Abstrict
An electric heater (1) arranged beneath a glass-ceramic cook top
(2) is controlled with the aid of a temperature sensor (7) for monitoring
temperature at or adjacent to the glass-ceramic cook top. The sensor
provides an electrical output as a function of temperature and permits
monitoring, in time controlled manner, temperature at or adjacent
to the glass-ceramic cook top. In a first stage the temperature
of the glass-ceramic cook top (2) is permitted to exceed a predetermined
continuous safe level (Y) for up to a predetermined maximum time
period (X, W.sub.2, W) and such that a predetermined temporary safe
level (Z) of temperature, in excess of the predetermined continuous
safe level (Y), is not exceeded. In a second stage the heater (1)
is regulated in accordance with the monitored temperature to achieve
a selected temperature of the glass-ceramic cook top in a range
up to the predetermined continuous safe level (Y) of temperature.
Claims
I claim:
1. A method of providing electronic control of an electric heater
arranged beneath a glass-ceramic cook top, which method comprises
providing a temperature sensor for monitoring temperature in the
region of the glass-ceramic cook top, which sensor provides an electrical
output as a function of temperature and monitoring by means of the
sensor, in time controlled manner, temperature in the region of
the glass-ceramic cook top, wherein in a first stage the temperature
of the glass-ceramic cook top is permitted to exceed a predetermined
continuous safe level for up to a predetermined maximum time period
and such that a predetermined temporary safe level of temperature,
in excess of the predetermined continuous safe level, is not exceeded
and wherein in a second stage the heater is regulated in accordance
with the monitored temperature to achieve a selected temperature
of the glass-ceramic cook top in a range up to the predetermined
continuous safe level of temperature.
2. A method according to claim 1, wherein in the first stage the
temperature in the region of the glass-ceramic cook top is monitored
only after elapse of a predetermined time period.
3. A method according to claim 2, wherein the temperature in the
region of the glass-ceramic cook-top is monitored only after the
predetermined maximum time period.
4. A method according to claim 1, wherein in the first stage the
temperature in the region of the glass-ceramic cook top is substantially
continuously monitored and the heater regulated in accordance with
the monitored temperature such that the predetermined temporary
safe level of temperature is not exceeded.
5. A method according to claim 1, wherein the rate of rise of temperature
in the first stage is monitored and compared with a specific rate
of rise on the basis of which at least one of the predetermined
maximum time period and the predetermined temporary safe level of
temperature have been established, and at least one of the predetermined
maximum time period and the predetermined temporary safe level of
temperature is adjusted proportionate to the compared rate of rise
and specific rate of rise of temperature.
6. A method according to claim 1, wherein the temperature sensor
comprises a device having an electrical parameter which changes
as a function of temperature.
7. A method according to claim 6, wherein the electrical parameter
of the device which changes as a function of temperature is selected
from electrical resistance, inductance and capacitance.
8. A method according to claim 7, wherein the device comprises
a platinum resistance temperature detector.
9. A method according to claim 6, wherein the temperature sensor
comprises a thermoelectric device producing an electrical output
as a function of temperature.
10. A method according to claim 9, wherein the thermoelectric device
comprises a thermocouple.
11. A method according to claim 1, wherein the temperature sensor
is located in the heater in a position selected from between a heating
element in the heater and the glass-ceramic cook top and in contact
with the glass-ceramic cook top.
12. A method according claim 11, wherein the temperature sensor
is located inside a heat-withstanding housing.
13. A method according to claim 12, wherein the housing is of tubular
form.
14. A method according to claim 13, wherein the housing is made
of a material selected from a metal and an alloy.
15. A method according to claim 14, wherein the alloy is selected
from a stainless steel and an iron-chromium-aluminium alloy.
16. A method according to claim 1, wherein the temperature sensor
is electrically connected to a microprocessor-based control system
whereby the temperature in the region of the glass-ceramic cook
top is monitored in time controlled manner and the heater regulated
in accordance with the predetermined temporary safe level of temperature
and the predetermined continuous safe level of temperature.
17. A method according to claim 1, wherein regulation of power
to the heater is effected by way of means selected from a relay
and a solid state switch means.
18. A method according to claim 1, wherein a user-settable power
control means is additionally provided for the heater.
19. A method according to claim 18, wherein the user-settable power
control means is selected from a manually-adjustable cyclic energy
regulator and a multiple-position switch arrangement.
20. Apparatus for providing electronic control of an electric heater
arranged beneath a glass-ceramic cook top, which apparatus comprises
a temperature sensor for monitoring temperature in the region of
the glass-ceramic cook top, which sensor provides an electrical
output as a function of temperature, and means to monitor by the
sensor, in time controlled manner, temperature in the region of
the glass-ceramic cook top wherein means is provided operating in
a first stage to permit the temperature of the glass-ceramic cook
top to exceed a predetermined continuous safe level for up to a
predetermined maximum time period and such that a predetermined
temporary safe level of temperature, in excess of the predetermined
continuous safe level, is not exceeded, and wherein means is provided
operating in a second stage to regulate the heater in accordance
with the monitored temperature, to achieve a selected temperature
of the glass-ceramic cook top in a range up to the predetermined
continuous safe level of temperature.
21. Apparatus according to claim 20, wherein during operation in
the first stage the temperature in the region of the glass-ceramic
cook top is monitored only after elapse of a predetermined time
period.
22. Apparatus according to claim 21, wherein the temperature in
the region of the glass-ceramic cook-top is monitored only after
the predetermined maximum time period.
23. Apparatus according to claim 20, wherein during operation in
the first stage the temperature in the region of the glass-ceramic
cook top is substantially continuously monitored, means being provided
to regulate the heater in accordance with the monitored temperature
such that the predetermined temporary safe level of temperature
is not exceeded.
24. Apparatus according to claim 20, wherein the apparatus is adapted
to monitor the rate of rise of temperature in the first stage and
compare such rate with a specific rate of rise on the basis of which
at least one of the predetermined maximum time period and the predetermined
temporary safe level of temperature have been established, and to
adjust at least one of the predetermined maximum time period and
the predetermined temporary safe level of temperature proportionate
to the compared rate of rise and specific rate of rise of temperature.
25. Apparatus according to claim 20, wherein the temperature sensor
comprises a device having an electrical parameter which changes
as a function of temperature.
26. Apparatus according to claim 25, wherein the electrical parameter
of the device which changes as a function of temperature is selected
from electrical resistance, inductance and capacitance.
27. Apparatus according to claim 26, wherein the device comprises
a platinum resistance temperature detector.
28. Apparatus according to claim 25, wherein the temperature sensor
comprises a thermoelectric device producing an electrical output
as a function of temperature.
29. Apparatus according to claim 28, wherein the thermoelectric
device comprises a thermocouple.
30. Apparatus according to claim 20, wherein the temperature sensor
is located in the heater in a position selected from between a heating
element in the heater and the glass-ceramic cook top and in contact
with the glass-ceramic cook top.
31. Apparatus according to claim 30, wherein the temperature sensor
is located inside a heat-withstanding housing.
32. Apparatus according to claim 31, wherein the housing is of
tubular form.
33. Apparatus according to claim 32, wherein the housing is made
of a material selected from a metal and an alloy.
34. Apparatus according to claim 33, wherein the alloy is is selected
from a stainless steel and an iron-chromium-aluminium alloy.
35. Apparatus according to claim 20, wherein the temperature sensor
is electrically connected to a microprocessor-based control system
whereby the temperature in the region of the glass-ceramic cook
top is monitored in time controlled manner and the heater regulated
in accordance with the predetermined temporary safe level of temperature
and the predetermined continuous safe level of temperature.
36. Apparatus according to claim 20, wherein regulation of power
to the heater is effected by way of means selected from a relay
and a solid state switch means.
37. Apparatus according to claim 20, wherein a user-settable power
control means is additionally provided for the heater.
38. Apparatus according to claim 37, wherein the user-settable
power control means is selected from a manually-adjustable cyclic
energy regulator and a multiple-position switch arrangement.
Description This invention concerns a method and apparatus for controlling
an electric heater arranged beneath a glass-ceramic cook top in
a cooking appliance. More particularly, the invention is concerned
with a control method and apparatus for such a heater which adapts
to different requirements between an initial stage of operation
of the heater and a subsequent longer term equilibrium stage of
operation and results in faster heating to boiling on the cook top.
DESCRIPTION OF PRIOR ART
As is well known, heaters used in glass-ceramic top cooking appliances
usually incorporate a temperature limiter which operates to maintain
the temperature of the glass-ceramic within safe limits. Such a
temperature limiter generally comprises a differentially expanding
rod and tube assembly arranged at least partly across the heater
between a heating element in the heater and the glass-ceramic top
and operating a switch arrangement located outside the heater.
Temperature limiters of this kind are typically calibrated in accordance
with worst case steady state conditions to be encountered, namely
free radiation of the heater under the glass-ceramic cook top, that
is in the absence of a cooking utensil on the cook top. Since the
calibration of the temperature limiter is fixed, the switching condition
is, of necessity, a compromise. In order to ensure safe steady state
conditions of operation of an appliance, the switching response
has to be set at such a level that so-called nuisance switching
off and on of the heater occurs during the initial heating stage.
Such undesirable repetitive switching of the heater disadvantageously
increases the boiling time of the contents of a cooking utensil
on the cook top and is especially problematical with poor quality
cooking utensils, or large volumes of materials to be cooked, or
with heaters of high power. In fact, increasing the power of a heater
can fail to result in faster boiling times since the nuisance switching
effect increases in proportion to increasing power and counteracts
the intended benefit to be derived from an increase in heater power.
The function of a temperature limiter is to restrict the temperature
reached by the glass-ceramic to a safe level, that is a predetermined
level at which the glass-ceramic may be continuously operated, and
this predetermined level is hereinafter referred to as the predetermined
continuous safe temperature level. However, a higher temperature
level is acceptable for the glass-ceramic without significantly
reducing the life thereof, provided such higher temperature level
is experienced for a short period of time only. Such short term
higher temperature level is hereinafter referred to as the predetermined
temporary safe level of temperature, being permitted for a predetermined
maximum time period.
Attempts have been made to arrange for such a higher temperature
level to be attained in an initial heating stage by delaying the
response of the temperature limiter by means which temporarily shield
the temperature limiter from incident thermal radiation. However,
this effect predominantly only occurs at the first switching of
the temperature limiter and is not accurately controllable.
A further factor to be considered is that when a heating element
in a heater is energised, thermal gradients occur therein and these
are significantly different when the element is initially switched
on, compared with the element when heated to equilibrium conditions.
This is particularly significant for temperature sensing if the
sensor is not directly coupled to the glass-ceramic and leads to
a different relationship between sensor temperature and glassceramic
temperature during the initial stages of operation of a heater as
compared with long term operation under equilibrium conditions.
United Kingdom Patent Specification No. 1,514,736 describes the
control of electrically heated hot plates of cast iron or other
suitable metal where the problems addressed by the present invention
do not arise. According to UK Patent Specification No. 1,514,736
a cooking vessel is permitted to attain an initial temperature which
is higher than under steady state operation. The duration of the
initial temperature boost and the effect thereof on the hot plate
are not considered.
United Kingdom Patent Publication No. 2,199,999 describes a temperature
limiting arrangement for a glass-ceramic cooktop appliance in order
to provide protection against overheating of the glass-ceramic cooktop.
It is acknowledged there is a need for a means of limiting the temperature
of the glass-ceramic plate which satisfactorily protects the glass-ceramic
from overheating while minimizing any adverse effect on cooking
performance and heating unit longevity. This is accomplished by
monitoring the glass-ceramic temperature and the temperature rate
of change to detect an abnormal thermal load condition such as operating
the heating unit with no utensil on the cooktop surface, using badly
warped surfaces and operating the heating unit with an empty utensil.
The power level is then reduced to limit the temperature of the
glass-ceramic cooking surface so as to avoid damage.
United Kingdom Patent Publication No. 2,212,303 describes a power
control for a cooking appliance having a glass-ceramic cooking surface.
A temperature sensor is provided for sensing the temperature of
the glass-ceramic cooking surface and the power level of a heating
unit is responsive to the sensed temperature.
OBJECT OF THE INVENTION
It is an object of the present invention to provide a temperature
sensing and heater control system which is adaptive to the differences
between the initial operating conditions and longer term equilibrium
conditions of a heater in a glass-ceramic top cooking appliance.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided
a method of providing electronic control of an electric heater arranged
beneath a glass-ceramic cook top, which method comprises providing
a temperature sensor for monitoring temperature in the region of
(that is, at or adjacent to) the glass-ceramic cook top, which sensor
provides an electrical output as a function of temperature and monitoring
by means of the sensor, in time controlled manner, temperature in
the region of the glass-ceramic cook top, wherein in a first stage
the temperature of the glass-ceramic cook top is permitted to exceed
a predetermined continuous safe level for up to a predetermined
maximum time period and such that a predetermined temporary safe
level of temperature, in excess of the predetermined continuous
safe level, is not exceeded and wherein in a second stage the heater
is regulated in accordance with the monitored temperature to achieve
a selected temperature of the glass-ceramic cook top in a range
up to the predetermined continuous safe level of temperature.
According to another aspect of the present invention there is provided
an apparatus for providing electronic control of an electric heater
arranged beneath a glass-ceramic cook top, which apparatus comprises
a temperature sensor for monitoring temperature in the region of
(that is, at or adjacent to) the glass-ceramic cook top, which sensor
provides an electrical output as a function of temperature, and
means to monitor by the sensor, in time controlled manner, temperature
in the region of the glass-ceramic cook top, wherein means is provided
operating in a first stage to permit the temperature of the glass-ceramic
cook top to exceed a predetermined continuous safe level for up
to a predetermined maximum time period and such that a predetermined
temporary safe level of temperature, in excess of the predetermined
continuous safe level, is not exceeded, and wherein means is provided
operating in a second stage to regulate the heater in accordance
with the monitored temperature, to achieve a selected temperature
of the glass-ceramic cook top in a range up to the predetermined
continuous safe level of temperature.
In the first stage, the temperature in the region of the glass-ceramic
cook top may be monitored only after elapse of a predetermined time
period, which may be the predetermined maximum time period.
Alternatively, in the first stage the temperature in the region
of the glass-ceramic cook top may be substantially continuously
monitored and the heater regulated in accordance with the monitored
temperature such that the predetermined temporary safe level of
temperature is not exceeded.
The rate of rise of temperature in the first stage may be monitored
and compared with a specific rate of rise on the basis of which
the predetermined maximum time period and/or the predetermined temporary
safe level of temperature have been established, and the predetermined
maximum time period and/or the predetermined temporary safe level
of temperature may be adjusted proportionate to the compared rate
of rise and specific rate of rise of temperature.
The temperature sensor may comprise a device having an electrical
parameter, such as electrical resistance, inductance, or capacitance,
which changes as a function of temperature. By way of example, such
a device may comprise a platinum resistance temperature detector.
Alternatively the temperature sensor may comprise a thermoelectric
device, such as a thermocouple, producing an electrical output as
a function of temperature.
The temperature sensor may be located in the heater between a heating
element in the heater and the glass-ceramic cook top, or in contact
with the glass-ceramic cook top.
The temperature sensor may be located inside a heat-withstanding
housing, optionally of tubular form, such as of a metal or alloy.
A suitable alloy is a stainless steel or an iron-chromium-aluminium
alloy.
The temperature sensor may be electrically connected to a microprocessor-based
control system whereby the temperature in the region of the glass-ceramic
cook top is monitored in time controlled manner and the heater regulated
in accordance with the predetermined temporary safe level of temperature
and the predetermined continuous safe level of temperature.
Regulation of power to the heater may be effected by way of a relay,
or a solid state switch means.
A user-settable power control means may additionally be provided
for the heater. Such control means may comprise a manually adjustable
cyclic energy regulator or a multiple-position switch arrangement.
The invention is now described by way of example with reference
to the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of an electric heater connected to an electronic
controller according to the present invention, the electronic controller
being represented diagrammatically;
FIG. 2 is a section along line A--A of the heater of FIG. 1 arranged
beneath a glass-ceramic cook top; and
FIGS. 3 to 5 are graphs illustrating control and regulation of
glass-ceramic cook top temperature with time.
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to FIGS. 1 and 2, an electric heater 1 is provided arranged
beneath a glass-ceramic cook top 2 in a cooking appliance, such
as a smooth top cooker. The heater 1 comprises a metal dish 3 having
therein a base layer 4 of compacted microporous thermal insulation
material.
A heating element 5 is provided, supported on the base layer 4.
As shown, the heating element 5 comprises a corrugated metal ribbon
supported edgewise on the base layer 4 and secured by partial embedding
in the base layer 4. However, the heating element 5 could comprise
other forms, such as coiled wire or coiled ribbon or other arrangements
of ribbon, or one or more infra-red lamps. Any of the well-known
forms of heating element, or combinations thereof, could be considered,
the invention not being restricted to any particular form of heating
element.
A peripheral wall 6 of thermal insulation material is provided,
a top surface of which contacts the underside of the glass-ceramic
cook top 2.
A temperature sensor 7 is arranged to extend partially across the
heater, between the heating element 5 and the glass-ceramic cook
top 2. The temperature sensor 7 comprises a tube, such as of metal,
having therein a device which provides an electrical output as a
function of temperature. The tube may, for example, comprise a stainless
steel or iron-chromium-aluminium alloy. The device may have an electrical
parameter, such as electrical resistance or inductance, which changes
as a function of temperature. In particular, the device may comprise
a platinum resistance temperature detector or thermometer. Alternatively
the device in the temperature sensor 7 could comprise a thermoelectric
device, such as a thermocouple, producing an electrical output,
such as a voltage output, as a function of temperature.
As an alternative, a temperature sensor could be provided secured
in contact with the glass-ceramic cook top 2.
A terminal block 8 is provided at the edge of the heater and by
means of which the heating element 5 is arranged to be electrically
connected to a power supply 9 for energisation.
Control circuitry 10 is provided for the heater 1. Such control
circuitry comprises a microcontroller 11, which is a microprocessor-based
circuit. A cyclic energy regulator 12 is also provided, which has
a control knob 13 by means of which a plurality of user-selectable
energy settings of the heater can be achieved in known manner.
Power is supplied to the heater from the power supply 9 by way
of a relay 14, or by way of a solid state switch means, such as
a triac, transistor, FET, IGBT, or SCR.
The temperature at or adjacent to the glass-ceramic cook top 2
is monitored in time controlled manner by means of the temperature
sensor 7 in association with the microcontroller 11, to which the
sensor 7 is connected.
The glass-ceramic cook top 2 may be operated continuously without
damage at a predetermined temperature level which is herein referred
to as the predetermined continuous safe level of temperature. However,
in a first, or initial, stage, in order to achieve the fastest possible
boiling time for a food item in a cooking utensil located on the
glass-ceramic cook top 2, the predetermined continuous safe level
of temperature may be temporarily exceeded for a short period of
time with safety. The glass-ceramic cook top may therefore be operated
at a predetermined temporary safe level of temperature, in excess
of the predetermined continuous safe level of temperature, for up
to a predetermined maximum time period.
The predetermined temporary safe level of temperature and maximum
time period can be obtained for each heating element, dependent
on such factors as power loading, and such that under so-called
abuse conditions, where the heater is operated under free radiation
conditions without the presence of a cooking utensil on the glass-ceramic
cook top, the glass-ceramic is permitted to reach a higher temperature
than the predetermined continuous safe level of temperature permitted
under equilibrium operating conditions of the heater.
One method of control according to the invention is illustrated
in FIG. 3.
When the cooking appliance is operated, the temperature of the
glass-ceramic is monitored by the temperature sensor 7 and microcontroller
11 but in the first stage only after a time period X, such as 10
minutes, has elapsed. Such time period X may be the predetermined
maximum time period, referred to above, as determined for the particular
heater. The monitored temperature after the time period X is in
excess of the predetermined continuous safe level Y. Consequently
the microcontroller 11 adapts the power input to the heater 1, on
the basis of this monitored temperature, such that in a second stage
of operation of the appliance, subsequent to the initial time period
X, the heater operates in equilibrium conditions such that the predetermined
continuous safe level Y of temperature for the glass-ceramic cook
top 2 is not exceeded.
In another method of control, as illustrated in FIGS. 4 and 5,
the temperature at or adjacent to the glass-ceramic cook top 2 is
substantially continuously monitored by the temperature sensor 7
in association with the microcontroller 11. If, after a predetermined
initial time period W.sub.1 (less than the predetermined maximum
time period) has elapsed, the monitored temperature has not reached
the predetermined temporary safe level Z of temperature, the heater
may be allowed to continue to operate without further control for
a further period, as indicated by the dotted trace 15 until, at
time period W.sub.2, representing the predetermined maximum time
period, the microcontroller 11 adapts the power input to the heater
such that in a second stage of operation the heater operates in
equilibrium conditions such that the predetermined continuous safe
level Y of temperature is not exceeded. Alternatively, after the
predetermined initial time period W.sub.1 has elapsed, the power
input to the heater could be adapted, as shown by the continuous
trace 16, such that after W.sub.1 the heater is operated in equilibrium
conditions whereby the predetermined continuous safe level Y of
temperature is not exceeded.
With reference now to FIG. 5, a situation can arise where during
the initial or first stage of heating the monitored temperature
reaches the predetermined temporary safe level Z. When this occurs,
the microcontroller 11 adapts the power input to the heater so that
up to the predetermined maximum time period W the predetermined
temporary safe level Z of temperature is not exceeded. Thereafter,
in the second stage of operation, the microcontroller further adapts
the power input to the heater such that the predetermined continuous
safe level Y of temperature for the glass-ceramic cook top 2 is
not exceeded.
In situations where the monitored temperature in the first stage
rises at a slower or faster rate than predicted, the predetermined
maximum time period and/or the predetermined temporary safe level
of temperature can be arranged to be automatically adjusted such
that a higher temperature for a shorter period of time or a lower
temperature for a longer period of time is permitted. The rate of
rise of temperature in the first stage is monitored and compared
with a specific rate of rise on the basis of which the predetermined
maximum time period and the predetermined temporary safe level of
temperature have been established. The predetermined temporary safe
level of temperature and/or the predetermined maximum time period
may then be adjusted in proportion to the compared rates of rise
of temperature.
Instead of the cyclic energy regulator 12, a well known form of
multiple position switch control arrangement (not shown) could be
provided to control the heater 1.
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