Patent Publication Number: US-2022216770-A1

Title: Reluctance sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority benefit to U.S. Provisional Utility Application Ser. No. 63/134,845 (Docket No. 010222-20043A) filed on Jan. 7, 2021. The entire disclosure of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates in general to the field of electric machines including motors and generators, and in particular, the detection of position of a rotor in an electric machine. Example electric machines include DC motors and generators, alternators and synchronous motors, and induction motors or asynchronous machines. The electric machines include a rotating part, referred to as the rotor, and a stationary part, referred to as the stator. Control of the electric machine may require, or otherwise be improved by, detection of the relative positions of the rotor and the stator. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example electric machine. 
         FIG. 2  illustrates an example magnetic equivalent circuit for the example rotor position sensing system. 
         FIGS. 3 and 4  illustrate an example rotor position sensing system. 
         FIGS. 5, 6, and 7  illustrate additional example embodiments of rotor position sensing system. 
         FIG. 8  illustrates an example driving and filtering circuit to apply a constant direct current and to extract a signal for magnetic flux through the reluctance sensor. 
         FIG. 9  illustrates example waveforms produced by the circuit illustrated in  FIG. 8 . 
         FIGS. 10 and 11  illustrate an example driving and filtering circuit to apply a time-varying voltage and extract a signal indicating the length of the magnetic path. 
         FIGS. 12 and 13  illustrate an example waveform representing the current through the reluctance sensor. 
         FIG. 14  illustrates example waveforms from a rotor position sensing system associated with the time-varying drive circuit of  FIGS. 10 and 11 . 
         FIGS. 15, 16, 17 and 18  illustrate four positions of an example toothed wheel moving past a reluctance sensor. 
         FIGS. 19, 20, 21 and 22  illustrate four positions of an example toothed wheel as it moves past an example variable reluctance sensor. 
         FIGS. 23, 24, 25, 26, 27, 28, 29 and 30  illustrate positions of an example drilled wheel as it moves past an example variable reluctance sensor. 
         FIG. 31  illustrates an example controller for the rotor position sensing system. 
         FIG. 32  illustrates an example controller for the rotor position sensing system. 
     
    
    
     DETAILED DESCRIPTION 
     An electric machine may be an electromagnetic rotating machine that includes a rotor and a stator. The rotor and stator are positioned on opposite sides of an air gap through which a magnetic field is present and magnetic flux flows between the rotor and the stator. The magnetic field may be created by permanent magnets or by current flowing in a winding. While other examples are possible, the electric machine may be a motor or a generator. The generator, which may be referred to as an engine-generator set or a genset, may include a power source (e.g., an engine) and an alternator or another device for generating electrical energy or power from mechanical energy. The motor, on the other hand, receives electrical energy and converts it to mechanical energy by outputting torque. 
     Controlling an electrical machine often requires feedback of absolute or relative position of the rotating element, called the rotor. This position is often measured using a position sensor. The position sensor can utilize a variety of different techniques, such as mechanical sensors, mechanical commutators, optical sensors, variable reluctance sensors, and hall effect sensors. Electrical machines are often coupled to engines. In this case, the engine is often controlled by an engine control unit (ECU). The ECU often requires engine position information for appropriate sequencing of certain engine actuators, such as fuel injectors, ignition devices, emissions control devices, protective functions, or control of the engine speed. 
     The following embodiments include a rotor position sensing system based on a magnetic field and/or reluctance caused by the rotation of the rotor of the electric machine. In one example, the rotor of the electric machine includes rotor teeth (extension of steel or other metal) and slots (absence of steel or other metal) between the rotor teeth. A printed circuit board (PCB) includes a sensing coil. A current or voltage generated in the sensing coil of the PCB may fluctuate according to the magnetic field or reluctance through the sensing coil. As the rotor teeth pass near the sensing coil, the magnetic field or reluctance is modified. A controller (e.g., ECU) may monitor and detect fluctuations in the generated current or voltage in order to determine how many teeth are passing the sensing coil and/or the speed at which the teeth are passing the sensing coil in order to determine the position and/or speed of the rotor. In one example, the PCB also includes the stator windings for the electrical machine. The PCB may include an outer portion (e.g., outside the magnetic portion of the rotor) including the sensing coil and an inner portion (e.g., adjacent to the magnetic portion of the rotor). 
       FIG. 1  illustrates an example electric machine including a rotor position detection system  20  and a stator implemented by PCB  21  arranged in cooperation with a rotor  124  including permanent magnets. The permanent magnets define a magnetic region where the flux from the magnets varies significantly while the rotor is rotating. The magnetic region may reside radially between an inner diameter of the magnets and an out diameter of the magnets. The magnetic region may extend slightly outside the region defined by the inner and outer diameter of the magnets. A sensing coil  22  is located outside of the magnetic region defined by the inner and outer diameter of the magnets. The sensing coil  22  may be a predetermined distance (e.g., 1 inch, 3 centimeters, 10 centimeters, or another value) outside of the magnetic region. The predetermined distance may be selected according to one or more factors. One example factor is based on a rotating feature such as the tooth or hole. For example, the predetermined distance may be based on the size of relative permeability of the tooth or hole. Another example factor is based on a stationary feature such as the permeable element. For example, the predetermined distance may be based on the size of relative permeability of the permeable element. One example factor is the length of magnetic path when permeable feature is present. One example factor is the length of magnetic path when permeable feature is not present. One example factor may include mechanical tolerances in the machine (such as end play or manufacturing tolerance). 
     The area outside the region defined by the inner and outer diameter of the magnets may be referred to as the non-magnetic region, periphery, or outer sensor region. 
     The stator is supported in a stator clamshell  125  which is connected to an engine block  121 . The rotor  124  is supported by bearings connecting it to the stator clamshell  125  and an engine crankshaft  120 . The printed circuit board may include coils of wire, or traces, that are energized in response to relative movement of magnets through electromagnetic induction. The coils may be included in different layers of the printed circuit board. Because the coils reside on the printed circuit board, the armature inductance (caused by the permeability of the iron used to direct the magnetic flux through the windings) may be lower, which may decrease the voltage drop under load, improve the efficiency of the generator and decrease the commutation losses in semiconductor diodes connected to rectify the output of the machine to direct current. 
     The shape of the windings may include concentric circles, rectangles, trapezoids to match magnet shape, or another shape. The exciter windings may be formed from copper or another conductive material. The traces may exist on multiple layers of the PCB. The traces forming the exciter windings are configured to induce a field current in response to magnetic fields of the stator magnets. 
     Although embodiments illustrated herein relate to an axial air gap electrical machine, embodiments are contemplated relating to a radial air gap electric machine, an engine, a driven component, such as a drive shaft, a fan, a propeller or similar driven component. In addition, the scope of the disclosure disclosed herein is not limited to position sensing for a rotating element. One skilled in the art can apply the concepts herein to any situation where a physical quantity may be measured based on the reluctance of a magnetic path. Examples include but are not limited to, linear position measurement, pressure measurement, sound measurement, temperature measurement, magnetic saturation level, allotrope characteristics, metallurgical characteristics, force measurement, and torque measurement. 
       FIG. 2  illustrates an example magnetic equivalent circuit  10  for the example rotor position sensing system  20  illustrated in  FIGS. 3 and 4 . For the example magnetic equivalent circuit  10 , the current in the equivalent circuit  10  corresponds to the magnetic flux through the magnetic path  12  illustrated in  FIGS. 3 and 4 . The current in the magnetic equivalent circuit  10  is related to the total magneto-motive force supplied to the circuit and the total equivalent resistance along the magnetic path, represented by the sum of R g , R 1 , R 2  R a1 , and R a2 . 
     The example magnetic equivalent circuit  10  is supplied by a magneto-motive force (MMF) generated by a magnetic field generation device  11 . Typical examples of a magnetic field generating device  11  include an electrically conductive coil (e.g., one or more coil traces on a PCB), a single electrical conductor, a solenoid, a permanent magnet, and residual magnetism in a ferrous material, or others. 
     The resistance R g  corresponds to the reluctance added to the magnetic circuit by the air gap  29  between a first permeable element  23  and a second permeable element  24 . The resistance R 1  corresponds to the reluctance added to the magnetic circuit by the first permeable element  23 . The resistance R 2  corresponds to the reluctance added to the magnetic circuit by the second permeable element  24 . The resistance R a1  corresponds to the reluctance added to the magnetic circuit by a first air gap  25 . The resistance R a2  corresponds to the reluctance added to the magnetic circuit by a second air gap  26 . The resistance R r  corresponds to the reluctance added to the magnetic circuit by a ferrous rotating element  27 . 
     The rotating element  27  may include of multiple rotating elements connected by a magnetically permeable path, the rotating element  27  may include of a single rotating element. The rotating element  27  may have permanent magnets  28  connected to it by a securement device, such as an adhesive, a mechanical retainer, fusing of material, a weld, a press fit, a fastener, such as a screw, or another securement device. The permanent magnets  28  may be formed as a part of the rotating element. The permanent magnets  28  may be discrete pieces of permanent magnet material, such as a neodymium magnet or NdFeB, Samarium Cobalt, ferrite, ceramic, AlNiCo, or a different type of material. The permanent magnets may be locally magnetized portions of a disc composed of permanent magnet material. 
     The permanent magnets  28  may provide a magnetic flux that may flow through the printed circuit board  21 , potentially generating a voltage in windings distributed on the printed circuit board  21 . The voltage generated in windings distributed on the printed circuit board  21  may provide a substantial portion of the torque produced or consumed by an electrical machine. The rotor position detection system  20  may be used to determine a commutation time and sequence for current flowing in windings distributed on the printed circuit board  21 . 
     For each resistive element in the example magnetic equivalent circuit  10 , the reluctance is related to the length of the path and the relative permeability of the material though which the magnetic flux will pass. The relative permeability of ferrous materials may be significantly greater than the relative permeability of air, as an example, the relative permeability of silicon steel may be 5,000 or 10,000 time the permeability of air. 
     For the example rotor position sensing system  20 , the relative permeability of the first permeable element  23 , second permeable element  24  and ferrous rotating element  27  is approximately 6,000 times the permeability of air. Due to the geometry of the example rotor position sensing system  20 , R a1  and R a2  comprise about 70% of the reluctance in the magnetic circuit in  FIG. 3  and about 93% of the reluctance of the magnetic circuit in  FIG. 4 . 
     For the example rotor position sensing system  20 , the magnetic field generating device  11  is the coil  22 , comprised of traces on multiple layers of a printed circuit board (PCB). Due to magnetic induction, the voltage generated by the coil  22  is related to the rate of change of the magnetic flux through the coil. The magnetic flux through the coil depends on the current flowing through the coil  22  as well as the reluctance of the magnetic path  12 . With a constant current, the magnetic flux, and thereby the voltage, depends on the reluctance of the magnetic path  12 . 
     In addition, the inductance of a coil may be related to the magnetic path length for the flux flowing through the coil. The magnetic path length may be directly measured by directly measuring the inductance of the coil. The inductance of a coil may be measured by applying a voltage to the coil and monitoring the resulting behavior of the current. The voltage applied to the coil may be a time varying voltage. The resulting behavior of the current may be monitored for amplitude, rise rate, fall rate, phase shift, time constant, or other similar characteristics. 
       FIGS. 5, 6, and 7  illustrate additional example embodiments of rotor position sensing system  20 . In the example of  FIG. 5 , rotor position sensing system  20  may include a multi-element rotor with external teeth  31 , shown with teeth present in a first state  32  and teeth absent in a second state  33 . 
       FIG. 6  illustrates an example in which the rotor position sensing system  20  may include a single-element rotor with external teeth  34 , shown in a first state  35  with teeth present (in line with the sensor) and in a second state  36  with teeth absent (not in line with the sensor).  FIG. 7  illustrates an example in which the rotor position sensing system  20  may include a multi-element rotor with position sensing holes  37 , shown with a hole out of alignment with the sensor in state  38  and hole in alignment with the sensor in state  39 . 
     The magnetic field generation device  11  may provide a constant magneto motive force by applying a constant direct current to coil of the rotor position sensing system  20 .  FIG. 8  illustrates an example driving and filtering circuit to apply a constant direct current to coil of the rotor position sensing system  20  and to extract a meaningful signal indicating the rate of change in the magnetic flux through the magnetic path detected by the sensor. This signal may be further processed to provide a tooth indication signal. 
     In one example, the current to the coil of the rotor position sensing system  20  may be varied based on the speed of the rotating element (e.g., rotating element  27 ). For example, the current amplitude or an excitation frequency may be varied in the AC drive case. 
     The driving and filtering circuit illustrated in  FIG. 8  illustrates the coil providing the magneto-motive force to the magnetic equivalent circuit, such as the coil of the rotor position sensing system  20 , as inductive element  40 . The current through coil (inductive element  40 ) is controlled by current source  41  to a constant target set by resistive divider  42 . The current source  41  may be an integrated circuit, a field effect transistor (FET) controlled by an Op-Amp, a bipolar junction transistor (BJT) controlled by an Op-amp, a BJT with compensated drive current, or another type of current source. 
     Changes in the magnetic path length for the flux flowing through the inductive element  40  may cause a voltage to be generated across inductive element  40 . This voltage may be limited by protective device  43  to prevent damage to the components of the current source  41 . Protective device  43  may be a Zener diode, gas discharge tube, varistor, avalanche diode, combination of such elements or other protective device. Protective device  43  may be omitted. 
     The decoupling capacitor  44  removes the DC component of the voltage across the inductive element  40  from the incoming signal  45 , potentially improving immunity to changes in the DC voltage across the inductive element  40 , which may be a result of temperature fluctuations, manufacturing tolerance, physical positioning environmental effects, or other factors that do not relate to the magnetic path length and are not desirable to measure. Embodiments are contemplated where the decoupling capacitor  44  is not included. 
     The comparison voltage  46  represents a threshold indicating a typical inductive element  40  voltage when resting. The comparison voltage  46  may be provided by a resistive divider or may be derived from the incoming signal  45  through a filter. 
     The overvoltage protection circuit  47  may act to prevent the incoming signal  45  from exceeding the allowable input limits for the comparator  48 . The protective elements in the overvoltage protection circuit  47  may be Zener diodes, rectifier diodes, Schottky diodes, varistors, gas discharger tubes, avalanche diodes, or another protective device. The overvoltage protection circuit  47  may be omitted if unnecessary under all operating conditions. 
     Based on the operation of the comparator  48 , the output signal  49  may represent a digital signal resulting from the analog incoming signal  45  for use by digital circuitry such as a logic device, a microprocessor, a microcontroller, a complex programmable logic device (CPLD), a field programmable gate array (FPGA), or another device that solves digital logic. Embodiments are contemplated where the incoming signal  45  is fed directly into an analog circuit device or converted to a digital signal using an analog to digital converter or similar device. 
       FIG. 9  illustrates example waveforms produced by the circuit illustrated in  FIG. 4  when inductive element  40  represents the coil for the rotor position detection system  20  from  FIGS. 3 and 4 . Analog waveform  50  represents an example of incoming signal  45 . Digital waveform  51  illustrates an example of output signal  49 . Rotor tooth signal  52  illustrates presence of a tooth aligning with the first permeable element  23  and second permeable element  24 . Maximum alignment position  53  corresponds to  FIG. 3 . Minimum alignment position  54  corresponds to  FIG. 4 . 
     In most cases, a rotor position detection system  20  applying a constant magneto-motive force using a permanent magnet or a constant current through an inductive element  40 , such as the coil from the rotor position detection system  20 , may only detect position when the ferrous rotating element  27  is moving. When the ferrous rotating element  27  is stopped, the magnetic flux through the coil may remain substantially constant, likely resulting in a very small or zero rate of change for the flux, which may generate a very small or 0 voltage across the inductive element  40 . 
     The magnetic field generation device  11  may provide a time-varying magneto motive force by applying a time-varying current or a time varying voltage to coil  22 .  FIGS. 10 and 11  illustrate an example driving and filtering circuit to apply a time-varying voltage to an inductive element  60  and to extract a meaningful signal indicating the length of the magnetic path for flux passing through the inductive element  60 , first permeable element  23  and second permeable element  24 . This signal may be further processed to provide a tooth indication signal. 
     The time varying drive circuit shown in  FIGS. 10 and 11  contains an increasing current path (illustrated in  FIGS. 19-22 ) and a decreasing current path (illustrated in  FIGS. 23-30 ). The current  63  follows the increasing current path through the increasing current switches  61 , the inductive element  60 , and the current sensing resistor  64  when the increasing current switches  61  are active. The current  63  follows the decreasing current path through the alternate passive path components  62  and the inductive element  60  when the increasing current switches  61  are not active until the current  63  through the inductive element  60  reaches a lower threshold value, such as 0. 
     The increasing current switches  61  may be driven to switch at a frequency. This frequency may be a fixed frequency much greater than an intended measurement frequency, a variable frequency or another frequency. The increasing current switches may be driven at a 50% duty cycle, a duty cycle slightly less than 50% to prevent windup when a tooth is moving into alignment with the first permeable element  23  and the second permeable element  24 . The increasing current switches may be driven at a varying duty cycle or another duty cycle. 
     The increasing current switches  61  may be semiconductor switches, such as FETs, BJTs, insulated-gate bipolar transistors (IGBTs), or another semiconductor switch. The increasing current switches  61  may be other switches, such as MEMs switch elements, relays, contacts, or any other device capable of switching electrical current. The alternate passive path components  62  may be semiconductor devices, such as rectifier diodes, Schottky diodes, Zener diodes, avalanche diodes, or another passive electric current conductive element. The alternate passive path components may be a semiconductor switch, such as a FET, BJT or IGBT, or another current switching device. 
     When current flows through the increasing current path, this current will flow through current sensing resistor  64 . Current through current sensing resistor  64  will result in a voltage across the current sensing resistor  64 , proportional to the current through the current sensing resistor  64 . The voltage across current sensing resistor  64  may be filtered by a filter to provide a representative position waveform  65 . 
     One skilled in the art may see that the current sensing resistor  64  may be applied to any portion of the increasing or decreasing current path and still provide similar behavior. In addition, embodiments are contemplated where the current through the inductive element  60  is measured by a different current measurement technique, such as a closed-loop magnetic sensor, an open-loop magnet sensor, passive current sensing across one or more increasing current switch or other means of measuring current. In some cases, current sensing resistor may be omitted. 
     In addition, embodiments are contemplated where voltage is applied to the inductive element  60  using various circuit topologies. The embodiments disclosed herein may be driven by a wide variety of fixed and time-varying current or voltage sources. 
       FIGS. 12 and 13  illustrate an example waveform representing the current through the inductive element  60  where the inductive element  60  represents the coil of the rotor position detection system  20  represented by traces on a printed circuit board.  FIG. 12  illustrates an example waveform representing the current through the inductive element  60  when the rotor is in the position illustrated by  FIG. 3 .  FIG. 13  illustrates an example waveform representing the current through the inductive element  60  when the rotor is in the position illustrated by  FIG. 4 . 
     It is possible to determine the absence or presence of a tooth or hole based only on characteristics of the current through the inductive element  60  as sensed by current sensing resistor  64  in example system illustrated by  FIGS. 10 and 11 . In this case, it may be possible to determine the position of the ferrous rotating element  27  without the need for movement of the ferrous rotating element. Such a possibility may provide enough benefit over a constant magneto-motive force rotor position sensing technique, such as that shown in  FIG. 8 , to offset any drawbacks to a time-varying application of voltage or current, such as increased circuit cost, increase circuit control complexity, increased EMI consideration, or other drawback encountered when implementing a time-varying application of voltage or current, such as provided by the circuit illustrated in  FIGS. 10 and 11 . 
       FIG. 14  illustrates example waveforms from a rotor position sensing system  20  where the inductive element  60  of the time-varying drive circuit of  FIGS. 10 and 11  and represents coil  22  on the printed circuit board  21 . Coil Current  80  illustrates the current through inductive element  60 , representing the current through coil  22 . Position Indication  81  represents the representative position waveform  66  output from the time varying drive circuit. Rotor tooth signal  82  illustrates presence of a tooth aligning with the first permeable element  23  and second permeable element  24 . Maximum alignment position  83  corresponds to  FIG. 3 . Minimum alignment position  84  corresponds to  FIG. 4 . 
       FIGS. 15, 16, 17 and 18  illustrate four positions of an example toothed wheel  91  as it moves past an example variable reluctance sensor  92 . The variable reluctance sensor  92  contains a coil  93  and is driven by a constant magneto-motive force, produced by a permanent magnet  94 . 
     For the example illustrated in  FIG. 15 , the tooth  95  is in maximum alignment with the variable reluctance sensor  92 . The magnetic path length for flux through the coil  93  travels through the ferrous material of the tooth  95  and the magnetic flux is at a maximum because the reluctance is at a minimum. 
     For the example illustrated in  FIG. 16 , the tooth  95  is traveling out of alignment with the variable reluctance sensor  92 . The magnetic path length for flux through the coil  93  travels through the ferrous material of the tooth  95  on one side, but mostly through free air on the other side. The magnetic flux is falling at a maximum rate because the tooth  95  is traveling away from the center of the variable reluctance sensor  92 . 
     For the example illustrated in  FIG. 17 , the tooth  95  is at minimum alignment with the variable reluctance sensor  92 , while next tooth  96  begins to approach the variable reluctance sensor  92 . The magnetic path length for flux through the coil  93  travels through no ferrous material and the magnetic flux is at a minimum because the reluctance is at a maximum. 
     For the example illustrated in  FIG. 18 , the next tooth  96  is beginning to come into alignment with the variable reluctance sensor  92 . The magnetic path length for flux through the coil  93  travels through the ferrous material of the next tooth  96  on one side, most mostly through free air on the other side. The magnetic flux is rising at a maximum rate because the tooth  96  is traveling toward the center of the variable reluctance sensor  92 . 
       FIGS. 19, 20, 21 and 22  illustrate four positions of an example toothed wheel  101  as it moves past an example variable reluctance sensor  102 . The variable reluctance sensor  102  is excited by a coil, similar to the coil of the rotor position detection system  20 . 
     For the example illustrated in  FIG. 19 , the tooth  105  is in maximum alignment with the variable reluctance sensor  102 . The magnetic path length for flux through the coil travels through the ferrous material of the tooth  105  and the magnetic flux is at a maximum because the reluctance is at a minimum. 
     For the example illustrated in  FIG. 20 , the tooth  105  is traveling out of alignment with the variable reluctance sensor  102 . The magnetic path length for flux through the coil travels through the ferrous material of the tooth  105  on one side, but mostly through free air on the other side. The magnetic flux is falling at a maximum rate because the tooth  105  is traveling away from the center of the variable reluctance sensor  102 . 
     For the example illustrated in  FIG. 21 , the tooth  105  is at minimum alignment with the variable reluctance sensor  102 , while next tooth  106  begins to approach the variable reluctance sensor  102 . The magnetic path length for flux through the coil travels through no ferrous material and the magnetic flux is at a minimum because the reluctance is at a maximum. 
     For the example illustrated in  FIG. 22 , the next tooth  106  is beginning to come into alignment with the variable reluctance sensor  102 . The magnetic path length for flux through the coil travels through the ferrous material of the next tooth  106  on one side, most mostly through free air on the other side. The magnetic flux is rising at a maximum rate because the tooth  106  is traveling toward the center of the variable reluctance sensor  102 . 
       FIGS. 23-30  illustrate four positions of an example drilled wheel  111  as it moves past an example variable reluctance sensor  112 . One position is illustrated by a front view of  FIG. 23  and side view of  FIG. 24 . One position is illustrated by a front view of  FIG. 25  and side view of  FIG. 26 . One position is illustrated by a front view of  FIG. 27  and side view of  FIG. 28 . One position is illustrated by a front view of  FIG. 29  and side view of  FIG. 30 . 
     In  FIGS. 23 / 24 ,  25 / 26  and  27 / 28 , the variable reluctance sensor  112  is excited by a coil, similar to the coil of the rotor position detection system  20 , contained in printed circuit board  113 . For  FIGS. 29 / 30 , the variable reluctance sensor  112  is excited by a coil  114 , wound around the permeable path for the sensor. 
     For the example illustrated in  FIG. 23 , the hole  115  is in maximum alignment with the variable reluctance sensor  112 . The magnetic path length for flux through the coil travels through the ferrous material of the hole  115  and the magnetic flux is at a minimum because the reluctance is at a maximum. 
     For the example illustrated in  FIG. 25 , the hole  115  is traveling out of alignment with the variable reluctance sensor  112 . The magnetic path length for flux through the coil travels through the ferrous material of the hole  115  on one side, but mostly through free air on the other side. The magnetic flux is rising at a maximum rate because the hole  115  is traveling away from the center of the variable reluctance sensor  112 . 
     For the example illustrated in  FIG. 27 , the hole  115  is at minimum alignment with the variable reluctance sensor  112 , while next hole  116  begins to approach the variable reluctance sensor  112 . The magnetic path length for flux through the coil travels through no ferrous material and the magnetic flux is at a maximum because the reluctance is at a minimum. 
     For the example illustrated in  FIG. 29 , the next hole  116  is beginning to come into alignment with the variable reluctance sensor  112 . The magnetic path length for flux through the coil travels through the ferrous material of the next hole  116  on one side, most mostly through free air on the other side. The magnetic flux is falling at a maximum rate because the next hole  116  is traveling toward the center of the variable reluctance sensor  112 . 
       FIG. 31  illustrates an example controller  100 , which may be applied as rotor position controller or reluctance sensing circuit. The controller may include a processor  200 , a memory  201 , and a communication interface  203 . The communication interface  203  may communicate with a parallel input signal  210 , a sensor input signal  212 , a display device  214 , and/or an input device  204 . Additional, different, or fewer components may be included. 
       FIG. 32  illustrates an example flow chart for operation of the controller  100 . Additional, different, or fewer components may be used. 
     At act S 101 , the controller  100  receives data indicative of a current generated by a reluctance sensor of the electric machine. The current may originate with a magnetic flux through a magnetically permeable element positioned in proximity to a rotor of an electric machine. The data may be a voltage value determined by a voltage sensor connected to a reluctance coil configured to receive the magnetic flux. 
     At act S 103 , the controller  100  performs a comparison of the data indicative of the current in the reluctance sensor to a threshold. The current generated by the reluctance sensor is proportional to the reluctance of a magnetic circuit including at least one air gap and at least one permeable element. The current generated by the reluctance sensor is proportional to the reluctance of a magnetic circuit including a first permeable element, a second permeable element, and an air gap and between the first permeable element and the second permeable element. The data indicative of the current in the reluctance sensor indicates a presence or absence of rotor teeth spaced apart on the rotor. The data indicative of the current in the reluctance sensor indicates a hole in the rotor and configured to affect magnetic flux to the reluctance. 
     At act S 105 , the controller  100  determines a speed of the rotor or a position of the rotor based on the comparison. The controller compares the data indicative of the voltage generated by the reluctance coil in order to estimate a position or speed of the rotor. 
     At act S 107 , the controller  100  generates a command for the rotor based on the detected speed of the rotor or position of the rotor. For example, the controller  100  may determine or identify a speed setting (e.g., constant speed) as a target for the speed of the rotor. In another example, the controller  100  may determine or identify a position setting as a target for the position of the rotor (e.g., stepper motor). In another example the controller  100  may determine a speed setting corresponding to a target output (e.g., target output frequency) of a generator. The engine driving the generator may operate at a variable speed with the output voltage controlled by adjusting the engine speed according to the speed setting based on feedback from the reluctance sensor and the controller  100 . 
     The processor  200  may include a general processor, digital signal processor, an application specific integrated circuit (ASIC), field programmable gate array (FPGA), analog circuit, digital circuit, combinations thereof, or other now known or later developed processor. The processor  200  may be a single device or combinations of devices, such as associated with a network, distributed processing, or cloud computing. 
     The memory  201  may be a volatile memory or a non-volatile memory. The memory  201  may include one or more of a read only memory (ROM), random access memory (RAM), a flash memory, an electronic erasable program read only memory (EEPROM), or other type of memory. The memory  201  may be removable from the network device, such as a secure digital (SD) memory card. 
     In addition to ingress ports and egress ports, the communication interface  303  may include any operable connection. An operable connection may be one in which signals, physical communications, and/or logical communications may be sent and/or received. An operable connection may include a physical interface, an electrical interface, and/or a data interface. 
     The communication interface  203  may be connected to a network. The network may include wired networks (e.g., Ethernet), wireless networks, or combinations thereof. The wireless network may be a cellular telephone network, an 802.11, 802.16, 802.20, or WiMax network. Further, the network may be a public network, such as the Internet, a private network, such as an intranet, or combinations thereof, and may utilize a variety of networking protocols now available or later developed including, but not limited to TCP/IP based networking protocols. 
     While the computer-readable medium (e.g., memory  201 ) is shown to be a single medium, the term “computer-readable medium” includes a single medium or multiple media, such as a centralized or distributed database, and/or associated caches and servers that store one or more sets of instructions. The term “computer-readable medium” shall also include any medium that is capable of storing, encoding or carrying a set of instructions for execution by a processor or that cause a computer system to perform any one or more of the methods or operations disclosed herein. 
     In a particular non-limiting, exemplary embodiment, the computer-readable medium can include a solid-state memory such as a memory card or other package that houses one or more non-volatile read-only memories. Further, the computer-readable medium can be a random access memory or other volatile re-writable memory. Additionally, the computer-readable medium can include a magneto-optical or optical medium, such as a disk or tapes or other storage device to capture carrier wave signals such as a signal communicated over a transmission medium. A digital file attachment to an e-mail or other self-contained information archive or set of archives may be considered a distribution medium that is a tangible storage medium. Accordingly, the disclosure is considered to include any one or more of a computer-readable medium or a distribution medium and other equivalents and successor media, in which data or instructions may be stored. The computer-readable medium may be non-transitory, which includes all tangible computer-readable media. 
     In an alternative embodiment, dedicated hardware implementations, such as application specific integrated circuits, programmable logic arrays and other hardware devices, can be constructed to implement one or more of the methods described herein. Applications that may include the apparatus and systems of various embodiments can broadly include a variety of electronic and computer systems. One or more embodiments described herein may implement functions using two or more specific interconnected hardware modules or devices with related control and data signals that can be communicated between and through the modules, or as portions of an application-specific integrated circuit. Accordingly, the present system encompasses software, firmware, and hardware implementations. 
     In accordance with various embodiments of the present disclosure, the methods described herein may be implemented by software programs executable by a computer system. Further, in an exemplary, non-limited embodiment, implementations can include distributed processing, component/object distributed processing, and parallel processing. Alternatively, virtual computer system processing can be constructed to implement one or more of the methods or functionality as described herein. 
     Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and anyone or more processors of any kind of digital computer. Generally, a processor may receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer may also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. Computer readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     The illustrations of the embodiments described herein are intended to provide a general understanding of the structure of the various embodiments. The illustrations are not intended to serve as a complete description of all of the elements and features of apparatus and systems that utilize the structures or methods described herein. Many other embodiments may be apparent to those of skill in the art upon reviewing the disclosure. Other embodiments may be utilized and derived from the disclosure, such that structural and logical substitutions and changes may be made without departing from the scope of the disclosure. Additionally, the illustrations are merely representational and may not be drawn to scale. Certain proportions within the illustrations may be exaggerated, while other proportions may be minimized. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive. 
     While this specification contains many specifics, these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     One or more embodiments of the disclosure may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any particular invention or inventive concept. Moreover, although specific embodiments have been illustrated and described herein, it should be appreciated that any subsequent arrangement designed to achieve the same or similar purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all subsequent adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the description. 
     It is intended that the foregoing detailed description be regarded as illustrative rather than limiting and that it is understood that the following claims including all equivalents are intended to define the scope of the invention. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.