Patent Publication Number: US-7718931-B2

Title: Electric heater incorporating a device for detecting a cooking utensil

Description:
This invention relates to an electric heater incorporating a device for detecting a cooking utensil, such as a pan, supported on an upper surface of a cooking plate, such as of glass-ceramic material, overlying the heater, the electric heater incorporating at least one electric heating element. Such a device comprises at least one inductively-operating loop of electrically conductive material which is provided in the heater and is electrically connected to an electrical circuit, such as an oscillatory circuit, which operates to detect changes in electrical inductance in the loop of electrically conductive material resulting from placement and/or removal of the cooking utensil on and/or from the upper surface of the cooking plate, and to effect appropriate energising and/or de-energising of the at least one electric heating element. 
     DESCRIPTION OF PRIOR ART 
     For efficient operation of the detecting device, it has been found necessary to provide the inductively-operating loop as close as possible to the lower surface of the cooking plate and also such that it extends spaced from and at least partly overlying the electric heating element. This has hitherto been accomplished by providing the loop of a relatively thick rigid wire material so as to be self-supporting where it overhangs the heating element, and particularly to avoid deformation and sagging when subjected to normal operating temperatures in the heater. Furthermore, because such a loop is exposed to direct thermal radiation from the underlying heating element or elements and subjected to temperatures of the order of 750 degrees Celsius, it has been found necessary to provide the loop of a relatively expensive material which is not susceptible to thermal damage by the direct thermal radiation from the heating element or elements. 
     It has also been proposed to provide a wire-form inductively-operating loop stretched across the top of a heater between supports at the periphery of the heater, the wire-form loop being suitably arranged in the form of an open triangle. However, in order to avoid deformation and sagging of the loop during operation of the heater, it has been found necessary to provide one or more elastically-resilient supports for the loop, to maintain a suitable mechanical tension in the loop. Such an arrangement is inconvenient to implement. 
     OBJECT OF THE INVENTION 
     It is an object of the present invention to overcome or minimise the aforementioned problems. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an electric heater incorporating at least one heating element, at least one wall member upstanding in the heater, and a device for detecting a cooking utensil supported on an upper surface of a cooking plate overlying the heater, the device comprising at least one inductively-operating loop of electrically conductive material having a plurality of portions thereof adapted to extend over and spaced from the at least one electric heating element between fixed supporting regions on the least one wall member, the at least one loop of electrically conductive material being such that the plurality of portions are substantially incapable of self-support as such at normal operating temperatures of the electric heater, the plurality of portions being supported by elongate members of heat-withstanding material. 
     For the avoidance of doubt, it is to be understood that by the plurality of portions of the at least one loop of electrically conductive material being substantially incapable of self-support at normal operating temperatures of the electric heater, it is meant that the plurality of portions would undergo deformation, such as sagging, at normal operating temperatures of the electric heater, if not supported. 
     The at least one wall member may be adapted to contact the lower surface of the cooking plate and may comprise thermal and electrical insulation material. The fixed supporting regions may be provided at upper regions of the at least one wall member, such as in the form of recesses or channels for receiving intermediate and/or end portions of the at least one loop of electrically conductive material or ends of the elongate members, or in the form of regions of the at least one wall member into which the intermediate and/or end portions of the at least one loop of electrically conductive material, or the ends of the elongate members, are embedded or moulded. 
     The elongate members of heat-withstanding material may comprise beams or tubes. Such beams may comprise channels for receiving the portions of the at least one loop of electrically conductive material, and the beams may be arranged such that the channels are open towards the lower surface of the cooking plate. Such tubes may be of circular, oval or rectangular cross-section. 
     The elongate members of heat-withstanding material may comprise ceramic material, such as steatite, cordierite, alumina or a glass material such as quartz. 
     The elongate members may each be substantially linear and may be arranged in a substantially rectangular, triangular, trapezoidal or V-shaped array. 
     The elongate members may be interconnected or united at ends thereof to form a frame. 
     The at least one loop of electrically conductive material may comprise a thin wire or ribbon of metal or metal alloy and may comprise a single turn thereof. 
     The at least one loop of electrically conductive material may comprise a material which is susceptible to damage by direct thermal radiation from the at least one electric heating element, the plurality of portions of the at least one loop being substantially protected from such damage by being shielded by the elongate members from the direct thermal radiation. The electrically conductive material may comprise a ferritic or austenitic stainless steel or a copper alloy. 
     The electric heater may comprise at least two heating zones, each incorporating at least one heating element and each being surrounded by one of the wall members, at least two of the loops of the electrically conductive material being provided, the loops being associated with the at least two heating zones to detect different sizes of the cooking utensil supported on the upper surface of the cooking plate. 
     The at least one loop of electrically conductive material may have open ends thereof adapted to be electrically connected to circuit means for detecting a change in electrical inductance in the at least one loop associated with placement and/or removal of the cooking utensil on and/or from the upper surface of the cooking plate and for effecting appropriate energising and/or de-energising of the at least one electric heating element. 
     The open ends of the at least one loop of electrically conductive material may be electrically connected to the circuit means by way of electrical terminal means provided at a peripheral region of the electric heater. Such electrical terminal means may be provided on a terminal block, which may comprise or form part of a housing associated with a temperature-responsive device provided on the electric heater. 
     The device may be provided as a modular assembly for attaching to the electric heater. Such modular assembly may include the at least one wall member. 
     The at least one wall member may comprise bound vermiculite. 
     The cooking plate may comprise glass-ceramic material. 
     The electric heater may comprise a base layer of thermal and electrical insulation material relative to which is supported the at least one electric heating element. 
     The electric heater may comprise a dish-like support, such as of metal. 
     By means of the present invention an inductively-operating loop of electrically conductive material may be provided of relatively thin wire or ribbon material, which has portions thereof supported by elongate members overlying one or more heating elements in a heater, the loop also being protected by the elongate members against thermal radiation damage from the heating element or elements, thereby avoiding a requirement for very high temperature-withstanding material to be used for the loop. 
     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: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of an assembly of a cooking utensil supported on an upper surface of a cooking plate overlying an electric heater arranged in contact with a lower surface of the cooking plate, the electric heater being provided with an embodiment of a cooking utensil detection device according to the present invention; 
         FIGS. 2A ,  2 B and  2 C are cross-sectional views of alternative constructions of elongate members supporting portions of an inductively-operating loop of electrically conductive material in the detection device of  FIG. 1 ; 
         FIGS. 3A to 3D  are detailed perspective views of alternative arrangements of fixed supporting regions on a wall member, for the inductively-operating loop in the cooking utensil detection device of  FIG. 1 ; 
         FIG. 4  is a plan view of an integral arrangement of the elongate members supporting the portions of the inductively-operating loop, for use as an alternative arrangement in the detection device of  FIG. 1 ; 
         FIG. 5  is a perspective view of an alternative embodiment of cooking utensil detection device according to the present invention; and 
         FIG. 6  is an exploded perspective view of a further assembly of a cooking utensil supported on an upper surface of a cooking plate overlying an electric heater arranged in contact with a lower surface of the cooking plate, the electric heater having two heating zones and being provided with a further embodiment of cooking utensil detection device according to the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a glass-ceramic cooking plate  2  has an upper surface  4  for receiving a cooking utensil  6 , such as a pan. A lower surface  8  of the cooking plate  2  has an electric heater  10  supported in contact therewith. The electric heater  10  comprises a dish-like support  12 , such as of metal, in which is a base layer  14  of thermal and electrical insulation material, such as microporous thermal and electrical insulation material. A peripheral wall member  16  of thermal and electrical insulation material, such as bound vermiculite, is arranged upstanding in the dish-like support  12  and has an upper surface  18  contacting the lower surface  8  of the cooking plate  2 . 
     At least one radiant electrical resistance heating element  20  is supported relative to the base layer  14 . The heating element or elements  20  can comprise any of the well-known forms of heating element, such as wire, ribbon, foil or lamp forms, or combinations thereof. In particular, the heating element or elements  20  can be of corrugated ribbon form, supported edgewise on the base layer  14  of insulation material. 
     A terminal block  22  is provided at the edge of the dish-like support  12  and electrically connected to one end of the heating element or elements  20 . 
     A temperature-responsive device  24 , of known basic construction, has an elongate probe portion  26  extending at least partly across the heater and overlying the heating element or elements  20 . The temperature-responsive device  24  also has a housing portion  28  of electrically insulating material arranged externally of the heater and suitably containing one or more switch components of known form. The housing portion  28  supports electrical terminals, two of which are visible in  FIG. 1  and denoted by reference numerals  30  and  32 . The heating element or elements  20  is or are electrically connected to a power supply  34  by way of leads  36  and  38  and a control system  40 , which may include microprocessor-based circuitry. The lead  36  is connected to one end of the heating element or elements  20  by way of the terminal block  22 . The lead  38  is connected to the other end of the heating element or elements  20  by way of the terminal  30  on one side of the housing portion  28  of the temperature-responsive device  24 , a switch means (not shown) inside the housing portion  28 , and a further terminal (not shown) on the opposite side of the housing portion  28 . 
     A device  42  is provided for detecting the cooking utensil  6  when placed on and removed from the cooking plate  2  and operating to automatically energise and de-energise the heater  10  by way of the control system  40 . The device  42  comprises an inductively-operating loop  44  of electrically conductive material, in the form of thin wire or ribbon, having open end or terminal portions  46 ,  48 . The loop  44  of wire or ribbon suitably comprises a ferritic or austenitic stainless steel or a copper alloy. 
     The loop  44  has three portions  50 ,  52 ,  54  arranged in triangular form and extending over and spaced from the heating element or elements  20 , between fixed supporting regions  56 ,  58 ,  60  and  62  provided at upper regions of the wall member  16 . 
     The wire or ribbon material of the loop  44  is such that the wire or ribbon portions  50 ,  52 ,  54  of the loop are substantially incapable of self-support per se and if used alone would deform and sag, particularly under the high temperature operating conditions of the heater, which involves temperatures of up to about 750 degrees Celsius. Furthermore, the material of the loop  44  would be susceptible to harmful damage and degradation if subjected to direct thermal radiation from the heating element or elements  20 . These problems are overcome in the present invention by providing elongate members  64 ,  66 ,  68  of heat-withstanding material to support and shield the portions  50 ,  52 ,  54  of the loop  44 . The elongate members  64 ,  66 ,  68  suitably comprise ceramic material, such as steatite, cordierite, alumina or a glass material such as quartz. The elongate members  64 ,  66 ,  68 , are substantially linear and may be in the form of tubes, such as of circular or oval cross-section, as shown in  FIGS. 2A and 2B , or may be in the form of channels, as shown in  FIG. 2C . Such channels are suitably arranged to be open towards the lower surface  8  of the cooking plate  2 . 
     The fixed supporting regions  56 ,  58 ,  60  and  62  are suitably provided at upper regions of the wall member  16  and may be in the form of recesses or channels for receiving intermediate  70  and/or end  46 ,  48  portions of the loop  44 , for example as illustrated in  FIG. 3A , or for receiving ends  72 ,  74  of the elongate members  64 ,  66 ,  68 , such as illustrated in  FIG. 3B . 
     Alternatively, the fixed supporting regions  56 ,  58 ,  60  and  62  may comprise upper regions of the wall member  16  into which the intermediate  70  and/or end portions  46 ,  48  of the loop  44  are embedded or moulded, as illustrated in  FIG. 3C , or into which the ends  72 ,  74  of the elongate members  64 ,  66 ,  68  are embedded or moulded, as illustrated in  FIG. 3D . 
     As illustrated in  FIG. 4 , the elongate members  64 ,  66 ,  68  may be interconnected or united at ends thereof to form an integral frame  76 , particularly of channel form, in which the loop  44  is supported, the frame  76  being supported at fixed supporting regions on the wall member  16 , in similar manner as previously described with reference to the supporting regions  56 ,  58 ,  60  and  62 . 
     The terminal or end portions  46  and  48  of the loop  44  are arranged to exit the heater  10 , through appropriate apertures in the dish-like support  12 , and are conveniently connected to terminals provided on either side of the housing portion  28  of the temperature-responsive device  24 , although a separate terminal block could be provided if desired. Only one such terminal, denoted by reference numeral  32 , is visible in  FIG. 1 . These terminals have no electrical connection to the temperature-responsive device  24  and leads  78 ,  80  are provided therefrom to the control system  40 . 
     The control system  40  is adapted to operate in known manner to detect a change in electrical inductance in the loop  44  associated with placement and/or removal of the cooking utensil  6  on and/or from the upper surface  4  of the cooking plate  2  and for effecting appropriate energising and/or de-energising of the heating element or elements  20 . Such control system  40  may, for example, include an electrical oscillatory or resonant circuit. 
     Although a triangular arrangement of the loop  44  is shown in  FIG. 1 , other arrangements are possible. For example, one of the three portions  50 ,  52 ,  54  of the loop could be dispensed with and a simple V-shaped arrangement of the loop  44  provided. Alternatively, an arrangement of four or more portions of the loop could be provided, such as to provide a rectangular or trapezoidal arrangement of the loop portions. This is illustrated in  FIG. 5 , in which, in addition to the three portions  50 ,  52  and  54  of the loop, a further portion  82  is included, to provide the rectangular or trapezoidal arrangement. The construction, including the support of the loop on the wall member  16 , the mounting on the associated heater  10 , and the electrical connections and control system, are substantially as previously described with reference to  FIG. 1 . 
       FIG. 5  also illustrates a further feature of the present invention. The detection device  42  can be provided in modular form for easy fitting to an electric heater. The loop  44 , with whatever selected number of portions, is pre-assembled on the wall member  16  and may also be pre-assembled with its end terminal regions  46 ,  48  connected to electrical terminals  32  and  84  provided on the housing portion  28  of the temperature-responsive device  24 , or on a separate terminal block if desired. 
     The device of the present invention can also be applied to heaters having more than one heating zone, such as two or more concentrically-arranged heating zones, and used to energise one or more heating zones according to the size of cooking utensil  6  detected on the cooking plate  2 . This is illustrated in  FIG. 6 . Here an electric heater  10 , with a metal dish-like support  12  and a base layer  14  of insulation material is provided with two concentric heating elements  20 A and  20 B, which are arranged such that inner element  20 A can be either energised alone or together with outer element  20 B. The detection device  42  comprises an inner wall member  16 A concentrically arranged with an outer wall member  16 B. The inner wall member  16 A provides the heater with two separated heating zones  86  and  88 , the outer wall member  16 B serving as a peripheral wall on the heater  10 . A tunnel  90  of thermal insulation material extends across the outer heating zone  88 . Terminal portions  92 ,  94  of the inner heating element  20 A pass through the tunnel  90  for connection to a terminal block  22  and/or terminals on a housing portion  28  of a temperature-responsive device  24 . An elongate probe portion  26  of the temperature-responsive device  24  also passes through the tunnel  90 . Two inductively-operating loops  44 A and  44 B are provided, each constructed in substantially the same manner as previously described with reference to  FIGS. 1 to 5 . Loop  44 A has its portions  50 A,  52 A,  54 A and  82 A supported and shielded by their associated elongate tubular or channel-form members and extending in the inner heating zone  86  between the fixed supporting regions on the wall member  16 A. Loop  44 B has its portions  50 B,  52 B,  54 B and  82 B supported and shielded by their associated elongate tubular or channel-form members and extending in the outer heating zone  88  between the fixed supporting regions on the wall member  16 B. 
     The two loops  44 A and  44 B have a common end terminal region  96  extending outside of the heater  10  and connected to a terminal  84  provided on the housing  28  of the temperature-responsive device  24 . The other end terminal regions  98 ,  100  of the loops  44 A and  44 B respectively, also extend outside of the heater  10  and are connected to further separate terminals, only one of which 32 is shown, provided on the housing portion  28  of the temperature-responsive device  24 . In substantially the same way as previously described with reference to  FIG. 1 , leads are provided, extending from the terminals on the housing portion  28  and the terminal block  22 , to a control system (such as the control system  40  of  FIG. 1 ), to enable the electrical inductance in the loops  44 A and  44 B to be monitored. 
     The control system monitors the electrical inductance of the loops  44 A and  44 B, the inductance values varying according to whether a large cooking utensil  6  is located on the cooking plate  2  and covering both heating zones  86 ,  88  of the heater  10 , or whether a small cooking utensil  6  is located on the cooking plate  2  and covering substantially only the inner heating zone  86 . When a small cooking utensil  6  is located, the control system operates to automatically energise only the heating element  20 A in the inner heating zone  86 . When a larger cooking utensil  6  is located, the control system operates to automatically energise the heating elements  20 A and  20 B in the inner and outer heating zones  86  and  88 .