Abstract:
An input circuit for a tractor includes one or more switches having open and closed positions, based on operating characteristics of the tractor, and a power transistor drawing a threshold current through the switches during short intervals during which the switch states are sensed by a microcontroller.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to electrical circuits on tractors, grass mowing machines, and utility vehicles, and more specifically to safety circuits that are used to stop or interrupt power for specified vehicle functions under certain conditions, and controller board inputs that read switches used in those circuits. 
   BACKGROUND OF THE INVENTION 
   Tractors, grass mowing machines, and utility vehicles may have electrical circuits that stop or interrupt power if an input switch is opened and certain conditions are met. These circuits may be used for safety purposes. For example, one such circuit may stop rotation of cutting blades in the absence of a seated operator. 
   These circuits typically use low cost, unsealed switches. As a result, water may enter the switch body during vehicle operation. Water entering the switch body may contain fertilizer, organic material, or other dirty contaminants. Empirical tests of switches having water in the switch bodies shows they typically exhibit less than 1000 ohms of resistance, depending on the impurities in the water. 
   In the past, these circuits have powered relays drawing about 100 mA. The relays have a pull in voltage of about 6 volts, and a drop out voltage of about 1 volt. This provides the relay good hysteresis for input state monitoring. For example, it takes 6 volts at 100 mA to turn on a seat interlock relay. Water in the seat input switch may not provide a false indication (i.e., the switch will not appear closed) unless the resistance drops below about 60 ohms. 
   More recently, microcontrollers are used instead of relays in these circuits. Several problems may be encountered, as described below. 
   First, water in the input switch may appear to a microcontroller as a valid switch closure input. This is because a microcontroller in such a circuit may have high impedance inputs, typically about 10K ohms or higher. A 150 ohm resistor placed across each input switch (in order to draw approximately 100 mA at 14.5 volts) must have a continuous rating of 1.5 Watts. However, it is preferable to use resistors rated at about 0.125 Watts. 
   Second, multiple inputs may continuously dissipate heat into the electronics enclosure, which can be detrimental. For example, mowing machines with multiple inputs that may dissipate high wattage (i.e., 20 Watts or more) inside the control enclosure. Due to the heat, component life may be shortened significantly. 
   Third, if battery voltage is low, a circuit with a microcontroller may sense the input switch as open when the switch is closed. For example, if battery voltage drops to about 5 volts during cold cranking of an engine, a relay may stay energized due to its 1 volt drop out. However, a microcontroller with an input that is voltage based to a 6 volt threshold may stop cranking the engine because it incorrectly senses the input switch is open. Setting the voltage threshold lower can alleviate this problem, but makes the input more sensitive to water in the switch. 
   Attempts have been made to address the problems described above. For example, an analog to digital converter on each input may be used to evaluate the voltage being returned from the input switch. This is not a preferred solution because it requires costly analog circuits and wiring. Alternatively, some tractors address the problem through use of a positive temperature coefficient thermistor on each input switch to draw higher currents when the switch is first closed, then reduce the current. This also is not an optimal solution, because positive temperature coefficient thermistors are expensive and do not address the problem of an input switch that is properly closed, then opened with water inside it to falsely maintain the circuit. Additionally, the ptc current may be used more as a “cleaning pulse” to remove oxides or corrosion from switch contacts going into high impedance inputs on the controller. 
   An improved input circuit is needed for tractors, mowing equipment and utility vehicles that will reject water in an open input switch. A low cost input circuit is needed that will improve the operation of circuits used to ensure safe vehicle operation. 
   SUMMARY OF THE INVENTION 
   The present invention provides an improved circuit for tractors, mowing equipment and utility vehicles that will reject water in an open input switch. The input circuit is low cost and helps ensure safe vehicle operation. 
   The circuit includes one or more switches sensing various operating conditions of the tractor or vehicle, and a microcontroller sensing the state of the switch or switches and performing one or more functions based at least in part on the switch state. A power transistor may be connected through a diode and resistor to the switch or switches, the power transistor normally being in an off condition, and being powered during short intervals to an on condition to increase current to a threshold through the switch. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of an interlock circuit according to a first embodiment of the invention, that rejects water as an input. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In one embodiment of the invention shown in  FIG. 1 , circuit  100  may be included in a tractor, grass mowing machine or off-road utility vehicle. The circuit includes input switches  101 ,  102 ,  103 . The state of each switch may depend on one or more operating conditions of the vehicle. Each switch may have two positions; i.e., an open position and a closed position. For example, an input switch may be an operator presence switch that is closed only if an operator is seated, a parking brake switch that is closed if the parking brake is locked, or another switch sensing one or more other vehicle conditions relating to safety or performance. The input switch may be a low-cost, open body switch that may be subject to adverse operating conditions. 
   In one embodiment, each switch  101 ,  102 ,  103  may draw a small amount of nominal current most of the time, and a substantially higher threshold current for short intervals. For example, each switch may draw about 10 to 20 milliamps most of the time, while the threshold current may be about 100 to 200 milliamps for short intervals. The short intervals may be about one millisecond every fifty milliseconds, and may be controlled by microprocessor  105 . 
   In one embodiment, the higher current draw may be accomplished by turning on the power transistor  104  for short intervals. The power transistor may be normally off and may be connected to switches  101 ,  102 ,  103  through diodes  106 ,  107 ,  108  and 47 ohm resistors  109 ,  110 ,  111 . The input switches may be read for the short interval of less than about one millisecond, and then power transistor  104  may be turned off. 
   In one embodiment, when power transistor  104  is on, the state of each input switch may be transferred through resistors  119 ,  120 ,  121  to an input mask in microcontroller  105 . The microcontroller also may receive input regarding one or more other operating conditions of the vehicle. The microcontroller may be programmed to provide a signal or signals to activate or deactivate one or more functions of the vehicle, such as stopping rotation of cutting blades or turning off a power take off, if the switch state along with other information satisfies certain specified conditions. 
   In one embodiment, the normally off power transistor may be a bipolar junction transistor (BJT) or field effect transistor (FET) connected through a resistor and diode to each input switch. Alternatively, several power transistors may be used, with one power transistor connected to each input switch. 
   In one embodiment, each input switch  101 ,  102 ,  103  may be connected to a visible indicator such as a light emitting diode (LED)  112 ,  113 ,  114  and resistor  115 ,  116 ,  117  to indicate if the switch is in the open or closed state. When power transistor  104  is off, only LED current (about 10 to 20 milliamps) may flow through each of the switches. When the power transistor is on, significantly higher current (about 100 to 200 milliamps) may flow through each of the switches. 
   Circuit  100  is low cost and has low power dissipation. The average power draw of circuit  100  may be quite low. The circuit includes a microprocessor or microcontroller that controls the timing of the current draw through each switch in a duty cycle. The circuit inputs reject water as an input in an open body switch.