Connector for NOx sensor

Details Connector for NOx sensor Connector for NOx sensor

1999-01-28

2002-03-05

Warden, Jill (Department: 1744)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S406000, C123S693000, C439S620040

Reexamination Certificate

active

06352632

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a connector for an NOx sensor equipped with input and output terminals for inputting a signal to and outputting a signal from an NOx sensor that measures an NOx concentration.
NOx sensors for measuring the concentration of nitrogen oxides (NOx) contained in gas to be measured (hereinafter referred to as a measurement gas) are disclosed in, for example, EP-A1-0678740 and SAE Paper No. 960334, pp. 137-142, 1996. Such a conventional NOx sensor is composed of oxygen-ion conductive solid electrolyte layers that form a first measurement chamber and a second measurement chamber. The first measurement chamber communicates with a measurement gas via a first diffusion-controlling layer, and the second measurement chamber communicates with the first measurement chamber via a second diffusion-controlling layer. A first oxygen-pumping cell and an oxygen-concentration-measuring cell are formed adjacent to the first measurement chamber and are each composed of a solid electrolyte layer sandwiched between porous electrodes. A second oxygen-pumping cell is formed adjacent to the second measurement chamber and is composed of a solid electrolyte layer sandwiched between porous electrodes.
BACKGROUND OF THE INVENTION
In order to obtain the concentration of NOx contained in a measurement gas, such as exhaust from an internal combustion engine, without influence of other gas components (such as oxygen, carbon monoxide, and carbon dioxide) contained in the measurement gas, the above Nox sensor undergoes the following control.
Electrical current (hereinafter referred to as first pump current) is caused to flow to the first oxygen-pumping cell such that an output voltage from the oxygen-concentration-measuring cell attains a predetermined value, thereby controlling the concentration of oxygen contained in the first measurement chamber to a very low level (for example, about 1000 ppm). The measurement gas whose oxygen concentration has been thus controlled to the low level enters the second measurement chamber. A constant voltage is applied to the second oxygen-pumping cell in such a direction that oxygen is pumped out from the second measurement chamber. Then, by virtue of a catalytic function of the porous electrodes of the second oxygen-pumping cell, the second oxygen-pumping cell decomposes NOx contained in the measurement gas into nitrogen and oxygen and pumps out the thus-obtained oxygen from the second measurement chamber. At this time, on the basis of current flowing through the second oxygen-pumping cell, the NOx concentration of the measurement gas is obtained.
The first pump current is controlled such that the concentration of oxygen contained in the first measurement chamber attains low level, but not zero, for the following reason. If the concentration of oxygen contained in the first measurement chamber is controlled to zero, the porous electrodes that constitute the first oxygen-pumping cell cause decomposition of NOx contained in the measurement gas that has entered the first measurement chamber. This disables measurement of NOx concentration through use of the second oxygen-pumping cell.
The above NOx sensor can obtain not only the concentration of NOX contained in the measurement gas from the second pump current but also the concentration of oxygen contained in the measurement gas from the first pump current.
However, a characteristic indicative of the relationship between NOx concentration and second pump current and that between oxygen concentration and first pump current somewhat differ among NOx sensors. If, on the assumption that NOx sensors have certain common characteristics indicative of the relationships, the NOx concentration and the oxygen concentration of a measurement gas are obtained from the second and first pump currents, respectively, detected by the NOx sensor, some NOx sensors may fail to provide sufficiently high measurement accuracy.
An object of the present invention is to provide sufficiently high accuracy in measurement by an NOx sensor regardless of variations in characteristics among NOx sensors.
According to the present invention there is provided a connector for an NOx sensor, which connector comprises input and output terminals for inputting a signal to and outputting a signal from the NOx sensor. The NOx sensor comprises a first measurement chamber and a second measurement chamber. The first measurement chamber comprises a first oxygen-pumping cell and an oxygen-concentration-measuring cell and communicates with a measurement gas via a first diffusion-controlling layer. The first oxygen-pumping cell and the oxygen-concentration-measuring cell are each formed of an oxygen-ion conductive solid electrolyte layer sandwiched between porous electrodes. The second measurement chamber comprises a second oxygen-pumping cell and communicates with the first measurement chamber via a second diffusion-controlling layer. The second oxygen-pumping cell is formed of an oxygen-ion conductive solid electrolyte layer sandwiched between porous electrodes. A first pump current is caused to flow to the first oxygen-pumping cell such that an output voltage from the oxygen-concentration-measuring cell is maintained at a constant value so as to control the concentration of oxygen contained in the first measurement chamber to a constant level. A constant voltage is applied to the second oxygen-pumping cell in such a direction that oxygen is pumped out from the second measurement chamber. There is detected a second pump current that flows through the second oxygen-pumping cell as a result of the application of the constant voltage and according to the concentration of NOx contained in a measurement gas.
The connector for an NOx sensor is equipped with a label resistor having resistance corresponding to at least either a relationship between the concentration of oxygen contained in the measurement gas and the first pump current or a relationship between the concentration of NOx contained in the measurement gas and the second pump current, and is equipped with a pair of label signal output terminals connected to opposite ends of the label resistor.
The thus-configured connector for an NOx sensor is used in the following manner. The resistance of the label resistor is detected via the label signal output terminals to thereby identify characteristics of the NOx sensor concerned. Subsequently, the first pump current is caused to flow to the first oxygen-pumping cell such that an output voltage from the oxygen-concentration-measuring cell becomes constant, i.e., such that the concentration of oxygen contained in the first measurement chamber becomes constant. Also, a constant voltage is applied to the second oxygen-pumping cell in such a direction that oxygen is pumped out from the second measurement chamber. The NOx concentration and the oxygen concentration of the measurement gas are respectively obtained from the second pump current, which flows through the second oxygen-pumping cell according to the concentration of NOx contained in the measurement gas, and the first pump current, which flows according to the concentration of oxygen contained in the measurement gas, as well as on the basis of a characteristic(s) of the NOx sensor identified by the resistance of the label resistor.
SUMMARY OF THE INVENTION
Accordingly, through use of the connector of the present invention for an NOx sensor, a characteristic (at least either relationship between NOx concentration and second pump current or relationship between oxygen concentration and first pump current) of an NOx sensor can be identified by the resistance of the label resistor. Through employment of the identified characteristic(s) of the NOx sensor, the NOx concentration and the oxygen concentration of the measurement gas can be obtained at a sufficiently high degree of accuracy.
Through identification of the relationship between NOx concentration and second pump current, variations in an obtained value of NOx concentration among NOx sensors can be compensated for. Through iden

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