Abstract:
The present invention can diagnose a potential discrepancy in electrical operating characteristics of an electric motor by generating two independent torque estimates using a plurality of current sensors and optionally a shaft position sensor. The invention provides a strategy to generate two independent torque estimates of a three phase electric motor comprising first and second systems to determine current in each motor phase, first and second systems to generate a first and second estimate of motor shaft position, and first and second systems to generate first and second estimates of motor torque using the first and second systems to determine current in each motor phase and the first and second estimates of motor shaft position. The present invention detects also an electrical operating characteristic discrepancy in an electric motor-propelled vehicle&#39;s electrical components and subsystems, including single subsystem discrepancies between the two independent torque estimates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 10/063,345, filed Apr. 12, 2002 and abandoned Aug. 20, 2003, entitled “Diagnostic Method for an Electric Motor Using Torque Estimates,” which is assigned to the assignee of the present invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates generally to an electrically powered vehicle, such as an electric vehicle (EV), a hybrid electric vehicle (HEV) or a fuel cell vehicle (FCV). More specifically, the invention relates to a strategy to diagnose a potential deviation in operating characteristics of an electric motor. The present invention can determine two independent electric motor torque estimates using a plurality of current transducers and optionally a shaft position sensor for the traction motor.  
           [0004]    2. Background Art  
           [0005]    The invention may be used in a hybrid electric vehicle of the type schematically shown in FIG. 1 of co-pending application Ser. No. 09/683,026, filed Nov. 9, 2001; in FIG. 1 of co-pending application Ser. No. 09/712,436, filed Nov. 14, 2000; as well as in co-pending applications Ser. No. 10/063,345, filed Apr. 12, 2002; and Ser. No. 09/966,612, filed Oct. 1, 2001. Each of these co-pending applications is assigned to the Assignee of the present invention.  
           [0006]    The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.  
           [0007]    The HEV is described in a variety of configurations. Many HEV patents disclose systems in which an operator is required to select between electric motor and internal combustion engine operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.  
           [0008]    Other configurations include, for example, a series hybrid electric vehicle (SHEV) configuration. A series hybrid vehicle has an engine (typically an ICE) connected to an electric motor/generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor can function as the sole source of wheel torque. There is no direct mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery using power produced by the ICE.  
           [0009]    A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations. It sometimes is referred to as a “split-power” configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the motor/generator, is connected to a sun gear. The ICE is connected to a planetary gear carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, motor/generator and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed, compared to conventional vehicles, by using the generator to control engine speed.  
           [0010]    The desirability of combining an ICE with electric motors is clear. There is potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or driveability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operation of the vehicle with the engine shut down.  
           [0011]    One such area of development for optimizing potential benefits of a hybrid electric vehicle involves calculating torque estimates delivered by an electric motor or motors. An effective and successful HEV design (or any vehicle powertrain propelled by electric motors and optionally capturing regenerative braking energy) requires reliable operation that can be improved through careful diagnosis of electric motor operation. Thus there is a need for a strategy to effectively detect potential discrepancies in electrical operating characteristics in an electric motor propelled vehicle&#39;s electrical components and sub-systems.  
           [0012]    Previous efforts have used rotor position sensors or estimates as part of the control strategy for an electric motor. For example, Jones et al. (U.S. Pat. No. 6,211,633) disclose an apparatus for detecting an operating condition of a machine by synchronizing sampling instants with the machine condition so that reliability data are obtained. The operating condition may be the position of the rotor, in which case estimates of the rotor position and rotor velocity at each of the sampling instants are developed.  
           [0013]    Lyons et al. (U.S. Pat. No. 5,864,217) disclose an apparatus and method for estimating rotor position and commutating a switched reluctance motor (SRM), using both a flux/current SRM angle estimator and a toothed wheel generating a magnetic pickup. Phase errors can be compensated by adjusting the angle input to the commutator as a function of estimated speed. Alternately, the flux/current SRM angle estimator can be run in background mode to tune the toothed wheel interrupt angle signal at different speeds.  
           [0014]    Drager et al. (U.S. Pat. No. 5,867,004) disclose a control for operating an inverter coupled to a switched reluctance machine that includes a relative angle estimation circuit for estimating rotor angle for a phase in the switched reluctance machine.  
           [0015]    Lyons et al. (U.S. Pat. No. 5,107,195) disclose a method and apparatus for indirectly determining rotor position in a switched reluctance motor that is based on a flux/current model of the machine, which model includes multi-phase saturation, leakage, and mutual coupling effects.  
           [0016]    Lastly, Acarnley (U.S. Pat. No. 6,005,364) discloses a motor monitoring and control circuit that calculates a value parameter for a position of the motor at given instants. The same parameter (which may be position or speed of a rotor) is then measured at subsequent instants. These values are used to compute a future value of the parameter.  
           [0017]    The use of two independent torque estimates to diagnose a potential deviation in the operating characteristics of the electric motor in an electric motor propelled vehicle is unknown in the prior art.  
         SUMMARY OF THE INVENTION  
         [0018]    Accordingly, the present invention provides a strategy to effectively detect the operating conditions in an electric-motor-propelled vehicle&#39;s electrical components and subsystems by creating two independent torque estimates of an electric motor for a hybrid electric vehicle (HEV) using a plurality of current transducers and, optionally, a shaft position sensor. Discrepancies between the two independent torque estimates or the signals used to create the two independent torque estimates can be indicative of potential discrepancies in electrical operating conditions such as a stray current leakage condition.  
           [0019]    More specifically, the invention provides a strategy to generate two independent torque estimates for a three phase electric motor comprising first and second systems to determine current in each motor phase, third and fourth systems to generate a first and second estimate of motor shaft position, and fifth and sixth systems to generate first and second estimates of motor torque using the first and second systems to determine current in each motor phase and the first and second estimates of motor shaft position.  
           [0020]    The strategy uses four current sensors to generate four measured currents, which are used for the first and second systems to determine current in each motor phase. The estimate of motor shaft position can be made using Kalman filters. Alternatively the motor shaft position estimate can be made using a resolver.  
           [0021]    Other objectives and features of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description, taken in conjunction with the accompanying figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The foregoing objects, advantages, and features, as well as other objects and advantages, will become apparent with reference to the description and figures below, in which like numerals represent like elements and in which:  
         [0023]    [0023]FIG. 1 illustrates a general hybrid electric vehicle (HEV) configuration of the type disclosed in the co-pending application identified in the foregoing “Background of Invention”, which may incorporate the present invention.  
         [0024]    [0024]FIG. 2 illustrates an electric traction motor for the hybrid electric vehicle shown in FIG. 1.  
         [0025]    [0025]FIG. 3 illustrates electric motor stator windings connected in a “wye” configuration for use in the traction motor of FIG. 1.  
         [0026]    [0026]FIG. 4 illustrates an arrangement of four current sensors having two sensors in each of two phases, which is used in practicing the present invention.  
         [0027]    [0027]FIG. 5 illustrates an alternate arrangement of four current sensors, which may be used in practicing the present invention.  
         [0028]    [0028]FIG. 6 illustrates the strategy of the present invention in block diagram form. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    The present invention relates to electric motors. For demonstration purposes and to assist in understanding the present invention, it is described in a hybrid electric vehicle (HEV) application. FIG. 1 demonstrates just one possible HEV configuration, specifically a parallel/series hybrid electric vehicle configuration.  
         [0030]    In a basic HEV, a planetary gear set  20  mechanically couples a carrier  22  to an engine  24  via a disconnect clutch  26 . The planetary gear set  20  also mechanically couples a sun gear  28  to a generator-motor  30  and a ring (output) gear  32 . The generator-motor  30  may be braked by a generator brake  34  to provide reaction torque for carrier  22 . It is electrically linked to a battery  36 . A traction motor  38  is mechanically coupled to the ring gear  32  of the planetary gear set  20  via a second gear set  40  and is electrically linked to the battery  36 . The ring gear  32  of the planetary gear set  20  and the traction motor  38  are mechanically coupled to drive wheels  42  via an output shaft  44 .  
         [0031]    The planetary gear set  20  splits the output energy of engine  24  into a series path from the engine  24  to the generator motor  30  and a parallel path from the engine  24  to the drive wheels  42 . Engine  24  speed can be controlled by varying the split to the series path while maintaining the mechanical connection through the parallel path. The traction motor  38  augments the engine  24  power to the drive wheels  42  on the parallel path through the second gear set  40 . The traction motor  38  also provides an opportunity to use energy directly from the series path, essentially running off power created by the generator-motor  30 . This reduces losses associated with converting electrical energy into and from chemical energy in the battery  36  and allows all energy of engine  24 , minus conversion losses, to reach the drive wheels  42 .  
         [0032]    A vehicle system controller (VSC)  46  controls many components in this HEV configuration by connecting to each component&#39;s controller. An engine control unit (ECU)  48  connects to the engine  24  via a hardwire interface. All vehicle controllers can be physically combined in any combination or can stand as separate units. They are described as separate units here because they each have distinct functions. The VSC  46  communicates with the ECU  48 , as well as a battery control unit (BCU)  50  and a transaxle management unit (TMU)  52 , through a communication network such as a controller area network (CAN)  54 . The BCU  50  connects to the battery  36  via a hardwire interface. The TMU  52  controls the generator motor  30  and traction motor  38  via a hardwire interface.  
         [0033]    A basic diagram of the traction motor  38  is illustrated in FIG. 2. The traction motor  38  has a stator  100 , having slots  104  and teeth  106 . Motor windings  108  carry electric current through the traction motor  38 . The windings are connected in a “wye” configuration, as illustrated in FIG. 3. Interior to stator is the rotor  102 . The illustrated rotor  102  has permanent interior magnets  110 . The motor shaft  112  passes through the rotor  102 . A resolver  114  can be connected to the motor shaft  112 .  
         [0034]    The windings  108  of a three phase electric motor can be represented as being arranged in a “wye.” Each of the three phases, commonly referred to as phases “a,” “b” and “c,” are represented by one leg of the “wye.” The “wye” configuration is illustrated in FIG. 3. Phase “a”  120  would have a corresponding electric current, current (I a ) 122, passing through it. Similarly, phases “b” 124 and “c”  128  would have corresponding electric currents, current (I b )  126  and current (I c )  130 , respectively, passing through them as well. Measurement or estimation of all three motor phase currents ( 122 ,  126 , and  130 ) and the motor shaft  112  position angle is required to calculate the motor torque.  
         [0035]    In the present invention the VSC  46  can detect the motor&#39;s operating condition generally by two procedures (shown in FIGS. 4 and 5) using alternate types of independent estimations of machine torque. For the embodiments presented, four current sensors per electric motor are used. Many other types of configurations are possible. Sensor output can be sent to the VSC  46  where appropriate action may be taken, such as lighting an indicator lamp or sounding an indicator tone to notify the operator of a potential system electrical deviation in operating characteristics.  
         [0036]    [0036]FIG. 4 shows a first embodiment of the present invention. FIG. 4, like FIG. 3, shows the “wye” configuration of the three phases of the electric motor. In practice, any individual leg of the “wye” can be any of the individual phases. In FIG. 4, the phases will be referred to as phases “x,” “y” and “z,” where phases “x, ” “y” and “z” can be any ordering of phases “a,” “b” or “c.” Phase “x”  140  would having a corresponding electric current, current “x” (I x )  142 , passing through it. Similarly, phases “y”  144  and “z”  148  would have corresponding electric currents, current “y” (I y )  146  and current “z” (I z )  150 , respectively, passing through them as well.  
         [0037]    Added to the “wye” configuration are four current sensors. The first current sensor  152  gives a measured current “x ” (i x ) . The second current sensor  154  gives a second measured current “x” (i x ′). The third current sensor  156  gives a measured current “y” (i y ). The fourth current sensor  158  gives a second measured current “y” (i y ′). These sensors can be of any type known in the art for measuring motor phase current, such as a resistive shunt or non-contacting current transducers. They can be either active or passive.  
         [0038]    [0038]FIG. 5 shows an alternative arrangement of four current sensors on the legs of the “wye” configuration representing the phases of the electric motor. In this embodiment, the first current sensor  152  gives a measured current “x” (i x ) . The second current sensor  154  gives a second measured current “x” (i x ′). The third current sensor  156  gives a measured current “y” (i y ) . The fourth current sensor  160  gives a measured current “z” (i z ′).  
         [0039]    [0039]FIG. 6 illustrates a possible strategy using the present invention in block diagram form. An inverter control for operating a switched reluctance machine  178  includes resolver  114  coupled by a motive power shaft  184  to the rotor  102  of the switched reluctance machine  178 . Excitation is provided by a resolver excitation circuit  188 . The resolver  114  develops first and second signals over lines  192  and  194  that have a phase quadrature relationship (also referred to as sine and cosine signals). A resolver-to-digital converter  190  is responsive to the magnitudes of the signals on the lines  192  and  194  and develops a digital output representing the position of the rotor  102  of the switched reluctance machine  178 . The position signals are supplied along with a signal representing machine rotor  102  velocity to a control and protection circuit  170 . The rotor  102  position signals are also supplied to a commutation circuit  180  and a current control circuit  172  having an input coupled to an output of the control and protection circuit  170 . Circuits  170  and  172  further receive phase current magnitude signals as developed by an inverter  176 . The circuits  170  and  172  develop switch drive signals on lines  174  for the inverter  176  so that the phase currents flowing in the windings of the switched reluctance machine  178  are properly commutated.  
         [0040]    A position estimation circuit or subsystem  182  is responsive to the phase current magnitudes developed by the inverter  176 , switch control or drive signals for switches in the inverter  176  and DC bus voltage magnitude to develop position and velocity estimate signals for the control and protection circuit  170 . In addition, the position estimate signals are supplied to the commutation circuit  180 . The current control circuit  172  is responsive to the phase current magnitudes developed by the inverter  176 , as well as phase enable output signals developed by the commutation circuit  180  and a reference current signal developed by the control and protection circuit  170 . The current control circuit  172  produces the switch control or drive signals on lines  174  for the inverter  176 . Measurements from these systems allow the development of strategies to estimate normal traction motor  38  torque.  
         [0041]    The resolver  114 , known in the prior art, is a direct measurement of rotor  102  position angle. A Kalman filter based estimation method, also known in the art, can generate a second independent calculation of the rotor  102  position angle in electric and hybrid-electric vehicles.  
         [0042]    Currents “a”  122 , “b”  126 , and “c”  130  in the three phases of the “wye” {“a”  120 , “b”  124 , and “c”  128 } are actively switched at high frequency by the three phase inverter  176  between the motor windings  108  and a direct current voltage source, such as the battery  36 .  
         [0043]    The traction motor  38  has the ideal torque “T” characteristic as follows: Equation 1:  
       T   =       3   4          p        [         MI   f          I   q       +       (       L   d     -     L   q       )          I   d          I   q         ]                               
 
         [0044]    where:  
         [0045]    p is the number of motor poles (known),  
         [0046]    M is the rotor to stator mutual inductance (known),  
         [0047]    I f  is the “equivalent” current corresponding to the permanent magnet magnetic flux (known),  
         [0048]    L d  is the direct axis inductance (known),  
         [0049]    L q  is the quadrature axis inductance (known),  
         [0050]    I d  is the “direct” axis current (estimated from measured and other values), and  
         [0051]    I q  is the “quadrature” axis current (estimated from measured and other values).  
         [0052]    To generate relative currents {I d ,I q } in a frame that rotates at the rotor velocity, one can write:  
         [0053]    Equation 2:  
         I   d     =       2   3          [         I   a        cos                 θ     +       I   b          cos        (     θ   -   γ     )         +       I   c          cos        (     θ   +   γ     )           ]                             
 
         [0054]    Equation 3:  
         I   q     =     -       2   3          [         I   a        sin                 θ     +       I   b          sin        (     θ   -   γ     )         +       I   c          sin        (     θ   +   γ     )           ]                               
 
         [0055]    where:  
         [0056]    I a , I b , I c  are the state “wye” coil current  122 ,  126 , and  130 ,  
         [0057]    ⊖is the rotor position angle, and  
         [0058]    Υis the electrical phase angle between stator coils, and  
         [0059]    where:  
       γ   =         2   3        π     =     120                   deg   .                               
 
         [0060]    To generate two independent estimates of electrical machine torque by using Equation 1, two independent ways to find I d , and I q  are required. These currents in turn each depend upon two signal sets:  
         [0061]    1. The “wye” connected stator phase coil currents {I a    122 , I b    126 , I c    130 }, and  
         [0062]    2. The motor shaft  112  position angle ⊖.  
         [0063]    At least two independent strategies are described to independently estimate each of these two signal sets. For the first strategy, assume each of the three legs of the stator coil has current flowing in that leg. The machine winding neutral at the center of the “wye” is not connected, which is true for the case of inverter driven motors. Because Kirchoff&#39;s current law, known to those skilled in the art, applies to the “wye” connected circuit, the currents {I a    122 , I b    126 , I c    130 } obey the relationship: Equation 4:  
           I   a   +I   b    +I   c =0  
         [0064]    Only two currents need to be known to estimate the third current.  
         [0065]    For example, if {I a , i b , i c } represent current sensor outputs measuring the currents {I a    122 , I b    126 , I c    130 }, then by measuring any two, for example {i a , i b }, one can estimate the third i c  as:  
         [0066]    Equation 5:  
         {circumflex over ( i )} c =−( i   a   +i   b )  
         [0067]    where î c  represents an estimated, not measured, output signal. By using two current sensors, we have estimated the three phase stator currents as {(i a ,i b ,î c }).  
         [0068]    To generate a redundant and completely independent second strategy to estimate stator currents, we cannot rely on either sensor indicating {i a , i b }. Instead, we can redundantly measure {i a , i b } with two additional sensors {i a ′}, as in FIG. 4, and apply Equation 5 to generate the second estimate of i c ′ as:  
         {circumflex over ( i )} c ′=−( i   a ′+ i   b ′).  
         [0069]    Alternatively, we might choose to measure i c ′ directly, as in FIG. 5, and either of {i a ′, i b ′} directly, and then apply Equation 5 to estimate the remaining current as:  
         {circumflex over ( i )} b ′=−(i a ′+i c ′), or {circumflex over ( i )} a ′=−(i b   ′+i   c ′)  
         [0070]    This dual stator current estimation is summarized in Table 1, where {“x” “y” “Z”} are any ordering of the stator coils {“a” “b” “c”}.  
                                                   TABLE 1                           Alternate Ways to Estimate One of the Three Stator Currents                Independent               Strategy 1:       Actual   Use Sensors and   Independent Strategy 2: Use any Column of       Current   Estimators   Sensors and Estimators                    I x  142   i x     i x ′   i x ′   −(i y ′ + i z ′)       I y  146   i y     i y ′   −(i x ′ + i z ′)   i y ′       I z  150   −(i x  + i y )   −(i x ′ + i y ′)   i z ′   i z ′                  
 
         [0071]    Referring to the Table 1, the far left column for Independent Strategy  2  redundantly measures the same two phase currents {“x” 142, “y” 146} as does Independent Strategy  1 . Putting two current sensors in the same leg may simplify the sensor packaging if two sensors {x 152, x′154}, for example, can share any of their non-critical components. Such non-critical components can include passive parts such as a sensor housing, mounting fasteners, ferrite core and electrical connector housing. In this case, Equation 4 can be validated as Equation 7 as follows:  
           i   X   +i   y +−( i   x   ′+i   y ′)=0.  
         [0072]    Furthermore, sensors in the same leg can be cross-checked as Equation 8 as follows:  
         ( i   x   −i   x ′)=0, 
         ( i   y   −i   y ′)=0. 
         [0073]    Any stray current leakage in coil “c” (due to short circuit deviations in operating characteristics in wiring to the coil, the coil drivers, and between the coil windings and the stator core) is not explicitly sensed.  
         [0074]    Alternatively, the right two columns of Independent Strategy  2  redundantly measure only one of the two phase currents I x    142  or I y    146  as measured in Independent Strategy  1 . The other phase current I z  ,  150 , has a separate sensor  160  to generate signal i z ′, resulting in three unique signals {i x , i y , i z ′} to verify Equation 4 as Equation 9 as follows:  
           i   x   +i   y + i   z ′=0.  
         [0075]    If either of the last two columns in the table is selected, any stray current leakage in stator coil c is explicitly sensed, which may enable detection of additional faults causing current leakage in stator coil c.  
         [0076]    In using a total of four current sensors on two or three legs of the traction motor&#39;s “wye” windings as in FIGS. 4 and 5, all three current measurements can be generated in two independent ways, and cross-checked to detect whether any one or more measurements should be faulted.  
         [0077]    All present inverter motor control technologies require the rotor  102  position ⊖ according to Equations 2 and 3. Motor shaft  112  angle ⊖ can be measured directly by a sensor called the resolver  114 , or estimated using an observer or Kalman filter based upon the measured motor currents.  
         [0078]    An alternate embodiment of the present invention adds the resolver 114 to the embodiment described above. Traditionally, the inverter torque motor controls use the resolver  114 , composed of a “toothed” ring consisting of a plurality of teeth rotating with the motor shaft  112  being measured, and one or more stationary “tooth” sensors of some technology, be it optical, variable reluctance, Hall effect, or other technology known in the art. If one “toothed” ring and one sensor are used, the resolver  114  is also called a “tone wheel.” The tone wheel measures relative position, and it is not capable of sensing direction of travel. Some “tone wheels” omit a tooth as a reference absolute position, but measurement is only relative, so measurement during changes of direction is impossible. If two “tooth” sensors are used, the resolver  114  can sense direction, but it still cannot measure absolute position. If more than two “tooth” sensors are used, the resolver  114  can sense direction and absolute position. Some drawbacks of resolvers are their expense, high failure rates, and requirement of a high speed interface at the microprocessor that receives their output signals.  
         [0079]    Methods have been developed to estimate the motor shaft  112  position. The estimate being derived not from a resolver  114 , but from implicit characteristics of the motor. One such characteristic of an inductance motor is the mutual inductance between the stator coils and the induced current in the rotor  102 , which is dependent upon the relative angle between the two and can be estimated from the motor phase currents {I a    122 , I b    126 , I c    130 }. Another characteristic that can be used to estimate motor shaft  112  position is the back EMF of the motor, known to those skilled in the art as a voltage across the coil that increases with motor speed.  
         [0080]    There are well-documented methods that capitalize on these position dependent motor characteristics to estimate the motor shaft  112  relative position. One method is an observer. Another method is a special case of observer called a Kalman filter. In general, the observer will compute by Equation 10:  
         {circumflex over (θ)}= F ( s )( I   a   ,I   b   ,I   c )  
         [0081]    where F(s) is the observer transfer function.  
         [0082]    To generate separate and independent estimates of {circumflex over (θ)} motor shaft  112  position, a first estimate is generated using the stator current estimation approach of Independent Strategy  1  given above, and a second estimate using the Independent Strategy  2 . The combined current and motor shaft  112  position measuring method can detect all single point deviations in desired operating characteristics and is robust in that it can enable safe, if not complete, operation even when such deviations occur and are detected.  
         [0083]    Alternatively, one independent motor shaft  112  angle may be measured with a resolver  114 , and a second independent motor shaft  112  angle may be estimated using the proposed observer or Kalman filter and either of the phase current measuring proposals.  
         [0084]    The above-described embodiments of the invention are provided purely for purposes of example. Variations, modifications, and applications of the invention may be made by persons skilled in the art without departing from the scope of the invention. All such variations, modifications and applications, as well as equivalents thereof, are intended to be covered by the following claims.