Electric heating – Heating devices – With power supply and voltage or current regulation or...
Reexamination Certificate
2002-09-26
2004-06-22
Paik, Sang Y. (Department: 3742)
Electric heating
Heating devices
With power supply and voltage or current regulation or...
C219S510000
Reexamination Certificate
active
06753511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to temperature control systems for cookware and, particularly, to a novel thermal limiting system and method for controlling application of thermal energy to a burner element of a cookware apparatus.
2. Discussion of the Prior Art
The life of the glass ceramic material forming a cooking surface or burner in a cookware apparatus is dependent on the temperature it is subjected to. Therefore, the power to a burner must be limited to prevent premature failure of the glass. The temperature of the glass is a function of time, burner power and the properties of the cooking utensil place on it (e.g. flatness, reflectivity, contents, etc.) consequently a method of dynamically adjusting the power to prevent overheating is needed, i.e. thermal limiting control.
In conventional systems, the temperature is limited in two ways: 1) by using of a temperature switch that interrupts power to the burner at excessive temperatures such as described in U.S. Pat. No. 6,150,641, the whole contents and disclosure of which is incorporated by reference as if fully set forth herein; or, 2) by directly sensing the temperature and applying appropriate feedback control such as described in U.S. Pat. No. 6,285,012, the whole contents and disclosure of which is incorporated by reference as if fully set forth herein.
The first thermal limiting approach
10
, as described in U.S. Pat. No. 6,150,641, and illustrated in FIG.
1
(
a
), includes implementing a thermal switch and bang-bang thermal limiting to control the temperature
18
of the cookware burner
12
, and incorporates a power control component
14
receiving the power command signal
16
which, in this approach, constitutes the user power command signal. This approach is inexpensive but results in large swings in power and temperature of the cooking utensil. That is, in this first approach, a thermal switch is used to provide bang-bang temperature control when the temperature exceeds the predetermined limit. This type of control results in the frequent cycling of the power causing corresponding swings in the pan temperature.
FIG.
2
(
a
) illustrates an example simulation of bang-bang thermal control implemented for a ceramic burner. In the example simulation, the thermal switch is modeled as a relay with an arbitrary 30° C. of hysteresis, and the thermal response of the burner (e.g., glass temperature output) is modeled as a first order linear model (derived empirically). Initially, as shown in FIG.
2
(
a
), the user-demanded power setting (user power command signal) is about one-half (50%)of the maximum power. At this initial setting, thermal limiting does not engage as indicated in FIG.
2
(
b
). At the time indicated at
141
, the user increases the power to 100% (FIG.
2
(
a
)) causing the conduction state
145
of the thermal switch (e.g., bi-metallic switch) to change in accordance with bang-bang thermal limiting at time indicated as time
142
. In FIG.
2
(
b
), the conduction on/off states, i.e., engagement of bang-bang thermal limiting, is represented as the plot
145
. At this setting, the glass temperature of the burner increases to the thermal limit
182
, e.g., the safety thermal limit of a glass burner, as shown in FIG.
2
(
c
). Finally, the user reduces the power back to its initial one-half power level and thermal limiting ceases, as indicated at time
143
in FIG.
2
(
a
).
The second thermal limiting approach
20
, as described in U.S. Pat. No. 6,285,012, and illustrated in FIG.
1
(
b
), includes implementing a thermal limiting controller component
22
that limits thermal heating of burner
12
′ in accordance with the user power command signal
16
′, a predetermined thermal limit signal
25
, and an instantaneous sensed temperature
28
that is feedback from a temperature sensor element included with the burner
12
′. As described in U.S. Pat. No. 6,285,012, the controller includes proportional plus integral control, minimum selector and anti wind-up control elements (not shown) to provide thermal limiting for a burner
12
′ implementing a sensor. The output
15
of the thermal limit controller
22
is input to a further power control unit for adjusting, e.g., quantizing the thermal limiter power output. This approach provides for very smooth power and temperature profiles but the temperature sensor is often expensive.
It would thus be highly desirable to provide a thermal limiting system and method for providing thermal limiting control to a cooktop burner of an electric cooking device, that provides for very smooth power without the use of an expensive thermal sensor.
SUMMARY OF THE INVENTION
A system and method for smoothly limiting the temperature of a burner of a cooking appliance, e.g. a stove ceramic burner, without the use of a temperature sensor. The method includes the steps of sensing the conduction state of a thermal switch in a bang-bang thermal limiting burner, and feeding back a signal representing this switch conduction state to control duty-cycle (and thus “on” time) of the applied power. The power to the burner is reduced until the sensed duty-cycle cycling is reduced (lower frequency and amplitude) resulting in smoother power and temperature control.
Preferably, this sensed duty-cycle cycling is increased to near 100%, i.e., the thermal switch conducting state is almost always on, i.e., off-time is reduced.
REFERENCES:
patent: 3783366 (1974-01-01), Cox
patent: 4496829 (1985-01-01), Black et al.
patent: 4777350 (1988-10-01), Crocket et al.
patent: 5708256 (1998-01-01), Montagnino et al.
patent: 6108183 (2000-08-01), Beene
patent: 6150641 (2000-11-01), Wilkins
patent: 6285012 (2001-09-01), Connolly et al.
patent: 6417496 (2002-07-01), Bates et al.
patent: 6577109 (2003-06-01), Dancy et al.
Glaser John Stanley
Mathews, Jr. Harry Kirk
Smolenski Joseph Lucian
General Electric Company
Paik Sang Y.
Scully Scott Murphy & Presser
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