GB2470292A - Grill or toaster with thin film heating elements incorporated into transparent front panel - Google Patents

Abstract

A grill or toaster for cooking food 100, 600 comprises front 105 and rear 106 panels, where the front panel comprises a substantially transparent panel through which food is visible and the rear panel is non-transparent. The transparent front panel is comprised of at least one substantially transparent substrate 607 bearing an electrical heating element 610 and an outer substantially transparent panel 609. An inner transparent panel 608 may also be present, situated between the substrate and the outer panel. The heating element may be a thin film coating located on the outer face of the substrate. The inner panel and the substrate may form a sealed chamber to reduce heat losses, and may be angled so as to compensate for heat build-up at the top of the grill. The rear panel may also include a heating element 604, which may be of the thin film type or resistance wire. Air vents 601 may be provided to arrange air flow in a single direction to aid uniform cooking.

Description

FOOD HEATING APPARATUS

Field of the Invention

This invention relates to electrical apparatus for the thermal processing of food, particularly although not exclusively to improvements in cooking apparatus such as food grills.

Background to the Invention

Background prior art can be found in the following documents: Patent Number Comments US 2140158 Shows viewing windows in an end wall and a (hinged) front wall of a toaster.

US 2444510 The base of a receiving pocket, in which bread is placed for toasting, is moveable to allow the toast to slide downwards from the pocket to a collection point.

The toaster is also provided with viewing windows.

FR 2050772 Reflectors and peripherally mounted heaters are used to permit toast to be viewed through windows in both long sides of a toaster.

GB 2013874 A viewing window for a cooking appliance such as a toaster is coated with a film of reflective material such as indium oxide to help retain heat in the cooking chamber.

DE 90 15300 u Gebrauchsmuster showing toaster with large window in front surface.

DE 42 14 297 Al Toaster with window.

FR 2707858 Long, glass-walled toaster.

DE 295 18 363 u 1 Gebrauchsmuster showing toaster with windows.

Wa 9724967 A toaster with a viewing window contains a source of EP 0873074 B illumination designed so that the toast color is US 5960702 accurately represented WO 9852451 A resistance heating element is provided adjacent a GB 2332349 B transparent substrate to allow viewing of toast US61 25234 W02001 26523 Toaster with transparent walls uses tubular heaters EP 1223841 B running along the upper and lower long sides of the US 6657168 toaster and has means protecting the lower tubes from crumbs EP 1125539 A Toaster with transparent sides containing heating US 6341554 elements configured to allow monitoring of the toasting process.

WO 2002 51291 Toaster includes heater rods and grill elements that EP 1353592 B are hinged to a base and glass "cloche" and move US 6530309 together using a reciprocating slide in the base.

W02002 51292 Bread toaster with transparent housing.

EP1 345517 U5671 7110 Further background prior art can be found in W02007/140652 (which describes a toaster with film-on-glass heating and groups of electrodes in pairs which divide the film into many heating zones); and also in GB2,367,482 (and US equivalent U56,543,337); AU652608B; W02009/1 28673; and FR2783689.

There is an ongoing motivation and need for improvements in grill design, in particular to achieve satisfactory performance and safety.

Summary of the Invention

According to a first aspect of the invention there is provided a food grill having front and back sides (and end portions/walls), wherein said front side of said grill comprises a substantially transparent side panel through which toasted or grilled food is visible and wherein said back side of said food grill is non-transparent, said substantially transparent side panel comprising: at least one substantially transparent substrate bearing an electrical heating element; an inner substantially transparent panel; and an outer substantially transparent panel; wherein said inner substantially transparent panel is located between said substantially transparent substrate and said outer substantially transparent panel such that, in use, said toasted or grilled food is visible by a user through said outer and inner panels and said substrate.

Embodiments of the above described arrangement provide a thermal and safety barrier; preferably the heating element is on a face of the substrate facing towards the face of the inner panel, to restrict user access, and preferably this substrate and inner panel between them define a substantially sealed chamber to reduce heat loss. It will be appreciated that in general a majority, but not all of the side of the food grill will be transparent -preferably most of the side is transparent to provide good visibility of the food inside, but in general at least end portions of the grill will not be transparent.

In embodiments the transparent outer panel defines one or more external side walls of the food grill. In some embodiments the inner and outer panels and substrate are hinged at the bottom to enable the grill to be opened, for example for cleaning; in other embodiments the outer panel may be lifted away from the grill so that the inner panel and substrate may then be hinged down.

In embodiments the outer panel may comprise a substantially transparent sheath around the grill, more particularly defining a pair of opposite, substantially transparent sides of the grill and a pair of end walls of the grill (which may also be transparent). In this way the outer panel may define an outer "cool wall" for the device.

In embodiments, as previously mentioned, a side wall of the device may be hinged down to facilitate access to the food-receiving space. In embodiments the hinging action may also enable access to a crumb-receiving space, in embodiments a chamber within a base or hinge/pivot at the bottom of the grill. In embodiments the hinging action may also wipe crumbs from an inner base of the food-receiving space into the crumb chamber or tray, for example by arranging for an inner base of the food-receiving space to be mechanically connected to the side walls so that when the side wall is hinged downwards the inner base is moved to open an aperture into the crumb-receiving chamber, preferably a surface of the inner base being wiped or scraped as the inner base is moved, in order to wipe crumbs into the crumb chamber/tray.

In embodiments a hinging side wall may have a portion which extends at least partially over the top of the device, more particularly over the food receiving space. In embodiments such a grill is configured such that, in operation, the top of the food receiving space is fully covered; this then defines a (vented) chamber.

In some embodiments of the grill although only one side panel is transparent both substrates may be substantially transparent, and both may bear a thin film coating to provide the electrical heating element. Such a film coating may comprise a semi-conducting oxide, more particularly doped tin oxide, for example antimony tin oxide, fluorine-doped tin oxide or some other substantially transparent conducting doped oxide material. In other implementations one of the two substrates may use a resistance wire electrical heating element to thereby form a hybrid system.

In embodiments one or both of the substrates may be configured to provide a pattern of browning on the food, for example by means of a pattern of bumps or undulations.

In some preferred embodiments a base portion of the device, more particularly a region immediately beneath the food receiving space, is provided with a set of air vents so that, in use, air flows in a substantially single direction from the base towards an upper opening of the food-receiving space. The air vents may, in embodiments, take the form of a set of parallel slots extending transversally across the short width of the food-receiving space, spaced at intervals along the (interior) base of the food-receiving space along the long edge of the space. This facilitates a generally parallel air flow and helps to achieve a relatively even temperature horizontally.

Preferred embodiments also include a system for achieving an approximately even vertical temperature distribution in particular by configuring the substrates to achieve this. Thus in some embodiments the substrates are angled outwards towards the upper region of the space. Additionally or alternatively the film coatings on one or both substrates may be dividing into a plurality of generally horizontal bands configured to be electrically heated such that in use a lower band becomes hotter than an upper band. This may be achieved by selection of the physical dimensions of the bands, for example, by defining one or both of the width of a band and the thickness of a coating of electrical resistance material of the band, and optionally by choosing difference materials for the band (although this less preferable). It will be appreciated that although in embodiments the bands are generally horizontal, they may undulate and/or have a shape at one or both ends of a band to compensate for end-effects of the heating process, in particular to compensate for temperature variations caused by end effects at the end edges of the substrates.

To achieve even toasting/grilling it is preferable to hold the food approximately in a central position between a pair of substrates bearing heating elements. Thus preferred embodiments of the device incorporate a food centering mechanism, in particular with two configurations, first open configuration to receive food, and the second, closed to configuration in which portions of the mechanism close on the food one or either side, preferably substantially symmetrically.

In embodiments the electrical heating element, in particular where it comprises a thin film, for example a layer of semiconducting material, may itself be used as a temperature sensor. In this case a signal maybe modulated onto the electrical power, typically DC or low-frequency AC, supplying the heating element, to enable this signal to be detected by demodulation. For example a higher frequency AC signal than a frequency of an AC current providing power for heating the heating element may be employed. Additionally or alternatively a region of the film maybe defined to be dedicated to temperature sensing and being provided with at least one separate electrode connection (optionally sharing one electrode connection with the heating element). It will be appreciated that such an arrangement maybe employed other than in the context of a transparent sided food grill.

In another aspect the invention provides food grill having just a single transparent side and having a pair of substrates each bearing an electrical heating element, and defining a toasting or grilling space for said food between said substrates, the food grill further comprising at least one side door, hinged at the bottom to enable the door to be opened to enable access to said cooking or heating space, and wherein a portion of said door extends over the top of said cooking or heating space such that said space is at least partially enclosed during operation of the device.

In embodiments a pair of side doors maybe provided on opposite sides of the device closing above the device, preferably to define a substantially completely enclosed (but vented) space in which food may be toasted or grilled.

In embodiments a pair of doors may be provided on each side of the device. In embodiments an outer "cool" wall panel of cover may be provided around or above the top of the toasting/grilling space to further protect a user from the heat.

In a food cooking device as described above an electrical power control device or switch maybe incorporated into a layer semiconducting material forming the heating element itself. For example by applying a gate electrode over an insulating layer on a portion of the semiconducting layer an FET (Field Effect Transistor) switch may be fabricated. Such a device may be fabricated in a dedicated, separately defined region of the semiconducting layer or may be incorporated into the heating element, for example extending along the length of an electrode connection to the heating element.

In embodiments the portion of the layer of semiconducting material comprising the electrical power control device is connected in series the material defining the electrical heating element, and in embodiments the semiconductor device and heating element may be part of the same, substantially continuous layer of serniconducting material (rather than needing to be defined in a separate, dedicated region of the semiconducting layer).

The device may comprise a diode, in particular a diode using a metal-semiconductor junction. Alternatively p-type and n-type doped regions of the layer maybe employed to fabricate a bipolar transistor. Alternatively, as previously described, an insulated gate field effect transistor FET (or junction FET) maybe fabricated. In general the power controlled semi-conductive device comprises a field effect transistor FET, bipolar transistor, insulated gate bipolar transistor IGBT, thyristor, silicon controlled rectifier (SCR), triode for alternating current (TRIAC), or other device. In embodiments the device and heating element and substrate may be substantially transparent.

Suitable materials include, but are not limited to, tin oxide, for example doped with antimony or fluorine, indium tin oxide, and silicon carbide.

In a grill embodiment the grill may have a hinged upper lid and a base portion, the upper lid comprising the substantially transparent front side of said grill and the base portion comprising the non-transparent back side of said grill.

In a further aspect the invention provides a food cooking device having a pair of substrates each bearing an electrical heating element, and defining a cooing of heating space for said food between said substrates, the device having front and back sides, wherein one of said front and back sides is substantially transparent and the other is non-transparent, the device further comprising one or more of: i) a set of air vents in a base portion of said device opening into a cooking or heating space for said food to, in operation, provide an air flow through said cooking or heating space in which air flows in substantially a single direction from said base towards an upper, food-receiving opening of said space; ii) a configuration wherein said substrates are configured to heat said food more at a lower region of said space that at an upper region of said space; iii) a food centering mechanism to receive and hold food to be toasted or grilled substantially in a central position between said pair of substrates, and wherein said food centering mechanism has two configurations, a first, open configuration in which said mechanism is open to receive food, and a second, closed configuration in which positions or said mechanism close on said food from either side; iv) a configuration wherein one of said substrates has an electrical heating element comprising a film coating and wherein the other of said substrates has an electrical heating element comprising resistance wire; v) a configuration wherein one of said substrates is substantially transparent, and a thermal warning indicator to indicate when said substrate is not at an elevated temperature, wherein said thermal warning indicator comprises a symbol displayed on said on said transparent side which is substantially invisible when said substrate is not at said elevated temperature; vi) a configuration wherein at least one of said substrates is hingeably mounted to enable the substrate to be pivoted to fluctuate access to a cooking or heating space for said food.

vii) a substrate bearing a layer of semiconducting material comprising said heating element, and wherein a said device includes a temperature sensor comprising a portion of said layer of semiconducting material.

A further aspect provides a food heating device comprising a front panel comprising a first heating element; a rear panel comprising a second heating element; and a food heating chamber positioned between said front and rear panels; wherein said front panel and said first heating element are each substantially see -through, so as to allow a user to view inside said food heating chamber; and said rear panel is substantially non see through.

Other aspects are as recited in the claims herein.

Brief Description of the Drawings

For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Figure 1 shows in perspective view a food heating device according to a first specific embodiment; Figure 2 shows a schematic illustration of a vertical cross section through a food cooking device according to a second specific embodiment, with a substantially transparent side panel; Figure 3 illustrates an undesired air convection flow within a cooking device; Figure 4 illustrates a cooking device configured to provide an improved convection air flow; Figure 5 illustrates schematically one embodiment of a centering mechanism for a cooking device; Figure 6 illustrates schematically in view from one side a further embodiment cooking device configured to provide an improved convection air flow; Figure 5 illustrates angled heating panels configured to provide an improved, more even vertical temperature distribution; Figure 7 illustrates an example electrode configuration for a thin film heating element, configured to enable power to be applied to selected combinations of electrodes in order to be able to selectively control heating of different regions of a thin film coated substrate; Figure 8 shows a temperature distribution for the a thin film heating element of Figure 7; Figure 9 illustrates a film coating heating element divided into a plurality of horizontal bands for heating control; Figure 10 illustrates schematically an example of a food heating device having a thermal lock in combination with a hinged side panel; Figures 11 a and 11 b illustrate examples of elevated temperature warning indicators; Figure 12 illustrates schematically an embodiment cooking device having and external outer cover for providing additional heat insulation; Figure 13 illustrates schematically a further embodiment cooking device having an upwardly moveable transparent heat insulating wall; Figure 14 illustrates an embodiment of a hybrid heating element cooking device incorporating a thin film heating element and a resistance wire heating element, in which the food-receiving space is at an acute angle to the vertical; Figures 15, shows respectively first and second patterned transparent substrate configurations for providing a patterned browning effect; Figure 16a shows schematically a food position centering device for centering food in heating chambers; Figure 1 6b illustrates schematically operation of the food centering device as Figure 16a; Figure 17 illustrates heating element power control schemes; Figure 18 illustrates temperatures of the cooking device in substantially continuous operation; Figure 19 shows one preferred example of food (bread) placement, illustrating air flow; Figure 20 illustrates a first use of a thermochromic ink as a temperature indicator; Figure 21 illustrates a second use of a therrriochrornic ink as a temperature sensor; and Figure 22 illustrates a glass sided cooking device according to a further specific embodiment.

Detailed Description

There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to

unnecessarily obscure the description.

The specific embodiments presented herein are primarily suitable for cooking food items including but not limited to bread, crumpets, buns, sandwiches, waffles, bagels, burgers, snacks, and the like, including food items which can be cooked by application of direct heat to their surfaces, or radiated heat from heating surface spaced apart from the food.

Referring to Figure 1 herein, a food heating device 100 comprises a first side 101; a second side 102; a hinge mechanism 103 connecting the first and second sides together so that the first and second sides can be opened and closed relative to each other in the manner similarly to a book or a suitcase; and a base stand 104.

The first side 101 comprises an electric heating element extending over a heating surface 105. The second side 102 comprises a transparent or translucent glass pane 106 to which is formed a transparent or translucent thin film heating element. In the best mode, the thin film heating element is a doped metal oxide, for example doped tin oxide. The dopants may comprise flourine, antimony or indium in the best mode, although other known equivalents may be used.

The base 104 is formed in cross sectional view, in the shape of a "X", or similar shape. The base has a plurality (in the embodiment shown, four) separate elongate edges 107, allowing the device to be positioned upright as shown in Figure 1, or alternatively positioned to lie flat, for example on first side 101.

As shown in Figure 1 when placed in the upright configuration, the area of the heating surface 105 on the first side and the area of the corresponding heating element on the glass pane 105 which lies opposite on the second side, may be of sufficient size to accept either one or a plurality of bread slices in a tiled arrangement. When in the upright position, the device may be placed on a worktop surface, and a user may swing down the second side 102, similarly to opening a bottom hinged door, place one or more slices of bread in an upright tiled arrangement adjacent the heating surface 105 of the first side, close the front of the device (the second side 102), turn the device on and watch the food being cooked through the transparent or partially transparent glass pane 106 which acts as a window.

When laid in the flat position, the device may open up in a manner similar to a notebook or laptop type computer, the second side becoming a lid, and the first side becoming a base. . Food may be placed on the lower side 101. The second side 102 forms the lid, which is closed on top of the food. In this configuration, a user can see through the transparent glass pane 106 to watch the food item heating up.

When used in the laid-flat configuration, the cooking device may be suitable for use as a sandwich maker. Sandwich fillings, e.g. sauces and other liquid or fluid foods between the bread slices may be contained within the first and second sides (which form a base and lid respectively when the device in placed laid flat).

The heating surface 105 as shown in Figure 1 may be recessed with respect to an outer frame 108 of the first side, so that when the first side is laid flat, there is no dripping or leakage of sauce or other fluids from the sandwiches, outside the frame on to the worktop or other surface on which the device is placed.

The heated surfaces 105, 106 may have ridges or undulations in order to provide a griddled effect on one or both sides of the bread or other food item being heated.

Referring to Figure 2 herein, there is illustrated schematically in view form one end, a glass sided cooking device 200 according to a second specific embodiment. The glass sided cooking device makes use of clear thin film resistive coating 202 on a glass substrate 204 as a heating element to provide a mechanism for browning toast. The heating mechanism can be exclusively thin film technology or a combination of thin film and nichrome or other wire (a "hybrid").

Benefits have been identified when using thin film technology as a heating mechanism. These include even browning and clear visibility of the cooking process. While the temperature of the coating operates above the caramelization temperature (200°C), the main method of cooking is through conduction and convection rather than radiation. Hence, in order to achieve these benefits, certain performance and safety requirements may be realized as follows: Three pane system A three pane I panel system is used so as to achieve cool wall touch temperatures on the outer wall. This system is designed to optimize efficiency at the same time as reducing energy loss. Figure 2 illustrates a cross section through panels. The inner glass panel 204 is coated with a clear resistive coating. The second panel 206 provides both a thermal and safety barrier. It is sealed so as to reduce heat loss and at the same time prevent user access to the live surface. The outer wall, third panel 208 provides a final thermal barrier, which induces a convection draft that draws in cool air and can keep the outer wall temperature with in desired/required standards.

There may be provided a plurality of vents 210 at the base of the device to allow air into a cavity between the outer wall 208 and the second glass panel 202, and to promote an upward flow of air to conduct heat away from the outer wall 208.

Convection optimization Given that the heating mechanism is based upon conduction and convection, it is important that the convection in the device is correctly optimized. Failure to create a laminar convection flow will result in toast being burnt in the middle or sides as illustrated in Figure 3 herein. This can be improved by the introduction of the parallel flow vents 400 periodically spaced apart from each other along on the base of the device as shown in Figure 4.

This generates a horizontally even temperature. The vertical temperature can then be made even by adjusting the coating temperature, as described later.

Centering mechanism Referring to Figure 5 herein, using thin film technology as the heating mechanism, is preferable for the bread slice to be uniformly centered to achieve an even browning. A centering mechanism may be provided to center the bread slices between a first heating plate and a second heating plate, wherein one of the heating plates comprises a glass heating plate having a metal oxide heating element.

The centering mechanism should preferably not only center the bread slices symmetrically between the two heating panels, but should preferably maintain the bread slice in a vertical orientation and inhibit the bread slice from bend ing or twisting. The regular spacing between the heated glass surface and the surface of the bread slice may result in uneven browning.

To achieve the above requirement, a mechanism using a parallelogram concept has been devised. This allows the cage mechanism 500 to close onto the bread slices equally from both sides and maintain the bread slices in an almost perfect vertical plane to prevent the slice from bending under its own weight. The spring mechanism 503 allows for appropriate pressure to be applied to the bread on loading.

The cage itself comprises a metal frame manufactured of thin wire. The thin wire reduces or prevents shadowing on the bread from the radiant heat either side of the bread slices.

Since accurate positioning of the bread adjacent to a glass plate with thin film heating element is important, there are two ways of approaching the problem of accurately positioning the bread relative to the heating plate.

In a first method, the bread carrying cage may be static and the glass heating panel may be moved towards the bread.

In a second method, the glass heating panel may be static, and the bread may be moved towards the panel.

In a device having a conventional heating element for example nichrome wire on one side, and a glass heating element with a metal oxide heating element on the other side, the mechanism shown in Figure 5 may be used whereby one side of the mechanism lies in a permanent fixed relationship to the glass heating plate, that is, one side of the bread cage is always in a fixed position and spaced a fixed distance apart from the glass heating element, whereas the other side of the bread cage may be moveable, so that as the glass is dropped into the bread cage, and the bread cage is lowered into the heating chamber, the side of the bread cage which is nearest the metal wire heating element clamps onto the bread, and any movement in the cage occurs on the side of the cage closest to the wire heating element, whereas the side of the bread cage adjacent the glass heating element is non-moveable and always in a fixes spatial relationship to the glass heating plate.

Even Browning With evenly coated thin film heating panels in a vertical plane it is found that the top edge of the panel tends to get hotter than the bottom edge through convection. The result of this is that the bread slice(s) brown quicker at the top than at the bottom and do not achieve and even toasting appearance. To alleviate this, the features described below are applied.

Angle panels Referring to Figure 6 herein, there is shown schematically a further embodiment cooking device having an angled heating plate. The device 600 comprises a base 601; a back plate 602 extending upwardly form the base; a first insulating plate 603 extending substantially parallel to a first heating plate 604 and having a lower edge secured in the base 601; heating plate 604 extending parallel to the back plate and insulating plate, the heating plate having a lower edge secured in the base 601 and comprising a plurality of heating elements 605; a food centering mechanism 606 comprising a wire grid or cage for holding the bread in an upright orientation; a second heating plate 607 in the form of a glass plate extending at an acute angle to a plane substantially parallel to the first heating plate 604, a lower edge of the second heating plate being secured in the base 601; a transparent or translucent second insulating plate 608, positioned substantially parallel to the first heating plate 604 and! or the back plate 602; and a transparent or translucent front plate 609 which lies spaced apart from and parallel to the insulating plate 608.

The second heating plate 607 has formed on it a see through transparent or translucent heating element 610 formed of a resistive thin film metal oxide of the type described elsewhere in this disclosure, e.g. a metal oxide such as tin oxide doped with an n -type or p-type dopant.

The second insulating plate 608, the second heating plate 607 and the front plate 609 are each see -through so that a user can view the food through those layers, and can see the food being browned from a position level with the sides of the device. The angling of the whole plate assembly to the vertical assists in viewing the food when cooking, so that the user views the food at a in a line of sight transverse to and preferably substantially perpendicular to a main plane of the see -through plates.

As shown in Figure 6, a plurality of air vents 611 are provided in the base so as to allow air to circulate in a cavity between the second insulating sheet I plate 608 and the front plate! wall 609.

The thin film heating panels are angled so that they are further apart at the top than at the bottom, as shown in Figure 6 herein. This compensates for the excessive heat build up at the top of the panels allowing the top of the bread slice to toast I brown at the same speed and time as the bottom creating an evenly toasted slice.

A disadvantage of this method of controlling the browning is that the energy efficiency is perhaps less than desired, since a lot of warm air escapes from the top of the cooking chamber.

In use the bread is placed in a chamber defined between the first and second heating plates, and is centered in the chamber to provide even heating of both sides of the bread, by the centering mechanism 606. The user is protected from the heat of the heating plates at the rear by the first insulating plate 603, and at the front by the second insulating plate 606, which is spaced apart form the second heating element 607, and which forms a sealed unit with the second heating plate 607 enclosing a cavity 612. The cavity 612 may be filled with an inert gas or with air.

Thin Film Coating patterns to achieve heat distribution Through different means of manipulation the thin film coating pattern and placement of bus bars (electrodes) 700 it is possible to heat different areas of the coated glass panel. Figure 7 shows a means of directing heat into the bottom corners. This can be achieved by switching power from the two main bus bars, I and 4, to the other bus bars 2 and 3. This can be done in different combinations to obtain the desired result: Switching between I and 4, 1 and 2, 3 and 4, 2 and 4 and I and 3.

Figure 7 shows how by manipulating the bus bars on the edges of the panels one can increase the temperature on the sides of the panels. It was found that there was excessive heat loss on the side of the heating panels resulting in the centre of the panel having a higher temperature than that of the sides. By simply reducing the length of the bus bar and leaving the thin film coating at its original dimension hot spots are created at the edge of the bus bar from increased current density. The increase heat at the edges of the panel compensates for the losses experienced and contributes to move even supply of heat to the bread slice.

Figure 9 herein shows how one can manipulate the heat distribution by dividing the thin film resistive coating into different band widths 120 across the length of the coated panel between the two bus bars. By increasing the width of each band 120 (and/or changing the coating resistance, for example the coating thickness) one reduces the resistance for that band between the two bus bars. With a reduction in resistance there would be an associated increase in power and hence increase in heat output. Having wider bands at the bottom of the panel than at the top will produce higher heat output at the bottom of the panel than at the top. This will compensate the heat increase at the top edge of the panel from convection. In embodiment the resistivity across all bands may be the same in Ohms I square for the different resistance of each band (the benefit of this is that these bands can be coating is a single process and would not require a separate process for each individual band).

Side opening Referring to Figure 10 herein, there is illustrated schematically in view from one end a food heating/cooking device having a locking mechanism to prevent opening of first moveable side panel, when the side panel is hot enough to cause injury to a person. A thin metal oxide film coating can provide a temperature sensor, or in alternative embodiments conventional temperature sensor can be used to determine the heat of a frontal glass plate 1004. A locking mechanism 1022 at the hinge prohibits opening of the door when temperature of the front panel is hot enough to potentially cause injury to a person.

Thermal warning When the glass panels are hot, there is no visible indication that they are warm. Hence, it is desirable to show the consumer that they are still warm.

Thermal indication can been shown in a number of ways, including: Thermochromic inks placed/printed on the sides of the device, however these degrade with use and UV light.

Referring to Figure ha herein, there is shown schematically a light emitting diode back light in view from the front, the rear and in cut away view.

An LED back light 1132 warning uses lensing 1130, such that the light is not visible when hot -Figure ha. Another warning light may use an LED edge lighting the edge of an etched glass block 134 to scatter the light, using the block as a light guide -Figure 11 b.

Hybrid heating system A hybrid system involves using a combination of both Thin Film Resistive Coated Heating Elements and Nichrome Wire Heating Elements within the same cooking device. The ability to electrically and mechanically control the cooking times, power input and distance from the bread allow for comparative browning levels from both the conventional wire heated side and the thin film resistive coated side. A preferred embodiment of a hybrid system has the food held at a (slight) angle, for example <300, 20°, or 10° to the vertical, for example by a cage or frame. By slightly angling the slots for the bread backward the bread can be allowed to naturally sit on a cage that is offset from the nichrome wire heating elements allowing the thin film resistive coated heating element to be offset at front of the piece of bread with minimal restraint (see Figure 12).

Griddle effect To create a pattern effect on the food product Thin Film Resistive Coated Heating Elements can be screen printed into patterns onto the substrate to allow for area heating, thus allowing food to have controlled and griddle effects on the desired surfaces.

Different tracks of resistive thin film having different power densities may be patterned onto the glass heating surface so as to produce a differential searing effect on the food items being cooked. Different tracks of different power densities may produce different amounts of heating and/or browning of the food items to produce a seared surface effect on the cooked food.

Referring to Figure 13 herein, an alternative method of achieving a griddle effect is to through the use of bumps and! or ridges or other like protrusions molded on to the glass heating plate.

Energy efficiency and safety doors A device for cooking food using clear glass panels with an active clear coated heating substrate has been described. Because many conventional toasters rely on the toast rising out of the top of the product, this area loses much of the rising heat and energy straight out of the device. A requirement therefore arose for a means to improve the efficiency in a cooking device whilst simultaneously allowing safe and easy access to the bread.

Thus in one aspect there is provided a cooking device with just a single transparent side and with a pivoting door that opens from the side of the device. In embodiments the or each access door wraps around the full side and over the top of the device to the mid point. On a two slice device there will preferably be two doors on both sides of the device, thus enabling a substantially fully enclosed environment when closed. In preferred embodiments venting is provided to allow some hot air to be evacuated at convenient points. Preferably this will allow the chamber, where the bread is toasting, to retain much of the previously lost heat and therefore add to the energy efficiency (heat loss). Once both doors are closed the opportunity to access hot and electrical parts internally may thus be substantially reduced.

In embodiments food items for toasting that are normally shallower in profile, like crumpets and buns etc can be fully accessed when the door is fully opened over 90 degrees unlike many conventional toasters that do not allow access of such food products (they reside below the access point of toast and tend to sit within the toasting unit). This also enhances the safety advantages of this door access as consumers might otherwise place something into the dangerous areas of the device to access such food products below the exit slot.

Preferably, once opened, any power is cut to the heated glass panels and therefore providing a safer environment for the user.

In embodiments this principle can also allow the doors to automatically (pop') open when required browning levels are reached allowing an immediate cooling and power off to the heated glass panels. A browning level may be determined by any of a range of methods including temperature, time, a combination of these, or by use of an ionizing sensor (as described in GB2,367,482). When the door is fully open the user is well away from any hot zones generated from the warm air rising. In embodiments the device is not able to be activated until both doors are closed and complete the electrical circuit.

Thermal sensor Typical thin film coatings for use as thermal sensors and br heating elements are (intrinsic) semiconductors. For example, SiC and tin oxide are both semiconductors with large band gaps (typically -3.2eV). By doping the semiconductor can be made to be n-type or p-type. Typically, impurities make the thin film an n-type semiconductor. For example, ATO (antimony tin oxide) is an n-type semiconductor. However p-type semiconductors can also be produced.

Typical thin film materials hence have a reversible resistance -temperature characteristic and thus the heating element itself can be used as a thermal sensor to measure the temperature of the heating element or substrate.

Alternatively, a separate area of thin film which does not constitute part of the heating element, but which is placed on the same substrate close to the element can be used to measure the temperature using a separate low voltage/low current circuit. The area can be manufactured using a masking process when the main heating element is being created.

It is preferred to detect the resistance change using a low voltage / low current so that the sensitivity is improved and the semiconductor is not saturated, hence a separate area for thermal detection is preferred rather than using the bulk element itself. Should the bulk element need to be used, then a high frequency signal multiplexed on to the DC or low frequency AC bias can be used to detect variation in resistance, without the requirement to measure high voltages or currents.

Switching mechanism In embodiments the heating elements are required to be switched on and off. This may achieved using a manual switch, a relay or a solid state switching device, generally separated from the heating element itself. However this can add extra cost to the overall system.

Hence, it is desired to create a system by which the heating element switch is included within the heating element. Given that the thin film technology is a semiconductor, it is possible to create at the same time as the heating element different types of semiconductor switch or rectifier. In particular, one can produce a FET device by overlaying a thin insulator, such as mica or silicon dioxide on top so an area of the thin film element (typically where the current enters or leaves the element). On top of the insulator a metallization layer is created to which a voltage can be applied to switch the element. Further devices are possible: for example, at the metal -thin film junction a Schottky diode is created, further using n-type and p-type variants of silicon carbide or tin oxide it is possible to create a rectifying diode or bipolar transistor. Because the material can withstand high temperatures, there is no need for a heat sink and any heat losses are directly used in the heater, thus increasing efficiency as well as reducing cost. Many of these devices can be transparent and hence can be used within the device to switch the elements to provide different heating levels and control.

Further embodiments Referring to Figure 11, this shows one example of an outer "cool wall" to restrict user access to hot surfaces of the device (in the illustration, front and back surfaces, referred to earlier as side surfaces). As illustrated, preferably the wall is removable for cleaning; the front sealed glass panel maybe pivoted open, again for cleaning; the rear wall may be fixed upright.

Figure 12 illustrates schematically a modification to the cooking device/grill of Figure 1 herein, comprising an additional clear plastics or other transparent material external wall 1200. The transparent additional outer wall slides into a locating slot at the base of the first side of the device. A rear part of the outer wall slides horizontally into another locating slot at the rear of the device. The wall can be designed such that in order to open the device the wall must be slid horizontally out of the slots, so as to allow opening of the device for cleaning, as shown in Figures 12(a)-(c). The outer sleeve may be completely removable to allow separate cleaning of the sleeve, and cleaning of the interior of the food preparation device. In use, a user can view the heating chamber within the device through the front of the transparent outer sleeve 1200, and through the front wall of the device.

Referring to Figure 13 herein, there is illustrated schematically a further embodiment cooking device. In this case, the device comprises a base portion 1300, a rear side 1301, a front side 1302, the front side being hinged with respect to the rear side so as to permit opening up of the device for cleaning; and a vertically removable transparent tubular sleeve which can be slid downwardly over the first and second sides to seat on a projecting rim 1303 of the base portion 1300.

In use, the devices used with the outer transparent sleeve 1303 in place.

The outer sleeve provides additional protection against the surfaces of the front and rear sides of the device, which may become hot. There is an air gap between the front and rear sides, and the inner wall of the tubular sleeves 1303.

In use, food items to cook are inserted in a slot 1304 at the top of the device, and may be transported up and down a food heating chamber inside the device using a carriage mechanism.

Figure 15 shows examples of glass substrates with bumps and ridges, for example formed by molding. These may be used to pattern the food.

Figure 16a shows an example cage and Figure 16b illustrates operation of the cage, closing side wings to hold bread as the apex of the cage is inserted into the base of the device. Figures 16c-e illustrate example forms of the cage; in embodiments the cage is colored or coated white so that it does not appear to be a heating element thereby breaking the illusion that the glass is the heating substrate.

Either proportional or on/off power control may be employed, but on-off control is preferred as this is faster; an example element power is 1100W. the distance of the food to the element effects cooking time; direct contact enables comparable heating time to a standard conventional toaster at 840W.

Figure 17 illustrates the effect of power control on temperature over time at different locations within an example device.

Figure 18 shows temperatures at the top of the glass substrate (upper trace), the top of the outer panel (middle trays) and at the middle of the outer panel (lower trace). The temperature control was set for 350°C, regulated at the edge of the glass panel; the top edge of the outer panel stabilized at 100°C; increasing the distance between the outer panels substantially reduces the outer temperature.

Figure 19 illustrates that it is preferable to lift the bread slices so as not to block the air vents that promote a more uniform convection cell.

Figures 20 and 21 illustrate the use of thermochromic inks for temperature indication.

In embodiments the elements may be switched by TRIAC switching, with differential choke filtering (for example, a differential mode filter of l8mH) for EMC capability.

Referring to Figure 22 herein, there is illustrated schematically in two perspective views, one from the front and one from the rear a further embodiment glass sided cooking device.

The device 2200 comprises a front side 2201; a rear side 2202; an a base portion 2203. Between the front and rear side its formed a cavity within which pieces of bread may be inserted. The cavity is open at an upper end, so that bread may be inserted into the top of the device. Within the cavity is provided adjacent the back side, a heating plate having a wire heating element, for example a nichrome heating element, and at the front side is provided a transparent or see through glass plate having formed thereon a transparent or see through heating element. The heating element may be formed of a doped metal oxide, for example antimony tin oxide, fluorine tin oxide or indium tin oxide.

Parallel to the see through heating plate is provided a transparent wall, forming the outer surface of the front of the device.

In use, a user can view the bread being heated through the transparent front wall, through the see through glass heating pane. The device is provided with a vertical ejection mechanism for carrying the toast into and out of the device. In the embodiment shown, the bread slices are placed side by side in tiled arrangement.

The cooking device of Figure 22 herein may have vents and apertures for conduction and/or convection of heat and to promote uniform air convection within the device similarly as described with reference to Figures 4 and 19 herein.

No doubt many other effective alternatives will occur to the skilled person.

It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims (21)

1. Claims 1. A food grill having front and back sides, wherein said front side of said food grill comprises a substantially transparent side panel through which food is visible; and wherein said back side of said food grill is substantially non-transparent, said substantially transparent side panel comprising: at least one substantially transparent substrate bearing an electrical heating element; and an outer substantially transparent panel; wherein in use, said food is visible by a user through said outer panel and said substrate.
2. 2. A food grill as claimed in claim 1, further comprising: an inner substantially transparent panel, wherein said inner substantially transparent panel is located between said substantially transparent substrate and said outer substantially transparent panel; and wherein in use, said food is visible by a user through said outer panel and said inner panel and said substrate.
3. 3. A food grill as claimed in claim 2, wherein said heating element is disposed on a face of said substrate facing said inner panel.
4. 4. A food grill as claimed in claim 2 or 3, wherein said substrate and said inner panel define opposite faces of a substantially sealed chamber.
5. 5. A food grill as claimed in claim any one of the preceding claims, wherein said outer panel defines an external side wall of said food grill.
6. 6. A food grill as claimed in an one of the preceding claims, wherein said front side is moveable with respect to said back side, such as to allow access to a food heating chamber located between said front side and said back side.
7. 7. A food grill as appendant to claim 2, wherein said substrate and said inner panel are hingeably mounted to enable said substrate and said inner panel to be pivoted to open said transparent side to facilitate access to heating or cooking space for said food.
8. 8. A food grill as claimed in any preceding claim, further comprising a thermal warning indicator to indicate when said substrate is not at an elevated temperature, wherein said thermal warning indicator comprises a symbol displayed on said transparent side which is substantially invisible when said substrate is not at said elevated temperature.
9. 9. A food grill as claimed in any preceding claim, comprising a second substrate, substantially parallel to said substantially transparent substrate, and wherein one of said substrates has an electrical heating element comprising a film coating and wherein the other of said substrates has an electrical heating element comprising resistance wire.
10. 10. A food grill as claimed in any preceding claim, wherein one or both of said substantially transparent substrate and said electrical heating element is configured to provide a pattern of browning on said food.
11. 11. A food grill as claimed in any preceding claim, further comprising a set of air vents in a base portion of said grill opening at a heating or cooking space for said food to, in operation, provide an air flow through a heating or cooking space in which air flows in substantially a single direction from said base towards an upper opening of said space.
12. 12. A food grill as claimed in any preceding claim, having a pair of substrates each bearing an electrical heating element and defining a toasting or grilling space for said food between said substrates, and wherein said substrates are configured to heat said food more at a lower region of said space that at an upper region of said space.
13. 13. A food grill as claimed in claim 12, wherein said substrates are angled towards said upper region of said space.
14. 14. A food grill as claimed in any one of the previous claims comprising a pair of substrates positioned opposite each other and with a food cooking space positioned there between; each said substrate comprising an electrical heating element; said pair of substrates being angled with respect to each other to as to diverge away from each other towards and upper portion of said cooking space.
15. 15. A food grill as claimed in claim 12, 13 or 14, wherein a said electrical heating element comprises a film coating on said substrate, and wherein said film coating is electrically divided into a plurality of generally horizontal bands, said bands being configured to be electrically heated such that, in use, a lower said band becomes hotter than an upper said band.
16. 16. A food grill as claimed in any preceding claim, having at least one side, hinged at the bottom to enable the side to be opened to enable access to said heating or cooking space; and wherein a portion of said side extends over the top of said heating or cooking space such that said space is at least partially enclosed during operation of the food grill.
17. 17. A food grill as claimed in any preceding claim, wherein said substrate bears a layer of semiconducting material comprising said heating element, and wherein said food grill includes a temperature sensor comprising a portion of said layer of semiconducting material.
18. 18. A food grill as claimed in any preceding claim, wherein having a hinged upper lid and a base portion, wherein upper lid comprises said substantially transparent front side of said grill and wherein said base portion comprises said non-transparent back side of said grill.
19. 19. A food grill having a pair of substrates each bearing an electrical heating element, and defining a cooking space for said food between said substrates, the food grill having front and back sides, wherein one of said front and back sides is substantially transparent and the other is non-transparent, the grill further comprising one or more of: (i) a set of air vents in a base portion of said grill opening into a heating or cooking space for said food to, in operation, provide an air flow through said cooking space in which air flows in substantially a single direction from said base towards an upper, food-receiving opening of said space; (ii) a configuration wherein said substrates are configured to heat said food more at a lower region of said space that at an upper region of said space; (iv) a configuration wherein one of said substrates has an electrical heating element comprising a film coating and wherein the other of said substrates has an electrical heating element comprising resistance wire; (v) a configuration wherein one of said substrates is substantially transparent, and a thermal warning indicator to indicate when said substrate is not at an elevated temperature, wherein said thermal warning indicator comprises a symbol displayed on said on said transparent side which is substantially invisible when said substrate is not at said elevated temperature; (vi) a configuration wherein at least one of said substrates is hingeably mounted to enable the substrate to be pivoted to fluctuate access to a heating or cooking space for said food, and a thermally operated mechanical lock to inhibit said pivoting when said substrate is at an elevated temperature; (vii) a substrate bearing a layer of semiconducting material comprising said heating element, and wherein a said food includes a temperature sensor comprising a portion of said layer of semiconducting material.
20. 20. A food grill comprising: a front panel comprising a first heating element; a rear panel comprising a second heating element; and a food heating chamber positioned between said front and rear panels; wherein said front panel and said first heating element are each substantially see through, so as to allow a user to view inside said food heating chamber; and said rear panel is substantially non see through.
21. 21. A food grill as claimed in claim 20, further comprising: a holding means for holding food items in said food heating chamber; a positioning means for positioning said food in said heating chamber at a predetermined distance from said plate heating element.