In-situ infrared thermal imaging detection device based on low-frequency modulation magnetic field excitation

Abstract

The invention belongs to the technical field of infrared thermal imaging defect detection, and particularly discloses an in-situ infrared thermal imaging detection device based on low-frequency modulation magnetic field excitation, which comprises a magnet yoke, an infrared thermal imager, a computer, a modulation power supply and an excitation coil group, the excitation coil group at least comprises a first excitation coil, a second excitation coil, a third excitation coil and a fourth excitation coil; the modulation power supply respectively introduces low-frequency alternating currents with different phases and direct currents with different directions into the first excitation coil, the second excitation coil, the third excitation coil and the fourth excitation coil; through composite excitation of an orthogonal direct-current magnetic field and a low-frequency rotating alternating-current magnetic field, the device can form a rotating low-frequency alternating-current magnetic field in a to-be-detected pipeline, the rotating magnetic field uniformly sweeps a detection area in a plurality of periods, the influence of eddy current distribution direction sensitivity is eliminated, and large-area uniform heating is realized; the problem of low efficiency of a traditional spot scanning or line scanning mode is avoided, and the coverage area of single detection is remarkably increased.

Description

Technical Field

[0001] The present invention belongs to the technical field of infrared thermal imaging defect detection, and in particular relates to an in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation. Background Art

[0002] As core infrastructure for industrial transportation, pipelines under test are prone to internal and external surface defects due to corrosion and stress during long-term service. If these defects are not detected in a timely manner, they can lead to major accidents such as leaks and explosions, threatening personnel safety and causing economic losses.

[0003] The eddy current thermal imaging detection method has the characteristics of direct imaging visualization, non-contact, and large lift-off, and is widely used in the quality inspection process of the pipeline to be tested. In the traditional eddy current thermal imaging method, a high-frequency AC coil is used to excite induced eddy currents on the surface of the pipeline to be tested. Due to the obstruction of the defect, the induced eddy current distribution will be distorted, thereby generating a distorted temperature distribution on the surface of the pipeline to be tested. Infrared image processing algorithms are further used to identify and quantify the defects. Due to the skin effect of high-frequency induced eddy currents, the detection of internal surface defects is limited. At present, there is a three-way orthogonal magnetization electromagnetic thermal imaging detection method for the pipeline to be tested. The three-way orthogonal magnetization coil is used to generate a rotating magnetic field to magnetize the pipeline to be tested, thereby causing the magnetic permeability distribution of defects in any direction to be distorted, and causing non-uniform surface temperature distribution under high-frequency AC excitation, thereby realizing the detection and quantitative evaluation of internal surface defects of the pipeline to be tested. However, the high-power excitation power supply and cooling system make thermal imaging inspection devices bulky, power-hungry, and non-portable. Due to the concentration and skin-like nature of high-frequency induced eddy currents, coverage inspection of the pipeline surface must be achieved by driving the pipeline in motion, making in-situ inspection of large areas impossible. Furthermore, deeper defects can only be detected under the influence of a DC magnetic field. Furthermore, the directional sensitivity of high-frequency eddy current distribution limits this method's ability to detect defects parallel to the eddy current flow direction. Therefore, the biased magnetization high-frequency eddy current thermal imaging method is not suitable for full-coverage inspection of multi-morphological defects in in-service pipelines. Summary of the Invention

[0004] In response to the above-mentioned shortcomings of the prior art, the present invention provides an in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation, which solves the problems of large device size and high power consumption in the existing method, and the difficulty in achieving large-area in-situ detection and the inability to ensure the same detection sensitivity for defects with different directions.

[0005] To achieve the above object, the technical solution adopted by the present invention is: Provided is an in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation, comprising a magnetic yoke, an infrared thermal imager, a computer, a modulation power supply, and an excitation coil assembly; the infrared thermal imager is arranged above the magnetic yoke and connected to the computer; The excitation coil group includes at least a first excitation coil, a second excitation coil, a third excitation coil and a fourth excitation coil, and the magnetic yoke is connected to the excitation coil group to form a closed magnetic circuit; The modulated power supply respectively passes low-frequency alternating currents of different phases and direct currents of different directions into the first excitation coil, the second excitation coil, the third excitation coil and the fourth excitation coil; The DC current is a square wave current with the same amplitude and a phase difference of 90°, which forms a periodic orthogonal magnetic field perpendicular to the axis of the pipeline to be tested. The calculation formula of the DC current is:

[0006] in, is the initial DC current amplitude, T is the period, t is the time, n is an integer ≥ 0, is the DC current amplitude of the first excitation coil, is the DC current amplitude of the second excitation coil, is the DC current amplitude of the third excitation coil, is the DC amplitude of the current in the fourth excitation coil; The low-frequency AC current is a sinusoidal current with a phase difference of 90°, which generates a dynamic rotating magnetic field with the center of the magnetic yoke as the rotation axis, thereby generating an induced eddy current rotating around the center. The calculation formula for the low-frequency AC current is:

[0007] in, is the initial low-frequency AC current amplitude, T is the period, t is the time, is the low-frequency current amplitude of the first excitation coil, is the low-frequency current amplitude of the second excitation coil, is the low-frequency current amplitude of the third excitation coil, is the low-frequency current amplitude of the fourth excitation coil; The total current after the superposition of DC current and low-frequency AC current forms a vector superposition of orthogonal DC magnetic field and rotating AC magnetic field inside the pipeline to be tested, which stimulates uniformly distributed induced eddy currents and distorted temperature fields at the defects. The calculation formula of the total current is:

[0008] in, is the current of the first excitation coil after superposition, is the current of the second excitation coil after superposition, is the current of the third excitation coil after superposition, is the current after superposition of the fourth excitation coil.

[0009] The beneficial effects of adopting the above technical solution are as follows: the magnetic yoke is connected to the excitation coil group to form a closed magnetic loop, so that the required magnetic field can be generated inside the test piece below the magnetic yoke, and the infrared thermal imager arranged above the magnetic yoke can extract the temperature information of the surface of the pipeline to be tested in the internal area of ​​the magnetic yoke to capture the distorted temperature field at the defect, and realize the identification and quantitative analysis of defects in any direction. The computer connected to the infrared thermal imager can perform data processing and result display on the temperature information transmitted by the thermal imager, which is beneficial for the operator to intuitively understand the test results, thereby improving the detection efficiency and accuracy; and the modulated power supply is used to respectively pass low-frequency AC currents of different phases and DC currents of different directions to the first excitation coil, the second excitation coil, the third excitation coil and the fourth excitation coil, to provide the required current for the excitation coil, to achieve different magnetic field excitation effects to meet the detection requirements; when the first excitation coil, the second excitation coil, the third excitation coil and the fourth excitation coil are respectively passed with the same amplitude and a phase difference of 90° When a square wave DC current is applied, a periodic orthogonal magnetic field perpendicular to the axial direction of the pipeline to be tested is formed, which can excite DC magnetic fields for defects in any direction, thereby forming distorted magnetic permeability distribution and achieving the purpose of detecting deeper defects. When sinusoidal low-frequency AC currents with a phase difference of 90° are respectively applied to the first excitation coil, the second excitation coil, the third excitation coil and the fourth excitation coil, a dynamic rotating magnetic field with the center of the magnetic yoke as the rotation axis is generated, forming a skin depth layer that penetrates the pipe wall. The rotating magnetic field can make the eddy current field uniformly distributed, uniformly heat the defect-free area inside the magnetic yoke, and induce local temperature field anomalies at the defects. The infrared thermal imager can capture temperature changes, thereby achieving high-sensitivity and high-accuracy defect detection. At the same time, the total current after the DC current and the low-frequency AC current are superimposed forms a vector superposition of orthogonal DC magnetic field and rotating AC magnetic field inside the pipeline to be tested, exciting uniformly distributed induced eddy currents and distorted temperature fields at the defects, further improving the ability and accuracy of defect detection.

[0010] This in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation can form a rotating low-frequency AC magnetic field in the pipeline to be tested through the composite excitation of an orthogonal DC magnetic field and a low-frequency rotating AC magnetic field. The rotating magnetic field uniformly sweeps the detection area within multiple cycles, eliminating the influence of the directional sensitivity of the eddy current distribution and achieving uniform heating over a large area. It solves the problem of insufficient sensitivity of traditional methods for defect detection parallel to the direction of the induced eddy current, avoids the low efficiency of traditional point scanning or line scanning methods, and significantly improves the coverage area of ​​a single detection.

[0011] Furthermore, the frequencies of the DC current and the low-frequency AC current satisfy:

[0012] in, is the DC current frequency, is the frequency of the low-frequency AC current, and n is an integer ≥0.

[0013] The beneficial effect of adopting the above technical solution is: when the DC current magnetic field and the rotating low-frequency AC current magnetic field frequency are integer multiples, it can ensure that the AC excitation heating is uniform under the action of the same DC magnetic field, and the excitation effect on defects with different directions is consistent.

[0014] Furthermore, the first excitation coil, the second excitation coil, the third excitation coil and the fourth excitation coil have the same winding direction and the same number of turns, and are symmetrically distributed around the pipeline to be tested.

[0015] The beneficial effects of adopting the above technical solution are: when the winding directions of the first excitation coil, the second excitation coil, the third excitation coil and the fourth excitation coil are consistent, it can be ensured that the directions of the magnetic fields generated by them can form an orderly and controllable magnetic field distribution when superimposed, so that defects in any circumferential direction of the pipeline to be tested can be covered, which helps to reduce the interference of the magnetic field, thereby improving the stability and controllability of the magnetic field; and the first excitation coil, the second excitation coil, the third excitation coil and the fourth excitation coil have the same number of turns, and can generate the same magnetic field strength under the same current, thereby ensuring the uniformity of the magnetic field distribution and avoiding the difference in magnetic field strength caused by different numbers of turns; in addition, the four excitation coils are symmetrically distributed around the pipeline to be tested, so that the magnetic field can be evenly distributed around the pipeline to be tested, and large-area in-situ detection can be achieved without moving the pipeline to be tested, greatly improving the detection efficiency and reducing the detection time and cost.

[0016] Furthermore, the distance between the magnetic yoke and the surface of the pipe to be measured is less than 5 mm.

[0017] The beneficial effect of adopting the above technical solution is that when the distance between the magnetic yoke and the surface of the pipe to be tested is less than 5mm, non-contact detection can be achieved and the magnetic field can be better concentrated to form a magnetic circuit, thereby improving the detection efficiency.

[0018] Furthermore, the infrared thermal imager includes an infrared camera, which is arranged at the center of the magnetic yoke.

[0019] The beneficial effect of adopting the above technical solution is that the infrared camera arranged at the center of the magnetic yoke can capture the abnormal temperature signal generated by the local temperature field abnormality caused by eddy current distortion, thereby improving the accuracy and sensitivity of defect detection.

[0020] In summary, the in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation provided by the present invention has the following beneficial effects: (1) The device can form a rotating low-frequency AC magnetic field in the pipeline to be tested through the composite excitation of an orthogonal DC magnetic field and a low-frequency rotating AC magnetic field. The rotating magnetic field uniformly sweeps the detection area in multiple cycles, eliminating the influence of the directional sensitivity of the eddy current distribution and achieving uniform heating over a large area. It solves the problem of insufficient sensitivity of traditional methods for defect detection parallel to the direction of the induced eddy current, avoids the problem of low efficiency of traditional point scanning or line scanning, and significantly improves the coverage area of ​​a single detection.

[0021] (2) By supplying square wave DC currents with the same amplitude and a phase difference of 90° into the excitation coil groups in the device, a periodically changing vertical orthogonal DC magnetic field can be generated inside the pipeline to be tested. The DC magnetic field can be excited to defects in any direction, thereby forming distorted magnetic permeability, thereby achieving the purpose of detecting deeper defects.

[0022] (3) When a sinusoidal low-frequency alternating current with a phase difference of 90° is passed through the excitation coil group in the device, a dynamic rotating magnetic field with the center of the magnetic yoke as the rotation axis is generated, forming a skin depth layer that penetrates the pipe wall. The rotating magnetic field can make the eddy current field evenly distributed, uniformly heat the defect-free area inside the magnetic yoke, and cause local temperature field anomalies at the defect site. The infrared thermal imager can capture the temperature changes, thereby achieving high-sensitivity and high-accuracy defect detection.

[0023] (4) This device breaks through the limitation of the bias magnetization high-frequency eddy current thermal imaging method on the direction of defects, and can realize large-area in-situ detection of all-morphological defects in the pipeline to be tested. The device is small in size and low in power consumption. It can be used for online detection at the service site of the pipeline to be tested, playing an important role in the service safety and life prediction of the pipeline to be tested. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of an in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation according to the present invention; Figure 2 Schematic diagram of periodic orthogonal DC magnetic field distribution; Figure 3 It is a schematic diagram of low-frequency AC rotating magnetic field and induced eddy current; Among them, 1. pipeline to be tested; 2. first excitation coil; 3. magnetic yoke; 4. second excitation coil; 5. thermal imager; 6. computer; 7. modulation power supply; 8. third excitation coil; 9. fourth excitation coil. DETAILED DESCRIPTION

[0025] The specific embodiments of the present invention are described below to facilitate understanding of the present invention by those skilled in the art. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the appended claims, these changes are obvious, and all inventions and creations utilizing the concepts of the present invention are protected.

[0026] like Figure 1 As shown, the in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation provided by the present invention includes a magnetic yoke 3, an infrared thermal imager 5, a computer 6, a modulation power supply 7 and an excitation coil group; the infrared thermal imager 5 is arranged above the magnetic yoke 3, and the infrared thermal imager 5 is connected to the computer 6; the excitation coil group includes at least a first excitation coil 2, a second excitation coil 4, a third excitation coil 8 and a fourth excitation coil 9, and the magnetic yoke 3 is connected to the excitation coil group to form a closed magnetic loop; and the excitation coil group is respectively connected to the modulation power supply 7, and with the modulation power supply 7 as the excitation source, low-frequency alternating currents of different phases and direct currents of different directions are respectively passed into the first excitation coil 2, the second excitation coil 4, the third excitation coil 8 and the fourth excitation coil 9, and the frequencies of the direct current and the low-frequency alternating current satisfy:

[0027] When the frequency of the DC current magnetic field is an integer multiple of the frequency of the rotating low-frequency AC current magnetic field, it can ensure that the AC excitation heating is uniform under the action of the same DC magnetic field, and the excitation effect on defects with different directions is consistent.

[0028] In this embodiment, the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9 have the same winding direction, the same number of turns, and are symmetrically distributed around the pipeline 1 to be tested. When the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9 have the same winding direction, it can be ensured that the directions of the magnetic fields generated by them can form an orderly and controllable magnetic field distribution when superimposed, thereby covering defects in any circumferential direction of the pipeline 1 to be tested, helping to reduce magnetic field interference, thereby improving the stability and controllability of the magnetic field. Moreover, the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9 have the same number of turns and can generate the same magnetic field strength under the same current, thereby ensuring the uniformity of the magnetic field distribution and avoiding differences in magnetic field strength caused by different numbers of turns. In addition, the four excitation coils are symmetrically distributed around the pipeline 1 to be tested, so that the magnetic field is evenly distributed around the pipeline 1 to be tested, and large-area in-situ testing can be achieved without moving the pipeline 1 to be tested, greatly improving detection efficiency and reducing detection time and cost.

[0029] In this embodiment, the distance between the magnetic yoke 3 and the surface of the pipe 1 to be tested is less than 5 mm, which can not only realize non-contact detection, but also better concentrate the magnetic field to form a magnetic circuit, thereby improving detection efficiency.

[0030] In this embodiment, the initial DC current amplitude is Can be 20A, initial low frequency AC current amplitude Can be 20A, low frequency AC current frequency It can be 50Hz; by setting the initial DC current amplitude and the initial low-frequency AC current amplitude Both can be set to 20A, which can generate a magnetic field of sufficient strength inside the pipeline 1 to be tested, thereby effectively exciting the induced eddy current and distorted magnetic field distribution, reducing the magnetic field changes caused by current fluctuations, thereby improving the detection sensitivity of defects and ensuring the accuracy and reliability of the test results; and the 50Hz low-frequency AC current frequency The generated rotating magnetic field can form a uniform eddy current distribution inside the pipeline 1 to be tested, thereby achieving uniform heating over a large area, helping to reduce the temperature gradient caused by uneven heating and improving the accuracy of thermal imaging detection.

[0031] In this embodiment, the infrared thermal imager 5 includes an infrared camera placed at the center of the magnetic yoke 3; the resolution of the infrared camera is greater than or equal to 640*480, the temperature measurement range is [-40°C, 650°C], and the thermal sensitivity is less than 0.1K.

[0032] The specific working principle and process of the present invention are: like Figure 1 As shown, the detection device is placed above the pipeline to be tested 1, with a distance from the first excitation coil 2, the second excitation coil 4, the third excitation coil 8 and the fourth excitation coil 9 less than 5 mm, and the first excitation coil 2, the fourth excitation coil 9 and the second excitation coil 4, the third excitation coil 8 are distributed symmetrically on both sides of the center line of the pipeline to be tested 1.

[0033] like Figure 1 and Figure 2 As shown, the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9 are connected to the modulation power supply 7. The modulation power supply 7 respectively passes square wave currents with the same amplitude and a phase difference of 90° into the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9, forming a periodic orthogonal magnetic field perpendicular to the axial direction of the pipeline 1 to be tested ( H DC ), the calculation formula of the DC current of the exciting coil group is:

[0034] in, is the initial DC current amplitude, T is the period, t is the time, n is an integer ≥ 0, is the DC current amplitude of the first excitation coil 2, is the DC current amplitude of the second excitation coil 4, is the DC current amplitude of the third excitation coil 8, is the DC current amplitude of the fourth excitation coil 9; like Figure 1 and Figure 3 As shown, the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9 are connected to the modulation power supply 7. The modulation power supply 7 respectively passes sinusoidal low-frequency alternating currents with the same amplitude and a phase difference of 90° into the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9, thereby generating a dynamic rotating magnetic field with the center of the magnetic yoke 3 as the rotation axis ( H AC ), thereby generating an induced eddy current rotating around the center ( I x ), the calculation formula of the low-frequency AC current of the exciting coil group is:

[0035] in, is the initial low-frequency AC current amplitude, T is the period, t is the time, is the low-frequency current amplitude of the first excitation coil 2, is the low-frequency current amplitude of the second excitation coil 4, is the low-frequency current amplitude of the third excitation coil 8, is the low-frequency current amplitude of the fourth excitation coil 9; According to the principle of vector superposition, the orthogonal DC magnetic field and the dynamically rotating AC magnetic field form a loop when the magnetic yoke 3 is connected to the pipeline 1 to be tested. Any defect in the detection area of ​​the pipeline 1 to be tested will form a magnetic field distribution distortion under the action of the DC magnetic field, thereby causing a distorted distribution of magnetic permeability, enabling the detection of deeply buried defects. In addition, the low-frequency AC magnetic field can form a skin depth layer that penetrates the pipe wall, and the rotating magnetic field can make the eddy current field uniformly distributed, uniformly heating the defect-free area inside the magnetic yoke 3. The calculation formula for the total current after vector superposition is:

[0036] in, is the current after superposition of the first excitation coil 2, is the current of the second excitation coil 4 after superposition, is the current after superposition of the third excitation coil 8, is the current after superposition of the fourth excitation coil 9.

[0037] According to the principle of vector superposition, a complete magnetic field loop is formed by the magnetic yoke 3 and the pipeline 1 to be tested. A DC magnetizing magnetic field with orthogonal periodic changes in direction and a low-frequency rotating AC magnetic field rotating about the center of the magnetic yoke 3 are generated inside the pipeline 1 to be tested below the magnetic yoke 3. The DC magnetizing magnetic field can excite defects in any direction. Defects of different depths on the inner and outer surfaces will produce magnetic field distortion and magnetic permeability disturbance distribution within the skin depth layer of the pipeline 1 to be tested under the excitation of the periodic orthogonal DC magnetic field. In addition, defects on the inner and outer surfaces of different directions are excited by the low-frequency AC rotating magnetic field on the basis of the distorted magnetic permeability distribution, achieving uniform heating of a large area except for the defect location. The thermal distribution results are picked up by the thermal imager 5 placed above the magnetic yoke 3 and transmitted to the computer 6 for temperature processing and result display, thereby realizing large-area, low-power in-situ detection of defects in all directions.

[0038] In summary, the in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation provided by the present invention can form a rotating low-frequency AC magnetic field in the pipeline 1 to be tested through the composite excitation of an orthogonal DC magnetic field and a low-frequency rotating AC magnetic field. The rotating magnetic field uniformly sweeps the detection area within multiple cycles, eliminating the influence of the directional sensitivity of the eddy current distribution, and realizing uniform heating over a large area. It solves the problem of insufficient sensitivity of traditional methods for defect detection parallel to the direction of the induced eddy current, avoids the problem of low efficiency of traditional point scanning or line scanning methods, and significantly improves the coverage area of ​​a single detection.

Claims

1. An in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation, characterized by: It comprises a magnetic yoke (3), an infrared thermal imager (5), a computer (6), a modulation power supply (7) and an excitation coil group; the infrared thermal imager (5) is arranged above the magnetic yoke (3), and the infrared thermal imager (5) is connected to the computer (6); The excitation coil group comprises at least a first excitation coil (2), a second excitation coil (4), a third excitation coil (8) and a fourth excitation coil (9), and the magnetic yoke (3) is connected to the excitation coil group to form a closed magnetic circuit; The modulation power supply (7) respectively passes low-frequency alternating currents of different phases and direct currents of different directions into the first excitation coil (2), the second excitation coil (4), the third excitation coil (8) and the fourth excitation coil (9); The DC current is a square wave current with the same amplitude and a phase difference of 90°, forming a periodic orthogonal magnetic field perpendicular to the axis of the pipeline (1) to be tested. The calculation formula of the DC current is: in, is the initial DC current amplitude, T is the period, t is the time, n is an integer ≥ 0, is the DC current amplitude of the first excitation coil (2), is the DC current amplitude of the second excitation coil (4), is the DC current amplitude of the third excitation coil (8), is the DC amplitude of the current of the fourth excitation coil (9); The low-frequency alternating current is a sinusoidal current with a phase difference of 90°, which generates a dynamic rotating magnetic field with the center of the magnetic yoke (3) as the rotation axis, thereby generating an induced eddy current rotating at the center. The calculation formula of the low-frequency alternating current is: in, is the initial low-frequency AC current amplitude, T is the period, t is the time, is the low-frequency current amplitude of the first excitation coil (2), is the low-frequency current amplitude of the second excitation coil (4), is the low-frequency current amplitude of the third excitation coil (8), is the low-frequency current amplitude of the fourth excitation coil (9); The total current after the DC current and the low-frequency AC current are superimposed forms a vector superposition of an orthogonal DC magnetic field and a rotating AC magnetic field inside the pipeline to be tested (1), which stimulates uniformly distributed induced eddy currents and a distorted temperature field at the defect. The calculation formula of the total current is: in, is the superimposed current of the first excitation coil (2), is the superimposed current of the second excitation coil (4), is the superimposed current of the third excitation coil (8), is the current after superposition of the fourth excitation coil (9).

2. The in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation according to claim 1, characterized in that: The frequencies of the direct current and the low-frequency alternating current satisfy: in, is the DC current frequency, is the frequency of the low-frequency AC current, and n is an integer ≥0.

3. The in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation according to claim 1, characterized in that: The first excitation coil (2), the second excitation coil (4), the third excitation coil (8) and the fourth excitation coil (9) have the same winding direction, the same number of turns and are symmetrically distributed around the pipeline (1) to be tested.

4. The in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation according to claim 1, characterized in that: The distance between the magnetic yoke (3) and the surface of the pipe to be measured (1) is less than 5 mm.

5. The in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation according to claim 1, characterized in that: The infrared thermal imager (5) includes an infrared camera, and the infrared camera is arranged at the center of the magnetic yoke (3).

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