Patent ID: 6635161
Filing Date: 2003-10-21
Classification: G01N

Abstract:
An NOx sensor control circuit unit adapted for use by connection to an NOx sensor, said NOx sensor comprising:a first processing chamber which is an internal chamber of the NOx sensor, wherein a measurement gas containing NOx and oxygen is introduced into the first processing chamber via a first diffusion-controlling passage; a second processing chamber which is an internal chamber of the NOx sensor, wherein a gas contained in the first processing chamber is introduced into the second processing chamber via a second diffusion-controlling passage; an oxygen concentration detection element formed of an oxygen-ion conductive solid electrolyte sandwiched between porous electrodes and adapted to measure the oxygen concentration of gas contained in the first processing chamber, said oxygen concentration detection element having an internal resistance that varies with element temperature; a first pump element formed on an oxygen-ion conductive solid electrolyte sandwiched between porous electrodes and adapted to pump out oxygen from the first processing chamber; a second pump element formed of an oxygen-ion conductive solid electrolyte sandwiched between porous electrodes and adapted to pump out oxygen from the second processing chamber; and a heating element for heating the first pump element, the oxygen concentration detection element and the second pump element, said NOx sensor control circuit unit comprising: a first pump element control circuit for controlling a voltage applied to the first pump element to thereby control the partial pressure of oxygen in the first processing chamber such that an output voltage from the oxygen concentration detection element becomes substantially constant; a first pump current detection circuit for detecting a first pump current flowing through the first pump element and for outputting a first pump current detection signal indicative of the detected current flowing through the first pump element; a second pump element control circuit for applying a constant voltage to the second pump element in a direction so as to pump out oxygen from the second processing chamber; a second pump current detection circuit for detecting a second pump current flowing through the second pump element and for outputting a second pump current detection signal indicative of the detected current flowing through the second pump element; an internal-resistance measurement control circuit for measuring the internal resistance of the oxygen concentration detection element; a heating control circuit for controlling said heating element; a microprocessor comprising heating control instruction means for instructing the heating control circuit to control heating performed by the heating element, based on the internal resistance of the oxygen concentration detection element as measured by the internal-resistance measurement control circuit, such that the temperature of the first pump element, the temperature of the oxygen concentration detection element and the temperature of the second pump element approach a target temperature; and oxygen concentration information generation means for determining the oxygen concentration of the measurement gas based on the first pump current detection signal and NOx concentration information generation means comprising a microprocessor including a data storage element containing a two-dimensional numeric table relating NOx concentration to the first and second pump current detection signals for determining the NOx concentration of the measurement gas based on the first and second pump current detection signals, wherein the first pump element control circuit, the first pump current detection circuit, the second pump element control circuit and the second pump current detection circuit are accommodated within a single housing.