Gas concentration measuring apparatus designed to enhance response of sensor

Abstract

A gas concentration measuring apparatus is provided which may be employed in measuring the concentration of oxygen contained in exhaust emissions of automotive engines. The apparatus includes a gas sensor equipped with a solid electrolyte body and a pair of electrodes affixed to opposed surfaces of the solid electrolyte body. The apparatus includes an applied voltage controller which works to apply the voltage to the electrodes of the gas sensor to create an electric current as a function of a concentration of gas to be measured. The applied voltage controller is designed to have an enhanced gain of the voltage to the value of the current, as produced by the gas sensor, in a predetermined frequency band, thereby increasing the response of the gas sensor without inducing the oscillation of the voltage to be applied to the gas sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.

In the drawings:

FIG. 1 is a circuit diagram which shows an electric structure of a sensor control circuit of a gas concentration measuring apparatus according to the invention;

FIG. 2 is a transverse sectional view which shows a sensor element used in the gas concentration measuring apparatus as illustrated in FIG. 1;

FIG. 3 shows an example of an applied voltage-to-output current map for use in determining a target voltage to be applied to the sensor element as illustrated in FIG. 2;

FIG. 4 is a graph which shows frequency characteristics of a sensor element, a sensor control circuit, and a combination thereof;

FIG. 5 is a graph which shows changes in gain of output of two types of sensors;

FIG. 6 is a graph which shows increases in gain of frequency characteristics of a sensor control circuit, and a combination of a sensor element and the sensor control circuit;

FIG. 7 is a circuit diagram which shows the first modification of a sensor control circuit of a gas concentration measuring apparatus;

FIG. 8( a) is a graph which shows increases in gain of frequency characteristics of the sensor control circuit, as illustrated in FIG. 7, and a combination of a sensor element and the sensor control circuit;

FIG. 8( b) is a graph which shows an increase in level of an output of a sensor current meter of the sensor control circuit representing the current, as produced by the sensor element in FIG. 7;

FIG. 9 is a circuit diagram which shows the second modification of a sensor control circuit of a gas concentration measuring apparatus;

FIG. 10 is a circuit diagram which shows the third modification of a sensor control circuit of a gas concentration measuring apparatus;

FIG. 11( a) is a graph which shows increases in gain of an output of a sensor current meter of the sensor control circuit, as illustrated in FIG. 10; and

FIG. 11( b) is a graph which shows an increase in level of an output of the sensor current meter of the sensor control circuit representing the current, as produced by the sensor element in FIG. 10.

Claims

1. A gas concentration measuring apparatus comprising: a gas concentration sensor which includes a solid electrolyte body and a pair of electrodes affixed to opposed surfaces of the solid electrolyte body, said gas concentration sensor being responsive to application of voltage to the electrodes to produce a sensor current that is an electric current as a function of a concentration of a selected component of gas;a current measuring circuit working to measure the sensor current, as produced by said gas concentration sensor; andan applied voltage controller working to change the voltage to be applied to said gas concentration sensor as a function of a value of the sensor current, as measured by said current measuring circuit, said applied voltage controller being designed to have an enhanced gain of the voltage to the value of the sensor current in a given frequency band.

2. A gas concentration measuring apparatus as set forth in claim 1, wherein said applied voltage controller has a low-pass filter circuit installed at an output stage thereof from which the voltage is outputted and applied to said gas concentration sensor, the low-pass filter being designed to have a dual filter structure.

3. A gas concentration measuring apparatus as set forth in claim 2, wherein a cut-off frequency of the low-pass filter circuit is approximately 10 Hz.

4. A gas concentration measuring apparatus as set forth in claim 1, wherein said applied voltage controller has a band-pass filter installed at an output stage thereof from which the voltage is outputted and applied to said gas concentration sensor.

5. A gas concentration measuring apparatus as set forth in claim 4, wherein the band-pass filter has a low-frequency side cut-off frequency of approximately 1 Hz and a high-frequency side cut-off frequency of approximately 5 Hz.

6. A gas concentration measuring apparatus as set forth in claim 1, wherein said applied voltage controller has a current path through which an electric current that is a function of the sensor current flows, said applied voltage controller also including a capacitor connected in parallel to the current path.

7. A gas concentration measuring apparatus as set forth in claim 6, wherein said applied voltage controller includes a parallel circuit which is made of the capacitor and a resistor connected to the capacitor in parallel and connected in series with the current path, and wherein the parallel circuit serves as a high-pass filter whose cut-off frequency is approximately 5 Hz.

8. A gas concentration measuring apparatus comprising: a gas concentration sensor which includes a solid electrolyte body and a pair of electrodes affixed to opposed surfaces of the solid electrolyte body, said gas concentration sensor being responsive to application of voltage to the electrodes to produce a sensor current that is an electric current as a function of a concentration of a selected component of gas;a current measuring circuit working to measure the sensor current, as produced by said gas concentration sensor, to output a gas concentration signal representing the concentration of the selected component of the gas; anda differential amplifier installed at an output stage of said current measuring circuit from which the gas concentration signal is outputted, said differential amplifier being designed to have an enhanced gain of output of the gas concentration signal in a given frequency band.

9. A gas concentration measuring apparatus as set forth in claim 8, wherein said current measuring circuit includes an input resistor leading to an input of said differential amplifier and a series circuit connected in parallel to the input resistor, the series circuit being made up of a capacitor and a resistor which are connected in series.

10. A gas concentration measuring apparatus as set forth in claim 9, wherein the series circuit serves as a high-pass filter whose cut-off frequency is approximately 1 Hz.