Abstract:
A method and apparatus is provided for operating a self-cleaning oven in which a gas sensor, such as for measuring concentrations of CO gas, is located remote from, yet in gas communication with, an exhaust flue of the oven. The gas sensor is located at the end of an outlet tube which extends from the exhaust flue. A sample gas flow is provided to the gas sensor through the outlet tube to isolate the sensor from the heat of the oven and a filter device is located in the outlet tube for filtering the sample gas flow. A valve may be provided at the inlet to the outlet tube. The rate of change (slope) of successive readings of gas concentrations may be used to determine when the combustion of food material is complete, in order to terminate a self-cleaning cycle. As a back up method to the gas sampling system, a count of lengths or numbers of baking cycles and broiling cycles performed since a last self-cleaning cycle can be kept, and a look up table consulted to determine a length of time required for a cleaning cycle. A display is provided to advise the user that a cleaning mode is in operation and to inform the user of the amount of time remaining in the cleaning cycle.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to self-cleaning ovens and in particular, to a system for controlling the operation of a self-cleaning oven. 
     During the use of an oven of an electric or gas range, deposits will generally accumulate as a result of spills, boil overs and other unintended release of foods from their cooking containers. In order to ease the cleaning of the spillage, provision is made in some ranges, known as “self-cleaning” ranges, to raise the temperature of the cooking cavity well above that which would be used in cooking in order to carbonize or burn out the residue. In general, this is achieved by the selection through the range&#39;s controls of a self-clean cycle. Initiation of this cycle typically sets a high control temperature for the range, locks the oven door at some predetermined time or temperature and proceeds to heat the cavity to a relatively high temperature for a predetermined time before ending the cycle, allowing cooling to occur and then releasing the door lock as an end to the cycle. 
     Typically, the time period set for this self-clean cycle is determined by the assumption of a worst case cycle. During the cycle, odors or even smoke may be released in the range environment and significant energy is used to hold the cooking cavity at a high temperature. Because of odor and smoke release, users are advised to open windows and will frequently leave the kitchen area for an extended period of time while self-clean is performed. 
     If a method can be devised which adjusts the time of self-cleaning to that needed for the existing degree of soil accumulation, then cycle times and their negative impact on kitchen enviromnent and energy usage can be minimized. 
     U.S. Pat. No. 4,954,694 discloses a self-cleaning oven which incorporates a heat controlled unit which is responsive to a gas signal from a gas sensor located in the exhaust passage. The gas sensor measures humidity or carbon dioxide levels. The heat control samples the gas signal at a given time interval to detect a variation of amount of the gas component and detect a first inflection point from decreasing to increasing or visa versa in a gas-component variation and a second inflection point from decreasing to increasing or vice versa in the gas component variation after detection of the first inflection point. The heat control means determines the heating time period for cleaning in correspondence with the second inflection point. An oxidizing catalyst is provided in the exhaust passage, upstream of the gas sensor. 
     SUMMARY OF THE INVENTION 
     It is generally recognized that the combustion of food product will generate various gases or gas components. This invention is generally directed to controlling the operation of a self cleaning oven wherein the duration of a self clean cycle wherein foods are combusted is controlled by monitoring the “signature” response of gas components resulting from the combustion of food soils in an oven cavity. More particularly, the time period of the self clean cycle is responsive to the amount of soil accumulation in the oven. 
     The present invention controls the duration of a self-clean cycle by monitoring a gas component produced by food combustion and by determining a rate of change between successive gas component signals. Termination of the self clean cycle is initiated once the determined rate of change is maintained below a minimum preset rate of change value for a predetermined length of time. The self clean cycle may be terminated, for example, a predetermined time after the rate of change is maintained below a minimum preset rate of change value for a predetermined length of time. In one embodiment, the measured gas component may be carbon monoxide. 
     The present invention includes a gas sensor or sensor mechanism to detect gas concentrations found in the exhaust gas during a self cleaning operation. The gas sensor is located remote from the oven and remote from the flue passage, but in communication therewith through a flue gas delivery system. This system comprises a relatively small diameter outlet tube or tubing which branches off from the main flue gas passage and which delivers flue gases to the sensor mechanism. A valve may be optionally provided on the inlet to the small diameter tubing to limit the gas sensor&#39;s exposure to flue gases. 
     In order for the user of the range to be made aware of the status of the self-cleaning cycle, a display may be provided. During the initial evaluation period, while the control is determining the extent of cleaning required, an icon, such as an hour glass, can be displayed on an electronic display located on the range console to symbolize that the clean cycle is in process. Once the self-cleaning duration is determined by the control system, a count down timer can be displayed in lieu of the icon, indicating to the user the time remaining for the completion of the cleaning cycle. 
     In a further embodiment, the present invention may include an alternate method for determining the amount of clean time needed to perform the self-cleaning cycle wherein the number or length of bake and broil cycles the user has performed since the last self-clean cycle is counted. The number of days since a self-clean cycle has been run is also counted. A minimum clean base time based on these factors could then be determined. Thus, when the user selects and starts a clean cycle, the number or length of bake and broil cycles and the number of days the oven has not been cleaned, are retrieved and used to determine the appropriate clean time. The calculated clean time is displayed to the user to show the length of the clean cycle. This method could be used in lieu of using a gas sensor, or as a back up method in the event of sensor malfunction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an oven embodying the principles of the present invention. 
     FIG. 2 is a schematic side section of an oven incorporating the principles of the present invention. 
     FIG. 3 is a graphic illustration of the gas concentration levels in the oven exhaust during a self-cleaning cycle. 
     FIG. 4 is an enlarged view of section A from FIG. 2, illustrating the filter and gas sensor. 
     FIG. 5 is a graphical illustration of measured gas component concentration levels in an oven with and without using a carbon filter. 
     FIG. 6 is a flow chart for describing an example of a cleaning time control operation for the clean cycle in accordance with the principles of the present invention. 
     FIG. 7 is a flow chart for describing an example of a gas concentration detection algorithm in accordance with the principles of the present invention. 
     FIG. 8 is a flow chart for describing an example of a backup algorithm to be used in the event of sensor failure or when no sensor is used, in accordance with the principles of the present invention. 
     FIG. 9 is a flow chart describing a cleaning time control operation for the clean cycle in accordance with the principles of the present invention. 
     FIG. 10 is a graphic illustration of bake and broil cycles vs. weeks since last self-clean cycle vs. time for self-clean cycle. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 illustrate an electric range  10  having a self-cleaning oven  12  adapted to be controlled by a microprocessor based control system  14  and a method in accordance with the principles of the present invention. Although an electric range  10  is illustrated, it should be understood that a gas range may implement the features of the present invention. 
     The range  10  includes a plurality of control knobs  16  for controlling a respective plurality of conventional electric (or gas) burners  18 . In addition, the range  10  includes a control knob  20  for controlling a mode of operation of the oven  12 . For example, an OFF mode, a bake mode, a broil mode and a clean mode of operation may be selected by the control knob  20  (as indicated at  20 C in FIG.  2 ). In addition, a control knob  22  is conventionally provided to select a desired oven temperature within the oven  12  (as indicated at  22 C in FIG.  2 ). A timer knob may optionally be provided in the event that the control permits a user override to individually control the length of time for a cleaning process. Disposed within a cavity  24  of the oven  12  are a conventional broiling element  26  and a conventional heating element  28 . Furthermore, positioned within the cavity  24  of the oven  12  is a conventional temperature sensor  30 , such as, for example, a standard oven temperature sensing probe. 
     The microprocessor based control system  14  includes a microprocessor  32  suitably programmed to effect the desired control of the range  10 . Conventionally, the microprocessor  32  includes an analog-to-digital (a/d) converter  34  for receiving analog voltage input signals from, for example, the temperature sensor  30 , and for providing digital output pulses or signals to a controller section  36  within the microprocessor  32 . Also, conventionally, the microprocessor  32  includes a memory  38  for retaining programmed instructions for operating the control system  14  including a desired oven temperature control algorithm for controlling the temperature of the oven  12 , particularly during the clean mode of operation. 
     The control system  14  also includes a power switching relay  40  having a pair of relay contacts  42  and  44  for switching power to a heating element, for example, the baking element  28 , from a constant voltage (e.g. 240 volts) source  46  of alternating current electric power under the control of the controller  36 . For simplification, only the baking element  28  and the power relay  40  therefore have been illustrated in FIG. 2 in the control system  14 . In an actual commercial embodiment, however, the broiling element  26  could, of course, be a part of the control system  14  along with its own power switching relay to interconnect the broiling element  26  to the source  46 . The broiling element  26  is used in conjunction with a heating element  28  during the broil mode of operation of the oven  12  and may further be used during the bake and clean modes of the oven  12  to provide sufficient heat to the oven  12  under the control of the controller  36 . 
     Above the oven cavity  24  is an exhausting passage or flue passage  50  through which atmosphere within the oven cavity  24  may be exhausted to the ambient atmosphere. In a preferred, although not necessary, arrangement, an outlet tube  54  is provided which communicates at a first inlet end  56  with the flue passage  50  and has a second end  58  which preferably is located in or near a console  59  of the stove on which the various control knobs  16 ,  20 ,  22  are mounted. A gas sensor  60  is connected to the second end  58  of the outlet tube  54 . With the sensor located in or near the console  59  the sensor will be isolated from the high temperatures of the oven cavity  24 . 
     The sensor  60  may be an infrared (IR) type gas sensor wherein infrared light is emitted from an infrared source and directed through a sample chamber to an infrared detector. The sensor  60  is interconnected with a sensor controller  62  for providing readings of selected gas concentration levels. The sensor controller  62 , along with the other control components may also be located within the console  59 . It can be understood by one skilled in the art that the sensor  60  may be mounted directly to a circuit board which also supports the sensor control  62 . In a preferred arrangement, there is the main controller  32  and a separate sensor control  62 —each being separately mounted printed circuit boards (PCBs). However, the main controller  32  and the sensor controller  62  may also be combined into single controller or mounted on a single PCB. The control system for the range  12  may be generally referred to as the control system  14 —including both the controller  32  and the sensor controller  62 . 
     Although the shape and arrangement of the outlet tube  54  can be varied, in a preferred arrangement the outlet tube  54  includes a portion that has a continuous upward slope from its inlet end  56  to its outlet end  58  such that any condensation from gases flowing therein will drip back into the flue passage  50  and will not collect in the outlet tube  54  which might otherwise block the tube  54 . 
     A gate valve  70  may be provided at the inlet  56  to the outlet tube  54  for controlling the flow of exhaust gas into the outlet tube  54 . The gate valve  70  may be formed of a bi-metal plate provided at the inlet  56  to the tubing  54  where it branches off from the flue passage  50 . The bi-metal operating temperature is designed for a high activation temperature such that only during the self-clean mode will the bi-metal plate open, permitting flue gases to flow through the tubing  54  to the sensor  60 . The bi-metal plate would remain shut at lower temperatures, such as during baking or broiling. In this manner, exposure of the sensor  60  to flue gases is significantly reduced, which in turn prolongs sensor life and performance. 
     As discussed above, the present invention may be practiced by measuring various gas components which result from the combustion of food in an oven cavity. FIG. 3 illustrates measured concentration of a gas component, such as CO or CO 2 , over time. Although different foods will provide different absolute levels gas components as measured by the sensor  60 , and different time frames will be involved for different amounts of spilled food products, a somewhat bell shaped curve of measured gas concentration will occur during the cleaning process. 
     In a first time period A, the measured amount of gas will be fairly negligible as the cooking cavity is heated up to the combustion temperature. If the range is a gas range, and if the concentration of CO is being sensed, there may be an initial spike of CO readings during this time, representing the combustion bi-products of the gas being combusted for heating. This initial spike should be ignored by the controller which can be effected by waiting an initial time period, at least as long as time period A, before beginning any gas readings. 
     During time period B there will be significant readings by the sensor  60  first with each successive reading generally being greater than the prior reading and then, following the peak, with each successive reading generally being less than the prior reading. 
     During time period C the readings will continue to diminish, however the difference between successive readings will become much smaller. Hence, the slope of the curve will diminish until it reaches a very small number, essentially zero. Applicants have determined that this changing slope of the curve can be used to determine the conclusion of the cleaning process. That is, when the downside slope becomes small, this signifies that the gas component, such as CO or CO 2 , is no longer being generated, meaning that combustion is essentially complete. 
     Accurately measuring gas component concentrations resulting from soil combustion in an oven cavity can be difficult to achieve. During the cleaning process in pyrolytic ovens, the combustion of accumulated soils produces various gas components but also moisture, grease-laden air and some amount of particulate matter. Additional moisture is produced in gas ranges as a result of normal combustion. These undesired products—moisture, grease laden air and particulates—can interfere with accurately measuring the gas components also produced from the soil combustion, particularly when using an IR type gas sensor. Moisture has a spectrum adsorption wavelength very close that of CO and CO 2  such that an IR sensor can misread the quantity of CO or CO 2  present. Moreover, grease contamination on reflective surfaces in an IR sensor can decrease the instrument sensitivity. 
     To address this concern regarding undesired products, the present invention includes a filter  64  provided in line with the outlet tube  54 , as best shown in FIG.  4 . Although different filters could be used, an activated carbon filter is preferred. Activated carbon is a very porous material capable of adsorbing water vapor. As the sample gas flow passes through the charcoal pellets in the filter  64 , it is forced to change direction may times causing the water to separate. This redirection also traps the grease and particulate matter before it reaches the gas sensor  60 . FIG. 5 illustrates the improved performance that is achieved through the use of a filter  64 . 
     Turning now to FIG. 6, when the clean cycle is selected by the user, via mode selection  20 C (FIG.  2 ), the control will start the clean cycle as indicated at step  200 . Step  202  designates that a timer is initiated, step  204  designates that a display is turned on to indicate that the clean cycle is in operation, step  206  indicates that the door to the oven is locked and step  208  indicates that a start signal is sent from the main control  36  to the sensor control  62 . This could occur as by sending a high voltage (5 volts) on line  66  from the main control  36  to the sensor control  62 . Each of the steps  202 - 208  can occur relatively simultaneously and in any selected order. The display in step  204  could be as simple as a lit lamp, such as an incandescent bulb, a neon bulb or an LED. Alternatively, an electronic display may be provided which initially could be an icon indicating that the cleaning cycle is in progress, and once the time required for the cleaning cycle is determined, a count down timer could be displayed indicating the time remaining for the cleaning cycle. 
     Step  210  indicates that the sensor control  62  conducts a self check. First, the sensor control  62 , upon receiving the start signal on line  66  from the main control  62 , will return a signal back to the main control on line  68 . Upon conclusion of the successful determination that the sensor is operable, the sensor control will send a signal, to the main control, as indicated in step  212 . Both of these steps should occur relatively quickly and before the timer, which is being monitored in step  225 , reaches a time indicating a missed signal. If both the high and low signals have been sent to the control  36 , control moves to step  214  where one or both heating elements in the oven cavity are energized to raise the temperature in the oven cavity to a cleaning temperature. Also in step  214  the timer will be checked to determine when an initial period, for example one minute, has passed which allows initial start up transients to settle before beginning any readings by the sensor  60 . Once the time has passed, control moves to step  216  where CO levels are checked and compared in accordance with the algorithm set forth below with respect to FIG.  7 . 
     Once that algorithm has been completed, control moves to step  218  to wait for an additional predetermined time, such as 45 minutes, following the sensed completion of the cleaning operation. Then control passes to step  222  wherein the sensor control  62  sends a signal, such as a low voltage (such as zero volts), to the main control  36  on line  68  and, as indicated in step  224 , the main control  36  terminates the cycle by terminating the input of any heat to the cooking cavity and allows for time for the cooking cavity to cool sufficiently before unlocking the oven door. At this point the signal from the main control  36  on line  66  would return to a low voltage (such as zero volts). 
     Between steps  210  and  212 , at step  225 , the main control  36 , after passage of a predetermined time period, may recognize that it has not received a first high signal from the sensor control  62 , indicating that the sensor control is not operational. If this is the case, control will then pass to step  226  wherein once the initial timer has been satisfied (one minute), heat will begin to be applied to the oven cavity through energization of one or both elements, as described above, by the main control to initiate the self-cleaning operation, and a backup algorithm will begin operation, such as the backup algorithm set forth below with respect to FIG. 9, or alternatively, a predetermined time period may be programmed for operating the heating device for the self-cleaning operation. Once the algorithm is completed or time for the self-cleaning operation has passed, control will pass to step  218  to continue as described above, or directly to step  224  to end the cycle. 
     If the predetermined time at step  225  has passed and, although there was an initial signal, such as the high voltage, sent by the sensor control  62  indicating that the sensor control was operational, but no second signal, such as a low voltage, indicating that the sensor  60  itself was operational, control will also pass to step  226  to initiate the self-cleaning operation in accordance with the procedures of step  228  and, upon their completion, control will pass to step  224  to end the self-clean cycle as described above. 
     FIG. 7 illustrates in detail the CO sensing control operation for determining the proper length of time for the self-cleaning step represented as step  216  in FIG.  6 . In step  240 , the CO sensor  60  is read and the value is stored as variable R. In step  241 , the timer is checked to determine whether a maximum time period since the start of the cleaning process in step  200  has passed. If the maximum time has not passed, then control passes to step  242 . If the maximum time for a cleaning operation has passed, it could indicate that the sensor has failed during the cleaning cycle, even though it was initially indicated to be operational, and control will be passed immediately to step  224  to end the cycle. Thus, the maximum time to be checked at step  241  would be a maximum worst case cleaning period. 
     In step  242  the variable SUM is incremented by the value of R. In step  244  there is a check to determine whether the number of readings is equal to some predetermined number of readings. If not, control passes to step  246  where the number of readings is incremented by one and then control passes to step  248  where the control waits a predetermnined interval of time before passing control back to step  240  to take an additional reading. Once the number of readings has reached the predetermined number in step  244 , control passes to step  250  where the sum of the readings is divided by the predetermined number to achieve an average reading which is stored in variable CR as the current reading. In step  252  the prior reading PR is subtracted from the current reading CR and that value is divided by a time interval T since the prior reading and that value is stored as variable S which comprises the slope of the line between the prior reading and the current reading. In step  254 , the slope S is checked to determine whether it is less than a predetermined final slope SF. If the slope S is not yet below the predetermined final slope, then control is passed to step  256  where the prior reading PR is replaced with the current reading CR, the number of readings N is reset to zero and the counter CN is reset to zero. After a time period T has passed, control passes back to step  240  to repeat the above process. 
     If the slope S is determined to be less than the final slope in step  254 , then control passes to step  258  where the counter is incremented by one and then control passes to step  260  where it is determined whether the counter CN exceeds a predetermined total count CT. If the counter has not yet exceeded the total count, then control is passed to step  262  where the prior reading PR is replaced by the current reading CR, the number of readings N is reset to zero and again the time T is passed before control returns to step  240  to repeat the above process. Once the loop passing through step  258  repeats a sufficient number of times without control branching to step  256 , the counter CN will exceed the total count CT in step  260  signifying that the slope has been maintained below the predetermined final slope over a sufficient time period and control with then pass to step  218  for the additional time to pass, as indicated above in connection with the flow chart of FIG.  7  and the method will proceed in accordance with the previous description following step  218 . 
     If the range  10  is a gas range, step  254  will need to be modified slightly due to the fact that the gas burners in the oven cavity will be operated periodically to maintain the cavity at the proper cleaning temperature. As this occurs, there will be a temporary increase in the CO levels which are not indicative of spilled food combusting, as so should be ignored. Therefore, a further counter K could be employed, and only if the slope is greater than the minimum for a consecutive number of readings would control be passed to step  256  to reset counter CN. If K has not reached the minimum number, control would pass to step  258 , even though the slope is (perhaps temporarily) higher than SF. In such a situation, clearly, the maximum permitted value for K would be a number smaller than CT. 
     FIG. 8 illustrates, in somewhat greater detail than FIG. 6, the cleaning time control operation for a self cleaning cycle in accordance with the principles of the present invention. In step  300  the user activates the self-clean cycle, this can be done by operation of the control knob  20  on the console  59 . In step  302  the main controller section  36  sends a signal to the sensor control  62  indicating that the self-clean cycle has begun. In step  304  an inquiry is made to determine whether the sensor control  62  has received the self-clean initiated signal from the main controller section  36 . If the signal has not been received, then control moves to step  306  where a backup algorithm is run to operate the self-clean procedure, such as described with respect to step  228  described above. 
     If the sensor control in step  304  did receive the signal, then control is passed to step  308  where the sensor control  62  performs a self-check and sends an acknowledgment signal back to the main control  36 . In step  310  an inquiry is made to determine whether the main control received the acknowledgment signal. If no acknowledgment signal was received, then control passes to step  306  to run the backup algorithm as described above. 
     If the main control in step  310  did receive acknowledgment, then control is passed to step  312  where the main control begins the maximum and minimum clean count down timers. Control then passes to step  314  where the cleaning is in progress and is detected by the sensor. Periodically an inquiry is made as in step  316  to determine whether the maximum clean count down timer has reached zero. If it has, then control passes to step  318  and the clean cycle is ended. If the count down timer has not reached zero in step  316 , then control is passed to step  320  where it is determined whether the sensor control has sent an end clean signal to the main control. If it has not, then control passes back to step  314  to continue the cleaning and sensing the cleaning step. 
     Once it has been determined in step  320  that the sensor control has sent the end clean signal to the main control, then control passes to step  322  where an inquiry is made to determine whether the main control has received the end clean signal. If it has not, control passes back to step  314  as to continue the cleaning process as described above. Once it has been determined in step  322  that the main control has received the end clean signal, then control passes to step  324  where the main control starts a clean add-on timer count down. This timer is utilized to provide an additional amount of cleaning time even beyond the detection of the end of the cleaning cycle to insure that all materials are combusted and the oven is cleaned. 
     Control then passes to step  326  where the add-on cleaning process is in progress. Periodically an inquiry is made in step  328  as to whether the maximum clean count down timer, which was initiated in step  312 , has reached zero. If it has, then control passes to step  330  to end the cleaning cycle. If the maximum clean count down timer has not reached zero in step  328 , then control passes to step  332  where an inquiry is made to determine whether the clean add on timer has reached zero. If it has not, then control passes back to step  326  to continue the add on cleaning process. 
     Once it has been determined in step  332  that the cleaning add-on timer has reached zero, control is passed to step  334  to inquire whether the minimum clean count down timer has reached zero. If it has not, then control is passed back to step  326  to continue with the add-on cleaning process. This will insure that at least a minimum amount of time for the self-clean process occurs. If the result of the inquiry in step  334  is affirmative, that the minimum clean count down timer has reached zero, then control is passed to step  336  to end the self-clean cycle. 
     As mentioned above, the present invention may be implemented using a gas or electric range. If the invention is practiced using CO measurements, it can be appreciated by one skilled in the art that for an electric range, the heating system does not contribute to the CO level in the cavity during cleaning. During the cleaning process, the CO level rises as combustion of spill material begins and falls off as the combustion is completed. The same results occur in a gas range, however with varying absolute levels of CO due to the gas burner contribution to the CO level. Gas ranges will show a characteristic rise of CO level at initiation of burner combustion (as the heating source for the cavity) as is seen during self-clean combustion. However, most gas burner ovens are designed such that in the cavity, this peak is reached well before the characteristic self-clean increase begins, that is, the cavity must receive significant energy from the burner before combustion of spilled material begins. 
     As indicated above with respect to FIG.  6  and FIG. 8, in step  228  or  306 , respectively, a backup algorithm may need to be employed in the event that the sensor control  62  or sensor  60  itself are not functional or not sending appropriate signals. In such case, a predetermined time may be selected for operation of the self-cleaning cycle in which only a timer need be employed. Alternatively, and still to gain some benefit from reduced energy usage based upon actual need for properly cleaning, an alternative algorithm which does not require the use of a gas sensor may be utilized. 
     FIG. 9 illustrates an alternative or back-up algorithm that may be used to control the time for a self-cleaning cycle. In such an algorithm, a first counter  72  (FIG. 2) counts or measures the actual run times for the broil and bake operations since the last self clean operation. Alternatively, the counter may count the number of bake cycles and broil cycles which have occurred since the last clean cycle. The counter  72  may be associated with the control  36  for measuring the time the oven has been operated since the previous self clean cycle or the counter  72  may be associated with the control selection knobs  20 ,  22  to count the number of times and/or duration of the bake or broil modes since the previous self cleaning cycle. A second counter or timer  74  is used to determine the length of time, in days or weeks, since the last cleaning cycle. 
     Once the backup algorithm is selected at step  228 , control passes to step  280  where the total oven operation time since the last self cleaning cycle is retrieved from the first counter  72 . The total oven operation time since the last self cleaning cycle may be expressed in minutes or hours. Alternatively, the number of baking cycles, or total baking times, may be retrieved and the number of broiling cycles, or total broiling times, may be retrieved. In step  284 , the total time since the last clean cycle is retrieved from the second counter  74 . The total time since the last self cleaning cycle may be expressed in days or weeks. The control  36  then references a lookup table, as shown in step  286 , to determine the oven clean time which corresponds to the measured oven operation duration and total time since the last oven cleaning. In step  292  a timer is initiated to operate the cleaning cycle for the selected oven clean time and, once the selected time has passed, control passes to step  224  to end the cycle. At the end of such self cleaning cycle, the oven operation duration counter  72  and the total time since last cleaning cycle counter  74  would be reset to zero. 
     FIG. 10 graphically illustrates values that could be placed into a look up table which is checked in step  286  as described above. This graph extends in three dimensions and along two perpendicular horizontal axes lists the hours of total use since the last cleaning cycle and the number of weeks representing a period of time since the last cleaning cycle has occurred. The vertical axis represents a period of time for the self-clean cycle which are values that would be experimentally determined for each particular type of oven cavity. Shown suspended in the graph is a surface  294  that extends horizontally but also is angled vertically starting from a low point at the leftmost corner  295 , representing the lowest number of hours of use and fewest number of weeks since the last cleaning and a high point at the rightmost corner  296  representing the highest number of hours of use and greatest number of weeks since the last cleaning. This surface  294  can be divided into grid pieces  297  for particular numerical values being the average of the position of each grid piece, or it can be divided into large segments  298 , such as the three illustrated, representing a quantity of time x or a multiple of that quantity. Thus, the control can either provide finally divided time differences for the cleaning cycle based upon the value of each grid piece  297  or could provide fewer different cycle times based upon the larger segments  299 . These values could be stored in a look-up table for the control to check in step  290 . 
     As is apparent from the foregoing specification, the invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. It should be understood that we wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of our contribution to the art.