Patent Publication Number: US-5298838-A

Title: Sensorless brushless DC motor starting system and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of brushless DC motors, and more particularily to a method for starting sensorless brushless DC motors. 
     2. Background Art 
     In a brushless DC motor, commutation is provided by selectively applying DC current to the motor&#39;s field windings. Whether and in what direction current is applied to a particular winding depends on the physical and operational configuration of the motor and the rotational position of the motor&#39;s armature. Each motor has a discrete number of commutation states, depending on the number of windings or phases of the motor and the motor&#39;s mode of operation (e.g. unipolar, bi-polar, etc.). A three-phase motor in unipolar operation has three different commutation states. A three-phase motor in bi-polar operation has six different commutation states. The commutation states for a three-phase, bipolar motor are shown in Table 1. 
     
                       TABLE 1                                                     
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COMMUTATION STATES FOR THREE-PHASE                                        
BIPOLAR DC MOTOR                                                          
Commutation State                                                         
            Winding 1  Winding 2  Winding 3                               
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1           Positive   Negative   Open                                    
2           Positive   Open       Negative                                
3           Open       Positive   Negative                                
4           Negative   Positive   Open                                    
5           Negative   Open       Positive                                
6           Open       Negative   Positive                                
and repeating:                                                            
1           Positive   Negative   Open                                    
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     where: 
     Positive means that a positive potential is applied to the indicated winding. 
     Negative means that a negative potential is applied to the indicated winding. 
     Open means that no potential is applied to the indicated winding 
     For continuous operation of the motor, the commutation applied to the motor must be advanced to successive commutation states as the motor rotates. Each commutation state is applicable over a finite angular displacement of the motor&#39;s armature. This finite angular displacement can be calculated from the formula: 
     
         THETA=(2*PI)/(POLES*PHASES) 
    
     where: 
     THETA=angular displacement in radians 
     PI=3.14159265 
     POLES=number of poles on armature 
     PHASES=number of phases 
     For a three-phase motor having a 12-pole armature, the above equation results in a value for THETA of 0.1745 radians or 10 degrees. Anotherwords, each commutation state applies for 1/36th of a revolution. The applied commutation must be advanced to the succeeding commutation state after every 10 degrees of the armature&#39;s rotation. For one complete revolution, 36 successive commutation states must be applied, corresponding to six cycles through the six commutation states listed on Table 1. 
     For the proper timing of commutation advancement, a means for determining the angular position of the motor&#39;s armature is required. In sensored brushless DC motors of the prior art, optical or hall effect position sensors are used to determine the angular position of the motor&#39;s armature. In sensorless brushless DC motors, however, the angular position of the armature is not measured directly. Instead, it is deduced from a characteristic polarity reversal that occurs in back EMF induced by the rotation of the armature in the motor&#39;s undriven field coil windings. Sensorless brushless DC motors have the advantage of simplicity and reduced cost since the need for separate angular position sensors is eliminated. However, the armature must be rotating at a significant angular velocity before sufficient back EMF is produced to allow back EMF sensing. It has heretofore been difficult to provide effective commutation of a sensorless brushless DC motor during the motor&#39;s start up period when the motor is spinning below this speed. 
     One method that has been used in the prior art is generating a fixed frequency timing pulse that triggers successive commutation states. The frequency of the timing pulse must be greater than the frequency at which commutation states advance at a rotational speed of the motor at which sufficient back EMF is generated to allow effective back EMF sensing. 
     This starting method has several drawbacks. Upon initial start-up, the timing pulses advance the applied commutation states more quickly than the angular position of the armature requires. As a result of this incorrect timing, the current supplied to the motor&#39;s field windings by the successively applied commutation states at times will oppose the motor&#39;s rotation, causing erratic and inefficient rotational accelleration of the motor and long start-up times. In the worst case, reverse rotation of the motor can occur. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention consists of a system and method for generating a timing pattern for advancing commutation states during start-up of a sensorless brushless DC motor based on the physical design characteristics of the motor. An initial sequence of timing values is generated based on the motor&#39;s electrical and physical characteristics and the magnitude of the applied current. The commutation applied to the motor is advanced through successive commutation states according to these calculated timing values. At the end of the timing value sequence, the speed of the motor is checked to determine whether the motor has accellerated sufficiently to allow commutation through back-EMF sensing. If the motor&#39;s speed is adequate, the motor&#39;s back-EMF sensing commutation system is implemented. If the motor&#39;s speed is not adequate, the commutation rate is successively decreased, and the starting cycle is re-initiated. This speed checking/timing-value-incrementation cycle is repeated until either the required operating speed has been obtained or until a predetermined number of starting cycles have been run, at which point the starting procedure is aborted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a first embodiment of the timing algorithm of the present invention. 
     FIG. 2 is a block diagram of the preferred embodiment of the timing algorithm of the present invention including an adaptive loop. 
     FIG. 3 is a schematic block diagram of a back-EMF sensing cummutation advancement system. 
     FIG. 4 is a schematic block diagram of a prior art sensorless brushless DC motor starting system. 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     A sensorless, brushless DC motor starting system and method is described. In the following description, numerous specific details, such as number of phases, motor speed, etc., are set forth in detail in order to provide a more thorough description of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well known features have not been described in detail so as not to obscure the present invention. 
     The present invention uses the physical equations of motion for a brushless DC motor to calculate timing intervals for advancing the applied commutation states during the start-up phase of the motor. The resulting sequence of timing values is referenced by a motor controller. The controller sends triggering signals to a commutation generator at the time intervals corresponding to the timing values. The commutation generator advances the commutation applied to the motor from the current commutation state to the next succeeding commutation state each time a triggering signal is received. If the motor is not up to the required speed after the sequence of timing values is exhausted, the starting procedure is started anew. This time, however, the timing values in the timing sequence are increased by multiplying each of them by a &#34;timing adjustment factor&#34;. If, after the sequence of timing values has again been exhausted, the motor is still not up to speed, the adjustment factor is increased. This process is repeated until either the motor is up to speed or a predetermined cycling limit is exceeded, in which case motor start-up is aborted. 
     Calculation of the Start-up Timing Values 
     The sequence of timing values is calculated by using the basic physical equations of motion for the motor to calculate the incremental time it should theoretically take for the armature of the motor to rotate through an angular displacement equal to the angular distance between successive commutation states of the motor. The following equations are used: ##EQU1## Where: ALPHA=angular accelleration of rotor in radians/sec 2   
     KT=motor torque constant in oz-in/Amp 
     I=applied current in Amps 
     J=motor and load inertia in oz-in-sec 2   
     PI=3.14159265 
     POLES=number of motor poles 
     PHASES=number of motor phases 
     THETA 0  =angular displacement between commutation states in radians 
     THETA=angular displacement in radians 
     T=time in seconds 
     W=angular velocity in radians/second 
     T n  =time in seconds till next commutation state 
     Equation 101 is the equation for the angular accelleration (&#34;ALPHA&#34;) of an electric motor as a function of the applied current (&#34;I&#34;), the motor&#39;s characteristic torque constant (&#34;KT&#34;), and the motor&#39;s and the attached load&#39;s moment of inertia (&#34;J&#34;). For a given motor, KT and J will ordinarily be constants. ALPHA will therefore be dependent only on the applied current I. Since it is desired to accellerate the motor up to its self-commutating speed as quickly as possible, in the preferred embodiment of the present invention I is held constant at the maximum current available from the motor&#39;s power supply. Since I in this case is also constant, equation 101 will result in a fixed value for the motor&#39;s angular accelleration ALPHA. 
     Equation 102 is used to calculate the angular displacement THETA 0  between successive commutation states. The number of commutation states per revolution is equal to the product of the PHASES and POLES of the motor. In equation 102, the angular displacement THETA 0  between commutation states is calculated by dividing the angular displacement corresponding to one revolution (2 PI radians) by the number of commutation states per revolution (POLES times PHASES). Since the number of POLES and PHASES for a motor are constants determined by the motor&#39;s design, THETA 0  will be a constant for a particular motor. 
     Equation 103 is the equation for calculating the angular displacement THETA over a time interval T for an object rotating at an initial angular velocity of W and accellerating and an angular accelleration of ALPHA. By substituting THETA 0  for THETA and solving for time T, this equation can be used to calculate the length of time that the applicable commutation state is applied before the subsequent commutation state should be applied. The resultant formula for T n , the time period over which a particular commutation state S n  is applicable, constitutes equation 104. 
     Equation 104 gives T n  as a function of W n-1  (the angular velocity at the beginning of time interval n, which is equivalent to the angular velocity at the end of time internal n-1), ALPHA and THETA 0 . As explained above, ALPHA and THETA 0  will generally be constants for a given motor design. Accordingly, T n , the time interval over which commutation state S n  applies, will be a function basically of W n-1 , the angular velocity of the motor at the beginning of the interval. 
     Since the motor is starting from rest, W 0 , the angular velocity of the motor at the beginning of the first time interval T 1 , is equal to zero. For subsequent intervals T n , the initial angular velocity W n-1  at the beginning of the interval is calculated using equation 105. 
     Table 2 shows a BASIC computer program that can be used to calculate the time intervals T for a predetermined number of commutation states N. The program asks for values for the variables KT, I, J, POLES, PHASES and the number of successive commutation states N for which values of T are desired. The program then uses equations 101-105 to calculate T for each commutation state 1 through N. The program prints out values for N, T, total time, and final RPM for each interval. After the values for the Nth state have been calculated, the program assigns a final value of &#34;0&#34; to T n+1  to indicate the end of the list of calculated commutation state timing intervals. Table 3 illustrates a sample print-out from the program of Table 2 for N equal to 20 for a 12-pole, 3-phase motor having a torque constant KT of 3.7 oz in/amp, a effective rotational inertia J of 0.024 oz-in-sec 2 , and an applied current I of 1 amp. As shown in Table 3, the time interval T between successive commutation states ranges from 47 milliseconds for the first interval down to 5 milliseconds for the twentieth interval. The total time to cycle through the listed 20 commutation states is 212 milliseconds, and the projected speed of the motor at the end of the 20th commutation state is 313 RPM. 
     Using the Calculated Timing Values 
     A simplified block diagram of a typical back-EMF-sensing commutation system for a sensorless brushless DC motor is illustrated in FIG. 3. The system consists of a back-EMF transition detector 210, a delay 220, and a commutation generator 225. The system works as follows. 
     Back-EMF transition detector 210 detects a transition in the back-EMF generated in an appropriate undriven leg of of the motor across a predetermined reference value. In a three-phase motor operating in a bipolar mode, at any time there are two driven legs (one positive, one negative) and one undriven leg. The back EMF induced in the undriven leg will be either rising from a negative to a positive or falling from a positive to a negative, according to the commutation state sequence shown in Table 1. The transition point detected is typically the point at which the back-EMF passes the neutral point and changes from negative to positive or vice-versa. When such a transition is detected, the back-EMF detector 210 generates a clock signal that is sent to commutation generator 225 by means of delay 220. 
     Delay 220 is used because the optimum time for commutation state advancement actually occurs somewhat after the transition point detected by back-EMF transition detector 210. Delay 220 compensates for this time discrepancy by delaying the clock signal generated by back-EMF transition detector 210 by a predetermined amount of time. This delay is typically on the order of a few microseconds. 
     Commutation generator 225 generates the commutation states necessary for operation of the motor, advancing the applied commutation from one commutation state to the next every time it receives a delayed clock signal from back-EMF transition detector 210. For example, for a motor having the commutation states listed in Table 1, the first clock signal received by commutation generator 225 causes it to generate commutation state 1: a positive potential is applied to winding #1 a negative potential is applied to winding #2, while no potential is applied to winding #3. The next clock signal advances commutation generator 225 to commutation state 2: the positive potential remains applied to winding #1, but this time winding #3 receives a negative potential, while the negative potential applied to winding #2 during the previous commutation state is removed. Each succeeding clock signal advances commutation generator 225 to the next succeeding commutation state. Commutation generator 225 cycles through commutation states 1 through 6, and then starts again at state 1. 
     During initial start up of the motor, the speed of the motor is too slow to generate the amount of back-EMF needed for operation of back-EMF transition detector 210. An alternative means must therefore be used to provide the clock signals needed by commutation generator 225. In the prior art, a simple variable frequency clock signal generator, designated item 235 in FIG. 4, has been used. Clock signal generator 235 generates clock signals at an increasing rate unrelated to motor and load characteristics. Each clock signal advances the commutation provided by commutation generator 225 to the next succeeding commutation state. 
     The motor, since it is starting from rest, is at first moving much slower than 300 RPM. The clock signals generated by clock signal generator 235 therefore initially advance the commutation states generated by commutation generator 225 more quickly than the speed of the motor requires. As a result, the commutation applied to the motor will often be inappropriate for the given rotational position of the motor. The resulting torque exerted on the motor may slow down the motor, cause erratic accelleration, or in the worst case even reverse the motor&#39;s direction. 
     In the present invention clock signals are not generated at a constant rate as in the prior art. Instead, the sequence of timing values calculated by the program of Table 2 are used to generate clock signals at an accellerating rate corresponding to the theoretically predicted rate of accelleration of the motor. For example, using the timing values listed in Table 3, first, an initial clock signal is generated. A second clock is generated 47 milliseconds after the first. A third is generated 19 milliseconds after the second, a forth 15 milliseconds after the third, and so on according to the timing value sequence. 
     FIG. 1 shows an example of a &#34;timing loop&#34; algorithm that can be used by a motor controller&#39;s microprocessor to generate clock signals according to the timing values in the timing value sequence. In this example, the timing value sequence is calculated off line and stored in the microprocessor&#39;s memory. 
     The timing loop starts at block 300. First, a &#34;ramp index&#34; is initiallized at block 305 to keep track of the position of the currently used timing value in the timing value sequence. Next, an initial clock signal is sent to the commutation generator at block 310 to advance the commutation generator one commutation state to insure that positive torque is applied to the motor. The first timing value is read at block 315. The timing value is checked at block 320 to see whether it is equal to zero. If the value is zero (which indicates that the end of the sequence of timing values has been reached), the timing loop is exited at block 325. If the value is not zero, a waiting period commences at block 330. The waiting period continues for a length of time that corresponds to the current timing value. In FIG. 1, the waiting period is measured by counting the timing signals produced by a timing clock at block 335. The correct time interval will be reached after a number of pulses of the timing clock equal to the current timing value have been counted. For example, the first timing value in Table 3 is &#34;47&#34;. If a 1 khz timing clock frequency is used, the waiting period for this interval will continue until 47 1 khz pulses have been counted. For a clock operating at a different frequency, the number of pulses counted during each time interval are adjusted accordingly. In the preferred embodiment of the present invention, a clock frequency of 2 khz is used. In this case, the timing values listed in Table 3 would have to be multiplied by two to get the number of pulses that must be counted during each interval. 
     After the waiting period has expired, a clock signal is generated at 340 and sent to the commutation generator to advance the commutation applied by the commutation generator to the next commutation state. After the clock signal has been sent, the ramp index is incremented at 360 and the next timing value is read at 315. Steps 315 through 360 are repeated until all the timing values in the sequence have been used. Assuming 1) that a sufficient number of commutation states have been applied to the motor and 2) the motor accellerates according to its optimum theoretical capabilities, the motor will then be running fast enough to switch to the motor&#39;s normal back-EMF sensing mode of commutation. 
     In practice, a motor sometimes does not perform up to its optimum theoretical capabilities. If the applied commutation states are advanced at a rate based on the motor&#39;s optimum theortetical accelleration, while the motor is actually accellerating more slowly, there will be a mismatch between the applied commutation states and the commutation states actually required by the motor. As a result, the motor may not accellerate sufficiently to have the speed necessary for back-EMF sensing at the end of the timing value sequence. To compensate for this gap between the predicted and actual performance of the motor, the motor starting method of the present invention uses an &#34;adaptive loop&#34; to adjust the timing values in the sequence to reflect the motor&#39;s actual performance. 
     FIG. 2 shows this adaptive loop added to the basic timing loop algorithm illustrated in FIG. 1. As shown in FIG. 2, the adaptive loop operates between block 320 and block 325 of the timing loop of FIG. 2. The adaptive loop is entered after the end of the stored sequence of timing values has been reached, which is indicated by a timing value of &#34;0&#34;. The adaptive loop first checks to see whether the motor is spinning at a speed that is high enough to allow back-EMF commutation. This spin check is done at block 370 in FIG. 2. If the speed of the motor is adequate, the adaptive loop is exited at block 325 and the motor&#39;s back-EMF sensing commutation system takes over. If the speed of the motor is not adequate, the adaptive loop is continued. A loop count index that keeps track of how many times the adaptive loop is run is incremented at block 375, and the loop index is checked to see whether it exceeds a predetermined loop limit at block 380. In the preferred embodiment, a loop limit of eight is used. If the loop limit is exceeded, the starting procedure is aborted at block 385. 
     If the loop limit has not been exceeded, the adaptive loop continues. As mentioned above, if the motor is not up to speed when all the timing values in the sequence have been used, it means that the applied commutation states are being advanced too quickly for the rotational speed of the motor. To compensate for the motor&#39;s less-than-theoretically-maximum accelleration, the adaptive loop slows down the rate at which the applied commutation states are advanced. A &#34;timing adjustment factor&#34; is multiplied by each timing value as it is read in the timing loop to give adjusted values that compensate for the slower accelleration of the motor. This multiplication occurs at a new block 365 inserted into the timing loop between blocks 320 and 330. The adjustment factor initially has a value of 1, such that there is no adjustment of the timing values the first time the timing loop is run. Each time the adaptive loop is entered, the adjustment factor is increased. In the preferred embodiment, the adjustment factor is increased by a factor of 1.25 during each cycle of the adaptive loop. This amount of increase is not required, however, and other values may be used. After the new value for the adjustment factor is calculated at block 390, the ramp index is once again initialized and the timing loop is run again. During this cycle of the timing loop, the length of the time between successive commutation states will be increased over the previous cycle by a factor equal to the adjustment factor. Accordingly, the applied commutation states will be advanced at a slower rate, reflecting the slower-than-theoretically-maximum rate of accelleration of the motor. 
     After the timing loop is completed using the adjusted index values, the adaptive loop is once again entered at decision block 320. If the motor is still not up to speed, the adaptive loop is repeated. The loop is repeated until the motor starts or until the loop count limit is exceeded. 
     A sensorless brushless DC motor start algorithm has thus been presented. The present invention uses the physical equations of motion of the motor to calculate commutation advancement rates during the motor&#39;s start-up period. The algorithm initially calculates the commutation advancement rates for the motor&#39;s maximum theoretical accelleration. The commutation applied to the motor is advanced through a predetermined number of commutation states using these maximum rates. After the predetermined number of commutation states have been applied, the speed of the motor is checked to determine whether the motor is spinning at a high enough speed to permit use of the motor&#39;s normal commutation state advancement system. If the motor is not up to speed, the rate at which the commutation states are advanced is successively decreased until satisfactory operation of the motor is obtained or until motor failure is diagnosed and starting is aborted. By starting at a high commutation state advancement rate and incrementally decreasing the commutation advancement rate until proper accelleration of the motor is attained, the present invention continuously adapts the rate of advancement of the applied commutation states to the condition of the motor, thereby automatically compensating for changes in the motor&#39;s performance over time. 
     
                                           TABLE 2                                 
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 10 PRINT &#34;INPUT KT&#34;: INPUT KT                                            
 15 PRINT &#34;INPUT I&#34;: INPUT I                                              
 20 PRINT &#34;INPUT J&#34;: INPUT J                                              
 25 PRINT &#34;INPUT POLES&#34;: INPUT POLES                                      
 30 PRINT &#34;INPUT PHASES&#34;: INPUT PHASES                                    
 35 PRINT &#34;INPUT N&#34;: INPUT N                                              
 37 DIM W(N), T(N + 1), TM(N + 1)                                         
 40 ALPHA = (KT*I)/J                                                      
 45 THETA0 = (2*3.14159265)/(POLES*PHASES)                                
 50 W(0) = 0; TT = 0                                                      
 55 PRINT &#34;INTERVAL&#34;, &#34;T(msec)&#34;, &#34;TTotal(msec)&#34;, &#34;RPM&#34;                    
 57 PRINT &#34;---------------------------------------------------------------
------------------------&#34;                                                 
 60 FOR K = 1 TO N                                                        
 65 T(K) = (-W(K - 1) + SQR((W(K - 1) 2) - (4*(ALPHA/2)*(-THETA0))))/     
 ALPHA                                                                    
 70 W(K) = W(K - 1) + (ALPHA*T(K))                                        
 75 TT = TT + T(K)                                                        
 80 TTM = FIX(1000*TT)                                                    
 85 TM(K) = FIX(1000*T(K))                                                
 90 RPM = FIX((ALPHA*TT*60)/(2*3.14159265))                               
 95 PRINT K, TM(K), TTM, RPM                                              
100 NEXT K                                                                
105 T(N + 1) =  0: TM(N + 1) = 0                                          
110 PRINT &#34;END&#34;, TM(N + 1)                                                
115 END                                                                   
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                       TABLE 3                                                     
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RUN                                                                       
INPUT KT                                                                  
? 3.7                                                                     
INPUT I                                                                   
? 1                                                                       
INPUT J                                                                   
? .024                                                                    
INPUT POLES                                                               
? 12                                                                      
INPUT PHASES                                                              
? 3                                                                       
INPUT N                                                                   
? 20                                                                      
INTERVAL    T(msec)   TTOTAL(msec)  RPM                                   
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 1          47         47            70                                   
 2          19         67            99                                   
 3          15         82           121                                   
 4          12         95           140                                   
 5          11        106           156                                   
 6          10        116           171                                   
 7          9         125           185                                   
 8          8         134           198                                   
 9          8         142           210                                   
10          7         150           221                                   
11          7         157           232                                   
12          7         164           242                                   
13          6         171           252                                   
14          6         177           261                                   
15          6         184           271                                   
16          6         190           280                                   
17          5         196           288                                   
18          5         201           297                                   
19          5         207           305                                   
20          5         212           313                                   
END         0                                                             
Ok                                                                        
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