WO2001084693A1

WO2001084693A1 - Full levitation bearing system with improved passive radial magnetic bearings

Info

Publication number: WO2001084693A1
Application number: PCT/US2001/013951
Authority: WO
Inventors: Christopher W. Gabrys; Co Si Huynh; Patrick T. Mcmullen
Original assignee: Indigo Energy, Inc.; Innovative Magnetic Technologies Inc.
Priority date: 2000-05-01
Filing date: 2001-05-01
Publication date: 2001-11-08
Also published as: AU2001259283A1

Abstract

A full levitation magnetic bearing system for support of a rotating body (71), such as a flywheel (92), about an axis of rotation includes passive radial magnetic bearings (70) that generate passive radial centering forces to counteract displacements of the rotating body (71) from its axis of rotation during rotation, and an axial actuator (102) for stabilizing the axial position of the rotating body (71). The passive radial magnetic bearings (70) include multiple concentric, radially spaced apart, axially magnetized ring magnets (73) on a stationary stator (72) and magnetically co-operating concentric pole rings (74) on a ferromagnetic end portion of a rotor (71). The rotor pole rings (74) are axially aligned with the magnet rings (73) when the rotor (71) is radially centered on the stator (72). Magnetic flux from the permanent magnet rings (73) passes directly across the airgap (78) between the axially facing surfaces of the magnet rings (73) and the rotor pole rings (74) and through rotating ferromagnetic section of the rotor (71), thereby generating both an axial attractive force and a passive radial centering force from the pole rings (74) tending to align with the stationary magnetized rings (73).

Description

Full Levitation Bearing System with Improved Passive Radial Magnetic Bearings

This invention pertains to a full levitation bearing system that uses passive radial and axial magnetic bearings and an active axial magnetic thrust actuator for centering the levitated rotor at an axially balanced position. The passive radial magnetic bearings generate high radial stifmess and inherent radial damping with minimized tilt moment instability and have improved high speed operating capability.

Background of the Invention Magnetic bearings are in many cases the desired bearings for support of rotating objects especially where high-speeds, non-contamination with lubricant or bearing wear products, or long life is required. Magnetic bearings can be designed with varying actively controlled degrees of freedom between one degree and five degrees. In some emerging applications, such as flywheel energy storage systems, minimizing the amount of required control is preferable for minimizing the system costs and extending the operating life. Single degree actively controlled designs employ passive radial magnetic bearings to maintain radial stability. Such bearings typically have an inherently low radial stiffness, however they allow very simple, reliable and long life control.

To date several different configurations of passive radial magnetic bearings have been developed and each has advantages as well as drawbacks. Fig. 1 is a schematic representation of one type of magnetic bearing element of the prior art having a passive radial magnetic bearing 30 having a rotor 31 and a stator 32, and having adjacent alternating polarity permanent magnets attached to both the rotor and stator for a fully levitating systemTwo matching permanent magnet rings 35 cooperate with the rotating magnets 33 on the rotor 31 across an axial airgap between the opposing faces of the pairs of magnets to create a passive radial centering force. Ferromagnetic return paths 34 and 36 are sometimes included to increase the flux density and hence radial stiffhess. Multiple axial layers of stator and rotor magnets (not shown) can also be used to further increase radial stifmess. This design provides very high radial stifmess as a result of the permanent magnet axial faces on the rotating and stationary portions. High energy permanent magnets, such as NdFeB, have very low magnetic permeability approaching that of an airgap. This helps provide high radial stifmess because the magnetic flux cannot redistribute across the magnet's surfaces during a radial displacement of the rotor. Therefore, the magnetic flux path necessarily becomes elongated, generating significant radial restoring/centering force. This radial centering force is yet further increased by the effects of cross repulsion of opposing polarity magnets on the rotor and stator. Although this design effectively provides high radial stifmess, it has several drawbacks, which include the use of permanent magnets on the rotating portion. High energy-product magnets have very low tensile strengths and require difficult and expensive reinforcement for high speed operation. The magnets can also in some cases limit the maximum operating speed depending on the design since at high speeds they require radial support that is capable of resisting high hoop tensile and radial compressive stresses. The second drawback of this design is that it has extremely little inherent radial damping. Because of the lack of flux redistribution on the surface of the permanent magnets, eddy current and hysteresis damping is not significant! A damper mechanism is required for this bearing system to function properly, which increases both cost and complexity. Another prior art configuration for a passive radial magnetic bearing, represented schematically at 40 in Fig. 2 includes a rotor 41 and a stator 42. On the rotor 41, radially magnetized permanent magnet rings 43 are attached along with inner and outer ferromagnetic pole rings 44 for focusing the flux axially. Cooperating permanent magnets 45 and pole rings 46 are also attached to the stator 42. The result is both an axial force as well as generation of a radial centering force, as the ferromagnetic pole rings 44 on the rotor 41 and the pole rings 46 on the stator 42 tend to align. The radial stiffness of this design is less than the previously described design for a given amount of permanent magnet material and bearing diameter due to the poles 44 and 46 allowing some flux redistribution when there is a radial displacement of the rotor 41, and also due to the absence of cross repulsion forces. This design does have higher internal damping during radial displacement due to hysterisis and eddy currents induced in the ferromagnetic axial surfaces of the pole rings 44 and 46. The main drawbacks with this configuration are that the bearing uses permanent magnets and added pole rings assemblies on the rotor 41 which need special reinforcement to resist centrifugal forces during high rotational speed. Also, the level of damping is likely still inadequate to be used alone without an added damper, and the inherent tilt moment instability generated from the bearing is high. The higher tilt moment instability arises from the use of radially thin ferromagnetic poles 44 and 46 for achieving appreciable radial stiffness, which increase the flux density in the airgap. Because the axial force increases with the square of the flux density and only linearly with area, the axial forces significantly increase with the use of thin poles 44 and 46. This also provides generation of a higher unstable tilting moment for an individual bearing that must be overcome by an opposing end radial bearing for complete levitation. In some cases the magnetic bearing diameter must be limited and multiple axially distributed bearings are used to achieve sufficient radial stiffness per unstable tilt moment. The use of radial magnetized permanent magnet rings is also less desirable due to more complicated and expensive manufacturing processes and lower availability. A third passive radial magnetic bearing configuration, represented schematically at 50 in Fig. 3, includes a rotor 51 and a stator 52. The rotor 51 has multiple concentric ferromagnetic pole teeth 53 that can be made integral with the rotor structure. The rotor pole rings 53 cooperate with matching pole rings 54 and 55 on the stator 52. A permanent magnet 56 on the stator 51 provides the magnetic flux for the bearing 50. However, the material at both surfaces of the axial airgaps between the rotor pole teeth 53 and the stator pole teeth 54 and 55 is made of ferromagnetic material, which results in low radial stiffness, due in part to flux redistribution. The radial stiffness can be increased by making the ferromagnetic pole teeth 53, 54 and 55 radially thin, but this results in a high level of tilt moment instability for given level of radial stiffness. This can limit its application for full levitation to relatively long rotating objects where the unstable tilt moments can be overcome by a counteracting correcting moment equal to the axial length of the rotor multiplied by its radial stiffness. Fig. 4 shows the flux redistribution at the surface regions of the pole teeth of this design that occurs when there is radial displacement of the rotor. During radial displacement, the flux 64 in the airgap between the rotor and stator poles can easily redistribute to opposite radial sides of the opposing pole rings because of the low magnetic permeability of the pole rings 55 and 53. This redistribution increases the inherent radial damping but it prevents maximum elongation of the airgap flux 64 and hence prevents development of maximum radial centering forces. This flux redistribution can also result in greater tilt moment instability because of increased flux density caused by concentration of the flux between reduced overlapping portions of the opposing pole rings. Summary of the Invention

The invention provides a full levitation magnetic bearing that uses improved passive radial and axial magnet bearings and an active axial magnetic thrust actuator for axial stabilization. The passive radial magnetic bearings are especially suited for full levitation magnetic bearing systems, because they allows full levitation support with simple, low cost single axis active control. The bearings use multiple axially magnetized, concentric magnet rings or, alternatively in another embodiment, concentric magnetized ring portions of a single block of magnetic material attached to the stationary portion, cooperating with pole rings on the axial face of a ferromagnetic rotating portion. This cooperation creates both an axial attractive force and passive radial centering forces, as the pole rings tend to align with the stationary permanent magnet rings. In a preferred embodiment, the concentric permanent magnet rings are arranged with alternating axial polarities for generation of maximum radial stiffness.

The radial stiffness of the bearings is higher than some previous passive radial magnetic bearing designs because the magnet rings have an axially facing surface that forms one boundary of the airgap, so the exit and entrance points of the flux on the stationary portion is fixed due to the flux exiting and entering the airgap directly from the permanent magnets. Permanent magnets have very low magnetic permeability, similar to an air gap, so flux cannot easily redistribute to either the inner or outer diameter of an individual magnet ring section when the cooperating ferromagnetic pole ring is displaced radially. The result is a generation of a larger radial centering force for the same radial displacement with a given magnetic flux density in the radial magnetic bearing. Because the flux density for a given radial stiffness can be made significantly lower, the axial attractive force generated between the stationary portion and rotating portion will also be lower. The direct benefit of the lower axial attractive forces is significantly reduced tilt moment instability caused by an individual bearing. This allows the passive radial bearings to be easily employed in full levitation magnetic bearing systems where stability can be achieved with only active axial control. The lower tilt moment instability allows axially shorter objects to be stably levitated for a given radial bearing diameter and stiffness. Likewise, the axial force generated is nearly linear over typical operating airgap dimensions for magnetic bearings, which facilitates simple control for the active magnetic thrust bearing. The axially magnetized permanent magnets can also be made axially tall to allow for an increased effective operating airgap, if desired. Although the actual airgap dimension is the same, the very low magnetic permeability of the permanent magnets give the magnetic circuit a large effective airgap dimension, so small fluctuations of airgap dimension during operation of the rotor result in small percentage changes in the effective airgap dimension, resulting in small changes in axial force exerted by the passive magnetic bearing and reduced tilt moment instability.

Another benefit of the invention is that the permanent magnets of the radial bearings drive flux through a relatively large surface area of the ferromagnetic and electrically conducting rotating portion in the magnetic circuit. The large flux penetration area generates higher inherent radial damping by virtue of hysterisis and eddy current losses than previous bearing designs. Depending on the system design, additional radial dampers may be unneccessary.

The invention also allows operation to higher rotational speeds than previous magnetic bearing designs. With full levitation, mechanical drag and wear of contacting elements are eliminated. The use of permanent magnets on the stationary portion acting in cooperation with a rotating ferromagnetic portion allows increased speed over previous passive radial magnetic bearings having magnets on the rotating portion. Because the magnets do not rotate, they do not need special reinforcement nor do they limit the operating speed by virtue of their own structural weakness.

The invention is also believed to be less prone to dynamic whirl instabilities. Whirl instabiUties in full levitation magnetic bearings systems with passive radial magnetic bearings are believed to be caused by drag forces that act in the direction perpendicular to the direction of a radial displacement. Although this invention generates a level of inherent radial damping, the ratio of tangential drag to radial drag is believed to be lower in comparison with previous designs due to the magnet surface forming one boundary of the airgap, so the flux flows directly between the airgap and the permanent magnets on the stationary portion of the radial bearings. Because the permanent magnets have a significantly lower electrical conductivity than steeland the flux cannot redistribute radially or circumferentially across the surface of the permanent magnets, tangential drag is reduced. The ferromagnetic portion in the magnetic bearing also does not need to operate in or near saturation, which would result in higher tangential drag from higher localized hysteresis losses The permanent magnets in the bearings are also of axial magnetization, which is believed to produce a higher circumferential field uniformity than previous designs using radially magnetized permanent magnets. In a further embodiment of the invention, a method and structure is disclosed to eliminate whirl instabilities if they do occur in a particular system.

Another embodiment of the invention includes an integrated magnetic bearing actuator for providing both passive radial with axial and active axial forces from a single stationary structure. The magnetic bearing with integrated actuator allows for design standardization as well as simple implementation capability of magnetic bearings for many applications. Two electrical control circuit methods for providing stabilization control are also disclosed. Because the bearing with combined actuator works with the axial faces of the levitated object, the rotor need not have shafts.

Description of the Drawings

Fig. 1. is a schematic drawing of a prior art passive radial magnetic bearing using adjacent alternating permanent magnets on the rotor and stator; Fig. 2. is a schematic drawing of a prior art passive radial magnetic bearing using radially magnetized permanent magnets and ferromagnetic pole rings on both the rotor and stator;

Fig. 3. is a schematic drawing of a prior art passive radial magnetic bearing using ferromagnetic poles on both the rotor and stator with a stationary permanent magnet; Fig. 4. is schematic drawing of prior art ferromagnetic pole radial magnetic bearing rings showing magnetic flux redistribution during radial displacement;

Fig. 5. is a schematic drawing of a preferred configuration of a passive radial magnetic bearing in accordance with this invention;

Fig. 5 A. is a schematic drawing of an alternate preferred configuration of a passive radial magnetic bearing in accordance with this invention;

Fig. 6. is a schematic drawing of a portion of the bearing shown in Fig. 5, showing the stretching of flux that produces a radial centering force;

Fig. 6A is a schematic sectional elevation of a portion of the bearing shown in Fig. 5, showing a shim in the airgap; Fig. 7. is a schematic drawing of a fully levitated flywheel energy storage system using a passive radial magnetic bearings with active axial control in accordance with this invention; Fig. 8. is a schematic drawing of an alternate configuration of a fully levitated flywheel energy storage system using a passive radial magnetic bearings with active axial control in accordance with this invention;

Fig. 9. is a schematic drawing of a second alternate configuration of a fully levitated flywheel energy storage system using passive radial magnetic bearings with active axial control in accordance with this invention;

Fig. 10. is a schematic drawing along lines 10-10 in Fig. 5 of the array of ferromagnetic pole rings, with radial dissymmetry;

Fig. 10A is a plan view along lines 10A-10A in Fig. 5, showing the magnet rings on the stator;

Fig. 10B is a developed view of one of the ferromagnetic pole rings, illustrating one technique that may be used to achieve radial dissymmetry of mechanical properties;

Fig. 11. is a schematic diagram of a preferred electrical circuit for providing axial control in a full levitation magnetic bearing system with opposing electromagnetic coils; and Fig. 12. is a schematic diagram of an alternate preferred electrical circuit for providing axial control in a full levitation magnetic bearing system with opposing electromagnetic coils.

Description of the Preferred Embodiment

Turning to the drawings wherein like characters designate identical or corresponding parts, Fig. 5 is a schematic drawing showing a preferred configuration of a passive radial magnetic bearing 70, in accordance with the invention, having a rotor 71 and a stator 72. The rotor 71 has a rotor end portion 76 that is constructed of ferromagnetic material, with steel preferred for high strength, low cost, high permeability and electrical conductivity, although other materials could be used. The ferromagnetic rotor end portion 76 has multiple pole rings 74 that cooperate with multiple concentric, radially spaced apart, axially magnetized ring magnets 73 that are attached to the stator 72. The stator 72 can be made of various materials, however it is preferred that it be made of ferromagnetic material or have a ferromagnetic insert adjacent the magnets 73 so as to provide a return path for magnetic flux and hence maximum performance. As shown, radially adjacent ring magnets 73 are arranged with axially alternating polarities. This is the preferred configuration because it allows the shortest return paths for all the magnetic fluxes. However, the bearing will also function if they are not arranged axially alternating consecutively.

The passive radial magnetic bearing of the invention does not achieve the maximum radial stiffness or axial load carrying capacity per amount of magnet material. In fact, the configuration shown in Fig. 1 can achieve approximately four times as much stiffness for a given amount of magnet material at a given bearing diameter. Moreover, rare earth magnet material, preferably used for maximum stiffness, has been expensive and thin large diameter rings less available. For these and other reasons, this design would initially be disfavored by the industry. However, the bearings of the invention provide increased high speed operation. The bearings also simultaneously have high inherent radial stiffness and damping and a relatively simple construction. The permanent magnet rings of the invention can be fabricated as complete rings or in smaller individual arc segments, as illustrated in Fig. 10A, and the arc segments can be assembled in to precut grooves in the stator to maintain concentricity as shown in Fig. 5. Because the rotor end portion 76 of the rotor 71 is constructed of only steel without permanent magnets, it can be rotated to extremely high speed. The use of permanent magnet rings having surfaces which define one of the boundaries of the axial airgap ensures that the magnet surfaces themselves will be the direct exit and entrance surfaces for the flux across the airgap. This provides increased radial stiffness over prior art designs of Figs. 2 and 3 because the magnets prevent flux redistribution on the stator side and therefore forces the flux field to elongate as shown in Fig. 6. Ferromagnetic shims could be added to the surface of the magnet rings as shown in Fig. 6 A to smooth the flux circumferentially at arc magnet joints when the rings are made of individual sectors to build the rings. Use of shims is generally not necessary as the tangential drag is very low and the axial airgap distance facilitates the smoothing of the flux between magnet arcs. Such shims would not adversely affect the advantageous resistance to flux redistribution because of the very small thickness, and thus the magnets with such shims should still be construed as having magnet surfaces defining one of the boarders of the airgap.For this reason, we intend that the claims herein to magnet rings having magnet surfaces defining one of the boundaries of the airgap be interpreted to cover magnet rings with such shims over the surfaces facing the airgap. The pole teeth 74 on the rotor end portion 76 can be made radially thicker than prior art designs which facilitates easier^' manufacturing and also higher speed operation as well as a lower operating flux density. The axial force versus axial deflection becomes more linearized and axial forces are greatly reduced, thereby reducing the unstable tilt moment that is generated. Axially shorter rotors can be levitated. Because of the large axial surface area of the magnetic bearing and the ferromagnetic pole rings on the rotor end portion 76, a high level of inherent radial damping is generated. Testing of the damping has resulted in values of approximately 0.1%, which facilitates smooth operation. An added damper may be unnecessary in some cases, which simplifies overall construction and reduces costs. The pole rings can protrude from the axial face of the rotor or more preferably for manufacturing, concentric grooves can be cut into the axial rotor face that define the inner and outer diameters of the rotor pole rings.

In an alternate preferred configuration 70 A of the passive radial magnetic bearing of the invention, shown in Fig. 5 A, the magnet pole rings are magnetized concentric annular zones in a continuous block 80 of magnetic material fixed to the stator 72 instead of separate individual magnet rings. The magnet pole rings in this embodiment are separately magnetized zones 84 separated by non-magnetized or low magnetized zones 86 on the block 80. As indicated by the arrows in the magnetized zones 84, the magnet ring zones 84 can be alternately axially polarized on radially adjacent ring zones. This use of a single block of selectively magnetized magnetic material allows for reduced manufacturing and assembly costs but with a possible loss of some radial stiffness. Fig. 6 is a schematic drawing showing how the magnets at the airgap maintain the flux distribution at their faces to produce an elongation of the flux path regardless of the radial displacement of the rotor 71 relative to the stator 72. As shown, the flux is generally evenly distributed across the face of the permanent magnets 73. This causes the flux in the airgap to elongate from radial rotor displacement and hence generates the high radial centering force. The redistribution of flux on the rotor pole rings 74 generates radial damping from eddy currents and hysteresis in the ferromagnetic material of pole rings 74. The pole rings 74 are preferably of the same radial thickness as the permanent magnets or slightly thinner for achieving maximum stiffness. Pole teeth with a significantly larger radial thickness than the permanent magnets would potentially give rise to a dead region where no restoring force is generated when nearly aligned. The passive radial magnetic bearings of the invention are well suited for high speed, full levitation applications and as such can be used for vacuum pumps, alternators and motors. High temperature applications such as turbines are another potential apphcation, provided that the magnets are protected from the high temperature by cooling. A particularly appealing apphcation is for use in energy storage flywheels. Many designs of flywheels exist and could be used with the invention. Fig. 7 is a schematic drawing of one such flywheel energy storage system 90 using two passive radial/axial magnetic bearings and an active magnetic thrust bearing actuator to achieve full levitation. The flywheel system 90 includes a flywheel 91 having a high speed carbon fiber/epoxy composite flywheel rim 92 pressed onto a solid steel hub 93. The flywheel 91 is support axially and radially on upper and lower passive magnetic bearings 94 and 95. The bearings 94 and 95 include stator portions 96 and 99, which have axially polarized permanent magnet rings 97 and 100 attached. The permanent magnet rings 97 and 100 cooperate with pole rings 98 and 101 located on opposite ends of the hub 93, exerting an attractive force on opposite ends of the hub 93 and thereby putting it into axial tension. A stability point, referred to herein as the "the position of metastable axial force equilibrium" exists axially where the upward force exerted by the top passive bearing 98 is equal to the weight of the flywheel 91 plus the downward force exerted by the lower passive bearing 95. The active axial thrust bearing 102 can be used to position the rotor such that zero axial force is exerted by the axial thrust bearing 102, thereby consuming little power and producing little heat. The flywheel system 90 can alternately be designed such that the active axial thrust bearing 102 always carries load. Several types of active axial magnetic thrust bearings exist and could be used in this flywheel system 90. As shown, the active thrust bearing 102 includes a thrust disk 103 attached to an axial shaft 104 attached to or integral with the hub 93. The disk is acted upon by upper and lower electromagnets 105 and 106, which are actuated by coils 107 and 108 driven by a power supply under control of a controller (not shown) known in the art. To accelerate and decelerate the flywheel 91, a brushless motor/generator 109 is also preferably attached to the hub 93. Auxiliary touchdown bearings, not shown, are preferably included to support the flywheel 91 when the active thrust bearing 102 is not operating. An alternate configuration of a flywheel energy storage system 110 using the magnetic bearing system of the invention is shown in Fig. 8. The flywheel system 110 has a composite flywheel rim 111, the bore of which is loaded with multiple permanent magnet pieces 112 at its inner diameter. The magnet pieces are used to produce a strong dipole field across the center for use as a motor/generator in an arrangement known as the Halbach magnet array named after its originator, Klaus Halbach. A stationary motor/generator stator 113 is located in the center of the flywheel and attached to a fixed shaft 114 for support. The flywheel is supported by upper and lower passive radial/axial magnetic bearings 119 and 120. Upper and lower bearing rotor end portions 116 and 118 are attached to the inner diameter of the flywheel rim 111 with use of strain matching assemblies 115 and 117 of known design. The upper and lower bearing rotor end portions 116 and 118 are made of ferromagnetic material, preferably high strength steel, and include ferromagnetic pole rings 125 and 126. These pole rings 125 and 126 cooperate with permanent magnet rings 121 and 123 attached to upper and lower bearing stator portions 122 and 124. In this system design, axial stability is maintained using a permanent magnet biased magnetic thrust actuator 127. The actuator 127 includes a ferromagnetic yoke 128 that surrounds a thrust disk 131 attached to the flywheel shaft 133. The actuator includes upper and lower coils 130 and 129 as well as a radial magnetized permanent magnet 132 for supplying bias flux. The advantage of this actuator is the permanent magnet flux bias with an efficient flux path design allows for very high forces with minimal power consumption. The only drawback however is the one radial airgap that will generate some radial destabilizing forces that must be overcome by the lower radial magnetic bearing 120. A sensor (not shown) detects the axial position of the flywheel relative to the stator and provides the position signal to a controller (not shown) for controlling electrical power from a power supply (not shown) to the coils 129 and 130 to correct the position of the flywheel whenever it deviates from its position of metastable axial force equilibrium. Another flywheel energy storage system 140, shown in Fig. 9, uses another embodiment of the magnetic bearing system of the invention. This embodiment has a combined passive radial and active axial magnetic bearing with an integrated axial actuator. The combined passive radial and active axial magnetic bearing is useful for allowing easier implementation of magnetic bearings for many applications. A levitated object needs only to have pole teeth cut into the axial faces. The illustrated flywheel system 140 uses a low cost composite flywheel 141 having a rim made of multiple material layers including a lower elastic modulus inner layer 143 such as E-glass/epoxy inside a higher elastic modulus outer layer 142 such as carbon fiber/epoxy. The flywheel is supported axially and radially by upper and lower passive axial/radial magnetic bearings 147 and 148. The flywheel 141 is attached to the upper and lower rotor hub end portions 145 and 144 through use of a tubular expansion member 146 that maintains coupling between the rim 142, 143 and the flywheel hub 144-146 as the rim grows radially under centrifugal force at high rotational speeds. The upper and lower rotor hub end portions 145 and 144 are preferably constructed of ferromagnetic material such as high strength steel to withstand the high rotational speeds during operation, and include integral or attached ferromagnetic pole rings 152 and 154 which cooperate with permanent magnet rings 151 and 153 of stationary bearing stator portions 149 and 150. The bearing stator portions are actually combined passive radial and active axial magnetic bearing actuators, as they include upper and lower electromagnets 157 and 159 in the same structure as the passive radial bearings and they operate on the same rotor portions. The electromagnets 157 and 159 include coils 158 and 160, which are used to maintain the flywheel 141 axially at its axial metastable equilibrium point. As shown the electromagnets act on the ferromagnetic rotor hub end portions 145 and 144 rather than on pole rings on the rotor portions, however extra pole rings could be added for this purpose. An axial position sensor 161 detects the axial displacement of the flywheel shaft 162 and produces a signal that is transmitted to a controller 165 which selectively adjusts current to the coils 158 or 160 of the electromagnets 157 and 159 to control the axial position of the flywheel. The controller 165 is powered from a power source 166 which can be provided from the flywheel system. With this design, it is preferable to position the flywheel at the axial metastable equilibrium point to iriinimize electric power consumption by the electromagnets. Because the axial actuators are not permanent magnet flux biased, they will generate a higher level of unstable tilt moments. The moments however become insignificant when the force exerted by the actuators is zero or small, such as when the axial passive forces and flywheel weight balance out. The axial actuator can also be constructed with the electromagnet poles with a smaller diameter than the passive radial bearing magnets. The result is significantly less unstable tilt moment generated by the active axial portion. A constant bias current can also be applied to each coil 158 and 160 to provide response linearity. A brushless motor/generator 164 is preferably used for storing and retrieving energy. For full levitation to be achieved using two sets of passive radial magnetic bearings with one on each axial end of the levitated portion, the axial axis position must be maintained and the design must be such that unstable tilting moments generated by individual radial bearings are overcome by the radial stiffness of the opposite bearing. A single radial bearing will generate an unstable axial force, a stable radial centering force and an unstable tilting moment. The unstable tilting moment of one bearing must be less than stable moment generated by the radial centering force multiplied by the axial length between the bearings. At minimum, the stable tilting moment must be greater than the unstable tilting moment of the other bearing when the radial displacement is equal to its maximum allowable value that still prevents physical contact. Typical values for maxim radial displacements are usually around 0.010 inch, (2.5 x 10^"4) m. Preferably the stable tilting moments of both bearings are greater than the unstable tilting moments of the opposing bearings for all radial displacements. This allows the system to be installed with an axis differing from perfectly vertical, where the radial bearings would also carry some extra loading from gravity acting on the levitated body. One issue that has arisen in the past with use of passive radial magnetic bearings is a phenomenon known as whirl instability. The whirl is believed to be caused by drag forces being produced on the rotating body that are perpendicular to the direction of the given radial displacement. The tangential forces tend to drive the system into a whirl when a certain speed is reached for a given system. We believe that this invention has a lower tendency toward this whirl instability for several reasons, including the minimal flux redistribution during radial displacement of the rotor, and the lower electrical conductivity in the permanent magnet surface of the stationary portion and also the lower flux density in the pole teeth of the rotating portion. However, if whirl instabilities are encountered for a particular system, radial dissymmetry of mechanical properties, such as damping or stiffness, may be provided to ininimize the whirl. The pole rings 74 of Fig. 5 are shown in Fig. 10, with the pole rings divided into four quadrants 172, 173, 174 and 175. One method to provide dissymmetry is to make the radial stiffness vary around the circumference. This can be accomplished, as illustrated in Fig. 10B, by grinding the height of one or several of the pole rings over opposite quadrants such as 172 and 173 to produce taller portions 74t and shallower portions 74s to give different stiffness at different angular positions around the circumference of the bearing. The variation in the height of the pole rings 74 could also be varied smoothly or sinusoidally instead of abruptly as shown. The dissymmetry of magnetic properties occurs on the rotating portion in the invention rather than adding magnetic dissymmetry to the stator portion. A second method is to provide dissymmetry in the radial damping. This can be accomplished by putting electrically conductive, non-ferromagnetic inlays such as copper radially between poles over adjacent quadrants. Inlays could also be provided between the stationary ring magnets but with a lesser effect. A third method is to make one or more pole rings thicker in opposing quadrants to provide a partial "dead zone" of reduced stiffness in those quadrants. Grinding the heights of the permanent magnets could also be done but this would increase tangential drag. Other methods exist and could be employed but probably not with as much ease as suggested.

The control of the magnetic bearing system can be done with analog circuits but is preferably done with digital circuitry for repeatable manufacturing without tuning and lower cost. Fig. 11 shows one possible control method 190, for a system using combined actuators and using a single amplifier. A digital signal processor, DSP 191 takes the input position signal 192 and calculates the correct output response voltage and polarity, which is sent to an amplifier 193. The amplifier 193 outputs voltage to the top and bottom coils 196 and 197. Diodes 198, 199 are used so that negative amplifier output voltage turns on the bottom coil 197, and positive output voltage turns on the upper coil 196. The diodes will cause a deadband where no coil is activated when the amplifier 193 output is between roughly -0.7 volt and 0.7 volt. Therefore, the DSP 191 can be programmed to skip that region. The DSP 191 is typically powered by a low voltage supply 200 and the amplifier 193 uses a high voltage supply 201. Both can be powered by converted power from the motor/generator output if desired. The control preferably operates such it initially pulls the rotor to the axial center when switched on. The DSP 191 then continually activates/adjusts the coils 196, 197 so as to position the rotor where it is in axial equilibrium and hence essentially no current is required for levitation.

An alternative method of control 210 is shown in Fig. 12 where two output amplifiers are used. A DSP 211 takes the input position signal 212 from the axial position sensor. The DSP 211 then outputs selectively to either one of two separate amplifiers 213 and 214, which separately control the top and bottom coils 215, 216. The DSP 211 operates from a low voltage supply 217 and the amplifiers from a high voltage supply 218. The control of the magnetic bearing system is again nonlinear. It can be made linear by applying a continuous amplifier offset current to the coils 215, 216 to create bias flux in both actuators. Other methods for axial control can be employed including other actuators and other electrical circuits.

Obviously, numerous modifications and variations of the preferred embodiment described above are possible and will become apparent to those skilled in the art in light of this specification. Moreover, many functions and advantages are described for the preferred embodiment, but in many uses of the invention, not all of these functions and advantages would be needed. Therefore, we contemplate the use of the invention using fewer than the complete set of noted features, benefits, functions and advantages. Moreover, several species and embodiments of the invention are disclosed herein, but not all are specifically claimed, although all are covered by generic claims. Nevertheless, it is our intention that each and every one of these species and embodiments, and the equivalents thereof, be encompassed and protected within the scope of the following claims, and no dedication to the public is intended by virtue of the lack of claims specific to any individual species. Accordingly, it is expressly intended that all these embodiments, species, modifications and variations, and the equivalents thereof, are to be considered within the spirit and scope of the invention as defined in the following claims, wherein we claim:

Claims

1. A full levitation magnetic bearing system for support of a rotating body about an axis of rotation, said bearing system having passive radial magnetic bearings that generate passive radial centering forces to counteract displacements for said rotating body from said axis of rotation during rotation, and having axial stabilization of said rotating body by an axial magnetic force generating actuator, wherein: said passive radial magnetic bearings consist of a stationary portion and a rotating portion; said stationary portion includes multiple concentric, radially spaced apart, axially magnetized permanent magnet rings attached to a fixed stator body and having free axially facing surfaces; said rotating portion includes a ferromagnetic section an axial end face and with magnetically cooperating concentric pole rings on said axial end face that are axially aligned with said magnet rings when said rotating portion is radially centered on said stationary portion; said concentric pole rings having axially facing surfaces axially spaced apart from said axially facing surfaces of said magnet rings and defining therebetween an axial airgap; whereby, said magnetic flux from said permanent magnet rings passes directly across said airgap and through said rotating ferromagnetic section thereby generating both an axial attractive force and a passive radial centering force from said pole rings tending to align with said stationary permanent magnet rings.

2. A full levitation magnetic bearing system as defined in claim 1, wherein said permanent magnet rings have axially alternating polarities on radially adjacent magnet rings.

3. A full levitation magnetic bearing system as defined in claim 2 wherein the number of said permanent magnetic rings acting on a single axial surface of said rotating portion exceeds two.

4. A full levitation magnetic bearing system as defined in claim 1 wherein said fixed stator body is constructed of ferromagnetic material where said permanent magnets are attached.

5. A full levitation magnetic bearing system as defined in claim 1 wherein said pole rings in said ferromagnetic rotating portion are formed by cutting concentric, radially spaced axial grooves in said axial end face of said rotating portion, leaving raised portions between said grooves to comprise said pole rings.

6. A full levitation magnetic bearing system as defined in claim 1 wherein the bearing system is used for support of an energy storage flywheel operating in a vacuum.

7. A full levitation magnetic bearing system as defined in claim 1, wherein: said bearing system includes a first set of said passive radial magnetic bearings at an axial upper end of said body, and a second set of said passive magnetic bearings at an opposite axial end of said body; said radial magnetic bearing set on one end of said rotating body generates a radial centering force equal to R Newtons/meter when displaced radially from the centerline position by an amount M meters, and said radial magnetic bearing set on said opposite end simultaneously generates a tilting moment equal to X Newton-meters; wherein X is less than R multiplied by the axial length between the radial bearings in meters when M is equal to (2.5 x 10^"4) m.

8. A full levitation magnetic bearing system as defined in claim 1 wherein: the radial magnetic bearing set on one end of said rotating body generates a radial centering force equal to R Newtons/meter when displaced radially from the centerline by an amount M meters, and the radial magnetic bearing set on the other end simultaneously generates a tilting moment equal to X Newton-meters; wherein X is less than R multiplied by the axial length between the radial bearings in meters for all values of M between zero and the maximum possible value allowable by physical constraints of contact.

9. A full levitation magnetic bearing system as defined in claim 1 wherein said axial airgap is free of ferromagnetic material.

10. A full levitation magnetic bearing system as defined in claim 1 wherein said rotating body has a center of mass , and said magnetic bearing system includes an upper set of passive radial magnetic bearings located axially above said center of mass and a lower set of passive radial magnetic bearings located axially below said center of mass of said rotating body.

11. A full levitation magnetic bearing system as defined in claim 10 wherein said passive radial magnetic bearings put said rotating body into axial tension.

12. A full levitation magnetic bearing system as defined in claim 10 wherein: said rotating body has a position of metastable axial force equilibrium at which upward forces exerted by said upper set of said passive bearings equals the weight of said rotating body plus downward forces exerted by said lower set of passive bearings; and said bearing system maintains said rotating body near said position of metastable axial force equilibrium.

13. A full levitation magnetic bearing system as defined in claim 1 wherein said permanent magnet rings are constructed from multiple individual arc segments.

14. Multiple individual arc segments as defined in claim 13 wherein the arc segments are aligned concentrically by placing them in grooves cut into said fixed stator body.

15. A passive radial magnetic bearing comprising a stationary portion and a rotating portion, said stationary portion includes multiple concentric, radially spaced apart, axially magnetized permanent magnet rings attached to an axially facing surface on a fixed stator body; said rotating portion includes a ferromagnetic section having an axial surface, with magnetically cooperating concentric pole rings on said axial surface, said axial surface of said rotating portion being free of magnets; whereby magnetic flux from said permanent magnet rings passes through said rotating ferromagnetic section, thereby generating both an axial attractive force and a passive radial centering force from said pole rings tending to align with said stationary permanent magnet rings.

16. A full levitation magnetic bearing system as described in claim 15 wherein said fixed stator body constructed of ferromagnetic material where said permanent magnets are attached.

17. A passive radial magnetic bearing as defined in claim 15 wherein said axially magnetized permanent magnet rings have axially alternating polarities on radially adjacent magnet rings.

18. A passive radial magnetic bearing as defined in claim 17 wherein said stationary portion has at least four of said permanent magnetic rings acting on a single axial surface of said rotating portion.

19. A passive radial magnetic bearing as defined in claim 18 wherein an axial gap defined between said permanent magnets of said stationary portion and said ferromagnetic section of said rotating portion is free of ferromagnetic material.

20. A passive radial magnetic bearing as defined in claim 15 wherein said bearing has radial dissymmetry of mechanical properties.

21. A passive radial magnetic bearing as defined in claim 20 wherein damping in two orthogonal radial directions is different.

22. A passive radial magnetic bearing as defined in claim 20 wherein the stiffness in two orthogonal radial directions is different.

23. A passive radial magnetic bearing as defined in claim 22 wherein said different radial stiffness is achieved by varying the axial height of at least one of said ferromagnetic poles around its circumference.

24. A passive radial magnetic bearing as defined in claim 15, further comprising: a thin shim in said airgap attached to said magnet rings or said rotor pole rings.

25. A passive radial magnetic bearing with inherent radial dissymmetry that is the result of a uniform characteristic stationary portion that cooperates with a rotating portion that has characteristics that varies with circumferential position.

26. A magnetic bearing system for full levitation of a rotating body using active axial thrust bearings at opposing ends of said body wherein the thrust bearings apply forces in opposite directions and are driven independently through use of diodes that direct positive voltage to one thrust bearing and negative voltage to the other.

27. A magnetic bearing for suspension of a rotating body, comprising: a stator portion having structure supporting permanent magnet rings that impart passive radial, passive axial and active axial forces to said rotating body, an electromagnetic coil with at least two ferromagnetic pole rings for generating said active axial force, said ferromagnetic pole rings are constructed in the same structure as the stationary portion of the passive radial force generating means.

28. A full levitation magnetic bearing system for support of a rotating body about an axis of rotation, said bearing system having passive radial magnetic bearings that generate passive radial centering forces to counteract displacements for said rotating body from said axis of rotation during rotation, and having axial stabilization of said rotating body by an axial magnetic force generating actuator, wherein: said passive radial magnetic bearings includea stationary portion and a rotating portion; said stationary portion includes fixed stator body having attached thereto a permanent magnet with multiple concentric, radially spaced apart, axially magnetized ring portions and a free axially facing surface; said rotating portion includes a ferromagnetic section having an axial end face and with magnetically cooperating concentric pole rings on said axial end face that are axially aligned with said magnetized ring portions when said rotating portion is radially centered on said stationary portion; said concentric pole rings having axially facing surfaces axially spaced apart from said axially facing surface of said magnet and defining therebetween an axial airgap; whereby, said magnetic flux from said permanent magnet magnetized ring portions passes directly across said airgap and through said rotating ferromagnetic section thereby generating both an axial attractive force and a passive radial centering force from said pole rings tending to align with said stationary magnetized ring portions.

29. A passive radial magnetic bearing comprising: a stationary portion and a rotating portion; said stationary portion includes fixed stator body having attached thereto a permanent magnet with multiple concentric, radially spaced apart, axially magnetized ring portions; said rotating portion includes a ferromagnetic section having an axial surface and with magnetically cooperating concentric pole rings on said axial surface, whereby magnetic flux from said permanent magnet magnetized ring portions passes through said rotating ferromagnetic section, thereby generating both an axial attractive force and a passive radial centering force from said pole rings tending to align with said stationary magnetized ring portions.