Electricity: measuring and testing – Determining nonelectric properties by measuring electric...
Reexamination Certificate
2000-02-24
2001-10-09
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Determining nonelectric properties by measuring electric...
C324S425000, C324S378000, C204S424000
Reexamination Certificate
active
06300754
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a circuit for an NOx measurement sensor.
In order to measure the NOx concentration in a gas, for example in the exhaust gas of an internal combustion engine, it is known to employ a thick-layer measurement sensor. Such a measurement sensor is described, for example, in the publication by N. Kato et al., titled “Thick Film ZrO
2
NOx Sensor for the Measurement of Low NOx Concentration”, Society of Automotive Engineers, Publication Number 980170, 1989, or in the publication by N. Kato et al., titled “Performance of Thick Film NOx Sensor on Diesel and Gasoline Engines”, Society of Automotive Engineers, Publication Number 970858, 1997. The measurement sensor has two measurement cells and is composed of a zirconium oxide that conducts oxygen ions. It implements the following measurement concept: in a first measurement cell, to which the gas to be measured is fed via a diffusion barrier, a first oxygen concentration is established by use of a first oxygen ion pump current, the intention being that no decomposition of NOx takes place. In a second measurement cell, which is connected to the first via a diffusion barrier, the oxygen content is lowered further by use of a second oxygen ion pump current. The decomposition of NOx at a measurement electrode leads to a third oxygen ion pump current, which is a measure of the NOx concentration. The entire measurement sensor is in this case brought to an elevated temperature, for example 750° C., by an electric heater.
In order to measure the third pump current, representing the measure of the NOx concentration, the current is normally fed through a measurement resistor, and the voltage drop across the resistor is tapped off and measured. In order to achieve the required accuracy, this measurement has to be carried out with a relatively high resolution. If it were wished to use a microcontroller that is normally employed for such measurement tasks, it has been shown that an analog/digital converter with a resolution of 8 bits is not adequate for this. The difficulties of current measurement are therefore to be seen essentially in the necessary resolution of the analog/digital converter and in measurement errors in the converter (offset, gain, etc.). It is therefore necessary to employ more expensive microcontrollers, of which only a few models are available on the market, so that the choice for the production or configuration is restricted here.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a circuit for an NOx measurement sensor which overcomes the above-mentioned disadvantages of the prior art devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for a measurement sensor that registers an NOx concentration in a gas, the measurement sensor having an external electrode to be exposed to the gas to be measured, measurement cells including a first measurement cell and a second measurement cell connected to the first measurement cell, a measurement electrode disposed in the second measurement cell, a reference electrode exposed to ambient air, a solid-state electrolyte having the measurement cells disposed therein and the measurement electrode and the reference electrode making contact with the solid-state electrolyte, the circuit containing:
a first circuit configuration for establishing in the first measurement cell an oxygen concentration differing from an oxygen concentration in the gas to be measured;
a second circuit configuration for establishing in the second measurement cell an oxygen concentration differing from the oxygen concentration in the first measurement cell; and
a third circuit configuration for pumping oxygen ions formed from NOx out of the second measurement cell from the measurement electrode to the external electrode in a pump current, the third circuit configuration including:
a controller connected to the measurement electrode and the reference electrode and generating a setting voltage; and
a voltage-controlled current source for driving the pump current and receiving the setting voltage generated by the controller, the controller taping off a Nernst voltage between the measurement electrode and the reference electrode and regulating the setting voltage such that a predicted Nernst voltage is established, a value of the setting voltage being a measure of the pump current and consequently of the NOx concentration to be measured.
According to the invention, the measurement of the pump current itself is entirely dispensed with. Instead, the circuit according to the invention provides a voltage-controlled current source that drives the pump current in question. The measurement signal which is registered is then no longer the driven current but the setting voltage of the voltage-controlled current source. Since the setting voltage is predefined directly by a controller, for example a microcontroller, it no longer has to be measured, but is defined by the microcontroller and is therefore known to the latter.
The use of the voltage-controlled current source therefore has a number of advantages: the measurement of the pump current, which is normally needed in each circuit configuration, via a measurement resistor and a suitable voltage tap, can therefore be dispensed with. Normally, an analog/digital port on a microcontroller would have to be employed to measure each pump current. Furthermore, the analog/digital port would have to have a high resolution, in order to be able to measure the current with sufficient accuracy. The fact that the current measurement is dispensed with and the known setting voltage occurs in its place results in that an 8-bit microcontroller, which is more cost-effective and available in many models, can be employed.
Also dispensed with are any measurement errors in the determination of the pump currents.
It is advantageous that all the pump currents which have to be driven in order to operate the NOx measurement sensor are generated by the voltage-controlled current source whose setting voltage is predefined by the controller, so that the measurement of all the currents can be dispensed with.
In an advantageous refinement, the setting voltage is generated by the microcontroller from a pulse-width-modulated signal, which is applied to a switching transistor which connects a reference circuit to a low-pass filter which, from the signal, uses the pulse duty factor of the pulse-width-modulated signal to provide a proportional output voltage, which serves as the setting voltage.
Finally, the driven pump current is independent of the impedance of the pump current circuit and is determined only by, for example, the known and accurate pulse duty factor of the pulse-width-modulated signal from the microcontroller.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit for an NOx measurement sensor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
REFERENCES:
patent: 6214207 (2001-04-01), Miyata et al.
“Thick Film ZrO2 NOx Sensor for the Measurement of Low NOx Concentration”, Nobuhide Kato et al., Society of Automotive Engineers, 1998, pp. 69-77.
“Performance of Thick Film NOx Sensor on Diesel and Gasoline Engines”, Nobuhide Kato et al., Society of Automotive Engineers, 1997, pp. 199-206.
Greenberg Laurence A.
Lerner Herbert L.
Leroux E P
Metjahic Safet
Siemens Aktiengesellschaft
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