GB2156103A - Direct ignition gas burner control system - Google Patents

Abstract

A direct ignition gas burner control system includes an igniter, two serially-arranged gas valves for controlling the flow of gas to the burner, and a microcomputer and related circuitry for controlling energizing of the igniter and valves. The microcomputer also contains instructions to provide a check of both the ROM and RAM during each demand for burner operation.

Description

SPECIFICATION Direct ignition gas burner control system CROSS-REFERENCE TO RELATED APPLICATIONS This application contains subject matter common to application Serial No. 470,309, filed Feburary 28, 1983, and constitutes, as to new matter, a continuation in part thereof.

BACKGROUND OF THE INVENTION This invention relates to electrically operated control systems for controlling operations of a main gas burner wherein the burner is directly ignited.

Due to the ever-increasing need for conservation of energy, direct ignition gas burner control systems, wherein a main burner is directly ignited by sparks or an electrical resistance igniter thereby eliminating the conventional standing pilot, are becoming more widely used. While the prior art discloses various such systems which appear to provide the required controlling functions, they are generally quite comlex and costly.

The advancements in microcomputer technology have made it economically attractive to construct a direct ignition gas burner control system utilizing a microcomputer. The microcomputer and related circuitry not only enable a considerable cost savings in providing system functions heretofore provided by discrete electrical and mechanial components, but also enable a versatility not found in prior systems.

SUMMARY OF THE INVENTION It is, therefor, a primary object of this invention to provide a generally new and improved direct ignitiion gas burner control system utilizing a microcomputer.

In accordance with the present invention, a direct ignition gas burner control system comprises an igniter, two serially-arranged gas valves for controlling the flow of gas to the burner, and a microcomputer and related circuitry for controlling energizing of the igniter and valves.

In the preferred embodiment, a microcomputer is programmed to provide various desired system functions and to provide a choice of certain system functions. Specifically, not only is the microcomputer programmed to provide a desired sequence of system operation, but it is also programmed to respond to the connection or non-connection of circuitry to various pins thereof so as to selectively provide for a pre-purge time of 30 seconds or no pre-purge; a warm up time of 25 seconds or 45 seconds for an electrical resistance igniter; and a single attempt at ignition or three attempts.

A particular feature of the present invention is that the ROM (read only memory) of the microcomputer also contains instructions to provide a check of both the ROM and the RAM (random access read/write memory) during each demand for burner operation. This dynamic self-check provides for the system to lock out if any bit in ROM or RAM is incorrect, thereby ensuring safe system operation in the event that the chip of the microcomputer should become defective.

The above mentioned and other objects and features of the present invention will become apparent from the following description when read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING Figures 1A and 1B, when combined, is a diagrammatic illustration of a burner control system constructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The diagrammatic illustration of the burner control system of the present invention is obtained by placing Fig. 1 A to the left of Fig. 1 B. When so combined, the connecting points Al, A2, and A3 of Fig. 1A are aligned with the connecting points Al, A2, and A3 of Fig. 1B.

Referring to Fig. 1A, the control system of the present invention includes a voltage step-down transformer 10 having a primary winding 1 2 connected to terminals 14 and 1 6 of a conventional 120 volt alternating current power source. The secondary winding 18 of transformer 10 provides a 24 volt alternating current power source and is connected at one end through a thermostat 20 and a pressure switch 22 to one side of a primary winding 24 of a voltage step-up isolation transformer 26. The other end of secondary winding 1 8 of transformer 10 is connected to the other end of primary winding 24 of transformer 26.

A fan 28 is connected across power source terminals 14 and 1 6 through a set of normallyopen relay contacts 30. Relay contacts 30 are controlled by a relay coil 32 which is connected across secondary winding 1 8 of transformer 10 through thermostat 20. Thus, whenever thermostat 20 is calling for heat, fan 28 is energized. When fan 28 is energized, pressure switch 22 senses the flow of air and closes its contacts.

Fan 28 and pressure switch 22 are generally positioned in the flue of a furnace (not shown) so as to be in air-flow communication with the combustion chamber of the furnace. Fan 28 provides the air required for obtaining a combustible air-gas mixture by inducing air into the combustion chamber, and provides a positive means for forcing the products of combustion out of the combustion chamber through the flue. As will be described hereinafter, fan 28 is also selectively energizable before initiation of energizing of the igniter and is always energized between unsuccessful attempts at ignition, and after a loss of burner flame, to purge the combustion chamber of any accumulated unburned fuel or products of combustion. The utilization of fan 28 is required for direct ignition burner control systems in which the combustion chamber of the furnace is sealed.It is to be understood, however, that there are other applications of direct ignition burner control systems, such as in some natural-draft furnaces, in which the fan 28 is not required and can be omitted.

A first valve winding 34 is connected across secondary winding 1 8 of transformer 10 through thermostat 20, pressure switch 22, and a set of normally-open relay contacts 36. A second valve winding 38 is connected in parallel with the first valve winding 34. First valve winding 34 controls a first valve 40 and second valve winding 38 controls a second valve 42. Valves 40 and 42 are connected fluidically in series in a gas conduit 44 leading from a gas source (now shown) to a gas burner 46. Burner 46 is grounded at 48.

Both valves 40 and 42 must be open to enable gas to flow to burner 46. It is to be understood that valves 40 and 42 can be separate devices as illustrated or a unitary device.

Utilization of a redundant valve arrangement, wherein two serially-connected valves control the flow of gas to a main burner, is well known in the art.

One end of the secondary winding 50 of isolation transformer 26 is connected to chassis common C which is isolated from earth ground. The other end of secondary winding 50 is connected through a lead 52 to connecting point A2 and provides an alternating current power source of approximately 34 volts between lead 52 and common C. A resistor R1 is connected between lead 52 and earth ground at 54 and prevents the flow of excessively high current through secondary winding 50 if common C should accidentally be connected to earth ground.

Secondary winding 50 of isolation transformer 26 has a winding tap 56 which is connected through a controlled rectifier CR1 to a junction 58. A filter capacitor C1 is connected between junction 58 and common C and cooperates with the unidirectional flow through rectifier CR1 to provide a filtered unidirectional power source of approximately 1 5 volts on a lead 60 connected between junction 58 and connecting point Al. This 1 5 volt source is also applied to the input of a commercially-available 5 volt regulated power supply 62 which in turn provides a + 5 volt power source between a terminal 64 and common C. A capacitor C2 is connected between terminal 64 and common C. A capacitor C2 is connected between terminal 64 and common C and cooperates with power supply 62 to stabilize the + 5 volt power source.

A flame rectification circuit is shown generally at 66 in Fig. 1A. Circuit 66 includes a PNP transistor Ol having its emitter connected to the + 5 volt power source and its collector connected through a resistor R2 to common C. The collector of transistor Q1 is further connected to a lead 68 which is connected to connecting point A3. Series connected to the base of transistor Ol are resistors R3 and R4 and a flame probe 70. Probe 70 is positioned so as to be impinged by a burner flame 72 emitted from burner 46. A capacitor C3 and a resistor R5 are connected in parallel with each other between the + 5 volt source and the junction of resistors R3 and R4.

In the absence of burner flame 72, the air-gap impedance between probe 70 and burner 46 is sufficiently great to prevent transistor Q1 from being biased to a conducting state. Thus, in the absence of burner flame 72, transistor Q1 is off and the signal on lead 68 is low.

When burner flame 72 exists, transistor Ol is biased on through the burner flame 72 during the half-cycle that current freely flows through the burner flame 72, and by the discharge of capacitor C3 during the non-conducting half-cycle of burner flame 72. Specifically, during the half-cycle in which current freely flows through burner flame 72, transistor Q1 is biased on from the + 5 volt source through the emitter-base circuit of transistor Q1, resistors R3 and R4, probe 70, flame 72, burner 46, ground 48, ground 54, resistor R1, lead 52, and secondary winding 50 of isolation transformer 26 to common C. Capacitor C3 is also charged during this half-cycle.

When the voltage on secondary winding 50 reverses, burner flame 72 essentially blocks the reverse polarity current. During this reverse polarity half-cycle, capacitor C3 discharges through the emitter-base circuit of transistor Ol and resistor R3 to keep transistor Q1 biased on. With transistor Q1 on, its collector is high, causing the signal on lead 68 to be high. Thus, in the presence of burner flame 72, transistor Q1 is on and the signal on lead 68 is high. The values of the circuit components are such that capacitor C3 can maintain transistor Q1 conductive for approximately 80 milliseconds. This 80-millisecond time period is sufficiently greater than a half-cycle of the 60 Hz source so as to insure continued conduction of transistor Q1 when burner flame 72 blocks current flow, and, as will hereinafter be described, is sufficiently small so as not to interfere with a time-based means for monitoring the existence of flame 72.

Referring to Fig. 1 B, shown therein is a single component 8-bit microcomputer M1.

Contained within microcomputer M1 are an 8-bit CPU (central processing unit), a 1 k X 8 ROM (read only memory), a 64 X 8 RAM (random access read/write memory), 27 1/O (input/output) lines, a clock, and an 8-bit timer/event counter. The pins of microcomputer M1 are designated VCC, VDD, Vss, P1 through P17, P29 through P27, DB, through DB7, Tf and T1, PROG, XTAL1 and XTAL2, RESET, SS, INT, EA, RD, PSEN, WR, and ALE.

Pin Vcc of microcomputer M1 is connected to the + 5 volt power source and functions as the main power supply input to microcomputer M1. A capacitor C4 is connected between pin Vcc and common C to suppress any high frequency transients that may come in on the + 5 volt power source or be generated by microcomputer M1. Also connected to the + 5 volt source are pins VDD and INT.

Pin Vss is connected to common C and functions as the connection of microcomputer M 1 to common C potential. Also connected to common C are pin EA and pin PROG.

An external timing control circuit for the on-chip oscillator comprises a capacitor C5 connected between pin XTAL1 and common C, a capacitor C6 connected between pin XTAL2 and common C, and an inductor connected across pins XTAL1 and XTAL2. The values of capacitors C5 and C6 and inductor L1 are such that the on-chip oscillator provides a cycle speed of approximately 5 microseconds.

A reset circuit for initializing microcomputer M1 is shown generally at 74 in Fig. 1 B. Reset circuit 74 includes a capacitor C7 connected between the RESET pin and common C. Reset circuit 74 further includes the series connection of a rectifier CR2, a resistor R6, a buffer 76, and a rectifier CR3 connected through a lead 78 and lead 52 in series with secondary winding 50 of isolation transformer 26, and a resistor R7 and a capacitor C8 connected in parallel with each other between the cathode of rectifier CR2 and common C.

When power is applied to the reset circuit 74, capacitor C8 is completely discharged so that the input, and thus the output, of buffer 76 is initially low. With the output of buffer 76 low, the voltage at RESET pin is low, causing microcomputer M1 to reset. Capacitor C8 quickly charges, causing the input, and thus the output, of buffer 76 to go high. When the output of buffer 76 is high, rectifier CR3 blocks, enabling capacitor C7 to begin to be charged by the + 5 volt power source through an internal pull-up resistance provided between the + 5 volt power source and the RESET pin in microcomputer M1. After a time period sufficient for the + 5 volt power source to have become stable, capacitor C7 charges sufficiently to make the RESET pin high. With RESET pin high, the microcomputer M1 enters its run mode.

On a momentary power interrupton, capacitor C8 quickly discharges through resistor R7, causing the input, and thus the output, of buffer 76 to go low. This low enables capacitor C7 to discharge and thus causes RESET pin to go low which causes microcomputer M1 to reset.

When power is resumed, microcomputer M1 resets and enters its run mode as described above.

Connected in series between lead 78 and pin P27 are a resistor R8 and a buffer 80. Resistor R8 and buffer 80 function to convert the 60 Hz alternating current signal on lead 78 to a 60 Hz square wave signal, thus providing a real time base for microcomputer M1.

An electrical resistance igniter 82, preferably a negative temperature coefficient silicon-carbide igniter, is connected through a set of normally-open rselay contacts 84 to terminals 86 and 88 of a conventional 1 20 volt alternating current power source. Although not specifically illustrated as such, igniter 82 is positioned adjacent burner 46 so that, when sufficiently heated, it is effective to ignite the gas emitted from burner 46. Preferably, igniter 82 is not impinged by the burner flame 46.

A relay winding 90 for controlling relay contacts 84 is connected at one end to lead 60 and at its other end to common C through an NPN transistor Q2. A rectifier CR4 is connected across relay winding 90 to suppress any back EMF generated by relay winding 90 so as to protect transistor Q2 from any high voltages or high currents due to such EMF generation. The base of transistor Q2 is connected through a resistor R9 to pins DB6 and DB7 of microcomputer M1, and through resistor R9 and a resistor R10 to common C.

When it is desired to enertize relay winding 90, a current for biasing transistor Q2 on is provided for microcomputer M1 at pins DB6 and DB7. With transistor 02 on, relay winding 90 is energized by the 1 5 volt filtered unidirectional power source on lead 60.

A relay winding 92 for controlling the relay contacts 36 of Fig. 1A is connected at one end to lead 52 through a pair of resistors R11 and R12, a lead 94, and a rectifier CR5, and at its other end to common C through an NPN transistor 03. A filter capacitor C9 is connected between lead 94 and common C and cooperates with the unidirectional flow through rectifier CR5 to provide a filtered unidirectional power source of approximately 30 volts on lead 94.

Relay winding 92 is also connected to lead 94 through a rectifier CR6, a lead 96, and a pair of resistors R 1 3 and R 1 4. A voltage regulator VR 1 is connected between lead 96 and common C and functions to maintain lad 96 at approximately 5.1 volts. A rectifier CR7 is connected across relay winding 92 to suppress any back EMF generated by relay winding 92. A capacitor Clb for effecting energizing of relay winding 92 is connected from common C through a resistor R15 to a junction 98 between rectifier CR6 and relay winding 92.

The base of transistor Q3 is connected through a resistor R16, a buffer 100, a rectifier CR8, and a capacitor C11 to pins Ply', Pal 1, and P12 of microcomputer My. A high-pass filter arrangement of a parallel-connected resistor R17 and capacitor Cl 2 is connected between common C and a junction 102 between rectifier CR8 and buffer 1 00. A rectifier CR9 is connected between common C and a junction 104 between capacitor C11 and rectifier CR8.

The collectors of two NPN transistors Q4 and Q5 are connected to junction 98, and through a resistor R18 and a buffer 106 to pin P25 of microcomputer M1. The emitters thereof are connected to common C.

The base of transistor Q4 is connected through a resistor R19, a buffer 108, a rectifier CR10, and a capacitor C13 to pins P14, P15, P16, and P17 of microcomputer M1. A resistor R20 is connected between the + 5 volt source and the input of buffer 108 at a junction 110. A capacitor C14 is connected between junction 110 and common C. A rectifier CR11 is connected between the + 5 volt source and a junction 112 between rectifier CR10 and capacitor C13.

The base of transistor OS is connected through a resistor R21 and a lead 114 to a pin P 1 3 of microcomputer Ml, and through resistor R21 and a resistor R22 to lead 96.

Connected in series between lead 96 and pin P26 of microcomputer M1 are a resistor R23 and a buffer 11 6. A resistor R24 is connected between common C and a junction 118 between resistor R23 and buffer 11 6.

It is noted that capacitor C10 is in parallel with series-connected relay winding 92 and transistor Q3, and in parallel with both transistors Q4 and Q5. As will hereinafter be described, when energizing of relay winding 92 is not desired, microcomputer M1 provides for conduction of transistors 04 and Q5 and non-conduction of transistor 03, and when energizing of relay winding 92 is desired, provides for non-conduction of all three transistors 03, Q4, and Q5 for a sufficient time period to enable capacitor C10 to charge, and then provides for conduction of transistor 03 and continued non-conduction of transistors 04 and Q5 to enable energizing of relay winding 92 by the discharging of capacitor ClO through relay winding 92 and transistor Q3.

When conduction of transistor Q4 is desired, microcomputer M1 provides a constant digital high signal at pins P14 and through P17. Under this condition, capacitor Cl 4 is charged by the + 5 volt source through resistor R20, putting a high on the input of buffer 108. The output of buffer 108 is therefore high and transistor Q4 is biased on. Capacitor C13 and rectifier CR10 prevent the constant high signal at pins P14 through P17 from changing the state of buffer 108.

When conduction of transistor Q4 is not desired, microcomputer M1 provides a high frequency digital signal of approximately 1k Hz at pins P14 through P17. When the signal is low, capacitor C14 discharges through rectifier CR10, capacitor C13, and pins P14 through P 1 7 of microcomputer M1, causing the input, and thus the output, of buffer 1 08 to go low.

With the output of buffer 108 low, transistor Q4 is biased off. When the signal at pins P14 through P17 goes high, capacitor C14 begins to charge, but is prevented by the high resistance value of resistor R20 and the short time duration of the high portion of the 1 k Hz signal from charging sufficiently to change the state of buffer 108.

When conduction of transistor Q5 is desired, microcomputer M1 provides a constant digital high signal at pin P13. With a constant high at pin P13, transistor Q5 is biased on through resistor R21. When conduction of transistor Q5 is not desired, microcomputer M1 provides a constant digital low signal at pin P13, causing transistor Q5 to turn off.

When conduction of transistor Q3 is not desired, microcomputer M1 provides a constant digital high signal at pins PlO' through P12. When the constant high exists, capacitor C11 blocks the constant high signal and capacitor Cl 2 is discharged, making the input of buffer 100 low. With the input of buffer 100 low, the output thereof is low and transistor 03 is therefore biased off.

When conduction of transistor Q3 is desired, microcomputer M1 provides a high frequency digital signal of approximately 1k Hz at pins P10/ through P12. When the signal first goes low, basically nothing happens. When the signal goes high, capacitor C12, which is in parallel with a 1 megohm resistor R17, is charged to a sufficiently high voltage to cause the input, and thus the output, of buffer 100 to go high. With the output of buffer 100 high, transistor Q3 is biased on. When the 1k Hz signal goes low, capacitor C12 discharges through resistor R17, thue time constant being sufficiently long to keep the input of buffer 100 high, and thus keep transistor Q3 biased on, for the duration of the low portion of the 1 k Hz signal.

To effect energizing of relay winding 92, it is necessary to charge capacitor C10 to a voltage sufficiently high to effect pull-in of relay winding 92 upon discharge thereof. Since capacitor C10 is in parallel with series-connected relay winding 92 and transistor Q3, and in parallel with transistors Q4 and Q5, it is necessary that all three transistors 03, 04, and Q5 be off in order to enable capacitor C10 to charge.

When all three transistors Q3, 04, and Q5 are off, capacitor C10 is charged through two circuits. The first circuit includes resistors R14 and R13, rectifier CR6, and resistor R15. As previously stated, voltage regulator VR 1 limits the voltage on lead 96 to approximately 5.1 volts. Thus the voltage at junction 98, when rectifier CR6 is conducting, is approximately 4.5 volts. Relay winding 92 requires at least 6 volts to effect pull-in thereof, so this first circuit cannot effect pull-in. The second charging circuit for capacitor C10 includes resistors R12, R11, and R15. This second circuit enables charging capacitor C10 with the voltage source of approximately 30 volts appearing on lead 94.When capacitor C1 Oa is charged by this voltage, it is rendered capable of pulling in relay winding 92. The values of resistors R11, R12, R13, R14, and R15 are such that capacitor C10 is charged to the required pull-in voltage level within 2 seconds. Therefore, when transistor Q3 is turned on after being off for at least 2 seconds, and transistors Q4 and Q5 remain off, capacitor C10 discharges through resistor R15, relay winding 92, and transistor 03, effecting pull-in of relay winding 92. As will be hereinafter described, although only 2 seconds are required, 4 seconds are actually provided.

Once relay winding 92 is pulled in, the voltage at junction 98 decreases due to the impedance of relay winding 92 being considerably less than the combined impedance of resistors R 11 and R 1 2. However, due to voltage regulator VR 1, the voltage at junction 98 is held at approximately 4.5 volts, a level sufficient to maintain energizing of relay winding 92 which can be held in which approximately 3.4 volts. Thus, once relay winding 92 is pulled in, it is held in through resistors R14 and R13 and rectifier CR6.

It should be noted that transistors Q4 and Q5, while not essential, are provided to enhance the safety and reliability of the control system. Specifically, the provision of transistors Q4 and OS negates the development of any unsafe condition should transistor O3 be operated improperly, such as, for example, by becoming conductive, due to a false signal from microcomputer M1, before igniter 82 is sufficiently heated to gas ignition temperature. To determine that transistors Q4 and Q5 are capable of providing their safety function, they are checked by microcomputer M1 during each burner cycle.Specifically, during the time period while igniter 82 is heating and transistors Q4 and OS are on, microcomputer M1 provides a constant digital low signal at pin P13 for 1 second, causing transistor Q5 to turn off. Pin P25 of microcomputer M1 is monitored to determine if the input thereto is high or low. If the input is high, the system goes into lockout since a high would indicate that transistor Q4, which should still be on, is off. That is, with transistor OS off and transistor Q4 on, the signal through resistor R18 and buffer 106 to pin P25 must be low, not high.After this 1-second low signal at pin P13, microcomputer M1 provides the 1 k Hz signal at pins P14 through P17 for 1 second, causing transistor Q4 to turn off. If the input signal to pin P25 is high, the system goes into lockout since a high would indicate that transistor Q5 is off when it should be on.

The lockout condition referred to above is a condition wherein all outputs of microcomputer M1 which control igniter 82 and gas valves 40 and 42 are in such a mode so as to prevent energizing of igniter 82 and opening of valves 40 and 42. The lockout condition can be removed by opening thermostat 20.

Also enhancing the safety of the control system of the present invention is the provision of voltage regulator VR1. Should transistors Q4 and Q5 be off and transistor Q3 be on at some time other than when capacitor C10 is to effect pull-in of relay winding 92, current would flow through relay winding 92 from lead 94 through resistors R11 and R12, and through resistors Rl 3 and R14 and rectifier CR6. Because of the relatively low impedance of winding 92 with respect to resistors R11 and R12, the voltage across relay winding 92 due to the current flow through resistors R 11 and R 1 2 would be considerably less than 6 volts required to pull it in.

However, since the resistance values of resistors R 1 3 and R 1 4 are relatively low, the voltage across relay winding 92 due to the current flow through resistors R13 and R14 and rectifier CR6 could conceivably be high enough to pull in relay winding 92 were it not for voltage regulator VR1 which limits the voltage at junction 98 to approximately 4.5 volts.

To determine that regulator VR1 is functional, microcomputer M1 monitors the signal at pin P26 at a time when transistors Q4 and OS are off. Resistors R23 and R24 function as a voltage divider to provide a signal at junction 118 on the input of buffer 116. If regulator VR1 is functional, the signal on the input, and thus the output, of buffer 11 6 will be low; if the regulator VR1 is non-functional, the signal will be high, causing the system to lock out.

Another factor enhancing the safety of the burner system is that transistors Q3, Q4, and Q5 are biased to the required modes by diverse signals from a single port of microcomputer M 1.

Specifically, for transistors Q4 and OS to be off and transistor Q3 to be on, a condition for effecting pull-in of relay winding 92 by the discharging of charged capacitor C10, the signal on port P1, bits 4 through 7, has to be the 1k Hz signal previously described, the signal on port P1, bit 3 has to be a constant low, and the signal on port P1, bits 0/ through 2, has to be the 1k Hz signal. It is believed extremely unlikely that a malfunction of microcomputer M1 could cause such a diverse condition to develop.

Yet another factor enhancing the safety of the burner control system of the present invention is a self-check of the ROM and RAM in microcomputer M1 to ascertain that the chip in microcomputer M1 has not become defective. The 1 k X 8 ROM is programmed in 4 pages, each page being 256 words. Each page utilizes a small number of words to provide instructions for implementing the self-check of ROM. For self-check of ROM, the instructions are to add up each bit of each remaining 8-bit word on each page, to compare the resulting 8-bit word sum with a calculated 8-bit word sum which is stored in one of the instructions, to perform this function on subsequent pages if the previous page showed that the checked sum agreed with the calculated sum, and to enter lockout if the checked and calculated sums on any page do not agree.For self-check of RAM, portions of ROM subject to the ROM self-check provide instructions to place a binary 1 in each bit of each 8-bit word in the 64 x 8 RAM, except in several counters which implement and terminate the ROM and RAM self-check, to compare each resulting 8-bit word with a single 8-bit word therein, to repeat the process with a binary 0 in each bit, and to enter lockout if any comparison shows that any bit is incorrect. This selfcheck is performed during each power-up of microcomputer M 1.

Four resistors R25, R26, R27, and R28 are shown in Fig. 1 B, some of which are connected to various pins of microcomputer M1 and others of which, as indicated by dashed lines instead of solid lines, are not connected. As will be hereinafter described, the connection or nonconnection determines certain system functions.

For example, in the program of microcomputer M1, a digital high at pin P21 provides a warm-up time for igniter 82 of 45 seconds, and a digital low at pin P21 provides a warm-up time of 25 seconds. Therefore, the resistor R26 not connected, as shown in Fig. 1 B, pin P21 is high and the warm-up time is 45 seconds. If resistor R26 were connected, pin P21 would be coupled to common C through resistor R26 and would be low so as to establish the 25-second warm-up time. This means for providing selective igniter warm-up times simplifies the adapting of the system to various types of igniters having different heating characteristics or to unusual conditions, such as a consistently low or high voltage supply, so as to provide sufficient time to heat the igniter to gas ignition temperature and yet prevent an unnecessarily long warm-up time.

A digital high at pin P2(V establishes a pre-purge time of 30 seconds, and a digital low establishes that there will be no pre-purge, pre-purge being the time period between the closing of the contacts of thermostat 20 and the initiation of energizing of igniter 82. When fan 28 is provided, it operates during this pre-purge to purge the combustion chamber; when fan 28 is not provided, the pre-purge provides a time period for the combustion chamber to vent itself.

With resistor R25 not connected, as shown in Fig. 1 B, pin P20/ is high and the 30-second prepurge is therefore established. It is to be noted that not all burner systems require pre-purge.

Therefore, this means for providing or not providing a pre-purge enables the system to be readily adapted for use in a wide variety of applications.

A digital high at pins P22 and P23 establishes that there will be one attempt at ignition before lockout; a digital low establishes that there can be three attempts before lockout. In Fig.

1 B, resistors R27 and R28 are shown connected to pins P22 and P23, respectively, so that pins P22 and P23 are low and thus three ignition attempts can be made.

Microcomputer M1 is programmed to provide a time period of 4 seconds for each ignition attempt. Also, microcomputer M1 is programmed to provide a purge time of 60 seconds after a first and after a second unsuccessful ignition attempt and after a loss of flame. This 60-second time period is in addition to the selected pre-purge time of 30 seconds.

Microcomputer M1 is also programmed to enter lockout if pins P22 and P23 are not the same, that is, if they are not both high or both low. This feature further enhances the safety of the system. Specifically, if only one of the pins P22 and P23, for example pin P22, were utilized to indicate the number of ignition attempts, and the system was intended to provide only one attempt at.ignition and provide no pre-purge, pin P2(V would have resistor R25 connected to it making pin P2(V low, while pin P22 would have no resistor connected to it, thus making pin P22 high.If pin P24/ should go high for any reason, for example, due to a malfunction in the chip or due to the disconnection of resistor R25, a pre-purge time period would subsequently be provided, causing a delay in initiation of energizing of igniter 82 which is not an unsafe condition. If thereafter, however, pin P22 should go low for any reason, repeated attempts at ignition would be indicated. Since the system itself might not tolerate repeated ignition attempts, either because of the type of gas used or because of the fan 28 not being provided, an unsafe condition could then develop wherein a large amount of unburned fuel accumulates in the combustion chamber. Providing the two pins P22 and P23, which must be the same at all times or lockout will occur, greatly reduces the likelihood of such an unsafe condition from developing.

OPERATION Operation of the system will hereinafter be described based on the illustrated circuitry.

Specifically, fan 28 is provided, resistors R27 and R28 are connected so as to provide for three attempts at ignition, resistor R25 is not connected so as to provide for a 30-second pre-purge, and resistor R26 is not connected so as to provide for a 45-second warm-up time period for igniter 82.

Prior to a call for heat, the only circuit component energized is primary winding 1 2 of transformer 10. When thermostat 20 closes its contacts on a call for heat, relay winding 32 is energized, causing its contacts 30 to close. With contacts 30 closed, fan 28 energized, drawing air from a source external of the furnace, such source being either ambient air when the combustion chamber is not sealed from ambient, or from a separate source, such as outside the dwelling, when the combustion chamber is sealed. The drawn-in air is forced through the combustion chamber and out the flue, thus purging the combustion chamber of any unburned fuel or products of combustion therein. When pressure switch 22 senses the forced flow of air through the flue, it closes its contacts and enables isolation transformer 26 to be energized.

With isolation transformer 26 energized, power is supplied to microcomputer M1, causing it to reset and then go into the run mode. Microcomputer M1 then monitors pin T1 to determine if it is low, as it should be in the absence of burner flame 72. If pin T1 is not low, the system goes into lockout. Possible causes for pin T1 not being low would be a shorted transistor Q1, which would cause a constant high at pin T1, or the condition of probe 70 touching burner 46, which would cause a 60 Hz signal on pin T1.

Microcomputer M1 also scans pins P2V through P23 to determine if they are high or low.

Since pin P20/ is high due to resistor R25 not being connected, microcomputer M1 sets an internal timer (counter) for 30 seconds. Concurrently, microcomputer M 1 establishes a constant high at pin P13 to turn on transistor OS, a constant high at pins P14 through P17 to turn on transistor Q4, and a constant high at pins P10/ through P12 to turn off transistor Q3.

Concurrently, microcomputer M1 latches pins DB6 and DB7 low to hold transistor Q2 off. Thus, for 30 seconds, fan 28 is operational and relay windings 90 and 92 remain de-energized.

After the 30-second timer for pre-purge is timed out, microcomputer M1 establishes a high at pins DB6 and DB7. This high turns transistor Q2 on, enabling relay winding 90 to be energized and effect the closing of its contacts 84. With contacts 84 closed, igniter 82 is energized by the 120 volt alternating current source at terminals 86 and 88. Since pin P21 is high due to resistor R26 not being connected, microcomputer M1 sets an internal timer for 45 seconds.

When the 45-second timer is at 4 seconds before timing out, microcomputer M1 effects the turning off of transistors Q4 and Q5. Specifically, microcomputer M 1 establishes a constant low at pin P13, turning off transistor Q5, and a 1k Hz signal at pins P14 through P17, turning off transistor Q4. Since transistor Q3 is also off, the turning off of transistors Q4 and Q5 enables capacitor C10 to charge. As previously described, although 4 seconds are provided, capacitor C10 is charged to the pull-in voltage of relay winding 92 within 2 seconds.

When the 45-second timer times out, microcomputer M1 establishes a 1 k Hz signal at pins Pl (V through P12, effecting the turn on of transistor Q3. With transistor 03 on, capacitor C10 discharges through resistor R 1 S, relay winding 92, and transistor 03, enabling the level of energizing of relay winding 92 required to effect the closing of its controlled contacts 36. With contacts 36 closed, valve windings 34 and 38 are energized, effecting the opening of valves 40 and 42 so as to enable gas to flow to burner 46. Preferably, microcomputer M1 is programmed to compensate for the inherent time delay between the energizing of relay winding 92 and the closing of its contacts 36 by providing the enabling 1 k Hz signal 2 milliseconds before the real time signal, as detected at pin P27, goes low.This 2-millisecond time period enables the closing of relay contacts 36 by relay winding 92 to occur near the zero cross-over of the alternating current source provided by secondary winding 1 8. It is to be noted that closing of relay contacts 36 near zero cross-over minimizes the wear thereon.

Microcomputer M1 then provides for a trial ignition time period by setting an internal timer for 4 seconds. Under normal operation, igniter 82, which has been heated for 45 seconds, will be sufficiently hot to ignite the gas at burner 46.

If ignition does not occur within the 4-second trial ignition time period, microcomputer M1, due to the connection of resistors R27 and R28 to pins P22 and P23, respectively, will again attempt ignition. Specifically, if ignition does not occur within 4 seconds, the 4-second timer times out and microcomputer M1 provides the required signals to effect de-energizing of relay windings 90 and 92, which de-energizing terminates heating of igniter 82 and the flow of gas to burner 46. Microcomputer M1 then sets an internal timer for 60 seconds and checks and responds to the selected pre-purge time of 30 seconds indicated by the non-connection of resistor R25 to allow fan 28 to purge the combustion chamber for a total of 90 seconds.

Microcomputer M1 also increments a retry-sustained ignition counter in RAM to indicate that this was a first attempt at ignition. After the 90-second time period, microcomputer M1 initiates a second attempt at ignition in the same manner as the first attempt, except that an extra 10 seconds is added to the 45 seconds provided for heating igniter 82.

If ignition does not occur during the 4-second trial ignition time period of the second ignition attempt, the above-described 90-second purge time period is again provided, the retry-sustained ignition counter in RAM is incremented to indicate that this was a second attempt, and then a third attempt at ignition is initiated. For the third attempt, the extra 10 seconds is again provided so that, again, igniter 82 is heated for 55 seconds. If ignition does not occur during the 4-second trial ignition time period of the third ignition attempt, the retry-sustained ignition counter in RAM is incremented to indicate that this was a third attempt and the system goes into lockout.While the system can be removed from this lockout condition by momentarily opening the contacts of thermostat 20, it is advisable that the cause of repeated failures at ignition be found and corrected before so doing.

When ignition does occur, the existence of burner flame 72 enables the biasing on of transistor Q1. With transistor Q1 on, a high signal is transmitted through lead 68 to pin T1.

When T1 is high, microcomputer Ml terminates the 4-second timer and establishes a low at pins DB6 and DB7 to effect the turning off of transistor 02. With transistor 02 off, relay winding 90 is de-energized, effecting the opening of its contacts 84 and thus de-energizing of igniter 82.

When the high signal first appears at pin T1, microcomputer M1 sets an internal timer for 30 seconds and another internal timer for 700 milliseconds. As previously described, capacitor C3 in the flame rectification circuit 66 can maintain transistor Q1 conductive for approximately 80 milliseconds. Thus, should flame 72 fail to impinge flame probe 70 for a time period less than 80 milliseconds due to, for example, a slight flame flicker or distortion, transistor Ol remains on and pin T1 remains high. Should flame 72 fail to impinge probe 70 for a time period greater than 80 milliseconds, capacitor C3 can no longer maintain conduction of transistor Q1 so that transistor Q1 turns off and pin T1 becomes low.Microcomputer M1, which is programmed to monitor pin T1 every 50 milliseconds, detects the low at pin T1 at its first 50-millisecond check, and decrements the 700-millisecond timer for 50 milliseconds. It is noted that since the monitoring of pin T1 is biased on real time, this first 50-millisecond decrementing can occur very near the time pin T1 goes low or almost 50 milliseconds after it goes low. If the absence of flame 72 continues to be indicated, the 700-millisecond timer is decremented every 50 milliseconds until it reaches zero. If flame 72 is again indicated to be present before the 700millisecond timer reaches zero, the timer is reset for 700 milliseconds and system operation continues.

If the 700-millisecond timer does reach zero, burner flame 72 has thus been indicated as being absent for a maximum of 780 milliseconds. Generally, this would imply that flame 72 is extinguished rather than just flickering. At this point, that is, when the 700-millisecond timer times out, microcomputer M1 effects de-energizing of relay winding 92 so as to cause valves 40 and 42 to close. Concurrently, the retry-sustained ignition counter in RAM is incremented to indicate that this was the first occurrence of a failure to sustain flame 72 for at least 30 seconds after it had been established. Microcomputer M1 then checks the counts in the retry-sustained ignition counter. If the count is one, the system returns back to the same program loop that it executes for a second attempt at ignition.That is, the purge fan 28 would operate for 90 seconds, and then microcomputer M1 would initiate an attempt at ignition in the same manner as the first attempt except for a 55-second warm-up period for igniter 82. If the count is two, the system returns back to the same program loop that it executes for a third attempt at ignition.

If the count is three, the system goes into lockout.

It is to be noted that a count of three in the retry-sustained ignition counter could be indicative of three unsuccessful attempts to ignite, or three failures to sustain flame 72 for at least 30 seconds, or a combination thereof totaling three. Thus, the connection of resistors R27 and R28 will always allow three attempts at normal burner operation regardless of whether the counts in the retry-sustained ignition counter are due to unsuccessful attempts at ignition or failures to sustain flame for at least 30 seconds. It is to be noted that when resistors R27 and R28 are not connected, indicating that only one attempt at ignition can be made, the retrysustained ignition counter is effective to also provide three attempts at normal burner operation but only if the sole problem is that flame 72 is not sustained for at least 30 seconds.That is to say, when resistors R27 and R28 are not connected, a failure to ignite results in lockout; however, if ignition occurs but flame 72 is not sustained for at least 30 seconds, the system can attempt two more times to sustain flame 72.

The 60-second time period described above ensures safe operation on those systems having no fan 28, such as natural draft systems. Specifically, in such systems, if flame 72 is extinguished rather quickly after ignition, there could be a large amount of unburned fuel in the combustion chamber. In the absence of a pre-purge time, igniter 82 would be immediately reenergized, were it not for the 60-second time period, which could cause an undesirable ignition.

Thus, the 60-second time period provides a time period for igniter 82 to cool down and for the combustion chamber to vent itself of any unburned fuel.

The above-described means for monitoring flame 72 continues until the 30-second timer initiated by the appearance of the high signal at pin T1 times out. When this 30-second timer times out, the retry-sustained ignition counter is cleared. Thereafter, monitoring of flame 72 continues in essentially the same manner except that if the 700-millisecond timer times out, the retry-sustained ignition counter is not incremented. Specifically, if flame 72 is lost, microcompu ter M1 effects the closing of valves 40 and 42, and the system returns to the same program loop it executes for a first attempt at ignition.

When thermostat 20 is satisfied, it opens it contacts, de-energizing all circuit components except transformer 1 0.

The reason for not incrementing the retry-sustained ignition counter if flame failure occurs after the 30-second time period is that the existence of flame 72 for a relatively long period of 30 seconds establishes that the system is functioning properly. A flame failure after 30 seconds would generally be caused by a transient condition, such as an abrupt change in gas pressure, which may never occur again.

The reason for allowing more than one attempt at normal burner operation if ignition occurs but flame 72 is not sustained for at least 30 seconds is that again, the failure may be due to some transient condition, and to go into lockout under such a condition could be a nuisance.

The reason for limiting such attempts to three is that such flame failure could be an indication of some problem, such as an improperly positioned flame probe 70, a defective fan 28, or a clogged orifice in burner 46, which problems would not necessarily correct themselves on subsequent attempts at normal burner operation. Under such conditions, if the retry-sustained ignition counter were not incremented, the system would continue to attempt normal burner operation as long as thermostat 20 remained closed and ignition occurred, thus causing a potentially large number of cycles on various circuit components, such as the relays, which cycles could shorten the effective life of the system.

It is noted that the above-described means for monitoring flame 72 relies mainly on the accurate timing inherent in microcomputer M1 rather than on the stability of numerous discrete components. Such a method, therefor, can safely tolerate longer flame-flicker time periods so as to prevent unnecessary recycling or possible nuisance lockouts caused by flame flicker, and yet ensure that the system will automatically recycle within a required time period of 800 milliseconds if flame 72, after being established for some reasonable period of time, is prematurely lost.

While the above-described system uses electrical resistance igniter 82, it should be noted that the system could be readily modified to use spark ignition. Specifically, if spark ignition were used, the provision of an igniter warm-up time of 25 or 45 seconds would be omitted.

Microcomputer M1 would preferably be programmed to inihibit energizing of the spark circuitry for a short time period, for example, 4 seconds, during which time capacitor C10 would be permitted to charge to the level required to effect pull-in of relay winding 92. In such a modified system, the various safety features such as the provision of transistors Q4 and Q5, the parity check provided at pins P21, P22, and P23, and the self-check of ROM and RAM, would still be utilized.

It is to be noted that microcompuer M 1 could be programmed for values different from those stated for the various time periods such as the 25 and 45 seconds for igniter warm-up, the 4second trial ignition time period, and the various purge times.

The following components have been found to be suitable for use in the system described herein.

COMPONENT TYPE r M1 8048 (Intel Corporation) L1 100 Micro-henries VR1 IN4734A Q1 JE9015C Q2, Q3, Q4, Q5 JE9100C CR1, CR2, CR through CR7 IN4004 CR3, CR8 through CR11 IN4150 R1 10k R2, R4, R6, R8, R18, R24 100k R3, R17, R20 1M R5 3.9M R7 270k R9, R11, R12, R16, R19 4.7k R10 47k R13, R14 750 ohms R15 47 ohms R21 1k R22 8.2k R23 390k R25 through R28 1.8k C1 1000 Mfd.

C2 22 Mfd.

C3 .022 Mfd.

C4 .1 Mfd.

C5, C6 20 Pfd.

C7 2.2 Mfd.

C8 .047 Mfd.

C9, C10 47 Mfd.

011,013 .0047Mfd.

C12, C14 .0022Mfd.

While the invention has been illustrated and described in detail in the drawings and foregoing description, it will be recognized that many changes and modifications will occur to those skilled in the art. It is therefore intended, by the appended claims, to cover any such changes and modifications as fall within the true spirit and scope of the invention.

Claims (7)

1. In a gas burner control system, a burner; an igniter for igniting said burner; valve means for controlling flow of gas to said burner; a flame probe for detecting presence of a burner flame; first circuit means for controlling energizing of said igniter; second circuit means for controlling operation of said valve means; third circuit means for responding to impingement and non-impingement of said flame probe by said burner flame; and a microcomputer for controlling operation of said first, second, and third circuit means, said microcomputer including a read only memory (ROM) which is programmed to provide a desired sequence of burner operation and which is further programmed to provide a self-check thereof during each demand for said burner operation.

2. The control system claimed in claim 1 wherein said microcomputer further includes a random access read-write memory (RAM), and wherein said ROM is further programmed to provide a check of said RAM during said each demand for said burner operation.

3. The control system claimed in claim 1 wherein said igniter is an electrical resistance igniter and wherein said first circuit means includes a first controlled solid state switch connected in series with the winding of a first relay, the contacts of said first relay being effective to connect said igniter across a first power source, said first switch being biased into conduction by said microcomputer when energizing of said igniter is to begin, and further including means selectively connected to said microcomputer for determining when said first switch is to be rendered conductive, and means selectively connected to said microcomputer for determining a time duration of said conduction of said first switch for providing a warm-up time period for said igniter.

4. The control system claimed in claim 3 wherein said valve means comprises two valves connected fluidically in series with said burner, each of said valves having a controlling electrical winding, and wherein said second circuit means includes a second controlled solid state switch connected in series with the winding of a second relay, and a capacitor connected in parallel with said series-connected second switch and second relay winding, said second switch being biased into conduction by said microcomputer upon expiration of said warm-up time period for said igniter so as to enable said second relay winding to be pulled in by the discharging of said capacitor therethrough, the contacts of said second relay being effective to simultaneously connect both said windings of said valves across a second power source whereby gas flows to said burner for ignition by said igniter.

5. The control system claimed in claim 4 wherein said second circuit means further includes third and fourth controlled solid state switches connected in parallel with each other and with said series-connected second switch and second relay winding, said third and fourth switches being biased into conduction by said microcomputer during a large portion of said warm-up time period for said igniter so as to ensure de-energizing of said second relay winding, and being biased into non-conduction by said microcomputer near the end of said warm-up time period so as to enable said capacitor to charge.

6. In a gas burner control system, a burner; an electrical resistance igniter for igniting said burner; first relay means for effecting energizing of said igniter; two valves connected fluidically in series with said burner, each of said valves having a controlling electrical winding; second relay means for effecting concurrent energizing of said electrical windings of said valves to enable gas to flow to said burner; a flame probe for detecting presence of a burner flame when impinged thereby; a microcomputer including a read only memory (ROM) programmed to control operation of said system; means, including said microcomputer, responsive to a demand for burner operation for determing when energizing of said first relay means is to begin, and thereafter effecting energizing thereof for a preselected time period sufficiently long to enable said igniter to attain gas ignition temperature; means, including said microcomputer, for inhibiting energizing of said second relay means during said preselected time period and for subsequently effecting energizing thereof upon expiration of said preselected time period so that said burner flame can be established; and means, including said microcomputer, responsive to impingement of said flame probe by said burner flame for terminating energizing of said first relay means, and to the absence of said impingement for terminating both said first and second relay means, said ROM being further programmed to provide a self-check thereof during each said demand for burner operation.

7. The control system claimed in claim 6 further including means selectively connected to said microcomputer for effecting repeated attempts at establishing said burner flame when a first attempt fails, said means including a pair of resistors selectively connected to a respective pair of input pins of said microcomputer, said microcomputer being effective to provide said repeated attempts only when both said pins have the same digital input.