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...
C324S378000, C324S425000, C204S424000
Reexamination Certificate
active
06300753
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention lies in the field of fluid sensors. The invention relates to a circuit for a 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 conventional to employ a thick-layer measurement sensor. Such a measurement sensor is described, for example, in the publication by N. Kato et al., “Thick Film ZrO
2
NOx Sensor for the Measurement of Low NOx Concentration”, Society of Automotive Engineers, Publication number 980170, 1989, or in N. Kato et al., “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. The sensor implements the following measurement concept: in a first measurement cell, to which the gas to be measured is fed through a diffusion barrier, a first oxygen concentration is established by 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 cell through a diffusion barrier, a second oxygen ion pump current lowers the oxygen content further. 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 brought to an elevated temperature, for example 750° C., by an electric heater.
In order to establish the oxygen ion pump currents, the Nernst voltage is tapped off in the respective measurement cells. Digital microcontrollers are typically employed for the controllers. Two A/D ports on the A/D converter of the microcontroller are needed for one Nernst voltage because the voltage can be measured only with respect to the reference potential of the microcontroller. Accordingly, the voltage across the measurement electrode is typically measured with respect to the reference potential of the microcontroller. In addition, the voltage across the reference electrode is measured with respect to the reference potential of the microcontroller. The Nernst voltage is obtained by forming the difference between these two voltages. If an 8-bit converter and 5 V range are employed, the resolution is then 20 mV. However, the accuracy of the detection of the reference variable is inadequate in the control loop.
Therefore, microcontrollers with 10-bit converters are needed. Such microcontrollers are relatively expensive and available only in a few models. As such, the choice for the production or configuration is restricted.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a circuit for a NOx measurement sensor that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that detects a NOx concentration in a gas while avoiding the difficulties and disadvantages resulting from the detection of the Nernst voltage.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a circuit for a measurement sensor detecting a NOx concentration in a gas, including a first measurement cell, a second measurement cell connected to the first measurement cell, the first and second measurement cells disposed in a solid-state electrolyte, a first circuit configuration tapping off a first Nernst voltage serving as a first reference variable for establishing an oxygen concentration in the first measurement cell differing from an oxygen concentration in a gas to be measured, a second circuit configuration tapping off a second Nernst voltage serving as a second reference variable for establishing an oxygen concentration in the second measurement cell differing from an oxygen concentration in the first measurement cell, a third circuit configuration tapping off a third Nernst voltage serving as a third reference variable for driving a pump current of oxygen ions originating from NOx out of the second measurement cell, at least one conditioning circuit, and at least one digital controller having at least one control point, at least one of the first, second, and third Nernst voltages being fed to the at least one controller through the at least one conditioning circuit for shifting and amplifying the at least one of the first, second, and third Nernst voltages such that at the at least one controller at least one of the first, second, and third reference variables lies in a range around the at least one control point of the at least one controller, and a reference potential of a conditioned at least one of the first, second, and third Nernst voltages equals that of the at least one controller.
According to the invention, a conditioning circuit conditions the Nernst voltage tapped off before it is fed to the microcontroller. The conditioning circuit amplifies the Nernst voltage and shifts it such that it is fed to the A/D port of the digital controller in a range around the control point of the controller. Given a gain of, for example, a factor of 10, the range of interest of about 500 mV is widened to 5 V and shifted. As a result, even using an 8-bit converter, a resolution of, for example, 2 mV can be achieved.
Such resolution is adequate for the reference variable. The conditioning circuit is advantageously constructed as an analog circuit, which performs both the formation of the difference between the voltages across the measurement electrode and the reference electrode and also performs the shifting and the amplification. Depending on the configuration of the circuit, the shift is made in either a positive or negative voltage direction.
In accordance with another feature of the invention, the at least one conditioning circuit includes an output, an electrode, a reference electrode, an operational amplifier with an operational amplifier output connected to the output, a conditioned at least one of the first, second, and third Nernst voltages being present at the output, an inverting input, and a non-inverting input, a first resistor connected to the inverting input and to the electrode for tapping off a first potential, a second resistor connected to the inverting input and to the operational amplifier output, a third resistor connected to the non-inverting input and to the reference electrode, and a fourth resistor connected to the non-inverting input and to ground.
In accordance with a concomitant feature of the invention, the at least one controller has at least one A/D converter converting a conditioned at least one of the first, second, and third Nernst voltages from analog to digital with 8-bit resolution.
The configuration results in a two-part advantage, on one hand, one A/D port on the A/D converter of the digital controller is saved, and, on the other hand, a cost-effective 8-bit converter is adequate for use.
Other features that 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 a NOx measurement sensor, it is nevertheless not intended to be limited to the details shown, because 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.
“Performance of Thick Film NOx Sensor on Diesel and Gasoline Engines”, Nobuhide Kato et al., Society of Automotive Engineers, Inc., Publication No. 970858, 1997, pp. 199-206.
“Thick Film ZrO2 NOx Sensor for the Measurement of Low NOx Concentration”, Nobuhide Kato et al., Society of Automotive Engineers, Inc., Publication No. 980170, 1998, pp.
Chemisky Eric
Walde Tim
Greenberg Laurence A.
Lerner Herbert L.
Leroux E P
Metjahic Safet
Siemens Aktiengesellschaft
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