Abstract:
A presently-preferred magnetic bearing comprises a rotor disk having a first plurality of concentric teeth extending from a surface thereof, and a stator disk having a second plurality of concentric teeth extending from a surface thereof. The first and the second plurality of concentric teeth are spaced apart by a gap that permits a primary magnetic flux to flow between the first and the second plurality of concentric teeth substantially in a first direction. Then magnetic bearing also comprises a plurality of flux focusing magnets fixedly coupled to at least one of the surface of the rotor disk and the surface of the stator disk. The flux focusing magnets produce a secondary magnetic flux that flows substantially in a second direction substantially opposite the first direction.

Description:
“This application is a continuation of Ser. No. 09/960,044 filed Sep. 22, 2001, now abandoned the entirety of which is incorporated herein by reference.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to magnetic bearings for levitating or suspending a rotatable component. More specifically, the invention relates to a magnetic bearing that provides radial positioning of a rotatable component on a passive basis to facilitate rotation of the component about a predetermined axis. 
     BACKGROUND OF THE INVENTION 
     Magnetic bearings are commonly used to levitate or suspend rotatable components, e.g., flywheels, and thereby facilitate rotation of the component about a predetermined axis. Magnetic bearings provide substantial advantages in relation to mechanical bearings. For example, magnetic bearings facilitate substantially friction-free operation, and thus function without most of parasitic energy losses that occur in virtually all mechanical bearings. 
     Magnetic bearings are classified as “active” or “passive.” Active magnetic bearings usually comprise one or more electromagnets that create return forces. A typical active magnetic bearing also comprises one or more position sensors that operate in conjunction with a servo control system. The servo control system varies the current passing through the electromagnets in a manner that causes the return forces to suspend and align the rotatable member along a desired axis of rotation. 
     Passive magnetic bearings typically comprise one or more permanent magnets fixed to the rotating or static components of the bearing. The permanent magnets produce attractive or repulsive forces that bias the rotating component toward or along a desired axis of rotation. Passive magnetic bearings, in general, are lighter, smaller, less complex, less expensive, and more reliable than active bearings of similar capability. A passive magnetic bearing, however, cannot provide stable positioning of the rotatable member in the radial and axial directions, i.e., with respect a set of orthogonal axes one of which extends along the desired axis of rotation. Passive magnetic bearings, therefore, are typically used in conjunction with one or more active bearings. 
     So-called “centering bearings” represent a particular type of passive magnetic bearing. Centering bearings exert a radial force on a rotatable member that biases the rotatable member toward a desired axis of rotation. One possible embodiment of a conventional centering bearing  100  is depicted in cross-section in FIGS. 5 and 6. 
     The bearing  100  comprises a first stator disk  102  and a second stator disk  104 . The bearing  100  further comprises a rotor disk  106 . The rotor disk  106  is fixedly coupled to a shaft  109  that supports a rotatable component such as a flywheel. 
     The stator disks  102 ,  104  and the rotor disk  106  are each formed from a soft ferromagnetic material. The stator disk  102  includes a major surface  102   a  having a plurality of concentric raised portions, or teeth  102   b , formed thereon. The teeth  102   b  each form a continuous ring, i.e., the teeth  102   b  each extend through a continuous arc of 360 degrees. The stator disk  104  likewise includes a major surface  104   a  having a plurality of concentric teeth  104   b  formed thereon. 
     The rotor disk  106  has a first surface  106   a  and a second surface  106   b . The first surface  106   a  has a plurality of concentric teeth  106   c  formed thereon. The second surface  106   b  likewise has a plurality of concentric teeth  106   d  formed thereon. The geometry, i.e., the size and shape, of each tooth  106   c  substantially matches that of a corresponding tooth  102   b  on the stator disk  102 . The geometry of each tooth  106   d  substantially matches that of a corresponding tooth  104   b  on the stator disk  104 . 
     The rotor disk  106  is positioned between the stator disks  102 ,  104 , as shown in FIG.  5 . More particularly, the rotor disk  106  is positioned so that the first surface  106   a  faces the surface  102   a  of the stator disk  102  across an axial gap  114 . The second surface  106   b  likewise faces the surface  104   a  of the stator disk  104  across an axial gap  116 . 
     The bearing  100  further comprises a ring-shaped permanent magnet  110  having a north pole  110   a  and a south pole  110   b . The magnet  110  is fixed to a non-magnetizable mounting surface  108 . In addition, the magnet  110  is fixedly coupled to the stator disks  102 ,  104  so that the north pole  110   a  is positioned proximate the stator disk  104 , and the south pole  110   b  is positioned proximate the stator disk  102 . 
     The noted arrangement of the magnet  110 , stator disks  102 ,  104 , and rotor disk  106  produces a magnetic-flux circuit within the bearing  100 . The primary direction of flow of the magnetic flux is denoted by arrows  112  included in FIG. 5 (the arrows  112  are not depicted in the lower portion of FIG. 5, for clarity). The magnetic flux flows from the north pole  110   a  into the stator disk  104 . The magnetic flux travels through the stator disk  104 , and is at least partially focused in the teeth  104   b . The magnetic flux flows from the teeth  104   b , across the gap  116 , and into to the teeth  106   d.    
     The magnetic flux flows through the rotor disk  106 , and is at least partially focused in the teeth  106   c . The magnetic flux flows from the teeth  106   c , across the gap  114 , and into the teeth  102   b  on the stator disk  102 . The magnetic flux subsequently flows through the stator disk  102  and into south pole  110   b  of the magnet  110 , thereby completing the magnetic circuit. 
     The noted flow of magnetic flux through the magnetic bearing  100 , in conjunction with the geometry and arrangement of the stator disks  102 ,  104  and the rotor disk  106 , produces a centering effect on the shaft  109 . More particularly, the magnetic flux causes the teeth  102   b  on the first stator disk  102  to substantially align with the teeth  106   c  on the rotor disk  106 . The magnetic flux likewise causes the teeth  104   b  on the second stator disk  104  to substantially align with the teeth  106   d  on the rotor disk  106 . This phenomenon is based on the principle that the magnetic flux seeks a path of minimum reluctance. 
     Minimum reluctance in the flux circuit is achieved when the gaps  114 ,  116  are minimized, i.e., when the distances that the flux must travel to reach the first stator disk  102  from the surface  106   a  of the rotor, or to reach the rotor  106  from the surface  104   a  of the stator disk  104 , are minimized. Minimization of the gap  114  occurs when the teeth  102   b  are substantially aligned with the teeth  106   c . Minimization of the gap  116  likewise occurs when the teeth  104   b  are substantially aligned with the teeth  106   d  (as shown in FIG.  5 ). 
     Hence, the magnetic flux flowing through the bearing  100 , in attempting to define a flow path of minimal reluctance, produces a magnetomotive force that urges each of the teeth  106   c ,  106   d  into substantial alignment with a corresponding tooth  102   b ,  104   b . Aligning the teeth  102   b ,  104   b ,  106   c ,  106   d  suspends the shaft  109  and substantially aligns the shaft  109  with a predetermined axis extending in the “z” direction, thereby permitting the shaft  109  to rotate about that axis (the noted axis is denoted “C 1 ,” and the direction of rotation is indicated by the arrow  126  in FIG.  5 ). The resistance of the shaft  109  to radial displacement away from the predetermined axis is commonly referred to as the “stiffness” of the bearing  100 , and is proportionate to the above-noted magnetomotive produced by the flow of magnetic flux through the teeth  102   b ,  104   b ,  106   c ,  106   d.    
     The magnetic-flux circuit in the bearing  100  is subject to various losses. In other words, only a portion of the magnetic flux available from the permanent magnet  110  is available to suspend and align the shaft  109 . The teeth  102   b ,  104   b ,  106   c ,  106   d  represent one source of flux loss. In particular, a portion of the magnetic flux that enters each tooth  102   b ,  104   b ,  106   c ,  106   d  escapes into the space between adjacent teeth  102   b ,  104   b ,  106   c ,  106   d.    
     For example, FIG. 6 is a magnified view depicting a plurality of the teeth  102   b ,  106   c . Adjacent one of the teeth  102   b  define valleys  118  located between the adjacent teeth  102   b . Adjacent teeth  106   c  likewise form valleys  118  located between the adjacent teeth  102   b . A portion of the magnetic flux passing through the teeth  102   b ,  106   c  escapes from the teeth  102   b ,  106   c  and into the neighboring valleys  118 . This flux leakage is denoted by the arrows  120  included in FIG.  6 . The magnetic flux that leaks or escapes from each of the teeth  106   c  in this manner does flow directly to a corresponding tooth  102   b  on the stator disk  102 . Hence, this flux does not contribute substantially to the suspension and centering of the shaft  109 . The capacity of the permanent magnet  110  must therefore be greater than otherwise required to account for the noted flux leakage. 
     Increasing the capacity of a permanent magnet in a magnetic bearing typically results in a corresponding increase in the size, weight, and expense of the permanent magnet (and the magnetic bearing). Hence, minimizing the flux leakage from the magnetic circuit of a bearing can lead to substantial reductions in the size, weight, and cost of the bearing. An ongoing need therefore exists for a passive radial magnetic bearing having features that minimize the leakage of magnetic flux therefrom. 
     SUMMARY OF THE INVENTION 
     A presently-preferred embodiment of radial magnetic bearing comprises a rotor disk having a first plurality of concentric teeth extending from a surface thereof, and a stator disk having a second plurality of concentric teeth extending from a surface thereof. The second plurality of concentric teeth is spaced apart from the first plurality of concentric teeth by a gap that permits a primary magnetic flux to flow between the first and the second plurality of concentric teeth substantially in a first direction. 
     The magnetic bearing also comprises a primary magnet magnetically coupled to at least one of the rotor disk and the stator disk and being adapted to provide the primary magnetic flux. The magnetic bearing further comprises a plurality of flux focusing magnets fixedly coupled to at least one of the surface of the rotor disk and the surface of the stator disk and producing a secondary magnetic flux that flows substantially in a second direction substantially opposite the first direction. 
     Another presently-preferred embodiment of radial magnetic bearing comprises a rotor disk adapted to rotate about a predetermined axis and having a first and a second circumferentially-extending raised portion projecting from a surface thereof, and a stator disk axially spaced from the rotor disk and positioned around the predetermined axis. The stator disk has a third and a fourth circumferentially-extending raised portion projecting from a surface thereof. The radial magnetic bearing also comprises a permanent magnet magnetically coupled to at least one of the rotor disk and the stator disk and providing a primary magnetic flux, a first ring-shaped magnet positioned between the first and the second raised portions, and a second ring-shaped magnet positioned between the third and the fourth raised portions. 
     Another presently-preferred embodiment of radial magnetic bearing comprises a rotor disk having a first plurality of circumferentially-extending raised portions projecting from a major surface thereof, and a stator disk having a major surface that faces the major surface of the rotor disk. The major surface of the stator disk has a second plurality of circumferentially-extending raised portions projecting therefrom. The radial magnetic bearing also comprises a plurality of flux focusing magnets fixedly coupled to at least one of the major surfaces of the rotor disk and the stator disk. 
     Another presently-preferred embodiment of radial magnetic bearing comprises a rotor disk adapted to rotate about an axis of rotation and having a first plurality of circumferentially-extending raised portions formed thereon for conducting a primary magnetic flux substantially in a first direction. The radial magnetic bearing also comprises a stator disk positioned around the axis of rotation and axially spaced from the rotor disk. The stator disk has a second plurality of circumferentially-extending raised portions formed thereon for conducting the primary magnetic flux substantially in the first direction. 
     The radial magnetic bearing further comprises a primary magnet magnetically coupled to at least one of the rotor disk and the stator disk and being adapted to provide the primary magnetic flux. The radial magnetic bearing also comprises a first plurality of flux focusing magnets each being positioned between adjacent ones of the first plurality of raised portions and each being polarized in a direction substantially opposite the first direction, and a second plurality of flux focusing magnets each being positioned between adjacent ones of the second plurality of raised portions and each being polarized in the direction substantially opposite the first direction 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, the drawings show an embodiment that is presently preferred. The invention is not limited, however, to the specific instrumentalities disclosed in the drawings. In the drawings: 
     FIG. 1 is a diagrammatic side view of a passive radial magnetic bearing in accordance with the present invention; 
     FIG. 2 is a diagrammatic cross-sectional view of the passive radial magnetic bearing shown in FIG. 1, taken along the line “ 2 — 2 ” of FIG. 1; 
     FIG. 3 is a magnified view of the area designated “A” in FIG. 2; 
     FIG. 4 is a magnified view of the area designated “B” in FIG. 2, rotated ninety degrees from the perspective of FIG. 2; 
     FIG. 5 is a diagrammatic side view, in longitudinal cross section, of a conventional passive radial magnetic bearing; and 
     FIG. 6 is a magnified view of the area designated “D” in FIG.  5 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1-4 depict a presently-preferred embodiment of a passive radial magnetic bearing  10 . The figures are each referenced to a common coordinate system  8  depicted therein. The magnetic bearing  10  is adapted to suspend a rotatable component such as a flywheel, and to align the rotatable component with a predetermined axis of rotation (the axis of rotation is “C” in FIG.  2 ). The magnetic bearing  10 , in a typical application, would be used in conjunction with one or more active magnetic bearing to provide stable rotation of the rotatable component about the axis of rotation. Details concerning these additional bearings are not necessary to an understanding of the invention, however, and therefore are not included herein. 
     The bearing  10  comprises a first stator disk  12  and a second stator disk  14 . The bearing  10  farther comprises a rotor disk  16 . The rotor disk  16  is fixedly coupled to a shaft  19  that supports a rotatable component such as a flywheel. The direction of rotation of the shaft  19  is denoted by the arrow  31  included in FIG.  2 . 
     The stator disks  12 ,  14  and the rotor disk  16  are preferably formed from a soft ferromagnetic material such as 2 vanadium permadur, very pure iron, or a high-permeability nickel-iron. The stator disk  12  includes a major surface  12   a  having a plurality of concentric raised portions, or teeth  12   b , formed thereon. The teeth  12   b  each form a continuous ring, i.e., the teeth  12   b  each extend through a continuous arc of 360 degrees. Adjacent teeth  12   b  define a space, or valley  24  located between the adjacent teeth  12 . The stator disk  14  likewise includes a major surface  14   a  having a plurality of concentric teeth  14   b  formed thereon, with adjacent teeth  14   b  defining one of the valleys  24 . The significance of the valleys  24  is explained below. 
     The rotor disk  16  has a first surface  16   a  and a second surface  16   b . The first surface  16   a  has a plurality of concentric teeth  16   c  formed thereon. The second surface  16   b  likewise has a plurality of concentric teeth  16   d  formed thereon. The geometry, i.e., the size, shape, and relative position, of each tooth  16   c  substantially matches that of a corresponding tooth  12   b  on the stator disk  12 . The geometry of each tooth  16   d  substantially matches that of a corresponding tooth  14   b  on the stator disk  14 . Adjacent ones of the teeth  16   c ,  16   d  define one of the valleys  24 . 
     The rotor disk  16  is positioned between the stator disks  12 ,  14 , as shown in FIG.  2 . More particularly, the rotor disk  16  is positioned so that the first surface  16   a  faces the surface  12   a  of the stator disk  12  across an axial gap  13 . (The “axial” direction, as referenced throughout the specification and claims, refers to the “z” direction denoted on the coordinate system  8 .) The second surface  16   b  likewise faces the surface  14   a  of the stator disk  14  across an axial gap  17 . 
     The bearing  10  further comprises a ring-shaped permanent magnet  11  having a north pole  11   a  and a south pole  11   b . The magnet  11  is fixedly coupled to a non-magnetizable mounting surface  18 . An inner circumferential surface  11   c  of the magnet  11  is fixedly coupled to a respective outer circumferential surface of the stator disk  12  and the stator disk  14  by conventional means such as bonding (see FIG.  1 ). The magnet  11  is positioned so that the so that the north pole  11   a  is located proximate the stator disk  14 , and the south pole  11   b  is located proximate the stator disk  12 . Note: The magnetic bearing  10  may include an outer casing or cover that houses the above-noted components; this casing or cover is not depicted in the figures, for clarity. 
     The noted arrangement of the magnet  11 , stator disks  12 ,  14 , and rotor disk  16  produces a magnetic-flux circuit within the bearing  10 . The primary direction of flow of the magnetic flux is denoted by arrows  15  included in FIGS. 2 and 3 (the arrows  15  are not depicted in the lower portion of FIG. 2, for clarity). The magnetic flux flows from the north pole  11   a  into the stator disk  14 . The magnetic flux travels through the stator disk  14 , and is at least partially focused in the teeth  14   b . The magnetic flux flows from the teeth  14   b  to the teeth  16   d  of the rotor disk  16  via the gap  17 . 
     The magnetic flux subsequently flows through the rotor disk  16 , and is at least partially focused in the teeth  16   c . The magnetic flux flows from the teeth  16   c  to the teeth  12   d  of the stator disk  12  via the gap  13 . The magnetic flux flows through the stator disk  12  and into south pole  11   b  of the magnet  11 , thereby completing the magnetic circuit. Note: The above-described magnetic circuit is hereinafter referred to as the “primary magnetic circuit” of the bearing  10 . 
     The noted flow of magnetic flux through the magnetic bearing  10 , in conjunction with the geometry and arrangement of the stator disks  12 ,  14  and the rotor disk  16 , produces a centering effect on the shaft  19 . More particularly, the flux through the primary magnetic circuit causes the teeth  12   b  on the first stator disk  12  to substantially align with the teeth  16   c  on the rotor disk  16 . The magnetic flux likewise causes the teeth  14   b  on the second stator disk  14  to substantially align with the teeth  16   d  on the rotor disk  16 . This phenomenon, as explained previously, is due to the fact that the magnetic flux seeks a path of minimum reluctance. The noted alignment of the teeth  12   b ,  14   b ,  16   c ,  16   d  suspends the shaft  19  and substantially aligns the shaft  19  with the axis of rotation “C,” thereby facilitating rotation of the shaft  19  (and the rotor disk  16 ) in relation to the stator plates  12 ,  14  and the mounting surface  18 . 
     The magnetic bearing  10  further includes a plurality of flux focusing magnets  20 . The flux focusing magnets  20  are positioned on the surfaces  12   a ,  14   a ,  16   a ,  16   b  of the respective stator disks  12 ,  14  and rotor disk  16 , and within the valleys  24  formed by the teeth  12   b ,  14   b ,  16   c ,  16   d . The flux focusing magnets  20 , as explained in detail below, minimize flux leakage from the teeth  12   b ,  14   b ,  16   c ,  16   d.    
     For clarity, the flux focusing magnets  20  are hereinafter described with reference to the flux focusing magnets  20  located on the second stator disk  14 . This description, unless otherwise noted, applies equally to the flux focusing magnets  20  positioned on the first stator disk  12  and the rotor disk  16 . 
     The flux focusing magnets  20  are preferably formed as continuous rings each having a substantially square cross-section (see FIGS.  2  and  3 ). Each flux focusing magnet  20  is adapted to fit within a corresponding valley  24  with minimal clearance between the flux focusing magnet  20  and the adjacent surfaces of the teeth  14   b . The flux focusing magnets  20  are fixedly coupled to the surface  14   a  by a suitable means such as bonding. The flux focusing magnets  20  may be formed from magnetic materials such neodimium iron boron or sumarium cobalt. 
     The magnetization vector of each flux focusing magnet  20  is oriented substantially in the axial (“z”) direction (the magnetization vector is represented by the arrows  26  included in FIG.  3 ). Furthermore, the flux focusing magnets  20  are positioned so that the magnetization vector acts in a direction opposite the local magnetic flux in the primary magnetic circuit In other words, the magnetic flux produced by each flux focusing magnet  20  is oriented in a direction opposite the direction of the magnetic flux in the adjacent teeth  14   b . Note: The optimal value for the magnetic flux produced by the flux focusing magnets  20  is application-dependent. Hence, a specific value for this parameter is not provided herein. 
     Applicant has found that the use of the flux focusing magnets  20  in the above-described manner substantially reduces the leakage of magnetic flux from the primary flux circuit of the bearing  10 . In particular, the flux focusing magnets  20  inhibit leakage of the primary magnetic flux from the teeth  14   b  by creating a localized magnetic field that acts in a direction opposite the primary magnetic field flowing through the teeth  14   b . This localized magnetic field, in effect, focuses the primary magnetic flux in the desired direction, i.e., toward the gap  17  and the teeth  16   d  of the rotor disk  16 . 
     Applicant has determined through experimentation that the radial stiffness of a magnetic bearing such as the bearing  10  can be doubled though the use of the flux focusing magnets  20 . In other words, the magnetomotive force generated by the flow of magnetic flux between the teeth  12   b ,  14   b ,  16   c ,  16   d  can be can be substantially increased by inhibiting the leakage of flux therefrom using the flux focusing magnets  20 . 
     The use of the flux focusing magnets  20  in a magnetic bearing such as the magnetic bearing  10  can thus increase the amount of radial stiffness achievable with a given level of primary magnetic flux. Alternatively, the flux focusing magnets  20  permit a given radial stiffness to be achieved with a lower level of primary magnetic flux. Hence, the permanent magnet that supplies the primary flux can be downsized, leading to potential reductions in the size, weight, and expense of the bearing. 
     It is to be understood that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of the parts, within the principles of the invention. 
     For example, the magnetic bearing  10  has been described in detail for illustrative purposes only. The principles of the invention can be applied to passive radial magnetic bearings of virtually any configuration. For example, the principles of the invention can be applied to bearings having a different number and arrangement of stator disks, rotor disks, and permanent magnets in comparison to the magnetic bearing  10  described herein. The principles of the invention can also be applied to bearings in which the permanent magnet that provides the primary magnetic flux is fixed to the rotor disk rather than the stator disks.