A nominal closed-loop temperature control system and method for a self-cleaning oven is adapted, upon recalibration of the bake temperature of the oven, to operate in an open-loop manner during the CLEAN mode of operation of the oven, Specifically, prior to recalibration of the oven, an average value of the duty cycle of the oven heating elements used during a self-cleaning operation is measured and stored. Subsequently, upon a positive recalibration or increase in the nominal values of the bake temperatures of the oven, the duty cycle of the heating elements during a self-cleaning operation is slightly increased to increase the average value of the self-cleaning temperature of the oven. Correspondingly, upon a negative recalibration or decrease in the nominal values of the bake temperatures, the duty cycle of the heating elements during a self-cleaning operation is slightly decreased to decrease the average value of the self-cleaning temperature of the oven. For safety and performance reasons, any such temperature increase or decrease occurs in an open-loop manner by being limited to a value less than that capable of being detected by the closed-loop operation of the oven temperature control system.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
The present invention generally relates to temperature control systems and 
methods, and, more particularly, to a new and improved open-loop 
self-cleaning oven temperature control system and method. 
B. Description of the Prior Art 
Self cleaning ovens and temperature controls therefor are old and well 
known in the prior art as exemplified by U.S. Pat. Nos. 3,121,158; 
3,122,626; 3,310,654; 3,327,094; 3,353,004; 3,569,670; 3,648,012; 
3,738,174; 3,924,101; 4,166,268; 4,214,224; and 4,369,352. Conventionally, 
the bake temperature controls for many prior art self-cleaning ovens are 
capable of being recalibrated in service to compensate for oven components 
that deviate from design specifications or to accommodate individual user 
preferences. See, for example, the above-identified '670 patent and the 
'101 patent and the '352 patent. Some prior art temperature control 
systems for self-cleaning ovens are designed to maintain a constant clean 
temperature even though the bake temperatures have been recalibrated and 
offset by a predetermined amount from nominal values. Recalibration of the 
bake temperatures in other prior art systems necessarily affect the clean 
temperature. 
With the advent of digital electronic microprocessor based control systems 
for controlling the temperature in self-cleaning ovens, specific operating 
characteristics are inherent in a particular design of the control system. 
For example, in a specific prior art electric range commercially made and 
sold by the assignee of the present invention, the digital electronic 
microprocessor based closed-loop control circuit for controlling the 
temperature of a self-cleaning oven of the range provides a much finer or 
greater level of temperature control resolution in the BAKE mode than in 
the CLEAN mode. Therefore, a particular self-cleaning oven that runs too 
hot or too cool due, for example, to a faulty oven temperature sensor, 
cannot easily have its clean temperature adjusted during recalibration of 
the bake temperatures. A need therefore exists to enable the clean 
temperature of a self-cleaning oven to be adjusted when the bake 
temperatures of the oven are recalibrated, while limiting the change 
produced in the clean temperature to a value smaller than that capable of 
being detected by the closed-loop temperature control system. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a new and improved 
self-cleaning oven temperature control system and method. 
Another object of the present invention is to provide a new and improved 
temperature control system and method for a self-cleaning oven in which 
the clean temperature may be adjusted and maintained in an open-loop 
manner after the bake temperatures of the oven have been recalibrated, the 
adjustment of the clean temperature being less than that capable of being 
detected by the nominal closed-loop operation of the control system. 
Briefly, the present invention constitutes a new and improved digital 
electronic microprocessor based self-cleaning oven temperature control 
system and method. The temperature control system and method operate in a 
conventional closed-loop manner to control the amount of heat energy 
supplied to the oven by the electrically energized oven heating elements. 
In this manner, the oven is heated until the desired or preselected oven 
temperature is reached. Thereafter, energy to the heating elements may be 
removed until the oven temperature falls below a predetermined value, at 
which time energy is resupplied to the heating elements to maintain the 
temperature of the oven at the desired or preselected value. Because the 
one or more oven heating elements have fixed resistances that are 
energized by a constant voltage source of alternating current electrical 
power, the temperature of the oven is controlled by controlling the duty 
cycle of the heating elements, i.e., the percentage of time the heating 
elements are ON or energized during the operation of the oven. 
In accordance with an important feature of the present invention, upon 
recalibration of the bake temperatures of the oven, the oven temperature 
control system and method are capable of operating in an open-loop manner 
during the CLEAN mode of operation of the oven to compensate for 
degradation in the performance of one or more oven components, such as a 
conventional oven temperature sensor. For example, if the bake 
temperatures have been increased during a recalibration operation, then 
during the CLEAN mode of operation of the oven, the heating elements of 
the oven are controlled in an openloop manner by slightly increasing their 
duty cycle, thereby increasing the average temperature of the oven during 
the CLEAN mode of operation. Correspondingly, if during a recalibration 
operation, the bake temperatures are decreased, then during the CLEAN mode 
of operation of the oven, the heating elements of the oven are controlled 
in an open-loop manner by slightly decreasing their duty cycle, thereby 
decreasing the average temperature of the oven during the CLEAN mode of 
operation. In either event, the temperature of the oven is continually 
sensed to ensure that the closed-loop nominal or rereference clean 
temperature is not exceeded in the positive or negative directions by more 
than the above-mentioned open-loop offset value. In this manner, the clean 
temperature may be adjusted in accordance with the recalibration of the 
bake temperatures and independently of the resolution of the 
microprocessor based closed-loop temperature control system and method.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing and specifically to FIGS. 1-3 thereof, therein is 
illustrated an electric range 10 having a self-cleaning oven 12 adapted to 
be controlled by a new and improved digital electronic microprocessor 
based control system 14 and method in accordance with the principles of 
the present invention. The range 10 includes a plurality of four control 
knobs 16 for respectively controlling a plurality of four conventional 
electric burners 18. In addition, the range 10 includes a control knob 20 
for controlling the mode of operation of the oven 12, for example, the OFF 
mode, the BAKE mode, the BROIL mode and the CLEAN mode of operation. In 
addition, the range 10 includes a control knob 22 to enable the desired 
oven temperature to be selected by the user of the oven 12. Disposed 
within a cavity 24 of the oven 12 are a conventional broiling element 26 
and a conventional heating element 28. Finally, suitably positioned within 
the cavity 24 of the oven 12 is a conventional temperature sensor 30, for 
example, a standard oven temperature sensing probe. 
The digital electronic control system 14 includes a conventional 
microprocessor 32 capable of being suitably programmed to effect the 
desired control of the range 10 and, more particularly with respect to the 
present invention, the oven 12. 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. Conventionally, the microprocessor 32 
includes a memory 38 for retaining the programmed instructions for 
operating the control system 14 including a desired oven temperature 
control algorithm for controlling the temperature of the oven 12. 
The control system 14 further includes an offset signal circuit 40 for 
providing a desired temperature offset signal to the controller 36 of the 
microprocessor 32 during a recalibration operation. For example, the 
offset signal circuit 40 conventionally could take the form of three 
digital input signals to the controller 36. The three digital input 
signals may be used to enable a recalibration of the bake temperature in 
three 7.degree. F. steps for a maximum bake temperature offset during 
recalibration of .+-.21.degree. F. Specifically, a first one of the three 
digital input signals may be used to indicate a desired positive bake 
temperature offset when, for example, that input signal is low and a 
desired negative bake temperature offset when, for example, that digital 
input signal is high. A second one of the three digital input signals may 
be used to indicate an offset of the bake temperatures of 7.degree. F. 
when, for example, that input signal is high; and the third input signal 
may be used to indicate a desired bake temperature offset of 14.degree. F. 
when, for example, that input signal is high. 
The control system 14 also includes a power switching relay 42 that 
includes a pair of relay contacts 44 and 46 for switching power to the 
heating element 28 from a constant voltage (e.g., 240 volts) source 48 of 
alternating current electric power, under the control of the controller 
36. For simplification, only the heating element 28 and the power relay 42 
therefor have been illustrated in FIG. 2 in the control system 14. In an 
actual commercial embodiment, however, the broiling element 26 would 
obviously also be part of the control system 14 along with its own power 
switching relay to interconnect the broiling element 26 to the source 48 
under the control of the controller 36. The broiling element 26 would 
obviously be used in conjunction with the heating element 28 during the 
BROIL mode of operation of the oven 12 and may also be used during the 
CLEAN and BAKE modes of operation of the oven 12 to provide sufficient 
heat to the oven 12 under the control of the controller 36. 
During the BAKE mode of operation, the heating element 28 is energized by 
the source 48 through the relay 42 under the control of the controller 36 
to heat and raise the temperature of items to be cooked within the oven 
cavity 24 of the oven 12. The sensor 30, typically disposed within the 
oven cavity 24, is used to provide an output analog voltage signal as an 
input to the A/D converter 34. That analog input signal is converted to a 
digital output signal and is supplied to the memory 38 and the controller 
36 for controlling the ON-OFF state of the relay 42 and, thereby, the 
energization of the heating element 28. 
As is conventional, a user of the range 10 selects by means of the control 
knob 20 the desired mode of operation of the oven 12, which mode selection 
is provided as an input signal to the microprocessor 32 by a conventional 
mode selection circuit 20c. If the BAKE mode of operation of the oven 12 
has been selected, the user also selects a desired bake temperature by 
means of the control knob 22, which desired temperature is also provided 
as an input signal to the microprocessor 32 by a conventional desired 
temperature circuit 22c. The microprocessor 32 then, through the 
controller 36, controls the state of the power relay 42 to energize or 
deenergize the heating element 28 as a function of the actual oven 
temperature as sensed by the sensor 30 and of the desired temperature as 
provided by the desired temperature circuit 22c. The broiling element 26 
may be similarly controlled to provide additional heat during the BAKE 
mode. 
As a result of a recalibration operation, one or the other of the above 
temperature signals may be modified by a temperature offset signal from 
the offset signal circuit 40, the adjusted temperature signal being 
compared to the other temperature signal by a conventional preprogrammed 
temperature control algorithm to control, through the controller 36, the 
duty cycle of the heating element 28, that is, the percentage of time that 
the heating element 28 is ON or energized during a particular oven 
operation. 
During the BROIL mode of operation, the broiling element 26 is energized or 
deenergized through an associated power relay under the control of the 
controller 36 of the microprocessor 32. If the CLEAN mode of operation is 
selected, the energization of one or both of the oven heating elements 26 
and 28 occurs under the control of the microprocessor 32 to maintain the 
temperature of the oven 12 at a relatively high level, for example, 
870.degree. F. As depicted in simplified form in FIG. 3, during a "Nominal 
Cleaning Cycle", the heating elements 26, 28, assuming both are used, are 
cycled ON and OFF under the control of the microprocessor 32 to maintain 
the temperature of the oven 12, as detected by the sensor 30, at the 
relatively high clean temperature. 
A prior art commercial embodiment of the range 10 manufactured and sold by 
the assignee of the present invention uses an A/D converter 34 having a 
relatively broad output temperature signal resolution in the CLEAN mode as 
compared to its resolution in the BAKE mode, the output signal of the A/D 
converter 34 being incremented in the CLEAN mode one count only after the 
oven cavity 24 has changed temperature by more than, for example, 
10.degree. F. Temperature differences in the CLEAN mode less than that 
amount (10.degree. F.) could not be read by that conventional closed-loop 
control system, which operated essentially the same as the description 
heretofore of the control system 14. 
The duty cycle of the heating elements 26, 28 controlled by the control 
system 14 acting in a conventional or nominal closed-loop manner is 
depicted in simplified form in the waveform in FIG. 3 labeled "Nominal 
Cleaning Cycle". As would be understood by those of ordinary skill in this 
art, the waveforms shown in FIG. 3 may represent directly the ON and OFF 
states of the heating elements 26, 28 or may represent the envelope of 
heating element activation within which the actual ON and OFF states of 
the heating elements 26, 28 may vary according to a preprogrammed timed 
duty cycling scheme. In the latter case, a FIG. 3 waveform high level 
indicates that timed duty cycling is occurring, while a low level 
indicates that the heating elements 26, 28 are not cycling, i.e., are in 
the OFF state, and hence are not providing heat to the oven cavity 24. 
As depicted in the above-mentioned waveform, the heating elements 26, 28 
are maintained energized or ON until the microprocessor 32 determines from 
the sensor 30 that the temperature of the oven cavity 24 has exceeded the 
preprogrammed, clean temperature of, for example, 870.degree. F. Upon 
reaching that temperature, as sensed by the sensor 30, the microprocessor 
32 through the controller 36 deenergizes the heating elements 26, 28 by 
switching OFF the power relay 42 and the power relay associated with the 
broiling element 26. As depicted in FIG. 3, the heating elements 26, 28 
are maintained OFF until the output signal of the A/D converter 34 
decrements one count in response to the temperature of the oven cavity 24 
falling below the preprogrammed clean temperature. 
While the closed-loop temperature control system discussed heretofore has 
been found to be effective in commercial prior art ranges, it would be 
desirable to control the clean temperature of self-cleaning ovens more 
accurately because too high a clean temperature may result in excessive 
oven or range surface temperatures, a safety concern, while too low a 
clean temperature may not clean the oven cavity 24 effectively. Therefore, 
a need exists to enable the clean temperature to be changed when the bake 
temperatures of the oven 12 are recalibrated to account for degradation or 
variation in the operation of or the sensitivity of one or more oven 
components, for example, the sensor 30, while limiting the change to the 
clean temperature to a value smaller than that capable of being detected 
by the control system 14 operating in a closed-loop manner. 
In accordance with an important feature of the present invention, upon 
recalibration of the bake temperatures of the oven 12, the control system 
14 adjusts the duty cycle of the heating elements 26, 28 in accordance 
with the nature of the recalibration, enabling the control system 14 to 
operate in an open-loop manner during the self-cleaning mode of operation 
of the oven 12. Specifically, upon recalibration, the microprocessor 32 
adjusts the duty cycle of the heating elements 26, 28 involved in heatin 
the oven cavity 24 in the CLEAN mode of operation in accordance with the 
middle and lower waveforms of FIG. 3 respectively labeled "Hi-Temp 
Recalibrated Clean Cycle" and "Lo-Temp Recalibrated Clean Cycle". More 
particularly, prior to open-loop operation, the microprocessor 32 measures 
and stores in the memory 38 a predetermined number of the values of the ON 
times and intervening OFF times of the heating elements 26, 28 utilized in 
a nonrecalibrated self-cleaning operation. After measuring a predetermined 
successive number of such cycle times, an average value of the duty cycle, 
i.e., a percentage, is determined. Such an average value may be 
continuously updated, prior to recalibration of the bake temperatures, by 
storing an immediately preceding predetermined number of such ON and 
intervening OFF values, the average values of which are continuously 
updated and the resultant average duty cycle stored by the microprocessor 
32 in a non-volatile portion of the memory 38 which retains such ON and 
OFF average values should power to the microprocessor 32 be discontinued. 
Thereafter, upon recalibration of the bake temperatures of the oven 12, 
the duty cycle is adjusted. The use of non-volatile memory may be 
precluded by determining such ON and OFF values at the beginning of each 
CLEAN mode. In such a case, nonrecalibrated closed-loop operation is 
maintained only until a predetermined number of ON and OFF values have 
been compiled. Open-loop operation is then initiated to alter the average 
value of the temperature of the oven cavity 24 during the CLEAN mode of 
operation. Specifically, after recalibration, the controller 36 detects 
the temperature offset signal from the offset signal circuit 40. If the 
temperature offset signal is positive, representative, for example, of an 
increase in the baking temperatures of +7.degree. F. or +14.degree. F. or 
+21.degree. F., then the duty cycle of the heating elements 26, 28 (and of 
their associated power relays) is increased slightly by an empirically 
determined amount as depicted in the curve of FIG. 3 labeled "Hi-Temp 
Recalibrated Clean Cycle". Such an increase preferably is determined 
empirically because most different models of electric ranges have their 
own individual temperature characteristics due to the use of different 
insulation systems, different oven cavity 24 configurations, different 
heating elements 26, 28 and different sensors 30. Because the temperature 
of the oven cavity 24 is directly related to the product of the power 
provided by the heating elements 26, 28 and their duty cycle, slightly 
increasing the duty cycle of the heating elements 26, 28 during the CLEAN 
mode of operation results in an increase in the average temperature of the 
oven cavity 24 during a self-cleaning operation. Preferably, for safety 
reasons, such an increase in the average temperature would be limited to a 
value less than that detectable by the closed-loop operation of the 
control system 14, i.e., in the above example, less than 10.degree. F., 
the resolution of the A/D converter 34 in the CLEAN mode of operation. 
Such an average temperature increase enables the control system 14 to 
operate in an open-loop manner to account for variations in the operation 
of one or more oven components, for example, the temperature sensor 30, 
while limiting the increase in the self-cleaning temperature to a value 
smaller than that capable of being detected by the closed-loop operation 
of the control system 14. 
Correspondingly, if an offset temperature signal from the offset signal 
circuit 40 is negative, representative, for example, of a decrease in the 
nominal values of the bake temperatures of -7.degree. F. or -14.degree. F. 
or -21.degree. F., then the duty cycle of the heating elements 26, 28 and 
of their associated power relays is decreased by a second empirically 
determined amount thereby decreasing the average temperature of the oven 
cavity 24 during a self-cleaning operation. The decrease in the duty cycle 
of the heating elements 26, 28 is illustrated in the curve of FIG. 3 
labeled "Lo-Temp Recalibrated Clean Cycle". In order not to seriously 
affect the self-cleaning performance of the oven 12, the decrease in the 
average temperature preferably is also limited to a value less than that 
capable of being detected by the closed-loop operation of the control 
system 14. In the above example, such a temperature decrease would 
therefore be less than 10.degree. F. In any event, the output signal from 
the sensor 30 as provided to the A/D converter 34 is continuously 
monitored by the microprocessor 32 to ensure that the nominal, closed-loop 
reference self-cleaning temperature is not departed from by more than the 
open-loop clean temperature offset value obtained by increasing or 
decreasing the duty cycle of the heating elements 26, 28 as discussed 
hereinabove. 
In this manner, the self-cleaning temperature of the oven 12 may be changed 
in accordance with a recalibration of the bake temperatures to account for 
variations in the operation of one or more oven components, for example, 
the sensor 30, while limiting for safety and performance reasons, any such 
change to values smaller than those capable of being detected by the 
nominal closed-loop operation of the control system 14. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. While for the sake of 
simplicity, the controls 16, 20 and 22 are depicted in FIG. 1 as rotatable 
control knobs, those controls may obviously assume other conventional 
forms, such as touch sensitive electronic switches or switch panels. Thus, 
it is to be understood that, within the scope of the appended claims, the 
invention may be practiced otherwise than as specifically described 
hereinabove.