Liquid level sensor for internal combustion engine

• US 6,239,709 B1
• Filed: 02/11/2000
• Issued: 05/29/2001
• Est. Priority Date: 02/18/1999
• Status: Expired due to Fees

• Current Assignee: Briggs & Stratton Corporation
• Original Assignee: Briggs & Stratton Corporation
• Inventors: Dykstra, Richard, Mitchell, Robert K.
• Primary Examiner(s): Lieu, Julie
• Application Number:
  US09/502,631
• Time in Patent Office: 473 Days
• Field of Search: 340/618, 340/450, 340/450.3, 340/620, 73/290 R, 73/291, 73/304 R
• US Class Current: 340/618
• CPC Class Codes: G01F 23/247 for discrete levels

Abstract

The inexpensive sensor detects a low oil or no oil condition in an internal combustion engine and shuts down the engine. The sensor includes two inductors having different resistances disposed in a sensing bridge. The inductors are heated by a steady low voltage. A periodic higher voltage is applied to the inductors by means of a power winding disposed on the same frame as the ignition coil. When at least one of the inductors is disposed in air and not in oil, an imbalance is created in the sensing circuit that is used to short out the ignition primary. Alternatively, the sensor can be used to provide a visual or audible indication of a low oil level without stopping the engine.

Claims

1. A sensor that detects a low liquid level condition in an engine, comprising:

a sensing circuit, including

a first inductor that is at least partially disposed in air during a low liquid level conditions;

a second inductor connected in series with said first inductor at a first point;

a first resistor;

a second resistor connected in series with said first resistor at a second point;

a first circuit that applies a first voltage to said first and second inductors;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors; and

a third circuit that senses a voltage difference between said first point and said second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value.

2. The sensor of claim 1, wherein said sensing circuit further includes a third resistor that reduces electrical noise.

3. The sensor of claim 1, wherein the engine includes an ignition coil primary winding, said sensor further comprising a fourth circuit that short circuits the ignition coil primary winding to shut off the engine in response to said signal.

4. The sensor of claim 1, further comprising:

a visual indicator that a low liquid level exists.

5. The sensor of claim 1, further comprising:

an audible indicator that a low liquid level exists.

6. The sensor of claim 1, wherein said first circuit includes a voltage source, a third resistor, and a capacitor, wherein said voltage source is connected in circuit to charge said capacitor with a charging voltage, and wherein said capacitor is connected in circuit to discharge through said third resistor to provide said first voltage.

7. The sensor of claim 6, wherein said first circuit further includes a diode and a zener diode, wherein said diode is connected in circuit to rectify the charging voltage supplied by said voltage source, and wherein said zener diode is connected in circuit to limit the charging voltage.

8. The sensor of claim 7, wherein the engine includes a flywheel magnet, and wherein said voltage source includes a power winding that generates a winding voltage when the flywheel magnet passes said power winding.

9. The sensor of claim 1, wherein the engine includes a flywheel magnet, and wherein said second circuit also includes a capacitor and a power winding, said power winding generating a charging voltage when the flywheel magnet passes said power winding to thereby charge said capacitor.

10. The sensor of claim 1, wherein said first inductor and said second inductor have different respective resistances.

11. The sensor of claim 10, wherein said respective resistances differ by at least 150 percent.

12. The sensor of claim 10, wherein said second inductor has a higher resistance than said first inductor.

13. The sensor of claim 1, wherein said first inductor and said second inductor have different respective inductances.

14. The sensor of claim 13, wherein the respective inductances differ by at least 150 percent.

15. The sensor of claim 13, wherein said second inductor has a larger inductance than said first inductor.

16. The sensor of claim 1, wherein said third circuit includes a programmable unijunction transistor, and wherein said signal switches ON said programmable unijunction transistor when said predetermined value is reached.

17. The sensor of claim 1, wherein said first voltage is between 5 and 6 volts.

18. The sensor of claim 1, wherein said second voltage is at least 30 volts greater than said first voltage.

19. The sensor of claim 1, wherein said second inductor is disposed physically at a higher level in said engine than said first inductor.

20. The sensor of claim 1, wherein said first and second inductors are disposed substantially adjacent to each other in said engine.

21. The sensor of claim 1, wherein said first circuit includes a voltage source, a capacitor and a diode, wherein said voltage source is connected in circuit to charge said capacitor with a charging voltage, and wherein said capacitor is connected in circuit to discharge through said diode to provide said first voltage.

22. The sensor of claim 1, wherein said third circuit includes:

a third resistor electrically connected to said second resistor at a first node;

a transistor having a base, an emitter and a collector, said base is electrically connected to said third resistor at said first node and said emitter is electrically connected to said second inductor at a second node; and

a capacitor electrically connected to said transistor at said second node.

23. The sensor of claim 1, wherein said third circuit includes a transistor and a programmable injunction transistor (PUT) connected in circuit, wherein said signal switches ON said PUT, and wherein turning ON said PUT turns OFF said transistor.

24. The sensor of claim 1, wherein said first and second voltages being applied to said inductors results in a composite voltage, wherein said composite voltage has a root-mean-square (RMS) value, and said sensor further comprises:

a voltage regulating circuit that adjusts said composite voltage such that the RMS value of said composite voltage remains substantially constant.

25. The sensor of claim 24, wherein said voltage regulating circuit adjusts said first voltage such that said RMS value of said composite voltage remains substantially constant.

26. The sensor of claim 25, wherein said voltage regulating circuit decreases said first voltage.

27. The sensor of claim 1, wherein said second circuit includes a voltage source, a diode and a third resistor connected in circuit to provide said second voltage.

28. The sensor of claim 27, wherein said second voltage has a peak amplitude, and wherein said second circuit includes a voltage limiting device connected in circuit to limit said peak amplitude of said second voltage.

29. The sensor of claim 27, wherein a third voltage develops across said third resistor, wherein said first and second voltages being applied to said inductors results in a composite voltage, and wherein said sensor further comprises:

a voltage regulating circuit that adjusts said composite voltage to said inductors, said voltage regulating circuit includes a capacitor, a diode, a voltage limiting device, and a silicon-controlled rectifier (SCR), said capacitor is connected in circuit and charges when a sufficient voltage develops across said third resistor allowing current flow through said diode and said voltage limiting device, and said SCR is connected in circuit to be controlled by said capacitor and said composite voltage.

30. The sensor of claim 29, wherein said first voltage has an amplitude, and wherein said amplitude of said first voltage decreases when said SCR switches on.

31. The sensor of claim 1, wherein said second voltage is a periodic signal.

32. The sensor of claim 31, wherein the engine includes a flywheel and a magnet, and wherein said periodic signal is generated using said flywheel and said magnet.

33. A method of measuring a low liquid level condition in an engine, the method comprising;

providing a predetermined value;

providing a sensing circuit including a first inductor that is at least partially disposed in air during a low liquid level condition, a second inductor connected in series to said first inductor at a first node, a first resistor, and a second resistor connected in series to said first resistor at a second node;

applying a first voltage to said sensing circuit;

applying a second voltage to said sensing circuit, said second voltage being greater than said first voltage;

sensing a voltage difference between said first and second nodes that indicates an imbalance in the sensing circuit; and

generating a signal indicative of a low liquid level when said voltage difference reaches said predetermined value.

34. A method as set forth in claim 33, further comprising:

providing a first circuit electrically connected to said sensing circuit;

providing a second circuit electrically connected to said sensing circuit;

generating said first voltage in said first circuit; and

generating said second voltage in said second circuit.

35. A method as set forth in claim 34, wherein said steps of applying said first and second voltages to said sensing circuit result in a composite voltage having a root-mean-square (RMS) value, the method further comprising:

providing a voltage regulating circuit electrically connected to said first and second circuits; and

adjusting said composite voltage such that said RMS value remains substantially constant.

36. A method as set forth in claim 35, wherein said adjusting step comprises:

reducing said first voltage such that said RMS value of said composite voltage remains substantially constant.

37. A method as set forth in claim 33, wherein the engine includes an ignition coil primary winding that generates a supply voltage to the engine and wherein the supply voltage assists the engine in generating power, the method further comprising:

preventing the supply voltage from assisting the engine to generate power when said signal is generated.

38. A method as set forth in claim 37, wherein said preventing step comprises:

short circuiting said ignition coil primary winding to shut off said engine.

39. A method as set forth in claim 33, further comprising:

providing a visual indication that low liquid level exists when said signal is generated.

40. A method a set forth in claim 33, further comprising:

providing an audible indication that low liquid level exists when said signal is generated.

41. A sensor that detects a low liquid level condition in an engine, the engine including an ignition coil primary winding, comprising:

a sensing circuit, including a first inductor that is at least partially disposed in air during a low liquid level condition, and including a second inductor;

a first circuit that applies a first voltage to said first and second inductors;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors;

a third circuit that senses a voltage difference between a first point and a second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value; and

a fourth circuit that short circuits the ignition coil primary winding to shut off the engine in response to said signal.

42. The sensor of claim 41, wherein said second voltage is a periodic signal.

43. The sensor of claim 42, wherein the engine includes a flywheel and a magnet, and wherein said periodic signal is generated using said flywheel and said magnet.

44. A sensor that detects a low liquid level condition in an engine, the engine including a flywheel magnet, comprising:

a sensing circuit, including a first inductor that is at least partially disposed in air during a low liquid level condition, and including a second inductor;

a first circuit that applies a first voltage to said first and second inductors, including

a voltage source, including a power winding that generates a winding voltage when the flywheel magnet passes said power winding;

a resistor;

a capacitor, wherein the voltage source is connected in circuit to charge said capacitor with a charging voltage, and wherein said capacitor is connected in circuit to discharge through said resistor to provide said first voltage;

a diode;

a zener diode, wherein said diode is connected in circuit to rectify the charging voltage supplied by said voltage source, and wherein said zener diode is connected in circuit to limit the charging voltage;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors; and

a third circuit that senses a voltage difference between a first point and a second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value.

45. A sensor that detects a low liquid level condition in an engine, the engine including a flywheel magnet, comprising:

a sensing circuit, including a first inductor that is at least partially disposed in air during a low liquid level condition, and including a second inductor;

a first circuit that applies a first voltage to said first and second inductors;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors, including

a capacitor;

a power winding that generates a charging voltage when the flywheel magnet passes said power winding to thereby charge said capacitor; and

a third circuit that senses a voltage difference between a first point and a second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value.

46. A sensor that detects a low liquid level condition in an engine, comprising:

a sensing circuit, including a first inductor that is at least partially disposed in air during a low liquid level condition, and including a second inductor that is physically disposed at a higher level in said engine than said first inductor;

a first circuit that applies a first voltage to said first and second inductors;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors; and

a third circuit that senses a voltage difference between a first point and a second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value.

47. A sensor that detects a low liquid level condition in an engine, comprising:

a sensing circuit, including

a first inductor that is at least partially disposed in air during a low liquid level condition, and including a second inductor electrically connected to said first inductor;

a first resistor;

a second resistor electrically connected to said first resistor;

a first circuit that applies a first voltage to said first and second inductors;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors; and

a third circuit that senses a voltage difference between a first point and a second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value, wherein said third circuit includes

a third resistor electrically connected to said second resistor at a first node;

a transistor having a base, an emitter and a collector, wherein said base is electrically connected to said third resistor at said first node and said emitter is electrically connected to said second inductor at a second node; and

a capacitor electrically connected to said transistor at said second node.

48. A sensor that detects a low liquid level condition in an engine, comprising:

a sensing circuit, including a first inductor that is at least partially disposed in air during a low liquid level condition, and including a second inductor;

a first circuit that applies a first voltage to said first and second inductors;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors; and

a third circuit that senses a voltage difference between a first point and a second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value, said third circuit including

a first transistor; and

a programmable unijunction transistor (PUT) connected in circuit to said first transistor, wherein said signal switches ON said PUT, and wherein turning ON said PUT turns OFF said transistor.

49. A sensor that detects a low liquid level condition in an engine, comprising:

a sensing circuit, including a first inductor that is at least partially disposed in air during a low liquid level condition, and including a second inductor;

a first circuit that applies a first voltage to said first and second inductors;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors, wherein the application of said first and second voltages to said first and second inductors results in a composite voltage having a root-mean-square (RMS) value;

a voltage regulating circuit that adjusts the composite voltage such that the RMS value remains substantially constant; and

a third circuit that senses a voltage difference between a first point and a second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value.

50. The sensor of claim 49, wherein said voltage regulating circuit adjusts said first voltage such that said RMS value remains substantially constant.

51. The sensor of claim 50, wherein said voltage regulating circuit decreases said first voltage.

52. A sensor that detects a low liquid level condition in an engine, comprising:

a sensing circuit, including a first inductor that is at least partially disposed in air during a low liquid level condition, and including a second inductor;

a first circuit that applies a first voltage to said first and second inductors;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors, said second circuit including

a voltage source, a diode and a resistor connected in circuit to provide said second voltage, said second voltage having a peak amplitude;

a voltage limiting device connected in circuit to limit said peak amplitude of said second voltage; and

a third circuit that senses a voltage difference between a first point and a second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value.

53. A sensor that detects a low liquid level condition in an engine, comprising:

a sensing circuit, including a first inductor that is at least partially disposed in air during a low liquid level condition, and including a second inductor;

a first circuit that applies a first voltage to said first and second inductors;

a second circuit that applies a second voltage, greater than said first voltage, to said first and second inductors to yield a composite voltage, said second circuit including

a voltage source, a first diode, and a resistor connected in circuit to provide said second voltage, wherein a third voltage is developed across said resistor;

a voltage regulating circuit that adjusts said composite voltage to said inductors, said voltage regulating circuit includes a capacitor, a second diode, a voltage limiting device, and a silicon-controlled rectifier (SCR), wherein said capacitor is connected in circuit and charges when a sufficient voltage develops across said resistor allowing current flow through said second diode and said voltage limiting device, and wherein said SCR is connected in circuit to be controlled by said capacitor and said composite voltage; and

a third circuit that senses a voltage difference between a first point and a second point that indicates an imbalance in the sensing circuit, and that provides a signal indicative of a low liquid level when the voltage difference reaches a predetermined value.

54. The sensor of claim 53, wherein said first voltage has an amplitude, and wherein said amplitude of said first voltage decreases when said SCR switches on.

55. A method of measuring a low liquid level condition in an engine, the method comprising;

providing a predetermined value;

providing a sensing circuit including a first node, a second node, a first inductor that is at least partially disposed in air during a low liquid level condition and a second inductor;

providing a first circuit electrically connected to said sensing circuit;

generating a first voltage in said first circuit;

applying said first voltage to said sensing circuit;

providing a second circuit electrically connected to said sensing circuit;

generating a second voltage in said second circuit;

applying a second voltage to said sensing circuit, said second voltage being greater than said first voltage;

wherein said first and second voltage applying steps results in a composite voltage having a root-mean-square (RMS) value;

providing a voltage regulating circuit electrically connected to said first and second circuits;

adjusting said composite voltage such that said RMS value remains substantially constant;

sensing a voltage difference between said first and second nodes that indicates an imbalance in the sensing circuit; and

generating a signal indicative of a low liquid level when said voltage difference reaches said predetermined value.

56. The method of claim 55, wherein said adjusting step comprises:

reducing said first voltage such that said RMS value remains substantially constant.

57. A method of measuring a low liquid level condition in an engine, the engine including an ignition coil primary winding that generates a supply voltage to the engine so that the supply voltage assists the engine in generating power, the method comprising;

providing a predetermined value;

providing a sensing circuit including a first node, a second node, a first inductor that is at least partially disposed in air during a low liquid level condition, and a second inductor;

applying a first voltage to said sensing circuit;

applying a second voltage to said sensing circuit, said second voltage being greater than said first voltage;

sensing a voltage difference between said first and second nodes that indicates an imbalance in the sensing circuit;

generating a signal indicative of a low liquid level when said voltage difference reaches said predetermined value; and

short circuiting the ignition coil primary winding to shut off the engine.