Abstrict
A radiant electric heater (2) is arranged for location underneath
and against a cooking plate (12) and incorporates a heating element
(14) spaced from the cooking plate and a temperature sensor assembly
(26). The temperature sensor assembly (26) comprises a beam (28)
of ceramic material provided within the heater and extending at
least partially across the heater over the at least one heating
element (14). The beam (28) has a substantially planar upper surface
(32) arranged to face the cooking plate (12), in contact with the
cooking plate or in close proximity to it, and an under surface
(34) arranged for exposure to direct radiation from the heating
element (14). Provided on the planar upper surface (32) is an electrical
component (36), such as of film or foil form, having an electrical
parameter which changes as a function of temperature of the cooking
plate.
Claims
1. A cooking apparatus comprising a radiant electric heater (2)
and electronic control apparatus (20) the heater being arranged
for location underneath and against a cooking plate (12) and incorporating
a heating element (14) spaced from the cooking plate and a temperature
sensor assembly (26), wherein the temperature sensor assembly comprises
a beam (28) of ceramic material provided within the heater (2) and
extending at least partially across the heater over the heating
element (14), the beam having a substantially planar upper surface
(32) arranged to face the cooking plate (12) and an under surface
(34) arranged for exposure to direct radiation from the heating
element, the planar upper surface having provided thereon an electrical
component (36) having an electrical parameter which changes as a
function of temperature of the cooking plate, the electrical component
(36) being electrically connected by means of electrical leads (54)
to the electronic control apparatus (20), which electronic control
apparatus receives input signals from at least one electrical component
(36) on the upper surface of the beam (28) and also input signals
from a manual control switch device (22). the input signals from
the at least one electrical component being processed by a fail-safe
circuit (70) having a fixed threshold temperature such that at least
one heating element (14) is arranged to be de-energised at a temperature
above such fixed threshold.
2. An apparatus as claimed in claim 1, wherein the temperature
sensor assembly (26) is located in a central region of the heater
(2).
3. An apparatus as claimed in claim 1, wherein the temperature
sensor assembly (26) is secured at least at one end region thereof
to the heater (2) at a periphery of the heater.
4. An apparatus as claimed in claim 3, wherein at least one end
region of the beam (28) extends outside the-heater (2).
5. An apparatus as claimed in claim 4, wherein the beam (28) is
secured, at one end region thereof, to the heater (2) by means of
a bracket (44) which securely receives the one end region of the
beam and is fixed to an external region of the heater.
6. An apparatus as claimed in claim 5, wherein the bracket (44)
is selected from metal, ceramic and plastics.
7. An apparatus as claimed in claim 3, wherein the beam (28) is
secured, at one end region thereof, to the heater (2) by securely
passing through an aperture (56) in a peripheral wall (8) of the
heater.
8. An apparatus as claimed in claim 1, wherein the peripheral wall
(8) comprises a substantially rigid material.
9. An apparatus as claimed in claim 8, wherein the peripheral wall
(8) comprises bound vermiculite.
10. An apparatus as claimed in claim 3, wherein a terminal block
(50) is provided in a-position selected from at, and adjacent to,
one end region of the beam (28).
11. An apparatus as claimed in claim 1, wherein the beam (28) is
supported with spring biasing (58) towards the cooking plate (12).
12. (Cancelled)
13. (Cancelled)
14. (Cancelled)
15. An apparatus as claimed in claim 1, wherein the input signals
from the manual control switch device (22), and the input signals
from the at least one electrical component (36), are processed by
a signal processing circuit (74) of a, form selected from analog
and digital form which is in terfaced with a switch means (76) for
controlling energising of the at least one heating element (14).
16. An apparatus as claimed in claim 15, wherein the signal processing
circuit (74) is arranged to compare sensed temperature with position
of the manual control switch device (22) and carry out a function
selected from energising and de-energising the at least one heating
element (14), depending upon whether the sensed temperature is respectively
at a temperature selected from below and above that set by the manual
control switch device.
17. An apparatus as claimed in claim 15, wherein the signal processing
circuit (74) is a digital circuit, comprising a microprocessor interfaced
with the at least one electrical component (36) by way of an analog
to digital converter and interfaced with the manual control switch
device (22) by way of a digital output driver.
18. An apparatus as claimed in claim 15, wherein the signal processing
circuit (74) is an analog circuit comprising an analog signal processing
integrated circuit which compares input signals from the manual
control switch device (22) with input signals from the at least
one electrical component (36) and controls energising of the at
least one heating element (14) in a manner proportional to the difference
between the two input signals.
19. An apparatus as claimed in claim 18, wherein the control of
energising of the at least one heating element (14) is effected
by way of an output signal tailored to specific control requirements
of a solid state switch device (76) which operates to control energising
of the at least one heating element.
20. An apparatus as claimed in claim 1, wherein control of the
at least one heating element (14) is effected in closed loop manner.
21. An apparatus as claimed in claim 1, wherein the upper surface
(32) of the beam (28) is arranged to be in contact with the cooking
plate (12).
22. An apparatus as claimed in claim 1, wherein the upper surface
(32) of the beam (28) is arranged to be in close proximity to the
cooking plate (12).
23. An apparatus as claimed in claim 22, wherein the substantially
planar upper surface (32) of the beam (28) is arranged to face the
cooking plate (12) at a distance of substantially not more than
3.5 mm therefrom.
24. An apparatus as claimed in claim 23, wherein the substantially
planar upper surface (32) of the beam (28) is arranged to face the
cooking plate (12) at a distance of from 0.5 to 3.5 mm therefrom.
25. An apparatus as claimed in claim 24, wherein the substantially
planar upper surface (32) of the beam (28) is arranged to face the
cooking plate (12) at a distance of from 0.5 to 2.0 mm therefrom.
26. An apparatus as claimed in claim 1, wherein the electrical
component (36) having an electrical parameter which changes as a
function of temperature is selected from film and foil form.
27. An apparatus as claimed in claim 26, wherein the electrical
component (36) has electrical conductors selected from film and
foil form extending therefrom to one end region of the beam (28).
28. An apparatus as claimed in claim 26, wherein the electrical
component (36) comprises an electrical resistance component whose
electrical resistance changes as a function of temperature.
29. An apparatus as claimed in claim 28, wherein the electrical
resistance component comprises platinum.
30. An apparatus as claimed in claim 26, wherein the electrical
component (36) is of thick film form.
31. An apparatus as claimed in claim 1, wherein a protective layer
(42) is provided over the electrical component (36).
32. An apparatus as claimed in claim 31, wherein the protective
layer (42) is selected from glass and ceramic.
33. An apparatus as claimed in claim 1, wherein a layer of thermal
radiation reflective material is provided on the under surface (34)
of the beam (28).
34. An apparatus as claimed in claim 1, wherein the beam (28) is
structurally reinforced.
35. An apparatus as claimed in claim 34, wherein the beam (28)
has a shape selected from a T-shaped and H-shaped cross section.
36. An apparatus as claimed in claim 1, wherein the material of
the beam (28) is selected from steatite, alumina and cordierite.
37. An apparatus as claimed in claim 1, wherein a plurality of
heating zones (66, 68), each with a heating element (14A, 14B),
are provided substantially side-by-side in the heater (2), a corresponding
plurality of the electrical components (36A, 36B) being provided
on the substantially planar upper surface (32) of the beam (28),
each of the electrical components being located in a corresponding
heating zone, whereby temperature of the cooking plate (12) is able
to be monitored.
38. An apparatus as claimed in claim 37, wherein the plurality
of heating zones (66, 68) are provided in concentric arrangement.
39. An apparatus as claimed in claim 37, wherein means (20) is
provided to determine the difference in temperature between the
plurality of heating zones (66, 68) in cooperation with the plurality
of electrical components (36A, 36B) and used to determine features
selected from placement and position of a cooking vessel (24) on
the cooking plate (12) and size of a cooking vessel on the cooking
plate and/or curvature of a base of a cooking vessel on the cooking
plate.
Description [0001] The present invention relates to a cooking apparatus incorporating
a radiant electric heater and a temperature sensor assembly, the
heater being arranged for location underneath a cooking plate, such
as of glass-ceramic. More particularly, the present invention relates
to such an apparatus in which a sensing element is provided having
an electrical parameter which changes as a function of temperature.
[0002] Radiant electric beaters are very well known, provided underneath
and in contact with a cooking plate, particularly of glass-ceramic
material, It is common practice to provide such heaters with thermal
sensors of electromechanical or electronic form, the purpose of
which is to limit maximum temperature of the upper surface of the
cooking plate.
[0003] WO 95/16334 describes the use of a one- or two-dimensional
thermoelectrical sensor based on radiation from a vitroceramic surface
to control the temperature of the vitroceramic surface, the sensor
possibly being shielded from direct radiation from the heating elements.
[0004] U.S. Pat. NO. 4,103,275 describes a means for measuring
resistance for a resistance thermometer consisting of an insulating
former as a carrier and a thin platinum layer as resistance material,
the carrier for the platinum layer being made of a material having
a greater thermal coefficient of expansion than platinum over the
range between 0 degrees Celsius and 1000 degrees Celsius.
[0005] In current technology, a temperature sensing probe is located
in a space between a heating element and the underside of the cooking
plate.
[0006] A disadvantage of such an arrangement is that the temperature
attained by the sensing probe is significantly influenced by direct
radiation from the heating element and does not accurately reflect
the temperature of the upper surface of the cooking plate. The probe
temperature can typically be 100 to 200 degrees Celsius higher than
the corresponding temperature of the upper surface of the cooking
plate, As a result, there are two temperature gradients between
the sensing probe and the upper surface of the cooking plate, namely
one temperature gradient between the sensing probe and the underside
of the cooking plate and another temperature gradient between the
underside of the cooking plate and its upper surface. These temperature
gradients may vary as a result of, for example, changes in heater
power density, heater temperature profile, and thermal loading by
a selected cooking vessel located on the upper surface of the cooking
plate. Such a cooking vessel affects the temperature of the upper
surface of the cooking plate.
[0007] A temperature sensor used in such heaters is arranged to
de-energise the heating element at a preset temperature value. Such
preset temperature value is a compromise value to maintain acceptable
maximum temperatures of the cooking plate under the requirements
of abnormal load conditions (for example: no cooking vessel load;
boil dry; offset cooking vessel on cooking plate; cooking vessel
with a concave base, brought to a boil condition), while minimising
the probability of de-energising the heating element under bring-to-boil
conditions in respect of a load in the form of a cooking vessel
located on the cooking plate. Repeated switching of the heating
element under the latter conditions is undesirable, since the time
to boil is increased.
[0008] When electronic temperature sensors are employed, the probability
of such undesirable switching of the heating element occurring may
be significantly reduced by incorporating an intelligent control
profile within a dedicated `fast boil` control setting. This necessitates
use of intelligent, usually digital, microprocessor controllers,
which are expensive.
[0009] The tolerance range of the preset temperature value of the
temperature sensor is critical, as it compounds the aforementioned
variables. Electromechanical temperature sensors yield a tolerance
range of typically 50 to 60 degrees Celsius as a result of constraints
imposed by materials, design and manufacturing technology. Currently
available electronic temperature sensors exhibit much lower tolerance
ranges, but these devices, together with their required control
circuits, cost significantly more than electromechanical temperature
sensor systems.
[0010] Furthermore, due to the aforementioned variables and temperature
gradients, an electronic temperature sensor, as previously described,
is only useful as a maximum temperature control device, such that
it de-energises the heating element at a predetermined temperature
value. Such an electronic temperature sensor is unable to support
a control system that may control the temperature of the cooking
plate in accordance with a required cooking duty cycle, involving
`closed loop` control temperature regulation, Current temperature
regulation systems for cooking appliances having glass-ceramic cooking
plates are `open loop` in nature. Such temperature regulation systems
cannot account for variations in cooking vessel material and geometry,
cooking vessel mass, mass and thermal capacity of a food item in
a cooking vessel, and most importantly, change in temperature gradient
as the cooking vessel and contents heat up, accompanied by evaporation
of water. Constant adjustment of the heater is required by a user,
especially at low settings.
[0011] Furthermore, it is anticipated that the maximum operating
temperatures for glass-ceramic cooking plates will be increased
in the near future by as much as 40 degrees Celsius, as a result
of materials and process development. The objective of this increased
temperature is to provide opportunity for higher temperatures to
be reached before switching of the heating element occurs, thereby
reducing the probability of de-energising of the heater during a
bring-to-boil cycle. Currently available temperature sensors may
require further development in order to withstand the resulting
higher maximum temperatures encountered during service, because
of the constraints imposed by existing sensor element and enclosure
materials. This could lead to increased cost of the sensor system.
[0012] It is an object of the present invention to overcome or
minimise one or more of the aforementioned problems.
[0013] According to the present invention there is provided a cooking
apparatus comprising a radiant electric heater and electronic control
apparatus, the heater being arranged for location underneath and
against a cooking plate and incorporating a heating element spaced
from the cooking plate and a temperature sensor assembly, wherein
the temperature sensor assembly comprises a beam of ceramic material
provided within the heater and extending at least partially across
the heater over the heating element, the beam having a substantially
planar upper surface arranged to face the cooking plate and an under
surface arranged for exposure to direct radiation from the heating
element, the planar upper surface having provided thereon an electrical
component having an electrical parameter which changes as a function
of temperature of the cooking plate, the electrical component being
electrically connected by means of electrical leads to the electronic
control apparatus, which electronic control apparatus receives input
signals from the or each electrical component on the upper surface
of the beam and also input signals from a manual control switch
device, the input signals from the or each electrical component
being processed by a fail-safe circuit having a fixed threshold
temperature such that the or each heating element is arranged to
be de-energised at a temperature above such fixed threshold.
[0014] The temperature sensor assembly may be located in a central
region of the heater.
[0015] Alternatively, the temperature sensor assembly may be secured
at least at one end region thereof to the heater at a periphery
of the heater. At least one end region of the beam may extend outside
the heater.
[0016] In such a case the beam may be secured, at one end region
thereof, to the heater by means of a bracket which securely receives
one end region of the beam and is fixed to an external region of
the heater. The bracket may be of metal, ceramic or plastics.
[0017] Alternatively, the beam may be secured, at one end region
thereof, to the heater by securely passing through an aperture in
a peripheral wall of the heater. Such peripheral wall may comprise
a substantially rigid material, such as bound vermiculite.
[0018] A terminal block may be provided at, or adjacent to, one
end region of the beam.
[0019] The beam may be supported with spring biasing towards the
cooking plate,
[0020] The input signals from the manual control switch device,
and the input signals from the or each electrical component, may
be processed by a signal processing circuit of analog or digital
form, which is interfaced with a switch means for controlling energising
of the or each heating element. The signal processing circuit may
be arranged to compare sensed temperature with position of the manual
control switch device and energise or de-energise the or each heating
element, depending on whether the sensed temperature is respectively
below or above that set by the manual control switch device.
[0021] When the signal processing circuit is a digital circuit,
it may comprise a microprocessor interfaced with the or each electrical
component by way of an analog to digital converter and interfaced
with the manual control switch device by way of a digital output
driver.
[0022] When the signal processing circuit is an analog circuit,
it may comprise an analog signal processing integrated circuit which
compares input signals from the manual control switch device with
input signals from the or each electrical component and controls
energising of the or each heating element in a manner proportional
to the difference between the two input signals. Such control of
energising of the or each heating element may be effected by way
of an output signal tailored to specific control requirements of
a solid state switch device which operates to control energising
of the or each heating element.
[0023] Control of the or each heating element may be effected in
closed loop manner.
[0024] The upper surface of the beam may be arranged to be in contact
with or in close proximity to the cooking plate. The substantially
planar upper surface of the beam may be arranged to face the cooking
plate at a distance of substantially not more than 3.5 mm therefrom.
The substantially planar upper surface of the beam may preferably
be arranged to face the cooking plate at a distance of from 0.5
to 3.5 mm therefrom and more preferably at a distance of from 0.5
to 2.0 mm therefrom.
[0025] The electrical component having an electrical parameter
which changes as a function of temperature may be of film or foil
form. The electrical component of film or foil form may have electrical
conductors of film or foil form extending therefrom to one end region
of the beam which is adapted to be secured to the heater at the
periphery thereof. The electrical component of film or foil form
may comprise an electrical resistance component whose electrical
resistance changes as a function of temperature. Such electrical
resistance component may comprise platinum. The electrical component
may be of thick film form.
[0026] A protective layer, such as of glass or ceramic, may be
provided over the electrical component.
[0027] A layer of thermal radiation reflective material may be
provided on the under surface of the beam.
[0028] The beam may be structurally reinforced, such as by having
a T-shaped or H-shaped cross section.
[0029] The beam may comprise steatite, alumina or cordierite.
[0030] A plurality of heating zones, each with a heating element,
may be provided substantially side-by-side in the heater, such as
in concentric arrangement, a corresponding plurality of the electrical
components being provided on the substantially planar upper surface
of the beam, each of the electrical components being located in
a corresponding heating zone, whereby temperature of the cooking
plate in each heating zone is able to be monitored.
[0031] Difference in temperature between the plurality of heating
zones may be determined by the electronic control apparatus in cooperation
with the plurality of electrical components and used to determine
placement and/or position of a cooking vessel on the cooking plate
and/or size of a cooking vessel on the cooking plate and/or curvature
of a base of a cooking vessel on the cooking plate.
[0032] The cooking apparatus of the present invention is advantageous
in that the temperature-sensitive electrical component or components,
for example of film or foil, on the upper surface of the beam is
or are directed towards the cooking plate and not exposed to direct
radiation from the heating element or elements. The beam shields
the temperature-sensitive component or components from the heating
element or elements.
[0033] The temperature sensor assembly is constructed such that
it can be in proximity with the underside of the cooking plate,
thereby ensuring that the temperature gradient between the temperature
sensor assembly and the underside of the cooking plate is significantly
reduced. A resulting increased distance between the heating element
or elements and the temperature sensor assembly reduces the influence
of direct radiation from the heating element or elements on the
sensor assembly.
[0034] As a result of a lower temperature differential between
the cooking plate and the temperature sensor assembly, such assembly
can be made to be more reliable at the higher maximum temperatures
of the cooking plate which are being introduced. Average maximum
temperatures of 590 to 630 degrees Celsius are expected to be introduced
for glass-ceramic cooking plates, compared with the present maximum
temperatures of 550 to 590 degrees Celsius.
[0035] A further advantage resulting from the present invention
is that a heater system may be set to provide a lower cooking plate
temperature under free radiation conditions, without incurring unwanted
switching of the heating element or elements during boiling cycles
with a cooking vessel. Having lower cooking plate temperatures under
free radiation conditions reduces heat loss under these conditions,
thereby resulting in increased efficiency of a cooking appliance.
[0036] The improved thermal coupling between the temperature sensor
assembly and the cooking plate allows the use of low-cost electronic
control technology, avoiding the need for special high temperature
excursion profiles applied through software-based algorithms programmed
into microcontrollers.
[0037] Maximum temperature control can be provided through a single
predetermined set point, allowing the use of a low-cost integrated
circuit, embodying analog or digital technology, and combining temperature
limiting and heater energy regulating functions.
[0038] There is also the potential to apply closed loop control
of the cooking plate temperature by regulation of heater input energy.
The cooking plate temperature can be applied as an input to the
regulator control system, enabling more consistent and predictable
power control to be achieved.
[0039] For a better understanding of the present invention and
to show more clearly how it may be carried into effect, reference
will now be made, by way of example, to the accompanying drawings
in which:
[0040] FIG. 1 is a plan view of one embodiment of a cooking apparatus
according to the present invention including a radiant electric
heater provided with an embodiment of a temperature sensor assembly
and provided with electronic control apparatus shown in schematic
form;
[0041] FIG. 2 is a cross-sectional view of the heater of FIG. 1,
beneath a cooking plate;
[0042] FIGS. 3 and 4 are perspective views from different angles
of the temperature sensor assembly as provided in the heater of
FIG. 1;
[0043] FIG. 5 is a detail showing an alternative mounting arrangement
for the temperature sensor assembly in the heater of FIG. 1;
[0044] FIG. 6 is a cross-sectional view of a further embodiment
of a radiant electric heater forming part of the present invention
and incorporating a temperature sensor assembly;
[0045] FIGS. 7 and 8 are perspective views from different angles
of the temperature sensor assembly as provided in the heater of
FIG. 6;
[0046] FIG. 9 is a plan view of a radiant electric heater forming
part of the present invention having dual heating zones and provided
with a temperature sensor assembly; and
[0047] FIG. 10 is a schematic diagram of an embodiment of electronic
control apparatus for use with the radiant electric heater forming
part of the present invention.
[0048] Referring to FIGS. 1 and 2, a radiant electric heater 2
comprises a metal dish-like support 4 having therein a base 6 of
thermal and electrical insulation material, such as microporous
thermal and electrical insulation material, and a peripheral wall
8 of thermal and electrical insulation material, such as bound vermiculite.
The peripheral wall 8 is arranged to contact the underside 10 of
a cooking plate 12, suitably of glass-ceramic material.
[0049] At least one radiant electric heating element 14 is supported
in the heater relative to the base 6. The heating element or elements
14 can comprise any of the well-known forms of element, such as
wire, ribbon, foil or lamp forms of element, or combinations thereof.
In particular, the heating element or elements 14 can comprise one
or more corrugated ribbon heating elements supported edgewise on
the base 6.
[0050] The at least one heating element 14 is connected by way
of a terminal block 16 at the edge of the heater to a power supply
18, through electronic control apparatus 20 provided with a manually
operated control switch device 22. The control switch device 22
has a rotatable knob arranged to provide selected heating settings
for the at least one heating element 14 in the heater 2.
[0051] The cooking plate 12 is arranged to receive a cooking vessel
24 on an upper surface 25 thereof.
[0052] A temperature sensor assembly 26, as shown in detail in
FIGS. 3 and 4, is provided in the heater 2. The temperature sensor
assembly 26 comprises a beam 28 of ceramic material supported at
one end region 30 thereof at the periphery of the heater and extending
at least partially across the heater over, and spaced from, the
at least one heating element 14. The beam 28 has a substantially
planar upper surface 32 arranged to face the underside 10 of the
cooking plate 12, in contact therewith or in close proximity thereto.
It is preferred that the upper surface 32 of the beam 28 should
be spaced from the underside 10 of the cooking plate 12 by at least
0.5 mm, but by not more than about 3.5 mm and preferably by not
more than about 2.0 mm.
[0053] The beam 28 suitably comprises steatite, alumina or cordierite
ceramic material and is structurally reinforced to minimise risk
of bending and/or fracture. Such structural reinforcement is suitably
achieved by providing the beam 28 of H-shaped cross-section, as
shown in FIG. 3, or of T-shaped cross-section, as shown in an embodiment
in FIG. 7 to be described later.
[0054] The beam 28 has an under surface 34 arranged for exposure
to direct radiation from the at least one heating element 14. The
under surface 34 of the beam 28 may be provided with a layer of
thermal radiation reflective material, such as silver, to minimise
absorption, by the beam 28, of heat from the direct radiation of
the at least one heating element 14.
[0055] The substantially planar upper surface 32 of the beam 26
has provided thereon at least one electrical component 36 of film
or foil form having an electrical parameter which changes as a function
of temperature of the cooking plate 12. The at least one electrical
component 36 has electrical conductors 38, of film or foil form,
extending therefrom to terminal lands 40 at the end region 30 of
the beam 28.
[0056] The at least one electrical component 36 of film or foil
form preferably comprises at least one electrical resistance component
whose electrical resistance changes as a function of temperature.
Such at least one electrical resistance component suitably comprises
platinum.
[0057] The at least one electrical component 36 and the leads 38
are suitably of thick film form, provided by screen-printing and
firing onto the upper surface 32 of the beam 28.
[0058] A protective layer 42, such as of glass or ceramic, may
be provided over the at least one electrical component 36 of film
or foil form.
[0059] The beam 28 is arranged to extend through an aperture in
the peripheral wall 8 and rim of the dish-like support 4 of the
heater 2 and such that the end region 30 of the beam 28 is located
outside the heater 2.
[0060] The beam 28 is secured to the heater 2 by means of a bracket
44, suitably of metal, ceramic or plastics material, which is fixed
to the end region 30 of the beam 28 and secured to the rim of the
dish-like heater support 4 by a threaded fastener 46 passing through
a hole 48 in the bracket 44. The beam 28 may have its end region
30 insert-moulded into the bracket 44, when the bracket 44 is of
plastics material. When the bracket 44 is of ceramic material, it
may be secured to the end region 30 of the beam 28 such that a dovetailed
interconnection is formed therebetween.
[0061] A terminal block 50 is suitably secured to the bracket 44
and connected by lead wires 52 to the terminal lands 40 on the end
region 30 of the beam 28. Since the end region 30 of the beam is
located outside the heater, the lead wires 52 need not have a high-temperature-withstandi-
ng capability and may comprise a material such as copper.
[0062] When the bracket 44 comprises plastics or ceramic material,
the terminal block 50 can be formed integral therewith.
[0063] The temperature sensor assembly 26 is electrically connected
to the electronic control apparatus 20 by electrical leads 54.
[0064] As shown in FIG. 5, instead of a bracket 44 being provided
to secure the beam 28 at the end region 30 thereof to the heater
2, the beam 28 has its end region 30 secured in an aperture 56 of
complementary shape, provided in the peripheral wall 8 of the heater.
Such peripheral wall 8, particularly of rigid bound vermiculite,
can be arranged to provide satisfactory securing of the beam to
the heater 2.
[0065] FIGS. 6, 7 and 8 illustrate an alternative embodiment which
is substantially similar to that of FIGS. 1 to 4, with the main
exception that the beam 28, which is here provided of T-shaped cross-section,
is spring-loaded against the underside 10 of the cooking plate 12.
Such spring-loading allows contact between the upper surface 32
of the beam 28 and the underside 10 of the cooking plate 12, while
minimising risk of mechanical damage to the cooking plate 12 and/or
the temperature sensor assembly 26, when subjected to mechanical
shock load conditions.
[0066] The spring loading is achieved by incorporating one or more
coil springs 58 cooperating between the end supporting bracket 44,
which is suitably of metal, such as nickel-plated steel, and the
underside of the end region 30 of the beam 28. A strut 60 is also
provided at a central region of the heater 2, the strut 60 extending
downwardly from the beam 28, through an aperture provided in the
base 6 and the metal dish-like support 4, and into an aperture provided
in a metal bracket 62 secured to the dish-like support 4. A coil
spring 64 is arranged to co-operate between the strut 60 and the
metal bracket 62.
[0067] The radiant electric heater 2 constructed according to the
present invention is advantageous in that the temperature sensor
assembly 26 is thermally closely-coupled to the cooking plate 12,
thereby ensuring that any temperature gradient between the assembly
and the underside 10 of the cooking plate 12 is minimised. The at
least one temperature-responsive electrical component 36 of film
or foil form on the upper surface 32 of the beam 28 is or are screened
from the direct radiation from the at least one heating element
14 by the thickness of the beam 28 and the at least one temperature-responsive
electrical component 36 responds primarily to the temperature of
the cooking plate 12. This enables simplified electronic control
apparatus 20 to be employed.
[0068] The close proximity of the temperature sensor assembly 26
to the cooking plate 12 results in increased distance between the
at least one heating element 14 and the temperature sensor assembly
26 and this, coupled with the optional feature of a reflective layer
on the underside of the beam 28, reduces the influence of direct
radiation from the at least one heating element 14 on the temperature-responsive
electrical component or components 36 of the temperature sensor
assembly 26.
[0069] Referring now to FIG. 9, a radiant electric heater 2 is
constructed in similar manner to the heater of FIGS. 1 and 2 with
the exception that multiple heating zones are provided. As shown,
two heating zones 66 and 68 are provided, although more than two
could be considered. The heating zones 66, 68 are concentrically
arranged and each is provided with at least one heating element
14A, 14B. The heating zone 66 is arranged to be energised alone
or together with the heating zone 68.
[0070] A temperature sensor assembly 26A is provided, substantially
as previously described for the temperature sensor assembly 26,
with the exception that two separate temperature-responsive electrical
components 36A and 36B of film or foil form are provided on the
upper surface 32 of the beam 28. The component 36A monitors the
temperature of a region of the cooking plate above the heating zone
66 and the component 36B monitors the temperature of a region of
the cooking plate above the heating zone 68.
[0071] A change in differential temperature between the cooking
zones 66, 68 can be monitored to detect the size of a cooking vessel
(such as the cooking vessel 24 shown in FIG. 2) located on the cooking
plate over the heater. If the temperature of the cooking plate above
the outer heating zone 68 increases relative to that above the inner
heating zone 66, this would indicate that a small cooking vessel
has been placed over the inner heating zone 66, but with both heating
zones 66, 68 energised. If both heating zones 66, 68 are detected
as becoming excessively hot, this would indicate both zones 66,
68 being energised under free-radiation conditions, that is without
a cooking vessel being present.
[0072] If the temperature of the cooking plate above the inner
zone 66 is detected as being high relative to that above the outer
zone 68, this could indicate that a cooking vessel having a bowed
base has been placed on the cooking plate.
[0073] FIG. 10 illustrates an embodiment of electronic control
apparatus 20 for use in the present invention. The apparatus 20
is arranged to receive input signals from the at least one temperature-responsive
component 36 of film or foil form of the temperature sensor assembly
26 and also input signals from the manual control switch device
22. The input signals from the temperature sensor assembly 26 are
processed by a fail-safe circuit 70 having a fixed threshold for
temperature, and such that the at least one heating element 14 is
arranged to be de-energised by operation of a main switch 72, such
as a relay, at a temperature above such fixed threshold for temperature.
[0074] The input signals from the manual control switch device
22, and the input signals from the temperature sensor assembly 26
are processed by a signal processing circuit 74, of analog or digital
form, which is interfaced with a solid-state control switch 76,
such as a triac, for controlling energising of the at least one
heating element 14. The signal processing circuit 74 is arranged
to compare temperature sensed by the sensor assembly 26 with position
of the manual control switch device 22 and energise or de-energise
the at least one heating element 14, depending upon whether the
sensed temperature is respectively below or above that set by the
manual control switch device 22.
[0075] When the processing circuit 74 is a digital circuit, it
suitably consists of a microprocessor interfaced with the temperature
sensor assembly 26 by way of an analog to digital converter, and
also interfaced with the manual control switch device 22 by way
of a digital output driver.
[0076] When the processing circuit 74 is an analog circuit, it
suitably comprises an integrated circuit adapted for analog signal
processing, which compares input signals from the manual control
switch device 22 with input signals from the temperature sensor
assembly 26 and controls energising of the at least one heating
element 14 in a manner proportional to the difference between the
two input signals. Such control of energising of the at least one
heating element 14 is effected by way of an output signal tailored
to specific control requirements of the solid-state switch 76.
[0077] Control of the at least one heating element 14 is thus able
to be effected in a manner which is known as closed-loop control.
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