Note: Descriptions are shown in the official language in which they were submitted.
~L2~7~'7
BACKGROUND ART
This invention pertains to ignition systems for gas
fired devices, and in particular to automatic ignition and heat
detection systems for such devices.
In many conventional gas fired appliances, such as
boilers, clothes dryer, ovens and the like, it is customary
to provide heat by igniting gas emanating from a main burner.
Commonly, gas flows thxough the main burner when the device is
activated, the gas being ignited by a nearby pilot flame which
is constantly burning. Recognizing the inefficiency and danger
of a constantly burning pilot flame, automatic ignition systems
which rely upon heat from a resistive element to ignite the
main burner have been substituted for constantly burning pilot
systems, the resistive element being energi~ed onl~ when the
device calls for heat. In such systems, it is ~nown to employ
a silicone carbide resistive element having a negative tempera-
ture characteristic (i.e. the resistance of silicone carbide
deereases with increasing temperature) as the igniter. One
such prior art system is described in U. S. Patent No. 3,282,324.
In the system disclosed in ~. S. Patent No. 3,282,324,
a solenoid activated gas valve is employed, the solenoid winding
being in a eircuit with the igniter element. Because silieone
carbide has a negative temperature eharacteristic, when the
device calls for heat, current flow through the igniter heats the
igniter thereby dropping its resistance. This continues until
current flow through the eircuit incorporating the solenoid
winding inereases suffieiently to energize the solenoid and open
the gasvalve.
s';~.
To close the gas valve in the event of a flameout, the
system includes a circuit which deenergizes the igniter element
after the gas valve is opened. The igniter element then operates
as a heat detector, the gas valve being closed if current flow
through the igniter element drops below a predetermined value
considered indictive of a sufficient drop in temperature to
confirm a flameout.
It will be apparent that in both the ignition and heat
detection modes, the system disclosed in U.S. Patent No.
3,282,324 is based on the assumption that current flow through
the igniter element, and hence its resi~tance, is an accurate
indication of the igniter element tempexature. Unfortunately,
this assumption ignores the reality that the
resistance/temperature characteristic for different silicone
carbide igniter elements varies from one igniter element to the
next. That is, one igniter element might display one temperature
at a particular resistance, while another igniter element might
display a quite different temperature at that resistance.
Accordingly, relying on a predetermined igniter element
resistance level as an indication that its temperature is
sufficient to ignite gas results in a potentially inaccurate
systemO Furthermore, the time required for the system discussed
in the patent to open and close the gas valve is relatively slow~
In U.S. Patent No. 3,784,351, an electric igniter
serves to ignite gas flowing through a bur~er, provided that the
igniter is sufficiently hot. To determine when the igniter has
reached ignition temperature~ a radiant sensor comprising a
bimetallic element is positioned in proximity to the igniter.
When the igniter is activated, current flow therethrough heats
the igniter until it reaches the transition temperature of the
- 2 -
~2~
bimetallic element. At that point, the bimetallic element
moves from one contact to another, thereby initiating the
ignition sequence which results in gas flow through the burner
~or ignition by the igniter. Thus this patent employs a bimetal
to sense the temperature of the igniter element and initiates
the ignition sequence when the predetermined transition
temperature o~ the bimetal is reached.
DISCLOSURE OF THE INVENTION
According to the present invention I have developed
an improved automatic ignition system for gas fired devices
of the type including a resistance type igniter. The
improved ignition system is capable of determining when the
igniter is sufficiently hot to ignite gas, taking into account
that different igniters have different temperature characterist-
i~s.
The invention in its broader aspect comprehends animproved automatic fuel ignition s~stem for gas fired devices
having a burner provided with an outlet, a power source, a
first normally closed fuel valve for controlling the gas flow
to the burner, and means for opening the valve. The system
is of the type inc~uding a variab~e resistance ignition means
having a particular temperature characteristic disposed in
proximity to the burner outlet for igniting gas flowing there-
through when the ignition means is energized by operative
connection to the power source. The improvement comprises
detection means for repeatedly measuring the resistance of
the varible resistance ignition means at predetermined intervals
and for comparing the measurements. Activating means is op-
eratively connected to the detection means and the valve opening
means for activating the valve opening means to open the
.~
~2~t7~
valve when the difference between measurements establishes
that the variable resistance ignition means is in ~he portion
of its temperature characteristic where the temperature thereof
is sufficient to ignite the gas.
Preferably, there is incorporated into the system a
microcomputer operatively connected to-the igniter, the
microcomputer being programmed to repeatedly measure the
resistance of the igniter at predetermined intervals and to
compare successive measurements. In this manner, the micro-
computer is capable of determining when the igniter has
reached the flattened portion of its defined temperature/
resistance curve where the igniter is known to be sufficiently
hot to ignite gas.
Preferably, the microcomputer is programmed to
conduct two separate tests to confirm that the igniter has
reached the flattened portion of its temperature characteristic.
In the first test, referred to hereinbe~ow as the "warm" test,
the microcomputer first establishes a t~reshold based on the
resistance of the igniter element priox to energization. The
- 4 -
:, .
~2~7~
igniter is then energized whereupon the microcomputer, after a
predetermined delay, again measures the resistance of the
igniter. If the new reading is below the threshold, the warm
- test is passed, and this indicates that the igniter is in the
relatively steep portion of its temperature characteristic where
increases in temperature result in relatively large decreases in
resistance. lf the warm test is not passed, the microcomputer,
again after a predetermined delay, remeasures the igniter
resistance for comparison with the threshold. This continues
until either the warm heat is passed or a predetermined time
interval expires. In the latter event, the system enters a F~UI.T
mode to be described hereinafter. In the second test, referred
to as the "hot" test, the microcomputer compares successive
resistance measurements until the difference batween successive
measurements is below a predetermined maximumu This confirms that
the igniter has reached the flattened portion of its temperature
characteristic where its resistance changes relatively slightly
with increasing temperatures. As noted, in the flattened portion
o~ the temperature characteristic, the igniter is sufficiently
hot to ignite gas. ~pon confirming that the igniter has reached
ignition temperature, the microcomputer is programmed to open the
gas valve thereby e~fecting i~nition as gas passes through the
burner in the vicinity of the igniter. The igniter element is
then deenergized.
In the preferred system, the microcomputer is also
prograrnmed to detect a flameout. This is also prefera~ly
accomplished by comparing successive resistance measurements of
the the igniter. Specifically, in the flame detection mode, the
microcomputer is programmed to conduct two separate tests, each
of which is independently capable of confirming a ~lameout. In
~2~
-the first test, referred to hereinbelow as the threshold or level
test, the microcomputer establishes a resistance threshold based
on the resistance of the igniter just prior to ignition. Once
the igniter is deenergized after ignition, the microcomputer
continuosly monitors the igniter resistance at regular intervals.
If the igni~er resistance exceeds the threshold, a flameout is
confirmed, as this indicates that the temperature in the vicinity
of the igniter is no longer sufficient to maintain the igniter on
the flattened portion of its temperature characteristic. In the
second test, referred to below as the "rate" test, the
microcomputer compares successive resistance measurements and
determines if the rate of change of the igniter resistance
exceeds a predetermined rate. The rate is selected such that, if
exceeded, a ~lameout is confirmed. In the event of a flameout,
the microcomputer is preferably programmed for corrective ac-tion.
The preferred system also includes means for
confirming that the gas pressure is above a predetermined minimum
considered safe. For this purpose, the system preferably
includes two independently operable, serially arranged gas
valves. ~ conduit having a pair of spaced apart electrically
conductive contacts at one end thereof communicates with the flow
path between the valves, and a conductive member is disposed for
sliding movement in the conduit. When the gas valve on the inlet
side of the conduit is opened, gas flows into the conduit and, if
gas pressure is sufficient, urges the conductive member upward
until it completes an electrical circuit between ~he contactsO
This is detected by the microcomputer, which is operatively
connected to the contacts. If gas pressure is low, the
electrical circuit between the contacts will not be completed by
the conductive member, and this condition will also be detected
by the microcomputer. In the event of a low gas pressure
condition, the microcomputer is preferably program~ed for
corrective action. Preferably, the microcomputer monitors gas
pressure both before and after ignition.
Another feature of the preferred system is the
provision of means for indicating the status of the system and
the nature of any malfunction. Preferably, such means comprises
a self-powered module having a digital display thereon, the
module being removably connectable to the microcomputer. When
the module is connected to the microcompu~er, the number on the
digital display indicates the existence and nature of the
particular system malfunction, or simply the status of the
system. For example, the different numbers on the display may be
utilized to indicate a faulty igniter, faulty valve circui-try,
flameout, failure of the igniter to reach ignition ~emperature,
etc. It is presently contemplated that the module will be
utilized by service personnel during system inspection and
repair.
The use of a microcomputer to control system
2~ operations also results in a reduction of system response time
and therefore greater overall fuel efficiency and safety. Also,
by reducing the number of moving parts, system reliability is
increased. The preferred system also preferably includes means
for modifying the programming of the microcomputer ~or particular
applications, preferably by ungrounding specific inputs to the
microcomputer~
In the following description the preferred system is
described for use in connection with a gas-fired boiler.
However, it will be apparent to those skilled in the art that it
may be utilized for controlling a wide range of gas-fired
devices, such as domestic ranges, dryers, and the like, and that
the system may be retrofitted on such gas-flred devices.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a diagrammatic illustration of the preferred
automatic ignition and heat detection system in accordance with
the present invention;
FIG. 2 is an elevational view illustrating the
preferred manner for supporting the igniter element in proximity
to the burner;
FIGS. 3A and 3B schematically illustrate the preferred
system shown in FIG. l; and
FIGS. 4-9 are system logic flow diagrams for the
preferred system, Fig. 9 appears with Fig. 7.
DESCRIPTION OF THE P~EFERRED EMBODIMENT
Referring initially to FIG. 1 of the drawings, the
presently preferred embodiment of the fuel ignition and heat
detection system in accordance with the present invention is
generally designated by the reference number 10. As shown, the
2~ system 10 preferably includes a microcomputer integrated circuit
chip 12, such as a COP411L, manufactured and distributed by
National Semiconductor Corp., which conventionally contains a
microcprocessor, associated input/output devices, a read only
memory and random access memory all in one chip. Such a
~ 8~
microcomputer chip 12 is conventionally programmable in the
associated machine language used with the chip such as, by way of
example, what is termed COP assembly language.
The other components of the preferred system 10
illustrated in FIG. 1, the functions of which will be explained
in greater detail hereinafter, are a burner 14, an igniter 16, a
redundant valve arrangement comprised of a pilot valve assembly
18 and a secondary valve assembl~ 20, a pressure sensiti~e switch
' 22, a thermostat control 24, a high limit switch 26, a diagnostic
plug-in module 2~, and a power source 30. As diagrammatically
illustrated in FIG. 1, the microcomputer 12 is supported within a
module or housing 32. The housing 32 also contains the circuitry
interfacing the microcomputer 12 with the other components of the
system 10. This interfacing circuitry will be described in
greater detail hereinafter with reference to FIG. 3, wherein the
system 10 is schematically illustrated. Mounted on the housing
32 are an indicator light 34 and a reset switch 36, both of which
are also connected to the microcomputer 12. The functions o~ the
light 34 and switch 36 will also be explained in greater detail
hereinafter. Power is supplied to the housing 32 and from there
to the various components in the system 10 by a power source 30
which preferably comprises a standard 117 volt AC power line.
The housing 32 is preferably mounted, as by screws, on a control
panel adjacent the controlled apparatus, which may be a boiler.
The burner 14 is conventional and may comprise, for
example, a burner of the type used ~in gas fired boilers. As
usual, it comprises a tubular member 38 having a plurality of
apertures 40 therein. Referring to FIGS. 1 and 2, the igniter 16
comprises an element 17 secured at one end in an insulating block
42 which is mounted, as by screw 44, on a bracket 46 extending
from the burner 14. In this fashion, -the element 17 is supported
near the burner 14 so that the element can perform its dual
functions of igniting the gas flowing from the burner and sensing
the heat of a resulting flame. A pair of leads 48 extending from
the other end of the insulating block 42 connect the igniter
element 17 with the module 32.
Igniter elements 17 suitable for incorporation in the
system 10 are commercially available. The element 17 is
comprised, for example, of silicon carbide, which has a negative
temperature characteristic, i.e. the resistance oE silicon
carbide decreases with increasing temperature. Generally, the
igniter element 17 is commercially available as a package
including the insulating block 42 and leads 48. By way of
example, the model no~ 767A silicon carbide igniter manufactured
by the White Rodgers division of Emerson Electric Company is
suitable for incorporation in the system 10.
~ hose skilled in the art will appreciate that the
temperature characteristic varies from one igniter element to the
ne~t. ~hat is, one igniter element will exhibit a particular
resistance at a temperature of 100 degrees F, while another
igniter element may exhibit a different resistance at that
temperature. ~ccordingly, for an automatic ignition system to be
compatible with different igniter elements, it must be able to
compensate for these differences. As will be explained in
greater detail hereinafter, the system 10 is fully capable of
doing so.
Before en-tering the burner 14, gas first flows through
the redundant valve arrangement comprised of pilot valve assembly
18 and secondary valve assembly 20, such as the redundant valve
arrangement model no~ 36C84 manufactured by the White Rodgers
- 10 -
~2~3~7~L~7
division of Emerson Electric Co. As shown, the valve assemblies
18 and 20 are preferably of the solenoid variety and thus include
cores 50, 52 and actuating coils 54, 56, respectively. As will
be more fully apparant from FIG. 3, the coils 54, 56 are actuated
by relays supported within the housing 32 and interfaced with the
microcomputer 12. As shown, valves 58 and 60 are connected,
respectively, to the cores 50, 52. The valve seats 62, 64 for
the valves 58, 60 are formed in a preferably casted chamber 66
. which defines the flow pa-th for the incoming gas. It will be
apparent from FIG. 1 that before incoming gas enters the burner
14, it must first flow through the openings defined by both of
the valve seats 62, 64. In FIG. 1, the valves 58, 60 are shown
in their closed positions wherein gas 1OW through the chamber 66
to the burner 14 is blocked. When the valves 58 and ~0 are
opened, gas flows into the burner 14 through a metered orifice
68.
The pressure sensitive switch 22 includes a conduit 70
which communica-tes with the gas 1OW path defined by chamber 66
between the valve seata 62 and 64. The conduit 70 opens into a
2~ larger chamber 72 in which a diaphragm 74 is slidably supported.
The diaphra~m 74 has a conducting elemen-t 76 secured thereon
which connects the contacts 78, 80 when the diaphragm 74 is in
its uppermost position. The significance of this wi:ll be more
fully apparent hereinafter. At this point, suffice it to say
that the diaphragm will assume its uppermost position whenever
the valve 58 is open and gas pressure is above a predetermined
minimum considered safe.
The thermostat 24 may be of the type conventionally
used for regulating the activation and deactivation of gas fired
boilers and the like. As will be explained in greater detail
- 11 -
~2~7~3~
hereinafter, when the thermostat 24 calls for heat, -the system 10
is activated and the ignition sequence is commenced. The high
llmit switch 26 is a safety feature comprising a temperature
sensitive switch preferably disposed to sense the temperature in
the boller chamber~ As will be more fully explained hereinafter,
if, for example, the temperature in the chamber is too hot, which
may~ for example, be caused by a fan malfunction, the high limit
switch 26 opens, whereupon the microcomputer 12 automatically
. closes the valves 58 and 60 thereby blocking the further flow of
gas to the burner 14.
The diagnostic plug-in module 28, which is
connectable to the housing 32 via the receptacle 82, contains a
digital display ~. As will be explained hereinafter, when the
plug-in module 28 is connected to the housing 32, the display 84
provides information indicative of the status of the system 10,
including the existence and nature of a malfunctionO if any. The
presence of a fault or malfunction in the system 10 is also
indicated by the lighting of the indicator light 34.
A schema~ic representation of the system 10 is
illustrated in FIG. 3 ~comprising FIGS. 3A and 3B) wherein
typical component values and circuit elements are indicated. A
detailed description of the schematic is deemed unnecessary, as
the operation of the illustrated circuit will be fully apparent
to the skilled art worker from this description. As regards the
2S microcomputer 12, and as previously noted, it is preferably a
. conventional COP411L microcomputer of the type distributed by
National Semiconductor Corp., which is conventionally programmed
in COP assembly language.
The operation of the system 10 will now be described
with particular reference to the flow charts illustrated in FIGS.
- 12 -
~2~7~
4-9. In describing the operation of the system 10, it will be
assumed that the flame is initially off. In this state, the
microcomputer 12 maintains the system 10 in an IDLE mode (FIG.
4). In the IDLE mode, the microcomputer 12 continuously monitors
the pilot valve relay driver input as well as the input connected
to the thermostat 24 and high limit switch 26. As shown in FIG.
3, the thermostat 24 and high ]imit switch 26 are connected in
series to a single input of the microcomputer 12.
~ As is apparent from FIG. 4, the microcomputer 12
maintains the system 10 in the IDLE mode until either the pilot
valve relay driver fails shorted or the thermostat/high limit
input becomes active, i.e. calls for heat. If the pilot relay
shorts, the microcomputer 12 will enter a FAULT mode. The
operation of the system 10 in the FAULT mode will be explained in
lS greater detail hereinafter.
As shown, the microcomputer 12 is preferably
programmed to establish a predetermined thermostat threshold
current which must be exceeded beEore the microcomputer will
attempt ignition. F'or example, a current threshold of 100
milliamperes may be set. This is done to accommodate
programmable setback thermostats which "steal" current from the
power circuit that might otherwise provide a false ignition
signal to the microcomputer.
Assuming no fault occurs and the thermostat calls for
heat, the microcomputer 12 effects a prepurge delay during which
commencement of the ignition sequence is delayed for preferably
thirty seconds. At the expiration of the thirty second delay,
the microcomputer 12 enters the IGNITION mode (FIG. 5). Upon
entering the IGNITION mode, the microcomputer 12 reduces the
thermoctat threshold current and then tests the igniter element
~2~7~7
17 for a short by measuring its resistance. If the igniter
element 17 is shorted, the microcompu-ter 12 en-ters the FAULT
mode. Assuming no fault, the microcomputer activates the pilot
valve relay drlver circuit thereby opening the pilot valve 58.
After a preferably one second delay, the microcomputer 12 again
monitors the thermostat/high limit input. If the thermostat/high
limit input is open, thereby indicating that heat is no longer
called for, the microcomputer 12 enters the OFFGAS mode. As will
be more fully apparent from the description of FIG. 8 below, when
-the system enters the OFFGAS mode, the microcomputer, after a ten
second delay, returns to the IDLE mode whereupon the pilot valve
58 is closed. Assuming the thermostat/high limit input is still
active, the microcomputer 12 next checks the gas pressure by
monitoring the pressure sensitive switch 22. If gas pressure is
normal, gas flow through the pilot valve 5~ into the conduit 70
and connected chamber 72 will move diaphragm 74 upward until the
conducting element 76 makes contact with the contacts 78, 80
thereby closing the circuit to the microcomputer 12. If the
microcomputer 12 detects that the contacts 78, 80 are open, the
2~ microcomputer enters a LOWPRS mode. The operation of the system
10 in the LOWPRS mode will be explained in greater detail below.
Assuming gas pressure is verified, the microcomputer 12 enters
the T~RNON mode.
At this point the microcomputer 12 readies the system
z5 lo for gas ignition. This requires activating the igniter
element relay driver circuit to energize the igniter element 17
and then opening gas flow to the burner 14 when the igniter
element is sufficiently hot to effect gas ignition. As
previously noted, to determine whether a particular igniter
3`0 element has reached ignition temperature, the microcomputer ~12
- 14 -
~2~ ¢3'7
must be capable of distinguishing between different elements
having different temperature characteristics. As shown in FIG.
6, to determine whether the element 17 has reached ignition
temperature, the microcomputer 12 conducts two tests - the "warm"
test and the "hot" test. First, the microcomputer establishes a
threshold based on the resistance of the element 17 before the
igniter 16 is energized, i.e. when the element 17 is still cold.
The igniter 16 is then energized for preferably two seconds and
the resistance of the element 17 again measured. If the
resistance reading is below the threshold, the warm test is
passed. If the warm test is not passed, the igniter 16 is again
energized for preferably two seconds, the resistance of the
element 17 is measured, and the new reading is compared to the
reference cold reading. This process continues until either the
warm test is passed or preferably one minute elapses. I~ the
warm test is not passed after one minute, the microcomputer 12
enters the FAU~T mode.
Assuming the warm test is passed, the microcomputer 12
next conducts the hot test. In this test, the microcomputer
compares two consecutive resistance measurements of the element
17. I~ the difference between these readings is less than a
predetermined value, the hot test is passed, as this indicates
that the flat, i.e. high temperature, portion of the temperature
characteristic for the element 17 has been reached. If the hot
test is not passed, the element 17 is again energized for
preferably two seconds, whereupon both the warm and hot tests are
again conducted. This continues until the hot test is passed or
until the one minute period expires. If the hot test is not
passed within one minute, the microcornputer 12 enters the FAULT
mode. By utilizing the above technique to confirm ignition
- 15 -
7~
temperature, ignition is achievable despite reduced line voltage.
When ignition temperature is confirmed, the
microcomputer 12 enters the IGNOK mode (FIG. 7~ whereupon the
secondary valve relay driver is activated to open the secondary
valve 60. Simultaneously, the element 17 is energized~ At this
point, gas flows through the chamber 66 and the metered orifice
68 into the burner 14 whereupon the gas is ignited as it passes
through the apertures 40 in the vicinity of the element 17.
Preferably four seconds later, the element 17 is deenergized.
~0 At this point, the element 17 is utilized as a heat
detector. To this end, the microcomputer 12 monitors the
resistance o~ the element 17 ~or the presence or absence of a
~lame. As will be explained below, if a flameout occurs, the
microcomputer is programmed for corrective action. For the
microcomputer 12 to determine whether a flameout has occurred
based on the resistance of the element 17, the microcomputer 12
must be capable of compensating for variations in the temperature
characteristics of different igniter elements. To determine if a
~lameout occurs, the microcomputer 12 conducts two tests - a
level test and a rate test. Referring to FIGS. 7 and 8,
preferably two seconds a~ter the igniter is deenergized the
microcomputer 12 establishes a threshold resistance based on the
measured resistance of the igniter element 17 just prior to
ignition, i.e. when the igniter element is hot. The threshold
resistance is preferably equal to the measured resistance
increased by a predetermined value. That is, the threshold
resistanc2 is established such that i~ the resistance of the
element 17 exceeds the threshold, this will indicate that the
temperature in the vicinity of the element 17 has dropped
su~ficiently to confirm the occurrence of a flameout. The rate
- 16 -
tes-t is accomplished by comparing the rate of change of the
resistance of the igniter element 17 with a preestablished rateO
As preferred and shown in FIG. 8, if the rate of change of the
igniter element resistance exceeds the preestablished rate twice
in a row, thereby indicating a continuing drop in temperature in
the vicinity of the igniter element, this too establishes a
flameout.
In the event of a ~lameout, the microcomputer 12,
after a ten second delay, returns to the IDLE mode. Assuming the
thermostat 24 is still calling for heat, the microcomputer then
repeats the ignition sequence described hereinabove in an effort
to again ignite the flame. If flameout occurs three times in a
row, as indicated by the flameout counter, the microcomputer 12
enters the FAULT mode.
When the microcompu-ter is in the MONITOR mode (FIG.
8), the microcomputer 12, in addition to monitoring the flame,
also continuously monitors the igniter element 17, the
thermostat/high limit input, the input from the pressure switch
22, and the pilot rela~ driver circuitry. As shown in FIG. 8, if
the igniter element 17 shorts or the pilot valve relay driver
circuitry fails, the mlcrocomputer 12 enters the FA~LT mode. If
either the thermostat 24 or hiyh limit switch 26 opens, the
microcomputer 12 returns the system to the IDLE mode ~FIG. 4)
thereby closing the pilot valve 58 and shutting the flame. If
the switch 22 opens, thereby indicating that gas pressure is low,
the microcomputer enters the LOWPRS mode (FIG. 5).
In the LOWPRS mode, the microcompu-ter 12 closes the
pilot and secondary valves 58, 60. As shown in FIGS. 4 and 5,
after a thirty second delay, tae microcomputer 12 then enters the
IGNITION mode whereupon the microcomputer 12 runs through the
- 17 -
3~ '7
ignition sequerlce more fully described above. This sequence
concludes with a gas pressure check. As long as the pressure
remains low, this sequence is repeated. As shown~ when the
switch 22 closes, thereby indicating that gas pressure has been
restored, the microcomputer 12 enters the T~RNON mode. Operation
of the system l0 in the TURNON mode is more fully discussed
above.
As is described above, the microcomputer 12 enters the
FAULT mode in response to a malfunction, e.gO if the igniter
element 17 fails shorted or open, the pilot valve relay driver
circuitry shorts, three consecutive flameouts occur, etc. The
flow chart for the ~AULT mode is illustrated in FIG. 9. ~s
shown, in the FA~LT mode the pilot valve 58 and secondary valve
60 are closed to turn off the gas flow, and the igniter element
17 is deenergized. Assuming the external power source remains
operative, these conditions will prevail until the reset switch
36, which preferably comprises a push button switch, is
depressed, whereupon the microcomputer 12 is returned to START
(FIG. 4). Of course, if the fault persists, the microcomputer 12
will re-enter the FAULT mode when the microcomputer again checks
the faulty component. Whenever the system enters the FAULT mode,
the indicator light 34 lights thereby visually indicatlng the
presence of a fault. However, the light 34 may not light if the
system 10 is completely down, which may result from a total loss
of power.
In the event of a fault, it is preferable to provide
means for indicating the nature of the fault. This function is
accomplished by the plug-in fault analyzer 28 which also
indicates the status of the system 10. The analyzer 28 is
presently contemplated for use by service personnel. As
18 -
~L%~'7~3'7
previously noted, the analyzer 28 incorporates a conventional
seven segment digital display 84. Referring to FIGS. 1 and 3,
the self-powered analyzer 28 is connected to the microcomputer 12
by plugging the analyzer into the receptacle 82. The number on
the digital display 84 then indicates the type of fault or a
particular system status. In the above described preferred
system 10, and referring to FIGS. 4-9, a reading of ~ero
indicates that the power level is insufficient to operate the
. system~ a reading of one indicates that the system is in the IDLE
mode, a reading of two lndicates that the thermostat/high limit
input is active; a reading of three indicates that the the system
is in the IGNITION mode, a reading of four indicates that the
system is in the IGNOK mode, a reading of five indicates that the
gas pressure is low, a readin~ of six indicates that the pilot
valve is improperly open in the IDLE mode, and a reading of seven
indicates an igniter malfunction.
To accommodate particular applications, the preferred
system 10 preferably incorporates means for modifying the
functioning of the microcomputer 12 for altering the mode of
operation described above. Referring to FIG. 3, the functioning
of the microcomputer 12 is preferably modifiable by ungrounding
specific inputs to the microcomputer provided for this purpose.
As shown in FIG. 3, such ungrounding is preferably accomplished
by providing a removable conductive member ~"strap a" in FIG. 3)
which connects the input to ground. The conductive member is
preferably factory installed and forms part of the interfacing
circuitry within the housing 32. When strap a is removed, the
mode of operation described above is modified as shown in the
flow chart, FIGS. ~-9. Specifically, when strap a is removed,
3~ the thirty second prepurge delay before the microcomputer 12
- 19 -
enters the i~nition mode is bypassed (see FIG. 4). This
modification would be used, for example, where immediate heat is
required~ When strap a is removed, the microcomputer also
preferably enters the FAULT mode in response to a single
flameout, as opposed -to three flameouts (FIG. 8). This prevents
the accumulation of gas which might otherwise occur if ignition
is attempted three times without a thirty second delay between
attempts. If desired, the microcomputer may be programmed for
still other options which would take effect upon removal of other
straps not shown~
Throughout the specification and claims, reference is
made to measuring the resistance of the igniter element 17 for
determining the temperature characteristic thereof. Those
skilled in the art will appreciate, however, that the relevant
information may be obtained not only by actually measuring the
resistance of the igniter element, but also by measuring the
current flow through the igniter element or the voltage drop
across the igniter element. Accordingly, the phrase "measuring
the resistance" or like phrases, when applied to the igniter
element, should be understood throughout as contemplating current
or voltage measurements which also yield information defining the
temperature characteristic of the igniter. In the preferred
system 10, information defining the temperature characteristic of
the igniter element is obtained by measuring the voltage drop
across the igniter element.
While I have herein shown and described the preferred
embodiment of the present invention and have suggested certain
modifications thereto, it will be apparent that further changes
and modifications may be made without departing from the spirit
and the scope of the invention.
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