There are many common examples of heating and/or heat maintaining apparatus for heating and maintaining heat in articles, e. g. pie warmers, heat lamp serveries, steam heaters, delayed service storage devices and the like.

Foodstuffs have specific cooking temperatures. Once a foodstuff reaches the level where the food begins to cook it can be readily overcooked and spoiled if the temperature exceeds that level. If cooking temperatures are lowered or varied during or after cooking, spoilage of the foodstuff will also occur.

Thus the best cooking and heat maintenance environment occurs when even heat is transferred to a foodstuff.

W09413184 describes a cabinet with the dual function of both heating and cooling foods. The apparatus is said to provide consistent and uniform heating or cooling of food contained therein using conduction with good contact to the food through the uniform temperature achieved all over the shelf and good contact to the food on the shelf.

W09221272 describes a cabinet which cooks and heats food articles or maintains same at a constant temperature. The heating of the surface of the shelves is uniform and what is described as unintended temperature gradients along the surface are said to be eliminated.

FR2738136 describes a heating element which is said to heat up more quickly and retain heat longer than current heater with minimum energy usage.

Whilst the abovementioned inventions recognise the importance of applying even heat to foodstuffs to cook and maintain the foodstuffs at a constant temperature one problem not addressed is the effect on an internal environment of temperature changes which occur when relatively inefficient thermostatic controls or forced drafts are used to control

heat transfer from heating elements, and when access doors are opened and shut.

Conventional heating elements and control systems tend not react quickly enough to prevent heat losses to below a desirable level and often in attempting to restore the environment overheating occurs resulting in drying and dehydration of the foodstuffs.

The insulation properties of objects and apparatus for maintaining temperatures are dependent to a large extent on temperature gradients throughout the whole of the body of an apparatus and, very importantly, the surface areas of same.

It is an object of the present invention to provide a method and apparatus for heating and maintaining the heat in foodstuffs or other objects with minimal heat variation occurring during periods when the heated object or foodstuff is maintained at a predetermined temperature for later consumption or other purposes.

Further objects and advantages of the present invention will become apparent from the ensuing description which is given by way of example.

DISCLOSURE OF INVENTION According to a first aspect of the present invention there is provided a heating apparatus comprising a body, means of access to the interiors of the body and at least one heating element comprising an internal shelf or wall which is a laminate or sandwich of at least two panels and an intermediate sheet of metalised plastics film adapted for connection to a power source.

The said at least two panels can be glass panels.

The apparatus can include an electrical controller interposed between the intermediate sheet and a power source which is programmable to measure temperatures of the said at least one internal shelf or wall and to provide variable currents to the intermediate sheet.

The intermediate sheet can be a one-piece unitary member which is of substantially the same area as each of the major surfaces of the

said at least one shelf or wall.

The intermediate sheet can be adhered to one or both of the glass panels of each shelf or wall.

An air gap can be provided between one of the glass panels and the intermediate sheet.

According to a further aspect of the present invention there is provided a method of heating comprising ; (a) providing a plurality of heat zones within a heating cabinet, (b) placing an item or items to be heated on or adjacent to a shelf or wall as aforesaid.

(c) monitoring the surface temperature of the shelf or wall which underlies or is adjacent to the item to be heated, and (d) maintaining the temperature of said at least one internal shelf or wall within predetermined ranges.

The monitoring of the surface temperature of the said at least one internal wall of the cabinet can be by a bi-metallic measuring device.

Electrical signals from the measuring device can be received and processed by a controller adapted to adjust the level of current fed to the elements of the said at least one internal wall.

BRIEF DESCRIPTION OF THE DRAWINGS Aspects of the present invention will now be described by way of example only with reference to the accompanying drawings in which Figure 1 is a perspective view of a typical heating and heat maintenance apparatus to which the present invention relates, and Figures 2 and 2a are section drawings of a shelf for a heating apparatus according to aspects of the present invention, and Figure 3 is a diagrammatic drawing showing the imposition of a controller between the power supply and a heating element of a shelf of the apparatus of the present invention, Figure 4 of the drawings is a general outline of a microcomputer

based temperature controller in accordance with one possible aspect of the present invention, and Figure 5 of the drawings is a diagrammatic drawing of an apparatus of the present invention and serves to illustrate how one or more heat zones can be provided with an apparatus.

Figure 6 is an electronic schematic showing the power supply, microcomputer and software protection sections of the controller.

Figure 7 is an electronic schematic showing the current sensing circuit and sixteen identical outputs for controlling the heater elements.

Figure 8 is an electronic schematic showing thirteen identical temperature measurement inputs for monitoring the shelf and air temperatures.

Figure 9 of the drawings is a diagrammatic elevational view of electrical connectors for heated panels of an apparatus according to the present invention.

With respect to the drawings figure 1 illustrates a typical apparatus to which the present invention relates. A heating environment is provided within a body generally indicated by arrow 1 with access to the interiors of the body being provided by a door or doors 2. The environment may be sub-divided by partitions such as shelves 3 upon which objects to be heated can be rested. The shape of the body may be varied to suit design or other criteria and may include curved portions (not shown).

Figures 2 and 2a illustrate possible structures for shelving in accordance with the present invention.

In each case the shelves may comprise a laminate or sandwich of toughened glass panels 4 and an intermediate sheet of electrically resistive material 5.

In the case of the figure 2 embodiment an air gap exists between the panels 4 facilitated by the provision of peripheral insulating spacers 6. The sheet 5 is fixed to the lower panel of the sandwich.

In the case of the figure 2a embodiment the panels 4 and sheet 5 are laminated.

Sheets 5 are by preference unitary sheets of metalised plastics film. The film may be polyethylene or polyester films which can be applied to the panels by adhesives or heating. Some advantage may be derived from using corona treated films which are known to have superior adhesive qualities.

Electrical connections'C'can be made to the sheets 5 (see figure 3) connecting the sheet to a power source 8 via a controller 7. The controller 7 shown in box form in figure 3 may include: (a) rectification and transformation means; (b) means to vary current supplie to the sheets 5; (c) a programmer which enables heat levels to be set for specific objects to be heated; (d) tamper proofing facilities which ensure the program is not incorrectly reset; (e) alarm/fault systems.

One form of controller may consist of a microcomputer based temperature controller of the general outline illustrated by figure 4.

Thermocouples are used as temperature sensors to achieve high accuracy temperature sensing without requiring individual calibration.

The amplifiers are low drift switched capacitor high gain precision devices and amplify the low level signal (40uV/oC) from the thermocouples to a level suitable for use by the digital to analog converter.

The digital to analogue converter takes signals from each of the thirteen temperature inputs converting these into 10 bits of digital information providing about 0.20 resolution. This information is available to the software running in the microcomputer for the purpose of stabilising the heating surfaces of the foodwarming cabinet and to provide and air temperature display during normal operation.

There are 16 individual outputs which can individually control up to 16 different heated surfaces in the cabinet. Outputs are switched at the zero crossing points to minimise the electromagnetic interference generated by the cabinet.

The electronics provides two digits of LED display which can display the air temperature within the cabinet over the range 0 to 99oC. This display is also used to allow the electronic controller to be programmed to suit differing cabinet configurations and the different conditions required by different food types that may be preserved by the cabinet using a key switch.

The software running in the microcomputer allows the cabinet to be configured with up to 12 surfaces being temperature controlled by fixing a thermocouple to a heated surface to a precision of better than 10C. These precisely controlled outputs are used to fix the storage shelf temperatures within the cabinet regardless of the ambient temperature or food loading applied.

Heating outputs not configured for temperature controlled operation can be operated in a controlled power mode whereby the level of heating power provided can be preset. Outputs operated in this mode are normally the outer surfaces (top, bottom and walls) and act primarily as a buffer between the precision controlled storage shelves and the ambient temperature.

A power supply is provided which produces the 5V required by the control and computer electronics from the main supply available to the controller.

As aforesaid the internal panels of the walls of the body 1 may be constructed similarly to the shelves which will ensure that all internal surfaces radiate or transfer programmed and even heat to the environment within the body.

With respect to figure 5 of the drawings the present invention enables the environment with a cabinet to be subdivided into a plurality of heat zones A. The subdivisions may be on a tiered basis as illustrated or in vertical and horizontal rows. Items to be heated B may be placed in each of the zones adjacent to a heated wall or walls C of the cabinet. The walls C as described herein can be a single wall as described herein or, top and bottom walls in the case of a tiered configuration of heating zones.

The surface temperatures and/or air-space temperatures within

each zone can be carefully and accurately monitored and if necessary quickly restored when the internal environment is disturbed, for example when a cabinet door is opened in order to gain access to the interiors of the cabinet.

Such accurate monitoring and temperature control could not be achieved with existing heating and warming equipment which generally have large airspaces and heating systems which tend to over-react or react slowly when an internal temperature fall is detected.

There are numerous ways in which the body 1 may be designed and numerous shapes and configurations are possible. The techniques used to form double glazed windows may be adopted to provide made to order apparatus.

With respect to figures 6 to 8 of the drawings, mains supply alternating current enters the controller via P3 and is protected from short circuit by the action of the fuse F1. Relay REL1 switches the mains supply to the heater element output stages under the control of the microcomputer U41 and its associated software.

Step down transformer T1 reduces the mains voltage to 9V AC as appropriate for the solid state electronics employed in the controller.

Rectifier bridge B1 and capacitor C1 convert the 9V output from T1 to approximately 12V DC.

Transistors Q1, Q2, Q3 and integrated circuit U24 form a "software protection"scheme commonly known as a"watchdog". This circuit switches the 12V DC available to the voltage regulator integrated circuit U23 off and on and prevents the main relay REL1 from being turned on via Q3 unless the microcomputer regularly toggles the MXD signal line shown entering pin 1 of U24. The purpose of this circuit is to reset the microcomputer in the event the software is not running correctly. Preventing the mains alternating current is the output to the heating element controlling outputs is a safety precaution.

With respect to figure 7 mains alternating voltage switched by the relay REL1 (figure 6) as described above passes through the current sensing circuit of figure 8 comprising D2, D3, D4, resistors R102, R103, R104

and opto-coupler integrated circuit U40. The voltage drop produced by current flow through resistors R103 and R104 activates the LED section of the opto-coupler U40 which causes the output transistor within the device to conduct and pull the output line labeled ISENSE to a low logic level. By this action the microcomputer detects the presence or absence of current flow in the heater elements. This information is used by the software to detect failed components in the heater controlling circuitry or broken film and glass in the heated surfaces of the cabinet. The software removes the dangerous voltages from the heater elements if a fault is detected in these areas to prevent accidenta injury to persons using the equipment.

One of a plurality of identical heater element control outputs is illustrated by figure 7. The construction and operation of each of the outputs is identical. Resistors R45, R46, R47, integrated circuit opto-coupler U15 and triac T15 form one of the heater control outputs and can be seen in the top right of figure 7.

When the control line from the microcomputer and its associated circuitry shown in figure 1 pulls the signal line marked TR13 low, current flows through R45 and lights the LED section of the opto-coupler integrated circuit U15. This causes the sensing section of U15 to conduct at the next zero crossing point of the mains alternating voltage and switch on triac T15. The opto-coupler employed performs the switch action at the zero crossing point so as to minimise the switching noise that is produced when the heater elements are switched on and off.

Each heater element is turned on or off by the microcomputer and its software as required to increase or decrease the temperature of that element respectively. The temperature of the elements is determined by temperature sensing devices processed by the temperature measurement circuits illustrated by figure 8.

K-type thermocouple temperature sensors are used to measure the temperature of the controller heating surfaces of the cabinet and the air within the cabinet. Integrated circuit U22 compensates for the cold junction of the thermocouple sensor formed where the thermocouple wiring connects to

the printed circuit board housing the controller electronics.

One of the thirteen (13) identical temperature measurement inputs is now described in detail. The construction and operation of each of the inputs is identical. Resistors R82, R83, capacitors C13, C41 and integrated circuit operational amplifier U33 form on the temperature measurement circuits and can be seen in the top right of figure 8.

Integrated circuit operational amplifier U33 and resistors R82 and R83 form a precision amplifier with a gain of approximately one thousand (1000) times. It is essential that the operational amplifier employed has a offset voltage drift of less than forty micro-volt (40ß, V) over the operating temperature and life of the cabinet so that the temperature measurement error is kept below one degree Celsius (10C).

Capacitors C13 and C41 offer a high rejection at the frequency of the mains alternating voltage operating the electronics and heater elements of the cabinet. This filtering is essential to prevent the high level of noise coupled into the heater elements (which are in close proximity to the heater elements) from effecting the temperature measurement.

The amplified signal from the thermocouple, labeled as signal TC13, is connection to analogue to digital converter integrated circuit U18 shown in figure 6 where is converted into a digital representation of temperature for use by the microcomputer and software.

With respect to figure 9 of the drawings current can be distributed to the resistive sheets 5 by the busbar type network of wires R.

Such an arrangement promotes even heat distribution.

It will be appreciated that the apparatus and methodology described can be adapted for use in experimental and laboratory work, for maintaining a heating environment for medical or other purposes.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims.