A high thrust density voice coil motor with auxiliary enhancement magnetic circuit

Abstract

This invention discloses a high thrust density voice coil motor with an auxiliary enhanced magnetic circuit, comprising a stator assembly and a mover assembly. The stator assembly includes a yoke portion, a basic magnetic circuit, and a first auxiliary magnetic circuit. The basic magnetic circuit includes a plurality of radially magnetized first inner ring permanent magnets and first outer ring permanent magnets, with each first inner ring permanent magnet spaced apart and sleeved on a second yoke, and each first outer ring permanent magnet spaced apart and disposed on the inner wall of the first yoke. The first auxiliary magnetic circuit includes a plurality of axially magnetized second inner ring permanent magnets and second outer ring permanent magnets, with each second inner ring permanent magnet alternately embedded between two adjacent first inner ring permanent magnets, and each second outer ring permanent magnet alternately embedded between two adjacent first outer ring permanent magnets. This invention, applied to the field of motors, not only increases the magnetic flux density in the working air gap of the coil but also helps reduce magnetic flux density fluctuations, thereby improving the output force and stability of the voice coil motor.

Description

Technical Field

[0001] This invention relates to the field of motor technology, specifically to a high thrust density voice coil motor with an auxiliary enhanced magnetic circuit. Background Technology

[0002] A voice coil motor (VCM) is a DC drive motor that achieves high-speed reciprocating motion over a short stroke. It boasts advantages such as high thrust, fast response, high precision, and ease of control, and is widely used in active vibration control. Traditionally, increasing the thrust or reducing thrust fluctuation has been achieved by altering the permanent magnet array configuration and increasing the number of coil groups. However, the inherent end effect of permanent magnets cannot be avoided, creating a conflict between the effective stroke of the mover, the thrust magnitude, and thrust fluctuation. Therefore, under constraints of volume and a fixed effective stroke, optimizing the magnetic circuit design to improve air gap flux utilization, thereby increasing the thrust density and reducing thrust fluctuation of the voice coil motor, has become a major challenge for both academia and engineering. Summary of the Invention

[0003] To address the shortcomings of the prior art, this invention provides a high thrust density voice coil motor with an auxiliary enhanced magnetic circuit. This motor not only has a simple structure but also increases the magnetic flux density in the working air gap where the coil is located, and helps to reduce magnetic flux density fluctuations, thereby improving the output force and stability of the voice coil motor.

[0004] To achieve the above objectives, the present invention provides a high thrust density voice coil motor with an auxiliary enhanced magnetic circuit, comprising a stator assembly and a mover assembly. The stator assembly includes a yoke portion, a basic magnetic circuit and a first auxiliary magnetic circuit, and the mover assembly includes a winding frame, a first coil and a second coil.

[0005] The magnetic yoke portion includes a first magnetic yoke and a second magnetic yoke. The first magnetic yoke is a cylindrical structure with one end open, and the second magnetic yoke is coaxially disposed inside the first magnetic yoke.

[0006] The basic magnetic circuit includes a plurality of radially magnetized and axially aligned first inner ring permanent magnets and first outer ring permanent magnets. Each of the first inner ring permanent magnets is axially spaced and sleeved on the second magnetic yoke, and each of the first outer ring permanent magnets is axially spaced on the inner wall of the first magnetic yoke.

[0007] The first auxiliary magnetic circuit includes a plurality of axially magnetized and axially aligned second inner ring permanent magnets and second outer ring permanent magnets. Each second inner ring permanent magnet is alternately embedded in the gap between two adjacent first inner ring permanent magnets and the bottom wall of the first yoke. Each second outer ring permanent magnet is alternately embedded in the gap between two adjacent first outer ring permanent magnets and the bottom wall of the first yoke, in order to improve the thrust density of the voice coil motor.

[0008] An air gap is formed between the first inner ring permanent magnet, the first outer ring permanent magnet, the second inner ring permanent magnet, and the second outer ring permanent magnet. The winding frame is disposed within the air gap. The first coil and the second coil are axially spaced on the winding frame, and the winding directions of the first coil and the second coil are opposite.

[0009] In one embodiment, the stator assembly further includes a second auxiliary magnetic circuit disposed on the second yoke for reshaping the air gap magnetic field of the voice coil motor to reduce thrust fluctuations without reducing thrust density.

[0010] In one embodiment, the second auxiliary magnetic circuit includes an axially magnetized first cylindrical permanent magnet and a second cylindrical permanent magnet;

[0011] The first cylindrical permanent magnet is embedded in the second magnetic yoke, or the first cylindrical permanent magnet is located between the bottom of the second magnetic yoke and the first magnetic yoke, and the bottom end of the first cylindrical permanent magnet is flush with the top end of the bottommost first inner ring permanent magnet and the first outer ring permanent magnet.

[0012] The second cylindrical permanent magnet is embedded in the second magnetic yoke, and the second cylindrical permanent magnet is located above the first cylindrical permanent magnet.

[0013] In one embodiment, the first inner ring permanent magnet, the first outer ring permanent magnet, the second inner ring permanent magnet, the second outer ring permanent magnet, the first cylindrical permanent magnet, and the second cylindrical permanent magnet are made of permanent magnet materials, including but not limited to AlNiCo permanent magnet materials, NdFeB permanent magnet materials, IronChromiumCo permanent magnet materials, ferrite permanent magnet materials, rare earth permanent magnet materials, or composite permanent magnet materials.

[0014] In one embodiment, the winding frame, the first coil, and the second coil are fixed together by resin injection.

[0015] In one embodiment, the first yoke and the second yoke are made of a soft magnetic metallic material or a soft magnetic ferrite material.

[0016] Compared with the prior art, the present invention has the following beneficial technical effects:

[0017] This invention optimizes the magnetic circuit design of the stator assembly, employs a bipolar basic magnetic circuit to improve the utilization rate of permanent magnets and yokes, and further concentrates and homogenizes the air gap magnetic flux distribution by adding an auxiliary magnetic circuit. This achieves high thrust density and low thrust fluctuation in the voice coil motor, and features a simple structure, low cost, and convenient installation. It can improve the thrust density and reduce thrust fluctuation of the motor without increasing the overall volume of the voice coil motor, thereby achieving precise control of the voice coil motor. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the surface structure of a high thrust density voice coil motor with an auxiliary enhancement magnetic circuit in an embodiment of the present invention;

[0020] Figure 2 This is a graph showing the air gap magnetic flux density characteristic curve of the voice coil motor in an embodiment of the present invention.

[0021] Figure 3 This is a graph showing the output thrust characteristic curve within the stroke range of the voice coil motor in an embodiment of the present invention.

[0022] Reference numerals in the attached diagram: 1. Magnetic yoke, 101. 2. Magnetic yoke, 102. 3. Basic magnetic circuit, 201. 4. 5. 6. 7. 8. 7. 8. 9. 101. 102. 103. 104. 105. 106. 107. 108. 109. 100. 100. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109 ...

[0023] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0026] Furthermore, in this invention, descriptions involving "first," "second," etc., 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. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0027] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixed," etc., should be interpreted broadly. For example, "fixed" can mean a fixed connection, a detachable connection, or an integral part; it can mean a mechanical connection, an electrical connection, a physical connection, or a wireless communication connection; it can mean a direct connection or an indirect connection through an intermediate medium; it can mean the internal communication of two elements or the interaction between two elements, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0028] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0029] like Figure 1 The image shows a high thrust density voice coil motor with an auxiliary enhanced magnetic circuit disclosed in this embodiment. It mainly includes a stator assembly and a mover assembly. The stator assembly includes a yoke part 1, a basic magnetic circuit 2 and a first auxiliary magnetic circuit 3. The mover assembly includes a winding frame 4, a first coil 5 and a second coil 6.

[0030] In this embodiment, the magnetic yoke portion 1 includes a first magnetic yoke 101 and a second magnetic yoke 102. The first magnetic yoke 101 is a cylindrical structure with one open end. The second magnetic yoke 102 is coaxially disposed on the inner bottom wall of the first magnetic yoke 101, and the outer wall of the second magnetic yoke 102 and the inner wall of the first magnetic yoke 101 form a cavity capable of housing the basic magnetic circuit 2, the first auxiliary magnetic circuit 3, and the mover assembly. Both the first magnetic yoke 101 and the second magnetic yoke 102 can be made of metallic soft magnetic materials or ferrite soft magnetic materials. For example, they can be made of metallic soft magnetic materials such as electrical pure iron, silicon steel, permalloy, iron-aluminum alloy, iron-silicon-aluminum alloy, iron-cobalt alloy, amorphous soft magnetic materials, nanocrystalline soft magnetic materials, and soft magnetic composite materials, or ferrite soft magnetic materials such as manganese-zinc ferrite and nickel-zinc ferrite.

[0031] In this embodiment, the basic magnetic circuit 2 includes a plurality of radially magnetized and axially aligned first inner ring permanent magnets 201 and first outer ring permanent magnets 202. Specifically, each first inner ring permanent magnet 201 is axially spaced and sleeved on the second yoke 102, with a gap between the bottom end of the lowermost first inner ring permanent magnet 201 and the bottom inner wall of the first yoke 101, and the top end of the uppermost first inner ring permanent magnet 201 is aligned with the top ends of the first yoke 101 and the second yoke 102. Each first outer ring permanent magnet 202 is axially spaced on the inner wall of the first yoke 101, and similarly, a gap between the bottom end of the lowermost first outer ring permanent magnet 202 and the bottom inner wall of the first yoke 101, and the top end of the uppermost first outer ring permanent magnet 202 is aligned with the top ends of the first yoke 101 and the second yoke 102. Furthermore, the first inner ring permanent magnet 201 and the first outer ring permanent magnet 202, which are at the same height, are magnetized in the same direction in the radial direction, while the two first outer ring permanent magnets 202 and the two first inner ring permanent magnets 201 connected in the axial direction are magnetized in opposite directions in the radial direction.

[0032] In this embodiment, the first auxiliary magnetic circuit 3 includes a plurality of axially magnetized and axially aligned second inner ring permanent magnets 301 and second outer ring permanent magnets 302. Specifically, each second inner ring permanent magnet 301 is axially spaced and sleeved on the second yoke 102, while each second inner ring permanent magnet 301 is alternately embedded in the gaps between two adjacent first inner ring permanent magnets 201 and between the bottommost first inner ring permanent magnet 201 and the bottom wall of the first yoke 101. Similarly, each second outer ring permanent magnet 302 is axially spaced on the inner wall of the first yoke 101, while each second outer ring permanent magnet 302 is alternately embedded in the gaps between two adjacent first outer ring permanent magnets 202 and between the bottommost first outer ring permanent magnet 202 and the bottom wall of the first yoke 101. Among them, the second inner ring permanent magnet 301 and the second outer ring permanent magnet 302, which are at the same height, are magnetized in opposite directions along the axial direction. The two adjacent second inner ring permanent magnets 301 are magnetized in opposite directions along the axial direction, and the two adjacent second outer ring permanent magnets 302 are magnetized in opposite directions along the axial direction.

[0033] In this embodiment, an air gap is formed between the first inner ring permanent magnet 201, the first outer ring permanent magnet 202, the second inner ring permanent magnet 301, and the second outer ring permanent magnet 302. The winding frame 4 is disposed within the air gap. The first coil 5 and the second coil 6 are axially spaced on the winding frame 4, and the winding directions of the first coil 5 and the second coil 6 are opposite. Specifically, the first coil 5 can be configured as a long coil, and the second coil 6 as a short coil. The first coil 5 is located on the winding frame 4 near the opening of the first magnetic yoke 101, and the second coil 6 is located on the winding frame 4 near the bottom wall of the first magnetic yoke 101. The number of turns and the winding length of the first coil 5 and the second coil 6 can be designed and determined according to requirements. In specific implementation, the winding frame 4, the first coil 5, and the second coil 6 are fixedly connected by injecting epoxy resin to form a mover assembly.

[0034] In a preferred embodiment, the stator assembly further includes a second auxiliary magnetic circuit 7, which is disposed on the second yoke 102 to reshape the air gap magnetic field of the voice coil motor and reduce thrust fluctuation without reducing thrust density. Specifically, the second auxiliary magnetic circuit 7 includes an axially magnetized first cylindrical permanent magnet 701 and a second cylindrical permanent magnet 702. The first cylindrical permanent magnet 701 is embedded in the second yoke 102, or the first cylindrical permanent magnet 701 is located between the bottom of the second yoke 102 and the first yoke 101, and the bottom end of the first cylindrical permanent magnet 701 is flush with the top end of the bottommost first inner ring permanent magnet 201 and the first outer ring permanent magnet 202. The second cylindrical permanent magnet 702 is embedded in the second yoke 102, and the second cylindrical permanent magnet 702 is located above the first cylindrical permanent magnet 701. That is, the first cylindrical permanent magnet 701 and the second cylindrical permanent magnet 702 divide the second yoke 102 into multiple parts.

[0035] In the specific implementation process, the first inner ring permanent magnet 201, the first outer ring permanent magnet 202, the second inner ring permanent magnet 301, the second outer ring permanent magnet 302, the first cylindrical permanent magnet 701, and the second cylindrical permanent magnet 702 are made of permanent magnet materials, such as AlNiCo permanent magnet materials, Neodymium Iron Boron permanent magnet materials, IronChromium Cobalt permanent magnet materials, ferrite permanent magnet materials, rare earth permanent magnet materials, or composite permanent magnet materials.

[0036] The working principle of the voice coil motor in this embodiment is as follows:

[0037] When the first coil 5 and the second coil 6 of the mover assembly located in the air gap magnetic field are energized, they are subjected to Ampere force and generate linear motion. The speed and direction of the mover assembly are controlled by controlling the current. Since the magnetic field generated in the air gap of the voice coil motor by the traditional single-pole permanent magnet array has an end effect, the magnetic induction intensity at both ends of the permanent magnet in the air gap is much lower than that at the middle part. Therefore, this embodiment designs a bipolar permanent magnet array composed of a first inner ring permanent magnet 201 and a first outer ring permanent magnet 202. The basic magnetic circuit 2 formed by this array compensates for the leakage magnetic field at the end of the voice coil motor and reduces the uneven distribution of magnetic flux in the air gap of the voice coil motor. Meanwhile, in order to allow more magnetic flux to pass through the working air gap of the voice coil motor and obtain a higher thrust density, and to reduce the leakage magnetic resistance of the upper part of the basic magnetic circuit 2, a first auxiliary magnetic circuit 3 composed of a second inner ring permanent magnet 301 and a second outer ring permanent magnet 302 was designed. It can not only compensate for the end leakage magnetic flux of the permanent magnet in the basic magnetic circuit 2, but also increase the air gap magnetic flux of the basic magnetic circuit 2 by reducing the saturation of the yoke. As a result, the magnetic flux of the upper part of the magnetic circuit will increase. After adding the first auxiliary magnetic circuit 3, the magnetic flux in the lower part of the magnetic circuit will decrease, thereby making the magnetic flux density distribution in the yoke more uniform. At the same time, the magnetic saturation in the middle and bottom will also decrease, so that the air gap can have a larger magnetic flux. Considering that the addition of the first auxiliary magnetic circuit 3 may cause local magnetic flux concentration due to the magnetic aggregation effect of the axial permanent magnet, resulting in uneven magnetic flux density distribution in the air gap, this embodiment further designs a second auxiliary magnetic circuit 7 to compensate for the insufficient reduction of air gap magnetic flux caused by the introduction of the first auxiliary magnetic circuit 3, so as to reduce thrust fluctuation and achieve precise and stable motion control of the voice coil motor.

[0038] The high thrust density voice coil motor with auxiliary enhancement magnetic circuit in this embodiment will be further explained below with specific examples.

[0039] In this example, the number of the first inner ring permanent magnet 201, the first outer ring permanent magnet 202, the second inner ring permanent magnet 301, and the second outer ring permanent magnet 302 are all two. Figure 1As shown. The first inner ring permanent magnet 201 and the first outer ring permanent magnet 202, located at the top, have a height of 5.7 mm and are both magnetized in a radially inward direction. The first inner ring permanent magnet 201 and the first outer ring permanent magnet 202, located at the bottom, have a height of 16 mm and are both magnetized in a radially outward direction. The second inner ring permanent magnet 301 and the second outer ring permanent magnet 302, located at the top, have a height of 7.5 mm, with the second inner ring permanent magnet 301 magnetized in the negative y-axis direction and the second outer ring permanent magnet 302 magnetized in the positive y-axis direction. The second inner ring permanent magnet 301 and the second outer ring permanent magnet 302, located at the bottom, have a height of 1 mm, with the second inner ring permanent magnet 301 magnetized in the positive y-axis direction and the second outer ring permanent magnet 302 magnetized in the negative y-axis direction. The height of both the first cylindrical permanent magnet 701 and the second cylindrical permanent magnet 702 is 2 mm. The first cylindrical permanent magnet 701 is magnetized along the positive y-axis, and the second cylindrical permanent magnet 702 is magnetized along the negative y-axis. The portion of the second yoke 102 above the second cylindrical permanent magnet 702 has a height of 18.2 mm, and the portion of the second yoke 102 between the first and second cylindrical permanent magnets 701 has a height of 7 mm. The height of the first yoke 101 is 32.2 mm. The first inner ring permanent magnet 201, the first outer ring permanent magnet 202, the second inner ring permanent magnet 301, the second outer ring permanent magnet 302, the first cylindrical permanent magnet 701, and the second cylindrical permanent magnet 702 are all made of neodymium iron boron (NdFeB) permanent magnet material, grade N45SH. Both the first yoke 101 and the second yoke 102 are made of soft iron and soft magnetic material. The first coil 5 has a coil length of 13.2 mm, and the second coil 6 has a length of 8.5 mm.

[0040] The air gap magnetic flux density characteristic curves of three voice coil motors were obtained through COMSOL finite element simulation, such as... Figure 2 As shown, the air gap magnetic flux density of a traditional single-pole permanent magnet array voice coil motor is relatively small throughout the entire air gap length, with an effective working air gap magnetic flux density of only 0.2T. However, the air gap magnetic flux density of the voice coil motor with the addition of the first auxiliary magnetic circuit 3 is significantly improved throughout the entire air gap length, with a maximum increase of 0.63T. In particular, the air gap magnetic flux density is increased uniformly in the upper half of the air gap. The improved dual-auxiliary magnetic circuit array voice coil motor proposed in this embodiment (i.e., the voice coil motor with the addition of the first auxiliary magnetic circuit 3 and the second auxiliary magnetic circuit 7) not only significantly improves the air gap magnetic flux density, but also exhibits smaller air gap magnetic flux density fluctuations in the upper half of the stroke compared to the voice coil motor with only the first auxiliary magnetic circuit 3, which is beneficial for improving the thrust stability of the motor.

[0041] like Figure 3The figure shows the thrust characteristic curves of three voice coil motors throughout their effective stroke. The average thrust of the traditional single-pole permanent magnet array voice coil motor is 8.46 N, the maximum thrust is 8.76 N, and the thrust fluctuation rate is 6.2%. The voice coil motor with the addition of the first auxiliary magnetic circuit 3 has an average thrust of 9.99 N, a maximum thrust of 10.34 N, and a thrust fluctuation rate of 7.0%. Compared with the traditional magnetic circuit motor, the thrust is improved, but the thrust fluctuation is increased. The voice coil motor with the addition of the first auxiliary magnetic circuit 3 and the second auxiliary magnetic circuit 7 reduces the end magnetic leakage, significantly increases the end thrust, and reduces the thrust fluctuation. The average thrust is 10.15 N, the maximum thrust is 10.5 N, and the thrust fluctuation rate is 6.0%.

[0042] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. All equivalent structural transformations made by using the contents of the present invention description and drawings under the inventive concept of the present invention, or direct / indirect application in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A high thrust density voice coil motor with an auxiliary enhanced magnetic circuit, characterized in that, It includes a stator assembly and a mover assembly. The stator assembly includes a yoke portion, a basic magnetic circuit and a first auxiliary magnetic circuit. The mover assembly includes a winding frame, a first coil and a second coil. The magnetic yoke portion includes a first magnetic yoke and a second magnetic yoke. The first magnetic yoke is a cylindrical structure with one end open, and the second magnetic yoke is coaxially disposed inside the first magnetic yoke. The basic magnetic circuit includes a plurality of radially magnetized and axially aligned first inner ring permanent magnets and first outer ring permanent magnets. Each of the first inner ring permanent magnets is axially spaced and sleeved on the second magnetic yoke, and each of the first outer ring permanent magnets is axially spaced on the inner wall of the first magnetic yoke. The first auxiliary magnetic circuit includes a plurality of axially magnetized and axially aligned second inner ring permanent magnets and second outer ring permanent magnets. Each second inner ring permanent magnet is alternately embedded in the gap between two adjacent first inner ring permanent magnets and the bottom wall of the first yoke. Each second outer ring permanent magnet is alternately embedded in the gap between two adjacent first outer ring permanent magnets and the bottom wall of the first yoke, in order to improve the thrust density of the voice coil motor. An air gap is formed between the first inner ring permanent magnet, the first outer ring permanent magnet, the second inner ring permanent magnet, and the second outer ring permanent magnet. The winding frame is disposed in the air gap. The first coil and the second coil are axially spaced on the winding frame, and the winding directions of the first coil and the second coil are opposite. The stator assembly also includes a second auxiliary magnetic circuit, which is disposed on the second yoke to reshape the air gap magnetic field of the voice coil motor and reduce thrust fluctuations without reducing thrust density. The second auxiliary magnetic circuit includes a first cylindrical permanent magnet and a second cylindrical permanent magnet that are axially magnetized; The first cylindrical permanent magnet is embedded in the second magnetic yoke, or the first cylindrical permanent magnet is located between the bottom of the second magnetic yoke and the first magnetic yoke, and the bottom end of the first cylindrical permanent magnet is flush with the top end of the bottommost first inner ring permanent magnet and the first outer ring permanent magnet. The second cylindrical permanent magnet is embedded in the second magnetic yoke, and the second cylindrical permanent magnet is located above the first cylindrical permanent magnet.

2. The high thrust density voice coil motor with auxiliary enhancement magnetic circuit according to claim 1, characterized in that, The first inner ring permanent magnet, the first outer ring permanent magnet, the second inner ring permanent magnet, the second outer ring permanent magnet, the first cylindrical permanent magnet, and the second cylindrical permanent magnet are made of permanent magnet materials, including AlNiCo permanent magnet materials, NdFeB permanent magnet materials, IronChromiumCo permanent magnet materials, ferrite permanent magnet materials, rare earth permanent magnet materials, or composite permanent magnet materials.

3. The high thrust density voice coil motor with auxiliary enhancement magnetic circuit according to claim 1 or 2, characterized in that, The winding frame, the first coil, and the second coil are fixed together by resin injection.

4. The high thrust density voice coil motor with auxiliary enhancement magnetic circuit according to claim 1 or 2, characterized in that, The first magnetic yoke and the second magnetic yoke are made of soft magnetic metallic material or soft magnetic ferrite material.

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