A kind of magnetic flux leakage detection system and method based on helmholtz coil

Abstract

This invention discloses a magnetic flux leakage detection system and method based on a Helmholtz coil, belonging to the field of nondestructive testing technology. The method utilizes a Helmholtz coil to generate a highly uniform and stable excitation magnetic field in the central region, saturating the entire specimen with magnetization. A two-dimensional moving platform drives a gaussmeter probe to scan the specimen surface, simultaneously acquiring spatial coordinates and tangential and normal magnetic field components. Background correction is performed using a defect-free specimen as a baseline to extract the pure magnetic flux leakage increment caused by defects. The depth is inverted using the linear calibration relationship between the tangential magnetic flux leakage increment and the defect depth, and the width is inverted using the linear calibration relationship between the normal magnetic flux leakage peak-valley spacing and the defect width. Defects are located by combining this with anomaly location analysis. This invention features uniform magnetization, good scanning repeatability, and non-contact, nondestructive testing, making it suitable for the quantitative detection of near-surface micro-defects in permalloy.

Description

Technical Field

[0001] This invention belongs to the field of nondestructive testing technology, specifically relating to a magnetic flux leakage nondestructive testing system and method for near-surface defects in ferromagnetic metal components, and particularly to a general magnetic flux leakage testing method based on Helmholtz coils to achieve uniform magnetization and combined with two-dimensional scanning and magnetic field inversion. Background Technology

[0002] Ferromagnetic engineering components are subjected to complex service environments such as alternating loads, humid heat corrosion, media erosion, and material fatigue aging over long periods, making them highly susceptible to initial damage such as near-surface microcracks and corrosion pits. These defects are characterized by their small size and high degree of concealment. If they are not detected and addressed in a timely manner, they will continue to expand and evolve over time, inducing serious engineering accidents such as component cracking, media leakage, and structural instability, posing a significant threat to the operational safety of industrial equipment and the stability of engineering structures.

[0003] Magnetic flux leakage (MFL) nondestructive testing technology, with its significant advantages such as high detection sensitivity, strong adaptability, non-contact testing, and the ability to identify minute defects, has been widely applied to the quality inspection and safe operation and maintenance of ferromagnetic components in key fields such as transportation, energy, and high-end equipment manufacturing. However, traditional MFL testing methods mainly suffer from the following technical shortcomings:

[0004] (a) Uneven magnetization problem. Traditional magnetic flux leakage detection often uses local electromagnets or permanent magnets for magnetization. The uneven magnetic field distribution leads to inconsistent magnetization states in different areas of the specimen, which seriously affects the stability and comparability of the test results.

[0005] (ii) Poor signal stability. In traditional methods, it is difficult to accurately repeat the magnetization position and orientation, and there are differences in magnetic field conditions between different detections, which makes it difficult to obtain a reliable benchmark for the quantitative analysis of defect leakage magnetic signals.

[0006] (iii) Insufficient quantitative accuracy of minute defects. Existing methods struggle to accurately separate the coupling effects of defect depth and width on leakage magnetic signals, resulting in limited quantitative inversion accuracy for defect size, particularly for shallow minute defects. More importantly, existing calibration models are only applicable to specific materials and working conditions. When the material, thickness, or detection parameters of the workpiece change, the calibration model needs to be rebuilt, lacking a universal calibration framework.

[0007] (iv) Low scanning and positioning accuracy. Traditional manual scanning methods cannot guarantee the accurate positioning and repeatability of measurement points, introducing large random errors. Summary of the Invention

[0008] The purpose of this invention is to address the technical challenges of uneven magnetization, poor signal stability, and lack of universality in calibration models in existing magnetic flux leakage detection technologies. It provides a magnetic flux leakage detection system and method based on Helmholtz coils, aiming to establish a universal detection method framework that can be adapted to different ferromagnetic materials and different specimen thicknesses. Through a unified calibration process, it achieves accurate positioning and quantitative inversion of near-surface defects in ferromagnetic metal components.

[0009] To achieve the above objectives, the present invention provides a leakage magnetic flux detection system based on a Helmholtz coil, comprising:

[0010] The Helmholtz coil assembly is used to generate a uniform and stable excitation magnetic field in the central region. The ferromagnetic component under test is set in the central region of the Helmholtz coil assembly by a non-magnetic fixture.

[0011] A magnetic field measuring probe is used to measure the spatial magnetic field component signals in the X-axis and Y-axis directions of the ferromagnetic component under test. The X-axis direction is horizontal and parallel to the magnetic field direction of the Helmholtz coil, and the Y-axis direction is vertical and perpendicular to the magnetic field direction of the Helmholtz coil.

[0012] A two-dimensional moving platform is capable of precisely displacing the magnetic field measuring probe along the X-axis and Y-axis in the central region of the Helmholtz coil assembly; the two-dimensional moving platform is located outside the Helmholtz coil assembly, and the magnetic field measuring probe is fixedly mounted on the two-dimensional moving platform by a non-magnetic clamp.

[0013] The data acquisition and processing system is connected to the two-dimensional mobile platform and the magnetic field measurement probe to synchronously record spatial position coordinates and magnetic field component data, and to perform data processing and defect size inversion calculation.

[0014] A DC regulated power supply, electrically connected to the Helmholtz coil assembly, is used to provide an adjustable and stable excitation current to the Helmholtz coil.

[0015] Preferably, the equivalent diameter of the Helmholtz coil is 200mm to 500mm, and the number of coil turns is 400 to 800.

[0016] Preferably, the magnetic field measuring probe is a gaussmeter probe, a Hall element probe, or a magnetoresistive sensor probe.

[0017] Based on the above detection system, the present invention also provides a method for detecting magnetic flux leakage based on a Helmholtz coil, comprising the following steps:

[0018] Step 1: Set up the detection system:

[0019] A pair of Helmholtz coils are coaxially fixed to form a uniform excitation magnetic field in their central region. The specimen is fixed in the central effective uniform magnetic field region of the Helmholtz coil assembly using a non-magnetic clamp. The magnetic field measuring probe is mounted on a two-dimensional moving platform using a non-magnetic clamp, keeping the lifting distance between the probe and the specimen surface constant. A fixed correspondence is established between the placement direction of the specimen, the coil axis, and the scanning coordinates of the magnetic field measuring probe, thus establishing a unified geometric positioning reference on the specimen.

[0020] Step 2: Baseline Calibration

[0021] Using a defect-free specimen of the same material and thickness as the ferromagnetic component to be tested as a reference specimen, a stable excitation current is supplied to the Helmholtz coil. The excitation current is adjusted to the set value, and the reference specimen is uniformly saturated with magnetization throughout by the uniform magnetic field generated in the central region of the Helmholtz coil assembly. Measurement begins after the central magnetic field of the Helmholtz coil assembly stabilizes. The two-dimensional moving platform is controlled to scan the surface of the reference specimen according to the preset scanning path, scanning step size, and number of sampling points. The scanning is performed close to the surface of the specimen along the X and Y axes with settable step sizes (1 mm, 0.1 mm, or 0.01 mm), and data is recorded simultaneously. Spatial coordinates (X, Y) and the tangential magnetic field component B are also acquired. x and normal magnetic field component B y A defect-free background baseline was obtained; the scan was repeated several times under the same detection conditions.

[0022] Step 3: Establish the calibration model

[0023] Replace the reference specimen with an array of gradient calibration specimens of the same material and thickness as the ferromagnetic component under test, which have known gradient defects. Keep the excitation current, specimen reference position, probe attitude, lift-off height, and scanning parameters unchanged, and perform scanning measurements under the same testing conditions, simultaneously acquiring spatial coordinates (X, Y) and the tangential magnetic field component B. x and normal magnetic field component B y A defect-free background baseline is obtained; the scan is repeated several times under the same testing conditions; using the scan data of the defect-free reference specimen as the background magnetic field data, the gradient calibration specimen data is aligned with the background magnetic field of the defect-free reference specimen according to a uniform relative coordinate and the background is subtracted to obtain the pure leakage magnetic field increment caused by the defect; B is extracted. x The mean pure leakage flux increment ΔB corresponding to the unipolar maximum at the defect center. x and B y The peak-to-valley spacing ΔX between the positive and negative peaks along the scan line p-p Establish the depth calibration relationship and width calibration relationship between the characteristic parameters of the leakage magnetic field and the geometric dimensions of the defect under the corresponding working conditions;

[0024] Step 4: Defect Location

[0025] The ferromagnetic component to be tested is fixed in the central effective uniform magnetic field region of the Helmholtz coil assembly using a non-magnetic clamp, with the placement orientation and position the same as the reference specimen. The two-dimensional moving platform is then moved point-by-point from the preset scanning starting point, according to the preset scanning path, scanning step size, and number of sampling points, scanning the ferromagnetic component to be tested. The defect location is determined based on the spatial coordinates of the magnetic field anomaly. The tangential magnetic field component B is then used to... x Local anomaly peak location and normal magnetic field component B y By cross-verifying the positions of peaks and valleys or changes in sign, the defect can be located near the defect center.

[0026] Step 5: Defect Size Inversion

[0027] ΔB x Substitute the pre-defined depth calibration relationship to calculate the defect depth D0, and then use ΔX p-p Substitute the pre-defined width calibration relationship to calculate the defect width w, and solve for the actual depth and width of the defect.

[0028] Preferably, step three specifically includes the following sub-steps:

[0029] (1) Scan the array of gradient calibration specimens with different defect depths, subtract the background magnetic field from the measurement data, and obtain the mean tangential pure leakage magnetic flux increment ΔB corresponding to defects of different depths. x ;

[0030] (2) Using an array of gradient calibration specimens with different defect widths, the normal leakage magnetic signal B is extracted. y The distance between the two peaks, one positive and one negative;

[0031] (3) Using the defect depth as the independent variable and the mean of the tangential pure leakage magnetic flux increment as the dependent variable, perform least squares linear fitting to establish the depth calibration relationship and the depth inversion relationship; using the defect width as the independent variable and the positive and negative bi-peak spacing of the normal leakage magnetic flux signal By as the dependent variable, perform fitting to establish the width calibration relationship and the width inversion relationship.

[0032] Furthermore, the depth calibration relationship described in step three is: ΔB x = a·D0 + b;

[0033] Where, ΔB x , where is the mean of the tangential pure leakage flux increment, D0 is the defect depth, and a and b are the condition specificity coefficients obtained by fitting using the least squares method, respectively.

[0034] The depth inversion formula is: D0=(ΔB x +b) / a;

[0035] The width calibration relationship is: ΔX p-p= c·w + d;

[0036] Where, ΔX p-p denoted as the positive and negative bipeak spacing of the normal leakage magnetic signal, w as the defect width, and c and d as the condition specificity coefficients obtained by least squares fitting, respectively.

[0037] The width inversion formula is: w=(ΔX) p-p -d) / c.

[0038] The depth and width calibration relationships are working condition-specific calibration relationships. For workpieces of different materials or thicknesses, this step needs to be repeated to generate the corresponding calibration relationships.

[0039] Preferably, the scanning step size is 0.1mm~1.0mm, and the single scan length is 30mm~100mm.

[0040] As a preferred method, the number of repeated scans under the same detection conditions is 3 to 10.

[0041] Preferably, the number of gradient calibration specimens is no less than 3 sets.

[0042] Preferably, the ferromagnetic component to be tested is permalloy, carbon steel, silicon steel or other ferromagnetic metal material, and the thickness of the specimen is 1mm to 10mm.

[0043] Working principle of the invention:

[0044] When a ferromagnetic specimen is magnetized to near-saturation flux in a uniform external excitation magnetic field, if near-surface defects exist, the defect region is dominated by air, and its magnetic reluctance is much greater than that of the base material. This significantly reduces the effective magnetic cross-section of the specimen and obstructs the internal magnetic flux transmission path. Most of the magnetic flux cannot propagate continuously within the specimen and will penetrate the surface to diffuse outward, forming a leakage magnetic field. This leakage magnetic field signal can be accurately acquired using a magnetic field measurement probe. The greater the defect depth, the smaller the effective magnetic cross-section, and the stronger the leakage magnetic response. The defect width mainly affects the spatial distribution and peak-valley spacing of the leakage magnetic field. Uniform saturation magnetization is achieved using a Helmholtz coil. The spatial magnetic field distribution is obtained through two-dimensional scanning. Background is subtracted from the defect-free baseline. Depth is inverted using tangential pure leakage magnetic field increments, and width is inverted using normal peak-valley spacing.

[0045] Based on the theory of magnetic medium transport and the saturation magnetization boundary condition, a theoretical correlation model of the leakage magnetic field response of a ferromagnetic sample under test defect can be established. Since there is no free current in the quasi-static leakage magnetic field detection space, according to Maxwell's equations:

[0046]

[0047] Where B is the magnetic flux density and H is the magnetic field strength. For the Hamiltonian operator, a scalar magnetic potential is introduced. :

[0048]

[0049] Given that the boundary conditions at the interface between the sample and air are tangentially continuous (H) and normally continuous (B), we can obtain:

[0050]

[0051] in Let n be the interface tangent vector, and n be the interface normal vector. The magnetic field strength inside the sample to be tested. The external magnetic field strength of the sample to be tested. The magnetic flux density within the sample to be tested. The magnetic flux density is the external magnetic field strength of the sample to be tested.

[0052] The magnetic induction intensity inside the magnetized sample and in air can be defined as:

[0053]

[0054] in, For the applied magnetization field, To demagnetize the magnetic field, The magnetization intensity, The magnetic permeability of the sample to be tested. The permeability is the vacuum permeability. Because the scalar potentials on both sides of the interface are continuous... The normal derivative exhibits a jump, from which we can obtain:

[0055]

[0056] By treating the defect wall as an equivalent magnetic charge source, we can define the surface magnetic charge density. Then the integral formula for the leakage magnetic field at any point in space is:

[0057]

[0058] In the formula, r is the vector from the source point to the field point, r= ; This represents the coordinates of the field point in the leakage magnetic field. The starting coordinates representing the defect width dimension. The endpoint coordinates representing the defect width dimension. This represents the coordinates of the source point of the surface magnetic charge source.

[0059] The magnetic charge density distribution mechanism of the equivalent magnetic charge source in the defect wall is that the normal component of M is uniformly distributed, while the air side... The tangential component accumulates along the depth; taking the depth element dz of the defect wall, the total external field at the source point can be divided into two parts: the normal component of magnetization. lateral air field Among them, the magnetization normal component is:

[0060]

[0061] In the formula The source magnetization intensity. Tangential depth integral accumulation:

[0062]

[0063] In the formula The source point magnetic induction intensity;

[0064] Substituting (7) and (8) into (4) yields:

[0065]

[0066] Where k is a constant determined by the magnetic permeability, representing the ratio of the tangential air field to the magnetization normal component. By the conservation of cross-sectional magnetic flux: the magnetic flux in the defect-free region equals the magnetic flux of the remaining matrix cross-section at the defect location.

[0067]

[0068] Where t is the specimen thickness and w is the specimen defect width. The depth of the defect in the specimen. For being without defects, Let (10) be the matrix field at the bottom of the defect. Simplifying equation (10), we get:

[0069]

[0070] The defect depth coefficient is the magnetic potential gradient difference on both sides of the defect wall, which is determined by the external field applied inside the substrate. From equation (11), it can be seen that the leakage magnetic field and the defect depth are approximately linearly related.

[0071]

[0072] Combining (5) and (12), we can obtain:

[0073]

[0074] Substituting (11) into (13) gives:

[0075]

[0076] Therefore, it can be seen that the equivalent surface magnetic charge density is related to the defect depth coefficient. and external magnetic field Proportional and The bigger The larger.

[0077] The total equivalent magnetic charge term Substituting into the formula for the magnetic charge integral, we can obtain the relationship between magnetic flux density and defect size:

[0078]

[0079] When a defect exists in the specimen, because the magnetic permeability of air is much lower than that of the specimen, some magnetic field lines will escape into the air and then return to the specimen, forming a leakage magnetic field. At one edge of the defect, the magnetic field lines exit the specimen and enter the air, and the radial leakage magnetic field component exhibits one polarity. At the other edge of the defect, the leakage magnetic field lines return to the specimen, and the radial component exhibits the opposite polarity, thus forming two peaks, one positive and one negative. Therefore, the peak spacing... It is linearly related to the defect width w.

[0080] However, the specific slope and intercept of the above linear relationship are affected by the following factors: (1) the permeability and saturation magnetization of the material being tested; (2) the thickness of the specimen; (3) the excitation magnetic field strength; and (4) the probe lift-off distance. Therefore, recalibration is required for different working conditions, but the calibration method framework is universal.

[0081] The beneficial effects of this invention are:

[0082] 1. The testing method is universal and transferable. This invention is not limited to specific ferromagnetic materials or specific specimen thicknesses, but provides a complete framework for a universal testing method. For workpieces of different materials and thicknesses, accurate testing can be achieved simply by re-establishing the calibration relationship according to a unified calibration procedure. This makes the technical solution of this invention widely applicable to various ferromagnetic materials such as permalloy, carbon steel, and silicon steel, and has extremely strong industrial application value.

[0083] 2. Stable and uniform magnetization conditions. This invention utilizes a Helmholtz coil to generate a highly uniform and stable excitation magnetic field in the detection area, achieving uniform saturation magnetization across the entire specimen. This overcomes the drawbacks of traditional local electromagnet / permanent magnet magnetization methods, such as uneven magnetic field distribution and poor detection stability. During the detection process, only the sample under test is replaced, without changing the reference positions of the coil, probe, fixture, and platform, and maintaining consistent excitation current and scanning parameters. This ensures that the magnetic field variations of different defect specimens mainly stem from differences in defect geometric parameters, resulting in excellent lateral comparability.

[0084] 3. High detection accuracy and good repeatability. This invention combines a precision two-dimensional moving platform driven by a stepper motor with a high-precision magnetic field measurement probe to achieve synchronous and automated acquisition of magnetic field signals and position coordinates. Scanning is completed with an adjustable step size of 0.1mm to 1.0mm, and each scan curve contains dozens to hundreds of measurement points. Under the same conditions, the main magnetic field curves show a basically consistent trend in multiple repeated scans, demonstrating excellent mechanical repeatability and data acquisition stability.

[0085] 4. The calibration model is flexible and accurate. This invention establishes a condition-specific calibration relationship through multiple sets of gradient calibration specimens, effectively separating the coupling effect of defect width and depth on the leakage magnetic field signal, and achieving accurate quantitative inversion of defect size. Specific implementation examples verify that the average relative error for depth detection is approximately 2.07% in the 0.7-2.5mm range, and approximately 1.03% in the core range of 1.3-2.5mm, with a relative combined uncertainty of approximately 1.92%-2.14%; the arithmetic mean relative error for width detection is approximately 5.36% in the 2.0-6.0mm range.

[0086] 5. The testing process is non-contact and non-destructive. A certain lifting distance is maintained between the probe and the specimen, and the measurement process does not require cutting, corroding, or damaging the specimen, meeting the basic requirements of non-destructive testing.

[0087] 6. The device has a simple structure, controllable cost, and good scalability. The detection system of this invention has a simple structure, standardized process, and good repeatability; it can be expanded to two-dimensional surface scanning by increasing the number of scanning lines, or a more sensitive magnetic field sensor can be replaced or a third-party directional movement mechanism can be added to achieve more precise three-dimensional magnetic field measurement. It can be used for experimental teaching and near-surface defect detection of small ferromagnetic components. Attached Figure Description

[0088] Figure 1 This is a schematic diagram of the overall structure of the magnetic flux leakage detection system based on Helmholtz coils of the present invention;

[0089] Among them, 1 is a Helmholtz coil, 2 is a two-dimensional moving platform, 3 is a two-dimensional moving platform control panel, 4 is a gaussmeter, 5 is a thermometer, 6 is a DC regulated power supply, 7 is a magnetic field measuring probe, 8 is a non-magnetic fixing frame, and 9 is a non-magnetic clamp.

[0090] Figure 2 This is a schematic diagram of the magnetic flux leakage field distribution of the sample under magnetic saturation.

[0091] Among them, 1 is a Helmholtz coil, 7 is a magnetic field measuring probe, 10 is a test piece, and 11 is a defect;

[0092] Figure 3 This is the curve showing the change in radial magnetic flux density of the leakage magnetic field as a function of the y-axis.

[0093] Figure 4 A physical diagram of the overall experimental setup including the Helmholtz coil, the test specimen, and the two-dimensional moving platform.

[0094] Figure 5 For the defect test specimen, see the actual image;

[0095] Figure 6 This is a schematic diagram of the defect depth inversion curve;

[0096] Figure 7 This is a schematic diagram of the defect width inversion curve. Detailed Implementation

[0097] The technical solution of the present invention will be further described in conjunction with the accompanying drawings and embodiments.

[0098] Example 1

[0099] like Figure 1 As shown, this embodiment provides a leakage magnetic flux detection system based on a Helmholtz coil, including:

[0100] The Helmholtz coil assembly consists of two Helmholtz coils 1 arranged coaxially and parallel to generate a uniform and stable excitation magnetic field in the central region. The ferromagnetic component to be tested is placed in the central uniform magnetic field region of the Helmholtz coil assembly through a non-magnetic fixing bracket 8.

[0101] The magnetic field measuring probe 7, in this embodiment, is a high-precision gaussmeter probe, connected to the gaussmeter 4, and is used to measure the spatial magnetic field component signals in the X-axis and Y-axis directions of the ferromagnetic component under test. The X-axis direction is horizontal and parallel to the magnetic field direction of the Helmholtz coil, and the Y-axis direction is vertical and perpendicular to the magnetic field direction of the Helmholtz coil.

[0102] A two-dimensional moving platform 2 is capable of precisely displacing the magnetic field measuring probe 7 along the X-axis and Y-axis in the central region of the Helmholtz coil assembly. The two-dimensional moving platform 2 is located outside the Helmholtz coil assembly, and the magnetic field measuring probe 7 is fixedly mounted on the two-dimensional moving platform 2 via a non-magnetic clamp 9. In this embodiment, the two-dimensional moving platform 2 includes an X-axis moving guide parallel to the central axis of the Helmholtz coil 1 and a vertical Y-axis moving guide. The Y-axis moving guide is located on the X-axis moving guide and performs reciprocating translational motion along the X-axis in the X direction. The non-magnetic clamp 9 is L-shaped, with one end connected to the Y-axis moving guide, enabling reciprocating translational motion along the Y-axis. The other end of the non-magnetic clamp 9 extends into the central part of the Helmholtz coil assembly and is connected to the magnetic field measuring probe 7, driving the magnetic field measuring probe 7 to move. The two-dimensional moving platform 2 is connected to a two-dimensional moving platform control panel 3, which controls the movement of the two-dimensional moving platform 2.

[0103] The data acquisition and processing system is connected to the two-dimensional mobile platform 2 and the magnetic field measurement probe 7. It is used to synchronously record spatial position coordinates and magnetic field component data, and to perform data processing and defect size inversion calculation.

[0104] A DC regulated power supply 6 is electrically connected to the Helmholtz coil assembly and is used to provide an adjustable and stable excitation current to the Helmholtz coil.

[0105] It also includes a thermometer 5, which is connected to the Helmholtz coil assembly to detect the real-time temperature of the Helmholtz coil assembly.

[0106] Example 2

[0107] This embodiment provides a leakage magnetic flux detection method based on a Helmholtz coil, which includes the following steps:

[0108] Step 1: Setting up the detection system

[0109] A pair of Helmholtz coils were coaxially fixed on the experimental platform, maintaining coaxiality. The coil parameters were 533 turns, wire diameter 2.24 mm, and equivalent diameter 300 mm. A DC regulated power supply provided a stable excitation current to the Helmholtz coils, with a power supply range of 0-100V / 0-10A. An 80Ni permalloy specimen, measuring 50 mm × 50 mm × 3 mm, was fixed within the effective uniform magnetic field region at the center of the coils. A TM5100 gaussmeter was used as the magnetic field measurement probe, rigidly mounted to the sliding table of the two-dimensional moving platform using a non-magnetic clamp, maintaining a constant lifting distance between the probe and the specimen surface. The two-dimensional moving platform was driven by a stepper motor, enabling precise X and Y axis displacement. A fixed correspondence was established between the specimen placement orientation, coil axis, defect direction, and scanning coordinates, which remained unchanged during the experiment.

[0110] Step 2: Preparation of Gradient Defect Specimens

[0111] Two sets of specimens with gradient defects were machined using 80Ni permalloy. The first set of specimens all had a thickness of 3 mm, a fixed defect width of 5 mm, and defect depths of 0.5, 0.7, 1.0, 1.3, 1.5, 1.8, 2.0, 2.2, and 2.5 mm, respectively. A defect-free specimen was used as a baseline. The second set of specimens all had a thickness of 3 mm, a fixed defect depth of 2 mm, and defect widths of 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0 mm, respectively. Defects were machined into rectangular grooves. Specimens were numbered and their dimensions recorded. Each time a specimen was changed, only the sample being tested was replaced; the coil, probe, fixture, and platform baseline positions remained unchanged.

[0112] Step 3: Scanning and Data Acquisition

[0113] First, install a defect-free reference specimen, ensuring its center is located within the effective uniform field region of the coil. Confirm the specimen placement orientation, probe posture, scanning coordinate system, and lift-off height. Connect the Helmholtz coil power supply and adjust the excitation current to the set value. Once the magnetic field at the coil center and the gaussmeter reading stabilize, begin measurement. The two-dimensional moving platform performs point-by-point scanning with a scanning length of 50 mm, a scanning step size of 0.5 mm, and 101 sampling points per scan, simultaneously saving the position coordinates and the tangential magnetic field component B. x and normal magnetic field component B y After completing the defect-free baseline measurement, only the test piece is replaced, keeping the coil current, test piece reference position, probe direction, lift-off height, and scanning parameters unchanged. Scanning samples of different defect types and gradient parameters is performed sequentially according to the test piece number. Measurements are repeated 5 times under the same conditions, each repetition starting from the same relative starting point. After each scan, the number of data points, coordinate sequence, and magnetic field components are checked for completeness.

[0114] Step 4: Data Processing and Feature Extraction

[0115] The raw data retains spatial coordinates and magnetic field components. During data processing, the number of scan points and coordinate directions are first checked for consistency, and then the five repeated measurements are aligned using uniform relative coordinates. Data from defect-free specimens is used to determine the background field, edge effects, and fixed bias; data from defective specimens are compared to highlight local magnetic field anomalies near defects. The defective specimen data is subtracted from the defect-free background baseline to obtain the pure magnetic flux leakage increment caused by the defect. The tangential magnetic flux leakage component B is extracted. x The mean pure leakage flux increment ΔB corresponding to the unipolar maximum at the defect center. x Extracting the normal leakage magnetic component B y The peak-to-valley spacing ΔX between the positive and negative peaks along the scan line p-p The defect location is based on B. x Local abnormal peak location and B y The positions of peaks and valleys or changes in sign are mutually verified and determined.

[0116] Step 5: Depth and Width Inversion

[0117] Depth inversion calibration: An 80Ni permalloy specimen with a thickness of 3.0 mm and a fixed groove width of 5.0 mm was used. Samples with depths of 1.0 mm, 1.5 mm, and 2.0 mm were selected. After subtracting the defect-free background, the mean tangential pure leakage magnetic flux increment ΔBx was obtained. Linear fitting using the least squares method was employed to obtain: ΔB x =22.870D0-1.309, R 2 =0.9996; Depth inversion formula: .

[0118] ΔB of the independent verification specimen xSubstituting into the above formula, the average relative error is 2.07% in the depth range of 0.7-2.5mm, and as low as 1.03% in the core range of 1.3-2.5mm.

[0119] Width inversion calibration: With a fixed defect depth of 2.0 mm, samples with defect widths of 2.5 mm, 3.5 mm, and 4.5 mm were selected, and the normal leakage magnetic signal B was extracted. y The distance between the positive and negative double peaks ΔX p-p Using the least squares linear fitting method, we obtain: ΔX p-p =0.750w + 3.075, R 2 =0.9868; Width inversion formula: w=(ΔX p-p -3.075) / 0.750.

[0120] ΔX of the independently verified sample p-p Substituting into the above formula, the arithmetic mean relative error is 5.36% within the width range of 2.0-6.0mm.

[0121] The uncertainty assessment for this embodiment is as follows:

[0122] Each specimen was subjected to n=5 independent repeated scans. Type A standard uncertainty:

[0123]

[0124]

[0125] Type B standard uncertainty: Gaussmeter instrument tolerance According to uniform distribution assessment:

[0126]

[0127] Combined standard uncertainty and relative combined uncertainty:

[0128] Combined uncertainty in magnetic field measurement:

[0129]

[0130] Geometric dimension inversion combined uncertainty:

[0131]

[0132] Relative uncertainty:

[0133]

[0134] The inversion calculation values ​​of the independently verified specimens agree well with the actual depth. The average relative error within the main detection range is (exist The interval average relative error is as low as The relative combined uncertainty remains stable across the entire depth range. The range indicates that the measurement system has good repeatability and high-precision inversion capability.

[0135] After removing the derived samples, from arrive (including) and The independent verification data points and the inversion curves of each group showed good agreement. The relative combined uncertainty remained within a certain range. The smaller interval strongly validates the bimodal spacing of the normal extreme values. Independent decoupled characterization capability for defect width.

[0136] The experiment involved 5 repeated scans for each of 18 defect conditions and 5 defect-free baseline scans, totaling 95 scan data points. Results showed that when defects were present in the 80Ni permalloy specimen, the magnetic field curve near the defect exhibited local anomalies different from those of the defect-free specimen, indicating the device's ability to detect groove-shaped defects. With increasing defect depth, the magnetic field characteristic quantity near the defect center generally increased significantly; with increasing defect width, the magnetic field characteristic quantity near the defect boundary generally moved away from the center; and defect width and depth had a coupled influence on the leakage magnetic response. This invention integrates stable magnetization, automatic scanning, background correction, feature extraction, defect localization, and depth / width inversion. It features a simple structure, standardized process, and good repeatability, providing a technical reference for magnetic flaw detection experimental teaching and non-destructive testing of near-surface defects in small ferromagnetic components.

Claims

1. A leakage flux detection system based on a Helmholtz coil, characterized in that, include: The Helmholtz coil assembly is used to generate a uniform and stable excitation magnetic field in the central region. The ferromagnetic component under test is set in the central region of the Helmholtz coil assembly by a non-magnetic fixture. A magnetic field measuring probe is used to measure the spatial magnetic field components in the X-axis and Y-axis directions of a ferromagnetic component under test. The X-axis direction is horizontal and parallel to the central axis of the Helmholtz coil, and the Y-axis direction is vertical and perpendicular to the central axis of the Helmholtz coil. A two-dimensional moving platform is capable of precisely displacing the magnetic field measuring probe along the X-axis and Y-axis in the central region of the Helmholtz coil assembly; the two-dimensional moving platform is located outside the Helmholtz coil assembly, and the magnetic field measuring probe is fixedly mounted on the two-dimensional moving platform by a non-magnetic clamp. The data acquisition and processing system is connected to the two-dimensional mobile platform and the magnetic field measurement probe to synchronously record spatial position coordinates and magnetic field component data, and to perform data processing and defect size inversion calculation. A DC regulated power supply, electrically connected to the Helmholtz coil assembly, is used to provide an adjustable and stable excitation current to the Helmholtz coil.

2. The leakage flux detection system based on a Helmholtz coil according to claim 1, characterized in that: The equivalent diameter of the Helmholtz coil is 200mm~500mm, and the number of coil turns is 400~800.

3. A method for detecting magnetic flux leakage based on a Helmholtz coil, characterized in that: The leakage flux detection system based on a Helmholtz coil as described in claim 1 includes the following steps: Step 1: Set up the detection system: A pair of Helmholtz coils are coaxially fixed to form a uniform excitation magnetic field in their central region; the specimen is fixed in the central effective uniform magnetic field region of the Helmholtz coil assembly by a non-magnetic clamp; the magnetic field measuring probe is installed on a two-dimensional moving platform by a non-magnetic clamp, keeping the lifting distance between the probe and the surface of the specimen constant. Step 2: Baseline Calibration Using a defect-free specimen of the same material and thickness as the ferromagnetic component to be tested as a reference specimen, a stable excitation current is passed through a Helmholtz coil to achieve uniform saturation magnetization over the entire surface of the specimen. Measurement begins after the central magnetic field of the Helmholtz coil assembly stabilizes. The two-dimensional moving platform is controlled to scan the surface of the reference specimen according to a preset scanning path, scanning step size, and number of sampling points, while simultaneously acquiring spatial coordinates (X, Y) and the tangential magnetic field component B. x and normal magnetic field component B y A defect-free background baseline was obtained; the scan was repeated several times under the same detection conditions. Step 3: Establish the calibration model Replace the reference specimen with an array of gradient calibration specimens of the same material and thickness as the ferromagnetic component under test, which have known gradient defects. Keep the excitation current, specimen reference position, probe attitude, lift-off height, and scanning parameters unchanged, and perform scanning measurements under the same testing conditions, simultaneously acquiring spatial coordinates (X, Y) and the tangential magnetic field component B. x and normal magnetic field component B y A defect-free background baseline is obtained; using the scanning data of the defect-free reference specimen as the background magnetic field data, the gradient calibration specimen data and the background magnetic field of the defect-free reference specimen are aligned with a uniform relative coordinate and the background is subtracted to obtain the pure leakage magnetic field increment caused by defects; B is extracted. x The mean pure leakage flux increment ΔB corresponding to the unipolar maximum at the defect center. x and B y The peak-to-valley spacing ΔX between the positive and negative peaks along the scan line p-p Establish the depth calibration relationship and width calibration relationship between the characteristic parameters of the leakage magnetic field and the geometric dimensions of the defect under the corresponding working conditions; Step 4: Defect Location The ferromagnetic component to be tested is fixed in the central effective uniform magnetic field region of the Helmholtz coil assembly using a non-magnetic clamp, with the placement orientation and position the same as the reference specimen. The two-dimensional moving platform is then moved point-by-point from the preset scanning starting point, according to the preset scanning path, scanning step size, and number of sampling points, scanning the ferromagnetic component to be tested. The defect location is determined based on the spatial coordinates of the magnetic field anomaly. The tangential magnetic field component B is then used to... x Local anomaly peak location and normal magnetic field component B y By cross-verifying the positions of peaks and valleys or changes in sign, the defect can be located near the defect center. Step 5: Defect Size Inversion ΔB x Substitute the pre-defined depth calibration relationship to calculate the defect depth D0, and then use ΔX p-p Substitute the pre-defined width calibration relationship to calculate the defect width w, and solve for the actual depth and width of the defect.

4. The leakage flux detection method based on a Helmholtz coil according to claim 3, characterized in that: Step three specifically includes the following sub-steps: (1) Scan the array of gradient calibration specimens with different defect depths, subtract the background magnetic field from the measurement data, and obtain the mean tangential pure leakage magnetic flux increment ΔB corresponding to defects of different depths. x ; (2) Using an array of gradient calibration specimens with different defect widths, the normal leakage magnetic signal B is extracted. y The distance between the two peaks, one positive and one negative; (3) Using the defect depth as the independent variable and the mean of the tangential pure leakage magnetic flux increment as the dependent variable, perform least squares linear fitting to establish the depth calibration relationship and the depth inversion relationship; using the defect width as the independent variable and the positive and negative bi-peak spacing of the normal leakage magnetic flux signal By as the dependent variable, perform fitting to establish the width calibration relationship and the width inversion relationship.

5. The leakage flux detection method based on a Helmholtz coil according to claim 3, characterized in that: The depth calibration relationship mentioned in step three is: ΔB x = a·D0 + b; Where, ΔB x , where is the mean of the tangential pure leakage flux increment, D0 is the defect depth, and a and b are the condition specificity coefficients obtained by fitting using the least squares method, respectively. The deep inversion formula is: D0 = (ΔB) x +b) / a; The width calibration relationship is: ΔX p-p = c·w + d; Where, ΔX p-p denoted as the positive and negative bipeak spacing of the normal leakage magnetic signal, w as the defect width, and c and d as the condition specificity coefficients obtained by least squares fitting, respectively. The width inversion formula is: w = (ΔX) p-p -d) / c.

6. The leakage flux detection method based on a Helmholtz coil according to claim 3, characterized in that: The scanning step size is 0.1mm to 1.0mm, and the length of a single scan is 30mm to 100mm.

7. The leakage flux detection method based on a Helmholtz coil according to claim 3, characterized in that: Under the same testing conditions, the number of repeated scans is 3 to 10.

8. The leakage flux detection method based on a Helmholtz coil according to claim 3, characterized in that: The number of gradient calibration specimens shall not be less than 3 sets.

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