Method for expanding measurement uniformity area of flat magnetic moment detection coil

Abstract

The invention belongs to the field of magnetic measurement, and particularly discloses a method for expanding a measurement uniform region of a flat magnetic moment detection coil, and the method comprises the steps: arranging a first coil, a second coil and a third coil which are coaxially nested, connected in series and wound from inside to outside; when a to-be-detected sample is arranged in the first coil and the to-be-detected sample is acted by a background magnetic field, the first coil, the second coil and the third coil sense related magnetic flux so as to detect the magnetic moment of the to-be-detected sample under the action of the background magnetic field; determining the number of turns and the average turn area of the first coil to the third coil according to the variable quantity of the magnetic flux caused when the middle section between the first coil to the third coil and the sample to be detected deviates from a preset distance, so that the first coil to the third coil can compensate the background magnetic field and the variable quantity of the magnetic flux introduced by the deviation of the middle section; the first coil, the second coil and the third coil are located within the action range of the background magnetic field. According to the invention, the measurement uniformity of the detection coil can be effectively optimized, and the influence of the position change of the sample on the test result is reduced.

Description

Technical Field

[0001] The present application belongs to the field of magnetic measurement, and more specifically, relates to a method for expanding a uniform measurement area of ​​a flat magnetic moment detection coil. Background Art

[0002] Permanent magnets are widely used in modern life, such as industrial production, power conversion, electronic products, data storage, etc. Pulsed Field Magnetometer (PFM) has a simple principle, is relatively cheap, can achieve a higher magnetizing field, is suitable for the growing demand for magnet coercivity, and has a good development prospect.

[0003] In the pulse magnetic measurement process, there is an air gap in the magnetic circuit, and the internal magnetic flux lines of the magnetic sample are unevenly distributed, resulting in different placement positions of the sample relative to the detection coil, which will cause obvious measurement errors, that is, the detection coil is too dependent on the sample position. In 2001, Eckert et al. from the Institute of Solids and Materials in Dresden, Germany proposed a method to optimize the geometric structure of the detection coil, which can reduce the impact of the axial displacement of the sample on the measurement signal. The coil structure is a fully enclosed magnetic moment detection coil, and the coil can wrap the entire sample.

[0004] Another issue that needs to be considered in the pulse magnetic measurement process is the demagnetization effect of the sample. Magnetic samples will generate a demagnetization field in an open magnetic circuit, so a demagnetization correction process is required. Demagnetization correction requires the use of a demagnetization factor, including the body demagnetization factor N m , Planar demagnetization factor N f , which are suitable for fully enclosed and flat magnetization detection coils respectively. In industrial applications, fully enclosed magnetization detection coils are widely used because they can effectively reduce the impact of sample spatial position changes on measurement signals through structural design, that is, they have a better measurement uniformity area.

[0005] However, since the height of the fully enclosed magnetization detection coil is greater than the sample, the degree to which the coil wraps the sample is different if the height of the coil is not uniform. Therefore, the N measured by the fully enclosed magnetization detection coil is not reliable. m The value is difficult to unify; the values ​​provided by different manufacturers are different, and in principle, they are not as good as the N detected by the flat magnetization detection coil. f It is accurate, but the flat magnetization detection coil is more sensitive to the displacement of the sample because its height is lower than the sample height. When the sample and the cross section of the coil are offset, it is more likely to cause a larger measurement error than the aforementioned coil structure, which means that the sample has limited deviation space within the error index range, that is, the measurement uniformity area is small. When the two are not aligned, the measurement error is large. Therefore, the position accuracy of the sample is required to be high within the measurement error range.

[0006] In order to apply a uniform and accurate planar demagnetization factor, it is necessary to study how to expand the measurement uniformity area of ​​the flat magnetic moment detection coil and propose a design structure suitable for the flat coil. Summary of the invention

[0007] In view of the defects of the prior art, the purpose of this application is to provide a method for expanding the measurement uniform area of ​​a flat magnetic moment detection coil, aiming to solve the problem that the existing flat magnetic moment detection coil is sensitive to sample displacement and has high requirements for the position accuracy of the sample within the measurement error range.

[0008] To achieve the above objectives, in a first aspect, the present application provides a method for expanding a uniform measurement area of ​​a flat magnetic moment detection coil, comprising:

[0009] A first coil, a second coil and a third coil are provided which are coaxially nested and wound in series from the inside to the outside, wherein the first coil and the second coil are wound in opposite directions, and the first coil and the third coil are wound in the same direction; when a sample to be tested is provided inside the first coil and the sample to be tested is subjected to a background magnetic field, the first coil to the third coil induce relevant magnetic fluxes to detect the magnetic moment of the sample to be tested under the action of the background magnetic field;

[0010] The number of turns and the average turn area of ​​the first to third coils are determined according to the change in the magnetic flux caused when the mid-section between the first to third coils and the sample to be tested deviates from a preset distance, so that the first to third coils can compensate for the change in the magnetic flux introduced when the background magnetic field and the mid-section deviation degree are within the preset distance range; the first to third coils are within the range of action of the background magnetic field.

[0011] It is understandable that the present application realizes background compensation and displacement compensation at the same time by adjusting the inner diameter and number of turns of the three coaxially distributed coils. The design of the magnetic moment detection coil needs to consider the needs of background compensation and displacement compensation, which are used to offset the influence of the background magnetic field and weaken the influence of sample displacement, respectively, and can effectively optimize the measurement uniformity of the detection coil and reduce the influence of sample position changes on the test results.

[0012] In one example, the number of turns and the average turn area of ​​the first to third coils are determined by the following formula:

[0013] N A S A -N B S B +N C S C =0

[0014]

[0015] in, are the changes in magnetic flux caused by the mid-section between the first coil to the third coil and the sample to be tested deviating from the preset distance, N A 、N B 、N C are the turns of the first coil to the third coil, S A , S B , S C are the average turn areas of the first to third coils respectively.

[0016] In one example, the preset distance is determined by the range of the magnetic moment detection coil expansion measurement uniformity area to be expanded;

[0017] When the mid-section deviation is within the range near the preset distance, the measurement result of the magnetic moment detection coil is relatively uniform; the relative uniformity means that the compensation for the background magnetic field is greater than the first preset value, and the mid-axis plane deviation error is less than the second preset value.

[0018] It can be understood that the above compensation for the background magnetic field is the basic background magnetic field compensation requirement, and the mid-axis deviation error can be understood as a measurement error; the above-mentioned first preset value and the second preset value can be preset targets, and technical personnel in this field can design the average turn area and number of turns of the first coil to the third coil according to the preset targets to meet the above-mentioned preset targets, so that the measurement uniformity area of ​​the flat magnetic moment detection coil is expanded to a certain extent.

[0019] In one example, the thickness of the first to third coils is not greater than the height of the sample to be tested.

[0020] In a second aspect, the present application provides a magnetic moment detection device, comprising: a first coil, a second coil, and a third coil, which are coaxially nested and wound in series from the inside to the outside;

[0021] The first coil and the second coil are wound in opposite directions, and the first coil and the third coil are wound in the same direction;

[0022] When a sample to be tested is arranged inside the first coil and is acted upon by a background magnetic field, the first to third coils sense relevant magnetic fluxes to detect the magnetic moment of the sample to be tested under the action of the background magnetic field.

[0023] In one example, the number of turns and the average turn area of ​​the first coil to the third coil are based on the change in the magnetic flux caused when the mid-section between the first coil to the third coil and the sample to be tested deviates from a preset distance, so that the first coil to the third coil can compensate for the change in magnetic flux introduced when the background magnetic field and the mid-section deviation degree are within the preset distance range; the first coil to the third coil are within the range of action of the background magnetic field.

[0024] In one example, the average turn area of ​​the first coil is smaller than the average turn area of ​​the second coil, and the average turn area of ​​the second coil is smaller than the average turn area of ​​the third coil; and / or when the mid-section deviates from a preset distance, the change in the magnetic flux in the first coil is greater than the change in the magnetic flux in the second coil, and the change in the magnetic flux in the second coil is greater than the change in the magnetic flux in the third coil.

[0025] In one example, an average turn area of ​​the second coil is greater than an average turn area of ​​the first coil, and the number of turns of the second coil is greater than or equal to the number of turns of the first coil.

[0026] In one example, the number of turns and the average turn area of ​​the first to third coils are determined by the following formula:

[0027] N A S A -N B S B +N C S C =0

[0028]

[0029] in, are the changes in magnetic flux caused by the mid-section between the first coil to the third coil and the sample to be tested deviating from the preset distance, N A 、N B 、N C are the turns of the first coil to the third coil, S A , S B , S C are the average turn areas of the first to third coils respectively.

[0030] In one example, the thickness of the first to third coils is not greater than the height of the sample to be tested.

[0031] It can be understood that the beneficial effects of the second aspect mentioned above can be found in the relevant description of the first aspect mentioned above, and will not be repeated here.

[0032] In general, the above technical solutions conceived by this application have the following beneficial effects compared with the prior art:

[0033] The present application provides a method for expanding the measurement uniformity area of ​​a flat magnetic moment detection coil, which realizes background compensation and displacement compensation at the same time by adjusting the inner diameter and number of turns of three coaxially distributed coils. The design of the magnetic moment detection coil needs to consider the needs of background compensation and displacement compensation, which are used to offset the influence of the background magnetic field and weaken the influence of sample displacement, respectively, and can effectively optimize the measurement uniformity of the detection coil and reduce the influence of sample position changes on the test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1( a ) is a schematic diagram of the positions of a sample to be tested and a fully enclosed magnetic moment detection coil according to an embodiment of the present application;

[0035] FIG1( b ) is a schematic diagram of the positions of the sample to be tested and the flat magnetic moment detection coil according to an embodiment of the present application;

[0036] FIG2( a ) is a schematic diagram showing the change in magnetic flux when the coil deviates from the cross section of the magnetic sample by 1 mm according to an embodiment of the present application;

[0037] FIG2( b ) is a schematic diagram of a normalized curve of a magnetic flux change relative to the middle cross section of a magnetic sample when the coil deviates 1 mm from the middle cross section according to an embodiment of the present application;

[0038] Figure 3 It is a cross-sectional structural diagram of an ABC type three-coil detection system according to an embodiment of the present application. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0040] The embodiments of the present application are described below in conjunction with the drawings in the embodiments of the present application.

[0041] This application proposes a method for expanding the measurement uniform area suitable for flat magnetic moment detection coils. The technical solution adopted is: by adjusting the inner diameter and number of turns of the three coaxially distributed coils A, B, and C, background compensation and displacement compensation are achieved simultaneously. The design of the magnetic moment detection coil needs to consider the needs of background compensation and displacement compensation, which are used to offset the influence of the background magnetic field and weaken the influence of sample displacement, respectively. For cylindrical magnetic samples, under the measurement condition of an open magnetic circuit, the magnetic flux at the middle section is the largest, and the optimal coil position is located at the middle section of the sample.

[0042] However, when the axial relative position between the coil and the sample changes, the magnetic flux passing through the coil becomes smaller, and the change in magnetic flux causes the induced voltage to change, affecting the final test results. For the magnetic moment detection coil, it is generally a structure in which two coils AB are wound in reverse series. The two are designed to have equal turn areas to offset the induced signal of the background magnetic field and only retain the magnetic moment information of the sample. The axial position offset of the sample will cause the reduction of the induced magnetic flux of coils A and B at the same time, and the reduced magnetic flux of A is greater. However, since the inner diameter of coil A is smaller than that of coil B and the number of turns of coil A is greater than that of coil B, the total magnetic flux of coil A is significantly greater than that of coil B. Even after the reverse series cancellation process, there will still be a large change in the induced signal. In order to minimize the change in magnetic flux caused by position change, it is necessary to reasonably adjust the number of turns and inner diameter of coils A and B, and introduce coil C to further adjust the change in magnetic flux. The magnetic flux caused by the background magnetic field will be called the background magnetic flux, and the change in magnetic flux caused by sample displacement will be called the displacement magnetic flux.

[0043] For the AB series coil, the inner diameter of coil A is small, the number of turns is large, and the displacement flux is large. A and B are connected in reverse series, and the displacement flux of coil B will have a certain compensation effect on the displacement flux of coil A, but the compensation is insufficient. In this regard, the number of turns of coil B can only be increased to compensate for the flux change of coil A as much as possible, but this leads to a worse compensation effect for the background flux, that is, the displacement compensation effect is optimized by sacrificing a certain background compensation effect, and the background compensation can only be performed by signal differential.

[0044] For the ABC series coil, the C coil is coaxial and coplanar with A and B, has the largest diameter, and has a smaller displacement flux than the B coil. The C coil is connected in series to the loop of the AB coil and is wound in the same direction as the A coil. In order to meet the background compensation condition, the C coil is added to enhance the positive background flux and compensate for the excessive reverse B coil background flux; in order to meet the displacement compensation condition, the C coil only provides a smaller positive displacement flux. Therefore, while meeting the larger background flux requirement, it only provides a smaller positive displacement flux, so that the above two conditions are met at the same time.

[0045] The present application provides a structural design method suitable for a flat magnetic moment detection coil, which can effectively optimize the measurement uniformity of the detection coil and reduce the impact of sample position changes on the test results.

[0046] It should be noted that, in the conventional AB series flat magnetic moment detection coil, the radius of the B coil is larger than the radius of the A coil, but the number of turns of the B coil is smaller than the number of turns of the A coil; while in the present application, an ABC series flat magnetic moment detection coil is used, the radius of the B coil is larger than the radius of the A coil, and the number of turns of the B coil is also greater than or equal to the number of turns of the A coil.

[0047] The present application embodiment provides a structural design of a flat magnetic moment detection coil. The variables involved in the method include: the single-turn coil magnetic flux of coils A, B, and C at the 0mm position The change in magnetic flux caused by the deviation of coils A, B, and C from the middle section by 1 mm Number of turns of coils A, B, and C N A 、N B 、N C , the average turn area S of coils A, B, and C A , S B , S C , total magnetic flux Ф at 0mm position 0 , total magnetic flux Ф at 1mm position 1 .

[0048] In this example, this method is used to expand the measurement uniform area of ​​the magnetic moment detection coil. Figure 1(a) and Figure 1(b) show the structures of the fully enclosed magnetic moment detection coil and the flat magnetic moment detection coil, which correspond to the body demagnetization factor N m And the plane demagnetization factor N f For application conditions, refer to the following formula:

[0049] N m =-∫∫∫H d dv / ∫∫∫Mdv (1)

[0050] N f =-∫∫H d ds / ∫∫Mds (2)

[0051] Among them, N m With N f In industrial applications, it is often called the magnetometer demagnetization factor and fluxmeter demagnetization factor. The magnetic moment detection coil is generally made of A and B coils coaxially wound in reverse series. Considering that it is difficult to achieve complete compensation for the B coil, it is often necessary to connect a C coil with a polarity switch and a sliding rheostat to further compensate for the background signal. At this time, the number of turns of the C coil is very small and it only plays an auxiliary adjustment role. The magnetic flux induced by the coil is mainly provided by the A and B coils.

[0052] For the above conventional AB series coil, the structural design method provided in this application can achieve a certain degree of displacement compensation effect, but it will sacrifice some background compensation effect. In order to achieve the background compensation effect, the AB type coil needs to meet the following conditions:

[0053] N A S A -N B S B =0 (3)

[0054] Figure 2(a) shows the relative offset between the sample and the coil plane. The position change causes the magnetic flux change, which in turn causes the induced signal change. The position where the cross section of the sample coincides with the coil plane is defined as the 0mm position, and the relative offset of 1mm is defined as the 1mm position. At 0mm, the single-turn magnetic flux of coils A and B are At 1mm, the magnetic flux is The total magnetic flux at the 0mm and 1mm positions are as follows:

[0055]

[0056]

[0057] If the displacement compensation conditions need to be met, then:

[0058] Φ 0 =Φ 1 (6)

[0059]

[0060] It is understandable that the flux changes etc. can be determined by finite element simulation, and the ratio of magnetic flux changes is only related to the sample size and has nothing to do with factors such as sample material.

[0061] At the same time, based on a cylindrical sample with a height of 9 mm and a diameter of 8 mm, the variation of the magnetic flux offset with the radius position shown in Figure 2(b) is plotted. The radius of 4 mm corresponds to the surface position of the sample. Considering the thickness of the coil skeleton, the radius of coil A generally needs to be greater than 5 mm. In the area greater than 5 mm, the magnetic flux offset generally shows a trend of decreasing with the radius. Therefore, coils A and B meet the following conditions:

[0062]

[0063] Obviously, the limitation of formula (8) cannot satisfy both conditions (3) and (7) at the same time. Table 1 is a design reference that only satisfies the background compensation condition, which can be referred to as AB1 type magnetic moment detection coil. It can be seen that after considering the displacement compensation condition, the measurement deviation caused by the axial deviation of the sample is improved.

[0064] Table 1

[0065]

[0066] Furthermore, if both conditions (3) and (7) are considered at the same time, the background compensation condition shown in (3) can only be sacrificed to improve the displacement compensation condition shown in (7). According to this idea, the magnetic moment detection coil shown in Table 2 is designed, which can be referred to as the AB2 type magnetic moment detection coil. The AB2 type structure considering displacement compensation can be used in some occasions where the requirements for background compensation conditions are not very strict, and is used to improve its measurement uniformity area.

[0067] Table 2

[0068]

[0069] For the above AB type coil which cannot realize the background compensation and displacement compensation conditions at the same time, the present application also provides an embodiment Figure 3 The ABC type three-coil structure shown in the figure satisfies two compensation conditions at the same time by adding the C coil. In order to meet the background compensation condition, there are:

[0070] N A S A -N B S B +N C S C =0 (9)

[0071] Similarly, let the magnetic flux of coil C at the 0mm position and the 1mm position be Coils A and C are connected in series in the same direction, and the total magnetic flux at the 0mm position and the 1mm position is:

[0072]

[0073]

[0074] If the displacement compensation conditions need to be met, then:

[0075]

[0076] For coils A, B, and C, we have:

[0077]

[0078] By reasonably adjusting the positions and turns of coils A, B, and C, both conditions (9) and (12) can be satisfied. Based on this idea, the magnetic moment detection coil shown in Table 3 is designed.

[0079] Table 3

[0080]

[0081] The deviation and compensation degree when the system position is offset by 1mm and 2mm obtained by the three coils with different structures are summarized in Table 4, where AB1 represents the coil shown in Table 1, and AB2 represents the coil shown in Table 2. It can be seen that the coil with AB2 structure improves the measurement uniformity area, but loses the degree of background compensation. For the ABC type coil structure, both the background compensation and displacement compensation conditions can be met.

[0082] Table 4

[0083]

[0084] Referring to Table 4, it can be seen that the measurement deviation caused by the change in sample position is significantly reduced. For example, when the sample is axially offset by 1 mm, the measurement deviation can be suppressed from 2% to within 0.5%, while meeting the compensation requirements for the background magnetic field.

[0085] It should be noted that the above axial offset can be designed according to actual needs to ensure that the measurement deviation is small enough within the above axial offset range and can also meet the compensation requirements for the background magnetic field. For example, the above axial offset can be determined based on the assembly error of the sample to ensure that the measurement results of the flat magnetic moment detection coil are relatively uniform within the sample assembly error range.

[0086] It should be understood that expressions such as "including" and "may include" that may be used in the present application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In the present application, terms such as "including" and / or "having" may be interpreted as indicating specific characteristics, numbers, operations, constituent elements, components, or combinations thereof, but may not be interpreted as excluding the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0087] In addition, in the present application, the expression "and / or" includes any and all combinations of the associated listed words. For example, the expression "A and / or B" may include A, may include B, or may include both A and B.

[0088] In the description of the embodiments of the present application, it should be noted that, unless otherwise clearly specified and limited, the term "connection" should be understood in a broad sense. For example, "connection" can be a detachable connection or a non-detachable connection; it can be a direct connection or an indirect connection through an intermediate medium. Among them, "fixed connection" means that the relative position relationship after connection remains unchanged. "Rotational connection" means that the two are connected to each other and can rotate relative to each other after connection. "Sliding connection" means that the two are connected to each other and can slide relative to each other after connection. The directional terms mentioned in the embodiments of the present application, such as "top", "bottom", "inside", "outside", "left", "right", etc., are only reference directions of the accompanying drawings. Therefore, the directional terms used are for better and clearer explanation and understanding of the embodiments of the present application, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0089] In addition, in the embodiments of the present application, the mathematical concepts mentioned are symmetry, equality, parallelism, verticality, etc. These limitations are all for the current state of the art, rather than being absolutely strict definitions in a mathematical sense, and allow a small amount of deviation, approximation to symmetry, approximation to equality, approximation to parallelism, approximation to verticality, etc. are all possible. For example, A and B are parallel, which means that A and B are parallel or approximately parallel, and the angle between A and B can be between 0 and 10 degrees. A and B are perpendicular, which means that A and B are perpendicular or approximately perpendicular, and the angle between A and B can be between 80 and 100 degrees.

[0090] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A method for expanding the measurement uniformity area of ​​a flat magnetic moment detection coil, characterized in that: include: A first coil, a second coil and a third coil are provided which are coaxially nested and wound in series from the inside to the outside, wherein the first coil and the second coil are wound in opposite directions, and the first coil and the third coil are wound in the same direction; when a sample to be tested is provided inside the first coil and the sample to be tested is subjected to a background magnetic field, the first coil to the third coil induce relevant magnetic fluxes to detect the magnetic moment of the sample to be tested under the action of the background magnetic field; The number of turns and the average turn area of ​​the first to third coils are determined according to the change in the magnetic flux caused when the mid-section between the first to third coils and the sample to be tested deviates from a preset distance, so that the first to third coils can compensate for the change in the magnetic flux introduced when the background magnetic field and the mid-section deviation degree are within the preset distance range; the first to third coils are within the range of action of the background magnetic field.

2. The method according to claim 1, characterized in that The number of turns and the average turn area of ​​the first to third coils are determined by the following formula: N A S A -N B S B +N C S C =0 in, are the changes in magnetic flux caused by the mid-section between the first coil to the third coil and the sample to be tested deviating from the preset distance, N A 、N B 、N C are the turns of the first coil to the third coil, S A , S B , S C are the average turn areas of the first to third coils respectively.

3. The method according to claim 1 or 2, characterized in that: The preset distance is determined by the range of the magnetic moment detection coil expansion measurement uniform area to be expanded; When the mid-section deviation is within the range near the preset distance, the measurement result of the magnetic moment detection coil is relatively uniform; the relative uniformity means that the compensation for the background magnetic field is greater than the first preset value, and the mid-axis plane deviation error is less than the second preset value.

4. The method according to claim 1 or 2, characterized in that: The thickness of the first coil to the third coil is not greater than the height of the sample to be tested.

5. A magnetic moment detection device, characterized in that: include: A first coil, a second coil and a third coil are coaxially nested and wound in series from the inside to the outside; The first coil and the second coil are wound in opposite directions, and the first coil and the third coil are wound in the same direction; When a sample to be tested is arranged inside the first coil and is acted upon by a background magnetic field, the first to third coils sense relevant magnetic fluxes to detect the magnetic moment of the sample to be tested under the action of the background magnetic field.

6. The device according to claim 5, characterized in that The number of turns and the average turn area of ​​the first to third coils are based on the change in the magnetic flux caused when the mid-section between the first to third coils and the sample to be tested deviates from a preset distance, so that the first to third coils can compensate for the change in the magnetic flux introduced when the background magnetic field and the mid-section deviation degree are within the preset distance range; the first to third coils are within the range of action of the background magnetic field.

7. The device according to claim 6, characterized in that The average turn area of ​​the first coil is smaller than the average turn area of ​​the second coil, and the average turn area of ​​the second coil is smaller than the average turn area of ​​the third coil; and / or when the mid-section deviates from the preset distance, the change in the magnetic flux in the first coil is greater than the change in the magnetic flux in the second coil, and the change in the magnetic flux in the second coil is greater than the change in the magnetic flux in the third coil.

8. The device according to any one of claims 5 to 7, characterized in that The average turn area of ​​the second coil is greater than the average turn area of ​​the first coil, and the number of turns of the second coil is greater than or equal to the number of turns of the first coil.

9. The device according to any one of claims 5 to 7, characterized in that The number of turns and the average turn area of ​​the first to third coils are determined by the following formula: N A S A -N B S B +N C S C =0 in, are the changes in magnetic flux caused by the mid-section between the first coil to the third coil and the sample to be tested deviating from the preset distance, N A 、N B 、N C are the turns of the first coil to the third coil, S A , S B , S C are the average turn areas of the first to third coils respectively.

10. The device according to any one of claims 5 to 7, characterized in that The thickness of the first coil to the third coil is not greater than the height of the sample to be tested.

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