Abstract:
A method ( 100 ) is provided for controlling torque of an electric motor ( 12 ) including the step ( 106 ) of supplying drive current to drive the electric motor. The speed of the electric motor is monitored ( 108 ) and from the monitored speed, it is determined ( 110 ) when the motor has reached a steady state condition. The supply current is turned off ( 118 ) if the monitored torque of the electric motor exceeds a predetermined value ( 114 ) after the electric motor has reached the steady state condition.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to electric motors and is more particularly directed to a method and apparatus for limiting torque in an electric drive motor. 
       BACKGROUND OF THE INVENTION 
       [0002]    Electric motors are often used to drive a work piece. The connection between the output of the motor and the driven work piece may be through drive linkage. The output force of the electric drive motor can be significant. If the work piece should encounter excessive resistance to movement or have its movement stopped by an obstacle, the output force of the drive motor may result in damage to the connection linkage. 
       SUMMARY OF THE INVENTION 
       [0003]    This invention monitors for an excess amount of output torque from an electric drive motor only after the output speed of the electric motor has reached a steady state drive condition, i.e., a steady state drive velocity. 
         [0004]    In accordance with one example embodiment of the present invention, a method is provided for controlling torque of an electric motor including the step of supplying drive current to drive the electric motor. The speed of the electric motor is monitored and from the monitored speed, it is determined when the motor has reached a steady state condition. The supply current is turned off if the monitored torque of the electric motor exceeds a predetermined value after the electric motor has reached the steady state condition. 
         [0005]    In accordance with another example embodiment of the present invention, an apparatus is provided for controlling torque of an electric motor comprising a drive circuit connect to the motor for supplying drive current to drive the electric motor. A speed sensor is connected to the motor for sensing speed of the electric motor and a torque sensor is connected to the motor for sensing output torque of the electric motor. A motor controller is connected to the motor, to the drive circuit, and to the speed sensor for (1) determining from the sensed speed of the electric motor when the motor has reached a steady state condition, and (2) turning off the drive circuit if the monitored torque of the electric motor exceeds a predetermined value after the electric motor has reached the steady state condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing and other features and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which: 
           [0007]      FIG. 1  is a schematic block diagram of a motor drive circuit having a torque limiting motor drive feature in accordance with one example embodiment of the present invention; and 
           [0008]      FIG. 2  is a flow diagram of a torque limiting motor control process in accordance with one example embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Referring to  FIG. 1 , a motor drive circuit  10 , made in accordance with one example embodiment of the present invention, is connected to an electric motor  12 . The electric motor is operatively connected to drive a work piece  14  through appropriate connection linkage  16 . The linkage  16  is connected to the output shaft (not shown) of the electric motor  12  and to the work piece  14  so that when the output shaft of the electric motor  12  is rotated in one direction, the work piece  14  rotates about a pivot point  18  in a first direction and when the output shaft of the electric motor  12  is rotated in a second direction, the work piece  14  rotates about the pivot point  18  in a second direction. 
         [0010]    The motor drive circuit  10  includes an electronic control unit (“ECU”)  20  connected to the electric motor  12  through drive circuits  22 ,  24 . An input command interface  30  is connected to the ECU  20  and provides command signals to the ECU to indicate the desired position of the work piece  14 . The ECU can be formed using discrete circuitry, an application specific integrated circuit (“ASIC”), or could be a microcomputer. In accordance with one example embodiment of the present invention, the work piece  14  could be a movable damper in a vehicle heating, ventilating, and air conditioning (“HVAC”) system. Such systems are also known as climate control systems. The movable damper is used to control air flow in air duct (not shown) of the system. If something were to become lodged in the duct and prevent normal movement of the damper  14 , the driving force of the electric motor  12  could result in damage to the linkage  16 . 
         [0011]    A motor speed sensor  40  is operative connected to the output of the motor  12  for sensing the output speed of the output shaft of the motor and for providing an electrical output signal having a characteristic indicative of the sensed motor speed. The output of the motor speed sensor  40  is connected to an input of the ECU  20  so that the ECU can monitor the motor speed. 
         [0012]    Motor speed can be sensed in any of several ways. In accordance with one example embodiment of the present invention, motor speed can be sensed by using an encoder connected to the motor&#39;s output shaft that provides a predetermined number of electrical pulses per shaft revolution. By counting the pulses per unit time, motor speed can be determined. In accordance with another example embodiment, a disk can be attached to the end of the motor shaft with a plurality of holes in it with a light source on one side and an optical detector aligned with the light source on the other side of the disk. As the disk turns, pulses will occur in the detector. Pulses per unit time sensed by the optical detector will be an indication of motor speed. Again, those skilled in the art will appreciate other ways of sensing motor speed, any of which may be used with the present invention. 
         [0013]    A motor torque sensor  44  is operative connected to the motor  12  for sensing the output torque of the motor and for providing an electrical output signal having a characteristic indicative of the sensed motor torque. The output of the motor torque sensor  44  is connected to an input of the ECU  20  so that the ECU can monitor the motor torque. 
         [0014]    Motor torque can be sensed in any of several ways. In accordance with one example embodiment of the present invention, motor torque can be sensed by monitoring the motor current. As monitored motor current increases, the torque increases. In accordance with another example embodiment, increased torque can be determined by monitoring a deceleration of the motor when no motor deceleration has been commanded by the ECU  20 . If the motor is decelerating, the motor torque is increasing since an increase load on the work piece  14  must be causing the motor deceleration. Again, those skilled in the art will appreciate other ways of sensing motor torque. For example, a mechanical type torque sensor per se could be coupled between the output shaft of the motor and the work piece  14  of a type that mechanically responds to the increased torque and provides an electrical signal indicative thereof. 
         [0015]    Referring to  FIG. 2 , a control process  100  to limit torque in an electric motor in accordance with one example embodiment of the present invention will be appreciated. The process begins at step  102  where memories are cleared and reset, flags are set to initial conditions, etc. At step  104 , a determination is made as to whether to motor  12  is to be energized, i.e., has a command been received from the input command  30 ? If the determination in step  104  is negative, the process loops back upon itself and waits for a command to be received. Once the determination in step  104  is affirmative, the process proceeds to step  106  where motor current is applied to the motor  12  through the drive circuits  22 ,  24 . 
         [0016]    When motor drive current is first applied to the motor  12 , there is an in-rush of current. One way to detect excessive motor torque being applied by the motor  12  is to monitor for motor current greater than a predetermined value. To prevent a false determination of an excessive torque over-load condition during motor start-up as a result of a normal in-rush current event, in accordance with an example embodiment of the present invention, is to wait until the motor has reached a steady state operating speed before the motor current is monitored. In step  108 , motor speed is monitored. In step  110 , a determination is made as to whether motor  12  has reached a steady state speed, i.e., has the motor  12  stopped accelerating? If the determination in step  110  is negative, the process loops back to step  108  and continues in the  108 ,  110  loop until the motor stops accelerating. Once the determination in step  110  is affirmative, the process proceeds to step  112  where motor torque is monitored. As mentioned, the motor torque can be monitored by monitoring the motor current or the motor torque could be monitored in other ways such as by monitoring the deceleration of the motor output or with use of a torque sensor per se. 
         [0017]    From step  112 , the process proceeds to step  114  where a determination is made as to whether the output torque of the motor  12  has exceeded a predetermined threshold? If torque is being measured by motor current, then torque being greater that a threshold is determined by the current being greater than a threshold. If motor deceleration is being monitored to detect a torque overload, then deceleration greater than a predetermined value when no deceleration has been commanded by the ECU is indicative of a torque overload. If the determination in step  114  is negative, the process proceeds to step  116  where a determination is made as to whether the motor has driven the work piece  14 , i.e., the position of the output shaft of the motor, to its desired position. If the determination is negative, the process loops back to step  112  and the motor is continued to be driven. If the determination is affirmative, the process goes to box  118  where the motor is turned off and the process loops back to step  104 . If the determination in step  114  is affirmative, then the drive current is turned off in step  120  and the process loops back to step  104 . 
         [0018]    From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.