Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-02-11
2001-05-29
Lieu, Julie (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S620000, C340S450000, C340S450300, C073S29000R, C073S291000, C073S30400R
Reexamination Certificate
active
06239709
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus which senses the level of a liquid in an internal combustion engine, and more particularly to low oil sensors for such engines.
Many types of liquid level sensors are known for internal combustion engines. One type of sensor is an electromechanical float switch. In a typical float switch, the float rides on the surface of the lubricating fluid, and is electrically connected to the engine ignition primary winding. A second contact is stationary, and is electrically connected to ground. When the lubricating fluid level falls below a pre-determined value, the two electrical contacts engage each other, and the ignition primary winding is grounded to shut off the engine.
Such float switches contain several mechanical and electrical components, and are relatively expensive to manufacture, assemble and maintain. Moreover, they tend to be inaccurate in that there is a great deal of turbulence in the engine fluid, causing the float to rise or fall substantially even though the level of the lubricating fluid is adequate.
SUMMARY OF THE INVENTION
A sensor is disclosed that detects a low liquid or a no liquid level condition in an internal combustion engine. The sensor includes a sensing circuit having first and second inductors, with at least one of the inductors partially disposed in air above the liquid level during a low liquid level condition. A first circuit applies a first voltage to the first and second inductors to heat the inductors. A second circuit applies a second voltage, greater than the first voltage, to the first and second inductors. The second voltage is preferably a periodic voltage signal generated by a power winding. The sensor includes a third circuit that senses a voltage difference between a first point and a second point in the sensor, and that provides a control signal when the voltage difference reaches a predetermined value. This control signal may be used to shut off the engine, or to provide a visual or audible indicator that a low liquid level condition exists. Moreover, the sensor can include a voltage-regulating circuit connected to the first and second circuits. The voltage-regulating circuit adjusts the voltage being applied to the sensing circuit such that the applied voltage has a constant root-mean-square value.
The first and second inductors have different resistances so that they heat differently when one of them is at least partially disposed in air. When a low liquid condition exists, the second inductor is at least partially disposed in air. When a no liquid level condition exists, both of the inductors are at least partially disposed in air. Since the inductors have different resistances and heat at different rates, having at least one of the inductors at least partially disposed in air creates an imbalance in the sensing circuit. This imbalance is used to generate an output signal indicative of a low or no liquid level condition.
In each of the embodiments, the power winding is used to provide the necessary power to generate the first voltage and the second voltage. In this way, the inductors may be constantly heated. The first voltage may be generated without the use of a battery, although a battery could be used.
In one embodiment, the second inductor, having a higher resistance and typically a higher inductance, is disposed at a physically higher level in the engine than the first inductor. In alternate embodiments, the positions of the two inductors are reversed or indeed the inductors may be adjacent to each other, depending upon the engine configuration. For example, if the inductors are disposed in a confined space such as a tube with an open bottom, it may be beneficial to place the inductor having the higher resistance near the bottom of the tube so that the liquid will flow over the inductor. The placement of the respective inductors is not critical since the sensor is designed to generate an output signal when at least one of the inductors is disposed in air.
Inductors are preferred over thermistors or other resistors in the present invention because the resistance of a copper wire used with typical inductors changes only about 2 to 1 over the typical engine operating range of −50° to 350° Fahrenheit. The resistance of thermistors changes on the order of 10,000 to 1 for this engine operating range, thereby necessitating expensive, complex circuitry for the sensor. Of course, copper wound resistors could also be used in the present invention in place of the inductors.
It is a feature and advantage of the present invention to use the inherent resistance of inductors in a low liquid level or low oil sensor.
It is another feature and advantage of the present invention to heat the inductors with a low voltage, thereby allowing inductors to be used without the need of a battery.
It is yet another feature and advantage of the present invention to at least periodically apply a higher voltage to the inductors, thereby generating a control signal in the sensing bridge in the event that at least one of the inductors is no longer disposed in liquid.
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Dykstra Richard
Mitchell Robert K.
Briggs & Stratton Corporation
Lieu Julie
Michael & Best & Friedrich LLP
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