Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For nitrogen or nitrogen containing compound
Reexamination Certificate
2000-06-21
2003-09-23
Nguyen, Nam (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For nitrogen or nitrogen containing compound
C205S784000, C204S425000, C204S426000, C204S427000, C204S001001, C204S001001
Reexamination Certificate
active
06623618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas sensor and a method for controlling the same 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
Recently, an oxygen sensor is widely known, for measuring a specified gas component, for example, oxygen, in which the voltage or the current is controlled to apply it to an oxygen pump based on the use of an oxygen ion-conductive member composed of a solid electrolyte of ZrO
2
so that oxygen is pumped in or pumped out under a predetermined diffusion resistance to measure a limiting current obtained during this process (see, for example, Japanese Laid-Open Patent Publication No. 8-271476).
Another sensor is also known, in which a proton pump is constructed by using an oxygen-proton ion-conductive member so that the limiting current is measured on the basis of the same principle as that used in the oxygen sensor to measure H
2
and H
2
O.
A NOx sensor
200
as shown in
FIG. 15
is also known, which is used to measure, for example, NOx as a specified gas component.
The NOx sensor
200
is operated as follows. That is, a measurement gas is introduced into a first hollow space
204
via a first diffusion rate-determining section
202
. A first oxygen-pumping means
212
, which is constructed by an inner pumping electrode
206
, an oxygen ion-conductive member
210
, and an outer pumping electrode
208
, is used to pump out or pump in oxygen contained in the measurement gas in such a degree that the measurement gas is not decomposed. Subsequently, the measurement gas is introduced into a second hollow space
216
via a second diffusion rate-determining section
214
. A second oxygen-pumping means
226
, which is constructed by a measurement gas-decomposing electrode
218
arranged in the second hollow space
216
, an oxygen ion-conductive member
220
, and a reference electrode
224
arranged in a reference air section
222
, is used to pump out oxygen which is produced by decomposition effected by the catalytic action of the measurement gas-decomposing electrode
218
. The sensor measures the value of current which is required to pump out the oxygen.
In other words, the foregoing gas sensors are operated such that the specified gas component is detected by using the ionic current, and the concentration of the predetermined gas is ensured in the internal space of the sensor by controlling the ionic current value.
However, the gas sensors as described above are disadvantageous as follows. That is, when the concentration of the measurement gas is low, the pumping current is decreased. As a result, it is difficult to perform the detection in some cases, and the accuracy is greatly deteriorated by the external electric noise in other cases.
For example, in the case of the NOx sensor
200
shown in
FIG. 15
, when the NOx concentration in the measurement gas is 10 ppm, the signal level is in a degree of about 0.05 &mgr;A. As a result, it is difficult to perform the detection. Further, it is feared that the measurement accuracy is greatly deteriorated due to the external electric noise.
In order to accurately control the oxygen concentration in the second hollow space
216
, the present applicant has suggested a NOx sensor
250
as shown in FIG.
16
. The NOx sensor
250
comprises an auxiliary pump
252
which is provided for the second hollow space
216
. The controlled oxygen concentration in the first hollow space
204
is corrected so that the current, which flows through the auxiliary pump
252
, is constant (see, for example, Japanese Laid-Open Patent Publication No. 9-113484 and European Patent Publication No. 0 807 818 A2).
In the case of the NOx sensor
250
, the auxiliary pumping current is not more than several &mgr;A which is small. Therefore, it has been revealed that the controlled oxygen concentration in the second hollow space
216
cannot be corrected at the desire of a user in some cases.
On the other hand, in the case of the sensors as described above, the limiting current is utilized to control the concentration of the gas component and measure the concentration thereof. Therefore, if the limiting current value is changed, the output is changed. In this context, for example, the limiting current value involves dispersion among individual sensors. At present, in order to correct the dispersion among individual sensors, a shunt resistor is provided, or a voltage divider resistor is provided.
FIG. 17
shows an arrangement of such a countermeasure. When the current, which flows to an oxygen pump
260
, is detected by using a current-detecting resistor Ra, the current supply from a variable power source
262
to the oxygen pump
260
is shunted by the aid of an adjusting resistor Rb (shunt resistor).
For example, when the gas sensor has a large limiting current, the shunt resistor Rb is decreased so that the amount of shunt is increased. Thus, the amount of current, which is detected by the current-detecting resistor Ra, is decreased to be a predetermined value. On the contrary, when the gas sensor has a small limiting current, the amount of shunt is decreased so that the current, which is detected by the current-detecting resistor Ra, is adjusted to be the predetermined value.
Another method is also available such that the voltage, which is generated between the both terminals of the current-detecting resistor Ra, is subjected to voltage division by using a voltage divider circuit to obtain a predetermined output voltage.
However, when the foregoing methods (the shunt resistor system and the voltage divider resistor system) are adopted, one extra lead wire is required, in accordance with which it is necessary to use a multiple terminal connector system for connecting the control circuit and the sensor, resulting in a problem concerning the cost.
SUMMARY OF THE INVENTION
The present invention has been made considering the problems as described above, an object of which is to provide a gas sensor and a method for controlling the gas sensor which make it possible to highly accurately measure a predetermined gas component while scarcely being affected by the electric noise or the like.
Another object of the present invention is to provide a gas sensor and a method for controlling the gas sensor which are advantageous in view of the production cost and which make it possible to compensate the dispersion among individual sensors without increasing the number of terminals, in addition to the requirement described above.
A gas sensor according to the present invention comprising:
a main pumping means for pumping-processing oxygen contained in a measurement gas introduced from external space, comprising solid electrolyte contacting with said external space, and an inner pumping electrode and an outer pumping electrode formed on inner and outer surfaces of said solid electrolyte; and
a measuring pumping means for decomposing a predetermined gas component contained in said measurement gas after being pumping-processed by said main pumping means by the aid of a catalytic action and/or electrolysis, and pumping-processing oxygen produced by said decomposition via said outer pumping electrode of said main pumping means, wherein:
a concentration of oxygen is controlled and/or the predetermined gas component is measured by allowing a pulse-shaped current to flow through said measuring pumping means;
the gas sensor further comprising:
a electromotive force-measuring circuit for constantly measuring the electromotive force corresponding to a difference between an amount of oxygen produced by said decomposition of said predetermined gas component and an amount of oxygen contained in a reference gas;
a frequency control means for controlling a frequency of said pulse-shaped current corresponding to a difference between an the electromotive force measured by said electromotive force-measuring circuit and a comparing
Hamada Yasuhiko
Kato Nobuhide
Kokune Nobuyuki
Burr & Brown
NGK Insulators Ltd.
Nguyen Nam
Olsen Kaj K.
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