Abstract:
A method and system for providing sensorless brushless DC motor control using predictive switch timing requires connecting a stator coil in a bridge configuration, applying a positive excitation voltage across the coil for a predetermined time period, deactivating the excitation voltage, and monitoring the voltage (V EMF ) generated due to electro-motive force (EMF) across the coil. The polarity of V EMF  changes when the rotor has moved a known distance—typically 90°. After detecting a polarity change, a negative excitation voltage is applied across the coil, deactivated, and V EMF  monitored to detect a polarity change. This sequence is repeated to maintain the rotation of the rotor. The motor is preferably set into motion using a start-up routine, which also determines the predetermined time period used during steady-state operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to the field of motors, and more particularly, to circuits and methods for determining the position of a brushless DC motor&#39;s rotor. 
         [0003]    2. Description of the Related Art 
         [0004]    Brushless DC motors generally comprise a stator and a permanent magnet rotor. The stator includes at least one coil; an excitation voltage is periodically applied across the coils, and the resulting electromagnetic field causes the rotor to rotate with respect to the stator. 
         [0005]    To ensure that the application of the excitation voltage is correctly timed, it is necessary to know the position of the rotor with respect to the stator coils. This is conventionally accomplished with the use of one or more Hall effect sensors, with the stator coils activated by drive electronics that are cued by signals received from the sensors. One motor system of this type is illustrated in  FIG. 1 . Here, a motor  10  includes stator coils  12  and  14  and a Hall sensor  16 . A controller  18  operates switches  20  and  22 , here bipolar transistors, to periodically apply an excitation voltage VDD across coils  12  and  14 , respectively. The position of the rotor is sensed with Hall sensor  16 , and reported to controller  18  via an amplifier  20 ; with this position information, controller  18  can operate switches  20  and  22  as necessary to maintain the rotation of the motor&#39;s rotor, and to control its speed if desired. 
         [0006]    However, the approach depicted in  FIG. 1  has a number of drawbacks. The need for Hall sensors contributes a significant cost to the motor. In addition, the sensors must be placed within the motor along with other electronics, which limits the design of the motor structure, and may degrade its reliability. 
         [0007]    One alternative to the use of Hall sensors is employed in some multiple-phase DC brushless motors. Here, the electro-motive force (EMF) generated in a passive coil while another coil is energized is measured and used to determine the position of the rotor. 
       SUMMARY OF THE INVENTION 
       [0008]    A method and system for providing sensorless brushless DC motor control using predictive switch timing are presented which overcome the problems noted above, providing accurate rotor position information without the need for Hall sensors or a passive coil. 
         [0009]    The present method determines the position of a brushless DC motor&#39;s permanent magnet rotor by applying an excitation voltage having a first polarity across at least one stator coil for a predetermined ON-time period, deactivating the excitation voltage, monitoring the voltage (V EMF ) across the coil generated by the electro-motive force (EMF) induced by the motor&#39;s rotor when the excitation voltage is deactivated, and detecting when V EMF  changes polarity. The polarity of V EMF  changes when the rotor has moved by a known distance which depends on the number of rotor poles. The stator coil is connected in a full-bridge configuration. This arrangement enables the excitation voltage to be applied and the V EMF  polarity to be monitored across the same coil. 
         [0010]    To keep the rotor spinning, after detecting that V EMF  has changed polarity, an excitation voltage having a polarity opposite that of the first polarity is applied across the coil for a predetermined ON-time period, at which point it is deactivated and V EMF  again monitored to detect when it changes polarity. This sequence of events is continuously repeated to maintain the rotation of the rotor. 
         [0011]    The motor is initially set into motion using a start-up routine, which also serves to determine the predetermined ON-time period used during steady-state operation. The start-up routine comprises exciting the coil for a fixed ON time with an excitation voltage having a first polarity, which is then deactivated and V EMF  monitored. If V EMF  has not changed polarity, the coil is excited again with a voltage of the same polarity, which is again deactivated and V EMF  monitored. This is repeated until V EMF  changes polarity. Then an excitation voltage having a second polarity opposite the first is applied across the coil for a fixed ON time, after which it is deactivated and V EMF  monitored. If V EMF  has not changed polarity, the coil is excited again with a voltage of the second polarity, deactivated, and V EMF  monitored; this is repeated until V EMF  changes polarity. The start-up routine is terminated when the excitation voltage need only be applied for one fixed ON time before V EMF  changes polarity. 
         [0012]    The present control method and system are suitably used to control DC brushless motors. One possible application for such a motor is to drive a fan blade for a fan designed to cool an integrated circuit. 
         [0013]    These and other features, aspects, and advantages of the present invention will become better understood with reference to the following drawings, description, and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is block/schematic diagram of a known motor controller. 
           [0015]      FIG. 2  is a timing diagram illustrating the principles of the sensorless motor control method of the present invention. 
           [0016]      FIG. 3  is a block/schematic diagram of a sensorless motor control system per the present invention. 
           [0017]      FIG. 4  is a block/schematic diagram of another embodiment of a sensorless motor control system per the present invention. 
           [0018]      FIG. 5   a  is a schematic and corresponding plan view of a rotor and stator illustrating the operation of one step of a start-up routine per the present invention. 
           [0019]      FIG. 5   b  is a schematic and corresponding plan view of a rotor and stator illustrating the operation of another step of a start-up routine per the present invention. 
           [0020]      FIG. 5   c  is a schematic and corresponding plan view of a rotor and stator illustrating the operation of another step of a start-up routine per the present invention. 
           [0021]      FIG. 5   d  is a schematic and corresponding plan view of a rotor and stator illustrating the operation of another step of a start-up routine per the present invention. 
           [0022]      FIG. 5   e  is a schematic illustrating the operation of another step of a start-up routine per the present invention. 
           [0023]      FIG. 5   f  is a schematic and corresponding plan view of a rotor and stator illustrating the operation of another step of a start-up routine per the present invention. 
           [0024]      FIG. 5   g  is a schematic illustrating the operation of another step of a start-up routine per the present invention. 
           [0025]      FIG. 5   h  is a schematic and corresponding plan view of a rotor and stator illustrating the operation of another step of a start-up routine per the present invention. 
           [0026]      FIG. 6   a  is a timing diagram illustrating the operation of a portion of a start-up routine per the present invention. 
           [0027]      FIG. 6   b  is a timing diagram illustrating the operation of another portion of a start-up routine per the present invention. 
           [0028]      FIG. 6   c  is a timing diagram illustrating the operation of another portion of a start-up routine per the present invention. 
           [0029]      FIG. 7  is a plan view of a rotor and stator in which the stator is formed such that the rotor has a preferred direction of rotation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    The present invention is a method and means for determining the position of a brushless DC motor&#39;s permanent magnet rotor which is induced to rotate with a stator that includes at least one coil. The method requires that at least one stator coil be excited with a voltage having a first polarity for a given period of time, and then deactivated. The rotor continues to spin because of inertia, and also generates a voltage (V EMF ) due to EMF in the deactivated coils. V EMF  is monitored, and when it changes polarity, the rotor has moved by a known distance with respect to its position at the time of its previous change of polarity. The known distance depends on the number (n) of rotor poles, and is given generally by (360/n)°. Thus, for a typical four pole rotor, the known distance is 90°, for an eight pole rotor, the known distance is 45°, etc. At this point, the at least one stator coil is excited with a voltage of a second polarity opposite the first polarity for a given period of time. The coil is again deactivated and V EMF  again monitored to detect when it changes polarity. In this way, the position of the rotor can be tracked. The stator coil is connected in a full-bridge configuration. This arrangement enables the excitation voltage to be applied and V EMF  to be monitored across the same coil, thereby obviating the need to monitor EMF across a passive coil as in prior art designs. 
         [0031]    This process is illustrated in  FIG. 2 . An excitation voltage ( 30 ) having a first polarity (here, positive) is applied across the coil. After a predetermined “ON” time ( 32 ), the coil is deactivated. The voltage across the deactivated coil is that which results from EMF that is induced in the coil by the spinning rotor. This V EMF  voltage is monitored while the coil is deactivated ( 34 ). When V EMF  changes polarity, this indicates that the rotor has moved a known distance since the last change of V EMF  polarity was detected; for this example, assume a four pole rotor, and a known distance of 90°. 
         [0032]    When a change of V EMF  polarity is detected, an excitation voltage ( 36 ) having a second polarity (here, negative) is applied across the coil. After a predetermined “ON” time, the coil is deactivated and V EMF  monitored. A change in the polarity of V EMF  indicates that the rotor has moved another 900. 
         [0033]    To maintain the rotation of the rotor, the sequence of events described above is continuously repeated. By detecting the change in the polarity of V EMF , the position of the rotor becomes known, and the timing of the excitation voltage pulses can be properly controlled—without the use of costly Hall sensors as are found in prior art methods. 
         [0034]    A basic system for implementing the control method described above is shown in  FIG. 3 . Here, the stator includes two coils  40 ,  42  connected in parallel between a node  44  and a node  46 . As noted above, the stator coils are connected in a full-bridge configuration: a switching network includes switches S 1  and S 2  connected between a supply voltage VDD nodes  44  and  46 , respectively, and switches S 3  and S 4  connected between a circuit common point  48  (typically, but not necessarily, ground) and nodes  44  and  46 , respectively. 
         [0035]    A first comparator C 1  has its inputs connected to node  44  and ground, and a second comparator C 2  has its inputs connected to node  46  and ground. The outputs of the comparators are provided to a digital control block  50 , which provides control signals  52 ,  54 ,  56 ,  58  to operate switches S 1 , S 2 , S 3  and S 4 , respectively. 
         [0036]    Digital control block is arranged to operate the switches as needed to apply a positive excitation voltage (by closing S 1  and S 4 ) or a negative excitation voltage (by closing S 2  and S 3 ). When so arranged, the system of  FIG. 3  operates as follows: 
         [0037]    1. Switches S 1  and S 4  are turned ON, making current flow from S 1  to S 4  and generating a positive excitation voltage across coils  40  and  42 . 
         [0038]    2. After a predetermined ON-time period, switch S 1  is turned OFF while S 4  remains ON. 
         [0039]    3. The V EMF  across coils  40  and  42  is monitored by comparator C 1 ; when V EMF  changes polarity, the output of C 1  toggles, which is detected by digital control block  50 . 
         [0040]    4. The controller turns switch S 4  OFF, and turns switches S 2  and S 3  ON, thereby generating a negative excitation voltage across coils  40  and  42 . 
         [0041]    5. After the predetermined ON-time period, switch S 2  is turned OFF while S 3  remains ON. 
         [0042]    6. The V EMF  across the coils is monitored by comparator C 2  during this OFF-time period; when V EMF  changes polarity, the output of C 2  toggles, which is detected by digital control block  50 . 
         [0043]    7. The control block turns S 3  OFF and the cycle is repeated from step 1. 
         [0044]    Switches S 1 -S 4  are preferably implemented with transistors. This is illustrated in  FIG. 4 , in which S 1 -S 4  are implemented with respective field-effect transistors (FETs)  60 ,  62 ,  64 ,  66 . 
         [0045]    At start-up, the position, direction of rotation and the time taken by the rotor to move 90° (assuming a four pole rotor) is unknown. The present method preferably includes a start-up routine which is used to accelerate the rotor from rest, and to start the rotor spinning in a desired direction. One possible start-up routine is illustrated in  FIGS. 5   a - 5   h , which depicts the excitation or deactivation of a multiple-coil stator via switches S 1 -S 4  for each step. The resulting angular relationship between an exemplary rotor  70  and multiple-coil stator  72  is also shown in  FIGS. 5   a ,  5   b ,  5   c ,  5   d ,  5   f  and  5   h  (there is no change in angular relationship in  FIGS. 5   e  and  5   g ), and  FIGS. 5   d ,  5   f  and  5   h  depict V EMF  and the output of comparator C 1  during their respective steps. In this example, first and second coils  74  and  75  lie along a first axis of stator  72 , and third and fourth coils  76  and  77  lie along a stator axis which is perpendicular to the first axis. In practice, for both the start-up routine and steady-state operation, all four coils are connected in parallel, and the excitation voltage is applied across all four simultaneously. The rotor shown in  FIGS. 5   a - 5   h  has two N poles and two S poles; this four pole arrangement causes EMF polarity to change when the rotor moves by 90°. 
         [0046]    The start-up routine proceeds as follows: 
         [0047]    1. In  FIG. 5   a , switches S 1  and S 4  are turned ON, making current flow from S 1  to S 4  and generating a positive excitation voltage across the stator coils. This forces rotor  70  to become aligned with a coil (here, coil  74 ) on stator  72 . 
         [0048]    2. In  FIG. 5   b , the coils are deactivated for a brief period, during which rotor  70  displaces itself in a “preferred direction of rotation”. This is explained in more detail below. 
         [0049]    3. In  FIG. 5   c , switches S 2  and S 3  are turned on for a fixed ON time, and rotor  70  begins to rotate. Then in  FIG. 5   d , the fixed ON time expires, S 2  is switched OFF, and V EMF  is monitored. In this example, rotor  70  has not yet rotated by 90°, so V EMF  is positive and the output of C 1  has not toggled. If V EMF  does not switch polarity during the fixed OFF time, the coil is excited again for the fixed ON time 
         [0050]    4. Step 3 is repeated until V EMF  switches polarity. For example, as shown in  FIGS. 5   e  and  5   f , S 2  and S 3  are again turned on for a fixed ON time, rotor  70  continues to rotate, the fixed ON time expires and S 2  is switched OFF, and V EMF  is monitored. However, rotor  70  still has not rotated by 90°, so V EMF  remains positive and the output of C 1  has not toggled. 
         [0051]    5. In  FIGS. 5   g  and  5   h , the coils are excited ( 5   g ) and then deactivated ( 5   h ) and V EMF  finally changes polarity, thereby causing the output of comparator C 1  to toggle. 
         [0052]    6. Steps 3, 4 and 5 are repeated for the opposite direction of the current (not shown), with S 1  and S 4  turned on for fixed ON time periods such that a negative excitation voltage is repeatedly generated across the coils until V EMF  changes polarity. 
         [0053]    The fixed ON time is selected so that, when the rotor first begins to turn, more than one excitation pulse is required before V EMF  changes polarity. However, as the rotor starts to accelerate, fewer excitation pulses will be required to achieve a change in V EMF  polarity. The start-up routine continues as described above until the rotor has picked up enough speed so that only one excitation pulse is needed to effect a change in V EMF  polarity. Then, the ON and OFF times of the single excitation pulse are increased or decreased as desired to achieve a desired steady-state motor speed. 
         [0054]    Steps 3, 4 and 5 are illustrated with the timing diagram shown in  FIG. 6   a . An excitation voltage ( 80 ) having a first polarity (here, positive) is applied across the coil. After a fixed “ON” time ( 82 ), the coil is deactivated and the V EMF  voltage monitored ( 84 ). This is repeated until V EMF  changes polarity, indicating that the rotor has moved 90° since the last change of V EMF  polarity. 
         [0055]    Step 6 is illustrated with the timing diagram shown in  FIG. 6   b . A negative excitation voltage ( 90 ) is applied across the coil. After a fixed “ON” time ( 92 ), the coil is deactivated and the V EMF  voltage monitored ( 94 ). This is repeated until V EMF  changes polarity. 
         [0056]    In  FIG. 6   c , a positive excitation voltage ( 100 ) is again applied for a fixed “ON” time ( 102 ), after which the coil is deactivated and V EMF  monitored ( 104 ). Here, only a single excitation pulse was required to effect a change in V EMF  polarity, so the start-up routine may terminate. At this point, the ON and OFF times of the single pulse may be increased or decreased as desired to achieve a desired steady-state motor speed. The ON and OFF times of the single pulse required to achieve a desired steady-state motor speed are used to establish the initial predetermined ON and OFF times used during steady-state operation of the motor. 
         [0057]    Once steady-state operation is achieved, there are many ways in which a constant rotor speed could be maintained. One possible technique proceeds as follows: 
         [0058]    1. During steady-state operation, measure the time taken for the rotor to move 90° (assuming a four pole rotor). Save this time as “T 1 ”. 
         [0059]    2. Measure the time taken for the rotor to move another 90°. Save this time as “T 2 ”. 
         [0060]    3. After these initial T 1  and T 2  values are saved: for every 90° rotation of the rotor, the measured time is saved as T 2  and the old T 2  is saved as T 1 . Thus, T 1 =T 2   old , and T 2   new =T measured . Then, the excitation voltage&#39;s predetermined ON time is set equal to 90% of T 2   new , and its OFF time is set equal to 10% of T 2   new . 
         [0061]    4. If T 2 &lt;T 1 , the motor is accelerating; if T 2 &gt;T 1 , the motor is decelerating. To restore a constant rotor speed, the excitation voltage pulse must be adjusted. For example, when it is detected that the motor is accelerating, the ON time can be set equal to 90% of T 2   new -a small fixed value. Similarly, when it is detected that the motor is decelerating, the ON time can be set equal to 90% of T 2   new +a small fixed value. In this way, the rotor speed should be maintained in a narrow range around a desired value. 
         [0062]    Note that the methods and/or systems of the present invention could be implemented in many different ways. It is only essential that at least one stator coil be connected in a full-bridge configuration, that an excitation voltage be applied across the coil for a predetermined ON-time period and then deactivated, and that the voltage (V EMF ) across the coil generated by the EMF induced the coil by the rotor be monitored while the excitation voltage is deactivated to detect when it changes polarity. 
         [0063]    In a two-phase motor, it is very difficult to determine the direction of the rotor; hence the physical shape of the stator is preferably changed to have the preferred direction of rotation. One possible stator-rotor design is shown in  FIG. 7 . The hammer-like shape of the stator  110  ensures that the rotor  112  will move in a particular direction; for the design shown, the preferred direction of rotation is counter-clockwise (CCW). When the stator is not energized, the rotor aligns itself to the stator in such a way that the center of the mass of the stator is closest to the pole of the rotor; i.e., in  FIG. 7 , instead of aligning perfectly with the stator, the rotor tends to move a little CCW. This misalignment ensures that when the coils are excited, the rotor will tend to move CCW as desired. 
         [0064]    A digital control block suitable for realizing the motor control and start-up method described herein could be implemented in many possible ways. One approach is to implement the digital block as a state machine. 
         [0065]    The present method has been described as it might be used with multiple stator coils connected in parallel, as would commonly be found on a 2-phase motor. However, the invention could also used with a single coil, with the excitation voltage applied and V EMF  measured across the same coil. In this case, no “spare” or second coil is needed. The single coil would lie along one axis of the stator, with a first segment on one side of the stator hub and a second segment on the opposite side of the hub. The two segments would be connected in parallel. Assuming that the coil is initially aligned with two of the rotor&#39;s N poles, when an excitation voltage is applied across the coil, it generates an N-N field, forcing the rotor to rotate until the coil is aligned with two of the rotor&#39;s S poles, at which point the EMF voltage changes polarity. Applying an excitation voltage of the opposite polarity causes the rotor to move until the coil is again aligned with two of the rotor&#39;s N poles. For a four pole rotor, each rotor movement is 90°. If the rotor had, for example, eight poles (with N and S poles alternating around the rotor), the rotor would move 45° each time the excitation voltage polarity was reversed. 
         [0066]    The present control method could be used with a variety of DC brushless motor types, which could in turn be used in a wide variety of applications. One possible application is that of a cooling fan designed for mounting on the surface of an integrated circuit. 
         [0067]    The embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.