Abstract:
An adaptive torque disturbance cancellation method and motor control system for rotating a load are described. The system has: (i) a speed controller for receiving a first input signal indicating a desired motor speed and, in response, for outputting a motor control signal; (ii) current sensing circuitry for sensing current through a motor that rotates in response to the speed controller; (iii) circuitry for storing, into a storage device, history data representative of the current through a motor when the motor operates to rotate the load; and (iv) circuitry for modifying the motor control signal in response to the history data.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority to, the benefit of the filing date of, and hereby incorporates herein by reference, U.S. Provisional Patent Application 62/195,495, entitled “Adaptive Noise Cancellation for Electric Motors,” and filed Jul. 22, 2015. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The preferred embodiments relate to electric motor systems and methods and, more particularly, to adaptive torque disturbance cancellation in such systems and methods. 
         [0004]    Electric motors are implemented in numerous systems and, in many applications, drive a load with a strong periodic torque disturbance. For example, loads that create such a disturbance include a reciprocating compressor, a washing machine, a piston load, and so forth. The changing load torque affects motor speed, which in turn affects current demand in responding to the altered motor speed. Specifically, a load torque increase tends to slow down the motor speed, and, in the prior art, the reduced speed is detected (e.g., by a speed controller) and, in response, current to the motor is increased to counteract the change in load torque. The effectiveness of the speed controller to regulate speed under these conditions, however, may fall short of the value required to achieve the desired speed regulation in most cases, as further detailed below. 
         [0005]      FIG. 1  illustration a simplified, abstracted view of a conventional motor control system  10 , by way of further introduction. System  10  includes a motor  12  that rotates a shaft  12   SH  to turn a load  12   L , where load  12   L  is shown merely as a plate on shaft  12   SH  with the understanding that a larger device may be connected thereto, to rotate either in 1:1 relationship with shaft  12   SH  or as some multiple, such as through some gearing or other translational system. Motor  12  is connected in a feedback configuration that in general includes a velocity loop and within the velocity loop is a torque loop, both shown in dashed rectangles. A commanded velocity V C  signal represents a desired velocity of motor  12  and is input to an error generator  14 , which also receives a feedback input as an estimated velocity V E  from a velocity estimator  16 . Velocity estimator  16  receives one or more signals S M  from a sensor(s)  12   SR , at motor  12 ; these signals may represent a motor shaft mechanical (i.e., angular) position θ m , or the signals may be current and voltage, from which velocity estimator  16  can estimate the angular velocity V E . The output of error generator  14  is connected to an input of a speed controller  18 , which typically applies a function, such as a proportional-integral (PI) control function, to the error signal between commanded velocity V C  and estimated velocity V E , with that error signal provided by error generator  14 . The output of speed controller  18  is a commanded current I C , which is connected as an input to an error generator  20 , which also receives a feedback input as a measured current I M  from sensors  12   SR  at motor  12 . The output of error generator  20  is connected to an input of a torque controller  22 , which typically applies a function, also such as a proportional-integral control function, to the error signal between commanded current I C  and measured current I M , with that error signal provided by error generator  20 . In response, torque controller  22  outputs a drive signal S D  (or plural signals) which may comprise data signals that ultimately become the drive signal to motor  12 . 
         [0006]    The general operation of system  10  is readily understood in the art. Commanded velocity V C  is input with a goal toward operating motor  12  at that velocity, and the feedback and PI operations thus endeavor toward satisfying that goal. As further background to the preferred embodiments, however, introduced earlier is the notion that a motor load with a strong periodic torque disturbance affects the speed of motor  12 . Thus, in  FIG. 1 , velocity estimator  16  provides estimated velocity V E  as feedback that will indicate the reduced speed from the torque disturbance, thereby increasing the error indicated by error generator  14  and causing speed controller  18  to increase the commanded current I C . The effectiveness of speed controller  18  to regulate speed, however, is dependent on its gain, where higher gains result in more constant motor speed. But the speed controller gain is usually limited by the stability requirements of the system, so that limitation may cause speed regulations that still vary undesirably or fail to reach desired levels in certain applications. Further, when the load reaches a different point in the mechanical cycle, the torque decreases, which causes speed controller  18  to increase motor speed. But once again, speed controller  18  cannot respond as quickly as desired, resulting in speed overshoot. This process repeats over and over again, resulting in a cyclical speed disturbance. The derivative of speed is acceleration, and the derivative of acceleration is jerk, which is directly proportional to the mechanical shock to the system. As jerk increases, so does the audible noise and mechanical wear of the system. 
         [0007]    Given the preceding discussion, while the prior art approaches may be acceptable in certain implementations, some applications may have requirements that are not sufficiently addressed by the prior art. Thus, the present inventors seek to improve upon the prior art, as further detailed below. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    In a preferred embodiment, there is a motor control system for rotating a load. The system comprises a speed controller for receiving a first input signal indicating a desired motor speed and, in response, for outputting a motor control signal. The system also comprises current sensing circuitry for sensing current through a motor that rotates in response to the speed controller. The system also comprises circuitry for storing, into a storage device, history data representative of the current through a motor when the motor operates to rotate the load. The system also comprises circuitry for modifying the motor control signal in response to the history data. 
         [0009]    Numerous other inventive aspects are also disclosed and claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0010]      FIG. 1  illustration a simplified, abstracted view of a conventional motor control system  10 . 
           [0011]      FIG. 2  illustrates a preferred embodiment motor control system with feed-forward compensation that implements an adaptive methodology that learns a periodic noise signature created by a pulsating torque load and applies a correction signal to cancel the noise. 
           [0012]      FIG. 3  illustrates an electrical block diagram with additional details of feed-forward compensator  32  from  FIG. 2 . 
           [0013]      FIG. 4  illustrates a timing diagram of the operation of system  30 , with time in units of seconds along the horizontal axis and rotational speed of motor  12  in units of revolutions per minute (RPM) in the vertical axis. 
           [0014]      FIG. 5  illustrates a timing diagram of the operation of system  30 , with time in units of seconds along the horizontal axis, and along the vertical axis are current values affecting the speed of motor  12 . 
           [0015]      FIG. 6  illustrates a more detailed illustration of feed-forward compensator  32  as implemented in a FOC controller system  30 ′ for controlling the operation of an AC motor  12 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0016]      FIG. 1  was described in the Background of the Invention section of this document and the reader is assumed familiar with the principles of that discussion. 
         [0017]      FIG. 2  illustrates a simplified block diagram of a preferred embodiment motor system  30 . System  30  is shown in simplified form so as to focus the discussion on various preferred embodiment aspects, with a more detailed illustration of a full field-oriented control (“FOC”) system shown later in  FIG. 6 . In any event, by way of introduction, system  30  improves on the prior art by including a feed-forward compensation that implements an adaptive methodology that learns a periodic noise signature created by a pulsating torque load and applies a correction signal to cancel the noise. Prior to discussing those aspects, however, note that system  30  also includes the same items shown in system  10  of  FIG. 1 , so for those items the same reference numbers from  FIG. 1  are carried forward into  FIG. 2 . Briefly revisiting those items, therefore, system  30  includes a motor  12  receiving a drive signal S D  (or plural signals) from a torque controller  22 . The speed of motor  12  is estimated by a velocity estimator  16 , in response to one or more signals S M  from sensor(s)  12   SR , those signals representing a shaft mechanical angular position θ m , or the signals may be current and voltage, from which velocity estimator  16  can estimate the angular velocity V E . The velocity estimate V E  from the velocity estimator  16  is connected as one input to an error generator  14 , the other input of which is connected to a commanded velocity V C , where the difference between those two signals is input to speed controller  18 , which applies a function, such as a proportional-integral (PI) control function, to the error signal. 
         [0018]    System  30  of  FIG. 2  differs from the prior art system  10  in that system  30  includes an additional feed-forward compensator  32 , which receives plural inputs and provides a feed-forward current component i ff  to an adder  34 , which is connected between speed controller  18  and error generator  20 . More particularly, in a preferred embodiment, feed-forward compensator  32  receives the following four inputs: 
         [0019]    (1) θ m : angular position of motor  12  (i.e., rotational angle of its shaft  12   SH ), either measured directly from motor  12  or estimated by an angle observer; 
         [0020]    (2) θ adv : an angle advance signal that accounts for timing delays between a read from a look-up table (LUT) of feed-forward compensator  32  and the application of the torque controller command, S D , to the motor  12 ; 
         [0021]    (3) LR: a learning rate coefficient, which indicates a rate at which feed-forward compensator  32  updates its LUT so as to converge its feed-forward current component i ff  to a relatively consistent periodic waveform; and 
         [0022]    (4) I M : the measured current from sensors (not separately shown in  FIG. 2 ), which is also connected, as described earlier, to error generator  20 . 
         [0023]    As will be detailed below, in response to the above four inputs, feed-forward compensator  32  adjusts the feed-forward current component i ff  and that signal is added to the output signal of speed controller  18  by adder  34 , with the sum of those two signals representing the commanded current I′ C  input to error generator  20 . Thus, feed-forward compensator  32  may modify the commanded current I C  input to adder  34  so as to provide an output current I′ C  to error generator  20 , which thereafter operates as described earlier and understood to one skilled in the art. The modification to I C , therefore, reduces the effects of periodic torque disturbances from the load  12   L , as further understood below. 
         [0024]      FIG. 3  illustrates an electrical block diagram with additional details of feed-forward compensator  32  from  FIG. 2 . The LUT is a data storage device (e.g., memory) that stores data representing current magnitude at differing angles of shaft  12   SH . The data value labeled FETCH 1  is located in the memory address location denoted by first pointer PTR 1  and the data value labeled FETCH 2  is located in the memory address location denoted by second pointer PTR 2 . FETCH 1  is used to denote the LUT data value being updated, and FETCH 2  is used to denote the LUT data value being used as the feed-forward current value i ff . As will become evident, below, the LUT data is updated from real-time measurements so as to provide an output signal representing a desired amount of current to apply to motor  12 . More specifically, the θ m  input provides a memory address or index for the first pointer PTR 1  to the LUT and is also connected as a first input to an adder  36 . The θ adv  input is connected as a second input to adder  36 , and the output of adder  36  provides an address or index for the second pointer PTR 2  to the LUT. The LR input is connected to a storage element  38 , which outputs the value of LR as a control input to a first scaling device  40  for applying LR as a scale factor, and also to a second scaling device  42  for applying 1-LR as a scale factor. The input of first scaling device  40  is connected to receive the measured current I M  and the output of first scaling device  40  is connected to a first input of an adder  44 , which has a second input connected to the output of second scaling device  42 . The output of adder  44  is fed back as an updated value to the LUT at the memory address denoted by first pointer PTR 1 , and the data output of the LUT, whose memory address is denoted by second pointer PTR 2 , provides the feed-forward current component i ff . 
         [0025]    The operation of feed-forward compensator  32 , in the context of system  30  is now described. Initially, the data in the LUT is zeroed and a commanded velocity V C  signal is applied to system  30 , which, prior to any feedback will pass to speed controller  18  and adder  34 ; meanwhile, the value of a feed-forward current component i ff  is selected as the FETCH  2  value from the LUT, but because the LUT is initially zeroed, then the feed-forward current component i ff  is also zero and therefore provides no offset to the output of speed controller  18 . Hence, initially adder  34  provides a commanded current I′ C  based solely on the speed controller output signal I C . The current command is also initially unaffected through error generator  20  as there is not yet a feedback measured current I M , so the unchanged value is processed by torque controller  22  to provide the signal(s) S D  to start motor  12  operating. Once such operation occurs, however, measured current I M  starts to rise, and that value is fed back not only to error generator  20 , but also as an input to feed-forward compensator  32 . 
         [0026]    As measured current I M  is fed back to feed-forward compensator  32 , it is used to update the LUT at the measured angle value θ m  of shaft  12   SH . For example, if I M =x amps at θ m =y degrees, then the LUT is updated with a scaled version of those x amps, at a table entry indicated by pointer PTR 1  for the address corresponding to y degrees. More particularly, as measured current I M  is fed back into feed-forward compensator  32 , it is scaled according to the learning rate LR, which preferably is a value between zero and one. Moreover, the larger the value of LR, the more quickly feed-forward compensator  32  begins to influence the signal applied to motor  12 . Such influence occurs first by way of first scaling device  40 , which scales I M  by LR, after which the scaled value is added, by adder  44 , to one minus that LR scale factor times a FETCH  1  output from the LUT, where the FETCH  1  output is taken from the memory location in the LUT denoted by pointer PTR 1 , corresponding to the measured angle θ m . The sum determined by adder  44  is then stored in the LUT, that is, it provides an update to the LUT, as a data point for x scaled amps at the same table location corresponding to θ m =y degrees. Pointer PTR 1  is included in  FIG. 3  to depict that it may move along the horizontal angle axis so as to point to the FETCH  1  and update LUT location in this manner, that is, by point to the location of θ m =y degrees, so as to read and store at that the same memory address location containing the original x scaled amps value. In this regard, therefore, one skilled in the art should appreciate that over time, as shaft  12   SH  rotates through full rotations and incurs a periodic load waveform, then measured current I M  will rise and fall with that waveform. Thus, as the LUT is updated as described above, eventually the plot PL I  of the LUT values, which depicts stored history data representative of the current waveform magnitude as the load torque is incurred, will converge to the periodic current values needed to compensate for the periodic torque disturbance. 
         [0027]    Preferred embodiments benefit from the LUT data converging in a stable fashion. In this regard, the update equation for the LUT current values, i LUT , can be written as the following Equation 1: 
         [0000]        i   LUT   [m,n+ 1]= LR·i[m,n ]+(1− LR )· i   LUT   [m,n]   Equation 1
 
         [0000]    where, 
         [0028]    m is a mechanical angle index 
         [0029]    n is the sample number 
         [0030]    LR is a convergence constant, with 0≦LR≦1, and LR=0 means that the LUT current value is not updated while LR=1 means that the LUT current value is equal to the measured current value (as to the latter, if one substitutes LR=1 into Equation 1, the result is i LUT [m,n+1]=i[m,n], thereby indicating there is no averaging, as the new LUT value is the same as the measured value). The update Equation 1 is guaranteed to be stable over the LR range indicated above. Smaller LR values ensure more averaging at each mechanical angle. 
         [0031]    Further, as mentioned above, at initialization/startup time, the LUT values are all initialized to zero. The angle resolution of the LUT table, Δθ, can be determined from the following Equation 2: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     θ 
                   
                   = 
                   
                     
                       360 
                        
                       ° 
                     
                     N 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0000]    where, N is the number of points in the LUT for one mechanical revolution. 
         [0032]    To determine a particular mechanical angle index, the following Equation 3 can be used: 
         [0000]    
       
         
           
             
               
                 
                   m 
                   = 
                   
                     mod 
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         
                           
                             θ 
                             m 
                           
                           
                             Δ 
                              
                             
                                 
                             
                              
                             θ 
                           
                         
                         , 
                         N 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
         [0000]    where mod( ) is the modulo operator. 
         [0033]    Given the preceding discussion, as the LUT is updated as described above and its waveform converges on a same shape as the current level that is proportional to the torque disturbance waveform, then at the same time it provides a FETCH  2  output as the compensated feed-forward current component i ff . Thus, the feed-forward current component i ff  will converge to an angle dependent waveform whose values can be used to anticipate the change in load torque as shaft  12   SH  rotates and compensating on the fly with a historic record of how that change will occur, with that record being the waveform in the LUT. In this regard, it is recognized in connection with a preferred embodiment that as shaft  12   SH  is at a measured angle θ m , then a value of current i LUT  could be selected at that angle and used as the current to be instantaneously applied to motor  12 . However, it is further recognized that there are delays due to table reading, computation time, electronic delay, etc. Thus, to determine which LUT current value to use for the feed-forward component, the latency of the system should be considered when determining the correct LUT current value to use. This time latency, ΔT lat , contributes to an angle advance needed to read the correctly addressed value from the LUT as the motor rotates at a given mechanical speed, ω m , which has units of rad/sec. The angle advance can be calculated using the following Equation 4: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       θ 
                       adv 
                     
                      
                     
                       [ 
                       n 
                       ] 
                     
                   
                   = 
                   
                     
                       
                         ω 
                         m 
                       
                       · 
                       Δ 
                     
                      
                     
                         
                     
                      
                     
                       
                         T 
                         lat 
                       
                       · 
                       
                         ( 
                         
                           
                             180 
                              
                             ° 
                           
                           π 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   4 
                 
               
             
           
         
       
     
         [0034]    The angle used to read the LUT current value for feedforward compensation can be determined using the following Equation 5: 
         [0000]      θ ff   [n]=θ   m   [n]+θ   adv   [n]   ‘Equation 5
 
         [0000]    where, 
         [0035]    θ ff  [n] is angle used to read the LUT current value for feedforward compensation and corresponds to the memory address denoted by pointer PTR 2 ; 
         [0036]    θ m [n] is the mechanical angle of motor  12 ; and 
         [0037]    θ adv  [n] is the mechanical angle advance to account for the system latency. 
         [0038]    As a result, the θ adv  input provides an angular advancing offset, so as to advance the read location in the LUT into the future by a small amount relative to the present location of the motor shaft, so that the resulting current read from the LUT will be very close to the desired value needed to compensate the torque disturbance as the mechanical angle reaches that point in time when the control command is actually applied to the motor; this advancement is shown in  FIG. 3  as the pointer PTR 2 , which also is able to read from any angle in the LUT, and by way of adder  36 , the read occurs at the added offset of θ adv , relative to the present input value of θ m . For example, assume at a given time that θ m =y degrees, and assume that offset of θ adv =4 degrees. Thus, when the value of current for feed-forward current component i ff  is read from the LUT for a current input of θ m =y degrees, that current will be read at the data location for y+θ adv =y+4 degrees. Thus, the read value will represent the angular location of shaft  12   SH  at a time that is 4 degrees in the future, so that when the feed-forward current component i ff  for that future position is read from the LUT and its effect eventually propagates into the influence of signal S D  in  FIG. 2 , by then shaft  12   SH  will have advanced by θ adv =4 degrees and will therefore be at the angular location corresponding to the position read from the LUT at the location of pointer PTR 2 . Note also that θ adv  is a function of motor speed, and best cancellation can be obtained if this value is updated as a function of commanded velocity. For most practical applications, in one preferred embodiment, this input only needs to be set once at initialization. An alternative preferred embodiment, however, contemplates updating θ adv , such as at fixed intervals or in response to some type of change in conditions. In any event, θ adv  thusly directs the ultimate selection of the feed-forward current, which mathematically can be shown by transforming Equation 5 into mechanical angle index as in the following Equation 6: 
         [0000]    
       
         
           
             
               
                 
                   
                     mod 
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         
                           
                             
                               θ 
                               ff 
                             
                              
                             
                               [ 
                               n 
                               ] 
                             
                           
                           
                             Δ 
                              
                             
                                 
                             
                              
                             θ 
                           
                         
                         , 
                         N 
                       
                       ) 
                     
                   
                   = 
                   
                     mod 
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         
                           
                             
                               
                                 θ 
                                 m 
                               
                                
                               
                                 [ 
                                 n 
                                 ] 
                               
                             
                             + 
                             
                               
                                 θ 
                                 adv 
                               
                                
                               
                                 [ 
                                 n 
                                 ] 
                               
                             
                           
                           Δθ 
                         
                         , 
                         N 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   6 
                 
               
             
           
         
       
     
         [0039]    to yield the following Equation 7: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       m 
                       ff 
                     
                      
                     
                       [ 
                       n 
                       ] 
                     
                   
                   = 
                   
                     mod 
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         
                           
                             
                               
                                 θ 
                                 m 
                               
                                
                               
                                 [ 
                                 n 
                                 ] 
                               
                             
                             + 
                             
                               
                                 θ 
                                 adv 
                               
                                
                               
                                 [ 
                                 n 
                                 ] 
                               
                             
                           
                           
                             Δ 
                              
                             
                                 
                             
                              
                             θ 
                           
                         
                         , 
                         N 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   7 
                 
               
             
           
         
       
     
         [0040]    Thus, the feedforward current component at sample number n can be written as the following Equation 8: 
         [0000]        i   ff   [n]=i   LUT   [m   ff   [n],n]   Equation 8
 
         [0041]    From the above, note that as feed-forward compensator  32  operates, its LUT will become populated with values that resemble the measured current waveform at a rate determined by the learning rate LR input, with that input therefore defining the aggressiveness of how fast compensator  32  stores data (i.e., “learns”) representing distortion in current due to the periodic torque disturbance. If the torque disturbance waveform is fairly repeatable from cycle to cycle, then the learning rate can be set to a higher value, which results in noise cancellation almost immediately. However, if the torque disturbance waveform has a basic shape but is not exactly the same from cycle to cycle, then the learning rate should be set to a lower value, which results in the LUT waveform representing the average shape of the torque disturbance over several cycles. In a preferred embodiment, therefore, in most applications, the learning rate LR input should only need to be set once at initialization. 
         [0042]    In another aspect of a preferred embodiment, the LUT methodology update can be periodically enabled and disabled, for example so as to save power (e.g., eliminating processing MIPS) at times when the LUT data is sufficient so as to cancel torque disturbance beyond a particular threshold. In this regard, in a preferred embodiment, an error measure, e[n] can be defined as shown in the following Equation 9: 
         [0000]        e[n]=i   LUT   [m,n+ 1]− i   LUT   [m,n]   Equation 9
 
         [0000]    Given Equation 9, in this preferred embodiment, if an absolute value of e[n] satisfied (e.g., falls below) a threshold (e.g., 0.001 A=1 mA), the LUT update process is disabled, such as for a period of time. During this disabled time period, therefore, processing power is saved. The process may be re-enabled later, in response to one more conditions (e.g., passage of time; new torque load; a stop and re-start of motor  12 , etc.). 
         [0043]      FIG. 4  illustrates a timing diagram of the operation of system  30 , with time in units of seconds along the horizontal axis and rotational speed of motor  12  in units of revolutions per minute (RPM) in the vertical axis. At time t=1 second, feed-forward compensator  32  is turned on and, thus, per the above discussion, the LUT begins to populate given the sensed current I M , that is, in relation to the load torque disturbance. In the example illustrated, the learning rate LR is equal to 0.5 and θ adv =4 degrees. Initially, from time t=1 to approximately t=1.1, the diagram indicates that motor  12  speed varies between a high level between approximately 1025 to 1030 RPM to a low level of approximately 965 to 970 RPM. However, by time 1=1.25 seconds, the variation is reduced such that motor  12  speed varies between a high level of approximately 1012 down at low level of approximately 997. Still further, note that by t=2 seconds, the variation is down within ±2 RPM. Thus, a preferred embodiment provides an improved variance in motor speed over time even under the disturbance of the torque load, as feed-forward compensator  32  refines its LUT and its feed-forward current value i ff  correctly matches or approximates the torque load demand. 
         [0044]      FIG. 5  illustrates a timing diagram of the operation of system  30 , with time in units of seconds along the horizontal axis, and along the vertical axis are three current values: (i) I C  from speed controller  18 ; i ff  from feed-forward compensator  32 ; and (iii) a summed current I SUM  which is the total of I C  and i ff . At time t=1 second, feed-forward compensator  32  is turned on and, thus, per the above discussion, the LUT begins to populate given the sensed current I M ; in the meantime, I C  from speed controller  18  is shown as the dominant (if not sole) current until approximately time t=1.1 seconds, at which time i ff  from feed-forward compensator  32  begins to contribute to the overall summed current I SUM  that is driving motor  12 . Thereafter, feed-forward compensator  32  continues to gradually provide more and more of the overall total of I SUM . Moreover, by t=2 seconds, current i ff  from feed-forward compensator  32  provides virtually all of the motor driving current, that is, compensator  32  in approximately one second has learned the current level corresponding to the torque disturbance and is thereby able to achieve a steady-state operating condition where, for a given commanded velocity V C , compensator is supplying all of the commanded current waveform. When this condition is met, then the output signal from speed controller  18  itself must approximate or be zero, which means that the speed error signal must also be zero and the measured speed must very closely approximate or be exactly equal to the commanded speed. Furthermore, since the periodic variation of measured speed is zero (or close to zero), the load acceleration caused by load torque distortions will also be zero (or close to zero), and the mechanical jerk (the derivative of acceleration), which is responsible for mechanical shock, wear, and audible noise, is also significantly reduced. Still further, it is anticipated from testing that an unexpected, but beneficial characteristic of the operation of the compensator  32  is that since the x-axis of the LUT represents angle and not time, when the commanded speed V C  changes, the output waveform (including any adaptive learning which is in progress) stays synchronized with the load distortion. The implication, therefore, is system  30  does not have to zero out the LUT and relearn the distortion every time a change in V C  occurs. 
         [0045]      FIG. 6  illustrates a more detailed illustration of feed-forward compensator  32  as implemented in a FOC (Field-Oriented-Control) controller system  30 ′ for controlling the operation of an AC (Alternating Current) motor  12 . In this architecture, speed controller function  18  is a control function that receives a desired motor speed input signal V C  along with a feedback rotor velocity estimate V E  from a velocity estimator  16 . Speed controller  18  typically applies a function, such as a proportional-integral control function, to a difference (i.e., error signal) between motor speed input signal V C  and velocity estimate signal V E  to generate the desired quadrature phase reference current signal Iq d  for application to summation block  50   q . Summation block  50   q  sums I′ q  from a summer  32   q , which is the difference (i.e., error signal) between the quadrature phase feedback current Iq and the feed-forward current value i ff , and the output from the speed controller, I c . The output of summation block  50   q  produces the feedforward compensated current signal I′ c  that is applied to a proportional-integral (PI) Iq controller  52   q . Similarly, error generator  50   d  receives a desired direct phase reference current Id d  and direct phase feedback current I d , and produces error signal εd (i.e., Id d  minus I d ) that is applied to proportional-integral (PI) Id controller  52   d . Controllers  52   q  and  52   d  each apply a conventional control function, for example a combination of proportional and integral functions, to the current signal I′ q  and error signal εd, respectively, to produce corresponding respective direct phase control signal V q  and quadrature phase control signal V d , both of which are applied to an inverse Park transform function  54 . Inverse Park transform function  54  transforms the d and q phase control signals V d , V q  to spatially fixed a and p phase control signals V α  and V β  (shown in  FIG. 6  as V αβ ) and applies those signals to a space vector generator  56 , which in turn produces three-phase data signals T a , T b , T c  (shown in  FIG. 6  as T abc ) that represent duty cycles (i.e., pulse widths) for corresponding motor phases; these data signals are applied to PWM drivers  58  that generate the pulses applied to the stator windings of motor  12  via power driver  60  and inverter  62 . PWM drivers  58  commonly include timers that control the start and stop times of each drive pulse, in response to data signals T a , T b , T c . 
         [0046]    The feedback side of the control loop in system  30 ′ receives feedback signals from multiple sources, all such sources based on sensors at motor  12  and corresponding sense circuits  12   SR . One source of feedback signals is a set of current sample streams produced by ADC  64  based on output from sense circuits  12   SR , and which represent currents induced in the stator windings by the rotor of motor  12  as it rotates. Current sample streams from ADC  64  are applied to a Clarke transform function  66  to produce spatially fixed and phase feedback signals transforming the three-phase time invariant system of differential equations into a two coordinate time variant system. These transformed feedback signals are input to a Park transform function  68 , which in turn produces direct and quadrature phase feedback currents Id, Iq, respectively. An N pp  (number of pole pairs of the motor) block  70  transforms the mechanical angle, θ m , to an electrical angle θ e , which is connected to Park transform function  68  and to an inverse Park transform function  54 . 
         [0047]    Given the preceding, both the generalized  FIG. 2  and the more detailed  FIG. 6  preferred embodiments provide motor systems and methods that adaptively cancel noise with an LUT that learns a current waveform profile in response to torque disturbance. Thus, a preferred embodiment learns a periodic noise signature created by pulsating torque loads and applies a correction signal to cancel the noise. Preferred embodiments, therefore, provide numerous advantages and benefits. For example, a preferred embodiment has been successfully implemented including a compressor product using FOC with a permanent magnet synchronous motor. The result was a dramatic reduction in audible noise from the system. As another benefit, more broadly the inventive scope allows it to be applied to any motor with a periodic pulsating load torque, where noise reduction is desired. As a result, market demand is expected, particularly from customers desiring to have motor-based products run quieter. Moreover, preferred embodiments achieve these benefits with smooth motor operation by reducing or eliminating the periodic variations in speed, without the prior art concern of increasing the gain of the speed controller. Still further, quieter motor operation is achieved without any additional hardware required. And, since no additional hardware is required, preferred embodiments may be achieved by retrofitting the inventive scope into existing motor control systems. Still further, quieter operation results in less wear and tear on the mechanics of a system, which increases efficiency and extends the life of the product. Additionally, the preferred embodiment teachings can work with many if not all motor systems (not just field-oriented systems), as long as the load distortion periodicity with respect to motor shaft angle is known ahead of time. Since the learning and compensation processes are done spatially instead of temporally, they will not be interrupted during changes in the desired speed. Still further, implementation with a sampled system on an embedded controller is extremely straightforward with no complex calculations required. Moreover, no load characterization or tuning is required since the technique is adaptive. As yet another benefit, power consumption is reduced because the torque controller does not have to track the periodic variations in current due to the torque disturbance. Lastly, no motor movement at startup is required to synchronize the shaft angle to the LUT angle, since the preferred embodiment methodology is adaptive. From the above, therefore, one skilled in the art should further appreciate that while some embodiments have been described in detail, various substitutions, modifications or alterations can be made to the descriptions set forth above without departing from the inventive scope, as is defined by the following claims.