Bidirectional motor stall protection circuit

[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 may tend to degrade with time. Circuit breakers are thermal devices that 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 through internal heating interrupts the supply of current to the motor. The circuit breaker cannot be reset until its temperature drops and resets the circuit breaker which may be perceived as a malfunction by the seat occupant.

[0006] Applicants' 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' 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' invention is summarized below.

[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 is 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.

[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 10A, a stall current detection circuit portion 10B and latching circuit portion 10C.

[0018] The motor drive circuit portion 10A 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 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 10B 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 10A. 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 10B determines that the motor 14 is stalled. The NOR gate provides a signal to the latching circuit portion 10C to engage the latch. A capacitor 48 is provided for the output of the NOR gate 46 for noise suppression.

[0021] Latching circuit portion 10C 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 70A, a stall current detection circuit portion 70B and latching circuit portion 70C.

[0023] The motor drive portion 70A 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 70B, voltages V_A and V_B are taken from opposites sides of the motor 74. Input V_C 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_A 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 70B. The outputs of the 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 in a similar manner. 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 70C. The output of the NOR gate 116 may be provided with a capacitor 120 for noise suppression.

[0026] Latching circuit portion 70C 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.