Homopolar permanent-magnet-biased action magnetic bearing with an integrated rotational speed sensor

This application is claims the benefit of priority to U.S. Provisional Application Ser. No. 61/869,001, filed on Aug. 22, 2013, the contents of which are hereby incorporated by reference.

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.

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.

Description of Drawings

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.

FIG. 2 schematically illustrates a typical voltage waveform on the output of the Hall-effect sensor in FIG. 1 when the rotor spins.

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.

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.

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.

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.

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.

Like reference symbols in the various drawings indicate like elements.

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).

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”.

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.

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.

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.

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.

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 .

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.

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.

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.

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.

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 .

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.

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.

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 .

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.

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).

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.

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 .

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 .

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.

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 .

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.

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 .

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.

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 .

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.

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.

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 .

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.

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.