Abstract:
An apparatus for detecting a gas having distinct infrared radiation absorption characteristics. The apparatus includes a spectral source/bolometer for conducting an electrical current and for producing an infrared radiation. The source/bolometer is disposed along an axis and has a temperature and a characteristic resistance, and the characteristic resistance is a predetermined function of the temperature. A return reflector is disposed along the axis beyond the gas such that at least a portion of the infrared radiation passing through the gas is reflected back through the gas to the source/bolometer. The apparatus also includes a driver/detector for driving a current through the source/bolometer, for determining the characteristic resistance, and for detecting the gas from a variation of the characteristic resistance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is based on provisional application, U.S. Serial No. 60/067,713, filed on Dec. 4, 1997 by William Andrew Bodkin, entitled INFRARED SPECTRAGRAPHIC SYSTEM, and U.S. Provisional Patent Application Serial No. 60/094,602, filed on Jul. 30, 1998, by Edward A. Johnson.  
         [0002]    This application is related to U.S. Provisional Patent Application Serial No. 60/096,133 which is hereby incorporated by reference. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
         [0003]    Not Applicable  
         REFERENCE TO MICROFICHE APPENDIX  
         [0004]    Not Applicable  
         FIELD OF THE INVENTION  
         [0005]    The field of the invention is electro-optical radiation sources and detectors, and more particularly, to an apparatus that simultaneously functions as an electro-optical radiation source and an electro-optical radiation detector.  
         BACKGROUND OF THE INVENTION  
         [0006]    Non-dispersive Infrared (NDIR) techniques utilizing the characteristic absorption bands of gases in the infrared have long been considered as one of the best methods for composite gas measurement. These techniques take advantage of the fact that various gases exhibit substantial absorption at specific wavelengths in the infrared radiation spectrum. The term “non-dispersive” refers to the type of apparatus incorporating this particular measurement technique, typically including a narrow band pass interference filter (as opposed to a “dispersive” element, such as a prism or a diffraction grating) to isolate and pass radiation in a particular wavelength band from a spectrally broad band infrared source. The gas concentration is discerned from the detected intensity modulation of source radiation that is passed by the filter coincident in wavelength with a strong absorption band of the gas to be measured.  
           [0007]    A prior art NDIR gas analyzer typically includes a discrete infrared source with a motor-driven mechanical chopper to modulate the source so that synchronous detection may be used to discriminate spurious infrared radiation from surroundings; a pump to push gas through a sample chamber; a narrow band-pass interference filter; a sensitive infrared detector, and infrared optics/windows to focus the infrared energy from the source onto the detector. Although the NDIR gas measurement technique is recognized as one of the most effective methodologies for composite gas measurement available, it has not enjoyed wide application because of its complexity and high cost of implementation.  
           [0008]    Infrared absorption instruments traditionally contain a source of infrared radiation, a means of spectral selection for the gas under study, an absorption cell with associated gas sample handling and/or conditioning, any necessary optics, a sensitive infrared detector, and associated signal processing electronics. A typical source of infrared radiation includes an incandescent filament or a thin film conductor. The emissions spectrum of the infrared source may be tailored via surface texturing techniques, as are described in U.S. Pat. No. 5,838,016. The invention simplifies and reduces the cost of an infrared instrument by integrating the function of the infrared source and infrared detector into a single self-supporting thin-film bolometer element. This element is packaged with inexpensive molded plastic optics and a conventional spectral filter to make a transistor-size “sensor engine.” Combined with a simple reflector plate to define the gas sampling region, this sensor engine provides a complete gas sensor instrument which is extremely inexpensive and which will approach the sensitivity of conventional infrared absorption instruments.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is an apparatus for detecting a gas having a distinct infrared radiation absorption characteristics. The apparatus includes a spectral source/bolometer for conducting an electrical current and for producing an infrared radiation. The source/bolometer is disposed along an axis and has a temperature and a characteristic resistance; the characteristic resistance is a predetermined function of the temperature. The apparatus further includes a concentrating reflector for directing the infrared radiation along the axis, first through a spectral filter and then through the gas. The apparatus also includes a return reflector disposed along the axis beyond the spectral filter and the gas, such that at least a portion of the infrared radiation passing through the filter and the gas is reflected back through the gas and the filter to the source/bolometer. The apparatus further includes a driver/detector for driving a current through the source/bolometer, for determining the characteristic resistance, and for detecting the gas from a variation of the characteristic resistance.  
           [0010]    In one embodiment, the source/bolometer includes a thin-film conductor.  
           [0011]    In another embodiment, the source/bolometer includes a filament conductor.  
           [0012]    In another embodiment, the source/bolometer includes surface texturing so as to tailor a spectral characteristic of the infrared radiation.  
           [0013]    In a further embodiment, the concentrating reflector is disposed about the axis so as to form a first aperture along the axis and a second aperture along the axis, the source/bolometer is disposed at the first aperture and the spectral filter is disposed at the second aperture.  
           [0014]    In another embodiment, the concentrating reflector forms a compound parabolic concentrator.  
           [0015]    In another embodiment, the return reflector defines a gas sampling region.  
           [0016]    In another embodiment, the return reflector includes a flat reflective surface disposed substantially perpendicular to the axis.  
           [0017]    In another embodiment, the return reflector includes a contoured reflective surface disposed substantially about the axis.  
           [0018]    In one embodiment, the contoured reflective surface includes a parabolic surface.  
           [0019]    In another embodiment, the spectral filter substantially passes infrared radiation within a first passband and substantially blocks infrared radiation outside of the first passband.  
           [0020]    In a further embodiment, the spectral filter includes a micromesh reflective filter.  
           [0021]    In another embodiment, the micromesh reflective filter is fabricated using micro-electro-mechanical systems technology.  
           [0022]    In yet another embodiment, the driver/detector includes a Wheatstone bridge circuit having a first resistor pair and a second resistor pair, wherein a first resistor of the first resistor pair includes the source/bolometer.  
           [0023]    In another embodiment, a second resistor of the first resistor pair includes a blind source/bolometer being identical to the source/bolometer and filtered at a second passband.  
           [0024]    In another embodiment, a ratio of the first resistor pair is substantially equal to a ratio of the second resistor pair. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which:  
         [0026]    [0026]FIG. 1 illustrates one preferred embodiment of a combined infrared source and sensor;  
         [0027]    [0027]FIG. 2 illustrates another embodiment of the combined infrared source and sensor shown in FIG. 1;  
         [0028]    [0028]FIG. 3 illustrates a Wheatstone bridge used to drive the source/bolometer component of the source and sensor shown in FIG. 1; and,  
         [0029]    [0029]FIG. 4 shows a test configuration that incorporates the Wheatstone bridge of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]    Referring now to the drawings, FIG. 1 illustrates one preferred embodiment of a combined infrared source and sensor  100  including a spectral source/bolometer  102 , a concentrating reflector  104 , a spectral filter  106 , a return reflector  108  and a driver/detector circuit  110 . The concentrating reflector  104  is disposed substantially symmetrically about an axis  112  so as to form a first aperture  114  and a second aperture  116 . The source/bolometer  102  is disposed along the axis  112  at the first aperture  114  so as to direct the infrared radiation from the source/bolometer  102  along an axis  112  toward the second aperture  116 . The return reflector  108  is disposed along the axis  112  such that the infrared radiation from the source/bolometer  102  directed along the axis  112  is reflected back along the axis  112  through the second aperture  116  toward the source/bolometer  102 . The spectral filter  106  is disposed along the axis  112  at the second aperture  116 . Infrared radiation passing through the second aperture  116  (either from the source/bolometer  102  to the return reflector  108 , or vice versa) passes through and may be modified by the spectral filter  106 . The driver/detector circuit  110  is electrically coupled to a first terminal  118  and a second terminal  120  of the source/bolometer  102  via a first electrical conductor  122  and a second electrical conductor  124 , respectively.  
         [0031]    The spectral source/bolometer  102  may include a filament, a thin-film element or other infrared radiating components known to those in the art. The first terminal  118  and the second terminal  120  are electrically coupled to the source/bolometer  102  such that an external driver (e.g., the driver/detector circuit  110 ) can apply a voltage across the source/bolometer  102  via the first terminal  120  and the second terminal  122 , thereby inducing current flow through the source/bolometer. In one preferred embodiment, the surface of the source/bolometer may be textured so as to selectively tailor the infrared emissions spectrum to substantially match the absorption characteristics of the target gas to be detected.  
         [0032]    In the illustrated embodiment of the invention, the concentrating reflector  104  includes a parabolic reflector, although other reflector shapes (e.g., spherical, conical and custom contoured) may be used to adequately direct the infrared radiation from the source/bolometer  102  along the axis  112 . Similarly, although the embodiment illustrated in FIG. 1 includes a flat reflector, other reflector shapes may be use. The spectral filter  106  may include any one of several conventional designs known to those in the art to achieve tight spectral control of the infrared emission. In general, the spectral filter  106  passes only infrared radiation that is within a predetermined passband. The predetermined passband is chosen as a function of the target gas to be detected.  
         [0033]    The electrical resistance R of the source/bolometer  102  varies as a function of its equilibrium temperature T, i.e., R=f{T}. The function f{T} may be determined empirically or analytically for a particular source/bolometer  102 . For a given amount of input power applied to the source/bolometer  102 , the equilibrium temperature T of the source/bolometer  102  is dependent upon how fast it cools, and the cooling rate of the source/bolometer  102  is dependent on the optical absorption characteristics of its immediate environment. In general, different gases are known to each exhibit distinct optical absorption characteristics. The spectral filter  106  may be selected such that the infrared source and sensor  100  forms a tuned cavity band emitter corresponding to the absorption characteristics of the gas under study. Thus, the gas may be detected in the presence of the source/bolometer  102  by monitoring the resistance R of the source/bolometer  102 .  
         [0034]    [0034]FIG. 2 illustrates another embodiment of the present invention, that forms an infrared gas monitoring component  200  of an integrated on-board exhaust NOx meter (where x is a positive non-zero integer). This embodiment utilizes silicon micro-machining technology to construct a sensor that is radically simpler than conventional infrared absorption instruments. This embodiment simplifies and reduces the cost of an infrared absorption instrument by integrating the function of the infrared source and infrared detector into a single self-supporting thin-film source/bolometer  102 . The source/bolometer  102  includes inexpensive molded plastic optics and a conventional spectral filter  106  to make a transistor-size sensor engine  202 . Combined with a simple reflector plate to define the gas sampling region, this sensor engine provides a complete gas sensor instrument which is extremely inexpensive and which will approach the sensitivity of conventional infrared absorption instruments.  
         [0035]    The embodiment of FIG. 2 illustrates a novel, low-cost infrared gas sensor using a thin-film source/bolometer  102  in an open path atmospheric gas measurement. As described herein, the source/bolometer  102  reaches radiative equilibrium with its surroundings at a slightly lower temperature if gas absorption frustrates light re-imaging source/bolometer  102 . The concentrating reflector  104 , in this case a compound parabolic concentrator, defines a relatively narrow illumination cone (+/−15 degrees about the axis  118 ) and the passive return reflector  108  is contoured to provide a pupil-image of the spectral filter  106  onto itself. The entire sensor engine  202  can be mounted in a substantially small package, e.g., on a TO-8 transistor header.  
         [0036]    Tight spectral control of the infrared emission is important in making the source/bolometer  102  work well. The device is particularly effective if the amount of radiation absorbed by gas molecules under study is measurably large in terms of the overall thermal budget of the bolometer surface. Thus, a tuned cavity band emitter is preferably constructed with spectral resolution (dl/l) around 0.1, roughly the performance achieved to date with micromesh reflective filters. This increases the conversion efficiency to nearly 15% for the NOx application. This level of surface topology (and therefore spectral) control, is achieved through micro-electro-mechanical systems (MEMS) technologies. An individual emitter die is packaged, together with individual infrared detector pixel elements and thin film interference filter windows in TO-8 transistor cans using standard process equipment.  
         [0037]    The embodiment illustrated in FIG. 2 uses drive and readout schemes having a microprocessor controlled, temperature-stabilized driver to determine resistance from drive current and drive voltage readings. The current and voltage information shows that incidental resistances (temperature coefficients in leads and packages and shunt resistors, for instance) do not overwhelm the small resistance changes used as a measurement parameter. The Wheatstone bridge  300  shown in FIG. 3, a straightforward analog control circuit, is used to drive the source/bolometer  102  and determine the incremental resistance values. The Wheatstone bridge is simple and accurate, is substantially insensitive to power supply variations and is relatively insensitive to temperature. The circuit is “resistor” programmable, but depends for stability on matching the ratio of resistors. In one form of the invention, an adjacent “blind” pixel, i.e., an identical bolometer element (a blind source/bolometer), filtered at some different waveband, is used as the resistor in the other leg of the bridge, allowing compensation for instrument and component temperatures and providing only a difference signal related to infrared absorption in the gas. The Wheatstone bridge provides a simple computer interface, and since it is implemented with relatively robust analog parts, it is not susceptible to radiation damage at high altitudes or in space. For the Wheatstone bridge  300  shown in FIG. 3, bridge is balanced when the ratio of the resistor pair R 1  and R 2  is substantially equal to the ratio of the resistor pair R 3  and R 4  (i.e., R 1 /R 2 =R 3 /R 4 ), and to first order, temperature coefficients of R 1  and R 2  can be neglected if resistors are matched.  
         [0038]    The temperature coefficient of R 3  is important but should have negligible effect across the relatively small change in temperature of the bolometer caused by the gas absorption. Preferably, the resistors are chosen so that the bridge is substantially balanced at the target operating temperature. The estimated errors from an analog readout of this circuit come from the amplifier input offset and input bias currents which introduce offset voltage or error term. FIG. 4 shows a test configuration that incorporates the Wheatstone bridge  300 . Note that the component reference designations FIG. 4 do not correspond to those in FIG. 3.  
         [0039]    An optics test bed has been used to evaluate different configurations and perform measurements of this embodiment. In an elevated ambient temperature environment (e.g., automotive), the device is operated as instrumented tube furnaces and to calibrate the infrared readings against a conventional gas analyzer.  
         [0040]    The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.