Abstract:
A magnetic thrust bearing having a high speed rotation capability and low cost construction. The magnetic thrust bearing has permanent magnets to provide bias flux. The magnetic circuits of the control flux and bias fluxes are substantially non-coincident, which allows for a low reluctance and efficient path for the control flux. The flux paths of the permanent magnets are completely defined with minimized airgaps for achieving higher forces and efficiency and very low control currents produce extremely large forces. No radially magnetized permanent magnets are required and no permanent magnets are attached to the rotor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a magnetic thrust bearing and more particularly a magnetic thrust bearing that uses permanent bias flux with a simplified construction to allow for highly efficient force generation, high speed rotation capability and low cost o construction.  
           [0003]    2. Description of Related Art  
           [0004]    Magnetic thrust bearings were originally constructed by using a single ferromagnetic disk attached to a rotating shaft. The thrust disk is then acted upon by electromagnets with a C-shaped cross-section located above and below the disk. This offers a very simple and low cost construction but has a very low efficiency along with requiring complex nonlinear control.  
           [0005]    The next advancement uses the same mechanical construction, however the electronics employs a large constant current to each coil to generate a bias flux. A small control current is added on top of the bias currents to control the bearing. The result of using a bias flux is simplified control because the relationship of force to control current becomes linearized. Linearization is provided because the force is proportional to the square of the flux density. This functions by adding the control flux to one coil&#39;s bias flux and at the same time subtracting the same control flux from the other. The force generated is then directly related to the difference of the squares of the net top and net bottom fluxes, and this varies linearly with control current. The drawback of this bearing configuration is the steady-state electrical inefficiency from having to electrically maintain the bias currents. FIG. 1 shows the configuration  30 . The thrust disk  32  is attached to the shaft  31  and acted upon by an upper C-core ring  34  and a lower C-core ring  33 . An upper coil  36  and lower coil  35  are used to generate magnetic flux. A bias current is applied to each coil  35 ,  36  to generate bias fluxes  37  and  38 . A control current is then applied in superposition to the bias currents in each coil  35 ,  36  which generates control fluxes  39  and  40 . In FIG. 1, the upper control and bias fluxes  40 ,  38  add and the lower control and bias fluxes  39 ,  37  subtract. The net result is force exerted upward on the disk  32  that varies linearly with the control current. A non-dimensionalized example on the linearization is as follows. If the bias fluxes have an arbitrary value of  5  and a control flux is superposed with a value of 1, the flux on the top side of the disk becomes 6 and on the bottom side becomes 4. The net force is then (6^ 2−4^ 2) or 20. Because of the bias flux, the relation of force to control current becomes both linearized and amplified. With a control flux of 2, the resulting force would then be double, 40. Without the bias flux, control flux would only be applied to one core at a time to generate force and a control flux of 1 and 2 would result in forces of only 2 and 4. Two amplifiers would also be required for operation.  
           [0006]    An improved design of magnetic thrust bearings places permanent magnets in series with the electromagnets so that the bias flux is generated without use of electric power. U.S. Pat. Nos. 3,937,148 and 5,003,211 show variations using this concept. This design increases the steady-state electrical efficiency, however the permeability of high energy permanent magnets is very low. Therefore, the electromagnets require much more control current to generate the same control flux because of the higher reluctance of the magnetic circuits. FIG. 2 shows the configuration  50 . The thrust disk  52  is attached to the shaft  51  and is acted upon by an upper C-core ring  54  and a lower C-core ring  53 . Permanent magnets  57  and  58  generate the bias fluxes  59  and  60 . Opposed control currents in coils  55  and  56  generate the control fluxes  61  and  62 . As before, the control and bias fluxes are additive in one core  54  and subtractive in the other  53 , resulting in an upward force on the disk  32 . Unfortunately, magnets  57  and  58  have permeability comparable to an airgap. Therefore, the required control current to generate the equivalent control fluxes  61  and  62  as in the configuration  30  control fluxes  39  and  40  is much higher.  
           [0007]    A further improvement is to use permanent magnets for generating bias flux but the permanent magnet flux paths are made non-coincident with the path of the electromagnet flux. The permanent magnets are not in series with the electromagnets but instead share only a portion of the same paths that include the airgaps. The result is a greatly improved design that allows for both linear and highly efficient control. U.S. Pat. No. 3,890,019 is one configuration and this is shown in FIG. 3. The thrust disk  72  is attached to the shaft  71  and is acted upon by a single external C-core yoke ring  73 . A single coil  74  is used to generate the control flux  79 . Permanent magnets  75  and  76  generate the bias fluxes  77  and  78 . Superposition of the control and bias fluxes  79 , 77 , 78  cause an upward force on the disk  72 . The only drawback with this configuration is that  1402782  it does not achieve the highest possible force capability or efficiency because of ill-defined large airgaps in the permanent magnetic flux paths  77  and  78 .  
           [0008]    U.S. Pat. No. 3,865,442 is a more efficient design using the same concept of non-coincident control and bias flux paths. FIG. 4 shows the configuration  130 . Three thrust disks  132 ,  133 ,  134  are attached to the shaft  131 . The thrust disks  132 ,  133 ,  134  are acted upon by a single external C-core ring  135  with a control coil  136  for producing control flux  141 . Permanent magnets  137  and  138  attached to the shaft  131  generate the bias fluxes  139  and  140 . The drawbacks of this design are the use of rotating permanent magnets, which limit the high speed rotation capability due to their low strength, and the complexity. The use of three thrust disks is also undesirable.  
           [0009]    U.S. Pat. No. 3,955,858 discloses an improved thrust bearing design in which the permanent magnet is stationery. The configuration  90  is shown in FIG. 5. Two thrust disks  92  and  93  are attached to the shaft  91  and are acted upon by stator rings  94  and  95 . A radially magnetized permanent magnet  96  generates the bias flux  99 . The control flux  100  is generated by the control coils  97  and  98 . As shown, superposition of the fluxes results in an upward force on the disks  92  and  93 . The design unfortunately has a more complicated than desired construction, including a radially magnetized permanent magnet and two thrust disks. U.S. Pat. No. 5,315,197 describes the same configuration but also discloses a modified version, allowing for use of only one thrust disk. The drawback to this design is the inclusion of a radial airgap in the magnetic circuit. The radial airgap causes generation of radially destabilizing forces. A similar configuration, U.S. Pat. No. 5,514,924, adds multiple radial control coils to the same design.  
           [0010]    U.S. Pat. No. 5,250,865 shows further improved thrust bearing configuration by only requiring one thrust disk and all permanent magnets are stationery. Unfortunately, the invention is complicated and requires use of four permanent magnets with eight airgaps. The bearing also requires assembly of multiple precision pieces for generation of the five flux paths.  
           [0011]    More recently, U.S. Pat. No. 5,804,899 discloses a magnetic bearing with a biased thrust actuator. This invention is same thrust bearing as disclosed in U.S. Pat. No. 5,317,197 but only with a large structure added and some separate permanent providing some radial centering force. A radially magnetized permanent magnet and two thrust disk portions are again required.  
           [0012]    There still exists a need for a high force, high efficiency magnetic thrust bearing that can allow for high speed rotation and also has a simple, low cost construction  
         SUMMARY OF THE INVENTION  
         [0013]    The invention is an improved magnetic thrust bearing that uses permanent magnets to provide bias flux. The magnetic circuits of the control flux and bias fluxes are substantially non-coincident but they do share the same path over some portions which include axial airgaps. This allows for a low reluctance and efficient path for the electromagnets flux. The flux paths of the permanent magnets are completely defined with minimized airgaps for achieving higher forces and efficiency and very low control currents produce extremely large forces. The design uses a single coil and amplifier for simplicity and only a single thrust disk is required. Likewise, no radially magnetized permanent magnets are required and no permanent magnets are attached to the rotor that would require reinforcement.  
           [0014]    Specifically the present invention is an electromagnetic bearing for a thrust member having a distal region extending outwardly from a support comprising: at least one ferrous member, such as an upper and lower yoke having a coil, the ferrous member straddles the distal region of the thrust member, confronting surfaces of at least two extrusions of the ferrous member and thrust member defining control flux air gaps on opposite sides of the thrust member, and generating an electromagnetic control flux path through the air gaps whereby to axially position the ferrous member relative to the thrust member; confronting surfaces of at least one permanent magnet and either the thrust member or the ferrous member, defining at least one magnetic air gap spaced from at least one of the control flux air gaps, and generating a bias flux path parallel and non-coincident with the control flux path for a substantial portion of its length, wherein the permanent magnet is outside the control flux path and the length of each air gap in said bearing is limited to the physical separation of the confronting surfaces.  
           [0015]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and accompanying drawings 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic of a prior art magnetic bearing configuration using bias flux generated from electromagnets;  
         [0017]    [0017]FIG. 2 is a schematic of a prior art magnetic bearing configuration using permanent magnets in series with electromagnets for generating bias flux;  
         [0018]    [0018]FIG. 3 is a schematic of a prior art magnetic bearing using permanent magnets for generation of bias flux with a non-coincident electromagnetic flux path and ill-defined permanent magnet flux paths;  
         [0019]    [0019]FIG. 4 is a schematic of a prior art magnetic bearing using permanent magnets for generation of bias flux with a non-coincident electromagnetic flux path and three rotating ferromagnetic thrust disks;  
         [0020]    [0020]FIG. 5 is a schematic of a prior art magnetic bearing using permanent magnets for generation of bias flux with a non-coincident electromagnetic flux path and two rotating ferromagnetic thrust disks;  
         [0021]    [0021]FIG. 6 is a schematic of a magnetic bearing of the present invention having a single rotating ferromagnetic thrust disk;  
         [0022]    [0022]FIG. 7 is a schematic of a magnetic bearing of the present invention including rotating permanent magnets;  
         [0023]    [0023]FIG. 8 is a schematic of a magnetic bearing as in FIG. 6 having reversed positions of the permanent magnet rings and ferromagnetic pole rings;  
         [0024]    [0024]FIG. 9 is a schematic of a magnetic bearing of the present invention showing a thrust disk or member with raised poles or centering extrusions, which provide passive radial centering;  
         [0025]    [0025]FIG. 10 is a schematic of a magnetic bearing of the present invention showing a thrust disk or member with circumferential grooves, which provide passive radial centering;  
         [0026]    [0026]FIG. 11 is a schematic of a magnetic bearing of the present invention showing passive radial centering from teeth;  
         [0027]    [0027]FIG. 12 is a schematic of a magnetic bearing invention alternate configuration with reversed radial positions of the rotor and stator;  
         [0028]    [0028]FIG. 13 is a schematic of a magnetic bearing invention alternate configuration with single permanent magnet ring; and  
         [0029]    [0029]FIG. 14 is a schematic of a magnetic bearing invention alternate configuration for use as a translation bearing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Turning to the drawings wherein like characters designate identical or corresponding parts, FIG. 6 shows a preferred configuration of the magnetic thrust bearing  10 . A thrust disk  111  is attached to the shaft  112  and is acted upon by a lower yoke ring  113  and an upper yoke ring  114 . A single coil  115  provides the control flux  122 . An upper permanent magnet ring  116  provides the upper bias flux  118  and the lower permanent ring  117  provides the lower bias flux  119 . The magnets  116  and  117  as well as the yokes  113  and  114  could be made as only a segment and not a complete ring. However, this would cause eddy currents in the thrust disk  111  during rotation so it is not desirable for rotating shaft applications.  
         [0031]    The control flux  122  and bias fluxes  118  and  119  have substantially non-coincident paths but do share the same paths through the pole rings  120  and  121  and in the airgaps to the thrust disk  111 . By superposition, as shown in FIG. 6, the control and upper bias flux  122 ,  118  subtract and the control and lower bias flux  122 ,  119  add. The result is a net force downward on the thrust disk  111 . Reversing the current in the control coil  115 , results in a net force upward. Because of the bias fluxes  118  and  119 , the force generated is linear with the current in control coil  115 . The force is also greatly amplified. The invention uses small airgaps in the bias flux paths and in the control flux path.  
         [0032]    The low permeability permanent magnets are also not included in the electromagnetic flux path  122 . Therefore, the bearing achieves maximum efficiency of force to applied control current. The bearing  110  works according to the following non-dimensionalized example. If the bias flux is  12  and a control flux of  8  is applied, the flux in the upper steel pole and airgap would be  20  and the flux in the lower steel pole and airgap would be  4 . The force is proportion to the square of the flux density. Therefore the upward force exerted on the thrust disk would be (20^ 2−4^ 2) or 384. The force is linear with control current. However, if a control current is applied such that its flux becomes larger than the bias flux, the combined flux on one side of the thrust disk will become negative and hence start to attract the disk again. Designs with permanent magnets in series with the electromagnet such as FIG. 2 would be significantly less due to inability to generate large control fluxes. Likewise, designs like FIG. 3 with large airgaps in the bias flux path would also have significantly lower force per control current due to less bias flux. Accordingly with the invention, a digital signal processor and single amplifier, not shown, can provide control or an analog circuit could be employed. An axial position sensor such as an inductive proximity sensor, not shown, can provide feedback.  
         [0033]    [0033]FIG. 7 shows a modified version of the invention. The thrust disk  151  is attached to the shaft  152  and is acted upon by the upper yoke ring  154  and lower yoke ring  153 . The control coil  155  provides the control flux  162 . The permanent magnets  156  and  157  for producing bias fluxes  158  and  159  are attached to the thrust disk  151 . The operation of the bearing  150  is the same as before with superposition of the control and bias fluxes  162 ,  158  and  159 . This design is however less favorable due to the rotating permanent magnets  156  and  157 , which have low tensile strength.  
         [0034]    [0034]FIG. 8 shows an alternate version of the invention in which the placement of the permanent magnet rings and the steel pole rings are switched. In this configuration  170 , o the thrust disk  171  is attached to the shaft  172  and is acted upon by upper and lower yoke rings  173  and  174 . A control coil  175  generates the control flux  180 . Permanent magnet rings  176  and  177  generate the bias fluxes  178  and  179 .  
         [0035]    [0035]FIG. 9 shows an alternate version of the invention that provides passive radial centering force. In this configuration  190 , the thrust disk  191  is attached to the shaft  192  and is again acted upon by the upper and lower ferromagnetic yoke rings  193  and  194 . Permanent magnets  196  and  197  provide bias flux. To achieve a passive radial centering force, the thrust disk contains raised centering rings that line up with the steel yoke pole rings  198  and  199  and the permanent magnet rings  196  and  197 . The rings will attempt to stay lined up as this is the position of minimum reluctance in the magnetic paths.  
         [0036]    [0036]FIG. 10 shows a modified version of the design depicted in FIG. 9. In this configuration  210 , the thrust disk  211  is attached to the shaft  212  and is acted upon by upper and lower yoke rings  213  and  214 . The control coil  215  provides control flux and the permanent magnet rings  216  and  217  provide bias flux. In this design passive radial centering is achieved by cutting grooves  220  in to the thrust disk  211 .  
         [0037]    [0037]FIG. 11 shows an alternate version of the invention for providing maximum passive radial centering. In this configuration  230 , the thrust disk  231  is attached to the shaft  232  and is acted upon by the upper and lower yoke rings  233  and  234 . The control coil  235  provides control flux and the permanent magnets  236  and  237  provide the bias flux. Maximum passive radial centering force is achieved by cutting multiple teeth  240  into the thrust disk  231 . These teeth try to stay aligned with matching teeth on the steel pole rings  238  and  239  and on magnet cap pieces  243  and  244 . The teeth could alternately be cut into the magnet faces but this may result in cracking of the magnets due to being brittle.  
         [0038]    [0038]FIG. 12 shows an alternate version of the same invention in which the stator portion is located in the center. In this configuration  250 , the thrust disk  252  with central hole is attached to the rotating tube  251 . The disk  252  is acted upon by upper and lower yokes  254  and  253 . A control coil  257  is wound around the central ferrous shaft  258  which acts as a yoke. Permanent magnet rings  255  and  256  provide the bias flux. Yoke pole rings  259  and  260  provide paths for the combined control and bias fluxes.  
         [0039]    [0039]FIG. 13 shows a version of the invention for providing primarily more force in one direction. In this configuration  280 , the thrust disk  281  is attached to the shaft  282  and is acted upon by upper and lower yokes  284  and  283 . A control coil  285  provides the control flux. An upper ring magnet  286  provides upper bias flux. The bearing  280  can exert forces both upward and downward but the maximum upward force capability is higher. Such a design may be useful in some applications to reduce size, cost or weight. Opposite actuators could also be used on opposite ends of the shaft  282 .  
         [0040]    [0040]FIG. 14 shows an alternate version of the invention for use in linear bearing applications. In this configuration  270 , the basic principle and design is the same except modified for linear motion. A linear ferromagnetic track  271  is fixed and the upper and lower yokes  272  and  273  move. A control coil  274  is wound around the ferrous member  272 , 273  and provides control flux and permanent magnet bars or cubes  275  and  276  provide the bias flux. The yoke poles  277  and  278  provide the path for the combined control and bias fluxes. Such a design could be useful for conveyors or magnetic levitation trains. The permanent magnets could also be replaced with superconductor magnets for generation of very high forces if required.  
         [0041]    The following references are incorporated herein by reference: U.S. Pat. Nos. 3,937,148 and 5,003,211; U.S. Pat. No. 3,890,019; U.S. Pat. No. 3,865,442; U.S. Pat. No. 3,955,858; U.S. Pat. No. 5,315,197; U.S. Pat. No. 5,514,924; U.S. Pat. No. 5,250,865; and U.S. Pat. No. 5,804,899.  
         [0042]    Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity and understanding, it will be obvious that various modifications and changes which are within the knowledge of those skilled in the art are considered to fall within the scope of the appended claims.