EP0652688B1 - Method of controlling the heating power of a vitroceramic hob according to required cooking conditions - Google Patents

Description

The invention relates to a method according to claim 1.

A method for adjusting the heating power of a ceramic hob is known from DE-A1 35 30 403. That there The method described is for automatic control of the parboiling process an electric heating plate while cooking food and to avoid overcooking at the end of the cooking process. At the start of the parboiling process, the parboiling heat output becomes one electric hotplate set to a selectable constant value. The temperature rise of the bottom of the cooking vessel is continuous determined over time. At a pot bottom temperature above of approx. 70 ° C the actual gradient value with a predetermined target gradient value compared and in the event of deviations, the parboiling power Approximation of the actual gradient value to the target gradient value switched. At a pot bottom temperature above approx. 90 ° C, when kinking the boiling heat output of the gradient of the pot bottom temperature off.

This method has the disadvantage that initially it is constant Heating rate is regulated and therefore when it reaches the desired desired maximum temperature to an undesirable the temperature is coming.

DE-A1 40 22 846 describes the power control and limitation with a heating surface made of glass ceramic. There are several independent of each other Switchable and controllable heating elements are provided, which by means of Temperature sensors, which are arranged in the area of the heating zone, if necessary can be switched on and off. However, this procedure is only able to avoid local temperature increases. An adjustment of the heating output to a cooking or cooking process is not possible.

Electrically heated hobs, the heating surface of which is made of glass ceramic, have become increasingly popular in kitchens in recent years Household found. The hob is heated using heaters arranged below the glass ceramic cooktop, e.g. B. electrical resistance heating elements or radiant heating elements. A Induction hobs also play a special role.

In the case of a domestic hob in which the heating output is Default set by the user or by a selectable Time program controlled electronically or electromechanically is one Adaptation to the heating power actually required is only possible roughly. Corresponding controls are for example in the patent DE-PS 36 39 186 A1. To maintain a fixed temperature, a Regulator used, as known for example from DE-OS 33 14 501 is.

The development of the temperature sensors has advanced further, since the temperature control is as free of hysteresis as possible and without time Delay should occur. It is from the German patent DE-PS 21 39 828 known, on glass ceramics one of the temperature dependent electrical resistance. This resistance exists a conductor track made of a material with a steep resistance-temperature characteristic, similar to the well-known NTC resistors. In Connection with appropriate wiring are in DE-OS 37 44 372 these temperature sensors are designed as parallel conductor tracks, which monitor the cooking surface along half a diameter.

In addition to monitoring the temperature in the middle of the hob and if necessary, their distribution over the hob is also one Monitoring the position of the pot with respect to the heater from growing interest. It is often the case that the pots are not centric put on and therefore energy is lost on the one hand, on the other the pots can be very overheated on one side. Have hobs typically a ceramic that is permeable to heat radiation, which is characterized by an infrared heater is heated. For obvious reasons wishes that the plate turns on when setting up a pot, but switches off again if the pot is not placed centrally.

For this purpose, inductive sensors have been developed, which can be found under the ceramic plate of the hob are arranged. Such an inductive Sensor, for example, consists of two or more electrodes below the ceramic plate, which together with a pan bottom two or form several capacitors whose capacitance can be queried. Out Capacity differences can then be due to an asymmetry with respect to the Pot position closed and thus the heating output can be interrupted. A hob with automatic pan detection in the form of a capacitive occupancy sensor is from the europ. Patent application EP-A1 0 374 868. The electrodes form concentric circles which are part of an oscillator circuit.

The feedback of this oscillator circuit is adjustable in such a way that this circuit does not start when there is no pan on the hob. In addition, the conductivity of the glass ceramic ensures that the ohmic losses in the glass ceramic the capacitance of the measuring capacitor arrangement downsize so that the resonance condition no longer increases is fulfilled. At higher temperatures, the oscillator becomes too no longer swing.

The arrangements that have been described so far can be found in generally only monitor and serve certain operating conditions general for the detection of operating errors or very high Hob temperatures. So far it is not possible to To control the temperature so that the overheating does not occur, by giving a signal as soon as a pot is cooked empty.

Another example of a method for adjusting the heating power a hob to those required for food preparation Kerm sizes is known from patent document EP-B-0 074 108.

The invention is therefore based on the object of specifying a method which enables completely safe operation of the hob, by causing damage to the glass ceramic, the pot or the Food overheating detected in good time and switched off become. Furthermore, temperature detection regardless of the type of pot, the pot size and the aging of the sensor can be carried out. Another object of the invention is to use electrical energy like this use as sparingly as possible and thus energy consumption compared to conventional methods.

This object is achieved according to the invention by the Features contained claim 1 solved.

Further developments of the invention are in the subclaims characterized.

The invention will be particularly applicable where large Security arrives and you cannot assume that the Cooking process is constantly and properly monitored. A fault tolerant Cooking is common in all households, in commercial kitchens and in old people's homes of great interest.

The advantages of this method are that both metal pots also ceramic pots with regard to their content, their temperature and their Position on the stove can be monitored. So it is possible to avoid damage due to incorrect operation as well as energy save up.

An advantage of the invention is that the change over time Resistance of two temperature sensors is evaluated electronically. On The sensor is an attached to the underside of the ceramic plate Trace, the other sensor is the resistance of the ceramic field on the hot spot between two conductor tracks. The fact becomes exploited that temperature curves during heating and cooking entirely have characteristic properties. These become electronic detected and fed to a computer. This calculator also processes the pot size measurement and the measurement of the centricity of the pots on the Ceramic plate. With defined threshold values, after exceeding the one or another limit either the temperature is lowered or but the hob or the hob is switched off.

The invention is explained in more detail with reference to the drawing.

Fig. 1

das Kochfeld mit den Kapazitäts- und Temperatursensoren;

Fig. 2

den Temperaturverlauf der Leiterbahn- und des Kochfeldsensors;

Fig. 3

den Verlauf der ersten Ableitung der Temperatur nach der Zeit und

Fig. 4

ein Blockschaltbild der Auswerteelektronik.

It shows:

Fig. 1

the hob with the capacity and temperature sensors;

Fig. 2

the temperature profile of the conductor track and the hob sensor;

Fig. 3

the course of the first derivative of the temperature over time and

Fig. 4

a block diagram of the evaluation electronics.

Fig. 1 shows a ceramic hob 1 with the for carrying out the Invention required electrodes. The two middle interconnects 2a and 2b have a double function here. On the one hand, they consist of one conductive material with high temperature coefficient. By measuring resistance one can determine a measured value of the temperature. This Temperature measured value, for example between the two end points the electrode 2a, gives approximately the average temperature under the hotplate on. The temperature in the saucepan is of course behind this Temperature lag when you're not in a steady state equilibrium has reached. A second type of temperature measurement results from the Resistance of the glass ceramic between the two electrodes 2a and 2b.

If the ceramic plate gets too hot, the corresponding one switches Heating off. It is also provided that if the temperature advance is too great the temperature measurement - with the ceramic material between Electrodes 2a and 2b and the temperature measurement with one of the metallic conductors 2a or 2b - what to do with a bad cookware increased energy consumption indicates an optical signaling (eco signal) for the user.

With the help of electrodes 3a, 3b, 4a and 4b the position and size the pot on the hob recognized. By measuring capacity versus the middle lines from both halves of the inner and outer ring electrodes is due to a detected asymmetry of the Heating process not started at all. For example, a Error message, supported by an acoustic warning become. The heating remains interrupted until the pot is placed somewhat symmetrically on the hob. After that the Heating with more or less output depending on the recognized pot size switched on. The capacity measurement allows a fine adjustment with at least four levels.

In the case of a glass pot, it can also be determined by the capacitive measurement whether there is content in the pot or not. If no content is in the pot, this also leads to the shutdown. Another The measurement provides the possibility of content recognition even with metal pots of the temperature curve during the heating process. Have filled pots a slower temperature rise than full.

Empty metal pots, on the other hand, are recognized at the same temperature due to their falling electrical heating output. The controller only has to replace the radiation losses from the pot in order to keep the temperature constant. The additional energy for evaporating the water (heat of vaporization) is eliminated, so that the total output drops from 20 - 50% below 10%.
After the pot size has been precisely identified by the capacity measurement and the pot content by a further measurement of the temperature rise during the boiling phase, the heating output is set at 100%. The further regulation of the heating output is then made dependent on the further temperature profile. A cooking process based on FIG. 2 is considered here as an example. First, the "Cooking" function is selected. A pot filled with water, for example, is placed on the plate in question and if it has good soil quality and is approximately in the middle of the plate, 100% of the power is supplied to it. At the same time, the temperature is measured on the underside of the glass ceramic.

One of the hotplates is in operation on the glass ceramic hob 5 and its hottest temperature is measured as the resistance of the ceramic material between the electrodes 2a and 2b. The temperature detection 6 takes place (FIG. 2) by means of resistance measurement and leads to a measured value of the temperature ϑ, from which the first derivative of the temperature (FIG. 3) is formed over time using a difference method. This gradient value is compared in logic unit 8 with a limit value. If the slope (at constant power) is greater than the preselected limit, it remains that 100% heating power is still present on the hob. If the increase value falls below the limit value, the temperature value ϑ _s , at which the limit value is undershot, is stored in a memory 9 in the computer. This temperature means that the boiling point has been reached. The true temperature of the ceramic plate below the pot can vary between 250 and 400 ° C. The temperature ϑ _s on the underside of the cooktop is fed to a controller as the target temperature. The heating power is reduced accordingly. Typical are 20 - 50% power, which the pot assigns due to its properties by the temperature controller. Typical are about 30 - 40% of the performance, which the pot has been awarded due to its degree of filling.

A one-button operation turns the "Cooking" function into one Function "melting" can be selected. Not one Temperature of the boiling point is specified but a temperature which can be specified individually in the range of approx. 70 - 90 ° C.

Another function would be "steaming", which corresponds to cooking, only after briefly boiling the temperature is immediately lowered to about 5 - 10 ° C below the boiling point.
A target temperature is also specified for roasting. The heating output is adapted to the amount of roast and regulated.

When steaming and cooking, it is guaranteed that if there is a drop in performance below a limit due to the loss of water or one empty cooked pot the energy supply is cut off immediately and it too there is no rise in temperature of the glass ceramic.

Since the energy is preferably using low-mass radiant heaters this interruption is very quickly possible and a Damage to the ceramic plate and the pot excluded. That too Cooking and continuing cooking is possible without supervision. So that is security and increased reliability and a relief and increased comfort for enables the operator.

Claims (11)

1. Method of setting the heating power supplied to a ceramic hob, in particular a glass ceramic hob, to the heating power necessary for food preparation, wherein

   a) during a parboiling process the heating power of the hob is set to a parboiling power,

   b) the temperature of the hob is measured continually, preferably at equal intervals, and the value of the rise in temperature over time is determined,

   c) the temperature rise determined is compared to a temperature rise limit value, and

   d) if the temperature rise is not short of the limit value, the heating power is kept at the parboiling heating power or

   e) if the rise in temperature is short of the limit value, the corresponding value of the measured temperature is stored as a standard temperature and

   f) from the stored standard temperature a reference temperature is calculated, which is lower than the standard temperature, and

   g) the temperature of the hob is regulated following the parboiling process during a continuing cooking process by setting the heating power to the calculated reference temperature.
2. Method according to claim 1, wherein a programme is selectable from a plurality of specified cooking programmes, in particular a boiling, steaming, melting or frying programme, and each programme comprises an associated temperature reduction for calculating the reference temperature for the continuing cooking process from the standard temperature.
3. Method according to claim 1 or claim 2, wherein the content, empty or full, of a pan on the hob is recognised in that the time curve of the temperature rise is monitored during the parboiling process and if a preselected curve is exceeded, the supply or heating power is interrupted.
4. Method according to one of the preceding claims, wherein by means of a capacitance measurement with electrically symmetrical electrodes, the pan size is established and if a pan is placed in the wrong position, the heating power supply is interrupted.
5. Method according to claim 4, wherein the pan position is determined relative to the position in the case of optimum symmetry by capacitive measurements against an earthed neutral conductor.
6. Method according to claim 4 or 5, wherein the pan size is determined via a capacitance measurements of concentrically arranged segments against one another and/or against an earthed neutral conductor.
7. Method according to one of the preceding claims, wherein the temperature of the hob is determined by means of a temperature-dependent electrical resistance of an electrical conductor.
8. Method according to one of the preceding claims, wherein the temperature of the hob is determined by measuring the resistance of the ceramic, in particular glass ceramic, between two electrodes.
9. Method according to claim 7 and 8, wherein an optical signal is effected if the temperature determined by the electrical conductor is higher than the temperature determined in the ceramic, in particular glass ceramic, by an amount which exceeds a specified minimum value,
10. Method according to one of the preceding claims, wherein during the continuing cooking process, pans which boil dry during the continued cooking process due to the heat output which drops below a specified power at the same temperature are recognised and then the heating power supply is interrupted.
11. Method according to one of the preceding claims, wherein temperature regulation is carried out continuously during the continuing cooking process.

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