Abstract:
A flame sensor for sensing flame produced by the burner of a furnace. The sensor includes a closed end tube having a photosensitive element located therein. The photosensitive element is sealed off from any external light so as to only respond to infrared wavelengths of light emitted from the end of the tube when subjected to a flame produced by the burner of the furnace.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the sensing of the presence of a flame produced by a burner within a furnace and in particular to the optical sensing of such a burner flame. 
     Photo optical sensors have heretofore been used to sense the presence of a flame produced by a burner of a furnace. These photo optical sensors are positioned relative to the burner so as to respond to the light emitted by the flame produced by the burner. It is to be appreciated that such sensing normally occurs under a variety of background lighting conditions. These background conditions may introduce a significant level of noise into any electrical signal generated by the photo sensor. This background noise must be dealt with by the circuitry associated with the photo sensor so as to produce a signal that can be relied upon to indicate the presence of a flame. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sensor which eliminates the impact of background lighting when optically sensing the presence of a flame. The sensor includes a hollow tube that is preferably closed at each end. One closed end is preferably formed during fabrication of the hollow tube from a substantially low thermally conductive material. An electrically conductive photo sensitive element is preferably located at one end of the tube and is oriented towards the opposing end of the tube. The closed end tube creates a substantially dark environment for the photo sensitive element. 
     The sensor is mounted relative to the burner in a furnace so as to position the opposing end of the tube in the red hot portion of the flame produced by the burner. Heat produced by the red hot portion of the flame will cause this end of the tube to rapidly heat up so as to produce infrared wavelengths of light within the tube that impinge upon the surface of the photo sensitive element located at the other end of the tube. The photo sensitive element changes its electrical conductance as a result of experiencing the particular infrared light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be apparent from the following description with the accompanying drawings, in which: 
     FIG. 1 is an exploded view of an optical flame sensor comprising a closed end hollow tube which receives an infrared photo sensitive element; 
     FIG. 2 is a cross-sectional view of the optical flame sensor shown relative to a portion of a furnace burner producing a flame to be sensed by the photo sensor; 
     FIG. 3 is an electrical circuit for sensing the presence of a flame when the infrared photo sensitive element is a photo diode; and 
     FIG. 4 is an example of an electrical circuit for sensing the presence of a flame when the infrared photo sensitive element is a photo transistor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a hollow opaque closed end tube 10 is illustrated relative to an encapsulated infrared photo sensitive element 12. The closed end tube 10 is preferably fabricated as a single piece part from a non-oxidizing material such as a stainless steel alloy. Such a material is substantially thermally non-conductive so as to essentially conduct little or no heat along the length of the tube. The hollow closed end tube 10 may also be manufactured out of a thermally conductive ceramic material that also produces little heat transfer along the length of the hollow tube. The closed end tube 10 may alternatively be fabricated as two separate parts with the closed end portion being formed as a separate part that is attached to one end of a hollow tube. 
     The infrared photo sensitive element 12 may be a photo diode, a photo transistor, or other photosensitive electrical device. The encapsulated infrared photosensitive element is preferably mounted to a back end plate 14. If the photosensitive clement is a photo diode, it will have two electrical conductors 16 and 18 extending outwardly from the black end plate 14. The back end plate 14 is preferably circular with a diameter greater than the interior diameter of the hollow closed end tube 10. The infrared photo sensitive element is itself preferably of a diameter slightly less than the interior diameter of the hollow closed end tube 10 so as to extend completely into the hollow closed end tube. The closed end tube 10 is preferably connected to a mounting bracket 20, which allows the closed end tube to be mounted in the furnace adjacent to the burner of the furnace. 
     Referring to FIG. 2, the thus mounted hollow closed end tube 10 is illustrated relative to a portion of a burner 22 of a furnace. The back end plate 14 is preferably sealed against the open end of the hollow closed end tube 10 by a non-conductive epoxy or adhesive 24 which completely seals this end of the tube from any light. The light sensitive surface 26 of the encapsulated infrared photo sensitive element is preferably oriented toward the closed end 28 of the tube. 
     The burner portion 22 is seen to produce a flame 30 shown in dotted outline form, which completely encompasses the closed end 28 of the hollow end tube. The tube 10 is mounted within the furnace so as to position the closed end 28 in what is commonly referred to as the &#34;red hot portion&#34; of the flame 30. 
     The closed end 28 will produce infrared wavelengths of light when heated by the flame 30. These wavelengths of light will impinge upon the light sensitive surface of the infrared photo sensitive element 12. This will cause the photo sensitive element 12 to preferably change its electrical conductance in a particular manner depending on what type of photosensitive element it is. 
     Referring to FIG. 3, a flame sensing circuit is illustrated wherein the photo sensitive element is a photo diode 32. The photo diode 32 becomes decreasingly resistive to the flow of current from a voltage source, &#34;V&#34;, through a reference resistor 34 when exposed to infrared light from the heated closed end 28. The photo diode 32 hence defines an input voltage to a comparator amplifier 36 that falls when its light sensitive surface is exposed to light. The thus decreasing voltage is compared to a threshold voltage for the comparator amplifier 36 defined by a pair of reference resistors 38 and 40 relative to the voltage source, V. The output from the comparator amplifier 36 may be provided to the input of a microcontroller 42, which responds to the change in voltage output from the comparator 36 so as to note the presence of a flame. 
     Referring to FIG. 4, an electrical sensing circuit is illustrated wherein the infrared photo sensitive element is a photo transistor 44. The particularly depicted photo transistor 44 becomes increasingly conductive when exposed to infrared light produced by a heated closed end 28. The photo transistor 44 is seen to define a path to ground from a voltage supply source, V, through a reference resistor 46. The voltage downstream of the reference resistor 46 in this particular circuit will fall with increasing conductivity of the photo transistor 44. This voltage is applied to an operational amplifier 48 having a feedback resistance 50 that defines a reference voltage, V r  to the operational amplifier. The output of the operational amplifier is preferably connected to the input of a microcontroller 52. The microcontroller is programmed to recognize the operational amplifier output produced by the conductive photo transistor as indicating the presence of a flame at the closed end 28. 
     It is to be appreciated that a particular embodiment of the invention has been described. Alterations, modification and improvements thereto will readily occur to those skilled in the art. Accordingly, the foregoing description is by way of example only and the invention is to be limited by the following claims and equivalents thereto.