Calibration method of alternating current magnetometer calibration device

Abstract

The invention provides a calibration method for an alternating-current magnetometer calibration device, relates to the technical field of calibration methods for alternating-current magnetometer calibration devices, and aims at specifying some technical parameters of the alternating-current magnetometer calibration device, improving and perfecting a traditional calibration method, and carrying out unified specification on value traceability of an alternating-current magnetometer. The technical guarantee is provided for the value traceability of the alternating current magnetometer; when the alternating-current magnetometer calibration device works, the excitation current indication error, the excitation current stability, the excitation current distortion degree, the magnetic field coil constant and the magnetic field uniformity of the alternating-current magnetometer calibration device are calibrated at the same time, and the metering performance of the alternating-current magnetometer calibration device can be comprehensively, uniformly and accurately evaluated through the error conditions of various parameters.

Description

Technical Field

[0001] The invention relates to the technical field of calibration of a magnetic field generation calibration instrument, and in particular to a calibration method for an AC magnetometer calibration device. Background Art

[0002] With the development of science and technology, AC magnetometers are being used more and more widely, ranging from high-speed rail track inspection, GPS navigation systems, metal material flaw detection, safety inspection, electromagnetic compatibility testing, position measurement and other fields. JJG 1049-2009 "Calibration Procedure for Weak Magnetic Field Alternating Magnetometers" is the only guiding technical regulation for AC magnetometers. It sets requirements for the metrological performance of the main metrological standards of the calibration device, such as AC constant current sources, frequency meters, AC standard resistors and AC voltmeters. This technical regulation is 15 years old. With the advancement of technology, manufacturers of AC magnetometer calibration devices have modularly integrated standard resistors, frequency meters and AC voltmeters into alternating current sources. A single alternating current source can output high-precision alternating current with adjustable amplitude and frequency, and can be used with computer software to automatically control the output current of the alternating current source through a program, greatly reducing the deviation caused by human operation. The schematic diagram of the typical structure of an AC magnetometer calibration device is shown below. Figure 1 As shown, the structure of the AC magnetometer calibration device has changed. It is no longer a pile of several complicated metrological standards. After modular integration, it has higher accuracy, faster response and higher practicality.

[0003] The calibration device of an AC magnetometer generally consists of an alternating current source, a magnetic field coil (Helmholtz coil or solenoid coil) and a probe position positioning device. The alternating current source and the magnetic field coil are generally calibrated separately, such as the alternating current source only calibrates the indication error of the alternating current, and the magnetic field coil only calibrates the coil constant. This leads to some problems: (1) Section 7.1.2.1 of JJG 1049-2009 "Calibration Procedure for Weak Magnetic Field Alternating Magnetometers" requires that "within 30 minutes, the change in current amplitude should not be greater than one tenth of the absolute value of the maximum allowable error of the alternating magnetometer being tested", which puts forward requirements for the stability of the alternating current source. It is necessary to measure the stability of the alternating current source within a certain period of time; (2) When the alternating current source outputs alternating current, there are certain harmonics. It is necessary to limit the content of the harmonics to avoid the high harmonic content affecting the magnetic induction intensity generated by the magnetic field coil at a certain frequency. The usual method is to use a distortion tester to directly measure the output current of the alternating current source. This approach has a disadvantage. For example, when the magnetic field is weak, the current in the circuit is small and the inductive reactance effect obviously causes the distortion to increase. Therefore, it is meaningless to measure the distortion without using the actual circuit. (3) When the alternating current source is used, it is connected to the magnetic field coil. The current generated by the circuit is inductive. However, when calibrating the alternating current source, the standard meter method or the current-to-voltage conversion method with a resistor in series is used. In fact, only the alternating current source is measured, not the actual circuit. (4) The magnetic field coil generally adopts a Helmholtz coil or a solenoid coil. The magnetic field generated by these two coils is more uniform. The size of the uniform area is affected by the coil size. It is necessary to give the uniformity of the central area of ​​the coil to minimize the influence of the magnetic field inhomogeneity on the calibration results of the alternating magnetometer.

[0004] In view of the above, the present invention provides a calibration method for an AC magnetometer calibration device to solve the above problems. Summary of the invention

[0005] The present invention aims to specify some technical parameters of an AC magnetometer calibration device, improve and perfect the traditional calibration method, provide a calibration method for an AC magnetometer calibration device, and provide technical guarantee for the value traceability of the AC magnetometer.

[0006] A calibration method for an AC magnetometer calibration device is intended to specify some technical parameters of the AC magnetometer calibration device, improve and perfect the traditional calibration method, and standardize the traceability of the value of the AC magnetometer. The method is characterized in that it includes calibration of the excitation current indication error, the excitation current stability, the excitation current distortion, the magnetic field coil constant, and the magnetic field uniformity. The metrological performance of the AC magnetometer calibration device can be comprehensively, uniformly and accurately evaluated through the error conditions of various parameters.

[0007] The significant progress of the present invention is that it standardizes and unifies the value traceability method of the AC magnetometer calibration device, uniformly calibrates the alternating current source and the magnetic field coil as a whole, stipulates some technical parameters of the AC magnetometer calibration device, improves and perfects the traditional calibration method, provides technical guarantee for the value traceability of the AC magnetometer, and accurately and comprehensively evaluates the metrological performance of the AC magnetometer calibration device. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 This is a schematic diagram of the structure of a typical weak magnetic field AC magnetometer calibration device;

[0009] Figure 2 This is a schematic structural diagram of an alternating magnetometer calibration device using a Helmholtz coil according to the present invention;

[0010] Figure 3 This is a schematic structural diagram of an alternating magnetometer calibration device using a solenoid according to the present invention;

[0011] Figure 4 This is a schematic diagram of the error of the excitation current indication measured by the standard meter method of the present invention;

[0012] Figure 5 A schematic diagram of the error of the excitation current indication measured by the current-voltage conversion method of the present invention;

[0013] Figure 6 Schematic diagram of an equivalent circuit of an induced voltage measurement circuit of the present invention;

[0014] Figure 7 This is a schematic diagram of current distortion measurement of the present invention;

[0015] Figure 8 This is a principle block diagram of the direct induction method of the present invention;

[0016] Fig. 9 A schematic diagram of the measurement points of the uniform area of ​​the Helmholtz coil of the present invention;

[0017] Fig.10 Schematic diagram of measurement points in the magnetic field uniformity zone of the solenoid coil of the present invention. DETAILED DESCRIPTION

[0018] The present invention provides a calibration method for an AC magnetometer calibration device, and simultaneously calibrates the excitation current indication error, excitation current stability, excitation current distortion, magnetic field coil constant, and magnetic field uniformity. In order to make the purpose, features, and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0019] The selection of frequency points can refer to the technical indicators in the instruction manual of the calibrated alternating current source, and select 3 to 5 frequency points, taking both low and high frequency points into consideration. 50Hz (60Hz), 100Hz, 400Hz, and 1kHz are preferred; as needed, select no less than 5 calibration points within the output range of the excitation current, and the calibration points should include the minimum and maximum current output values.

[0020] 1. Excitation current indication error

[0021] like Figure 4 As shown, the standard meter method is used to connect the current output end of the calibration device in series with the magnetic field coil and the AC ammeter. The direct comparison measurement method is used to measure the current value output by the AC ammeter. The current indication error of the calibrated AC current source is expressed by formula (1):

[0022] Δ=I x -I s (1)

[0023] Among them, the unit of Δ indication value error is A, I x The unit of the calibrated alternating current source current output value is A, I s The unit of the current meter measurement is A; the relative error is expressed by formula (2):

[0024]

[0025] Among them, γ refers to the relative indication error.

[0026] like Figure 5 As shown, the current-voltage conversion method is used to connect the current terminal of the current-voltage converter to the current output circuit of the calibrated alternating current source, and connect the potential terminal of the current-voltage converter to the AC voltage input terminal of the AC measurement standard to convert the current into voltage. The AC resistance of the current-voltage converter is R s , AC voltage measurement value U of AC measurement standard s , the measured current output value of the calibrated alternating current source is I s =U s / R s The output current of the calibrated alternating current source is I x , then the indication error of the output current of the calibrated alternating current source is expressed by formula (3):

[0027] Δ=I x -U s / R s (3)

[0028] In the formula, the unit of Δ indication value error is A, I x The unit of the calibrated alternating current source current output value is A, U sRefers to the AC measurement standard. The AC voltage measurement value is in V, R s The unit of AC resistance of the current-to-voltage converter is Ω, and its relative error is expressed by formula (4):

[0029]

[0030] In the formula, γ refers to the relative indication error.

[0031] 2. Excitation current stability

[0032] According to the calibration method of the excitation current indication error, the calibration point is selected at the commonly used current output value of the alternating current source (such as the current value corresponding to 0.1mT or 0.5mT), and the indication error calibration is carried out simultaneously. The calibration environment conditions are required to remain unchanged, and the output of the alternating current source is not adjusted. The maximum and minimum values ​​of the output indication of the calibration point within the specified time interval (generally 1min, about 60 samples) are recorded, and the stability is calculated using formula (5):

[0033]

[0034] Where S refers to the stability of the calibration point output within a specified time interval, A max The maximum value of the calibration point output indication within the specified time interval is in A. min It refers to the minimum value of the output indication of the calibration point within the specified time interval, with the unit being A. A0 refers to the setting value of the calibrated point, with the unit being A.

[0035] 3. Excitation current distortion

[0036] like Figure 7 As shown, the current output end of the calibration device is connected in series with the magnetic field coil and the current-voltage converter. The calibration point is selected at the commonly used current output value of the alternating current source (such as the current value corresponding to 0.1mT or 0.5mT). Each frequency is tested and the current distortion is measured by a distortion meter through a current-voltage converter.

[0037] 4. Magnetic field coil constant

[0038] Adopt direct induction method, the principle block diagram is as attached Figure 8As shown, the AC power supply is connected to the calibrated magnetic field coil and outputs current. The magnetic field generated by the calibrated magnetic field coil is directly measured with a standard measuring coil. The measurement is performed according to the following steps: a standard measuring coil with a diameter and frequency range that matches the calibrated magnetic field coil is selected, and the standard measuring coil is placed in the center of the calibrated magnetic field coil so that their magnetic axes coincide; the output current of the AC power supply is selected according to the instruction manual of the calibrated magnetic field coil, and 90% of the upper limit of the working range of the calibrated magnetic field coil is taken; the magnetic field coil to be calibrated generates a magnetic field according to the selected frequency point and current intensity; the output voltage of the standard magnetic field coil, the output current of the AC power supply and the output frequency are recorded; the magnetic field coil constant K is calculated according to formula (6) B0 Values:

[0039]

[0040] Where U x Refers to the output voltage effective value of the standard measuring coil in mV, f x The frequency value of the AC power output is in Hz. x Refers to the peak current output of the AC power supply in A, K sw Refers to the coil constant of the standard measuring coil in m 2 , θ refers to the angle between the measured magnetic field coil and the axis of the standard measuring coil, the unit is °, and the relative error of the calibrated magnetic field coil constant is calculated according to formula (7):

[0041]

[0042] In the formula, K B0 Refers to the calculated value of the calibrated magnetic field coil constant in mT / A, K Bx It refers to the nominal value of the calibrated magnetic field coil constant in units of mT / A, and τ refers to the relative error of the calibrated magnetic field coil constant.

[0043] It should be emphasized that the magnetic field coil constant is different in DC mode and AC mode. In AC mode, the magnetic field coil constant needs to consider the frequency response effect. In AC mode, the standard measurement coil K sw There are inductance and distributed capacitance. The equivalent circuit of the induced voltage measurement loop is shown in the attached figure. Figure 6 As shown, the measured voltage U f Not the induced voltage U of the standard measuring coil s , but the voltage divided by the standard measuring coil impedance at the input impedance of the voltmeter. The impedance values ​​at different frequency points will change. In order to accurately measure the induced voltage, the standard measuring coil needs to be corrected at each frequency. The calibration method of the correction coefficient is: first use an AC voltage signal source with a stable amplitude in series with the standard measuring coil to simulate the induced voltage signal, and use a voltmeter to measure the output voltage U of the signal source respectively. s And the voltage U after series connectionf , calculate U s with U f The ratio of is the correction coefficient α of this frequency point f0 , changing the signal source voltage frequency and repeating the above measurement steps can obtain the correction coefficient of each frequency point. After the correction by the above method, the frequency response measurement calculation formula (8) of the coil is expressed as:

[0044]

[0045] The frequency response of the magnetic field coil constant at 40Hz is very small, so the coil constant at 40Hz can be defined as K f0 , calculate the relative K at other frequencies f0 The ratio of the standard measurement coil constant at different frequencies can be calculated. After testing, it was found that when the resonant frequency is 100 times lower than the standard measurement coil, the relative change of the correction coefficient is less than 1×10 -4 Therefore, in the actual frequency response measurement calibration process, when the frequency changes from low to high, the correction coefficient changes close to 1×10 -4 To set the limit, replace the standard measuring coils with different turn areas. When the frequency is low, the standard measuring coil with a large turn area needs to be replaced. When the frequency is high, the standard measuring coil with a small turn area needs to be replaced. This can save the calculation of the correction coefficient of the standard measuring coil.

[0046] 5. Magnetic field uniformity

[0047] like Figure 2 As shown, for all windings of the Helmholtz coil, the coil constant values ​​at different points in its uniform region are measured at the same frequency. The frequency can be selected from the typical operating frequency or the intermediate operating frequency of the calibrated Helmholtz coil. According to the steps of the direct induction method in Section 4, the coil constant K of each winding at the measuring point is calculated by formula (6). Bi Coil constant K with center point B0 , and then calculate K Bi With K B0 The relative difference is taken, and the maximum value of the relative difference is taken as the uniformity of the Helmholtz coil.

[0048] The method for determining the positions of different points in the uniform area specifically includes: firstly selecting the measurement points according to the instructions of the calibrated Helmholtz coil; if the instructions do not provide the measurement points, then measuring at the following 7 points, as shown in the attached figure. Fig. 9 As shown: the center of the Helmholtz coil (point O); the two intersection points of the magnetic axis of the Helmholtz coil and the edge of the uniform zone (points 1 and 2); the four intersection points of the two orthogonal lines that are orthogonal to the magnetic axis and pass through the center of the Helmholtz coil and the edge of the uniform zone (points 3, 4, 5, and 6). Determine the position of each measurement point by measuring with a ruler.

[0049] like Figure 3 As shown, for a solenoid coil, within a given magnetic field uniformity region, as shown in the attached Fig.10 As shown in the figure, the magnetic induction intensity values ​​of five points evenly distributed between the two intersection points of the magnetic axis and the edge of the magnetic field uniform area are measured, and the relative difference between each measuring point and the central magnetic induction intensity measurement value is calculated according to formula (9):

[0050]

[0051] In the formula, δ i Refers to the relative difference between the magnetic induction intensity measured at the i-th point and the center point of the calibrated solenoid coil, B i The magnetic induction intensity of the calibrated solenoid coil is measured in mT at the i-th point, and B0 is the magnetic induction intensity of the center point of the calibrated solenoid coil. The unit is mT. i The maximum value of is taken as the uniformity of the solenoid coil. The coil constant K at each measuring point can also be calculated by following the steps of the direct induction method in Section 4 through formula (6): Bi Coil constant K with center point B0 , and then calculate K Bi With K B0 The relative difference of the two coils is taken, and the maximum value of the relative difference is taken as the uniformity of the solenoid coil.

[0052] The five calibration points are generally selected at more than 50% of the maximum output magnetic field of the solenoid coil. The angle of the alternating magnetic field meter probe is adjusted so that the top plane of the axial probe is perpendicular to the magnetic axis of the solenoid coil. At this time, the alternating magnetometer displays the maximum reading. The position of each measuring point is then determined by measuring with a ruler.

[0053] In summary, through the above method, the alternating current source and magnetic field coil of the AC magnetometer calibration device during operation can be calibrated uniformly, comprehensively and accurately as a whole, providing technical guarantee for the traceability of the value of the AC magnetometer. At the same time, the metrological performance of the AC magnetometer calibration device is comprehensively evaluated through the error conditions of various parameters, and the corresponding module in the instrument that causes inaccurate calibration is accurately located so that it can be adjusted, repaired or replaced in time.

[0054] The above description is only for illustrating the present invention. It should be understood that the present invention is not limited to the above embodiments, and various variations that conform to the concept of the present invention are within the protection scope of the present invention.

Claims

1. A calibration method for an AC magnetometer calibration device is intended to standardize the technical parameter indicators of the AC magnetometer calibration device, improve and perfect the traditional calibration method, and standardize the traceability of the value of the AC magnetometer. It is characterized in that: Including the calibration of the excitation current indication error, excitation current stability, excitation current distortion, magnetic field coil constant, and magnetic field uniformity. The measurement performance of the AC magnetometer calibration device can be comprehensively, uniformly and accurately evaluated through the error conditions of various parameters.

2. The calibration method of an AC magnetometer calibration device according to claim 1, characterized in that: The frequency points of the excitation current during the calibration process can be selected by referring to the technical indicators in the instruction manual of the calibrated alternating current source, and 3 to 5 frequency points can be selected, taking both low and high frequency points into consideration, with 50Hz (60Hz), 100Hz, 400Hz, and 1kHz being preferred; as needed, no less than 5 calibration points should be selected within the output range of the excitation current, and the calibration points should include the minimum and maximum current output values.

3. The calibration method of an AC magnetometer calibration device according to claim 1, characterized in that: The calibration method for the indication error of the excitation current specifically includes: using the standard meter method, connecting the current output terminal of the calibration device in series with the magnetic field coil and the AC ammeter, using the direct comparison measurement method, and using the AC ammeter to measure the current value output by the alternating current source. The current indication error output by the calibrated alternating current source is expressed by formula (1): Δ=I x -I s (1) Among them, the unit of Δ indication value error is A, I x The unit of the calibrated alternating current source current output value is A, I s The unit of the current meter measurement is A; the relative error is expressed by formula (2): Among them, γ refers to the relative indication error. The current-to-voltage conversion method is used to connect the current terminal of the current-to-voltage converter to the current output circuit of the calibrated alternating current source, and the potential terminal of the current-to-voltage converter is connected to the AC voltage input terminal of the AC measurement standard to convert the current into voltage. The AC resistance of the current-to-voltage converter is R s , AC voltage measurement value U of AC measurement standard s , the measured current output value of the calibrated alternating current source is I s =U s / R s The output current of the calibrated alternating current source is I x , then the indication error of the output current of the calibrated alternating current source is expressed by formula (3): Δ=I x -U s / R s (3) In the formula, the unit of Δ indication value error is A, I x The unit of the calibrated alternating current source current output value is A, U s Refers to the AC measurement standard. The AC voltage measurement value is in V, R s The unit of AC resistance of the current-to-voltage converter is Ω, and its relative error is expressed by formula (4): Where, γ refers to the relative indication error.

4. The calibration method of an AC magnetometer calibration device according to claim 1, characterized in that: The calibration method for the excitation current stability specifically includes: according to the calibration method for the excitation current indication error, the calibration point is selected at the common current output value of the alternating current source, and the current value corresponding to 0.1mT or 0.5mT can be selected, and the calibration is performed simultaneously with the indication error calibration, requiring the calibration environment conditions to remain unchanged, and the output of the alternating current source is not adjusted in any way, recording the calibration point within a specified time interval, which can be selected as 1min sampling 60 times, outputting the maximum and minimum values ​​of the indication, and calculating the stability using formula (5): Where S refers to the stability of the calibration point output within a specified time interval, A max The maximum value of the calibration point output indication within the specified time interval is in A. min It refers to the minimum value of the output indication of the calibration point within the specified time interval, with the unit being A. A0 refers to the setting value of the calibrated point, with the unit being A.

5. The calibration method of an AC magnetometer calibration device according to claim 1, characterized in that: The calibration method for the excitation current distortion specifically includes, as shown in FIG7 , connecting the current output end of the calibration device in series with the magnetic field coil and the current-voltage converter, selecting the calibration point at the commonly used current output value of the alternating current source (such as the current value corresponding to 0.1 mT or 0.5 mT), testing each frequency, and measuring the current distortion with a distortion meter through a current-voltage converter.

6. The calibration method of an AC magnetometer calibration device according to claim 1, characterized in that: The method for calibrating the magnetic field coil constant specifically includes: using a direct induction method, an AC power supply is connected to the calibrated magnetic field coil and outputs current, and a standard measuring coil is used to directly measure the magnetic field generated by the calibrated magnetic field coil, and the measurement is performed according to the following steps: selecting a standard measuring coil whose diameter and frequency range match those of the calibrated magnetic field coil, and placing the standard measuring coil in the center of the calibrated magnetic field coil so that their magnetic axes coincide; selecting the output current of the AC power supply according to the instruction manual of the calibrated magnetic field coil, and taking 90% of the upper limit of the working range of the calibrated magnetic field coil; generating a magnetic field by the calibrated magnetic field coil according to the selected frequency point and current intensity; recording the output voltage of the standard magnetic field coil, the output current of the AC power supply and the output frequency; and calculating the magnetic field coil constant K according to formula (6): B0 Values: Where U x Refers to the output voltage effective value of the standard measuring coil in mV, f x The frequency value of the AC power output is in Hz. x Refers to the peak current output of the AC power supply in A, K sw Refers to the coil constant of the standard measuring coil in m 2 , θ refers to the angle between the measured magnetic field coil and the axis of the standard measuring coil, the unit is °, and the relative error of the calibrated magnetic field coil constant is calculated according to formula (7): In the formula, K B0 Refers to the calculated value of the calibrated magnetic field coil constant in mT / A, K Bx It refers to the nominal value of the calibrated magnetic field coil constant in units of mT / A, and τ refers to the relative error of the calibrated magnetic field coil constant.

7. The calibration method of an AC magnetometer calibration device according to claim 6, characterized in that: The magnetic field coil constant is different in DC mode and AC mode. In AC mode, the magnetic field coil constant needs to consider the frequency response effect. In AC mode, the standard measurement coil K sw There are inductance and distributed capacitance. Through the equivalent circuit of the induction voltage measurement loop, it can be seen that the measured voltage U f Not the induced voltage U of the standard measuring coil s , but the voltage divided by the standard measuring coil impedance at the input impedance of the voltmeter. The impedance values ​​at different frequency points will change. In order to accurately measure the induced voltage, the standard measuring coil needs to be corrected at each frequency. The calibration method of the correction coefficient is: first use an AC voltage signal source with a stable amplitude in series with the standard measuring coil to simulate the induced voltage signal, and use a voltmeter to measure the output voltage U of the signal source respectively. s And the voltage U after series connection f , calculate U s with U f The ratio of is the correction coefficient α of this frequency point f0 , changing the signal source voltage frequency and repeating the above measurement steps can obtain the correction coefficient of each frequency point. After the correction by the above method, the frequency response measurement calculation formula (8) of the coil is expressed as: The frequency response of the magnetic field coil constant at 40Hz is very small, so the coil constant at 40Hz can be defined as K f0 , calculate the relative K at other frequencies f0 The ratio of the standard measurement coil constant at different frequencies can be calculated. After testing, it was found that when the resonant frequency is 100 times lower than the standard measurement coil, the relative change of the correction coefficient is less than 1×10 -4 Therefore, in the actual frequency response measurement calibration process, when the frequency changes from low to high, the correction coefficient changes close to 1×10 -4 To set the limit, replace the standard measuring coils with different turn areas. When the frequency is low, the standard measuring coil with a large turn area needs to be replaced. When the frequency is high, the standard measuring coil with a small turn area needs to be replaced. This can save the calculation of the correction coefficient of the standard measuring coil.

8. The calibration method of an AC magnetometer calibration device according to claim 7, characterized in that: The calibration of the magnetic field uniformity includes measuring the magnetic field coil constant values ​​at different points in the uniformity zone of all windings of the Helmholtz coil at the same frequency. The frequency can be selected from the typical operating frequency or the intermediate operating frequency of the calibrated Helmholtz coil. According to the steps of the direct induction method, the magnetic field coil constant K of each winding at the measuring point is calculated by formula (6). Bi The magnetic field coil constant K at the center point B0 , and then calculate K Bi With K B0 The relative difference of the relative difference is taken, and the maximum value of the relative difference is taken as the uniformity of the Helmholtz coil; the method for determining the position of different points in the uniform area specifically includes: preferentially selecting the measurement points according to the provisions of the instruction manual of the calibrated Helmholtz coil, if the instruction manual does not provide the measurement points, then measuring at the following 7 points: the center of the Helmholtz coil, the 2 intersection points of the magnetic axis of the Helmholtz coil and the edge of the uniform area, the 4 intersection points of two orthogonal lines orthogonal to the magnetic axis and passing through the center of the Helmholtz coil and the edge of the uniform area, and determining the position of each measurement point by measuring with a ruler.

9. The calibration method of an AC magnetometer calibration device according to claim 7, characterized in that: Furthermore, the calibration of the magnetic field uniformity also includes, for the solenoid coil, measuring the magnetic induction intensity values ​​of 5 points evenly distributed between the magnetic axis and the two intersection points of the edge of the magnetic field uniformity zone within a given magnetic field uniformity zone, and the relative difference between each measuring point and the central magnetic induction intensity measurement value is calculated according to formula (9): In the formula, δ i Refers to the relative difference between the magnetic induction intensity measured at the i-th point and the center point of the calibrated solenoid coil, B i The magnetic induction intensity of the calibrated solenoid coil is measured in mT at the i-th point, and B0 is the magnetic induction intensity of the center point of the calibrated solenoid coil. The unit is mT. i The maximum value of is taken as the uniformity of the solenoid coil. The coil constant K at each measuring point can also be calculated by following the steps of the direct induction method through formula (6): Bi Coil constant K with center point B0 , and then calculate K Bi With K B0 The relative difference is taken, and the maximum value of the relative difference is also taken as the uniformity of the solenoid coil; the five points are generally selected as calibration points of more than 50% of the maximum output magnetic field of the solenoid coil, and the angle of the alternating magnetic field meter probe is adjusted so that the top plane of the axial probe is perpendicular to the magnetic axis of the solenoid coil. At this time, the alternating magnetometer displays the maximum reading, and then the position of each measuring point is determined by measuring with a ruler.

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