Patent Document

This application claims benefit of provisional application Ser. No. 60/102,846, filed Oct. 2, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to switched reluctance motor drive systems; and in particular, to an apparatus and method for commutation of a switched reluctance motor. 
     BACKGROUND OF THE INVENTION 
     Due to high system efficiency over a wide range of speed, multiphase switched reluctance motors (SRMs) are favored for their variable-speed drive applications. A conventional SRM includes salient rotor and stator poles, as shown in FIG.  1 . Such motors typically have a fixed stator structure comprising one or more phase windings, and a rotor structure. As shown, each pair of diametrically opposite stator pole windings is connected in series or in parallel to form an independent phase winding of the multiphase SRM. Direct current is selectively switched to pass through the phase windings. Resultant electromagnetic fields induced by the windings interact with the fixed fields of the rotor in a manner resulting in a rotary force or torque which causes the rotor to rotate relative to the stator. 
     Illustrated in FIG. 2, when the stator and rotor poles are aligned, maximum inductance exists. Applying the definition of torque        (       i   .   e   .              Ta               L          θ            i   2       )                          
     and noting the relationship between inductance, angular position and phase in FIG. 3, when the poles are aligned, the change of inductance, dL, is positive. Thus, torque, T, is positive. In order to maintain positive torque, however, it is necessary to switch the applied current off at some reference angle θ ref  just prior to the change of inductance dL of the phase becoming negative which corresponds to the phase at an unaligned position. Thus, switching current from one phase winding to the next in a predetermined sequence that is synchronized with the angular position of the rotor continuously generates positive torque. As such, SRMs require angular rotor position sensing devices to determine the position of the rotor and thereby, maintain positive torque of the rotor. 
     In conventional SRMs, a shaft angle transducer, such as an encoder or a resolver, generates a rotor position signal and a controller reads this rotor position signal. In an effort to improve reliability while reducing size and cost, various approaches have been previously proposed to eliminate the shaft position sensor by determining the reference commutation angle. These approaches implement indirect rotor position sensing by monitoring terminal voltages and currents of the motor. 
     One approach is disclosed in U.S. Pat. No. 4,959,596, issued to S. R. MacMinn, et al., on Sep. 25, 1990 which patent is incorporated by reference herein. As disclosed, a method of indirect motor position sensing involves applying voltage sensing pulses to one unenergized phase. The result is a change in phase current which is proportional to the instantaneous value of the phase inductance. Proper commutation time is determined by comparing the change in phase current to a reference current, thereby synchronizing phase excitation to rotor position. Phase excitation can be advanced or retarded by decreasing or increasing the threshold, respectively. Due to the unavailability of inactive phases during higher speeds, this commutation method which makes use of the inactive phases of the SRM are limited to low speeds. Furthermore, although current and torque levels are relatively small in an inactive phase, they will contributed to a loss in SRM efficiency. 
     Another such approach is disclosed in U.S. Pat. No. 5,140,243, issued to J. P. Lyons, et al., on Sep. 25, 1990 which patent is incorporated by reference herein. As disclosed, a method of indirect motor position sensing involves using a flux-current map of a given SRM, such as the one illustrated in FIG.  2 . Utilizing this flux-current map, measured phase voltage, phase current and phase resistance and estimated flux provided necessary data to determine a reference angle. Comparison of the estimated phase flux to the reference flux is the basis for commutating the motor. The disadvantage of this approach is that the flux-current characteristics of a motor are not readily known; thereby, requiring costly calibration measurements. Additionally, these characteristics exhibit change over time, requiring recalibration of the SRM. Therefore, this commutation method is costly. 
     Although the above-cited patent advantageously provides a method for indirectly determining rotor position so that a conventional rotor position sensor is not required, it is desirable to provide a method which does not require the need for prior knowledge of the flux-current characteristics of the SRM. 
     A control system and method for a multiphase switched reluctance motor (SRM) provides commutation of the motor operable at high speeds, requiring no rotor position sensor not detailed prior knowledge of the SRM magnetic characteristics. This commutation method and system includes two routines: a calibration routine and a commutation routine. The calibration routine is a self-training calibration routine to determine the flux-current characteristics of each phase in an aligned position. During this calibration routine, voltage sensing pulses of current applied to an active phase (i.e. one producing torque) create a change in phase current which is inversely proportional to the instantaneous value of the phase inductance. Using the integral form of Faraday&#39;s law, each pulse of current provides the appropriate variables to determine phase flux. Subsequent interpolation of the data to fit a curve provides the necessary data for deriving flux-current characteristics at a reference angle for commutating the motor. During the commutation routine, a commutation algorithm commutates the SRM by measuring the flux in an active phase and comparing the flux to one approximated for the reference angle. 
     The method of the invention is particularly well-suited for relatively heavy duty loading applications, such as a fan. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of a conventional four-phase SRM; 
     FIG. 2 is a flux-current-angle mapping relationship of a conventional SRM; 
     FIG. 3 is a variable inductance profile for the four-phase SRM of FIG. 1; 
     FIG. 4 is a flux-current-angle mapping relationship in accordance with the present invention; 
     FIG. 5 is a SRM calibration routine flow chart in accordance with the present invention; 
     FIG. 6 is a SRM commutation routine flow chart in accordance with the present invention; 
     FIG. 7 is a block diagram of SRM controller, using position feedback; and 
     FIG. 8 is a block diagram of position sensorless SRM controller. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention improves the prior art by providing an approach for commutating an SRM, without a position sensor, while removing the requirement of a predetermined mode of a flux-control relationship of the SRM. Accordingly, a method and apparatus providing indirect estimation of instantaneous rotor angular position is disclosed. 
     FIG. 1 shows a conventional SRM drive configuration. By way of example, SRM  10  is illustrated as a four-phase machine. As shown, SRM  10  includes a rotor  12  rotatable in either a forward or reverse direction within a stationary stator  14 . Rotor  12  has three pairs of diametrically opposite rotor poles  16   a - 16   b,    18   a - 18   b  and  20   a - 20   b.  Stator  14  has four pairs of diametrically opposite stator poles  22   a - 22   b,    24   a - 22   b,    26   a - 26   b  and  28   a - 28   b.  Stator pole windings  30   a - 30   b,    32   a - 32   b,    34   a - 34   b  and  36   a - 36   b,  respectively, are wound on stator pole pairs  22   a - 22   b,    24   a - 24   b,    26   a - 26   b  and  28   a - 28   b  forming four phases A, B, C, and D. As illustrated, the rotor is in the aligned position for phase A and the unaligned position for phase C. Conventionally, the stator pole windings on each pair of opposing or companion stator pole winds comprising each companion pair  30   a - 30   b,    32   a - 32   b,    34   a - 34   b  and  36   a - 36   b  are connected in series with each other and with an upper and lower current switching devices, respectively. Each phase winding is further coupled to a dc source, such as a battery or rectified ac source, such as a return diode. At the end of each conduction interval of each phase, stored magnetic energy in the respective phase winding is returned, through each respectively coupled diode to the dc source. 
     Typically, as shown in FIG. 1, a shaft angle transducer  38 , e.g. an encoder or a resolver, is coupled to rotor  12  for providing rotor angle feedback signals to machine controller  40 . An operator command, such as a torque command, is also generally supplied as an input signal to controller  40 . The controller  40  provides firing signals to the stator windings for energizing the machine phase windings in a predetermined sequence, depending upon the particular quadrant of operation. To improve reliability of the SRM while reducing size and cost, it is desirable to eliminate the rotor position sensor. Accordingly, the purpose of this invention is to provide a useful approach for operating the SRM, while eliminating the need for a rotor position sensor. 
     In an effort to eliminate the rotor position sensor, it is necessary to determine a reference flux ψ ref  measurement of the SRM at which point a control means in the present invention will provide voltage to the stator windings sufficient to energize the next machine phase. Thus, an analysis of the flux-current characteristics of the winding linkages is a precursor to elimination of the rotor position sensor. 
     As shown in FIG. 2, phase flux ψ is proportional to current I for different values of rotor angle θ. The current I in one phase winding of a SRM and the flux linked ψ by that winding are related by the winding inductance L according to the following expression: ψ=L I. Thus, if phase flux linkage ψ is plotted against phase current I, the slope of the resulting graph is the phase inductance, The bending of the curves at higher values of flux ψ is caused by magnetic saturation of the iron in the motor. Curve ψ a , which has the steepest initial slope, represents the ψ-I curve for the excited phase when the stator poles of that phase are aligned with rotor poles, the rotor angle corresponding thereto being designated as θ a . On the other hand, curve ψ a , which has the smallest initial slope represents the ψ-I-curve for the excited phase when the stator poles of that phase are at the point of maximum unalignment with rotor poles of the SRM, the rotor angle corresponding thereto being designated as θ a . The curves falling between curves ψ a  and ψ a  represent intermediate inductance values corresponding to varying slopes of the curves monotonically decreasing as the rotor advances from the aligned position to the unaligned position. Curve ψ ref  represents the inductance value corresponding to the position of the rotor when the SRM is commutated. Note the flux estimated ψ a  at an aligned rotor position is greater than the reference flux ψ ref . 
     Additionally, as illustrated in FIG. 2, a number of current levels from i min  to i max  are established in the phase winding, where i min  is the minimum current level below which the flux-current curves of the SRM exhibits very low resolution with respect to rotor position and i max  is the maximum rated phase current of the motor. 
     As illustrated in FIG. 3, phase inductance for a four-phase SRM as viewed from the stator phase windings is a strong function of rotor position. Specifically, phase inductance ranges from a maximum value L max , corresponding to alignment of rotor poles with the stator poles of the respective phase, to a minimum value L min , corresponding to maximum unalignment of rotor poles with the stator poles of the respective phase. 
     Ideal phase inductance is a function of rotor angle θ, in electrical degrees. A given inductance value occurs once as the rotor poles are moving toward alignment with stator poles of a respective phase, and again as the poles are moving away from alignment. From the given equation for phase flux ψ, it is apparent that this value of inductance can be determined by corresponding measurements of phase flux ψ and phase current I. Phase flux ψ can be made by employing the relationship between phase flux ψ, phase current I, and phase voltage V according to the following expression: 
     
       
         V=Ir+dψ/dt 
       
     
     where r is the phase winding resistance. An estimate of flux can thus be determined from 
     
       
         ψ est =∫(V−Ir)dt 
       
     
     As is well known, the torque developed in any phase follows the relationship:        T   ∝            L          θ              i   2     .                              
     While the change in inductance, dL, with respect to the change in rotor angle, dθ, is positive, the SRM will generate a positive torque. It is clear from FIG. 3, however, that where the change in rotor angle, dθ, is negative, the SRM will generate a negative torque. Thus, in order to maintain positive torque, at some θ ref  the controller  40  must energize the next phase; for example, energizing phase B while de-energizing phase A. 
     The commutation algorithm according to the present invention starts with a calibration routine, followed by a commutation routine. The inventive method for commutation of a SRM requires neither direct rotor position sensing nor detailed prior knowledge or motor magnetic characteristics. 
     The two routines of commutation method, a calibration routine and a commutation routine, are set forth in FIGS. 5 and 6, respectively. During the calibration routine, the rotor is energized such that each motor phase is aligned and the corresponding phase flux-current characteristics of the SRM are estimated. During the commutation routine, these estimated characteristics are used to define the position sensorless operation of the motor. 
     By exciting any arbitrary phase with sufficient current to generate torque in excess of the starting friction of the motor, the rotor aligns with the aligns with the stator in the position of the energized phase. For example, when the first phase in the sequence, phase A, is energized, the rotor rotates and aligns itself with the stator poles of phase A. The direction of rotation, however, depends on the initial position of the rotor. Typically, the initial position of the rotor is unknown; thus, the initial direction of the motor is unpredictable. In certain applications, e.g. computer hard disk drives, this uncertain behavior is undesirable. In order to determine a calibration routine that produces rotation in a known direction, the technique, as described in U.S. Pat. No. 5,051,680 issued Sep. 24, 1991 to D. J. Belanger, may be implemented and is herein incorporated by reference. A control means applies small voltage pulses to all the motor phases and measures the resulting phase currents. These measurements determine the initial position of the rotor and thus, the phase of the SRM can be controllably energized in a specified 
     Thereafter, each phase is energized in a predetermined sequence, i.e. for a four phase SRM, the sequence may be A, B, C, D, A, B, . . . Accordingly, at each aligned position, the calibration routine applies desired levels of current to each motor phase at a series of discrete test points. 
     At each current test point, estimated phase flux ψ est  is calculated using the integral from of Faraday&#39;s law,                ψ        (   k   )       =       (       ∑     k   =   1     N            [       v        (   k   )       -       i        (   k   )            R        (   k   )           ]        T       )     +     ψ        (   0   )                 [   1   ]                                
     where, 
     ψ(k)=estimated phase flux at time, k 
     v(k)=estimated voltage across the phase winding 
     i(k)=measured phase current 
     R(k)=estimated phase resistance 
     T=sampling rate 
     N=number of intervals in the estimation period 
     ψ( 0 )= 0   
     At the end of the estimation period (k=N), flux-current data pairs, [ψ(N), i(N)], are stored. 
     Before calibrating the next phase, a curve fitting technique (e.g., least squares curve fit) is sued on these data points to minimize the effect of measurement noise. Moreover, curve fitting at this stage also minimizes memory requirements by allowing simultaneous computation for subsequent phases. Furthermore, instead of storing large number of data points, only polynomial coefficients need be stored. As an example, a linear curve fit of the data is expressed as: 
     
       
         ψ j   α ( i )= L   j   α   i   j   
       
     
     has a positive coefficient L j   a  which represents the slope of a straight line. Only this coefficient L j   α  is stored. Thus, the present invention not only reduces memory usage but also eliminates time consuming table lookups. 
     Upon completion of the calibration sequence and subsequent curve fitting of the data, the magnetic characteristics of the motor at the reference angle θ ref  are derived using the expression: 
     
       
         ψ(θ ref ,f)=α(i)g(i) 
       
     
     where g(i)=ψ(θ aligned ,i) and 0≧α(i)≦1. This calibration routine estimates the curve ψ a  for each phase and then uses a advance coefficient α proportional to the desired speed to define a reference flux level. The advance coefficient α is defined as a function of the estimated speed ω. 
     
       
         α=α 0 −k 60  |{circumflex over (ω)}(t)| 
       
     
     where k α  and α 0  are positive constants. Hence, as the actual speed increases, each phases is subsequently energized at shorter interval of time than its predecessor. When the estimated flux in the active phase exceeds the reference level, the next phase is energized. 
     The calibration procedure is repeated for each phase of the motor. The coefficients representing the curve can be stored in a non-volatile memory to avoid calibration after every power down. 
     FIG. 6 illustrates a flux-current diagram including an flux ψ a  at the aligned angle and reference flux ψ ref  generated during the calibration routine of FIG. 5 in accordance with the present invention. 
     FIG. 5 sets forth a flow chart of firmware steps performed during the calibration routine, the initial operation mode  100  during which reference flux ψ ref  is determined. The calibration routine  100  is entered at step  102 . A step  104  initializes a phase counter variable P to 1. At step  106 , the control means (not shown) applies a voltage across the windings of the SRM energizing phase P generating a step energizing current. 
     A step  108  initializes a wait period to wait for the rotor to cease oscillations prior to data collection. The time period, WAIT # 1 , must be greater than or equal to the time when the rotor settles after being energized in response to the step energizing current. Oscillation of the rotor is a factor of friction, inertia of the rotor and of the load. “Analysis of single-step damping in a multistack variable reluctance stepping motor,” A. P. Russell and I. E. D. Pickup, IEE Proceedings on Electric Power Applications, January 1996, pp. 95-107, a non-patent publication, is incorporated by reference herein. Russell et al outline a method by which an analytical approach to defining a period of time to implement the proper wait period. Analysis may include factors of variation of load inertia and friction. 
     A step  110  initializes the count of the flux-current data pair, variable j, to 1. A step  112  initializes phase current to 0. A step  114  initializes another wait cycle of time period WAIT # 2  for to wait for the current to dissipate. A step  116  then initializes the flux integrator and time k to 0. Step  116  also sets the phase current to the i th  value. The voltage is either measured or more preferably computed inside the controller software e.g. in a system using pules-width modulated current controller. The average voltage across the phase is calculated according to the following expression: 
     
       
         v j =d j V but   
       
     
     where d j  is the duty ratio of the phase transistor switch and V bus  is the inverter bus voltage. The phase resistance can be readily measured by using an Ohm meter. 
     A step  118  initializes another wait cycle which responds to an interrupt signal from the DSP which defines the interval time sampling period of the algorithm T. A step  120  increments the time variable k to k+1. At this step, phase current is measured by a control means. Accordingly, phase flux is estimated using equation [1]. Control signals are applied to the phase windings causing current to flow through the windings for determining the initial position of the rotor with respect to the stator from a determination of relative magnitude of the current flow through the phase windings prior to activation of the phase windings to start rotation of the rotor. The estimated flux at k+1 is derived from the summation of estimate flux at k and the new measured values of voltage, current and resistance for the j th  flux-current data pair. 
     A logical node  122  determines if the length of the estimation period, N is less than k. If it is not, a return is made to step  120 . Steps  120  and  122  are repeated until k is greater than the length of estimation period, N. This represents the entire estimation period over which the phase current is read. A step  124  records the current-flux data pair and increments the count i by 1. A logical node  126  determines if j is greater than the number of current test points M. If it is not, a return is made to step  112  where the phase current at phase P is reinitialized to 0. If j is greater than M, all data samples have been determined and recorded, a step  128  reinitializes the phase current to 0. 
     A step  130  interpolates the data to a curve for a particular phase P. To estimate the flux at the next phase, the variable P representing the count for representative phases of the SRM is incremented in this step. A logical node  132  determines whether the count of phases P is greater than the number of possible phases in the SRM. If not, a return is made to step  106  and the phase is energized. Steps  108  through steps  132  are repeated until all current-flux data sample pairs are determined and recorded for each phase. If the count of phase P is greater than the possible number of phases in the SRM, a stop is initiated in step  134 . This completes the rotor position sense routine. 
     FIG. 6 sets forth a flowchart of firmwave steps performed during operation mode  200  during which the four phase SRM is commutated from one phase to the next. This commutation routine follows the calibration routine of FIG. 5 which determines calibrates the SRM having a known rotor position. The commutation routine is entered at step  202 . A step  204  initializes the estimated velocity ω est  of the rotor to zero. A step  206  reads the desired speed command velocity ω des  of the SRM. A logical node  208  determines whether the desired velocity ω des  is greater than 0. If not, step  212  commutates the phases of the SRM in reverse order, using the sequence D-C-B-A. If in logical node  208  the desired velocity ω des  is greater than 0, a step  210  commutates the phase of the SRM in a forward direction, using the sequence A-B-C-D. A step  214  determines the speed loop compensation. A logical node  216  determines whether the current applied to the rotor is greater than the minimum operable current I min  for the SRM. If not, a step  220  initializes the applied current I to equal the minimum operable current I min . If the current applied to the rotor is greater than the minimum operable current I min  for the SRM, then a step  220  waits for an interrupt signal from the DSP. By controlling the desired phase current i j   des , the developed torque and speed are controlled. For example, a simple proportional controller is defined as, 
     
       
           j   des   =k   p (ω d ( f )={circumflex over (ω)}( t )) 
       
     
     where k p  is the proportional gain. 
     A step  222  reads the phase current. Additionally at step  222 , current loop compensation converts the desired current into switch commands. The PWM commands for selecting the proper PWM waveform are updated as well. A step  224  estimates the flux according to the equation cited above. 
     
       
         ψ est ( k )=ψ( k− 1)+( v ( k )− i ( k ) R )· T ), where ψ(0)=0. 
       
     
     Additionally, the reference flux ψ ref  is determined at this step according to the equation: 
     
       
         ψ ref ( k )=α(·) g ( i ( k )). 
       
     
     A logical node  226  compares each respective phase flux estimate ψ est  with the phase switching reference flux ψ ref  and generating a first logic level signal when the actual rotor angle is closer to axial alignment of the respective stator and rotor poles than the rotor angle reference, and generating a second logic level signal when the actual rotor angle is farther from axial alignment than the rotor angle reference. If not, a return is made to step  214  and steps  214  through  226  are repeated until the estimated flux ψ ref  is greater than the reference flux ψ ref . If the estimated flux ψ ref  is greater than the reference flux ψ ref , step  228  updates the estimated velocity ω ref . Steps  208  through  228  are repeated until the SRM is disabled. 
     As is exemplified for conventional systems in FIG. 7, a block diagram of an SRM controller using position feedback is illustrated. The circuit  300  essentially includes a digital signal processor (DSP)  320 , an inverter  340 , a SRM  360  and optocouplers  380 . The DSP  320  includes a first summer  321 , a velocity controller  322 , a torque to current device  323 , an advance angle calculator  324 , a second summer  325 , a current controller  326 , a pulse-width modulator (PWM)  327 , an analog-to-digital converter (ADC)  328 , a position estimator  329 , and velocity estimator  330 . The DSP  320  is coupled to the inverter  340  which is connected to the SRM  360  having two output signals. The first output signal of the SRM  360  is coupled to the opto-couplers  380 . The second is coupled to the DSP  320 . The output of the opto-coupler is coupled to the DSP  320 . 
     During operation, the DSP  320  receives a speed command  310 . This signal is summed in first summer  321  with the signal generated by the velocity estimator  330 . The summation is received by the velocity controller  322 . The velocity controller  322  generates a torque command, which is received by the torque to current device  323 . The current generated i emd  by the torque-to-current device  323  is send to the second summer  325  and to the advance angle calculator  324 . At the second summer, the current signal received from the SRM  360  decrements the current i cmd . The summation is received by the current controller  326 . The signal generated by the current controller  326  is received by PVM  327  for generating a PWM command. The signal is received by the inverter  340  for inverting the signal to an analog signal readable by the SRM. The signal generated by the inverter  340  is received by the SRM  360 . The SRM  360  generates a current signal and a rotor position signal. The current signal is coupled to the DPS  320  at the switch  328 . The rotor position signal is coupled to the opto-couplers  380 . It generates a signal that is fed into the DSP at the position estimator  329  and velocity estimator  330 . The signal generated by the velocity estimator  330  is sent to the position estimator  329 , the advance angle calculator  324  and the first summer  321 . The position estimator  329  generates a signal fed to the advance angle calculator  324 . The advance angle calculator  324  is coupled to a DC voltage Bus. The advance angle calculator  324  generates commutation angles for the rotor in the SRM  360 . 
     FIG. 8 illustrates the block diagram of a position sensorless SRM controller  400  in accordance with the principles of the present invention. The position sensorless SRM controller  400  essentially includes a DSP  420 , an inverter  440  and a SRM  460 . The DSP  420  is coupled to the inverter  440 . The inverter  440  is coupled to the SRM  460 . The DSP  420  includes a first summer  421 , a velocity controller  422 , a torque to current device  423 , a second summer  424 , a current controller  425 , a pulse-width modulator (PWM)  426 , an analog-to-digital converter (ADC)  427 , a first storage device  428 , a second storage device  429 , a third storage device  430 , a multiplier  431 , a third summer  432 , an integrator  433 , a comparator  434 , a clock signal generator  436  and a velocity estimator  437 . 
     Accordingly in operation the DSP  420  receives a speed command signal  410 . The DSP generates signals controlling the state of the power devices in the inverter  440 . The SRM  460  reads the signal from the inverter  440  to control phase sequence commutation. The output current signal generated by the SRM  460  is measured by ADC  427 . The first summer  431  receivers two inputs: the speed command signal  410  and the signal generated by the velocity estimator  437 . The summed output is received by the velocity controller  422 . The signal generated by the velocity controller  422  is fed into the torque-to-current device  423  which generates a current signal i cmd . The second summer  424  sums input of the i cmd , an output current signal generated by the SRM  460  and a signal stored in a third storage device  430 . The summed output is received by the current controller  425  which generates a signal fed to the PWM  426 . The output of the PWM  426  is fed to the inverter  440 . Accordingly, the output current signal generated by the SRM  460  is measured by ADC  427 . The first storage device filter  428  the signal with g(i). This signal is fed to a filter  429  of α(i) to generate reference flux ψ ref . The reference flux signal ψ ref  is fed to comparator  434 . The estimated flux is calculated by summing the output of the current controller  425  and the DC bus voltage in the multiplier  431 . The third summer  432  sums the product with the output of the third storage device  430  containing the estimated resistance of the SRM, R, in the third summer  432 . The summed signal is fed to integrator  433  which ultimately generates the estimated flux ψ est . The estimated flux ψ ref  is fed to the comparator  434  which compares both the reference flux ψ ref  and the estimated flux ψ ref . The comparator  434  generates a commutation signal. The commutation signal is stored in a latch  435  which is driven by counter  436 . The latched signal is fed to the velocity estimator  437 . 
     Those skilled in the art to which the invention relates will appreciate that various substitutions, modifications and additions can be made to the described embodiments, without departing from the spirit and scope of the invention as defined by the claims.

Technology Category: h