Abstract:
A microprocessor-controlled triac switching circuit for an igniter, having line voltage and igniter current input signals to a microprocessor for determining power to the igniter. Upon determining the line voltage, the microprocessor selects from a look-up table a corresponding triac-switching sequence intended to drive the igniter to a power level proven to ignite gas. The igniter current resulting from the switching sequence is fed to the microprocessor, which determines the actual power and an offset value. The offset enables adjustment by shifting sequences, to achieve the pose level at which the igniter is proven to ignite gas.

Description:
FIELD OF THE INVENTION 
     This invention relates to systems for controlling the energizing of electrical resistance igniters for proven ignition systems. 
     BACKGROUND OF THE INVENTION 
     In many gas-fired devices, standing pilots have been replaced by either hot surface igniters or spark ignition devices. While spark ignition devices provide rapid ignition, they generate undesirable electrical and acoustical noise. In applications that use a microprocessor, such electrical noise is undesirable since it can adversely affect operation of the microprocessor. In addition, spark devices may be difficult to predict as to generation of an arc proven to ignite gas. Hot surface ignition does not generate such electrical or acoustical noise, but does require careful control of its temperature to prevent damage to the igniter. 
     An igniter known in the art that is capable of warming up quickly is a silicon nitride igniter, an igniter constructed of a tungsten alloy heater element embedded in a silicon nitride insulating material. While such an igniter is desirable because of its mechanical strength and durability, it has a critical temperature limitation, which must be adhered to. Specifically, the silicon nitride igniter must remain below approximately 1300° C. If the igniter temperature repeatedly approaches 1300° C., the igniter will prematurely fail, such failure generally consisting of the opening of the tungsten heater element. Since a temperature over 1100° C. is required to reliably ignite gas, the igniter must operate within a narrow temperature span between the lowest temperature that will reliably ignite gas and the highest temperature that the igniter can withstand. 
     U.S. Pat. No. 4,925,386, assigned to the assignee of the present invention, discloses a learning routine for energizing an igniter to a temperature above the minimum ignition temperature, and successively energizing the igniter to a slightly lower temperature during each successive cycle until it fails to ignite the gas. After an unsuccessful ignition attempt, energy to the igniter is increased so that the igniter operates just above the lowest possible ignition temperature, which prolongs igniter life. While an occasional unsuccessful ignition attempt is generally acceptable in many applications, it is not acceptable in commercial applications of high BTU output that require a proven method for igniting gas. 
     U.S. Pat. No. 5,725,368, assigned to the assignee of the present invention, discloses a system for determining the level of power to be applied to the igniter based on the value of the voltage available to energize the igniter and on the value of the resistance of the igniter. A triac in series with the igniter is fired in an irregular firing sequence, which is determined from a look-up table in a microcomputer. Specifically, the look-up table enables selecting a firing sequence based on the determined value of line voltage and the determined “cold” resistance value of the igniter. But due to manufacturing tolerances, the “cold” resistance of the igniters can vary considerably from igniter to igniter. In addition, the “cold” resistance of each igniter can vary considerably from cycle to cycle as a result of residual heat in the igniter. Such variances are difficult to accurately compensate for. While prior art systems are useful for the purposes described, there is still a need for a method of accurately controlling the power and temperature of an igniter that is used where a proven source of igniting gas is required before the gas supply is opened. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a microprocessor-controlled triac-switching circuit for a silicon nitride igniter, having line voltage and igniter current input means to the microprocessor for determining power to the igniter. Upon determining the line voltage, the microprocessor uses a look-up table to select a corresponding triac-switching sequence to drive a igniter to a power level proven to ignite gas via empirical testing on the application. The igniter current resulting from the switching sequence is fed to the microprocessor, which determines the actual power level and an offset value. The offset functions as a compensation means, shifting the sequences in the look-up table to achieve the desired power to the igniter. After a predetermined warm-up time, the microprocessor sends a signal indicating whether the igniter is at a power level proven to ignite gas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an igniter controlling system in accordance with the present invention; 
     FIG. 2 is a flowchart outlining an igniter controlling system in accordance with the present invention for use in the ignition controlling system of FIG. 1; 
     FIG. 3 is a graph of the temperature curve of an igniter powered by an igniter controlling system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The igniter controlling system in accordance with the present invention is indicated generally at  10  in FIG.  1 . Igniter controlling system  10  includes a microprocessor M 1  for controlling a triac switching circuit  20  for providing power to an igniter  40 , a voltage measuring means  30  for determining the value of line voltage, and a current measuring means  50  for determining the current through the igniter  40 . The igniter controlling system  10  may be incorporated into an integrated furnace control or integrated boiler control that controls a supply of gas to a burner in a gas furnace or boiler. However, the igniter controlling system of the present invention is not so limited, and can be incorporated into devices for controlling ignition in other gas applications. 
     The traic switching circuit shown generally at  20  essentially comprises a transistor  22  that switches power from supply  24  to the input of an opto-isolator  26 , which gaits a triac Q 1  for switching power to the igniter  40 . The power supply  24  of the present invention is a 24 volt dc supply with a dropping resistor to reduce the voltage to the opto-isolator  26 , but may optionally be a 5 volt dc supply or other equivalent supply means. Opto-isolator  26  switches  120  VAC (line voltage) across a dropping resistor to supply a gait current to traic Q 1 . The opto-isolator  26  preferably is of a type that switches at the zero crossing of the AC voltage source, so as to gait the traic Q 1  and switch power to the igniter when the voltage level is lowest to minimize electrical noise. An equivalent circuit that will switch the triac Q 1  at approximately the zero crossing of the AC voltage source may optionally be substituted for the opto-isolator  26  above. 
     The voltage measurement means shown generally at  30  comprises a redundant set of resistor branches in parallel with the triac Q 1 . The branches have a resistance value significantly higher than that of the igniter  40 , such that when the triac Q 1  is open the line voltage is effectively dropped across the resistor branches. The branches are comprised of two resistors R 32  and R 34  in series, and two resistors R 36  and R 38  in series. Resistors R 32 , R 34 , R 36  and R 38  are all of equal resistance. Voltage is taken at a point between the resistors R 32  and R 34 , and at a point between R 36  and R 38 , for input to the microprocessor M 1  for determining the value of the line voltage. 
     The igniter  40  preferably is a Kyocera WRS-6 Silicon Nitride igniter, and is effective, when sufficiently heated, to ignite gas. Silicon Nitride igniter  40  has a resistance range of 12 to 60 ohms depending on temperature, and must remain below approximately 1300° C. to prevent premature failure of the igniter. Igniter  40  is also in series with a fuse  42  to protect against shorting of the igniter. 
     The igniter current measurement means shown generally at  50  comprises a set of current sampling resisters R 52  and R 54  in series with the igniter  40 . Voltage is taken on the high side of the resistors R 52  and R 54 , for input to the microprocessor M 1  for determining the current through the igniter  40 . 
     A microprocessor M 1  is effective for controlling operation of igniter controlling system  10 . Preferably, microprocessor M 1  is a Microchip PIC16 F87 X device. Included within microprocessor M 1  are a CPU, a ROM (read only memory), a RAM (random access read/write memory), and a plurality of I/O (input output) pins. Such I/O pins include pins RA 0 , RA 1 , RA 2 , RA 3 , RB 1 , RB 4 , RB 6 , and RB 7 . Microprocessor M 1  controls the switching of triac Q 1  via transistor  22  through input impedance resistor R 61  and pin RB 7 . Line voltage measurements are input to the microprocessor M 1  through input impedance resistors R 62  and R 63  and pins RA 0  and RA 2  respectively. Igniter current sampling measurements are input to the microprocessor through input impedance resistors R 64  and R 65  and pins RA 1  and RA 3  respectively. 
     It is to be understood that microprocessor M 1  has a plurality of other pins (not shown) that are connected to other circuitry not shown in FIG.  1 . However, a description of such other pins and other circuitry is not believed to be essential to provide an enabling disclosure of the present invention and is therefore omitted. 
     In operation as shown in FIG. 2 step  100 , microprocessor M 1  switches triac Q 1  through pin RB 7  to provide power to the igniter  40 , and starts a warm-up timer. The traic Q 1  when closed has a voltage drop of approximately 1.7 volts, as do the resister branches in parallel with triac Q 1 . Thus, at step  110  when the microprocessor M 1  reads line voltage inputs, it essentially reads line voltage less 0.85 volts across resistors R 32  and R 36 . If the redundant line voltage inputs at step  130  do not differ more than a predetermined amount, the microprocessor M 1  proceeds in step  140  to look up an index number corresponding to the averaged line voltage inputs in a look-up table. Microprocessor M 1  then increases or decreases the index number by an offset number and an EEPROM number in step  150 . These numbers function as compensation means to adjust power to the igniter  40 , and initially have no value. At step  160 , the microprocessor M 1  selects a switching sequence in the look-up table corresponding to the index number, and initiates the switching sequence. The look-up table essentially comprises a plurality of switching sequences, or on and off times corresponding to known line voltage values. The amount of on-time verses off-time increases as the line voltage level decreases, to increase the on-time for applying voltage to the igniter  40 . The microprocessor M 1  determines the RMS voltage to igniter  40  based on the line voltage value and the on-off duty ratio of the select switching sequence. Upon determining the RMS voltage to the igniter  40  in step  160   a , the microprocessor M 1  reads igniter current inputs through pins RA 1  and RA 3  in step  170 . Due to the resistance variations of production igniters, the measured current may vary from igniter to igniter, as may the power. If the current inputs at step  180  do not differ more than a predetermined amount, the microprocessor M 1  determines the product of the RMS voltage and average current to obtain the actual power to the igniter. Upon obtaining the actual power level at step  190 , the microprocessor M 1  proceeds in step  200  to look up an offset number corresponding to the actual power level in the look-up table. The offset number is used to shift switching sequences within the look-up table to change the on verses off time and adjust the RMS voltage to the igniter  40  in response to the actual power level. While operating within the predetermined warm-up time of step  210 , microprocessor M 1  will return to step  110  to repeat voltage and current readings, and offset the index number at step  150  to shift switching sequences and tune the power to the igniter to the desired level. When the predetermined igniter warm-up time is reached, the microprocessor M 1  will proceed to send a signal through pin RB 7  or RB 1  as to whether the igniter has been powered to a level proven to ignite gas. If the power to the igniter  40  is at a level proven to ignite gas for an application, the offset number used to compensate the index number is stored in step  220  in EEPROM (electrically erasable programmable read only memory) to immediately shift the index number at step  150  on the next start up. If at step  220  the power to the igniter  40  is not at a level proven to ignite gas for an application, the microprocessor M 1  will send a signal to an integrated furnace control or device, so as to prevent switching on of the gas supply to the igniter  40 . 
     Igniter  40  is constructed of a tungsten heater element embedded in a silicon nitride insulator material. The surface area of the silicon nitride insulator portion is relatively constant, even though the electrical resistance may vary from igniter to igniter. Specifically, FIG. 3 shows two temperature curves for a Kyocera Silicon Nitride igniter powered to 95 watts and 105 watts, which define a range below which the igniter is known to not ignite gas and above which the igniter is known to experience reduced life. This controlled range for powering such an igniter has been proven to ignite gas in the Genesis boiler application. Resistance variations are overcome by operating the igniter within a specific range of power, which dissipates from the relatively constant surface area to produce a consistent heat source proven to ignite gas in a given application. For example, a power range of 95 to 105 watts as shown in FIG. 3 is proven to ignite gas in a maximum airflow, minimum air temperature setting of an application such as a Genesis series Boiler manufactured by A. O. Smith Corporation. Other applications having greater airflow, such as a Legend series Boiler manufactured by A. O. Smith, may require a specified power range of 105 to 115 watts to reliably ignite gas. Therefore, it should be understood that the specified power range proven to ignite gas may vary depending on application. The specified power range also ensures the igniter will operate at a temperature below the 1300° C. critical temperature of the silicon nitride igniter  40 , so as to prolong the life of the igniter. 
     With respect to the values of the component parts, (e.g., resistors transistors, and opto-isolators) as indicated in FIG.  1  and listed below, it should be noted that these values may be adjusted as required or desired depending upon the particular application and igniter assembly construction. 
     
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 OPTO-ISOLATOR MOC 3031 
               
               
                   
                 26 
                 (manufactured by Fairchild) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 R28 
                   
                 56K 
                 Ohm 
               
               
                   
                 R32 
                   
                 47K 
                 Ohm 
               
               
                   
                 R34 
                   
                 47K 
                 Ohm 
               
               
                   
                 R36 
                   
                 47K 
                 Ohm 
               
               
                   
                 R38 
                   
                 47K 
                 Ohm 
               
               
                   
                 R52 
                   
                 ½ 
                 Ohm 
               
               
                   
                 R54 
                   
                 ½ 
                 Ohm 
               
               
                   
                 R61 
                   
                 10K 
                 Ohm 
               
               
                   
                 R62 
                   
                 10K 
                 Ohm 
               
               
                   
                 R63 
                   
                 10K 
                 Ohm 
               
               
                   
                 R64 
                   
                 10K 
                 Ohm 
               
               
                   
                 R65 
                   
                 10K 
                 Ohm 
               
               
                   
                   
               
             
          
         
       
     
     Those skilled in the art will recognize that the inventive igniter controlling system of this invention may be useful in many applications and for control of many different types of gas appliances. Inasmuch as many modifications within the spirit of the invention will be apparent to those skilled in the art, the scope of the invention should be determined by reference to the claims appended below and the full scope of equivalents as provided by applicable laws.