CA1203868A - Method and apparatus for flame detection - Google Patents
Method and apparatus for flame detectionInfo
- Publication number
- CA1203868A CA1203868A CA000388544A CA388544A CA1203868A CA 1203868 A CA1203868 A CA 1203868A CA 000388544 A CA000388544 A CA 000388544A CA 388544 A CA388544 A CA 388544A CA 1203868 A CA1203868 A CA 1203868A
- Authority
- CA
- Canada
- Prior art keywords
- flame
- probe
- signal
- conductivity
- voltage
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23N—REGULATING OR CONTROLLING COMBUSTION
- F23N5/00—Systems for controlling combustion
- F23N5/02—Systems for controlling combustion using devices responsive to thermal changes or to thermal expansion of a medium
- F23N5/12—Systems for controlling combustion using devices responsive to thermal changes or to thermal expansion of a medium using ionisation-sensitive elements, i.e. flame rods
- F23N5/123—Systems for controlling combustion using devices responsive to thermal changes or to thermal expansion of a medium using ionisation-sensitive elements, i.e. flame rods using electronic means
Abstract
Abstract Method and apparatus are provided for burner-flame detection. The flame-envelope conductivity is converted to an inversely proportional representative voltage, and the flame generated ac-signal level is converted to a proportional representative voltage, or vice versa. Both representative voltages are effectively summed to provide a contineous decision whether they are dynamically inversely proportional, and to indicate flame presence if they are and continue to be.
Description
`- ~2~3~fi~
Field of the In~ention The present invention relates to the detection and indication of flame presence. More particularly it relates to novel method and apparatus for utilizing hitherto unknown or neglected burner flame characteristics for the detection and indication of the presence of such flame.
Background and Prior Art of the Invention United States patent No. 4,157,506 issued June 5, 1979 to the present inventor and assigned to Combustion Engineering, Inc., Windsor, Conn., U.S.A. is indicative of and discusses the general prior art of the present invention. The said prior art patent states in column 1 thereof as follows:
"Various types of sensors are used to detect flames in furnaces. For example, electromagnetic-radiation sensors of various types can be employed to detect the ~nfrared, visible, or ultraviolet radiation emitted from flames and thereby detect flame presence. As another example, flame probes can be used to impress potential differences across the flame area in order to detect currents that are permitted to flow across the potential difference by flame-induced ionization. But the mere presence of ionization current or electromagnetic radiation is not a reliable indication o~ flame presence, so there remains the question of what to do with the sensor signal in order to be assured of flame presence.
~''.
12(~3~S8 One method of processing the sensor signal is to provide circuits that test the signal for proper amplitude.
I~ enou~h light or enough current is sensed, then an indication of flame is produced. Though there is some correlation ~etween the output of such a circuit and the presence of flame, the indication that results ~rom this circuit is not entirely satisfactory. Light is not produced only by flame; it can also be produced by glowing furnace walls or ambient light. The flow of current can also be caused by sources other than flame;
sometimes there is shorting of the probe to the grounded ignitor horn, causing a large current flow without flame presence. Accordingly, the mere senslng a DC level is not the best means of processing the signal.
Because of the difficulties with the DC signal, some schemes sense alternating currents. Canadian Pat. No. 801,250, for instance, uses a logarithmic detector that indicates flame presence when the ratio o AC-signal amplitude to DC-si~nal amplitude is sufficiently high~ The idea behind this method is ambient ultraviolet light is less likely to flicker than ultraviolet light caused by the presence of a flame. Accord-ingly, if there is sufficient flicker, than the source of the ultraviolet light must be a flame rather than, say, glowing walls. As a safeguard, this scheme ~NDs the logarithmic-detect-or output with a DC magnitude determination.
Whatever the virtues o~ this scheme for ultraviolet radiation, it has a drawback when used in a flame-probe 3131~8 apparatus because AC signals can be produced by non-flame sources such as electric ignitors. Canadian Pat. No. 748,015 contains a possible solution to this problem because it proposes detecting AC only in a limited frequency band, a properly selected frequency band should avoid the effect of ignition noise. However, the very limiting of the frequency band could make the requirements of the signal too stringent.
Unless the fuel supply is purposely modulated, a flame cannot be counted on always to supply frequency components within the predetermined band."
The invention in U.S. Patent ~,157,506 itsel~, while an improvement over its own prior art, goes only as far as utilizing the dc-value (or conductivity) of the probe current conjunctively with the presence of an ac-signal within a predetermined frequency band. It is true that the adding of the two variables is not simple conjunction, but involves two logic levels and exhibits hysteresis-like behaviour. Neverthe-less, the detection logic does not in its principle of operation go beyond the verification of one type of signal ~dc) through another (ac).
Summary of the Invention ~he present invention endeavors to improve the reliability of flame detection and indication and to mitigate flase operation.
~203~
In the cou~se o e~orts to achieve the herein-above enunciated broad objective, interesting characterestics of flame-envelopes that were either unknown or disregarded by the prior art were encountered. The most salient of these electrical characterestrics of flame-envelopes are:
(1) High frequency variations in conductivity are predominant at the root of the flame-envelope;
Field of the In~ention The present invention relates to the detection and indication of flame presence. More particularly it relates to novel method and apparatus for utilizing hitherto unknown or neglected burner flame characteristics for the detection and indication of the presence of such flame.
Background and Prior Art of the Invention United States patent No. 4,157,506 issued June 5, 1979 to the present inventor and assigned to Combustion Engineering, Inc., Windsor, Conn., U.S.A. is indicative of and discusses the general prior art of the present invention. The said prior art patent states in column 1 thereof as follows:
"Various types of sensors are used to detect flames in furnaces. For example, electromagnetic-radiation sensors of various types can be employed to detect the ~nfrared, visible, or ultraviolet radiation emitted from flames and thereby detect flame presence. As another example, flame probes can be used to impress potential differences across the flame area in order to detect currents that are permitted to flow across the potential difference by flame-induced ionization. But the mere presence of ionization current or electromagnetic radiation is not a reliable indication o~ flame presence, so there remains the question of what to do with the sensor signal in order to be assured of flame presence.
~''.
12(~3~S8 One method of processing the sensor signal is to provide circuits that test the signal for proper amplitude.
I~ enou~h light or enough current is sensed, then an indication of flame is produced. Though there is some correlation ~etween the output of such a circuit and the presence of flame, the indication that results ~rom this circuit is not entirely satisfactory. Light is not produced only by flame; it can also be produced by glowing furnace walls or ambient light. The flow of current can also be caused by sources other than flame;
sometimes there is shorting of the probe to the grounded ignitor horn, causing a large current flow without flame presence. Accordingly, the mere senslng a DC level is not the best means of processing the signal.
Because of the difficulties with the DC signal, some schemes sense alternating currents. Canadian Pat. No. 801,250, for instance, uses a logarithmic detector that indicates flame presence when the ratio o AC-signal amplitude to DC-si~nal amplitude is sufficiently high~ The idea behind this method is ambient ultraviolet light is less likely to flicker than ultraviolet light caused by the presence of a flame. Accord-ingly, if there is sufficient flicker, than the source of the ultraviolet light must be a flame rather than, say, glowing walls. As a safeguard, this scheme ~NDs the logarithmic-detect-or output with a DC magnitude determination.
Whatever the virtues o~ this scheme for ultraviolet radiation, it has a drawback when used in a flame-probe 3131~8 apparatus because AC signals can be produced by non-flame sources such as electric ignitors. Canadian Pat. No. 748,015 contains a possible solution to this problem because it proposes detecting AC only in a limited frequency band, a properly selected frequency band should avoid the effect of ignition noise. However, the very limiting of the frequency band could make the requirements of the signal too stringent.
Unless the fuel supply is purposely modulated, a flame cannot be counted on always to supply frequency components within the predetermined band."
The invention in U.S. Patent ~,157,506 itsel~, while an improvement over its own prior art, goes only as far as utilizing the dc-value (or conductivity) of the probe current conjunctively with the presence of an ac-signal within a predetermined frequency band. It is true that the adding of the two variables is not simple conjunction, but involves two logic levels and exhibits hysteresis-like behaviour. Neverthe-less, the detection logic does not in its principle of operation go beyond the verification of one type of signal ~dc) through another (ac).
Summary of the Invention ~he present invention endeavors to improve the reliability of flame detection and indication and to mitigate flase operation.
~203~
In the cou~se o e~orts to achieve the herein-above enunciated broad objective, interesting characterestics of flame-envelopes that were either unknown or disregarded by the prior art were encountered. The most salient of these electrical characterestrics of flame-envelopes are:
(1) High frequency variations in conductivity are predominant at the root of the flame-envelope;
(2) The ac-component varies in frequency from a low of 20Hz up to 1000 Hz, with robust flames, having good fuel/air rations, rapid fuel/air mixing and rapid combustion, generate high frequency signals generally in excess of 250 Hz; and
(3) Robust flames exhibit an impedance of 3 MOhms or less. Gas flames, for instance, may exhibit impedances as low as 50,000 Ohms. (Impedance being the inverse of conductivity).
But the most important characteristic of all is the fact that flame conductivity is inversely proportional to the ac-signal level generàted. Thus, as conductivity increases (i.e. impedance decreases) the ac-signal component decreases, and vice-versa.
The method and apparatus of the present invention ~3~
utiliz.e the inverse proportionality of conductivity and ac-signal to provide reliable contineous flame detection.
Accordingly, the present invention provides an improved method of electronic flame detection comprising the steps of: -(a) sensing flame conductivity;
(b) sensing a flame generated alternating current signal (ac-signal~;
(c) indicating flame presence when and as long as said flame conductivity and said level are inversely proportional within predetermined bounds.
It is important in this respect to point out that, due to the fact that a flame and its envelope are not static entities but vary dynamically with time, inverse proportion-ality ~etween conductivity and ac-signal is a contineous condition of flame existence. Thus, as soon as such inverse proportionality ceases, there is a high probability that the flame has altogether ceased to exist. Hence, the dynamic existence of inverse proportionality is to be taken, according to the invention, as indication of flame presence.
Thus, in its broadest scope, the present invention provides an improved method of flame detection CHAP~CTERI ~ED
BY indicating flame presence as long as an inversely ~ 5 --proportional relationship between conductivity and ac-signal level within predetermined bounds continues in time.
Correspondingly, the electronic apparatus provided according to the present invention contineously monitors or senses the conductivity and ac-signal, contineously detects whether they are inversely proportional or not, and if yes, contineously indicates flame presence.
It must be understood, of course, that conductivity and ac-signal are synomynous with steady (dc) and varying (ac) components, respectively, of electrical current conduction (or resistance thereto) through the flame envelope.
More specifically, an improved method oE electronic flame detection is provided, wherein flame conductivity and a flame generated alternating currant signal (ac-signal) are sensed. CHARACTERIZED BY indicating flame presence only if one of the following conditions obtains:
(i) Said flame conductivity is high and said ac-signal level is low;
(ii) Said flame conductivity is low and said ac-signal level is high; and (iii) Said flame conductivity is average and said ac-signal level is average, where low, average and high conductivity and ac-signal level are approximate, pre-determined, and characteristic of flame type.
~2~3~
~ ore specifically still, an improved electronic appartus for 1ame detection is provided comprising:
(a) a probe adapted for effecting electrical contact with a flame-envelope;
(b) first sensing means for sensing flame conductivity between said probe and a common terminal;
(c) second sensing means for sensing an alternating current signal (ac-signal) supplied by said probe;
(d) detecting means for detecting inverse proportion-ality between voltages representative of said flame conductivity and said ac-signal; and 4e) indicating means for indicating flame presence in response to said detecting means.
The present invention will be better understood through the detailed description of the following preferred embodiment in conjunc~ion with the attached drawing figure, which is a circuit schematic of an electronic apparatu~ according to the invention.
Detailed Description of the Preferred Embodiment:
Referring to the circuit schematic in the arawing, a burner flame 1 is schematically illustrated having a probe 2 in contact with the envelope of the flame 1, preferra~ly close to :~%~31~.~8 TM
its root. The probe 2 is pxe~errably made of KAN~HAL ,a high temperature metal that resists oxidation and droop. A phantom resistance R~ is shown connected between the probe 2 and the common terminal of the total system (ground). The resistance Rf is the flame-envelope resistance between the probe 2 and ground, which varies and is "modulated" by the flame yielding a dc-component and an ac-component. The inverse of the dc-component is termed the flame conductivity, while the ac-component results in "generated" frequencies between appr. 20 Hz and 1000 Hz. A relatively low value resistor Rl connects the probe 2 to the input of a dc-amplifier 3, and via a dc-blocking capacitor Cl to an ac-preamplifier 4, which itself feeds an ac-filter S driving a rectifier (or ac-detector) 6~ The outputs of the ac-detector 6 and of the dc-amplifier drive a differential amplifier or comparator 7 r which, accordingly, generates at its output 8 a signal to drive transistor Ql via LED 9 into conduction, which in turn causes relay Kl in the collector circuit of the transistor Ql to energize. The output 8 will activate the LED 9 and Ql only when the output of ac-detector 6 is slightly more positive than the output of the dc-amplifier 3. While thus the action of the comparater 7 is binary, the effective operation of the total circuit is to yield an indication at the output 8 only when, and as long as, the two dc and dc signal values are inversely proportional within bounds d~termined by the biasing and dimensioning of the circuit components. For instance, the term; nA] 10 haYing ~4.8 volts applied thereto approximately determines the lower bound of the output of the ac-detector 6, while its upper bound ., is approximately ~ 11 volts, or slightly less than the power supply voltagè (+12 volts~ of the apparatus. Thus, the non-inverting input to the comparator 7 may have a voltage between ~4.8 and ~11 volts. This voltage would normally vary as long as a flame is burning, and it increases proportional to an increase in ac-singal level~ On the other hand, the output of the dc-amplifier 3 decrease sharply from a maximum ~12 volts as the flame conductivity increases, so that as the ac-amplifier 6 output increases toward ~11 volts, the dc-amplifier 3 output decreases from ~12 volts. Throughout flame presence, these two voltages vary dynamically in opposite directions, and as long as they do the output 8 of the comparator 7 will be sufficient to keep Ql conducting, the indicator LED 9 on, and the relay Kl energized indicating flame presence.
Thus, the circuit is contineously detecting the inverse proportionality of flame conductivity and ac-signal level, as long as such inverse proportionality exist5 within certain bounds.
It is a matter of the nature of the burner flame type and to some extent the designers choice, what these bounds ara to be.
In the preferred embodiment, the dc-amplifier 3 has component values as follows:
R2 = 220 kOhm R3 : 220 kOhm R4 : 1000 kOhm R~ - 1000 kOhm R6 : 2 kOhm ~.~
_ g _ ~ 203~
R7 - 1000 kOhm C2 : 3.3 microfarad (eleetralytie) A diode 11 is an LED and indicates, when radiating, the existenee of ionization (i.e. a certain minimum conductivity) in the flame-envelope. Diodes 12 are merely for proteeting the opertional amplifier Al. The resistor Rl has a value of 20 kOhm and would limit any damaging currants through a fault in the probe wiring.
In the ac-preamplifier, the values are as follows:
R8 ~ 100 k~hm R9 : 100 kOhm R10 : 220 kOhm The dc blocking eapacitor Cl is 0.1 mierofarad.
In the ae-filter S the values are as follows:
Rll - 2.4 kOhm R12 : 390 Ohm R13 ~ 100 kOhm C3 ~ 0.1 microfarads C4 = 0.1 mierofarads C5 - 0.1 microfarads 3~
In the ac-detector the values are as follows:
R14 20 kOhm R15 - 20 kOhm R16: 1000 kOhm C6 . 0.47 microfaraas (electrolytic) C7 : 3.3 microfarads (electrolytic) Diodes 13 and 14 together with operational amplifier A4 10 approximate an ideal full-wave rectifier, with the capacitor C7 providing adequate ripple suppression.
Circuit Operation The junction of the flame-envelope resistance R
in series with Rl (negligible) with the resistor R2 is applied to the non-inverting input of the operational amplifier Al, the gàin of which is 4 as determined by the quotient R4/R3.
In the "No Flame" condition, no current flows through R2 since Rf is infinite (or e~tremely high). Thus, the non-inverting input o~ Al has the supply voltage ~12 volts applied thereto.
Accordingly, the output of the total dc-amplifier 3 is ~12 volts.
As the flame-envelope developes, its resistance Rf decreases and draws currant through R2, the voltage drop across which causes a proportional reduction (four fold) at the output of the amplifier Al, although the final output of the dc-amplifier 3 does not go below ~6 volts due to R5 and R7 With the component values given, the output of the amplifier Al reaches its minimum of ~1.5 volts as the flame-envelope i~
~3~
resistance drops to 2000 kOhm tequivalent to a conductance of 0.5 micro Mho). ~n~ further increase in conductance does not lead to a change in the output of the dc-amplifier 3. This extablishes a lower bound on conductance. Such lower bound may be altered by altering the gain of the dc-amplifier 3.
The ac-singal generated by the flame-envelope is picked up at the junction of Rl and R2 and coupled, via Cl, to the non-inverting input of operational amplifier A2 in the ac-preamplifier 4, the gain of which (determined by the quotient R10/R9~ is appr. 2.2, and the output of which derives the ac-filter 5. The latter is a high-pass active filter pro-viding C. 60 dB of attenuation to frequencies in the ac-signal below 250 Hz. The ac-detector or full-wave rectifier 6 has its-operational amplifier A4 connected at its inverting input to the output of the filter 5. In addition to rectification, the ac-detector has a gain of appr. 25 (determined by the quotient R16/R14, due to ripple capacitor C7).
As explained previously, the voltages representative of the conductivity and ac-signal level are "summed" in differential amplifier or comparator 7. While the comparator 7 is utilized as a binary output device, its function could be broken down into two units. The first unit would be to produce an algebraic sum of the two imput voltages, and the second unit to provide a threshold circuit to indicate flame presence once a certain threshold is exceeded, or to indicate flame absence once a certain threshold is undercut. As it is -3i!~
in the preferred embodiment, ~lame presence is indicated once ac-level is larger than conductivity by a small voltage difference just sufficient to trigger the relay driver Ql.
As those skilled in the art will immediately realize, there are an indeterminate number of circuit realizations to implement the principles of the present invention. As an example of said alternate realization, a representative voltage that is inversely proportional to the ac-signal level may be developed, while a proportional representative voltage i5 developed for the conductivity.
But the most important characteristic of all is the fact that flame conductivity is inversely proportional to the ac-signal level generàted. Thus, as conductivity increases (i.e. impedance decreases) the ac-signal component decreases, and vice-versa.
The method and apparatus of the present invention ~3~
utiliz.e the inverse proportionality of conductivity and ac-signal to provide reliable contineous flame detection.
Accordingly, the present invention provides an improved method of electronic flame detection comprising the steps of: -(a) sensing flame conductivity;
(b) sensing a flame generated alternating current signal (ac-signal~;
(c) indicating flame presence when and as long as said flame conductivity and said level are inversely proportional within predetermined bounds.
It is important in this respect to point out that, due to the fact that a flame and its envelope are not static entities but vary dynamically with time, inverse proportion-ality ~etween conductivity and ac-signal is a contineous condition of flame existence. Thus, as soon as such inverse proportionality ceases, there is a high probability that the flame has altogether ceased to exist. Hence, the dynamic existence of inverse proportionality is to be taken, according to the invention, as indication of flame presence.
Thus, in its broadest scope, the present invention provides an improved method of flame detection CHAP~CTERI ~ED
BY indicating flame presence as long as an inversely ~ 5 --proportional relationship between conductivity and ac-signal level within predetermined bounds continues in time.
Correspondingly, the electronic apparatus provided according to the present invention contineously monitors or senses the conductivity and ac-signal, contineously detects whether they are inversely proportional or not, and if yes, contineously indicates flame presence.
It must be understood, of course, that conductivity and ac-signal are synomynous with steady (dc) and varying (ac) components, respectively, of electrical current conduction (or resistance thereto) through the flame envelope.
More specifically, an improved method oE electronic flame detection is provided, wherein flame conductivity and a flame generated alternating currant signal (ac-signal) are sensed. CHARACTERIZED BY indicating flame presence only if one of the following conditions obtains:
(i) Said flame conductivity is high and said ac-signal level is low;
(ii) Said flame conductivity is low and said ac-signal level is high; and (iii) Said flame conductivity is average and said ac-signal level is average, where low, average and high conductivity and ac-signal level are approximate, pre-determined, and characteristic of flame type.
~2~3~
~ ore specifically still, an improved electronic appartus for 1ame detection is provided comprising:
(a) a probe adapted for effecting electrical contact with a flame-envelope;
(b) first sensing means for sensing flame conductivity between said probe and a common terminal;
(c) second sensing means for sensing an alternating current signal (ac-signal) supplied by said probe;
(d) detecting means for detecting inverse proportion-ality between voltages representative of said flame conductivity and said ac-signal; and 4e) indicating means for indicating flame presence in response to said detecting means.
The present invention will be better understood through the detailed description of the following preferred embodiment in conjunc~ion with the attached drawing figure, which is a circuit schematic of an electronic apparatu~ according to the invention.
Detailed Description of the Preferred Embodiment:
Referring to the circuit schematic in the arawing, a burner flame 1 is schematically illustrated having a probe 2 in contact with the envelope of the flame 1, preferra~ly close to :~%~31~.~8 TM
its root. The probe 2 is pxe~errably made of KAN~HAL ,a high temperature metal that resists oxidation and droop. A phantom resistance R~ is shown connected between the probe 2 and the common terminal of the total system (ground). The resistance Rf is the flame-envelope resistance between the probe 2 and ground, which varies and is "modulated" by the flame yielding a dc-component and an ac-component. The inverse of the dc-component is termed the flame conductivity, while the ac-component results in "generated" frequencies between appr. 20 Hz and 1000 Hz. A relatively low value resistor Rl connects the probe 2 to the input of a dc-amplifier 3, and via a dc-blocking capacitor Cl to an ac-preamplifier 4, which itself feeds an ac-filter S driving a rectifier (or ac-detector) 6~ The outputs of the ac-detector 6 and of the dc-amplifier drive a differential amplifier or comparator 7 r which, accordingly, generates at its output 8 a signal to drive transistor Ql via LED 9 into conduction, which in turn causes relay Kl in the collector circuit of the transistor Ql to energize. The output 8 will activate the LED 9 and Ql only when the output of ac-detector 6 is slightly more positive than the output of the dc-amplifier 3. While thus the action of the comparater 7 is binary, the effective operation of the total circuit is to yield an indication at the output 8 only when, and as long as, the two dc and dc signal values are inversely proportional within bounds d~termined by the biasing and dimensioning of the circuit components. For instance, the term; nA] 10 haYing ~4.8 volts applied thereto approximately determines the lower bound of the output of the ac-detector 6, while its upper bound ., is approximately ~ 11 volts, or slightly less than the power supply voltagè (+12 volts~ of the apparatus. Thus, the non-inverting input to the comparator 7 may have a voltage between ~4.8 and ~11 volts. This voltage would normally vary as long as a flame is burning, and it increases proportional to an increase in ac-singal level~ On the other hand, the output of the dc-amplifier 3 decrease sharply from a maximum ~12 volts as the flame conductivity increases, so that as the ac-amplifier 6 output increases toward ~11 volts, the dc-amplifier 3 output decreases from ~12 volts. Throughout flame presence, these two voltages vary dynamically in opposite directions, and as long as they do the output 8 of the comparator 7 will be sufficient to keep Ql conducting, the indicator LED 9 on, and the relay Kl energized indicating flame presence.
Thus, the circuit is contineously detecting the inverse proportionality of flame conductivity and ac-signal level, as long as such inverse proportionality exist5 within certain bounds.
It is a matter of the nature of the burner flame type and to some extent the designers choice, what these bounds ara to be.
In the preferred embodiment, the dc-amplifier 3 has component values as follows:
R2 = 220 kOhm R3 : 220 kOhm R4 : 1000 kOhm R~ - 1000 kOhm R6 : 2 kOhm ~.~
_ g _ ~ 203~
R7 - 1000 kOhm C2 : 3.3 microfarad (eleetralytie) A diode 11 is an LED and indicates, when radiating, the existenee of ionization (i.e. a certain minimum conductivity) in the flame-envelope. Diodes 12 are merely for proteeting the opertional amplifier Al. The resistor Rl has a value of 20 kOhm and would limit any damaging currants through a fault in the probe wiring.
In the ac-preamplifier, the values are as follows:
R8 ~ 100 k~hm R9 : 100 kOhm R10 : 220 kOhm The dc blocking eapacitor Cl is 0.1 mierofarad.
In the ae-filter S the values are as follows:
Rll - 2.4 kOhm R12 : 390 Ohm R13 ~ 100 kOhm C3 ~ 0.1 microfarads C4 = 0.1 mierofarads C5 - 0.1 microfarads 3~
In the ac-detector the values are as follows:
R14 20 kOhm R15 - 20 kOhm R16: 1000 kOhm C6 . 0.47 microfaraas (electrolytic) C7 : 3.3 microfarads (electrolytic) Diodes 13 and 14 together with operational amplifier A4 10 approximate an ideal full-wave rectifier, with the capacitor C7 providing adequate ripple suppression.
Circuit Operation The junction of the flame-envelope resistance R
in series with Rl (negligible) with the resistor R2 is applied to the non-inverting input of the operational amplifier Al, the gàin of which is 4 as determined by the quotient R4/R3.
In the "No Flame" condition, no current flows through R2 since Rf is infinite (or e~tremely high). Thus, the non-inverting input o~ Al has the supply voltage ~12 volts applied thereto.
Accordingly, the output of the total dc-amplifier 3 is ~12 volts.
As the flame-envelope developes, its resistance Rf decreases and draws currant through R2, the voltage drop across which causes a proportional reduction (four fold) at the output of the amplifier Al, although the final output of the dc-amplifier 3 does not go below ~6 volts due to R5 and R7 With the component values given, the output of the amplifier Al reaches its minimum of ~1.5 volts as the flame-envelope i~
~3~
resistance drops to 2000 kOhm tequivalent to a conductance of 0.5 micro Mho). ~n~ further increase in conductance does not lead to a change in the output of the dc-amplifier 3. This extablishes a lower bound on conductance. Such lower bound may be altered by altering the gain of the dc-amplifier 3.
The ac-singal generated by the flame-envelope is picked up at the junction of Rl and R2 and coupled, via Cl, to the non-inverting input of operational amplifier A2 in the ac-preamplifier 4, the gain of which (determined by the quotient R10/R9~ is appr. 2.2, and the output of which derives the ac-filter 5. The latter is a high-pass active filter pro-viding C. 60 dB of attenuation to frequencies in the ac-signal below 250 Hz. The ac-detector or full-wave rectifier 6 has its-operational amplifier A4 connected at its inverting input to the output of the filter 5. In addition to rectification, the ac-detector has a gain of appr. 25 (determined by the quotient R16/R14, due to ripple capacitor C7).
As explained previously, the voltages representative of the conductivity and ac-signal level are "summed" in differential amplifier or comparator 7. While the comparator 7 is utilized as a binary output device, its function could be broken down into two units. The first unit would be to produce an algebraic sum of the two imput voltages, and the second unit to provide a threshold circuit to indicate flame presence once a certain threshold is exceeded, or to indicate flame absence once a certain threshold is undercut. As it is -3i!~
in the preferred embodiment, ~lame presence is indicated once ac-level is larger than conductivity by a small voltage difference just sufficient to trigger the relay driver Ql.
As those skilled in the art will immediately realize, there are an indeterminate number of circuit realizations to implement the principles of the present invention. As an example of said alternate realization, a representative voltage that is inversely proportional to the ac-signal level may be developed, while a proportional representative voltage i5 developed for the conductivity.
Claims (14)
1. An improved method of electronic flame detection comprising the steps of:
(a) sensing flame conductivity;
(b) sensing a flame generated alternating current signal (ac-signal) level;
(c) indicating flame presence when and as long as, said flame conductivity and said level are inversely proportional within predetermined bounds.
(a) sensing flame conductivity;
(b) sensing a flame generated alternating current signal (ac-signal) level;
(c) indicating flame presence when and as long as, said flame conductivity and said level are inversely proportional within predetermined bounds.
2. An improved method of electronic flame detection, wherein flame conductivity and a flame generated alternating currant signal (ac-signal) level are sensed, CHARACTERIZED BY
indicating flame presence only if one of the following conditions continues to obtain:
(i) siad flame conductivity is high and said ac-signal level is low;
(ii) said fIame conductivity is low and said ac-signal level is high; and (iii) said flame conductivity is average and said ac-signal level is average, where low, avergae and high conductivity and ac-signal level are approximate, pre-determined, and characteristic of flame type.
indicating flame presence only if one of the following conditions continues to obtain:
(i) siad flame conductivity is high and said ac-signal level is low;
(ii) said fIame conductivity is low and said ac-signal level is high; and (iii) said flame conductivity is average and said ac-signal level is average, where low, avergae and high conductivity and ac-signal level are approximate, pre-determined, and characteristic of flame type.
3. The improved method as defined in claim 1, wherein said step (c) obtains only if a sum of analog representations of said flame conductivity and said ac-signal level exceeds a predetermined value.
4. The improved method as defined in claims 1, 2 and 3, carried out by analog, electronic processing except for the step of indicating flame presence.
5. The improved method as defined in claims 1,2 or 3, said flame conductivity being sensed by impressing a dc-voltage on a probe in contact with the flame and amplifying the probe-current, and said ac-signal being sensed by amplifying, filtering and rectifying ac-signals picked up by said probe.
6. The improved method as defined in claims 1, 2 or 3, said flame conductivity being sensed by impressing a dc-voltage on a probe in contact with the flame and amplifying the probe-current, and said ac-signal being sensed by amplifying, filtering and rectifying ac-signals picked up by said probe, further CHARACTERIZED BY algebraically summing a voltage representative of said probe-current and a voltage representative of the rectified ac-signal, and indicating flame presence only when, and as long as, said summing step yields a predetermined voltage sufficient to trigger a flame indicating means.
7. An improved electronic apparatus for flame detection comprising:
(a) a probe adapted for effecting electrical contact with a flame envelope;
(b) first sensing means for sensing flame conductivity between said probe and a common terminal;
(c) second sensing means for sensing an alternating current signal (ac-signal) supplied by said probe;
(d) detecting means for detecting inverse proportionality between voltages representative of said flame conductivity and said ac-signal;
and (e) indicating means for indicating flame presence in response to said detecting means.
(a) a probe adapted for effecting electrical contact with a flame envelope;
(b) first sensing means for sensing flame conductivity between said probe and a common terminal;
(c) second sensing means for sensing an alternating current signal (ac-signal) supplied by said probe;
(d) detecting means for detecting inverse proportionality between voltages representative of said flame conductivity and said ac-signal;
and (e) indicating means for indicating flame presence in response to said detecting means.
8. The improved electronic apparatus as defined in claim 7, said detecting means comprising summing means responsive to said voltages.
9. The improved electronic apparatus as defined in claim 8, said indicating means responding to said summing means when said voltages sum to within a predetermined range.
10. The improved electronic apparatus as defined in claims 7, 8 or 9, said first sensing means comprising means for impressing a dc-voltage across said probe and said common terminal.
11. The improved electronic apparatus as defined in claims 7, 8 or 9, said sensing means comprising means for impressing a dc-voltage across said probe and said common terminal, and said second sensing means comprising an ac-preamplifier, an ac-filter and an ac-detector.
12. The improved electronic apparatus as defined in claims 7, 8 or 9, said first sensing means comprising means for impressing a dc-voltage across said probe and said common terminal, and dc-amplifier means for generating an output voltage inversely proportional to current flow between said probe and said common terminal.
13. The improved electronic apparatus as defined in claims 7, 8 or 9, said first sensing means comprising means for impressing a dc-voltage across said probe and said common terminal, and dc-amplifier means for generating an output voltage inversely proportional to current flow between said probe and said common terminal, and said second sensing means generating an output voltage proportional to said ac-signal level.
14. The improved electronic apparatus as defined in claims 7, 8 or 9, said first sensing means comprising means for impressing a dc-voltage across said probe and said common terminal, dc-amplifier means for generating an output voltage inversely proportional to current flow between said probe and said common terminal, said second sensing means comprising an ac-preamplifier, an ac-filter and an ac-detector, and said second sensing means generating a dc-output voltage proportion-al to said ac-signal level.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000388544A CA1203868A (en) | 1981-10-22 | 1981-10-22 | Method and apparatus for flame detection |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000388544A CA1203868A (en) | 1981-10-22 | 1981-10-22 | Method and apparatus for flame detection |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1203868A true CA1203868A (en) | 1986-04-29 |
Family
ID=4121238
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000388544A Expired CA1203868A (en) | 1981-10-22 | 1981-10-22 | Method and apparatus for flame detection |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1203868A (en) |
-
1981
- 1981-10-22 CA CA000388544A patent/CA1203868A/en not_active Expired
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