Spatial orthogonal differential eddy current probe based on magnetic flux directional extraction and detection device

Abstract

The invention discloses a spatial orthogonal differential eddy current probe based on magnetic flux directional extraction and a detection device, the probe is formed by vertically arranging four groups of excitation coils and detection coils with the same parameters in a rectangular-ambulatory-plane manner to form two groups of differential coils; differential signals are taken during detection, if double peaks appear, the defect is judged, the center of the double peaks is the defect position, the amplitude difference delta Vpp is equal to the depth d, the speed v meets the conditions that d = kd * delta Vpp + bd and delta Vpp = kv * v + bv, the array probe works independently, and through real-time calculation of a multi-channel A / D and an FPGA, cracks with the width of 0.5 mm and the positioning error lt can be detected at the speed of 2 m / s under the lift-off of 0.5 mm; 2 mm, the depth error lt; compared with a traditional probe signal, the signal is improved by 3.3 times, the support can be fixed or held by hand, and the device is suitable for online nondestructive testing of metal plates and pipelines.

Description

TECHNICAL FIELD

[0001] The application relates to a spatial orthogonal differential eddy current probe based on magnetic flux distribution extraction and a detection device, and belongs to the technical field of nondestructive testing of metal plate surface defects. BACKGROUND

[0002] Metal plates or metal tubes are key structural materials of electrical equipment, and millimeter-level micro-defects on the surfaces of the metal plates or metal tubes usually occur at key positions. The key positions include inner walls of GILs, boiler pipelines of power plants and busbar connection areas of distribution boxes. The micro-defects may become starting points of electric field stress concentration, cause partial discharge and overheating, and finally cause equipment failure or power failure accidents.

[0003] Common nondestructive testing (NDT) methods include machine vision detection, ray detection, ultrasonic detection, magnetic flux leakage detection and eddy current detection. However, these methods have certain limitations in actual application. Although the machine vision detection technology can efficiently identify surface defects, it cannot obtain depth information of the defects. The ray detection method can effectively detect internal defects by penetrating materials, but it may cause radiation hazards to the detection personnel during the operation process, thereby limiting its use in some scenarios. The ultrasonic detection technology is sensitive to material surface roughness, and needs to use a coupling agent, and the efficiency is relatively low in the detection of metal surface defects. The magnetic flux leakage detection method is only suitable for ferromagnetic materials, and has slow detection speed and insufficient resolution for micro-defects. In comparison, the eddy current detection technology has unique application potential in the detection of metal surface defects due to its high sensitivity, wide range of applicable materials, no need for a coupling agent and fast detection speed.

[0004] The structure design of the eddy current probe is the most fundamental method to improve the defect detection precision. Usually, the eddy current is concentrated by changing the spatial position of the excitation coil or the shape of the magnetic core, thereby increasing the detection signal strength. However, due to the parallel arrangement of the excitation coil and the detection coil, the induced electromotive force obtained by the detection coil is still limited. By arranging the excitation coil and the detection coil vertically, a higher induced electromotive force can be obtained in the detection coil. However, the position relationship between the coil axis and the defect is not studied, and the response of the eddy current at the defect is insufficient. Moreover, the defect is not positioned. In the aspect of defect positioning, the differential structure is an effective method to realize accurate positioning of the defect, but needs good structure design to reduce the volume. Therefore, it is very meaningful to design an eddy current probe which is non-contact, compact in structure, has higher precision in defect detection and can accurately position the defect. SUMMARY

[0005] In order to effectively detect micron-level damage to the insulation of metal plates and metal tubes, the present invention explores the correlation mechanism between the detection voltage signal and the directional correlation between the defect eddy current flux component, and designs a spatial orthogonal differential eddy current probe and its detection device based on flux direction extraction. This solves the problem that when using eddy current detection to detect surface defects of metal plates or metal tubes, the detection signal of the existing probe structure is weak, and the detection signal strength and detection speed cannot be taken into account at the same time.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a spatially orthogonal differential eddy current probe based on magnetic flux direction extraction includes four excitation coils L1 to L4 with the same parameters and four detection coils D1 to D4 with the same parameters; the axes of the excitation coils and the axes of the detection coils are spatially perpendicular to each other, and the whole is compactly arranged in a circular shape; wherein: The first detection coil D1 and the second detection coil D2 form a differential coil group A; The third detection coil D3 and the fourth detection coil D4 form a differential coil group B; The differential output signal V is given by equation (1): V=(V D2 –V D1 )+(V D4 –V D3 ) (1); Among them, V is the differential eddy current probe detection signal, V D1 、V D2 、V D3 、V D4 are the detection voltages corresponding to the detection coils D1, D2, D3, and D4 in the improved spatial orthogonal differential eddy current probe; When V shows a double-peak waveform, it is determined that there are defects on the surface of the metal being tested.

[0007] Furthermore, the middle moment t0 of the double-peak waveform satisfies t0=(t1+t2) / 2, where t1 and t2 are the double-peak peak moments respectively, and the middle position corresponds to the defect center coordinate x0=v·t0, where v is the travel speed of the metal plate under test.

[0008] Furthermore, at a constant speed v, the defect depth d and the double peak amplitude difference ΔV pp Satisfying the empirical relationship: d=k d ΔV pp +b d (2); Among them, k d 、b d is the system calibration constant; When the defect size is fixed, the velocity v and ΔV pp satisfy: ΔVpp =k v ·v+b v (3); Among them, k v 、b v is the system calibration constant.

[0009] Furthermore, the detection coil induced voltage V coil Given by Faraday's law: (4); Among them, N is the number of coil turns, μ is the magnetic permeability, H1 and H2 are the primary magnetic fields of the excitation coil, and H yz is the eddy current J x The secondary magnetic field generated.

[0010] Furthermore, the -Z axis eddy current component J caused by the defect z satisfy: ∇×H xy =J z e z (5); Among them, H xy It represents the component of magnetic field intensity H in the xy plane; J z represents the component of the current density J in the z plane; e z is the unit vector in the z direction; The spatial orthogonal arrangement enables the detection coils to H xy The magnetic flux component φ x It has the highest sensitivity, thus increasing the amplitude of the defect signal.

[0011] The present invention provides a detection device for a spatially orthogonal differential eddy current probe based on magnetic flux directional extraction, characterized in that M×N probes are fixed on a bracket in an array form, and each probe is connected to a data acquisition device through an independent lock-in amplifier and A / D channel; Differential signal of each probe V mn The independent calculations by formula (1) are not connected in series or in parallel to achieve parallel detection.

[0012] Furthermore, the data acquisition device includes: Multi-channel synchronous A / D converter; Field Programmable Gate Array (FPGA) for executing the calculations of equations (1), (2) and (3) in real time; The host computer is used to store, display and fit the relationship curve between the defect position x0, depth d and velocity v.

[0013] Furthermore, the bracket is a fixed structure, one end of which is fixed to the base, and the other end of which is mounted on the probe arm via a screw-nut pair; The probe arm can move along the Z direction to adjust the lifting distance h between the probe and the metal plate to be measured. The value range of h is 0.5mm≤h≤10mm.

[0014] Furthermore, the bracket can also be handheld, with the probe and data acquisition device connected by a flexible coaxial cable, allowing manual scanning of large or heavy metal parts; In handheld mode, the velocity v is measured in real time by the built-in inertial measurement unit and substituted into equation (3) to correct ΔV pp .

[0015] Furthermore, the defect width w and the detection voltage V satisfy: V=k w / w+c w (6); Among them, k w 、c w is the system calibration constant; Under the same defect width w, the detection voltage V increases linearly with the speed v: V=k s ·v+c s (7); Among them, k s 、c s is the system calibration constant; Through parallel detection of array probes, rapid imaging of the width, depth and position of surface defects on metal plates or metal tubes can be achieved.

[0016] Compared with the prior art, the present invention has the following beneficial effects: the present invention significantly improves the detection sensitivity, positioning accuracy and detection efficiency of small defects on metal surfaces through the spatial orthogonal differential eddy current probe and its array detection device. First, the probe adopts a circular-shaped, spatially vertically arranged excitation-detection coil structure, which can maximize the extraction of the orthogonal magnetic flux component generated by the eddy current at the defect, significantly improving the detection signal amplitude compared with the traditional runway-type probe, and effectively suppressing background noise. Second, by using the differential operation of formula (1) and the calibration relationship of formulas (2) and (3), the defect depth can be directly inverted from the double-peak amplitude at a constant speed, and the higher the speed, the more significant the signal, taking into account the requirements of high-speed detection and high resolution. Furthermore, the array independent working mode is combined with FPGA parallel computing and IMU speed compensation to achieve online identification of 0.5mm lift-off and 0.5mm wide defects, solving the problem that large and heavy metal parts cannot be moved or are difficult to contact for detection. The handheld and fixed dual-mode bracket further expands the applicable scenarios of the device, ensuring that the detection process is contactless, free of coupling agent, and free of radiation, safe and reliable, and has wide industrial promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further described below with reference to the accompanying drawings.

[0018] Figure 1 Schematic diagram of the position distribution of the detection coils of the present invention.

[0019] Figure 2 Schematic diagram of horizontal comparison of differential voltages of probe detection coils in an embodiment of the present invention.

[0020] Figure 3 1 is a comparison diagram of detection voltages of different probes in an embodiment of the present invention.

[0021] Figure 4 This is the original waveform diagram of the differential detection coil detection in the embodiment of the present invention.

[0022] Figure 5 1 is a waveform diagram of detection voltage at different detection speeds in an embodiment of the present invention.

[0023] Figure 6 Comparison diagram of detection voltages for different widths at different detection speeds in an embodiment of the present invention. DETAILED DESCRIPTION

[0024] The present invention will be further described below with reference to specific embodiments.

[0025] like Figures 1-6 As shown in the figure, the spatial orthogonal differential eddy current probe based on magnetic flux direction extraction is used to detect 1mm×2mm×1mm surface cracks online at a speed of 2m / s.

[0026] 1. Test objects and working conditions Test object: Q235 carbon steel plate, size 1000mm×500mm×5mm, with one artificial crack preset on the upper surface: length L=2mm, width w=1mm, depth d=1mm.

[0027] Working conditions: The steel plate passes through the fixed detection device at a constant speed of v=2m / s, and the probe is lifted h=0.5mm.

[0028] 2. Probe Arrangement The U-shaped spatial orthogonal differential probe described in claim 1 is used (the excitation coils L1 to L4 and the detection coils D1 to D4 are at 90 degrees to each other).

[0029] The probe is installed at the end of the probe arm of the fixed bracket. The probe arm is locked after adjustment by the screw-nut to ensure the lifting accuracy of ±0.05mm.

[0030] Array size: 1×4 probes in parallel (arranged along the width direction), with a center distance of 15mm between adjacent probes, ensuring that a single scan covers a 60mm bandwidth.

[0031] 3. Working Principle and Signal Flow Excitation: Function generator output 200 kHz, 5Vpp sine wave through power amplifier to four excitation coils, to establish the primary magnetic field H1+H2.

[0032] Defect-free area: Steel plate surface forms a circular eddy current J x , the detection coil D1, D2 (and D3, D4) in the symmetric position inducts equal voltage amplitude, opposite phase, after difference according to formula (1) V≈0.

[0033] Defect area: Crack hinders X-direction eddy current, generates vertical component J z , and further generates secondary magnetic field H xy . H xy The magnetic flux component ϕ x in D2, D4 induces additional electromotive force, and the difference signal appears double-peak waveform, as shown in Figure 2 .

[0034] Signal processing: Each channel analog signal enters FPGA through 16-bit synchronous A / D (sampling rate 2MS / s); FPGA real-time calculates V=(V D2 –V D1 )+(V D4 –V D3 ); The host computer calculates the depth according to formula (2): d=k d ·ΔV pp +b d , this calibration k d =0.32mm / V, b d =–0.05mm; According to formula (3): ΔV pp =k v ·v+b v , the amplitude is corrected using real-time speed v=2m / s, k v =0.18V·s / m, b v =0.02V.

[0035] Measured ΔV pp =0.38V, substitution: d=0.32×0.38–0.05≈0.97mm, error≤3%.

[0036] Defect positioning: Calculate the defect center position x0=v·t0=250mm at the double-peak center time t0=0.125s (given by encoder synchronization), with a deviation of <2mm from the preset position 248mm.

[0037] Four, results and verification All four probes of the array successfully captured defect signals, with no missed detections; Compared with the defects of 0.5mm, 1mm and 2mm in width, it is verified that the wider the defect is, the lower the detection voltage is, which conforms to formula (6): V=k w / w+c w ; At the speeds of 0.5m / s, 1m / s and 2m / s, the detection voltage linearly increases with the speed, which conforms to formula (7): V=k s ·v+c s ; Compared with the traditional runway-type differential probe, the probe of the embodiment can still identify the crack of 0.5mm in width at the speed of 2m / s, the resolution is improved by 1 times, and the signal amplitude is improved by 3.3 times.

[0038] V. Conclusion The embodiment verifies that the spatial orthogonal differential eddy current probe can accurately detect the micron-level surface crack under the working condition of high speed and small lift-off, the positioning error is less than 2mm, the depth error is less than 3%, and the demand of the power industry, the metallurgical industry and the like for the online detection of the surface defects of the metal plate is met.

[0039] The embodiments of the application are described in detail above in combination with the drawings, but the application is not limited to the above embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the purpose of the application.

Claims

1. A spatially orthogonal differential eddy current probe based on magnetic flux direction extraction, characterized in that: It includes four excitation coils L1 to L4 with the same parameters and four detection coils D1 to D4 with the same parameters; The axis of the exciting coil is perpendicular to the axis of the detecting coil, and the whole is arranged compactly in a U-shaped pattern. in: The first detection coil D1 and the second detection coil D2 form a differential coil group A; The third detection coil D3 and the fourth detection coil D4 form a differential coil group B; The differential output signal V is given by equation (1): V=(V D2 –V D1 )+(V D4 –V D3 ) (1); Among them, V is the differential eddy current probe detection signal, V D1 、V D2 、V D3 、V D4 are the detection voltages corresponding to the detection coils D1, D2, D3, and D4 in the improved spatial orthogonal differential eddy current probe; When V shows a double-peak waveform, it is determined that there are defects on the surface of the metal being tested.

2. The spatially orthogonal differential eddy current probe based on magnetic flux direction extraction according to claim 1 is characterized in that: The middle moment t0 of the double-peak waveform satisfies t0=(t1+t2) / 2, where t1 and t2 are the double-peak peak moments respectively. The middle position corresponds to the defect center coordinate x0=v·t0, and v is the travel speed of the metal plate being tested.

3. The spatially orthogonal differential eddy current probe based on magnetic flux direction extraction according to claim 1 or 2, characterized in that: At a constant speed v, the defect depth d and the double peak amplitude difference ΔV pp Satisfying the empirical relationship: d=k d ·ΔV pp +b d (2); Among them, k d 、b d is the system calibration constant; When the defect size is fixed, the velocity v and ΔV pp satisfy: ΔV pp =k v ·v+b v (3); Among them, k v 、b v is the system calibration constant.

4. The spatially orthogonal differential eddy current probe based on magnetic flux direction extraction according to claim 1 is characterized in that: Detection coil induced voltage V coil Given by Faraday's law: （4）； Among them, N is the number of coil turns, μ is the magnetic permeability, H1 and H2 are the primary magnetic fields of the excitation coil, and H yz is the eddy current J x The secondary magnetic field generated.

5. The spatially orthogonal differential eddy current probe based on magnetic flux direction extraction according to claim 4 is characterized in that: -Z-axis eddy current component J caused by defects z satisfy: ∇×H xy =J z have been z (5); Among them, H xy It represents the component of magnetic field intensity H in the xy plane; J z represents the component of the current density J in the z plane; e z is the unit vector in the z direction; The spatial orthogonal arrangement enables the detection coils to H xy The magnetic flux component φ x It has the highest sensitivity, thus increasing the amplitude of the defect signal.

6. A detection device based on the spatial orthogonal differential eddy current probe based on magnetic flux direction extraction according to any one of claims 1 to 5, characterized in that: M×N probes are fixed on a bracket in an array, and each probe is connected to a data acquisition device through an independent lock-in amplifier and A / D channel; Differential signal of each probe V mn The independent calculations by formula (1) are not connected in series or in parallel to achieve parallel detection.

7. The detection device of the spatial orthogonal differential eddy current probe based on magnetic flux direction extraction according to claim 6 is characterized in that: The data acquisition device includes: Multi-channel synchronous A / D converter; Field Programmable Gate Array (FPGA) for executing the calculations of equations (1), (2) and (3) in real time; The host computer is used to store, display and fit the relationship curve between defect position x0, depth d and velocity v.

8. The detection device of the spatial orthogonal differential eddy current probe based on magnetic flux direction extraction according to claim 6 or 7, characterized in that: The bracket is a fixed structure, one end of which is fixed to the base, and the other end is mounted on the probe arm through a screw-nut pair; The probe arm can move along the Z direction to adjust the lifting distance h between the probe and the metal plate to be measured. The value range of h is 0.5mm≤h≤10mm.

9. The detection device of the spatial orthogonal differential eddy current probe based on magnetic flux direction extraction according to claim 6, characterized in that: The bracket can also be handheld, with the probe and data acquisition device connected by a flexible coaxial cable, allowing manual scanning of large or heavy metal parts; In handheld mode, the velocity v is measured in real time by the built-in inertial measurement unit and substituted into equation (3) to correct ΔV pp .

10. The detection device of the spatial orthogonal differential eddy current probe based on magnetic flux direction extraction according to any one of claims 6 to 9, characterized in that: The defect width w and the detection voltage V satisfy: V=k w / w+c w (6); Among them, k w 、c w is the system calibration constant; Under the same defect width w, the detection voltage V increases linearly with the speed v: V=k s ·v+c s (7); Among them, k s 、c s is the system calibration constant; Through parallel detection of array probes, rapid imaging of the width, depth and position of surface defects on metal plates or metal tubes can be achieved.

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