The invention relates to an apparatus for establishing the thermal load on a cooking zone heated by radiation and/or convection from a heating element or elements placed at some distance from the lower surface of the cooking zone and comprising temperature sensing elements and evaluation circuit- It has been determined that modern cooking zones which are areas of in particular glass or ceramic top plates require far more sophistication in their control than mere wattage adjustment. It is desirable to be able to control the temperature, possibly within specific ranges, to determine whether a load is applied to a cooking zone and to determine fault conditions which should cause the power supply to shut down.

From EP-A-0 467 133 and EP-A-0467 134 it is known to determine the thermal load on a glass ceramic cooking zone by evaluating the temperature dependent specific electrical resistance of resistors deposited on the lower side of a glass ceramic cooking zone. On the one hand this solution is good in that measurements may be performed anywhere on a cooking zone (or rather on the cooking surface) , on the other hand modifications of cooking zones according to the wishes of a customer are exceedingly difficult because extensive modifications in the connections to the deposited resistors must be performed. This fact makes the solution less desirable for flexible production.

Other techniques have been used, such as the mechanical action of the expansion of rods supported at one end only and extending into the space between a heating element and the lower surface of the cooking zone, which rods will activate on/off switches, in

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particular at the maximum allowable temperature. Such controllers relying on mechanical switches are difficult to apply consistently without calibration after fitting. Furthermore the hardware used for fitting these controllers distorts the heat distribution in them which reduces their usefulness, in particular during heating and cooling. Hence they are mostly used as overload switches.

Detection of a load being applied to a cooking zone has been performed according to EP 0 553 425 Al by determining the influence on the high frequency impedance of measuring wires by the proximity of pots or pans placed on the cooking zone. However, this requires a complex oscillator function and also means for ensuring that thermal expansion of the wires does not distort their precision of positioning in order to avoid false signals.

It is a purpose of the invention to provide an apparatus of the kind mentioned above in which the disadvantages of the known art are avoided and which is universally adaptable to cooking zones in which there is a distance between the source of heat energy and the cooking zone itself.

This is obtained by an apparatus according to the invention which is peculiar in that at least one continuous temperature signal is obtained in the space between the heating element or elements and the lower surface of the cooking zone, related to a subarea of the cooking zone large enough to provide signals representative of the temperature and the thermal load of the whole cooking zone for providing an input signal or signals to the evaluation circuit.

It has been determined that the measurement of the thermal load on the cooking zone as it expresses itself in the temperature variation near the lower side of the cooking zone is useful as an input to the evaluating

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circuit. In this respect the heating element or elements are regarded as sources of heat, the intervening airspace is regarded as an impedance to radiant heat transfer as well as a conductance with respect to e.g. convective heat transfer. The load on a cooking zone is regarded as a drain, and by measuring the temperature at more than one location it is possible to determine the flow of energy into the load. The result of the evaluation will be used to control the supply of energy to the cooking zone by methods some of which are standard in the field, in their simpelst form as a thermostat.

In order to enable an evaluation of the whole cooking zone by means of simple measurements, according to claim 2 this will be obtained when the temperature sensing elements are influenced by contributions from at least two domains related to the subarea and subjected to different thermal load influences.

In order to avoid a swamping influence on the measurements by direct radiation from the heating elements, a preferred embodiment specified in claim 3 uses thermal shielding interspersed between the actual sensors and the heating elements.

A further embodiment according to claim 4 maximises the influence from the thermal load in that the temperature sensing elements are in close proximity to the cooking zone.

A further embodiment simplifies calibration and the evaluation circuit according to claim 5 by the temperature sensing elements being disposed in a plane essentially parallel to the cooking zone.

A further advantageous embodiment according to claim 6 further simplifies evaluation by letting each temperature sensing element provide an average of the temperatures prevalent in the neighbourhood of each temperature sensing element. This average may be

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obtained by letting the temperature sensing elements have an appreciable physical extent or by letting them be in close thermal contact with surfaces or bodies covering an area interspersed between the heat source arid the heat drain.

An efficient way to obtain this is by making the temperature sensing elements comprise a rod with a diameter sufficient to attain a temperature which is related to the average of the temperatures it is subjected to along its lenght according to a further embodiment of the invention specified in claim 7.

Although various sensing means are applicable for the electrical temperature measurement used in the present invention, such as a PTC or NTC element, a preferred embodiment according to claim 8 relies on the difference of the thermoelectric voltages of conductors or semiconductors, e.g. as used in thermocouples, in that each end of the rod is a part of a thermoelectric junction, the other parts being constituted by the leads connecting the ends of said rod to the evaluation circuit.

Using this approach, evaluation becomes still more simplified when according to a further preferred embodiment of claim 9 the characteristic thermoelectric voltage of the rod material lies between the characteristic thermoelectric voltages of the lead materials.

A further advantageous simplification is obtained according to claim 10 when the characteristic thermoelectric voltage of the rod falls within the range + 10% of the mean value of the characteristic thermal voltages of the lead materials. The errors obtained are compensatable, but in the case where the characteristic thermoelectric voltage is exactly midway between the characteristic thermal voltages of the lead materials there is no need for compensation.

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A preferred embodiment provides a simple and efficient way of obtaining two sensing elements as above described from one and the same rod by according to claim 11 letting the rod be connected at its midpoint to a further lead which establishes a connection to the evaluation circuit.

An efficient integral construction according to claim 12 which is self-compensating is obtained by combining the thermal shield with the sensors in that the rod is made of an electrically conductive material with essentially the same thermal expansion coefficient as the shield.

A further advantageous embodiment obtains an additional signal representative of other areas below the cooking zone surface by combining the features of claim 11 and 12 in claim 13 in that the further lead is not directly connected to the midpoint but to a thermally shielded further rod going from the midpoint into another area in the plane of the sensing elements.

The model of the energy transport expressed as a distributed source of energy, a cross section in which energy flows and a drain is useful, because the flow will be determined by the area or part of the cross section which has a lower temperature and which will hence absorb relatively more heat. In connection with the determination of the thermal load a point measurement of temperature is meaningless, because the temperature at a single point is determined only by a fraction of the total load. Hence the present invention is based on the realization that although the average temperature is useful for a simple thermostat function, the utilization of several temperature measurement contributions enables a much more precise control of energy supply in response to the acutal load conditions.

The invention will be described in detail with reference to the drawing, in which

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Fig. 1 shows the principle underlying the present invention,

Fig. 2 shows a preferred placement of the elements of the invention,

Fig. 3 shows a further preferred and more detained representation of the elements shown in Fig. 2,

Fig. 4a shows in perspective the structure of a preferred sensor element for an apparatus according to the invention,

Fig. 4b shows a side view of a preferred sensor according to the invention,

Fig. 5 shows an embodiment similar to that of Fig. 3 but utilizing a sensor element according to Fig. 4,

Fig. 6 shows the same seen from above,

Fig. 7 shows the principle of a detector reaching over a larger area, and

Fig. 8 shows a sensor element covering a larger area based on the principle shown in Fig. 4 and 6.

Fig. 1 shows a schematic section through a cooking zone according to invention. A cooking zone 1 which may be part of the ceramic top plate has the edge 3 of a heating chamber 5 resting against it. The material of the heating chamber is usually a soft ceramic fibrous material. The chamber 5 defines a heat transfer volume V in which a series of heating elements 7 transfer heat indicated by a dotted Q to the hot zone 1.

In the volume V a temperature sensor 8 with at least one sensing element is provided in close approximation to the cooking zone. The sensor delivers an electrical signal e(th) in dependence of the temperature th. The temperature sensor is mounted such that thermic drain resistances, such as Rth are as large as possible. Thereby the influence on the temperature measurement of spurious heat transfer shown as dotted QST are minimized. The temperature sensor may be an NTC or a PTC resistor or a thermocouple or thermojunction.

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According to Fig. 2 the thermic resistor represented by a support for the temperature sensor will be as large as possible when the lenght of the support is maximum and subjected to as constant a temperature as possible which in practice means a disposition in parallel to the cooking zone and extending to the periphery.

One embodiment shown in Fig. 3 uses two temperature sensors disposed as thermocouples 9a and 9b in the volume between the heating elements and the cooking zone.

A preferred embodiment shown in Fig. 4a and Fig. 4b uses two series coupled thermojunctions obtained between the lead 13 and the rod 14, and the rod 14 and the lead 15. The rod performs an averaging of temperatures it is subjected to, and the leads 13 and 15 have a diameter which is so small that they contribute very little to the temperature distribution in the rod 14. The leads 13 and 15 are of different materials and are insulated with a heat resistant insulation. The junction between the rod and the wires is by spot welding in order not to introduce extraneous materials. The lead 15 is connected to the far end of the rod 14 and is taken below the whole lenght of the rod inside a thermal shield 16. In the practical embodiment the wires are clamped in the shield and serve to support the rod in the position shown. The shield 16 is provided with prongs 18 for easy engagement with the soft edge material of the insulation at 3 (Fig. 1) . The material of the rod and wires are preferably suitable alloys of Cr and Ni, the exact proportion of which is simple to determine by the person skilled in the art according to known relationships with the characteristic thermal voltages. However other combinations of material may be found which are equally useful. It is advantageous to make the thermal shield in the same material as the rod or at least with the same

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linear thermal expansion coefficient, as this will stress minimally the spot welds used to connect the leads 13, 15 and which support the rod at each end.

In Fig. 5 is shown how the temperature sensor is disposed in conjunction with the cooking zone, and on Fig. 6 the arrangement is seen from above.

In Fig. 7 it is shown how 3 elements A, B, and C may be interconnected such that thermocouples are created at their junctions 20, 21, and 22. The outputs 24a and 24b provide the sum of the thermovoltages of the elements 20 and 21, while at the thermocouple 22 a single thermovoltage is available at the two outputs 24c. Another sum is available beetween the outputs 24c and 24a. By such an arrangement it is possible in a flexible manner to obtain the temperature distribution for a whole cooking zone without having to move any sensor physically.

In Fig. 8 a construction very similar to that of Fig. 4 is shown, however in the view of Fig. 6. It is an extension of the concept of the sums of thermovoltages which uses a star connection rather than a delta connection as described in conjunction with Fig. 7. A further rod 14 is spot welded to the midpoint of the previously described rod 14, and a further insulated lead 13 is available for a signal for evaluation.

The concept of the measurement used in the apparatus according to the invention is not limited to thermocouples, as the main concept on which it is based is that of the need to provide temperatures for the sensors which depend on larger parts of the area of the cooking plate in order to determine with precision the thermal load conditions on the upper surface of the cooking plate.