Note: Descriptions are shown in the official language in which they were submitted.
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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.
~''.
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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.
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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
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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
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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.
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~ 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
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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
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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
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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~
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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
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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.
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