A hierarchical gradient magnetic assembly

Abstract

The application relates to a hierarchical gradient magnetic assembly, which comprises at least two groups of magnetic concentration units and at least one intermediate magnet arranged along a preset arrangement path; each group of magnetic concentration units comprises a first magnet, a second magnet and a magnetic attraction sheet, the first magnet and the second magnet are arranged side by side along the arrangement path, the magnetization directions of the first magnet and the second magnet are parallel to the arrangement path, the adjacent end faces are the same magnetic poles, and the magnetic attraction sheet is clamped between the first magnet and the second magnet; the intermediate magnet is arranged between two adjacent groups of magnetic concentration units, and the magnetization direction of the intermediate magnet is perpendicular to the arrangement path. The magnetic lines of force of the same-pole opposite magnets are collected and led out through the magnetic attraction sheet, a high-gradient strong magnetic region with high surface magnetic intensity and short magnetic attraction distance is formed, a wide diffusion magnetic field region with low surface magnetic intensity and long magnetic attraction distance is formed through the vertically magnetized intermediate magnet, the two regions are alternately arranged, and the ferromagnetic microparticles far from the surface in the fluid are first guided and gathered by the long-distance magnetic force, and then are adsorbed and fixed by the short-distance strong magnetic force.

Description

Technical Field

[0001] This invention relates to the field of magnetic filtration technology, and more specifically to a graded gradient magnetic focusing component. Background Technology

[0002] Magnetic filtration devices are widely used for separating and removing ferromagnetic impurities from liquid or slurry materials, with magnetic rods and strips being common core functional components. Existing magnetic rods or strips typically consist of built-in permanent magnets. The magnetic field generated by the permanent magnets applies a magnetic attraction force to ferromagnetic particles passing through the fluid, causing them to be attracted to the surface of the magnetic rod, thereby achieving impurity separation.

[0003] In existing technologies, to improve the adsorption efficiency of magnetic rods, a common approach is to arrange multiple radially magnetized permanent magnets sequentially along the axial direction of the rod, or to use magnetic sheets to separate adjacent magnets, thereby forming multiple axially distributed magnetic field peak regions on the surface of the rod. These structures increase the magnetic field strength on the surface of the rod to some extent, enhancing local adsorption capacity. However, in this type of design, the magnetic field function of each region on the surface of the rod is relatively singular, typically relying solely on increasing the surface magnetic intensity to improve adsorption force. For ferromagnetic particles in the fluid that are slightly farther from the surface of the rod, the magnetic force often decays rapidly, resulting in insufficient guidance, pre-aggregation, and early collection capabilities before capture. Particles can easily bypass the rod in the fluid without being intercepted, limiting further improvements in filtration efficiency. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a graded gradient magnetic focusing component.

[0005] The objective of this invention can be achieved through the following technical solution: a graded gradient magnetic focusing component, comprising at least two sets of magnetic focusing units and at least one intermediate magnet arranged sequentially along a preset arrangement path; each set of magnetic focusing units includes a first magnet, a second magnet, and a magnetic attracting sheet, the first magnet and the second magnet are arranged side by side along the preset arrangement path, and the magnetization direction of both is parallel to the preset arrangement path; the end faces of the first magnet and the second magnet adjacent to each other are of the same magnetic pole; the magnetic attracting sheet is clamped and fixed between the first magnet and the second magnet; the intermediate magnet is disposed between two adjacent sets of magnetic focusing units, and the magnetization direction of the intermediate magnet is perpendicular to the preset arrangement path.

[0006] Preferably, the arrangement path of the magnetic focusing units and the intermediate magnet is a straight line, and multiple sets of magnetic focusing units and intermediate magnets are arranged alternately along the straight line to form a linear array structure.

[0007] Preferably, the arrangement path of the magnetic focusing units and the intermediate magnet is an arc or a complete circle, and multiple sets of magnetic focusing units and intermediate magnets are arranged alternately along the circumference to form an arc or ring array.

[0008] Preferably, when the arrangement path of the magnetic focusing unit and the intermediate magnet is a linear array or an arc array, end magnetic guide plates are respectively provided at both ends of the linear array or the arc array.

[0009] Preferably, along the extension direction of the arrangement path, the thickness of the intermediate magnet is less than the thickness of the first magnet and the second magnet along that direction.

[0010] Preferably, the thickness of the intermediate magnet is d1, the thickness of the first magnet is d2, and d1 = (0.3~0.6)d2.

[0011] The beneficial effects of this invention are as follows: by using magnetic attracting sheets to gather and concentrate the magnetic lines of force of opposing magnets with the same poles on both sides, a high-gradient strong magnetic region with high surface magnetic intensity and short magnetic attraction distance is formed on the working surface. At the same time, a wide-range diffused magnetic field region with low surface magnetic intensity and long magnetic attraction distance is formed on the working surface by a vertically magnetized intermediate magnet. The two regions are alternately arranged along the arrangement path, so that ferromagnetic particles in the fluid that are far from the surface are first migrated and gathered to the surface of the component by the long-distance magnetic force of the intermediate magnet region, and then finally adsorbed and fixed by the close-distance strong magnetic force of the adjacent magnetic attracting sheet region. This forms a graded processing mode of first guiding and then capturing, which is beneficial to improving the overall capture efficiency of ferromagnetic particles in the fluid. Attached Figure Description

[0012] The present invention will be further described with reference to the accompanying drawings, but the embodiments in the drawings do not constitute any limitation on the present invention. For those skilled in the art, other drawings can be obtained based on the following drawings without creative effort.

[0013] Figure 1 This is a schematic diagram of the structure of a linear array of graded gradient magnetic components according to the present invention.

[0014] Figure 2 This is a schematic diagram of the structure of a graded gradient magnetic focusing magnetic component arc array according to the present invention.

[0015] Figure 3 This is a schematic diagram of a circular array of graded gradient magnetic components according to the present invention.

[0016] The labels in the figure represent: 1. Magnetic focusing unit; 2. Intermediate magnet; 3. First magnet; 4. Second magnet; 5. Magnetizing sheet; 6. End magnetic guide plate. Detailed Implementation

[0017] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indicators will also change accordingly.

[0018] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. If the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0019] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0020] See Figures 1 to 3As shown, the structure of the present invention is as follows: a graded gradient magnetic focusing component, comprising at least two sets of magnetic focusing units 1 and at least one intermediate magnet 2 arranged sequentially along a preset arrangement path; each set of magnetic focusing units 1 includes a first magnet 3, a second magnet 4, and a magnetic attracting sheet 5, the first magnet 3 and the second magnet 4 are arranged side by side along the preset arrangement path, and the magnetization direction of both is parallel to the preset arrangement path; the end faces of the first magnet 3 and the second magnet 4 adjacent to each other are of the same magnetic pole; the magnetic attracting sheet 5 is clamped and fixed between the first magnet 3 and the second magnet 4; the intermediate magnet 2 is disposed between two adjacent sets of magnetic focusing units 1, and the magnetization direction of the intermediate magnet 2 is perpendicular to the preset arrangement path. The region where the magnetic attractor sheet 5 is located forms a capturing magnetic field zone with high surface magnetic intensity and a short effective magnetic attraction distance. The region where the intermediate magnet 2 is located forms a guiding magnetic field zone with lower surface magnetic intensity and a longer effective magnetic attraction distance than the capturing magnetic field zone. These two magnetic field zones alternately form a graded gradient magnetic field along the arrangement path. Specifically, in the surface region corresponding to the magnetic attractor sheet 5, because the first magnet 3 and the second magnet 4 are arranged opposite each other with the same poles, the magnetic lines of force generated by the magnets on both sides cannot directly cross along the arrangement path due to the mutual repulsion of the like magnetic poles. Instead, they are collected and redistributed through the magnetic attractor sheet 5, whose magnetic permeability is much higher than that of air, and finally concentrated and emitted from the surface of the magnetic attractor sheet 5 toward the working surface. This magnetic flux concentration effect makes the magnetic line density in this region extremely high, which is manifested as a surface magnetic intensity significantly higher than other regions of the component; however, the magnetic lines of force diverge rapidly after leaving the surface of the magnetic attractor sheet 5, and the magnetic field intensity decreases exponentially with increasing distance. Therefore, the effective magnetic attraction distance in this region is short, and the area of ​​strong magnetic effect is limited to the surface of the magnetic attractor sheet 5 and a small area adjacent to it. In the surface region corresponding to the intermediate magnet 2, since the magnetization direction of the intermediate magnet 2 is perpendicular to the arrangement path and orthogonal to the horizontal magnetization direction of the two magnetic focusing units 1, the intermediate magnet 2 forms a wide-range diffused magnetic field region with a single magnetic pole as the main component and uniformly distributed, facing the working surface of the fluid. The magnetic field lines in this region are relatively uniformly distributed, and the divergence range of the magnetic field lines in space is large. Therefore, its surface magnetic intensity is lower than the concentrated magnetic field at the magnetic attracting sheet 5, but the decay rate of the magnetic field intensity with increasing distance is significantly slower, and the effective magnetic attraction distance is longer. It can generate a magnetic effect on a spatial range far from the component surface, and the magnetic field coverage area is larger than the strong magnetic area at the magnetic attracting sheet 5. When the fluid to be filtered, containing ferromagnetic particles, flows along the component surface, the above two functional areas work together. Ferromagnetic particles in the fluid that are far from the component surface first enter the effective range of the magnetic field generated by the intermediate magnet 2. Although the surface magnetic field of the intermediate magnet 2 region has a low magnetic intensity, its effective distance is sufficient to exert a magnetic force on distant particles. This causes the particles to be subjected to a magnetic force component pointing towards the component surface during fluid movement, gradually deviating from their original streamline direction and migrating and accumulating towards the component surface. Particles that have migrated to the vicinity of the component surface continue to move along the surface under the influence of the fluid, entering the effective range of the high surface magnetic intensity region at the adjacent magnetic sheet 5.Because the magnetic flux is highly concentrated in the region of the magnetic attractor plate 5, the spatial magnetic field gradient is much greater than that in the region of the intermediate magnet 2. When particles approach this region, they are subjected to a sharply increased magnetic attraction force and are quickly adsorbed from the fluid and fixed to the surface position corresponding to the magnetic attractor plate 5. Thus, the intermediate magnet 2 region completes the function of guiding and pre-gathering distant particles, while the magnetic attractor plate 5 region completes the function of finally capturing and fixing nearby particles. The two functional regions are alternately arranged along the arrangement path, forming a multi-stage series gradient filtration structure along the fluid flow direction, realizing the continuous and hierarchical adsorption and separation of ferromagnetic particles in the fluid. Meanwhile, this structure, through the alternating arrangement of a high magnetic flux density capture magnetic field region and a wide-range uniform guiding magnetic field region, can form a periodic and stable alternating magnetic coupling magnetic field. It can also be applied to power transmission fields such as magnetic drive, magnetic coupling, and non-contact torque transmission. The high-gradient magnetic focusing region can provide high-intensity magnetic tangential coupling force, ensuring high torque and high stability power output. The wide-range diffusion magnetic field region can effectively buffer magnetic field abrupt changes, reduce hysteresis loss and transmission pulse error, making non-contact magnetic drive operation more stable, efficient, and low-noise. It realizes the dual functions of fluid magnetic purification and precision magnetic drive of a single magnetic component, broadening the product's applicable scenarios and technical application value.

[0021] like Figure 1 As shown, the arrangement path of the magnetic focusing unit 1 and the intermediate magnet 2 is a straight line. Multiple sets of magnetic focusing units 1 and intermediate magnets 2 are arranged alternately along the straight line to form a linear array structure. Specifically, the overall component arrangement path is set to a straight line to form a linear permanent magnet array, which is suitable for the installation conditions of conventional straight cylindrical filter pipes, straight-insertion fluid filter chambers with matching magnetic rods, and long flat plate filter magnetic strips. This linear array can continuously arrange multiple sets of magnetic focusing units 1 and intermediate magnets 2 along the unidirectional flow direction of the fluid. When the fluid flows in a straight line in a unidirectional direction along the pipe, it will continuously pass through the high magnetic flux adsorption area corresponding to the magnetic focusing unit 1 and the wide diffusion magnetic field area corresponding to the intermediate magnet 2 in sequence. Ferromagnetic impurities inside the fluid can be continuously captured in layers and stages, effectively avoiding the problem of impurity escape caused by the fluid flowing through the filter component at high speed for a short time. Meanwhile, the linear array structure is regular, and the number of magnetic focusing units 1 and intermediate magnets 2 can be flexibly increased or decreased according to the actual length of the filter pipe, making it suitable for high-flow-rate and long-channel fluid filtration scenarios. The segmented and zoned magnetic field generated by the linear array is evenly distributed along the fluid flow direction, which can provide stable and continuous adsorption and filtration capabilities throughout the process, ensuring long-term and stable magnetic purification effect under continuous high-flow-rate fluid conditions.

[0022] like Figure 2 , Figure 3As shown in the figure, the arrangement path of the magnetic focusing unit 1 and the intermediate magnet 2 is an arc or a full circle. Multiple sets of magnetic focusing units 1 and intermediate magnets 2 are arranged alternately along the circumference to form an arc or ring array. Specifically, when the arrangement path is further extended into an arc, multiple sets of magnetic focusing units 1 and intermediate magnets 2 are arranged alternately along the arc direction to form an arc-shaped permanent magnet array. The basic filtration mechanism of this array is consistent with that of the linear permanent magnet array. In this configuration, the magnetization direction of the intermediate magnet 2 is arranged radially along the arc. The side of the intermediate magnet 2 facing the fluid working surface forms a wide diffusion magnetic field region extending along the arc path. This magnetic field covers the corresponding central angle range along the arc direction and can uniformly apply traction magnetic force to the fluid flowing inside the arc space. The corresponding position of the magnetic attracting sheet 5 forms a high gradient strong magnetic adsorption region radially. The arc array is adapted to filter devices with flow channels bent into arc shapes. It can complete the far-field guidance and graded capture of ferromagnetic particles inside the arc flow channel with a defined central angle. The effective range of the magnetic field is determined by the central angle corresponding to the arc array. When the arrangement path is a closed circle, multiple sets of magnetic focusing units 1 and intermediate magnets 2 are arranged alternately along the circumference to form a ring-shaped permanent magnet array. In this configuration, the intermediate magnets 2 are magnetized radially along the pipe, and the wide-amplitude diffusion magnetic fields corresponding to each section of the intermediate magnets 2 are continuously connected along the circumference, forming a ring-shaped diffusion magnetic field layer that radiates radially towards the center of the flow channel and uniformly covers the entire circumference. The ring-shaped wide-amplitude diffusion magnetic field can continuously apply radial magnetic force to the ferromagnetic particles inside the fluid, pointing towards the working surface of the permanent magnet component, within the entire circumferential range, achieving particle guidance and pre-gathering and early collection without dead angles. The positions of each magnetic attracting plate 5 are uniformly arranged along the circumference, forming a continuous ring of high-gradient strong magnetic capture points, which complete the final adsorption and locking of ferromagnetic particles that migrate radially towards the component. The ring array is particularly suitable for cylindrical fluid channel structures such as tubular magnetic filters, relying on the 360° surrounding zoned magnetic field to achieve magnetic iron removal and separation of the fluid across the entire cross-section of the pipe without dead angles.

[0023] like Figure 1 , Figure 2 As shown, when the magnetic focusing unit 1 and the intermediate magnet 2 are arranged in a linear array or an arc array, end magnetic guide plates 6 are respectively provided at both ends of the linear array or arc array. Specifically, the high magnetic permeability of the end magnetic guide plates 6 is used to gather and close the diverging magnetic lines of force at the end of the array, effectively suppressing magnetic leakage at the end and improving the overall effective utilization rate of the magnetic field. At the same time, it forms a magnetic adsorption and limiting effect on the end magnets, balancing the unilateral magnetic repulsion force at the end of the array, preventing the magnets from shifting and misaligning, and improving the overall structural stability. In addition, the end magnetic guide plates 6 can form a closed magnetic field barrier at both ends of the array, making up for the magnetic field gap at both ends of the open array, preventing ferromagnetic particles from escaping with the fluid from the weak areas at the end, and further improving the overall graded filtration and iron removal effect.

[0024] Furthermore, along the extension direction of the arrangement path, the thickness of the intermediate magnet 2 is less than the thickness of the first magnet 3 and the second magnet 4 along this direction. Specifically, the thickness of the intermediate magnet 2 along the extension direction of the arrangement path is significantly smaller than the thickness of the first magnet 3 and the second magnet 4 along this direction. This thickness difference is not a simple dimensional adjustment, but a core design feature that directly controls the overall magnetic field spatial distribution efficiency and fluid filtration performance of the magnetic focusing assembly. If the thickness of the intermediate magnet 2 is too large, although the coverage area of ​​its wide-range diffused magnetic field will increase, it will encroach on the axial arrangement space of the array, reducing the density of high-gradient strong magnetic capture points per unit length. Conversely, if the thickness of the intermediate magnet 2 is too small, its total magnetic flux is insufficient, the effective extension distance of the magnetic lines of force in the fluid is greatly shortened, and the function of guiding and pre-gathering ferromagnetic particles at a distance fails. Through multiple sets of magnetic circuit simulations and fluid iron removal experiments, it was verified that setting the intermediate magnet 2 as a narrow structure with a thickness smaller than that of the first magnet 3 and the second magnet 4 can lay out high-density, high-flux, strong magnetic capture points within a limited axial length, while maintaining a sufficient wide-range diffusion magnetic field transition area between adjacent capture points. Even under conditions of high fluid flux and fast ferromagnetic particle movement, impurity particles can still achieve far-field aggregation and near-field adsorption and fixation in sequence, effectively preventing ferromagnetic particles from flowing out with the fluid before completing aggregation in the weak magnetic transition area, thus avoiding the phenomenon of tailing and escape.

[0025] like Figure 1 As shown, the thickness of the intermediate magnet is d1, and the thickness of the first magnet is d2, where d1 = (0.3~0.6)d2. Specifically, if the intermediate magnet 2 is too narrow, the overall magnetic flux of the intermediate magnet 2 will be insufficient, the effective range of the far-field wide-width guiding magnetic field will be greatly shortened, and the ability to gather and pull distant fine ferromagnetic particles in the fluid will be significantly reduced. At the same time, if the axial width of the intermediate magnet 2 is too narrow, the corresponding magnetic field coverage area will be insufficient, the overall effective adsorption area will be reduced, and a large number of far-field particles will not be able to be pulled to the strong magnetic capture area in advance, making them very easy to escape with the fluid, resulting in a significant reduction in filtration capture efficiency. If the intermediate magnet 2 is too thick, the intermediate magnet 2 will occupy too much array arrangement space, the number of magnetic gathering units 1 that can be arranged under the same total length will be reduced, and the density of high-gradient strong magnetic capture points will decrease. Moreover, when the thickness of the intermediate magnet 2 is too large, the overall magnetic induction intensity of the diffusion magnetic field formed by its own orthogonal magnetization will be diluted, the magnetic force intensity of the working surface will be difficult to meet the stable guidance requirements, and the pre-gathering of particles will not be completed efficiently, resulting in the failure of the graded magnetic field collaborative filtration effect. Therefore, by controlling the thickness of the intermediate magnet 2 within the range of 0.3 to 0.6 times the thickness of the first magnet 3, sufficient far-field guiding adsorption area and high-density near-field strong magnetic capture points can be taken into account within a limited array length, so that the low-intensity long-range guiding magnetic field and the high-gradient short-distance capture magnetic field form an optimal match.

[0026] In practical use, during operation, the magnetic focusing units 1 and the intermediate magnets 2, arranged alternately along a preset path, cooperate to complete the graded magnetic filtration operation. In each magnetic focusing unit 1, the first magnet 3 and the second magnet 4, with their magnetization direction parallel to the arrangement path and their opposite end faces having the same magnetic poles, force the magnetic lines of force on both sides to be unable to directly penetrate along the arrangement path through the repulsion of their like poles. This concentrates all magnetic flux to the magnetically attracting sheet 5 sandwiched between the two and redistributes it, creating a localized strong magnetic capture area on the working surface of the magnetically attracting sheet 5 with high density, high surface magnetic field, and high magnetic field gradient, but with a short effective distance and concentrated effective range. Meanwhile, the intermediate magnets 2, positioned between adjacent magnetic focusing units 1 and with their magnetization direction orthogonal to the arrangement path, can form a wide-range diffused magnetic field area on the fluid's working surface, characterized by uniform magnetic field distribution, wide coverage, gradual attenuation, and a longer effective distance. When the fluid contains ferromagnetic particles... As the fluid continues to flow along the working surface of the module, the dispersed ferromagnetic particles far from the module surface in the flow channel are first within the wide-range diffusion magnetic field of the intermediate magnet 2. Under the long-distance magnetic traction, they gradually deviate from the original fluid flow lines and continue to migrate and gather towards the working surface of the module, completing the pre-gathering and aggregation of the long-distance particles. As the fluid continues to flow, the ferromagnetic particles that have come close to the module surface enter the high-gradient strong magnetic region corresponding to the magnetic attractor 5 in sequence. Under the action of the sudden increase in magnetic attraction in this region, they are quickly adsorbed and fixed on the surface of the magnetic attractor 5, realizing the precise capture and locking of the particles. Relying on the dynamic cooperation of the alternating wide-distance weak magnetic guiding region and the close-distance strong magnetic capture region, the module forms a continuous multi-level gradient filtration system along the fluid flow direction. The entire process realizes a graded, continuous, and escape-free magnetic separation and purification process for the ferromagnetic particles in the fluid, from far-field gathering and aggregation to near-field precise adsorption.

[0027] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A graded gradient magnetic focusing component, characterized in that: It includes at least two sets of magnetic focusing units (1) arranged sequentially along a preset arrangement path and at least one intermediate magnet (2); each set of magnetic focusing units (1) includes a first magnet (3), a second magnet (4) and a magnetic attracting sheet (5), the first magnet (3) and the second magnet (4) are arranged side by side along the preset arrangement path, and the magnetization direction of both is parallel to the preset arrangement path; the end faces of the first magnet (3) and the second magnet (4) adjacent to each other are magnetic poles of the same polarity; the magnetic attracting sheet (5) is clamped and fixed between the first magnet (3) and the second magnet (4); the intermediate magnet (2) is arranged between two adjacent sets of magnetic focusing units (1), and the magnetization direction of the intermediate magnet (2) is perpendicular to the preset arrangement path.

2. The magnetizing permanent magnet assembly according to claim 1, characterized in that: The magnetic focusing unit (1) and the intermediate magnet (2) are arranged in a straight line, and multiple sets of the magnetic focusing unit (1) and the intermediate magnet (2) are arranged alternately along the straight line to form a linear array structure.

3. The magnetizing permanent magnet assembly according to claim 1, characterized in that: The arrangement path of the magnetic focusing unit (1) and the intermediate magnet (2) is an arc or a circle, and multiple sets of the magnetic focusing unit (1) and the intermediate magnet (2) are arranged alternately along the circumference to form an arc or ring array.

4. The magnetizing permanent magnet assembly according to claim 2 or 3, characterized in that: When the arrangement path of the magnetic focusing unit (1) and the intermediate magnet (2) is a linear array or an arc array, end magnetic guide plates (6) are respectively provided at both ends of the linear array or the arc array.

5. The magnetizing permanent magnet assembly according to claim 1, characterized in that: Along the extension direction of the arrangement path, the thickness of the intermediate magnet (2) is less than the thickness of the first magnet (3) and the second magnet (4) along the same direction.

6. The magnetizing permanent magnet assembly according to claim 5, characterized in that: The thickness of the intermediate magnet (2) is d1, and the thickness of the first magnet (3) is d2, and d1 = (0.3~0.6)d2.

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