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Title:
MULTI-POLE RING MAGNET FOR ROTATING ELECTRICAL MACHINES
Document Type and Number:
WIPO Patent Application WO/1998/027638
Kind Code:
A1
Abstract:
A multi-pole ring magnet (10) including axially mutually interfitting disks (12, 14) of permanently magnetized material magnetized in radially opposing directions. The disks include annular support hubs (16) having an inner diameter, an outer diameter, and first (24) and second (26) axially opposed faces, and pole shoes having segments (18) extending radially outwardly from the outer diameters of the hubs and axially from the first faces. The pole shoes form alternating magnetic poles around the circumference of the ring magnet. Support members (40, 42) including flat disks (44) of nonmagnetic material extend radially along the pole shoes on opposite axial sides of the ring magnet. The support members include a lip (46) extending axially over a circumferential portion (50) of the segments to prevent radially outward expansion of the ring magnet, and a central portion (46) extending axially along the inner diameter of the annular support hubs.

Inventors:
GEBHART STEVEN A
Application Number:
PCT/US1997/014770
Publication Date:
June 25, 1998
Filing Date:
August 22, 1997
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
GLOBE MOTORS (US)
International Classes:
H02K1/27; H02K11/00; H02K29/08; (IPC1-7): H02K1/22; H02K11/00; H02K21/12
Foreign References:
US4513216A1985-04-23
US3508095A1970-04-21
US5200661A1993-04-06
US4503391A1985-03-05
Attorney, Agent or Firm:
Folkerts, Michael D. (Biebel & French, 35 East First Street Dayton, OH, 45402, US)
Download PDF:
Claims:
-CLAIMS--
1. A multipole ring magnet comprising: a plurality of mutually axially interfitting disks, wherein each said disk comprises an annular support hub; and at least one pole shoe extending radially outwardly from said hub; wherein at least one of said pole shoes comprises permanently magnetized material magnetized in a radial direction.
2. The ring magnet recited in claim 1 wherein said plurality of axially interfitting disks includes a first disk and a second disk, each said disk including a plurality of pole shoes supported on a respective annular support hub and comprising permanently magnetized material magnetized in a radial direction.
3. The ring magnet recited in claim 2 wherein said pole shoes of said first disk are formed integrally with said annular support hub of said first disk and said pole shoes of said second disk are formed integrally with said annular support hub of said second disk.
4. The ring magnet recited in claim 2 wherein said plurality of pole shoes of said first disk are magnetized in a first radial direction, and said plurality of pole shoes of said second disk are magnetized in a second radial direction opposite to said first radial direction.
5. The ring magnet recited in claim 4 wherein said pole shoes of said first disk comprise circumferentially spaced first segments including axially extending edges, said pole shoes of the second disk comprise circumferentially spaced second segments including axially extending edges, and said axially extending edges of said first segments engage said axially extending edges of said second segments in an interference fit.
6. The ring magnet recited in claim 5 wherein each of said first segments have substantially the same circumferential dimension and each of said second segments have substantially the same circumferential dimension.
7. The ring magnet recited in claim 5 wherein each said annular support hub comprises an inner diameter, an outer diameter, and first and second axially opposed faces, said first and second segments extend radially outwardly from respective outer diameters of said annular support hubs of said first and second disks, and said first and second segments extend axially from said first faces of said first and second disks, respectively.
8. The ring magnet recited in claim 7 wherein said first segments extend axially from said first face of said first disk a distance substantially equal to a distance between said first and second faces of said second disk, and said second segments extend axially from said first face of said second disk a distance substantially equal to a distance between said first and second faces of said first disk.
9. The ring magnet recited in claim 1 further comprising a support member extending around an outer circumference of said pole shoes preventing movement of said pole shoes radially outwardly.
10. The ring magnet recited in claim 9 wherein said support member comprises a disk plate formed of a nonmagnetic material extending radially along said pole shoes and including an axially extending lip extending over an outer circumferential portion of said pole shoes.
11. The ring magnet recited in claim 10 wherein said disk plate includes an annular central portion extending axially within at least one of said annular support hubs.
12. The ring magnet recited in claim 10 wherein said outer circumferential portion comprises a groove formed in said pole shoes for receiving said lip.
13. The ring magnet recited in claim 10 wherein said support member comprises two of said disk plates located at axially opposed sides of said ring magnet.
14. A rotating electrical machine comprising: a stationary housing; a rotor assembly including a ring magnet supported for rotation relative to said stationary housing; and means for detecting magnetic flux, said means for detecting positioned in operative relation to said ring magnet; said ring magnet including a plurality of mutually axially interfitting disks, wherein each said disk comprises an annular support hub, and at least one pole shoe extending radially outwardly from said hub, and wherein at least one of said pole shoes comprises permanently magnetized material magnetized in a radial direction.
15. The electrical machine recited in claim 14 wherein said plurality of axially interfitting disks includes a first disk and a second disk, each said disk including a plurality of pole shoes comprising permanently magnetized material magnetized in a radial direction, said plurality of pole shoes of said first disk are magnetized in a first radial direction, and said plurality of pole shoes of said second disk are magnetized in a second radial direction opposite to said first radial direction.
16. The electrical machine recited in claim 15 wherein said pole shoes of said first disk comprise circumferentially spaced first segments formed integrally with said annular support hub of said first disk, said first segments including axially extending edges, said pole shoes of said second disk comprise circumferentially spaced second segments formed integrally with said annular support hub of said second disk, said second segments including axially extending edges, wherein said axially extending edges of said first segments engage said axially extending edges of said second segments in an interference fit.
17. The electrical machine recited in claim 16 wherein each said annular support hub comprises an inner diameter, an outer diameter, and first and second axially opposed faces, said first and second segments extend radially outwardly from respective outer diameters of said annular support hubs of said first and second disks, respectively, and said first segments extend axially from said first face of said first disk a distance substantially equal to a distance between said first and second faces of said second disk, and said second segments extend axially from said first face of said second disk a distance substantially equal to a distance between said first and second faces of said first disk.
18. The electrical machine recited in claim 15 further comprising a support member extending around an outer circumference of said pole shoes preventing movement of said pole shoes radially outwardly, said support member including a first disk plate and a second disk plate located at axially opposed sides of said ring magnet, said first disk plate and second disk plate formed of a nonmagnetic material extending radially along said pole shoes of said first disk and second disk, respectively, said first disk plate and second disk plate each including an axially extending lip extending over an outer circumferential portion of said pole shoes, and an annular central portion extending axially within said annular support hubs of said first disk and second disk, respectively.
19. The electrical machine recited in claim 14 wherein said means for detecting comprises a Hall sensor device.
20. The electrical machine recited in claim 19 wherein said Hall sensor device comprises a unipolar Hall sensor device.
21. The electrical machine recited in claim 14 wherein the machine comprises a magnetic encoder.
22. The electrical machine recited in claim 21 wherein said plurality of axially interfitting disks includes a first disk and a second disk, each said disk including a plurality of pole shoes comprising permanently magnetized material magnetized in a radial direction, said plurality of pole shoes of said first disk are magnetized in a first radial direction, and said plurality of pole shoes of said second disk are magnetized in a second radial direction opposite to said first radial direction.
23. The electrical machine recited in claim 22 wherein said pole shoes of said first disk comprise circumferentially spaced first segments formed integrally with said annular support hub of said first disk, said first segments including axially extending edges, said pole shoes of said second disk comprise circumferentially spaced second segments formed integrally with said annular support hub of said second disk, said second segments including axially extending edges, wherein said axially extending edges of said first segments engage said axially extending edges of said second segments in an interference fit.
24. The electrical machine recited in claim 14 wherein the machine comprises a brushless DC motor including a rotor supported for rotation with said ring magnet, and stator windings supported on said housing.
25. The electrical machine recited in claim 24 wherein said plurality of axially interfitting disks includes a first disk and a second disk, each said disk including a plurality of pole shoes comprising permanently magnetized material magnetized in a radial direction, said plurality of pole shoes of said first disk are magnetized in a first radial direction, and said plurality of pole shoes of said second disk are magnetized in a second radial direction opposite to said first radial direction.
26. The electrical machine recited in claim 25 wherein said pole shoes of said first disk comprise circumferentially spaced first segments formed integrally with said annular support hub of said first disk, said first segments including axially extending edges, said pole shoes of said second disk comprise circumferentially spaced second segments formed integrally with said annular support hub of said second disk, said second segments including axially extending edges, wherein said axially extending edges of said first segments engage said axially extending edges of said second segments in an interference fit.
Description:
MULTI-POLE RING MAGNET FOR ROTATING ELECTRICAL MACHINES Background of the Invention This invention relates to the field of rotating electrical machines, and more particularly relates to electrical motors and magnetic encoders. More specifically, the invention relates to a radially magnetized multi-pole permanent ring magnet, comprising mutually axially interfitting disks, for use in electrical motors and magnetic encoders.

Permanent magnets are widely used in motors and encoders. In particular, they are advantageously used as a commutation disk in direct current (DC) brushless motors to provide rotor position information to an electronic circuit that varies the applied voltage to a stator winding. For this purpose, a typical commutation disk employs multiple magnetic north and south poles.

Constructing a magnet with multiple north and south poles is well known in the art. For example, U.S. Pat. No. 3,898,599 issued to Reid et al., discloses a toroidal permanent magnet, comprised of two concentric rings, and having radially oriented magnetic poles. One ring is a support ring made of paramagnetic material having a relatively low magnetic retentivity and conformally shaped to fit the other ring. The other ring comprises a radially polarized permanent magnetic ring made from sintered powder material. After the rings are formed by compaction and sintering, the integral structure is placed in a magnetic field for permanently magnetizing the magnet with radial magnetic poles. However, magnetizing the integral structure in this manner does not form well-defined north and south poles. The transition in magnetic flux density between adjacent poles occurs gradually through a finite distance.

Another prior art multiple-pole permanent magnet, U.S. Pat. No.

5,258,735 issued to Allwine, Jr., discloses a composite permanent magnet rotor having axially oriented magnetic north and south poles. Allwine, Jr., discloses a magnet comprising two pieces of permanent magnet material, each piece having a complementary pattern of holes and/or projections. The two pieces are magnetized in opposite axial directions and mated together in interlocking relationship. The

composite magnet exhibits a pattern of well-defined magnetic north and south poles in an axial direction. However, the pieces of the composite magnet of Allwine, Jr., have axially incompatible magnetic polarities, repel each other when brought together to be joined, and do not provide well-defined radially oriented magnetic poles.

It is generally desirable to provide a permanent ring magnet having well-defined multiple poles, in combination with at least one magnetic flux sensor, in order to improve the determination of rotor position in rotating electrical machines.

Accordingly, there continues to exist a need for a permanent ring magnet having sharp, well-defined radially polarized magnetic poles.

Summarv of the Invention The present invention provides a ring shaped permanent magnet for use in DC brushless motors, stepper motors, and encoders, wherein the permanent ring magnet includes a plurality of mutually axially interfitting disks defining an annular support hub and at least one pole shoe extending radially outwardly from the hub.

In a preferred embodiment, the pole shoes are made of wedge shaped permanently magnetized material, magnetized in a radial direction. All of the pole shoes of a first disk are magnetized in a first radial direction, and all of.the pole shoes of a second disk are magnetized in a second radial direction opposite to the first radial direction. The disks are then axially interfitted to define contiguous alternating radially magnetic poles having substantially equal circumferential dimensions.

Also in the preferred embodiment, there are three unipolar Hall effect sensors disposed at predetermined circumferentially spaced apart positions about an annular sensor housing with their faces oriented radially inward. The sensor housing is disposed concentrically about the permanent ring magnet with the Hall effect sensors held in adjacent radial relationship to the magnet.

Therefore, it is an object of the present invention to provide a permanent ring magnet including mutually axially interfitting disks having at least one radially magnetized pole shoe.

It is a further object of the invention to provide such a permanent ring magnet wherein a first disk includes pole shoes magnetized in a first radial direction and a second disk includes pole shoes magnetized in a second radial direction opposite to the first radial direction.

It is yet another object of the invention to provide a permanent ring magnet including radially magnetized pole shoes defining contiguous alternating well-defined magnetic poles.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

Brief Description of the Drawings Fig. 1 is an exploded view of a permanent ring magnet of the present invention; Fig. 2 is a perspective view of an assembled permanent ring magnet of the present invention; Fig. 3 is a top plan view of a disk of the present invention; Fig. 4 is a cross-sectional view of a disk of the present invention; Fig. 5 is a bottom plan view of a disk of the present invention; Fig. 6 is a top plan view of a support member of the present invention; Fig. 7 is a cross-sectional view of a support member of the present invention; Fig. 8 is a bottom plan view of a support member of the present invention; and Fig. 9 is a partial exploded view of a rotating electrical machine incorporating the present invention.

Detailed Description of the Preferred Embodiment Referring to Figs. 1-5, the permanent ring magnet 10 of the present invention includes a plurality of disks 12, 14 of substantially identical construction having an annular support hub 16, and a plurality of pole shoes defined by circumferentially spaced segments 18 extending from an outer circumference of the

annular support hubs 16. The annular support hubs 16 each define an inner diameter 20, an outer diameter 22, and axially opposed first and second faces 24 and 26. The segments 18 each include axially extending substantially planar edges 30. When disk 12 is brought into axially interfitting engagement with disk 14, the axially extending edges 30 of disk 12 engage the axially extending edges 30 of disk 14 in an interference fit, thus ensuring a tight fit and thereby minimizing the transition distance between adjacent poles.

In the preferred embodiment, the segments 18 of the disk 12 extend axially from the first face 24 a distance substantially equal to the distance between the first face 24 and the second face 26 of the disk 14. Likewise, the segments 18 of the disk 14 extend axially from the first face 24 a distance substantially equal to the distance between the first face 24 and the second face 26 of the disk 12. Thus, the preferred form of the ring magnet 10 has substantially flat, parallel, axially opposite sides. In addition, the disks 12 and 14 interfit to position inner faces 29 of the segments 18 in engagement with outer circumferential faces 31 of the hubs 16 such that the flat sides of the assembled disks 12, 14 are defined by contiguous surfaces across the interfaces between the disks 12, 14.

The annular support hub 16 and the segments 18 are made of a permanently magnetized material exhibiting a low degradation of magnetic flux density, and are preferably formed of a barium neodymium ferrite blend magnet material in moldable form, for example 1062B from Arnold Engineering Company.

However, any suitable material may be used including, but not limited to neodymium ferrite, neodymium boron, rare-earth materials, or ceramic materials. Preferably, the disks 12, 14 are formed in a molding operation with the annular support hub 16 and the segments 18 formed integrally. In addition, the disks 12, 14 are preferably permanently magnetized in a radial direction during a molding operation for forming the disks 12, 14, with the disks 12, 14 magnetized in opposite radial directions. For example, the disk 12 may be radially magnetized with north poles oriented radially inward and south poles radially outward, while the disk 14 may be magnetized with north and south poles in opposite radial direction to that of the disk 12. When the disks 12, 14 are brought together with their support hubs 16 in axial alignment, their

respective segments 18 interfit with their axially extending edges 30 in tight abutment to form the ring magnet 10 having a contiguous outer circumferential surface having alternating radially magnetic poles. The interference fit between axially extending edges 30 ensures that the transition between adjacent magnetic poles is sharp and well-defined to thereby provide a steep magnetic flux density slope between adjacent poles.

It should be observed that as the disks 12, 14 are initially brought together, inwardly directed north poles of the segments 18 of the disk 12 are repelled by the outwardly directed north poles of the annular support hub 16 of the disk 14.

However, because the forces exerted on the disks 12, 14 are radially symmetrical and equal, the net radial force of repulsion between the disks 12, 14 is zero. Moreover, the adjacent oppositely polarized outer circumferential portions of the segments 18, and the axially adjacent oppositely polarized inner diameter portions of the annular support hubs 16, form a magnetic circuit which tends to attract the disks 12, 14 toward each other when they are fully assembled together. Thus, the disks 12, 14 are mutually axially attracted toward each other and held together tightly. In the preferred embodiment, no adhesive is necessary to join the disks 12, 14 together, although one may be used if so desired.

Also shown in Figs. 1 and 2, and in more detail in Figs. 6-8, are support members 40 and 42. It will be seen that the support members 40, 42 are of substantially identical construction and include a disk plate 44, an axially extending lip 46, and an axially extending central portion 48. The support members 40, 42 preferably are formed of nonmagnetic stainiess steel, but may be formed of any suitable material.

When the support members 40, 42 are assembled together with the disks 12, 14, it can be seen that the disk plate 44 extends radially along the segments 18. The axially extending lip 46 extends over an outer circumferential portion of the segments 18 to engage a peripheral groove 50 located at opposing axial sides of the segments 18 (Figs. 3-5). In its intended application, the ring magnet 10 may be spun at high rates of rotational velocity while subjected to elevated temperatures, possibly causing the ring magnet to expand radially outwardly. The engagement of the lip 46

with the groove 50 prevents the ring magnet 10 from expanding radially outwardly.

The axially extending central portions 48 of the support members 40, 42 extend into the region bounded by the inner diameter 20 of the disks 12, 14.

Preferably, the outer diameter of the central portion 48 is substantially equal to the inner diameter 20 of the annular support hub 16 so that the central portion 48 engages the support hub 16 in an interference fit. Further, the inner diameter of central portion 48 is dimensioned so as to fit on a shaft 60, as may be seen in Fig. 9.

Also, in the preferred embodiment, the central portion 48 extends axially a distance substantially equal to the distance between the first face 24 and the second face 26 of the annular support hub 16.

Referring to Fig. 9, it can be seen that the permanent ring magnet 10 may be mounted on a rotor shaft 60 of a rotating electrical machine 61, such as a motor or an encoder, such that the ring magnet 10 is disposed in concentric relationship with a sensor module 62 comprised of a sensor housing 64, at least one magnetic flux sensor 66, and a printed circuit board 68. Sensor housing 64 is preferably formed of plastic, but may be made of any suitable material. Within the sensor housing 64 is at least one magnetic flux sensor 66 in radially adjacent relationship to the ring magnet 10. The sensor module 62, ring magnet 10, and rotor shaft 60 are all mounted in operative relationship within stationary housing 70.

The magnetic flux sensor 66 may be mounted on a printed circuit board 68 and connected to external circuitry (not shown). Preferably, there are three magnetic flux sensors 66 (only one of which is shown), each of which is a unipolar Hall effect sensor. One advantage of using Hall effect sensors with a permanent ring magnet is that as temperature increases, the Hall effect sensor sensitivity increases while the magnetic flux density decreases, thus permitting accurate detection even as temperature increases. Also, unlike optical sensors, Hall effect sensors are capable of operating at high temperatures, typically to about 150 "C.

In the preferred embodiment, two of the Hall effect sensors 66 are disposed in circumferential relationship forty degrees apart, while the third Hall effect sensor 66 is disposed a multiple of forty degrees apart from the other two sensors 66. However, it should be understood that the number and arrangement of

the magnetic flux sensors 66 may be selected depending upon the particular application for the present invention.

In a DC brushless motor, for example, the permanent magnet 10 functions as a commutation disk. In an especially preferred embodiment, each of the disks 12, 14 of the ring magnet 10 are made with three segments 18. As the rotor shaft 60 rotates, the magnetic poles of the permanent ring magnet 10 rotate past the Hall effect sensors 66. The sensors 66 detect the sharp, well-defined transitions between magnetic poles to provide shaft position information to an electronic commutation circuit (not shown) for controlling flow of electricity to the windings of a stator 72 to rotatably drive a rotor 74 supported on the shaft 60.

Typical unipolar Hall effect sensors turn on or off when the magnetic flux density impinging upon them is above or below about 100 gauss. In order to achieve unipolar Hall effect sensor 66 switching at precise sixty degree intervals of rotation of the rotor shaft 60, the segments 18 of the disk 14, magnetized with south poles directed radially outwardly, are circumferentially wider by a predetermined amount. The corresponding segments 18 of the disk 12, magnetized with north poles directed radially outwardly, are circumferentially narrower by a corresponding amount. For example, the segments 18 ofthe disk 14 are each formed witha circumferential dimension corresponding to about a 60.6 degree arc, and the segments 18 of the disk 12 are each formed with a circumferential dimension corresponding to a 59.4 degree arc. This is necessary since the unipolar Hall effect sensors 66 switch on the south poles of the disk 14. In order to achieve a sufficient magnetic flux density to switch the sensors 66 every 60 degrees, the south pole segments 18 of the disk 14 must define a slightly longer circumferential dimension than the segments 18 of the disk 12. Thus, the points where the magnetic flux density is about 100 gauss for switching the sensors 66 are positioned about every sixty degrees around the circumference of the ring magnet 10.

The adjacent magnetic poles of the ring magnet provide sharp, well- defined transitions between fields of oppositely polarized magnetic flux density. The magnetic flux density radially outwardly from north pole segments is preferably about -600 gauss or less, while the magnetic flux density radially outwardly from

south pole segments is about +600 gauss or more. The tight interference fit between adjacent pole segments means that the magnetic flux density changes by about 1200 gauss, or more, within a very small distance. Further, as a result of the present ring magnet construction, the slope of the change in magnetic flux density between adjacent poles per degree of arc is so steep that the unipolar Hall effect sensors 66 typically switch state in about one-half degree of arc of rotation, or less. The resultant accurate detection of rotor shaft position means that external commutation circuitry can more precisely control a stator winding magnetic field to drive a motor with less torque ripple and greater efficiency.

Similarly, when the present invention is incorporated in a rotating electric machine in the form of a magnetic encoder, the rotational position of the rotor shaft 60 can be accurately determined using the Hall effect sensors 66. In such applications, the ring magnet may have fifty to sixty pole segments, or more.

While the form of apparatus herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.