Measuring and testing – Gas analysis – With compensation detail
Reexamination Certificate
1999-06-01
2001-09-18
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Gas analysis
With compensation detail
C073S023310, C073S025010, C073S031050, C204S424000
Reexamination Certificate
active
06289719
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for operating a gas sensor having two sensor electrodes mounted in an ion-conducting ceramic which is heatable by an electric heater. The output signals of the sensor electrodes are evaluated in an evaluation circuit connected downstream of the sensor electrodes.
BACKGROUND OF THE INVENTION
Such ceramic gas sensors have been known for a long time. These gas sensors are used for detecting toxic exhaust gases, for example, of automobile exhaust gases or burner exhaust gases. The ion-conducting ceramic must be heated for ensuring a proper function and this ceramic can, for example, be zircon oxide (ZrO
2
). The heater is embedded in an insulating layer which, in most cases, comprises aluminum oxide (Al
2
O
3
). The insulation of the heater is, however, problematic especially at high temperatures because leakage resistances occur from the heater to the sensor electrodes in the order of magnitude of several megaohms. In this case, the leakage current, which flows from the heater, is superposed on the actual measuring current and causes relatively large errors. This is so because in gas sensors of this type, the measuring result lies only at a few microamperes.
Furthermore, in gas sensors, there is a large scattering thereof and a significant deterioration occurs. For this reason, the leakage current cannot be compensated simply by a fixed corrective quantity.
For this reason, it is, for example, known to use an insulating amplifier for measuring the measurement current. With this insulating amplifier, the actual measurement circuit is operated so as to be separated with respect to potential and the measurement quantity is transmitted in an insulated manner for further signal processing. The potential separation can, for example, be achieved via transformers or optoelectronically. In this case, the leakage current cannot flow off and therefore vanishes. As a consequence of this, only the wanted measurement current is measured. The high requirement of expensive and non-integratable components or components which are only integratable with difficulty is disadvantageous.
Furthermore, a method and an arrangement for detecting a fault state of a &lgr;-probe and the measures taken as a consequence of a fault signal are presented in EP 0 403 615 B1. This fault signal is outputted for a detected fault state. In the method, impermissibly large fault causes are diagnosed by a correlation method during operation of the &lgr;-probe. Shunt currents exist only during operation of the heater. For this reason, the probe heater is switched off in this measuring method for detecting a fault state because, in this case, no shunt voltage and therefore no incorrectness of the measuring result is present. The difference between the probe voltage, which is measured for a switched-on heater and a switched-off heater, yields the shunt voltage. This method is relatively complex and requires a rather complex circuit.
SUMMARY OF THE INVENTION
The invention is therefore based on the object to provide a method for operating a gas sensor with a precision of detection of the measurement current as precise as possible with the least amount technical complexity, that is, a method which makes possible a very substantial elimination of a disturbance caused by a leakage current.
The object is achieved in a method for operating a gas sensor of the kind described above in that a pulsewidth modulated heater voltage signal is applied to the heater and that a compensation signal is superposed on the output signal of the sensor electrodes. The compensation signal is essentially counterclocked (inverted) to the pulsewidth modulated heater voltage signal.
One can eliminate the disturbance signal in an especially simple manner by applying a pulsewidth modulated heater voltage signal and by superposing a compensation signal on the output signal of the sensor electrode which defines essentially a counterclocked signal to the pulsewidth modulated heater voltage signal. The disturbance signal is coupled to the sensor signal via the heater voltage. This disturbance signal has essentially the shape of the pulsewidth modulated heater voltage signal with capacitive in-coupling peaks and is substantially compensated by the compensating signal which is formed as a counterclocked signal, that is, as an inverse signal to this heater voltage signal.
The superposition of the compensation signal can, in principle, take place in the most different way. An advantageous embodiment provides that the compensation signal is superposed on the output signal before this output signal is supplied to the evaluation circuit. In this case, the superposition takes place in a manner easy to realize with analog circuitry and one need only process an already compensated signal in the evaluation circuit.
In another advantageous embodiment, it is provided that the compensation signal is superposed on the output signal of the evaluation circuit. In this case, the superposition can likewise be superposed, that is, added with analog circuitry in a simple manner.
In a further advantageous embodiment, it is provided that the compensation signal is superposed on the sensor signal in the evaluation circuit, preferably digitally. This embodiment has especially the advantage that the evaluation circuit, which is anyway present, can be utilized for processing the compensation signal so that additional circuit components for superposing the compensation signal are not required.
So far, no details have been given with respect to the pulsewidth modulated heater voltage. An especially advantageous embodiment provides that the pulsewidth modulated heater voltage signal has flat flanks. This advantageous embodiment especially makes possible a further reduction of the disturbance signal. Because of these flat flanks of the pulsewidth modulated heater voltage signal or, stated otherwise, because the pulsewidth modulated heater voltage with flat flanks is clocked, high frequencies are avoided in the heater voltage signal which could lead to disturbances. This is explained in greater detail below. The flat flanks are slowly increasing and slowly falling flanks of the pulsewidth modulated heater voltage signal.
The generation of a heater voltage signal of this kind having flat flanks can take place in the most different ways. An advantageous embodiment provides that the pulsewidth modulated heater voltage signal with flat flanks is generated by a MOSFET switch having a gate input line. A series resistor is connected in this input line. An RC-lowpass is formed by this series resistor in the gate input line together with the gate capacitors. The RC-lowpass makes possible a slow switching of the pulsewidth modulated heater voltage signal and therefore a reduction of the high frequency components in the signal.
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Bloemer Bernhard
Schumann Bernd
Cygan Michael
Larkin Daniel S.
Ottesen Walter
Robert & Bosch GmbH
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