Abstract:
An electromagnetic actuator includes a body and first and second poles residing apart from the body. The first and second poles communicate magnetic flux across a gap with opposing end facing surfaces of the body. The body, the first pole, and the second pole are magnetically coupled and define an axial magnetic control circuit. A plurality of radial poles reside apart from the body, adjacent a lateral facing surface of the body, and communicate magnetic fluxes with the lateral facing surface. The body and the plurality of radial poles define a plurality of radial magnetic control circuits. The plurality of radial poles communicate magnetic fluxes with the lateral facing surface and at least one of the first pole or the second pole, and the body, the plurality of radial poles, and at least one of the first pole or the second pole define a magnetic bias circuit.

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 U.S.C. §119 to U.S. Provisional Patent Application Ser. No. 61/292,746, filed on Jan. 6, 2010, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to generating electromagnetic forces through an electromagnetic actuator, and, more particularly, to generating radial and axial electromagnetic forces using a combination radial/axial electromagnetic actuator with an improved axial bandwidth. 
     BACKGROUND 
     Equipment and machinery often contain moving (e.g., rotating, translating) members, which require support during operation. A bearing, or similar device, may be used to support the moving member. Although many types of bearings require direct contact with the member to provide the necessary support, some applications benefit from non-contact, or nearly non-contact support for the member. A magnetic bearing uses an electromagnetic actuator to apply a controlled electromagnetic force to support the moving member in a non-contact, or nearly non-contact, manner. The non-contact or nearly non-contact support provided by the magnetic bearing can provide frictionless or nearly frictionless movement of the member in both the axial and radial directions. 
     SUMMARY 
     In certain implementations, an electromagnetic actuator may include a body with a rotational axis. A first pole may reside apart from the body, the first pole may be adjacent a first end facing surface of the body and adapted to communicate magnetic flux across a gap with the first end facing surface of the body. A second pole may reside apart from the body, the second pole may be adjacent a second end facing surface of the body and adapted to communicate magnetic flux with the second end facing surface of the body. The body, the first pole, and the second pole may be magnetically coupled and define an axial magnetic control circuit. A plurality of radial poles may reside apart from the body, the plurality of radial poles adjacent a lateral facing surface of the body and adapted to communicate magnetic fluxes with the lateral facing surface of the body. The body and the plurality of radial poles define a plurality of radial magnetic control circuits, the plurality of radial poles adapted to communicate magnetic fluxes with the lateral facing surface of the body and at least one of the first pole or the second pole. The body, the plurality of radial poles, and at least one of the first pole or the second pole define a magnetic bias circuit. 
     In certain implementations, an electric machine system includes a stator and a rotor, the rotor having a rotational axis configured to move relative to the stator. The system may also include an electromagnetic actuator subassembly that includes a cylindrical actuator target rigidly mounted on the rotor. A first axial pole may reside apart from the actuator target, the first axial pole adjacent a first end facing surface of the actuator target and adapted to communicate magnetic flux across a gap with the first end facing surface of the actuator target. A second axial pole residing apart from the actuator target, the second axial pole adjacent a second end facing surface of the actuator target and adapted to communicate magnetic flux with the second end facing surface of the actuator target. An axial back-iron may magnetically link the first axial pole and the second axial pole. The actuator target, the first axial pole, the second axial pole and the axial back-iron may be magnetically coupled and define an axial magnetic control circuit. An axial control conductive coil may be adapted to produce a magnetic flux in the axial magnetic control circuit. A magnetically permeable annual element located concentric to the rotational axis and including a plurality of radial poles and an electrically isolating radial gap interrupting a conductive path around the rotational axis, the magnetically permeable annual element including a plurality of radial poles residing apart from the actuator target, the plurality of radial poles adjacent a lateral facing surface of the actuator target and adapted to communicate magnetic fluxes with the lateral facing surface of the actuator target, the actuator target and the plurality of radial poles defining a plurality of radial magnetic control circuits, the plurality of radial poles adapted to communicate magnetic fluxes with the lateral facing surface of the actuator target and at least one of the first axial pole or the second axial pole, the actuator target, the plurality of radial poles and at least one of the first axial pole or the second axial pole defining a magnetic bias circuit. Radial control conductive coils may be wound around the radial poles and adapted to produce a magnetic flux in the radial magnetic control circuit. One or more position sensors may be configured to sense a position of the actuator target. At least one control electronics package may be configured to control the electric currents in the axial control conductive coil and radial control conductive coils. 
     In certain implementations, a method for exerting axial and radial forces on a cylindrical body having a symmetry axis includes communicating a first bias magnetic flux through a first axial facing surface of the body. The method may also include communicating a second bias magnetic flux through a second axial facing surface of the body. The method may also include communicating combined the first and the second bias magnetic fluxes through a lateral surface of the cylindrical body. In addition, the method may include communicating an axial control magnetic flux through the first and the second axial facing surfaces of the body. Communicating a radial control magnetic flux diagonally across the body may be accomplished by a stationary radial pole assembly located around the body and separated from it. Electrical currents may be suppressed in the stationary radial pole assembly by introducing isolating interrupts of the conductive paths. 
     In certain implementations, the end facing surface of the body is orthogonal to the rotational axis. In some implementations, the body incorporates a magnetically permeable actuator target, the actuator target adapted to communicate a magnetic flux. 
     In some embodiments, a magnetic element may be configured to produce magnetic bias flux in the magnetic bias circuit. An axial coil may be adapted to produce a magnetic flux in the axial magnetic control circuit and a plurality of radial coils adapted to produce magnetic fluxes in the plurality of radial magnetic control circuits. 
     In certain implementations, the magnetic flux entering the end facing surface of the body exerts an axial force on the body and the magnetic fluxes entering the lateral surface of the body exert radial forces on the body. In certain instances, the axial force is proportional to the magnetic flux in the axial magnetic control circuit and the radial forces are proportional to the magnetic fluxes in the radial magnetic control circuits. 
     In implementations, the plurality of radial poles is defined by a first annular lamination and a second annular lamination, the first and second annular laminations defining an annular lamination stack coaxial to the rotational axis. In some instances, the first and the second annular laminations comprise a magnetically permeable material. In certain implementations, the first and the second annular laminations are electrically isolated from each other. The first annular lamination may be a first disjointed annular element defining a first air gap between disjoined segments of the annular element and the second annular lamination may be a second disjointed annular element defining a second air gap between disjoined segments of the second annular element. The first air gap may reside misaligned from the second air gap in the annular lamination stack. 
     In certain embodiments, the rotor may be coupled to a driven load, the driven load comprising at least one of a flywheel, a generator, or an expander. The rotor may be coupled to a driver, the driver comprising at least one of a motor, an engine, or a turbine. 
     The magnetic fluxes exert electromagnetic forces on the actuator target. The electronic control package is further configured to energize the control coils around each of the plurality of radial poles with control currents in response to changes of signals from the position sensors so that the rotor is supported by electromagnetic forces without a mechanical contact with the stator. 
     In certain instances, the stationary radial pole assembly may be composed of magnetically-permeable laminations made of electrical steel stacked together along the body symmetry axis. The isolating interrupts may be introduced in each lamination. In certain instances, circumferential locations of the insolating interrupts may vary from lamination to lamination across the lamination stack. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partial cross-sectional schematic of an electromagnetic actuator in accordance with the present disclosure and illustrates generating an axial force. 
         FIG. 2  is a radial sectional schematic of an electromagnetic actuator in accordance with the present disclosure and illustrates generating a radial force. 
         FIG. 3  is an example schematic of a current induced in a radial control pole assembly during production of a time-varying axial control force in accordance with the present disclosure. 
         FIG. 4  is a cross-sectional schematic of a radial control pole assembly in accordance with the present disclosure. 
         FIG. 5  is a cross-sectional schematic of stacked laminations for radial poles of a magnetic actuator in accordance with the present disclosure. 
         FIG. 6  is a cross-sectional schematic of an example of an AMB system in an electric rotational machine. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to generating electromagnetic forces through an electromagnetic actuator and, more particularly, to generating radial and axial electromagnetic forces through a combination radial/axial electromagnetic actuator with an improved axial bandwidth. 
     Permanent-Magnet-Biased Homopolar Combination Axial/Radial Electromagnetic Actuators offer advantages over arrangements of separate radial and axial actuators including smaller part count, smaller size and weight, and shorter axial length. One of the important applications of such an actuator is in Active Magnetic Bearings (AMBs) providing non-contact support of objects using electromagnetic forces. In particular, when an AMB system is used in rotating machinery, the combination actuator allows achieving better rotordynamic response due to a more compact design than a combination of separate radial and axial actuators. However, the axial channel of a combination actuator may exhibit lower bandwidth characteristics as compared to a dedicated axial actuator. This may complicate the axial control of an AMB system and degrades its performance. In conventional axial electromagnetic actuators, the bandwidth limitation is caused by eddy currents induced in the components of the axial control magnetic circuit, which are made metallic for practical reasons, when an alternating axial control current is applied. These currents result in both amplitude attenuation and a phase lag of the magnetic control flux, which subsequently affect the control force. In addition, in the combination actuator, the bandwidth may be further limited by the currents induced in the stator lamination stack, a part of the radial control magnetic circuit. 
       FIG. 1  is a partial cross-sectional schematic of an electromagnetic actuator  100  in accordance with the present disclosure and illustrates generating an axial force  32 . Bias magnetic flux  1 , generated by permanent magnet  3 , is directed by axial pole  5  to axial gap  7 . The bias flux  1  passes through axial gap  7  and enters the actuator target  9 . Likewise, magnetic flux  2 , generated by permanent magnet  4 , is directed by axial pole  6  to axial gap  8 . The bias flux  2  passes through axial gap  8  and enters the actuator target  9 . Bias fluxes  1  and  2  merge together and exit through the radial gaps  10   a  through  10   d  (shown in  FIG. 2 ) into the radial actuator pole assembly  11 . 
     The coil  12  carries axial control current  30  flowing around the actuator axis  40 . This current  30  produces magnetic axial control flux  13  which propagates through the axial pole  5 , axial gap  7 , actuator target  9 , axial gap  8 , axial pole  6  and axial back-iron  14 . The magnitude and direction of the flux  13  can be changed by changing the current  30  in the coil  12 . If the axial control flux  13  is zero, the bias flux  1  in the axial gap  7  is equal or near equal to the bias flux  2  in the axial gap  8  and the net axial electromagnetic force acting on the actuator target  9  is zero or near zero. If there is a non-zero axial control flux  13  flowing in the direction shown in  FIG. 1 , the control flux  13  adds to the bias flux  1  in the axial gap  7 , but subtracts from the bias flux  2  in the axial gap  8 . Because of the differences in the flux densities on the actuator target sides facing gaps  7  and  8 , there will be an axial force F ax    32  directed along the Z-axis  17  towards the axial pole  5  (positive Z-direction). Reversing direction of the current  30  in the control coil  12  reverses the direction of the force F ax    32 . Since the actuator target  9  is rigidly mounted on the machine shaft  15 , all the forces exerted on it are directly transferred to the shaft  15 . 
     The magnetic actuator  100  also provides radial forces on the same actuator target  9 . The mechanism of the radial force generation is explained in  FIG. 2 .  FIG. 2  is a radial sectional schematic of an electromagnetic actuator in accordance with the present disclosure and illustrates generating a radial force. To produce radial forces in multiple (or all) directions within a radial plane, the radial pole assembly  11  is equipped with at least three radial control poles and control windings around these poles. For example,  FIG. 2  shows four radial control windings  16   a  through  16   d  located in slots between the poles  38   a - 38   d . The bias fluxes  1  and  2  generated by the magnets  3  and  4  add up in the radial air gaps  10   a  through  10   d  and flow radially within the radial poles  38   a - 38   d . When the target  9  is in the central position and there are no currents in windings  16   a  through  16   d , the bias flux density under each pole associated with windings  16   a - 16   d  is the same or similar because of the system symmetry. Therefore, the net radial force is zero or close to zero. By energizing the radial control coils  16   a - 16   d , the flux distribution can be altered so that a radial force would develop. For example,  FIG. 2  shows coils  16   a  and  16   c  being energized with control currents  20   a  and  20   c , respectively. These currents produce radial control flux  22 . In the air gap  10   a  under the pole  38   a  associated with the control coil  16   a  control flux  22  adds to the combined bias fluxes  1  and  2 , while in the air gap  10   c  under the pole associated with the control coil  16   c  it subtracts. Since the flux density will be higher at the top of the target  9  than at the bottom, there will be a radial force F Y    24  acting on the target, directed along the Y-axis  19  upwards in  FIG. 2  (positive Y-direction). Similarly, by energizing windings  16   b  and  16   d  a force can be produced in the direction of the X-axis  18 . 
     For practical reasons the radial actuator pole assembly  11  and the actuator target  9  may be assembled of magnetically permeable and electrically conductive laminations (e.g. steel laminations) stacked axially and electrically isolated from each other. The isolation reduces eddy currents in these components induced when the radial control windings  16   a - 16   d  are energized with time-varying currents in order to produce time-varying radial forces. An issue with this construction arises when the axial control current  30  changes in time in order to produce a time-varying axial force F z    32 . In this case, the axial control flux  13  may also be varying in time. 
     According to Faraday&#39;s Law, the time varying magnetic flux induces time-varying electromotive forces around the flux. Furthermore, if there is a closed conductive loop surrounding the time-varying magnetic flux, the above electromotive forces will induce electrical currents in that loop. In particular, there will be current  26  induced in the radial actuator pole assembly  11  as shown in  FIG. 3 .  FIG. 3  is an example schematic of a current induced in a radial control pole assembly during production of a time-varying axial control force in accordance with the present disclosure. Having the radial actuator pole assembly  11  composed of electrical steel laminations stacked in the Z direction and electrically isolated from each other cannot prevent current  26  in  FIG. 3  from flowing in the lamination plane. Current  26  induces a magnetic flux of its own  28 , which becomes superimposed on the original axial control flux  13 , affecting the value of the force F ax    32  acting on the actuator target  9 . If the axial control current  30  is a harmonic function of time (e.g., a sinusoidal function), then magnetic flux  13 , the current  26  and the magnetic flux  28  will also be harmonic functions in the first approximation. If there were no current  26 , the control current  30 , the magnetic fluxes  13  and the resulting force  32  would be harmonic functions and they would be in phase. Because of the presence of the induced current  26 , the superposition of the magnetic fluxes  13  and  28  will be a harmonic function with a smaller amplitude than the original flux  13  and lagging it in time. Consequently, the net axial force  32  exerted on the actuator target  9  will be smaller than it would be without the current  26  and it will be lagging the axial control current  30  in time. This makes producing and controlling the axial force  32  more difficult. 
       FIG. 4  is a cross-sectional schematic of a radial control pole assembly in accordance with the present disclosure. Mitigating current  26  in the radial control pole assembly  11  includes introducing a radial slot  34 . In certain implementations, slot  34  can affect magnetic radial control fluxes such as flux  22  in  FIG. 2 . Moreover, one slot would make magnetic reluctances of the magnetic paths within the radial actuator pole assembly  11  including this slot to be higher than reluctances of the paths bypassing the slot, which would result in different radial force values in different directions even when the control windings  16   a - 16   d  are energized with identical currents. Having more than one slot  34 , would be difficult without violating the structural integrity of the radial actuator pole assembly  11 . Even having a single slot  34  deteriorates the rigidity of the assembly  11 . 
     Reduction of the circular current  26  when the radial actuator pole assembly  11  is composed of individual electrically isolated laminations stacked together in the axial direction may be achieved by introducing a slot in each lamination and rotating them during the stacking so that the slots in any two neighboring laminations do not overlap. This method of preventing a current in the radial control pole assembly is illustrated in  FIG. 5 .  FIG. 5  is a cross-sectional schematic of stacked laminations for radial poles of a magnetic actuator in accordance with the present disclosure. In  FIG. 5 , each lamination  36   a  through  36   e  has a radial slot  35   a  through  35   d , and each subsequent lamination is rotated with respect to the previous lamination by 90 degrees so that the slots in any two neighboring laminations do not overlap. In a general case of an arbitrary number of poles, the rotation angle can be calculated as 360 degrees divided by the number of poles—four in the example shown in  FIG. 5 . It is also not necessary to rotate the laminations consequently by the same angle—any method of rotation would work as long as slot locations in any two neighboring laminations do not coincide. Furthermore, a variety of shapes and locations of the slots can be utilized as long as they completely interrupt the closed current loop around the lamination axis. 
     In some aspects, the proposed homopolar combination axial/radial magnetic actuator  100  may be utilized as a part of an Active Magnetic Bearing (AMB) system to support a rotor of a rotational machine without a mechanical contact. The rotational machine can be, for example, an electric pump including an electric motor driving an impeller mounted directly on the motor shaft. The electric motor may have a rotor and a stator. Alternatively, the impeller can be driven by a flow of gas or liquid and spin the rotor attached to it through the shaft. In this case, the motor can be used as a generator. In embodiments, the rotor of the electric machine can be supported without mechanical contact by means of, for example, a combination axial/radial AMB and a radial AMB located on the opposite ends of the rotor. The combination axial/radial AMB utilizes the combination axial/radial electromagnetic actuator per present invention to exert radial and axial forces on an actuator target firmly mounted on the rotor in response to rotor displacements from the desired non-contact position measured with a set of sensors included in the AMB. 
       FIG. 6  is a cross-sectional schematic of an example of an AMB system in an electric rotational machine  600 . The rotational electric machine  600  can be, for example, an electric pump consisting of an electric motor  604  driving an impeller  606  mounted directly on the motor shaft  608 . The electric motor  604  shown in  FIG. 6  has a rotor  610  and a stator  612 . Alternatively the impeller  606  can be driven by a flow of gas or liquid and spin the rotor  610  attached to it through the shaft  608 . In this case the motor  604  can be used as a generator which would convert the mechanical energy of the rotor  610  into electricity. In embodiments, the rotor  610  of the electric machine  600  can be supported radially and axially without mechanical contact by means of front and rear radial AMBs  614  and  616 . The front AMB  614  provides an axial suspension of the entire rotor  610  and a radial suspension of the front end of the rotor, whereas the rear AMB  616  provides only radial suspension of the rear end of the rotor  610 . When the AMBs  614  and  616  are not working, the rotor rests on the mechanical backup bearings  620  and  622 . The front backup bearing  620  may provide the axial support of the entire rotor  610  and a radial support of the rotor front end, whereas the rear backup bearing  622  may provide radial support of the rear end of the rotor  610 . There are sufficient radial clearances between the inner diameters of the mechanical backup bearings  620 ,  622  and the outer diameters of the rotor portions interfacing with those bearing to allow the rotor  610  to be positioned radially without touching the backup bearings  620 ,  622  when the AMBs  614  and  616  are activated. Similarly, there are sufficient axial clearances between the backup bearings  620 ,  622  and the portions of the rotor  610  interfacing with those bearings to allow the rotor  610  to be positioned axially without touching the backup bearings  620  and  622  when the AMBs  614  and  616  are activated. 
     The front AMB  614  consists of a combination radial and axial electromagnetic actuator  601  per the concepts described herein, radial position sensors  624 , axial position sensor  626  and control electronics  632 . The electromagnetic actuator  601  in accordance with the concepts described herein may be capable of exerting radial and axial forces on the actuator target  609  firmly mounted on the rotor  610 . The axial force is the force in the direction of Z-axis  617  and the radial forces are forces in the direction of X-axis  618  (directed into the page) and the direction of Y-axis  619 . The actuator may have several sets of coils corresponding to each of the axes and the forces may be produced when the corresponding coils are energized with control currents produced by control electronics  632 . The position of the front end of the rotor in space is constantly monitored by non-contact position sensors  624  and  626 . The non-contact position sensors  624  can monitor radial position of the rotor whereas the position sensor  626  monitors the axial position of the rotor. 
     Signals from the position sensors  624  and  626  may be input into the control electronics  632 , which may generate currents in the control coils of the electromagnetic actuator  601  when it finds that the rotor is deflected from the desired position such that these currents may produce forces pushing the rotor back to the desired position. 
     In certain instances, smaller axial gain attenuation with frequency and smaller phase difference between the actuator force and the control current in the combination actuator  601  per the concepts described herein compared to conventional designs can result in a larger axial load capacity at any particular frequency and simplify control design. 
     The rear AMB  616  consists of an electromagnetic actuator  628 , radial non-contact position sensors  630 , and control electronics  632 . It may function similarly to the front AMB  614  except that it might not be configured to control the axial position of the rotor  610  because this function is already performed by the front AMB  614 . Correspondingly, the electromagnetic actuator  628  may not be able to produce controllable axial force and there may be no axial position sensor. 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the concepts described herein. Accordingly, other embodiments are within the scope of the following claims.