Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B3/00—Ohmic-resistance heating
  • H05B3/68—Heating arrangements specially adapted for cooking plates or analogous hot-plates
  • H05B3/74—Non-metallic plates, e.g. vitroceramic, ceramic or glassceramic hobs, also including power or control circuits
  • H05B3/746—Protection, e.g. overheat cutoff, hot plate indicator
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D23/00—Control of temperature
  • G05D23/19—Control of temperature characterised by the use of electric means
  • G05D23/20—Control of temperature characterised by the use of electric means with sensing elements having variation of electric or magnetic properties with change of temperature
  • G05D23/22—Control of temperature characterised by the use of electric means with sensing elements having variation of electric or magnetic properties with change of temperature the sensing element being a thermocouple

Definitions

• a device for determining the thermal load of a cooking zone A device for determining the thermal load of a cooking zone.
• 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.
• 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.
• 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
• SUBSTITUTESHEET 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.
• 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.
• thermoelectric voltages of conductors or semiconductors e.g. as used in thermocouples
• 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.
• 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.
• SUBSTITUTESHEET 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.
• a point measurement of temperature is meaningless, because the temperature at a single point is determined only by a fraction of the total load.
• 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.
• SUBSTITUTESHEET 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
• 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.
• 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.
• 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.
• thermocouples 9a and 9b are used 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
• TITUTESHEET 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.
• 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.
• 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.
• 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.

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• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Automation & Control Theory (AREA)
• Chemical & Material Sciences (AREA)
• Ceramic Engineering (AREA)
• Measuring Temperature Or Quantity Of Heat (AREA)
• Electric Stoves And Ranges (AREA)

Applications Claiming Priority (5)

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• 1994
  • 1994-12-06 EP EP95901364A patent/EP0740895A1/de not_active Withdrawn
  • 1994-12-06 WO PCT/DK1994/000457 patent/WO1995016334A1/en not_active Application Discontinuation
  • 1994-12-06 AU AU10642/95A patent/AU1064295A/en not_active Abandoned

Non-Patent Citations (1)

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