Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-01-07
2001-09-18
Warden, Sr., Robert J. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S406000
Reexamination Certificate
active
06290828
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a circuit in which a limited electric current of a limited electric current type oxygen sensor is measured to decide an oxygen concentration.
BACKGROUND OF THE INVENTION
The limited electric current type oxygen sensor is a sensor which can widely detect an oxygen concentration by using an oxygen ion pump function of a zirconia solid electrolyte system. Its basic principle is shown in FIG.
1
. When a cell voltage is applied between electrodes sandwiching the zirconia solid electrolyte, oxygen molecules supplied to a negative electrode on a detective gas side through a diffusion resistance layer become oxygen ions on the negative electrode surface, and an electric current appears by movement of the oxygen ions inside the solid electrolyte from cathode to anode. This electric current value is limited by quantity of oxygen molecules diffused in the diffusion resistance layer.
Therefore, because the quantity of oxygen molecules diffused in the diffusion resistance layer is proportional to the oxygen concentration of the detective gas, the relationship between applied cell voltage and electric current shows a limited electric current characteristic in which every output currents are saturated by constant values as shown in
FIG. 2
, when the oxygen concentration is regulated. These saturated current values become values each proportional to oxygen concentrations in the detective gas, and oxygen concentrations can be detected by reading these electric current values.
In the conventional circuit for measuring oxygen concentration generally used, a constant voltage is supplied to an output of a grounded limited electric current type oxygen sensor through a load resistor and an electric current value flown through an oxygen sensor is detected as a voltage drop in the load resistor to obtain its oxygen concentration. This circuit is found to be difficult to detect accurate oxygen concentration because an output voltage or a cell voltage is changed by changing an oxygen concentration.
FIG. 3
shows a concentration detecting circuit of oxygen sensor disclosed in JP-A-9-166,573 official gazette. In the drawing, a variable reference voltage source
12
is connected between a non-inverting input of an operational amplifier
10
and ground. Moreover, an oxygen sensor
14
is connected between its inverting input and ground, and a detection resistor
16
is connected between the inverting input and its output.
Therefore, a constant cell voltage is intend to be applied the oxygen sensor
14
to obtain its electric current value due to the voltage drop of the detection resistor
16
. However, to obtain the voltage drop corresponding to the oxygen concentration, a subtracter subtracting the cell voltage of oxygen sensor from the output voltage of the operational amplifier
10
is necessary.
The subtracter disclosed in this laid-open official gazette seems to be incomplete invention because it discloses the unnecessary resistor
20
connected to a non-inverting input of a differential amplifier
18
while it does not provide any feedback resistor between an output and an inverting input of the differential amplifier
18
. The addition of this differential amplifier
18
causes measurement errors increase by a resistance value variation due to the deviation of the offset or the temperature change and aged deterioration. Moreover, components of the detection circuit are increased, and it is needless to say that their costs are raised.
Furthermore, a rush current sometimes flows to the oxygen sensor when turning on or off a power supply of the detection circuit is repeated. The higher the rush current is, the longer a recovery time of the sensor to a normal condition is. The oxygen sensor is also likely to be broken down in rarely by the rush current. Therefore, in
FIG. 3
, there is a certain limit to increase the value of the detection resistor
16
in view of the existence of the differential amplifier
18
. In other words, the smaller this resistance value is, the more accurate the output of the differential amplifier is but the more difficult a protection against the rush current of the oxygen sensor is.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a circuit for measuring oxygen concentration having extremely reduced measurement error, improved reliability and low cost.
The circuit for measuring oxygen concentration according to the invention is characterized in that a non-inverting input of an amplifier is grounded; a resistor is connected between an inverting input and an output of said amplifier; and a limited electric current type oxygen sensor is connected between said inverting input of said amplifier and a reference power supply.
In this amplifier, the voltage of the inverting input is operated to be always identical to the ground electric potential. Then, the cell voltage applied to the oxygen sensor is always constant even if the limited electric current value is changed by oxygen concentration. Because an input impedance of the operational amplifier is very high, the limited electric current flown through the oxygen sensor flows almost all through the resistor. Therefore, the output voltage of the operational amplifier accurately corresponds to the limited electric current value of oxygen sensor (oxygen concentration).
REFERENCES:
patent: 4359030 (1982-11-01), Sone et al.
patent: 4664773 (1987-05-01), Suzuki et al.
patent: 4799018 (1989-01-01), Ichikawa et al.
patent: 5423963 (1995-06-01), Fletcher et al.
Hopgood, Calimafde Judlowe & Modolino LLP
Kabushiki Kaisha Riken
Olsen Kaj K.
Warden, Sr. Robert J.
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