U.S. patent number 3,861,854 [Application Number 05/220,788] was granted by the patent office on 1975-01-21 for flame monitoring system.
This patent grant is currently assigned to Walter Kidde & Company, Inc.. Invention is credited to Lyman H. Walbridge.
United States Patent |
3,861,854 |
Walbridge |
January 21, 1975 |
FLAME MONITORING SYSTEM
Abstract
Disclosed is a flame monitoring system having a sensing
capacitor connected between the "hot" line of an a.c. power supply
and a flame electrode in the flame and current isolated from the
neutral line. Current rectified by flame charges the capacitor
providing a flame indicating signal that is coupled into a control
circuit fed by the a.c. power supply.
Inventors: |
Walbridge; Lyman H. (Ashland,
MA) |
Assignee: |
Walter Kidde & Company,
Inc. (Clifton, NJ)
|
Family
ID: |
22824980 |
Appl.
No.: |
05/220,788 |
Filed: |
January 26, 1972 |
Current U.S.
Class: |
431/80 |
Current CPC
Class: |
F23N
5/123 (20130101); F23N 5/12 (20130101); F23N
5/242 (20130101); F23N 5/203 (20130101); F23N
2227/36 (20200101); F23N 2229/12 (20200101); F23N
2227/32 (20200101); F23N 2223/06 (20200101); F23N
2229/00 (20200101) |
Current International
Class: |
F23N
5/24 (20060101); F23N 5/20 (20060101); F23N
5/12 (20060101); F23n 005/10 () |
Field of
Search: |
;431/80,78 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Favors; Edward G.
Attorney, Agent or Firm: Toupal; John E.
Claims
What is claimed is:
1. Circuit apparatus for monitoring flame generated by a fuel
burner and comprising:
first and second input terminals for connection to a source of a.c.
power;
a flame sensing electrode for disposition adjacent a fuel burner in
the region occupied by flame emanating therefrom;
direct current sensing means connected between said first terminal
and said flame sensing electrode; said direct current sensing means
being electrical current isolated from said second input terminal
and adapted to produce a flame signal in response to current flow
supported by a flame emanating from the burner;
ground circuit means electrically isolated from said first and
second terminals, said ground circuit means adapted for connecting
a ground wire of a three-wire supply with a second electrode means
spaced from said first electrode in said region occupied by flame;
and
control means connected between said first and second terminals so
as to be powered by the source of a.c. power connected thereto, and
coupled to said sensing means so as to be responsive to said flame
signal.
2. Circuit apparatus according to claim 1 wherein said direct
current sensing means comprises a capacitive element for storing
the current flow supported by the flame.
3. Circuit apparatus according to claim 1 wherein said direct
current sensing means comprises a signal voltage source means that
provides a d.c. signal voltage with respect to an a.c. voltage
applied to said first terminal in response to current flow
supported by the flame.
4. Circuit apparatus according to claim 3 wherein said signal
voltage source means comprises a discharge capacitor for storing
energy in response to current flow through the flame and switching
means for periodically discharging said discharge capacitor to
produce said signal voltage.
5. Circuit apparatus according to claim 4 wherein said direct
current sensing means comprises a storage capacitor for storing
energy in response to current flow through the flame, said storage
capacitor being coupled to said discharge capacitor so as to
provide electrical energy thereto.
6. Circuit apparatus according to claim 5 wherein said control
means comprises a load and an SCR for controlling energization
thereof, said SCR being coupled to said sensing means so as to be
gated by said signal voltage.
7. Circuit apparatus according to claim 6 wherein said load
comprises a solenoid for controlling a fuel supply valve.
8. Circuit apparatus according to claim 1 wherein said control
means comprises an electrical operator for a fuel supply valve.
9. Circuit apparatus according to claim 1 wherein said control
means comprises spark electrode means for supporting ignition
sparks in said region, and a transformer having a winding for
providing ignition pulses to said spark electrode means.
10. Circuit apparatus according to claim 9 wherein said spark
electrode means comprises said flame sensing electrode.
11. Circuit apparatus according to claim 10 wherein said second
electrode comprises the burner providing the flame, and said ground
circuit comprises an electrically conductive housing.
12. Circuit apparatus according to claim 2 wherein said direct
current sensing means comprises energy storage means for storing
energy in said direct current; and including sampling means for
periodically sampling the energy level stored in said energy
storage means, and timing means coupled to said sampling means and
adapted to produce sampling thereby at substantially zero-crossings
of said a.c. power source.
13. Circuit apparatus according to claim 12 wherein said timing
means produces said samplings only on particular zero-crossings
during which said a.c. power source is proceeding to produce said
direct current.
14. Circuit apparatus for monitoring flame generated by a fuel
burner and comprising
valve means for controlling flow of fuel to the burner;
energy source means for providing a level of energy dependent upon
the presence of flame at the burner;
electrical sampling means for detecting the energy level provided
by said source means, said sampling means drawing power only from
said source means and providing a periodic output signal indicative
of flame at the burner; and
control means for closing said valve to prevent fuel flow to said
burner in response to the absence of a minimum energy level sampled
by said sampling means.
15. Circuit apparatus according to claim 14 wherein said energy
source means comprises flame responsive means for providing a
direct current in response to the presence of flame at the burner,
and energy storage means for storing energy in said direct
current.
16. Circuit apparatus according to claim 15 wherein said control
means comprises amplifier means for amplifying the energy sampled
by said sampling means.
17. Circuit apparatus according to claim 16 wherein said energy
storage means comprises a discharge capacitance charged in response
to flow of said direct current and discharged into said amplifier
means by said sampling means.
18. Circuit apparatus according to claim 17 wherein said energy
storage means further comprises a storage capacitance charged by
said direct current so as to store energy therein and coupled to
said discharge capacitor so as to provide charging current thereto
and including current limiting means connected between said storage
capacitance and said discharge capacitance.
19. Circuit apparatus according to claim 18 wherein said sampling
means comprises a timer electronic switch for discharging said
storage capacitance into said amplifier means.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to fuel burners and, more
particularly, to fuel control systems for fuel burners.
Extensive efforts have been directed toward the improvement of fuel
control systems for fuel burners such as gas and oil burners and
the like. Increased system safety and reliability have been primary
objectives of such efforts. These objectives, however, generally
conflict with an obvious desire to limit the cost and physical size
of the systems.
Most burner systems employ fuel supply valves that are
automatically controlled by some type of flame sensing mechanism
that automatically interrupts fuel flow in response to a
predetermined loss of flame condition. According to one common
technique, the presence of a flame is indicated by a signal current
which is rectified by the flame as a result of the well known
ionization phenomena. Although flame rectification provides a
relatively effective method of sensing flame, prior systems of this
type have suffered from certain disadvantages including the
requirement for expensive isolation transformers for isolating the
flame sensing circuitry from the power lines. Other problems of
prior systems are associated with the necessity for isolating the
d.c. flame rectification signal from a.c. component present
therewith. In many poor flames the detection of directional
conduction is marginal because of leakage in both directions and
amplification does not fully solve the problem in that it is
susceptible to a.c. pickup particularly when the amplifier is
connected to the "hot" side of the line.
The above noted problems are avoided to some extent in the system
disclosed in U.S. Pat. No. 3,441,356. In that system a single
polarity supply is utilized to produce the flame responsive current
and the relative conduction from a positive electrode is compared
with that from a negative electrode to establish the presence of
flame. However, in that type of system an inadvertent short circuit
to the flame sensing electrode will produce a d.c. current that
cannot be distinguished from a flame supported signal. Other common
problems of this as well as other burner control systems are
associated with the electronic elements used to monitor the signals
produced by the flame rectification current. Typically, an
electronic switching element such as a silicon controlled rectifier
is gated by the flame signal to produce a desired control signal.
False triggering of such conductor devices by transients is
relatively common and reduces overall system reliability.
The object of this invention therefore, is to provide an improved
flame responsive control system for fuel burners that is both
reliable and of reasonable cost.
SUMMARY OF THE INVENTION
The invention is characterized by the provision of a flame
monitoring circuit in which a storage capacitor is connected
between the "hot" line of an a.c. power supply and a flame
electrode disposed so as to be bathed in the flame being monitored.
The storage capacitor is electrical current isolated from the
neutral line of the power source so as to pass only that current
circulating between the "hot" line and the grounded burner
providing the flame being monitored. Because of its rectification
properties, the flame causes a flow of direct current that charges
the storage capacitor providing a flame indicating signal voltage.
A control circuit powered by the a.c. source is coupled to the
storage capacitor so as to respond to either the presence or
absence thereon of a d.c. signal voltage with respect to the "hot"
line. By isolating the flame rectified current in a ground loop and
utilizing the resultant d.c. signal voltage with respect to the
"hot" line, the signal to noise ratio of the system is greatly
enhanced without extensive filtering and the requirement for an
isolation transformer is eliminated.
In a featured embodiment of the invention above described, the
control circuit includes a silicon-controlled rectifier (SCR) that
is gated by the flame indicating signal voltage to supply power to
suitable load. Generally the load consists of an electrical
operator for controlling a valve that supplies fuel to the burner
being monitored. The load can also include a pulse transformer for
providing ignition pulses to electrodes disposed so as to ignite
fuel emanating from the burner. In that case, one of the spark
electrodes is preferably utilized to function also as the flame
sensing electrode that carries the flame rectified current.
Another feature of the invention constitutes a sampling circuit for
periodically sampling the energy level stored in the storage
capacitor. The sampling circuit includes a discharge capacitor
coupled to the storage capacitor so as to receive charging current
therefrom and a complementary silicon-controlled rectifier is
periodically activated to dump the energy from the discharge
capacitor into the gate circuit of the silicon controlled
rectifier. Preferably, the complementary silicon-controlled
rectifier is fired at zero-crossings of the a.c. power source
immediately preceding those half cycles during which flame
rectified current is produced. This insures that the signal level
at time of discharge is dependent only upon flame rectified current
flow and not upon any temporary charge produced by alternating
current flow through the high impedance path provided by the flame.
Also, the possibility of inadvertent firings of the
silicon-controlled rectifier by stray signals is substantially
reduced.
DESCRIPTION OF THE DRAWINGS
These and other objects and features of the invention will become
more apparent upon a perusal of the following description taken in
conjunction with the accompanying drawings therein:
FIG. 1 is a schematic circuit diagram showing a preferred
embodiment of the invention;
FIGS. 2a, 2b, 2c are graphs showing various waveforms present in
the circuit of FIG. 1; and
FIG. 3 is a schematic circuit diagram of another preferred
embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1 there is shown a circuit 11 for monitoring
the presence or absence of flame in a region 12 directly adjacent a
fuel burner 13. The monitoring network 11 is retained by an
electrically conductive housing 14 and includes a sensing circuit
15 connected between a control circuit 16 and a flame electrode 17
disposed in the flame region 12. Power is supplied to the network
11 by connection of a first terminal 18 and a second terminal 19 to
a conventional a.c. source. The terminal 18 is connected to a line
20 while a third terminal 21 is connected to the conductive housing
14. Fuel such as natural gas or oil, for example, is supplied to
the burner 13 through a supply pipe 22 and a solenoid controlled
valve 23.
The sensing circuit 15 includes a storage capacitor C1 and Resistor
R1 connected in series between the terminal 18 and the flame
electrode 17. Also included in the sensing circuit 15 is a series
combination of a filter resistor R2, a discharge capacitor C2 and a
signal resistor R3 connected across the storage capacitor C1. A
further element of the sensing circuit 15 is a sampling circuit
comprising a complementary silicon-controlled rectifier (CSCR)
having an anode connected to the line 20 and a cathode connected to
a junction between the resistor R2 and the capacitor C2. The gate
electrode of the CSCR is connected to the line 20 by a diode D1 and
to the terminal 19 by a resistor R4.
Included in the control circuit 16 are a parallel combination of a
solenoid relay winding 31 and a filter capacitor C3 connected to
the line 20 by a capacitor C4 and to the terminal 19 by a diode D2.
Additional elements of the control circuit 16 are an ignition
transformer T1 and a silicon controlled rectifier (SCR1). The
primary winding 32 of the transformer T1 is connected between the
capacitor C4 and the anode of the SCR, the cathode of which is
connected to the line 20. Signal, coupling between the sensing
circuit 15 and the control circuit 16 is provided by a connection
between the gate of the SCR1 and the junction between the discharge
capacitor C2 and the signal resistor R3. The secondary winding of
the transformer T1 is connected between a spark electrode 34
positioned in the flame region 12 and the flame electrode 17 which
also serves as a second spark electrode.
A start-up circuit 41 is also included in the network 11 shown in
FIG. 1. The start-up circuit 41 includes a parallel combination of
a filter capacitor C5 and a relay winding 42 connected in series
with a resistor R5 and a diode D3 across the input terminals 18 and
19. Actuated by the winding 42 is a switch with a movable contact
43 connected to the input terminal 19. With the winding 42
deenergized, the movable contact 43 engages a stationary contact 44
connected to the junction between capacitor C1 and resistor R1 by a
resistor R6 and a diode D4. Energization of the winding 42 moves
the contact 43 into engagement with a stationary contact 45
connected to one end of a solenoid 37 that controls the fuel supply
valve 23. The other end of the solenoid 37 is connected to a
normally open switch 36 that is activated by the winding 31.
During installation, the network 11 is connected to a conventional
115 volt power source with terminal 18 connected to the "hot" line
and terminal 19 connected to the neutral wire. Assuming a typical
three wire supply, the terminal 21 is connected to the ground wire
assuring that the housing 14 is at the ground potential of the
supply pipe 22 and the burner 13.
To initiate operation of the burner 13, an on-off switch 47 is
closed so as to produce on line 20 the sine wave voltage
illustrated in FIG. 2a. Because of a time delay provided by the
capacitor C5 the winding 42 is not immediately energized and the
contacts 43 and 44 remain engaged to provide a current path through
the storage capacitor C1, the resistor R6 and the diode D4 during
positive half cycles on the line 20. Consequently, there is built
up in the storage capacitor C1 a charge having the polarity
indicated in FIG. 1. This charge on storage capacitor C1 which is,
for example, in the range of about 10 volts is illustrated by the
wave-form in FIG. 2b shown in time alignment with the waveform in
FIG. 2a. The diode D1 biases the gate of the CSCR negative when the
voltage on line 20 is positive causing it to conduct and biases the
gate positive by the amount of forward drop in the diode D1 while
the voltage on line 20 is negative insuring non-conduction of the
CSCR. Thus, as the CSCR becomes conductive during a portion of each
positive half cycle on line 20 so as to shunt the capacitor C2
through resistor R3 and the capacitor C1 through resistor R2.
However, during negative half cycles on line 20, the CSCR in
non-conductive and discharge capacitor C2 is charged through R2 in
accordance with the charge remaining on storage capacitor C1. As
the voltage on line 20 goes positive to fire the CSCR, any
appreciable charge on discharge capacitor C2 appears as a positive
pulse across the resistor R3 to fire the SCR 1.
Triggering of the SCR 1 allows the capacitor C4 which on the
previous half cycle was charged with the polarity indicated in FIG.
1 to discharge through the primary winding 32 of the transformer
T1. This produces a high voltage pulse in the secondary 33 and a
resultant spark between the electrodes 17 and 34. During the next
negative half cycle on line 20 the capacitor C4 is again charged by
current flow through the diode D2 and the relay winding 31. This
operation continues producing during each cycle on line 20 an
ignition spark in the region 12 between electrodes 17 and 34 and a
surge of current through the relay winding 31 maintaining
energization thereof and resultant closure of the switch 36. As
described above, the winding 42 is not immediately energized
because of the time constant exhibited by the capacitor C5. After a
certain delay, however, the winding becomes energized moving the
contact 42 into engagement with the fixed contact 45.
Simultaneously, contacts 43 and 44 are opened to terminate the
supply of line current to the storage capacitor C1 which, however,
retains sufficient charge to continue firing the SCR1 for a given
ignition period in the manner described above. During that period
both switch 36 and contacts 44, 45 are closed to energize the valve
solenoid 37. In response to energization to solenoid 37, the valve
23 opens initiating fuel flow to the burner 13.
Assuming that the fuel fed to the burner 13 is ignited, a flame
appears in the region 12 occupied by the electrodes 17 and 34. As
is well known, that flame acts as an imperfect diode that may be
represented (as shown dotted in FIG. 1) by a perfect diode with a
high resistance in series and another high resistance in parallel
with the combination. The flame produced diode is polarized such
that the greater current flow occurs through the flame on the
positive half cycles illustrated in FIG. 2a thus maintaining on
storage capacitor C1 the charge illustrated in FIG. 2b. Thus, the
maintenance of a charge on the storage capacitor C1 is indicative
of current flow supported by flame in the region 12. The presence
of charge on the capacitor C1 insures continued operation of the
sensing circuit 15 and control circuit 16 in the manner described
above to insure continued flow of fuel through the valve 23.
If at any time the flame in region 12 is extinguished, the network
11 tries for re-ignition during a brief ignition period. As
described above, this period is provided by the storage
capabilities of the capacitor C1 which continues to supply current
to the discharge capacitor C2 for a limited period even in the
absence of continuing flame rectified current flow. However, if the
flame is not re-ignited within the short ignition period, the
absence of flame rectified current flow will result in discharge of
the capacitor C1 and eliminate periodic charging current flow to
the discharge capacitor C2. Consequently, no further pulses will be
produced across the resistor R3 to fire the SCR1, which will remain
nonconductive terminating periodic discharge of the capacitor C4.
This in turn will eliminate energizing current flow through the
winding 31 to open the switch 36 and de-energize solenoid 37.
Thus, a prolonged loss of flame in the region 12 automatically
results in closing of valve 23 go prevent further fuel flow to the
burner 13. Furthermore, because the winding 42 in the start-up
circuit 41 remains energized to prevent engagement of contacts 43
and 44 and, accordingly, charging current flow through the storage
capacitor C1, a new try for ignition can be initiated only by a
loss of power between the terminals 18 and 19. That occurence
caused for example by opening the switch 47, will de-energize
winding 42 allowing contact to be made between contacts 43 and 44
and producing another try for ignition in the manner described
above. It will be appreciated that this re-ignition in the manner
described required also in the event that ignition is not initially
achieved within the ignition period provided by retained charge in
the storage capacitor C1.
It will be noted with regard to the network 11 shown in FIG. 1,
that the sensing circuit 15 is current isolated from the control
circuit 16. Any current available for charging the storage
capacitor C1 and accordingly the discharge capacitor C2 must be
supported by flame in the region 12 which completes a path to the
ground circuit including the burner 13 and the ground terminal 21.
Thus, any energy available in the discharge capacitor C2 for
producing a flame signal across the resistor R3 that in turn
triggers SCR 1 can result only from current flow through a flame in
region 12. Furthermore, by utilizing as a flame signal a pulse of
stored energy having a level dependent upon the flame condition
being sensed, no signal amplification is required. For these
reasons highly reliable signal information is provided and, in
addition, circuit isolating the neutral terminal 19 from the ground
circuit eliminates the need for isolation transformers.
Of further note is the utilization of the flame signal applied to
the SCR1 in the control circuit 16 with respect to the hot line 20
thereby substantially reducing the effect on the sensing signal of
a.c. present within the system.
It will be appreciated however, that on an instantaneous basis some
a.c. effects are present in the measuring circuit 15. As noted
above the flame acts as an imperfect rather than a perfect diode
and does support a small component of a.c. This is demonstrated in
FIG. 2b wherein the negative flame voltage on capacitor C1
increases during the positive half cycle on signal line 20 and then
decreases to its steady state value at the conclusion of the
negative half cycle on line 20. The present invention reduces this
a.c. effect by the above described sampling of the charge energy
stored in the discharge capacitor C2 at a particular time during
the a.c. cycle. The sampling diode D1 triggers the CSCR to
discharge the capacitor C2 at each positive going transition of the
voltage on line 20. Those are the particular a.c. zero crossings
which initiate a.c. current flow in the same sense as the d.c.
flame current through the storage capacitor C1. As illustrated FIG.
2b, it is at those particular times that the opposite half cycle
a.c. effects on the capacitor C1 have compensated each other
leaving a steady state d.c. signal responsive only to the d.c.
current provided by the sensed flame condition.
Referring now to FIG. 3, there is illustrated another embodiment 51
in which components identical to those shown in FIG. 1 are given
corresponding reference numerals. A sensing circuit 52 is identical
to the sensing circuit 15 of FIG. 1 except that a primary winding
53 of a transformer T2 replaces the resistor R3. A control circuit
54 includes a silicon-controlled rectifier SCR2 and a load resistor
RL connected in series across input terminals 18 and 19. Also
included in the control circuit 54 is a secondary winding 55 of the
transformer T2 connected between the gate of the SCR2 and the
junction between the resistor RL and the cathode of the SCR2.
The operation of the embodiment 51 is similar to that described
above for embodiment 11. However, in this case the presence of
flame in region 12 is indicated at each discharge of the discharge
capacitor C2 by a pulse through the primary winding 53. The
resultant pulse in the secondary winding 55 fires the SCR2 so as to
provide energizing current for the load RL. It should be noted that
SCR2 is poled opposite to that shown in embodiment 11 and may be
powered directly from the line instead of from a previously charged
capacitor. It will be obvious that the load RL could include, for
example, a valve controlling relay or an ignition transformer as in
embodiment 11. Also, the starter circuit 41 shown in FIG. 1 could
be similarly employed in embodiment 51.
Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore, to be understood that within the scope of the appended
claims the invention can be practised otherwise than as
specifically described.
* * * * *