Medium-high-frequency high-field-intensity alternating magnetic field measurement method based on electromagnetic induction

Abstract

The present invention relates to the field of magnetic field measurement, and provides a medium-high-frequency high-field-intensity alternating magnetic field measurement method based on electromagnetic induction. In the present invention, an alternating induced electromotive force generated by the electromagnetic induction effect of a closed metal coil under the action of an alternating magnetic field is used, a designed probe element is applied to convert an alternating magnetic field parameter into a synchronously-varying electrical signal, and the frequency and amplitude of the electrical signal are measured to characterize the frequency and field intensity of an alternating magnetic field to be measured, thereby broadening the measurable frequency and field intensity range of the alternating magnetic field. In addition, the device in the present invention is highly tolerant to measurement environment requirements, compact, convenient to carry, and easy and convenient to operate.

Description

A method for measuring medium-high frequency and high field strength alternating magnetic fields based on electromagnetic induction Technical Field

[0001] The present invention relates to the field of magnetic field measurement technology, and more particularly to a method for measuring medium-high frequency and high field strength alternating magnetic fields based on electromagnetic induction. Background Art

[0002] Medium-high-frequency and high-field-strength alternating magnetic fields are widely used in magnetic hyperthermia, induction cladding, chemical catalysis and other fields, in which accurate monitoring of magnetic field parameters (field strength, frequency, etc.) is crucial.

[0003] Currently, devices used to measure alternating magnetic fields primarily include Hall sensors, fluxgates, anisotropic magnetoresistive (AMR) sensors, and Josephson junctions. The Hall element used in Hall sensors is a semiconductor magnetoelectric device whose electrical performance parameters are significantly affected by temperature. When measuring medium- and high-frequency alternating magnetic fields, the Hall element can experience temperature drift due to induced current heating, making it difficult to accurately measure magnetic field strength. Fluxgates utilize the hysteresis effect of a high-permeability core in an applied magnetic field to measure magnetic field strength by differentially detecting coil signals. However, due to the transformer effect, high-frequency excitation sources can couple into the feedback winding, generating noise interference, making them unsuitable for measuring strong medium- and high-frequency magnetic fields. Anisotropic magnetoresistive sensors are constructed of ferromagnetic metals or alloys. After linearization, AMR sensors exhibit high sensitivity to low-frequency and low-field-strength magnetic fields. However, due to magnetic field perturbations, measurement accuracy and sensitivity decrease rapidly with increasing field strength and frequency, making them unsuitable for measuring medium- and high-frequency alternating magnetic fields. Josephson junctions utilize the changes in the maximum superconducting current in a superconducting Josephson junction to measure magnetic fields. Josephson junctions are usually used to measure low-frequency magnetic fields or static magnetic fields. This is because eddy current losses will be generated in the Josephson junction in medium and high frequency alternating magnetic fields, causing its temperature to rise and unable to maintain a stable superconducting state, making the measurement data inaccurate.

[0004] In summary, it can be seen that current magnetic field measurement devices can measure alternating magnetic fields with low frequency and high field strength or medium and high frequency and low field strength, but there is still a lack of alternating magnetic field measurement devices with medium and high frequency and high field strength (magnetic field frequency f≥100kHz, magnetic field strength H≥200Oe).

[0005] Summary of the Invention

[0006] The present invention provides a device that can quickly measure the parameters of medium-high-frequency and high-field-strength alternating magnetic fields. By using the principle of alternating induced electromotive force generated by the electromagnetic induction effect of a closed metal coil under the action of an alternating magnetic field, the alternating magnetic field parameters are converted into synchronously changing electrical signals using designed probe elements. The frequency and amplitude of the electrical signals are measured to characterize the frequency and field strength of the alternating magnetic field to be measured, thereby broadening the measurable frequency and field strength range of the alternating magnetic field.

[0007] To achieve the above objectives, the present invention adopts the following technical solutions:

[0008] A method for measuring medium-high frequency and high field strength alternating magnetic fields based on electromagnetic induction. This method is based on the principle of electromagnetic induction and uses a designed magnetic field measurement probe element to convert alternating magnetic field parameters into synchronously changing electrical signals. The frequency and amplitude of the electrical signal are measured to characterize the frequency and field strength of the alternating magnetic field to be measured. The magnetic field measurement device mainly includes: a magnetic field measurement probe device, a signal acquisition and processing module, and an output module. Among them, the magnetic field measurement probe is used to convert the alternating magnetic field parameters into a measurable electrical signal, which is transmitted to the signal acquisition and processing module after voltage division. The frequency and field strength of the alternating magnetic field processed by the signal acquisition and processing module are displayed through the output module. The specific steps are as follows:

[0009] The first step is to convert the alternating magnetic field parameters into synchronously changing electrical signals through a magnetic field probe.

[0010] The magnetic field measurement probe is fixedly placed at the designated location of the magnetic field to be measured. The magnetic field measurement probe includes a long-axis device 3 and a cube box 4 for fixing the coil position. The cube box 4 is a hollow structure, and a horizontal coil 1 and a vertical coil 2 are installed perpendicular to each other inside. The horizontal coil 1 and the vertical coil 2 are connected to the voltage divider via a transmission line.

[0011] The horizontal coil 1 is designed with an insulating layer to increase resistivity and reduce heating caused by eddy currents. Its horizontal projection is a completely symmetrical shape (e.g., circular, rectangular, or hexagonal). The coil radius ranges from 1mm to 5mm, and the diameter of the copper wire used for winding the coil ranges from 1mm to 5mm. The number of turns ranges from 1 to 10, with each layer tightly packed. The intersection of the normal extensions of the center points of the horizontal coil 1 and the vertical coil 2 is the center point of the cube box 4. The axial extension of the long-axis device 3 also passes through the center of the cube box to ensure accurate and convenient rotation. The vertical coil 2 is identical in structure to the horizontal coil 1, differing only in its placement. The vertical coil 2 can only be placed at the intersection of the long-axis device 3 and the cube box 4, or opposite the intersection, to ensure that the measured values ​​do not change before and after rotation. Furthermore, the two coils should not interfere with each other in space. The long-axis device 3 is a hollow rod to facilitate installation of the transmission line.

[0012] In addition, in order to reduce the voltage and current amplitude changes and phase lag along the line caused by the distributed parameter effect during high-frequency signal transmission, the transmission line chooses to use a coaxial signal line or twisted pair with a diameter of 1mm to 5mm.

[0013] In the second step, the signal acquisition and processing module processes the continuous periodic electrical signal converted by the probe into a discrete time series, and then obtains the alternating magnetic field frequency and field strength by processing the discrete time series.

[0014] The signal acquisition and processing module includes a printed circuit board and a clock generator arranged on the circuit board (enabling the device to collect signals), a high-sampling-rate analog-to-digital converter (converting the collected electrical signals into digital signals), a central processing unit (processing the converted digital signals), input and output terminals, and a casing (which must have strong magnetic field shielding properties).

[0015] The electrical signal after the first step is reduced by the analog-to-digital converter, which then discretizes the signal. The central processing unit then processes the discretized signal, amplifies it to the original signal size, and finally outputs it. The signal processing process is as follows:

[0016] Step 2.1: Take the sampling frequency as F s , the number of data points is N, and the electrical signal collected by the magnetic field measurement probe is subjected to fast Fourier transform.

[0017] Step 2.2: The central processing unit compares the collected data points and obtains the maximum value K among all scattered points in one cycle. max , the frequency corresponding to the maximum value is obtained according to the following formula (1), which is the frequency of the electrical signal to be measured. Since the measured signal changes synchronously with the alternating magnetic field, this frequency is also the frequency f of the alternating magnetic field.

[0018] Step 2.3: Take the maximum value point K among all the scattered points determined in step 2 max The arithmetic mean K of the previous and next scatter points - max , then take the maximum value point K max and the arithmetic mean K of the next scatter point + max , and then find the ratio of these two arithmetic means Finally, the coefficient r is calculated according to the following formula (2).

[0019] Step 2.4: Calculate the amplitude U of the electrical signal in the coil at this time according to the following formula (3).

[0020] Step 2.5: After obtaining two voltage measurement data (horizontal position coil data U1 and vertical position coil data U2), rotate the front end of the probe ninety degrees and measure again to obtain two voltage measurement data again (horizontal coil data U3 after position change and vertical position coil data U4 whose position has not changed).

[0021] Step 2.6: According to the principle of electromagnetic induction, the voltage U is proportional to the alternating magnetic field strength B, the alternating magnetic field angular frequency ω = 2πf, the induction coil area S, the number of induction coil turns N, and the cosine value cosθ of the angle between the magnetic flux lines and the coil normal vector. U = NBSωcosθ (4)

[0022] Assume that during measurement, the angle between the horizontal coil and the magnetic flux lines passing through the coil is α, the angle between the vertical coil and the magnetic flux lines is β, and the angle between the horizontal coil and the magnetic flux lines after rotation is γ. At this time, U1, U2, U3, and U4 are: U1 = N1BS1ωcosα (5) U2 = U4 = N2BS2ωcosβ (6) U3 = N1BS1ωcosγ (7)

[0023] Then the sum of the squares of the cosines of the angles between a vector and the three coordinate axes of the spatial rectangular coordinate system is 1. That is: cos 2 α+cos 2 β+cos 2 γ=1(8)

[0024] The alternating magnetic field intensity B can be calculated according to the above formula. The alternating magnetic field frequency f and the amplitude of the measured electrical signal U are obtained by steps 2.2 and 2.4, and the induction coil area S and the number of turns N are limited according to actual needs.

[0025] The third step is to display the measured alternating magnetic field frequency and field strength through the output module.

[0026] The output module is a liquid crystal display, which is connected to the PBC circuit board GPIO using an I2C communication method and is mainly used to display the measured alternating magnetic field frequency and field strength.

[0027] Furthermore, when selecting an A / D converter, it should be noted that during the analog / digital signal conversion process, the sampling frequency should be 4 to 10 times the highest frequency of the signal, so that the collected digital signal can effectively retain the original signal information.

[0028] Furthermore, to ensure that the signal acquisition and processing module is not affected by the alternating magnetic field to be measured when processing data, low resistance and high magnetic permeability materials should be used when making the housing;

[0029] Furthermore, to ensure circuit safety, the voltage input to the PCB should be less than its rated voltage (e.g., 3.3V). Two non-inductive resistors of different sizes are connected in series and then connected in parallel across the coil probe to create a voltage divider circuit. The circuit board is then connected in parallel across the small resistor to achieve voltage control. The resistance ratio of the two resistors is controlled within a range of 1 to 100, with multiple selectable settings available based on the resistance ratio. For example, five resistance ratios are designed: 1:1, 1:9, 1:29, 1:59, and 1:99. This not only ensures circuit safety during measurement but also improves measurement accuracy.

[0030] The benefits of the alternating magnetic field measuring device of the present invention are:

[0031] The present invention is based on the principle of electromagnetic induction. By sampling and analyzing the electrical signal generated by the induction coil, the frequency and field strength of the medium and high frequency alternating magnetic field to be measured are obtained, thereby broadening the measurable frequency and field strength range of the alternating magnetic field. The present invention uses two coils for measurement to reduce the influence of the self-inductance and mutual inductance of the coils in the high-frequency magnetic field on the measurement data. In addition, the two measurements can make the measurement more accurate by comparing the measurement data of the coils whose positions do not change. The invention directly processes the electrical signal to effectively reduce the error in the measurement caused by the temperature rise of the coils in the medium and high frequency alternating magnetic field for a long time. In addition, the device of the present invention is tolerant to the measurement environment requirements, the device is small in size, easy to carry, and easy to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG1 is a schematic diagram of the components of the device of the present invention;

[0033] FIG2 is a schematic diagram of a measuring probe of the present invention;

[0034] FIG3 is a schematic diagram of an alternating magnetic field generating coil;

[0035] Figure 4 is a graph showing magnetic field intensity and frequency at different current levels;

[0036] In the figure: 1 horizontal position coil; 2 vertical position coil; 3 rotation axis; 4 cube box. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings and specific preferred embodiments.

[0038] The portable medium- and high-frequency alternating magnetic field measurement device designed in this example mainly includes: an STM32 microcontroller, a coil probe, a shell made of nickel silver as the main material, and two pairs of twisted-pair cables.

[0039] Specific steps:

[0040] Step 1: Based on Figure 3, the maximum field strength generated by the alternating magnetic field generating coil is 450 Oe. Select two copper coils with a diameter of 4.5 mm and 1.5 turns. Connect them with twisted-pair wires and secure them in a cube box made of acrylic sheet to create the front end of the coil probe, as shown in Figure 2. Pass the twisted-pair wires through the carbon fiber rods, connect them to the voltage divider, and then to the microcontroller.

[0041] Step 2: Since the frequency of the magnetic field measured in this example is around 100kHz, an STM32 microcontroller with a sampling frequency of 14MHz is directly selected as the acquisition and processing module. The program is burned into the microcontroller and can be run. The microcontroller is installed in the prepared housing and connected to a display. Finally, the measurement probe is connected to the signal acquisition component.

[0042] Step 3: Keep the probe fixed at the center of the high-frequency alternating magnetic field generating coil in Figure 3. Then turn on the measuring device and the magnetic field generating device, save the stable data, and rotate the carbon fiber rod 90 degrees. ° , recording the measured magnetic field strength and frequency. Then, change the coil current, save the data, and rotate the carbon fiber rod back to its original position to obtain the second reading. Repeat this process, varying the current from 150A to 800A, with each experiment consisting of 50A increments.

[0043] Step 4: Data processing: For a finite length solenoid, the theoretical value of the magnetic field intensity B (in Oe) at the center position can be calculated according to formula (5).

[0044] Among them, N = 12 is the number of turns of the alternating magnetic field coil; I is the current passing through the coil; d = 0.2m is the diameter of the alternating magnetic field generating coil; l = 0.22m is the total height of the alternating magnetic field generating coil.

[0045] The results obtained by integrating the measured and theoretical values ​​of the magnetic field strength at 14 measurement points are shown in Figure 4.

[0046] The above-described embodiments merely express the implementation methods of the present invention, but should not be understood as limiting the scope of the patent of the present invention. It should be pointed out that for those skilled in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1. A method for measuring medium - high frequency and high - field - strength alternating magnetic fields based on electromagnetic induction, characterized in that, for the method for measuring medium - high frequency and high - field - strength alternating magnetic fields, first, the alternating magnetic field parameters are converted into measurable electrical signals that change synchronously through a magnetic field measurement probe; second, after voltage division, they are transmitted to a signal acquisition and processing module, and the frequency and amplitude of the electrical signals are measured by the signal acquisition and processing module to characterize the frequency and field strength of the alternating magnetic field to be measured; finally, the measurement results are displayed through an output module.

2. The method for measuring medium - high frequency and high - field - strength alternating magnetic fields based on electromagnetic induction according to claim 1, characterized in that, it includes the following steps: The first step, the alternating magnetic field parameters are converted into measurable electrical signals that change synchronously through a magnetic field measurement probe; The magnetic field measurement probe is fixedly placed at a designated position in the magnetic field to be measured; the magnetic field measurement probe includes a long - axis - type device (3) and a cube box (4) for fixing the coil position. The cube box (4) is a hollow structure, and a horizontal coil (1) and a vertical coil (2) perpendicular to each other are installed inside it. The horizontal coil (1) and the vertical coil (2) are connected to a voltage - dividing part through a transmission line; The second step, the signal acquisition and processing module processes the continuous periodic electrical signals converted by the probe into discrete time series, and obtains the frequency and field strength of the alternating magnetic field through the processing of the discrete time series; specifically: The signal acquisition and processing module includes a printed circuit board and a clock generator, a high - sampling - rate analog - to - digital converter, a central processing unit, an input - output terminal, and a housing with strong magnetic field shielding arranged on the circuit board; the reduced electrical signal is discretely processed by the analog - to - digital converter through the signal acquisition and processing module, and then the central processing unit processes the discrete signal and amplifies it back to the original signal size, and finally outputs it; the signal processing process is as follows: Step 2.1: With a sampling frequency of F s , and the number of data points being N, perform a fast Fourier transform on the electrical signals collected by the magnetic field measurement probe; Step 2.2: Compare the collected data points through the central processing unit to obtain the maximum value K among all the scattered points within one period. max , obtain the frequency corresponding to the maximum value according to the following formula (1), which is the frequency of the electrical signal to be measured; since the signal to be measured changes synchronously with the alternating magnetic field, this frequency is also the frequency f of the alternating magnetic field; Step 2.3: Take the arithmetic mean K of the scatter point immediately before and the scatter point immediately after the maximum point K determined in Step 2 max Then, take the arithmetic mean K of the maximum point K - max and the scatter point immediately after it max Then, calculate the ratio of these two arithmetic means + max ​ Finally, the coefficient r is calculated according to the following formula (2); Step 2.4: Calculate the amplitude U of the electrical signal in the coil at this time according to the following formula (3); Step 2.5: The horizontal position coil data U can be obtained in Step 2.4 1 and the vertical position coil data U 2 . After obtaining the two voltage measurement data, rotate the front part of the magnetic field measurement probe by ninety degrees and measure again. Two voltage measurement data are obtained again, that is, the data U of the horizontal coil after the position change is obtained 3 and the vertical position coil data U with unchanged position 4 ; Step 2.6: According to the principle of electromagnetic induction, the voltage U is proportional to the alternating magnetic field strength B, the alternating magnetic field angular frequency ω = 2πf, the area S of the induction coil, the number of turns N of the induction coil, and the cosine value cosθ of the included angle between the magnetic induction line and the normal vector of the coil; U = NBSωcosθ (4) Assume that when measuring, the angle between the horizontal-position coil and the magnetic induction line passing through the coil is α, the angle between the vertical-position coil and the magnetic induction line is β, and the angle between the horizontal-position coil and the magnetic induction line after rotation is γ; the U at this time 1 、U 2 、U 3 、U 4 are respectively: U 1 = N 1 BS 1 ωcosα (5) U 2 = U 4 = N 2 BS 2 ω cos β (6) U 3 = N 1 BS 1 ωcosγ (7) Furthermore, the sum of the squares of the cosines of the included angles formed by a vector with the three coordinate axes of a space rectangular coordinate system is 1; that is: cos 2 α + cos 2 β + cos 2 γ = 1 (8) According to the above formula, the alternating magnetic field strength B can be calculated; among them, the alternating magnetic field frequency f and the amplitude U of the electrical signal to be measured are obtained from steps 2.2 and 2.4, and the area S and the number of turns N of the induction coil are limited according to actual needs; The third step, the measured alternating magnetic field frequency and field strength are displayed through an output module.

3. The method for measuring medium - high frequency and high - field - strength alternating magnetic fields based on electromagnetic induction according to claim 2, characterized in that, in the magnetic field measurement probe described above: The horizontal position coil (1) is provided with an insulating layer, and its horizontal projection shape is a completely symmetric figure, with a radius of 1 mm to 5 mm. The diameter of the copper wire used for winding the coil is 1 mm to 5 mm, and the number of turns of the coil is 1 to 10 turns and is closely arranged in each layer during winding; the intersection point of the normal extension lines of the center points of the horizontal position coil (1) and the vertical position coil (2) is the center point of the cube box (4), and the two coils should not interfere with each other in space; the axial extension line of the long-axis type device (3) also passes through the center of the cube box; the structure of the vertical position coil (2) is exactly the same as that of the horizontal position coil (1), except for the arrangement position. The arrangement position of the vertical position coil (2) can only be the intersection surface of the long-axis type device (3) and the cube box (4) or the opposite surface of the intersection surface, ensuring that the measured values before and after rotation do not change.

4. A method for measuring medium and high frequency and high magnetic field strength alternating magnetic fields based on electromagnetic induction according to claim 3, characterized in that the long-axis type device (3) is a hollow rod; the transmission line is selected from coaxial signal lines or twisted pairs with a diameter of 1 mm to 5 mm.

5. A method for measuring medium and high frequency and high magnetic field strength alternating magnetic fields based on electromagnetic induction according to claim 2, characterized in that the output module in the third step is a liquid crystal display, which is connected to the GPIO of the PBC circuit board by using the I2C communication method and is used to display the measured alternating magnetic field frequency and field strength.

6. A method for measuring medium and high frequency and high magnetic field strength alternating magnetic fields based on electromagnetic induction according to claim 2, characterized in that during the process of the A / D converter converting analog / digital signals, the sampling frequency is 4 to 10 times the highest frequency of the signal.

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