Engine misfire or roughness detection method and apparatus

• US 5,287,735 A
• Filed: 12/10/1990
• Issued: 02/22/1994
• Est. Priority Date: 12/10/1990
• Status: Expired due to Fees

• Current Assignee: Sensortech LP
• Original Assignee: Sensortech LP
• Inventors: Vigmostad, Erik B., Klauber, Robert D.
• Primary Examiner(s): MYRACLE, JERRY
• Application Number:
  US07/625,222
• Time in Patent Office: 1,170 Days
• Field of Search: 73/116, 73/117.2, 73/117.3, 73/660, 73/862.23, 73/DIG. 2, 364/431.08, 74/859
• US Class Current: 73/114.02
• CPC Class Codes: B60G 17/019 characterised by the type of sensor or the arrangement thereof B60G17/01941 takes precedence
• CPC Class Codes: B60G 2204/11 Mounting of sensors thereon
• CPC Class Codes: B60G 2400/30 Propulsion unit conditions
• CPC Class Codes: B60G 2400/60 Load
• CPC Class Codes: B60G 2400/91 Frequency
• CPC Class Codes: B60G 2401/172 Hall effect
• CPC Class Codes: B60G 2600/602 high pass
• CPC Class Codes: B60G 2600/604 low pass
• CPC Class Codes: B60G 2800/802 Diagnostics
• CPC Class Codes: G01L 23/223 using magnetic or magnetostrictive means
• CPC Class Codes: G01M 15/042 by monitoring a single specific parameter not covered by groups G01M15/06 - G01M15/12

Abstract

A method and apparatus for comparing torsional stress/strain states of a power transmitting member of an internal combustion engine to detect an abnormal combustion condition, including misfire or roughness. A first signal is obtained relating to the torsional stress in the member produced by a first combustion event or series of events. A second signal is obtained which relates to the torsional stress in the member produced by a second combustion event or series of events. The first and second signals are compared to detect the abnormal combustion event in the operation of the engine. A magnetostrictive sensor may preferably be used to obtain the signals relating to the torsional stress in the power transmitting member, such as the engine crankshaft.

Claims

1. A method for detecting an abnormal combustion event in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one combustion event in the internal combustion engine;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one combustion event in the internal combustion engine,

wherein the first and second signals are derived, at least in part, from magnetostrictive sensing means; and

comparing said first stress related signal to said second stress related signal to detect the abnormal combustion event in the operation of the engine.

2. The method of claim 1 wherein said steps of obtaining said first and second signals are performed sufficiently close in time to one another so as to obtain said first and second signals under substantially constant engine ambient conditions including temperature.

3. The method of claim 1 wherein said step of obtaining said second signal comprises the step of obtaining said second signal related to the torsional stress in the member produced during a second at least one combustion event which follows said first at least one combustion event in the series of combustion events.

4. The method of claim 1 wherein all of said first at least one combustion event is different from all of said second at least one combustion event.

5. The method of claim 1 further comprising the step of storing said first signal.

6. The method of claim 1 wherein said step of comparing said first and second signals comprises the steps of subtracting said second signal from said first signal to produce a difference value, and comparing said difference value to a threshold value.

7. The method of claim 6 wherein said step of comparing said difference value comprises the step of comparing said difference value to a threshold to detect misfire.

8. The method of claim 6 wherein said step of comparing said difference value comprises the step of comparing said difference value to a threshold to detect roughness.

9. The method of claim 1 wherein said step of comparing said first and second signals comprises the step of dividing said first and second signals to calculate a ratio value, and comparing said ratio value to a threshold.

10. The method of claim 9 wherein said step of comparing said ratio value comprises the step of comparing said ratio value to a threshold, to detect misfire.

11. The method of claim 9 wherein said step of comparing said ratio value comprises the step of comparing said ratio value to a threshold to detect roughness.

12. The method of claim 1 wherein said steps of obtaining said first and second signals each comprises the step of obtaining at least one of an instantaneous value, a summation of instantaneous values, an integrated value, an average value, at least one Fourier component value, and a filtered value related to the torsional stress in the member produced from said first at least one combustion event and said second at least one combustion event respectively.

13. The method of claim 1 wherein said steps of obtaining said first and second signals each comprise the step of sensing said signals with at least two sensors positioned adjacent the power transmitting member.

14. The method of claim 1 wherein said steps of obtaining said first and second signals each comprise the step of sensing said signals with at least one sensor positioned adjacent the power transmitting member.

15. The method of claim 14 wherein said sensing step comprises sensing with at least one magnetostrictive sensor excited by at least one of a constant amplitude current, a constant amplitude voltage, a constant amplitude magnetic flux, a constant phase, a direct current, a direct voltage, and a direct flux.

16. The method of claim 15 wherein said sensing with said at least one magnetostrictive sensor comprises operating said sensor at resonance.

17. The method of claim 16 wherein said steps of obtaining said first and second signals each comprise the step of triggering said at least one sensor responsive to at least one of an ignition system signal, a cam shaft position signal, a crankshaft position signal, a flywheel position signal, a rod position signal, a drive shaft position signal, and a cylinder position signal.

18. The method of claim 1 wherein said step of obtaining said first signal comprises the step of obtaining at least one signal produced by a first combustion chamber in the internal combustion engine, and wherein said step of obtaining said second signal comprises the step of obtaining at least one signal produced by a second combustion chamber in the internal combustion engine.

19. The method of claim 1 wherein said steps of obtaining said first and second signals each comprise the step of obtaining said first and second signals produced by a first combustion chamber in the internal combustion engine.

20. The method of claim 1 wherein said step of generating said first signal comprises the steps of:

sensing a signal produced by a peak portion of said first at least one combustion event;

sensing a signal produced by a non-peak portion of said first at least one combustion event to produce a reference signal; and

subtracting one of said non-peak reference signal and said peak signal from the other signal to produce said first signal.

21. The method of claim 20 wherein said step of obtaining said second signal comprises the steps of:

sensing a signal produced by a peak portion of said second at least one combustion event;

sensing a signal produced by a non-peak portion of said second at least one combustion event to produce a reference signal; and

subtracting one of said non-peak reference value and said peak signal from the other to produce said second signal.

22. The method of claim 1 wherein said step of obtaining said first signal comprises the step of obtaining a series of signals produced by a series of combustion events of a first combustion chamber in the internal combustion engine.

23. The method of claim 22 wherein said step of comparing said first and second signals comprises the steps of:

calculating a mean and a standard deviation from said first signal series, and

comparing the difference between said second signal and said mean to a multiple of said standard deviation.

24. The method of claim 22 wherein said step of comparing said first and second signals comprises the steps of:

calculating a mean and a variance from said first signal series, and

comparing the square of the difference between said second signal and said mean to a multiple of said variance.

25. The method of claim 22 wherein said step of obtaining said first signal comprises the steps of calculating a scale value from a maximum value and a minimum value of said series of combustion events, and wherein said step of comparing said first and second signals comprises the step of calculating a ratio between said second and first signals.

26. The method of claim 1 wherein said step of obtaining said first signal comprises the steps of calculating a scale value from a maximum value and a minimum value from a first series of combustion events for a combustion chamber, and calculating a ratio between a current signal related to the torsional stress in the member produced by said combustion chamber and the scale value to generate said first signal;

and wherein said step of obtaining said second signal comprises the steps of calculating a scale value from a maximum value and a minimum value from a second series of combustion events for said combustion chamber, and calculating a ratio between a current signal related to the torsional stress produced by said combustion chamber and the scale value to generate said second signal.

27. The method of claim 1 wherein said step of obtaining said first signal comprises the steps of calculating a scale value from a maximum value and a minimum value from a first series of combustion events for a first combustion chamber, and calculating a ratio between a current signal related to the torsional stress in the member produced by said first combustion chamber and the scale value to generate said first signal;

and wherein said step of obtaining said second signal comprises the steps of calculating a scale value from a maximum value and a minimum value from a second series of combustion events for a second combustion chamber, and calculating a ratio between a current signal related to the torsional stress produced by a second combustion chamber and the scale value to generate said second signal.

28. The method of claim 1 wherein said step of obtaining said first signal comprises the steps of calculating a scale value from series of peak values and a non-peak values obtained at predetermined times from a first series of combustion events for a combustion chamber, and calculating a ratio between a current signal related to the torsional stress in the member produced by said combustion chamber and the scale value to generate said first signal;

and wherein said step of obtaining said second signal comprises the steps of calculating a scale value from a series of peak values and a non-peak values at predetermined times from a second series of combustion events for said combustion chamber, and calculating a ratio between a current signal related to the torsional stress produced by said combustion chamber and the scale value to generate said second signal.

29. The method of claim 1 wherein said step of obtaining said first signal comprises the steps of calculating a scale value from peak values and non-peak values obtained at predetermined points from a first series of combustion events for a first combustion chamber, and calculating a ratio between a current signal related to the torsional stress in the member produced by said first combustion chamber and the scale value to generate said first signal;

and wherein said step of obtaining said second signal comprises the steps of calculating a scale value from peak values and non-peak values at predetermined points from a second series of combustion events for a second combustion chamber, and calculating a ratio between a current signal related to the torsional stress produced by a second combustion chamber and the scale value to generate said second signal.

30. The method of claim 1 further comprising setting a flag for the occurrence of a first number of abnormal combustion events, and giving an abnormal combustion event indication for a second number of flags occurring during at least one of a predetermined period of time, a predetermined mileage and a predetermined number of cylinder firings.

31. A method for detecting a torque producing event in a power transmitting member which experiences a series of torque producing events, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one torque producing event experienced by the power transmitting member;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one torque producing event experienced by the power transmitting member,

wherein the first and second signals are derived, at least in part, from magnetostrictive sensing; and

comparing said first signal to said second signal to detect the torque producing event.

32. The method of claim 31 wherein said power transmitting member is an axle of a vehicle and wherein said method further comprises the step of providing an indication of the torque producing event to a traction control system.

33. The method of claim 31 wherein the power transmitting member is operatively coupled to at least one of an engine transmission, a braking system, and a vehicle steering system.

34. An apparatus for detecting an abnormal combustion event, such as a misfire or roughness, in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said apparatus comprising:

means for obtaining a first signal and a second signal, said first signal being related to stress in the member produced by a first at least one combustion event, said second signal being related to stress in the member produced by a second at least one combustion event,

wherein at least part of the signal obtaining means comprises a magnetostrictive sensor; and

processing means for comparing said first and second signals to detect the abnormal combustion event.

35. The apparatus of claim 34 further comprising timing trigger means cooperating with said signal obtaining means for generating said first and second signals wherein said timing trigger means provides a means for determining when to sample a magnetostrictive based signal from the magnetostrictive sensor.

36. The apparatus of claim 35 wherein said timing trigger means comprises a sensor positioned adjacent a flywheel coupled to the engine, and wherein said flywheel sensor is adapted to generate a timing pulse for a predetermined amount of rotation of the flywheel.

37. The apparatus of claim 35 wherein said timing trigger means comprises a sensor located adjacent a portion of a power transmitting assembly, and wherein said sensor is adapted to generate a timing signal for a predetermined amount of rotation of the portion of the power transmitting assembly.

38. The apparatus of claim 34 wherein said signal obtaining means comprises at least two magnetostrictive sensors positioned adjacent the power transmitting member.

39. The apparatus of claim 34 wherein said signal obtaining means comprises at least one magnetostrictive sensor positioned adjacent the power transmitting member.

40. The apparatus of claim 39 wherein said magnetostrictive sensor comprises at least one of a cross design sensor, a C-core sensor, a single branch sensor, a dual branch sensor, and a multi-branch sensor.

41. The apparatus of claim 39 wherein said at least one magnetostrictive sensor is excited by at least one of a constant amplitude current, a constant amplitude voltage, a constant amplitude magnetic flux, a constant phase, a direct current, a direct voltage, and a direct flux.

42. The apparatus of claim 39 wherein said at least one magnetostrictive sensor is operating at resonance.

43. The apparatus of claim 39 wherein said at least one magnetostrictive sensor is operating at above resonance.

44. The apparatus of claim 39 wherein said at least one magnetostrictive sensor is operating at below resonance.

45. The apparatus of claim 39 wherein said at least one magnetostrictive sensor is positioned adjacent a bearing cap.

46. The apparatus of claim 39 wherein the power transmitting member is the crankshaft of the engine, and wherein said at least one magnetostrictive sensor is positioned within a bearing cap adjacent the crankshaft of the engine.

47. The apparatus of claim 46 wherein said at least one magnetostrictive sensor is a cross design sensor.

48. The apparatus of claim 39 wherein the engine has a back end connected to a drive shaft and a front end opposite said back end, and wherein said magnetostrictive sensor is positioned adjacent said front end of the engine.

49. The apparatus of claim 34 wherein the engine is coupled to a transmission and wherein at least part of said signal obtaining means is positioned adjacent said transmission.

50. The apparatus of claim 34 further comprising means for identifying a cylinder for a combustion event.

51. The apparatus of claim 50 wherein said cylinder identifying means comprises a cam shaft position sensor.

52. The apparatus of claim 34 wherein said processing means comprises an electronic microprocessor operating under stored program control.

53. The apparatus of claim 52 wherein said processing means includes means for subtracting said second signal from said first signal to produce a difference value, and for comparing said difference value to a threshold value.

54. The apparatus of claim 53 wherein said processing means includes means for comparing said difference value to a threshold to detect misfire.

55. The apparatus of claim 52 wherein said processing means includes means for comparing said difference value to a threshold to detect roughness.

56. The apparatus of claim 52 wherein said processing means includes means for dividing said first and second signals to calculate a ratio value, and for comparing said ratio value to a threshold.

57. The apparatus of claim 56 wherein said processing means includes means for comparing said ratio value to a first threshold to detect misfire, and for comparing said ratio value to a second threshold to detect roughness.

58. The apparatus of claim 52 wherein said processing means includes means for calculating a mean and a standard deviation from a series of said first signals, and for comparing a difference between said second signal and said mean to a multiple of said standard deviation.

59. The apparatus of claim 52 wherein said processing means includes means for calculating a mean and a variance from a series of said first signals, and for comparing the square of a difference between said second signal and said mean to a multiple of said variance.

60. The apparatus of claim 34 wherein said processing means comprises a neural network.

61. A method for detecting an abnormal combustion event, such as a misfire or roughness, in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining at least one first pair of signals from an at least one first pair of successive combustion events, each signal related, at least in part via magnetostriction to the torsional stress in the member produced by a respective one of the combustion events;

calculating a difference between each signal in the at least one first pair of signals;

obtaining a second pair of signals from a second pair of successive combustion events, each signal related, at least in part via magnetostriction to the torsional stress in the member produced by a respective one of the second pair of successive combustion events;

calculating a difference between each signal in the second pair of signals; and

comparing the difference between the at least one first pair of signals to the difference between the second pair of signals to detect an abnormal combustion event in the operation of the engine.

62. The method of claim 61 wherein the step of obtaining the at least one first pair of signals comprises the step of obtaining a plurality of pairs of signals from a corresponding plurality of pairs of combustion events, and further comprising the step of obtaining an average difference for the plurality of pairs of signals to compare to the difference between the second pair of signals.

63. A method of detecting an abnormal combustion event in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one combustion event in the internal combustion engine;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one combustion event in the internal combustion engine; and

comparing said first stress related signal to said second stress related signal to detect the abnormal combustion event in the operation of the engine,

wherein said step of obtaining said first signal comprises the steps of;

sensing a signal produced by a peak portion of said first at least one combustion event;

sensing a signal produced by a non-peak portion of said first at least one combustion event to produce a reference signal; and

subtracting one of said non-peak reference signal and said peak signal from the other to produce said first signal.

64. The method of claim 63 wherein said step of obtaining said second signal comprises the steps of:

sensing a signal produced by a peak portion of said second at least one combustion event;

sensing a signal produced by a non-peak portion of said second at least one combustion event to produce a reference signal; and

subtracting one of said non-peak reference signal and said peak signal from the other to produce said second signal.

65. A method for detecting an abnormal combustion event in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one combustion event in the internal combustion engine;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one combustion event in the internal combustion engine; and

comparing said first stress related signal to said second stress related signal to detect the abnormal combustion event in the operation of the engine,

wherein said step of obtaining said first signal comprises the step of obtaining a series of signals produced by a series of combustion events of a first combustion chamber in the internal combustion engine, and

wherein said step of comparing said first and second signals comprises the steps of;

calculating a mean and a standard deviation from said first signal series, and

comparing the difference between said second signal and said mean to a multiple of said standard deviation.

66. A method for detecting an abnormal combustion event in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one combustion event in the internal combustion engine;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one combustion event in the internal combustion engine; and

comparing said first stress related signal to said second stress related signal to detect the abnormal combustion event in the operation of the engine,

wherein said step of obtaining said first signal comprises the step of obtaining a series of signals produced by a series of combustion events of a first combustion chamber in the internal combustion engine, and

wherein said step of comparing said first and second signals comprises the steps of;

calculating a mean and a variance from said first signal series, and

comparing a square of a difference between said second signal and said mean to a multiple of said variance.

67. A method for detecting an abnormal combustion event in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one combustion event in the internal combustion engine;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one combustion event in the internal combustion engine; and

comparing said first stress related signal to said second stress related signal to detect the abnormal combustion event in the operation of the engine,

wherein said step of obtaining said first signal comprises the step of obtaining a series of signals produced by a series of combustion events of a first combustion chamber in the internal combustion engine, and the steps of calculating a scale value from a maximum value and a minimum value of said series of combustion events, and

wherein said step of comparing said first and second signals comprises the step of calculating a ratio between said second and first signals.

68. A method for detecting an abnormal combustion event in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one combustion event in the internal combustion engine;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one combustion event in the internal combustion engine, and

comparing said first stress related signal to said second stress related signal to detect the abnormal combustion event in the operation of the engine,

wherein said step of obtaining said first signal comprises the steps of calculating a scale value from a maximum value and a minimum value from a first series of combustion events for a combustion chamber, and calculating a ratio between a current signal related to the stress in the member produced by said combustion chamber and the scale value to generate said first signal; and

wherein said step of obtaining said second signal comprises the steps of calculating a scale value from a maximum value and a minimum value from a second series of combustion events for said combustion chamber, and calculating a ratio between a current signal related to the stress produced by said combustion chamber and the scale value to generate said second signal.

69. A method for detecting an abnormal combustion event in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one combustion event in the internal combustion engine;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one combustion event in the internal combustion engine; and

comparing said first stress related signal to said second stress related signal to detect the abnormal combustion event in the operation of the engine,

wherein said step of obtaining said first signal comprises the steps of calculating a scale value from a maximum value and a minimum value from a first series of combustion events for a first combustion chamber, and calculating a ratio between a current signal related to the stress in the member produced by said first combustion chamber and the scale value to generate said first signal; and

wherein said step of obtaining said second signal comprises the steps of calculating a scale value from a maximum value and a minimum value from a second series of combustion events for a second combustion chamber, and calculating a ratio between a current signal related to the stress produced by a second combustion chamber and the scale value to generate said second signal.

70. A method for detecting an abnormal combustion event in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one combustion event in the internal combustion engine;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one combustion event in the internal combustion engine; and

comparing said first stress related signal to said second stress related signal to detect the abnormal combustion event in the operation of the engine,

wherein said step of obtaining said first signal comprises the steps of calculating a scale value from a series of peak values and non-peak values obtained at predetermined times from a first series of combustion events for a combustion chamber, and calculating a ratio between a current signal related to the stress in the member produced by said combustion chamber and the scale value to generate said first signal; and

wherein said step of obtaining said second signal comprises the steps of calculating a scale value from a series of peak values and a non-peak values at predetermined times from a second series of combustion events for said combustion chamber, and calculating a ratio between a current signal related to the stress produced by said combustion chamber and the scale value to generate said second signal.

71. A method for detecting an abnormal combustion event in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said method comprising the steps of:

obtaining a first signal related to stress in the power transmitting member produced by a first at least one combustion event in the internal combustion engine;

obtaining a second signal related to stress in the power transmitting member produced by a second at least one combustion event in the internal combustion engine; and

comparing said first stress related signal to said second stress related signal to detect the abnormal combustion event in the operation of the engine,

wherein said step of obtaining said first signal comprises the steps of calculating a scale value from peak values and non-peak values obtained at predetermined points from a first series of combustion events for a first combustion chamber, and calculating a ratio between a current signal related to the stress in the member produced by said first combustion chamber and the scale value to generate said first signal; and

wherein said step of obtaining said second signal comprises the steps of calculating a scale value from peak values and non-peak values at predetermined points from a second series of combustion events for a second combustion chamber, and calculating a ratio between a current signal related to the stress produced by a second combustion chamber and the scale value to generate said second signal.

72. As apparatus for detecting an abnormal combustion event, such as a misfire or roughness, in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said apparatus comprising:

means for obtaining a first signal and a second signal from the power transmitting member, said first signal being related to stress in the power transmitting member produced by a first at least one combustion event, said second signal being related to stress in the power transmitting member produced by a second at least one combustion event; and

processing means for comparing said first and second signals to detect the abnormal combustion event wherein said processing means comprises an electronic microprocessor operating under stored program control,

wherein said processing means includes

means for dividing said first and second signals to calculate a ratio value, for comparing said ratio value to a first threshold to detect misfire, and for comparing said ratio value to a second threshold to detect roughness.

73. An apparatus for detecting an abnormal combustion event, such as a misfire or roughness, in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said apparatus comprising:

means for obtaining a first signal and a second signal from the power transmitting member, said first signal being related to stress in the power transmitting member produced by a first at least one combustion event, said second signal being related to stress in the power transmitting member produced by a second at least one combustion event; and

processing means for comparing said first and second signals to detect the abnormal combustion event wherein said processing means comprises an electronic microprocessor operating under stored program control,

wherein said processing means includes

means for calculating a mean and a standard deviation from a series of said first signals, and for comparing a difference between said second signal and said mean to a multiple of said standard deviation.

74. An apparatus for detecting an abnormal combustion event, such as a misfire or roughness, in an internal combustion engine which produces a series of combustion events and which is coupled to a power transmitting member, said apparatus comprising:

means for obtaining a first signal and a second signal from the power transmitting member, said first signal being related to stress in the power transmitting member produced by a first at least one combustion event, said second signal being related to stress in the power transmitting member produced by a second at least one combustion event; and

processing means for comparing said first and second signals to detect the abnormal combustion event wherein said processing means comprise an electronic microprocessor operating under stored program control,

wherein said processing means includes

means for calculating a mean and a variance from a series of said first signals, and for comparing a square of a difference between said second signal and said mean to a multiple of said variance.