Magnetic bias alternating current detection method based on infrared thermal imaging

Abstract

The invention belongs to the technical field of current detection, and particularly relates to a magnetic bias alternating current detection method based on infrared thermal imaging, which comprises the following steps of: assembling a permanent magnet magnetizer to form an outer magnetic circuit with air; placing a ferromagnetic metal wire in the outer magnetic circuit, and applying a bias magnetic field to form a closed total magnetic circuit; analyzing the magnetic parameter structure to obtain the radial magnetic field intensity of the ferromagnetic metal wire; analyzing electrical parameters of the electrified equivalent circuit by combining the magnetic parameters; alternating current is connected, an electric signal is monitored, and an infrared thermal imager is used for collecting an original thermal image sequence; on the basis of various parameters and signals, using Fourier transform to extract a same-frequency temperature amplitude; fitting a temperature amplitude curve by using one-dimensional Fourier series; and measuring the alternating current according to the fitting result. According to the scheme, based on the infrared thermal imaging technology, alternating current long-distance detection is achieved in a non-contact mode, the method can be used for electric power transmission and distribution high-voltage line inspection, personnel safety can be guaranteed, inspection efficiency can be improved, and workload and risks can be reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of current detection, and particularly relates to a magnetic-biased alternating current detection method based on infrared thermal imaging. Background Art

[0002] The detection technology of alternating current is widely used in power systems, industry, transportation, medical care, environmental protection and other fields. The existing current detection technologies are divided into invasive and non-invasive measurements. Among them, invasive measurement is based on Ohm's law, and the measured current is obtained by the voltage generated by the measured current on the sampling resistor. Non-invasive measurement methods are based on Faraday's law of electromagnetic induction and measure the magnetic field generated by the measured current. However, the above current detection methods all require the sensor to have direct physical contact with the circuit or be arranged around the circuit to be measured, with a short detection distance and problems such as potential safety hazards. In fields such as energy and power, remote current detection is very important. For example, in the field of power transmission and distribution, the inspection of high-voltage lines requires remote detection of their current to improve the inspection efficiency, reduce the workload and risks while ensuring safety. Therefore, it is of great significance to develop a method for remotely detecting alternating current.

[0003] At present, after rapid development, infrared thermal imaging technology has been able to detect the temperature distribution of the measured target with high precision at a long distance, and at the same time, the response speed is very fast. When the current on the metal conductor is converted into heat through Joule heat, the temperature distribution on the metal surface includes both the accumulation of Joule heat and the heat diffusion process. The heating power shows a periodic fluctuation synchronized with the excitation signal and is manifested through the change of the surface temperature of the material. The existing infrared thermal imaging technology can capture this temperature signal that changes with the current in real time. However, since the frequency of the temperature fluctuation is twice the frequency of the alternating current, the frame rate of the infrared thermal imaging technology needs to be greater than four times the frequency of the alternating current to satisfy the sampling theorem, so as to extract the temperature fluctuation caused by the alternating current. Although the frame rate of the current infrared thermal imaging technology has been greatly improved, high-frame-rate thermal imagers require high costs and are not suitable for large-scale use. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the existing current detection technology, and provide a magnetic-biased alternating current detection method based on infrared thermal imaging, which realizes the remote detection of alternating current through a non-contact detection method based on infrared thermal imaging.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A magnetic-biased alternating current detection method based on infrared thermal imaging, comprising the following steps:

[0007] S1. Assemble a permanent magnetizer. A permanent magnetizer and the air therein form an external magnetic circuit.

[0008] S2. Place a ferromagnetic metal wire in the external magnetic circuit. Use the permanent magnetizer to apply a bias magnetic field to the ferromagnetic metal wire. The ferromagnetic metal wire and the external magnetic circuit together form a closed total magnetic circuit.

[0009] S3. Analyze the magnetic parameter structure of the closed total magnetic circuit to obtain the magnetic field strength H in the radial direction of the ferromagnetic metal wire b ;

[0010] S4. Analyze the electrical parameter structure of the equivalent circuit with current flowing through the ferromagnetic metal wire in combination with the magnetic parameter structure.

[0011] S5. Pass an alternating current through the ferromagnetic metal wire and monitor the electrical signal. Under the action of the bias magnetic field of the permanent magnetizer, use an infrared thermal imager to collect the original thermal image sequence S of the ferromagnetic metal wire during the current-carrying stage.

[0012] S6. Based on the current signal and the infrared thermal image sequence S, in combination with the magnetic parameter structure and the electrical parameter structure, use Fourier transform to extract the temperature amplitude at the same frequency for each current amplitude.

[0013] S7. Use one-dimensional Fourier series to fit the temperature amplitude curves for different current amplitudes.

[0014] S8. Measure the alternating current on the ferromagnetic metal wire according to the fitting result.

[0015] Preferably, in step S1, assembling the permanent magnetizer includes the following steps:

[0016] S11. Prepare a magnetic yoke and two permanent magnet groups. Each permanent magnet group includes several single permanent magnets.

[0017] S12. Place the two permanent magnet groups parallel to each other so that the magnetization directions of the two permanent magnet groups are opposite.

[0018] S13. Open a through hole in the magnetic yoke for releasing heat, and then place the magnetic yoke flatly and closely between the two permanent magnet groups so that the magnetization directions of the two permanent magnets are perpendicular to the axis direction of the yoke, thus forming a permanent magnetizer. The permanent magnetizer and the air therein together form an external magnetic circuit.

[0019] Preferably, in step S3, analyzing the magnetic parameter structure of the closed total magnetic circuit includes the following steps:

[0020] S31. According to the geometric parameters and magnetic properties of each component in the closed total magnetic circuit, obtain the magnetic potential F i and the magnetic resistance R i , and the specific calculation formula is expressed as:

[0021] F i = H c l i (Equation 1);

[0022]

[0023] where i represents the components in the closed magnetic circuit, i.e., i = m, a, b, c; m represents the permanent magnet; a represents the magnetic conducting armature; b represents the ferromagnetic metal wire; c represents the air; F i represents the magnetomotive force of component i; H c represents the coercive force of the permanent magnet; l i represents the length of component i in the magnetic flux direction; μ i represents the magnetic permeability of component i, S i represents the projected area of component i perpendicular to the magnetic flux direction; R i represents the magnetic resistance of component i;

[0024] S32, taking the ferromagnetic metal wire as a magnetic thermal radiation sensor, whereby the external magnetic circuit magnetic resistance R s and the magnetic thermal radiation sensor magnetic resistance R b together constitute the total magnetic resistance of the closed total magnetic circuit; the external magnetic circuit magnetic resistance R s and the magnetic thermal radiation sensor magnetic resistance R b are calculated as follows:

[0025]

[0026] where l s represents the length of the external magnetic circuit in the magnetic flux direction; μ s represents the average magnetic permeability of the external magnetic circuit; S represents the average cross-sectional area of the external magnetic circuit; d represents the length of the ferromagnetic metal wire in the magnetic flux direction, which is also its own width; μ b represents the average magnetic permeability of the ferromagnetic metal wire; S b represents the average cross-sectional area of the ferromagnetic metal wire; μ0 represents the vacuum magnetic permeability;

[0027] S33, according to Kirchhoff's law to describe the magnetic flux distribution in the closed total magnetic circuit, i.e.:

[0028] F = Φ(R s + R b ) = H s l s + H b d (Equation 5);

[0029] where F represents the total magnetomotive force of the closed total magnetic circuit; Φ represents the total magnetic flux of the closed total magnetic circuit, which is also the magnetic flux on the ferromagnetic metal wire; Hb H5 represents the average magnetic field strength on the ferromagnetic metal wire, and H5 represents the average magnetic field strength of the external magnetic circuit;

[0030] S34, without considering the leakage flux, the magnetic flux in the entire closed total magnetic circuit is continuous, that is:

[0031] μ b μ0S b H b =μ s μ0H5S (Formula 6);

[0032] S35, combine the above (Equation 1) to (Equation 6) to obtain the average magnetic field strength of the ferromagnetic metal wire, which is the radial magnetic field strength H of the ferromagnetic metal wire. b , then:

[0033]

[0034] Among them, l m Represents the length of the permanent magnet in the direction of magnetic flux.

[0035] Preferably, in step S4, analyzing the electrical parameter structure of the equivalent circuit on the ferromagnetic metal wire includes the following steps:

[0036] S41, according to Kirchhoff's voltage law, the equivalent circuit of the ferromagnetic metal wire is expressed as:

[0037] U = IR - e - e0 (Formula 8);

[0038]

[0039] I=I m cos(2πft) (Eq. 11);

[0040] Among them, U represents the circuit voltage, I represents the circuit current, R is the resistivity of the ferromagnetic metal wire, e0 is the leakage electromotive force, e is the induced electromotive force caused by the bias magnetic field on the ferromagnetic metal wire; L represents the equivalent inductance in the energized equivalent circuit; Φ represents the total magnetic flux of the closed total magnetic circuit, which is also the magnetic flux on the ferromagnetic metal wire; I m represents the amplitude of the alternating current; f represents the frequency of the alternating current; t represents the time for passing the alternating current into the ferromagnetic metal wire;

[0041] S42, ignoring the leakage electromotive force e0, then: U = IR-e (Formula 12);

[0042] S43, combined with the magnetic parameter structure, the magnetic flux Φ on the ferromagnetic metal wire is derived, that is:

[0043]

[0044] Among them, μ b represents the average magnetic permeability of the ferromagnetic metal wire; S b represents the average cross-sectional area of the ferromagnetic metal wire; μ s represents the average magnetic permeability of the external magnetic circuit; S represents the average cross-sectional area of the external magnetic circuit; H c represents the coercive force of the permanent magnet; l m represents the length of the permanent magnet in the magnetic flux direction; d represents the length of the ferromagnetic metal wire in the magnetic flux direction; l s represents the length of the external magnetic circuit in the magnetic flux direction;

[0045] S44, by combining the above (Equation 8) to (Equation 13), the circuit voltage, that is, the voltage U on the magnetic metal wire, is obtained, and then:

[0046]

[0047] Among them, E represents the absolute value of the induced electromotive force caused by the bias magnetic field on the ferromagnetic metal wire, that is, E = |e|

[0048] Preferably, in the step S5, the acquisition of the original thermal image sequence S of the ferromagnetic metal wire during the energization stage includes the following steps:

[0049] S51, use a signal generator to generate a sinusoidal alternating current with a set frequency, and then amplify the alternating current through a power amplifier to apply an alternating current with a frequency of 50 Hz and an amplitude of 2.5 A to the ferromagnetic metal wire;

[0050] S52, use a six-and-a-half-digit digital multimeter to monitor the current amplitude I m on the ferromagnetic metal wire, and at the same time use the method of cooperating an oscilloscope with a current clamp to monitor the current frequency f on the ferromagnetic metal wire;

[0051] S53, use an infrared thermal imager with a frame rate of 200 fps and a resolution of 640×120 to collect the surface temperature of the ferromagnetic metal wire, so as to obtain the infrared thermal image sequence;

[0052] S54, perform infrared recording through FLIR Research software. On its operation page, by selecting the detection point, the temperature amplitude on the ferromagnetic metal wire and the temperature-time curve in the time domain can be viewed in real time.

[0053] Preferably, in the step S6, the extraction of the temperature amplitude at the same frequency under each current amplitude by using Fourier transform includes the following steps:

[0054] S61, based on the narrow and thin planar structure characteristics of the ferromagnetic metal wire, the temperature distribution after its energization is represented by a one-dimensional heat transfer model, and the specific formula is:

[0055]

[0056] Among them, let the length of the ferromagnetic metal wire be expressed as \(l = 2a\), and a one-dimensional linear coordinate system is established with the center of the ferromagnetic metal wire as the origin \(O\); \(-a\) and \(a\) respectively represent the left and right two endpoints on the one-dimensional linear coordinate system of the ferromagnetic metal wire; \(x\) represents any point on the one-dimensional linear coordinate system of the ferromagnetic metal wire; \(T()\) represents the temperature of the spatial point in the one-dimensional linear coordinate system on the one-dimensional linear coordinate system at the energization time \(t\); \(k\) represents the thermal conductivity of the ferromagnetic metal wire; \(c\) represents the specific heat capacity of the ferromagnetic metal wire, \(\rho\) represents the mass density of the ferromagnetic metal wire, \(P(t)\) represents the heat source caused by the alternating current; \(C\) represents the end temperature of the ferromagnetic metal wire; \(T_0\) represents the initial temperature of the ferromagnetic metal wire;

[0057] S62. Obtain the heat source \(P(t)\) caused by the alternating current based on the electrical parameter structure of the equivalent circuit energized on the ferromagnetic metal wire, that is:

[0058]

[0059]

[0060] Among them, \(U\) represents the circuit voltage; \(I\) represents the circuit current; \(R\) is the resistance of the ferromagnetic metal wire;

[0061] \(E\) represents the absolute value of the induced electromotive force caused by the bias magnetic field on the ferromagnetic metal wire; \(I\) m represents the alternating current amplitude; \(f\) represents the alternating current frequency; \(t\) represents the time when the alternating current is applied to the ferromagnetic metal wire; \(\varPhi\) represents the total magnetic flux of the closed total magnetic circuit and is also the magnetic flux on the ferromagnetic metal wire; \(\rho_0\) represents the resistivity of the ferromagnetic metal wire; \(S_0\) represents the cross-sectional area of the ferromagnetic metal wire; \(\mu\) b represents the average magnetic permeability of the ferromagnetic metal wire; \(\mu\) s represents the average magnetic permeability of the external magnetic circuit; \(S\) represents the average cross-sectional area of the external magnetic circuit; \(S\) b represents the average cross-sectional area of the ferromagnetic metal wire; \(H\) c represents the coercive force of the permanent magnet; \(l\) m represents the length of the permanent magnet in the magnetic flux direction; \(d\) represents the length of the ferromagnetic metal wire in the magnetic flux direction; \(l\) s represents the length of the external magnetic circuit in the magnetic flux direction;

[0062] S63. Combine the above (Equation 15) and (Equation 16), and obtain the analytical solution of the one-dimensional heat transfer model based on the superposition method, that is expressed as:

[0063]

[0064] Among them, T f represents the temperature amplitude with the same frequency as the current; T 2f represents the temperature amplitude twice the frequency of the alternating current; T x represents the temperature at point x on the ferromagnetic metal wire; T s is used to describe the instantaneous process between the initial state and the steady state of the temperature and converges to a constant in the form of an exponential function; e represents the base of the exponential function; n represents a set of natural numbers;

[0065] S64. Based on the analytical solution of the one-dimensional heat transfer model, analyze the structural correlation and influencing factors of the temperature parameters on the ferromagnetic metal wire, and thereby obtain a quantitative detection formula for the amplitude of the alternating current under the DC bias magnetic field:

[0066]

[0067] Among them, T m represents the amplitude of the temperature with the same frequency as the current T f of the amplitude, that is, the amplitude of the single-frequency temperature fluctuation, then there is:

[0068] S65. According to the magnetic permeability characteristics of the external magnetic circuit and the ferromagnetic metal wire, ignore the influence of the term, then rewrite the formula as:

[0069] Preferably, in the step S7, using the one-dimensional Fourier series to fit the temperature amplitude curves at different current amplitudes includes the following steps:

[0070] S71. The relationship between the temperature amplitude and the current amplitude is expressed according to the one-dimensional Fourier series as:

[0071] T m = a0 + a1cos(w·I m ) + b1sin(w·I m ) (Equation 20);

[0072] Among them, a0, a1, and b1 represent Fourier coefficients; w represents the angular frequency; I m represents the amplitude of the alternating current; T m represents the amplitude of the single-frequency temperature fluctuation;

[0073] S72. Solve the inverse function of (Equation 20), that is:

[0074] T m – a0 = a1cos(w·I m ) + b1sin(w·I m ) (Equation 21);

[0075] S73. According to the properties of trigonometric functions, rewrite the inverse function as:

[0076]

[0077] where Z represents the amplitude of the function, and represents the phase angle of the function, satisfying

[0078]

[0079] S74. According to the interval properties of the function, rewrite the above (Equation 22) as:

[0080]

[0081] S75. Combining the domain analysis of the arcsin() function in (Equation 23), obtain the expression for remote detection of alternating current based on infrared thermal imaging:

[0082] Preferably, in the step S8, the method for measuring the alternating current on the ferromagnetic metal wire according to the fitting curve is as follows: increase the current in steps of 0.15 A, apply a power frequency alternating current of 0.1 A - 4.95 A to the metal wire. During this process, extract the temperature amplitudes at the same frequency under different current amplitudes; substitute the temperature amplitudes at the same frequency under different current amplitudes extracted into the expression for remote detection of alternating current to obtain the measured current amplitude.

[0083] Preferably, in the step S7, using the one-dimensional Fourier series to fit the temperature amplitude curves under different current amplitudes further includes verification of the fitting degree, that is, calculating the determination coefficient R 2 and the root mean square error RMSE, where:

[0084]

[0085] where y i represents the actual value of T m , represents the predicted value of T m ; represents the mean value of the actual value of T m , M represents the number of samples of T m ; i represents the sample number of T m ; the regression sum of squares

[0086] represents the sum of the squares of the errors between the predicted value and the actual value; the total sum of squares

[0087] represents the sum of the squares of the differences between the actual value and its mean value.

[0088] Advantages of the present invention:

[0089] 1) Realize long-distance non-contact detection: Compared with traditional current detection technologies, invasive methods rely on Ohm's law to measure voltage through a sampling resistor, and non-invasive methods are based on Faraday's law of electromagnetic induction to measure magnetic fields. Both methods have the problems that the sensor needs to be in direct physical contact with the circuit or be arranged around the circuit to be measured, resulting in a short detection distance and potential safety hazards. The present invention is based on infrared thermal imaging technology and achieves long-distance detection of alternating current through a non-contact detection method. For example, in the inspection scenario of high-voltage power transmission and distribution lines, it can not only ensure the safety of operators but also effectively improve the inspection efficiency, significantly reducing the workload and risks. At the same time, the present invention uses an infrared thermal imager as the sensor, avoiding direct contact with the conductor to be measured. During detection, no special magnetic core material is required, and the conductor to be measured does not need to pass through the detection coil, greatly relaxing the restrictions on the detection position and area of the conductor to be measured.

[0090] 2) Reduce the frame rate requirement for the thermal imager: When using infrared thermal imaging to detect current, generally, the temperature fluctuation frequency is twice the frequency of the alternating current. Therefore, usually, the frame rate of the thermal imager needs to be greater than four times the frequency of the alternating current to meet the sampling theorem for extracting temperature fluctuations. However, high-frame-rate thermal imagers are costly and not suitable for large-scale use. The present invention, through a series of technical means such as applying magnetic bias, deeply combines the analysis of magnetic parameters and electrical parameters of ferromagnetic metal wires, and establishes a one-dimensional heat transfer model, etc., to successfully extract the temperature amplitude at the same frequency as the current. This innovative method significantly reduces the stringent requirement for the frame rate of the thermal imager to a certain extent, strongly promoting the popularization and application of this detection method. Specifically, the present invention adopts a mathematical model from the thermal field to the electric field under the influence of a bias magnetic field, makes full use of the energy conversion between the thermal field and the electric field, and ingeniously realizes the measurement of alternating current based on infrared thermal imaging, thereby reducing the demand for the frame rate of the infrared thermal imager.

[0091] 3) Rigorous theoretical analysis and calculation: The present invention conducts detailed theoretical analysis and formula derivation for each key step. From the careful assembly of the permanent magnet type magnetizer, to the in-depth analysis of the magnetic parameter structure of the closed total magnetic circuit and the electrical parameter structure of the equivalent circuit with current on the ferromagnetic metal wire, to the accurate acquisition of the temperature amplitude, precise curve fitting, and accurate current measurement, etc., all have precise formulas and calculation methods. Moreover, the goodness of fit is strictly verified. By calculating the coefficient of determination and the root mean square error, the scientificity and accuracy of the detection method are fully guaranteed. The present invention uses a permanent magnet type magnetizer to provide a bias magnetic field, accurately extracts the temperature response signal through Fourier transform, and uses the one-dimensional Fourier series fitting method to inversely calculate the alternating current on the metal wire, effectively improving the signal-to-noise ratio at the system method level.

[0092] 4) The method is specific and operable: For each step of the present invention, detailed and specific implementation manners are given, covering the assembly steps of the permanent magnet type magnetizer, the calculation formulas of various parameters, the specific equipment and operations for collecting the thermal image sequence, the temperature amplitude extraction method, the curve fitting process, and the current measurement method, etc. This makes the detection method highly operable and provides reliable technical support for practical applications.

[0093] 5) Principle innovation and performance optimization: The alternating current detection method proposed by the present invention is based on the principle of the same frequency of temperature and current under the magnetic bias effect, and combines the heat conduction theory and Joule's law to create a new non-contact alternating current detection method. This method not only overcomes the shortcoming of introducing a bypass in the traditional contact current detection method and avoids the influence of the traditional contact measurement circuit on the circuit to be measured, but also improves the situation where it is difficult to balance the measurement range and detection accuracy in the non-contact magnetic detection method. At the same time, while retaining the advantage of long-distance detection of the infrared thermal imaging technology, it also has the characteristics of large-area detection and high resolution, and has a wide range of application prospects. Brief Description of the Drawings

[0094] Figure 1 is the basic implementation flowchart of the present technical solution;

[0095] Figure 2 is the schematic assembly structure diagram of the permanent magnet type magnetizer;

[0096] Figure 3 is the axonometric structure diagram of the permanent magnet type magnetizer;

[0097] Figure 4 is the equivalent circuit diagram after the ferromagnetic metal wire is energized;

[0098] Figure 5 is the schematic layout principle diagram of the electrothermal detection system;

[0099] Figure 6 is the infrared thermal image of the ferromagnetic metal wire when passing through a 50Hz, 2.5A alternating current;

[0100] Figure 7 is the time-domain - frequency-domain diagram of the temperature fluctuation example;

[0101] Figure 8 is the relationship curve diagram between the temperature amplitude at the same frequency as the current and the bias magnetic field strength;

[0102] Figure 9 is the schematic diagram of the relationship between the temperature amplitude and the current amplitude;

[0103] Figure 10 is the comparison diagram between the measured current amplitude and the standard current amplitude;

[0104] Figure 11Schematic diagram of relative error between measured current and standard current.

[0105] In the figure:

[0106] 1. Permanent magnet group; 2. Magnetic conducting armature; 3. Ferromagnetic metal wire; 4. Insulating ceramic; 5. Infrared thermal imager; 6. Permanent magnet type magnetizer; 7. Alligator clip; 8. Function waveform generator; 9. Power amplifier; 10. Digital multimeter; 11. Oscilloscope; 12. Current clamp; 13. Computer; 14. Through hole. Specific implementation mode

[0107] To make the objectives, technical solutions and advantages of the invention more clear, the technical solutions in the present invention will be clearly and completely described below with reference to the accompanying drawings in the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments.

[0108] Therefore, the detailed description of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without creative efforts fall within the scope of protection of the present invention.

[0109] Embodiment 1

[0110] This embodiment discloses a magnetic bias alternating current detection method based on infrared thermal imaging. As a basic implementation solution of the present technical solution, as Figure 1 shown, it includes the following steps:

[0111] S1. Assemble a permanent magnet type magnetizer. Based on the permanent magnet type magnetizer and the air therein, an external magnetic circuit is formed, laying a foundation for applying a bias magnetic field to the ferromagnetic metal wire later and providing a stable magnetic field environment. Specifically: Select a permanent magnet material with appropriate specifications and precisely assemble it into a permanent magnet type magnetizer according to specific design requirements. The atomic magnetic moments inside the permanent magnet present an orderly arrangement state due to the crystal structure and electron spin characteristics of the material, thus being able to generate a stable magnetic field. When the permanent magnet is combined with the surrounding air, according to Ohm's law of magnetic circuits, the distribution of magnetic flux is inversely proportional to the magnetic permeability. The magnetic permeability of air is approximately 1, while the magnetic permeability of the permanent magnet is much greater than 1, which makes the magnetic force lines tend to flow in the path formed by the permanent magnet and air, thereby forming an external magnetic circuit.

[0112] S2. Place a ferromagnetic metal wire in an external magnetic circuit through an insulating ceramic, and apply a bias magnetic field to the ferromagnetic metal wire using a permanent magnetizer. The ferromagnetic metal wire and the external magnetic circuit together form a closed total magnetic circuit. Specifically, there are a large number of magnetic domains in the internal structure of the ferromagnetic metal. Under the action of an external magnetic field, these magnetic domains will quickly adjust their directions and tend to be consistent with the direction of the external magnetic field, thereby significantly increasing the magnetic flux inside the metal wire. When the ferromagnetic metal wire is placed in the external magnetic circuit, the bias magnetic field generated by the permanent magnetizer will further increase the magnetic flux inside the ferromagnetic metal wire, and the magnetic force lines form a closed loop through the ferromagnetic metal wire and the external magnetic circuit, ultimately forming a closed total magnetic circuit. In this embodiment, the ferromagnetic metal wire is made of martensitic stainless iron, and its dimensions are 90 mm × 2 mm × 0.03 mm (length × width × height). Place the ferromagnetic metal wire in a specific magnetic field environment and make full use of its high magnetic permeability characteristics to create favorable conditions for the subsequent current detection process.

[0113] S3. Analyze the magnetic parameter structure of the closed total magnetic circuit, including parameters such as magnetic permeability and magnetic flux, to obtain the magnetic field strength H in the radial direction of the ferromagnetic metal wire. b . Professional magnetic field analysis software, such as ANSYS Maxwell, can be used to model and simulate the closed total magnetic circuit. At the same time, combined with high-precision magnetic field measurement instruments, such as Hall effect magnetic field sensors, the actual magnetic field distribution can be measured and verified. According to Ampere's circuital law, the line integral of the magnetic field strength along a closed path is equal to the algebraic sum of the currents enclosed by the path. In the closed total magnetic circuit, by analyzing parameters such as magnetic permeability and magnetic flux, combined with Ampere's circuital law and the magnetic characteristic curve of the material (such as the B-H curve), the magnetic field strength distribution in the radial direction of the ferromagnetic metal wire can be accurately calculated. Obtaining the magnetic field strength in the radial direction of the ferromagnetic metal wire is a key link connecting magnetism and electricity. This parameter is of great significance for subsequent analysis of the electrical parameter structure and establishing the relationship between current and other physical quantities.

[0114] S4. Analyze the electrical parameter structure of the equivalent circuit when the ferromagnetic metal wire is energized in combination with the magnetic parameter structure. Based on electromagnetic theory, use circuit analysis software, such as Multisim, to establish an equivalent circuit model after the ferromagnetic metal wire is energized. Through the simulation analysis of the model and combined with the actually measured magnetic parameters, electrical parameters such as resistance and inductance in the equivalent circuit are determined. When the ferromagnetic metal wire is energized, the current will generate a magnetic field, and this magnetic field interacts with the external bias magnetic field. At the same time, the ferromagnetic metal wire itself has electrical characteristics such as resistance and inductance. According to Faraday's law of electromagnetic induction and Ohm's law, by analyzing the magnetic parameter structure, an equivalent circuit model when energized can be established, and then electrical parameters such as resistance and inductance can be calculated. Combine the magnetic and electrical characteristics organically to comprehensively and deeply understand the physical process of the ferromagnetic metal wire in the energized state. This lays a solid theoretical foundation for subsequent detection using the relationship between current and temperature.

[0115] S5. Pass an alternating current through the ferromagnetic metal wire and monitor the electrical signal. Under the action of the bias magnetic field of the permanent magnetizer, use an infrared thermal imager to collect the original thermal image sequence S of the ferromagnetic metal wire during the power-on stage. A high-precision AC power supply can be used to provide a stable alternating current for the ferromagnetic metal wire. Use an oscilloscope to monitor the electrical signal in real time, including parameters such as the waveforms and amplitudes of the current and voltage. Adopt a high-resolution infrared thermal imager to collect the original thermal image sequence of the ferromagnetic metal wire at set time intervals. According to Joule's law, heat is generated when an electric current passes through a conductor, and the heat is proportional to the square of the current, the resistance, and the time. After passing an alternating current through the ferromagnetic metal wire, it will heat up, and the surface temperature distribution will change. The infrared thermal imager works based on the principle of thermal radiation of an object. The thermal radiation intensity of an object is related to the temperature. By detecting the infrared radiation intensity on the surface of the ferromagnetic metal wire, it can be converted into a temperature distribution image, thus collecting the original thermal image sequence. Obtain accurate current signals and thermal image data, and these original data are important bases for subsequent extraction of temperature amplitudes and measurement of current.

[0116] S6. Based on the current signal and the infrared thermal image sequence S, combine the magnetic parameter structure and the electrical parameter structure, and use Fourier transform to extract the temperature amplitude at the same frequency for each current amplitude. Digital signal processing technology can be used, and software platforms such as MATLAB are used to process the collected current signal and infrared thermal image sequence. By writing a special algorithm, the Fourier transform of the signal is realized, and the temperature amplitude at the same frequency for each current amplitude is extracted. The essence of Fourier transform is to decompose a function in the time domain or spatial domain into the superposition of sine and cosine functions of different frequencies. In this step, both the current signal and the temperature change signal contain multiple frequency components, but the temperature change frequency related to the current is the same as the current frequency. Through Fourier transform, the complex time-domain signal can be converted into a frequency-domain signal, so as to accurately extract the temperature amplitude at the same frequency for each current amplitude. Establish a quantitative relationship between current and temperature, providing key data support for subsequent current measurement.

[0117] S7. Use one-dimensional Fourier series to fit the temperature amplitude curves at different current amplitudes. Professional data fitting software, such as Origin, can be used to analyze and fit the temperature amplitude curves at different current amplitudes. By adjusting the fitting parameters, the fitting results can reflect the variation law of the actual data as accurately as possible. One-dimensional Fourier series can represent a periodic function as a linear combination of a series of sine and cosine functions. The temperature amplitude curves at different current amplitudes have certain periodicity and regularity. Through Fourier series fitting, this relationship can be described by a mathematical model, and the coefficients of the curve can be determined, thus obtaining an accurate mathematical model. Transforming complex experimental data into a concise mathematical model facilitates subsequent analysis and calculation. This not only improves the accuracy and reliability of measurement but also provides convenience for further theoretical research and application development.

[0118] S8. Measure the alternating current on the ferromagnetic metal wire according to the fitting results, realizing the conversion from temperature measurement to current measurement and achieving the ultimate goal of this detection method. Specifically, the obtained mathematical model can be applied to the actual measurement process. By measuring the temperature amplitude of the ferromagnetic metal wire and using the pre-established mathematical relationship, the magnitude of the corresponding alternating current can be calculated. Through the previous steps, the mathematical relationship between the temperature amplitude and the current amplitude is established. According to this relationship, when the temperature amplitude is measured, the magnitude of the alternating current on the ferromagnetic metal wire can be deduced through mathematical calculation. Realizing the conversion from temperature measurement to current measurement and achieving the ultimate goal of this detection method provides accurate current data for the operation monitoring of power systems and electrical equipment.

[0119] In summary, this magnetic bias alternating current detection method based on infrared thermal imaging, through the coordinated action of multiple steps, from magnetic field construction to data acquisition, processing, and analysis, comprehensively uses a variety of advanced technical means, deeply explores the basic principles of electromagnetism and thermology, and finally realizes the accurate measurement of alternating current. This method has the advantages of innovation, high efficiency, and accuracy, and has broad application prospects in the fields of electric power, electronics, etc.

[0120] Embodiment 2

[0121] This embodiment discloses a magnetic bias alternating current detection method based on infrared thermal imaging. As a preferred implementation of this technical solution, that is, based on Embodiment 1, in step S1 of assembling the permanent magnet type magnetizer, the following steps are included:

[0122] S11. Before starting to assemble the permanent magnet magnetizer, it is necessary to accurately prepare suitable materials first. Selecting suitable magnetic yokes and permanent magnets can ensure the formation of a stable and required magnetic field environment subsequently, providing a basis for the entire detection process. Specifically, prepare a magnetic yoke with excellent magnetic conductivity and two groups of permanent magnets. Each group of permanent magnets consists of multiple single permanent magnet pieces with stable performance and consistent specifications. Professional material analysis instruments such as X-ray diffractometer (XRD) can be used to detect the crystal structure of the materials, and a vibrating sample magnetometer (VSM) can be used to measure the magnetic performance parameters of the permanent magnets, so as to strictly screen the magnetic yokes and permanent magnets. In this embodiment, the size of the single permanent magnet piece is 70mm×45mm×10mm (length×width×height). Using a high-precision tesla meter, the magnetic field intensity at the center position is measured to be 4000 Oe, and at the edge position is 2800 Oe. The performance parameters such as remanence and coercivity must highly conform to the design requirements. The magnetic yoke is made of pure iron with high magnetic permeability, and its size is 90mm×20mm×5mm (length×width×height). Through precise machining processes such as numerical control milling and wire cutting, the dimensional accuracy is ensured. Its function is to guide the magnetic flux and provide a magnetic field intensity parallel to the axis inside the ferromagnetic metal wire.

[0123] S12. After the materials are prepared, as Figure 2 and Figure 3 shown, place the two groups of permanent magnets parallel in a specific manner, and ensure that the magnetization directions of the two groups of permanent magnets are in opposite states. This setting of the reverse magnetization direction can form a unique magnetic field line distribution in the subsequently constructed magnetic field, laying a foundation for the entire detection method.

[0124] S13. Use high-precision numerical control drilling equipment to finely drill through holes on the magnetic yoke according to the pre-designed positions and sizes. The positions and sizes of the drilling are accurately calculated, which can not only ensure that the magnetic conductivity is not affected, but also effectively release the heat generated during the operation of the magnetic circuit, facilitating subsequent temperature acquisition on the surface of the ferromagnetic metal wire.

[0125] S14. After completing the through-hole processing of the magnetic yoke, place the magnetic yoke flatly between the two groups of permanent magnets, and ensure that the magnetization directions of the two permanent magnets are both perpendicular to the axis direction of the yoke. In this way, a permanent magnet magnetizer with a stable structure and reliable performance is assembled, and its internal magnetic field interacts with the air to jointly form an external magnetic circuit.

[0126] In this technical solution, the assembly principle of the permanent magnet magnetizer takes into account the characteristics of permanent magnets, magnetic field distribution, and the formation of the external magnetic circuit, where:

[0127] Properties of permanent magnet: Due to the crystal structure and electron spin characteristics of the permanent magnet material, the atomic magnetic moments are arranged in an orderly state, thus generating a stable magnetic field. High remanence ensures that the permanent magnet can still maintain a strong magnetic field without external excitation, and high coercivity makes the permanent magnet more resistant to external magnetic field interference. In this embodiment, the magnetic field parameters of a single permanent magnet determine the stability and intensity range of the generated magnetic field.

[0128] Magnetic field distribution: When two groups of permanent magnets with opposite magnetization directions are placed in parallel, a specific magnetic field distribution area will be formed between them. The magnetic force lines start from the N pole of one group of permanent magnets, pass through the air and the magnetic conducting armature, and enter the S pole of the other group of permanent magnets, forming a closed magnetic circuit. Due to the high magnetic permeability of the magnetic conducting armature, it can guide the magnetic force lines to concentrate on a specific path, enhancing the magnetic field intensity and stability. The through holes are opened on the magnetic conducting armature because when the permanent magnet type magnetizer works, heat will be generated due to magnetic hysteresis loss and eddy current loss, etc. The through holes can promote air circulation and heat dissipation, prevent the temperature from being too high and affecting the magnetism of the permanent magnet and the performance of the entire magnetizer, and at the same time create conditions for accurately collecting the temperature of the ferromagnetic metal wire later.

[0129] Formation of external magnetic circuit: According to Ohm's law of magnetic circuit, the magnetic flux distribution is inversely proportional to the magnetic permeability. The magnetic permeability of air is approximately 1, while the magnetic permeabilities of the permanent magnet and the magnetic conducting armature are much greater than 1, making the magnetic force lines more inclined to flow in the path composed of the permanent magnet, the magnetic conducting armature and air, thus forming an external magnetic circuit.

[0130] In summary, this technical solution constructs a stable and excellent permanent magnet type magnetizer and external magnetic circuit environment. The stable magnetic field condition is the key to applying a stable bias magnetic field to the ferromagnetic metal wire later. Only in such a magnetic field environment can the accuracy and reliability of the entire detection process be ensured.

[0131] Embodiment 3

[0132] This embodiment discloses a method for detecting magnetic bias alternating current based on infrared thermal imaging. As a preferred implementation of this technical solution, that is, based on Embodiment 2, in step S3, analyzing the magnetic parameter structure of the closed total magnetic circuit includes the following steps:

[0133] S31, obtain the magnetomotive force and magnetic resistance of each component. Let i represent the components in the closed magnetic circuit, that is, i = m, a, b, c; m represents the permanent magnet; a represents the magnetic conducting armature; b represents the ferromagnetic metal wire; c represents the air; F i represents the magnetomotive force of component i; H c represents the coercivity of the permanent magnet; l i represents the length of component i in the magnetic flux direction; μ i represents the magnetic permeability of component i, S i represents the projected area of component i perpendicular to the magnetic flux direction; Ri represents the magnetic resistance of component i. The geometric parameters and magnetic properties of each component jointly determine the magnetic potential and magnetic resistance. Then, according to the geometric parameters and magnetic properties of each component in the closed total magnetic circuit, the magnetic potential F of each component is obtained. i and the magnetic resistance R i , and the specific calculation formula is expressed as:

[0134] F i = H c l i (Equation 1) and

[0135] The magnetic potential is similar to the electromotive force in an electric circuit and is the source that drives the magnetic flux, which is determined by the magnetomotive force and the magnetic circuit length; the magnetic resistance is similar to the resistance in an electric circuit and hinders the passage of the magnetic flux, which is related to the magnetic permeability, the magnetic circuit length, and the cross-sectional area. According to the basic theory of the magnetic circuit, through the geometric parameters and magnetic properties of each component, the magnetic potential and magnetic resistance can be calculated using Equations (1) and (2). High-precision measuring instruments, such as laser scanners, can be used to obtain the accurate geometric parameters of each component (permanent magnet, magnetic conducting armature, ferromagnetic metal wire, air) in the closed total magnetic circuit, including the length, cross-sectional area, etc. Professional material analysis equipment, such as a vibrating sample magnetometer (VSM), is used to measure the magnetic properties of each component, such as the magnetic permeability, etc. Substitute these measurement data into Equations (1) and (2) to accurately calculate the magnetic potential and magnetic resistance of each component.

[0136] S32, calculate the total magnetic resistance. Let l s represent the length of the outer magnetic circuit in the direction of the magnetic flux; μ s represent the average magnetic permeability of the outer magnetic circuit; S represents the average cross-sectional area of the outer magnetic circuit; d represents the length of the ferromagnetic metal wire in the direction of the magnetic flux, which is also its own width; μ b represent the average magnetic permeability of the ferromagnetic metal wire; S b represent the average cross-sectional area of the ferromagnetic metal wire; μ0 represents the magnetic permeability of vacuum. Taking the ferromagnetic metal wire as a magnetic thermal radiation sensor, the magnetic resistance R s of the outer magnetic circuit and the magnetic resistance R b of the magnetic thermal radiation sensor together constitute the total magnetic resistance of the closed total magnetic circuit; the calculation formulas for the magnetic resistance R s of the outer magnetic circuit and the magnetic resistance R b of the magnetic thermal radiation sensor are: and

[0137] The outer magnetic circuit and the ferromagnetic metal wire form a closed total magnetic circuit, and their magnetic resistances jointly determine the total magnetic resistance. According to the series characteristic of magnetic resistance, the magnetic resistance of the outer magnetic circuit and the magnetic resistance of the magnetic thermal radiation sensor are added to obtain the total magnetic resistance, and the calculation is carried out using Equation (3) and Equation (4). Thus, regarding the ferromagnetic metal wire as a magnetic thermal radiation sensor, a high-precision resistance measuring instrument and a magnetic field measuring device are used to measure the relevant parameters of the outer magnetic circuit and the ferromagnetic metal wire, such as length, magnetic permeability, cross-sectional area, etc., and then substitute them into Equation (3) and Equation (4) to calculate the magnetic resistance of the outer magnetic circuit and the magnetic resistance of the magnetic thermal radiation sensor, and further obtain the total magnetic resistance of the closed total magnetic circuit.

[0138] S33. Let F represent the total magnetomotive force of the closed total magnetic circuit; Φ represent the total magnetic flux of the closed total magnetic circuit, which is also the magnetic flux on the ferromagnetic metal wire; H b represent the average magnetic field intensity on the ferromagnetic metal wire, and H s represent the average magnetic field intensity of the outer magnetic circuit. According to Kirchhoff's law, describe the magnetic flux distribution in the closed total magnetic circuit, that is:

[0139] F = Φ(R s + R b ) = H s l s + H b d (Equation 5).

[0140] Kirchhoff's law is also applicable in the magnetic circuit, which describes the relationship between magnetomotive force, magnetic flux and magnetic resistance in the magnetic circuit. Through Equation (5), the magnetic flux distribution in the closed total magnetic circuit can be clearly expressed. Thus, according to Kirchhoff's law, with the help of circuit analysis software, such as Multisim, an equivalent circuit model of the closed total magnetic circuit is established. Through the simulation analysis of the model and combined with the measurement data, substitute them into Equation (5) to accurately describe the magnetic flux distribution in the closed total magnetic circuit.

[0141] S34. Under the condition of not considering the leakage magnetic flux, the magnetic flux in the entire closed total magnetic circuit is continuous, that is:

[0142] μ b μ0S b H b = μ s μ0H s s (Equation 6).

[0143] Ideally, when the leakage flux is not considered, the magnetic flux in the closed total magnetic circuit is continuous, that is, the magnetic flux entering a certain section is equal to the magnetic flux leaving the section, which conforms to the basic law of conservation of magnetic flux, expressed by Formula (6). Therefore, in the experimental environment, the influence of leakage flux is minimized by optimizing the experimental device and measurement method. Using high-precision flux measurement equipment, such as Hall effect flux sensors, the magnetic flux in the closed total magnetic circuit is measured at multiple points to verify the continuous characteristics of the magnetic flux in Formula (6).

[0144] S35, combine the above (Equation 1) to (Equation 6) to obtain the average magnetic field strength of the ferromagnetic metal wire, which is the radial magnetic field strength H of the ferromagnetic metal wire. b , then: Among them, l m Represents the length of the permanent magnet in the direction of magnetic flux.

[0145] By combining the previous formulas and eliminating the intermediate variables, the average magnetic field strength calculation formula (Formula 7) of the ferromagnetic metal wire is obtained, which is derived based on the basic theory of magnetic circuits and the relationship between the various parts. Therefore, the various parameters and data obtained in the previous steps are substituted into Formulas (Formula 1)-(Formula 6), and mathematical calculation software, such as Mathematica, is used to perform a combined solution to obtain the average magnetic field strength of the ferromagnetic metal wire, that is, its radial magnetic field strength, as shown in Formula (Formula 7).

[0146] Example 4

[0147] This embodiment discloses a magnetic bias alternating current detection method based on infrared thermal imaging, as a preferred implementation scheme of this technical solution, that is, based on Example 1, 2 or 3, in step S4, the equivalent circuit of the ferromagnetic metal wire is as follows: Figure 4 As shown in the figure, analyzing the electrical parameter structure of the equivalent circuit with electricity flowing through the ferromagnetic metal wire includes the following steps:

[0148] S41, let U represent the circuit voltage, I represent the circuit current, R represent the resistivity of the ferromagnetic metal wire, e0 represent the leakage electromotive force, e represent the induced electromotive force caused by the bias magnetic field on the ferromagnetic metal wire; L represent the equivalent inductance in the equivalent circuit of the current; represents the total magnetic flux of the closed total magnetic circuit, which is also the magnetic flux on the ferromagnetic metal wire; I m represents the amplitude of the alternating current; f represents the frequency of the alternating current; and t represents the time for which the alternating current is passed through the ferromagnetic metal wire. According to Kirchhoff's voltage law, the equivalent circuit of the ferromagnetic metal wire is expressed as:

[0149] U = IR - e - e0 (Formula 8);

[0150]

[0151] I = I m cos(2πft) (Equation 11).

[0152] Kirchhoff's law states that at any given moment, the algebraic sum of the potential differences (voltages) across all elements in a closed loop is equal to zero. Using high-precision voltage and current measuring instruments, such as digital oscilloscopes and power analyzers, the voltage and current in the equivalent circuit of a ferromagnetic metal wire when it is energized are monitored in real time. In the equivalent circuit of the ferromagnetic metal wire when it is energized, based on this law and combined with the characteristics of each element in the circuit, such as resistance, inductance, electromotive force, etc., with the help of circuit analysis software, such as Multisim, the circuit is modeled to establish Equations (8) - (11), which describe the relationship between the voltage and current in the circuit and accurately represent the equivalent circuit of the ferromagnetic metal wire when it is energized.

[0153] S42, under the condition of neglecting the leakage electromotive force e0, then: U = IR - e (Equation 12). The leakage electromotive force is usually generated by factors such as stray capacitance and electromagnetic interference in the circuit. When it is found through experimental measurement and analysis that its influence on the main parameters of the circuit is extremely small and within the allowable error range, it can be ignored, thereby simplifying the circuit equation to obtain (Equation 12), which is convenient for subsequent analysis and calculation. This technical solution evaluates the magnitude of the leakage electromotive force and its influence on the entire circuit through multiple experimental measurements and data analyses. After confirming that its influence can be ignored, based on the experimental results and theoretical analysis, the formula is simplified to obtain (Equation 12).

[0154] S43, let μ b represent the average magnetic permeability of the ferromagnetic metal wire; S b represent the average cross-sectional area of the ferromagnetic metal wire; μ s represent the average magnetic permeability of the external magnetic circuit; S represents the average cross-sectional area of the external magnetic circuit; H c represent the coercive force of the permanent magnet; l m represent the length of the permanent magnet in the magnetic flux direction; d represents the length of the ferromagnetic metal wire in the magnetic flux direction; l s represent the length of the external magnetic circuit in the magnetic flux direction. Combining the magnetic parameter structure, the magnetic flux Φ on the ferromagnetic metal wire is deduced, that is: The magnetic flux is related to factors such as magnetic field strength, magnetic permeability, and cross-sectional area. In the closed total magnetic circuit, based on the basic theory of the magnetic circuit and the magnetic relationship between each part, combined with the magnetic parameter structure obtained in step S3, using professional magnetic field analysis software, such as ANSYS Maxwell, the closed total magnetic circuit is analyzed in depth. Substituting the relevant magnetic parameters of the ferromagnetic metal wire and the external magnetic circuit, such as average magnetic permeability, average cross-sectional area, and the length of each part, into Equation (13), the magnetic flux on the ferromagnetic metal wire is accurately deduced.

[0155] S44. Use a mathematical calculation software such as Mathematica to simultaneously solve equations (8)-(13). Through precise mathematical operations, eliminate the intermediate variables to obtain the circuit voltage, that is, the voltage expression on the ferromagnetic metal wire:

[0156] Among them, E represents the absolute value of the induced electromotive force caused by the bias magnetic field on the ferromagnetic metal wire, that is, E = |e|. By simultaneously solving multiple equations, unknown quantities can be solved using known quantities. In this step, the equations describing the relationship of circuit electrical parameters and the equations related to magnetic parameters are simultaneously solved to eliminate the intermediate variables, thereby obtaining the voltage expression (14) on the ferromagnetic metal wire. This is a comprehensive derivation based on mathematical operations and physical principles.

[0157] This technical solution comprehensively and deeply understands the electrical parameter structure of the ferromagnetic metal wire in the energized state. By analyzing the relationships among parameters such as circuit voltage, current, and magnetic flux, the magnetic and electrical characteristics are organically combined, laying a solid theoretical foundation for subsequent detection using the relationship between current and temperature, and also providing a basis for further optimizing the detection method and improving the detection accuracy.

[0158] Example 5

[0159] This example discloses a method for detecting magnetic bias alternating current based on infrared thermal imaging. As a preferred implementation of this technical solution, that is, based on Example 1, 2, 3, or 4, in step S5 of it, collecting the original thermal image sequence S of the ferromagnetic metal wire during the energized stage includes the following steps:

[0160] S51. According to the layout shown in Figure 5 , deploy the energized thermal detection system to ensure the stable and correct connection between devices. The electrothermal detection system includes an infrared thermal imager, a computer, a power amplifier, a signal generator, a digital multimeter, a current clamp, and an oscilloscope. Use professional communication cables to achieve a reliable communication connection between the computer and the infrared thermal imager. Install the infrared thermal imager precisely above the permanent magnetizer. By adjusting parameters such as the focal length and angle of the thermal imager, it can clearly capture the infrared radiation signal of the ferromagnetic metal wire. Select a function waveform generator with excellent performance as the signal generator, connect its output end to the power amplifier using a standard signal transmission line, and connect the output of the power amplifier to one end of the ferromagnetic metal wire through an alligator clip. Use a current clamp, tightly connect it to the other end of the ferromagnetic metal wire through an alligator clip, and correctly connect the current clamp to the oscilloscope. Select a six-and-a-half-digit digital multimeter, and use special test leads to connect its two ends to the power amplifier and the current clamp respectively to ensure the accuracy of measurement.

[0161] Among them, the energization principle of the energized thermal detection system is as follows: The sinusoidal alternating current signal generated by the signal generator has its frequency and amplitude that can be precisely controlled through setting. The function of the power amplifier is to amplify the relatively weak electrical signal output by the signal generator so that it has sufficient power to drive the ferromagnetic metal wire. According to the principle of electromagnetic induction, a changing current will generate a magnetic field around the ferromagnetic metal wire. The current clamp utilizes the principle of electromagnetic induction to induce a voltage signal proportional to the current in the wire through the coil wound around the ferromagnetic metal wire, thereby realizing the measurement of the current. The oscilloscope analyzes the voltage signal output by the current clamp to measure the frequency and waveform of the current. The six-and-a-half-digit digital multimeter, based on Ohm's law and the principle of electromagnetic induction, measures parameters such as resistance and voltage in the circuit and calculates the current amplitude through internal calculation.

[0162] S52. Based on the energization principle of the energized thermal detection system, turn on the signal generator and precisely set it on its operation interface to generate a sinusoidal alternating current signal with a frequency of 50 Hz according to the experimental requirements. Input this signal into the power amplifier and amplify the input alternating current by adjusting parameters such as the gain of the power amplifier to make it meet the requirement of passing an alternating current with a frequency of 50 Hz and an amplitude of 2.5 A through the ferromagnetic metal wire. During this process, use the oscilloscope to monitor the waveform of the electrical signal output by the power amplifier in real time to ensure its stability and accuracy.

[0163] S53. Set the six-and-a-half-digit digital multimeter to the current measurement mode to accurately monitor the current amplitude I on the ferromagnetic metal wire m . At the same time, the oscilloscope and the current clamp work together. The oscilloscope is set to the frequency measurement function and analyzes the current signal transmitted by the current clamp to monitor the current frequency f on the ferromagnetic metal wire in real time. During the measurement process, read and record the measurement data multiple times to ensure the reliability of the measurement results.

[0164] S54. Turn on an infrared thermal imager with a frame rate of 200 fps and a resolution of 640×120, and perform initialization settings on it, including adjustments of parameters such as temperature measurement range and emissivity correction. The thermal imager detects the infrared radiation emitted from the surface of the ferromagnetic metal wire, converts it into an electrical signal, and after the internal signal processing circuit amplifies and filters the signal, etc., the electrical signal is then converted into a surface temperature signal through an algorithm. These temperature signals are sent to a computer in real time through a communication cable after further data analysis and processing. Any object with a temperature higher than absolute zero will emit infrared radiation. After the ferromagnetic metal wire is energized with alternating current, heat is generated due to the Joule heat effect, causing its surface temperature to rise, and thus infrared radiation is emitted. The detector inside the infrared thermal imager can sense this infrared radiation and convert it into an electrical signal. The working principle of the detector is based on the photoelectric effect or thermal effect of certain materials on infrared radiation. For example, a mercury cadmium telluride (HgCdTe) detector uses the photoelectric effect to convert infrared radiation into electron-hole pairs, and then generates an electrical signal. Through the processing and conversion of these electrical signals, the temperature distribution image on the surface of the ferromagnetic metal wire, that is, the infrared thermal image sequence, is finally obtained.

[0165] S55. Install and open the FLIR Research software on the computer, and establish a communication connection with the infrared thermal imager through this software. On the software operation page, accurately select the detection points according to the position and shape of the ferromagnetic metal wire. The software receives the temperature data sent by the infrared thermal imager in real time, generates and displays the temperature amplitude on the ferromagnetic metal wire and the temperature-time curve in the time domain. At the same time, the software has a data storage function, and stores the collected infrared thermal image sequence and related temperature data for subsequent analysis. The FLIR Research software communicates with the infrared thermal imager, receives and analyzes the temperature data sent by the thermal imager. The software has powerful data processing and visualization functions, and can intuitively display the temperature data in the form of images and curves. After selecting the detection points, the software analyzes the temperature data at this point, and through mathematical algorithms such as Fourier transform, converts the temperature signal in the time domain into a frequency domain signal, so as to obtain the amplitude information of the temperature signal at different frequencies, providing key data for subsequent calculation of the current amplitude.

[0166] In this embodiment, through a series of rigorous technical operations, accurate current signals and thermal image data are obtained. Monitoring the current amplitude and frequency is to ensure that the alternating current passed through the ferromagnetic metal wire meets the experimental setting requirements, guaranteeing the consistency and repeatability of the experimental conditions. The infrared thermal image sequence is collected because the heat generated by the current passing through the ferromagnetic metal wire will cause its temperature to change, and there is a specific relationship between the temperature change and the current. By analyzing the infrared thermal image sequence, the temperature amplitude at the same frequency as the current is extracted, providing an important basis for establishing the quantitative relationship between the current and the temperature and measuring the current subsequently, which is a key step in realizing the detection of magnetic bias alternating current based on infrared thermal imaging.

[0167] Through the above operations, the infrared thermal image of the ferromagnetic metal wire to be measured, as shown in Figure 6 , is obtained when a 50 Hz, 2.5 A alternating current is passed through it. By selecting the average temperature within the 2×9 region in the through-hole of the magnetic conducting armature shown in the figure for Fourier transform, the Figure 7 time-domain - frequency-domain diagram of the temperature fluctuation as shown can be obtained. Figure 7 presents the infrared thermal sequence obtained after 5 s of acquisition in the acquisition area by infrared thermal imaging. The figure reflects the amplitude information at the same frequency (50 Hz) as the current signal of the temperature signal. Using this temperature amplitude, the current amplitude can be obtained through the calculation method proposed in the patent. Further, by adjusting the number of single permanent magnets to change the bias magnetic field intensity, the relationship curve between the temperature amplitude at the same frequency as the current and the bias magnetic field intensity is as shown in Figure 8 , indicating that the maximum temperature amplitude can be extracted from the ferromagnetic metal wire under a bias magnetic field intensity of 614 Oe. Figure 8 shows the change of the temperature amplitude signal at the same frequency as the current on the ferromagnetic metal wire when the number of permanent magnets is increased to increase the bias magnetic field intensity in this embodiment. It reflects that under the magnetic saturation condition, the temperature amplitude on the ferromagnetic metal wire is the largest and the effect of calculating the current amplitude is the best. This series of experimental data and analysis results provide an important reference for optimizing the alternating current detection method, contributing to improving the accuracy and sensitivity of the detection.

[0168] Example 6

[0169] This embodiment discloses a method for detecting magnetic bias alternating current based on infrared thermal imaging. As a preferred implementation of this technical solution, that is, based on Example 1, 2, 3, 4 or 5, in step S6, using Fourier transform to extract the temperature amplitude at the same frequency for each current amplitude includes the following steps:

[0170] S61. Due to the narrow and thin planar structure characteristics of the ferromagnetic metal wire, the change in its temperature distribution in the length direction is relatively obvious, while the changes in other directions can be ignored. According to the basic laws of heat conduction, such as Fourier's law, the transfer of heat is proportional to the temperature gradient. Combining the physical properties and boundary conditions of the ferromagnetic metal wire, a one-dimensional heat transfer model is established to describe its temperature distribution after energization. Specifically: Let the length of the ferromagnetic metal wire be expressed as l = 2a, and a one-dimensional linear coordinate system is established with the center of the ferromagnetic metal wire as the origin O; -a and a respectively represent the left and right endpoints on the one-dimensional linear coordinate system of the ferromagnetic metal wire; x represents any point on the one-dimensional linear coordinate system of the ferromagnetic metal wire; T() represents the temperature of the spatial point in the one-dimensional linear coordinate system at the energization time t; k represents the thermal conductivity of the ferromagnetic metal wire; c represents the specific heat capacity of the ferromagnetic metal wire, ρ represents the mass density of the ferromagnetic metal wire, P(t) represents the heat source caused by the alternating current; C represents the end temperature of the ferromagnetic metal wire; T0 represents the initial temperature of the ferromagnetic metal wire. Based on the narrow and thin planar structure characteristics of the ferromagnetic metal wire, its temperature distribution after energization is represented by a one-dimensional heat transfer model, and the specific formula is:

[0171]

[0172] S62. According to Joule's law, when an electric current passes through a conductor, heat is generated, and the amount of heat is proportional to the square of the current, the resistance, and the time. At the same time, under the action of a bias magnetic field, an induced electromotive force will be generated in the ferromagnetic metal wire, which will also affect the generation of heat. Combining the electrical parameter structure of the equivalent circuit energized on the ferromagnetic metal wire and considering these factors comprehensively, the calculation formula for the heat source caused by the alternating current is derived. Specifically: Let U represent the circuit voltage; I represent the circuit current; R be the resistance of the ferromagnetic metal wire; E represent the absolute value of the induced electromotive force caused by the bias magnetic field on the ferromagnetic metal wire; I m represents the amplitude of the alternating current; f represents the frequency of the alternating current; t represents the time when the alternating current is passed into the ferromagnetic metal wire φ; represents the total magnetic flux of the closed total magnetic circuit, which is also the magnetic flux on the ferromagnetic metal wire; ρ0 represents the resistivity of the ferromagnetic metal wire; S0 represents the cross-sectional area of the ferromagnetic metal wire; μ b represents the average magnetic permeability of the ferromagnetic metal wire; μ s represents the average magnetic permeability of the external magnetic circuit; S represents the average cross-sectional area of the external magnetic circuit; S b represents the average cross-sectional area of the ferromagnetic metal wire; H c represents the coercive force of the permanent magnet; l m represents the length of the permanent magnet in the magnetic flux direction; d represents the length of the ferromagnetic metal wire in the magnetic flux direction; l sRepresents the length of the external magnetic circuit in the direction of magnetic flux. Based on the electrical parameter structure of the equivalent circuit passing through the ferromagnetic metal wire, the heat source P(t) caused by the alternating current is obtained, that is:

[0173]

[0174] S63, superposition method is a commonly used mathematical method, which is applicable to linear systems. In the one-dimensional heat transfer model, the temperature changes caused by different factors are considered separately, and then these temperature changes are superimposed to obtain the temperature distribution of the entire system. Based on this, mathematical software such as Mathematica or MATLAB is used to combine the above (Equation 15) and (Equation 16), and the analytical solution of the one-dimensional heat transfer model is obtained based on the superposition method combined with the symbolic calculation function of the software, so that the composition and change law of temperature can be analyzed more clearly. The analytical solution of the one-dimensional heat transfer model is expressed as:

[0175]

[0176] Among them, T f Indicates the temperature amplitude at the same frequency as the current; T 2f Represents the temperature amplitude twice the frequency of the AC current; T x represents the temperature at point x on the ferromagnetic metal line; T e It is used to describe the instantaneous process between the initial temperature state and the stable temperature state, and converges to a constant in the form of an exponential function; e represents the base of the exponential function; n represents a set of natural numbers.

[0177] S64, based on the analytical solution of the one-dimensional heat transfer model obtained, the data analysis software is used to conduct an in-depth analysis of the correlation and influencing factors of the temperature parameter structure on the ferromagnetic metal wire. By studying the variables in (Equation 17), the influence of different variables on temperature is clarified. According to the analysis results, a quantitative detection formula for the amplitude of AC current under a DC bias magnetic field is derived. Among them, the analytical solution of the one-dimensional heat transfer model is analyzed to clarify the degree and manner of influence of different variables on temperature. Through research, it is found that for a specific metal wire, some terms are only related to spatial variables, while some terms are related to current amplitude and frequency. In a stable state, there is a specific relationship between the change in temperature and the current amplitude. Specifically: From (Equation 17), it can be seen that for a specific metal wire, T x is only determined by the spatial variable x, while T e Represents the fluctuation trend of temperature and is determined by two variables: time variable t and space variable x. f With T 2fThese two items are independent of the spatial variable x, but are related to the current amplitude and frequency. At the same time, during the dynamic heat transfer process from the initial state to the new steady state, these four parts in (Equation 17) do not interfere with each other. Therefore, if the temperature change T is accurately measured f , and the current frequency is known, then the amplitude of the current under the DC bias magnetic field can be quantitatively detected by the following quantitative detection formula:

[0178]

[0179] where T m represents the amplitude of the temperature with the same frequency as the current, that is, the amplitude of the single-frequency temperature fluctuation, then there is: f S65. By analyzing in detail the permeability characteristics of the ferromagnetic metal wire and the external magnetic circuit, and combining the relevant parameters of the external magnetic circuit and the ferromagnetic metal wire obtained from experimental measurements, such as permeability, cross-sectional area, length, etc., to judge whether the influence of the item in the formula

[0180] can be ignored. After determining that the influence of this item can be ignored, the formula is simplified and rewritten, that is: Generally, the average cross-sectional area of the vertical magnetic flux in the external magnetic circuit is larger than that of the ferromagnetic metal wire. In addition, because the external magnetic circuit contains a magnetic conducting armature, its permeability generally varies in the range of 1000 - 5000 H / m, while the permeability of ferromagnetic metal wires such as stainless iron only varies in the range of 1.2 - 1.4 H / m. Therefore, the permeability of the external magnetic circuit is three orders of magnitude larger than that of the ferromagnetic metal wire, but the length of the external magnetic circuit in the magnetic flux direction is equivalent to the length of the metal wire in the magnetic flux direction. Therefore, the influence of the item in the formula can be ignored. Thus, when ignoring the influence of the item , the rewritten formula is: Then the formula is rewritten as:

[0181]

[0182] The Fourier transform is a mathematical method for converting a time-domain signal into a frequency-domain signal. In this step, the collected temperature signal is a time-domain signal, which contains information of different frequency components. Through the Fourier transform, the temperature signal can be decomposed into sine and cosine components of different frequencies, so as to extract the amplitude of the temperature fluctuation at the same frequency as the current. This is because under the action of alternating current, the temperature change of the ferromagnetic metal wire will contain components with the same frequency as the current, and the amplitude information of this frequency component can be accurately obtained through the Fourier transform. Thus, based on the above steps, the Fourier transform is performed to obtain the temperature amplitude. Using professional signal processing software, such as the FFT function in MATLAB, the Fourier transform is performed on the collected temperature signal. Through the Fourier transform, the amplitude of the temperature fluctuation at the same frequency as the current is extracted for each current amplitude, and the relationship curve between the temperature amplitude and the current amplitude is obtained, such as Figure 9 shown.

[0183] In summary, through a series of technical means and theoretical analyses, this technical solution establishes a quantitative relationship between the temperature amplitude and the current amplitude. Specifically, establishing a one-dimensional heat transfer model and solving its analytical solution is to deeply understand the temperature distribution law and influencing factors of the ferromagnetic metal wire under the condition of being energized. Analyzing the temperature parameter structure and deriving the quantitative detection formula is to find a method for accurately measuring the amplitude of alternating current. By extracting the temperature amplitude at the same frequency for each current amplitude through the Fourier transform, the relationship curve between the temperature amplitude and the current amplitude is obtained, providing key data support and theoretical basis for subsequent fitting the temperature amplitude curve using one-dimensional Fourier series and finally measuring the alternating current on the ferromagnetic metal wire, realizing the conversion from temperature measurement to current measurement.

[0184] Embodiment 7

[0185] This embodiment discloses a magnetic bias alternating current detection method based on infrared thermal imaging. As a preferred implementation of this technical solution, that is, based on any one of Embodiments 1 to 6, in step S7, using one-dimensional Fourier series to fit the temperature amplitude curves under different current amplitudes includes the following steps:

[0186] S71, with the help of professional mathematical analysis software, such as Mathematica or MATLAB, input the data pairs of the temperature amplitude and the current amplitude, and these data are from the same-frequency temperature fluctuations at different current amplitudes obtained through the Fourier transform in step S6. According to the one-dimensional Fourier series theory, set the corresponding parameters in the software, and represent the relationship between the temperature amplitude and the current amplitude according to the one-dimensional Fourier series as:

[0187] T m =a0 + a1cos(w·I m ) + b1sin(w·I m) (Equation 20).

[0188] Among them, a0, a1, and b1 are expressed as Fourier coefficients, and the initial values of the Fourier coefficients a0, a1, and b1 can be preliminarily determined using the calculation function of the software. w represents the angular frequency; I m represents the amplitude of the alternating current; T m represents the amplitude of the single-frequency temperature fluctuation.

[0189] S72. To solve for the current amplitude I m , using the symbolic operation function of the above software, perform an inverse function solution operation on (Equation 20), that is, input the corresponding instructions in the software, and transform the equation through the built-in algorithm to obtain the expression of the current amplitude with respect to the temperature amplitude and other parameters:

[0190] T m -a0 = a1cos(w·I m ) + b1sin(w·I m ) (Equation 21).

[0191] S73. According to the basic properties and identity transformation rules of trigonometric functions, manually further transform (Equation 21) in the mathematical software, that is: Among them, Z represents the amplitude of the function, and represents the phase angle of the function, satisfying

[0192] S74. According to the interval properties of the function, rewrite the above (Equation 22). Specifically: Draw the graph of the function represented by (Equation 21) through the mathematical analysis software, and visually observe the monotonicity and periodicity of the function. According to the period of the sine function being this characteristic, it is found that for any real number (T m -a0), (Equation 21) has infinitely many solutions, that is, (Equation 20) does not have a unique inverse function. To solve this problem, in the software, by adjusting the domain, restrict it to the interval . Within this interval, redraw the function graph to confirm that the function is monotonically increasing and has a unique inverse function. Then, perform a targeted transformation on Equation (22) in the software to obtain:

[0193] S75. Combining the domain analysis of the arcsin() function in (Equation 23), obtain the expression for the remote detection of alternating current based on infrared thermal imaging. Specifically: Since the domain of the arcsin() function is [-1, 1], it is required that the value of T m -a0 is between [-R, R]. Then, substitute into the above formula to obtain (Equation 20) in The inverse function on it is obtained, and then the expression for remote detection of alternating current based on infrared thermal imaging is obtained; The current amplitude I is obtained according to this formula m Thus, the remote detection of alternating current based on infrared thermal imaging is realized.

[0194] In summary, this technical solution fits the temperature amplitude curve using the one-dimensional Fourier series, solves the corresponding inverse function, and obtains the expression for remote detection of alternating current based on infrared thermal imaging. Its core purpose is to utilize the internal connection between temperature and current, and through mathematical means, convert the temperature amplitude data into a calculation expression of current amplitude. In this way, in practical applications, only by obtaining the temperature amplitude information of the ferromagnetic metal wire through infrared thermal imaging technology, the alternating current amplitude can be accurately calculated with the help of this expression, realizing the remote and non-contact detection of alternating current, and providing a convenient, efficient and accurate detection method for fields such as power system monitoring and electrical equipment fault diagnosis.

[0195] Embodiment 8

[0196] This embodiment discloses a method for detecting magnetically biased alternating current based on infrared thermal imaging. As a preferred implementation of this technical solution, that is, based on Embodiment 7, in step S8, the method for measuring the alternating current on the ferromagnetic metal wire according to the fitting curve is as follows: Based on the theoretical basis established in the previous steps, especially the quantitative relationship between the temperature amplitude and the current amplitude established through Fourier transform in step S6, and the expression for remote detection of alternating current obtained by fitting with Fourier series in step S7, it is clear that there is a specific mathematical relationship between the temperature amplitude and the current amplitude. When a temperature amplitude is given, according to this mathematical expression, the corresponding current amplitude can be calculated. Therefore, a system composed of a high-precision signal generator and a power amplifier is used to increase the current in steps of 0.15 A, and a power frequency (50 Hz) alternating current of 0.1 A - 4.95 A is passed through the metal wire. When different current amplitudes are loaded each time, with the help of the infrared thermal imaging system and related data processing software as described above, the temperature amplitude at the same frequency of the ferromagnetic metal wire at this current amplitude is accurately extracted.

[0197] The temperature amplitudes at the same frequency under different current amplitudes extracted are successively substituted into the expression for remote detection of alternating current obtained in step S7. Using professional calculation software (such as MATLAB), by writing a program to achieve automatic calculation, the measured current amplitude corresponding to each temperature amplitude can be quickly obtained.

[0198] Take the current result measured by a six-and-a-half-digit digital multimeter as the standard current amplitude. Compare the measured current amplitude of this technical solution with the standard current amplitude. Also use software such as MATLAB to calculate the relative error of each measurement, and through the plotting function, generate a comparison chart of the measured current amplitude and the standard current amplitude as shown in Figure 10 and a relative error chart of the measured current and the standard current as shown in Figure 11 . The relative error is an important indicator to measure the accuracy of the measurement result. By calculating the relative error between the measured current amplitude and the standard current amplitude, the deviation degree between the measured current of this technical solution and the true value can be intuitively understood. In an ideal situation, the smaller the relative error, the more accurate the measurement method.

[0199] This technical solution aims to achieve a non-contact and long-distance alternating current detection method based on infrared thermal imaging technology. In the actual operation environment of a power system or electrical equipment, without directly contacting the measured line, by detecting the temperature change of a ferromagnetic metal wire, the amplitude information of the alternating current can be obtained using this technical solution, providing an effective technical means for the monitoring and maintenance of the power system and the fault diagnosis of electrical equipment. It can detect alternating currents with different amplitudes within the range of relative error ±3.2%, which proves that this technical solution meets the expected detection accuracy requirements and successfully realizes the magnetic bias alternating current detection based on infrared thermal imaging.

[0200] In addition, by comparing the measured current amplitude with the standard current amplitude and analyzing the relative error, the accuracy and reliability of the magnetic bias alternating current detection method based on infrared thermal imaging are verified. If this method can accurately measure alternating currents with different amplitudes within a certain error range, it indicates that this method is feasible in practical applications.

[0201] Example 9

[0202] This example discloses a magnetic bias alternating current detection method based on infrared thermal imaging. As a preferred implementation of this technical solution, that is, based on Example 7 or 8, in step S7, using the one-dimensional Fourier series to fit the temperature amplitude curve under different current amplitudes also includes the verification of the fitting degree, that is, including calculating the determination coefficient R of (Equation 20) 2 and the root mean square error RMSE. RMSE is also called the standard error and is used to measure the deviation between the observed value and the true value. Among them:

[0203]

[0204] where y i represents the actual value of T m , represents the predicted value of T m ; Denote T m The mean of the actual values, M denotes T m The number of samples; i denotes T m The sample serial number; the regression sum of squares Denotes the sum of squares of the errors between the predicted values and the actual values; the total sum of squares Denotes the sum of squares of the differences between the actual values and their mean value.

[0205] In actual operation, after obtaining the data pairs of the temperature amplitude and the current amplitude, these data are sorted according to the sample serial number. The actual value of the temperature amplitude in each sample is clarified, as well as the predicted value calculated by (Equation 20). At the same time, the mean of all actual values is calculated to determine the number of samples. Using data analysis software (such as the Pandas library of Python or the array operation function of MATLAB), the regression sum of squares SS res And the total sum of squares SS tot Are calculated by programming.

[0206] According to the coefficient of determination R 2 Its essence is to measure the explanatory ability of the regression model to the data. The total sum of squares SS tot Represents the total variation degree of the observed data, that is, the dispersion degree of the data itself. The regression sum of squares SS res Represents the sum of squares of the errors between the model predicted values and the actual values, reflecting the part of the data variation that the model fails to explain. The coefficient of determination R 2 The meaning of the calculation formula is the proportion of the part of the data variation that the model can explain in the total variation. When approaching 1, it indicates that the model can explain most of the data variation, that is, the fitting degree of the model to the data is very high.

[0207] The root mean square error RMSE measures the average deviation between the predicted value and the true value by averaging the squares of the errors between the predicted value and the actual value of each sample and taking the square root. It takes into account the error situation of each sample and gives a greater weight to larger errors. Because the influence of larger errors is amplified after squaring the errors, it can more intuitively reflect the deviation degree between the model predicted value and the actual value. When approaching 0, it indicates that the predicted value is very close to the actual value and the fitting effect is good.

[0208] To sum up, by calculating the coefficient of determination R 2 And the root mean square error RMSE, the effect of fitting the temperature amplitude curve at different current amplitudes using the one-dimensional Fourier series is quantitatively evaluated. These two indicators reflect the degree of fit between the fitting model and the actual data from different angles, helping to judge whether this fitting method can accurately describe the relationship between the temperature amplitude and the current amplitude. In this technical solution, the coefficient of determination R 2= 0.9998, root mean square error RMSE = 0.0661, that is, the coefficient of determination R 2 is relatively high and the root mean square error RMSE is relatively low, indicating that using the one-dimensional Fourier series to fit the temperature amplitude curve has a high degree of fitting. This further verifies the reliability of the established expression for remote detection of alternating current based on this fitting method, provides strong support for accurately measuring the amplitude of alternating current through this expression subsequently, and enhances the credibility of the entire magnetic bias alternating current detection method based on infrared thermal imaging.

Claims

1. A method for detecting magnetic-biased alternating current based on infrared thermal imaging, characterized in that The following steps are involved: S1, assembling a permanent magnet magnetizer, and forming an external magnetic circuit based on the permanent magnet magnetizer and the air therein; S2, placing a ferromagnetic metal wire in an external magnetic circuit, applying a bias magnetic field to the ferromagnetic metal wire using a permanent magnet magnetizer, and the ferromagnetic metal wire and the external magnetic circuit together form a closed total magnetic circuit; S3. Analyze the magnetic parameter structure of the closed total magnetic circuit to obtain the magnetic field strength H in the radial direction of the ferromagnetic metal wire b ; S4, combined with the magnetic parameter structure, analyze the electrical parameter structure of the equivalent circuit passing through the ferromagnetic metal wire; S5, passing alternating current to the ferromagnetic metal wire and monitoring the electrical signal, and using an infrared thermal imager to collect an original thermal image sequence S of the ferromagnetic metal wire during the power-on stage under the action of the bias magnetic field of the permanent magnet magnetizer; S6, based on the current signal and infrared thermal image sequence S, combined with the magnetic parameter structure and the electrical parameter structure, the temperature amplitude at the same frequency under each current amplitude is extracted by Fourier transform; S7, using one-dimensional Fourier series to fit the temperature amplitude curves at different current amplitudes; S8, measuring the AC current on the ferromagnetic metal wire according to the fitting results.

2. The magnetic bias alternating current detection method based on infrared thermal imaging according to claim 1, wherein, In step S1, assembling the permanent magnet magnetizer includes the following steps: S11, preparing a magnetic armature and two permanent magnet groups, each permanent magnet group including a plurality of permanent magnets; S12, placing two groups of permanent magnets in parallel so that the magnetization directions of the two groups of permanent magnets are opposite; S13, a through hole is opened on the magnetic armature for releasing temperature, and then the magnetic armature is placed flatly between two sets of permanent magnets so that the magnetization directions of the two permanent magnets are perpendicular to the axis direction of the armature, thus forming a permanent magnet magnetizer; the permanent magnet magnetizer and the air therein together form an external magnetic circuit.

3. The magnetic bias alternating current detection method based on infrared thermal imaging according to claim 2, characterized in that, In step S3, analyzing the magnetic parameter structure of the closed total magnetic circuit includes the following steps: S31. Obtain the magnetomotive force F and reluctance R of each component according to the geometric parameters and magnetic characteristics of each component in the closed total magnetic circuit. The specific calculation formula is expressed as: i and reluctance R i , specifically, the calculation formula is expressed as: F i = H c l i (Equation 1); Among them, i represents the components in the closed magnetic circuit, that is, i = m, a, b, c; m represents the permanent magnet; a represents the magnetic conducting armature; b represents the ferromagnetic metal wire; c represents the air; F i represents the magnetomotive force of component i; H c represents the coercive force of the permanent magnet; l i represents the length of component i in the magnetic flux direction; μ i represents the magnetic permeability of component i, S i represents the projected area of component i perpendicular to the magnetic flux direction; R i represents the magnetic resistance of component i; S32. Use a ferromagnetic metal wire as a magnetic thermal radiation sensor. The external magnetic circuit magnetoresistance R s and the magnetoresistance R of the magnetic thermal radiation sensor b together form the total magnetoresistance of the closed total magnetic circuit; the external magnetic circuit magnetoresistance R s and the magnetoresistance R of the magnetic thermal radiation sensor b are calculated as follows: where l s represents the length of the outer magnetic circuit in the magnetic flux direction; μ s represents the average magnetic permeability of the outer magnetic circuit; S represents the average cross-sectional area of the outer magnetic circuit; d represents the length of the ferromagnetic metal wire in the magnetic flux direction, which is also its own width; μ b represents the average magnetic permeability of the ferromagnetic metal wire; S b represents the average cross-sectional area of the ferromagnetic metal wire; μ0 represents the vacuum magnetic permeability; S33, according to Kirchhoff's law, the magnetic flux distribution in the closed total magnetic circuit is described, that is: F = Φ(R s + R b ) = H s l s + H b d (Equation 5); Among them, F represents the total magnetomotive force of the closed total magnetic circuit; Φ represents the total magnetic flux of the closed total magnetic circuit, which is also the magnetic flux on the ferromagnetic metal wire; H b represents the average magnetic field strength on the ferromagnetic metal wire, H s represents the average magnetic field strength of the external magnetic circuit; S34, without considering the leakage flux, the magnetic flux in the entire closed total magnetic circuit is continuous, that is: μ b μ0S b H b = μ s μ0H s S (Equation 6); S35, combine the above (Equation 1) to (Equation 6) to obtain the average magnetic field strength of the ferromagnetic metal wire, which is the radial magnetic field strength H of the ferromagnetic metal wire. b , then: where l m represents the length of the permanent magnet in the magnetic flux direction.

4. The magnetic bias alternating current detection method based on infrared thermal imaging according to claim 1, wherein In step S4, analyzing the electrical parameter structure of the equivalent circuit on the ferromagnetic metal wire includes the following steps: S41, according to Kirchhoff's voltage law, the equivalent circuit of the ferromagnetic metal wire is expressed as: U = IR - e - e0 (Formula 8); I = I m cos(2πft) (Equation 11); Among them, U represents the circuit voltage, I represents the circuit current, R is the resistivity of the ferromagnetic metal wire, e0 is the leakage electromotive force, and e is the induced electromotive force caused by the bias magnetic field on the ferromagnetic metal wire; L represents the equivalent inductance in the energized equivalent circuit; Φ represents the total magnetic flux of the closed total magnetic circuit, which is also the magnetic flux on the ferromagnetic metal wire; I m represents the amplitude of the alternating current; f represents the frequency of the alternating current; t represents the time when the alternating current is applied to the ferromagnetic metal wire; S42, ignoring the leakage electromotive force e0, then: U = IR-e (Formula 12); S43, combined with the magnetic parameter structure, the magnetic flux Φ on the ferromagnetic metal wire is derived, that is: Among them, μ b represents the average magnetic permeability of the ferromagnetic metal wire; S b represents the average cross-sectional area of the ferromagnetic metal wire; μ s represents the average magnetic permeability of the external magnetic circuit; S represents the average cross-sectional area of the external magnetic circuit; H c represents the coercive force of the permanent magnet; l m represents the length of the permanent magnet in the magnetic flux direction; d represents the length of the ferromagnetic metal wire in the magnetic flux direction; l s represents the length of the external magnetic circuit in the magnetic flux direction; S44, combine the above (Equation 8) to (Equation 13) to obtain the circuit voltage, that is, the voltage U on the magnetic metal wire, then: Wherein, E represents the absolute value of the induced electromotive force caused by the bias magnetic field on the ferromagnetic metal wire, that is, E=|e|.

5. The magnetic bias alternating current detection method based on infrared thermal imaging according to claim 1, wherein In the step S5, collecting the original thermal image sequence S of the ferromagnetic metal wire in the power-on stage includes the following steps: S51, using a signal generator to generate a sinusoidal alternating current of a set frequency, and then amplifying the alternating current through a power amplifier to pass an alternating current with a frequency of 50 Hz and an amplitude of 2.5 A through the ferromagnetic metal wire; S52, Use a six-and-a-half-digit digital multimeter to monitor the current amplitude I on a ferromagnetic metal wire m , and at the same time use the method of cooperating an oscilloscope with a current clamp to monitor the current frequency f on the ferromagnetic metal wire; S53, using an infrared thermal imager with a frame rate of 200 fps and a resolution of 640×120 to collect the surface temperature of the ferromagnetic metal wire, thereby obtaining an infrared thermal image sequence; S54. Infrared recording is carried out through FLIR Research software. On its operation page, by selecting the detection points, the temperature amplitude on the ferromagnetic metal wire and the temperature-time curve in the time domain can be viewed in real time.

6. The magnetic bias alternating current detection method based on infrared thermal imaging according to claim 1, wherein In step S6, the steps of using Fourier transform to extract the temperature amplitude at the same frequency under each current amplitude include the following steps: S61. Based on the narrow and thin planar structure characteristics of the ferromagnetic metal wire, its temperature distribution after energization is represented by a one-dimensional heat transfer model. The specific formula is: Where, let the length of the ferromagnetic metal wire be expressed as l = 2a, and a one-dimensional straight coordinate system is established with the center of the ferromagnetic metal wire as the origin O; -a and a respectively represent the left and right endpoints on the one-dimensional straight coordinate system of the ferromagnetic metal wire; x represents any point on the one-dimensional straight coordinate system of the ferromagnetic metal wire; T() represents the temperature of the space point in the one-dimensional straight coordinate system at the energization time t; k represents the thermal conductivity of the ferromagnetic metal wire; c represents the specific heat capacity of the ferromagnetic metal wire, ρ represents the mass density of the ferromagnetic metal wire, P(t) represents the heat source caused by the alternating current; C represents the endpoint temperature of the ferromagnetic metal wire; T0 represents the initial temperature of the ferromagnetic metal wire; S62. Based on the electrical parameter structure of the equivalent circuit energized on the ferromagnetic metal wire, the heat source P(t) caused by the alternating current is obtained, that is: Among them, U represents the circuit voltage; I represents the circuit current; R is the resistance of the ferromagnetic metal wire; E represents the absolute value of the induced electromotive force caused by the bias magnetic field on the ferromagnetic metal wire; I m represents the amplitude of the alternating current; f represents the frequency of the alternating current; t represents the time when the alternating current is passed into the ferromagnetic metal wire; Φ represents the total magnetic flux of the closed total magnetic circuit, and also the magnetic flux on the ferromagnetic metal wire; ρ0 represents the resistivity of the ferromagnetic metal wire; S0 represents the cross-sectional area of the ferromagnetic metal wire; μ b represents the average magnetic permeability of the ferromagnetic metal wire; μ s represents the average magnetic permeability of the external magnetic circuit; S represents the average cross-sectional area of the external magnetic circuit; S b represents the average cross-sectional area of the ferromagnetic metal wire; H c represents the coercive force of the permanent magnet; l m represents the length of the permanent magnet in the magnetic flux direction; d represents the length of the ferromagnetic metal wire in the magnetic flux direction; l s represents the length of the external magnetic circuit in the magnetic flux direction; S63. Combine the above (Equation 15) and (Equation 16), and based on the superposition method, the analytical solution of the one-dimensional heat transfer model is obtained, that is expressed as: Among them, T f represents the temperature amplitude with the same frequency as the current; T 2f represents the temperature amplitude twice the frequency of the alternating current; T x represents the temperature at point x on the ferromagnetic metal wire; T e is used to describe the transient process between the initial state and the stable state of the temperature, and converges to a constant in the form of an exponential function; e represents the base of the exponential function; n represents a set of natural numbers; S64. Based on the analytical solution of the one-dimensional heat transfer model, analyze the correlation and influencing factors of the temperature parameter structure on the ferromagnetic metal wire, and thereby obtain the quantitative detection formula for the alternating current amplitude under the DC bias magnetic field: Among them, T m represents the amplitude of the temperature T f with the same frequency as the current, that is, the amplitude of the single-frequency temperature fluctuation. Then, there is: S65, according to the magnetic permeability characteristics of the external magnetic circuit and the ferromagnetic metal wire, ignoring the influence of the term, the rewritten formula is:

7. The magnetic bias alternating current detection method based on infrared thermal imaging according to claim 1, characterized in that In step S7, the steps of using the one-dimensional Fourier series to fit the temperature amplitude curves under different current amplitudes include the following steps: S71. The relationship between the temperature amplitude and the current amplitude is expressed according to the one-dimensional Fourier series as: T m = a0 + a1cos(w·I m ) + b1 sin(w·I m )(Equation 20); where a0, a1, and b1 are represented as Fourier coefficients; w represents the angular frequency; I m represents the AC current amplitude; T m represents the amplitude of the fundamental frequency temperature fluctuation; S72. Solve the inverse function of (Equation 20), that is: T m -a0 = a1cos(w·I m ) + b1sin(w·I m ) (Equation 21); S73. According to the properties of trigonometric functions, rewrite the inverse function as: where z represents the amplitude of the function, and represents the phase angle of the function, satisfying S74. According to the interval properties of the function, rewrite the above (Equation 22) as: S75, Combining with the domain analysis of the arcsin() function in the formula 23 ), the expression for the remote detection of alternating current based on infrared thermal imaging is obtained:

8. The magnetic bias alternating current detection method based on infrared thermal imaging according to claim 7, characterized in that In step S8, the method for measuring the alternating current on the ferromagnetic metal wire according to the fitting curve is: increase the current in steps of 0.15A, apply a power frequency alternating current of 0.1A - 4.95A to the metal wire. During this process, the temperature amplitudes at the same frequency under different current amplitudes are extracted; by substituting the temperature amplitudes at the same frequency under different current amplitudes into the expression for remote detection of alternating current, the measured current amplitude is obtained.

9. The magnetic bias alternating current detection method based on infrared thermal imaging according to claim 7, characterized in that, In the step S7, using the one-dimensional Fourier series to fit the temperature amplitude curves at different current amplitudes further includes the verification of the fitting degree, that is, calculating the determination coefficient R of (Equation 20) 2 and the root mean square error RMSE, where: Among them, y i represents the actual value of T m , represents the predicted value of T m ; represents the mean of the actual values of T m , M represents the number of samples of T m ; i represents the sample sequence number of T m ; The regression sum of squares represents the sum of the squares of the errors between the predicted value and the actual value; The total sum of squares represents the sum of the squares of the differences between the actual value and its mean.

Images