Method for Expanding Measurement Uniform Region of Flat Magnetic Moment Detection Coil

Abstract

This application belongs to the field of magnetic measurement, and specifically discloses a method for expanding the measurement uniform region of a flat magnetic moment detection coil, including: arranging a first coil, a second coil, and a third coil that are coaxially nested and wound in series from the inside out; when a sample to be measured is arranged inside the first coil, when the sample to be measured is affected by a background magnetic field, the first to third coils sense the relevant magnetic fluxes to detect the magnetic moment of the sample to be measured under the action of the background magnetic field; determining the number of turns and the average turn area of the first to third coils according to the change amount of the magnetic flux caused by the middle cross-section between the first coil to the third coil and the sample to be measured deviating from a preset distance, so that the first to third coils can compensate for the change amount of the magnetic flux introduced by the background magnetic field and the deviation of the middle cross-section; the first to third coils are within the action range of the background magnetic field. Through this application, the measurement uniformity of the detection coil can be effectively optimized, and the influence of the change in the sample position on the test result can be reduced.

Description

Technical Field

[0001] This application belongs to the field of magnetic measurement. More specifically, it relates to a method for expanding the measurement uniform region of a flat magnetic moment detection coil. Background Art

[0002] Permanent magnets are widely used in modern life, such as in industrial production, power conversion, electronic products, data storage, etc. The pulsed field magnetometer (PFM) has a simple principle and a relatively low price, can achieve a high magnetization field, meets the increasing demand for the coercivity of magnets, and has good development prospects.

[0003] For the pulsed magnetic measurement process, there is an air gap in the magnetic circuit, and the internal magnetic induction lines of the magnetic sample are unevenly distributed, resulting in obvious measurement errors caused by different placement positions of the sample relative to the detection coil, that is, the detection coil is too dependent on the sample position. In 2001, Eckert et al. from the Institute of Solid State and Materials Research in Dresden, Germany, proposed a method to optimize the geometric structure of the detection coil, which can reduce the influence 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 problem that needs to be considered in the pulsed magnetic measurement process is the demagnetization effect of the sample. The magnetic sample will generate a demagnetizing field in an open magnetic circuit, so a demagnetization correction process is required. Demagnetization correction requires the use of demagnetization factors, including the bulk demagnetization factor N m and the planar demagnetization factor N f , which are applicable to fully enclosed and flat magnetization detection coils respectively. In industrial applications, since the fully enclosed magnetization detection coil can effectively reduce the influence of the change in the spatial position of the sample on the measurement signal through structural design, that is, it has a good measurement uniform region, it is widely used.

[0005] However, since the height of the fully enclosed magnetization detection coil is greater than that of the sample, but the heights of the coils are not uniform, the degree of wrapping the sample by the coils is different. Therefore, the N m values measured depending on the fully enclosed magnetization detection coil are difficult to be unified; the values provided by each manufacturer are different, and in principle, they are not as accurate as the N f detected by the flat magnetization detection coil. However, since the height of the flat magnetization detection coil is lower than the height of the sample, it is relatively more sensitive to the displacement of the sample; when the sample is offset from the mid-section of the coil, it is easier to cause a larger measurement error compared to the aforementioned coil structure, which means that the allowable deviation space of the sample within the error index range is limited, that is, the measurement uniform region is small, and the measurement error is large when the two are not aligned. Therefore, the position accuracy requirement of the sample within the measurement error range is high.

[0006] In order to apply a unified and accurate planar demagnetization factor, it is necessary to study how to expand the measurement uniform region of the flat magnetic moment detection coil, and a design structure suitable for the flat coil is proposed. Summary of the Invention

[0007] Aiming at the defects of the prior art, the purpose of this application is to provide a method for expanding the measurement uniform region of a flat magnetic moment detection coil, aiming to solve the problems that the existing flat magnetic moment detection coil is sensitive to the displacement of the sample and has a high requirement for the position accuracy of the sample within the measurement error range.

[0008] To achieve the above object, in the first aspect, this application provides a method for expanding the measurement uniform region of a flat magnetic moment detection coil, including:

[0009] Set the first coil, the second coil, and the third coil that are coaxially nested and wound in series from the inside out. 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 measured is arranged inside the first coil, when the sample to be measured is affected by the background magnetic field, the first coil to the third coil sense the relevant magnetic fluxes to detect the magnetic moment of the sample to be measured under the action of the background magnetic field.

[0010] Determine the number of turns and the average turn area of the first coil to the third coil according to the change amount of the magnetic flux caused when the middle cross-section between the first coil to the third coil and the sample to be measured deviates from the preset distance, so that the first coil to the third coil can compensate for the change amount of the magnetic flux introduced when the background magnetic field and the degree of deviation of the middle cross-section are within the preset distance range; the first coil to the third coil are within the action range of the background magnetic field.

[0011] It can be understood that this application realizes background compensation and displacement compensation simultaneously by adjusting the inner diameters and the number of turns of the three coaxially distributed coils. The design of the magnetic moment detection coil needs to consider the requirements of background compensation and displacement compensation, which are used to offset the influence of the background magnetic field and weaken the influence of the sample displacement respectively, and can effectively optimize the measurement uniformity of the detection coil and reduce the influence of the sample position change on the test result.

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

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

[0014]

[0015] Wherein, are the change amounts of the magnetic fluxes caused by the middle cross-sections of the first to third coils deviating from the preset distance from the sample to be measured, respectively, N A 、N B 、N C are the number of turns of the first to third coils, respectively, 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 uniform measurement area extended by the magnetic moment detection coil to be extended;

[0017] When the middle cross-section deviates within the range near the preset distance, the measurement results of the magnetic moment detection coil are relatively uniform; the relative uniformity means that the compensation for the background magnetic field is greater than the first preset value, and the deviation error of the central axis plane 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 deviation error of the central axis plane can be understood as the measurement error; the above first preset value and second preset value can be preset targets, and those skilled in the art can design the average turn area and the number of turns of the first to third coils according to the preset targets to meet the above preset targets, so as to extend the measurement uniform area of the flat magnetic moment detection coil to a certain extent.

[0019] In one example, the thicknesses of the first to third coils are not greater than the height of the sample to be measured.

[0020] In a second aspect, the present application provides a magnetic moment detection device, including: a first coil, a second coil, and a third coil that are coaxially nested and serially wound from the inside out in sequence;

[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 measured is disposed inside the first coil, when the sample to be measured is affected by the background magnetic field, the first to third coils sense the related magnetic fluxes to detect the magnetic moment of the sample to be measured under the action of the background magnetic field.

[0023] In one example, the number of turns and the average turn area of the first to third coils are based on the change amount of the magnetic flux caused by the middle cross-section of the first to third coils deviating from the preset distance from the sample to be measured, so that the first to third coils can compensate for the magnetic flux change amount introduced when the background magnetic field and the deviation degree of the middle cross-section are within the preset distance range; the first to third coils are within the action range of the background magnetic field.

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

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

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

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

[0028]

[0029] where are the change amounts of the magnetic flux caused by the deviation of the middle cross-section between the first coil to the third coil and the sample to be measured from the preset distance, respectively, and N A 、N B 、N C are the number of turns of the first coil to the third coil, respectively, and S A 、S B 、S C are the average turn areas of the first coil to the third coil, respectively.

[0030] In one example, the thicknesses of the first coil to the third coil are not greater than the height of the sample to be measured.

[0031] It can be understood that the beneficial effects of the second aspect above can be referred to the relevant descriptions in the first aspect above, and will not be elaborated here.

[0032] Generally speaking, compared with the prior art, the above technical solution conceived by the present application has the following beneficial effects:

[0033] The present application provides a method for expanding the measurement uniform area of a flat magnetic moment detection coil, which realizes background compensation and displacement compensation simultaneously by adjusting the inner diameters and the number of turns of three coaxially distributed coils. The design of the magnetic moment detection coil needs to consider the requirements of background compensation and displacement compensation, which are used to offset the influence of the background magnetic field and weaken the influence of the sample displacement respectively, and can effectively optimize the measurement uniformity of the detection coil and reduce the influence of the change of the sample position on the test result. Description of the Drawings

[0034] Figure 1(a) is a schematic diagram of the position of the sample under test and the fully enclosed magnetic moment detection coil according to an embodiment of the present application;

[0035] Figure 1(b) is a schematic diagram of the position of the sample under test and the flat magnetic moment detection coil according to an embodiment of the present application;

[0036] Figure 2(a) is a schematic diagram of the magnetic flux change at a position 1 mm away from the mid - section of the magnetic sample according to an embodiment of the present application;

[0037] Figure 2(b) is a schematic diagram of the normalized curve of the magnetic flux change amount relative to the mid - section at a position 1 mm away from the mid - section of the magnetic sample according to an embodiment of the present application;

[0038] Figure 3 is a sectional structure diagram of the ABC - type three - coil detection system according to an embodiment of the present application. Detailed implementation manners

[0039] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to 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 will be described below with reference to the accompanying drawings in the embodiments of the present application.

[0041] The present application proposes a method for expanding the measurement uniform area applicable to flat magnetic moment detection coils. The technical solution adopted is: by adjusting the inner diameters and turns of three coaxial 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 requirements 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 a cylindrical magnetic sample, under the measurement condition of an open magnetic circuit, the magnetic flux at the mid - section is the largest, and the optimal coil position is at the mid - 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. The change in magnetic flux causes a change in the induced voltage, which affects the final test result. For the magnetic moment detection coil, it is generally a structure in which two coils, A and B, are wound in series in opposite directions. The two are designed to have equal turn areas to cancel out 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 simultaneously cause a decrease in the induced magnetic flux of both coils A and B, and the decrease in the magnetic flux of coil 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 change in magnetic flux of coil A is significantly greater than that of coil B. Even after the cancellation process of series connection in opposite directions, there will still be a relatively large change in the induced signal. In order to suppress the change in magnetic flux caused by the position change as much as possible, it is necessary to reasonably adjust the number of turns and the inner diameter of coils A and B, and at the same time introduce coil C to further adjust the change in magnetic flux. Subsequently, the magnetic flux caused by the background magnetic field will be referred to as the background magnetic flux, and the change in magnetic flux caused by the sample displacement will be referred to as the displacement magnetic flux.

[0043] For the AB series-connected coils, coil A has a small inner diameter, a large number of turns, and a large displacement magnetic flux. Coils A and B are connected in series in opposite directions. The displacement magnetic flux of coil B will have a certain compensation effect on the displacement magnetic flux of coil A, but the compensation is insufficient. In this regard, only by increasing the number of turns of coil B can we try to compensate for the change in magnetic flux of coil A, but this results in a worse compensation effect for the background magnetic flux, that is, by sacrificing a certain background compensation effect to optimize the displacement compensation effect, and only the signal difference method can be used for background compensation.

[0044] For the ABC series-connected coils, coil C is coaxial and coplanar with coils A and B, has the largest diameter, and has a smaller displacement magnetic flux than coil B. Coil C is connected in series to the circuit of coils A and B and is wound in the same direction as coil A. In order to meet the background compensation condition, coil C is added to enhance the forward background magnetic flux and compensate for the excessive reverse background magnetic flux of coil B; in order to meet the displacement compensation condition, coil C only provides a small forward displacement magnetic flux. Therefore, while meeting the requirement of a large background magnetic flux, only a small forward displacement magnetic flux is provided, so that the above two conditions are met simultaneously.

[0045] This application provides a structural design method applicable to a flat magnetic moment detection coil, which can effectively optimize the measurement uniformity of the detection coil and reduce the influence of sample position change on the test result.

[0046] It should be noted that for the conventional AB series-connected flat magnetic moment detection coil, the radius of coil B is larger than that of coil A, but the number of turns of coil B is smaller than that of coil A; while in this application, the flat magnetic moment detection coil using ABC series connection has a radius of coil B larger than that of coil A, and the number of turns of coil B is also greater than or equal to that of coil A.

[0047] The embodiments of the present application provide a structural design of a flat magnetic moment detection coil. The variables involved in this method include: the magnetic flux per turn of coils A, B, and C at the 0 mm position The change in magnetic flux caused by coils A, B, and C deviating 1 mm from the mid-section The number of turns N of coils A, B, and C A 、N B 、N C The average turn area S of coils A, B, and C A 、S B 、S C The total magnetic flux Ф0 at the 0 mm position and the total magnetic flux Ф1 at the 1 mm position

[0048] In this example, this method is used to expand the measurement uniform area of the magnetic moment detection coil. Figures 1(a) and 1(b) show the structures of the fully enclosed magnetic moment detection coil and the flat magnetic moment detection coil, corresponding to the application conditions of the volume demagnetization factor N m and the planar demagnetization factor N f respectively. Refer to the following formula

[0049] N m =-∫∫∫H d dv / ∫∫∫Mdv (1)

[0050] N f =-∫∫H d ds / ∫∫Mds (2)

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

[0052] For the above conventional AB series coils, the structural design method provided by the present application can achieve a certain degree of displacement compensation effect, but will sacrifice a part of the background compensation effect. In order to achieve the background compensation effect, the AB type coil needs to meet the condition

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

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

[0055]

[0056]

[0057] If the displacement compensation condition needs to be satisfied, then:

[0058] Φ0 = Φ1 (6)

[0059]

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

[0061] Meanwhile, 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. A radius of 4 mm corresponds to the sample surface position. Considering the thickness of the coil skeleton, the radius of coil A generally needs to be greater than 5 mm. In the region greater than 5 mm, the magnetic flux offset generally shows a trend of decreasing with the radius. Therefore, coils A and B satisfy:

[0062]

[0063] Obviously, the limitation of equation (8) cannot satisfy both conditions of equation (3) and equation (7) simultaneously. Table 1 is the design reference that only satisfies the background compensation condition, which can be simply referred to as the 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 of (3) and Equation (7) are considered simultaneously, only the background compensation condition shown in Equation (3) can be sacrificed to improve the displacement compensation condition shown in Equation (7). According to this idea, the magnetic moment detection coil shown in Table 2 is designed, which can be simply 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 the background compensation condition are not very strict to improve its measurement uniformity region.

[0067] Table 2

[0068]

[0069] For the above AB type coils, it is impossible to simultaneously achieve the background compensation and displacement compensation conditions. The embodiments of the present application also provide Figure 3 the ABC type three-coil structure shown in the figure, which can simultaneously meet the two compensation conditions by adding a C coil. To meet the background compensation condition, there is:

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

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

[0072]

[0073]

[0074] If it is necessary to meet the displacement compensation condition, then there is:

[0075]

[0076] For coils A, B, and C, there is:

[0077]

[0078] By reasonably adjusting the positions and turns of coils A, B, and C, the two conditions of Equation (9) and Equation (12) can be simultaneously met. According to this idea, the magnetic moment detection coil shown in Table 3 is designed.

[0079] Table 3

[0080]

[0081] The deviation degrees and compensation degrees brought about by the system position offset of 1 mm and 2 mm for the above three coils with different structures are tabulated in Table 4. Among them, 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 the AB2 structure improves the measurement uniform area but loses the background compensation degree. For the ABC-type coil structure, it can satisfy both the background compensation and displacement compensation conditions.

[0082] Table 4

[0083]

[0084] Referring to Table 4, it can be seen that the measurement deviation caused by the change of the 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 at the same time can meet the compensation requirements for the background magnetic field. For example, the above axial offset can be determined according to the assembly error of the sample to ensure that within the assembly error range of the sample, the measurement results of the flat magnetic moment detection coil are relatively uniform.

[0086] It should be understood that expressions such as "including" and "may include" used in this 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 this application, terms such as "including" and / or "having" can be interpreted as indicating a specific characteristic, number, operation, constituent element, component, or a combination thereof, but cannot be interpreted as excluding the existence or possibility of addition of one or more other characteristics, numbers, operations, constituent elements, components, or a combination thereof.

[0087] In addition, in this application, the expression "and / or" includes any and all combinations of the associated listed words. For example, the expression "A and / or B" can include A, can include B, or can 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 two are connected and the relative position relationship after connection remains unchanged. "Rotational connection" means that the two are connected and can rotate relative to each other after connection. "Sliding connection" means that the two are connected and can slide relative to each other after connection. The orientation terms mentioned in the embodiments of the present application, such as "top", "bottom", "inner", "outer", "left", "right", etc., are only references to the directions in the drawings. Therefore, the orientation terms are used to better and more clearly illustrate and understand 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 thus should not be construed as a limitation to the embodiments of the present application.

[0089] In addition, in the embodiments of the present application, mathematical concepts such as symmetry, equality, parallelism, and perpendicularity are mentioned. These limitations are all in view of the current technological level, rather than absolute strict definitions in the mathematical sense, and a small amount of deviation is allowed. Approximations to symmetry, equality, parallelism, perpendicularity, etc. are all acceptable. For example, when it is said that A is parallel to B, it means that A is parallel to B or approximately parallel to B, and the included angle between A and B can be between 0 degrees and 10 degrees. When it is said that A is perpendicular to B, it means that A is perpendicular to B or approximately perpendicular to B, and the included angle between A and B can be between 80 degrees and 100 degrees.

[0090] The above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of changes or substitutions, which should all be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for expanding the measurement uniform region of a flat magnetic moment detection coil, characterized in that, Comprising: A first coil, a second coil, and a third coil are coaxially nested and wound in series from the inside out. 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 measured is arranged inside the first coil, when the sample to be measured is affected by a background magnetic field, the first coil to the third coil sense relevant magnetic fluxes to detect the magnetic moment of the sample to be measured under the action of the background magnetic field. The number of turns and the average turn area of the first coil to the third coil are determined according to the change amount of the magnetic flux caused when the middle cross-section between the first coil to the third coil and the sample to be measured deviates from a preset distance, so that the first coil to the third coil can compensate for the change amount of the magnetic flux introduced when the background magnetic field and the degree of middle cross-section deviation are within the preset distance range. The first coil to the third coil are within the action range of the background magnetic field. The number of turns and the average turn area of the first coil to the third coil are determined by the following formula: wherein, φ1, φ2, and φ3 are respectively the change amounts of magnetic fluxes caused by the mid-sections of the first to third coils deviating from a preset distance from the sample to be measured, N A 、 N B 、 N C are respectively the number of turns of the first to third coils, S A 、 S B 、 S C are respectively the average turn areas of the first to third coils.

2. The method according to claim 1, characterized in that, The preset distance is determined by the range of the uniform measurement area expanded by the magnetic moment detection coil to be expanded. When the deviation of the middle cross-section 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 a first preset value, and the deviation error of the central axis plane is less than a second preset value.

3. The method according to claim 1, wherein The thickness of the first coil to the third coil is not greater than the height of the sample to be measured.

4. A magnetic moment detection device, characterized in that, Comprising: A first coil, a second coil, and a third coil are coaxially nested and wound in series from the inside out in sequence; 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 measured is arranged inside the first coil, when the sample to be measured is affected by a background magnetic field, the first coil to the third coil sense relevant magnetic fluxes to detect the magnetic moment of the sample to be measured under the action of the background magnetic field; The number of turns and the average turn area of the first coil to the third coil are determined according to the change amount of the magnetic flux caused when the middle cross-section between the first coil to the third coil and the sample to be measured deviates from a preset distance, so that the first coil to the third coil can compensate for the change amount of the magnetic flux introduced when the background magnetic field and the degree of middle cross-section deviation are within the preset distance range. The first coil to the third coil are within the action range of the background magnetic field; The number of turns and the average turn area of the first coil to the third coil are determined by the following formula: Among them, φ1, φ2, and φ3 are the change amounts of magnetic fluxes caused by the middle cross-sections of the first to third coils deviating from the preset distance from the sample to be measured, respectively. N A 、 N B 、 N C are the number of turns of the first to third coils, respectively. S A 、 S B 、 S C are the average turn areas of the first to third coils, respectively.

5. The device according to claim 4, characterized in that The average turn area of the first coil is smaller than that of the second coil, and the average turn area of the second coil is smaller than that of the third coil; and / or when the middle cross-section deviates from the preset distance, the change amount of the magnetic flux in the first coil is greater than that in the second coil, and the change amount of the magnetic flux in the second coil is greater than that in the third coil.

6. The device according to claim 4 or 5, characterized in that The average turn area of the second coil is greater than that of the first coil, and the number of turns of the second coil is greater than or equal to that of the first coil.

7. The device according to claim 4 or 5, characterized in that The thickness of the first coil to the third coil is not greater than the height of the sample to be measured.

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