Sensor control apparatus and sensor control system
A sensor control apparatus for a gas sensor which detects a particular gas component contained in a gas under measurement. In the case where humidity detection is performed using an electromotive-force-type gas sensor, a sensor control apparatus changes a target voltage of an electromotive force cell to a second target voltage for humidity detection and then to a third target voltage which is lower than a first target voltage which is a target voltage used for detecting the concentration of the particular gas component. In the case where humidity detection is performed using a limiting-current-type gas sensor, the sensor control apparatus changes the voltage applied to a pump cell to a second application voltage for humidity detection and then to a third application voltage which is lower than a first application voltage which is an application voltage used for detecting the concentration of the particular gas component.
1. A sensor control apparatus configured for connection to a gas sensor, the gas sensor including an electromotive force cell having a first solid electrolyte body and a pair of first electrodes formed on the first solid electrolyte body and a pump cell having a second solid electrolyte body and a pair of second electrodes formed on the second solid electrolyte body and which detects a particular gas component contained in a gas under measurement, the sensor control apparatus comprising:
current control means for controlling pump current flowing between the pair of second electrodes such that an electromotive force cell voltage generated between the pair of first electrodes becomes equal to a target voltage;
pump current detection means for detecting the pump current;
voltage setting means for setting the target voltage to a first target voltage, a second target voltage, and a third target voltage, in this order, the first target voltage being a voltage at which moisture contained in the gas under measurement does not substantially dissociate and which is used when the particular gas component is detected, the second target voltage being a voltage at which moisture contained in the gas under measurement dissociates, the third target voltage being lower than the first target voltage,
control means for detecting humidity of the gas under measurement based on a difference between the pump current when the target voltage is the first target voltage and the pump current when the target voltage is the second target voltage,
wherein the control means causes the pump current detection means to detect the pump current when a predetermined time has elapsed after the target voltage has been changed from the third target voltage to the first target voltage by the voltage setting means, and
wherein the first, second, and third target voltages are positive voltages.
2. The sensor control apparatus according to claim 1 , wherein the gas sensor further includes a hollow gas detection chamber having opposite wall surfaces defined by the first and second solid electrolyte bodies, and
the third target voltage is set to a value configured to cause oxygen to be pumped into the gas detection chamber of the gas sensor.
3. A sensor control system comprising:
a gas sensor which includes an electromotive force cell having a first solid electrolyte body and a pair of first electrodes formed on the first solid electrolyte body and a pump cell having a second solid electrolyte body and a pair of second electrodes formed on the second solid electrolyte body; and
the sensor control apparatus as claimed in claim 1 .
4. The sensor control system according to claim 3 , wherein the gas sensor further includes a hollow gas detection chamber having opposite wall surfaces defined by the first and second solid electrolyte bodies, and
the third target voltage is set to a value configured to cause oxygen to be pumped into the gas detection chamber of the gas sensor.