Abstract:
A method of operation for a motor-driven worm gear actuator coupled to a load device energizes the motor in a series of pulses during an initial period of motor operation when reversal of the motor is commanded. The pulsed energization produces a slow initial axial movement of the motor armature without moving the load device, minimizing the contact force between the armature shaft and a mechanical end stop of the motor. Rotation of the motor is determined by identifying and counting motor current pulses due to commutation, and the pulse count is adjusted at each reversal of motor rotation to compensate for motor current pulses due to the pulsed energization.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to the control of a motor-driven actuator, and more particularly to a motor control for reducing audible noise due to armature shaft end-play.  
         BACKGROUND OF THE INVENTION  
         [0002]    Control systems designed to position a load device frequently utilize an actuator coupled to a DC motor through a gear arrangement that provides a mechanical advantage. This is a common configuration because small permanent magnet DC motors may be produced at low cost, and actuator movement can be detected without position sensors by counting commutation pulses of the motor; see, for example, the U.S. Pat. No. 6,078,154 to Manlove et al., issued Jun. 20, 2000, and incorporated herein by reference. In many such systems, a worm gear is used to couple the armature of the motor to an output gear having an axis perpendicular to the armature; this is a popular arrangement because the worm gear can be formed or attached directly to the armature shaft, and because it provides a reasonably high mechanical advantage and virtually eliminates back-driving of the armature by the actuator. However, a drawback of this arrangement occurs due to axial shifting of the armature shaft each time the motor rotation is reversed. Mechanical stops are provided for limiting axial movement of the armature shaft, and clunking noises occur when the shaft forcibly impacts the stops. While the axial shifting and clunking noise can be virtually eliminated by minimizing the armature end-play, a certain amount of end-play is highly desirable from the standpoints of manufacturing cost and operating efficiency. Accordingly, what is needed is a way of minimizing the audible noise associated with axial shifting of the motor armature shaft while retaining the above-mentioned benefits of low cost and high efficiency.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention is directed to an improved method of operation for a motor-driven actuator in which the motor is coupled to a load device through a worm gear on an armature shaft of the motor, wherein the motor is energized in a series of pulses during an initial period of motor operation when reversal of the motor is commanded. The pulsed energization produces a slow initial axial movement of the motor armature without moving the load device, minimizing the contact force between the armature shaft and a respective mechanical end stop of the motor. Rotation of the motor is determined by identifying and counting motor current pulses due to commutation, and the pulse count is adjusted at each reversal of the motor to compensate for motor current pulses caused by the pulsed energization. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a diagram of a control system including a permanent magnet DC motor and worm gear drive arrangement, a pulse count circuit and a microprocessor-based motor control unit for carrying out the control method of this invention.  
         [0005]    [0005]FIG. 2 is a graph depicting a portion of the motor control carried out by the control unit of FIG. 1.  
         [0006]    [0006]FIG. 3 is a flow diagram representative of a software routine executed by the control unit of FIG. 1 according to this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0007]    Referring to FIG. 1, the control method of invention is disclosed in the context of an automotive actuator control system for positioning a load device  10 , which may be an air control door in an automatic climate control system, for example. The device  10  is mechanically coupled to an output gear  12 , as indicated by the broken line  14 , and the output gear  12  is maintained in meshing engagement with a worm gear  16  fastened to the armature shaft  18  of a permanent magnet DC motor  20 . The armature shaft  18  carries a pole and winding assembly  22  disposed within a set of permanent magnets  24  attached to the inner periphery of the motor case  26 , and the shaft  18  is radially constrained with respect to the motor case  26  by front and rear bushings  28  and  30 . A pair of washers  32  and  34  disposed on the shaft  18  limit axial movement of the shaft  18  relative to the case  26  by virtue of contact with the respective bushings  28  and  30 . This arrangement permits a limited axial movement of the shaft  18 , which allows the shaft  18  to rotate freely within bushings  28  and  30  without binding. A number of commutator segments  36  coupled to the armature windings are affixed to the shaft  18  between the pole and winding assembly  22  and the rear washer  34 , and a pair of brushes  38   a,    38   b  bonded to conductors  40   a,    40   b  contact opposing commutator segments  36 . The conductors  40   a,    40   b  pass through grommets  42   a,    42   b  disposed in the motor case, permitting activation of motor  20  by a motor control circuit, generally designated by the reference numeral  44 .  
         [0008]    The motor control circuit  44  includes a system controller  46 , a pulse count circuit (PCC)  48 , a microprocessor-based motor control unit  50 , and a motor drive circuit such as H-Switch (HS)  52 . The pulse count circuit  48 , which may be of the type described in the aforementioned U.S. Pat. No. 6,078,154 to Manlove et al., is capacitively coupled to the conductors  40   a,    40   b  via capacitors  54   a,    54   b,  and develops a PULSE_COUNT output on line  56  representative of a displacement of armature shaft  18 , and hence, device  10 . In general, the pulse count circuit  48  identifies and counts motor current pulses associated with commutation of the motor current to produce the output PULSE_COUNT. The system controller  46  develops a motor position command POS_DES on line  58 , and the motor control unit  50  activates the H Switch  52  via line  60  to bring the detected motor position into correspondence with POS_DES. Of course, the control is bi-directional—when the motor  20  needs to be driven in the forward direction, the motor control unit  50  activates H-Switch  52  to couple conductors  40   a  and  40   b  to battery voltage Vbat and ground, respectively, and vice-versa when the motor  20  needs to be driven in the reverse direction. Thus, the control signal on line  60  is also provided as an input to pulse count circuit  48  so that the identified commutation pulses increase PULSE_COUNT when the motor  20  is activated in the forward direction, and decrease PULSE_COUNT when the motor  20  is activated in the reverse direction.  
         [0009]    As explained above, a problem with the mechanical arrangement disclosed in FIG. 1 is that the motor armature shaft  18  shifts axially when the direction of motor rotation is reversed, producing audible noise as the front or rear washers  32 ,  34  forcibly contact the respective bushings  28 ,  30 , which act as end-stops for the armature  18 . According to the present invention, the motor control unit  50  uses pulse-width-modulation (PWM) to slowly initiate motor rotation each time the direction of motor rotation needs to be reversed, and then compensates the PULSE_COUNT output of pulse count circuit  48  for the effect of motor current pulses caused by the PWM. FIG. 2 graphically depicts the initial modulation of the motor energization according to a preferred embodiment this invention, where the motor windings are energized in a series of low duty cycle pulses in the time interval t 0 -t 1  prior to full energization of the motor windings. In a particular mechanization of this invention, for example, the motor energization signal was pulsed for six periods of PWM at a frequency of approximately 100 Hz, and a PWM duty cycle of approximately 12%. Obviously, various other PWM periods and duty cycles can be used, depending on the application. In general, however, the PWM duty cycle is chosen to produce sufficient torque to take up the mechanical lash of the actuator without moving the device  10 . The number of pulses is chosen to permit sufficient time for the initial armature movement, and the PWM frequency is chosen to prevent objectionable audible noise.  
         [0010]    [0010]FIG. 3 depicts a flow diagram of a software routine executed by motor control unit  50  for carrying out the above-described control. The reference numeral  70  designates a series of initialization instructions for setting various parameters and variables to predefined values, and for initializing the PULSE_COUNT output of pulse count circuit  56  by driving the motor  20  to a limit position of device  10 , for example, and then setting PULSE_COUNT to a corresponding value. The variables initialized at block  70  include an actual motor position term POS_ACT, which is typically initialized at the initial PULSE_COUNT value. Following initialization, the block  72  is executed to determine if POS_DES is equal to POS_ACT. If so, blocks  74  are  76  are executed to turn off motor  20 , and to set POS_ACT based on the PULSE_COUNT output of pulse count circuit  48 , whereafter block  72  is re executed as shown. If block  72  is answered in the negative, the block  78  determines if the motor  20  needs to be reversed relative to the previous direction of motor rotation. If not, significant axial shifting of the armature shaft  18  is not expected; in this case, the blocks  80  and  82  are executed to turn the motor  20  fully on and to set POS_ACT based on the PULSE_COUNT output of pulse count circuit  48 , whereafter block  72  is re-executed as shown. If block  78  is answered in the affirmative, axial shifting of the armature shaft  18  is expected to occur, and the blocks  84 - 92  are executed to effect a low duty-cycle PWM energization of the motor windings, which allows the armature shaft  18  to slowly shift axially, moving the front or rear washer  32 ,  34  into engagement with the respective bushing  28 ,  30 , as the worm gear  16  begins to exert torque on output gear  12 . The block  84  sets a PWM cycle counter to zero, the block  86  turns the motor on for a predetermined interval (TIME_ON), the block  88  turns the motor off for predetermined interval (TIME_OFF), and the block  90  increments the cycle counter. Block  92  compares the cycle counter to a threshold count THR_COUNT, and directs the re-execution of blocks  86 - 90  until the cycle counter has been incremented to THR_COUNT. At such point, the block  94  turns the motor  20  fully on, and the block  96  updates the variable POS_ACT by subtracting the predetermined number of PWM pulses (THR_COUNT) from the PULSE_COUNT output of pulse count circuit  48 , whereafter block  72  is re-executed as shown.  
         [0011]    In summary, the control of the present invention selectively utilizes PWM of the motor energization to eliminate audible noise due to axial shifting of the armature shaft  18  while retaining the cost and performance advantages associated with high motor efficiency and pulse count motor position feedback. While the present invention has been described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. For example, the PWM frequency developed by motor control unit  50  may be higher than the highest expected commutation pulse frequency, so that the pulse count circuit  48  or an auxiliary circuit can filter out or otherwise ignore the corresponding motor current pulses. Thus, it will be understood that control methods incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.