Abstract:
Drive systems and methods for energizing electronically commutated motors are provided. A drive system includes an inverter for providing pulse width modulated drive signals for energizing an electronically commutated motor in response to control signals and a controller for generating the control signals and for applying the control signals to the inverter such that winding pairs of the electronically commutated motor are energized in a selected sequence. The pulse width modulated drive signals include first and second pulses having first and second pulse widths, respectively, that are unequal. The average voltage applied to the energized winding pair is a function of the time difference between the first and second pulse widths.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to drive systems for electronically commutated motors and, more particularly, to improved pulse width modulated drive systems for electronically commutated motors.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electronically commutated motor systems include an electronically commutated motor and a control system for energizing the motor. An electronically commutated motor typically includes a rotor having permanent magnets and a stator having a balanced three phase coil in the form of three motor windings. The control system includes a controller for generating pulse width modulated control signals and an inverter for energizing selected pairs of motor windings in response to the control signals. By energizing the motor windings in sequence, the rotor is caused to rotate.  
           [0003]    A voltage applied to two of the motor windings produces a magnetic flux that interacts with the magnetic flux produced by the rotor, resulting in an electromagnetic torque which rotates the rotor. In the third open winding, the rotor magnetic flux induces an electromotive force that may be measured. The electromotive force is called “back emf”, and its amplitude depends on the motor speed and permanent magnet characteristics. The back emf is measured because it contains information regarding the rotor position and velocity.  
           [0004]    By applying a variable voltage to the electronically commutated motor, a variable torque may be obtained and therefore the motor speed may be changed. The variable voltage is obtained by utilizing pulse width modulation (PWM) techniques. Pulse width modulation is a method to obtain a variable voltage using a DC supply voltage. By varying the duty cycle of the pulse width modulated control signal, the average voltage varies. In this way, a variable voltage may be applied to the motor windings.  
           [0005]    A first prior art PWM modulation technique, known as soft chopping modulation, is shown in FIG. 9. A control period T is the time that each winding pair is energized during one rotation of the motor. Voltages V A  and V B  are the drive voltages applied to energized windings A and B, respectively. As shown, winding A is held at supply voltage V DC  during control period T, and winding B is pulsed to supply voltage V DC  for a time t 2 . It may be shown that the average motor voltage V m  is proportional to the time t 1  during the control period T when the pulse on winding B is off.  
           [0006]    The soft chopping modulation technique shown in FIG. 9 has the advantage that current passes through the inverter DC bus only during time t 1 , which results in low current ripple and low power consumption. However, the prior art soft chopping modulation technique has disadvantages. The instantaneous and average values of the back emf depend on the voltage applied to the motor. This means that the back emf signal must be filtered in order to eliminate PWM harmonics. In addition, the flyback, which is a period after the commutation during which the current in the open phase decreases to zero, is unbalanced.  
           [0007]    A second prior art PWM modulation technique, known as hard chopping modulation, is shown in FIG. 10. Winding A is pulsed to the supply voltage V DC  during time t 1 , and winding B is pulsed to supply voltage V DC  during time t 2 . The average voltage V m  applied to the motor is proportional to time t 1 −t 2 . The main difference between this modulation technique and the soft chopping modulation technique described above is that in the hard chopping modulation technique, current passes through the inverter DC bus at all times, because one winding is linked to supply voltage V DC  and the other winding is connected to ground during the entire control period.  
           [0008]    The hard chopping modulation technique has the advantage that the back emf in the open phase is always symmetric with respect to V DC /2. In addition, the flyback is constant during the entire revolution of the motor when the motor runs at constant speed and torque. A disadvantage of this approach is that current flows through the DC bus at all times, resulting in high current ripple and increased power consumption.  
           [0009]    All of the known prior art pulse width modulation techniques for driving electronically commutated motors have one or more disadvantages. Accordingly, there is a need for improved pulse width modulated drive systems and methods for electronically commutated motors.  
         SUMMARY OF THE INVENTION  
         [0010]    According to a first aspect of the invention, a drive system is provided for an electronically commutated motor. The drive system comprises an inverter for providing pulse width modulated drive signals for energizing an electronically commutated motor in response to control signals and a controller for generating the control signals and for applying the control signals to the inverter such that winding pairs of the electronically commutated motor are energized in a selected sequence. The pulse width modulated drive signals comprise first and second pulses having first and second pulse widths, respectively, that are unequal. The average voltage applied to the energized winding pair is a function of the time difference between the first and second pulse widths.  
           [0011]    Preferably, the midpoints of the first and second pulses are timed to occur approximately simultaneously, and the first and second pulses have the same polarity. Each of the winding pairs of the electronically commutated motor is energized for a control period T, and the first and second pulses may be substantially centered with respect to the control period T.  
           [0012]    The controller may further comprise means for varying the first and second pulse widths of the first and second pulses to adjust the average voltage applied to the electronically commutated motor.  
           [0013]    The inverter may comprise a circuit for connecting each winding of the electronically commutated motor to a supply voltage or to a reference voltage in response to the control signals.  
           [0014]    According to another aspect of the invention, a method is provided for energizing an electronically commutated motor. The method comprises energizing pairs of windings of an electronically commutated motor with the pulse width modulated drive signals in a selected sequence. The pulse width modulated drive signals comprise first and second pulses having first and second pulse widths, respectively, that are unequal. The average voltage applied to the energized winding pair is a function of the time difference between the first and second pulse widths.  
           [0015]    According to a further aspect of the invention, a motor system is provided. The motor system comprises an electronically commutated motor including a plurality of windings, an inverter for providing pulse width modulated drive signals for energizing the electronically commutated motor in response to control signals, and a controller for generating the control signals and for applying the control signals to the inverter such that winding pairs of the electronically commutated motor are energized in a selected sequence. The pulse width modulated drive signals comprise first and second pulses having first and second pulse widths, respectively, that are unequal. The average voltage applied to the energized winding pair is a function of the time difference between the first and second pulse widths. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    For better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:  
         [0017]    [0017]FIG. 1 is a schematic block diagram of an electronically commutated motor system in accordance with an embodiment of the invention;  
         [0018]    [0018]FIG. 2 is a timing diagram that illustrates winding voltages and back emfs of the electronically commutated motor in accordance with an embodiment of the invention;  
         [0019]    [0019]FIG. 3 is a equivalent circuit diagram of the electronically commutated motor;  
         [0020]    [0020]FIG. 4 is a timing diagram of one control period T, illustrating pulse width modulated drive signals in accordance with a first embodiment to provide a relatively low motor voltage;  
         [0021]    [0021]FIG. 5 is a timing diagram of one control period T, illustrating pulse width modulated drive signals in accordance with the first embodiment to provide an intermediate motor voltage;  
         [0022]    [0022]FIG. 6 is a timing diagram of one control period T, illustrating pulse width modulated drive signals in accordance with the first embodiment to provide a relatively high motor voltage;  
         [0023]    [0023]FIG. 7 is a timing diagram of one control period T, illustrating pulse width modulated drive signals in accordance with a second embodiment;  
         [0024]    [0024]FIG. 8 is a flow diagram of a process for generating pulse width modulated control signals in accordance with an embodiment of the invention;  
         [0025]    [0025]FIG. 9 is a timing diagram of one control period T, illustrating a prior art soft chopping modulation technique; and  
         [0026]    [0026]FIG. 10 is a timing diagram of one control period T, illustrating a prior art hard chopping modulation technique.  
     
    
     DETAILED DESCRIPTION  
       [0027]    A block diagram of an embodiment of an electronically commutated motor system  10  is shown in FIG. 1. System  10  includes an electronically commutated motor  12 , an inverter  14  for supplying drive signals to motor  12  and a controller  16  for supplying control signals to inverter  14 .  
         [0028]    Electronically commutated motor  12  includes a stator winding  20 , a stator winding  22  and a stator winding  24 . In FIG. 1, each winding is represented by an inductance  30  an a voltage generator  32 . The inductance  30  represents the stator resistance and the stator inductance. Motor  12  also includes a rotor (not shown) having rotor magnets. The voltage generator represents the back emf induced in the winding by the rotor magnets. The windings  20 ,  22  and  24  are connected to a common node  34 .  
         [0029]    Inverter  14  includes circuitry for connecting each motor winding to a supply voltage V DC  or to a reference voltage, such as ground. A DC voltage source  28  provides supply voltage V DC  and is connected between a first terminal  36  and a second terminal  38  of inverter  14 . Winding  20  may be connected by a power transistor  40  to supply voltage V DC  or may be connected by a power transistor  42  to ground. Winding  22  may be connected by a power transistor  44  to supply voltage V DC  or may be connected by a power transistor  46  to ground. Winding  24  may be connected by a transistor power  48  to supply voltage V DC  or may be connected by a power transistor  50  to ground. A free wheeling diode  52  is connected between the collector and the emitter of each of transistors  40 - 50 . A capacitor  60  is connected between the first terminal  36  and the second terminal  38  of inverter  14 , and a shunt resistor  62  is placed in the return path of inverter  14 .  
         [0030]    Transistors  40 - 50  are controlled by pulse width modulated control signals from controller  16  to energize motor  12 . In particular, winding pairs are energized in a selected sequence to produce motor rotation. For example, by turning on transistor  40  and transistor  46 , current flows through transistor  40 , winding  20 , winding  22  and transistor  46 . Current flows in the opposite direction through windings  20  and  22  when transistors  44  and  42  are turned on. By appropriate control of transistors  40 - 50 , winding pairs  20  and  22 ,  20  and  24 , and  22  and  24  can be energized in sequence. When a winding pair is energized, the third of the three windings is open and the two transistors connected to that winding are turned off. Thus, for example, when windings  20  and  22  are energized, transistors  48  and  50  are off and winding  24  is open.  
         [0031]    A timing diagram showing an example of winding drive voltages V A , V B  and V C , and corresponding back emfs e 1 , e 2 , and e 3  for one revolution of the rotor is shown in FIG. 2. Drive voltages V A , V B  and V C  represent the voltages applied to stator windings  20 ,  22  and  24 , respectively, and back emfs e 1 , e 2  and e 3  represent the back emfs in stator windings  20 ,  22  and  24 , respectively. Pulse width modulated winding voltages V A , V B  and V C  are applied in a predetermined sequence during each control period T. Thus, in control period  5 , windings  20  and  22  are energized; in control period  6 , windings  20  and  24  are energized; in control period  1 , windings  22  and  24  are energized; etc. The characteristics of the pulse width modulated signals applied to each energized winding pair determine the motor speed as described below. The back emf induced in each winding has a trapezoidal variation.  
         [0032]    Assume that each of stator windings  20 ,  22  and  24  has two poles. When the rotor makes one complete turn, the emf induced in the stator windings completes one period. If the windings in the stator each have four poles, the situation changes. When the rotor makes one complete turn, the back emf induced in the stator windings completes two periods. Similarly, for stator windings having six poles, the back emf completes three periods, etc. FIG. 2 illustrates one period of the back emfs e 1 , e 2  and e 3  This period may be divided into 6 sectors, each of 60°. In every sector, the pattern of the PWM pulses in different. For example, in sector  5  winding  24  is maintained open (the back emf induced in winding  24  varies in this sector) and voltages are applied to windings  20  and  22 , with voltage V A  greater than voltage V B  (because back emf e 1  is greater than back emf e 2 ).  
         [0033]    The sector time represents the time which the rotor stays in this sector, which may or may not represent 60° on the rotor, because the sector time is a function of the number of poles of the stator windings. If each winding has two poles, a back emf sector represents 60° on the rotor. If each winding has four poles, a back emf sector represent 30° on the rotor, etc. The sector time is a function of the rotor speed and therefore varies. Also, the number of PWM cycles in a sector varies as a function of rotor speed.  
         [0034]    An equivalent circuit diagram of the motor during control period  5  is shown in FIG. 3. Winding  20  is energized by voltage V A , winding  22  is energized by voltage V B , and winding  24  is open (transistors  48  and  50  in FIG. 1 are both off). Winding  20  is represented by a resistance R S , an inductance L S  and a back emf e 1 . Winding  22  is represented by a resistance R S , an inductance L S  and a back emf e 2 . Winding  24  is represented by a back emf e 3 , and the voltage between winding  24  and ground is represented by voltage V 30 . The back emf e 3  is measured because it contains information regarding the rotor position and velocity.  
         [0035]    Waveforms for driving an electronically commutated motor in accordance with a first embodiment of the invention are shown FIGS.  4 - 6 . A waveform  100  represents a drive voltage V A  applied to winding  20  (FIG. 1) during a control period T, and a waveform  102  represents a drive voltage V B  applied to winding  22  during the control period T. The control period illustrated in FIGS.  4 - 6  corresponds to control period  5  shown in FIG. 2. It will be understood that similar voltage waveforms are applied to other winding pairs during the other control periods as shown in FIG. 2. FIG. 4 represents a relatively low average motor voltage V m  (relatively small time t 2 ), FIG. 5 represents an intermediate average motor voltage V m  (intermediate time t 2 ), and FIG. 6 represents a relatively high average motor voltage V m  (relatively large time t 2 ).  
         [0036]    As illustrated in FIGS.  4 - 6 , winding  20  and winding  22  are both pulsed to supply voltage V DC  during a portion of control period T. In particular, waveform  100  includes pulse  110 , and waveform  102  includes pulse  112 . In FIGS.  4 - 6 , time t 3  represents the width of pulse  112  applied to winding  22 , and time t 2  represents the time difference between the widths of pulse  110  and pulse  112 . Time t 1  represents the time during control period T when pulse  110  is off, or at ground. During time t 1 , both windings  20  and  22  are connected to ground, and no current flows to or from DC voltage source  28 . Similarly, during time t 3 , both windings  20  and  22  are connected to supply voltage V DC , and no current flows to or from DC voltage source  28 . During time t 2 , winding  20  is connected to supply voltage V DC  and winding  22  is connected to ground, resulting in current flow from the inverter through the windings. Accordingly, current flows through the A and B windings and the inverter only during time t 2 , which corresponds to the time difference between pulse  110  and pulse  112 .  
         [0037]    Waveforms  100  and  102  may be generated by control signals applied to power transistors  40 - 50  as follows during times t 1 , t 2 , and t 3  of control period T. During time t 1 , transistors  40 ,  44 ,  48  and  50  are off, and transistors  42  and  46  are on. During time t 2 , transistors  42 ,  44 ,  48  and  50  are off, and transistors  40  and  46  are on. During time t 3 , transistors  42  and  46 ,  48  and  50  are off, and transistors  40  and  44  are on. It will be understood that the above description of control signals applies to control period  5  shown in FIG. 2 and that different combinations of transistors  40 - 50  are turned on and off in other control periods.  
         [0038]    As further illustrated in FIGS.  4 - 6 , pulses  110  and  112  may be centered with respect to control period T. That is, the midpoint of pulse  110  and the midpoint of pulse  112  both occur at the midpoint of control period T. This holds true as the pulse widths are varied to adjust the average motor voltage V m . As a result of the centering of pulses  110  and  112  in control period T, time t 2  is divided into two equal segments of time t 2 /2.  
         [0039]    The average voltages applied to windings  20  and  22  may be determined as follows.  
               V   A     =       V     D                 C                t   2     +     t   3       T               (   1   )                 V   B     =       V     D                 C              t   3     T               (   2   )                               
 
         [0040]    where V A  and V B  represent the average voltages applied to windings  20  and  22 , respectively. The motor voltage is given by  
               V   m     =         V   A     -     V   B       =       V     D                 C              t   2     T                 (   3   )                               
 
         [0041]    where V m  represents the average voltage applied to the motor. In a typical application, the required motor voltage V m  is given, and from this value the widths of pulses  110  and  112  are determined. Referring the equation (3), the time t 2 , which represents the time difference between pulses  110  and  112 , can be determined from the given motor voltage V m . Preferably, times t 1  and t 3  are made equal, and the values of times t 1  and t 3  are computed as follows:  
               t   1     =       t   3     =       T   -     t   2       2               (   4   )                               
 
         [0042]    In the case where times t 1  and t 3  are equal, the average value of the back emf in the open winding is determined as follows:  
               e   3     =       V   30     -       V     D                 C       2               (   5   )                               
 
         [0043]    where e 3  represents the back emf in the open winding and V 30  represents the voltage between the open winding and ground.  
         [0044]    The pulse width modulation technique shown in FIGS.  4 - 6  and described above has the advantage that current flows through the inverter DC bus only during time t 2 . This period is relatively small, so current ripple and power consumption are low. Another advantage is that the average value of the back emf in the open winding does not depend on motor voltage V m  and is symmetrical with respect to V DC /2. It may be observed that the soft chopping modulation technique described above is a particular case of the modulation technique of FIGS.  4 - 6  if time t 1  is zero or time t 3  is zero. A disadvantage of the pulse width modulation technique shown in FIGS.  4 - 6  is that the instantaneous value of the back emf in the open winding is not symmetrical, thus requiring filtering to obtain the average value.  
         [0045]    A second embodiment of the pulse width modulation technique in accordance with the invention is shown FIG. 7. As in FIGS.  4 - 6 , a single control period T is shown in FIG. 7. In the embodiment of FIG. 7, the pulses are inverted with respect to the pulses shown in FIGS.  4 - 6 . A pulse  120  is applied to winding  20 , and a pulse  122  is applied to winding  22 . Pulses  120  and  122  begin at the time when each winding is switched from supply voltage V DC  to ground. Pulse  120  has a pulse width of time t 3 , and pulse  122  has a pulse width of time t 2 +t 3 . As in the embodiment of FIGS.  4 - 6 , the average motor voltage is given by equation (3), and the back emf in the open winding is given by equation (5), for the case where time t 1  is equal to time t 3 .  
         [0046]    Controller  16  (FIG. 1) supplies pulse with modulated control signals to inverter  10  for applying pulse width modulated drive signals to electronically commutated motor  12 , as shown in FIGS.  4 - 6  or FIG. 7 and described above. The control signals control each of transistors  40 - 50  to energize a selected winding pair according to the pulse width modulated drive signals described above. In a preferred embodiment, controller  16  is implemented as a programmed digital signal processor which generates control signals corresponding to a desired average motor voltage V m . In one embodiment, controller  16  may be a type ADMCF328 sold by Analog Devices, Inc. However, it will be understood that different controller configurations may be utilized within the scope of the invention. For example, the control signals may be generated by any digital signal processor, microprocessor or microcontroller that is programmed to generate PWM signals. Furthermore, the control signals may be generated by special purpose or hardwired circuitry. These controller configurations are given by way of example only and are not limiting as to the scope of the invention.  
         [0047]    A flow chart of a process for generating pulse width modulated control signals is shown in FIG. 8. In step  200 , a signal measurement is acquired for controlling motor  12 . The signal measurement typically includes measurement of the voltage across shunt resistor  62  (FIG. 1) and the voltage on the open winding. The voltage across shunt resistor  62  represents the current through the return path of the inverter and is used to control the current in the motor windings. The voltage on the open winding is used to determine the velocity and position of the rotor. In step  202 , the process determines the required motor voltage V m  to achieve a desired control function. In step  204 , a pulse width modulation routine generates the pulse width modulated control signals to control power transistors  40 - 50  of inverter  10 . In particular, the pulse width of pulse  112 , which is time t 3 , and the pulse width of pulse  110 , which is time t 2 +t 3 , can be determined from equations (3) and (4) above, since supply voltage V DC  and control period T are known.  
         [0048]    Having described this invention in detail, those skilled in the art will appreciate that numerous modifications may be made of this invention without departing from its spirit. Therefore, it is not intended that the breadth of the invention be limited to the specific embodiment illustrated and described. Rather, the breadth of the invention should be determined by the appended claims and their equivalents.