GB2392499A

GB2392499A - Electric heater

Info

Publication number: GB2392499A
Application number: GB0219817A
Authority: GB
Inventors: Peter Ravenscroft Wilkins
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 2002-08-24
Filing date: 2002-08-24
Publication date: 2004-03-03
Anticipated expiration: 2022-08-24
Also published as: GB0219817D0; DE20313067U1; GB2392499B

Abstract

An electric heater (2) for use beneath a glass-ceramic cooking surface (4), adapted to receive a metallic cooking vessel (6), comprises at least one electric heating element (12; 14) supported relative to a base (10) of thermal and electrical insulation material. Upstanding ring-shaped walls (18; 20) of insulation material has an upper surface (22; 26) adapted to contact the underside of the cooking surface (4). At least one loop-form electrically inductive sensor means (32; 34) is provided at a region of the upper surface (22; 26) of the wall and is connected to circuitry for sensing location and/or size of the cooking vessel (6) on the cooking surface. The loop-form sensor means comprises flexible metal foil material, which may be ribbon form and corrugated.

Description

- 1 - ELECTRIC HEATER

This invention relates to an electric heater for use beneath a glassceramic cooking surface and provided with 5 sensor means for detecting the presence and/or size of a metallic cooking vessel located on the cooking surface.

It is known to provide a radiant electric heater comprising a dish-like support, such as of metal, having 10 therein at least one electric heating element. At least one upstanding ring-shaped wall of thermal and electrical insulation material is generally provided in the dish like support and having an upper surface thereof contacting the underside of the cooking surface. The 15 wall or walls define one or more heating zones of the heater. In order to detect the presence and/or size of a metallic cooking vessel located on the cooking surface overlying 20 the heater and to provide automatic energising of the one or more heating elements, as appropriate, it is known to provide one or more electrically inductive loop sensors at a region of the upper surface of the one or more walls of the heater. An inductive loop sensor provided for 25 this purpose generally comprises a metal wire, which is

- 2 - arranged to be connected to appropriate external processing circuitry.

For such an inductive loop sensor system to function 5 reliably, the sensor loop has to be located as close as possible to the underside of the glass-ceramic cooking surface. It is known to provide one or more wire-form inductive loop sensors in one or more pre-formed grooves or openings in the top surface of one or more walls of 10 the heater. However, a typical section of the wire material used is generally such that the resulting mass of the loop may be sufficient to cause damage to, or mechanical failure of, the glass-ceramic cooking-surface when a cooking appliance comprising the heater and the 15 cooking surface is subjected to mechanical shock, particularly under transit test conditions which the appliance is required to meet.

In order to reduce this problem, the wire-form sensor 20 loop has to be provided and installed with a high degree of dimensional and geometric accuracy in the heater, which leads to complications and high costs in assembly.

It is an object of the present invention to overcome or 25 minimise this problem.

r - 3 - According 0 the present invention there is provided an electric heater for use beneath a glass-ceramic cooking surface, the cooking surface being adapted to receive a metallic cooking vessel, the heater comprising: at least 5 one electric heating element supported relative to a base of thermal and electrical insulation material; at least one upstanding ring-shaped wall of thermal and electrical insulation material having an upper surface thereof adapted to contact the underside of the cooking surface; 10 and at least one loop-form electrically inductive sensor means provided at a region of the upper surface of the at least one wall and adapted to be connected to electric circuitry for sensing location and/or size of the cooking vessel on the cooking surface overlying the heater, the 15 at least one loop-form sensor means comprising flexible metal foil material.

The metal foil material may be arranged in the form of one or more ribbonform loops having a width greater than 20 a thickness thereof and which may be of substantially rectangular cross-section.

The one or more ribbon-form loops may be arranged flat or edgewise at the upper surface of the at least one wall 25 and may be received in one or more elongate grooves in the upper surface of the at least one wall.

l The one or more ribbon-form loops may be arranged supported flat on the upper surface of the at least one wall. 5 Alternatively, the one or more ribbon-form loops may be arranged edgewise in the one or more elongate grooves in the upper surface of the at least one wall, and may be of corrugated form.

10 The one or more ribbon-form loops may form an interference fit in the one or more elongate grooves.

The one or more elongate grooves may be of a depth such that the one or more ribbon-form loops received therein 15 does or do not substantially protrude from the upper surface of the at least one wall.

The one or more elongate grooves may be of meandering form in the at least one wall.

When the wall, or one of the walls, is provided at a periphery of the heater against a rim of a dish-like support, the ribbon-form loop in its elongate groove may be arranged such as to maximise distance between the 25 ribbon-form loop and the rim of the dish-like support.

The elongate groove receiving the ribbon-form loop may be

arranged to mear,de between one or more regions of the upper surface of the wall most remote from the rim of the dish-like support and a middle region of the upper surface of the wall.

The dish-like support may comprise metal.

The metal foil material may comprise a non-magnetic material, such as a stainless steel.

The metal foil material may have a thickness of between 0.05 and 0.5 mm.

The use of the flexible metal foil material for the at 15 least one electrically inductive sensor means results in a cost-effective sensor means which, if in contact with the glass-ceramic cooking surface, does not result in mechanical damage to, or mechanical failure of, the cooking surface under mechanical shock load conditions in 20 a cooking appliance.

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 25 accompanying drawings in which: .

- 6 - Figure 1 is a plan view of an embodiment of an electric heater according to the present invention; Figure 2 is a cross-sectional view of the heater of 5 Figure 1, shown beneath a glass-ceramic cooking surface supporting a cooking vessel; Figure 3 is a perspective view of ribbonform sensor loops of flexible metal foil material provided in the 10 heater of Figure 1; Figure 4 is a perspective view of part of one of the ribbon-form sensor loops of Figure 3 provided of corrugated form; Figure 5 is a plan view of part of an inner wall of the heater of Figure 1; Figures 6 and 7 are plan views of alternative 20 arrangements of an outer wall of the heater of Figure 1; Figure 8 is a cross-sectional view of a modification of the heater of Figure 1 in which an alternative arrangement of loop-form sensor means is provided;

figure 9 Is a plan view of the ioop-torm sensor means of the heater of Figure 8; and Figures 10 and 11 are plan views of loop-form sensor 5 means for use in further embodiments of electric heaters according to the present invention.

Referring to Figures 1 and 2, a radiant electric-heater 2 is provided for location beneath a glass-ceramic cooking 10 surface 9. The cooking surface 4 is arranged to receive a cooking vessel 6.

The heater 2 comprises a metal dish-like support 8 containing a base layer 10 of thermal and electrical 15 insulation material, such as microporous thermal and electrical insulation material.

Two electric heating elements are supported relative to the base layer 10 and spaced beneath the glass-ceramic 20 cooking surface. A central heating element 12 is surrounded by an outer heating element 14. The heating elements 12, 14 are connected to a terminal block 16 at the edge of the heater and by means of which the heater is arranged to be connected to a power supply for 25 -energising the heating elements. The heater is arranged whereby the central heating element 12 may be energised

- 8 - alone, for heating a small cooking vessel 6A, or together with the outer heating element 14, for heating a larger cooking vessel 6B.

5 Two upstanding ring-shaped walls 18, 20 of thermal and electrical insulation material are provided in the heater. An inner wall 18 surrounds the inner heating element 12 and has an upper surface 22 adapted to contact the underside of the glass-ceramic cooking surface 4. An 10 outer wall 20 peripherally surrounds the heater, against a rim 24 of the dish-like support 8, and has an upper surface 26 adapted.to contact the underside of the glass-

ceramic cooking surface 4. The walls 18, 20 suitably comprise bound vermiculite material, but may comprise any 15 of the well-known materials used for walls in heaters of this nature.

Part of the inner wall 18 is shown in detail in Figure 5 and is provided with a tunnel 28 integral therewith, 20 which extends to the outer wall 20 across a region of the base layer 10 not occupied by the outer heating element 14. A well-known form of rod-like temperature-responsive device 30 extends from a periphery of the heater to a central region of the heater, through the tunnel 28.

- 9 - In order- Lo uecect One presence andior size of the cooking vessel 6 located on the cooking surface 4 and to automatically energise the inner heating element 12 alone, or the inner 12 and outer 14 heating elements 5 together, according to whether a small 6A or large 6B cooking vessel is located on the cooking surface, an electrically inductive loop sensor arrangement is provided. A first loop-form inductive sensor 32 is provided at a region of the upper surface 22 of the inner 10 wall 18 and a second loop-form inductive sensor 34 is provided at a region of the upper surface 26 of the outer wall 20. The loop-form inductive sensors 32, 34 are electrically connected to external circuitry 35 and operate in known manner to sense changes in electrical 15 inductance resulting from placement of the cooking vessel 6 on the cooking surface 4 and to effect automatic energising of the inner heating element 12 alone, or the inner 12 and outer 14 heating elements together, depending upon whether a small cooking vessel 6A or a 20 larger cooking vessel 6B is detected.

As particularly shown in Figure 3, the first and second loop-form inductive sensors 32, 34 comprise flexible metal foil material arranged in the form of first and 25 second ribbon-form loops having a width greater than a thickness thereof and of substantially rectangular cross

- 10 -

section. The ribbon-form loops 32, 34 may be corrugated if desired, as shown in Figure 4, to increase their structural rigidity. The metal foil material suitably comprises a non-magnetic stainless steel material and has 5 a thickness suitably between about 0.05 mm and about 0.5 mm, and typically about 0.3 mm. The first ribbon-form loop 32 is arranged edgewise (upright), and forms an interference fit, in an elongate groove 36 in the upper surface 22 of the inner wall 18. The second ribbon-form 10 loop 34 is likewise arranged edgewise (upright), and forms an interference fit, in an elongate groove 38 in the upper surface 26 of the outer wall 20. The grooves 36, 38 are provided of a suitable depth such that the ribbon-form loops 32, 34 located therein do not 15 substantially protrude from the upper surfaces 22, 26 of the walls 18, 20.

There is a problem with inductive sensors used in heaters for cooking vessel detection in that drift in sensed 20 electrical inductance can occur due to small changes in electromagnetic properties of the dish-like support 8 with temperature. For this reason it is desirable that the ribbon-form loop 34 of foil material, located in the groove 38 in the wall 20 adjacent the rim 24 of the dish 25 like support 8, should be located as far as possible away from the rim 24 of the dish-like support 8. As shown in

Figures 6 and 7, this is achieved by arranging for the elongate groove 38 receiving the ribbon-form loop 34 to meander between one or more regions 40 of the upper surface of the wall 20 most remote from the rim 24 of the 5 dish-like support 8 and a middle region 42 of the upper surface of the wall 20. The flexible nature of the.

ribbon-form loop 34 permits easy insertion of the loop 34 into the meandering elongate groove 38.

10 Instead of providing the ribbon-form loops 32, 34 of flexible metal foil material inserted edgewise (upright) into grooves 36, 38 in the walls 18, 20, the ribbon-form loops 32, 34 of flexible metal foil material may be provided in a planar arrangement, as shown in Figures 8 15 and 9 and located flat on the upper surfaces 22, 26 of the inner and outer walls 18, 20 of the heater, the grooves 36 and 38 of Figure 2 being omitted. The heater 2 of Figure 8 is otherwise constructed in substantially the same way as described for the heater 2 of Figures 1 20 and 2.' As shown in Figure 9, the ribbon-form loops 32, 34 can be formed as an integral component, having a common terminal region, from a sheet of metal foil material. 25 Although the heater 2 of Figures 1, 2 and 8 is provided with two walls 18, 20, each provided with a loop-form

- 12 -

sensor means 32, 34, the present invention is not limited to such a heater and a heater with any desired number of ring-shaped walls could be contemplated. For example, a heater with a single heating zone may be provided having 5 only an outer wall 20 and a single loop-form metal foil sensor 34, an example of such a sensor 34 being shown in Figure 10.

A metal foil sensor arrangement for use in a two-zone 10 oval heater of well known form is shown in Figure 11 and comprises a first loop-form element 32A of metal foil material and a second loop-form element 34A, also of metal foil material. The elements 32A and 34A may be formed as an integral component from a sheet of metal 15 foil material.

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Claims

CLA1MS

1. An electric heater for use beneath a glass-ceramic cooking surface, the cooking surface being adapted to 5 receive a metallic cooking vessel, the heater comprising: at least one electric heating element supported relative to a base of thermal and electrical insulation material; at least one upstanding ring-shaped wall of thermal and electrical insulation material having an upper surface 10 thereof adapted to contact the underside of the cooking surface; and at least one loop-form electrically inductive sensor means provided at a region of the upper surface of the at least one wall and adapted to be connected to electric circuitry for sensing location 15 and/or size of the cooking vessel on the cooking surface overlying the heater, the at least one loop-form sensor means comprising flexible metal foil material.

2. A heater as claimed in claim 1, wherein the metal 20 foil material is arranged in the form of one or more ribbon-form loops having a width greater than a thickness thereof.

3. A heater as claimed in claim 2, wherein the one or 25 more ribbon-form loops is or are of substantially rectangular cross-section.

- 14

4. A heater as claimed in claim 2 or 3, wherein the one or more ribbon-form loops is or are arranged flat or edgewise at the upper surface of the at least one wall.

5 5. A heater as claimed in claim 4, wherein the one or more ribbon-form loops is or are received in one or more elongate grooves in the upper surface of the at least one wall. 10

6. A heater as claimed in claim 4 or 5, wherein the one or more ribbon-form loops is or are arranged supported flat on the upper surface of the at least one wall.

7. A heater as claimed in claim 5, wherein the one or 15 more ribbon-form loops is or are arranged edgewise in the one or more elongate grooves in the upper surface of the at least one wall.

8. A heater as claimed in claim 7, wherein the one or 20 more ribbon-form loops is or are of corrugated form.

9. A heater as claimed in claim 7 or 8, wherein the one or more ribbonform loops form(s) an interference fit in the one or more elongate grooves.

- 15

10. A heater as claimed in any of claims 7 to 9, wherein the one or more elongate grooves is or are of a depth such that the one or more ribbon-form loops received therein does or do not substantially protrude from the 5 upper surface of the at least one wall.

11. A heater as claimed in any of claims 7 to TO, wherein the one or more elongate grooves is or are of meandering form in the at least one wall.

12. A heater as claimed in any of claims 7 to 11, wherein the wall, or one of the walls, is provided at a periphery of the heater against a rim of a dish-like support and the ribbon-form loop in its elongate groove 15 is arranged such as to maximise distance between the ribbon-form loop and the rim of the dish-like support.

13. A heater as claimed in claim 12, as dependent upon claim 11, wherein the elongate groove receiving the 20 ribbon-form loop is arranged to meander between one or more regions of the upper surface of the wall most remote from the rim of the dish-like support and a middle region of the upper surface of the wall.

25

14. A heater as claimed in claim 12 or 13, wherein the dish-like support comprises metal.

- 16

15. A heater as claimed in any preceding claim, wherein the metal foil material comprises a non-magnetic material. 5

16. A heater as claimed in claim 15, wherein the metal foil material comprises a stainless steel

17. A heater as claimed in any preceding claim, wherein the metal foil material has a thickness of from 0.05 to 10 0.5 mm.

18. An electric heater substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.