Abstract:
The invention provides an electrically-driven kitchen machine including a food mixer or a food processor. The machine has a support platform for supporting a receptacle for foodstuffs to be mixed or otherwise processed. The machine has an electric motor used to drive mixing or processing tools disposed in the receptacle; and a heater for heating the foodstuffs in the receptacle. In order to ensure that the temperature of the foodstuffs is accurately determined, the base of the receptacle consists of multiple layers, including a first metallic material, such as aluminum, having a relatively high thermal conductivity, sandwiched between outer and inner skins of a second metallic material of lower thermal conductivity than the first metallic material. The machine is also provided with first and second probes, each protruding through the receptacle-supporting platform and urged upwardly to contact the inner skin and the first metallic material respectively. The second metallic material typically comprises stainless steel, and the heater typically comprises an induction heater, in which case the probes are constructed so as to resist absorption of heat from the energy field generated by the heater.

Description:
FIELD OF THE INVENTION 
     This invention relates to kitchen machines, and it relates in particular to such machines as are commonly referred to as food mixers, and food processors. 
     BACKGROUND OF THE INVENTION 
     Food mixers typically comprise stand mixers, by which is meant the kind of kitchen machine in which a receptacle such as a mixing bowl is supported on a pedestal which also supports an electric motor and a drive system including a drive outlet, overhead of the bowl, which permits a planetary mixing action to be imparted to tools suspended into the bowl from the overhead drive outlet. 
     Food processors, on the other hand, typically comprise machines in which receptacles equipped with rotatable blades or the like can be driven from beneath by means of a motor and drive system housed within a casing upon which the receptacle is supported. Frequently such food processors present two drive outlets (sometimes these are disposed coaxially) which can selectively drive the blades or the like associated with different receptacles, such as a bowl and a goblet, for mixing and blending respectively. 
     Both types of such kitchen machines are extremely versatile, and recent technical developments provide the possibility of still broader operational capability; such arrangements including heating means provided to permit, in accordance with user selection, ingredients to be heated whilst being subjected to a mixing or processing action. The degree of heating employed may, in accordance with user selection, be sufficient to completely cook, to partially cook, or to merely warm ingredients and it will be understood in this respect that the degree of heating is influenced both by the heater power (wattage) used in any given situation and the time for which the heat is applied. 
     Such added operational facilities, whilst opening up various more sophisticated possibilities for food preparation, place increased demands upon the sensors and associated controls used to regulate the operation of such machines. In particular, there is a critical need, with kitchen machines equipped with heating capabilities, for accurate measurement of the temperature of the ingredients being mixed or processed and/or of a receptacle containing the ingredients, and it is one object of this invention to provide a kitchen machine incorporating rugged and accurate means for sensing the temperature of a receptacle and/or ingredients therein. 
     SUMMARY OF THE INVENTION 
     According to the invention from one aspect there is provided a kitchen machine having a support platform for supporting a receptacle for ingredients to be mixed or otherwise processed therein; an electric motor and a drive system capable of imparting a mixing or other processing action to tool means disposed in the receptacle; and heating means to permit said ingredients in the receptacle to be heated; the receptacle being constructed, at least in a base region thereof, of a first metallic material, such as aluminium, having a relatively high thermal conductivity, disposed between outer and inner skins of a second metallic material of lower thermal conductivity than the first metallic material; the machine further comprising thermal sensing means including a first probe protruding through the receptacle-supporting platform and urged upwardly by resilient means to contact said inner skin and a second probe protruding through the receptacle-supporting platform and urged upwardly by resilient means to contact said first metallic material, and means utilising electrical signals developed by said probes to display and/or control the temperature of said ingredients. 
     In a preferred embodiment, the second metallic material comprises stainless steel. 
     Preferably the heating means comprises an induction heater mounted within said machine and at least one of said probes comprises a tubular metallic member, for example of aluminium, sleeved over a part at least of its length with a thermally resistive plastics material substantially unresponsive to the heat developed by said inductive heater. Optionally, an outer surface of said sleeve may be coated with, or incorporate, a non-stick substance, to facilitate repeated movement of the probe with sliding contact relative to an aperture in the receptacle-supporting platform as described in GB2441508 A. 
     In some preferred embodiments of the invention, at least one of the probes is adapted to resist absorption of heat from the energy field generated by the heater by reducing the diameter of the tubular metal member over a substantial part of its length and providing said sleeve around the part of reduced diameter. 
     In some such embodiments, the sleeve substantially compensates for the said reduction in diameter; thereby providing a probe which, with the sleeving in place, is of a substantially constant diameter throughout its length. Further preferably, the largest diameter part of the metallic member is at its head, in the region of contact of the probe with the receptacle. 
     Furthermore, and importantly in some embodiments, the invention may provide the function of a thermal fuse wherein a microprocessor is conditioned to utilise electrical signals provided by the probes to de-energise the heater if a monitored temperature exceeds a predetermined threshold value. The microprocessor may be dedicated to this specific function, or it may be conditioned to perform other functions, such as providing operational control of the heating process and/or producing temperature indications for display. 
     It is preferred in some embodiments of the invention to provide control means configured such that, once the temperature sensed by a probe exceeds a predetermined level, the electric drive motor is operated relatively slowly, in order to reduce the risk of splashing or otherwise ejecting hot ingredients out of the bowl or causing them to separate. 
     The control means may include a microprocessor and conveniently the aforementioned microprocessor or an additional such device may be configured to facilitate cooking operations by pre-programming therein a series of pre-determined operational cooking and mixing sequences, any of which can be selected by the user pressing one of a plurality of special function keys, thereby causing the machine to implement the desired operational programme without requiring the user to input all of the individual instructions. 
     Electrical signals derived from the probes may be utilised to provide a temperature value which may be displayed visually on the kitchen machine or elsewhere and may include textual or graphic indications conveyed to the user by means of a display, such as a liquid crystal display (LCD). 
     In either event, the electrical signals may be conveyed to the display by hard wiring or they may be transmitted to the display wirelessly, for example by means of radio frequency signalling techniques and/or by means of Bluetooth protocols. 
     In some preferred embodiments of the invention, the heating means can be automatically controlled by temperature feedback signals derived from the probes to comply with user-input instructions. 
     In preferred embodiments, the receptacle-supporting surface of the platform is constructed from a specifically chosen grade of plastic material, such as glass-filled PPS, which has been found in practice to provide a good, bearing-like property which accommodates the repeated rotational sliding movement between the base of the bowl and the platform, each time the bowl is fitted to or removed from the platform, typically by means of bayonet fittings. 
     If necessary, or if preferred, the receptacle-supporting surface of the platform may be coated with or impregnated by a non-stick material such as PTFE, thereby to further facilitate the repeated rotational sliding movements between the base of the bowl and the platform. 
     Some preferred embodiments of the invention further comprise an interlock configured to prevent energisation of the heating means unless the receptacle is in place on the receptacle-supporting platform and/or an interlock configured to prevent operation of the electric motor unless the receptacle is in place on the receptacle-supporting platform. 
     In still further embodiments of the invention, the machine may be further provided with a receiver for radio frequency (RF) control signals from a remote control handset (not shown) which permits the kitchen machine to be instructed to perform, or be conditioned to perform, some at least of its functions remotely. In other embodiments, remote control signals for the stand mixer are input by means of a suitable device into the mains wiring and sent to the stand mixer over that wiring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be clearly understood and readily carried into effect, certain embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: 
         FIG. 1  shows, in perspective view, a kitchen machine in accordance with one example of the invention; the machine comprising in this example a stand mixer; 
         FIG. 2  shows the stand mixer of  FIG. 1  from a frontal elevation; 
         FIG. 3  shows a side elevation of the stand mixer of  FIG. 1 ; and 
         FIGS. 4   a  and  4   b  show, in cross-sectional view and in separated and operational relationship respectively, a receptacle and associated temperature probes for use with the stand mixer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1 ,  2  and  3 , in which corresponding features carry the same reference numbers, a stand mixer  10  comprises a pedestal  20  which carries a bowl-supporting platform  30  and a housing  40 . The housing  40  encloses, in any convenient fashion, an electric drive motor (not shown) and gearing (not shown) which conveys the motive power supplied by the motor to a plurality of drive outlets to which various tools can be attached to perform a wide variety of tasks in the kitchen. 
     In this particular example, there is provided a high-speed blender drive outlet behind covers  41 ; a slow-speed drive outlet, behind cover  42 , which may be used, inter alia, to power a mincing attachment; and a planetary drive, overhead of the bowl location, at  43 . It will readily be appreciated that more, fewer and/or different drive outlets can be provided in accordance with desired functionality of the stand mixer. 
     A shanked mixing tool, attached as is conventional, to a socket  44  of the outlet  43 , will depend in use into a receptacle, in this case a mixing bowl, placed on the platform  30 , and is configured to rotate about both the axis of the socket  44  and the central axis  45  of the outlet  43 , thus performing a planetary mixing action. The necessary relationships between the relative shapes and dimensions of the bowl and the mixing tool to ensure thorough and repeatable mixing of ingredients are well known and established in use over many years. 
     As shown, the stand mixer  10  is, in this example, provided with a pair of latches  31 ,  32  within a recess  33  provided in the bowl platform  30 , which latches co-operate with components on the base of the bowl to form a bayonet latching system which ensures firm and ready location of the bowl on its platform. Other latching systems, such as screw-threading for example, can be used as an alternative to bayonet latching if preferred. 
     The upright part  46  of the housing  40  is configured with a break line  47 , and a suitable mechanism to permit the top part  48  of the stand mixer to be hinged away from the platform  30  end of the pedestal part  20 , in order to facilitate the insertion and removal of the mixing tools and the bowl. 
     A first example of an arrangement in accordance with the present invention will now be described with additional reference to  FIGS. 4   a  and  4   b , which show a bowl  60  configured and dimensioned to be received in the stand mixer  10  with its base  61  located in the recess  33  of the bowl platform  30 . The base  61  of the bowl  60  is formed with a diametrically opposed pair of shaped slots (not shown), respectively disposed and configured to co-operate with the latching elements  31 ,  32  of the stand mixer  10  to perform the aforementioned bayonet latching function. 
     As shown schematically by dashed outline in  FIG. 1 , a heater  71  is integrated into the base of the stand mixer  10 , beneath the platform  30 . The heater  71  is, in this example of the invention, an induction heater of a kind known per se. 
     The stand mixer  10  also has a thermal sensor for measuring the temperature of the bowl  60 , and thus of the contents thereof. The thermal sensor comprises, in this example, a first probe  80  protruding through an aperture  34  in the bowl-supporting platform  30  and urged upwardly by resilient means (not shown) to contact the base  61  of the bowl  60  in use of the arrangement. 
     It has been shown in practice that, in order to control the foodstuff temperature sufficiently to prevent burning of the foodstuff, and to optimise the heating time, it is necessary to measure not only the temperature of the foodstuff, but also the temperature of the heating surface itself. 
     The design and position of the temperature sensors has been developed so that their accuracy is not influenced by the heating inductive field, and to also provide reliable and accurate temperature control without compromising the integrity of the bowl. 
     The heat input to the bowl  60  is induced from the inductive coil  71  which generates heat within an outside skin  62  of stainless steel on the base  61  of the bowl. The heat generated typically is not constant across this stainless steel skin  62 , and the formation has been discovered in practice of areas of high localised heat (hot-spots), rendering it difficult to consistently measure the bowl temperature and therefore to control the input heating power sufficiently to prevent burning of the foodstuff. 
     Accordingly, the bowl  60  is modified by the inclusion of a layer  63  of highly thermally conductive material, in this case aluminium, in its base  61 . The aluminium layer  63  evenly distributes the heat across a relatively large area and almost completely eliminates burning under controlled conditions. The inside surface of the bowl  60  comprises a second, inner skin  64  of stainless steel; whereby the base  61  of the bowl  60  comprises the inner skin  64  of stainless steel, the aluminium inter-layer  63  and the outer skin  62 . It will be appreciated that the inner skin  64  is the food contacting surface. 
     The present invention employs two temperature sensors which co-operate with a specially formed base to the bowl. The probe  80  which, in this embodiment of the invention, comprises a negative temperature coefficient (NTC) probe, is located to pass through an aperture  65  in the outer skin  62  and the aluminium interlayer  63 , and is urged in use into firm thermal contact with the exposed underside of the inner skin  64  of stainless steel. A second, shorter, probe  81 , also (in this example) an NTC probe, offset from the probe  80  but generally parallel to it, is also provided as will be described below. 
     The two sensors  80  and  81  are, in this example, fitted within plastic sleeves and resiliently urged upwards to ensure good thermal end-on contact with the bowl and to accommodate any dimensional variations due to manufacture. The choice of materials is critical to ensure that the temperature measurements are not adversely affected by the inductive field, and to ensure the mechanical integrity is maintained at high temperatures throughout the life of the product. 
     As previously mentioned, the probe  80  is in contact with the inner stainless steel skin  64  through the aperture  65  in the aluminium interlayer  63 . When the bowl  60  contains foodstuff with high water content, the temperature at the contact area of probe  80  closely represents the temperature of the bowl contents. 
     Probe  81  is resiliently urged into good thermal contact with an area  66  of the aluminium interlayer  63  which is not covered by the outer skin  62  of stainless steel, and thus measures the temperature of the aluminium inter-layer  63  within the bowl base  61 . Since aluminium is a very good conductor of heat, probe  81  senses a temperature that is closely representative of the average foodstuff input temperature. For convenience, in this example, the area  66  comprises part of an annular area  67  which completely surrounds the aperture  65 , and from which the outer skin  62  has been omitted or removed. 
     The support surface, which lies between the bowl and the induction coil  71 , is made, in this example of the invention, from a specially chosen grade of plastic, such as glass-filled polyphenylene sulphide (“PPS”). This material has been found to suitably accommodate, and to provide a bearing-like surface for, the repeated rotational sliding movements between the base of the bowl and the platform, each time the bowl is fitted to or removed from the platform by means of the bayonet fittings. 
     Conventional induction cooking devices use a glass or ceramic plate and crudely measure temperature through this plate. This embodiment of the invention, on the other hand, utilises the plastic plate with temperature sensors that touch the cooking vessel. 
     Typically, the probes  80  and  81  comprise tubular aluminium members, each of which is respectively sleeved over a part at least of its length with a thermally resistive material, such as a plastics material, substantially unresponsive to the energy field generated by the induction heater  71 . 
     The probes  80  and  81  are adapted to resist absorption of heat from the energy field generated by the heater  71 . In this respect, and particularly as regards the longer probe  80 , its diameter is reduced over a substantial part of its length and the sleeve of plastics material is provided around the part of reduced diameter. 
     Conveniently, the radial thickness of the sleeve is such as to compensate for the reduction in diameter of the aluminium probe  80 ; thereby providing a probe which, with the sleeving in place, is of a substantially constant diameter throughout its length. 
     The probes  80  and  81  may, of course, be made of materials other than aluminium, provided that such materials (a) provide adequate thermal transfer to a thermal sensor (not shown) such as an NTC sensor, typically mounted within a hollowed internal region of the probe or otherwise associated therewith, to permit reliable temperature measurements to be made, and (b) can tolerate the mechanical and thermal environments in which they have to work in this context. 
     As is known, the supply of power to the inductive heater  71  may be interrupted briefly each time temperature measurements are taken from the probes  80  and  81 . 
     Temperature readings derived from electrical signals provided by the thermal sensors associated with the probes  80  and  81  are processed to provide useful parameters indicative of the temperature of the bowl  60  and/or its contents. These parameters are, in this example, used to automatically control the heating or cooking process. Alternatively, or in addition, the parameters, or features derived therefrom, may be displayed visually on the stand mixer  10  and/or elsewhere and may include textual or graphic indications conveyed to the user by means of a display, such as a liquid crystal display (LCD)  90  which, in this case, is carried by the stand mixer  10 . 
     Whether the display is carried by the stand mixer  10  or located elsewhere, the temperature readings may be conveyed thereto either by hard wiring or wirelessly, for example by means of radio frequency signalling techniques and/or by means of Bluetooth protocols. 
     In some embodiments of the invention, the operation of the heater  71  can be automatically controlled by temperature feedback signals derived by processing the electrical signals provided by the probes  80  and  81  to comply with user-input instructions for certain operational cooking programmes. 
     A thermal fuse (not shown) may also be provided in the stand mixer  10 . In a preferred embodiment, however, the function of a thermal fuse is performed by microprocessor conditioned to utilise electrical signals provided by the probes to de-energise the heater if the monitored temperature exceeds a predetermined threshold value. The microprocessor may be dedicated to this specific function, or it may be conditioned to perform other functions, such as providing operational control of the heating process and/or producing temperature indications for display. 
     It is preferred that an interlock is provided to prevent the energisation of the heater  71  unless the bowl  60  is in place. Indeed, an interlock may also prevent operation of the electrical drive motor of the stand mixer  10  unless the bowl is in place. 
     Referring again to  FIGS. 1 ,  2  and  3 , it will be noted that the stand mixer is provided with a switch pad  100  presenting user-operable power controls  101  and timer controls  102 . The controls  101  and  102  comprise, in this example, digital controls interacting with a microprocessor (not shown) housed at any convenient location within the stand mixer housing. Alternatively, the controls  101  and  102  may be analogue controls or one of them may use digital technology and the other analogue, depending upon design, cost and other criteria. 
     In this example, the power controls  101  provide control switches such as  103  to provide user-selected instructions to the microprocessor as to the heating power required and digital timing controls  102  provide control switches  104  for controlling timed operation of the motor-driven planetary drive of the stand mixer and control switches  106  for controlling timed operation of the heating element. The electrical signals provided by the probes  80  and  81  are also processed by the microprocessor to provide the required display and/or automatic control of the kitchen machine. 
     The various switches are used to convey user instructions to the microprocessor and it will be appreciated that the microprocessor may be configured to provide internal electronic interlocks to prevent certain combinations of instructions from being implemented. If an operation is prohibited, the user is warned that an inoperable or inadvisable combination of instructions has been inputted and that the user&#39;s operational strategy needs to be revised. In some embodiments of the invention, the warnings may be supplemented with, or replaced by, a visual display, such as a part of the liquid crystal display (LCD)  90  used to convey textual and/or graphic instructions to the user. Associated with the display  90  are typical resetting and function control buttons  91 . 
     When the planetary mixing action is to be used, continuously or intermittently, during a cooking process, some embodiments of the invention provide an automatic facility whereby, once the temperature of the ingredients in the bowl  60  has exceeded a predetermined threshold temperature, as indicated by the electrical signals provided by the probes  80  and  81 , the electric drive motor is caused to operate with reduced speed, in order to reduce the risk of splashing or otherwise ejecting hot ingredients out of the bowl. 
     In some embodiments, the stand mixer is provided with a receiver, for example in the form of an antenna formed on or coupled to a window  120  in the casing  20 , for radio frequency (RF) control signals from a remote control handset (not shown) which permits the stand mixer to be instructed to perform, or be conditioned to perform, some at least of its functions remotely. In other embodiments, remote control signals for the stand mixer are input by means of a suitable device into the mains wiring and sent to the stand mixer over that wiring. 
     In some embodiments of the invention, the bowl-supporting surface of the platform  30  may be coated with (or impregnated by) a non-stick material such as PTFE, thereby to further facilitate the repeated rotational sliding movement between the base  61  of the bowl  60  and the platform  30 , each time the bowl  60  is bayonet-fitted to, or removed from, the platform.