Patent Publication Number: US-6216683-B1

Title: Gas oven control

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a division of application Ser. No. 08/954,011, filed Oct. 20, 1997, now U.S. Pat. No. 6,030,205, which is a continuation-in-part of commonly assigned patent application Ser. No. 08/516,595, entitled “Gas Oven Fuel Control With Proof of Ignition,” filed Aug. 18, 1995, now U.S. Pat. No. 5,791,890 which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to gas ovens and more particularly to control and ignition systems for gas ovens. 
     Household gas ovens typically include at least a broil burner, typically positioned towards the top of an oven chamber, and a bake burner, typically positioned towards the bottom of the oven chamber. Conventional ignition systems for gas ovens typically include a hot surface ignitor, for example a glowbar, in conjunction with a thermally operated gas control valve. The thermally operated gas control valve opens so as to permit gas flow to the respective burner assembly only when a specified current has been established through the glowbar. The specified current corresponds to a glowbar temperature that will ignite the gas flow upon introduction. 
     Accordingly, a system user selects the type of gas oven operation needed, for example bake mode or broil mode, typically by manipulating a control knob. Once selected, the glowbar begins heating and the current increases until it reaches a steady state. After the current rises above the lower limit for ignition, the thermally operated gas control valve opens, the fuel is ignited, and a flame is established at the selected burner. 
     One current problem with the beforementioned ignition systems is cost. In the highly competitive household gas oven market, any unnecessary or excessive costs should be avoided. In the beforementioned ignition system both the thermostatic gas control valves and the hot surface ignitors are expensive components for a household gas oven system, and the hot surface ignitors are subject to frequent breakage. Additionally, misalignment of the hot surface ignitor relative to the thermostatic gas valve may delay or prevent burner ignition. 
     Another current problem with commercially available gas ovens is that once gas is issued through a burner element and an ignition attempt is made, there is no mechanism for ensuring the ignition attempt was successful. Additionally, even if the ignition attempt was successful, there is no mechanism for determining if there is a flameout at the burner element. 
     Therefore, it is apparent from the above that there exists a need in the art for improvements in safe, low cost gas oven ignition and detection systems. 
     SUMMARY OF THE INVENTION 
     A gas oven comprises at least a first burner element disposed within an oven cavity of the gas oven. A first control valve is disposed within a gas line connected to the burner element and to a gas source. The control valve controls gas flow to the burner element. A first temperature sensor is positioned so as to detect temperature about the burner element. A controller is electrically coupled to the temperature sensor and to the control valve wherein the controller ensures successful ignition of the burner element by monitoring the temperature signals generated from the temperature sensor to detect if the temperature signals increase at a rate that is greater than a predetermined ignition rate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmented side elevation view of an illustrative embodiment of the instant invention; 
     FIG. 2 is a schematic illustration of an ignition detection system in accordance with one embodiment of the instant invention; 
     FIG. 3 is a graph showing a comparison of thermocouple readings and change in thermocouple readings in accordance with one embodiment of the instant invention; and 
     FIG. 4 is an exemplary control logic flowchart in accordance with one embodiment of the instant invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary embodiment of a gas oven  10  includes an outer cabinet  12  with a top cooking surface  14  having at least one individual surface unit  16 , as shown in FIG.  1 . Although the present invention is described herein in connection with gas oven  10 , the present invention is not limited to practice with gas oven  10 . In fact, the present invention can be implemented and utilized with many other configurations. 
     Positioned within cabinet  12  is a cooking chamber  18  formed by a box-like oven liner  20  having vertical side walls  22 , a top wall  24 , a bottom wall  26 , a rear wall  28  and a front opening drop door  30 . Cooking chamber  18  is provided with a bake element  32 , typically positioned adjacent bottom wall  26 , and a broil element  34 , typically positioned adjacent top wall  24 . Bake element  32  and broil element  34  typically comprise heating units such as resistance heat elements or the like. 
     A control knob  40  extends outwardly from a backsplash  42  of gas oven  10 . Control knob  40  is provided such that a system-user can select the mode of operation for gas oven  10 . 
     Gas oven  10  further comprises a first control valve  44 , a second control valve  46 , a first ignitor  48 , a second ignitor  50 , a first temperature sensor  52  and a second temperature sensor  54 , each of which are electrically coupled to a controller  55 , as shown in FIG.  2 . 
     First control valve  44 , typically a solenoid valve, is disposed within a first gas line  56 , which first gas line  56  connects a gas source (not shown) to broil element  34 . Gas flow from the gas source is delivered to broil element  34  when first control valve  44  is disposed in an open position and conversely, gas flow is prevented to broil element  34  when first control valve  44  is disposed in a closed position. First ignitor  48  is positioned adjacent broil element  34  such that first ignitor  48  can provide ignition to the gas flow issuing from broil element  34  when first control valve  44  is disposed in an open position. 
     Second control valve  46 , typically a solenoid valve, is disposed within a second gas line  58 , which second gas line  58  connects a gas source (not shown) to bake element  32 . Gas flow from the gas source is delivered to bake element  32  when second control valve  46  is disposed in an open position and conversely, gas flow is prevented to bake element  32  when second control valve  46  is disposed in a closed position. Second ignitor  50  is positioned adjacent bake element  32  such that ignitor  50  can provide ignition to the gas flow issuing from bake element  32  when second control valve  46  is disposed in an open position. 
     Temperature sensors  52  and  54  typically comprise thermocouples or the like. Temperature sensors  52  and  54  are positioned adjacent broil element  34  and bake element  32  respectively, so as to sense temperature about each element. 
     For purposes of clarity, the operation of gas oven  10  will be discussed in terms of a BAKE MODE and a BROIL MODE. Although the exemplary embodiments will be discussed in terms of a BAKE MODE and a BROIL MODE, the invention is not limited to these modes. In fact, the present invention can be implemented and utilized with many other modes of operation. 
     During operation, a system-user manipulation of control knob  40  (FIG. 1) to the corresponding position, inputs either BAKE MODE or BROIL MODE. 
     If the system-user selects BAKE MODE, a preset temperature (T s ) is established, typically in the range between about 100° F. and 550° F. 
     Controller  55  (FIG. 2) generates a control signal to open control valve  46  such that a flow of gas is established through gas pipe  58  and is issued through bake element  32 . Additionally, controller  55  causes an ignition signal to be generated to activate ignitor  50  such that a spark or the like is generated by ignitor  50  to ignite the flow of gas issuing through bake element  32 . 
     Controller  55  receives temperature signals from temperature sensor  54  so as to monitor the temperature and temperature change about bake element  32 . 
     Controller  55  also receives temperature signals from a conventional oven thermometer  36  (FIG. 1) to monitor the overall oven temperature. If controller  55  (FIG. 2) senses from oven thermometer  36  that the oven temperature is greater than or equal to the preset temperature (T s ), heating is no longer required, and controller  55  generates a control signal to close control valve  46 . 
     One current problem with commercially available gas ovens is that once gas is issued through a burner element and an ignition attempt is made, there is no mechanism for ensuring the ignition attempt was successful. Additionally, even if the ignition attempt was successful, there is no mechanism for determining if there is a flameout at the burner element. 
     In accordance with one embodiment of the instant invention, controller  55  ensures ignition attempts are successful by monitoring the temperature signals generated from temperature sensor  54 . If the temperature signals generated by temperature sensor  54  increase at a rate that is greater than a predetermined ignition rate, the ignition attempt is determined to be successful. 
     In one embodiment, ignition is proven within 10 seconds of the opening of control valve  46  by detecting at least a 2.0 mV increase in the temperature signals generated by temperature sensor  54 . In another embodiment of the instant invention, ignition is proven within 60 seconds of the opening of control valve  46  by detecting at least a 3 degrees Fahrenheit increase in the temperature signals generated by temperature sensor  54 . 
     If the temperature signals sensed by temperature sensor  54  do not increase at a rate that is greater than a predetermined ignition rate, the ignition attempt is determined by controller  55  to have been unsuccessful, controller generates a control signal to close control valve  46 , and oven cavity  18  (FIG. 1) is allowed to purge itself during a predetermined time delay before another ignition attempt is made. During the delay, unburned fuel leaves the oven cavity  18 , and after the delay the ignition process is begun anew. 
     If controller  55  (FIG. 2) determines ignition was successful, controller  55  continues to monitor the temperature signals generated by temperature sensor  54  to detect if there is a premature flameout. 
     Controller  55  determines that there is a premature flameout if either, the temperature signals generated by temperature sensor  54  are decreasing at a rate that is greater than a predetermined flameout rate, or if the temperature signals generated by temperature sensor  54  are increasing at a rate that is less than a predetermined flame rate. 
     In one embodiment, the temperature signals generated by temperature sensor  54  are monitored at one second intervals. Controller  55  compares each temperature signal to the temperature signal detected 14 seconds earlier. The change in the temperature signal over that time period is compared with predetermined criteria. One representative embodiment of the instant invention would correspond with predetermined criteria as displayed in FIG. 3 If the current temperature signal plotted against the change in the temperature signal, over the time period, maps above the plotted predetermined criteria (one example of which is shown in FIG.  3 ), flame is proven. If the current temperature signal plotted against the change in the temperature signal, over the time period, maps below the plotted predetermined criteria, flameout is detected and controller  55  sends a control signal to close control valve  46 . 
     Thermocouples utilize a relationship that when two dissimilar metals are brought into intimate contact, a voltage is developed that depends on the temperature at the junction and the particular metals used. If two such junctions are connected in series with a voltage-measuring device, the measured voltage will be very nearly proportional to the temperature difference of the two junctions. 
     In one embodiment of the instant invention, type K thermocouples are utilized. The proportionality of a type K thermocouple [reference junction at 32° F.] is as follows: at about 32° F., the thermal electromotive force registered would be about 0 mV; at about 500° F., the thermal electromotive force registered would be about 10 mV; and at about 1000° F., the thermal electromotive force registered would be about 24 mV. 
     Utilizing this known proportionality, a flameout detection method is developed through controller  55 . As shown in FIG. 3, flameout detection criteria is inputted to controller  55 , for example by programming into memory of an application specific integrated circuit (ASIC) or other programmable memory device. The flameout detection criteria, as plotted in FIG. 3, is compared by controller  55  to the current thermocouple reading in mV against the change in thermocouple reading over the selected time frame. When an operational mode is selected and ignition is successful, the signals from the thermocouples are monitored. If the sensor is at a relatively low temperature, the thermocouple reading will be relatively low, for example, for a temperature of 250° F. the thermal electromotive force registered would be about 5 mV. If the sensor is at a relatively high temperature, the thermocouple reading will be relatively high, for example, for a temperature of about 750° F. the thermal electromotive force would be about 15 mV. 
     Now referring to FIG. 3, in this embodiment of the instant invention, if controller  55  detects that temperature sensor  54  is generating a temperature signal between 0 mV to about 5 mV, the oven is in the process of warming up towards the preset temperature (T s ). If controller  55  also detects that the change in the temperature signals over that time frame is not increasing at greater than a predetermined ignition rate, for example, the change in temperature signals is greater than +2 mV, flameout is detected, or a successful ignition is not proven. 
     If controller  55  detects that the thermocouple reading is between about 5 mV to about 15 mV, the sensor temperature is between about 250° F. and 750° F., the typical operating range for both BAKE MODE and BROIL MODE. If controller  55  detects that the change in thermocouple reading is decreasing at greater than a predetermined flameout rate, for example, the change in temperature signal is less than about 0, flameout is detected. 
     If controller  55  detects that the thermocouple reading is greater than 15 mV, the sensor temperature is greater than 750° F. Accordingly, within this temperature range, the oven temperature is greater than the typical operating range for both BAKE MODE and 750° F. If controller  55  also detects that the change in thermocouple reading is decreasing at greater than a predetermined flame rate, for example, the change in temperature signal is less than −2 mV, flameout is detected. 
     If the thermocouple signal mapped against the change in thermocouple signal, over the selected time frame, plots above this criteria, flame is detected and controller  55  continues to monitor. 
     If the system user selects BROIL MODE, a preset temperature (T s ) is established, typically in the range between about 550° F. to 800° 0  F. 
     Controller  55  generates a control signal to open control valve  44  such that a flow of gas is established through gas pipe  56  and the flow of gas is issued through broil element  34 . Additionally, controller  55  generates an ignition signal to activate ignitor  48  such that a spark or the like is generated by ignitor  48  to ignite the flow of gas issuing through broil element  34 . 
     Controller  55  also receives temperature signals from a conventional oven thermometer  36  (FIG. 1) to monitor the overall oven temperature. If controller  55  (FIG. 2) senses that the oven temperature is greater than or equal to the preset temperature (T s ), heating is no longer required and controller  55  generates a control signal to close control valve  44 . 
     In accordance with one embodiment of the instant convention, controller  55  ensures ignition attempts are successful by monitoring the temperature signals generated from temperature sensor  52 . If the temperature signals generated by temperature sensor  52  increase at a rate that is greater than a predetermined ignition rate, the ignition attempt is determined to be successful. 
     In one embodiment, ignition is proven within ten seconds of the opening of control valve  44  by detecting at least 2.0 mV increase in the temperature signals generated by temperature sensor  52 . In another embodiment of the instant invention, ignition is proven within 60 seconds of the opening of control valve  44  by detecting at least a 3 degrees Fahrenheit increase in the temperature signals generated by temperature sensor  52 . 
     If the temperature signals sensed by temperature sensor  52  do not increase at a rate that is greater than a predetermined ignition rate, the ignition attempt is determined by controller  55  to have been unsuccessful, controller  55  generates a control signal to close control valve  44 , and oven cavity  18  (FIG. 1) is allowed to purge itself during a predetermined time delay before another ignition attempt is made. During the delay, unburned fuel leaves the oven cavity  18 , and after the delay the ignition process is begun anew. 
     If controller  55  (FIG. 2) detects ignition was successful, controller  55  continues to monitor the temperature signals generated by temperature sensor  52  to detect if there is a premature flameout. 
     Controller  55  detects that there is a premature flameout if either, the temperature signals generated by temperature sensor  52  are decreasing at a rate that is greater than a predetermined flameout rate, or if the temperature signals generated by temperature sensor  52  are increasing at a rate that is less than a predetermined flame rate. 
     In one embodiment, the temperature signals generated by temperature sensor  52  are monitored at one second intervals. Controller  55  compares each temperature signal to the temperature signal from 14 seconds earlier. The change in the temperature signal is compared with predetermined criteria. If the current temperature signal plotted against the change in the temperature signal, over the time frame, maps above the plotted predetermined criteria, flame is proven. If, however, the current temperature signal plotted against the change in the temperature signal, over the time frame, maps below the plotted predetermined criteria, flameout is detected and controller  55  sends a control signal to close control valve  44 . 
     An exemplary control logic sequence for gas oven  10  is shown in FIG. 4. A system user initiates the control sequence at block  200  by selecting a mode of operation, for example, BAKE MODE, or BROIL MODE, and a preset temperature (T s ) is established. 
     At block  202 , the oven temperature (T) is monitored by controller  55  through oven thermostat  36 . The oven temperature is continuously monitored by controller  55  until the mode of operation is turned off, typically by a system user. 
     Next, at block  204 , controller  55  compares the current oven temperature (T) with the preset temperature (T s ). If the current oven temperature (T) is greater than or equal to the preset temperature (T s ), no further heating is necessary, and the control sequence returns to block  202  and continues to monitor the current oven temperature (T). If, however, the current oven temperature (T) is less than the preset temperature (T s ), further heating of the oven is necessary, and the control sequence advances to block  206 . 
     At block  206 , controller  54  energizes the appropriate control valve (control valve  42  for bake mode or control valve  52  for broil mode) and the appropriate ignitor (ignitor  50  for bake mode or ignitor  48  for broil mode), such that fuel flow to the appropriate burner is established and ignition is attempted. 
     Next, at block  208 , controller  54  monitors the sensor temperature with the appropriate temperature sensor, bake temperature sensor  56  or broil temperature sensor  58 . 
     If, the sensor temperature is not increasing at a rate that is greater than a predetermined ignition rate, the controller detects that ignition has been unsuccessful and the sequence continues to block  210 . 
     At block  210 , controller  55  monitors the elapsed time from when the appropriate valve was opened at block  206 . If the elapsed time is less than a predetermined time, for example 10 to 15 seconds, safe operation is ensured and the sequence returns to block  208  to continue the ignition process. If, however, the elapsed time is greater than or equal to a predetermined time, controller  55  generates a control signal to close the appropriate control valve as a safety precaution at block  212 . 
     After the control valve is closed at block  212 , the sequence enters a delay stage at block  214  to purge any unburned fuel that has accumulated within oven cavity  18  while the control valve was in an open position. Generally, the delay at block  214  will last in the range between about 15 seconds to about 100 seconds. 
     Next, after sufficient delay at block  214 , the control sequence returns to block  206  where the appropriate valve and ignitor are re-energized and ignition is re-attempted. 
     If the sensor temperature sensed by the appropriate temperature sensor is increasing at a rate that is greater than a predetermined ignition rate the controller determines ignition has been successful and the sequence continues to block  216 . 
     At block  216 , controller  55  monitors the burner for premature flameout. Controller  55  monitors the sensor temperature signals to detect if the temperature signals are decreasing at a rate that is greater than a predetermined flameout rate. If controller  55  detects that the temperature signals are decreasing a rate that is greater than a predetermined flameout rate, flameout is detected and the control valve is closed at block  218 , the sequence enters a delay stage at block  220 , and after sufficient delay the sequence returns to block  202  to monitor the oven temperature. 
     If controller  55  detects that the temperature signals are not decreasing at a rate that is greater than a predetermined flameout rate, the sequence advances to block  222 . 
     At block  222 , controller  55  continues to monitor the burner for premature flameout. Controller  55  monitors the sensor temperature signals to detect if the temperature signals are increasing at a rate that is less than a predetermined flame rate. 
     If controller  55  detects that the temperature signals are increasing at a rate that is less than a predetermined flame rate, flameout is detected and control valve is closed a block  218 . The sequence enters a delay stage at block  220  and after sufficient delay the sequence returns to block  202  and continues to monitor the current sensor temperature. 
     If controller  55  detects that the temperature signals are not increasing at a rate that is less than a predetermined flame rate, the sequence advances to block  224 . 
     At block  224 , controller  55  compares the current oven temperature (T) with the preset temperature (T s ). If the current oven temperature (T) is greater than or equal to the preset temperature (T s ), no further heating is necessary, the control valve is closed at block  218 , the sequence enters a delay stage at block  220  and after sufficient delay the sequence returns to block  202  and continues to monitor the current oven temperature (T). 
     If the current oven temperature (T) is not greater than or equal to the preset temperature (T s ), further heating is necessary and the sequence returns to block  216  for flameout monitoring. Thus the control sequence of the instant invention is a closed loop which continues until a system user turns off gas oven  10 . 
     While only certain features of the invention have been illustrated and described, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.