Patent Application
20080150740
This application claims the benefit of the filing date under 35 U.S.C. § 119(a)-(d) of German Patent Application No. 10 2006 059 652.8, filed Dec. 18, 2006.
The present invention relates to a method of processing an analog sensor signal, the method using an operational amplifier. In particular, the present invention relates to a method of this type for use in a gas sensor arrangement, to an associated gas sensor arrangement and a measured value processing device.
Known gas sensor arrangements comprise a radiation-emitting radiation source, a gas measuring chamber, which may be filled with a gas for measurement which contains at least one analyte to be measured, and at least one radiation-detecting detector device, which generates an output signal dependent on the presence and/or the concentration of the analyte. Such gas sensor arrangements are known for detecting the widest possible range of analytes, for example carbon dioxide or methane. Conventional gas sensors are based on the property of many polar gases to absorb radiation in the infrared range. In this case, the IR light is positioned to bring the molecules into excited states by inciting rotational and vibration oscillations through cooperation with the dipole moment of the polar molecule. In this way, the thermal energy of the IR light is transmitted to the gas and in the same way the intensity of an IR beam passing through the gas volume is reduced. In this respect, absorption occurs in accordance with the excitation states at a wavelength in each case characteristic of the relevant gas, in the case of CO2 for example at 4.25 μm.
Carbon dioxide detection in particular is currently growing in importance in a plurality of fields of application. Thus, for example, in the automotive field carbon dioxide detection may be used to monitor the CO2 content in the passenger compartment air for increasing the energy efficiency of the heating and air-conditioning systems, so that fresh air is supplied by means of corresponding fan vent actuation only when necessary, i.e. in the event of an elevated CO2 concentration. Furthermore, modern automotive air-conditioning systems are also based on CO2 as a coolant, such that CO2 gas sensors may fulfill a monitoring function in the automotive field in connection with CO2 leakage in the event of faults. In the automotive field in particular, gas sensors have to satisfy the most stringent requirements for robustness, reliability and miniaturizability. For safety applications, moreover, the response time of the sensor must not exceed given limit values.
German patent application DE102005032722 describes a gas sensor arrangement and a measurement method with early warning. In particular, this application relates to the radiation source, which emits radiation in the form of pulses. In addition, German patent application DE102006019705.4 relates to a method of processing time-discrete measured values, which may be described with regard to their time profile by means of a time function. The method according to this application uses a measured value filter to achieve a desired transient response.
In many gas sensor arrangements, “pyrosensors” are used as the detector device for evaluating the IR radiation. The analog sensor signal which is output by such a pyrosensor has a high offset voltage and only a small amplitude due to the nature of the measurement. For further processing and analysis of the signal it is helpful to remove this offset voltage, wherein, however, the amplitude of the analog sensor signal should be amplified.
In known arrangements for example, as shown in
The four curves in
The capacitance of the coupling capacitor 306 is highly temperature-dependent, however, which may cause difficulties in the automotive field, since a given operating temperature cannot be guaranteed. In the temperature range from −40° C. to +85° C. the capacitor changes its high capacitance for example, and moreover this property is not stable in the long term. This coupling capacitor 306 is also unsuitable for easy integration into assemblies because of its size, and is therefore of complex and expensive design.
An object underlying the present invention, among others, is to provide an improved method of processing analog sensor signals, which enables temperature-independent, rapid and robust DC voltage suppression.
A method of processing an analog sensor signal according to the invention includes first, amplification of the analog sensor signal by an operational amplifier. A measurement of the amplified analog sensor signal is then performed followed by a comparison of the amplified analog sensor signal with a threshold value. A DC voltage is generated as a function of a difference between the amplified analog sensor signal and the threshold value. A composite signal is then formed from the analog sensor signal and the DC voltage and the composite signal is amplified and output as an output signal.
The invention is explained more fully below with reference to exemplary embodiments illustrated in the attached drawings. Similar or corresponding details of the subject matter according to the invention are provided with the same reference signs. In the drawings:
The structure of the gas sensor arrangement according to the invention and the mode of operation of the method according to the invention for amplifying the analog sensor signal will be explained in greater detail below with reference to the Figures.
As shown in
Here, the emitted infrared radiation 116 is passed through the gas measuring chamber 104 to the detector unit 108. An optical filter is arranged on the detector unit 108, which filter allows through only the wavelength range in which the gas molecules to be detected are absorbent. Other gas molecules normally absorb at other specific wavelengths and therefore have no influence on the amount of radiation which reaches the detector unit 108. Any suitable infrared detectors may be used as the detector unit and the one according to the invention may be adapted to a particular detector type.
For example, the detector may be a pyroelement, an infrared thermopile or a photodiode. The respectively suitable detector should be selected in accordance with the respective requirements. For instance, a photodiode offers the advantage that it represents a comparatively inexpensive component, while the thermocouple pile, as the thermopile detector is also known, offers the advantage of a particularly elevated and uniform absorption of the radiation in the selected range of the spectrum. Finally, pyroelectric sensors have the advantage of highly elevated sensitivity and miniaturized production.
The infrared signal is pulsed by the radiation source 102, in order to generate a dynamic signal, since the pyroelectric sensor only responds to variations. Thus, the measured values supplied by the detector unit are present in the form of time-discrete values, which, as indicated in
A controller 120 on the one hand drives the radiation source 102 and on the other hand receives the detector signals from the detector unit 108 and further processes them according to the principles of the present invention. In particular, the controller comprises a filter unit 109, which performs conversion of the detector signal into an amplified output signal without offset.
As shown in
The mode of operation of the measured value processing device according to the invention is described below.
A microcontroller-controlled voltage is added to the analog sensor signal in the operational amplifier 402, and the difference is simultaneously amplified to generate an output signal. The microcontroller 406 measures the output signal, for which purpose an analog-to-digital converter 404 is used, which converts the amplified analog output signal from the operational amplifier 402 into a digital input signal for the microcontroller 406. The microcontroller 406 produces a signal correction by means of the digital input signal and the predetermined threshold value.
This threshold value may be determined by the operating voltage of the microcontroller 406, but other factors may also come into play. If the output signal of the operational amplifier 402 is not in the desired range, countermeasures are then taken using a pulse-width-modulated signal smoothed by a filter 410, and thus a DC voltage, in order to suppress the offset signal. The result is amplification of the pure analog signal without offset voltage, as may be seen in the time profile in
In the invention the analog sensor signal corresponds to the signal in
The microcontroller 406 may, for example, detect in the time profile that the voltage of the output signal is reaching its maximum operating voltage. The signal arrives at the analog-to-digital converter 404 internal to the microcontroller 406, which converter only admits signals up to the reference voltage. Then the pulse-width-modulated signal is varied in such a way (smaller pulse-width ratio) that a smaller constant voltage is generated downstream of the filter 410. The operational amplifier 402 forms the sum of the two signals, which has then in this case become smaller, and simultaneously amplifies the result in order to achieve better signal evaluation. The signal output of the operational amplifier 402 is then comparable with the signal “U OP2” in
During measured value recording over the time profile the pulse-width-modulated signal (pulse-width ratio) has naturally to be constant. After measurement, adjustment may optionally be performed (pulse-width ratio is varied), and then new measurement is started.
The operational amplifier then adds this direct voltage to the analog sensor signal in step 510 and amplifies this composite signal in step 512, in order then to output an output signal.
The greater the difference between the amplified analog sensor signal and the threshold value in step 508, the smaller should be the value of the DC voltage to be added, so that the output signal of the operational amplifier has only a small offset. Conversely, the DC voltage to be added should have a higher value if there is a big difference between the amplified analog sensor signal and the threshold value. The reason for this is that the output signal of the operational amplifier should be amplified to the greatest possible extent without the operating voltage of the microcontroller being exceeded. The available amplitude should thus be utilized. If the amplified analog sensor signal, i.e. the amplified output signal of the pyrosensor, is already close to the operating voltage of the microcontroller, then not much more may be added thereto, but if the amplified output signal of the pyrosensor is weak, a larger DC voltage may be added thereto before the maximum operating voltage of the microcontroller is achieved. It is here assumed that the reference variable is always the amplified analog sensor signal, i.e. the output signal of the operational amplifier.
For most applications in gas sensors it is not only the final value of the signal but rather in particular the slope of the signal which is the variable of interest.
This will be explained in greater detail with reference to
According to the invention, simple, temperature-independent, linear control is achieved over the entire voltage range. It is advantageous that according to the invention a small number of microcontroller lines are necessary, for example just one signal line, so making possible a selection of more favorable, smaller processors. This also means a low current load and that independent control is possible by means of autonomous microcontroller peripherals, in particular by means of an integrated UART (Universal Asynchronous Receiver/Transmitter). This results in relieving of the microcontroller CPU (Central Processing Unit), which in turn means that simple regulating mechanisms may be used with low program memory consumption.
Use according to the invention of a pulse-width-modulated signal for controlling the DC voltage has the advantage of time optimization due to the possibility of rapid range variation.
The solution according to the invention also has the advantage that, in the event of a sudden increase in gas concentration, more rapid processing of the analog sensor signal may be guaranteed.
The advantageous features of the measured value processing according to the invention may be exploited in particular in gas sensor arrangements which are used to detect carbon dioxide, for example in the automotive field, both for monitoring CO2 escaping from leakage points and for checking the air quality in the passenger compartment. It goes without saying, however, that the principles according to the invention may also be used in connection with the detection of any other desired gases and are of significance for all sensors in which a measurement signal with a high DC voltage content needs to be evaluated.
With the measured value processing according to the invention, it is possible in particular, in connection with gas sensors, to achieve a more rapid response by the entire sensor system to suddenly increased gas concentration values. Although the above description refers all the time to the particular case of an NDIR CO2 sensor, it is clear that the present invention may be adapted to all sensor systems in which an analog sensor signal with offset is present.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102006059652.8 | Dec 2006 | DE | national |
