System and method for controlling a self-heated gas sensor based on sensor impedance

• US 6,409,969 B1
• Filed: 06/01/1999
• Issued: 06/25/2002
• Est. Priority Date: 06/01/1999
• Status: Expired due to Term

• Current Assignee: Cummins Engine Company Incorporated (Cummins Incorporated)
• Original Assignee: Cummins Incorporated
• Inventors: Taylor, John E., Streicher, Steven R., Freeman, James A.
• Primary Examiner(s): Warden, Jill
• Assistant Examiner(s): SINES, BRIAN J
• Application Number:
  US09/323,199
• Time in Patent Office: 1,120 Days
• Field of Search: 422/94, 422/51, 422/90, 73/250.1, 602/74, 324/706, 219/497
• US Class Current: 422/94
• CPC Class Codes: G01N 27/123 for controlling the temperature temperature control per se G05D23/00

Abstract

The present invention contemplates a system and method for controlling or adjusting the accuracy of an exhaust gas sensor utilizing the impedance of the sensing element. In one embodiment, a periodic AC signal is superimposed over low frequency or DC output signal produced by the gas sensor. The AC current flowing through the gas sensor is a function of the actual impedance of the sensor, which is in turn a function of the temperature of the sensor. Thus, the invention further contemplates an impedance sensor circuit connected to an output of the gas sensor. The output of the impedance sensor circuit is a peak voltage that is indicative of the AC voltage drop across the sensor, and ultimately the impedance of the sensing element. This peak voltage is utilized to control the operation of the heating element in a closed loop control system in which the thermal output of the heating element is continuously varied as a function of the magnitude signal to accurately maintain a consistent temperature for the exhaust gas sensor. In a specific embodiment of the invention, the impedance sensor circuitry includes a bandpass filter centered around the frequency of the superimposed AC signal to eliminate spurious noise. The output from the bandpass filter is provided to a half-wave rectifier, the output of which is the peak voltage signal indicative of the sensor impedance.

Claims

1. A gas sensor system for use in an internal combustion engine, comprising:

a gas sensor having a sensing element for detecting a gas and operable to generate a gas level signal at an output of said sensor;

a heating element adjacent said gas sensor operable to generate a continuously variable amount of heat for warming said sensing element in response to variable power applied to said heating element;

an impedance sensor connected to said output of said sensing element and having circuitry generating an impedance signal as a function of the impedance of said sensor element; and

control means, connected between said impedance sensor and said heating element, for continuously varying the power applied to said heating element as a function of said impedance signal.

2. The gas sensor system according to claim 1, wherein said control means includes:

a variable voltage source connected to said heating element, wherein said heating element is responsive to variations in the voltage from said voltage source; and

means for varying said voltage from said voltage source as a function of said impedance signal.

3. The gas sensor system according to claim 2, wherein said control means includes a memory storing a table of voltage values to be applied to said heating element as a function of said impedance signal.

4. The gas sensor system according to claim 1, wherein:

said impedance sensor includes a signal source connected to said gas sensor to apply a periodic signal to said sensing element; and

said circuitry include components operable to determine the magnitude of a periodic output signal sensed at said output of said sensing element, said magnitude corresponding to said impedance signal.

5. The gas sensor system according to claim 4, wherein said circuitry of said impedance sensor includes a bandpass filter centered around the frequency of said periodic signal.

6. The gas sensor system according to claim 5, wherein said circuitry of said impedance sensor includes an amplifier connected to said bandpass filter to apply a gain to the output of said bandpass filter.

7. The gas sensor system according to claim 4, wherein said components of said impedance sensor include a half-wave rectifier.

8. The gas sensor system according to claim 4, wherein said signal source applies a periodic signal having a frequency substantially greater than the frequency of the gas level signal generated by said gas sensor.

9. The gas sensor system according to claim 1, wherein:

said sensing element of said gas sensor has an equivalent impedance; and

said gas sensor includes a voltage divider including said sensing element and a reference resistor, said output of said gas sensor corresponding to a node between said reference resistor and said sensing element.

10. The gas sensor system according to claim 9, wherein said circuitry of said impedance sensor is configured to generate said impedance signal as a function of the ratio of said sensing element impedance and the sum of said sensing element impedance and the value of said reference resistor.

11. The gas sensor system according to claim 1, wherein said control means includes means for comparing said impedance signal to a baseline signal to vary the power applied to said heating element as a function of the result of the comparison.

12. A method for controlling a self-heated gas sensor system, the gas sensor system including a heating element operable to generate a continuously variable amount of heat in response to variable power applied to the heating element, and a sensing element having an equivalent impedance that varies as a function of the temperature of the sensing element, the method comprising:

applying an AC signal to the sensing element;

determining the magnitude of an AC component of the applied signal across the sensing element; and

controlling the power applied to the heating element as a function of the magnitude of the AC component.

13. The method for controlling a self-heated gas sensor system of claim 12, wherein the determining step includes:

sensing the AC signal at an output of the sensing element;

passing the sensed AC signal through a bandpass filter centered about the frequency of the applied AC signal to eliminate spurious noise.

14. The method for controlling a self-heated gas sensor system of claim 12, wherein the determining step includes:

sensing the AC signal at an output of the sensing element;

passing the sensed AC signal through a capacitively coupled half-wave rectifier to generate a magnitude signal.

15. The method for controlling a self-heated gas sensor system of claim 12, in which the sensing element generates a low frequency output signal, wherein said step of applying an AC signal includes applying a signal having a frequency substantially greater than the low frequency output signal.

16. The method for controlling a self-heated gas sensor system of claim 12, wherein said step of controlling the power applied to the heating element includes:

comparing the magnitude of the AC component to a baseline magnitude; and

varying the power applied to the heating element as a function of the result of the comparing step.

17. The method for controlling a self-heated gas sensor system of claim 12, wherein said step of controlling the power applied to the heating element includes:

comparing the magnitude of the AC component to a baseline magnitude; and

determining the power to be applied to the heating element from a look-up table as a function of the result of the comparison step.

18. The method for controlling a self-heated gas sensor system of claim 12, wherein:

said step of controlling the power applied to the heating element includes comparing the magnitude of the AC component to a predetermined limit value indicative of degraded performance of the sensing element; and

the method further comprises the step of generating an error signal when the magnitude exceeds the limit value.

19. A method for detecting exhaust gases from an internal combustion engine, the method comprising:

positioning a gas sensor system, including a heating element operable to generate a continuously variable amount of heat in response to variable power applied to the heating element, and a sensing element having an equivalent impedance that varies as a function of the temperature of the sensing element, in a path of the exhaust gases;

applying an AC signal to the sensing element;

determining a magnitude of an AC component of the applied AC signal across the sensing element; and

applying a variable amount of power to the heating element, wherein the amount of power applied varies as a function of the magnitude of the AC component.

20. The method for detecting exhaust gases from an internal combustion engine of claim 19, wherein the determining step includes:

sensing the AC signal at an output of the sensing element; and

eliminating spurious noise by transmitting the sensed AC signal through a bandpass filter, wherein the bandpass filter is centered about the frequency of the applied AC signal.

21. The method for detecting exhaust gases from an internal combustion engine of claim 19, wherein the determining step includes:

sensing the AC signal at an output of the sensing element;

generating a magnitude signal by passing the sensed AC signal through a capacitively coupled half-wave rectifier.

22. The method for detecting exhaust gases from an internal combustion engine of claim 19, in which the sensing element generates a low frequency output signal, wherein said step of applying an AC signal includes applying a signal having a frequency substantially greater than the low frequency output signal.

23. The method for detecting exhaust gases from an internal combustion engine of claim 19, wherein said step of applying a variable amount of power to the heating element includes:

comparing the magnitude of the AC component to a baseline magnitude; and

varying the amount of power applied to the heating element as a function of the result of the comparing step.

24. The method for detecting exhaust gases from an internal combustion engine of claim 19, wherein said step of applying a variable amount of power to the heating element includes:

comparing the magnitude of the AC component to a baseline magnitude; and

determining the amount of power to be applied to the heating element by utilizing a function of the result of the comparison step as an index to a look-up table.

25. The method for detecting exhaust gases from an internal combustion engine of claim 19, wherein said step of applying a variable amount of power to the heating element includes:

comparing the magnitude of the AC component to a predetermined limit value, the predetermined limit value being indicative of degraded performance of the sensing element; and

generating an error signal when the magnitude of the AC component exceeds the predetermined limit value.