WO2018019766A1

WO2018019766A1 - Induction heated cooking vessel

Info

Publication number: WO2018019766A1
Application number: PCT/EP2017/068637
Authority: WO
Inventors: Glyn Hauser; Michael James; James Meddick
Original assignee: Morphy Richards Limited
Priority date: 2016-07-28
Filing date: 2017-07-24
Publication date: 2018-02-01
Also published as: GB2552534A; GB201613064D0; GB2553906A; GB201711738D0; GB2553906B

Abstract

An induction heated cooking vessel comprises a body portion which is intended to be in contact with a food item or a liquid to be heated, the body portion having an underside comprising a first region of a first thickness, and a second region of a second thickness, said second region having a reduced thickness relative to the thickness of said first region.

Description

Induction Heated Cooking Vessel

Field of the Invention

[0001] The present invention relates to an induction heated cooking vessel as configured for use with an induction cooking apparatus.

Background of the Invention

[0002] Induction hobs are well-known for domestic cooking. A known induction hob works by having an electromagnetic coil which generates an electromagnetic field which extends above a cooking surface, for example a glass or ceramic hob, to interact with a ferrous element of a cooking vessel such as a saucepan, frying pan or the like placed on the hob. The electromagnetic radiation induces eddy currents within the ferrous material, which leads to heating of the material and therefore heating of the pan or other cooking vessel.

[0003] Induction hobs are highly controllable and are more energy efficient than conventional electric hobs which rely on an electric heating element and thermal conduction from the electric heating element to a cooking vessel. [0004] A known induction hob has an electric coil to produce an electromagnetic field, a set of control electronics, a glass or ceramic hob, through which the electromagnetic waves pass to interact with an inductive material on the underside of a cooking vessel, and in some cases a cooling fan to blow air over the coil and control electronics for cooling. Heat generated in the inductive layer of the cooking vessel passes through thermal conduction to the base of the cooking vessel, and heats the contents of the cooking vessel.

[0005] In a known induction hob, a temperature sensor is provided which is placed in contact with the underside of the glass cover plate of the hob, in the heating area where the cooking vessel is placed over the electromagnetic coil. The temperature sensor measures the temperature of the underside of the glass, as being representative of the temperature of the base of the cooking vessel. [0006] In a known induction hob, the temperature sensor is provided as a safety measure to make sure that the pan does not overheat. [0007] There is an additional negative temperature coefficient (NTC) temperature sensor which is provided as a safety measure. In a typical known induction hob, cooking temperatures are set similarly to a conventional electric cooking hob, using heat controls which can be either manual rotatable rheostat type controls or touch sensitive electronic controls located on the upper surface of the hob adjacent heating areas.

[0008] Measurement of temperature in the known induction hob is made from underneath a glass plate, and underneath an induction heated ferritic part of the cooking vessel.

[0009] In order to produce an induction cooking hob which provides a level of automation of cooking procedures, it is desirable to measure the temperature inside a cooking vessel, and to provide a temperature signal which can be used to generate a temperature display to the user, and/or which can be used as an input to an automated or semi - automated cooking system which follows a predetermined recipe.

[0010] Known induction hobs do not generally have temperature sensors which are intended to measure the cooking temperature of food items within a cooking vessel.

[0011] One object of the embodiments described herein is to provide an accurate temperature measurement of cooking temperatures inside an induction heated cooking vessel which is heated on an induction powered hob. [0012] Another object of the embodiments described herein is to provide an induction heated cooking vessel which is adapted for improved temperature measurement of the contents of the cooking vessel. Summary of the Invention

[0019] According to a first aspect of the present invention there is provided an induction cooking vessel comprising: a body portion, said body portion having an underside comprising: a first layer of an electrically non-inductive material; a second layer of an electrically non - inductive material; said second layer being of a material having a higher thermal conductivity than said first layer; a third layer, said third layer being of an electrically inductive material; said underside comprising a first region of a first thickness, and a second region of a second thickness, said second region having a reduced thickness relative to the thickness of said first region; wherein said first region of said first thickness comprises said first layer, said second layer and said third layer; and said second region of relatively reduced thickness comprises said first layer.

[0020] Preferably, within said second region of relatively reduced thickness, an upper surface of said first layer forms a bottom interior surface of said vessel. [0021 ] Preferably, within said second region of relatively reduced thickness, a lower surface of said first layer is exposed.

[0022] Said first layer may comprise stainless steel 304.

[0023] Preferably a material of said second layer has a higher thermal mass than a material of said first layer or said third layer.

[0024] Preferably said second layer comprises aluminium or an aluminium alloy. Preferably the second layer has a higher thermal mass and/or a higher thermal conductivity than said first layer or said third layer.

[0025] Said third layer may comprise stainless steel 430. [0026] Preferably the second layer is bonded to said first layer.

[0027] Said third layer of inductive material is bonded to said second layer of non - inductive material. [0028] Preferably said reduced thickness region may comprise an indent or cavity in a bottom plate or underside of the cooking vessel.

[0029] Preferably an outer surface on the underside of the second region is substantially flat so that a flat upper surface of a temperature sensor can make good thermal contact with the region on the underside of the cooking vessel, assisting in making an accurate temperature measurement.

[0030] In one embodiment, the first region which is relatively thicker compared to the second region, comprises one or a plurality of areas of inductive material embedded in a non-inductive material. The inductive material is preferably steel, and the non-inductive material is preferably aluminium.

[0031 ] In another embodiment, the first region which is relatively thicker compared to the second region comprises comprises an upper layer of non- inductive material, and a lower layer of inductive material of lower thermal mass than the upper non - inductive layer. The lower layer of inductive material provides inductive heating, and the upper layer of non-inductive material provides efficient conductive heat transfer from the inductive material to the contents of the cooking vessel. The upper layer of non-inductive material and said lower layer of inductive material may each have an aperture surrounding the second relatively thinner region, so that temperature can be measured at an area of the cooking vessel which is not directly over an induction coil, and which is not directly heated by induction.

[0032] In one embodiment, the apertures in the lower inductive material layer and an intermediate said non-inductive material layer are closed off by a part of said body portion. [0033] In one embodiment, the body portion comprises steel; said layer of inductive material comprises a ferrous material; and there is a non - inductive layer of aluminium located between said body portion and said inductive material.

In another embodiment, the cooking vessel comprises a body of non- inductive material into which are embedded one or a plurality of regions of inductive material; and there being a region of said body of reduced thickness, surrounded by a region of increased thickness, there being no said inductive material in said region of reduced thickness. [0034] The induction heated cooking the vessel may comprise a pan; a griddle; a steamer; an egg poacher, or a like vessel. In the general case, the induction heated cooking vessel may comprise any cooking vessel type which can be inductively heated and fitted on top of an induction hob base unit. [0035] On the underside of the cooking vessel there is provided a layer of inductive material which heats up when exposed to an electromagnetic field. The inductive material is bonded to the underside of a body of the cooking vessel, so that heat generated in the inductive material is transferred by conduction to the floor and/or sides of the cooking vessel.

[0036] In one embodiment, the layer of inductive material surrounds at least one aperture, being an area where the inductive material is absent. Preferably the aperture area is located substantially centrally to the base of the cooking vessel.

[0037] On the base unit, there is provided a protruding housing region which protrudes above the surrounding substantially flat upper surface area of the induction hob onto which the inductive plate of the cooking vessel sits.

[0038] The raised housing contains a negative temperature coefficient thermistor (NTC). The negative temperature coefficient thermistor may be located within the aluminium housing with heat conductive paste being provided between the thermistor and the inside of the housing, to ensure efficient heat transfer, and accurate temperature measurement. An objective is to physically locate the negative temperature coefficient thermistor device itself as close as possible to the inside surface of the cooking vessel, whilst the thermistor remains part of the induction hob base unit, by arranging the housing to contact the underside of the cooking vessel, as closely as possible to the interior of the cooking vessel, and with thermal conduction between the material of the cooking vessel and the thermistor being as efficient as possible, with the minimum amount of temperature gradient between the thermistor and the material of the cooking vessel immediately adjacent the food to be cooked, and with a greater temperature gradient between the position at which temperature is measured and the inductive heating element.

[0039] In use, the housing engages a recessed region in the underside of the cooking vessel, in which there is no inductive material, or a relatively thinner region of non - inductive material such as steel. The housing engages the reduced thickness region, so that the upper part of the housing is in direct contact with the material of the floor of the cooking vessel, rather than in direct contact with the main lower layer of inductive material. This means that the temperature sensor is measuring the temperature of the cooking vessel at a position in the base of the cooking vessel which is heated substantially only by conduction of heat from the contents of the vessel, and which is not heated by induction. The temperature sensor is not measuring the temperature of the induction layer itself at a place where it is directly heated by induction.

[0040] This temperature measurement is a more accurate reflection of the temperature of the base or floor of the cooking vessel than is produced by measuring the temperature of the induction material layer only, at a position underneath the cooking vessel where the layer is directly overlying an inductive coil. [0041] In the embodiments herein, the thermistor housing is preferably aluminium. The aluminium shell of the thermistor housing projects through the upper surface plate of the base unit upon which a cooking vessel is placed, so that when a cooking vessel is present, the temperature sensing housing is in direct contact with the underside of the cooking vessel at a position where there is no inductive material.

[0042] In various embodiments, the aluminium housing of the temperature sensor is placed in direct contact with the non- inductive floor material of the cooking vessel in a region where there is no inductive material, for example in an aperture in the inductive layer of the cooking vessel. This enables the temperature sensor to be in direct contact with the part of the cooking vessel, the other side of which is in contact with the contents of the cooking vessel, and gives a more reliable measurement of cooking vessel temperature. [0043] The aluminium housing is preferably spring mounted, so that in the absence of any pressure the housing protrudes slightly above the surrounding upper surface of the base unit, ensuring that when the cooking vessel is placed on the base unit, there is a good contact between the underside of the cooking vessel and the temperature sensor. The full weight of the cooking vessel depresses the resiliently biased temperature sensor housing, so that the surrounding inductive layer rests upon the upper surface of the hob, and the temperature sensor remains urged against an in contact with the underside of the cooking vessel.

[0044] In embodiments presented herein, the NTC temperature sensor is used to measure cooking temperatures, rather than being used as a safety cut out temperature sensor. The required accuracy of temperature measurement is greater than for a prior art temperature sensor used solely to detect overheating of the cooking vessel for safety reasons.

[0045] In the presently disclosed embodiments, temperature information from the temperature sensor may be used in automated menus and cooking methods, and may be used as a data import to generate a temperature display on the base unit.

[0046] Preferably measurement of temperature is taken from a non-ferritic part of the cooking vessel.

[0047] Other aspects are as set out in the claims herein which are incorporated into this description by reference. Brief Description of the Drawings

[0048] 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 illustrates schematically a cooking system comprising an induction heater base unit and a plurality of induction heated cooking vessels; Figure 2 illustrates schematically an induction cooker base unit and an induction heated cooking vessel; Figure 3 illustrates schematically the induction cooker base unit in perspective view from the front and above;

Figure 4 illustrates schematically one example of an induction heated cooking vessel of the cooking system ;

Figure 5 illustrates schematically in cutaway view the induction heated cooking vessel of figure 5 herein, placed upon the induction cooker base unit;

Figure 6 herein illustrates schematically in cutaway view a temperature sensor comprising the induction heater base unit;

Figure 7 herein illustrates schematically an underside of the induction heated cooking vessel as shown in figure 2, turned upside down to show an inductive material ;

Figure 8 herein illustrates schematically in cutaway view the temperature sensor positioned in an aperture in a cover plate, having the bottom of a cooking vessel positioned on the cover plate of the hob, with the temperature sensor in touching contact with the base of the cooking vessel during a cooking operation ;

Figure 9 herein illustrates schematically a circuit diagram of electronic control components for the induction hob base unit as shown in figure 1 herein;

Figure 1 0 illustrates schematically a second base for an induction heated cooking vessel, the second base comprising a non-inductive material embedded with regions of inductive material, and having a temperature sensing region on the underside of the base; Figure 1 1 herein illustrates schematically a third type of base for an induction heated cooking vessel, the third type of base comprising a layer of inductive material comprising a vessel body, second layer of inductive material and a layer of non-duct inductive material there between, and showing a temperature sensing region on the underside of the base;

Figure 12 herein illustrates schematically an underside of a steam vessel showing a heating plate comprising a three layer construction in which a first region of inductive material comprises an annular area which overlies an area of a first layer of non - inductive material, with a non - inductive material between said first and third layers; and

Figure 13 herein illustrates schematically an underside of a further vessel showing a heating plate comprising a three layer construction, in which a third inductive layer forms an annular first region surrounding a recessed second region comprising a first non - inductive layer.

Detailed Description of the Embodiments

[0049] 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.

[0050] Referring to Figure 1 herein, there is illustrated schematically in perspective view, an induction cooking set comprising an induction hob 1 00, together with a set of induction powered cooking appliances or vessels 1 01 -

106, and a cooking apparatus controller unit 107. [0051 ] The induction hob unit 100 comprises one or a plurality of induction coils, positioned underneath an upper surface onto which the cooking appliances 101 - 106 can be placed. Numerous types of cooking appliances may be placed for use on the induction hob unit 1 00 and by way of example only the cooking vessels 1 01 - 1 06 include:

- a slow cooker 101 ;

- a rice steamer 1 02;

- a steamer 1 03;

- a grill 104;

- a soup cauldron 1 05; and

- an oven 1 06.

[0052] The cooking apparatus controller unit 107 can plug directly into the induction hob unit 1 00, for example by means of a USB connector or other similar or equivalent convenient electronic connector, or can communicate with the hob wirelessly using a known wireless protocol, for example Bluetooth or equivalent. The cooking apparatus controller unit 1 07 may alternatively be integrated with the induction hob unit 100. The induction hob unit 100 can be controlled remotely from an application (app) on a mobile phone or hand held device, to set or program cooking parameters of the hob as hereinafter described. The induction hob unit 1 00 can also be controlled by directly entering commands into the cooking apparatus controller unit 1 07 using an interface provided on the controller, for setting cooking times and selecting pre-stored cooking programs or menus.

[0053] Referring to figure 2 herein, there is illustrated schematically in perspective view from the front and above the induction hob base unit 100 having seated thereon a tray or pan shaped cooking vessel 200. The shape of the lower part of the cooking vessel 200 matches the shape of an upper surface of the base unit 1 00, so that the tray 200 locates securely on top of the induction base unit 1 00. [0054] Referring to figure 3 herein, there is illustrated schematically in perspective view from the front and above the induction cooker base unit of figures 1 and 2. The induction base unit 100 comprises an outer casing 300; an upper surface 301 upon which a cooking vessel may be placed, the upper surface 301 having an outer peripheral rim 302 which assists in locating the underside of the cooking vessel on top of the induction base unit 100; a central raised platform 303 which, in use, locates in a correspondingly opposite shaped recess in the underside of the cooking vessel, the central raised platform 303 housing thereunder a metallic coil which when energized using alternating current produces an alternating electromagnetic field which extends in the vicinity immediately above the central raised platform area 303 to interact with an inductive material on a lower part of a cooking vessel placed upon the induction heating hob; a central region 304 containing a temperature sensor which, in use, contacts the underside of a cooking vessel for measuring a temperature of the underside of the cooking vessel; the controller unit 107, in this case shown integrated with the casing 300, but in the general case which can be either integral with the base unit, or a separate unit attachable to and detachable from the outer casing 300; and within the casing 300, a power supply unit for supplying AC mains power from an external AC mains power supply. Controller unit 107 comprises a set of control electronics for controlling the unit; a set of display control electronics for controlling a liquid crystal display on the control unit 107. The base unit 100 comprises a set of drive electronics for driving the metallic induction coil.

[0055] Referring to figure 4 herein, there is illustrated schematically in perspective view an induction heated cooking vessel 400 comprising the cooking system of figure 1 herein. The vessel comprises a vessel body 401 having a perimeter sidewall extending around a vessel base 402;; first and second handles 403, 404 at opposite ends of the vessel; and a ceramic or glass lid 405 for closing off an upper opening of the vessel. [0056] The vessel base 402 comprises a vessel floor, the vessel floor having an inductive material, for example a ferritic material. When vessel 400 is placed upon the induction hob 100, and the electromagnetic coil in the induction hob is energized, the inductive material of the vessel base heats up and heat is transmitted by conduction to the vessel floor and to a lesser extent, to the sides 401 of the vessel, thereby heating the contents of the vessel.

[0057] The vessel base 402 has a region of reduced thickness in which the base material, suitably steel, is thinner than in the surrounding region. This region is designed to fit over a corresponding region of the base unit which houses the temperature sensor, so that when the vessel is placed on the induction hob base unit the reduced thickness region under the vessel sits over the temperature sensor, allowing the temperature sensor to contact the underside of the vessel at its reduced thickness region.

[0058] Referring to figure 5 herein, there is illustrated schematically in cutaway view from the side, the cooking vessel 400 of figure 4 as placed on top of the induction hob base unit 100 of figures 1 to 3. On the underside of the vessel floor 402, there is a layer of inductive material which heats up when exposed to electromagnetic radiation. The inductive layer has a recess or aperture 500 being a region in which the inductive material is thinner than the surrounding regions of inductive material, or in which the inductive material is absent altogether. [0059] The base unit comprises a platform area 502 having a central aperture 503 in which there is provided a temperature sensor 504 which projects through the aperture, beyond the upper surface of the raised platform 502 to contact an underside of the vessel floor 402 in the thinner region 500 of the base of the vessel where the inductive material is absent, or of reduced thickness relative to the inductive material surrounding that region. The temperature sensor

504 is resiliently biased, for example by means of being mounted on a compression spring, so that the sensor is always in contact with the reduced thickness region 500 of the vessel floor 402. In this manner, the temperature sensor measures the temperature of the vessel floor through contact with the vessel floor, at a position which is not directly heated by an immediately adjacent induction coil. Rather, the reduce thickness region 500 of the vessel floor 402 is heated by conduction through the surrounding material of the floor 402, and is not directly heated by induction. The temperature as measured in this region more accurately represents the temperature of the inner surface of the vessel floor 402 and vessel wall 400, than the temperature of the adjacent regions of the inductive layer 505, surrounding the thinned floor region 500.

[0060] Referring to figure 6 herein, there is illustrated schematically in cut away view the temperature sensor 504 of the base unit 100. The temperature sensor comprises a negative temperature coefficient (NTC) thermistor 600 mounted within an aluminium housing 601 . The aluminium housing 601 is surrounded by or substantially surrounded by a flexible silicone collar 602 which is movable in a vertical direction, but at the same time allows for a fluid tight seal between the lower parts 603 of the collar 602 and adjacent layer 502 of the induction platform of the induction hob base unit. The silicone collar 602 fits into an aperture in the cover plate 502. The silicone collar comprises a heat proof membrane which can withstand the cooking temperatures underneath the cooking vessel, and allows for a generally up/down movement of the temperature sensor, with a small degree of lateral movement or lateral play, and at the same time makes a fluid tight seal between the aluminium housing 601 of the temperature sensor, and between the outer portions of the silicone seal and the cover plate 502. The cover plate 502 of the base unit may be for example a glass or ceramic plate.

[0061] The aluminium housing 601 is movable in a vertical direction and is resiliently biased upwardly by means of a compression spring 605 or other like resilient biasing means which performs the same function, such that normally the housing 601 is urged upwardly. This means that when a cooking vessel, such as induction heated cooking vessel 400 of figure 4, is placed on top of the platform layer 502, the cooking vessel will depress the upwardly resiliently biased sensor housing 601 , with the compression spring 605 or other like resilient biasing means urging the aluminium housing 601 into direct contact with the lower part of the cooking vessel.

[0062] The aluminium housing 601 in use is pressed directly against the lower surface of the vessel floor 402 at a location which is not directly heated by an induction coil immediately underneath it. In the best mode embodiment, the upper part of the aluminium housing of the temperature sensor protrudes above the level of the upper surface of the surrounding cover plate 502, to a sufficient extent that the aluminium housing makes touching contact with a reduced thickness region of a cooking vessel placed upon the induction hob, the reduced thickness area having a reduced thickness of (or absence of) inductive material compared to the surrounding parts of the base of the cooking vessel, such that the part of the cooking vessel which the temperature sensor contacts is substantially not directly heated by heat generated from induction in the reduced thickness area, but rather the temperature of that region is determined by conduction of heat from surrounding areas of the base plate of the cooking vessel which have been directly inductively heated.

[0063] The protruding negative temperature coefficient thermistor or temperature sensor fits closely within the recess 500 in the cooking vessel base 402 when the cooking vessel is placed on the base unit. This means that the temperature sensor measures the temperature of the vessel, such as a pan, at a place where the pan material is heated only by convection from the surrounding inductive material and material of the pan itself, rather than at a position where the base of the pan is directly inductively heated. This therefore gives a more accurate measure of the actual pan temperature, compared to if the temperature sensor were abutted directly up to the inductively heated material 505.

[0064] Referring to figure 7 herein, there is illustrated schematically in perspective view and underside of the pan type cooking vessel 200 as shown in figure 2 herein, with the vessel turned upside down to better show the features of the underside of the cooking vessel.

[0065] The cooking vessel comprises a base portion 700; an upright perimeter wall portion 701 ; first and second handles 702, one on either end of the vessel. The base portion comprises a protruding perimeter region 703 which is shaped to fit into the corresponding moat like region 301 of the base unit, the protruding perimeter region 703 surrounding a recessed region 704 in which is located, in this example, a flat annular layer of inductive material 705, the annular layer of inductive material having a central region 706 where the inductive material is absent, or is of reduced thickness compared to the surrounding region.

[0066] The protruding perimeter region 703 is shaped such that it engages in the recessed region 301 of the cover plate, and such that the recessed region 704 on the underside of the cooking vessel fits closely over the protruding platform region 303 of the induction base unit 100. In the best mode, the cooking vessel may comprise one or a plurality of shaped interlock regions which engage with one or a plurality of protruding interlock features 305, 306 on the raised platform portion 303 of the base unit.

[0067] The temperature sensor 304, being resiliently biased automatically presses upwardly into the central region 706 on the underside of the cooking vessel such that a relatively large surface area of the aluminium housing of the temperature sensor contacts with the underside of the cooking vessel. In the best mode, the contact area on the top of the temperature sensor may be in the region 78 mm²to 700 mm², corresponding to a circular contact area having radius between 5 mm and 15 mm [0068] Referring to figure 8 herein, there is illustrated schematically in cutaway view a base portion 800 of a cooking vessel positioned on top of a temperature sensor as described herein before with reference to figure 6. The underside of the cooking vessel comprises a solid plate of inductive ferritic material having a circular recess region 801 in which the plate thickness is relatively thinner or reduced, compared to the regions 802 surrounding the recessed region 801 . The surrounding regions 802 lay directly over an induction coil 803, whilst the recessed region 801 , in use, with the cooking vessel on the cover plate 502 lies directly over the temperature sensor such that the aluminium housing 601 of the temperature sensor contacts and underside of the reduced thickness recess region 801 . Since the recess region 801 does not lie directly over an induction coil, the material in the recessed region is not directly heated by induction from an electromagnetic coil 803. There may be a residual amount of open overlap of the electromagnetic field generated by the coils 803 and the material in the thinned region 801 , but the induction heating effect in the thinned region is insignificantly small compared to the induction heating effect of the surrounding full thickness portions 802 of the bottom of the cooking vessel which lie directly over the electromagnetic coils 803, although there may be a small amount of direct induction heating due to overlap directly over the induction coils 803. In this way, thejemperature of the thinned region 801 is close to the temperature of the inner surface 804 of the bottom of the vessel. [0069] Direct contact between the aluminium housing 600 of the temperature sensor and the underside of the thinned region 801 means that the temperature sensor measures as closely as possible the temperature of the inner surface 804, that is the floor, of the cooking vessel. Compression spring 605 makes sure that the aluminium housing 601 of the temperature sensor is pressed against the underside of the recessed region 801 to ensure efficient contact between the temperature sensor and the underside of the cooking vessel for temperature measurement.

[0070] Referring to figure 9 herein, there is illustrated a schematic block diagram of an electric circuit 900 which, in a preferred embodiment, is comprised as part of the cooking hob base unit 100. Electric circuit comprises a controller unit 901 , to which are coupled a clock 902, a user interface 903 which includes a visual display 904 and a set of user controls 905; a memory module 906 which can be used to store a set of recipes in electronic format; an appliance interface module 907 for reporting back to the controller unit with parameters such as temperature and coil operation/activation; an electromagnetic induction coil 908; a coil driver unit 909 for energising the electromagnetic coil; a temperature sensor 910 containing a negative temperature coefficient thermistor.

[0071] In use, the control unit may carry out a cooking sequence, being a sequence of electronic instructions to drive the electromagnetic coil 908 in response to pre-stored recipe data stored in the memory module 906 and/or cooking instructions entered via the user interface 903. The control unit 901 sends instructions to the appliance interface module 907 and the coil driver 909 to energise the electromagnetic coil 908 in a manner appropriate to the cooking profiles for various recipes. Temperature is monitored by the temperature sensor 910 which feeds back a signal representing temperature to the appliance interface module, which forwards a temperature signal to the control unit 901 which is in overall control of energising the coil 908 via the electromagnetic coil driver circuit 909. [0072] The temperature sensor 910 measures a temperature which represents the temperature inside the cooking vessel, and therefore represents a cooking temperature of the contents of the cooking vessel. Having an accurate measurement of temperature of the cooking vessel enables the control unit 901 to follow a predetermined set of memory instructions representing a cooking operation, and to make variations of that cooking programme depending upon the real-time temperature measurements generated by the temperature sensor 910.

[0073] In addition to the temperature sensor 910, there may be provided an additional temperature sensor for the purposes of safety cut out in case of malfunction of the electromagnetic coil 908 or other electrical fault, as is known in the prior art. This additional temperature sensor is not shown in figure 9. Cooking Vessels

[0074] The cooking vessels may be of two basic types being firstly an aluminium body having a bonded ferritic inductive layer, and secondly a steel body having a ferritic layer of material which can be heated through induction. The ferritic layer is preferably steel.

[0075] In the second type of cooking vessel disclosed herein, the vessel comprises a steel body, on the base of which is provided a steel inductive layer. Between the steel inductive layer and the underside of the steel body is provided a layer of aluminium, which serves the purpose of distributing heat generated in the inductivelayer by conduction laterally across the base area of the cooking vessel, and upwardly towards the steel base of the cooking vessel to ensure more even heat distribution across the base of the cooking vessel.

[0076] Another possibility is the provision of a magnet detection cover which performs the function of housing the magnet aligning the pan on the base and some degree of heat insulation between the remainder of the hot pan and the base excluding the central portion which contains induction coil, and with the components made out of a high temperature plastic. By high temperature, it is meant a temperature of 270°C as an upper limit. This component is for use on the stainless steel or aluminium versions of the cooking vessel, and all on all versions which fit on the base unit. [0077] In general, the cooking vessels each comprise a main body portion, for example in the shape of a pan, bowl, wok, griddle or the like, and a layer of inductive material underneath the main body portion. The main body portion may be steel, for example stainless steel, or aluminium. [0078] The lower part of the cooking vessel comprises a body portion which is intended to be in contact with a food item or a liquid to be heated. The lower part of the body portion of the cooking the vessel comprises a region of reduced thickness relative to a region of said body portion which surrounds the reduced thickness region. The reduced thickness region can be made of an inductive material such as steel, or a non-inductive material such as aluminium. However this reduced thickness region is not intended to be a region which is directly heated by induction, since when the cooking vessel is placed upon the base unit the reduced thickness region does not directly sit upon an electromagnetic inductive coil, but rather the inductive coils surround the region of reduced thickness. [0079] Referring to figure 10 herein, there is illustrated schematically a base portion of a second embodiment of a base plate 1000 of a cooking vessel as is configured in accordance with the present invention. The base portion comprises a plate of non-inductive material 1001 , for example aluminium, having embedded therein a plurality of blocks of inductive material 1002, for example stainless steel. The blocks of inductive material may be formed within the plate of non-inductive material 1001 . During heating, the non-inductive material is not directly heated by the electromagnetic field of the energising coil, but rather is heated through convection of heat from the individual blocks of inductive material 902 which are directly heated by the electromagnetic induction coil. There is provided a recessed region 1003 having an underside 1004 where the non- inductive material is relatively thinner compared to the plate material surrounding the thinned region 1003. On the other side of the base plate 1000 of the cooking vessel the plate may be in direct contact with food items. In the relatively thinner region 1003, the temperature of the underside 1004 of the region more closely approximates the temperature on the upper surface 1005 of the base plate than the temperature underneath the other regions containing inductive material. Using the inductive hob having the novel temperature sensor described herein, the temperature of the underside of the cooking vessel can be measured, and represents fairly accurately the temperature of the upper surface 1005 upon which food is cooked, of the cooking vessel. [0080] Referring to figure 1 1 herein, there is illustrated schematically a preferred embodiment third base portion 1 1 00 of a third cooking vessel. The base portion comprises a non-inductive stainless steel plate 1 1 01 , and underneath the stainless steel plate 1 101 a second inductive stainless steel plate 1 1 02. Between the first and second stainless steel plates 1 101 , 1 102 there is provided a layer of non-inductive material 1 1 03, such as aluminium. The purpose of the non-inductive layer central layer 1 103 is to distribute heat more evenly through conduction of heat, across the area of the base from the second inductive layer 1 102 to the first non-inductive layer 1 1 01 . Aluminium has a higher thermal mass or thermal capacity than stainless steel which means that it conducts heat more evenly, and heats up and cools down less quickly than stainless steel.

[0081 ] There is provided an aperture region 1 1 04 in the second inductive layer 1 102 where the second inductive plate and the central non-inductive layer 1 1 03 are absent, and allowing direct access to the underside of the first stainless steel non-inductive layer 1 101 . In this region 1 1 04, the temperature of the underside of the first inductive layer 1 1 01 can be measured as hereinbefore described by a resiliently biased temperature sensor housing which is urged upwardly against the underside of the first non - inductive layer of the cooking vessel. When placed upon the cooking hob, the region of the base of the cooking vessel containing the recessed region 1 104 lays directly over the depressable resiliently biased temperature sensor and the temperature sensor urges against the underside of the first layer 1 101 to give an accurate temperature measurement of the first layer 1 101 .

[0082] Referring to figure 12 herein, there is illustrated schematically an underside of a vessel for steaming food. The steamer vessel comprises a pan shaped body 1200 having a base 1 201 shaped to fit on top of the hob base unit 100. The main body of the vessel itself may be made from a heat resistant plastics material, including a floor part 1202 to which a substantially circular induction heating plate 1 203 is attached. [0083] The heating plate 1 203 is of a three layer construction, substantially as set out with reference to figure 1 1 herein. A first (upper) layer 1204 comprises a non-inductive and relatively poorly heat conducting substantially flat sheet of metal, for example stainless steel SUS 304.

[0084] A second, intermediate layer forms an annular ring bonded to and in contact with the first layer 1204, and which is made of a relatively highly heat conducting material, which is electrically non - inductive, for example aluminium sheet or a sheet of an aluminium alloy.

[0085] A third (lower) layer 1 205 comprises an annular mesa shaped plate having a central substantially circular aperture. The third layer 1 205 is bonded to the first layer 1 204 around the perimeter of the third layer, with the result that the substantially annular shaped third layer encapsulates the annular shaped second layer. The third layer is electrically inductive, and provides heat generation upon being energised by the electromagnetic field generated by the induction hob unit 100. Heat generated in the third layer is distributed across the annular region (first region) transferring heat to the non-inductive first layer. Conduction of heat and distribution across the first layer is assisted by the relatively high heat conducting second layer. Preferably, the third layer is made of an electrically inductive sheet material for example stainless steel SUS 430. [0086] The second layer is made of a material which has a higher thermal conductivity than the materials of the first or third layers, and also has a higher thermal mass or thermal capacity of the materials of the first or third layers.

[0087] A circular central region (second region) is left exposed, and uncovered by the second and third layers, such that the first layer extends across a circular aperture residing in the central aperture of those second and third layers. The second region comprises an underside portion of the first layer 1 204 which extends across the aperture. [0088] The diameter of the annular first region is less than the maximum outer diameter of the circular first layer 1 204, so that between the perimeter edge of the third layer and the perimeter edge of the first layer, there is a relatively small radial dimension annular area of exposed first layer. This region is provided to reduce heat conduction between the third layer and the floor 1 202 of the vessel, which may be made of a plastics material. The outermost diameter of the third layer 1 205 is spaced apart from the edge of the plastics floor, being connected to the plastics floor via the relatively low heat conducting material of the first layer.

[0089] At the junctions and connections between the third layer and the first layer, the third layer is angled with a frusto-conical surface, and a flat annular ring which is bonded to the first layer. Around the outer perimeter, there is a first, relatively larger frusto conical angled surface 1 206. Surrounding the inner circular second region, the third plate comprises a second relatively smaller (in the radial direction) annular frusto - conical surface 1207, with a second flat annular region of the third layer, which is provided to bond to the first layer around the central aperture region.

[0090] In use, the internal lower surface or floor of the steam vessel comprises the plastics floor material which surrounds the upper part of the heating plate 1202, and the heating plate itself. In use and upper surface of the single sheet of non - inductive material comprising the first layer 1204 is in contact with the contents of the vessel, for example food or water, whereas the underside of the first layer 1 204 of non - inductive material is exposed underneath the vessel and can be contacted by the protruding temperature sensor housing 601 . [0091 ] Because the first layer 1 204 is relatively poorly heat conducting, lateral conduction of heat from the actively heated first region comprising the area outlined by the third layer is minimised. Heat transfer inwardly towards the centre of the part of the first layer which extends across the aperture in the third layer is minimised. This means that the temperature sensor is measuring as nearly as possible the temperature of the first non - inductive layer rather than the temperature of the inductively heated third layer, and therefore giving a more accurate representation of the temperature of the contents of the vessel which are in direct contact with the other (upper) side of the non - inductive first layer.

[0092] Referring to figure 1 3 herein, there is illustrated schematically an underside of a further embodiment vessel 1 300 being a "smart vessel" which can be used for sous vide, searing, or for containing high liquid content recipes such as soup or dishes containing sauces.

[0093] The further vessel comprises a relatively larger pan shaped receptacle 1301 similar in overall area footprint to the steamer of figure 1 2, but having higher sides, for example as shown as item 1 01 in figure 1 herein. The vessel body of the "smart vessel" is preferably made of a heat resistant metal material, for example aluminium. The underside of the smart vessel is substantially as described herein with reference to the steamer 1 200 with the exception that the surrounding floor portion 1302 of the vessel, surrounding the active heating element, is made of aluminium. The heating element comprises a first, upper layer 1 304 of non - inductive, relatively low heat conducting material ; a second intermediate layer of non - inductive, relatively high heat conducting material, and a third (lower) layer of inductive material 1303. This means that the third layer 1 303 of inductive material can extend to substantially the same diameter as the first layer 1 304 of non - inductive material, giving greater area overlap between the annular third layer and the substantially circular first layer.

Where the vessel body is made of metal, it is not as important to provide thermal isolation between the inductively heated third layer and the material of the floor of the vessel. [0094] The third layer 1303 is of a substantially annular mesa shape having a first frusto - conical side around its outer perimeter, and a second frusto conical side surrounding a circular central aperture. Each of the frusto - conical sides has at its lower end a corresponding respective flat annular ring connected thereto which enables the edge of the respective frusto conical side wall to connect to the upper first layer or plate 3004. [0095] Around an outer perimeter of the relatively larger outer frusto conical surface, the third layer is formed into thin annular ring 1305 enabling it to be bonded to the first layer. Similarly, second relatively thinner (in the radial direction) annular ring 1306 connects to the internal diameter of the inner frusto - conical wall. This inner flat annular ring is parallel to the lower surface of the third layer and abuts the first layer in the region immediately surrounding the central aperture, so as to bond the edges of the aperture of the third layer to the first layer.

[0096] In the smart vessel, typically the temperature of the contents of the vessel need to be measured accurately to within 1 °C. The vessel can be used for searing, poaching, slow cooking or similar operations, or can be filled with a liquid such as water, a sauce, or a soup. Therefore, it is important that the temperature measurement is as accurate as possible, and this is facilitated by having the first layer as the only layer at the base of the vessel between the temperature sensor on the inductive hob unit 100 and the contents of the vessel. Ideally, the thickness of the non - inductive first layer is made as thin as possible, to allow for heat conduction through the relatively poorly heat conducting non - inductive first layer in use in the vertical direction, along a main central axis of the vessel, from the contents of the vessel to the housing of the temperature sensor based on the hob unit 100. For example, the thickness of the first layer may be in the range 0.2 mm to 1 .5 mm.

[0097] On the other hand, the radial distance between the centre of the first layer 1304 in the central (second) region is much greater than the thickness of the first layer 1304 in the transverse direction along the main central axis of the vessel, (i.e. substantially vertically in use), meaning that heat generated in the inductive third layer is transmitted laterally by conduction to the centre of the second region to a much lower extent than heat transmitted vertically, because the laterally transmitted heat has further to travel through the material of the first layer, than heat transmitting from the upper surface of the first layer through the thin layer itself, to the lower surface of the first layer where the temperature sensor contacts the first layer to measure cooking temperature.

[0098] The cooking vessel may be typically pressed, formed from a metal sheet such as sheet steel or aluminium, together with a ferrous-based inductive heating member welded, brazed or otherwise fixed to an underside of the cooking vessel to form part of the base.

[0099] In the best mode, the region of the upper surface of the heating area of the induction hob in which the temperature sensor is located is substantially central, and surrounded by an induction coil underneath the cover plate. However, depending upon the architecture and layout of the heating area of the induction hob and the induction coil underneath the cover plate, the temperature sensor could be positioned offset towards the edge of the cooking area over which a given cooking vessel fits. In the best mode arrangement, the temperature sensor is located within the area upon which the cooking vessel sits on the hob at a position on the base of the cooking vessel which does not directly overlie an electromagnetic induction heating coil.

[00100] Embodiments described herein may find particular application in controlling the temperature of the contents of a cooking vessel which is inductively heated when seated on an induction cooking hob.

Claims

1 . An induction cooking vessel comprising: a body portion, said body portion having an underside comprising: a first layer of an electrically non-inductive material; a second layer of an electrically non - inductive material ; said second layer being of a material having a higher thermal conductivity than said first layer; a third layer, said third layer being of an electrically inductive material; said underside comprising a first region of a first thickness, and a second region of a second thickness, said second region having a reduced thickness relative to the thickness of said first region ; wherein said first region of said first thickness comprises said first layer, said second layer and said third layer; and said second region of relatively reduced thickness comprises said first layer.

2. The induction cooking vessel as claimed in claim 1 , wherein within said second region of relatively reduced thickness, an upper surface of said first layer forms a bottom interior surface of said vessel.

3. The induction cooking vessel as claimed in any one of the preceding claims, wherein within said second region of relatively reduced thickness, a lower surface of said first layer is exposed.

4. The induction cooking vessel as claimed in any one of the preceding claims, wherein said first layer comprises stainless steel 304.

5. The induction cooking vessel as claimed in any one of the preceding claims, wherein a material of said second layer has a higher thermal mass than a material of said first layer or said third layer.

6. The induction cooking vessel as claimed in any one of the preceding claims, wherein said second layer comprises aluminium.

7. The induction cooking vessel as claimed in any one of the preceding claims, wherein said third layer comprises stainless steel 430.

8. The induction cooking vessel as claimed in any one of the preceding claims, wherein said second layer is bonded to said first layer.

9. The induction cooking vessel as claimed in any one of the preceding claims, wherein said third layer of inductive material is bonded to said second layer of non - inductive material.

10. The induction cooking vessel as claimed in any one of the preceding claims, wherein an outer surface of said second region is substantially flat.

1 1 . The induction cooking vessel as claimed in any one of the preceding claims, wherein said first region comprises one or a plurality of areas of inductive material embedded in a non-inductive material.

12. The induction cooking vessel as claimed in any one of the preceding claims, wherein said first region comprises an upper layer of non- inductive material, and a lower layer of inductive material.

13. The induction cooking vessel as claimed in claim 5, wherein said second layer of non-inductive material and said third layer of inductive material each comprise an aperture surrounding said second region.

14. The induction cooking vessel as claimed in claim 1 2, wherein said apertures are closed off by a part of said body portion.

15. The induction cooking vessel as claimed in any one of the preceding claims wherein said body portion comprises a material selected from the set: a non - inductive steel ; a plastics material ; aluminium.

1 6. The induction cooking vessel as claimed in any one of the preceding claims, comprising a body of non-inductive material into which are embedded one or a plurality of regions of inductive material ; and there being a region of said body of reduced thickness, surrounded by a region of increased thickness, there being no said inductive material in said region of reduced thickness.

17. The induction cooking vessel as claimed in any one of the preceding claims, wherein said cooking device is selected from the set: a pan;

a griddle;

a steamer;

an egg poacher.

18. The induction cooking vessel as claimed in any one of the preceding claims, wherein said first region comprises a ferritic material, and said second region comprises a substantially non-ferritic material.

19. The induction cooking vessel as claimed in any one of the preceding claims comprising a side wall of a substantially non- ferritic material.

20. The induction cooking vessel as claimed in any one of the preceding claims wherein an underside of said second region is substantially smooth.

21 . The induction cooking vessel as claimed in any one of the preceding claims wherein said second region comprises a depression or recess in an otherwise substantially flat underside of said base portion.

22. The induction cooking vessel as claimed in any one of the preceding claims, wherein said first layer extends across an aperture formed in said second and third layers such that in use, an upper surface of said first layer is in direct contact with a food item to be cooked, and a lower surface of said third layer is exposed and accessible for measurement of temperature.