Abstract:
A method and apparatus for electronic commutation of a pulse width modulation (PWM) controlled motor involves temporarily increasing the frequency of one or more PWM drive signals applied to the motor upon the occurrence of an asynchronous commutation event.

Description:
TECHNICAL FIELD 
     The present application relates generally to electronic commutation, and more particularly to a method and apparatus to improve electronic commutation of a pulse width modulation (PWM) controlled electric motor. 
     BACKGROUND 
     Electronic commutation of electric motor phase windings is used to control the torque produced and the resulting rotation of the motor shaft. Torque about the rotor shaft is produced from the interaction of magnetic force fields generated by a magnetic rotor attached to the motor shaft and current flowing through the stator phase windings. Maximum torque occurs when the angle between stator and rotor magnetic field vectors is 90 electrical degrees and decreases as the vectors align during rotation. 
     To control shaft rotation, the stator phase windings can be energized in a sequence defined by the angular position of the magnetic rotor with respect to the stator phase windings. Angular position of the magnetic rotor can be detected using three stationary Hall effect sensors positioned on a radius about the shaft and a multi-pole ring magnet aligned with the rotor poles attached to the shaft. The Hall effect sensor outputs can collectively be used to determine sequential commutation states during rotation of the magnetic rotor. 
     For example, in a stator having three phase windings, a six-step commutation method may produce six possible stator magnetic field vectors over 360 electrical degrees. Each commutation state may represent 60 electrical degrees with one phase winding connected to a positive voltage, a second phase winding connected to a negative voltage and a third phase winding not connected. A commutation event may be defined as occurring when the rotor moves from a position associated with one commutation state to a position associated with a next commutation state, as determined by a change in output from the Hall effect sensors due to rotation of the rotor ring magnet. For a new commutation state, the stator phase windings may be energized with the correct voltage polarities. In a PWM motor control system, phase winding voltage polarity and average value are typically controlled by comparison of a duty cycle value with a dual-sloped linear ramp function driven by a controller. The comparison output is valid for the time the dual-sloped linear ramp function value exceeds the duty cycle value. 
     Each phase winding can be driven by a complementary pair of electronic switching devices controlled by PWM drive signals generated by the controller. For each phase winding, three possible voltage connections can be made using the electronic switching device pair: positive voltage when the HI device is powered on, negative voltage when the LO device is powered on and no voltage when both devices are powered off. Simultaneously powering on the HI and LO devices of a phase winding may result in a short circuit, allowing potentially destructive shoot through currents to flow from the positive to negative voltage supplies through the electronic switching device pair. Shoot through currents can be avoided by incurring a dead-time between powering off one electronic switching device and powering on the other electronic switching device. 
     The timing diagram of  FIG. 1  illustrates an ideal commutation response to a commutation event  10  represented by the transition from a first commutation state (State  1 ) to a second commutation state (State  2 ) for a three phase motor with a stator having three similar phase windings A, B and C. Prior to commutation event  10 , the phase A terminal is connected to positive voltage, the phase B terminal is connected to negative voltage and the phase C terminal is unconnected. Upon duty cycle completion, the phase A terminal is momentarily disconnected to incur dead-time  12  to avoid shoot through current. The phase A terminal is then connected to negative voltage to allow freewheeling load current to circulate. A resultant voltage will be impressed across the phase A and phase B terminals in direct proportion to the PWM duty cycle. 
     Upon occurrence of commutation event  10 , new voltage connections are required. The phase A terminal is disconnected, the phase B terminal remains connected to negative voltage and the phase C terminal is connected to positive voltage. A new PWM period or cycle is initiated and the average resultant voltage impressed across the phase C and phase B terminals would be equal to the average resultant voltage impressed across the phase A and phase B terminals for the previously completed PWM cycle prior to the commutation event. Although this ideal embodiment is technically feasible, many hardware implementations are incapable of implementing this ideal operation. 
     In some hardware implementations incapable of ideal operation, phase winding voltage polarities for a new commutation state cannot take effect until the current or ongoing PWM drive signal cycle is complete, thus delaying commutation. A six-step commutation method cannot maintain the angle between the rotor and stator magnetic fields at 90 electrical degrees for maximum torque. The actual angle varies from 60 to 120 electrical degrees. The commutation event is critical for its angular (time) accuracy and any deviation will cause torque ripple and speed variations. 
     The timing diagram of  FIG. 2  illustrates typical delayed commutation from State  1  to State  2  due to hardware limitations. After commutation event  10 , the voltage connections are changed only after initiation of the new PWM drive signal cycle at  14 . While common, this method results in a delay of up to one PWM drive signal cycle. 
     The timing diagram of  FIG. 3  illustrates a decreased resultant average voltage applied across the phase B and phase C terminals in response to commutation event  10 . After commutation event  10 , the ongoing PWM drive signal being applied to the phase A terminal is steered to the phase C terminal as shown. However, this technique results in application of an incorrect duty cycle to the new commutation state. 
     These delays ( FIG. 2 ) and incorrect duty cycles ( FIG. 3 ) in response to a change in commutation states create torque ripple and speed variations due to the deviation of the electrical angle between stator and rotor magnetic field vectors. There is a need for a motor system and method for its control that reduces or eliminates these shortcomings. 
     SUMMARY 
     The present application provides a method and apparatus for improving the commutation response of a PWM controlled electric motor so new phase winding voltage polarities take effect with minimal time delay, reducing torque ripple and speed variation by decreasing the angular deviation of the stator to rotor magnetic field vector relationship from the ideal 90 degrees. 
     In one aspect, a method of controlling an electronically commutated motor having multiple phase windings by applying PWM drive signals in accordance with a series of commutation states is provided, where a commutation event occurs when a rotor moves from a position associated with one commutation state to a position associated with another commutation state. The method involves the steps of: applying PWM drive signals at a set frequency on a cyclical basis and, responsive to occurrence of a commutation event that is asynchronous with an ongoing PWM drive signal cycle, increasing the frequency of PWM drive signals applied during the ongoing PWM drive signal cycle to reach the end of the ongoing PWM drive signal cycle more quickly than without such a frequency increase. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a timing diagram illustrating ideal commutation; 
         FIG. 2  is another timing diagram illustrating the conventional delayed response in commutation; 
         FIG. 3  is another timing diagram illustrating a known commutation technique that results in an incorrect duty cycle being applied during a new commutation state; 
         FIG. 4  is another timing diagram illustrating an embodiment of a technique for improving commutation response; 
         FIG. 5  is another timing diagram illustrating a second embodiment of a technique for improving commutation response; 
         FIG. 6  is a diagrammatic view of a three phase motor; 
         FIG. 7  is another diagrammatic view of the three phase motor; 
         FIG. 8  is a block diagram illustrating a motor control system; and 
         FIG. 9  is a schematic diagram of a three phase motor control system circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Commutation response in an electronically commutated motor is improved by increasing the PWM drive signal frequency immediately after a commutation event, thereby reducing the time spent at voltage connection polarities from the previous state and maintaining a more uniform average voltage across the commutated phase windings. 
     In the prior art delayed systems described above, applied torque is reduced during the time between an asynchronous commutation event and when a response occurs as the angle between the rotor and stator flux vectors is decreasing from 60 degrees to zero and a significant increase in winding current results from the rapidly decreasing back EMF. After a delayed response, the ability to inject current in the next commutation state winding connection is reduced because the generated back EMF is rapidly increasing. The reduced current injection causes lower torque production because of reduced current at the ideal 90 degrees torque angle. The commutation delay becomes increasingly significant at higher rotational speeds with higher pole count motors. To illustrate this condition, each commutation state of an 8 pole motor running at 7000 RPM is approximately 360 microseconds while the delay of one period of 20 kHz PWM drive signal frequency is 50 microseconds. 
     Referring to the timing diagram of  FIG. 4 , an improved method of commutation in a three phase motor system is shown in response to commutation event  10  that is asynchronous to an ongoing PWM drive signal cycle  32 . The standard PWM drive signal frequency F PWM  is increased to 3F PWM , as shown at  20 , until the end of the ongoing PWM drive signal cycle at  22 . The existing voltage connections are maintained until the end of this PWM drive signal cycle due to particular hardware limitations, and the increase in frequency allows the ongoing PWM drive signal cycle to complete more quickly. In some implementations, connection of the phase A terminal to negative voltage as shown at  24  may also be eliminated if hardware restrictions are such that application of the necessary dead-time  26  cannot be guaranteed. 
     For applied winding voltages controlled by PWM duty cycle generation, an additional PWM drive signal cycle  30 , here shown at 2F PWM , may be required to assure new duty cycle values are loaded into the controller to apply the desired voltage to the next commutation state winding connection. In this embodiment, the commutation event  10  occurs during ongoing PWM drive signal cycle  32 , and the particular hardware implementation requires a full PWM drive signal cycle before the controller registers new duty cycle values for the next commutation state winding connection. After PWM drive signal cycle  30  is complete, the PWM drive signal frequency returns to F PWM  as shown by cycle  34 . Thus, new phase winding voltage polarities take effect with reduced time delay, reducing torque ripple and speed variation. 
     Referring to the timing diagram of  FIG. 5 , an alternative improved method of commutation response to commutation event  10 , which is asynchronous with ongoing PWM drive signal cycle  40 , involves increasing the PWM drive signal frequency from F PWM  to 2F PWM  until reaching the end of the ongoing PWM drive signal cycle  40 . The existing voltage connections are maintained until the end of cycle  40 , and thus the transition to PWM cycle  42  occurs more quickly than without such a frequency increase. Note that drive signal frequency reverts to F PWM  in cycle  42 . Thus, new phase winding voltage polarities take effect with reduced time delay, reducing torque ripple and speed variation. 
     Referring to the timing diagrams of  FIGS. 4 and 5 , the objective is to obtain the maximum PWM drive signal frequency increase permitted by the power stage dead-time and switching limitations to reduce the delay after a commutation event  10 . The frequency increase may be an integer or non-integer multiple of the original PWM drive signal frequency. Hardware implementation limitations for a particular embodiment should be considered such that the previous commutation state winding voltage is removed as quickly as possible and the next commutation state winding voltage is completely applied to all phases as quickly as possible. One or more intermediate decreases in PWM drive signal frequency may be required to insure that the PWM drive signals for all phases are properly updated. The PWM drive signal frequency should return to the original PWM drive signal frequency once the applied winding voltage transition in response to commutation event  10  has been completed. 
     Referring to  FIGS. 6 and 7 , an exemplary three phase motor  44  has a magnetic rotor  46 , shown in this embodiment having N (positive) and S (negative) poles. The motor  44  has a stator with three phase windings  48 A (made up of A and A′),  48 B (made up of B and B′) and  48 C (made up of C and C′), surrounded by a sensor arrangement including three Hall effect sensors  50 - 1 ,  50 - 2  and  50 - 3 . 
     The three phase motor has six commutation states  52 A,  52 B,  52 C,  52 D,  52 E and  52 F, each commutation state comprising a 60 degree sector  45  of a circle representing rotation of the three phase motor  44  magnetic rotor  46 . Arrows originating from the center of  FIGS. 6 and 7  represent the location of commutation events  47  between adjacent sectors  45 . In this embodiment, the three phase motor  44  has three phase windings  48 A,  48 B and  48 C, which can be configured in a wye (Y) configuration, delta (Δ) configuration or any other suitable configuration. Other numbers of phase windings can also be used. 
     The magnetic rotor  46  is driven by connecting the phase windings  48 A,  48 B and  48 C to a positive voltage (V+), a negative voltage (V−) or no voltage (NC). Each of the Hall effect sensors  50 - 1 ,  50 - 2  and  50 - 3  is either triggered (e.g., closed) or not triggered (e.g., opened) depending upon the position of the magnetic rotor  46 . The phase windings  48 A,  48 B and  48 C are energized in a predetermined sequence utilizing PWM drive signals for the sectors  45  corresponding with each of the six commutation states  52 A,  52 B,  52 C,  52 D,  52 E and  52 F. Table I below illustrates an exemplary energizing sequence for the motor  44  of  FIGS. 6 and 7 . For Hall effect sensors  50 - 1 ,  50 - 2  and  50 - 3 , a “1” indicates a triggered sensor and a “0” indicates a non-triggered sensor. For the phase windings  48 A,  48 B and  48 C, NC indicates no voltage connection, V+ indicates a positive voltage connection and V− indicates a negative voltage connection. 
       FIG. 6  shows the motor during commutation State  52 A with the rotor magnetic flux vector  43  and resultant stator magnetic flux  49  vector at less than a 90 degree angle for the rotor position shown. In  FIG. 6  the resultant stator magnetic flux vector  49  is produced by the combined effect of the magnetic flux from A to A′ and the magnetic flux from B′ to B.  FIG. 7  shows the motor during commutation State  52 B with the magnetic flux vector  43  and resultant stator magnetic flux  49  vector less than 120 degrees for the rotor position shown. In  FIG. 7  the resultant stator magnetic flux vector  49  is produced by the combined effect of the magnetic flux from C to C′ and the magnetic flux from B′ to B. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Exemplary PWM Drive Signal Sequence 
               
             
          
           
               
                   
                 Hall 
                 Hall 
                 Hall 
                 Phase 
                   
                   
               
               
                 Commu- 
                 Effect 
                 Effect 
                 Effect 
                 Wind- 
                 Phase 
                 Phase 
               
               
                 tation 
                 Sensor 
                 Sensor 
                 Sensor 
                 ing 
                 Winding 
                 Winding 
               
               
                 State 
                 50-1 
                 50-2 
                 50-3 
                 48A 
                 48B 
                 48C 
               
               
                   
               
               
                 52A 
                 0 
                 0 
                 1 
                 V+ 
                 V− 
                 NC 
               
               
                 52B 
                 0 
                 1 
                 1 
                 NC 
                 V− 
                 V+ 
               
               
                 52C 
                 0 
                 1 
                 0 
                 V− 
                 NC 
                 V+ 
               
               
                 52D 
                 1 
                 1 
                 0 
                 V− 
                 V+ 
                 NC 
               
               
                 52E 
                 1 
                 0 
                 0 
                 NC 
                 V+ 
                 V− 
               
               
                 52F 
                 1 
                 0 
                 1 
                 V+ 
                 NC 
                 V− 
               
               
                   
               
             
          
         
       
     
     During each commutation state, one phase winding  48 A,  48 B or  48 C has a positive voltage, one phase winding has a negative voltage, and one phase winding is off. Thus, the attracting and repelling magnetic flux vectors caused by energizing phase windings  48 A,  48 B and  48 C drive rotation of the magnetic rotor  46 . 
       FIG. 8  is a block diagram of an exemplary motor control system having a motor  54  with a magnetic rotor. Control requests  56  arrive at controller  58  (e.g., requesting a specific speed of operation for the motor  54 ). The controller  58  sends one or more PWM drive initiation signals  59  at a standard frequency to one or more gate drives  60 . The gate drives  60  then step up the signal voltage to a level sufficient to effectively operate switching devices  62  associated with the motor phase winding terminals. For instance, the controller  58  may send PWM drive initiation signals  59  at 0-5V, which the gate drives  60  will then increase to 0-15V. Operation of the electronic switching devices  62  produces PWM drive signals  63  that energize the phase windings with the appropriate voltage (positive, negative or no voltage). 
     Sensor arrangement  50  detects commutation events as they occur. When a new commutation event occurs due to magnetic rotor rotation, the sensor output from the sensor arrangement  50  to the controller  58  changes (see Table I). In one embodiment, a change in output from the sensor arrangement acts as an interrupt to the controller, causing the controller to increase the frequency of the ongoing PWM drive signal. In another embodiment, the controller may analyze the sensor output to identify if the commutation event is asynchronous to an ongoing PWM drive cycle and, if so, modifies the PWM frequency in a manner such as those illustrated in  FIG. 4  or  FIG. 5 , thereby improving the commutation response. Other variations on the manner of increasing the frequency of the PWM signals upon the occurrence of asynchronous commutation events are possible. The controller  58  adjusts the frequency of the PWM drive initiation signals  59  it sends to the gate drives  60 , resulting in a corresponding frequency change in the PWM drive signals  63  applied to the motor phase windings. The PWM drive initiation signals  59 , and thus the PWM drive signals  63 , are later returned to a standard frequency per  FIGS. 4 and 5 . The temporary frequency increase may be repeated for each asynchronous commutation event. 
       FIG. 9  is a schematic diagram of a circuit showing electronic switching devices  62  of the control system as connected to motor  54 . Each phase winding  48 A,  48 B and  48 C has a corresponding HI side electronic switching device (e.g., at the positive voltage side of the voltage source)  70 A,  70 B and  70 C and LO side electronic switching device (e.g., at the negative side of the voltage source)  72 A,  72 B and  72 C, respectively. The gate drives (not shown) step up the voltage of the PWM drive initiation signals, then apply the increased voltage signals to the electronic switching devices  62  via circuit inputs  74 A,  74 B,  74 C,  76 A,  76 B and  76 C. 
     It is to be clearly understood that the above description is intended by way of illustration and example only and is not intended to be taken by way of limitation. For example, why  FIGS. 6 and 7  depict an embodiment implemented in a rotary motor, such as a brushless direct current motor or a switched reluctance motor, the PWM frequency control technique described above could readily implemented on any electronically commutated motor, including linear motors. Further, the exact level of frequency increase and manner of implementation could vary widely depending upon the specific implementation. Other changes and modifications could also be made.