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

Abstract

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

Description

Technical Field

[0001] This invention belongs to the field of infrared thermal imaging defect detection technology, specifically relating to an in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation. Background Technology

[0002] As a core facility in industrial transportation, the pipeline under test is prone to internal and external surface defects due to corrosion, stress, and other factors during long-term service. If these defects are not detected in time, they may lead to major accidents such as leaks and explosions, threatening personnel safety and causing economic losses.

[0003] Eddy current thermal imaging, characterized by direct imaging visualization, non-contact operation, and large lift-off capability, is widely used in the quality inspection of pipelines. Traditional eddy current thermal imaging methods use high-frequency AC coils to excite eddy currents on the surface of the pipeline. Due to the obstruction effect of defects, the distribution of these eddy currents is distorted, resulting in a distorted temperature distribution on the pipeline surface. Infrared image processing algorithms are then used for defect identification and quantification. However, the skin effect of high-frequency eddy currents limits the detection of internal surface defects. Currently, a triaxially orthogonally magnetized electromagnetic thermal imaging method for pipelines has been developed. This method uses triaxially orthogonally magnetized coils to generate a rotating magnetization field, magnetizing the pipeline and causing distortion of the magnetic permeability distribution of defects in arbitrary orientations. Under high-frequency AC excitation, this induces a non-uniform temperature distribution on the surface, enabling the detection and quantitative evaluation of internal surface defects in the pipeline. However, the high-power excitation power supply and cooling system result in large size, high power consumption, and lack of portability for thermal imaging detection devices. Due to the concentration and skin effect limitations of high-frequency induced eddy currents, it is necessary to drive the movement of the pipe under test to achieve surface coverage detection, which cannot achieve in-situ large-area detection, and deep defects can only be detected under the excitation of a DC magnetic field. In addition, due to the sensitivity of the high-frequency eddy current distribution direction, the detection method is limited in its ability to detect defects parallel to the eddy current flow direction. Therefore, the bias magnetized high-frequency eddy current thermal imaging detection method is not suitable for full-coverage detection of multi-morphological defects in in-service pipes. Summary of the Invention

[0004] To address the aforementioned shortcomings of existing technologies, this invention provides an in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation. This solves the problems of large device size and high power consumption in existing methods, as well as the difficulty in achieving large-area in-situ detection and the inability to guarantee the same detection sensitivity for defects with different orientations.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] An in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation is provided, including a magnetic yoke, an infrared thermal imager, a computer, a modulation power supply and an excitation coil group; the infrared thermal imager is disposed above the magnetic yoke and is connected to the computer.

[0007] 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;

[0008] The modulation power supply passes low-frequency alternating current of different phases and direct current of different directions into the first excitation coil, the second excitation coil, the third excitation coil and the fourth excitation coil respectively;

[0009] The direct current is a square wave current with the same amplitude and a 90° phase difference, forming a periodic orthogonal magnetic field perpendicular to the axis of the pipe under test. The formula for calculating the direct current is:

[0010]

[0011] in, Let T be the initial DC current amplitude, t be the period, and n be an integer ≥ 0. The magnitude of the DC current in the first excitation coil. The DC current amplitude of the second excitation coil. The DC current amplitude of the third excitation coil. This represents the DC amplitude of the current in the fourth excitation coil;

[0012] Low-frequency alternating current is a sinusoidal current with a 90° phase difference, which generates a dynamic rotating magnetic field with the center of the yoke as the axis of rotation, thereby producing induced eddy currents rotating around the center. The formula for calculating low-frequency alternating current is:

[0013]

[0014] in, The initial low-frequency alternating current amplitude is given by T, the period is given by t, and the time is given by t. The low-frequency current amplitude of the first excitation coil, This refers to the low-frequency current amplitude of the second excitation coil. The low-frequency current amplitude of the third excitation coil. This represents the low-frequency current amplitude of the fourth excitation coil;

[0015] The total current resulting from the superposition of the DC current and the low-frequency AC current creates a vector superposition of an orthogonal DC magnetic field and a rotating AC magnetic field inside the pipe under test. This excites uniformly distributed induced eddy currents and a distorted temperature field at the defect location. The formula for calculating the total current is:

[0016]

[0017] in, This is the current after the first excitation coil is superimposed. This is the current after the second excitation coil is superimposed. This is the current after the third excitation coil is superimposed. This is the current after the fourth excitation coil is superimposed.

[0018] The beneficial effects of the above technical solution are as follows: the magnetic yoke and the excitation coil group are connected to form a closed magnetic circuit, enabling the required magnetic field to be generated inside the test piece below the magnetic yoke. The infrared thermal imager set above the magnetic yoke can extract the temperature information of the surface of the pipe under test in the area inside the magnetic yoke to capture the distorted temperature field at the defect, realizing the identification and quantitative analysis of defects with arbitrary orientation. The computer connected to the infrared thermal imager can process the temperature information transmitted by the thermal imager and display the results, which is beneficial for operators to intuitively understand the test results, thereby improving the detection efficiency and accuracy. The modulation power supply is used to pass low-frequency AC currents of different phases and DC currents of different directions to the first, second, third, and fourth excitation coils respectively, providing the required current for the excitation coils to achieve different magnetic field excitation effects to meet the detection requirements. When the first, second, third, and fourth excitation coils are respectively supplied with low-frequency AC currents of 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 axis of the pipe under test can be formed. This allows for DC magnetic field excitation of defects in any orientation, resulting in distorted permeability and enabling the detection of deeper defects. When the first, second, third, and fourth excitation coils are respectively supplied with sinusoidal low-frequency AC currents with a 90° phase difference, a dynamic rotating magnetic field is generated around the center of the yoke. This forms a skin depth layer that penetrates the pipe wall. The rotating magnetic field can make the eddy current field uniformly distributed, uniformly heating the defect-free area inside the yoke and inducing local temperature field anomalies at the defects. The infrared thermal imager can capture these temperature changes, thus achieving high-sensitivity and high-accuracy defect detection. Simultaneously, the total current resulting from the superposition of the DC current and the low-frequency AC current forms a vector superposition of the orthogonal DC magnetic field and the rotating AC magnetic field inside the pipe under test, exciting uniformly distributed induced eddy currents and distorted temperature fields at the defects, further improving the defect detection capability and accuracy.

[0019] 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 inside the pipe under test by combining the 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 sensitivity of the eddy current distribution direction, realizing large-area uniform heating, solving the problem of insufficient sensitivity of traditional methods for detecting defects parallel to the direction of induced eddy current, avoiding the inefficiency of traditional point scanning or line scanning methods, and significantly improving the coverage area of ​​a single detection.

[0020] Furthermore, the frequencies of the direct current and the low-frequency alternating current satisfy:

[0021]

[0022] in, The frequency of the direct current. Where n is the frequency of the low-frequency alternating current, and n is an integer ≥ 0.

[0023] The beneficial effects of adopting the above technical solution are as follows: when the frequency of the DC current magnetic field and the rotating low-frequency AC current magnetic field are integer multiples, it can be ensured that the AC excitation heating is uniform under the action of the same DC magnetic field, and the excitation effect on defects with different orientations is consistent.

[0024] Furthermore, the first, second, third, and fourth excitation coils have the same winding direction, the same number of turns, and are symmetrically distributed around the pipe under test.

[0025] The beneficial effects of adopting the above technical solution are as follows: When the winding directions of the first, second, third, and fourth excitation coils are consistent, it can be ensured that the magnetic field directions generated by them can form an orderly and controllable magnetic field distribution when superimposed, thereby covering defects in any circumferential direction of the pipe under test, helping to reduce magnetic field interference, and thus improving the stability and controllability of the magnetic field; In addition, the first, second, third, and fourth excitation coils have the same number of turns, which 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; Furthermore, these four excitation coils are symmetrically distributed around the pipe under test, which can make the magnetic field uniformly distributed around the pipe under test, and can realize large-area in-situ detection without moving the pipe under test, greatly improving detection efficiency and reducing detection time and cost.

[0026] Furthermore, the distance between the magnetic yoke and the surface of the pipe to be tested is <5mm.

[0027] The beneficial effects of adopting the above technical solution are as follows: 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.

[0028] Furthermore, the infrared thermal imager includes an infrared camera, which is positioned at the center of the magnetic yoke.

[0029] The beneficial effects of adopting the above technical solution are as follows: the infrared camera set at the center of the magnetic yoke can capture the abnormal temperature signal generated by the local temperature field anomaly caused by eddy current distortion, thereby improving the accuracy and sensitivity of defect detection.

[0030] In summary, the in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation provided by this invention has the following beneficial effects:

[0031] (1) The device can form a rotating low-frequency AC magnetic field in the pipe under test by combining the excitation of the orthogonal DC magnetic field and the low-frequency rotating AC magnetic field. The rotating magnetic field sweeps the detection area uniformly in multiple cycles, eliminating the influence of the sensitivity of the eddy current distribution direction, realizing large-area uniform heating, solving the problem of insufficient sensitivity of traditional methods for detecting defects parallel to the direction of induced eddy current, avoiding the problem of low efficiency of traditional point scanning or line scanning methods, and significantly improving the coverage area of ​​a single detection.

[0032] (2) By passing square wave DC currents with the same amplitude and a phase difference of 90° into the excitation coil group in the device, a periodically changing vertical orthogonal DC magnetic field can be generated inside the pipe to be tested. The DC magnetic field can be used to excite defects of any direction, thereby forming distorted permeability and achieving the purpose of detecting deeper defects.

[0033] (3) When a sinusoidal low-frequency alternating current with a phase difference of 90° is passed into the excitation coil group in the device, a dynamic rotating magnetic field with the center of the yoke as the rotation axis is generated, forming a skin depth layer that penetrates the tube wall. The rotating magnetic field can make the eddy current field uniformly distributed, uniformly heating the defect-free area inside the yoke, and causing local temperature field anomalies at the defect. The infrared thermal imager can capture temperature changes, thereby achieving high sensitivity and high accuracy defect detection.

[0034] (4) This device breaks through the limitation of the bias magnetization high-frequency eddy current thermal imaging method on the defect direction, and can realize large-area in-situ detection of defects in the entire shape of the pipeline under test. The device is small in size and low in power consumption, and can be used for online detection at the service site of the pipeline under test, playing an important role in the service safety and life prediction of the pipeline under test. Attached Figure Description

[0035] 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.

[0036] Figure 2 This is a schematic diagram of a periodic orthogonal DC magnetic field distribution.

[0037] Figure 3 A schematic diagram of a low-frequency alternating rotating magnetic field and induced eddy currents;

[0038] Among them, 1. the pipe to be tested; 2. the first excitation coil; 3. the magnetic yoke; 4. the second excitation coil; 5. the thermal imager; 6. the computer; 7. the modulation power supply; 8. the third excitation coil; and 9. the fourth excitation coil. Detailed Implementation

[0039] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0040] 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 disposed above the magnetic yoke 3 and 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 circuit; and the excitation coil group is respectively connected to the modulation power supply 7, and the modulation power supply 7 is used as the excitation source to pass low-frequency alternating current of different phases and direct current 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, and the frequencies of the direct current and the low-frequency alternating current satisfy:

[0041]

[0042] 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 be ensured that the AC excitation heating is uniform under the action of the same DC magnetic field, and the excitation effect on defects with different orientations is consistent.

[0043] 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 pipe 1 under test. When the winding directions of the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9 are consistent, it can be ensured that the magnetic field directions generated by them can form an orderly and controllable magnetic field distribution when superimposed, thereby covering defects in any circumferential direction of the pipe 1 under test, helping to reduce magnetic field interference, and thus improving the stability and controllability of the magnetic field. In addition, 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, which 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. Furthermore, the symmetrical distribution of these four excitation coils around the pipe 1 under test can make the magnetic field uniformly distributed around the pipe 1 under test, enabling large-area in-situ detection without moving the pipe 1 under test, greatly improving detection efficiency and reducing detection time and cost.

[0044] In this embodiment, the distance between the magnetic yoke 3 and the surface of the pipe 1 to be tested is less than 5mm, which can achieve non-contact detection and better concentrate the magnetic field to form a magnetic circuit, thereby improving the detection efficiency.

[0045] In this embodiment, the initial DC current amplitude It can be 20A, initial low-frequency AC current amplitude. It can be 20A, low-frequency AC current. It can be 50Hz; by adjusting the initial DC current amplitude and the initial low-frequency alternating current amplitude Both can be set to 20A, which can generate a sufficiently strong magnetic field inside the pipe under test 1, thereby effectively exciting induced eddy currents and distorted magnetic field distribution, reducing magnetic field changes caused by current fluctuations, thus improving the detection sensitivity of defects and ensuring the accuracy and reliability of the detection results; while the 50Hz low-frequency AC current frequency The generated rotating magnetic field can form a uniform eddy current distribution inside the pipe 1 under test, thereby achieving large-area uniform heating, which helps to reduce the temperature gradient caused by uneven heating and improves the accuracy of thermal imaging detection.

[0046] 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℃, 650℃], and the thermal sensitivity is less than 0.1K.

[0047] The specific working principle and process of this invention are as follows:

[0048] like Figure 1As shown, the detection device is placed above the pipe 1 to be tested, with a distance of less than 5 mm from the first excitation coil 2, the second excitation coil 4, the third excitation coil 8 and the fourth excitation coil 9. The first excitation coil 2, the fourth excitation coil 9 and the second excitation coil 4 and the third excitation coil 8 are distributed on both sides of the centerline of the pipe 1 to be tested, and are symmetrically distributed.

[0049] 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 supplies square wave currents with the same amplitude and a phase difference of 90° to the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9, respectively, forming a periodic orthogonal magnetic field perpendicular to the axis of the pipe 1 under test. H DC The formula for calculating the DC current of the excitation coil group is:

[0050]

[0051] in, Let T be the initial DC current amplitude, t be the period, and n be an integer ≥ 0. The DC current amplitude of the first excitation coil 2, The DC current amplitude of the second excitation coil 4, The DC current amplitude of the third excitation coil 8, This refers to the DC amplitude of the current in the fourth excitation coil 9;

[0052] 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 supplies the first excitation coil 2, the second excitation coil 4, the third excitation coil 8, and the fourth excitation coil 9 with sinusoidal low-frequency alternating currents of the same amplitude and phase difference of 90°, respectively, to generate a dynamic rotating magnetic field with the center of the magnetic yoke 3 as the rotation axis. H AC ), thereby generating induced eddy currents rotating around the center ( I x The formula for calculating the low-frequency AC current of the excitation coil group is:

[0053]

[0054] in, The initial low-frequency alternating current amplitude is given by T, the period is given by t, and the time is given by t. The low-frequency current amplitude of the first excitation coil 2, This refers to the low-frequency current amplitude of the second excitation coil 4. The low-frequency current amplitude of the third excitation coil 8, This refers to the low-frequency current amplitude of the fourth excitation coil 9;

[0055] Based on the principle of vector superposition, the orthogonal DC magnetic field and the dynamic rotating AC magnetic field form a loop when the magnetic yoke 3 is connected to the pipe under test 1. Any defect within the detection area of ​​the pipe under test 1 will cause a distortion in the magnetic field distribution under the action of the DC magnetic field, thus causing a distortion in the magnetic permeability distribution, enabling the detection of deeply buried defects. Furthermore, 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 distribution uniform, uniformly heating the defect-free area inside the magnetic yoke 3. The formula for calculating the total current after vector superposition is:

[0056]

[0057] in, The current is the superimposed current of the first excitation coil 2. This is the current superimposed from the second excitation coil 4. This is the current superimposed from the third excitation coil 8. This is the current after the fourth excitation coil 9 is superimposed.

[0058] Based on the principle of vector superposition, a complete magnetic field loop is formed between the magnetic yoke 3 and the pipe under test 1. A DC magnetizing magnetic field with orthogonal periodic changes and a low-frequency rotating AC magnetic field rotating around the center of the magnetic yoke 3 are generated inside the pipe under test 1 below the magnetic yoke 3. The DC magnetizing magnetic field can excite defects of any orientation. Under the periodic orthogonal DC magnetic field excitation, defects of different depths on the inner and outer surfaces will generate magnetic field distortion and permeability perturbation distribution in the skin depth layer of the pipe under test 1. Furthermore, the defects on the inner and outer surfaces of different orientations are excited by the low-frequency AC rotating magnetic field based on the distorted permeability distribution, realizing large-area uniform heating except for the defect location. The heat distribution result is 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 orientations.

[0059] In summary, the in-situ infrared thermal imaging detection device based on low-frequency modulated magnetic field excitation provided by this invention can form a rotating low-frequency AC magnetic field in the pipe to be tested 1 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 sensitivity of the eddy current distribution direction, realizing large-area uniform heating, solving the problem of insufficient sensitivity of traditional methods for detecting defects parallel to the direction of induced eddy currents, avoiding the problem of low efficiency of traditional point scanning or line scanning methods, and significantly improving 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 in that: It 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 located 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). The first excitation coil (2), the fourth excitation coil (9), the second excitation coil (4), and the third excitation coil (8) are distributed on both sides of the centerline of the pipe (1) to be tested, and are symmetrically distributed. The magnetic yoke (3) is connected to the excitation coil group to form a closed magnetic circuit. The modulation power supply (7) supplies low-frequency alternating current of different phases and direct current of different directions to the first excitation coil (2), the second excitation coil (4), the third excitation coil (8) and the fourth excitation coil (9), respectively. The direct 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 pipe (1) under test. The formula for calculating the direct current is: in, Let T be the initial DC current amplitude, t be the period, and n be an integer ≥ 0. 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 in the fourth excitation coil (9); The low-frequency alternating current is a sinusoidal current with a 90° phase difference, which generates a dynamic rotating magnetic field with the center of the magnetic yoke (3) as the rotation axis, thereby generating induced eddy currents rotating around the center. The calculation formula for the low-frequency alternating current is: in, The initial low-frequency alternating current amplitude is given by T, the period is given by t, and the time is given by t. 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) The low-frequency current amplitude of the fourth excitation coil (9); The total current resulting from the superposition of the DC current and the low-frequency AC current forms a vector superposition of an orthogonal DC magnetic field and a rotating AC magnetic field inside the pipe under test (1), which excites uniformly distributed induced eddy currents and a distorted temperature field at the defect. The formula for calculating the total current is: in, The current is the superimposed current of the first excitation coil (2). The current is the superimposed current of the second excitation coil (4). The current is the superimposed current of the third excitation coil (8). The current is the superimposed current 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 DC current and the low-frequency AC current satisfy the following: in, The frequency of the direct current. Where n is the frequency of the low-frequency alternating 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 pipe (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 (1) to be tested is <5mm.

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, which is located at the center of the magnetic yoke (3).

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