Resonant eddy current probe structure for inhibiting lift-off effect and detection method

Abstract

The invention discloses a resonant eddy current probe structure for inhibiting a lift-off effect and a detection method. In a probe provided by the invention, an included angle between the short edge extension direction of an exciting coil and the short edge extension direction of a receiving coil is 45 degrees. According to the method provided by the invention, corresponding sampling signals are obtained by applying sweep frequency excitation signals at different positions, signal factors are obtained, a function relationship between the signal factors and frequencies is established, and then a plurality of characteristic parameters capable of reflecting signal factor change characteristics are selected; and establishing a function relationship between each characteristic parameter and the surface position of the defect sample, then obtaining the characteristics of the function to identify the defect position, and then obtaining an effective detection lift-off distance range through the capability difference of identifying the defect position under different lift-off distances. According to the method, a resonance eddy current detection means is adopted to facilitate defect identification, the signal processing method provided by the invention can rapidly identify existence of defects, and the whole process of the method enables a judgment result of an effective detection range to be more accurate.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic nondestructive testing technology, specifically relating to a resonant eddy current probe structure and testing method for suppressing lift-off effect. Background Technology

[0002] Carbon steel is a widely used material in industry. However, under long-term exposure to environments including high stress, high temperature, high pressure, and corrosion, carbon steel components are prone to various defects on their surface, including stress corrosion cracks (SCC). These defects, especially SCC, can reduce the strength of components and lead to accidents. To avoid accidents, regular inspection of components is essential. Resonant Eddy Current Testing (RECT) is a testing method that uses capacitors connected in parallel or series at the excitation and receiving coils to form a resonant circuit. The interaction between this resonant circuit and the test piece is then used for testing. Compared to traditional eddy current testing (ECT), this method can obtain richer signal characteristics, thereby improving testing accuracy.

[0003] However, like traditional ECT technology, RECT is also susceptible to lift-off effects, which can reduce detection accuracy. The existence of lift-off distance is often unavoidable in practical testing. For example, due to corrosion protection requirements, components often have non-conductive coatings. This structure creates a distance between the component and the probe, known as lift-off distance. The lift-off distance itself weakens the detection signal. Furthermore, unevenness on the coating or other surfaces causes lift-off distance fluctuations, generating lift-off noise, further reducing the signal-to-noise ratio of the detection signal.

[0004] Therefore, it is necessary to propose a probe structure based on RECT technology that can suppress the influence of lift-off on the detection signal within a certain lift-off distance range; in order to determine the application range of such a probe, i.e. the applicable lift-off distance range, it is also necessary to propose a detection method for the effective detection distance of the proposed probe. Summary of the Invention

[0005] The purpose of this invention is to provide a resonant eddy current probe structure and detection method for suppressing the lift-off effect, thereby solving the above-mentioned technical problems.

[0006] The technical solution adopted in this invention is as follows:

[0007] A resonant eddy current detection probe structure for suppressing lift-off effect is disclosed. This probe is a resonant eddy current probe, comprising a first excitation coil, a second excitation coil, a first receiving coil, and a second receiving coil. All coils are planar coils, each consisting of two long sides and two short sides. The first and second excitation coils are generally figure-eight shaped, with the two long sides of each coil intersecting and the two short sides parallel. The first excitation coil is stacked on top of the second excitation coil, with the long sides of the two coils coinciding and their corresponding short sides perpendicular to each other. Both the first and second receiving coils are rectangular coils located on the same plane, with the long sides of the two rectangular coils extending in parallel directions. An angle exists between the extension direction of the short side of any excitation coil and the extension direction of the short side of the receiving coil.

[0008] Further optimization resulted in an angle of 45° between the extension direction of the short side of the excitation coil and the extension direction of the short side of the receiving coil.

[0009] The probe structure described in this invention is a further study by the inventors based on the probe designed in previous research to suppress the lift-off effect. The inventors' previous research results have been published in the journal article: Jin Jianhui, Zhang Qing, Han Bo, Yang Shipin, Ren Yi. Research on Eddy Current Detection and Evaluation Method for Complex Cracks [J]. Journal of Instrumentation. 2024, 45(2):280-288. The structure of the excitation coil and receiving coil in the probe described in this invention is the same as that used in the excitation coil and receiving coil in the probe designed in the previous literature, but improvements have been made to the relative position distribution between the excitation coil and the receiving coil. The probe described in the literature has an angle of 45° between the direction of the induced current formed in the test piece when the magnetic field acts on the test piece and the direction of the long side extension of the receiving coil. This structure is beneficial to give the sampled signal richer signal characteristics. The probe described in this invention has the direction of the induced current formed in the test piece parallel to the direction of the long side extension of the receiving coil. This structure is beneficial to reduce the influence of common mode noise, increase the intensity of the induced current in the receiving coil, and thus increase the ability to suppress the lift-off effect.

[0010] The detection method based on the above-mentioned resonant eddy current probe structure for suppressing the lift-off effect specifically includes the following steps:

[0011] S1. Build a detection platform, the detection platform includes a detection module and an analysis module, the detection module performs defect detection through a probe, the probe structure is as described in claim 1, and the detection module and the analysis module are connected by a signal.

[0012] S2. Construct multiple defect samples based on actual working conditions. The defect samples are test pieces with surface defects. The test pieces are strip-shaped and have the same specifications in all defect samples. At the same time, determine multiple distance values ​​as the lift-off distance during detection.

[0013] S3. Based on the detection platform established in step S1, all defect samples are detected one by one using the probe at each lift-off distance. The complete detection process for a certain defect sample at a certain lift-off distance is called the detection cycle. The electrical signal applied to the excitation coil during detection is called the excitation signal, which is a frequency sweep signal. The signal collected by the receiving coil is called the sampling signal. In a certain detection cycle, the probe applies the same frequency sweep excitation signal at different positions and collects the corresponding sampling signals. All coils in the probe are always parallel to the surface of the defect sample. During the detection process, the detection module transmits the sampling signals to the analysis module in real time.

[0014] S4. The analysis module determines whether the defect can be effectively detected at a certain lift-off distance based on the sampled signal received at that lift-off distance, and derives the range of effective detection distances based on the judgment results corresponding to all lift-off distances.

[0015] Resonant eddy current testing is a detection method that allows electrical signals to contain a wealth of features. The characteristics of the sampled signal are most pronounced when the excitation signal frequency is close to the resonant frequency. Since the resonant frequency is determined by both the test piece itself and the defects located on its surface, the location of the defects on the test piece's surface can be inferred from the features contained in the sampled signal. The purpose of studying the effective detection distance of the probe is to determine the range at which the location of the defects can still be determined from the features contained in the obtained sampled signal.

[0016] Further optimization involves the following steps: In step S3, a frequency range needs to be determined before detection as the frequency coverage range of the sweep excitation signal during detection. Within this range, the frequency of the excitation signal changes continuously while maintaining a consistent amplitude. Before detection, a one-dimensional coordinate axis needs to be established on the surface of all defective samples, and the detection position and detection distance need to be determined. The extension direction of the one-dimensional coordinate axis is parallel to the extension direction of the defective sample. The detection position is the position where the probe is when the excitation signal is applied. The point formed by the vertical projection of the center point of the excitation coil onto the surface of the defective sample when the probe is at the detection position is called the detection point. All detection points are located on the same straight line, which is parallel to the extension direction of the defective sample, and the distance between two adjacent detection points is a constant value, which is the detection distance. The detection cycle consists of multiple detection stages. The process of any detection stage is as follows: at a certain detection position, the probe applies a sweep excitation signal that scans the entire frequency range and acquires the corresponding sampling signal. Then, the probe is moved to the next detection position for the next detection stage. The detection positions of the probe are different in different detection stages, and the number of detection stages is equal to the number of detection positions.

[0017] In resonant eddy current testing, the characteristics of the sampled signal are most pronounced when the excitation signal frequency is close to the resonant frequency. However, the magnitude of the resonant frequency is determined by the probe's topology and the electrical characteristics of both the probe and the test sample. It is difficult to accurately determine the specific value of the resonant frequency before testing. Therefore, a sweeping excitation signal covering a preset frequency range is applied during testing. By acquiring sampled signals across the entire frequency band and analyzing their amplitude variations, the signal closest to the resonant frequency can be determined. The characteristics of this sampled signal will be used for subsequent analysis. Since eddy current testing is a scanning process, the sampled signal from a single location can only reflect local information at that location and cannot fully characterize the location and characteristics of the crack. Therefore, to accurately determine the crack location, testing needs to be performed at multiple locations, and the changes in signals acquired from all locations need to be comprehensively analyzed.

[0018] Further optimization, the specific process of determining whether a defect can be effectively detected at a certain lift-off distance in step S4 includes the following steps:

[0019] S4.1. Based on the principle of electromagnetic induction and the electrical characteristics of the defect sample and the probe, a control equation is constructed to describe the electrical relationship between the probe and the defect sample; then, based on the received sampling signal and the parameters contained in the control equation, a signal factor is obtained, which is a parameter used to describe the relationship between the excitation signal and the sampling signal.

[0020] S4.2. Establish the functional relationship between the signal factor and the excitation signal frequency, and then select a variety of parameters that can reflect the characteristics of the signal factor changing with the excitation signal frequency as feature parameters. Establish the functional relationship between each feature parameter and the detection position. The function of the feature parameter with respect to the position change is called the position function.

[0021] S4.3. Take a certain feature of the position function as the identification feature. If the position of the defect in a certain defect sample can be obtained through the position corresponding to the identification feature of a certain position function, then it is considered that the defect in the defect sample can be identified through the position function. If the defect in all defect samples can be identified through a certain position function, then the feature parameter corresponding to the position function is considered to be a valid feature parameter.

[0022] S4.4. According to the process described in step S4.3, determine whether each feature parameter is a valid feature parameter at a certain lift-off distance, and then determine a threshold. If the number of valid feature parameters is not less than the threshold, it is considered that the probe can effectively detect defects at that lift-off distance.

[0023] The signal factor is an intermediate parameter that makes signal analysis more efficient. The functional relationship between the signal factor and the frequency of the excitation signal not only allows the information contained in the sampled signal to be fully reflected, but also makes the signal analysis process independent of the amplitudes of the excitation and sampled signals, thus improving signal processing efficiency. The best way to determine whether a defect can be identified through the sampled signal is to establish a functional relationship between the characteristic parameters and the location, and then observe whether the function at the location of the defect has a certain characteristic. If so, it indicates that the presence of the defect can be identified through the sampled signal.

[0024] Further optimization involves obtaining the signal factor in step S4.1 through the following steps:

[0025] S4.1.1. The first receiving coil and the second receiving coil are connected in reverse series to form a differential receiving coil, and the first excitation coil and the second excitation coil are connected in reverse series to form a differential excitation coil; both the excitation coil and the receiving coil are connected in parallel through a parallel capacitor to form a parallel resonant circuit. In the parallel resonant circuit, the excitation coil and the receiving coil are equivalent to two resistors connected in series and two inductors connected in reverse series on a branch. The test piece is equivalent to a loop with one inductor and one resistor connected in series. There is electromagnetic induction between any of the above coils and other coils, and the other coils are not in the same path as this coil.

[0026] S4.1.2. Based on the equivalent circuit and Kirchhoff's voltage law described in step S4.1.1, establish the control equations, which are expressed in the following form:

[0027]

[0028] Where, r s The internal resistance of the excitation circuit is given by j, where j is the imaginary unit and ω is the angular frequency. S For the excitation signal amplitude, U L Let I be the amplitude of the sampled signal, and I1 and I2 be the magnitudes of the currents in the excitation coil and the receiving coil, respectively. S R represents the magnitude of the induced current in the device under test; R1 and R2 are the resistances of the two coils in the equivalent circuit of the excitation coil, R3 and R4 are the resistances of the two coils in the equivalent circuit of the receiving coil, L1 and L2 are the inductances of the two coils in the equivalent circuit of the excitation coil, and L3 and L4 are the inductances of the two coils in the equivalent circuit of the receiving coil. S and L S C1 and C2 are the resistance and inductance in the equivalent circuit of the device under test, respectively, and the capacitors connected in parallel in the equivalent circuits of the excitation coil and the receiving coil, respectively; M ij For L i With L j The mutual inductance coefficient between them, M is For L i With LS The mutual inductance coefficient between them, M js For L j With L S The mutual inductance coefficients between them, i∈{1,2}, j∈{3,4}, and the circuit elements on both sides of || are respectively located in two parallel branches.

[0029] S4.1.3. The signal factor is the scattering coefficient S. 21 S 21 Defined by the following expression:

[0030]

[0031] U can be obtained from formula (1) S with U L The expressions are as follows:

[0032]

[0033] Where ΔR1-ΔR2 and ΔL1-ΔL2 are the total changes in resistance and inductance of the first and second excitation coils under the mutual inductance between the excitation coil and the test piece, respectively; ΔR3-ΔR4 and ΔL3-ΔL4 are the total changes in resistance and inductance of the first and second receiving coils under the mutual inductance between the receiving coil and the test piece, respectively. Since the excitation coil and the receiving coil are differential, the total changes in resistance and inductance are all differential values. ΔM TR This represents the total change in the mutual inductance coefficient between the excitation coil and the receiving coil.

[0034] ΔR1+ΔR2, ΔL1+ΔL2, ΔR3-ΔR4, ΔL3-ΔL4 and ΔM TR The values ​​are obtained using the following formulas:

[0035]

[0036] Substituting formulas (3) and (4) into formula (2) yields the following expression:

[0037]

[0038] It can be seen from formula (5) that ΔR1-ΔR2, ΔL1-ΔL2, ΔR3-ΔR4, ΔL3-ΔL4 and ΔM TR These are all parameters related to the mutual inductance between the test piece and the coil. Therefore, the specific values ​​of these five parameters are all related to the test piece. As can be seen from formula (6), the scattering coefficient S 21 It is related to all five parameters mentioned above, therefore, through S 21 Its features can accurately identify the location of defects.

[0039] Further optimization, step S4.2 specifically includes the following steps:

[0040] S4.2.1. Based on the sampling signal acquired at the same detection position under the action of the frequency sweep excitation signal, the scattering coefficient S is derived. 21 A first type of graph showing the functional relationship between the frequency of the excitation signal and the frequency of the excitation signal. The horizontal axis of the first type of graph corresponds to the frequency of the excitation signal, and the vertical axis corresponds to the scattering coefficient S. 21 The resulting curve is "M" shaped; then multiple detection positions are selected, and a corresponding first-type curve is plotted for each selected detection position to form a curve set.

[0041] S4.2.2. Obtain the set of curves corresponding to each defect sample at the same lift-off distance according to the process described in step S4.2.1.

[0042] S4.2.3. In the first type of curve graph, the maximum value closer to the vertical axis is denoted as the first resonant peak value, the maximum value farther from the vertical axis is denoted as the second resonant peak value, and the minimum value between the two maximum values ​​is denoted as the resonant valley value. The frequency values ​​corresponding to the first resonant peak value, the second resonant peak value, and the resonant valley value are denoted as f, respectively. peak1 f peak2 and f valley Then, at the first resonant peak, the second resonant peak, the resonant trough, and f... peak1 f peak2 f valley Multiple parameters are selected from six types as feature parameters, and the values ​​of each feature parameter corresponding to different detection positions are obtained from the set of curves. All detection positions are represented by the coordinates of their corresponding detection points. Then, a position function is fitted for each feature parameter, and the position function is described by a second type of curve graph.

[0043] S4.2.4. Based on the process described in step S4.2.3, obtain all the second type curves corresponding to each defect sample at the same lift-off distance.

[0044] The maxima and minima in the "M"-shaped curve are features that clearly reflect the information contained in the sampled signal. Therefore, all parameters related to the maxima and minima are considered as candidate feature parameters. In practical applications, the selection of which parameters to use as feature parameters is based on a comprehensive consideration of computational complexity and discrimination effect. Similarly, because the maxima and minima are relatively obvious features, the position function is also represented by a curve.

[0045] The beneficial effects of the method of the present invention are as follows:

[0046] 1. The method of the present invention uses resonant eddy current detection, which has more features than the sampling signal obtained by traditional eddy current detection, and is helpful for defect identification;

[0047] 2. This invention proposes a probe structure that can suppress the lift-off effect and a novel signal processing method, which can effectively process the sampled signal measured by resonant eddy current detection and quickly identify the presence of defects based on signal characteristics;

[0048] 3. The method of the present invention first determines whether the position function corresponding to each feature parameter can identify the existence of the defect, and then combines the identification results of all position functions to determine whether effective detection can be performed at the lift-off distance, so as to make the judgment result of the effective detection range more accurate. Attached Figure Description

[0049] Figure 1 Schematic diagram of the probe structure;

[0050] Figure 2 Cloud maps of induced current density in specimens under different lift-off distances for two types of probes;

[0051] Figure 3 Schematic diagram of the overall structure of the testing platform;

[0052] Figure 4 The overall equivalent circuit during the testing process;

[0053] Figure 5 Curve assembly pattern at a 2mm lift-off distance;

[0054] Figure 6 All Type II curves at a 2mm lift-off distance;

[0055] Figure 7 All Type II curves at a 4mm lift-off distance;

[0056] Figure 8 All Type II curves at a 6mm liftoff distance;

[0057] Figure 9 All Type II curves at an 8mm liftoff distance;

[0058] Figure 10 All Type II curves at a 10mm lift-off distance. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below through specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0060] Example 1:

[0061] A resonant eddy current detection probe structure for suppressing lift-off effect is disclosed. This probe is a resonant eddy current probe, comprising a first excitation coil, a second excitation coil, a first receiving coil, and a second receiving coil. All coils are planar coils, each consisting of two long sides and two short sides. The first and second excitation coils are generally figure-eight shaped, with the two long sides of each coil intersecting and the two short sides parallel. The first excitation coil is stacked on top of the second excitation coil, with the long sides coinciding and the corresponding short sides perpendicular to each other. Both the first and second receiving coils are rectangular coils located on the same plane, with the long sides of the two rectangular coils extending in parallel directions. The extension direction of any short side within the excitation coil is at a 45° angle to the extension direction of the short side within the receiving coil. The specific structure of the probe is as follows... Figure 1 As shown in the figure, 1 and 2 are the first excitation coil and the second excitation coil, respectively. The two excitation coils are completely identical. 3 and 4 are the first receiving coil and the second receiving coil, respectively.

[0062] The lift-off effect resistance of the probe described in this embodiment was simulated using COMSOL software. The simulation conditions were: a 1A current flowed through the excitation coil; the specimen was made of carbon steel and was strip-shaped; its surface had a groove-shaped crack; the direction of the crack's extension was considered vertical, with the direction of the crack's extension being considered transverse. The induced current density distribution in the specimen at different lift-off distances is shown below. Figure 2 As shown, Figure 2 (a)-(c) show the distribution of induced current density in the specimens under different lifting conditions of the probe of the present invention. Figure 2 (d)-(f) show the induced current density distribution of the probe used in the previous research paper: Jin Jianhui, Zhang Qing, Han Bo, Yang Shipin, Ren Yi. Research on Eddy Current Detection and Evaluation Method for Complex Cracks [J]. Journal of Instrumentation. 2024, 45(2):280-288. The data shown in the figure are the peak-to-peak values ​​of the induced current density, i.e., the difference between the maximum and minimum values. Figure 2 (a) Figure 2 (b) and Figure 2 (c) The peak-to-peak values ​​of the probe of the present invention at 1, 3, and 5 mm lift-off are 7.83 × 10⁻⁶, respectively. 6 A / m 2 27.6×10 5 A / m 2 9.98×10 5 A / m 2 , Figure 2 (d) Figure 2 (e) and Figure 2 (f) These are the peak-to-peak values ​​of the probe at 1, 3, and 5 mm lift-off, respectively, which are 7.56 × 10⁻⁶.6 A / m 2 23.6×10 5 A / m 2 8.47×10 5 A / m 2 Compared with the probe in the paper, it can be seen that the probe described in this embodiment can still produce a strong induced current in the specimen even with an increased lift-off distance, thus exhibiting good resistance to lift-off effects.

[0063] Example 2:

[0064] This embodiment describes a detection method based on the resonant eddy current probe structure that suppresses the lift-off effect. The defect sample is a strip-shaped carbon steel plate, and the defect to be detected is a groove-shaped crack. The method includes the following steps:

[0065] S1. Construct a testing platform, which includes a detection module and an analysis module. The detection module performs defect detection using a probe, the structure of which is as described in Embodiment 1. The analysis module includes a vector network analyzer and a computer, and the detection module and the analysis module are connected via a signal. Furthermore, the detection module also includes a three-dimensional scanning platform on which the test piece is fixed. The overall structure of the testing platform is as follows: Figure 3 As shown in the figure, 5 is the probe, 6 is the test piece, 7 is the 3D scanning stage, 8 is the vector network analyzer, and 9 is the computer.

[0066] S2. Construct multiple defect samples based on actual working conditions. These defect samples are test pieces with surface defects. Simultaneously, determine multiple distance values ​​as lift-off distances during detection. In this embodiment, three strip-shaped crack defects are constructed on the surface of the same test piece. These three defects are denoted as defect 1, defect 2, and defect 3, respectively. The corresponding depths of defects 1, 2, and 3 are 3mm, 2.5mm, and 2mm, respectively. Each defect has a length of 20mm and a width of 0.8mm, and their extension directions form the same angle with the extension direction of the test piece. In this embodiment, five distance values ​​are selected as lift-off distances: 2mm, 4mm, 6mm, 8mm, and 10mm.

[0067] S3. Based on the detection platform established in step S1, the probe is used to sequentially detect three defects on the defect sample at each lift-off distance. The complete detection process for a certain defect sample at a certain lift-off distance is called the detection cycle. The electrical signal applied to the excitation coil during detection is called the excitation signal, which is a frequency sweep signal. The signal collected by the receiving coil is called the sampling signal. In a certain detection cycle, the probe applies the same frequency sweep excitation signal at different positions and collects the corresponding sampling signals. All coils in the probe are always parallel to the surface of the defect sample. During the detection process, the detection module transmits the sampling signals to the analysis module in real time.

[0068] Before detection, a frequency range needs to be determined as the frequency coverage range of the sweep excitation signal during detection. Within this range, the frequency of the excitation signal changes continuously while the amplitude remains consistent. In this embodiment, the excitation signal amplitude is 1A, and the frequency range is between 0 and 20MHz. Before detection, a one-dimensional coordinate axis needs to be established on the surface of the defect sample, and the detection position and detection distance need to be determined. The extension direction of the one-dimensional coordinate axis is parallel to the extension direction of the defect sample. The detection position is the position of the probe when the excitation signal is applied. The point formed by the vertical projection of the center point of the excitation coil onto the surface of the defect sample when the probe is at the detection position is called the detection point. All detection points are located on the same straight line, which is parallel to the extension direction of the defect sample, and the distance between two adjacent detection points is a constant value, which is the detection distance. In this embodiment, the detection distance is 0.5mm. In this example, the origin of the coordinate axis is located at the edge of the surface. The coordinate axis is denoted as x. The center points of the openings of defects 1, 2, and 3 are located at x = 15mm, x = 65mm, and x = 115mm, respectively, and the first detection point is located at x = 0. The detection cycle consists of multiple detection stages. The process of any detection stage is as follows: at a certain detection position, the probe applies a frequency sweep excitation signal that scans the entire frequency range and acquires the corresponding sampling signal; then the probe is moved to the next detection position to proceed to the next detection stage; the detection positions of the probe are different in different detection stages, and the number of detection stages is equal to the number of detection positions.

[0069] S4. The analysis module determines whether a defect can be effectively detected at a certain lift-off distance based on the received sampling signal, and derives the range of effective detection distances based on the judgment results for all lift-off distances. The specific process of determining whether a defect can be effectively detected at a certain lift-off distance includes the following steps:

[0070] S4.1. Based on the principle of electromagnetic induction and the electrical characteristics of the defect sample and the probe, a control equation is constructed to describe the electrical relationship between the probe and the defect sample; then, based on the received sampling signal and the parameters contained in the control equation, a signal factor is obtained, which is a parameter used to describe the relationship between the excitation signal and the sampling signal; the signal factor is specifically obtained through the following steps:

[0071] S4.1.1. The first receiving coil and the second receiving coil are connected in reverse series to form a differential receiving coil; the excitation coil and the receiving coil are both connected in parallel via capacitors to form a parallel resonant circuit. In the parallel resonant circuit, the excitation coil and the receiving coil are equivalent to two resistors connected in series on a branch and two inductors connected in reverse series. The test piece is equivalent to a loop with one coil and one resistor connected in series. There is electromagnetic induction between any of the above coils and other coils. The other coils are not in the same path as this coil.

[0072] S4.1.2. Based on the equivalent circuit and Kirchhoff's voltage law described in step S4.1.1, establish the control equations, which are expressed in the following form:

[0073]

[0074] Where, r s The internal resistance of the excitation circuit is given by j, where j is the imaginary unit and ω is the angular frequency. S For the excitation signal amplitude, U L Let I be the amplitude of the sampled signal, and I1 and I2 be the magnitudes of the currents in the excitation coil and the receiving coil, respectively. S R represents the magnitude of the induced current in the device under test; R1 and R2 are the resistances of the two coils in the equivalent circuit of the excitation coil, R3 and R4 are the resistances of the two coils in the equivalent circuit of the receiving coil, L1 and L2 are the inductances of the two coils in the equivalent circuit of the excitation coil, and L3 and L4 are the inductances of the two coils in the equivalent circuit of the receiving coil. S and L S C1 and C2 are the resistance and inductance in the equivalent circuit of the device under test, respectively, and the capacitors connected in parallel in the equivalent circuits of the excitation coil and the receiving coil, respectively; M ij For L i With L j The mutual inductance coefficient between them, M is For L i With L S The mutual inductance coefficient between them, M js For L j With L S The mutual inductance coefficients between them, i∈{1,2}, j∈{3,4}, mean that the circuit elements on both sides of || are located in two parallel branches. The overall equivalent circuit of the coil and the device under test is as follows: Figure 4 As shown in the figure, the left circuit of the upper two circuits is the equivalent circuit of the excitation coil, the right circuit is the equivalent circuit of the receiving coil, and the lower circuit is the equivalent circuit of the defect sample.

[0075] S4.1.3. The signal factor is the scattering coefficient S. 21 S 21 Defined by the following expression:

[0076]

[0077] S4.2. Establish the functional relationship between the signal factor and the excitation signal frequency, and then select multiple parameters that can reflect the changes in the signal factor with the excitation signal frequency as feature parameters. Establish the functional relationship between each feature parameter and the detection position. The function of the feature parameter with respect to position change is called the position function. Specifically, this includes the following steps:

[0078] S4.2.1. Based on the sampling signal acquired at the same detection position under the action of the frequency sweep excitation signal, the scattering coefficient S is derived. 21 A first type of graph showing the functional relationship between the frequency of the excitation signal and the frequency of the excitation signal. The horizontal axis of the first type of graph corresponds to the frequency of the excitation signal, and the vertical axis corresponds to the scattering coefficient S. 21 The resulting curve is "M" shaped; then multiple detection positions are selected, and a corresponding first-type curve is plotted for each selected detection position to form a curve set.

[0079] S4.2.2. Obtain the set of curves corresponding to each defect sample at the same lift-off distance according to the process described in step S4.2.1.

[0080] S4.2.3. In the first type of curve graph, the maximum value closer to the vertical axis is recorded as the first resonance peak, the maximum value farther from the vertical axis is recorded as the second resonance peak, and the minimum value between the two maximum values ​​is recorded as the resonance valley value. The set of curves with a distance of 2mm is as follows: Figure 5 As shown in the figure, A, B, and C represent the first resonant peak, the second resonant peak, and the resonant trough, respectively. The frequencies corresponding to the first resonant peak, the second resonant peak, and the resonant trough are denoted as fi, ... peak1 f peak2 and f valley In this embodiment, the first resonant peak value, the second resonant peak value, the resonant valley value, and f are selected. peak1 f peak2 f valley All six parameters are used as feature parameters, and the values ​​of each feature parameter corresponding to different detection positions are obtained from the curve set. All detection positions are represented by the coordinates of their corresponding detection points. Then, a position function is fitted for each feature parameter. The position function is described by a second type of curve graph. The horizontal axis of the second type of curve graph corresponds to the position coordinates of the defect sample surface, and the vertical axis corresponds to the feature parameter values.

[0081] S4.2.4. Following the procedure described in step S4.2.3, obtain all the second-type curves corresponding to each defect sample at the same lift-off distance. The second-type curves corresponding to the six characteristic parameters at lift-off distances of 2mm, 4mm, 6mm, 8mm, and 10mm are shown below. Figure 6 , Figure 7 , Figure 8 , Figure 9 , Figure 10 As shown, in this embodiment, the three defects are located on the same test piece. Therefore, the second type curves corresponding to the three defect samples are combined into one graph. In this embodiment, all feature parameter values ​​are zero-referenced to ensure that the curves are located near the zero reference line in the graph. Figures 6-10 The vertical axis of the graph corresponding to all frequency values ​​is in MHz.

[0082] Figure 6 (a) is the second type of curve corresponding to the first resonance peak at a lift-off distance of 2mm. Figure 6 (b) f at a lift-off distance of 2mm peak1 The corresponding second type of curve, Figure 6 (c) is the second type of curve corresponding to the second resonance peak at a lift-off distance of 2mm. Figure 6 (d) is f at a lift-off distance of 2mm. peak2 The corresponding second type of curve, Figure 6 (e) is the second type of curve corresponding to the resonance valley value at a lift-off distance of 2 mm. Figure 6 (f) represents f at a lift-off distance of 2mm. valley The corresponding second type of curve. Figure 7 (a) is the second type of curve corresponding to the first resonance peak at a lift-off distance of 4mm. Figure 7 (b) f at a lift-off distance of 4mm peak1 The corresponding second type of curve, Figure 7 (c) is the second type of curve corresponding to the second resonance peak at a lift-off distance of 4mm. Figure 7 (d) is f at a lift-off distance of 4mm. peak2 The corresponding second type of curve, Figure 7 (e) is the second type of curve corresponding to the resonance valley value at a lift-off distance of 4 mm. Figure 7 (f) represents the lift-off distance of 4mm. valley The corresponding second type of curve. Figure 8 (a) is the second type of curve corresponding to the first resonance peak at a lift-off distance of 6mm. Figure 8 (b) f at a lift-off distance of 6mm peak1 The corresponding second type of curve, Figure 8 (c) is the second type of curve corresponding to the second resonance peak at a lift-off distance of 6mm. Figure 8 (d) is f at a lift-off distance of 6mm. peak2 The corresponding second type of curve, Figure 8 (e) is the second type of curve corresponding to the resonance valley value at a lift-off distance of 6 mm. Figure 8 (f) represents f at a lift-off distance of 6mm. valley The corresponding second type of curve. Figure 9 (a) is the second type of curve corresponding to the first resonance peak at a lift-off distance of 8mm. Figure 9 (b) f at a lift-off distance of 8mm peak1 The corresponding second type of curve, Figure 9 (c) is the second type of curve corresponding to the second resonance peak at a lift-off distance of 8mm. Figure 9 (d) is f at a lift-off distance of 8mm. peak2 The corresponding second type of curve, Figure 9 (e) is the second type of curve corresponding to the resonance valley value at a lift-off distance of 8 mm. Figure 9 (f) represents f at a lift-off distance of 8mm. valley The corresponding second type of curve. Figure 10 (a) is the second type of curve corresponding to the first resonance peak at a lift-off distance of 10 mm. Figure 10 (b) f at a lift-off distance of 10mm peak1 The corresponding second type of curve, Figure 10 (c) is the second type of curve corresponding to the second resonance peak at a lift-off distance of 10mm. Figure 10 (d) is f at a lift-off distance of 10mm. peak2 The corresponding second type of curve, Figure 10 (e) is the second type of curve corresponding to the resonance valley value at a lift-off distance of 10 mm. Figure 10 (f) represents f at a lift-off distance of 10mm. valley The corresponding second type of curve.

[0083] S4.3. A certain feature of the position function is used as a recognition feature. If the location of a defect in a defect sample can be determined through the position corresponding to the recognition feature of a certain position function, then the defect in that defect sample can be identified through that position function. If the defects in all defect samples can be identified through a certain position function, then the feature parameter corresponding to that position function is considered a valid feature parameter. In this embodiment, the maximum or minimum value in the second type of curve graph is used as the recognition feature. The center points of the openings of defects 1, 2, and 3 correspond to the positions in the second type of curve graph. Figures 6-10 The values ​​are marked to visually demonstrate whether defects can be identified through extreme points.

[0084] S4.4. Based on the process described in step S4.3, determine whether each feature parameter is a valid feature parameter at a given liftoff distance. From... Figure 6 It can be seen that all feature parameters are valid feature parameters at a lift-off distance of 2mm. Figure 7 It can be seen that all feature parameters are also valid feature parameters at a lift-off distance of 4mm. Figure 8 It can be seen that at a lift-off distance of 6mm, except for f peak1 f valley Other parameters besides these are valid feature parameters. Figure 9 It can be seen that at a lift-off distance of 8mm, only the first resonance peak, the second resonance peak, and the resonance valley remain effective characteristic parameters. Figure 10 It can be seen that at a lift-off distance of 10mm, only the second resonance peak is an effective characteristic parameter.

[0085] A threshold is then determined. If the number of effective feature parameters is not less than the threshold, it is considered that the defect can be effectively detected at that lift-off distance. In this embodiment, the threshold is set to 4. Therefore, the number of effective feature parameters at lift-off distances of 2mm, 4mm, and 6mm is not less than the threshold. Thus, it is considered that the probe used in this embodiment can effectively detect defects within a lift-off distance of no more than 6mm.

Claims

1. A resonant eddy current probe structure for suppressing lift-off effect, comprising a first excitation coil, a second excitation coil, and a first receiving coil, a second receiving coil, wherein each of the above-mentioned coils is a planar coil and is composed of two long sides and two short sides; wherein, The first excitation coil and the second excitation coil are in the shape of an 8, and the two long sides of each excitation coil are arranged in a cross shape, and the two short sides are parallel; the first excitation coil is stacked on the second excitation coil, and the long sides of the two excitation coils coincide, and the corresponding short sides of the two excitation coils are in different planes and perpendicular to each other; The first receiving coil and the second receiving coil are both rectangular coils and are located on the same plane, and the long sides of the two rectangular coils extend in parallel; The short sides of the excitation coils and the short sides of the receiving coil extend in different directions.

2. A resonant eddy current probe structure to suppress lift-off effects as claimed in claim 1, characterised in that: The angle between the short sides of the excitation coils and the short sides of the receiving coil is 45°.

3. The method of detecting a structure of a resonant eddy current probe that suppresses the lift-off effect according to any one of claims 1 and 2, characterized by, Specifically comprising the following steps: S1. Build a detection platform, the detection platform includes a detection module and an analysis module, the detection module detects defects through a probe, the probe structure is as claimed in claim 1, and the detection module and the analysis module are connected through a signal; S2. Construct a plurality of defect samples according to the actual working conditions, the defect sample is a test piece with defects on the surface, the test piece is in a strip shape, and the specifications of the test pieces in all defect samples are the same; meanwhile, a plurality of distance values are determined as lift-off distances during detection; S3. Based on the detection platform built in step S1, the probe is used to detect all defect samples one by one under each lift-off distance, and the complete detection process of a defect sample under a certain lift-off distance is called a detection cycle; the electric signal applied to the excitation coil during detection is called an excitation signal, the excitation signal is a sweep signal, and the signal collected by the receiving coil is called a sampling signal; in a certain detection cycle, the probe applies the same sweep excitation signal to different positions in sequence and collects the corresponding sampling signals, and all coils in the probe are always parallel to the surface of the defect sample; the detection module transmits the sampling signals to the analysis module in real time during detection; S4. The analysis module judges whether the defect can be effectively detected under a certain lift-off distance based on the received sampling signals, and obtains the range of the effective detection distance according to the judgment results corresponding to all lift-off distances.

4. The detection method of claim 3, wherein: In step S3, a frequency range needs to be determined as the frequency coverage range of the sweep excitation signal during detection before detection, the frequency of the excitation signal changes continuously and the amplitude remains the same within the range; a one-dimensional coordinate axis needs to be established on the surface of all defect samples before detection, and the detection position and the detection distance need to be determined; the extension direction of the one-dimensional coordinate axis is parallel to the extension direction of the defect sample, the detection position is the position of the probe when the excitation signal is applied, the center point of the excitation coil vertically projected on the surface of the defect sample when the probe is located at the detection position is called a detection point, all detection points are located on the same straight line, the straight line is parallel to the extension direction of the defect sample, and the distance between adjacent two detection points is a constant value, which is the detection distance; the detection cycle is composed of a plurality of detection stages, and the process of any detection stage is as follows: at a certain detection position, the probe applies a sweep excitation signal that scans the entire frequency range, and collects the corresponding sampling signals; then the probe is moved to the next detection position for the next detection stage. The detection positions of the probe in different detection stages are different, and the number of the detection stages is equal to the number of the detection positions.

5. The detection method of claim 4, wherein, The specific process of judging whether the defects can be effectively detected at a certain lift-off distance in the step S4 includes the following steps. S4.

1. Based on the principle of electromagnetic induction and the electrical characteristics of the defect sample and the probe, a control equation for describing the electrical relationship between the probe and the defect sample is constructed; then, based on the received sampling signal and the parameters contained in the control equation, a signal factor is derived, which is a parameter for describing the relationship between the excitation signal and the sampling signal; S4.

2. A functional relationship between the signal factor and the frequency of the excitation signal is established, and then a plurality of parameters capable of reflecting the characteristics of the signal factor changing with the frequency of the excitation signal are selected as characteristic parameters, a functional relationship between each characteristic parameter and the detection position is established, and the function of the characteristic parameter with respect to the position change is referred to as a position function; S4.

3. A certain characteristic of the position function is taken as an identification characteristic, if the position corresponding to the identification characteristic of a certain position function can derive the position of the defect in a certain defect sample, it is considered that the defect in the defect sample can be identified by the position function; if the defect in all defect samples can be identified by a certain position function, it is considered that the characteristic parameter corresponding to the position function is an effective characteristic parameter; S4.

4. According to the process of step S4.3, whether each characteristic parameter is an effective characteristic parameter at a certain lift-off distance is determined, and then a threshold is determined, if the number of effective characteristic parameters is not less than the threshold, it is considered that the probe can effectively detect the defects at the lift-off distance.

6. The detection method of claim 5, wherein, The signal factor in step S4.1 is derived by the following steps: S4.1.

1. The first receiving coil and the second receiving coil are connected in reverse series to form a differential receiving coil, and the first excitation coil and the second excitation coil are connected in reverse series to form a differential excitation coil; the excitation coil and the receiving coil form a parallel resonant circuit through a parallel capacitor, in the parallel resonant circuit, the excitation coil and the receiving coil are equivalent to two resistors connected in series and two inductors connected in reverse series on a branch, and the test piece is equivalent to a loop with an inductor and a resistor connected in series; electromagnetic induction exists between any coil and other coils, and the other coils are not in the same path as the coil; S4.1.

2. Based on the equivalent circuit and Kirchhoff's voltage law in step S4.1.1, a control equation is established, which is expressed in the following form: wherein, r s is the internal resistance of the excitation circuit, j is the imaginary unit, ω is the angular frequency, U S is the amplitude of the excitation signal, U L is the amplitude of the sampling signal, I1 and I2 are the current sizes in the excitation coil and the receiving coil, respectively, I S is the induced current size in the test piece; R1 and R2 are the resistances of the two coils in the equivalent circuit of the excitation coil, R3 and R4 are the resistances of the two coils in the equivalent circuit of the receiving coil, L1 and L2 are the inductances of the two coils in the equivalent circuit of the excitation coil, L3 and L4 are the inductances of the two coils in the equivalent circuit of the receiving coil, R S and L S are the resistance and inductance in the equivalent circuit of the test piece, respectively, C1 and C2 are the parallel capacitors in the equivalent circuits of the excitation coil and the receiving coil, respectively; M ij is the mutual inductance coefficient between L i and L j , M is is the mutual inductance coefficient between L i and L S , M js is the mutual inductance coefficient between L j and L S , i∈{1,2}, j∈{3,4}, the circuit elements located on the left and right sides are in two parallel branches, respectively; S4.1.

3. The signal factor is the scattering coefficient S 21 , S 21 is defined by the expression:

7. The detection method of claim 6, wherein, The step S4.2 specifically includes the following steps: S4.2.

1. Based on the sampling signal obtained at the same detection position under the action of the sweep excitation signal, a first type of curve describing the function relationship between the scattering coefficient S 21 and the excitation signal frequency is obtained, wherein the horizontal axis of the first type of curve corresponds to the excitation signal frequency, and the vertical axis corresponds to the scattering coefficient S 21 , and the shape of the obtained curve is "M"; then a plurality of detection positions are taken, and for each detection position taken, a corresponding first type of curve is drawn to form a curve set; S4.2.

2. According to the process in step S4.2.1, a curve set corresponding to each defect sample at the same lift-off distance is derived; S4.2.

3. The maximum value close to the longitudinal axis in the first type of curve is recorded as the first resonance peak value, the maximum value far from the longitudinal axis is recorded as the second resonance peak value, and the minimum value between the two maximum values is recorded as the resonance valley value. The frequency values corresponding to the first resonance peak value, the second resonance peak value, and the resonance valley value are recorded as f peak1 , f peak2 , and f valley respectively; then, multiple parameters are selected from the six parameters f peak1 , f peak2 , f valley as characteristic parameters, and the numerical values of each characteristic parameter corresponding to different detection positions are obtained from the curve set. All detection positions are represented by the coordinates of their corresponding detection points; then, a position function corresponding to each characteristic parameter is fitted, and the position function is described by the second type of curve. S4.2.

4. According to the process in step S4.2.3, all second type curve graphs corresponding to each defect sample at the same lift-off distance are derived.

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