Non-magnetic metal eddy current magnetic field measurement method

Abstract

The present application relates to non-magnetic metal eddy current magnetic field measurement method, belongs to eddy current magnetic field detection technical field. The method is based on magnetic variable simulation method, and the eddy current magnetic field generated by the movement of non-magnetic metal is equivalent to the eddy current magnetic field excited by the regular change of the applied magnetic field acting on non-magnetic metal. When the variable current is passed through the local magnetic simulation coil, the change of the magnetic field value measured by the sensor includes the applied magnetic field and the eddy current magnetic field. The eddy current magnetic field includes the background eddy current magnetic field and the non-magnetic metal eddy current magnetic field. The applied magnetic field and the background eddy current magnetic field are filtered from the sensor measurement value, and the eddy current magnetic field generated by the non-magnetic metal under the excitation of the external magnetic field is obtained. The present application avoids the accidental error caused by the repeatability of multiple energization, and ensures the simultaneity of current and magnetic field measurement value, and improves the measurement accuracy of non-magnetic metal eddy current magnetic field.

Description

Technical Field

[0001] This invention belongs to the field of eddy current magnetic field detection technology, specifically relating to a method for measuring eddy current magnetic fields in non-magnetic metals. Background Technology

[0002] Ship magnetic fields are an important component of the physical field surrounding a ship. Based on their generation mechanisms, they can be classified into induced magnetic fields, fixed magnetic fields, eddy current magnetic fields, and other stray magnetic fields. Among these, fixed and induced magnetic fields typically dominate the ship's magnetic field, and significant progress has been made in the measurement methods for fixed and induced magnetic fields. However, for some low-magnetic ships, eddy current magnetic fields will serve as the primary magnetic field source. Therefore, in-depth research on eddy current magnetic fields has important theoretical guiding significance for subsequent work such as ship magnetic compensation and shipborne geomagnetic field measurement.

[0003] The generation mechanism of eddy current magnetic fields in ships is as follows: During navigation, ships undergo rotations such as rolling, pitching, and bowing, causing changes in the magnetic flux passing through the ship. This induces an electromotive force on the conductive hull, which in turn generates eddy currents, thus exciting an eddy current magnetic field. Methods for detecting eddy current magnetic fields in ships are generally divided into mechanical oscillation measurement and magnetic simulation measurement. Early research on eddy current magnetic fields primarily relied on the mechanical oscillation method; for example, British researchers induced oscillation by manually moving the ship. However, this method is resource-intensive and has low measurement accuracy. In 1994, after completing facility expansion, the Yokosuka Demagnetization Station in Japan pioneered the use of magnetic simulation to systematically study eddy current magnetic fields in ships. Magnetic simulation equates the eddy current magnetic field generated during conductor motion to a regularly changing spatial magnetic field acting on the conductor, thus significantly improving experimental controllability and measurement efficiency. In the early 20th century, J.J. Holmes, based on magnetic simulation, theoretically derived and numerically simulated the eddy current magnetic field of a finite-length thin cylindrical shell model. Their research findings have propelled magnetic variation simulation to become the mainstream method for studying eddy magnetic fields in the international academic community. Birsan M et al. constructed a finite element model of the eddy magnetic field of the CFAV Quest magnetic ship, and the simulation results matched the actual ship test data, verifying the reliability of the model in predicting the distribution of eddy magnetic fields around the ship.

[0004] Research on eddy current magnetic fields in ships in China started relatively late. In 1982, Zhang Zhonglong pioneered the research on the generation mechanism and theoretical formula of eddy current magnetic fields, opening a new chapter in this field in China. In recent years, related research has continued to deepen. Wang Junmin et al. used a low-magnetic rotating ellipsoidal shell as a hull model and established a mathematical model of eddy current magnetic fields under the conditions of pitching motion of the ellipsoidal shell, laying an important theoretical foundation for the study of eddy current magnetic fields generated by pitching of ships made of low-magnetic materials. Zhang Ning et al. used the finite element method to numerically calculate eddy current magnetic fields; Zhu Wubing, Xu Zhiming et al. conducted numerical calculations of eddy current magnetic fields in ships based on the magnetic variation simulation method. The above studies mainly focus on the numerical calculation of eddy current magnetic fields, with relatively little experimental verification. Only Wen Haodong et al. conducted preliminary experimental research, constructing a zero-magnetic space in the laboratory, energizing a geomagnetic simulation coil to simulate changes in the external magnetic field, and passing the same current before and after placing an aluminum plate. They extracted the eddy current magnetic field of the aluminum plate by subtracting the magnetic field measurements with and without the aluminum plate. However, this method requires high repeatability of the excitation current, making it difficult to ensure accurate synchronization of the magnetic field acquisition times before and after placing the aluminum plate. Furthermore, there is no ideal zero magnetic space, which may be affected by the background eddy current magnetic field generated by the conductor in the space, thus introducing a large number of errors that affect the measurement accuracy.

[0005] Therefore, overcoming the shortcomings of existing technologies is an urgent problem to be solved in the field of eddy current magnetic field detection technology. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for measuring the eddy current magnetic field of non-magnetic metals.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for measuring the eddy current magnetic field of non-magnetic metals, characterized by comprising the following steps: Based on the magnetic variation simulation method, the eddy current magnetic field generated by the motion of non-magnetic metal is equivalent to an externally applied magnetic field that changes in a certain regularity acting on the eddy current magnetic field excited by the non-magnetic metal. The external magnetic field is obtained by passing a current with different patterns through the geomagnetic simulation coil according to the geomagnetic simulation method; When a changing current is passed through the magnetic field simulation coil, the change in the magnetic field value measured by the sensor includes both the applied magnetic field and the eddy current magnetic field. Eddy current magnetic fields include background eddy current magnetic fields and non-magnetic metallic eddy current magnetic fields; By filtering out the external magnetic field and background eddy current magnetic field from the sensor measurements, the eddy current magnetic field generated by the non-magnetic metal under the excitation of the external magnetic field is obtained.

[0008] Furthermore, the method for measuring the eddy current magnetic field of non-magnetic metals includes the following steps: Step (1), obtaining the external magnetic field: The change in the applied magnetic field is directly proportional to the current flowing through the geomagnetic simulation coil, that is: (1) In the formula: B mn I(t) represents the external magnetic field generated by the simulated coil after it is energized at time t, in nT; I(t) represents the time-varying current flowing through the geomagnetic simulation coil at time t, in A; k mn This is the magnetic field coefficient of the geomagnetic simulation coil, with units of nT / A; If the magnetic field coefficient k of the geomagnetic simulation coil can be obtained mn And by monitoring the time-varying current I(t), the applied magnetic field can be obtained; Step (2), obtaining the background eddy current magnetic field: By analyzing the eddy current proportionality coefficient and the background eddy current, the background eddy current magnetic field can be obtained. The eddy current magnetic field is proportional to the eddy current, and the calculation formula is as follows: (2) In the formula, B wx (t), B wy (t), B wz (t) represent the x, y, and z components of the eddy current magnetic field at time t, respectively, all in nT; i b (t) represents the eddy current at time t, in A; k 11 k 21 k 31 These are the longitudinal, transverse, and vertical vortex proportionality coefficients, respectively, in nT / A; Step (3): Arrange a magnetic sensor (i.e., sensor 2) above the non-magnetic metal, and filter out the external magnetic field and background eddy current magnetic field from the change in the measurement value of the magnetic sensor to obtain the non-magnetic metal eddy current magnetic field. The calculation formula is as follows: (3) In the formula, △B m B represents the change in the measured value of the magnetic sensor. mn B is the external magnetic field generated when the coil is energized. w B is the background eddy current magnetic field. wjs It is a non-magnetic metallic eddy current magnetic field.

[0009] Furthermore, the specific steps of step (1) are as follows: Step (1.1): Build an experimental platform in the geomagnetic simulation system and place a background magnetic sensor, denoted as sensor 1, at a distance from the experimental platform. There are no ferromagnetic materials or conductive materials around it. A constant current I is passed through the geomagnetic simulation coil, and the change in the magnetic field of sensor 1 before and after the energization is observed, i.e., the change in the applied magnetic field B.mn The magnetic field coefficient of the geomagnetic simulation coil at sensor 1 is calculated according to equation (1) and denoted as k. mnb ; From equation (1), we can obtain I(t) = B mn (t) / k mn Then, after any current is passed through the geomagnetic simulation coil, the applied magnetic field B measured by sensor 1 can be used to determine the magnetic field. mn (t) and the magnetic field coefficient k of the geomagnetic simulation coil mnb To calibrate the time-varying current I(t) passed through the geomagnetic simulation coil; Step (1.2): Place a non-magnetic metal inside the experimental platform, and arrange a magnetic sensor above the non-magnetic metal, denoted as sensor 2. Remove the non-magnetic metal from the experimental platform. Pass a certain constant current through the geomagnetic simulation coil, and calculate the magnetic field coefficient k of the geomagnetic simulation coil at sensor 2 by measuring the change in the magnetic field of sensor 2. mnc ; Based on the time-varying current I(t) calibrated by sensor 1, the applied magnetic field measured at sensor 2 is calculated according to equation (1). , and filter it out from the measurement value of sensor 2.

[0010] Furthermore, the specific steps of step (2) are as follows: Step (2.1), the equivalent process of eddies: If the background eddy current is equivalent to a multi-order RL dynamic circuit, then the time-varying characteristics of the background eddy current magnetic field are equivalent to the output characteristics of the RL dynamic circuit, and the terminal voltage u(t) of the equivalent circuit satisfies: (4) In the formula: B mnz (t) represents the applied magnetic field in the vertical direction, in nT; α is the terminal voltage coefficient; u(t) is the terminal voltage, in V; R and L are the equivalent resistance and equivalent inductance in the circuit, in Ω and H, respectively; The background eddy current i induced in the equivalent circuit b (t) is discretized into several units with a time interval of Δt; if Δt is sufficiently small, the rate of change of the magnetic field... Since is a constant, according to the formula for calculating the total response of a first-order dynamic circuit: (5) In the formula, i b (t0) represents the initial background eddy current at time t0, in A; the time constant τ is defined as L / R, in seconds; ΔB mnz α is the change in the applied magnetic field in the vertical direction, in nT; α is the terminal voltage coefficient; R is the equivalent resistance in the circuit, in Ω; Δt is the time interval. It is known that the applied magnetic field is directly proportional to the time-varying current flowing through the coil, that is: (6) Combining equations (5) and (6), and multiplying both ends by the eddy current proportionality coefficient k 11 k 21 k 31 ,but: (7) Where k1 is defined as the longitudinal vortex amplitude coefficient, derived from k mn α / R and longitudinal eddy current proportionality coefficient k 11 Multiplying them together yields the result; similarly, the transverse vortex amplitude coefficient k2 and the vertical vortex amplitude coefficient k3 are respectively k mn α / R and transverse vortex proportionality coefficient k 21 Vertical vortex proportionality coefficient k 31 Multiplying them together, all three units are in nT / A; B wx B wy B wz denoted as x, y, and z components of the background eddy current magnetic field; Δt is the time interval; I is the time-varying current; τ is the time constant; Step (2.2), eddy current amplitude coefficient: Before inserting non-magnetic metal, a slope current with a constant slope is passed through the geomagnetic simulation coil; Based on the measured values ​​of sensor 1 and the magnetic field coefficient k of the geomagnetic simulation coil mnb Determine the ramp current I(t) at each moment; The slope current value at any given time can be obtained by subtracting the background magnetic field value before energization from the magnetic field value measured by sensor 1 at that time, selecting the component in any direction, and then dividing it by the component of the magnetic field coefficient of the geomagnetic simulation coil at the location of sensor 1 in the same direction. Next, the magnetic field coefficient k of the geomagnetic simulation coil was used. mnc The external magnetic field is filtered out from the change in the measured value of sensor 2 by the ramp current I(t), resulting in the background eddy current magnetic field B. w ; (8) When Δt / τ is much greater than 1, the background eddy current magnetic field reaches a steady state, then: (9) After calibrating the time-varying current I(t) using sensor 1 in step (1.2), divide each component of the background eddy current magnetic field at each moment of the steady state by the corresponding current change rate, and take the average value of all moments of the steady state to obtain the eddy current amplitude coefficients k1, k2, and k3. Step (2.3), Identification of the time constant: The time constant τ is solved by function fitting. Then, the time constant τ is substituted into equation (7) and combined with the time-varying current I(t) and the eddy current amplitude coefficient to calculate the background eddy current magnetic field.

[0011] Further, in step (2.3), the time constant τ is cyclically taken in the interval [1,50] ms with a step size of 0.01 ms. This value is substituted into equation (7) and combined with the time-varying current I(t) and the eddy current amplitude coefficient to calculate the background eddy current magnetic field at this time, denoted as B. js ; The background eddy current magnetic field B at this time js With background eddy magnetic field B w The time constant τ is obtained by comparing the two and finding that the error between them is the smallest.

[0012] In this invention, the geomagnetic simulation system is an existing system, and this invention does not limit it.

[0013] In step (1) of this invention, sensor 1 is a background magnetic sensor, which is far away from the experimental platform and there are no ferromagnetic materials or conductive materials around it, thus ensuring that the change in the measured magnetic field value is only a change in the external excitation.

[0014] In step (1.1) of this invention, a background magnetic sensor is placed at a distance from the experimental platform. This invention does not have a specific limitation on the placement distance, as long as it can be ensured that the objects in the experimental platform do not affect the measurement of the magnetic sensor.

[0015] To address the shortcomings and deficiencies of the existing technology, this invention improves the eddy current magnetic field measurement method by using non-magnetic metals as the research object. The reason for using non-magnetic metals is that, compared to induced magnetic fields and fixed magnetic fields, the value of eddy current magnetic fields is relatively small; using non-magnetic metals as the research object can avoid interference from induced magnetic fields and fixed magnetic fields.

[0016] This invention places a background magnetic sensor at a distance from the experimental platform. The change in the sensor's measured value can be regarded as only a simulated magnetic field. Since the simulated magnetic field is proportional to the current flowing through the geomagnetic simulation coil, if the proportional relationship between the two is obtained, the current can be calibrated according to the change in the sensor's magnetic field measured value. This avoids random errors caused by insufficient repeatability of multiple power-on cycles and ensures the simultaneity of the current and magnetic field measured values.

[0017] Considering that the experimental environment is not an ideal space without conductive materials (such as the presence of aluminum alloy tracks, metal equipment and facilities, etc.), background eddy current magnetic fields will be generated, affecting the measurement accuracy. The background eddy current magnetic field is approximated as a first-order RL equivalent circuit. Its time-domain characteristic parameters and calibrated current values ​​are used to calculate and filter it, thereby improving the measurement accuracy of non-magnetic metal eddy current magnetic fields.

[0018] Compared with the prior art, the beneficial effects of this invention are as follows: (1) The present invention calibrates the time-varying current to avoid random errors caused by insufficient repeatability of multiple energizations; (2) The present invention ensures that the time of the applied time-varying current and the magnetic field measurement are consistent, thus satisfying the simultaneity requirement; (3) Based on the characteristics of eddy current magnetic field and eddy current, this invention analyzes the background eddy current magnetic field generated in space, avoids errors caused by insufficient zero magnetic space accuracy, and improves the measurement accuracy of non-magnetic metal eddy current magnetic field. In summary, the non-magnetic metal eddy current magnetic field measurement method proposed in this invention can reduce the measurement error of traditional eddy current magnetic field measurement methods from about 5% to less than 3%. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the operation of filtering with an external magnetic field; Figure 2 This is a schematic diagram of the first-order RL equivalent circuit; Figure 3 This is a schematic diagram of the z-component of the background eddy current magnetic field under slope magnetic field excitation; Figure 4 This is a schematic diagram of the background eddy current magnetic field acquisition; Figure 5 This is a schematic diagram of the background eddy current magnetic field fitting results; where the solid line represents the background eddy current magnetic field extracted from the measured values, and the dashed line represents the background eddy current magnetic field calculated based on the time-domain characteristic parameters and the current; Figure 6 This is a schematic diagram of the applied magnetic field and the z-component of the eddy current magnetic field of the aluminum plate under the downslope function in the z direction; where (a) represents the z-component of the applied magnetic field and (b) represents the z-component of the eddy current magnetic field of the aluminum plate. Figure 7 This is a schematic diagram of the z-components of the applied magnetic field and the eddy current magnetic field of the aluminum plate under a 5Hz sinusoidal magnetic field in the z-direction; where (a) represents the z-component of the applied magnetic field and (b) represents the z-component of the eddy current magnetic field of the aluminum plate. Figure 8 This is a schematic diagram of a half-model of the aluminum plate used in simulation verification. Figure 9 This is a schematic diagram of the mesh generation for finite element analysis of an aluminum plate. Figure 10 This is a schematic diagram of eddy currents in an aluminum plate; Figure 11 The z-components of the eddy current magnetic field of the aluminum plate at different measurement points under the excitation of the slope magnetic field are: (a) is the z-component of the eddy current magnetic field of the aluminum plate at measurement point B, and (b) is the z-component of the eddy current magnetic field of the aluminum plate at measurement point C. The dashed line represents the eddy current magnetic field of the aluminum plate extracted from the measurement value, the solid line represents the simulation result of the eddy current magnetic field of the aluminum plate, and the double colon line represents the error between the two. Figure 12 The z-components of the eddy current magnetic field of the aluminum plate under sinusoidal excitation at different frequencies at measurement point B are: (a), (b), (c), and (d) are the z-components of the eddy current magnetic field of the aluminum plate under sinusoidal excitation at frequencies of 1Hz, 2Hz, 5Hz, and 10Hz, respectively; the dashed line represents the eddy current magnetic field of the aluminum plate extracted from the measured value, the solid line represents the simulation result of the eddy current magnetic field of the aluminum plate, and the double colon line represents the error between the two. Figure 13 The z-components of the eddy current magnetic field of the aluminum plate under sinusoidal excitation at different frequencies at measurement point C are: (a), (b), (c), and (d) are the z-components of the eddy current magnetic field of the aluminum plate under sinusoidal excitation at frequencies of 1Hz, 2Hz, 5Hz, and 10Hz, respectively; the dashed line represents the eddy current magnetic field of the aluminum plate extracted from the measured value, the solid line represents the simulation result of the eddy current magnetic field of the aluminum plate, and the double colon line represents the error between the two. Figure 14 This is a schematic diagram of an experimental platform for measuring eddy current magnetic fields on an aluminum plate. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the embodiments.

[0021] Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in the field or according to the product instructions. Materials or equipment whose manufacturers are not specified are all conventional products that can be obtained by purchase.

[0022] Example 1 The method for measuring the eddy current magnetic field of non-magnetic metals includes the following steps: Based on the magnetic variation simulation method, the eddy current magnetic field generated by the motion of non-magnetic metal is equivalent to an externally applied magnetic field that changes in a certain regularity acting on the eddy current magnetic field excited by the non-magnetic metal. The external magnetic field is obtained by passing a current with different patterns through the geomagnetic simulation coil according to the geomagnetic simulation method; When a changing current is passed through the magnetic field simulation coil, the change in the magnetic field value measured by the sensor includes both the applied magnetic field and the eddy current magnetic field. Eddy current magnetic fields include background eddy current magnetic fields and non-magnetic metallic eddy current magnetic fields; By filtering out the external magnetic field and background eddy current magnetic field from the sensor measurements, the eddy current magnetic field generated by the non-magnetic metal under the excitation of the external magnetic field is obtained.

[0023] The method for measuring the eddy current magnetic field of non-magnetic metals includes the following steps: Based on the magnetic variation simulation method, the eddy current magnetic field generated by the motion of non-magnetic metal is equivalent to an externally applied magnetic field that changes in a certain regularity acting on the eddy current magnetic field excited by the non-magnetic metal. The external magnetic field is obtained by passing a current with different patterns through the geomagnetic simulation coil according to the geomagnetic simulation method; When a changing current is passed through the magnetic field simulation coil, the change in the magnetic field value measured by the sensor includes both the applied magnetic field and the eddy current magnetic field. Eddy current magnetic fields include background eddy current magnetic fields and non-magnetic metallic eddy current magnetic fields; By filtering out the external magnetic field and background eddy current magnetic field from the sensor measurements, the eddy current magnetic field generated by the non-magnetic metal under the excitation of the external magnetic field is obtained.

[0024] Specifically, the steps include the following: Step (1), obtaining the external magnetic field: The change in the applied magnetic field is directly proportional to the current flowing through the geomagnetic simulation coil, that is: (1) In the formula: B mn I(t) represents the external magnetic field generated by the simulated coil after it is energized at time t, in nT; I(t) represents the time-varying current flowing through the geomagnetic simulation coil at time t, in A; k mn This is the magnetic field coefficient of the geomagnetic simulation coil, with units of nT / A; If the magnetic field coefficient k of the geomagnetic simulation coil can be obtained mn And by monitoring the time-varying current I(t), the applied magnetic field can be obtained; Step (2), obtaining the background eddy current magnetic field: By analyzing the eddy current proportionality coefficient and the background eddy current, the background eddy current magnetic field can be obtained. The eddy current magnetic field is proportional to the eddy current, and the calculation formula is as follows: (2) In the formula, B wx (t), B wy (t), B wz (t) represent the x, y, and z components of the eddy current magnetic field at time t, respectively, all in nT; i b (t) represents the eddy current at time t, in A; k 11 k 21 k 31 These are the longitudinal, transverse, and vertical vortex proportionality coefficients, respectively, in nT / A; Step (3): Arrange the magnetic sensor 2 above the non-magnetic metal, and filter out the external magnetic field and background eddy current magnetic field in the change of the magnetic sensor measurement value to obtain the non-magnetic metal eddy current magnetic field. The calculation formula is as follows: (3) In the formula, △Bm B represents the change in the measured value of the magnetic sensor. mn B is the external magnetic field generated when the coil is energized. w B is the background eddy current magnetic field. wjs It is a non-magnetic metallic eddy current magnetic field.

[0025] The specific steps of step (1) are as follows: Step (1.1): Build an experimental platform in the geomagnetic simulation system and place a background magnetic sensor, denoted as sensor 1, at a distance from the experimental platform. There are no ferromagnetic materials or conductive materials around it. A constant current I is passed through the geomagnetic simulation coil, and the change in the magnetic field of sensor 1 before and after the energization is observed, i.e., the change in the applied magnetic field B. mn The magnetic field coefficient of the geomagnetic simulation coil at sensor 1 is calculated according to equation (1) and denoted as k. mnb ; From equation (1), we can obtain I(t) = B mn (t) / k mn Then, after any current is passed through the geomagnetic simulation coil, the applied magnetic field B measured by sensor 1 can be used to determine the magnetic field. mn (t) and the magnetic field coefficient k of the geomagnetic simulation coil mnb To calibrate the time-varying current I(t) passed through the geomagnetic simulation coil; Step (1.2): Place a non-magnetic metal inside the experimental platform, and arrange a magnetic sensor above the non-magnetic metal, denoted as sensor 2. Remove the non-magnetic metal from the experimental platform. Pass a certain constant current through the geomagnetic simulation coil, and calculate the magnetic field coefficient k of the geomagnetic simulation coil at sensor 2 by measuring the change in the magnetic field of sensor 2. mnc ; Based on the time-varying current I(t) calibrated by sensor 1, the applied magnetic field measured at sensor 2 is calculated according to equation (1). , and filter it out from the measurement value of sensor 2.

[0026] The specific steps for step (2) are as follows: Step (2.1), the equivalent process of eddies: If the background eddy current is equivalent to a multi-order RL dynamic circuit, then the time-varying characteristics of the background eddy current magnetic field are equivalent to the output characteristics of the RL dynamic circuit, and the terminal voltage u(t) of the equivalent circuit satisfies: (4) In the formula: B mnz (t) represents the applied magnetic field in the vertical direction, in nT; α is the terminal voltage coefficient; u(t) is the terminal voltage, in V; R and L are the equivalent resistance and equivalent inductance in the circuit, in Ω and H, respectively; The background eddy current i induced in the equivalent circuitb (t) is discretized into several units with a time interval of Δt; if Δt is sufficiently small, the rate of change of the magnetic field... Since is a constant, according to the formula for calculating the total response of a first-order dynamic circuit: (5) In the formula, i b (t0) represents the initial background eddy current at time t0, in A; the time constant τ is defined as L / R, in seconds; ΔB mnz α is the change in the applied magnetic field in the vertical direction, in nT; α is the terminal voltage coefficient; R is the equivalent resistance in the circuit, in Ω; Δt is the time interval. It is known that the applied magnetic field is directly proportional to the time-varying current flowing through the coil, that is: (6) Combining equations (5) and (6), and multiplying both ends by the eddy current proportionality coefficient k 11 k 21 k 31 ,but: (7) Where k1 is defined as the longitudinal vortex amplitude coefficient, derived from k mn α / R and longitudinal eddy current proportionality coefficient k 11 Multiplying them together yields the result; similarly, the transverse vortex amplitude coefficient k2 and the vertical vortex amplitude coefficient k3 are respectively k mn α / R and transverse vortex proportionality coefficient k 21 Vertical vortex proportionality coefficient k 31 Multiplying them together, all three units are in nT / A; B wx B wy B wz denoted as x, y, and z components of the background eddy current magnetic field; Δt is the time interval; I is the time-varying current; τ is the time constant; Step (2.2), eddy current amplitude coefficient: Before inserting non-magnetic metal, a slope current with a constant slope is passed through the geomagnetic simulation coil; Based on the measured values ​​of sensor 1 and the magnetic field coefficient k of the geomagnetic simulation coil mnb Determine the ramp current I(t) at each moment; The slope current value at any given time can be obtained by subtracting the background magnetic field value before energization from the magnetic field value measured by sensor 1 at that time, selecting the component in any direction, and then dividing it by the component of the magnetic field coefficient of the geomagnetic simulation coil at the location of sensor 1 in the same direction. Next, the magnetic field coefficient k of the geomagnetic simulation coil was used. mncThe external magnetic field is filtered out from the change in the measured value of sensor 2 by the ramp current I(t), resulting in the background eddy current magnetic field B. w ; (8) When Δt / τ is much greater than 1, the background eddy current magnetic field reaches a steady state, then: (9) After calibrating the time-varying current I(t) using sensor 1 in step (1.2), divide each component of the background eddy current magnetic field at each moment of the steady state by the corresponding current change rate, and take the average value of all moments of the steady state to obtain the eddy current amplitude coefficients k1, k2, and k3. Step (2.3), Identification of the time constant: The time constant τ is solved by function fitting. Then, the time constant τ is substituted into equation (7) and combined with the time-varying current I(t) and the eddy current amplitude coefficient to calculate the background eddy current magnetic field.

[0027] In step (2.3), the time constant τ is cyclically taken in the interval [1,50] ms with a step size of 0.01 ms. Substitute this value into equation (7) and combine it with the time-varying current I(t) and the eddy current amplitude coefficient to calculate the background eddy current magnetic field at this time, denoted as B. js ; The background eddy current magnetic field B at this time js With background eddy magnetic field B w The time constant τ is obtained by comparing the two and finding that the error between them is the smallest.

[0028] Example 3 This invention proposes a method for measuring the eddy current magnetic field of non-magnetic metals. Based on magnetic variation simulation, the eddy current magnetic field generated by the motion of a non-magnetic metal is equivalent to the eddy current magnetic field excited by a regularly changing external magnetic field acting on the non-magnetic metal. The external magnetic field can be obtained by passing different regularly changing currents through a geomagnetic simulation coil using geomagnetic simulation. When a changing current is passed through the geomagnetic simulation coil, the change in the magnetic field value measured by the sensor includes both the external magnetic field and the eddy current magnetic field. Since an ideal non-conductive space does not exist in a laboratory environment (e.g., the presence of aluminum alloy tracks, metal equipment, etc.), a certain background eddy current magnetic field is generated when the magnetic field in the space changes. Therefore, the eddy current magnetic field includes both the background eddy current magnetic field and the non-magnetic metal eddy current magnetic field. Clearly, only by filtering out the external magnetic field and the background eddy current magnetic field from the sensor measurements can the eddy current magnetic field generated by the non-magnetic metal under the excitation of an external magnetic field be obtained.

[0029] The specific operation process is as follows: Step (1), obtaining the external magnetic field: According to Biot-Sava's law, the change in the applied magnetic field is directly proportional to the current flowing through the geomagnetic simulation coil, that is: (10) In the formula: B mn I(t) represents the external magnetic field generated by the simulated coil after it is energized at time t, in nT; I(t) represents the time-varying current flowing through the geomagnetic simulation coil at time t, in A; k mn Let k be the magnetic field coefficient of the geomagnetic simulation coil, representing the change in the applied magnetic field when a unit current flows through the geomagnetic simulation coil, with units of nT / A. Therefore, if the magnetic field coefficient k of the geomagnetic simulation coil can be obtained... mn By monitoring the time-varying current I(t), the external magnetic field can be filtered out. The specific operation is as follows: (1.1) An experimental platform is built in the geomagnetic simulation system. A background magnetic sensor, denoted as sensor 1, is placed at a distance from the experimental platform. There are no ferromagnetic or conductive materials around it to ensure that the change in the measured magnetic field value is only a change in the external excitation. A constant current I is passed through the geomagnetic simulation coil, and the change in the magnetic field of sensor 1 before and after the current is applied is observed, i.e., the change in the external magnetic field B. mn At this time, B mn Since I(t) is constant, the magnetic field coefficient of the geomagnetic simulation coil at sensor 1 is calculated according to equation (10) and denoted as k. mnb From equation (10), we can obtain I(t) = B. mn (t) / k mn Then, after any current is passed through the geomagnetic simulation coil, the applied magnetic field B measured by sensor 1 can be used to determine the magnetic field. mn (t) and coefficient k mnb To calibrate the time-varying current I(t) passed through the geomagnetic simulation coil.

[0030] (1.2) Place a non-magnetic metal inside the experimental platform, and arrange sensor 2 above the non-magnetic metal. Remove the non-magnetic metal from the experimental platform. Similarly to (1.1), pass a constant current through the geomagnetic simulation coil, and calculate the magnetic field coefficient k of the geomagnetic simulation coil at sensor 2 by measuring the change in the magnetic field of sensor 2. mnc Based on the time-varying current I(t) calibrated by sensor 1, the applied magnetic field measured at sensor 2 can be calculated according to equation (10). Filter it out.

[0031] The operation process is as follows Figure 1 As shown.

[0032] Step (2), obtaining the background eddy current magnetic field: Considering that the eddy current magnetic field is proportional to the eddy current (since only the magnetic field is applied in the z-direction, we take the external excitation in the z-direction as an example): (11) In the formula, B wx (t), B wy (t), B wz (t) represent the x, y, and z components of the eddy current magnetic field at time t, respectively, all in nT; i b (t) represents the eddy current at time t, with units of A; k is defined as follows. 11 k 21 k 31 These are the longitudinal, transverse, and vertical eddy current proportionality coefficients, respectively, with units of nT / A. They represent the three components of the eddy current magnetic field generated at the sensor by a unit eddy current caused by a change in the external magnetic field. Therefore, by analyzing the eddy current proportionality coefficients and the background eddy current, the background eddy current magnetic field can be obtained.

[0033] (2.1) Equivalent process of eddy current From the perspective of the "path," eddy currents can be equivalent to a multi-order RL dynamic circuit. Therefore, the time-varying characteristics of the eddy current magnetic field can be equivalent to the output characteristics of the RL dynamic circuit. Based on the analysis, the model of the background eddy current is relatively simple and can be analyzed as a first-order RL circuit. Its equivalent circuit is as follows: Figure 2 As shown. Where u(t) is the terminal voltage in V; R and L are the equivalent resistance and equivalent inductance in the circuit, respectively, in Ω and H. Taking vertical external magnetic field excitation as an example, according to the law of electromagnetic induction, the terminal voltage u(t) of the equivalent circuit satisfies: (12) In the formula: B mnz (t) represents the applied magnetic field in the vertical direction, in units of nT; α is the terminal voltage coefficient.

[0034] The eddy current i induced in the equivalent circuit b (t) is discretized and divided into several units with a time interval of Δt for analysis. When Δt is sufficiently small, the rate of change of the magnetic field can be considered as... Since is a constant, according to the formula for calculating the total response of a first-order dynamic circuit: (13) In the formula, i b (t0) represents the initial background eddy current at time t0, in A; the time constant τ is defined as L / R, in seconds; ΔB mnz α is the change in the applied magnetic field in the vertical direction, in nT; α is the terminal voltage coefficient; R is the equivalent resistance in the circuit, in Ω; Δt is the time interval.

[0035] Assuming the initial eddy current is known, the eddy current after time Δt depends only on the time constant τ and the rate of change of the applied magnetic field. In step (1), it is known that the applied magnetic field is proportional to the time-varying current flowing through the geomagnetic simulation coil, therefore: (14) Combining equations (13) and (14), and multiplying both ends by the eddy current proportionality coefficient k. 11 k 21 k 31 Then it can be rewritten as: (15) Where k1 is defined as the longitudinal vortex amplitude coefficient, derived from k mn α / R and longitudinal eddy current proportionality coefficient k 11 Multiplying them together yields the result; similarly, the transverse vortex amplitude coefficient k2 and the vertical vortex amplitude coefficient k3 are respectively k mn α / R and transverse vortex proportionality coefficient k 21 Vertical vortex proportionality coefficient k 31 Multiplying them together, all three units are in nT / A; B wx B wy B wz denoted as x, y, and z components of the background eddy current magnetic field; Δt is the time interval; I is the time-varying current; and τ is the time constant.

[0036] (2.2) Eddy current amplitude coefficient Before inserting the non-magnetic metal, a sloped current with a constant slope is passed through the geomagnetic simulation coil, referring to the method described in step (1), based on the measured value of sensor 1 and the magnetic field coefficient k of the geomagnetic simulation coil. mnb The ramp current at each moment is determined, and then the magnetic field coefficient k of the geomagnetic simulation coil is used. mnc The external magnetic field is filtered out from the change in the measured value of sensor 2 by the slope current, resulting in the background eddy current magnetic field B. w ,like Figure 3 As shown (taking the z-component of the eddy current magnetic field as an example).

[0037] analyze Figure 3 It can be seen that when the eddy current magnetic field reaches its maximum, it will exhibit a period of approximately constant stability. The reason for this phenomenon is that when the applied excitation satisfies the ramp function, the slope is constant, and the rate of change of magnetic flux through the conductor cross section remains constant, so the eddy current will exhibit a period of stability, at which point the eddy current magnetic field is constant. According to equation (6), when Δt / τ is much greater than 1, the eddy current magnetic field reaches a stable state, at which point: (16) After calibrating the time-varying current using sensor 1 in step (1), divide each component of the background eddy current magnetic field at each moment of the steady state by the corresponding rate of change of current, and take the average value of all moments of the steady state to obtain the eddy current amplitude coefficients k1, k2, and k3.

[0038] (2.3) Identification of time constant From equation (15), it can be seen that the eddy current i b The only unknown in (t) is the time constant τ. The time constant τ is solved using a function fitting method, taking values ​​cyclically in the interval [1, 50] ms with a step size of 0.01 ms. These values ​​are then substituted into equation (15) and combined with the time-varying current I(t) and the eddy current amplitude coefficient to calculate the background eddy current magnetic field at this time, denoted as B. js , with background eddy current magnetic field B w Compare them.

[0039] Depend on Figure 3 It can be seen that the changes in the background eddy current magnetic field mainly consist of rising and falling segments. Selecting B... js and B w The rising and falling segments are compared, and the time constant τ is obtained when the error between the two is minimized.

[0040] Step (3): Arrange the magnetic sensor 2 above the non-magnetic metal, filter out the external magnetic field and background eddy current magnetic field in the change of the magnetic sensor measurement value, and obtain the non-magnetic metal eddy current magnetic field. The calculation formula is as follows: (17) In the formula, △B m B represents the change in the measured value of the magnetic sensor. mn For the external magnetic field, B w B is the background eddy current magnetic field. wjs It is a non-magnetic metallic eddy current magnetic field.

[0041] Thus, the time-domain characteristic parameters of the background eddy current magnetic field are identified. When the applied excitation is of arbitrary waveform, the background eddy current magnetic field B at each moment can be obtained by calibrating the current I(t) passed through the geomagnetic simulation coil by sensor 1 through equation (15). w (t).

[0042] The above method is used to filter out the applied magnetic field and background eddy current magnetic field from the change in the measured value of sensor 2, resulting in a non-magnetic metallic eddy current magnetic field. The specific operation is as follows: Figure 4 As shown.

[0043] Application Example: Aluminum Plate Eddy Current Magnetic Field Measurement According to the method of this invention, an aluminum plate is used as the research object to measure the eddy current magnetic field of the aluminum plate in the laboratory. Since the value of the eddy current magnetic field itself is relatively small, and the Earth's magnetic field fluctuates greatly during the day, affecting the acquisition of the eddy current magnetic field, this experiment is carried out in the early morning to ignore the influence of the Earth's magnetic field fluctuation.

[0044] Experiments were conducted using an existing geomagnetic simulation system and an aluminum plate. The aluminum plate, measuring 1.2m × 0.8m × 0.01m, was selected from 6061 aluminum profiles. A coordinate system was constructed with the center of the aluminum plate as the origin, the x-axis along the long side, the y-axis along the short side, and the z-axis pointing vertically upwards. A measurement platform was constructed along the y-axis, 4cm above the aluminum plate, using plastic profiles. Three magnetic sensors, A, B, and C, were placed at 10cm intervals from left to right, passing through the origin. Their coordinates are shown in Table 1. Using plastic profiles to construct the measurement platform avoids additional magnetic field interference. Sensor D, placed at a greater distance from the measurement platform, can be used to monitor current by neglecting the influence of eddy current magnetic fields. All sensors used were MAG-13 magnetic sensors. During ship motion, the eddy current magnetic field generated by rolling usually constitutes the majority. Based on the magnetic variation simulation method, external excitation needs to be applied in both the y and z directions to simulate the changing spatial magnetic field. Since the aluminum plate is only 10mm thick, the eddy currents generated by the conductor cross-section are not significant when the excitation is applied in the y direction. Therefore, this invention only analyzes the eddy current magnetic field generated by the aluminum plate when the external excitation changes in the z direction. Thus, only a vertical geomagnetic simulation coil is used in the experiment. The experimental setup is as follows: Figure 14 As shown.

[0045] Table 1. Coordinates of measurement points on aluminum plates in the laboratory Coordinates (m) (0,-0.1,0.04) (0,0,0.04) (0,0.1,0.04) The steps for measuring the eddy current magnetic field of an aluminum plate using the method proposed in this invention are as follows: (1) When the aluminum plate is not placed in the geomagnetic simulation coil, a constant current of 20A is passed through the geomagnetic simulation coil, and the changes in the measured values ​​of sensors A, B, and C, as well as the change in the measured value of sensor D, are obtained. The magnetic field coefficient k of the geomagnetic simulation coil is calculated according to the method of the present invention. mn The results are shown in Table 2. Table 2. Magnetic field coefficient k of the geomagnetic simulation coil at different sensor locations mn (2) Set the sampling frequency of the magnetic sensor to 1000Hz, and pass a ramp current with a slope of 10A / s and a maximum value of 10A through the geomagnetic simulation coil to obtain the changes in the measured values ​​of sensors A, B, and C, as well as the changes in the measured value of sensor D. First, subtract the measured value before power-on from the measured value at each moment to eliminate the background magnetic field; second, calibrate the ramp current using the method of this invention, and filter out the external magnetic field in the changes in the measured values ​​of sensors A, B, and C using the magnetic field coefficients of the geomagnetic simulation coil in Table 2, extract the background eddy current magnetic field, and solve the time-domain characteristic parameters of the background eddy current magnetic field according to the method in step (2). The results are shown in Table 3; among them, the fitting curve when solving the time constant τ using the method described in step (2.3) is as follows. Figure 5 As shown (taking the z-component of the background eddy current magnetic field at sensor B as an example).

[0046] Table 3. Time-domain characteristics of background eddy current magnetic field at different sensors (3) Place the aluminum plate into the geomagnetic simulation coil and pass a ramp current with a slope of 10A / s and a maximum value of 10A through the geomagnetic simulation coil; and sinusoidal currents with frequencies of 1Hz, 2Hz, 5Hz, and 10Hz and a peak value of 7.07A through the geomagnetic simulation coil. After each acquisition, record the magnetic field values ​​measured by sensors A, B, and C, and the change in the measured value of sensor D. First, eliminate the background magnetic field, calibrate the current I(t) flowing through the geomagnetic simulation coil according to the change in sensor D, and then filter out the external magnetic field; second, combine the time-domain characteristic parameters of the background eddy current magnetic field at different sensors in Table 3, and solve the background eddy current magnetic field according to the method of this invention; finally, filter out the background eddy current magnetic field from the measured value to obtain the eddy current magnetic field of the aluminum plate. The results of the eddy current magnetic field of the aluminum plate and the corresponding external magnetic field are as follows. Figure 6 , Figure 7 As shown (taking the z-component of the magnetic field at sensor B as an example, the magnetic field results at 5Hz are only listed for sinusoidal excitation).

[0047] (4) Simulation verification of eddy current magnetic field of aluminum plate The preceding section detailed the measurement method for eddy current magnetic fields in aluminum plates and demonstrated experiments conducted in the laboratory to extract eddy current magnetic fields under different excitations from sensors. The following section further uses the finite element method to simulate and analyze the eddy current magnetic field in aluminum plates. By comparing the simulated values ​​with the measured values, the accuracy of the measurement method for eddy current magnetic fields in aluminum plates is verified. The accuracy of the three-dimensional transient eddy current magnetic field numerical calculation based on the finite element method has already been verified in existing technologies.

[0048] (4.1) Simulation Modeling A simulation model was created using the aluminum plate selected in the experiment. The center of the aluminum plate was taken as the origin, and x, y, and z axes were constructed along the length, width, and height of the plate. The relative permeability of the aluminum was set to 1, and the electrical conductivity to 24.96 MS / m. A cuboid air domain cutoff boundary was constructed according to the appropriate proportions of the aluminum plate dimensions. Due to the symmetry of the eddy current magnetic field of the aluminum plate, the simulation model could be split along its symmetry plane, and half of the model could be simulated and analyzed. The mesh density of the aluminum plate model and its surrounding space was appropriately increased to improve computational efficiency and accuracy. This invention simulates and analyzes the portion where y ≥ 0. Based on the coordinates of the measurement points shown in Table 1, only measurement points B and C were selected during the simulation. Figure 8 As shown. The simulation model is meshed, and the mesh diagram is shown below. Figure 9 As shown.

[0049] The vector magnetic potential of the air domain cutoff boundary was set according to different excitations applied during experimental measurements, so that the entire air domain generated uniform ramp excitation and sinusoidal excitation in the z-direction, with the magnetic field direction along the positive z-axis. The ramp excitation simulation time was set to 0.28s. Since the instantaneous eddy current magnetic field generated by the ramp excitation changes significantly, and the eddy current magnetic field reaches a steady state after the ramp stabilizes, a variable step size setting was adopted. The step size for the first 0.08s was 0.001s, and the subsequent step size was 0.01s. The sinusoidal excitation simulation time was two cycles at the corresponding frequency, with step sizes of 0.02s, 0.01s, 0.004s, and 0.002s at frequencies of 1Hz, 2Hz, 5Hz, and 10Hz, respectively. Three-dimensional transient eddy current magnetic field numerical calculations were performed on an aluminum plate based on the finite element method. Taking a 5Hz sinusoidal excitation as an example, the eddy current generated by the ship at a certain moment was as follows... Figure 10 As shown, the eddy currents circulate clockwise, and the resulting eddy current magnetic field conforms to the distribution characteristics of the eddy current magnetic field of the aluminum plate under z-direction excitation.

[0050] (4.2) Results and Analysis After the simulation calculation is completed, the initial results obtained at the measurement point include both the eddy current magnetic field of the aluminum plate and the external magnetic field. In order to separate the eddy current magnetic field component, the conductivity of the aluminum plate material is set to 0, and the external magnetic field is obtained by simulation calculation under the same boundary conditions. Then, the external magnetic field is removed from the total magnetic field to obtain the simulation solution of the eddy current magnetic field.

[0051] Since the z-component of the eddy current magnetic field constitutes the majority of the data in this experiment, the simulation results and measured values ​​of the z-component of the eddy current magnetic field on the aluminum plate under different excitations are mainly compared. When the applied magnetic field is a ramp magnetic field, the starting moment when the ramp magnetic field is first applied is selected from the measured values, and the measured values ​​are compared with the simulated values. Figure 11 As shown.

[0052] When the applied magnetic field is sinusoidal, because the eddy current in the first cycle has a certain transition time, the simulation value is selected from the simulation result of the second cycle; the measured value is selected from one cycle of sinusoidal excitation from the time-varying current, and the eddy current magnetic field of the aluminum plate corresponding to that cycle is selected with the zero-crossing point as the starting time. The errors of the two are compared, such as... Figure 12 , Figure 13 As shown in Table 4, the maximum relative error between the simulated and measured values ​​under different excitations (taking the z-component as an example) is expressed as follows: (19) Table 4. Maximum relative error of z-component of eddy current magnetic field at measurement point on aluminum plate under different excitations. By comparing the simulated and measured values ​​of the eddy current magnetic field of the aluminum plate under different excitations, it can be seen that the two are in good agreement, with the maximum relative error not exceeding 3%, indicating that the proposed eddy current magnetic field measurement method has high measurement accuracy.

[0053] In summary, the measurement method employed in this invention can accurately measure the eddy current magnetic field of aluminum plates. In subsequent research on eddy current magnetic fields, this method can be used to measure the eddy current magnetic field of non-magnetic metals. It provides an important methodological reference for eddy current magnetic field measurement technology, and further application of this method to the measurement of eddy current magnetic fields of magnetic objects will improve the research on eddy current magnetic field measurement. This has significant theoretical guiding significance for in-depth research on eddy current magnetic fields, particularly for subsequent work such as ship magnetic compensation and shipborne geomagnetic field measurement.

[0054] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for measuring the eddy current magnetic field of non-magnetic metals, characterized in that, Includes the following steps: Based on the magnetic variation simulation method, the eddy current magnetic field generated by the motion of non-magnetic metal is equivalent to an externally applied magnetic field that changes in a certain regularity acting on the eddy current magnetic field excited by the non-magnetic metal. The external magnetic field is obtained by passing a current with different patterns through the geomagnetic simulation coil according to the geomagnetic simulation method; When a changing current is passed through the magnetic field simulation coil, the change in the magnetic field value measured by the sensor includes both the applied magnetic field and the eddy current magnetic field. Eddy current magnetic fields include background eddy current magnetic fields and non-magnetic metallic eddy current magnetic fields; By filtering out the external magnetic field and background eddy current magnetic field from the sensor measurements, the eddy current magnetic field generated by the non-magnetic metal under the excitation of the external magnetic field is obtained.

2. The method for measuring the eddy current magnetic field of non-magnetic metals according to claim 1, characterized in that, Includes the following steps: Step (1), obtaining the external magnetic field: The change in the applied magnetic field is directly proportional to the current flowing through the geomagnetic simulation coil, that is: （1） In the formula: B mn I(t) represents the external magnetic field generated by the simulated coil after it is energized at time t, in nT; I(t) represents the time-varying current flowing through the geomagnetic simulation coil at time t, in A; k mn This is the magnetic field coefficient of the geomagnetic simulation coil, with units of nT / A; If the magnetic field coefficient k of the geomagnetic simulation coil can be obtained mn And by monitoring the time-varying current I(t), the applied magnetic field can be obtained; Step (2), obtaining the background eddy current magnetic field: By analyzing the eddy current proportionality coefficient and the background eddy current, the background eddy current magnetic field can be obtained. The eddy current magnetic field is proportional to the eddy current, and the calculation formula is as follows: （2） In the formula, B wx (t), B wy (t), B wz (t) represent the x, y, and z components of the eddy current magnetic field at time t, respectively, all in nT; i b (t) represents the eddy current at time t, in A; k 11 k 21 k 31 These are the longitudinal, transverse, and vertical vortex proportionality coefficients, respectively, in nT / A; Step (3): Arrange the magnetic sensor 2 above the non-magnetic metal, and filter out the external magnetic field and background eddy current magnetic field in the change of the magnetic sensor measurement value to obtain the non-magnetic metal eddy current magnetic field. The calculation formula is as follows: （3） In the formula, △B m B represents the change in the measured value of the magnetic sensor. mn B is the external magnetic field generated when the coil is energized. w B is the background eddy current magnetic field. wjs It is a non-magnetic metallic eddy current magnetic field.

3. The method for measuring the eddy current magnetic field of non-magnetic metals according to claim 2, characterized in that, The specific steps of step (1) are as follows: Step (1.1): Build an experimental platform in the geomagnetic simulation system and place a background magnetic sensor, denoted as sensor 1, at a distance from the experimental platform. There are no ferromagnetic materials or conductive materials around it. A constant current I is passed through the geomagnetic simulation coil, and the change in the magnetic field of sensor 1 before and after the energization is observed, i.e., the change in the applied magnetic field B. mn The magnetic field coefficient of the geomagnetic simulation coil at sensor 1 is calculated according to equation (1) and denoted as k. mnb ; From equation (1), we can obtain I(t) = B mn (t) / k mn Then, after any current is passed through the geomagnetic simulation coil, the applied magnetic field B measured by sensor 1 can be used to determine the magnetic field. mn (t) and the magnetic field coefficient k of the geomagnetic simulation coil mnb To calibrate the time-varying current I(t) passed through the geomagnetic simulation coil; Step (1.2): Place a non-magnetic metal inside the experimental platform, and arrange a magnetic sensor above the non-magnetic metal, denoted as sensor 2. Remove the non-magnetic metal from the experimental platform. Pass a certain constant current through the geomagnetic simulation coil, and calculate the magnetic field coefficient k of the geomagnetic simulation coil at sensor 2 by measuring the change in the magnetic field of sensor 2. mnc ; Based on the time-varying current I(t) calibrated by sensor 1, the applied magnetic field measured at sensor 2 is calculated according to equation (1). , and filter it out from the measurement value of sensor 2.

4. The method for measuring the eddy current magnetic field of non-magnetic metals according to claim 3, characterized in that, The specific steps for step (2) are as follows: Step (2.1), the equivalent process of eddies: If the background eddy current is equivalent to a multi-order RL dynamic circuit, then the time-varying characteristics of the background eddy current magnetic field are equivalent to the output characteristics of the RL dynamic circuit, and the terminal voltage u(t) of the equivalent circuit satisfies: （4） In the formula: B mnz (t) represents the applied magnetic field in the vertical direction, in nT; α is the terminal voltage coefficient; u(t) is the terminal voltage, in V; R and L are the equivalent resistance and equivalent inductance in the circuit, respectively, with units of Ω and H. The background eddy current i induced in the equivalent circuit b (t) is discretized into several units with a time interval of Δt; if Δt is sufficiently small, the rate of change of the magnetic field... Since is a constant, according to the formula for calculating the total response of a first-order dynamic circuit: （5） In the formula, i b (t0) represents the initial background eddy current at time t0, in A; the time constant τ is defined as L / R, in seconds; ΔB mnz α is the change in the applied magnetic field in the vertical direction, in nT; α is the terminal voltage coefficient; R is the equivalent resistance in the circuit, in Ω; Δt is the time interval. It is known that the applied magnetic field is directly proportional to the time-varying current flowing through the coil, that is: （6） Combining equations (5) and (6), and multiplying both ends by the eddy current proportionality coefficient k 11 k 21 k 31 ,but: （7） Where k1 is defined as the longitudinal vortex amplitude coefficient, derived from k mn α / R and longitudinal eddy current proportionality coefficient k 11 Multiplying them together yields the result; similarly, the transverse vortex amplitude coefficient k2 and the vertical vortex amplitude coefficient k3 are respectively k mn α / R and transverse vortex proportionality coefficient k 21 Vertical vortex proportionality coefficient k 31 Multiplying them together, all three units are in nT / A; B wx B wy B wz denoted as x, y, and z components of the background eddy current magnetic field; Δt is the time interval; I is the time-varying current; τ is the time constant; Step (2.2), eddy current amplitude coefficient: Before inserting non-magnetic metal, a slope current with a constant slope is passed through the geomagnetic simulation coil; Based on the measured values ​​of sensor 1 and the magnetic field coefficient k of the geomagnetic simulation coil mnb Determine the ramp current I(t) at each moment; The slope current value at any given time can be obtained by subtracting the background magnetic field value before energization from the magnetic field value measured by sensor 1 at that time, selecting the component in any direction, and then dividing it by the component of the magnetic field coefficient of the geomagnetic simulation coil at the location of sensor 1 in the same direction. Next, the magnetic field coefficient k of the geomagnetic simulation coil was used. mnc The external magnetic field is filtered out from the change in the measured value of sensor 2 by the ramp current I(t), resulting in the background eddy current magnetic field B. w ; （8） When Δt / τ is much greater than 1, the background eddy current magnetic field reaches a steady state, then: （9） After calibrating the time-varying current I(t) using sensor 1 in step (1.2), divide each component of the background eddy current magnetic field at each moment of the steady state by the corresponding current change rate, and take the average value of all moments of the steady state to obtain the eddy current amplitude coefficients k1, k2, and k3. Step (2.3), Identification of the time constant: The time constant τ is solved by function fitting. Then, the time constant τ is substituted into equation (7) and combined with the time-varying current I(t) and the eddy current amplitude coefficient to calculate the background eddy current magnetic field.

5. The method for measuring the eddy current magnetic field of non-magnetic metals according to claim 4, characterized in that, In step (2.3), the time constant τ is cyclically taken in the interval [1,50] ms with a step size of 0.01 ms. Substitute this value into equation (7) and combine it with the time-varying current I(t) and the eddy current amplitude coefficient to calculate the background eddy current magnetic field at this time, denoted as B. js ; The background eddy current magnetic field B at this time js With background eddy magnetic field B w The time constant τ is obtained by comparing the two and finding that the error between them is the smallest.

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