Patent Publication Number: US-2009235720-A1

Title: Dome Gas Sensor

Description:
The present invention relates to gas sensing, in particular gas sensors such as non-dispersive infrared (NDIR) gas sensors having a radiation source, radiation detector and a reflector arranged to reflect radiation from the radiation source to the radiation detector. 
     In the field of gas sensing, there is a requirement for small, low cost gas sensors that can operate over a wide range of environmental conditions. This is driven by legislation directed to increasing safety and reducing emissions in a variety of applications. For example, in the automotive industry, sensing of the presence of automotive exhaust gases and CO 2  in vehicle cabins and engine management systems are applications where a small form factor as well as low cost and efficiency are desirable. The need for detection of CO 2  in vehicle cabins comes from the move towards CO 2  refrigerant based air conditioning systems away from the use of more environmentally harmful Fluorocarbon based refrigerants such as P134a. By providing CO 2  based air conditioning systems, automotive manufacturers will be able to avoid emission penalties applied to the disposal and recycling of hydrofluorocarbons. However, conventional gas sensors suitable for CO 2  and CO gas sensing are too large and too expensive for use in such automotive applications. Furthermore, in such applications, the gas sensor is required to operate over a wide range of temperatures. 
     Like the automotive industry, industrial heating, ventilation and air conditioning (HVAC) systems based on CO 2  refrigerants require low cost CO 2  gas sensors that operate in a variety of environments. The safety products that detect combustion or solvent gas leaks in many applications, from gas welding through automatic production processes to solvent cleaners, also require low-cost, efficient gas sensors. 
     In the domestic heating field, gas sensors are used to provide safety from carbon monoxide poisoning. Furthermore, combustion gas sensing provides safety from an explosion risk. 
     In gas sensing, infrared gas sensors have advantages compared with other technologies, including long lifetime and resistance to poisoning. However many infrared detectors use thermal components such as incandescent sources (e.g. bulbs) and pyro-electric or thermopile detectors, which themselves have several disadvantages. For example they may have a slow response or a limited wavelength range and may require explosion-proof housing to prevent the bulb acting as an ignition source. Replacing the incandescent sources and thermal detectors with high performance LEDs (Light Emitting Diodes) and photodiodes offers advantages including low power, fast response and intrinsic safety, for a greater range of gases. 
     Gas sensors may be made using a LED and a photodiode that are manufactured at matched frequencies such that they have stable and very narrow coincident optical bandwidths in operation. 
     In an NDIR gas sensor, light is emitted from a light source, passed through a gas and then measured by a light detector. For efficient detection of a gas, it is important to have a large interaction between the light and the gas and this is influenced by the length and volume of the interacting optical path, and the transport of the gas into and out of the interacting optical path. The problems with simply arranging a detector in front of an emitter are that when the light diverges from the source, only a small proportion of the light is incident upon the detector and the optical path length is merely the distance around the emitter and the detector. Therefore, there is a relatively small length and volume for the gas to interact with the light. Known approaches to improve this arrangement are to coat the internal walls of the sensor housing with reflective material and to fold the optical path by the use of mirrors. However, though the folded linear path the latter approach can increase the optical path length, it retains the problem that the path only sweeps out part of the available volume between the light source and the light detector, even using curved mirrors. Thus with a folded optical path, only a fraction of the available volume of the sensor housing is used for the interacting optical path. Also, a multiply folded optical path, for example in a zig-zag shape, requires multiple reflectors that need to be well aligned. 
     It is an object of the present invention to provide a compact, high-efficiency gas sensor. 
     According to the present invention, there is provided a gas sensor comprising:
         a radiation source;   a radiation detector; and   a reflecting means arranged to reflect radiation from the radiation source to the radiation detector along an optical path,
 
wherein the radiation source and the radiation detector are disposed side by side.
       

     Preferably the gas sensor further comprises a screen disposed in between the radiation source and the radiation detector. 
     Preferably the screen is disposed in line with the radiation source and the radiation detector. 
     Preferably the screen is configured to reflect radiation. 
     Preferably the reflecting means is arranged to reflect radiation divergent from the radiation source and to concentrate the reflected radiation onto the radiation detector. 
     Preferably, the reflecting means is arranged such that the optical path is defined at least in part by a cavity extending around the radiation source and radiation detector. 
     Preferably the cavity is bounded by a plane parallel to surfaces of the radiation source and the radiation detector. 
     Preferably, the reflecting means comprises a curved surface. 
     Preferably, the reflecting means comprises a dome. 
     Preferably, the reflecting means has a radial symmetry. 
     Preferably, the reflecting means comprises a hemispherical surface. 
     Alternatively, the reflecting means comprises a semi-ellipsoidal surface. 
     Preferably the reflecting means comprises a mirror. 
     Preferably, the reflecting means comprises a reflective surface of a housing. 
     Preferably, the housing has at least one aperture for permitting the transport of gas in and out of the gas sensor. 
     Preferably the radiation source is a light emitting diode having an emission bandwidth. 
     Preferably the gas sensor further comprises a filter in the optical path configured to filter at least a portion of the emission bandwidth. 
     Preferably, the radiation source and radiation detector are mounted on a common substrate. 
     Preferably the screen is mounted on the substrate. 
     Alternatively, the substrate comprises the screen. 
     Optionally, the substrate is configured to provide structural support for the radiation source and radiation detector within the gas sensor. 
     Preferably the substrate is configured to locate the radiation source and radiation detector in relation to the housing. 
     By acting a mechanical location means it therefore avoids the need for location adjustment during assembly. 
     Preferably the substrate is configured as an elongate member extending along a diameter of the housing. 
     Optionally, the gas sensor further comprises a temperature adjusting means for adjusting the temperature of the radiation source and radiation detector simultaneously. 
     Optionally, the gas sensor further comprises a temperature sensing means for sensing the temperature of the radiation source and radiation detector simultaneously. 
     Preferably, the temperature sensing means comprises a thermistor. 
     Optionally, the temperature sensing means uses the characteristics of the radiation source and/or detector to measure temperature. 
     Optionally, the substrate further comprises a signal processing means for processing signals relating to the radiation source. 
     Optionally, the substrate further comprises a signal processing means for processing signals relating to the radiation detector. 
     Optionally, the substrate further comprises a signal amplifying means for amplifying signals relating to the radiation detector. 
     Preferably, the radiation source and radiation detector are in thermal communication. 
     Preferably, the radiation source is operable to heat the radiation detector. 
     Preferably, the radiation detector is heated above the dew point of ambient gas. 
     Preferably, the gas sensor further comprises a radiation source reflector arranged to reflect radiation from the radiation source back into the radiation source. 
     Optionally, the radiation source reflector is applied to a surface of the radiation source. 
     Optionally the radiation source reflector is provided by the mounting of the radiation source. 
     Preferably, the gas sensor further comprises a radiation detector reflector arranged to reflect radiation from the radiation detector back into the radiation detector. 
     Optionally, the radiation detector reflector is applied to a surface of the radiation detector. 
     Optionally the radiation detector reflector is provided by the mounting of the radiation detector. 
     Preferably, the radiation source and radiation detector are fabricated from the same substrate. 
     Preferably, the reflecting means comprises a surface comprising a plurality of sub surfaces, each defined by an arc with a radius and a centre point, the arcs being swept out around an axis, and each sub surface being tangent to an adjacent sub surface and having a different radius and different centre point from the adjacent sub surface. 
     Preferably, the axis is in line with the radiation source and the radiation detector. 
     Optionally, the arc length tends to zero. 
     Preferably, the sub surface are semi-toroidal. 
     Preferably, the surface is configured such that radiation originating from a point on the radiation source is unfocussed as it converges on the radiation detector. 
     Preferably, the surface is configured to reflect radiation from the radiation source to a corresponding location on the radiation detector, irrespective of the radiation exit angle from the radiation source. 
     Preferably, the surface is configured to reflect radiation leaving the centre of the radiation source to the centre of the radiation detector, radiation leaving the outer side of the radiation source to the outer side of the radiation detector, and radiation leaving the inner side of the radiation source to the inner side of the radiation detector. 
     Preferably, the surface is configured to reflect radiation such that the length of the optical path is on average equal for each sub surface. 
     Preferably, the elongate member is adjustable so as to optimise the location of a reflected radiation pool on the radiation detector. 
     Preferably, the elongate member is adjustable by sliding of pins. 
     Preferably, the pins are electrical leads. 
     Preferably, the adjustable elongate member is lockable with respect to the reflecting means. 
     Preferably, the adjustable elongate member is lockable by gluing the pins to the reflecting means. 
     Preferably, the adjustable elongate member is lockable by soldering the pins. 
    
    
     
       The present invention will now be described by way of example only with reference to the figures in which: 
         FIG. 1  illustrates in schematic form a cross section of a first embodiment of a gas sensor; 
         FIG. 2  illustrates in schematic form a cross section of the radiation source and radiation detector assembly; 
         FIG. 3  illustrates in schematic form a cross section of a second embodiment of a gas sensor; 
         FIG. 4  illustrates in schematic form a perspective view of the second embodiment of a gas sensor; 
         FIG. 5  illustrates in schematic form a half cross section of the dome reflector; 
         FIG. 6  illustrates in schematic form rays of light being reflected between the centres and outer sides of the radiation source and radiation detector; 
         FIG. 7  illustrates in schematic form rays of light being reflected between the centres and inner sides of the radiation source and radiation detector; and 
         FIG. 8  illustrates in schematic form a cross section of an embodiment of the gas sensor having an adjustable bridge. 
     
    
    
     With reference to  FIG. 1 , a partial cross section of a gas-sensor in accordance with a first embodiment of the present invention is shown. The gas sensor has a screen  1  in between a LED radiation source  2  and a photodiode radiation detector  3  on a substrate  4  mounted within the gas sensor. The LED and photodiode are therefore side-by-side, with the screen in line and in between them. Only half of a housing  5  and the radiation path is shown in  FIG. 1 . The housing has radial symmetry centred on the LED/screen/photodiode assembly. The inner surface  6  of the housing is reflective. This may be achieved by applying a reflective coating to a moulded plastic housing. Light rays  8  that diverge from the LED are reflected from the inner surface of the housing. The housing is shaped such that the light emitted by the LED is reflected through the cavity extending around the LED and photodiode and the reflected light rays  9  are concentrated onto the photodiode. The cavity is bounded by the plane parallel to the main emitting and absorbing surfaces of the LED and photodiode respectively. The rays reflected from surface  6  may or may not be focused. The curved shape of the housing is arranged to provide an even, broad spread of the light throughout the cavity. The surface may be hemispherical or semi-ellipsoidal. The even spread of light through the cavity is generally characterised by avoidance of focus where light rays originating from a point on the source do not converge on a particular focal point, but none the less converge on the photodiode. The side-by-side geometry has the advantage of allowing a small housing with a maximum spread of the interacting light path throughout the available volume of the housing. This provides good optical absorption efficiency and minimises the risk of saturation of the gas&#39;s interaction with the light, all in a compact housing. The compact optical design enables the gas sensor to fit within a 200 mm diameter and 17 mm long form factor. These features improve the gas sensor&#39;s sensitivity for gas sensing and response and the compact size makes it suitable for use in a wide range of space sensitive applications, where large housings are not acceptable. 
     The LED has a narrow emission bandwidth, therefore using a LED and photodiode the narrow optical bandwidth required for gas sensing may be achieved without optical filters as are required for incandescent and other sources. However, the radiation source may be a LED that uses an optical bandpass filter to trim the optical emission profile but remove all other light frequencies that may cause error in the gas sensing process. Such an optical bandpass filter may remove no more than 25% of the emitted light form the LED, whereas in the prior art case of an incandescent source the vast majority of radiated light would be removed by the filter. Therefore, the LED radiates a precise and narrow bandwidth which is not post or pre optically filtered other than by simple bandwidth trimming. 
       FIG. 2  shows a cross section of the radiation source, screen and radiation detector is shown. With reference to  FIG. 2 , the LED radiation source  2  and the photodiode radiation detector  3  are side-by-side mounted on an interconnecting substrate  4 , with the screen  1  in between. The screen may be formed as part of the substrate and the LED and/or photodiode may abut the screen. The screen may be reflecting. The surfaces of the LED and/or photodiode facing the screen may be reflecting. Alternatively the screen may be a reflective coating on one or more of the surfaces of the LED or photodiode facing each other. 
     Both the radiation source and detector in this embodiment are based on the narrow band gap III-V material indium aluminium antimonide (In (1-x) Al x Sb), grown on a gallium arsenide (GaAs) substrate, the band gap of which can be tuned to a very narrow width to provide light emission and detection that is specific to carbon dioxide (CO 2 ) and carbon monoxide (CO gases) or other selected gases without the use of expensive optical filters and complicated differentiating circuitry. The LED and photodiode may be fabricated from the same semiconducting substrate. The LED and photodiode may also be fabricated from very similar substrates varying only by their epilayer thicknesses, which maybe tuned to enhance the performance of light emission in the case of the LED or collection in the case of the photodiode. In other embodiments, the radiation source and radiation detector may comprise one or more discrete LED or photodiode elements respectively. 
     The invention is not limited to this type of radiation source and radiation detector. For example, cadmium mercury telluride compounds are useful with ultraviolet frequencies. Although solid state radiation sources and detectors are convenient for miniaturised application, the present invention may also be implemented using incandescent sources and pyro-electric or thermopile detectors. 
     The interconnecting substrate  4  and/or screen is thermally conductive and provides thermal communication between the LED and the photodiode. The thermal communication allows the transfer of heat from the LED to the photodiode. This provides the advantage of reducing the temperature-difference between the LED and photodiode, thereby simplifying the compensation of any temperature dependent effects on the operation of the LED and/or photodiode. This approach is in contrast to most common electrical applications of conductive layers where the heat is transferred away from the semiconductors. The heating effect may be used to keep the photodiode at an elevated temperature when compared to its surroundings, thus keeping it on the positive side of the dew point of the ambient gas, therefore reducing the risk of condensation forming on the photodiode. 
     The substrate may have integrated in it or mounted on it a temperature control means  12  such as a heater or cooler (Peltier device or similar) that can be controlled and powered to affect the temperature of the LED and photodiode simultaneously. 
     Temperature detection maybe achieved by use of an additional device (not shown), which may be in or on the substrate, such as a thermistor, or may be detected by measuring the characteristics of either the emitter or detector. For example the forward voltage of the LED will vary with temperature. 
     The substrate provides a structural mounting for the LED and photodiode within the gas sensor. The substrate may be shaped to aid in locating the radiation source or detector for mounting on the substrate. The substrate may also provide mechanical features that serve to precisely locate the optical pair within the optical housing avoiding the need for adjustment or setting during the assembly process. 
     The LED and photodiode are each provided with optically reflective layers  10 ,  11  on their surfaces. These reflective layers may be included on one or other of the LED and photodiode, or not at all. The reflective layers may be part of the substrate or applied as a coating to the back and/or sides of the LED and/or photodiode. The optical reflection improves the efficiency of both the LED and the photodiode. A proportion of the light generated or detected can pass straight through either device without being absorbed, however the incorporation of the reflective layers functions to return the light back through the LED improving emission efficiency, or similarly in the case of the photodiode, it can significantly increase the absorption by reducing loss of light out of the back or sides of the photodiode. 
     With reference to  FIG. 3 , a cross section of a second embodiment of a gas sensor is shown. The elements are numbered as in  FIG. 1 . In comparison with the first embodiment illustrated in  FIG. 1 , the domed reflector has been inverted. In this embodiment the substrate is an elongate printed circuit board extending along a diameter of the domed reflecting housing, which has radial symmetry. This substrate allows the mounting of further components (not shown). These components can include a temperature sensor ( 12  in  FIG. 2 ) that because of the side-by-side mounting of the LED and photodiode allows the measurement simultaneously by one sensor of the temperature of both the LED and the photodiode. This has the advantage of reducing the component count. Another component or set of components that can be incorporated adjacent to the LED and photodiode are signal processing elements, including a preamplifier. For example, the preamplifier may be mounted on the substrate next to the LED/photodiode pair with the remaining electronics and processing components being located at the next available position on the substrate away from the LED/photodiode pair. In that case there is an advantage that the preamplifier and processor are located adjacent to both the LED and the photodiode. Any electrical modulation signals can be transmitted to the LED with minimised noise pickup. Furthermore, the same components can detect the signal from the photodiode, which may be in the nA range. These low level signals are similarly sensitive to noise pickup-effects and the location of the processing elements adjacent to the detector reduces the effect of such noise pickup. There is a further-advantage to having the signal amplification and processing components within a gas sensor housing, which is the shielding afforded by the metalised housing. The apertures in the housing may reduce such shielding, but the circuits containing the components may be designed to balance with any antenna affect of the housing in order to achieve a zero biased system. 
     With reference to  FIG. 4 , a perspective view of the second embodiment of a gas sensor is shown. Two gas filters  13 ,  14  are shown with the very narrow printed circuit board (PCB)  4  being located in the middle of them. As an alternative to the signal processing and temperature control components  15  being located on the emitter/collector PCB as discussed above, they are placed on a second PCB  16 . Having the preamplifier thus separated from the LED/photodiode pair has been enabled by the use of a centre tap connection in the centre of an array of discrete photodiode elements that make up the radiation detector connected along with the other two LED terminations by pins  17  to either: 
     a) two independent-transimpedance amplifiers and the outputs differentially amplified to combine the signals and cancel out any common noise; or
 
b) a differential transimpedance amplifier.
 
     A tubular external housing (not shown) may be placed around the assembly, which forms a Faraday cage between the metalised reflector, the external tube and a shielding layer built into the second PCB, so improving the electrical isolation of the components, as well as being a structural support. 
       FIG. 5  shows a cross section of half of the internal surface  6  of the dome in another embodiment of the present invention. The dome has an internal surface comprising a plurality of sub surfaces  51  to  59 , shown in section as arcs, each defined by a radius R9.3711 to R9.0104 respectively and a centre point  60 , each sub surface being tangent to an adjacent sub surface and having a different radius and different centre location from the adjacent subsurface. The offsets of the centre points  61 ,  62  from the perpendicular datum lines  63  and  64  respectively are also shown. The emitter and detector lie on the datum line  64 . The surface is defined by the arcs labelled  51  to  59  (and their reflection about datum line  63 ) being swept out by rotation by 180 degrees around datum line  64 . Another embodiment may have the arc length tending to zero, thus giving a continuously varying curve from datum line  63  to datum line  64 . 
     The internal sub surfaces of this embodiment are semi-toroidal. The internal surface therefore does not form a focused reflector. 
     The dome functions to reflect, as near as possible, the radiation from the emitter  2  through a single reflection to the identical mirrored location on the detector  3 , irrespective of the radiation exit angle from the emitter. This is illustrated by  FIGS. 6 and 7 . 
     With reference to  FIG. 6 , light rays  65 ,  66  leaving the centre  67  of the emitter  2  are reflected by the inner surface  6  as rays  68 ,  69  to the centre of the detector  70 . Light rays  71 ,  72  leaving the outer side  73  of the emitter  2  are reflected as rays  74 ,  75  to the outer side of the detector  76 . 
     With reference to  FIG. 7 , as for  FIG. 6 , light rays  65 ,  66  leaving the centre  67  of the emitter  2  are reflected by the inner surface  6  as rays  68 ,  69  to the centre of the detector  70 . Light rays  77 ,  78  leaving the inner side  79  of the emitter  2  are reflected as rays  80 ,  81  to the inner side of the detector  82 . 
     The emitter and detector are positioned on a common mounting a small distance apart, typically 3 mm centre to centre. The mutually tangent radii forming the swept profile of the toroid are constructed such that the path length from emitter and detector is on average equal. The number of tangent radii used to construct the swept profile of the toroid determines the variation in path length for each ray angle between emitter and detector, therefore the statement that the path length for each specific radii is on average equal refers to the average path length of each radius, which generally occurs at the radius mid point. Therefore, apart from one specific point in each radius, the path length varies continuously across the face of each radius and therefore the face of the toroidal swept curve, within the limits imposed by the number of radii selected in the construction of the swept curve. 
     This dome has the effect of transferring the image of the emitter to the detector, without a focus. The image may be concentrated towards the centre of the detector when the components become out of alignment. This increases the manufacturing tolerance of the assembly. 
     The surface is formed by silver coating an injection moulded feature that forms part of the sensor housing and does not provide a mounting for the emitter and detector. 
     With reference to  FIG. 8 , the emitter  2  and detector  3  are bonded to a bridge PCB (printed circuit board)  4  that serves to provide electrical connectivity, thermal path, and a mounting means that is adjustable on assembly to optimise the location of the reflected radiation pool on the detector. Other components are labelled as in previous figures. A typical surface emitting LED may have an emission surface that is 1 mm 2  in area, and the location of the bridge mounting the emitter and detector is adjusted to provide as near as possible a pool of radiation (nearly identical to that radiated) striking the detector photodiode (as the non-focussed lighting provides greater efficiency) that is the same size. If the bridge PCB is wrongly adjusted in either direction, at no time will the total emitted radiation focus to a single point. The adjustment is performed when the bridge. PCB is raised or lowered in the direction shown  83  in response to feedback, such as the detector received signal strength. When the adjustment is optimum, the location of the bridge  4  with respect to the dome  5  is locked, for example by gluing the PCB interconnect pins into the dome  5 . Soldering may also be used, for example by soldering the pins to lock the pins in position in the bridge PCB  4  or the base PCB  16 . Other forms of providing the adjustment to an optimum position may be used, for example the adjustment may move the bridge  4  with respect to the pins  17  and the position may be locked by affixing the bridge to the pins after adjustment. Soldering may also be used for locking, for example by soldering the pins in position in the bridge PCB  4  or the base PCB  16 . The bridge PCB stop  84  acts as a limit to the adjustment during assembly and prevents the assembly falling apart in case the locking means fails during use. 
     There is no limitation to the range of angles that light emitted from the LED can reflect within the housing with the exception of the natural optical reflectivity of the emitter and detector surfaces. Therefore typically radiation up to around 80 degree half angle may find its way from the emitter to a similar location on the detector. 
     In the case where multiple gases are to be detected with the single housing or multiple emitter or detector elements are required, these are grouped as previously described for the single LED and photodiode except that in the case where multiple frequency LEDs are required these will be clustered in an area that is equal to or less than the area of the detector. In this configuration radiation from the multiple emitters will strike the detector in a similar corresponding location to that of emission from the LED. In the case where multiple detectors are required the same principle will apply. Also for any combination of emitter and or detector numbers the same principle will apply. 
     Further modifications and improvements may be made without departing from the scope of the invention herein described by the claims.