Proof of igniter and flame sensing device and system

Abstract

A single device for proving both the presence of an igniter and flame in a fuel burner is provided, the device including means such as a ceramic coating for both transmitting actinic radiation emitted by the igniter and generating actinic radiation upon flame excitation. The ceramic or other means is associated with an actinic radiation transmitting element which delivers the radiation to a photoresponsive element whereby a signal is provided which triggers fuel delivery to the burner when the igniter is an igniting mode or thereafter when flame is present. A method of proving igniter and flame is also provided utilizing such device and electronic circuitry adapted to prove its own integrity.

Description

This invention relates to the art of detecting the presence of an igniter and flame in a fuel burner system, and especially to a system for sensing and proving both igniter and flame with a single device.

There is a need for a reliable, inexpensive electric ignition and flame detection system which can be applied to a broad range of appliance burners. This need results from the inadequacies of present ignition and flame sensing configurations.

The constant burning gas pilot is currently the most common means of igniting the main burner in an appliance. The pilot itself must be manually lit and it is proven by a thermocouple or mercury expansion flame switch. The main burner flame is not proven.

There are several pilot relight systems commercially available. These systems provide a spark to relight the pilot whenever it is accidentally extinguished. A few types of electric resistance igniters are also available. Two such devices are the molybdenum disilicide igniter and the silicon carbide glow plug. Additionally, there are some main burner ignition and flame detection systems available. The majority of these systems provide ignition by means of sparking electrodes. Some systems spark during the entire burner cycle; others spark for a short duration at the beginning of every cycle. Both types use one of the sparking probes or employ a separate high temperature metal probe for sensing flame. These systems use the properties of electrical conduction or rectification and conduction through the flame to prove its existence. Also, there are a limited number of systems which use thermal bi-metal type switches for sensing burner flame. There are significant drawbacks with each of these ignition and flame sensing systems.

Main burner spark ignition systems suffer from probe degradation, especially in high heat flux burners. The probe spark gap is critical. Oxidation of the probes and condensation on the probes can cause these systems to fail by preventing proper sparking or improper biasing of the flame signal. The ceramic insulator can cause problems by allowing water to condense in its pores, presenting an alternate path for conduction. The ceramic itself can begin to conduct at high temperatures and improperly bias the system. Radio frequency interference caused by some spark igniter systems is also undesirable.

The constant burning pilot system must be manually lit and usually manually relit if it goes out. Pilots are continually on, and are increasingly being criticized as a waste of energy. Pilots cannot usually be employed with powered burners. Proof of pilot does not necessarily assure proper and safe main burner operation.

The pilot relighter increases system reliability, but it has the drawback of all spark systems and does not perform well under conditions of high humidity. The pilot relighter is too costly for the benefit it provides, with the possible exception of commercial roof-top gas equipment.

Bi-metal flame sensing switches can only be applied to a limited number of burner designs. For example, bi-metal systems have been used for years on gas dryers, but the same systems cannot withstand the conditions of a higher heat flux burner, nor can they respond rapidly enough to meet ANSI Standards when applied on products such as residential furnances which retain large quantities of residual heat after flameout.

Electrical resistance igniters eliminate the problems of spark ignition. However, the molybdenum disilicide igniter is very fragile and cannot work at high flow rates or on high heat flux burners. The silicon carbide igniter has been successfully applied with a bi-metal sensor as a proof of ignition and flame system in certain gas dryers. But the system is especially modified to a specific burner design and has all the drawbacks previously associated with bi-metal sensing systems.

It is an object of the present invention to provide a sensing device which avoids the above-mentioned drawbacks.

A further object is the provision of a single sensing device which proves both igniter and flame.

A still further object is to provide a sensing system which is fail safe.

In one embodiment of this invention there is provided an optical sensing device adapted for use with a photoresponsive element sensitive to actinic radiation comprising an actinic radiation conducting element, said conducting element being in energizing relationship to said photoresponsive means, and cover means in actinic radiation transmissive association with said conducting element, said cover means being heat stable, ambient light opaque, actinic radiation transmissive and actinic radiation emissive upon subjection to flame energy.

In another embodiment, a process for detecting igniter and flame in a burner system is provided comprising

1. arranging the above-defined optical sensing device in actinic radiation receiving relationship to a fuel igniter, said fuel igniter being adapted to emit actinic radiation when in the igniting mode,

2. energizing said fuel igniter to said igniting mode wherein said fuel igniter emits actinic radiation at a level capable of being sensed by said device,

3. receiving said actinic radiation by said cover means associated with said actinic radiation conducting element,

4. transmitting said actinic radiation to said photoresponsive element via said actinic radiation conducting element whereby said photoresponsive element provdes a signal for actuating a valve means,

5. supplying fuel to said burner via said valve means for a time sufficient to cause ignition of said fuel by said fuel igniter in the igniting mode,

6. de-energizing said fuel igniter to a quiescent mode wherein said fuel igniter does not emit actinic radiation at a level capable of being sensed by said device,

7. arranging the device in a location providing direct contact between said device and a flame from said burner,

8. flame heating said cover means to a thermally excited mode wherein said cover means emits actinic radiation, and

9. transmitting said actinic radiation to said photoresponsive element via said actinic radiation conducting element whereby said photoresponsive element provides a signal maintaining said valve means open so long as said cover means is in said thermally excited mode.

These and other embodiments of this invention which will be disclosed hereinafter are better understood by reference to the accompanying drawings wherein:

FIG. 1 is a side elevational view of a burner system of this invention;

FIG. 2 is a side elevational view in section of a preferred embodiment of the sensing device of this invention;

FIG. 3 is a side elevational view in section of a portion of another embodiment of the sensing device of this invention;

FIG. 4 is a side elevational view in section of another embodiment of the sensing device of this invention;

FIG. 5 is a top plan view with block diagrams of the burner system of FIG. 1 together with a schematic representation of a control system usable therewith;

FIG. 6 is a schematic diagram of a circuit for use in conjunction with the sensing device of this invention;

FIG. 7 is a logic diagram for a burner system utilizing the sensing device of this invention; and

FIG. 8 is a schematic diagram of another circuit for use in conjunction with the sensing device of this invention.

Referring to FIG. 1, sensor 1 is mounted in bracket 3 above and in the flame region of burner 5 and also adjacent to and in actinic radiation-receiving relationship to igniter 7. Burner 5 may be a conventional burner providing a flame capable of thermally exciting the sensor 1 as hereinafter described. Burner 5 may utilize conventional fuels, especially gaseous fuels of hydrocarbon or hydrogen composition. Situated atop burner 5 is a grate 9. Igniter 7, also mounted to bracket 3, includes lead attaching means 13, ceramic housing 15 and igniter core 17 which holds ceramic spiral 18. Igniter 7 emits radiation which activates a photoresponsive element hereinafter described. Igniter 7 is of the electrical resistance or hot body type as opposed to the type of igniters known as spark igniters. Upon sufficient excitation such an igniter emits radiation in the visible and infrared wavelengths as well as in the thermal region. A preferred igniter is a silicon carbide igniter with a negative temperature coefficient and a power consumption of approximately 100-350 watts. Such an igniter is available under the tradename Carborundum.

Sensor 1 includes a forward portion 19 situated in close proximity to igniter 7 and burner 5 and a rearward portion 21 situated relatively remotely from the igniter and burner. Forward portion 19 is so located as to be directly within the flame of burner 5.

FIG. 2 depicts sensor 1 in greater detail. Sensor 1 includes an elongated actinic radiation conductive element 25 which is surrounded at forward portion by covering 29. Conductive element 25 includes a first segment 30 in the form of an integral, elongated rod, and a second segment 33 optically coupled thereto by optical coupling agent 37 to provide a path for actinic radiation received by the sensor 1 to a photoresponsive element hereinafter described. Surrounding actinic radiation conductive element 25 in the rearward portion is a covering 41 which serves primarily to shield conductive element 25 from thermal energy in the flame area and specious radiation especially ambient light such as from sources other than the burner and igniter which are out of the flame area.

Suitable actinic radiation elements 25 may be composed of materials such as quartz, sapphire, or optically transparent alumina or combinations of the foregoing materials. Such materials should have the ability to conduct actinic radiation, i.e., radiation to which the photoresponsive element is responsive. For purposes of this invention, actinic radiation is generally in the red visible and infrared region. Additionally, such materials should be adapted to function over prolonged periods of time at the temperatures to which it is subjected, generally at 1,200.degree. F. or higher. It should also be noted that the said conducting element will also generate as well as transmit actinic radiation at the hot end. Preferably, the first segment 30 of conductive element 25 is composed of the foregoing materials in an integral rod form. The second segment 33 may likewise be composed of the foregoing materials or may be composed of a fiber optic bundle of quartz or other suitable actinic radiation transmissive material. In the former case, where the first and second segments are of the same composition, conductive element 25 may be of a single unitary construction. The advantage of the fiber optic bundle is that it permits angular configurations of the second segment 33 so that the path of actinic radiation transmission can be bent as needed to conform to design demands of system. In particular, this allows the photoresponsive element to be located in a relatively thermally remote region to avoid deleterious thermal effects on the photoresponsive element.

A preferred optical coupling agent 37 is a high temperature, optical epoxy resin. The epoxy resin is stable at the elevated temperatures encountered in use, and has generally been found stable at temperatures in the order of 180.degree. C. or higher. The epoxy resin is very clear with optical properties similar to quartz and has high tensile and compressive strength. The coefficients of expansion of the coupling agent and elements to be optically coupled are sufficiently similar that the bond remains in tact at the elevated temperatures encountered in operation. A suitable epoxy resin is available commercially under the tradename "Isochem."

Covering 29 is situated in actinic radiation-transmissive relationship to element 25. In the embodiment of FIG. 2, covering 29 is adherably bonded to the surface of conductive element 25. Covering 29 is actinic radiation transmissive and also capable of generating actinic radiation in situ upon thermal excitation by the flame from burner 5. In addition, covering 29 is heat stable and opaque to ambient light. Covering 29 is preferably of a ceramic composition. A preferred composition, especially for bonding directly to conductive element 25, is a dry ceramic powder available under the tradename "Ceramacast 511" mixed with silicon carbide. It was found that 325 mesh silicon carbide improved the bonding, opaqueness, and actinic radiation emissiveness of the ceramic. The ceramic powder/silicon carbide mixture combined with as little water as possible to attain a thick creamy consistency and the mixture so obtained is then applied by rolling the conductive element, generally in rod form, in the mixture and thereafter curing the coated element at 1,000.degree. F. for 3 minutes. The silicon carbide may suitably be present in amounts of from 10% to 65% and preferably 30% to 35% by volume of ceramic powder.

Covering 29 is quite thin in cross-section, preferably on the order of 0.8 - .16 cm. in the case of a bonded coating. In other forms, such as a self-supporting case as depicted in FIG. 3, the thickness is about .16 cm. Other suitable materials for covering 29 are metals which are capable of withstanding the flame temperatures and will exhibit the other properties mentioned above. Stainless steel and inconel are two such suitable materials. As noted above, when the covering 29 is subjected to flame from the burner 5, it emits as well as transmits actinic radiation. This radiation includes radiation in the red visible and infrared regions. The actinic radiation so emitted is received by actinic radiation conductive element 25 for transmission to the photoresponsive element.

Covering 41 may, but need not be, of the same composition as covering 29, since actinic radiation transmission and emission are not required for this element of the invention. Covering 41 generally performs the function of shielding conductive element 25 from harmful temperatures and specious light and for the latter purpose is opaque to visible light. Suitable materials for covering 41 include ceramics, such as silicon carbide, silicon nitride, alumina or graphite.

FIG. 3 illustrates another form of the sensor 1 of this invention. Conductive element 25 is enclosed in a case 45 which in terms of functional properties is essentially the same as covering 29 with the exception that case 45 should be structurally self-supporting whereas covering 30 should be bondable to conductive element 25. The gap between conductive element 25 and case 45 is generally an air gap sufficiently small to allow efficient transmission of actinic radiation from case 45 to element 25. A gap of about .03 cm. has been found adequate. In this embodiment, it is seen that a covering analogous to covering 41 is provided by case 45.

FIG. 4 depicts still another embodiment of the sensor 1. Conductive element 25 includes first segment 30, second segment 33 and coupling agent 37 as in previous embodiments. These elements are enclosed in a metal case 49 having a shoulder portion 53. Case 49 is essentially the same functionally as case 45 of FIG. 3 and covering 29 of FIG. 2. Shoulder portion 53 of case 49 is bored to accept hardware to attach ceramic shield 61 (analogous in function to covering 41 of FIG. 2) to case 49.

In FIG. 5, the system of FIG. 1 is integrated with a control system depicted in block diagram form. In this system, thermostat 65 provides an exciting voltage, on lines 69 and 73, to sensing circuit 77 and conventional timing circuit 81. Timing circuit 81 energizes the hot body igniter 7. As igniter 7 self-heats, it emits radiation in visible and infrared wavelengths as well as in the thermal region. Red and infrared radiation is absorbed by covering 29 at the forward portion 19 of sensor 1. This radiation is transported through the sensor 1 via the first segment 30 of actinic radiation-conducting element 25 to the second segment 33, where it is transported to sensing circuit 77. Sensing circuit 77 energizes fuel valve 79, allowing fuel to enter burner 5, where contact with igniter 7 produces flame. The flame impinges on covering 29 sensor 1. After several seconds covering 29 begins to emit actinic radiation which is also transported as above to sensing circuit 77. Timing circuit 81 de-energizes igniter 7 which cools to a state wherein the actinic radiation emission is quantitatively insufficient to energize fuel valve 79. The sensing circuit 77 maintains power to fuel valve 79 as long as the flame is maintaining covering 29 in the excited state or until thermostat 65 disconnects the system.

Sensor 1 is connected as noted above in energizing relationship to a photoresponsive element included in sensing circuit 77. The photoresponsive element may be of a conventional type and may be photoconductive or photovoltaic in operation. Suitable photoconductive materials include cadmium selenide and cadmium sulfide. Suitable photovoltaic elements include selenium, silicon, and bismuth telluride. The photoresponsive element may be optically coupled to conducting element 25 by conventional means. Preferably, the second segment 33 of conductive element 25 is a fiber optic bundle connected at a thermally remote location to the photoresponsive element by an optical coupler such as the above-described epoxy resin.

FIG. 6 depicts a sensing circuit for performing the sensing operations described in connection with FIG. 5. This FIGURE represents the first of two embodiments of this sensing circuit. Since the timing sequence can be performed well by any number of conventional means, this has been left out of the detailed circuit schematics and its position relative to the sensing circuit as shown in FIG. 5. The function of the timing circuit is merely to de-energize the igniter 7 when the system is to begin its flame sensing mode.

Thermostat 65 provides 60 Hertz AC voltage at lines 69 and 73. The sensing circuit provides means for actuating fuel valve 79. The valve is energized by the control A1 of relay A. Relay A is turned on by the discharge of capacitor 89 and then held on by PUT 94, resistor 167 and resistor l05. The PUT 94 is triggered into conduction when the anode voltage exceeds the gate voltage by 0.2 to 0.6 volts. By design of the triggering elements, which consist of reference photoconductor 101, resistor 113, sensing photoconductor 85, resistors 121, 105 and 109, triggering is accomplished when the sensing photoconductor 85 receives sufficient actinic radiation. The actinic radiation level is sufficient when it is indicative that the igniter 7 has reached a state capable of igniting the fuel to be employed. By requiring photoresponsive element 85 to be less than the reference element 101 in resistance for the PUT 94 to trigger, several functions are accomplished.

The reference element 101 serves the function of setting the required level of actinic radiation to be received by photoresponsive element 85, as well as offsetting the effect of changes in the energizing voltage and the effect of temperature on the photoresponsive element 85. This is accomplished by illuminating the reference element 101 with sufficient radiation to establish a reference resistance (R.sub.101). The amount of illumination is selected such that the resistance R.sub.101 will be at a value slightly greater than the resistance R.sub.85 of the photoresponsive element 85 when the igniter 7 is in an igniting mode. The illumination is provided by lamp 139, the brilliance of which is controlled by resistor 143 and varible resistor 147. Reference element 101 also compensates for variations in input voltage since any variations will directly proportionately affect the brilliance of lamp 139 just as it will affect the igniter 7. While the effect on the igniter 7 and brilliance of lamp 139 will not in practice be precisely the same, the difference will not be significant in terms of operation of the circuit. Insofar as temperature effect on the resistance R.sub.85 is concerned, this too will be effectively compensated for by reference element 101 since the latter is of the same type as photoresponsive element 85 and reference element 101 is so physically situated that it will experience substantially the same temperature environment as photoresponsive element 85.

Resistors 105 and 109 are of equal value. This requires resistors 101 and 113 to be greater than resistors 85 and 121. Resistors 113 and 121 are also set equal and this in effect requires the sensing element 85 to be more conductive than the reference element 101 if the voltage at A (the programmable unijunction transistor anode) is to be greater than the voltage established by the photoresponsive elements at B (the PUT 94 gate terminal).

In order for fuel valve 79 to be energized, the cirlcuit must store enough energy in capacitor 89 to turn relay A on when it is discharged by transistor 99 and PUT 94. These transistors are brought into conduction by the previously explained triggering elements. Diode 129, resistors 151, 155 and 169 determine the final charge level and the time required for the charge to be established. Failure of key triggering components such as the photoresponsive elements or the transistors will cause premature discharge of capacitor 89 and the fuel valve 79 will not be energized. If such failures occur in the previous system cycle or during the timing period (i.e., when capacitor 89 is charging), the system will be protected from unsafe operation.

Capacitor 163, at the base of transistor 99, will charge more quickly than capacitor 89 to assure that transistor 99 will not discharge on negative half cycles due to the emitter voltage of transistor 99 being greater than the base voltage. Resistors 164 and 165 provide charging for capacitor 163.

In operation, after some predetermined time T, capacitor 89 will have reached an energy level sufficient to energize flame relay A. After some time greater than T, igniter 7 will provide a signal to photoresponsive element 85 causing it to become electrically conductive, thereby triggering unijunction 94, which will cause transistor 99 to saturate, thereby discharging capacitor 89 into the solenoid of the flame relay A. The flame relay A energizes and closes its contact A1. The photoresponsive element 85 continues conducting and PUT 94 is allowed to conduct current every positive half cycle. this alternating current is sufficient to hold flame relay A in the active or energized state by resistors 105 and 167. Diode 168 serves as a holding diode for flame relay A during negative half cycles.

If either resistor 109 becomes open circuited, or resistors 105 or 167 should short, there is not enough current present to turn on the flame relay A. The circuit is failsafe; any component can fail during a cycle, and the circuit will be safe on the succeeding cycle.

Under normal operation capacitor 89 charges. The igniter 7 reaches full brilliance. This brilliance is sufficient for the photoresponsive element 85 to be made more conductive than the photoresponsive element 101. The PUT 94 triggers into conduction--so too does transistor 99 trigger into conduction. Capacitor 89 is discharged into relay A and resulting in actuation of valve 79. The igniter 7 continues to glow thereby containing the triggering of PUT 94 on successive positive half cycles. The timing circuit 81 de-energizes igniter 7. If the burner system was functioning, the fuel ignited on contact with igniter 7 and the resultant flame was allowed to contact the sensing probe 1 during the time igniter 7 was left on.

After the burner flame impinges on covering 29 of sensor 1 for several seconds, the covering 29 will emit actinic radiation in situ which will further increase the conductivity of photoresponsive element 85. Hence, the triggering process will continue and fuel valve 79 will remain energized. When timing circuit 81 de-energizes igniter 7, radiation from the flame-activated sensor 1 will be sufficient to maintain triggering. Flame-out will cause the actinic radiation emission of sensor 1 to fall below the threshold amount needed to operatively actuate photoresponsive element 85 (i.e., the photoconductor will return to a relatively non-conducting state). The triggering process will stop, thereby de-energizing flame relay A and de-energizing fuel valve 79 by opening contact A1. Consequently, the fuel supply to burner 5 will be interrupted and the system will not automatically initiate another cycle.

The operation of the circuit of FIG. 6 can be further understood by reference to the logic diagram of FIG. 7. The symbols employed in the diagram have the following meaning: R.sub.85 is the resistance of the photoresponsive element 85; R.sub.101 is the resistance of the reference photoresponsive element 101; t.sub.0 is the time at which the system is energized; and t.sub.1 is the time allowed to test the photoresponsive element 85.

At the outset, a 60 Hertz AC voltage is applied by thermostat 79 to lines 69 and 73 of the sensing circuit 77 (Box 600). This occurs at time t.sub.0. Within a time t.sub.1 after t.sub.0, the resistance of photoresponsive element 85 undergoes testing. If R.sub.85 is less than the resistance R.sub.101 of the reference element 101 (Box 601), this means that element 85 is not working properly because it is in a radiation receiving state before the time when igniter 7 could reach its igniting mode. This condition is considered an unsafe failure condition and the circuit will lock out (Box 602) with the valve 79 off and the igniter 7 on. As a result the system will be de-energized (Box 603) and the system is then turned off (Box 604). This lockout occurs because the PUT 94 is triggered too early and continues to be triggered into conduction every positive half cycle. It receives its triggering voltage from the bridge circuit 125 composed of elements 105, 109, 101, 113, 85 and 121. The conductive paths to the flame relay A which include resistors 105 and 25, as well as resistors 151 and 159, are not sufficient to turn the flame relay A on.

If the photoresponsive element 85 is properly functioning, the resistance R.sub.85 is higher than the resistance R.sub.101 (Box 605). Thus, PUT 94 is not in a conducting or triggering mode.

If the photoresponsive element 85 has the proper resistance R.sub.85 (Box 605), lockout will occur (Box 602) if either one of two situations prevail: the signal from igniter 7 is insufficient (Box 606), or the signal from igniter 7 or from any other source, while sufficient, occurs at a time t prior to t.sub.1 (Box 607).

If a sufficient signal is received by the photoresponsive element 85 at a time t after t.sub.1 (Box 608), element 85 will drop in resistance below the value R.sub.101. Triggering of the PUT 94 will occur when the voltage at point A is 0.2 to 0.6 volts greater than the voltage at B. Since resistor 105 is of equal value to resistor 109, and since the input voltage is always much greater than 0.6 volts, this differential voltage is established by the photoresponsive element 85 being only slightly less resistance than reference photoconductor 101. This triggering point will be voltage dependent as is seen by the equation:

V.sub.AB = V/2 - V(R.sub.85 + R.sub.121)/(R.sub.85 + R.sub.121 + R.sub.101 + R.sub.133)

where V is the input voltage and V.sub.AB is the voltage at point A relative to point B. As V increases, the resistance R.sub.85 at which V.sub.AB equals 0.2 to 0.6 volts (triggering voltage) is higher. This is offset by the increased brilliance of lamp 139 at the higher voltage. This causes reference photoconductor 101 to be at a lower resistance because the lamp is brighter thereby requiring the same of the photoresponsive element 85 and cancelling out the effect of increasing voltage. The same thing holes true in reverse for low voltage. Also, since the igniter will glow more brilliantly at higher voltages, the increased brilliance of the lamp at these higher voltages makes it a good match for the effects of varying voltage on the igniter.

As previously explained, when the voltage at point A becomes slightly greater than that at B by the increased conduction of photoresponsive element 85, PUT 94 conducts on every positive half cycle. This causes charged capacitor 89 to discharge its energy into the solenoid of relay A, thereby turning relay A on. The continuation of photoresponsive element 85 in the low resistance state causes PUT 94 to conduct every positive cycle thereby holding relay A in the energized state through the conductive path of resistor 170 and resistor 105. The flame relay A turns the gas valve 79 on (Box 609) and holds it on as long as either the igniter or flame signal is received by the photoresponsive element 85. After allowing time for the sensor 1 to emit radiation upon flame excitation, the timer 81 turns the igniter off (Boxes 610, 611). If ignition did not occur (Box 612) or the flame is of insufficient intensity (Box 613), the circuit will again lockout (Box 614) and return to the nontriggering mode causing the valve 79 to be de-energized. The igniter 7 will remain off and the circuit locked out until the entire system is recycled by removing power and reapplying it after a few seconds (Boxes 604, 600). If the flame is sufficient (Box 615), the valve 79 will remain on (Box 616) until the thermostat 69 de-energizes the circuit (Box 603) or until flameout occurs (Box 617).

FIG. 8 illustrates another sensing circuit for use in the invention. Certain elements common to the circuits of FIGS. 6 and 8 are designated by like numerals.

The circuit is energized by applying 60 Hertz A.C. voltage at lines 69 and 73. The circuit can be designed to operate at any of the conventional voltages 24, 110, 117, 208, 240. Proper sizing of the elements is all that is required. Fuse 211 allows resistor 171 and diode 207 to charge capacitor 175. The R C time constant is chosen to provide some delay less than the minimum time for the igniter 7 to reach its glowing state. Resistors 183 and 187 are so large relative to resistor 171 that they do not effect the charging level or rate significantly. The purpose of resistor 183 and 187 is to provide a discharge path for capacitor 175 so that the circuit will reset in a few seconds after being de-energized by the total discharge of capacitor 175.

If resistor 171 short circuits it would fail to provide the required timing interval since capacitor 175 will reach its peak value almost instantaneously. If the resistor 171 short circuits, diode 204 will cause heavy conduction through fuse 211 and it will blow thereby de-energizing the circuit.

Capacitor 175 must fully charge before the proper operational sequence leading to the opening of fuel valve 79 can take place. If any key triggering elements 85, such as photoresponsive element reference photoconductor 101, or SCR 199 have failed in the preceding cycle or fail during the test period, the capacitor 175 will not charge if these elements have failed in a nonsafe mode. The nonsafe mode is such that triggering of SCR 199 to the on start would occur without the presence of actinic radiation. This nonsafe mode causes early triggering thereby discharging capacitor 175 prematurely, and thus, causing circuit lockout because insufficient energy had been stored in the capacitor 175. Capacitor 175 must charge to sufficient voltage to energize flame relay 179 by its discharge into the relay.

If the circuit elements are in a safe operating mode, capacitor 175 will become fully charged. When igniter 7 reaches full brilliance, the sensing photoconductor 85 will be at the critical value R.sub.101 set by lamp 139 and the reference photoconductor 101. This critical value is the resistance expected for the photoresponsive element 85 when it receives actinic radiation from the igniter 7. Resistor 143 is of the proper value to set lamp 139 at the proper radiation level to cause the reference photoconductor 101 to have a resistance value R.sub.101. The values of resistors 215, 219, 235 and 205 are such that the diac 195 will break into conduction when the photoresponsive element 85 is at the R.sub.101 resistance level. The resistors 215, 219, 235 and 205, in combination with the diac 195 and the photoresponsive elements 85 and 101, make up the triggering circuit which can cause SCR 199 to conduct. This triggering circuit receives energy on negative half cycles. Thus, it is on negative half cycles when the SCR can be brought into the conductive state. When the diac 195 breaks into conduction, it allows capacitor 101 to discharge through capacitor 205 and across the SCR gate biasing resistor 237. This effects a positive voltage at the gate of SCR 199 sufficient to cause SCR 199 to conduct. This conduction discharges capacitor 175 into relay 179 causing the contact A1 of said relay to close. The closing of contact A1 energizes fuel valve 79 and the highly conductive path of diode 203. The conductive path allows capacitor 175 to charge each positive cycle. During each positive cycle diode 239 acts as a holding diode for the coil of relay 179 preventing chattering of said relay. During each negative cycle SCR 199 will be triggered into conduction again causing discharge of capacitor 175 into relay 171. This charge, trigger, discharge sequence will continue as long as the sensing photocell is receiving sufficient actinic radiation.

Reference cell 101 provides much the same voltage and temperature compensation as it did in the circuit of FIG. 6. When the voltage to the circuit of FIG. 8 increases, the voltage across the diac increases proportionally. However, the increased voltage also causes lamp 139 to glow more brilliantly. This causes the back biasing voltage to diac 95 to increase. The net effect negates the voltage increase and allows the diac to trigger at the same photoresponsive element 85 resistance regardless of the voltage variations across the otherwise voltage-sensitive triggering circuit.

Most important, however, is the temperature compensation afforded by the photoresponsive reference element 101. The resistance of this element changes with temperature during its actinic radiation-receiving mode. This allows the voltage across the diac to be unaffected by the temperature effects of the sensing photocell resistance.

The safety of flame sensing may be enhanced by the addition of a nematic liquid crystal between the secondary conducting element 33 and photoresponsive element 85. This junction area could be temperature controlled to give both the photoresponsive element 85 and the liquid crystal further stability. Also, the liquid crystal can be used as a shutter to generate a pulsating signal to the photoresponsive element 85. Such a signal will assure immediate detection of the failure of photoresponsive element 85 by the lack of its continually changing resistance, the frequency of which is controlled by the liquid crystal shutter and its associated shutter control circuit.

The flame detection circuitry of this invention proves its own integrity at the beginning of each operating cycle, proves the igniter during the proof of ignition mode and then provides main burner proof of flame. This system provides greater safety in all functional modes than any system currently available for application to residential and commercial appliances.

A preferred sensor of the type depicted in FIG. 2 includes a 3 inch length of 2.4 MM quartz rod as the first segment of the actinic radiation-conductive element. The quartz rod can withstand temperatures in excess of 1,100.degree.C. and is very transparent in the visible and near infrared spectrum, but opaque to the infrared beyond 7.0 microns. The quartz rod is bonded to a fiber optic bundle with a high temperature, optical epoxy. The epoxy can operate to 180.degree.C. It is very clear with optical properties similar to quartz. It has high tensile and compressive strength. The fiber optics passes radiation between 0.44 microns and 1.3 microns and with an additional narrow passband around 1.5 microns. The junction at the fiber optic end and all but 1 inch of quartz are sheathed with a ceramic tube. The exposed quartz is coated with a thin coating of a ceramic cement. The cement is not transparent to visible light. The cement can maintain high operating temperature without deterioration. Since the coefficients of expansion of the quartz and cement are close, the cement adheres despite the rapid heating in a flame. Although the ceramic expands more than the quartz, its expansion is very small, and the structure of the ceramic allows this to occur without hazard to the bonding properties or to the ceramic itself.

When the probe is inserted in a flame, the thin ceramic coating begins to emit broadband radiation. The radiation travels down the quartz rod through the fiber optic bundle and is received by a photoresponsive device.

The system of this invention can withstand the severe environment of a high heat flux powered burner and also reliably prove flame in a much lower heat flux burner. It is sensitive enough to prove flame for a rangetop burner over a reasonable ambient temperature range using the sensor of FIG. 2 or FIG. 4.

The system of this invention enables proof of ignition source as well as proof of flame. It is applicable in any type of burner. It can easily be used on a ceramic burner tile which does not provide the ground return required by many conduction systems. It can be used from 20,000-2,000,000 BTU/HR/FT.sup.2 of burner surface. The system can get enough signal from burners as small as a 250 BTU/HR pilot burner. The resistance igniter is not affected by moisture and is not as position sensitive as a sparking igniter. The system is low in cost, simple and reliable. The system eliminates probe deterioration and probe gap problems prevalent in spark ignition systems. Carbon deposits which can prevent ignition and bias flame sensing on other systems do not effect this system.

Claims

What is claimed is:

1. A device for sensing the presence of a flame and an electrical resistance igniter in the igniting mode in a fuel burner system in combination with a photoresponsive element sensitive to infrared radiation, said device comprising infrared radiation-conducting means having a portion thereof adapted to be located directly within said flame, ceramic cover means associated with and substantially enclosing said portion of said infrared radiation-conducting means, said ceramic cover means being heat stable, visible light opaque, transparent to the passage of infrared radiation emitted by said igniter through said cover means whereby said igniter is sensed by said photoresponsive element and further said cover means emitting infrared radiation upon excitation by a flame whereby said flame is sensed by said photoresponsive element.

2. The device of claim 1 wherein said infrared radiation conducting means comprises quartz.

3. The device of claim 1 wherein said infrared radiation conducting element comprises a quartz rod, a fiber optic bundle, and bonding means for optically coupling said rod to said bundle.

4. The device of claim 1 wherein said cover means comprises a ceramic composition adherably bonded to said infrared radiation conducting means.

5. The device of claim 1 wherein said cover means comprises a ceramic composition in infrared radiation transmitting relationship to said conducting means.

6. The device of claim 1 wherein said infrared radiation conducting means is in optically coupled relationship to said photoresponsive element responsive to said infrared radiation.

7. The device of claim 6 wherein said photoresponsive element is photoconductive.

8. The device of claim 6 wherein said photoresponsive element is photovoltaic.

9. The device of claim 6 wherein a nematic liquid crystal is operatively interposed between said conducting means and said photoresponsive element.

10. The device of claim 3 wherein said optical bonding means comprises an epoxy adhesive optically transmissive to said infrared radiation.