Temperature controller for cooking appliance

In accordance with the present invention a cooking appliance 10 is disclosed comprising heating element 14 in thermal communication with a cooking medium 12 for cooking food, temperature sensing element 18 for detecting a cooking medium temperature and control 20 connected to the temperature sensing element 18 for receiving the cooking medium temperature, wherein the control 20 is operable to generate a working temperature which may differ from the cooking medium temperature in order to account for a thermal transit time between the heating element 14 and the temperature sensing element 18, and further operable for regulating the heating element 14 in accordance with the working temperature. Other devices, systems and methods are disclosed.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to a cooking appliance and more particularly 
to a cooking appliance including a temperature controller for controlling 
the temperature of the medium used for cooking. 
BACKGROUND OF THE INVENTION 
Heretofore in this field, automatic controllers have been used to regulate 
the temperature of a cooking medium, such as oil or shortening, in a 
cooking appliance, such as a deep fat fryer. Typical controllers are based 
on microprocessors which execute software programs stored in associated 
memory, such as read-only memory (ROM), for example. The microprocessor 
and ROM may be contained on the same integrated circuit, but this is not 
necessary. The microprocessor typically has a plurality of input/output 
(I/O) ports operable to receive data indicative of the state of the fryer 
and/or the cooking medium and further operable to output control signals 
to the fryer. In a typical system, the temperature of the cooking medium 
is sensed by a temperature sensing probe and this temperature is input to 
the control system. The control system then compares this temperature 
reading with the desired cooking medium temperature and outputs a control 
signal which turns on or turns off a burner or electrical heating element. 
The desired cooking medium temperature may be placed into the ROM during 
manufacture of the controller or during use in the field, if provision has 
been made in the design of the controller for programming by the user. 
This desired temperature is normally expressed as a "set point", or target 
temperature. 
Advanced fryers require relatively sophisticated automatic controllers to 
keep the cooking medium at the set point primarily because the temperature 
of the cooking medium is significantly lowered when uncooked food is 
placed into the cooking medium. This effect is particularly problematic 
when frozen food is placed into the cooking medium. The large temperature 
differential between the frozen food and the hot cooking medium enables a 
significant amount of thermal energy to be quickly transferred from the 
cooking medium to the food, lowering the temperature of the cooking medium 
below the set point. The amount of temperature reduction is unpredictable 
and depends on such factors as the quantity of food, the food temperature, 
the effective surface area of the food, the temperature of the basket 
which holds the food, and many other factors. The control system is 
designed to return the cooking medium temperature to the set point as 
rapidly as possible while minimizing the overshoot of the set point. 
A significant problem in prior art cooking device controllers has been the 
large amount of overshoot encountered while attempting to arrive at the 
set point. The problem arises when heat is applied to the fryer to 
compensate for a lowering of the cooking medium temperature caused by the 
uncooked food being placed into the cooking medium. The indicated 
temperature at the temperature sensing probe normally rises a significant 
amount of time after the heat is applied to the cooking medium. This lag 
time is due to the thermal transit time through the cooking medium from 
the heat source to the temperature sensing probe. During heating, this 
inertia in the temperature sensing system causes the indicated temperature 
of the cooking medium to be less than the actual temperature of the 
cooking medium closer to the heating element. Therefore, if the heat is 
removed from the cooking medium once the set point temperature is reached 
at the temperature sensing probe, the temperature lag time results in the 
temperature of the cooking medium at the probe location continuing to rise 
and overshooting the set point. The converse is also true. When heat is 
removed from the cooking medium, its temperature near the heating element 
falls faster than the indicated temperature of the temperature sensing 
probe. The indicated temperature of the cooking medium is therefore higher 
than the actual temperature. The inertia in the temperature sensing system 
will cause the indicated temperature of the cooking medium to continue to 
fall even after heat is reapplied to the system. The indicated temperature 
will not begin to rise again until all of the thermal masses between the 
heating element and the temperature sensing probe are thermally charged. 
A number of different prior art algorithms have been written using 
proportion control in an effort to limit temperature overshoot and still 
achieve a rapid return to the set point under different load conditions. 
These programs have had less than desirable success because of the slow 
response or necessarily poor location of the temperature sensing probe. 
SUMMARY OF THE INVENTION 
It is therefore the object of the present invention to provide a software 
control routine which overcomes most of the problems of the prior art 
cooking device temperature controllers associated with poor temperature 
sensing probe location, slow temperature sensing probe response and 
excessive heat storage characteristics of high capacity, high efficiency 
deep fat fryers. 
In accordance with the present invention a microprocessor controlled deep 
fat fryer is disclosed, comprising a frypot for storing a quantity of 
cooking oil, means for heating the cooking oil in order to cook a food 
product, means for sensing a sensed temperature of the cooking oil and a 
microprocessor controller operable to regulate the means for heating based 
on the sensed temperature and on other data, the regulation accounting for 
a thermal transit time between the means for heating and the means for 
sensing. 
In another form of the invention, a microprocessor controlled deep fat 
fryer is disclosed, comprising a frypot for storing a quantity of cooking 
oil, means for heating the cooking oil in order to cook a food product, 
means for sensing a sensed temperature of the cooking oil and a 
microprocessor controller connected to the means for sensing in order to 
receive the sensed temperature and further connected to the means for 
heating, and operable to generate a working temperature which may differ 
from the sensed temperature in order to account for a thermal transit time 
between the means for heating and the means for sensing, the 
microprocessor controller further operable to regulate the means for 
heating in accordance with the working temperature. 
In another form of the invention, a microprocessor controlled deep fat 
fryer is disclosed, comprising a frypot for storing a quantity of cooking 
oil, means for heating the cooking oil in order to cook a food product, 
means for sensing a sensed temperature of the cooking oil and a 
microprocessor controller connected to the means for sensing in order to 
receive the sensed temperature and further connected to the means for 
heating, and operable to generate a working temperature which may differ 
from the sensed temperature in order to account for a thermal transit time 
between the means for heating and the means for sensing, the 
microprocessor controller further operable to deactivate the means for 
heating until an end of a cook cycle if the cook cycle is within a preset 
number of seconds from ending and if the working temperature is within a 
preset number of degrees from a preset temperature. 
In another form of the invention, a microprocessor controlled deep fat 
fryer is disclosed, comprising a frypot for storing a quantity of cooking 
oil, means for heating the cooking oil in order to cook a food product and 
a microprocessor controller connected to the means for heating and 
operable to deactivate the means for heating for a preset time period 
after the food product is removed from the cooking oil. 
In another form of the invention, a method for operating a deep fat fryer 
is disclosed, comprising the steps of placing a food product into a frypot 
containing a quantity of cooking oil, providing a heating means in thermal 
communication with the cooking oil, sensing an indicated temperature of 
the cooking oil at a location, generating a working temperature which may 
differ from the indicated temperature in order to account for a thermal 
transit time between the heating means and the location and regulating the 
heating means in accordance with the working temperature and other data.

DETAILED DESCRIPTION OF THE DRAWINGS 
The present invention is directed towards an automatic temperature 
controller for use in conjunction with a cooking appliance, such as a deep 
fat fryer, employing a cooking medium, such as oil or shortening, into 
which food to be cooked is immersed. The fryers described in U.S. Pat. 
Nos. 4,848,318, 4,751,915 and 4,898,151, which are assigned to the same 
assignee as the present invention and hereby incorporated by reference, 
are representative of the type of fryer for which the automatic 
temperature controller of the present invention is contemplated. Referring 
to FIG. 1, such a cooking device is represented in schematic block diagram 
form and designated generally as 10. The cooking appliance includes a 
cooking medium 12 which is in thermal communication with heating means 14. 
Path 16 indicates the path of thermal communication between heating means 
14 and cooking medium 12. Heating means 14 will typically include a burner 
and burner radiator, or an electric heating element immersed in the 
cooking medium 12, for example. A temperature sensor 18 is also provided 
for sensing a temperature of cooking medium 12. Finally, a controller 20 
is provided which regulates the heating means 14 based upon temperature 
information supplied to it by temperature sensor 18. 
When the burner on a deep fat fryer is activated, the thermal energy 
produced must pass from the burner to, for example, the burner radiators, 
the cooking pot and the oil before it is transmitted to the food being 
cooked. Each of these transferring mediums have a particular thermal mass 
associated with them. As the thermal energy passes from one medium to the 
next, the transfer is not thermally efficient. The medium itself heats up 
and stores thermal energy in addition to passing it to the next medium. 
Therefore, each of the thermal transfer mediums must be thermally charged 
by the thermal energy source before efficient thermal transfer may 
proceed. This results in a delay between the time when the burner produces 
thermal energy and the time when the food begins to receive it. Also, the 
thermal masses closer to the burner will reach the burner temperature 
before the thermal masses which are more remote. A similar effect occurs 
in relation to the "hot spot" at the back of the frypot where the exhaust 
gas flue is located. 
The automatic temperature controller of the present invention is designed 
to anticipate the arrival of the delayed thermal energy before it actually 
arrives at the food. The controlling routine used to operate the fryer may 
perform many functions, but the present invention concerns only specific 
portions of the entire control routine, such as the portion which 
determines the temperature to be used in calculations by the rest of the 
routine (i.e. the working temperature), and the portions of the control 
routine which activate the burner. It is contemplated that the present 
invention as disclosed herein may be used in conjunction with any cooking 
appliance control routine, hence only the novel aspects are described 
herein. 
Referring to FIG. 2, a flowchart is shown which details the control routine 
of the first preferred embodiment of the invention. All degree 
measurements are in degrees Fahrenheit. The process begins at decision 
point 100 which determines if the main control routine has turned on the 
burner which applies thermal energy to the fryer. If the burner has been 
turned on, decision point 110 determines if the working temperature (the 
temperature value used in calculations performed by the main control 
routine) is not more than twelve degrees above the indicated temperature. 
The indicated temperature is the temperature reading of the physical 
temperature probe located somewhere in or on the fryer. If the working 
temperature is twelve degrees or more above the indicated temperature, the 
working temperature is not modified and the process returns to decision 
point 100. 
On the other hand, if decision point 110 determines that the working 
temperature is not more than twelve degrees above the indicated 
temperature, the process proceeds to step 120 where there is a delay of 
eight seconds. This delay is inserted into the process routine because it 
is desired that the working temperature only be increased every eight 
seconds. Due to the speed of the controller hardware, the entire routine 
may be executed in negligible time, therefore it is necessary to insert a 
wait state into the process. After a delay of eight seconds, the process 
adds one degree to the working temperature at step 130. This amounts to a 
periodic rate of increase in working temperature of about one-eighth of a 
degree per second. Next, decision point 140 determines if the working 
temperature is greater than the maximum allowable temperature. This 
maximum allowable temperature is the set point temperature plus the 
maximum allowable amount of overshoot temperature specified for the food 
being cooked. If the working temperature has not exceeded this limit, the 
process returns to decision point 100. If the working temperature has 
exceeded this limit, step 150 subtracts one degree from the working 
temperature. After this, decision point 160 determines if the working 
temperature is still greater than the indicated temperature. If it is, the 
process returns to decision point 140 to check whether the working 
temperature is still greater than the maximum allowable temperature. If, 
on the other hand, the working temperature is not greater than the 
indicated temperature, it will not be lowered any further and the process 
returns to decision point 100. Therefore, the working temperature will not 
exceed the maximum allowable temperature unless the working temperature 
has been decreased all the way to the indicated temperature and the 
indicated temperature itself is greater than the maximum allowable 
temperature. 
The above portion of the control routine attempts to anticipate that the 
actual temperature of the food being cooked in the fryer will continue to 
rise after the burner is turned off. This is because the thermal mass of 
the heat transfer chain has energy stored in it which will continue to be 
transferred to the food even after the thermal energy source (the burner) 
is removed from the thermal path. Therefore, when the main control routine 
turns on the burner to apply heat-to the fryer, the above process adds one 
degree to the working temperature ever eight seconds. This temperature 
addition continues as long as the burner is supplying thermal energy to 
the system and the working temperature is less than twelve degrees higher 
than the indicated temperature. The calibration of the working temperature 
is further limited by the provision that it not exceed the maximum 
overshoot temperature. Because this working temperature is used by the 
rest of the control routine to make decisions such as when to turn off the 
burner, the process of the first preferred embodiment of the present 
invention achieves more accurate results because the thermal energy stored 
in the thermal transit path is accounted 
An analogous process is employed when the burner has been deactivated. 
Referring again to FIG. 2, if decision point 100 determines that the 
burner has been turned off, the process moves to decision point 170 which 
determines if the working temperature is greater than the indicated 
temperature. If it is not, then no reduction in the working temperature 
will take place and the process returns to decision point 100. However, if 
the working temperature does exceed the indicated temperature, the process 
inserts a delay of twenty seconds at step 180. This delay is necessary 
because the process should not decrement the working temperature more 
often than every twenty seconds. After the delay at step 180, step 190 
subtracts one degree from the working temperature. This amounts to a 
periodic rate of decrease of working temperature of about one-twentieth of 
a degree per second. After this, the process returns to decision point 
100. 
In the above portion of the control process, which is active when the 
burner has stopped applying thermal energy to the thermal transit path, 
allowance is being made for the fact that the temperature of the food 
being cooked will become lower than the current indicated temperature 
because the thermal energy is being depleted in the thermal path between 
the food and the thermal energy source. Because there is some delay 
between the time of activation of the thermal energy source and the time 
the food actually receives some of that thermal energy, the food will 
continue to cool for a period of time after the burner has been activated. 
This is the time it takes to thermally charge the thermal path to its 
equilibrium temperature. By anticipating this drop in the temperature of 
the food, the control system will activate the burner sooner than it would 
without the compensation, and the food will be maintained nearer to the 
set point temperature. Consequently, while the burner is deactivated, the 
routine subtracts one degree of temperature from the working temperature 
every twenty seconds as long as the working temperature is greater than 
the indicated temperature. 
Appendix A reproduces a software listing of the present invention written 
in the C language. The code section which is exemplary of the first 
preferred embodiment of the present invention begins at the line "void 
process.sub.-- kfc(void)", in which the variable "counts" represents 
degrees of temperature. 
Referring now to FIG. 3, a process flowchart illustrating a second 
preferred embodiment of the present invention is shown. This process is 
intended to be executed before the main control routine determines if the 
burner should be activated. Beginning at decision point 200, the process 
determines if the end of the cook cycle is less than thirty seconds away. 
If it is not, then the process exits to the burner activation decision 
portion of the main control routine. However, if the process is within 30 
seconds of the end of the cook cycle, the process moves to decision point 
210 which determines if the working temperature is within ten degrees of 
the set point temperature. If it is not, the process once again exits to 
the burner activation decision portion of the main control routine. If, on 
the other hand, the working temperature is within ten degrees of the set 
point temperature, the process activates an override at step 220 which 
prevents the burner from being activated by any other portion of the 
control routine during the present cook cycle. 
The process of the second preferred embodiment of the present invention 
recognizes that the thermal mass of the fryer will keep the temperature of 
the food from being increased before the expiration of the cook cycle 
(thirty seconds) because the thermal energy from the burner will not reach 
the food before the end of the cycle. However, if the working temperature 
is more than ten degrees away from the set point temperature, the main 
control routine will turn the burner on to a high level which might apply 
enough thermal energy to the system to reach the food before the end of 
the cycle. 
The code section of Appendix A which is exemplary of the second preferred 
embodiment of the present invention begins at line "char sp.sub.-- 
ch.sub.-- last(void)" for a split vat and "char sp.sub.-- ch.sub.-- 
last2(void)" for a full vat, in which "post.sub.-- off.sub.-- time" 
represents the preset amount of time away from the end of the cook cycle. 
Referring now to FIG. 4, a process flowchart of a third preferred 
embodiment of the present invention is shown. This process is intended to 
compensate for thermal destabilization of the cooking medium after the 
food basket is removed from the oil. After the point in the main control 
routine which lifts the food basket out of the cooking medium, step 300 
inserts a fifteen second override which will not allow any other section 
of the main control routine to activate the burner. The process then 
continues in the main control routine at the step normally following the 
basket up command. 
The third preferred embodiment of the present invention compensates for the 
fact that when the food basket is pulled from the cooking medium at the 
end of the cook cycle, the ensuing turbulence brings cool oil up from the 
cold zone of the fryer. This cool oil would normally cause the main 
control routine to activate the burner, resulting in additional overshoot 
of the cooking medium idle temperature. The process of the third preferred 
embodiment enables a ten second override to prevent the burner from being 
activated. This overriding heat off command gives the cooking medium time 
to stabilize prior to returning to the normal control routine. 
The code section of Appendix A which is exemplary of the third preferred 
embodiment of the present invention begins at line "void set.sub.-- 
cook.sub.-- alarm(int chann)" in which "post.sub.-- off" is the variable 
which selects the duration (in seconds) of the heat off override. 
Although several preferred embodiments of the present invention have been 
described in detail herein, those skilled in the art will recognize that 
various substitutions and modifications may be made to the specific 
processes shown and described without departing from the spirit and scope 
of the invention as recited in the appended claims. For example, the 
recitation of specific temperature increments and decrements, as well as 
the recitation of specific time intervals are optimized to a specific 
fryer (Model H50, Frymaster Corp.). It will be obvious to those skilled in 
the art that appropriate temperature and time values will vary for other 
types and makes of fryers, but that such values may be determined as a 
matter of routine design once the novel processes disclosed herein and 
defined in the appended claims are understood.