Compensation method for radial magnetic field uniformity of magnetic field coil

Abstract

A compensation method for radial magnetic field uniformity of a magnetic field coil utilizes a compensation coil to compensate the radial magnetic field uniformity of a main coil, and comprises the following steps: respectively and symmetrically arranging a pair of magnetic moment coils in two radial directions of the main coil, and utilizing magnetic field difference values attenuated to different positions of a coil working area by the pair of magnetic moment coils to compensate the radial magnetic field uniformity of the main coil; therefore, the magnetic field difference of the working area of the main coil is reduced, and the magnetic field uniformity of the working area of the main coil is improved. According to the method, the uniformity of the horizontal radial direction and the vertical radial direction of the main coil can be compensated respectively, and the radial magnetic field uniformity of the main coil can be compensated within the full range of the magnetic field of the main coil.

Description

Technical Field

[0001] The present invention relates to the field of electromagnetic fields, and particularly relates to a method for compensating the radial magnetic field uniformity of a magnetic field coil. Background Art

[0002] In the field of magnetic metrology technology, when calibrating various types of magnetometers, a magnetic field coil is usually used as a magnetic field standard instrument to generate a standard magnetic field to calibrate the indication value of the magnetometer. When evaluating the measurement uncertainty of the calibration result of the magnetometer, the magnetic field uniformity in the working area of the used standard coil is an important source for calculating the measurement uncertainty. In addition, when conducting various types of magnetic field tests using a magnetic field coil, due to the different sizes of the devices under test and the deviation of the placement position of the device in the coil from the center, the magnetic field coil will give the magnetic field uniformity index within a certain area centered on the coil center after metrology and testing, for metrology professionals and experimenters to conduct data analysis on the conclusions obtained from working with the magnetic field coil. In the national metrology technical specification JJF1906-2021 "Calibration Specification for Constant Magnetic Field Coils", the definition of magnetic field uniformity is "the degree of uniformity of the magnetic field generated at each point after the magnetic field coil is energized within a given area, usually expressed by the degree of uniformity of the coil constants at different points". In the calibration method of magnetic field uniformity in this specification, first calculate the relative difference between the coil constant at each measurement point and the coil constant at the coil center point, and take the relative deviation with the largest absolute value as the uniformity of the magnetic field coil. The magnetic field uniformity in the working area, like the coil constant at the center point, is one of the core technical indicators of the magnetic field coil.

[0003] For improving the magnetic field coil uniformity index, generally three measures are adopted: selecting the structural form before coil manufacturing, quality control during the manufacturing process, and compensation after the coil is manufactured. Among them, selecting the structural form before manufacturing mainly involves selecting a coil combination form with higher uniformity under the same external dimensions. For example, compared with the simplest two-ring Helmholtz coil, selecting a three-ring, four-ring, or even six-ring coil can significantly improve the magnetic field uniformity, but a balance needs to be achieved between economy and uniformity; quality control during the manufacturing process mainly controls the frame size deviation, winding size deviation, and assembly deviation during the manufacturing process to make the actually manufactured coil as close as possible to the theoretical design value; after the coil is manufactured, if the uniformity cannot meet the requirements and downgrading is not desired, only the method of uniformity compensation can be used for remedy.

[0004] The patent "A Non-uniformity Compensation Winding and Compensation Method for an Elongated Toroidless Coil" (CN111044944A) proposes a method of adding compensation windings at both ends of a solenoid toroidless coil to compensate for the axial magnetic field uniformity of the coil. The patent "A Method for Compensating the Non-uniformity of a Magnetic Field Coil by Shunt Compensation" (CN201010610332) proposes a method of fine-tuning the current passing through each winding of a combined coil to compensate for the uniformity of the magnetic field generated by the coil. These two patents can only compensate for the axial magnetic field non-uniformity of the coil, while the uniformity measurement points in JJF1906-2021 are distributed three-dimensionally. In addition to the axial measurement, there are also measurement points in the horizontal radial and vertical radial directions participating in the uniformity calculation. Therefore, neither of these two patents can compensate for the radial magnetic field uniformity.

[0005] The patent "A Coil for Magnetic Field Compensation Inside a Magnetic Shielding Device" (CN202310209680) proposes to compensate for the magnetic field uniformity in the central region of a shielding chamber by designing a special-shaped planar coil in a near-zero magnetic field environment of the magnetic shielding chamber. This method is only applicable to the uniformity compensation of a fixed magnetic field and is not applicable to a magnetic field coil that needs to frequently adjust the magnetic field value. Summary of the Invention

[0006] In order to overcome the defects of the prior art, the present disclosure proposes a method for compensating the radial magnetic field uniformity of a main coil using a compensation coil.

[0007] The principle of this method is to symmetrically arrange two magnetic moment coils in the radial direction of the main coil, and use the magnetic field difference of a pair of magnetic moment coils decaying to different positions in the working area of the coil to reduce the magnetic field difference in the working area of the main coil, so as to improve the magnetic field uniformity in the working area of the main coil. It mainly includes the following steps:

[0008] Symmetrically arrange a pair of magnetic moment coils in each of the two mutually perpendicular radial directions of the main coil to be compensated for uniformity compensation, where: the two pairs of magnetic moment coils are both centrosymmetric about the center of the main coil, and the center connection lines of the two pairs of magnetic moment coils are perpendicular to each other; the two coils of each pair of magnetic moment coils are the same, and the direction of the magnetic field at the center inside them is parallel to the magnetic field at the center of the main coil.

[0009] Before designing the parameters of the compensation coil, first measure the coil constants of at least the edge points, center points, and necessary intermediate points in two radial directions within the working area of the main coil, and draw the coil constant curve in the radial direction of the working area of the main coil. Common classical coils such as Helmholtz, Barker, and Maxwell are all radially symmetric coils, and the coil constant curves in the radial direction of their internal working areas are all parabolas.

[0010] By using a pair of magnetic moment coils, a compensation coil constant parabola that is as close as possible to the coil constant curve of the main coil is generated radially in the working area. According to the principle of opposite parabola openings, the main coil and the auxiliary coil are connected in series, and the same current is passed through the main coil and the compensation coil, so as to realize the uniformity compensation of the radial magnetic field of the main coil.

[0011] Further, the specific method is as shown in the appendix Figure 1 As shown, the magnetic axis of the main coil 1 points to the X-axis, and Y and Z are the radial directions of the main coil 1. The magnetic moment coils 2 and 3 are a pair of compensation coils, the magnetic axis direction points to the X-axis, and they are arranged along the Y-axis direction. The magnetic moment coils 2 and 3 are symmetric about the center point O of the main coil 1, and are used to compensate the magnetic field uniformity of the X-direction magnetic field of the main coil 1 along the Y-axis direction. Another pair of magnetic moment coils 4 and 5, the magnetic axis direction also points to the X-axis, and are arranged along the Z-axis direction. The magnetic moment coils 4 and 5 are symmetric about the center point O of the main coil, and are used to compensate the magnetic field uniformity of the X-direction magnetic field of the main coil 1 along the Z-axis direction.

[0012] Among them, the magnetic moment coils 2 and 3 are a pair, with completely the same parameters and the same magnetic field direction; the magnetic moment coils 4 and 5 have completely the same parameters and the same magnetic field direction.

[0013] As shown in the appendix Figure 2 and 3 As shown, the ratio of the winding radius to the winding length of the magnetic moment coils 2, 3, 4, and 5 satisfies And when the distance D from a certain point on the outer radial of the magnetic moment coil to the center of the magnetic moment coil is D > 6r, the coil constant of a single magnetic moment coil at this point is calculated according to the following formula:

[0014]

[0015] In the formula:

[0016] K b ―― Coil constant at a certain point on the outer radial of the magnetic moment coil, T / A;

[0017] μ0 ―― Vacuum magnetic permeability, μ0 = 4π×10 -7 T·m / A;

[0018] K m ―― Magnetic moment constant of the magnetic moment coil, m 2 ;

[0019] D ―― Distance from this point to the center of the magnetic moment coil, m.

[0020] Taking the formula derivation and calculation of the pair of compensation coils of the magnetic moment coils 2 and 3 as an example for illustration, the design of the pair of magnetic moment coils 4 and 5 is similar.

[0021] As shown in the appendix Figure 2As shown in the figure, assume that the radial boundary points of the working area of the main coil 1 are A and B, and the center point is O. The distances from both points A and B to O are d (the size of the uniform area of the main coil is a known value). For any point P between line segment AB, assume its distance from point O is x. Then, the combined coil constant of a pair of magnetic moment coils 2 and 3 at point P is calculated according to the following formula:

[0022]

[0023] It can be calculated from formula (2) that when D > 2d, the combined coil constant K of a pair of magnetic moment coils bp The numerical change curve from A to B is a parabola.

[0024] Preferably, the matching of the parabola curve of the coil constants of the main coil 1 and a pair of compensation coils (2 and 3) can be achieved in a relatively simple way: Assume that the difference in coil constants between point A of the main coil 1 and point O is measured as ΔK b , and the simple method is to make the difference in coil constants between point A and point O of the magnetic moment coils 2 and 3 equal to ΔK b , and the formula is as follows:

[0025]

[0026] By transforming formula (3), the magnetic moment calculation formula of the magnetic moment coils 2 and 3 is obtained:

[0027]

[0028] Furthermore, let the half side length (or radius) of the winding of the main coil be L, and the value of D is limited to between 0.3L and 2L. For the main coil 1 with better uniformity, D can take a larger value in the interval; for the main coil 1 with poorer uniformity, D can take a smaller value in the interval. If the other parameters in formula (4) are all known, then the magnetic moment K of the magnetic moment coil can be calculated according to formula (4). m .

[0029] Furthermore, according to the magnetic moment calculation formula of the magnetic moment coil:

[0030] K m = n·π·r 2 (5)

[0031] In the formula:

[0032] K m ―― The magnetic moment constant of the magnetic moment coil, m 2 ;

[0033] n ―― The number of turns of the magnetic moment coil;

[0034] r ―― The average radius of the winding of the magnetic moment coil.

[0035] Among them, K m It is known that the number of turns n is a positive integer, and the radius r of the magnetic moment coil is < D / 6. Then, many combinations of the number of turns and radius of the magnetic moment coil can be obtained according to formula (5), and the parameters of the number of turns and radius of the magnetic moment coil that are convenient for processing and assembly can be selected.

[0036] Two pairs of magnetic moment coils (2, 3 and 4, 5) are respectively fixed at two radials of the main coil 1 according to the value of the distance D from the center. Taking the principle that the openings of the coil constant curves of the compensation magnetic moment coils (2, 3, 4, 5) and the main coil 1 face each other to determine the positive and negative directions of the current, all four magnetic moment coils (2, 3, 4, 5) are connected in series with the main coil 1, and the compensation of the uniformity of the two radial magnetic fields can be realized.

[0037] Compared with the prior art, the beneficial effects of the present disclosure are: (1) Compared with the existing technology, two pairs of magnetic moment coils can be used to compensate the uniformity of the horizontal radial and vertical radial of the main coil respectively; (2) The coil constant of the magnetic moment coil is used to compensate the coil constant of the main coil, and the main coil and the compensation coil are connected in series. When the current in the main coil is adjusted so that the magnitude of the standard magnetic field generated changes, the current passing through the compensation magnetic moment coil also changes accordingly, and the generated compensation magnetic field also changes in proportion. The radial magnetic field uniformity of the main coil can be compensated within the full range of the magnetic field generation of the main coil; (3) The implementation is simple and feasible. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] By describing the exemplary embodiments of the present disclosure in more detail in conjunction with the drawings, the above and other objects, features, and advantages of the present disclosure will become more obvious. Among them, in the exemplary embodiment mode of the present disclosure, the same reference numerals generally represent the same components.

[0039] Figure 1 It is a layout diagram of an exemplary main coil and a compensation magnetic moment coil according to the present disclosure;

[0040] Figure 2 It is a magnetic field and shaft system diagram of the main coil and the compensation magnetic moment coil;

[0041] Figure 3 It is a parameter diagram of the main coil and the compensation magnetic moment coil;

[0042] Among them, 1 represents the main coil, 2 and 3 are a pair of compensation coils, and 4 and 5 are another pair of compensation coils. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] The preferred embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the preferred embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure will be more thorough and complete, and the scope of the present disclosure can be fully conveyed to those skilled in the art.

[0044] This embodiment provides a method for compensating the radial magnetic field uniformity of a magnetic field coil, and its specific steps include:

[0045] Step 1: First, measure the difference ΔK in coil constants between the radial edge point A and the center point O in the working area of the main coil 1. b .

[0046] Step 2: Determine the value of D between 0.3L and 2L.

[0047] Step 3: Calculate the magnetic moment K of the magnetic moment coil according to formula (4). m .

[0048] Step 4: According to the principle of r < D / 6, determine the number of turns n and the winding radius r of the magnetic moment coil that are convenient for processing and assembly according to formula (5).

[0049] Step 5: Similarly, determine the parameters of the magnetic moment coil in another radial direction.

[0050] Step 6: According to the principle that the openings of the constant curves of the main coil and the compensating magnetic moment coil face each other, determine the positive and negative relationships between a pair of magnetic moment coils and the corresponding main coil, and connect both pairs of magnetic moment coils in series to the main coil.

[0051] Since then, the compensation of the magnetic field uniformity in two radial directions of the main coil can be achieved.

[0052] Application Example

[0053] For a square Helmholtz coil with a half-side length of the winding being 1500 mm and 100 turns of winding, within the radial range of ±300 mm, before compensation, the theoretical value of its radial magnetic field uniformity is calculated to be 6.6×10 -4 .

[0054] Step 1: Theoretically calculate the difference ΔK in coil constants between the radial edge point A and the center point O in its working area. b = 36 nT / A.

[0055] Step 2: Since its uniformity is poor, take D = 0.4L = 600 mm.

[0056] Step 3: Calculate the magnetic moment K of the magnetic moment coil from formula (4). m = 0.0124 m 2 .

[0057] Step 4: Determine that the number of turns n of the magnetic moment coil for easy processing and assembly is 10 turns according to formula (5), and the winding radius r is 20 mm.

[0058] After superimposing the calculated magnetic field of the magnetic moment coil and the magnetic field of the main coil, the theoretical value of the magnetic field uniformity is increased to 6.1×10 -5 , compared with the uniformity of 6.6×10 -4 without compensation, it is increased by one order of magnitude.

[0059] The above technical solution is only an exemplary embodiment of the present invention. For those skilled in the art, based on the application methods and principles disclosed in the present invention, it is very easy to make various types of improvements or deformations, not limited to the methods described in the above specific embodiments of the present invention. Therefore, the foregoing description is only preferred and does not have a restrictive meaning.

Claims

1. A method for compensating the radial magnetic field uniformity of a magnetic field coil, comprising the following steps: S1, symmetrically arrange a pair of magnetic moment coils on each of two mutually perpendicular radial directions of the main coil to be compensated for uniformity compensation, where: both pairs of magnetic moment coils are symmetric about the center of the main coil, the two coils of each pair are the same, and the direction of the central magnetic field inside them is parallel to the central magnetic field of the main coil; S2, determine the positive and negative relationships between a pair of magnetic moment coils and the corresponding main coil according to the principle that the openings of the constant curves of the main coil and the magnetic moment coils for compensation face each other, and connect both pairs of magnetic moment coils in series to the main coil.

2. The method according to claim 1, wherein The specific steps of step S1 include: S11, measure the difference in coil constants ΔK between the radial edge point A and the center point O of the main coil 1 in the working area b ; S12, determine the value of D between 0.3L and 2L according to the magnitude of the uniformity of the main coil, where L is the half side length or radius of the winding of the main coil, and D is the distance between the magnetic moment coil and the center of the main coil; S13. Let the difference in coil constants between a pair of magnetic moment coils at point A and point O also be equal to ΔK b , and calculate the magnetic moment K of this pair of magnetic moment coils m ; S14, determine the number of turns n and the winding radius r of the magnetic moment coil that are convenient for processing and assembly according to the principle that the winding length r of the magnetic moment coil is < D / 6; S15, similarly, determine the parameters of the magnetic moment coil in the other radial direction.

3. The method according to claim 2, wherein In step S12, the specific method for determining D includes: For the main coil with better uniformity, D takes the larger value in the interval; For the main coil with poorer uniformity, D takes the smaller value in the interval.

4. The method according to claim 2 or 3, characterized in that, In the step S13, the magnetic moment K m is calculated by the following method: Where μ0 is the magnetic permeability of vacuum; d is the distance between the radial boundary point A or B of the working area of the main coil and the center point O.

5. The method according to claim 2 or 3, characterized in that, The specific steps of step S14 include: According to formula K m = n·π·r 2 , select the number of turns n and radius r of the magnetic moment coil that are convenient for machining and assembly.

6. The method according to claim 2, wherein The following steps are also included in step S1: Measure the coil constants of the main coil in two radial directions within the working area, including at least the edge points, the center point, and the necessary intermediate points, and draw the coil constant curve in the radial direction of the working area of the main coil.

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