Method and apparatus for identifying gas sensor faults

1. A method for fault identification of gas sensors configured to measure concentrations of gas species in a gas mixture, comprising the steps of:

receiving a first output signal from a first gas sensor having an output that varies according to both the concentration of a first gas species in the gas mixture and the concentration of a second gas species in the gas mixture;

receiving a second output signal from a second gas sensor having an output that varies according to both the concentration of the first gas species in the gas mixture and the concentration of the second gas species in the gas mixture;

processing the first output signal and the second output signal in a diagnostic controller that implements a model of the first gas sensor and a model of the second gas sensor so as to identify a fault in the first gas sensor or the second gas sensor; and

indicating any identified faults;

wherein the gas mixture comprises an exhaust gas from an internal combustion engine having an exhaust system comprising means for controllably introducing ammonia into the gas mixture, and wherein the first gas species is NH 3 and the second gas species is NO x ; and

wherein the processing of the first output signal and the second output signal includes the steps of:

using the first output signal and the second output signal as inputs to the model to calculate the concentration of the first gas species in the gas mixture;

using the calculated concentration of the first gas species as an input to the model of the second gas sensor to calculate a predicted signal level from the second gas sensor;

indicating a first fault condition if the second output signal is not within a first predetermined range;

indicating a second fault condition if the predicted signal level is not within a second predetermined range;

indicating a third fault condition if the difference between the second output signal and the predicted signal level is not within a third predetermined range.

2. The method of claim 1 wherein the first output signal and the second output signal are measured at a time when the means for controllably introducing ammonia into the gas mixture is controlled so as to introduce ammonia into the gas mixture.

3. The method of claim 2 wherein at least one of the first predetermined range, the second predetermined range, and the third predetermined range is a function of the operating state of the internal combustion engine.

4. The method of claim 1 wherein the processing of the first output signal and the second output signal further includes the steps of:

indicating a fourth fault condition if the first output signal is not within a first predetermined range;

indicating a fifth fault condition if the second output signal is not within a second predetermined range.

5. The method of claim 4 wherein the first output signal and the second output signal are measured at a time when the means for controllably introducing ammonia into the gas mixture is controlled so as to prevent the introduction of ammonia into the gas mixture.

6. The method of claim 5 wherein at least one of the first predetermined range and the second predetermined range is a function of the operating state of the internal combustion engine.

7. The method of claim 1 wherein the step of indicating any identified faults comprises generating a signal operable to control an output device.

8. The method of claim 7 wherein the output device is a visual indicator or a memory storage device.

9. A fault identification system for gas sensors configured to measure concentrations of gas species in a gas mixture a gas mixture, comprising:

a first gas sensor providing an output that varies according to both the concentration of a first gas species in the gas mixture and the concentration of a second gas species in the gas mixture;

a second gas sensor providing an output that varies according to both the concentration of the first gas species in the gas mixture and the concentration of the second gas species in the gas mixture;

a diagnostic controller that is coupled to said first gas sensor and said second gas sensor that implements a model of the first gas sensor and a model of the second gas sensor so as to identify a fault in the first gas sensor or the second gas sensor, the diagnostic controller including a non-transitory computer readable medium containing instructions that, when executed, cause the diagnostic controller to:

use the first output signal and the second output signal as inputs to the model to calculate the concentration of the first gas species in the gas mixture;

use the calculated concentration of the first gas species as an input to the model of the second gas sensor to calculate a predicted signal level from the second gas sensor;

indicate a first fault condition if the second output signal is not within a first predetermined range;

indicate a second fault condition if the predicted signal level is not within a second predetermined range; and

indicate a third fault condition if the difference between the second output signal and the predicted signal level is not within a third predetermined range.

10. The system of claim 9 wherein the first gas species is NH 3 and the second gas species is NO x .

11. The system of claim 9 wherein the diagnostic controller is configured to indicate the first fault condition, the second fault condition, and the third fault condition by generating a signal operable to control an output device.

12. The system of claim 11 wherein the output device is a visual indicator or a memory storage device.

13. A method for fault identification of gas sensors configured to measure concentrations of gas species in a gas mixture, comprising the steps of:

receiving a first output signal from a first gas sensor having an output that varies according to both the concentration of a first gas species in the gas mixture and the concentration of a second gas species in the gas mixture;

receiving a second output signal from a second gas sensor having an output that varies according to both the concentration of the first gas species in the gas mixture and the concentration of the second gas species in the gas mixture;

processing the first output signal and the second output signal in a diagnostic controller that implements a model of the first gas sensor and a model of the second gas sensor so as to identify a fault in the first gas sensor or the second gas sensor, wherein the processing of the first output signal and the second output signal includes the steps of:

using the first output signal and the second output signal as inputs to the model to calculate the concentration of the first gas species in the gas mixture;

using the calculated concentration of the first gas species as an input to the model of the second gas sensor to calculate a predicted signal level from the second gas sensor;

indicating a first fault condition if the second output signal is not within a first predetermined range;

indicating a second fault condition if the predicted signal level is not within a second predetermined range;

indicating a third fault condition if the difference between the second output signal and the predicted signal level is not within a third predetermined range; and

indicating any identified faults.

14. The method of claim 13 wherein the step of indicating a first fault condition, the step of indicating a second fault condition, and the step of indicating a third fault condition each comprises generating a signal operable to control an output device.

15. The method of claim 14 wherein the output device is a visual indicator or a memory storage device.