Asymmetric thrust-disc-free three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing

Abstract

An asymmetric thrust-disc-free three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing. A rotor (2) is sleeved with two three-pole radial stators (31, 32) located on two sides of the ce

Description

The present disclosure relates to the technical field of non-contact magnetic bearing, and specifically to a three-degree-of-freedom hybrid magnetic bearing without a thrust disc, integrating both radial and axial functions. BACKGROUND A magnetic bearing is a new type of high-performance bearing characterized by the absence of mechanical contact between a rotor and a stator. By utilizing electromagnetic forces to suspend the rotor in space, the magnetic bearing offers numerous advantages, including no friction, long service life, high speed, and high precision. The magnetic bearing is widely applied in fields such as life sciences, flywheel energy storage, high-speed machine tools, and aerospace. The magnetic bearing can be classified into passive, active, and hybrid types based on the methods of magnetic flux generation. In the hybrid magnetic bearings, the control magnetic flux is generated by energized coils, while the bias magnetic flux is produced by a permanent magnet. The hybrid configuration helps reduce the number of coil turns and decreases the power losses of the drive circuit. Based on the degrees of freedom controlled by the rotor, the magnetic bearing can be further divided into axial magnetic bearings (single degree of freedom), radial magnetic bearings (two degrees of freedom), and radial-axial magnetic bearings (three degrees of freedom). Among these, a three-degree-of-freedom magnetic bearing that integrates radial and axial control offer a more compact structure, shorter axial length, improved critical rotor speed, and reduced air friction losses. Chinese Patent Publication No. CN101149077A, titled "Permanent magnet biased axial-radial magnetic bearing," discloses a design where a radial magnetic bearing is fixed inside a thrust magnetic bearing. The structure, which is symmetric on the radial plane, can simultaneously control two radial degrees of freedom and one axial degree of freedom, forming a radial-axial hybrid magnetic bearing. This design uses two radially magnetized annular permanent magnets to establish a static bias magnetic field. However, the structure has disadvantages such as a relatively long radial length, high cost, increased losses, poor dynamic response, and unfavorable heat dissipation. Chinese Patent Publication No. CN101832335A, also titled "Permanent magnet biased axial-radial magnetic bearing," discloses a structure where the radial stator consists of four teeth and two pairs of magnetic poles. This design requires the use of either two bipolar or four unipolar DC power amplifiers. However, permanent magnet biased axial-radial magnetic bearing has drawbacks such as a high number of power electronic switches, high cost, and low power density. SUMMARY An objective of the present disclosure is to overcome the shortcomings of the prior art by providing a novel asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without a thrust disc, characterized by compact structure, low cost, and high load-bearing capacity. The technical solution adopted by the asymmetric three-degree-of-freedom axial-radial dualdisc hybrid magnetic bearing without a thrust disc proposed by the present disclosure is as follows. An asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without a thrust disc comprises a rotor, two three-pole radial stators, and two three-pole axial stators. The rotor is externally sleeved with the two three-pole radial stators with an identical structure respectively positioned on two sides of a center point of the rotor and the two identical three-pole axial stators also located symmetrically on two sides of the center point of the rotor. On the same side of the center point of the rotor, one of the two three-pole radial stators and one of the two three-pole axial stators share a common stator yoke. A circular ring-shaped permanent magnet ring is fixedly embedded between the two stator yokes, with the permanent magnet ring magnetized axially. Projections of the six radial stator magnetic poles from the two three-pole radial stators are evenly arranged along the circumferential direction in the radial cross-section, the three axial stator magnetic poles of the three - pole axial stator and the three radial stator magnetic poles are evenly arranged in the circumferential direction. Each of the six radial stator magnetic poles is wound with a radial control coil, and an axial control coil is wound on the three axial stator magnetic poles on the same side of the center point of the rotor. A ring-shaped bias magnetic flux adjustment coil is wound between the permanent magnet ring and a rotor shaft. The bias magnetic flux adjustment coil is positioned inside the radial control coils. Further, the axial cross-section of the stator yoke portion of each axial stator is U-shaped. A circular ring-shaped axial control coils is arranged within each U-shaped yoke. The two axial control coils are connected in series in the same direction and are driven and controlled by a bidirectional direct current(DC) inverter. Further, the winding direction of the bias magnetic flux adjustment coils is the same as the winding direction of the axial control coils, and the bias magnetic flux adjustment coils is positioned inside the radial control coils. Further, the radial control coils projected onto the opposite radial stator poles on the radial crosssection have the same winding direction. These coils are connected in pairs in series and configured in a star shaped connection. The present disclosure has the following advantages: 1. The disclosure adopts an integrated structural design that combines axial and radial functionalities. Compared with the combined structure of a two-degree-of-freedom radial magnetic bearing and a single-degree-of-freedom axial magnetic bearing, this design reduces the axial length of the motor system supported by the magnetic bearing under the same power conditions. The structure is more compact, and the critical rotational speed is increased. 2. By utilizing a six-pole structure formed by two three-pole bearings with a rotational phase difference of 60 degrees, this disclosure optimizes the nonlinear issues caused by asymmetry in conventional three-pole structures. The six-pole structure reduces the nonlinearity of the suspension force and the coupling between the two radial degrees of freedom, thereby simplifying control complexity. The radial control coils on opposing radial stator magnetic poles are connected in series to form a set of three-phase coils. A three-phase inverter is used for driving, reducing the number of switching devices, power losses, and driver costs. 3. The disclosure proposes a thrust-disc-free design. By utilizing an asymmetric axial stator structure, the axial control part can directly act on the rotor. Unlike most radial-axial magnetic bearings that require an axial thrust disc, which increases the rotor's outer diameter and causes air friction issues, this structure effectively reduces the outer diameter of the rotor. The thrust-disc-free design improvement enhances the critical rotational speed and dynamic performance of the rotor. 4. The hybrid magnetic bearing in this disclosure uses an axially magnetized permanent magnet to provide bias flux for both radial and axial components while isolating the radial and axial control magnetic fields. This design achieves decoupling control, reduces power losses, and generates dynamic suspension forces through control flux provided by the control coils to counter external disturbances and loads. This improves load capacity, effectively reduces coil current, and lowers power consumption. 5. The disclosure includes a bias magnetic flux adjustment coil, which allows for the adjustment of the magnetization state of the permanent magnet. This adjustment modifies the bias flux from the permanent magnet, mitigating the effects of issues such as high-temperature demagnetization. In case of problems with the permanent magnet, the bias magnetic flux adjustment coil can temporarily substitute the permanent magnet to provide bias flux. This design enhances the fault tolerance of the bias flux and improves the operational stability of the magnetic bearing. BRIEF DESCRIPTION OF THE DRAWINGS In order to make the content of the present disclosure more obvious and understandable, the present disclosure will be described in detail below with reference to the accompanying drawings and specific embodiments. FIG. 1 is a perspective view of the asymmetric thrust-disc-free three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing of the present disclosure FIG. 2 is a semi-sectional schematic diagram of FIG. 1. FIG. 3 is a radial plane sectional schematic diagram of the present disclosure. FIG. 4 is an axial plane sectional schematic diagram along the D-D line in FIG. 3. FIG. 5 is a schematic diagram of the radial magnetic circuit of the present disclosure. FIG. 6 is a schematic diagram of the axial magnetic circuit of the present disclosure. In the drawings, 1: shaft; 2: rotor; 31, 32: three-phase radial stator; 311, 321: stator yoke; 312, 322: radial stator magnetic poles; 41, 42: radial control coils; 5: radial air gap; 61, 62: three-phase axial stator; 611, 621: axial stator magnetic poles; 71, 72: axial control coils; 81, 82: axial air gap; 9: permanent magnet ring; 10: bias magnetic flux adjustment coil; 11: bias magnetic flux; 12: radial control magnetic flux; 13: axial control magnetic flux. DETAILED DESCRIPTION OF THE EMBODIMENTS As shown in FIG. 1, FIG. 2, FIG. 3 and FIG. 4, the present disclosure comprises a shaft 1, a rotor 2, two three-pole radial stators 31 and 32, and two three-pole axial stators 61 and 62. The rotor 2 is fixedly sleeved at the middle position of the shaft 1. The rotor 2 is externally sleeved with two three-pole radial stators 31 and 32 and two three-pole axial stators 61 and 62. The two three-pole radial stators 31 and 32 are identical in structure and are symmetrically positioned on two sides of the center point of the rotor 2. The axial distances between the two three-pole radial stators 31 and 32 and the center point of the rotor 2 are equal. The two three-pole axial stators 61 and 62 are identical in structure and are symmetrically positioned on two sides of the center point of the rotor. The axial distances between the two three-pole axial stators 61 and 62 and the center point of the rotor are equal. On the same side of the center point of the rotor, a three-pole radial stator 31 and a three-pole axial stator 61 share a stator yoke 311, while the other three-pole radial stator 32 and three-pole axial stator 62 share a common stator yoke 321. The two stator yokes 311 and 321 are identical in structure and are symmetrically located on two sides of the center point of the rotor, with equal axial distances from the center point of the rotor. A circular ring-shaped permanent magnet ring 9 is fixedly embedded between the two stator yokes 311 and 321. The outer diameter of the permanent magnet ring 9 is equal to the outer diameter of the two stator yokes 311 and 321, while the inner diameter of the permanent magnet ring is greater than the inner diameter of the two stator yokes 311 and 321. The permanent magnet ring 9 is axially magnetized, enabling the separation of radial and axial control magnetic fields and achieving decoupling control of radial and axial directions. Three radial stator magnetic poles 312 of the first three-pole radial stator 31 are evenly arranged in the circumferential direction. The three radial stator magnetic poles 322 of the second three-pole radial stator 32 are evenly arranged m the circumferential direction. The two three-pole radial stators 31 and 32 form a total of six radial stator magnetic poles 312 and 322. The stator magnetic poles 312 of the first three-pole radial stator 31 are rotated by 60° along the centerline of the rotor 2 relative to the stator magnetic poles 322 of the second three-pole radial stator 32, creating a 60° phase difference between the two three-pole radial stators 31 and 32. The projections of these six radial stator magnetic poles 312 and 322 onto the radial crosssection are evenly distributed along the circumference, with adjacent projections spaced 60° apart. Each pair of adjacent radial stator magnetic pole projections includes one magnetic pole from the first three-pole radial stator 31 and one magnetic pole from the second three-pole radial stator 32. Similarly, the three stator magnetic poles 611 of the first three-pole axial stator 61 are evenly distributed along the circumference, as are the three stator magnetic poles 621 of the second three-pole axial stator 62. Together, the two three-pole axial stators 61 and 62 form a total of six axial stator magnetic poles 611 and 621. On the same side of the center point of the rotor 2, the three axial stator magnetic poles 611 and the three radial stator magnetic poles 312 are evenly distributed along the circumferential direction. In this arrangement, the projection centers of the three axial stator magnetic poles 611 on one side of the center point of the rotor 2 coincide with the projection centers of the three radial stator magnetic poles 312 on the other side in the radial cross-section. The axial stator magnetic poles 611 of the first three-pole axial stator 61 are rotated by 60° along the centerline of the rotor 2 relative to the axial stator magnetic poles 621 of the second three-pole axial stator 62, resulting in a 60° phase difference between the two three-pole axial stators 61 and 62. The projections of the six axial stator magnetic poles 611 and 621 onto the radial cross-section are evenly distributed along the circumference and align one-to-one with the projection centers of the six radial stator magnetic poles 312 and 322 in the radial cross-section. Adjacent projections of the axial stator magnetic poles 611 and 621 in the radial cross-section are spaced 60° apart. Each pair of adjacent proj ections includes one axial stator magnetic pole 611 from the first three-pole axial stator 61 and one axial stator magnetic pole 621 from the second three-pole axial stator 62. The six radial stator magnetic poles 312 and 322 of the magnetic bearing extend radially from the inner walls of the corresponding stator yokes 311 and 321 towards the rotor 2. There are identical radial air gaps 5 between the poles and the rotor 2, and the width of the radial air gaps 5 is approximately 0.5 mm. Each radial stator magnetic pole 312 and 322 is wound with radial control coils 41 and 42 of the same number of turns and specifications. The three radial stator magnetic poles 312 of the first three-pole radial stator 31 are wound with radial control coils 41, and the three radial stator magnetic poles 322 of the second three-pole radial stator 32 are wound with radial control coils 42. on the opposing radial stator magnetic poles 312 and 322, as projected onto the radial cross-section, have the same winding direction. The radial control coils 41 and 42 are connected in pairs in series and configured in a star connection to form a three-phase coils driven by a three-phase inverter. As shown in FIG. 3, the axial cross-section of each axial stator 61 and 62 at the stator yoke 311 and 321 portions is U-shaped, forming U-shaped yoke parts. Circular ring-shaped axial control coils 71 and 72 are installed inside the U-shaped yokes. The remaining portions of the stator yokes 311 and 321 form the circular ring-shaped yokes for the radial stators 31 and 32. The U-shaped side walls of the axial stators 61 and 62, far from the center of the rotor 2, extend axially to form the axial stator magnetic poles 611 and 621 on the end faces of the rotor 2. The six U-shaped side walls with axial stator magnetic poles 611 and 621 coincide with the projection centers of the six radial stator magnetic poles 312 and 322 on the radial crosssection in a one-to-one correspondence. The U-shaped side wall near the center of rotor 2 is fixedly connected to permanent magnet ring 9. Axial air gaps 81 and 82 are formed between the axial stator magnetic poles 611 and 621 and the end faces of the rotor 2, with the air gap width being approximately 0.5 mm. The radial distance between the axial stator magnetic poles 611 and 621 and the shaft 1 is significantly greater than the axial air gaps 81 and 82. Each U-shaped yoke of the three-pole axial stators 61 and 62 contains one axial control coil 71 and 72. The two sets of axial control coils 71 and 72 are circular ring-shaped, coaxially mounted outside the rotor 2, and do not contact the radial stator magnetic poles 312 and 322. The two axial control coils 71 and 72 are connected in series with the same winding direction and driven by a bidirectional DC inverter. A circular ring-shaped bias magnetic flux adjustment coil 10 is wound between the permanent magnet ring 9 and the shaft 2. The bias magnetic flux adjustment coil 10 is coaxially mounted outside the rotor 2, positioned between the permanent magnet ring 9 and the shaft 1, and has the same winding direction as the axial control coils 71 and 72. The bias magnetic flux adjustment coil 10 is located inside the radial control coils 41 and 42, without contact with them. The rotor 2 and radial stator magnetic poles 312 and 322 are made of laminated silicon steel sheets. The stator yokes 311 and 321 and axial stators 61 and 62 are made from the same magnetic material. The radial control coils 41 and 42, axial control coils 71 and 72, and the bias magnetic flux adjustment coil 10 are all wound using insulated copper wires of the same specification. The axial stator magnetic poles 611 and 621 are narrower in the tangential direction of the circumference than the radial stator magnetic poles 312 and 322. The end face of the rotor 2 is flush with the end faces of the radial stator magnetic poles 312 and 322. The inner diameter of the bias magnetic flux adjustment coil 10 is larger than the inner diameter of the radial stator magnetic poles 312 and 322. As shown in FIG. 3, FIG. 4, FIG. 5, and FIG. 6, the axially magnetized permanent magnet ring 9 provides bias magnetic flux for both the radial and axial components. Taking the Al direction as an example, the path of the bias magnetic flux 11 generated by the permanent magnet ring 9 is as follows: Starting from the N pole of the permanent magnet ring 9, the bias magnetic flux 11 passes through the stator yoke 311, radial stator magnetic pole 312, radial air gap 5, and rotor 2. the bias magnetic flux 11 then splits into two equal paths, passing through the radial air gap 5, axial stator 62, and axial air gap 82, reaching the axial stator magnetic pole 621, and finally returning to the S pole of the permanent magnet ring 9 to complete the loop. For the A2 direction, the bias magnetic flux 11 path is as follows: Starting from the N pole of the permanent magnet ring 9, bias magnetic flux 11 splits into two paths within the axial stator 61. One path passes through the axial stator magnetic pole 611 and axial air gap 81, while the other passes through the inner side of axial stator 61, radial air gap 5, rotor 2, radial air gap 5, and radial stator magnetic pole 322, finally returning to the S pole of the permanent magnet ring 9 to form a closed loop. When a positive or negative current is applied to the bias magnetic flux adjustment coil 10, the bias magnetic flux adjustment coil 10 generates a bias magnetic flux loop in the same or opposite direction as the bias magnetic flux 11 generated by the permanent magnet ring 9, thereby adjusting the magnetization state of the permanent magnet and modifying the bias field of the magnetic bearing. As shown in FIG. 5, when the rotor 2 is in the central position, the bias magnetic flux 11 generates suspension forces of equal magnitude and opposite direction at the radial air gaps 5 in directions Al and A2, maintaining the balance of rotor 2. When the rotor 2 is in a radially eccentric state (e.g., in the A1A2 direction), a positive current is applied to the radial control coils 41 in the Al direction. The path of the radial control magnetic flux 12 generated in the Al direction is as follows: The flux flows from the radial stator magnetic pole 31 in the Al direction through the radial stator yoke 311 and splits evenly into two paths, flowing counterclockwise and clockwise toward the radial stator magnetic poles in directions Bl and Cl. The radial control magnetic flux 12 then passes through the radial air gap 5 and rotor 2, circulates around the rotor 2 to the radial air gap in the Al direction, and finally returns to the radial stator magnetic pole 31 in the Al direction. When the same direction current is applied to the radial control coils 42 in the A2 direction, the radial control magnetic flux generated at the radial air gap 5 in the A2 direction has the same direction as the radial control magnetic flux at the Al radial air gap. At the Al radial air gap, the radial control magnetic flux 12 is superimposed with the bias magnetic flux 11, while at the A2 radial air gap, the radial control magnetic flux 12 cancels with the bias magnetic flux 11. The combination of flux changes generates a radial suspension force in the Al direction at rotor 2. When negative current is applied to the radial control coils 41 and 42 in the Al and A2 directions, the radial control magnetic flux 12 reverses direction, producing a radial suspension force in the A2 direction at rotor 2. By simultaneously adjusting the magnitude and direction of the current in the radial control coils 41 and 42, the magnetic flux generated by the two rotating three-pole bearings cancels out in unneeded directions. The combination of flux changes reduces the nonlinearity of the suspension force and the coupling between the two radial degrees of freedom, allowing control of the magnitude and direction of the radial suspension force and achieving stable suspension of rotor 2. As shown in FIG. 6, when the rotor 2 is in an axially eccentric state, the axial control coils 71 and 72 can be used to control the rotor 2 axially. Taking the axial coils 71 of axial stator 61 as an example, when a positive current is applied to the axial coils 71, the generated axial control magnetic flux 13 flows from the axial stator magnetic pole 611 through the axial air gap 81 to the rotor 2. The axial control magnetic flux 13 then flows through the rotor 2 and radial air gap 5 to return to the axial stator 61, forming a closed loop. At this time, the axial control magnetic flux 13 superimposes with the bias magnetic flux 11 at the axial air gap 81, generating an axial suspension force directed to the right on rotor 2. Meanwhile, the same current applied to the axial coils 72 generates an axial control magnetic flux at the radial air gap 5 that is opposite in direction to the flux generated by axial coils 71 at the radial air gap 5. The configuration of the axial control magnetic fluxes 13 with opposite directions at the radial air gap 5 ensures that the axial control does not interfere with the radial control, achieving decoupled structural control. When negative current is applied to the axial control coils 71 and 72, the axial control magnetic flux 13 cancels with the bias magnetic flux 11 at the axial air gap 82, generating an axial suspension force directed to the left on rotor 2. By employing the dual-layer axial stator magnetic poles 611 and 621, the axial magnetic flux directly acts on rotor 2 through axial air gaps 81 and 82, eliminating the need for an axial thrust disc as required in conventional axial magnetic bearing. The use of the dual-layer axial stator magnetic poles reduces the radial size of the rotor 2. By adjusting the magnitude and direction of the current in the axial control coils 71 and 72, the magnitude and direction of the axial suspension force can be controlled, achieving stable suspension of rotor 2.

Claims

1. An asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without a thrust disc, comprising a rotor (2), two three-pole radial stators (31, 32), and two three-pole axial stators (61, 62), wherein the rotor (2) is externally sleeved with the two three-pole radial stators (31, 32) with an identical structure respectively positioned on two sides of a center point of the rotor (2), and the two three-pole axial stators (61, 62) with an identical structure respectively positioned on the two sides of the center point of the rotor (2);one of the two three-pole radial stators (31, 32) and one of the two three-pole axial stators (61, 62) on the same side of the center point of the rotor (2) share a common stator yoke (311, 321);a circular ring-shaped permanent magnet ring (9) is fixedly embedded between the two stator yokes (311, 321), and the permanent magnet ring (9) is axially magnetized;the two three-pole radial stators (31, 32) form a total of six radial stator magnetic poles (312, 322), and projections of the six radial stator magnetic poles (312, 322) in a radial cross-section are evenly distributed in a circumferential direction; three axial stator magnetic poles (611, 621) and three of the six radial stator magnetic poles (312, 322) on the same side of the center point of the rotor (2) are evenly distributed in the circumferential direction; andeach of the six radial stator magnetic poles (312, 322) is wound with a radial control coil (41, 42), and each of the three axial stator magnetic poles (611, 621) on the same side of the center point of the rotor (2) is wound with an axial control coil (71, 72); a circular ring-shaped bias magnetic flux adjustment coil (10) is wound between the permanent magnet ring (9) and a shaft (1), and the bias magnetic flux adjustment coil (10) is positioned inside the radial control coil (41, 42).

2. The asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without the thrust disc according to claim 1, wherein the stator yoke (311, 321) of each axial stator (61, 62) has a U-shaped axial cross-section, and each U-shaped yoke (311, 321) contains a circular ringshaped axial control coil (71, 72), with the two axial control coil (71, 72) connected in series in thesame winding direction; and the two axial control coil (71, 72) are driven by a bidirectional direct current (DC) inverter.

3. The asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without the thrust disc according to claim 1, wherein the winding direction of a bias magnetic flux adjustment coil (10) is the same as the winding direction of the axial control coil (71, 72), and the bias magnetic flux adjustment coil (10) is positioned inside the radial control coil (41, 42).

4. The asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without the thrust disc according to claim 1, wherein the width of the axial stator magnetic poles (611, 621) in a tangential direction along the circumference is smaller than the width of the radial stator magnetic poles (312, 322) in the tangential direction along the circumference, and the end face of the rotor (2) is flush with the end face of the radial stator magnetic poles (312, 322), the inner diameter of the bias magnetic flux adjustment coil (10) is larger than the inner diameter of the radial stator magnetic poles (312, 322).

5. The asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without the thrust disc according to claim 1, wherein the axial distances between the two three-pole radial stators (31, 32) and the center point of the rotor (2) are equal, and the axial distances between the two three-pole axial stators (61, 62) and the center point of the rotor (2) are also equal.

6. The asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without the thrust disc according to claim 1, wherein the outer diameter of the permanent magnet ring (9) is the same as the outer diameter of the two stator yokes (311, 321), and the inner diameter of the permanent magnet ring (9) is greater than the inner diameter of the two stator yokes (311, 321).

7. The asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without the thrust disc according to claim 1, wherein the winding direction of the radial control coil (41, 42) on the opposing radial stator magnetic poles (312, 322) in a radial cross-section is the same, and the radial control coil (41, 42) on the opposing radial stator magnetic poles (312, 322) are connected in pairs in series and configured in a star connection.

8. The asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without the thrust disc according to claim 1, wherein the radial distance between the axial stator magnetic poles (611, 621) and the shaft (1) is greater than the axial air gap (81, 82).

9. The asymmetric three-degree-of-freedom axial-radial dual-disc hybrid magnetic bearing without the thrust disc according to claim 1, wherein the rotor (2) and six radial stator magnetic poles (312, 322) are made of laminated silicon steel sheets, two stator yokes (311, 321) and two axial stators (61, 62) are made of the same magnetic-conducting material, and the radial control coil (41, 42), the axial control coil (71, 72), and the bias magnetic flux adjustment coil (10) are all wound with an insulated copper wire of the same specification.

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