Abstract:
An oven temperature controller for regulating temperature to a temperature setpoint over a temperature range in an oven compartment comprising a microprocessor and memory storage means. The memory storage means stores temperature setpoint related information for temperature setpoints over the temperature range and this information is used by the microprocessor for establishing an upper control limit curve and a lower control limit curve. The upper control limit curve and lower control limit curve have no discontinuities over a majority of the temperature range and at least one of these curves is non-parallel to a temperature setpoint line, the temperature setpoint line formed in a stepwise linear manner between temperature setpoints as the temperature setpoint is varied over the temperature range.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/097,165, filed Aug. 19, 1998. 
    
    
     FIELD OF THE INVENTION 
     The instant invention relates to oven temperature control, and more particularly to electronic controllers and temperature sensors for controlling same. 
     BACKGROUND OF THE INVENTION 
     The maintenance of a consistent cooking temperature has long been a problem which has plagued appliance engineers, gourmet chiefs, and homemakers alike. The variation of temperature within the oven cavity from the center to the walls, as well as the variation of temperature over time during a cooking cycle may result in inconsistent cooking behavior. As an example, cake which should be cooked at 350° F. for 20 to 25 minutes may be nearly burned one time at 18 minutes, and may still be wet after 28 minutes another time. This problem is a result of many factors including the size of the oven cavity, the placement of the temperature sensor within the cavity, the type of heat source (gas versus electric), the amount of insulation used in the construction of the oven, convection currents within the cavity, as well as the sensor characteristics themselves. 
     Recognizing that some of these factors are beyond the appliance engineer&#39;s control, efforts were made to design a mechanism of temperature sensing and burner control which would minimize temperature variation within the cavity due to controller induced changes. In the past, oven temperature controllers utilized electromechanical controls which included at least one hot thermostat within the cavity which controlled the relays or solenoids which supplied the fuel (gas or electric) to adjust oven temperature. This control was a simple ON/OFF type control which operated the burners to maintain the sensed temperature within a hysteresis band defined, in large part, by the hysteresis of the temperature sensing element itself. Such a temperature control band is illustrated in FIG.  6 . 
     As may be seen with reference to FIG. 6, the set point line  10  indicates the desired temperature as set by the user. However, because the temperature sensor and control included hysteresis, the oven temperature control would actually not turn the burners off until the temperature had risen beyond the set point by a given hysteresis amount as illustrated by the line  12 . Once the burner control had turned off the burners, the interior temperature within the oven begin to fall. Unfortunately, due once again to hysteresis of the temperature sensing and control circuitry, the interior temperature would be allowed to fall beyond the set point  10  to a point along the line  14 . Once the temperature had fallen below line  14 , the burners would again be turned on and the temperature would begin to rise. This temperature rise would continue until line  12  was reached, and the cycle would continue. The temperature hysteresis band of these early oven temperature controls was typically as wide as 20° F., and was fairly constant for all temperature settings. 
     As electronic controls were introduced to appliance design, the operating characteristics of the electromechanical temperature control, including the hysteresis band, were emulated within the electronic controller. As with their electromechanical counterparts, a linear hysteresis band of approximately 20 F. was used throughout the set point band defined by line  10  of FIG.  6 . Unfortunately, utilizing a linear hysteresis band results in a large percent error at lower cooking temperatures, e.g. a 20  F. band at the 170° F. setting equates to a percent effort of +/−6%, while at the setting of 550° F. it equates to only a +/−2% error. 
     Recognizing this large disparity in the percentage error resulting from emulating the electromechanical sensors of the past, the next generation of electronic oven temperature controllers utilized a stepped turn on hysteresis limit  16  as illustrated in FIG.  7 . This stepped lower limit  16  allowed for the percent error allowed over the entire cooking cycle to be lowered to a more acceptable level. These next generation electronic controllers utilized three (3) to four (4) discrete lower limits as illustrated by line segments,  16 A,  16 B, and  16 C, resulting in three to four discrete hysteresis bands. Typically, these bands were set to 5° F., 10° F., and 15° F. for a three zone implementation, and to 5° F., 10° F., 15° F., and 20° F. for a four zone implementation. These discrete hysteresis zones greatly improved the cooking performance of the ovens in which these controllers were installed, especially when cooking delicate foods such as pastries, etc. 
     However, the non-linear nature of this lower hysteresis limit has also resulted in cooking control problems. Specifically, since a discontinuity exists between different cooking zones (e.g. defined by the upper hysteresis limit  12  and the first segment  16 A, the upper limit  12  and the second segment  16 B, and the upper limit  12  and the third segment  16 C), inconsistent cooking performance was observed when the oven was set at a temperature near the end point of two zones. This inconsistent cooking performance is a result of the controller oscillating between the two adjacent control zones of lower limit  16 . Attempts to stabilize this problem through software coding have met with limited success due to the limited code space available and the cost restraints imposed by the highly competitive appliance industry. As a result, this problem remains. 
     In addition to this problem, these next generation electronic controllers also suffer from a similar problem relating to initial turn on of the oven. When the oven is first turned on and a temperature is set by the user, the cavity temperature begins to climb. It is known in the oven art that the oven temperature will continue to climb once the burners are turned off during this initial pre-heat phase as illustrated by temperature curve  20  of FIG.  9 . Because of this effect, the controller utilizes a separate preheat turn off limit as illustrated in FIG. 8 as line  18   P  or  18   NP . The position of this preheat turn off limit  18   P  or  18   NP  in relation to the normal control hysteresis limits  12 ,  16  shown in FIG. 7 varies depending on many factors, including whether the oven is a pyro type (see line  18   P ) or a non-pyro type (see line  18   NP ). 
     Because a pyro type oven includes a self-cleaning cycle which raises the interior temperature to approximately 900° F., it includes much more insulation than a non-pyro type oven which does not include a self cleaning cycle. Because of this increased insulation, the pre-heat turn off limit  18   P  is typically lower than the pre-heat turn off limit  18   NP  in a non-pyro oven, and may be below the steady state burner turn on limit  16  of FIG.  7 . In a non-pyro type oven, the pre-heat turn off limit  18   NP  may actually be above the steady state turn off limit  12  shown in FIG. 7 to allow for the increased need to heat the walls of the oven (which contain relatively little insulation compared to a pyro type oven). In any event, the pre-heat turn off limit  18   P  or  18   NP  is set to minimize temperature overshoot and maximize temperature settling time within the steady state temperature control band  12 ,  16  of FIG.  7 . 
     However, the non-linear nature of this pre-heat turn off limit  18   P  or  18   NP  has also resulted in cooking control problems. Specifically, since a discontinuity exists between different cooking zones (e.g. in FIG. 7 defined by the upper hysteresis limit  12  and the first segment  16 A, the upper limit  12  and the second segment  16 B, and the upper limit  12  and the third segment  16 C), the pre-heat limit  18   P  or  18   NP  was also discontinuous. These discontinuities also resulted in inconsistent pre-heating performance when the oven was first turned on and set at a temperature near the end point of two zones. As with the above, this inconsistent pre-heating performance is a result of the controller oscillating between the two adjacent pre-heat zones of lower limit  18   P  or  18   NP . Attempts to stabilize this problem through software coding have met with limited success, also due to the limited code space available and the cost restraints imposed by the highly competitive appliance industry. As a result, this problem also remains. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to overcome these and other problems existing in the art. More particularly, it is an object of the instant invention to provide a new and improved electronic controller for oven temperature control which overcomes the above and other problems existing in the art. Specifically, it is an object of the instant invention to provide an electronic oven temperature controller which minimizes the software coding and expense for temperature control while increasing the performance and consistency of the temperature control. It is a further object of the instant invention to allow for individual oven characterization of the temperature control limits. It is an additional object of the instant invention to allow for re-characterization of the temperature control limits, both in absolute value and in shape. It is a further object of the instant invention to allow for re-programming of the temperature control limits. 
     These and other aims, objectives, and features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagrammatic illustration of a preferred embodiment of an electronic oven temperature controller of the instant invention; 
     FIG. 2 is a tabular representation of a memory storage table utilized in a preferred embodiment of the instant invention; 
     FIG. 3 is a graphical illustration of exemplary control curves appropriate for use in an embodiment of the instant invention; 
     FIG. 4 is a graphical illustration of additional exemplary control curves appropriate for use in an embodiment of the instant invention; 
     FIG. 5 is a tabular representation of an alternate memory storage table utilized in a preferred embodiment of the instant invention; 
     FIG. 6 is a graphic representation of control curves typical for electromechanical and prior electronic oven controllers; 
     FIG. 7 is a graphical representation of control curves utilized by next generation electronic controllers; 
     FIG. 8 is a graphical representation of exemplary preheat control curves for pyro and non-pyro type ovens; and 
     FIG. 9 is a graphical representation of oven temperature during initial preheat representing the necessity for a separate preheat control curve. 
     While the invention is susceptible of various modifications and alternative constructions, certain illustrative embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the invention as defined by the appended claims. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the instant invention is illustrated in block diagrammatic form in FIG.  1 . As may be seen from this Figure, the oven temperature controller of the instant invention comprises an electronic temperature controller  22  which receives as an input the set point  24  as selected by a user. The input circuitry to the set point  24  may be a standard potentiometer type temperature selector whose analog input is converted to digital form for processing by the electronic temperature controller  22  by analog to digital circuitry (not shown). Additionally, the set point temperature input  24  may be programmed digitally by a touch pad of other appropriate means as will be recognized by one skilled in the art. The electronic temperature controller  22  may be a standard industrial 4 or 8 bit microprocessor, although more powerful microprocessor may also be utilized, bounded mainly by cost considerations in the highly competitive appliance industry. 
     The output  26  from the electronic temperature controller  22  is preferably converted from digital to analog form by the digital to analog converter  28 . The converted burner control signal on line  20  is used to control the burner control driver  32  to turn the burner on or off. As will be recognized by one skilled in the art, the burner control driver  32  may be of any conventional type appropriate for driving the particular technology burner on and off. For example, the burner control driver  32  may be an electronic relay driver control circuit which, in a conventional fashion, closes and trips an electrical control relay for supplying electric power to the burner elements in an electric range. As will also be recognized by one skilled in the art, the burner control driver  32  may also be a solenoid driver which opens and closes a gas supply valve to allow gaseous fuel to flow in a gas range. Additionally, one skilled in the art will recognize that, depending on the burner control driver  32  input circuitry requirements, the digital to analog converter  28  may not be needed. Indeed, the particular use of the output burner control signal  26  is not dependent on, nor should be limited by the particular technology which ultimately utilizes this signal to control the burner element. 
     A preferred embodiment of the instant invention also includes a temperature sensor  34  which is placed preferably within the oven cavity to sense the interior temperature thereof. This temperature sensor may be of any known technology, so long as its sensing accuracy is within the ultimate control limits demanded by a customer specification. The output  36  of temperature sensor  34  may preferably be converted to digital form by analog to digital converter  38  for use by the electronic temperature controller  22 . However, depending on the particular implementation of the electronic temperature controller  22 , the necessity of having analog to digital converter  38  may be removed. In the preferred embodiment of the instant invention, however, the output  40  of analog to digital converter  38  is used by the electronic temperature controller  32  as a temperature feedback for use in its temperature control algorithm. One skilled in the art will recognize that the electronic temperature controller  22  may implement various control architectures including proportional, proportional integral (PI), proportional integral derivative (PID), etc. depending on the desired complexity and response of the system. However, because the temperature variation within the oven cavity of a typical oven varies at a relatively slow rate, a simple proportional feedback control loop may be implemented by the electronic temperature controller  22 . 
     Also as may be seen from this Figure, the electronic temperature controller  22  preferably utilizes a memory table  42  to provide control parameter inputs for use in controlling the oven temperature. While any type memory device may be utilized as element  42 , a preferred embodiment of the instant invention utilizes an electrically erasable programmable read-only memory (EEPROM). The use of an EEPROM allows rapid prototyping and control algorithm modification without the need of removing the memory device  42 . However, one skilled in the art will recognize that less expensive memory devices may be utilized in a mass production environment to reduce cost. 
     An exemplary memory storage table is illustrated in tabular structure form in FIG.  2 . As may be seen, the memory storage table  44  would coordinate the set point input  46  with the control parameters for the turn on temperature  48 , the turn off temperature  50 , and the pre-heat turn off temperature  52 . As this Figure illustrates, a pointer  54  would select a particular set point  46  as determined from the user input  24  (See FIG.  1 ). Once selected, the data from the corresponding row would be returned to the electronic temperature controller  22  to be utilized in setting the turn on temperature, the turn off temperature, and in, the case of initial pre-heating, the pre-heating temperature turn off point of the particular user set point. The electronic temperature controller  22  would then use these various control parameters, along with the temperature feedback signal  40  to generate the burner control signal output  26 . By utilizing such a lookup table  44 , the electronic temperature controller  22  can effectuate a piece wise linear upper and lower temperature limit to control the oven temperature. Additionally, since both the control parameters for the turn on and turn off temperature are stored within the table  44 , distinct individual curves for the turn on and turn off limits may be stored. 
     Unlike conventional systems which utilize an upper temperature hysteresis limit which is a straight parallel line offset from the temperature set point, the control system of the instant invention allows both the upper and lower hysteresis limits to be varied as desired. One such possibility is illustrated in graphical form in FIG.  3 . As may be seen, the upper hysteresis limit  56  varies from the temperature set point  58  in a non-parallel fashion. Additionally, the lower hysteresis limit  60  also varies in a stepwise linear, non-parallel fashion from the temperature set point  58 . By utilizing the system of the instant invention, the controller is able to maintain a proportional percent error throughout the entire temperature selection range. 
     Additionally, since the lookup table may contain a vast number of points, the number of which is limited solely by the resolution of the analog to digital converter used to condition the inputs utilized by the electronic temperature controller  22  and by the physical memory size of the memory table  42 , a nearly continuous limit may be obtained. This overcomes the serious problem existing in the prior art when a temperature is selected very near a control limit discontinuity as illustrated in FIG.  7 . This method also allows for control parameter limits to be instituted within the table  44 . Specifically, and with reference to FIG. 4, the upper control limit  56  may include a region  62  which will turn off the burner regardless of increasing temperature set point. For example, an upper limit safety region may be instituted at 570° F. Likewise, a lower temperature safety limit, as illustrated by region  64  on lower temperature limit  60  may also be instituted. For example, a minimum temperature allowance of 170° may be instituted to prohibit the growth of bacteria, regardless of further decreases of the temperature set point  58 . 
     The curves illustrated in FIG. 4 also illustrate the flexibility of the system of the instant invention in determining acceptable temperature bands over the selector temperature range. Specifically, FIG. 8 illustrates that the upper temperature turn off limit  56  and the lower temperature turn on limit  60 , in addition to including temperature safety regions  62 ,  64 , also provide a region of very small variance from the temperature set point  58 . This type of controller temperature hysteresis may be useful over the temperature range utilized in baking delicate foods such as pastries, etc., where larger temperature variations may damage or otherwise ruin the dish. 
     An alternate embodiment of the table  44  is illustrated in FIG.  5 . In this particular embodiment, which utilizes less memory than the embodiment illustrated in FIG. 2, only the upper temperature turn off limit points are stored in column  50 . Instead of storing the actual temperature turn on limit data and the pre-heat temperature turn off data in the table, an amplitude column  66  an a pre-heat offset column  68  are utilized. In this way, the electronic temperature controller receives values for the temperature turn off point, an amplitude and pre-heat offset value which must be combined with the temperature turn off point to determine the lower temperature turn on limit and the pre-heat offset limit. An exemplary set of values are included in FIG. 5 for the pointer  54  designating the set point temperature of 500° F. The numbers illustrated in the table  44  in the row corresponding to a set point temperature of 500° F. are the analog to digital numbers to be used by the electronic temperature controller  22  with the temperature feedback analog to digital converted signal  40 . As illustrated, the T off  value stored in table  44  is 225. The T on  value would then be calculated by subtracting the number stored in column  66  from the number stored in column  50 , i.e. 225−0.7=218. The value stored in the pre-heat offset column  68  is the analog to digital bit which is multiplied by a constant and subtracted from the value stored in the T off  column  50  to arrive at the pre-heat temperature turn off limit. As illustrated in this Figure, the calculation yields 225−(03×5)=210. A preferred embodiment of the instant invention utilizes temperature set points in column  46  which vary by 5° F. steps, and which may be read to approximately 1° F. accuracy. 
     The values stored in lookup table  44  may be individually measured during a calibration cycle, or may be interpolated between measured points on the oven sensor curve. These measured points can be adjusted at calibration to reflect the individual oven characteristics and, through the use of an EEPROM, reflected into the individual lookup table  44  for that particular oven. This results in a nearly continuous, smoothed representation of the oven characteristics for enhanced temperature control avoiding the large discontinuities of the prior control system. Additionally, both the upper temperature and lower temperature control limits may be varied in any fashion desired to allow accommodation of customer requirements throughout the entire cooking temperature range. 
     An alternate embodiment of the instant invention utilizes online calculation of the upper and lower temperature control points for any selected temperature set point. In this embodiment, a lookup table of discreet points along the curve is not required, but is replaced by the storage of the equations defining the upper and lower control curves. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode for carrying out the invention. Details of the structure may be varied substantially without departing from the spirit of the invention, and exclusive use of all modifications that come within the scope of the appended claims is reserved.