Abstract:
Radial poles are placed around a radial actuator target mounted on a body. The poles are separated from a cylindrical surface of the target by radial gaps and adapted to communicate a magnetic flux with it. The radial poles are equipped with electrical control windings and magnetically coupled to form magnetic control circuits. A flux return pole is adjacent to the body, separated from it by an air gap and adapted to communicate a magnetic flux with the radial actuator target. A permanent magnet generates a magnetic bias flux in the magnetic bias circuit formed by the radial actuator target, the radial poles and the magnetic flux return pole. A radial force is exerted on the actuator when the control windings are energized with a current. A Hall effect sensor measures bias magnetic field in the air gap between the magnetic flux return pole and the body. A feature on a body is adapted to produce a circumferentially local discontinuity in the magnetic field measured by the Hall effect sensor as the body rotates.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to measuring rotating speed, and, more particularly, to non-contact measuring rotating speed of rotors suspended without mechanical contact using homopolar permanent-magnet-biased active magnetic bearings. 
       BACKGROUND 
       [0002]    Active Magnetic Bearings (AMBs) are often used to support rotating members in magnetic fields without a mechanical contact. In such systems, a need often arises in non-contact measurement of a rotating speed of the member. 
     
    
     
       SUMMARY 
       Description of Drawings 
         [0003]      FIG. 1  illustrates design and operation of a Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator with an Integrated Rotational Speed Sensor in accordance with the present disclosure. 
           [0004]      FIG. 2  schematically illustrates a typical voltage waveform on the output of the Hall-effect sensor in  FIG. 1  when the rotor spins. 
           [0005]      FIG. 3  illustrates design and operation of a Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator with an Integrated Differential Rotational Speed Sensor in accordance with the present disclosure. 
           [0006]      FIG. 4  schematically illustrates a typical voltage waveform on the output of the differential arrangement of Hall-effect sensors shown in  FIG. 3  when the rotor spins. 
           [0007]      FIG. 5  illustrates design and operation of a Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator with a non-magnetic rotor and an Integrated Rotational Speed Sensor in accordance with the present disclosure. 
           [0008]      FIG. 6  illustrates design and operation of a Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator with Axial Force Offset and an Integrated Rotational Speed Sensor in accordance with the present disclosure. 
           [0009]      FIG. 7  is a cross-sectional schematic of an example of an electric machine on Active Magnetic Bearings utilizing a Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator with integrated speed sensor in accordance with the present disclosure. 
       
    
    
       [0010]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0011]    This disclosure relates to measuring rotating speed, and, more particularly, to non-contact measuring rotating speed of rotors suspended without mechanical contact using homopolar permanent-magnet-biased Active Magnetic Bearings (AMB). 
         [0012]    A magnetic bearing, such as an Active Magnetic Bearing (AMB), 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 such as spinning around an axis. In certain implementations electromagnetic actuators may use permanent magnets, and may be referred to as Permanent-Magnet-Biased Electromagnetic Actuators. Electromagnetic actuators may be referred to as “homopolar” if in the absence of radial loading, the magnetic polarity stays the same around the rotor at a given axial position. Examples of homopolar actuators are discussed in the U.S. Pat. No. 8,169,118 titled “High-Aspect Ratio Homopolar Magnetic Actuator” and U.S. Pat. No. 8,482,174 titled “Electromagnetic Actuator”. 
         [0013]    If an Active Magnetic Bearing system is used to support a rotating member, there is often a need to measure the rotational speed of the member without a mechanical contact. The concepts presented herein are directed to an arrangement and a method of measuring the rotational speed utilizing a Hall effect sensor integrated into a magnetic bias flux return pole of a radial homopolar permanent-magnet-biased electromagnetic actuator. Since the sensor is integrated into the actuator it does not require an additional space inside the machine, making it more compact and improving rotordynamic performance. 
         [0014]      FIG. 1  illustrates design and operation of a Rotational Speed Sensor integrated into a Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator in accordance with the present disclosure.  FIG. 1  shows one radial and two axial cross-sectional schematics of a Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator (actuator  100 ) with an integrated speed sensor in accordance with the present disclosure. Permanent magnet  2  is sandwiched between a radial actuator pole assembly  4  and a magnetic bias flux return pole  3 . More details of the radial actuator pole assembly  4  are shown in the cross-sectional view A-A on  FIG. 1 . The permanent magnet  2  generates a magnetic bias flux  5 , which is guided by the magnetic bias flux return pole  3  toward the air gap  7  which separates the magnetic bias flux return pole  3  from a soft magnetic shaft  10 . The magnetic bias flux  5  then is directed within the soft magnetic shaft  10  towards a radial actuator target  11 , exits the radial actuator target  11  radially through radial air gaps  12   a - 12   d , travels radially within radial magnetic poles  18   a - 18   d  towards the permanent magnet  2  where it completes the loop. In general, the positioning and composition of structural elements of the magnetic actuator  100  direct the magnetic flux  5  (generated by the permanent magnet  2 ) to propagate in accordance with the present disclosure. 
         [0015]    The magnetic bias flux return pole  3 , the shaft  10 , the radial actuator target  11  and the radial pole assembly  4  may include or be composed of soft-magnetic materials (e.g., carbon steels and/or other soft magnetic material) that more effectively conduct magnetic fluxes than other materials. 
         [0016]    The axial thickness of the magnetic bias flux return pole  3  may be chosen so that the pole material is magnetically saturated by the bias flux  5 . Since the magnetic saturation levels of ferrous alloys are known to be nearly independent of the temperature within a typical operating temperature range, this feature results in a bias flux  5  being nearly constant over a typical operating temperature range. 
         [0017]    The mechanism of the radial force generation in a Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator is explained in Section A-A of  FIG. 1 . To produce radial forces in multiple (or all) directions within a radial plane, the radial pole assembly  4  is equipped with at least three radial control poles and control windings around these poles. For example, Section A-A of  FIG. 1  shows four radial control windings  17   a - 17   d  located in slots between the poles  18   a - 18   d . The bias flux  5  generated by the magnets  2  flows radially through the radial air gaps  12   a - 12   d  and within the radial poles  18   a - 18   d . When the radial actuator target  11  is in the central position and there are no currents in windings  17   a - 17   d , the bias flux density under each pole  18   a - 18   d  associated with windings  17   a - 17   d  is the same or similar because of the system symmetry. Therefore, the net radial force may approach zero or be close to zero. By energizing the radial control coils  17   a - 17   d , the flux distribution can be altered so that a radial force would develop. For example,  FIG. 1  shows coils  17   a  and  17   c  being energized with control currents  19   a  and  19   c , respectively. These currents produce radial control flux  20 . In the air gap  12   a  under the pole  18   a  associated with the control coil  17   a , control flux  20  adds to the bias fluxes  5 , while in the air gap  12   c  under the pole  18   c  associated with the control coil  17   c , it subtracts. Since the flux density will be higher at the top of the radial actuator target  11  than at the bottom, there will be a radial force F Y    21  acting on the target, directed along the Y-axis  22  upwards in  FIG. 1  (positive Y-direction). Similarly, by energizing windings  17   b  and  17   d , a force can be produced in the direction of the X-axis  23 . 
         [0018]    The radial actuator target may include a lateral surface adjacent and spaced apart from the radial pole. In certain instances, the target may be concentric to the actuator (or rotational) axis  15 , and may have a cylindrical (precisely or substantially cylindrical) shape. 
         [0019]    In certain instances, the radial actuator pole assembly  4  and the radial actuator target  11  may be assembled of magnetically permeable and electrically conductive laminations (e.g., steel and/or other magnetically permeable and electrically conductive laminations) stacked axially and electrically isolated from each other. The isolation reduces eddy currents in these components induced when the rotor spins and/or the radial control windings  17   a - 17   d  are energized with time-varying currents to produce time-varying radial forces. 
         [0020]    To measure a rotational speed of the shaft  10 , the electromagnetic actuator  100  is equipped with a Hall-Effect sensor  30  embedded into the cylindrical surface of a magnetic bias flux return pole  3  adjacent to the shaft  10  or mounted in the air gap  7  separating the magnetic bias flux return pole  3  from the shaft  10 . The Hall-Effect sensor  30  is configured to measure a radial component of the magnetic bias field  5  in the radial gap  7 . Further, the shaft  10  has a feature, such as notch  31 , interrupting the continuity of its cylindrical surface and axially collocated with the Hall-effect sensor  30 . The notch produces a circumferentially local discontinuity in the magnetic field around the cylindrical surface of the shaft  10  in the air gap. The remainder of the magnetic field around the circumferential surface, axially collocated with the feature, is uniform (precisely or substantially) so that the discontinuity, magnetically speaking, is readily sensed by the Hall-effect sensor  30 . Thus, the magnetic field  5  sensed by the Hall-effect sensor  30  will be smaller when the sensor  30  faces the notch  31  rather than a continuous cylindrical surface.  FIG. 2  schematically illustrates a voltage pulse generated by a Hall-effect sensor when a notch on the rotor passes by. The sensor output voltage is equal to V 0  when the sensor faces a continuous surface of the rotor  10  and drops to a lower value V notch  when the notch passes by the sensor. Such pulses can be counted with an external counter and the number of pulses per unit time can be used to calculate the rotational speed of the rotor. 
         [0021]    Because the level of the magnetic bias flux  5  is maintained nearly constant due to the magnetic saturation of the magnetic bias flux return pole  3 , the magnetic field measured by the Hall-effect sensor  30  at each specific orientation of the notch  31  will be nearly the same regardless the operating temperature, control currents in the windings  17   a - 17   d  and other factors. Therefore, all those factors will not affect the sensor operation. 
         [0022]    The Hall-effect sensor can be of a programmable type, which parameters such as gain and output zero offset can be programmed even after the sensor was already installed into a machine. This feature may be used to eliminate affects of various parameter variations such as air gap  7 , orientation and location of the Hall-effect sensor  30 , depth of the notch  31 , etc. The sensor parameters can be programmed after the sensor was installed so that the sensor outputs will be nearly the same from machine to machine if the sensor looks at the continuous surface of shaft  10  or the notch  31 . 
         [0023]      FIG. 3  shows a different implementation of the proposed speed sensor using two Hall-Effect sensors  330   a  and  330   b  embedded into the cylindrical surface of a magnetic bias flux return pole  303  adjacent to the shaft  310  or mounted in the air gap  307  separating the magnetic bias flux return pole  303  from the shaft  310 . The associated electronics generate a difference between the outputs of two Hall effects sensors  330   a  and  330   b  which is used to measure the rotational speed of the rotor  310  similar to the pulse shown in  FIG. 2 . Such a difference is shown in  FIG. 4 . The advantage of using two Hall effect sensors  330   a  and  330   b  per  FIG. 3  instead of one Hall effect sensor  30  per  FIG. 1  is that the base signal level V 0  in  FIG. 2  may be affected by temperature and other factors (such as axial movements of the rotor  310 ), which may make detection of the pulse difficult, whereas in  FIG. 4  the base level will always be zero or near zero if two Hall-effect sensors  330   a  and  330   b  are the same and exposed to the same temperature and other factors. 
         [0024]      FIG. 5  shows a sensor implementation in a case when the shaft  510  is made of non-magnetic material. In this case a magnetically-permeable actuator target extension  508  can be added to the shaft  510  to conduct the bias magnetic flux  505  from the magnetic bias flux return pole  503  to the actuator target  511 . A feature  531  triggering the speed sensor can be added to the actuator target extension  508  instead of magnetically permeable shafts  10  and  310  shown in  FIGS. 1 and 3  respectively. 
         [0025]      FIG. 6  shows another implementation of the proposed speed sensor in which the magnetic bias flux return pole  603  faces an axial facing surface of an actuator target extension  608  instead of the cylindrical surface as in  FIGS. 5 . Such a configuration produces an uncontrollable axial force F z    615  always pulling the rotor  610  towards the axial pole  603 , which may be beneficial in some magnetic bearing applications, for example where there is a dominant external axial force pushing the rotor  610  away from the axial pole  603 , which may be offset by the force F z    615 . 
         [0026]    In more details, permanent magnet  602  is sandwiched between a radial actuator pole assembly  604  and a magnetic bias flux return pole  603 . The permanent magnet  602  generates a magnetic bias flux  605 , which is guided by the magnetic bias flux return pole  603  toward the axial air gap  609  which separates the magnetic bias flux return pole  603  from an axial face of an actuator target extension  608 . The magnetic bias flux  605  crosses the air gap  609 , enters an actuator target extension  608  travels to the radial actuator target  611 , then it exits the radial actuator target  611  radially through radial air gaps  612   a - 612   d , travels radially within the radial actuator pole assembly  604  towards the permanent magnet  602  where it completes the loop. In general, the positioning and composition of structural elements of the magnetic actuator  600  direct the magnetic flux  605  (generated by the permanent magnet  602 ) to propagate in accordance with the present disclosure. 
         [0027]    The magnetic bias flux return pole  603 , the shaft  610 , the radial actuator target  611 , an actuator target extension  608  and the radial pole assembly  604  may include or be composed of soft-magnetic materials (e.g., carbon steels and/or other soft magnetic material). 
         [0028]    The axial thickness of the magnetic bias flux return pole  603  may be chosen so that the pole material is magnetically saturated by the bias flux  605 . Since the magnetic saturation levels of ferrous alloys are known to be nearly independent of the temperature within a typical operating temperature range, this feature results in a bias flux  605  being nearly constant over a typical operating temperature range. 
         [0029]    The mechanism of the radial force generation in Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator  600  shown in  FIG. 6  is the same as in actuators shown in  FIGS. 1 and 3 . In addition, the actuator  600  also produces an uncontrollable and nearly constant force  615  due to presence of the bias magnetic flux  605  in the axial air gap  609 . 
         [0030]    To measure a rotational speed of the shaft  610 , the electromagnetic actuator  600  is equipped with a Hall-Effect sensor  630  embedded into the axial surface of the magnetic bias flux return pole  603  adjacent to the axial face of the actuator target extension  608 . Alternatively, the Hall-Effect sensor  630  may be mounted in the axial air gap  609  separating the magnetic bias flux return pole  603  from the actuator target extension  608 . Further, the axial face of the actuator target extension  608  adjacent to the Hall-effect sensor  630  has a feature, such as notch  631 , interrupting the continuity of the face and radially collocated with the Hall-effect sensor  630 . 
         [0031]    Because the level of the magnetic bias flux  605  is maintained nearly constant due to the magnetic saturation of the magnetic bias flux return pole  603 , the magnetic field measured by the Hall-effect sensor  630  at each specific orientation of the notch  631  will be nearly the same regardless the operating temperature, control currents in the windings  617   a - 617   d  and other factors. Therefore, all those factors will not affect the sensor operation. 
         [0032]    Similar to the speed sensor shown in  FIG. 1 , the speed sensor shown in  FIG. 5  can also benefit from using programmable Hall-effect sensors and a differential sensor arrangement similar to the one shown in  FIG. 3 . 
         [0033]    In some aspects, the proposed integrated rotational speed sensor may be used as a part of an Active Magnetic Bearing (AMB) system supporting a rotor of a rotational machine without a mechanical contact. In particular, when an AMB system is used in rotating machinery, the rotational speed sensor may deliver information about the rotational speed of the machine necessary for AMB operation and monitoring purposes. Since the sensor is integrated into a radial AMB design it does not require any additional space producing a more compact design with better rotordynamic characteristics.  FIG. 6  shows an example of using an AMB system with an integrated speed sensor in an electric rotational machine  700 . The rotational electric machine  700  can be, for example, an electric motor  704  driving an impeller  706  (e.g., liquid and/or gas impeller) mounted directly on the motor shaft  708 . The electric motor  704  shown in  FIG. 6  has a rotor  710  and a stator  712 . Alternatively, the impeller  706  can be driven by a flow of gas or liquid and spin the rotor  710  attached to it through the shaft  708 . In this case the motor  704  can be used as a generator which would convert the mechanical energy of the rotor  710  into electricity. 
         [0034]    In embodiments, the rotor  710  of the electric machine  700  can be supported radially and axially without mechanical contact by front and rear radial AMBs  714  and  716 . The front AMB  714  provides an axial suspension of the rotor  710  and a radial suspension of the front end of the rotor, whereas the rear AMB  716  provides only radial suspension of the rear end of the rotor  710 . The rear AMB  716  is equipped with a rotational speed sensor per the concepts herein which includes a Hall-effect sensor  730  embedded into the cylindrical surface of a magnetic bias flux return pole  703  adjacent to the shaft  710  or mounted in the air gap  707  separating the magnetic bias flux return pole  703  from the shaft  710 . Further, the shaft  710  has a feature, such as notch  731 , interrupting the continuity of its cylindrical surface and axially collocated with the Hall-effect sensor  730 . 
         [0035]    When the AMBs  714  and  716  are not working, the rotor rests on the mechanical backup bearings  720  and  722 . The front backup bearing  720  may provide the axial support of the rotor  710  and a radial support of the rotor front end, whereas the rear backup bearing  722  may provide radial support of the rear end of the rotor  710 . There are radial clearances between the inner diameters of the mechanical backup bearings  720 ,  722  and the outer diameters of the rotor portions interfacing with those bearing to allow the rotor  710  to be positioned radially without touching the backup bearings  720 ,  722  when the AMBs  714  and  716  are activated. Similarly, there are axial clearances between the backup bearings  720 ,  722  and the portions of the rotor  710  interfacing with those bearings to allow the rotor  710  to be positioned axially without touching the backup bearings  720  and  722  when the AMBs  714  and  716  are activated. 
         [0036]    In certain instances, the front AMB  714  may be a combination radial and axial electromagnetic actuator  701  per U.S. Pat. No. 8,482,174, combination radial/axial position sensors  724  and control electronics  750 . The electromagnetic actuator  701  may be capable of exerting axial forces on the axial actuator target  709  and radial forces on the radial actuator target  711 , both rigidly mounted on the rotor  710 . The axial force is the force in the direction of Z-axis  717  and the radial forces are forces in the direction of X-axis  718  (directed out of the page) and the direction of Y-axis  719 . The actuator may have three 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  750 . The position of the front end of the rotor in space is constantly monitored by non-contact position sensors, such as combination radial/axial position sensor  724 . 
         [0037]    Signals from the position sensors  724  may be input into the control electronics  750 , which may generate currents in the control coils of the combination electromagnetic actuator  701  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. 
         [0038]    The rear AMB  716  is an electromagnetic actuator  728 , radial non-contact position sensors  725 , and control electronics  752 . It may function similarly to the front AMB  714  except that it might not be configured to control the axial position of the rotor  710  because this function is already performed by the front AMB  714 . Correspondingly, the electromagnetic actuator  728  may not be able to produce controllable axial force and there may be no axial position sensor. 
         [0039]    The electromagnetic actuator  728  is equipped with a rotational speed sensor per the concepts herein which includes a Hall-effect sensor  730  embedded into the cylindrical surface of a magnetic bias flux return pole  703  adjacent to the shaft  710  or mounted in the air gap  707  separating the magnetic bias flux return pole  703  from the shaft  710 . Further, the shaft  710  has a feature, such as notch  731 , interrupting the continuity of its cylindrical surface and axially collocated with the Hall-effect sensor  730 . 
         [0040]    When the rotor  710  spins, the Hall effect sensor  730  will see changes in the magnetic field at the sensor location whenever the notch  731  passes it producing a pulse of a positive or negative polarity. Such pulses can be counted with an external counter, which may be a part of a control unit  752  and the number of pulses per unit time can be used to calculate the rotational speed of the rotor. 
         [0041]    The present disclosure describes embodiments of a Rotational Speed Sensor integrated into a Radial Homopolar Permanent-Magnet-Biased Electromagnetic Actuator. Other embodiments and advantages are recognizable by those of skill in the art by the forgoing description and the claims.