IR-based nitric oxide sensor having water vapor compensation

IS 2 199 229 T3

DESCRIPTION

Infrared based nitric oxide detector with water vapor compensation.

This invention relates in general to sensors that respond to specific chemical species and, in particular, to an optical sensor for detecting a concentration of nitric oxide (hereinafter NO) in an emission from a vehicle, a chimney and a similar source. .

Environmental pollution is a serious problem that is especially acute in urban areas. An important cause of this pollution is the exhaust gas emissions of motor vehicles. Official standards have been established to regulate the allowable amounts of pollutant species in automobile exhaust gases, and in some areas, periodic inspections and “toxic smoke checks” are required to ensure that vehicles meet these standards.

However, there are still large numbers of vehicles on public highways that do not meet the standards. Also, it has been determined that a disproportionately large amount of pollution is generated by a relatively small number of vehicles.

Very polluting vehicles can operate even in areas where periodic emissions inspections are required. There are some older vehicles and special types of vehicles that are exempt from inspections.

Anti-pollution devices that are required equipment in most modern vehicles serve their intended purpose of reducing pollution in vehicle exhaust gases to within prescribed levels. However, some vehicle owners have observed that anti-pollution equipment reduces engine performance.

For this reason, some vehicle owners with mechanical experience can carry out whatever service is necessary to get their vehicles in condition to pass the required inspections, and subsequently remove the anti-pollution devices and / or return the vehicles to their previous situation. with a concomitant increase in pollutant emissions for normal use.

Therefore, an anti-pollution program that relies entirely on mandatory periodic inspections carried out in fixed installations is inadequate. Vehicles that are actually operating in violation of prescribed emission standards need to be identified, and either required to be brought into compliance or removed from service.

A system for the remote detection of automobile exhaust gas emissions is described in an article entitled "ANALYTICAL SOLUTION - Long-way Infrared Photometry: A Remote Sensitive Tool for Automobile Emissions": by G. Bishop and collaborators, published in Analytical Chemistry 1989, 61, 617A. An infrared beam is transmitted through the mass of exhaust gases from a motor vehicle to a sensor unit that includes a beam splitter that splits the beam into a carbon dioxide (CO2) channel and a carbon monoxide channel (CO).

The beam contained in the CO2 channel passes through a band-pass filter that isolates the spectral region of absorption of carbon dioxide and strikes a photovoltaic detector. The beam contained in the CO channel passes through a rotating gas filter wheel, one half of which contains a mixture of CO and hydrogen (H2), and the other half of which contains nitrogen (N2). From the filter wheel, the CO channel beam passes through another band-pass filter that isolates the carbon monoxide absorption spectral region and strikes another photovoltaic detector.

The output signals of the detectors vary according to the transmittance of the vehicle's exhaust gas mass at the respective wavelengths, and therefore the concentrations of CO and CO2 in the mass of gases. The CO / H2 fraction from the filter wheel provides a reference output, while the N2 fraction provides a carbon monoxide output.

The baseline sensor outputs are obtained without any vehicle passing through the beam, and with the beam blocked by a vehicle prior to detection of the mass of gases. These values ​​are used as references to calibrate the outputs of the detectors when the mass of gases is actually detected. The outputs of the detectors, which correspond to the transmittances at the respective wavelengths, are then processed according to predetermined functions to determine the relative percentages of CO2 and CO in the mass of vehicle exhaust gases.

This system is said to be capable of detecting the composition of exhaust gases from moving vehicles, and is useful for identifying polluting vehicles for regulatory compliance purposes. However, it suffers from certain drawbacks.

For example, precise alignment is required to ensure that the beams in the two paths hit the detectors in an identical way. A small misalignment error can seriously degrade the accuracy of the measurement. The two photovoltaic detectors are distant from each other, and require separate cooling units to

ES 2 199 229 T3 regulate the temperature. A small difference in temperature, as well as small unevenness in the characteristics of the detectors, can also seriously degrade the accuracy of the measurement.

The rotating filter wheel is a mechanical unit that is expensive and prone to mechanical malfunction. The concentrations of the gases in the filter must be kept at precise values ​​in order to obtain accurate measurements. The system is also difficult to scale up to detect additional polluting species, since each new channel will require another beam splitter, another detector, etc., and will involve the problems described above.

US Patent Application Serial Number 08 / 119,788, commonly assigned 10/9/93, entitled "Optical Sensor Apparatus for Remote Measurement of Exhaust Gas Composition of Mobile Motor Vehicles" , by Michael D. Jack et al., later abandoned, teaches an infrared-based system that solves the problems discussed above. This system employs a number of adjacently spaced photodetectors that are sensitive to different wavelengths corresponding to the spectral absorption peaks of the constituents of the exhaust gas mass composition, including carbon monoxide, carbon dioxide, and hydrocarbons.

However, a particularly harmful pollutant that is not detected at all by known types of systems, or that is only detected inaccurately, is nitric oxide (NO).

Standard infrared techniques such as FTIR are not able to quantify NO present in the atmosphere due to the significant interference that results from the absorption of water in the absorption bands in the region around 5.2 μιη and 6.2 μm where NO absorbs. Attempts to subtract the water absorption band have not been successful due to the limited precision with which the water absorption signature can be modeled throughout the spectral region in which FTIRs, by their very nature, can explore.

Alternative infrared laser concepts using overtone techniques are also unsuitable, due, at least in part, to the requirement to compensate for water vapor absorption, and to the requirement to provide very precise temperature control.

An alternative solution uses absorption with ultraviolet (UV) radiation in the spectral region around 270 nm. Although NO absorption is strong in this spectral region, the application of this solution to moving vehicles is difficult due to the interference of natural pollutants present in automobile exhaust gases, namely the aromatic hydrocarbons benzene and toluene. The multiplicity of aromatic hydrocarbons emitted in a typical mass of exhaust gases, and the absorption caused by them, makes compensation for these aromatic hydrocarbons very difficult, and also limits the precision of the measurement.

WO-A-89 03029 describes a gas analyzer that determines the concentration of NO, H2O, and CO2 in a sample that is introduced into a sample cell in the analyzer. Use a cooling stage to remove H2 0.

In WO-A-92 12411 a combination of infrared and ultraviolet radiation is used to distinguish NO from H2 0.

It should be noted that many of these problems are compounded when the exhaust gas pollution detection system is required to be portable, and also capable of operating in less than ideal environments, such as when monitoring vehicles is desired. that are rolling down a road, such as a highway or highway.

Objects of this invention

Therefore, an object of this invention is to provide a system for accurately quantifying an amount of NO in a mass of gases that solves the problems discussed above and other problems inherent in the various solutions described above.

A further object of this invention is to provide a system and method based on infrared radiation for accurately quantifying a concentration of NO in a mass of gases, in which the system and method compensate for ambient water vapor and also for vapor. of water that is present in the mass of gases.

A further object of this invention is to provide a sealed gas cell having an ability to vary a path length product for use in the calibration of an exhaust gas emission system.

Summary of the invention

The above and other problems are solved and the objects of the invention are realized by means of a system (10) for determining the concentration of NO in a mass of exhaust gases (14), comprising:

a source (18, 22, 24) of electromagnetic radiation for directing radiation having a plurality of wavelengths along an optical path that passes through the mass of gases;

ES 2 199 229 T3 first sensor means (32, 32a) that give an output to indicate the amount of radiation absorption, within a band of first wavelengths, which is due to the presence of NO in the mass of gases including a first photodetector (32) and a first narrow band filter (30) interposed between the first photodetector and the mass of gases, wherein the first filter has a wavelength passband of predetermined width that includes a wavelength of 5.26 microns;

second sensor means (32, 32b) giving an output to indicate an amount of radiation absorption, within a band of second wavelengths, that is due to the presence of water along the optical path including a second photodetector and a second narrow band filter (30) interposed between the second photodetector (32) and the mass of gases, wherein the second filter has a wavelength passband of predetermined width that includes a wavelength of 5.02 microns; and means (38,42), having inputs coupled to the outputs of said first sensor means and of said second sensor means, to compensate for the indicated absorption within the first wavelength band according to the indicated absorption within of the second wavelength band and according to a predetermined modifying NO absorption factor.

An algorithmic manipulation of the signals obtained from the infrared detectors, in conjunction with a reference channel, compensates for variations in an infrared source, and also allows a precise compensation, in real time, of the water present both in the atmosphere and that emitted in the exhaust gases of the car. In this invention the background water concentration due to relative humidity (RH) and in the exhaust gases emitted by a vehicle is determined by measuring in an appropriate water absorption band, for example bands centered at 1998 cm^-1 or 2003 cm^-1 , using a narrow band filter that selects radiation contained only within a narrow spectral region that is very close to the resonant NO absorption peak (or peaks).

Compensation for the presence of water vapor is achieved using a look-up table that is made using models and measurements. The precision of this look-up table technique has been shown to be equal to or greater than 2%. It has been shown that with precise compensation for absorption by the water band, the final sensitivity of the system is limited only by the width of the spectral filter around the NO line. For example, with a spectral filter that has a width of 1%, a concentration of NO that is equal to or less than 500 ppm can be detected, even with high levels of water in the exhaust gases and with high levels of relative humidity (up to about 60% RH).

Thus, this solution provides an accurate quantitative measurement of NO and water vapor using infrared radiation channels in conjunction with narrow band filters. Narrowband compensation allows sensitivities of up to 100 ppm for automobile exhaust gases.

The teaching of this invention overcomes the deficiencies described above in FTIR (infrared) and ultraviolet measurement techniques, which are limited by inaccuracies due to incomplete compensation of water across the entire spectral band, and by imprecise compensation of absorbing species of interfering ultraviolet (eg aromatic hydrocarbons) that cannot be compensated a priori, respectively.

Thus, this invention provides in a first aspect thereof a system for detecting NO within a mass of exhaust gases. The system includes a source for generating an optical beam and for directing the optical beam through the mass of exhaust gases, the optical beam of which has wavelengths within a predetermined band of wavelengths contained in the spectrum of infrared radiation. (TO GO). The system further includes a radiation sensor / filter assembly having a first channel for determining a measured NO transmission value for a first predetermined band of wavelengths; a second channel for determining a measured water transmission value for a second predetermined band of wavelengths; a third channel for determining a reference transmission measured value for a third predetermined wavelength band selected such that it is not significantly absorbed by the mass of exhaust gases; and a fourth channel for determining a measured combustion by-product transmission value for a fourth predetermined band of wavelengths. A data processing equipment provides response to the channel outputs in order to determine an effective NO transmission value from the NO transmission measured value that is modified by (a) the water transmission measured value, (b) a predetermined factor that compensates for water absorption within the first predetermined wavelength band, and (c) the reference transmission value. The data processing equipment can further function to convert the effective NO transmission value into a relative NO concentration using a predetermined calibration factor, and to convert the NO transmit relative value into a NO concentration value using the measured value of combustion by-product transmission.

Furthermore, according to this invention a sealed calibration cell is constituted by a housing that is divided into two compartments by a pair of pistons transparent to infrared radiation. The housing has two windows transparent to infrared radiation at opposite ends thereof to allow a beam to pass through the housing, and also through the transparent pistons. A first compartment contains a hot mixture of gases of interest (eg, NO, CO2 and H2 0) in a predicted percentage. The mixture is kept at a desired temperature. The concentration path length product contained in the sealed cell is varied

ES 2 199 229 T3 redistributing the gas mixture between the first compartment and an adjoining hermetic tank of hot gas, which can also contain a source of water in liquid state.

The mechanical variation of the path length product through the calibration cell is accomplished by magnetically driving a first piston with a second piston by means of two permanent or electrically activated ring magnets. This is done with a pressurized gas source that is connected to a second compartment within the housing. The pressurization of the second compartment produces a movement of the second piston which, by exerting a magnetic force on the first piston, causes a displacement of the first piston and reduces the volume of the gaseous mixture contained in the first chamber. A restoring force is applied by the action of some springs.

Other embodiments of the dual chamber sealed cell employ a bellows to displace the first piston, or an electrostatically actuated first piston displacement. A further embodiment uses the bellows or electrostatic displacement in combination with an inflatable reservoir.

The use of the sealed calibration cell enables rapid on-site calibration of NO, H2O and CO2 detectors. Significantly, and since the cell is a sealed system, no gases are released into the atmosphere, and consequently there is no need to purchase and ship replacement calibration gas cartridges.

Brief description of the drawings

The aforementioned characteristics and other characteristics of the invention will become more apparent in the following detailed description thereof, read in conjunction with the attached drawings, in which:

Figure 1 is a block diagram of an exhaust gas pollution detection system that includes the multi-channel NO sensor embodiment of this invention;

Figure 2 graphically illustrates the close proximity of the water vapor absorption spectrum to the predicted NO absorption spectrum;

Figure 3 graphically illustrates the effect of different ideal filters on the measurement of the NO absorption band;

Figure 4 graphically illustrates two candidate NO spectral lines (5.2617 μ / m and 5.2629 μ / m) above water vapor baselines for different levels of relative humidity, and makes evident a requirement to subtract the water vapor background;

Figure 5 is a simplified cross-sectional view (not to scale) of a gas calibration cell in accordance with one aspect of this invention;

Figure 6 is a logic flow diagram illustrating a method of this invention for determining the concentration of NO in a mass of exhaust gas;

Figure 7 is a logic flow diagram illustrating a method of this invention for monitoring exhaust emissions from vehicles moving on a roadway;

Figures 8A and 8B are graphs illustrating the use of this invention in measuring an emission from a high emission vehicle (1875 ppm) and a relatively low emission vehicle, respectively, with Figure 8B further showing the compensation. of the water vapor contained in the mass of exhaust gases; Y

Figures 9A through 9C each illustrate a cross-sectional view of a different embodiment of a sealed calibration cell.

Detailed description of the invention

Figure 1 is an overall system diagram illustrating a multi-channel NO sensor system 10 of this invention for use with a vehicle 12 that emits a mass 14 of exhaust gases when the vehicle is traveling on a roadway 16. It should be understood that vehicle 12 is not part of system 10. Furthermore, it should be understood that this invention can also be used to quantify an emission of NO from a chimney and similar equipment, and that its use is not limited only to vehicles having internal combustion engines that emit NO as a constituent of their gases. exhaust. In general, this invention is applicable to the determination of the NO concentration in a mass of gases emitted by any natural or artificial source.

It should further be noted that the teaching of this invention can be used in a stand-alone manner to quantify only the concentration of NO in an emission, or it can be included or used in conjunction with a system of the type described in US Patent Application Serial No. USA previously cited and commonly assigned serial number 08/1 19,788, issued 10/9/93, entitled "Optical Detection Apparatus for Remote Measurement of Exhaust Gas Composition from Mobile Motor Vehicles", by Michael D. Jack et al.

IS 2 199 229 T3

System 10 includes an infrared source 18, preferably a broadband infrared source such as a glow stick, having a significant output of infrared radiation in the range of about 3 microns to about 6 microns. Infrared source 18 provides a beam 20 that can optionally be passed through a chopper 22 (nominally 200 cycles per second) and a beam former 24, such as a reflector. The beam 20 is arranged to pass through the mass 14 of exhaust gases of the vehicle 12 when the vehicle is moving on the road 16. The passage of the infrared beam 20 through the mass 14 of exhaust gases It results in the selective partial absorption of various wavelengths within the broadband beam, whose selective absorption occurs as a consequence of the presence of NO, water vapor, CO2, and other molecular species contained in the exhaust gases.

After passing through the mass 14 of gases, the beam 20 passes through an optional gas cell 26 transparent to infrared radiation (Figure 5), used for calibration purposes, and then passes through an integrator or diffuser 28 of do. The diffuse beam is applied to a plurality n of narrowband filters 30, where n is equal to a number of measurement channels of the system 10. Each filter 30 is selected so as to pass a predetermined narrow band of wavelengths to an associated detector of a plurality of infrared detectors 32. Each detector 32 outputs an electrical signal to an input of a corresponding measurement channel constituted by suitable analog electronic elements 34 (for example amplifiers), an analog-digital (A / D) converter 36 of n channels, and a device of data processing 38 having an associated output device 40. The data processing equipment 38 provides the required signal processing of the outputs of the A / D converter 36. The data processing equipment 38 is coupled to a look-up table 42 (hereinafter LUT), the use of which is described in detail below. LUT 42 is most easily implemented as a memory region (semiconductor and / or disk) that is accessible by data processing equipment 38. A suitable cooler 44, such as a thermoelectric device (TE), is employed to cool types of infrared detectors 32 that need to be cooled to an operating point below ambient temperature.

In a presently preferred embodiment of this invention, there are four spectral measurement channels. These are a spectral channel 32a of NO (which has a filter 30 with a passband centered at 5.26 μm), a spectral channel 32b of H₂O (having a filter 30 with a passband centered at 5.02 μm), a first reference channel, or spectral channel 32c of CO₂ (having a filter 30 with a passband centered at 4.2 gm), and a second reference spectral channel 32d (REF) (having a filter 30 with a passband centered at 3.8 μm). If desired, additional channels can be added to measure other pollutants.

In general, the spectral channel 32a of NO is located near the resonant absorption peaks in the vicinity of 5.2 μιη; the water vapor spectral channel 32b is in a region of intense water absorption where the fundamental lines do not saturate; the first spectral reference channel 32c is used to normalize pollutants to normal combustion products, that is, CO2; and the second reference spectral channel 32d (REF) is installed in a region where atmospheric gases or automobile emissions are not absorbed.

The REF spectral channel is installed to compensate the other three spectral channels for variations caused by: (a) fluctuations in the output of the infrared source 18; (b) particles in the form of road dust; and (c) particles in the mass 14 of exhaust gases, and by any other factors that may reduce the amount of illumination reaching the detectors 32. Thus, the REF spectral channel works to provide a baseline output that is independent of the molecular species (NO, H2O, and CO2) being measured. The output of spectral channel 32d REF is used to normalize, such as by division, the three spectral channels of molecular species 32a through 32c.

The detectors 32 are preferably made of high-detectivity (sensitivity) materials, and are also preferably fabricated on a common substrate or bonded to a common substrate. Suitable examples include, but are not limited to, a HgCdTe photoconductor (cooled by a thermoelectric device, TE); InSb that cools down to 77 ° K (liquid nitrogen temperatures); or uncooled detectors such as those based on bolometers, thermoelectric cells, pyroelectric detectors, and Pb salt detectors. Each detector 32 is configured with electronic circuitry 34 for amplification, and as such supplies an electrical signal to one of the four spectral measurement channels. Photovoltaic detectors can also be used.

Preferably, the detectors 32 are optically isolated from each other with an opaque material, such as alumina, to minimize optical crosstalk. The optical filters 30 have the predetermined passbands and are preferably formed on a transparent substrate and then adhered to the elements of the detectors 32 by an optically transparent adhesive. For example, filters 30 are formed on a substrate made of germanium (Ge), with filters 30 being formed as multilayer dielectric stacks that include multiple layers of zinc sulfide (ZnS).

Examples of dimensions for each of the detectors 30 are approximately 1 x 1 millimeters, but are not limited to. These dimensions are large enough to accommodate filters 32 and still achieve a high signal-to-noise ratio. The comprehensive design of the photodetector assembly (detectors and filters) ensures that the photodetectors operate isothermally, thereby eliminating inaccuracies due to temperature differences. If required, the temperature of detectors 32 is regulated by cooler 44.

IS 2 199 229 T3

The beam integrator 28 preferably includes a plano-convex lens portion 28a. A rectangular array of flat facets is formed on a convex surface of lens 28a. The facets refract light segments from the respective portions of incident beam 20 toward a central axis of symmetry, such that the refracted light segments overlap each other at photodetectors 32. The superimposed image incident on the detectors 32 is a refracted homogenized or averaged image of the facets, thereby representing the average intensity of the beam 20. A converging lens can optionally be used to reduce the size of the homogenized image on the photodetectors. 32.

The principles of a preferred embodiment of beam integrator 28 are described in US Patent No.

4,195,913, entitled "OPTICAL INTEGRATION WITH THREADED SUPPORTS", issued on April 1, 1980 to Mr.

Dourte. A beam integrator that is suitable for practicing the invention is commercially available from Spawr Optical Research, Inc. of Corona, CA.

It should be noted that the configuration of beam integrator 28 suitable for practicing the invention is not limited to the multi-faceted embodiment just described. For example, the beam integrator 28 can be realized by a convergent or divergent lens that produces an unfocused image of the beam 20 on the photodetectors 32. The beam integrator 28 can also be realized using a reflective rather than a refractive implementation.

One method of this invention, illustrated in Figure 6, includes the following steps:

(A) calibration of system 10 using cell 26; (B) Measurement of background water vapor concentration using water band absorption (for example, at wavelengths such as 1,998 cm^-1 or 2,023 cm^-1); (C) normalization of the background for absorption of water vapor in the NO band; and (D) real-time measurements of the transmission of automobile exhaust gases in spectral channels 32a to 32c of NO, water vapor and CO2, augmented by a subsequent step (E) of algorithmically compensating for the water emitted by the vehicle. 12 by using lookup table 42. As described above, look-up table 42 contains entries relating the water vapor absorption in the water spectral channel 32b to the water vapor absorption in the selected spectral band (or bands) of NO.

In this sense, Figure 2 shows a spectral region of interest around 5.26 µm, one of the preferred NO absorption lines. The water absorption band is shown at approximately 5.02 µm. The REF region has not been shown, at approximately 3.8 µm.

Figure 3 shows an expanded spectrum around the 5.26 µm NO line and illustrates the use of several different spectral widths (0.1%, 0.3%, and 1%) for the NO filter 30. The thick line (designated “A”) illustrates a case of 60% relative humidity with zero ppm NO in the exhaust gas mass 14, while the thinner line (designated “B”) illustrates the case of the 60% relative humidity with 1,000 ppm NO in exhaust gas mass 14. The graph in Figure 3 assumes "perfect" passband filters 30 having vertical faces and 100% transmittance. The term "% difference" is a measure of the effect of NO on the area under the curve.

Table 1 shows a fraction of NO absorption divided by water absorption as a function of relative humidity and spectral passband (determined by the filter of filters 30 that is used in particular).

TABLE 1

Relationship between absorption per 1,000 ppm of NO and absorption by water in a spectral band around 5.2 μm as a function of the filter spectral width and relative humidity

Relative humidity at 32 ° C (90 ° F)

<td>Filter width</td><td> 0%</td><td> 30%</td><td> 60%</td><td> 90%</td>

<td> 0,1%</td><td> 184%</td><td> 54%</td><td> 29%</td><td> 18%</td>

<td> 0,3%</td><td> 33%</td><td> 12%</td><td> 8%</td><td> 5%</td>

<td> 1,0%</td><td> 15%</td><td> 5%</td><td> 4%</td><td> 3%</td>

The entries in Table 1 also take into account the presence of water vapor in the mass 14 of exhaust gases. As shown, and for a relative humidity of 60% between, for example, an optical path length of 9 meters (30 feet) and a NO concentration of 1,000 ppm, the relationship between the relative absorption of the water line and the relative absorption due to NO varies from 29% for a 0.1% filter to 4% for a 1% filter. This relationship thus determines the precision of the field / laboratory calibration necessary to measure NO at a given sensitivity. For example, in the case of a 1% filter, the ratio of integrated NO absorption to spectral water absorption is 4%. Therefore, with 2% accuracy in the look-up table correction, 500 ppm NO is the limit of detection at 60% relative humidity at a 30-foot separation from the source.

ES 2 199 229 T3 infrared 18 and detectors 32. In contrast, with a NO filter 30 having a 0.1% passband the concentration ratio is 29%. For this case, with 2% accuracy in the look-up table correction, 100 ppm NO is the detection limit at 60% relative humidity and 30 feet (9 meters) apart.

The following is an example of the lookup table 42 for NO.

<td> %</td><td>CH absorption₂O (Band 1)</td><td>Transfer function</td><td>CH absorption₂O (B</td>

<td></td><td></td><td>T = CH₂O (Band 1) / CH₂O (Band 2)</td><td></td>

<td> 1,0</td><td> 0,0011014</td><td> 3,205</td><td> 0,00353012</td>

<td> 2,0</td><td> 0,00210755</td><td> 3,205</td><td> 0,00673933</td>

<td> 3,0</td><td> 0,0030484</td><td> 3,205</td><td> 0,00977024</td>

The graph in Figure 4 illustrates the importance of subtracting or compensating for absorption due to the water vapor background. In Figure 4, spectral plots for different amounts of relative humidity illustrate the monotonic deviation in the NO baseline as a function of relative humidity. This linear change, which corresponds to a flat top absorption integral, is compensated very precisely if a narrow band NO filter 30 is used.

Calibration with respect to variations in illumination provided by source 18 between the various detectors 32 is preferably carried out using hot calibration cell 26 which is depicted in Figure 5.

According to one aspect of this invention, the calibration cell 26 is constituted by a housing 50 which is divided into two compartments 26a and 26b by a pair of pistons 52 and 54 transparent to infrared radiation. Housing 50 has infrared transparent windows 50a and 50b at opposite ends thereof to allow beam 20 to pass through housing, and also through transparent pistons 52 and 54. Compartment 26b contains a hot mixture of the gases of interest (eg, NO, CO2, and H 2 O) in a predicted percentage such as, for example, 2%, 15%, and 15%, respectively. Other gases are typically provided, such as CO (15%) and various hydrocarbons (2%). The gas calibration mixture is maintained at a temperature, for example, around 100 ° C. The product of concentration path length within the sealed cell 26 is varied by redistributing the gas mixture between the optically transparent compartment 26b and an adjoining hermetic reservoir 56, which may also contain a source of water in a liquid state. The reservoir 56 includes a heater 56a, and the compartment 26a also includes a heater 26c to maintain the intended temperature.

The mechanical variation of the path length through the cell 26 is accomplished by magnetically driving the piston 54 with the piston 52 by means of two permanent or electrically activated ring magnets (52a and 52b). This is done with a pressurized gas (air) reservoir 50 and a conduit 60 that is connected to chamber 26a. Pressurization of compartment 26a causes piston 52 to move to the right in the drawing, thereby exerting a repulsive magnetic force on piston 54 via magnets 52a and 52b. This causes piston 54 to move to the right as well, thereby reducing the volume of gas mixture contained in chamber 26a through conduit 26a and plugged reservoir 56, and thereby varying the path length product within cell 26. By the action of springs 64 and 66 working in compression, a restoring force is applied.

The use of the calibration cell 26 allows the NO, H2O and CO2 detectors 32 to rapidly calibrate in situ with each other. Significantly, since cell 26 is a sealed system, no gases are released into the atmosphere, and as a result, it is not necessary to purchase and ship replacement calibration gas cartridges. Since NO is considered a toxic substance, this represents a significant advantage.

During the actual measurement of one or more masses of exhaust gases, the calibration cell 26 can be removed from the beam. Alternatively, calibration cell 26 can be controlled to reduce the volume of chamber 26a to zero. The minimal absorption of beam 20 as it passes through cell 26 is compensated for by the spectral channel measurement REF, as described above.

The use of other solutions, such as a bellows or rotating elements for linear feeds, is also within the scope of this invention.

As an example, reference is made to Figures 9A through 9C to illustrate additional embodiments of the calibration cell.

Figure 9A illustrates a calibration cell 70 that includes a bellows 72 that is propelled from a cylinder 74 of compressed air through valves 76a and 76b. Valve 76b is opened to vent bellows 72. Bellows 72 drives, by means of an infrared radiation transparent coupling 78, a piston 80. The piston 80 operates in a manner that varies the length of the path through a compartment. 82 for radiation propagating from a source 84 to a detector 86. This embodiment includes an expandable reservoir 88 for supplying the intended mixture of gases and also, preferably, steam to compartment 82.

IS 2 199 229 T3

Figure 9B shows an embodiment of a calibration cell 90 that employs electromagnets 92 to vary the length of the optical path through a compartment 94. The electromagnet assembly 92a is capable of movement and is adjustable attracted and repelled by the electromagnet. 92b. The moving electromagnet assembly 92a is loaded with springs 96. The expandable reservoir 88 is used, as in the embodiment of Figure 9A, to supply the intended gas mixture to the compartment 94.

Figure 9C shows an embodiment of a calibration cell 100 that operates with a reversible polarity electromagnet 102 and with an annular permanent magnet 104. For example, annular permanent magnet 104 may be a magnetized cobalt and samarium disk having a window. 104 transparent to infrared radiation and centrally arranged. Magnet 104 is contained within a bearing assembly 106. Reversing the polarity of the electromagnet 102 causes a movement of the magnet 104 that varies the length of the optical path through the compartment 108 containing the gases. Compartment 108 is also coupled to expandable reservoir 88, as in the embodiments of Figures 9A and 9B.

In the operation of the system 10, a signal processing routine executed by the data processing equipment 38 of Figure 1 carries out the method depicted in Figure 6. The data processing equipment normalizes the signal in the band of the NO both by the transmission determined in the REF channel and by the transmission determined in the water vapor channel, converted by the use of the look-up table 42 to a residual transmission of water in the NO band. This normalization is carried out continuously for both background measurements and dynamic emission measurements by applying the functional expression:

T (NO) eff = T (NO) / T (H2O) x [Query (H2O band with respect to NO band)] x T (REF), where T (NO) eff is the effective transmission of NO, T (NO) is the measured transmission of NO, T (H2O) is the measured transmission of water, Query (H2 Band = with respect to NO band) is a predetermined water / NO absorption correction obtained from lookup table 42 , and T (REF) is the measured transmission of the REF spectral channel.

The relative NO concentration in the exhaust gas mass 14 is obtained from the above expression using the transmission as a function of the NO detector 32 concentration established during factory calibration and updated during field calibration using the cell. 26. The actual concentration of NO in the mass of exhaust gases is determined as the ratio between the NO in the mass of gases and the measured concentration of CO2 in the mass of gases, multiplied by the relative concentration of CO2 in the exhaust gases determined using an effective ratio C : H for a “medium” fuel. For example, a CO2 concentration of 15% is a reasonable value for a mass of exhaust gases resulting from the combustion of a “medium” fuel.

In other words, the method first determines the effective transmission of NO from the measured transmission of NO modified by (a) the measured transmission of water, (b), the correction factor from the look-up table, and (c) the output of the REF channel that compensates for particles, dust and similar materials that can obscure the beam 20. Once the effective transmission of NO has been determined, this value is converted to a relative NO concentration using the results obtained from factory calibrations, last updated by using the known NO concentration within the calibration cell 26. Once the relative NO concentration is determined, this value is converted to an actual NO concentration using the measured CO2 concentration, and assuming the effective C: H ratio for a typical fuel.

Figures 8A and 8B are graphs illustrating the use of this invention in the measurement of an emission from a high emission vehicle (1,875 ppm) and a relatively low emission vehicle, respectively, Figure 8B further showing the vapor compensation. of water present in the mass of exhaust gases.

The flow chart in Figure 7 illustrates the overall operation of the system 10. The system 10 is suitable for operation alone or with attendant personnel, once the components have been configured and calibrated.

System 10 waits for a vehicle 12 to pass through beam 20. This is indicated by a sharp drop in the amplitudes of the output signals from photodetectors 32 when vehicle 12 blocks beam 20. This generates a trigger that initiates measurement of the NO concentration of the mass 14 of the vehicle's exhaust gases.

The amplitudes of the signals emitted by the detectors 32 will increase abruptly as the rear end of the vehicle 12 exits the beam 20. This indicates that the beam 20 is unlocked and propagates through the mass of the exhaust gases of the vehicle 12.

The data processing equipment 38 integrates the output signals from the detectors 32 during the intervals that the photodetectors 32 are unlocked by the trimmer 22. In this manner, the outputs from the photodetectors 32 are periodically sampled and processed.

The data processing equipment 38 then calculates the composition of the mass 14 of gases as a function of at least the percentage or concentration of the NO constituent, based on the amplitudes of the signals from the photodetectors 32. This data can be displayed visually, together with the video from camera 46, on monitor 40 as illustrated in Figure 1. This operation is performed for a predetermined period of time, for example half a second,

ES 2 199 229 T3 which is sufficient for the system to produce an accurate measurement. The data processing equipment 38 then determines whether the composition is within the tolerances specified in the standards. If so, the apparatus 10 reconfigures itself and waits for the next vehicle. If it is not, indicating that vehicle 12 is causing excessive pollution, the data processing equipment inputs a video frame from camera 46, the video signal of which includes a vehicle identification feature, such as an image of the license plate, superimposes at least the NO concentration data on the video frame, and saves the combined data and video frame on a mass memory device such as a hard disk.

The data can later be retrieved for executive use, such as sending a violation notice to the vehicle owner. It is also within the scope of the invention to save a combo table of data and video for each vehicle passing through the beam 20, rather than just for polluting vehicles, for applications such as generating a database of the composition of the exhaust gases. for different types and makes of vehicles.

A number of modifications can be made to system 10 that will be within the scope of this invention. As an example, calibration cell 26 may not be required if initial laboratory calibration of detectors has been deemed sufficient. Likewise, other detectors, with suitable filters, can be used to measure other molecular species of interest, such as CO and / or hydrocarbons.

Thus, although the invention has been shown in and described in particular with respect to a preferred embodiment thereof, those skilled in the art will understand that variations in form and details can be made therein without departing from the scope of the invention.