Gas sensor

Details Gas sensor Gas sensor

2000-01-19

2001-09-18

Warden, Sr., Robert J. (Department: 1744)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S401000, C204S408000, C204S426000

Reexamination Certificate

active

06290829

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas sensor for measuring oxides such as NO, NO
2
, SO
2
, CO
2
, and H
2
O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
Various measuring systems and apparatuses have been hitherto suggested in order to know the concentration of a desired gas component in a measurement gas.
For example, those known as the method for measuring NOx in a measurement gas such as combustion gas include a technique in which the NOx-reducing ability of Rh is utilized while using a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia to measure an electromotive force generated between the both electrodes.
The sensor as described above suffers the following problem. That is, the electromotive force is greatly changed depending on the change in concentration of oxygen contained in a combustion gas as a measurement gas. Moreover, the change in electromotive force is small with respect to the change in concentration of NOx. For this reason, the conventional sensor tends to suffer influence of noise. Further, in order to bring out the NOx-reducing ability, it is indispensable to use a reducing gas such as CO. For this reason, the amount of produced CO is generally smaller than the amount of produced NOx under a lean fuel combustion condition in which a large amount of NOx is produced. Therefore, the conventional sensor has a drawback in that it is impossible to perform measurement for a combustion gas produced under such a combustion condition.
A system has been disclosed, for example, in Japanese Laid-Open Patent Publication Nos. 63-38154 and 64-39545, in which a pair of electrochemical pumping cell and sensor cell comprising Pt electrode and an oxygen ion-conductive solid electrolyte are combined with another pair of electrochemical pumping cell and sensor cell comprising Rh electrode and an oxygen ion-conductive solid electrolyte to measure NOx in accordance with a difference between respective pumping current values.
Further, for example, Japanese Laid-Open Patent Publication Nos. 1-277751 and 2-1543 disclose the following method. That is, two pairs of electrochemical pumping cells and sensor cells are prepared. The limiting pumping current is measured at a partial pressure of oxygen at which NOx is not reduced, by using a sensor comprising one of the pairs of pumping cells and sensor cells, while the limiting pumping current is measured at a partial pressure of oxygen at which NOx is reduced, by using a sensor comprising the other pair of pumping cell and sensor cell so that the difference between the limiting pumping currents is determined. Besides, the difference in limiting current is measured by using a sensor comprising a pair of pumping cell and sensor cell, while switching the partial pressure of oxygen in a measurement gas between a partial pressure of oxygen at which NOx is reduced and a partial pressure of oxygen at which NOx is not reduced.
SUMMARY OF THE INVENTION
The present invention relates to the gas sensor as described above, an object of which is to provide a gas sensor which has a self-diagnosis function capable of quickly and reliably detecting whether or not the gas sensor has any trouble.
According to the present invention, there is provided a gas sensor comprising a main pumping means for pumping-processing oxygen contained in a measurement gas introduced from an external space into a processing space formed and comparted by solid electrolytes contacting with the external space; a main pumping control means for comparing a partial pressure of oxygen in the processing space with a first reference value to control the main pumping means so that the partial pressure of oxygen has a predetermined value; and an electric signal-generating conversion means for making conversion into an electric signal corresponding to an amount of oxygen contained in the measurement gas after being pumping-processed by the main pumping means; wherein a measurement gas component contained in the measurement gas is measured on the basis of the electric signal supplied from the electric signal-generating conversion means; the gas sensor further comprising an oxygen concentration-detecting means for detecting a concentration of oxygen contained in the measurement gas after being pumping-processed by the main pumping means; a correcting control means for correcting and controlling the main pumping control means on the basis of a difference between a detected value supplied from the oxygen concentration-detecting means and a second reference value to give a constant concentration of oxygen contained in the measurement gas after being pumping-processed by the main pumping means; and a self-diagnosis means for comparing the detected value supplied from the oxygen concentration-detecting means with a prescribed range to decide whether or not any trouble occurs, on the basis of a result of the comparison.
According to the present invention, at first, the oxygen, which is contained in the measurement gas introduced from the external space, is pumping-processed by the main pumping means, and the oxygen is adjusted to have a predetermined concentration. The measurement gas, which has been adjusted for the oxygen concentration by the main pumping means, is introduced into the electric signal-generating conversion means in the next step. The electric signal-generating conversion means decomposes the measurement gas component contained in the introduced measurement gas by means of catalytic action and/or electrolysis, to make conversion into the electric signal corresponding to the amount of oxygen produced by the decomposition. The measurement gas component contained in the measurement gas is measured on the basis of the electric signal supplied from the electric signal-generating conversion means.
The detecting operation described above is performed while heating at least the main pumping means and the electric signal-generating conversion means to predetermined temperatures by the aid of a heater. Therefore, the amount of the predetermined component is detected highly accurately by using the electric signal-generating conversion means.
The predetermined gas component includes, for example, NO, and the measurement gas component includes, for example, NOx.
When the electric signal-generating conversion means comprises a measuring pumping means, the measurement gas, which has been adjusted for the oxygen concentration by the main pumping means, is introduced into the measuring pumping means.
The measuring pumping means decomposes the measurement gas component after being pumping-processed by the main pumping means, by means of catalytic action and/or electrolysis, and it pumping-processes oxygen produced by the decomposition. The predetermined gas component corresponding to an amount of oxygen is measured on the basis of a pumping current generated in the measuring pumping means in accordance with the amount of oxygen pumping-processed by the measuring pumping means.
In another embodiment, the electric signal-generating conversion means comprises a concentration-detecting means. In this case, the measurement gas, which has been adjusted for the oxygen concentration by the main pumping means, is introduced into the concentration-detecting means in the next step. An electromotive force of an oxygen concentration cell is generated in the concentration-detecting means, which corresponds to a difference between an amount of oxygen contained in a reference gas and an amount of oxygen produced by decomposition of the predetermined gas component contained in the measurement gas. The predetermined gas component corresponding to the amount of oxygen is measured on the basis of the electromotive force.
During the period in which the measurement operation is performed for the predetermined gas component, the concentration of oxygen contained i

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