Patent Publication Number: US-2007114421-A1

Title: Gas Sensor Array with a Light Channel in the Form of a Conical Section Rotational Member

Description:
FIELD OF THE INVENTION  
      The present invention relates to a gas sensor array with at least one radiation source emitting radiation, a gas measuring chamber or light channel, which can be filled with a measuring gas that contains at least one analyte to be measured, and at least one radiation detector, which generates an output signal dependent on the presence and/or concentration of the analyte. In particular, the present invention relates to a miniaturized gas sensor array having the above-described elements that can be used, for example, in motor vehicles.  
     BACKGROUND OF THE INVENTION  
      Gas sensor arrays are known for the detection of a wide range of analytes, for example, methane or carbon dioxide, and are disclosed, for example, in European patent application EP 1 566 626 A1. These gas sensor arrays are based on the idea that many polyatomic gases absorb radiation, in particular in the infrared wavelength range. Such absorption occurs in a wavelength characteristic for the relevant gas, for example, at 4.24 μm in the case of carbon dioxide. With the help of such infrared gas sensors it is thus possible to determine the presence of a gas component and/or the concentration of this gas component.  
      Gas sensor arrays normally have a source of radiation, a gas measuring chamber or light channel, and a radiation detector. The intensity of radiation measured by the radiation detector is an indication of the concentration of the absorbing gas in the gas measuring chamber. It is either possible to use a broadband source of radiation with the wavelength of interest being adjusted via an interference filter or grid, or it is possible to use a selective source of radiation, for example a light-emitting diode (LED) or a laser, in combination with non wavelength-selective radiation receivers.  
      The detection of carbon dioxide is becoming increasingly important in the motor vehicle sector. This is partly due to the fact that in motor vehicles the carbon dioxide content of the interior air is monitored to increase energy efficiency in heating and air-conditioning. For example, when a high carbon dioxide concentration is detected, a supply of fresh air is initiated via a corresponding air vent control system. In modem air-conditioning systems, which are based on carbon dioxide as a coolant, on the other hand, the carbon dioxide gas sensors perform a monitoring function in association with escaping carbon dioxide in the event of possible defects. However, such sensors must satisfy extremely stringent requirements in terms of robustness, reliability, and above all size, especially in the motor vehicle sector.  
      In European patent application EP 1 566 626 A1, it is known that the detector and the radiation source are arranged in a housing in such a manner that inner surfaces of this housing, which are equipped with a reflective coating, form a light channel directing the light to the detector. Each radiation source is assigned a separate light channel formed by a hemispherical concave mirror and a tube. However, the array shown in this application has the disadvantage that the light efficiency is comparably low in the range of the maximum permissible angle of incidence diverging from a main axis of the detector.  
     BRIEF SUMMARY OF THE INVENTION  
      It is therefore an object of the present invention to provide a gas sensor array of the type specified above, which has an increased light efficiency and the highest possible selectivity while still being compact and low-cost to manufacture.  
      This and other objects are achieved by a gas sensor array comprising a housing having a gas measuring chamber. A detector at least partially arranged in the gas measuring chamber measures radiation and generates an output signal as a function of the measured radiation. The detector is arranged on a main axis of the housing. Radiation sources are at least partially arranged in the gas measuring chamber and direct radiation toward the detector. The radiation sources are arranged symmetrically to the main axis at a first focal point and have the same effective radiation path length to the detector. The gas measuring chamber has at least one concave mirror formed by inner walls of the housing. The inner walls form a rotational member produced by a conical section and are configured to bundle the radiation emitted from the radiation source at a second focal point proximate the detector.  
      This and other objects are achieved by a gas sensor array comprising a housing having a gas measuring chamber. A detector at least partially arranged in the gas measuring chamber measures radiation and generates an output signal as a function of the measured radiation. At least one radiation source at least partially arranged in the gas measuring chamber directs radiation toward the detector. The gas measuring chamber has at least one concave mirror formed by inner walls of the housing. The inner walls form a rotational member produced by a conical section and are configured to bundle the radiation emitted from the radiation source at a focal point proximate the detector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view of a gas sensor array according to a first embodiment of the invention;  
       FIG. 2  is a perspective view of a first half of a housing of the gas sensor array of  FIG. 1 ;  
       FIG. 3  is a top schematic view of the gas sensor array of  FIG. 1 ;  
       FIG. 4  is a partially cut away perspective view of a gas sensor array according to a second embodiment of the invention;  
       FIG. 5  is a partially cut away perspective view of the gas sensor array of  FIG. 4  showing the light rays;  
       FIG. 6  is a sectional view of the gas sensor array of  FIG. 4 ;  
       FIG. 7  is a top schematic view of the gas sensor array of  FIG. 4 ;  
       FIG. 8  is a diagrammatic view of the path of the light rays in a gas measuring chamber in the form of a rotational ellipsoid; and  
       FIG. 9  is a diagrammatic view of the path of the light rays in a gas measuring chamber partially in the form of a rotational paraboloid. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIGS. 1-3  show a gas sensor array  100  according to a first embodiment of the invention. As shown in  FIG. 1 , the gas sensor array  100  comprises a housing consisting of a first half  106  joined with a second half  112 . The housing may be formed, for example, from a plastic material using injection-molding. As shown in  FIG. 2 , infrared radiation sources  102 ,  104  are arranged in the first half  106  of the housing. The radiation sources  102 ,  104  may be, for example, lamps that emit broadband light spectrums or light-emitting diodes (LED), whereby the latter has the advantage that it is possible to dispense with filter arrays for wavelength selection. The radiation sources  102 ,  104  directs radiation or light rays  105  toward a detector  108  arranged in the first half  106  of the housing. The detector  108  may be, for example, a pyrodetector, which evaluates incoming radiation and supplies an electrical output signal as a function of the measured radiation. The detector  108  is provided with a shield  130  and a sensor  138  ( FIG. 3 ). The sensor  138  is positioned substantially parallel to a main axis  132  of the housing. It will be appreciated by those skilled in the art that although two radiation sources and one detector are shown in the illustrated embodiment, any number of radiation sources and/or detectors may be used.  
      The radiation sources  102 ,  104  may consist, for example, of a measuring radiation source and a reference radiation source, which operate on a differential measuring principle. The radiation sources  102 ,  104  are arranged symmetrically to the main axis  132  and the detector  108  is arranged on the main axis  132  in such a manner that the paths of the light rays  105  of the radiation sources  102 ,  104  have the same effective radiation path length to the detector  108 . Such a gas sensor array  100  array can be operated, for example, in such a manner that, as disclosed in German patent specification DE 199 25 196 C2, the reference radiation source is switched on at periodic intervals to check the ageing condition of the radiation source. Deviations in relation to the output signals of the detector  108  with the reference radiation source switched on and the measuring radiation source switched off provide information about ageing of the measuring radiation source and this can be compensated for as appropriate. This provides for a marked increase in the reliability and service life of the gas sensor array  100  particularly in the motor vehicle sector.  
      As shown in  FIG. 1 , the first half  106 , which includes the radiation sources  102 ,  104  and the detector  108 , is arranged on a first printed circuit board  122 . Terminals  126  extend from the detector  108  and are electrically connected to signal evaluation electronics arranged on a second printed circuit board  124 . The second half  112  of the housing is provided with a gas inlet  118 . The gas inlet  188  is equipped with a filter  120  configured for removing particles of dirt.  
      As shown in  FIG. 1 , an external housing  128  surrounds the first and second halves  106 ,  112  and the first and second printed circuit boards  122 ,  124 . The external housing  128  protects the entire gas sensor array  100  from dust, environmental influences, and undesirable scattered light. The external housing  128  allows the first and second halves  106 ,  112  of the housing to be manufactured with much thinner walls, as the mechanical stability is ensured by the external housing  128 . It is, however, possible to form the gas sensor array  100  without the external housing  128 .  
      As shown in  FIG. 1 , inner walls of the first and second halves  106 ,  112  form a light channel or gas measuring chamber  110 . In the illustrated embodiment, the inner walls of the gas measuring chamber  110  form a rotational ellipsoid. A gas containing an analyte, such as carbon dioxide, is contained in the gas measuring chamber  110 . The intensity of the radiation reaching the detector  108  depends on the composition of the gas contained in the gas measuring chamber  110 . The inner walls are coated with a reflective material. The reflective material may be, for example, a metal such as gold and may be deposited on the inner walls by, for example, sputtering, vapor-depositing, or electroplating. The inner walls thereby form a concave mirror and at least partially take the form of a rotational member produced by a conical section, which is designed in such a manner as to result in bundling of the light rays  105  at a region in which the detector  108  is arranged. The radiation sources  102 ,  104  are arranged at a first focal point  114 . The detector  108  is arranged proximate a second focal point  116 . As can be seen from the course of the light rays  105 , in accordance with the laws of optics, the shape of the gas measuring chamber  110  greatly improves bundling of the light rays  105  at the detector  108 . At the second focal point  116 , a tilted mirror (not shown) is provided that is positioned and configured to direct the light rays  105  to the sensor  138  of the detector  108 . The tilted mirror (not shown) may be, for example, aligned parallel to the main axis  132  of the housing. Alternatively, the detector  108  may be installed crosswise to the main axis  132  of the housing. A temperature sensor (not shown) may be provided for monitoring the temperature in the gas measuring chamber.  
      To ensure that each of the radiation sources  102 ,  104  is arranged at the first focal point  114 , a connecting region  134  is provided between the detector  108  and the radiation sources  102 ,  104 . The connecting region  134  extends between the radiation sources  102 ,  104  and the detector  108  and follows the curvature of the inner walls of the gas measuring chamber  110  in the direction of the main axis  132 , but is not curved transverse to the direction of the main axis  132 . In the embodiment shown, longitudinal limits  135 ,  136  of the connecting region  134  run substantially parallel to each other and the path of the light rays  105  of the two radiation sources  102 ,  104  also run substantially parallel to each other. A flat projection of the connecting region  134  has a substantially rectangular shape.  
      It can generally be demonstrated that for clear separation of the various frequency ranges of the radiation sources  102 ,  104 , only the proportion of the light rays  105  deviating from 0 degrees to a maximum permissible angle of incidence from the main axis  132  should be evaluated. This maximum permissible angle of incidence depends on such factors as, for example, the choice of the wavelength-selective filter before the detector  108 , which is selected according to the light frequency of interest depending on the analyte to be detected. In the case of the gas sensor array  100  shown, the maximum permissible angle of incidence is, for example, approximately 20 degrees, although other values are also possible. For this reason, in the embodiment shown in  FIG. 1 , the detector  108  is provided with the shield  130 , which prevents the incidence of the light rays  105  deviating more than about 20 degrees from the main axis  132 . In other words, the shield  130  is arranged around the detector  108  so that only the light rays  105  deviating between 0 degrees and approximately 20 degrees from the main axis  132  reach the detector  108 . However, other values for the maximum permissible angle of incidence are likewise possible as already mentioned, depending on the gas component to be detected. It is also possible to dispense with the shield  130 .  
      According to the first embodiment shown in  FIGS. 1-4 , the radiation sources  102 ,  104  are arranged next to each other and the longitudinal limits  135 ,  136  of the connecting region  134  extend substantially parallel to each other. Each of the radiation sources  102 ,  104  is thus located on one half of the first focal point  114  of the rotational ellipsoid of the gas measuring chamber  110  associated therewith. This variant represents a solution that is very simple to perform on assembly but has the disadvantage that bundling in the sensor  138  takes place at two places at the second focal point  116 .  
       FIGS. 4-7  show a second embodiment of a gas sensor array  100  according to the invention, which improves upon the gas sensor array  100  according to the first embodiment of the invention. As shown in  FIG. 7 , in the gas sensor array  100  according to the second embodiment, the connecting region  134  is formed so that the longitudinal limits  135 ,  136  of the connecting region  134  enclose an angle corresponding to an angle enclosed by center lines of the radiation sources  102 ,  104 . In other words, the connecting region  134  has longitudinal limits  135 ,  136  corresponding to a center line extending between each of the radiation sources  102 ,  104  and the detector  108 . This produces two rotationally elliptical regions of the gas measuring chamber  110 , which have different first focal points  114 ,  115  but only one second focal point  116 , which is located at the detector  108 . A flat projection of the connecting region  134  has a substantially trapezoidal shape.  
      As shown in  FIG. 4 , the inner walls of the gas measuring chamber  110  only partially take the form of a rotational ellipsoid. A substantially flat tilted mirror  140  is arranged at the second focal point  116  of the rotational ellipsoid. The tilted mirror  140  can be manufactured as a single piece from the first and second halves  106 ,  112  of the housing by applying a metal coating to the first and second halves  106 ,  112  of the housing. As shown in  FIGS. 5-6 , the tilted mirror  140  is arranged above the detector  108  so that the light rays  105 , which arrive at the second focal point  116 , are focused on the sensor  138 . To clarify the functional principle, both the real and the virtual paths of the light rays  105  are shown in  FIGS. 5-6 . The second focal point  116  is therefore a virtual focal point, whereas the light rays  105  for the embodiment shown in  FIGS. 1-3  also actually meet at the second focal point  116 , which is a real focal point.  
      As shown in  FIG. 4 , another tilted mirror  142  is provided in a region below the detector  108 . This tilted mirror  142  deflects the light rays  105  striking it to the opposite rotationally elliptical inner wall from where the radiation can then be focused on the tilted mirror  140 . The tilted mirror  142  thus further increases light efficiency.  
      The assembly of the gas sensor array  100  will now be described. The detector  108  and the radiation sources  102 ,  104  are mounted on the first printed circuit board  122 . The second printed circuit board  124 , on which other electronic components are mounted, such as those required for sensor signal evaluation and control of the infrared radiation sources, is connected to the terminals  126  of the detector  108  and accordingly also to the radiation sources  102 ,  104 .  
      The first half  106  of the housing is mounted on the first printed circuit board  122  so that the radiation sources  102 ,  104  and the detector  108  are held in corresponding recesses. To ensure overall installation space for geometrical extension of the measuring chamber  110  crosswise to the main axis  132 , a corresponding opening, into which the measuring chamber  110  can reach, is provided in the first printed circuit board  122 .  
      The second half  112  of the housing is positioned on the first half  106  of the housing and fixed in place, for example, using a screwed connection. If necessary, the external housing  128  can also be provided to ensure additional protection from mechanical stress and the penetration of scattered light that may cause interference. As shown in  FIGS. 4-7 , the external housing  128  may also be integrally formed with the first and second half halves  106 ,  112  of the housing. Although such integration of the first and second halves  106 ,  112  and the external housing  128  requires more material and thus also increases the weight of the housing, it simplifies the manufacturing process to a significant extent and also offers very high mechanical stability. A boundary layer between the first half  106  and the second half  112  of the housing may optionally be sealed with a suitable sealing device, as taught in EP 1 566 626 A1.  
      The present invention makes it possible to provide an optimized light channel, which is simple and provides a much greater light efficiency. By reducing the proportion of light outside the maximum permissible angle of incidence with reference to the main axis  132 , it is also possible to achieve a clearer separation of various frequency ranges. The gas sensor array  100  according to the invention is therefore suitable for use in motor vehicles sector.  
      Although  FIGS. 1-7  illustrate a rotationally elliptical design of the gas measuring chamber  110 , it is also possible to use other conical sections to produce the gas measuring chamber  110 .  FIGS. 8-9  show, for example, a diagrammatic comparison of the direction of the light rays  105  for a rotational ellipsoid ( FIG. 8 ) where the inner walls of the gas measuring chamber  110  take the form of a rotational paraboloid. According to  FIG. 9 , two parabolic mirrors are set up facing each other so that this embodiment also results in bundling of the radiation emitted at the first focal point  914  at a second focal point  916  at which the detector  108  can be arranged. One of the advantages of such a design is that a region of a parallel ray path  900  can be selected in terms of length according to the requirements placed on the sensitivity of the gas sensor array  100 . With very low detection limits, it may be necessary to extend the optical path length through the gas measuring chamber  110  to generate a sufficiently great detection signal.  
      The present invention is based on the fundamental idea that light efficiency can be significantly increased with simple geometry of the gas measuring chamber  110  and an array of components suitable for production when a housing containing the radiation sources  102 ,  104 , the gas measuring chamber  110  and the detector  108  has reflective inner walls, which form a concave mirror and at least partially take the form of a rotational member produced by a conical section, which is designed in such a manner as to result in bundling of the light rays  105  emitted at a region in which the detector  108  is arranged. In this way, a much greater light efficiency can be achieved with the same radiation source intensity. In addition, the proportion of light outside the maximum permissible angle of incidence can be reduced, thus allowing the various frequency ranges to be separated more clearly from each other. Here, the maximum permissible angle of incidence depends on such factors as the choice of the filter arranged before the detector  108  and may be about 20 degrees, for example. In terms of production technology such a housing shape can be manufactured with comparably simple tools.  
      The rotational member can be formed by a rotational member produced by a conical section such as a rotational ellipsoid, a rotational paraboloid or a rotational hyperboloid and also by parts of these bodies. In the geometrically simplest case, the radiation sources  102 ,  104  are located at the first focal point  114  of a rotational ellipsoid, while the detector  108  is located at the second focal point  116  of the rotational ellipsoid on which the radiation emitted by the radiation sources  102 ,  104  is focused. This gas sensor array  100 , however, has the disadvantage that the sensor  138  of the detector  108  has to be aligned crosswise to the main axis  132  of the housing and thus cannot be simply mounted on the same first printed circuit board  122  as the radiation sources  102 ,  104 . According to an advantageous development of the present invention, it is thus possible to provide, in addition to the rotationally elliptical shape of the gas measuring chamber, for the at least one tilted mirror  140  which deflects the bundled radiation once again so that it strikes the sensor  138  of the detector  108 . The tilted mirror  140  is preferably designed as a flat mirror. It is, however, clear that another concave mirror can also be provided if needed.  
      The gas sensor array according to the invention can be integrated in electronic systems in a particularly space-saving manner where it is designed so that it can be mounted on the printed circuit board as a module. This also offers the advantage that the necessary evaluation electronics, which, for example, are used for further processing of the output signal generated by the detector  108 , can be installed on the same printed circuit board.  
      The radiation sources  102 ,  104  are arranged so that they are positioned substantially next to each other and their light ray paths only enclose a comparably small angle. Thus, manufacture of the gas sensor array  100  can be simplified to a marked extent. In order to achieve the greatest possible bundling of the respective radiation at the detector  108 , the rotationally elliptical form of the gas measuring chamber  110  can be interrupted by the connecting region  134  between the radiation sources  102 ,  104  and the detector  108 . This connecting region  134 , according to the first embodiment, is shaped as part of an elliptical cylinder jacket, which in a longitudinal direction, i.e. in the direction of the connection between The radiation sources  102 ,  104  and the detector  108 , follows the curvature of the rotational ellipsoid but is not curved in a transversal direction, a flat projection of this connecting region  134  being rectangular. In this way, each of the radiation sources  102 ,  104  is located at the focal point of the rotationally ellipsoidal inner surface of the housing closest to it and its radiation is bundled particularly effectively.  
      The disadvantage of this gas sensor array  100  is, however, that two second focal points  116  likewise occur at the site of the detector  108 . To overcome this disadvantage, according to a second embodiment, the inner walls of the housing can be designed in such a manner that the connecting region  134  in the form of an elliptical cylinder jacket has a trapezoidal flat projection. Thus, each of the radiation sources  102 ,  104  is then located at the first focal point  114 ,  115  of the half of the rotational ellipsoid assigned thereto while the second focal points  116  coincide and lie on the sensor  138  of the detector  108 .  
      The advantageous properties of the gas sensor array  100  according to the invention are particularly useful for the detection of carbon dioxide, for example, in the motor vehicle sector, and for monitoring carbon dioxide leaks as well as for checking the air quality in an interior of a vehicle. However, the gas sensor array  100  according to the invention can of course also be used for the detection of any other gases.  
      The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.