Abstract:
A stall protection circuit is provided for bidirectional motor operation that is controlled by a half bridge switch that has forward, reverse and off positions. A method of protecting a motor is also disclosed. A Hall effect current sensor may be provided in a stall current protection circuit that engages a latching circuit. The latching circuit is operative to interrupt current flowing to the motor by disabling MOSFETs on opposite sides of the motor. Alternatively, FET voltage from drain to source may be used to measure the current supplied to the motor that is used to engage the latching circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to electrical circuits for protecting an electric motor from damage caused by stalling the motor.  
         [0003]     2. Background Art  
         [0004]     Small, low current electric motors are used in a myriad of applications in which a reversible drive is required. Examples of some applications include vehicle seat positioning systems, seat recliners, window regulator drives, mirror positioning systems, and the like. For example, a reversible motor may be used in a vehicle seat to control the fore-and-aft positioning of the vehicle seat in a track. The motor is used to adjust the seat forwardly or rearwardly to accommodate a seat occupant. Seats move within tracks between a maximum forward position and a maximum rearward position that is determined by stops defining the length of the track. When the motor drives the seat to either maximum position, the motor may stall if the switch controlling the motor is continued to be actuated after the seat reaches the maximum travel position. Stalling the electric motor may cause damage to the motor.  
         [0005]     Prior art solutions to the problem of damage caused to motors by stalling include providing positive temperature coefficient (PTC) devices or circuit breakers in the motor drive control circuit to protect the electrical motor. PTC devices tend to degrade with time. Circuit breakers are subject to hysteresis effects that may cause intermittent seat operation. For example, if the switch is held until the motor stalls after the seat has been moved to maximum travel position, the circuit breaker interrupts the supply of current to the motor. The circuit breaker cannot be reset until the current drops which may be perceived as a malfunction by the seat occupant.  
         [0006]     Applicants&#39; invention is directed to providing a simple and effective integrated motor control that protects the motor from damage in the event the motor stalls. Applicants&#39; invention provides a system that has an adjustable threshold current level and also provides a robust system that may be repeatedly activated without degradation or adverse hysteresis effects. Applicants&#39; invention is summarized below.  
       SUMMARY OF THE INVENTION  
       [0007]     According to one aspect of the present invention, a stall protection circuit is provided for a motor. The circuit comprises a manual switch having an off position, a forward position, and a reverse position that is used to control an electric motor. A current sensor is provided that senses the flow of current through the motor and produces a first signal representative of the level of current flowing through the motor. A stall current detection circuit receives the first signal and compares the first signal to a threshold value. If the first signal is above the threshold value, a second signal is generated. A latching circuit receives the second signal and shuts off the motor.  
         [0008]     According to another aspect of the present invention, a stall protection circuit is provided that includes a manually actuated switch that controls an electric motor. A switch controller senses the level of current flowing through the motor and provides a current signal representative of the sensed level of current. A stall current detection circuit receives the current signal and compares the current signal to a threshold value. A fault signal is provided if the current value exceeds the threshold value. A latch circuit receives the fault signal and opens at least one motor protection switch that is operatively connected to the motor.  
         [0009]     Other aspects of the invention relate to specific implementations of the above stall protection circuits. One such aspect is that the manual switch is a half bridge circuit. Another aspect of the stall protection circuit is that a Hall effect current sensor may be used to provide a voltage output that proportional to the current supplied to the motor. The latching circuit may be used to shut off the motor until the manual switch is in the off position. The stall protection circuit may comprise a pair of field effect transistors, each of which are operatively connected to one of two opposites sides of the motor. The field effect transistor drain to source voltage on each side of the motor are compared to detect stalling of the motor. The latch circuit may be powered by a power supply that is interrupted when current flow to the motor is interrupted. The latch circuit may be connected to a pair of field effect transistors on opposite sides of the motor.  
         [0010]     According to another aspect of the present invention, a method of protecting an electric motor against damage caused by stalling is provided. The method includes sensing the level of current supplied to the motor. The sensed level of current provided to the motor is then compared to a threshold value. An interrupt signal is sent to a latch circuit if the sensed level of current exceeds the threshold value. The flow of current to the motor is interrupted when the interrupt signal is received from the latch circuit to protect the motor from damage.  
         [0011]     According to other aspects of the invention as they relate to the method, the method may further comprise providing a manual switch actuator that is held in to cause the motor to turn in either a forward or a reverse direction. The motor may be caused to stall if the switch actuator is held in until the motor exceeds a maximum limit. The latch circuit remains latched until the switch is released. The latch circuit is powered by a power supply that is interrupted when current flow to the motor is interrupted.  
         [0012]     According to other aspects of the method, the sensing step may be performed by a Hall effect sensor that produces a voltage signal that is proportionate to the level of current supplied to the motor.  
         [0013]     Alternatively, the sensing step may be performed by field effect transistors each of which are operatively connected to one of two opposite sides of the motor. The field effect transistor drain to source voltage on each side of the motor are compared in the sensing step. A half bridge may be provided in the motor power circuit.  
         [0014]     Other aspects of the invention will be better understood in view of the attached drawings and following detailed description of the preferred embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a circuit diagram of a stall current detection circuit based on a Hall effect current sensor; and  
         [0016]      FIG. 2  is a circuit diagram of a stall current detection circuit based on FET voltage from drain to source for measuring current provided to the motor. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     Referring to  FIG. 1 , one embodiment of a stall current protection circuit is generally referred to by reference numeral  10 . The stall current protection circuit  10  includes a motor drive circuit portion  10 A, a stall current detection circuit portion  10 B and latching circuit portion  10 C.  
         [0018]     The motor drive circuit portion  10 A includes a half bridge switch  12  that has forward, reverse, and off positions. The half bridge switch  12  is a bipolar power-driver output. A motor  14  is controlled by half bridge switch  12  that switches the motor  14  for operation in a forward mode, reverse mode, and turns the motor off. A Hall effect current sensor  16  is a low current Hall effect current sensor that may sense current on PCB traces, and the like. The current sensor  16  senses the level of current provided to the motor  14  and produces a voltage output that is proportional to the sensed current.  
         [0019]     A pair of forward N channel MOSFETs  18  and  20  are provided on opposite sides of the motor  14 . The FETs  18  and  20  function as switches that are connected to each of the brushes of the motor  14 . Reverse diodes  24  and  26  are provided for FETs  18  and  20 . The reverse diodes conduct current, one at a time, with current flowing through the diode on the high side of the switch, through the motor, and through the low side FET. Depending on the polarity of the current supplied to the motor, the current flow operates the motor in either its forward or reverse mode. Zener diodes  30  are connected across the motor  14  to protect the motor from excess voltage on either side of the motor that may appear when the circuit is interrupted by one of the FETs. Voltage spikes are believed to be caused by the tendency of the motor to continue rotation after current flow to the motor is interrupted.  
         [0020]     The stall current detection circuit portion  10 B receives voltage from the current sensor  16  that is applied to comparators  36  and  38 . The voltage is provided to the negative pole of comparator  36  and the positive pole of the comparator  38 . By this arrangement, one of the comparators is operative regardless of the direction of current flow in the motor drive portion of circuit  10 A. The voltage in comparator  36  is compared to the output of voltage divider  40  that is used to set the threshold value. Similarly, comparator  38  is connected to voltage divider  42  that sets the threshold value. If the threshold values are exceeded in either of the comparators  36  and  38 , an output is provided to NOR gate  46 . Nor gate  46  provides a logic output based upon comparison of the output of the comparators  36  and  38 . If either of the outputs provided to the NOR gate  46  are high, it is determined that the current through the motor  14  has exceeded the threshold value and circuit  10 B determines that the motor  14  is stalled. The NOR gate provides a signal to the latching circuit portion  10 C to engage the latch. A capacitor  48  is provided for the output of the NOR gate  46  for noise suppression.  
         [0021]     Latching circuit portion  10 C is built around a 555 timer IC  50 . A latching circuit is required because once the detection circuit determines that the motor  14  is stalled, if the motor is turned off there is no current supplied to the motor and the detection circuit will no longer prevent motor operation. The latch circuit locks out the motor by holding the FETs off until the switch is released. IC  50  is connected to a capacitor  52  that is used to initialize the MOSFETs  18  and  20  to their on state. This also allows the reset pin of IC  50  to be charged. A PNP transistor  56  bridges the capacitor  52 . Transistor  56  is a bipolar junction transistor (BJT) that discharges the capacitor and resets the IC  50 . Upon receiving the output from the NOR gate  46 , the IC  50  produces an output to an inverter  64 . The output of the IC  50  is a high output that is inverted by the inverter to a zero that is provided to FETs  18  and  20 . This functions to turn off the MOSFETs  18  and  20  and interrupts the supply of current to the motor  14 .  
         [0022]     Referring to  FIG. 2 , another embodiment of a stall current protection circuit is generally referred to by reference numeral  70 . The stall current protection circuit  70  includes a motor drive circuit portion  70 A, a stall current detection circuit portion  70 B and latching circuit portion  70 C.  
         [0023]     The motor drive portion  70 A includes a half bridge switch  72  that has a forward, reverse, and off positions. A motor  74  is controlled by half bridge switch  72  that switches the motor  74 .  
         [0024]     Two MOSFETs  78  and  80  are provided on opposite sides of the motor  74 . The FETs  78  and  80  function as switches that are connected to each of the brushes of the motor  74 . Reverse diodes  84  and  86  are provided for FETs  78  and  80 . The reverse diodes conduct current through the high side of the diode  84  or  86 , through the motor  74 , and through the low side FETs  78  or  80 . Two Zener diodes  88  are connected across the motor  74  to protect the motor from excessive voltage on either side of the motor  74  that may appear when the circuit is interrupted by one of the FETs.  
         [0025]     In stall current detection circuit portion  70 B, voltages V A  and V B  are taken from opposites sides of the motor  74 . Input V A  is supplied to operational amplifier  90  while input V B  is supplied to operational amplifier  92 . Voltage from the switch  72  is provided to operational amplifier  94  at V C  and to operational amplifier  96  at V D . Operational amplifiers  90 ,  92 ,  94  and  96  act as unity gain buffers for the voltages received from the switch  72  and from opposite sides of the motor at V A  and V B . The operational amplifiers function to isolate the motor  74  from the sensing circuit  70 B. The output of operational amplifiers  90  and  94  are provided to operational amplifier  100  that compares voltage V A  to the switch voltage V C . Operational amplifier  100  subtracts the voltages and determines the voltage difference across FET  84 . Operational amplifier  102  determines the voltage difference across FET  86 . The voltage difference output of operational amplifier  100  is provided to comparator  106 . The voltage difference output of operational amplifier  102  is provided to comparator  108 . Comparators  106  and  108  compare the voltage differences to voltage divider networks  110  and  112 , respectively, that set the threshold for detecting stall current levels. If either of the outputs of comparators  106  and  108  is high, the outputs provided to the NOR gate  116  cause the NOR gate to signal the latching circuit portion  70 C. The output of the NOR gate  116  may be provided with a capacitor  120  for noise suppression.  
         [0026]     Latching circuit portion  70 C includes IC  124  connected to a capacitor  126  that is used to initialize the MOSFETs  78  and  80  to their on state. This also allows the reset pin of IC  124  to be charged. A PNP transistor  128  bridges the capacitor  126 . Transistor  128  discharges the capacitor and resets the IC  124 . Upon receiving the output from the NOR gate  116 , the IC  124  sends an output to an inverter  132 . The output of the IC  124  is high, but it is inverted by the inverter  132  to a zero that is provided to FETs  78  and  80 . This functions to turn off the MOSFETs  78  and  80  and interrupts the supply of current to the motor  74 .  
         [0027]     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.