Abstract:
A method of processing an analog sensor signal is disclosed. The method includes feeding the analog sensor signal into a first input of an operational amplifier, amplifying the analog sensor signal using the operational amplifier, measuring the amplified analog sensor signal, and comparing the amplified analog sensor signal with a threshold value. The method also includes generating a direct voltage depending on a difference between the amplified analog sensor signal and the threshold value, forming a difference signal from the analog sensor signal and the direct voltage, and amplifying the difference signal and outputting an output signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION DATA 
       [0001]    This application claims the benefit of the earlier filed parent German Patent Application DE 10 2006 054 164.2 having a filing date of Nov. 16, 2006. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a method of processing an analog sensor signal, a gas sensor arrangement, and to a measured value processing device. 
       BACKGROUND 
       [0003]    Known gas sensor arrangements include a radiation-emitting radiation source, a gas measurement space, which can be filled with a measurement gas which includes at least one analyte to be measured, and at least one radiation-detecting sensor device, which generates an output signal which depends on the absence and/or concentration of the analyte. Such gas sensor arrangements for proving the presence of a wide variety of analytes, e.g. carbon dioxide or methane, are known. Traditional gas sensors are based on the property of many polar gases that they absorb radiation in the infrared wavelength range. The IR light is capable of shifting the molecules into excited states by exciting rotation and vibration oscillations, by interacting with the dipole moment of the polar molecule. In this way, the heat energy of the IR light is transferred to the gas, and in the same way the intensity of an IR beam passing through the gas volume is reduced. Corresponding to the excitation states, the absorption occurs at a wavelength which is characteristic of the relevant gas, e.g. in the case of CO 2  at 4.25 μm. 
         [0004]    Currently, carbon dioxide detection is gaining increasing importance in many application fields. For instance, in the motor vehicle field, carbon dioxide detection can be used for monitoring the CO 2  content of the interior air to increase energy efficiency for heating and air-conditioning to cause a fresh airflow via an appropriate ventilator flap drive only when required, i.e. in the case of increased CO 2  concentration. Also, modern motor vehicle air-conditioning systems are based on CO 2  as the coolant, so that CO 2  gas sensors in the motor vehicle field can carry out a monitoring function in relation to escaping CO 2  in the case of any defects. Particularly in the motor vehicle field, gas sensors must fulfill the highest requirements for robustness, reliability and miniaturizability. Additionally, for safety applications, the response time of the sensor must not exceed specified limits. 
         [0005]    In German patent application DE102005032722, a gas sensor arrangement and a measurement method with early warning are described. In particular, this application refers to the radiation source that emits the radiation in the form of pulses. Also, German patent application DE102006019705.4 refers to a method of processing time-discrete measured values, the course of which over time can be described by means of a time function. The method according to this application uses a measured value filter to achieve a desired transient response. 
         [0006]    In many gas sensor arrangements, as the sensor device to analyze the IR radiation, so-called pyrosensors are used. The analog sensor signal that is output by such a pyrosensor has, depending on the measurement, a high offset voltage and only a small amplitude. For further processing and analysis of the signal, it is helpful to remove this offset voltage, but in this case the amplitude of the analog sensor signal should be amplified. 
         [0007]    For instance, in the case of known arrangements, as shown in Prior Art  FIG. 1 , the analog signal which the detector  302  outputs is amplified in an operational amplifier  304 . The offset voltage of this amplified signal is then removed by a capacitor  306 , before the now cleaned analog signal is amplified in another operational amplifier  308 . This amplified analog signal, without the offset voltage, is then further processed in a microcontroller  310 . 
         [0008]    The four curves in  FIGS. 2 to 4  show how the analog sensor signal is processed using the arrangement from Prior Art  FIG. 1 . It can be seen that the amplitude of the signal which is output by the detector in  FIG. 2  is only very small, and that it carries a large direct voltage part. The operational amplifier  304  amplifies the weak signal, but also the direct voltage part, so that the amplified signal possibly leaves the dynamic range of the subsequent analysis. The capacitor  306  blocks the direct voltage part out, see  FIG. 4 , and the operational amplifier  308  amplifies this cleaned signal until it can be measured well, see  FIG. 5 . 
         [0009]    However, the coupling capacitor  306  is very temperature-dependent, which can create difficulties in the motor vehicle field, since a specified operating temperature cannot be ensured. In the temperature range from −40° C. to +85° C., for instance, the capacitor changes its high capacitance, and it is also unstable in the long term. This coupling capacitor  306 , because of its size, cannot easily be integrated on modules, and is therefore resource-intensive and expensive to construct. 
       SUMMARY 
       [0010]    The present invention relates to a method of processing an analog sensor signal. The method includes feeding the analog sensor signal into a first input of an operational amplifier, amplifying the analog sensor signal using the operational amplifier, measuring the amplified analog sensor signal, and comparing the amplified analog sensor signal with a threshold value. The method also includes generating a direct voltage depending on a difference between the amplified analog sensor signal and the threshold value, forming a difference signal from the analog sensor signal and the direct voltage, and amplifying the difference signal and outputting an output signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention is explained in more detail below, on the basis of the advantageous versions which are shown in the attached drawings. Similar or corresponding details of the subject according to the invention are given the same reference symbols. 
           [0012]    Prior Art  FIG. 1  shows a circuit diagram of a conventional measured value processing device; 
           [0013]      FIG. 2  shows a first course of the signal over time in the arrangement from Prior Art  FIG. 1 ; 
           [0014]      FIG. 3  shows another course of the signal over time in the arrangement from Prior Art  FIG. 1 ; 
           [0015]      FIG. 4  shows another course of the signal over time in the arrangement from Prior Art  FIG. 1 ; 
           [0016]      FIG. 5  shows another course of the signal over time in the arrangement from Prior Art  FIG. 1 ; 
           [0017]      FIG. 6  shows a schematic representation of a gas sensor unit according to this invention; 
           [0018]      FIG. 7  shows a possible advantageous circuit diagram of the measured value processing device according to this invention; 
           [0019]      FIG. 8  shows a table of resistance values according to one embodiment of this invention; 
           [0020]      FIG. 9  shows a graph of a correction voltage depending on a total resistance according to the table in  FIG. 8 ; 
           [0021]      FIG. 10  shows another possible circuit diagram of the measured value processing device according to this invention; 
           [0022]      FIG. 11  shows a table of resistance values according to another embodiment of this invention; 
           [0023]      FIG. 12  shows a graph of a correction voltage depending on a total resistance according to the table in  FIG. 11 ; 
           [0024]      FIG. 13  shows a table of resistance values according to another embodiment of this invention; 
           [0025]      FIG. 14  shows a graph of a correction voltage depending on a total resistance according to the table in  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0026]    As shown in  FIG. 6 , a sensor  108  captures a gas concentration, which here can be taken approximately as a Heaviside function or step function. However, the input signal for the sensor  108  does not specifically have to be a gas concentration, but the output signal of any sensor can be processed according to the principles of this invention. The sensor  108  supplies a time-discrete analog sensor signal  109  that has an offset. 
         [0027]    As shown in  FIG. 6 , the gas sensor arrangement  100  according to the invention also includes a radiation source  102 , in this embodiment, a broadband infrared radiation source. In principle, the shown gas sensor arrangement  100  is a so-called NDIR (non-dispersive infrared) sensor. The gas sensor arrangement  100  further comprises the gas measurement space  104 , a wavelength filter  106  and an infrared sensor as the sensor  108 . The temperature can optionally be measured by a temperature sensor  118 . The measurement gas  110 , which is to be checked for the gas component to be detected, is pumped into the gas measurement space  104  or diffused into it, which is symbolized by the inlets  112  and outlets  114 . As explained above, the presence and/or concentration of the gas of interest can be determined electro-optically via the absorption of a specific wavelength in the infrared range. 
         [0028]    The emitted infrared radiation  116  is transmitted through the gas measurement space  104  into the sensor  108 . An optical filter that only lets through the wavelength range in which the gas molecules to be detected absorb is arranged at the sensor  108 . Other gas molecules normally absorb no light at this specific wavelength, and therefore, do not affect the quantity of radiation  116  that reaches the sensor  108 . Any suitable infrared sensor can be used as the sensor  108  and the signal processing method according to the invention can be adapted according to the appropriate sensor type. 
         [0029]    For instance, the sensor  108  can be a pyroelement, an infrared thermopile or a photodiode. The suitable sensor  108  in each case should be chosen according to the requirements in each case. The photodiode has the advantage of being a comparatively inexpensive component, whereas the thermopile sensor has the advantage of an especially high, even absorption of radiation  116  in the selected spectral range. Finally, pyroelectrical sensors have the advantage of very high sensitivity and the possibility of miniaturized production. 
         [0030]    The infrared signal is pulsed by the radiation source  102 , to be able to filter out thermal background signals from the desired signal. Thus, the measured values that the sensor supplies are present in the form of time-discrete values that essentially satisfy an exponential function. 
         [0031]    A controller  120 , on the one hand, activates the radiation source  102 , and on the other hand, receives the analog sensor signals  109  of the sensor  108  and processes them further according to the principles of this invention. In particular, the controller  120  includes a filter unit, which does the conversion of the analog sensor signal  109  into an amplified output signal without offset. 
         [0032]    For most applications of gas sensors, not only the final value of the signal, but above all, the gradient of the signal is the important magnitude. 
         [0033]    As shown in  FIG. 7 , the analog sensor signal is fed into one input of the operational amplifier  706 . The output signal of the operational amplifier is connected to the analog/digital converter  702 , which is controlled by the controller  708 . In this embodiment, the controller  708  is a microcontroller. The controller  708  also controls its own switch outputs  0  to  10 , which can be switched between the “negative supply voltage”, “open” and “positive supply voltage” states. A combination of resistors R 1  to R 11  are switched in different combinations between the switch outputs  0  to  10  of the controller  708 , each of which can have one of the three above-mentioned states, and the negative input of the differential amplifier  704 . The output of this differential amplifier  704  is fed through a resistor R 13  into the negative input of the differential amplifier  706 , which then amplifies the difference signal between the analog sensor signal and the offset, which was fed into the negative input. 
         [0034]    Below, the method of functioning of the measured value processing device according to the invention is described. 
         [0035]    In the amplifier  706 , a microcontroller-controlled voltage is subtracted from the analog sensor signal, and the difference is simultaneously amplified, to generate an output signal. The controller  708  measures the output signal, for which purpose an analog/digital converter  702 , which converts the amplified analog output signal of the operational amplifier  706  into a digital input signal for the controller  708 , is used. The controller  708  determines a signal correction on the basis of the digital input signal and the specified threshold value. 
         [0036]    This threshold value can be determined by the operating voltage of the controller  708 , but other factors may also play a part. If the output signal of the differential amplifier  706  is not in the desired range, it is counteracted with a direct voltage, to suppress the offset signal. The result is amplification of the pure analog signal without offset voltage, as can be seen in the course over time in  FIG. 5 . In this way, the amplified analog sensor signal is put into the active measurement range. In the invention, the analog sensor signal corresponds to the signal in  FIG. 3 , and the output signal of the operational amplifier  706  corresponds to the signal in  FIG. 5 . 
         [0037]    Referring again to  FIG. 7 , the direct voltage is generated as follows: the resistors R 15  and R 16  define the voltage value of the positive input signal of the operational amplifier  704 ; in the embodiment of  FIG. 7 , this voltage is set as a 0.1 V reference voltage. The negative input signal of the operational amplifier  704  is given by the combination of resistors R 1  to R 11 . Each of these resistors R 1  to R 11  can be connected to the switchable digital outputs  0  to  10  of the controller  708  at one of three selectable voltage values, namely positive or negative operating voltage or no voltage, i.e. open. 
         [0038]    Next, the values of the resistances between the negative operating voltage and the voltage at the negative input of the operational amplifier  704 , and the values of the resistances between the positive operating voltage and the voltage at the negative input of the operational amplifier  704 , can be switched individually or in parallel by the controller  708 . A parallel circuit of at least two resistors of the combination of resistors reduces the total resistance value. In this way, via relatively few resistors, many different voltage values can be reached, namely 2 n  or 3 n  combinations. This makes possible a variable setting of the value of the direct voltage that is applied to the negative input of the amplifier  704 . In this way, dynamic offset compensation in the amplifier branch is achieved. 
         [0039]    For instance, if the controller  708  in the course of time detects that the voltage of the output signal is reaching its maximum operating voltage—the signal is fed into the analog/digital converter  702 , which is integrated in the controller  708 , and which tolerates only a specified maximum voltage—the combination of resistors R 1  to R 11  are switched so that a greater constant voltage is generated. The differential amplifier  706  forms the difference of the two signals, which has become smaller, and simultaneously amplifies the result, to achieve a better signal analysis. The signal output of the amplifier  706  can now be compared with the signal “U OP2” in  FIG. 5 . 
         [0040]    During the measured value recording of the course of time for a radiation pulse, the resistance values must be constant. After the measurement, switching takes place if necessary, and a new measurement is then started. 
         [0041]    With reference to  FIGS. 7 to 14 , two embodiments of this invention are shown in more detail, their circuit diagrams being in  FIG. 7  (first embodiment) and  FIG. 10  (second embodiment). Additionally,  FIGS. 8 and 9  belong to the first embodiment,  FIGS. 11 to 14  give more information about the second embodiment. In the tables of  FIGS. 8 ,  11  and  13 , the used values of the resistors R 1  to R 11  or R 12  are shown. A “1” means that the resistor is connected to the operating voltage, whereas a “−1” shows that the resistor has been switched to the negative operating voltage. If no value is given in the relevant column, the switch within the controller  708  is open. The second row gives the values of the relevant resistor in ohms. From  FIGS. 8 and 11 , it can be seen that the resistors all have different values, and in  FIG. 11  only digital outputs  0  to  7  are populated with resistors. In  FIG. 13 , on the other hand, resistors R 1  to R 12 , which all have the same value, 20000 Ω, are given. 
         [0042]    In  FIG. 9 , the correction voltage is shown against the code number. In  FIGS. 12 and 14 , the total resistance compared with the correction voltage is shown. Code numbers can also be assigned to the various combinations of resistors, and they can be found here on the x-axis. Each code number stands for a specified combination of resistors, and they can be found in the rows of the tables of  FIGS. 8 ,  11  and  13 . The controller  708  stores the various combinations of resistors under these code numbers. 
         [0043]    It is pointed out that the embodiments in the tables in  FIGS. 8 ,  11  and  13  are only partly occupied, to ensure a clear representation; more intermediate steps are possible. In addition to as small a step width as possible, as constant as possible a total resistance of the arrangement is desirable. Additionally, the voltage difference between symmetrical code pairs should be approximately equal, which is achieved by not all of the combination of resistors R 1  to R 11  being connected to positive or negative operating voltage, but many of the digital inputs  0  to  10  of the controller  708  being “open”. This provides the further advantage that less memory space is required for a code assignment table in the controller  708 . In  FIG. 8 , only the first rows of the table are listed; the other values can easily be calculated. In principle, a count down takes place in a Boolean manner. 
         [0044]    It has also been shown that a linear course of the offset correction is possible, if it is taken into account that the digital outputs  0  to  10  of the controller  708  have a significant internal resistance. 
         [0045]    The amplification of the analog sensor signal without offset will be clarified further using  FIGS. 2 to 5 . The output signal of the pyrosensor is given in  FIG. 2 . As is made clear in  FIG. 3 , this signal is amplified, so that the amplitude of the signal is amplified, but the offset is also magnified. This signal in  FIG. 3  is the amplified analog sensor signal. The output signal of the operational amplifier  706  is then shown in  FIG. 5 , in which it can clearly be seen that the offset has been reduced and the amplitude of the signal has been amplified. This effect cannot be achieved with an Automatic Gain Control (AGC), for instance, since with it the offset would still be amplified. 
         [0046]    Obviously, the embodiments of the invention are not restricted to the above-mentioned values and numbers of resistors and other components. For instance, the number of resistors R 1  to R 11  is restricted only by the number of free switch outputs  0  to  10  of the controller  708 . 
         [0047]    When the pure analog sensor signal  109  is amplified without offset voltage, improved temperature behavior occurs if the offset voltage can be removed without using a capacitor. For this purpose, the output signal of the operational amplifier is fed back to minimize the offset. The use of resistors according to the invention simplifies miniaturization of the gas sensor arrangement. The result is also a cost saving compared with the known use of a large capacitor. By the above methods, a simple, temperature-independent, linear control is achieved over the whole voltage range. The above-described systems and methods make simpler production of the component possible and resource-intensive calibration unnecessary. Further, more precise measurements are made possible by the better signal resolution. 
         [0048]    The advantageous properties of the measured value processing according to the invention can be exploited, in particular, in the case of gas sensor arrangements which are used for detection of carbon dioxide, e.g. in the motor vehicle field, both for monitoring for CO 2  escaping from leaks and for checking the air quality in the passenger compartment. Obviously, the principles according to the invention can also be used in relation to detection of any other gases, and are important for all sensors where a measurement signal with an unreliably high direct voltage part is to be analyzed. 
         [0049]    The above-described methods of processing analog sensor signals makes temperature-independent, fast and robust direct voltage suppression possible. With the measured value processing according to the invention, in particular in relation to gas sensors, more precise, temperature-independent measurements, with long term stability, are possible because of the better signal resolution. Although the special case of an NDIR CO 2  sensor is always described above, it is clear that this invention can be adapted for all sensor systems in which an analog sensor signal with offset is present.