Abstract:
A circuit for measuring the relative absorption signals from an infrared detector to measure gas concentrations in the detector, wherein an absorption gas wavelength signal is compared to a reference gas wavelength signal, through circuits and a computer processor, together having automatic compensation for absorption variables and which permit the measurement of gas absorption by use of Beer&#39;s Law equations.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to infrared detectors used primarily for detecting gas concentrations in a confined space, usually defined as a fixed distance of space between an infrared source and an infrared detector. More particularly, the invention relates to electronic control circuits for receiving analog electrical signals from such infrared detectors, and automatically compensating for variables in signal and circuit parameters. 
     The measurement of gas concentrations by using infrared detectors is typically accomplished by application of Beer&#39;s Law, which can predict the amount of absorption of an infrared signal wavelength passing through a region of concentration of a particular target gas, according to the formula: 
     
       
           I=I   o    e   −γlc ; where 
       
     
     I is the signal after absorption; 
     I o  is the signal before absorption; 
     γis the absorption coefficient of the gas; 
     l is the path length through the gas; and 
     c is the gas concentration. 
     In designing such a measurement system, it is common to use two infrared channels, each developing an infrared signal at a different frequency (wavelength), where one wavelength is known to be particularly sensitive to absorption in the chosen target gas, and the other wavelength is known not to be particularly sensitive to absorption in the chosen target gas. Therefore, the concentration of the target gas can be measured by taking the ratio of the signal strength of the non-sensitive wavelength (the reference channel) to the signal strength of the sensitive wavelength (the analytical channel). 
     The two infrared channels can be developed using a single infrared source positioned at a fixed distance from two wavelength bandpass filters which are sensitive to various wavelengths as described above, and permitting the target gas to fill the space and distance between the source and the two filters. One channel is chosen so as to pass frequencies in the infrared band, but the bandpass filters are centered on a wavelength known to be highly absorbed in the target gas; this channel is known as the “analytical” channel. The other channel is chosen so as to pass frequencies in the infrared band, close to the bandpass frequencies of the first channel, with the bandpass filters centered on a wavelength which is not highly absorbed in the target gas; this channel is known as the “reference” channel. For example, if measuring a hydrocarbon gas such as methane, the reference channel sensitivity would be set to a wavelength of 3.9 microns, and the analytical channel sensitivity would be set to a wavelength of 3.4 microns. Most of the environmental factors which may be present will affect both the analytical and the reference channel in a similar manner, but the presence of a target gas affects primarily the analytical channel. A quotient, formed by dividing the reference signal value by the analytical signal value, will only change when a target gas is present in the optical path of the infrared channels; the amount of quotient change can be used to calculate the target gas concentration using Beer&#39;s Law. 
     There are a number of factors which can degrade accuracy in the foregoing scheme. The infrared sensors are necessarily analog devices, although the time-varying analog voltage signals they produce are frequently converted to digital values through the use of analog-to-digital converters (A/D converters) to obtain greater precision. If the analog voltage signals become too large they can exceed the bounds of the A/D converter; if the analog voltage signals become too small they produce poor resolution of the digital converted value, and both of these factors can lead to a reduction in accuracy of the measured values. Another cause of reduced accuracy is component tolerance variations, particularly in the photo elements, which make it difficult to control analog voltage levels from unit to unit; this can be addressed by providing adjustable potentiometers in the circuits, for making tolerance adjustments during the manufacturing process. Another cause of reduced accuracy, after units are placed into operation, is that the optical components of a system can become coated with dust or other substances which attenuate the light passing through, reducing the analog voltage signal levels produced by the optical detectors; similarly, the output light intensity level typically lowers as a light source ages, which also reduces the analog voltage signal level. These problems can been addressed by frequently cleaning the optical path components and using manual potentiometer adjustments as the equipment ages. 
     Another cause of inaccuracy is the variation in absorption coefficients of different gases, which causes the absorption response curve to vary from gas to gas, and also from one gas concentration level to another. This problem can be addressed by choosing specific, selectable, families of response curves for specific gases, and different analog voltage gain values for the electronic circuits used to detect the gas concentrations. Another cause of inaccuracy is temperature variations in the measurement environment, which can cause changes in circuit performance in the measurement system. 
     Most of the foregoing problems are more or less continuously present, and frequent readjustments of circuit parameters and frequent cleaning are inadequate for ensuring a smooth, continuously accurate measurement result. It is therefore a principal object of the present invention to provide a stable, continuously accurate measurement system for monitoring gas concentrations in a particular environment. Specifically, it is an object of the present invention to provide a circuit for automatically adjusting analog voltage signal levels to keep the permissible signal range constant, ie., to maintain the maximum permissible signal at a constant voltage span relative to the “zero” signal level; it is also an object of the invention to provide circuit compensation for variations or drift in the “zero” or reference voltage signal level, and for circuit gain variations caused by any of the aforementioned effects. The foregoing objects advantageously provide a circuit for automatically controlling variables related to measuring analog voltage signals from infrared gas detection devices, by providing a constant zero point for analog voltage signals, a constant voltage value representing the full-scale point for the same signals, an adjustable gain circuit to compensate for changes in overall circuit gain, and an absorption coefficient response curve for any known gas, which is accurate to a first order approximation. 
     SUMMARY OF THE INVENTION 
     A control and measurement circuit responsive to analog voltage signals produced by infrared sensors in a gaseous environment, including a computer processor connected to receive signals from the infrared sensors after the circuit&#39;s zero reference voltage has been calibrated by a BALANCE circuit, the circuit&#39;s gain has been adjusted by an AGC circuit and the circuit&#39;s voltage range has been set by a SPAN circuit, wherein each of these circuits has a digitally-controllable potentiometer as an input impedance, wherein each potentiometer is adjustable by binary voltage feedback signals generated by a computer processor. As a result, the analog voltages present within the circuit during the calibration process are converted to digital values which are used by software within the computer processor to generate digital feedback signals to the respective BALANCE, AGC, and SPAN circuits to adjust the respective circuit&#39;s analog voltage response characteristics. 
     The computer processor receives the analog voltage signals generated by the infrared sensors and converts them to digital values for internally comparing the digital values to a prestored Beer&#39;s Law curve for the particular gas being measured, for providing a measurement of the concentration of the gas being measured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects of the invention will become apparent from the following description of a preferred embodiment of the invention, and with reference to the appended drawings, in which: 
     FIG. 1 shows a symbolic block diagram of amplifier circuits having a digitally-adjustable potentiometer input circuit, wherein the circuit output is connected to a digital processor, which is part of a closed loop feedback circuit; 
     FIG. 2 shows a block diagram of the invention; 
     FIG. 3 shows a circuit diagram of the several amplifier circuits of the invention, each connected to an addressable dual digital potentiometer; and 
     FIG. 4 shows a typical gas concentration curve for a gas according to Beer&#39;s Law. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, there is shown a group of three amplifier circuits having a processor-addressable dual digital potentiometer  10  input circuit and a computer processor  20  output circuit, illustrating the essential circuit connections which form the heart of the present invention. The processor  20  output circuit is shown in FIG. 1 as a display  21 , but this is representative of any type of computer output circuit or device which is known in the art. The addressable dual digital potentiometer  10  is a commercially available component, as for example, a type DS1803 manufactured by Dallas Semiconductor Corporation. Each potentiometer  11 ,  12  represented within the box  10  consists of a resistor array having 256 selectable positions, formed by a series-connected plurality of semiconductor MOSFET devices, each of which are switchable “on” or “off” by a binary voltage signal, and an 8-bit register (not shown) to receive the binary voltage signals, and thereby to effectively set the “wiper”  13 ,  14  position to any of the 256 positions, depending on the digital value of the binary voltage combination loaded into the register. The potentiometers  11 ,  12  are connected together at their respective ends, and the top end connection is connected to an analog input voltage signal V IN  and the bottom end connection is connected to a voltage ground. 
     The device is addressable and controlled by a two-wire serial interface over the lines “data” and “clock,” which are connected to the computer processor  20  serial output port. The computer processor  20  provides data for loading into the potentiometer registers, under control of a clock signal also provided by the processor  20 . 
     Each potentiometer wiper  13 ,  14  is connected to a separate amplifier  15 ,  16  having essentially identical transfer characteristics. The amplifier is a non-inverting, fixed gain operational amplifier. The output signal from amplifier  15  is connected to an amplifier  17  input via a 1000-ohm (1K) resistor, and the output signal from amplifier  16  is connected to the same amplifier  17  input via a 256 K resistor, thereby providing an additive input signal, where the two input signals are summed in the ratio of 256/1 to the amplifier  17  input from the respective amplifiers  15 ,  16 . This provides an effective “high byte” and “low byte” contribution to the output voltage from amplifier  17 , where the output voltage is directly proportional to the input voltage, with a signal resolution of one part in 2 16 . The output signal from amplifier  17 , V OUT , is applied to an analog-to-digital input terminal of computer processor  20 , and therefore produces a digital value to the computer processor which is representative of the analog input voltage V IN , to a resolution of one part in 65,536 (2 16 ). 
     FIG. 2 shows a block diagram of the present invention, illustrating the major circuit functions, BALANCE, AGC, and SPAN, which are used to achieve the control desired herein, all in conjunction with the computer processor  20 . Each of the circuit functions contains circuit components as shown in FIG. 1, and each circuit function is operated under the control of the processor  20 . The BALANCE circuit  30  has an input connected to receive the analog voltage signal V A  from an infrared sensor, and an output connected to a switch terminal of switch  40 . Another switch terminal of switch  40  is connected to a source of reference voltage V REF , which could come from a second infrared sensor, and the third terminal of switch  40  is connected to an input of AGC circuit  50 . The output of AGC circuit  50  is connected to the input of SPAN circuit  60 , and the output of SPAN circuit  60  is connected to an A/D input connection to processor  20 . If two infrared sensors are used to connect to the switch  40 , they are typically sensors which are responsive to different wavelengths of light. The V A  signal is from a sensor monitoring the environment where a concentration of the gas desired for detection might be found, and the V REF  signal is from a sensor monitoring the same environment but which is non-responsive to the gas desired for detection. 
     FIG. 3 shows a simplified circuit diagram of the several circuits described above; it should be apparent that the circuit connections of each of the circuits is virtually identical to the other circuits, with the exception that the SPAN circuit is referenced to a +4 VOLT power supply rather than to circuit ground as are the AGC and BALANCE circuits. Each circuit has an addressable dual digital potentiometer associated with an amplifier as described earlier; namely, the BALANCE circuit  30  has a dual digital potentiometer  31  associated therewith, the AGC circuit  50  has a dual digital potentiometer  51  associated therewith, and the SPAN circuit  60  has a dual digital potentiometer  61  associated therewith. In each case, the computer processor  20  is connected, via a “DATA” and “CLOCK” signal line, to provide the register settings for the respective digital potentiometers. 
     In operation, the circuits are first manipulated through a calibration procedure to select the initial settings for the AGC, BALANCE and SPAN potentiometers. Because of the particular circuits selected for the preferred embodiment, the “zero” voltage reference is selected to be +4.0 volts, and the maximum gas concentration voltage is selected to be +0.5 volts. Since the infrared detector converts light intensity to voltage, this voltage selection translates into a Beer&#39;s Law equation of: 
     
       
         
           V=V 
           o 
           e 
           −γlc 
         
       
     
     
       
           V   o =+4.0 volts; therefore, 
       
     
     
       
           V =4.0  e   −γlc   
       
     
     Since the maximum, or full scale value of the Beer&#39;s Law calculation is +0.5 volts, we can calculate the value of the exponent at the maximum gas concentration level (cmax): 
     
       
         0.5=4.0 e   −γlcmax ; taking the log of each side: −Ln (0.5/4.0)=γlcmax=2.08 
       
     
     The foregoing calculations set the parameters for the Beer&#39;s Law equation in the context of the actual circuit parameters used for this invention; namely, a voltage indication of 4.0 volts when zero gas concentration exists in the sensor environment, and a voltage indication of 0.5 volts when a maximum gas concentration exists in the sensor environment. For all other concentrations of gas the following Beer&#39;s Law formula can be solved for any measured voltage “V”: 
     
       
           V =4.0  e   −γlc   
       
     
     The calibration procedure can proceed according to the following steps. First, the processor selects the switch  40  and sets switch  40  to select the reference channel, which is the V REF  position, and which corresponds to a zero gas concentration reference, and the AGC potentiometer is selected by the processor. The processor sends binary digital feedback signals to the input registers of the AGC digital potentiometer circuit to obtain an input voltage to the computer processor of 4.0 volts. Thus, the gain is set by the processor to reflect the zero gas concentration value to the processor. 
     Next, the processor selects the switch  40  and sets switch  40  to select the analytical channel, which is the V A  position, under conditions of zero gas concentration in the sensor environment. The processor sends binary digital feedback signals to the input registers of the BALANCE potentiometer circuit to obtain an input voltage to the computer processor of 4.0 volts. This provides a sensor signal value to the processor corresponding to the zero gas concentration value set in the previous step. 
     Next, a known gas of known concentration is injected into the infrared detector environment; the selected gas concentration is preferably ½ the maximum obtainable concentration level, or ½ cmax as derived above. This represents a gas concentration “c” equal to “cmax/2”. and the Beer&#39;s equation becomes: 
     
       
           V (½ gas concentration)=4 e   −1.04 =1.414 volts 
       
     
     The processor sends binary digital feedback signals to the SPAN potentiometer circuit to obtain an input voltage to the computer processor of 1.414 volts, which is the voltage associated with the target gas at ½ concentration (see FIG.  4 ). 
     Having determined the ½-concentration voltage, any other voltage (V M ) measured by the processor can be used to calculate any unknown gas concentration (C u ) using Beer&#39;s Law. The processor constructs the Beer&#39;s Law curve of FIG.  4  and prestores this curve in memory, either in the form of a table look-up or in any other convenient form useful to the processor, because three points along the exponential curve are known from the foregoing calibration steps, and therefore all other points along the curve can be plotted or calculated. Any subsequent voltage received by the processor can be applied to the known exponential curve, and the gas concentration corresponding to that voltage value can be determined from the prestored table or curve. 
     Once the device is calibrated it can then be operated in real time, with unknown concentrations of target gas passing into the infrared sensor device. In real time operation, the switch  40  is switched four times per second to regularly measure the reference voltage at the computer, and to change the binary digital feedback value to the AGC potentiometer if necessary to return the measured value to +4.0 VOLTS. This AGC potentiometer change is effective for adjusting the reference sensor voltage value received by the processor, which also adjusts the analytical sensor voltage value, on the theory that whatever outside influence caused the drift in the “zero” voltage value would also cause an identical drift in the analytical sensor value. 
     During the time the switch is connected to the sensor via the analytical channel, the actual gas concentration being measured is presented to the computer processor  20  input in the form of a voltage ranging between the zero concentration level (+4.0 VOLTS) and the full scale concentration level (+0.5 VOLTS); the actual measured voltage is applied to the curve of FIG. 4, or to a curve-fitting process residing in computer software, to derive the relative gas concentration being measured. The BALANCE potentiometer circuit and the SPAN potentiometer circuit are not subsequently readjusted, unless a new calibration procedure is necessary. This operating procedure provides a sensor reading to the processor four times a second, and also provides a gain adjustment, if necessary, four times a second. 
     The essential advantage of the invention is that the analog signals over the zero to full scale range are always identical regardless of the type of gas being measured, the range of concentrations of the gas, the amount of signal drift which occurs, and the signal degradation which may occur. It enables determination of gas concentrations using Beer&#39;s Law a matter of trivial complexity, involving only a single response curve, where the analog signals can&#39;t exceed the boundaries set for the device and the A/D resolution is constant. 
     The present invention may be embodied in other forms without departing from the spirit or essential attributes thereof; and it is, therefore, desired that the preferred embodiment be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the invention scope.