Abstract:
In a flame ionization sensor type gas combustion control apparatus, the sensor is provided with a power supply which will increase the voltage as contamination build up occurs on the in-flame sensor electrode thereby keeping a constant sensor current and enabling the sensor to perform as intended even though insulative contaminant build up is present on the electrode.

Description:
This application claims benefit of Ser. No. 60/181,005, filed Feb. 8, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to temperature probes, or sensor tips, of the type used for the control and safety monitoring of gaseous fuel burners as used in various heating, cooling and cooking appliances. In particular, the present invention relates to flame ionization sensor probes used in gas combustion control/safety environments where contamination coating of the in-flame sensor probe shortens the useful life of the sensor. 
     2. Discussion of the Related Art 
     Flame ionization sensing provides known methods and apparatus for monitoring the presence of a flame for a gaseous fuel burner. 
     It is known that hydrocarbon gas flames conduct electricity because charged species (ions) are formed by the chemical reaction of the fuel and air. When an electrical potential is established across the flame, the ions form a conductive path, and a current flows. Using known components, the current flows through a circuit including a flame ionization sensor, a flame and a ground surface (flameholder or ground rod). 
     FIG. 1 illustrates a flame ionization sensor system  10  with a typical sensor/burner circuit loop as may be used in accordance with the present invention. Flame ionization sensor  11  having a probe  12 , will be mounted near the burner  13 . The output  15  of sensor  11  will be fed into a computer-controller  17 . The sensor loop can provide information regarding the status of a flame  18  in the burner  13 . If there is no flame, then the sensor  11  will not generate a signal which will cause the controller  17  to instruct the system to shut off fuel flow. 
     In utilizing a flame sensor as previously described, a voltage, such as a 120 AC voltage  21 , will be applied across the sensor loop, with the flame holder, or burner  13 , serving as the ground electrode  20 . Flame contact between the sensor probe  12  and the burner  13  will close the circuit. The alternating current (AC) output of the sensor/ground circuit, can be rectified, if the ground electrode (flameholder or burner  13 ) is substantially larger in size than the positive electrode, since, due to the difference in electrode size, more current flows in one direction than in the other. 
     Flame ionization sensor probes  12  are thus electrodes, made out of a conductive material which is capable of withstanding high temperatures and steep temperature gradients. Hydrocarbon flames conduct electricity because of the charged species (ions) which are formed in the flame. Placing a voltage across the probe and the flameholder causes a current to flow when the flame closes the circuit. 
     Unfortunately it has been found that contaminants in the air stream of the fuel/air mixture can result in the build up of an insulating contamination layer on the probe, rendering it much less effective. At a certain level of coating, which prevents sufficient electron flow to the probe surface, the sensor is rendered useless, creating a premature or false system failure. Often these airborne contaminants are organosilicones found in personal and home care products which are oxidized by the flame  18  to silicon oxides (SiOx) which in turn build up through impact on the probe  12  providing the insulative contaminant coating. 
     It is thus desirable to find ways to increase the useful life of flame ionization sensor probes in spite of this insulative build up resulting from normal use of the flame ionization sensor system. 
     SUMMARY OF THE INVENTION 
     The voltage potential between the flame sensor and the burner (ground) is the driving force for the flame ionization signal, therefore an increased voltage should yield a higher signal in spite of the probe being covered with insulative contaminants. However, merely applying a higher sensor voltage will likely yield mixed results for increased sensor life, at least in part because the higher voltage may increase contaminant build up on the sensor probe. It was therefore determined that what was needed was a means of regulating the applied voltage to the minimum value needed for a desired flame signal, and incrementally increasing the applied voltage only as required, while limiting the circuit current overdrive ratio (ratio of maximum sensor signal to the minimum threshold detection level) and allowing for a low signal threshold equal to the baseline configuration of standard commercial sensor apparatus, despite the increasing voltage. 
     The present invention therefore provides a sensor circuit which maintains a reasonably constant current, e.g. 5 microamps, over the operating life of the furnace by incrementally increasing the sensor voltage as the contamination buildup increases. This circuit allows the flame sensor circuit to draw on a higher voltage as needed, without incurring circuit overdrive issues, thereby eliminating the need for raising the signal threshold. Laboratory trials have shown an improved time to failure of the sensor circuit of four to seven times the life of known, or baseline, sensor circuit models. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and objects of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein: 
     FIG. 1 illustrates the known arrangement of components for explanation of a flame ionization sensor circuit. 
     FIG. 2 is a block diagram of a constant current sensor circuit according to the present invention. 
     FIG. 3 is a timing diagram illustrating the operation of the embodiment of FIG.  2 . 
     FIGS. 4A and 4B show a schematic of an alternative circuit to the block diagram of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As mentioned above, the primary cause of failure for flame ionization sensors is believed to be SiOx contamination insulation of the sensor probe, or tip, which is exposed to the flame. The SiOx contamination problem was studied by accelerated life testing of an flame ionization sensor in various furnace units by introduction of organosilicone contaminants into the burner air stream through a compressed air bubbler. Dow 344 fluid available from Dow Chemical Co., consisting of ninety percent Dow D4 fluid and ten percent Dow D5 fluid was used in the contaminant vaporization apparatus. The organosilicones are oxidized in the burner flame to silicon oxides (SiOx) which are deposited by impact on the sensor probe surfaces. While the results mentioned are the result of the accelerated life testing, it is believed that all results may be validly extrapolated to the real time phenomena of flame ionization sensor failure. 
     Referencing FIG. 2, the circuit  23  comprises a two-tap 4:1 step up transformer  25  increasing the 120 volt AC line voltage  21  to a 480 volts output  29  for operation of the AC source of the flame ionization sensor  11  at a first tap  31 . The second tap  33  provides power through rectifiers  26 , filters  28  and regulators  30  in known fashion for the amplifier and integrated circuit component DC power requirements for the control circuitry as set forth below. 
     The high side  29  of the 480 V ac source is then switched on/off by a controllable semiconductor switch  34 , e.g. a field effect transistor or FET, at a rate that will provide an RMS voltage value just high enough to maintain the desired sensor current, e.g. 5 microamps. Alternatively, variable voltage may be obtained through use of a multi-tap transformer with selectable switching between taps of either the primary or secondary, a variac, a triac, or other known power control schemes or combinations thereof. 
     As the sensor resistance increases with SiOx buildup, the current feedback will cause the switching time to increase thereby increasing the RMS voltage driving the sensor until the reference current is reestablished. 
     FIG. 3 shows a sample timing diagram illustrating the switch timing and its effect on the sensor voltage. With reference to FIGS. 2 and 3, the stepped up line voltage wave form from the first tap  31  of the transformer  25  is shown at reference number  35 . For each positive going zero crossing of the wave form  35  the zero crossing detector  37  and its associated first one shot multivibrator  39  output a positive pulse  40 . For each negative going zero crossing of the wave form  35  an invertor with hysteresis  41 , receiving the output of the zero crossing detector  37 , and its associated second one shot multivibrator  43  output a positive pulse  42 . The positive pulse streams of  40  and  42  are then combined, as by wired OR, into a single stream  45  which is input to the trigger input  47  of a voltage-to-pulse width converter  49 . 
     The output  52  of a linear amplifier  51  comparing the sensed resistance across the flame ionization sensor  11  and the desired constant current reference  54 , e.g. five microamps, is fed to the control input  53  of the voltage-to-pulse width converter  49 . The sensed resistance is gathered from a sensing resistor  55  in series with the flame ionization sensor  11 , which is amplified with a large common mode amplifier  57 , i.e. an amplifier with large common mode voltage handling capability, and then filtered with a low pass one hertz (1 Hz) filter  59  to extract the DC component. The output  61  of the voltage-to-pulse width converter  49  is then fed to a FET driver  63  which drives the duty cycle, seen at reference number  67 , of the controllable switch, or FET  65 , at an increased RMS voltage level, e.g. 270 Vrms, in order to keep the flame ionization sensor current output at a level which compensates for the increasing resistivity of the sensor due to contaminant build up on the probe. 
     While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments, such as that of FIGS. 4A and 4B and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.