore, it is particularly important to separate the signal of the third stage from the entire signal and to find a suitable eddy current loop in the third stage for thickness calculation. This application employs the following series of methods to solve this problem.

[0065] For example, the characteristics of the second stage of receiving voltage signals are as follows: In the second stage of eddy current diffusion, the excitation magnetic field has been completely turned off, and the voltage signal of the receiving coil is generated solely by the eddy current field in the conductor specimen. In the second stage of eddy current diffusion, the voltage signal of the receiving coil conforms to an inverse power law function.

[0066] If the inverse power function is known... Equation (2-15); Where in the formula , It is a constant term, and >0; , Let be the independent and dependent variables in the Cartesian coordinate system, respectively. Taking the logarithm of both sides of the equation, we get... Equation (2-16); Substitute the variables in this expression. Equation (2-17); Equation (2-18); Substituting (2-17) and (2-18) into (2-16), and taking the logarithm of both sides of the equation, we obtain the power function as follows: Equation (2-19); As shown in formula (2-19), the received voltage attenuation curve in the second stage is a straight line in the double logarithmic domain. (Refer to Appendix...) Figure 7-8 As shown. Therefore, the curve characteristics of the second segment of the received voltage signal can be used to distinguish the third segment of received voltage attenuation from the entire signal.

[0067] In the embodiments of this application, reference is made to the appendix. Figure 11 As shown, in this embodiment of the application, based on the selection of the thickness quantitative time interval of the same eddy current ring, for each detected received signal, the inverse power law segment signal function of the detected signal is used as a reference, and the inverse power law function is multiplied by a certain coefficient α (0<α<1), which is called the α-power function.

[0068] Using a coaxial cylindrical probe, numerical simulations of pulsed eddy current detection were performed on specimens of the same material but different thicknesses. The locations of the maximum eddy current density in conductors of different thicknesses were obtained when the receiving coil voltage decayed to an α-power function. Table 4 shows the coordinates of the maximum eddy current modulus in the conductor's 1 / 4 cross-section at different α-power function locations for conductor specimens of the same material, when the conductor thickness varied in increments of 0.8 mm or multiples thereof. The coordinates in the table (…) , The coordinate system is represented as follows: with the center point of the upper surface of the conductor specimen as the origin, the coordinates of the maximum eddy current density point in the disk-plate conductor specimen are the abscissa value in the radial direction and the ordinate value in the thickness direction. The change in the abscissa value of the location of the maximum eddy current density represents the radial diffusion of the eddy current; the change in the ordinate value of the location of the maximum eddy current density represents the penetration of the eddy current along the thickness direction.

[0069] Table 4: Location coordinates of the maximum eddy current modulus of conductor materials with different thicknesses

[0070] Table 4: (continued)

[0071] In the initial stage of the three-stage transition, the location of the maximum eddy current density modulus in the conductor gradually changes from moving towards the depth of the conductor thickness to moving along the radial direction. This period is referred to as the transition segment of the third stage. Table 4 shows that when the conductor thickness is thin (2.4 mm to 6.4 mm), the location of the maximum eddy current density in the conductor moves radially, meaning the transition segment of the third stage is short. When the conductor thickness increases (greater than 6.4 mm), the location of the maximum eddy current density modulus not only moves radially but also towards the depth of the conductor. As the conductor thickness increases, the received voltage signal needs to decay to a lower α-power function value before the initial transition segment of the third stage ends. Therefore, as the conductor specimen thickness increases, the received signal at the end of the transition segment becomes a smaller α-power function.

[0072] From the location information of the maximum eddy current density modulus in the conductor in Table 4, it can be seen that after the transition section of the third stage, as the value of α gradually decreases, the location of the maximum eddy current density modulus is always located at the center of the conductor thickness and moves radially along the conductor. Table 4 also shows that when the conductor specimen thickness does not change significantly, the radial position of the maximum eddy current density modulus in conductor specimens of different thicknesses is basically the same at the same α power function. Therefore, the simulation results show that for conductor specimens with small thickness changes in the table, at the same α power function time between the 0.7 and 0.4 power functions of the received voltage signal, the radial position of the equivalent eddy current ring of conductor specimens of different thicknesses remains basically unchanged, with only the longitudinal depth of the equivalent eddy current ring changing.

[0073] Further simulations show that for the received signal corresponding to a conductor material with a thickness ranging from 2.4 mm to 16 mm, the eddy current density attenuation ratio is between 0.32 and 0.44 within the time range of 0.7 to 0.3 times the power function attenuation. This value is approximately equal to 1 / The magnitudes are close. When using the slope of the received voltage signal to invert the thickness calculation, the data used for thickness calculation is selected between 0.7 and 0.3 times the power function of the received voltage signal, and the difference in the power function multiples corresponding to the two ends of the data interval is 0.2, which can achieve good calculation results.

[0074] This application also discloses a cladding wall thickness pulse eddy current detection system based on multiple eddy current rings, used to implement the above-mentioned cladding wall thickness pulse eddy current detection method based on multiple eddy current rings, including: a signal generator, an excitation coil, a receiving coil, a data acquisition unit, and a processing unit.

[0075] The system includes a signal generator for generating a square wave excitation signal; an excitation coil for inducing eddy currents in the tested component; a receiving coil for acquiring the attenuation signal of the eddy current magnetic field; a data acquisition unit for acquiring the voltage signal of the receiving coil; and a processing unit configured to perform quantitative wall thickness calculations, including identifying the power function interval of the received signal, determining the quantitative thickness time interval, calculating the slope of the voltage signal in a logarithmic coordinate system, and performing quantitative wall thickness calculations based on a preset slope-wall thickness relationship.

[0076] In some implementations, the system also includes a probe, which is a coaxial cylindrical probe or a semi-circular focusing probe.

[0077] According to another specific implementation of the embodiments of this application, the testing equipment or platform of this application consists of a pulse eddy current analyzer, different types of test specimens, sensors, and auxiliary devices. The sensors include a coaxial cylindrical probe and a semi-circular focusing probe. In this example, the test specimen can be a Q235 flat plate specimen of different thicknesses, and a focusing probe is used.

[0078] This application's embodiments, by integrating dedicated hardware components and optimized processing algorithms, transform the aforementioned method into a stable and automated testing platform, thereby enabling rapid and accurate measurement of the wall thickness of coated structural components in engineering practice. Specifically, the system's inclusion of different sensor configurations, such as coaxial cylindrical probes or semi-circular focusing probes, enhances its adaptability to various testing scenarios and specimen specifications. Simultaneously, the complete testing platform, composed of a pulse eddy current analyzer, diverse specimens, and probes, provides a solid hardware foundation and experimental flexibility for the effective implementation and verification of this method.

[0079] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of this application, and this application is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and substance of this application, and these modifications and improvements are also considered to be within the scope of protection of this application.

Claims

1. A pulsed eddy current detection method for coating wall thickness based on multiple eddy rings, characterized in that, include: A square wave excitation signal is applied to the excitation coil, and eddy currents are induced in the clad structure. The voltage signal generated by the attenuating magnetic field generated by the eddy current is acquired by the receiving coil. The attenuation process of the voltage signal includes three stages: the first stage is the instantaneous response segment after the excitation magnetic field is turned off; the second stage is the segment in which the eddy current penetrates into the center region of the wall thickness; and the third stage is the segment in which the eddy current diffuses and attenuates radially. Identify the power function interval of the second stage to determine the start time of the third stage; Based on the equivalent model of time-sharing excitation of multi-vortex rings, a preset thickness quantitative time interval is selected within the third stage, with the start time of the third stage as the benchmark. Calculate the first derivative of the voltage signal with respect to time in a logarithmic coordinate system within the time interval for the thickness measurement, and use it to obtain the slope; The wall thickness of the clad structure is calculated based on the quantitative relationship between the slope and the wall thickness of the clad structure.

2. The method according to claim 1, characterized in that, The identification of the power function interval in the second stage includes: Take the logarithm of the voltage signal value and the corresponding time value respectively to obtain the voltage logarithmic vector and the time logarithmic vector; Data points are selected at fixed intervals on the time logarithm vector to perform linear fitting, and the linear distance from all data points to the fitted line is calculated. The interior points of the straight-line distance are filtered according to a preset distance threshold, and an interior point dataset is formed to find the interior point dataset corresponding to the maximum time difference. The least squares method was used to fit a straight line to the inlier dataset. The linear intervals in the double logarithmic coordinate system corresponding to the parameters of the fitted straight line are identified and defined as the power function intervals of the second stage.

3. The method according to claim 1, characterized in that, The thickness measurement time interval is the time interval during which the voltage signal decays to the point where the amplitude of the voltage signal is equal to a power function of α1 to α2. The possible combinations of values ​​for α1 and α2 include: α1 = 0.7 and α2 = 0.5; or α1 = 0.6 and α2 = 0.4; or α1 = 0.5 and α2 = 0.

3.

4. The method according to claim 1, characterized in that, The application of a square wave excitation signal to the excitation coil includes: Based on the conditions of a duty cycle of 50% and an excitation frequency within 100Hz, the square wave excitation signal is applied to the excitation coil.

5. The method according to claim 1, characterized in that, The slope is achieved through the following mathematical relationship: ； in, The slope is represented by the first derivative of the voltage signal with respect to time in a logarithmic coordinate system. This indicates taking the derivative with respect to time; This indicates that the voltage signal acquired by the receiving coil changes with time. A changing function.

6. The method according to claim 1, characterized in that, The mathematical formula for the quantitative relationship between the slope and the wall thickness of the cladding structure is as follows: ； ； in, Characteristic quantities used for thickness measurement; Indicates the slope; Indicates the wall thickness value of the structure with a cladding layer; Indicates proportional to; Indicates time The characteristic quantity of change; This indicates the start time of the third stage.

7. The method according to claim 1, characterized in that, Before calculating the first derivative of the voltage signal with respect to time in a logarithmic coordinate system within the specified thickness measurement time interval, the following steps are included: The voltage signal is subjected to nonlinear compression and smoothing filtering to suppress noise.

8. The method according to claim 6, characterized in that, The calculation of the wall thickness value of the clad structure based on the quantitative relationship between the slope and the wall thickness of the clad structure includes: The feature quantity is compared with the preset thickness and feature quantity calibration curve to obtain the wall thickness value through inversion. The preset thickness and characteristic quantity calibration curve is obtained by measuring the characteristic quantities of multiple standard specimens with known thicknesses under the same testing conditions and fitting the relationship between the characteristic quantities and the wall thickness value.

9. A pulsed eddy current detection system for coating wall thickness based on multiple eddy rings, used to implement the pulsed eddy current detection method for coating wall thickness based on multiple eddy rings as described in any one of claims 1 to 8, characterized in that, include: A signal generator is used to generate a square wave excitation signal; Excitation coils are used to induce eddy currents in the workpiece being tested; A receiving coil is used to acquire the attenuated signal of the eddy current magnetic field. A data acquisition unit is used to acquire the voltage signal from the receiving coil. The processing unit is configured to perform quantitative wall thickness calculation, including identifying the power function interval of the received signal, determining the quantitative thickness time interval, calculating the slope of the voltage signal in a logarithmic coordinate system, and performing quantitative wall thickness calculation based on a preset slope-wall thickness relationship.

10. The system according to claim 9, characterized in that, The system also includes a probe, which is a coaxial cylindrical probe or a semi-circular focusing probe.

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