EP3267113B1 - Method for operating a cooking hob - Google Patents

Description

APPLICATION AREA AND PRIOR ART

The invention relates to a method for operating a hob, in which a state given at the time of activation should be able to be maintained, in particular because an operator has triggered a corresponding holding function. This is particularly advantageous if the condition given at this point in time is considered by the operator to be desirable or advantageous for continued cooking or further operation of the hob for this cooking vessel.

From the EP 2330866 A2 it is known how temperature changes on the cooking vessel can be detected on an inductively heated hotplate with a cooking vessel on it. For this purpose, an exact absolute temperature does not have to be known or is not determined, since only changes in temperature are used or only changes in temperature can be recorded.

From the WO 2008/148529 A1 it is known to be able to recognize the influence of the radiation of heat from an erected pot in a hob with an inductive heating device. This is to enable control of the power of the inductive heating device.

From the DE 101 41 754 A1 Another cooktop with heating devices is known, in which either a specific heat output curve or a warming process can be specified. Either a constant heat output or a specific heat output curve can be used.

From the EP 2 574 143 A2 a method is known for heating liquid in a saucepan with a hob by means of an inductive heating device. The liquid contained in the cooking vessel should be heated at a constant temperature.

From the EP 1 732 357 A2 a hob is known with an inductive heating device in which the heating device is to be monitored to determine whether a cooking temperature has been reached. For this purpose, a frequency is set for the control of the inductive heating device, and a certain electrical variable is monitored and recorded over time. A performance of the inductive heating device can be measured in this way.

From the DE 10 2008 008 604 A1 Another hob is known with a temperature sensor to detect a temperature of a pot placed on the hob. A control unit of the hob can provide a continued cooking process, with which a food to be cooked can continue to be heated after a short decay time of a few minutes with a predetermined continued cooking time.

From the DE 102 53 198 B4 a method for operating a hob with an inductive heating device is known, in which the temperature of a set up pot is detected. A boiling point of water in the pot and an empty boiling of the pot can also be recorded.

From the DE 10 2013 108 646 A1 a hob with an inductive heating device is known, in which a sensor device is provided in order to be able to detect at least one variable that is directly dependent on a temperature on the pot. In this way, a temperature on the pot should be recognized.

TASK AND SOLUTION

The invention has for its object to provide a method for operating a cooktop, with which an advantageously usable option for an operator to maintain a given state at a certain point in time on an induction heated cooktop of a cooktop with a cooking vessel is possible, preferably with the method in the cooktop it should also be possible to react to different circumstances or states or changes in state.

This object is achieved by a method having the features of claim 1. Advantageous and preferred embodiments of the invention are the subject of the further claims and are explained in more detail below. The wording of the claims is incorporated in the present description by express reference.

It is envisaged that a cooktop is operated with a hotplate according to specifications, a cooking vessel being set up and being heated, advantageously being heated inductively. A certain power level has been specified either by a cooking program or advantageously by an operator and the cooking vessel heats up or remains hot. The cooking vessel is preferably filled or contains something, for example water or similar liquid or a solid food such as a steak or the like. This causes a change in temperature of the cooking vessel as a change in state, preferably with one according to the one mentioned EP 2330866 A2 known methods, in particular with an induction heated hob. It is therefore advantageously possible that the measured variable, which correlates with the cooking vessel temperature, is the period of the oscillating circuit of this hotplate and / or a different variable is derived from it.

Especially at the beginning of the operation of the hob, it can be assumed that the temperature will continue to rise, usually starting from room temperature. A heating process of the cooking vessel can be recorded, advantageously recorded from the beginning. This is done particularly advantageously by means of a control of the hob. In a similar form, the power supplied to the heating device or the cooking vessel and / or a change in temperature of the cooking vessel can be recorded and evaluated, in particular if these recorded variables are still changing. This can also apply to the temporal course of the output and / or temperature change of the cooking vessel. Here the term "capture" should be understood to mean the same as "observation".

At any point in time, an operator can trigger an activation of the hold, which is intended to keep the state at that point in time at the hotplate with the cooking vessel set up. In practice, this is relevant, for example, when a sauce simmers in the cooking vessel or simmers slightly at a rather low temperature with an optical appearance that is appropriate and desirable for the operator. So it shouldn't boil fizzy. Another example is the boiling of water in the cooking vessel with or without food in it. For example, when cooking potatoes or noodles, cooking with blistering is usually desired, but excessive blistering with the resulting splashing of water should usually be avoided. This is a special process at the boiling point of water.

Another example is the searing of meat in a pan as a cooking vessel at temperatures of usually above 200 ° C., for example if fat introduced into the pan shows a behavior that the operator seems to have in accordance with the desired temperature. At this temperature or in this condition, the meat should be prepared or fried in the pan.

In all of the aforementioned cases, it is desirable if the operator can maintain this state or freeze, so to speak, without having to reach the required performance level or pay attention to the resulting temperature. This is to offer the so-called hold function.

According to the present invention, the state at that point in time at the hotplate is divided into a process at the boiling point of water, in particular when water or a similar liquid is boiling. On the other hand, a distinction is made between a different process, which takes place at a different temperature and above all without a phase transition of water in the cooking vessel, this being possible both at temperatures below 100 ° C. and well above 100 ° C. Even at 100 ° C, such a process could be carried out as a second case if no water was involved, for example searing at this temperature.

In the former case, in which there is preferably a largely constant temperature at the cooking vessel immediately before activating the hold, a process is recognized at the boiling point of water, since the temperature at the boiling point of water is known to be relatively constant and relatively exactly 100 ° C. As an alternative to a largely constant temperature on the cooking vessel directly before activating the hold, the temperature can also rise or fall slightly, for example 1 ° C. to 5 ° C. Since the operator has already optically recognized the boiling of water in this case, it must already be present and this can be recognized by the hob due to the largely constant temperature on the cooking vessel. Then the power supply or an area power supply should be kept largely constant at this point in time, since after all it not only led to the boiling of water in the cooking vessel, but also to a desired appearance. Alternatively, a conventional power density for boiling water can be set, for example between 2 W / cm ² and 4 W / cm ² .

In the second case, in the case of a process with a temperature removed or different from the boiling point of water, the temperature of the cooking vessel is regulated to a largely constant temperature by adapting the power supply, specifically to the temperature that prevails or is prevailing at the time when the holding is activated, without this temperature being perceivable as an absolute value, regulated to this temperature by avoiding a temperature change. The temperature is kept constant. This is from the process according to the above EP 2330866 A2 known.

When making a decision in the first case, control is therefore carried out on a constant power supply or area power supply, in the second case on a constant temperature.

This is because a decision in the second case is based on the assumption that at temperatures different from 100 ° C, i.e. in a process away from the boiling point, various temperature changes can be corrected and thus a temperature can also be kept constant by changing the power supply as it is necessary. This is not possible directly at the boiling point of water in the first case, since with an increased power supply or area power supply no change in temperature would be detectable, the 100 ° C cannot be exceeded. Furthermore, it is assumed that, in the second case, a cooking pattern prevailing on the basis of a certain temperature is present and is recognized, and the operator would like to keep this cooking pattern, irrespective of a power supply or area power supply required for this.

In an advantageous embodiment of the invention, it is possible that a size of the cooking vessel set up is determined on the basis of a size of the hotplate or its heating device on which the cooking vessel is placed, which size is known in the hob. In the case of known discrete heating devices or induction heating coils as heating devices, their diameter and thus areas are known, so that the area power supplied can also be determined on the basis of a known supplied power. Alternatively, if a hob is present with a large number of smaller heating devices or induction heating coils, which are then operated together to form a hotplate for a cooking vessel, with a cooking vessel usually covering three to seven or nine heating devices, the size of the cooking vessel can also be based on the degree of coverage of the heating devices can be determined. This is for example from the EP 2945463 A1 and the WO 2009/016124 A1 known. An area power supply can then in turn be determined from this on the basis of the sum of the power supplied to the heating devices.

In a further embodiment of the invention can according to the above EP 2330866 A2 a change in temperature of the cooking vessel can be recorded from operating parameters for the inductive heating device. This is used as the basis for temperature control in accordance with the second case.

In one embodiment of the invention, the holding can either be held until an operator either switches it off or consciously and specifically changes an output at this hotplate or for this cooking vessel. Alternatively, it can be provided that the holding is ended automatically after a certain time, that is to say automatically. This time can be specified as an absolute time, for example 30 minutes to 60 minutes or even 90 minutes. Alternatively, the maximum duration until an automatic shutdown of the Depend on the level of an estimated temperature at the hotplate, which can be estimated via a level of a power supply or, above all, an area power supply. The maximum running time should be shorter the higher the area power supply or the higher an estimated temperature.

In a further embodiment of the invention it can be provided that a sudden drop in temperature is determined after the activation of the hold, in particular within two to ten or even 20 seconds. In practice, this can be triggered by placing cooler food or fried food in the cooking vessel, and finally also by adding water or similar liquids with boiling temperatures close to that of water.

If such a sudden drop in temperature is determined, it can be provided in an embodiment of the invention that the heating device or the hob or its control tries to raise the temperature again in the two cases mentioned at the beginning. In the case of an operation with a largely constant power supply or area power supply, this will take place anyway, since the food to be cooked or the liquid is also heated, which leads to a renewed rise in temperature. After all, the cooking process should continue with a high probability. Because of the constant power supply or area power supply, this will usually take a little longer. If the aforementioned case of regulation to a constant temperature of the cooking vessel is present, the power or area power is increased or even significantly increased for faster compensation of the temperature drop or the temperature change, preferably by 30% to 100% or even 200%. In this case, the case that had fundamentally prevailed up to that point should continue to be maintained, that is to say, during the compensation of the temperature drop and also afterwards, it should either continue to be heated with a constant power supply or be regulated to a previously prevailing constant temperature.

The duration and / or steepness can advantageously be recorded from the sudden drop in temperature to the compensation of the drop in temperature or the change in temperature. Depending on this duration and / or slope, it can be seen what triggered the temperature drop. In an embodiment of the invention, for example, the sudden drop in temperature with a duration of less than 10 seconds until compensation is considered to be the introduction of a food to be roasted or cooked into the cooking vessel. Then the cooking vessel continues to be heated with the previous or now reached temperature. This applies to both liquid food and solid food or food. The previous one The condition in the cooking vessel should, as previously explained, be maintained at the operator's request.

If, for example, it takes more than 10 seconds to equalize, the sudden drop in temperature is regarded as the introduction of water or a liquid food with a similar boiling temperature into the cooking vessel. Then a large amount of food to be cooked will usually have been introduced into the cooking vessel, which in most cases can only be water or a corresponding liquid. Thus, the cooking vessel with the previous power density or area power density continues to be heated or with a usual area power density for boiling water away. Alternatively, it is also possible to regulate to the previous temperature value, which then prevails again as the target temperature.

It is important here, however, that after the evaluation, which is dependent on the duration of the compensation of the temperature drop, the basic procedure can also change during the hold. In particular, from a previous regulation to a constant temperature away from the boiling point of water according to the second case, it is possible to switch to a corresponding constant power supply in order to continue a cooking process at the boiling point of water with a constant power supply or area power supply. This applies in particular if a roasting process with temperatures far above 100 ° C., in particular above 200 ° C., has previously occurred with a high probability due to a high power supply or area power supply, in which, for example, fried meat is quenched with liquid. Then the meat in the liquid should usually be brought to a boil again or at least simmer.

Particularly in measuring systems in which the magnetic properties of the cooking vessel are used as a measurement variable for the temperature, it can happen that a change in the signal of the hob control initially occurs as a change in temperature, but in reality it is a different influence. Moving the cooking vessel is particularly worth mentioning here. When moving, the surface coverage of the cooking vessel changes over an induction heating coil, and thus the measured inductance changes, just as if the permeability of the cooking vessel would change due to the temperature. To achieve a reliable function, this effect must be differentiated from actual temperature changes. In a further embodiment of the invention, it can therefore be provided that in the event of a signal change or temperature change lasting less than 5 seconds, only a displacement of the cooking vessel on the hob is detected and no actual temperature change on the cooking vessel. This is not considered a control deviation. In this case it is possible that the signal change is ignored and the new value is used as the new control value.

In a further embodiment of the invention, it is possible for an increase in the signal change or temperature change to be evaluated after the sudden temperature drop. In the aforementioned case of introducing water into the cooking vessel, this slope will increase more slowly after a few seconds than if roasting or cooking food is introduced into the cooking vessel.

After recognizing the introduction of additional water into the cooking vessel, the temperature profile is still monitored. This introduction of additional water can be recognized when the temperature profile becomes constant by the boiling point of water having been reached after the temperature drop has been compensated for. This can be recognized anyway by an established constant temperature.

If a process is recognized with a temperature at the boiling point of water, the heating device can be supplied with a constant power or area power, which can advantageously be between 0.5 W / cm ² and 5 W / cm ² . Especially between 2 W / cm ² and 4 W / cm ² , boiling of water is achieved with great certainty. A higher power supply or area power supply is possible, but generally not necessary to keep water boiling. Rather, only an unnecessarily large amount of energy would be consumed and, in addition, too much boiling of water could be brought about, which is then regarded as annoying because of excessive bubble formation and water splashes.

The physical measured variable in this invention is advantageously the period (Per) of the resonant circuit with the induction coil when it is excited for measurement purposes and swings freely, see FIG EP 2330866 A2 . It changes due to a change in the permeability of the cooking vessel with increasing temperature (T). So Per = f (T) applies.

At the same time, however, the period is also determined by the position of the cooking vessel. If, starting from a concentric arrangement of a round cooking vessel on a round induction coil with a similar diameter, the cooking vessel is pushed outwards, the period also changes. The measurement signal is also dependent on an eccentricity (e) of the cooking vessel to the coil. So Per = f (e) also applies.

If temperature control is now to be set up by measuring the period signal, the challenge is that this measured variable depends not only on the temperature of the cooking vessel itself, but also on its position Per = f (T, e). However, it is quite common for the user to move the cooking vessel during a cooking / roasting process. A method must therefore be found to distinguish a signal change by moving the cooking vessel from an actual temperature change.

In one possible method, empty boiling can be detected within processes at the boiling point if no more water covers the bottom of the pot and this makes it warmer than with covering water. This can be suitably signaled to an operator, advantageously acoustically and / or optically, and / or the power output can be reduced or ended.

In a possible further method, an operator can have the possibility during the holding process to adjust or fine-tune the actual level of the held temperature again. With this fine adjustment, the target temperature can be adjusted in the case of a temperature control and / or the set area power density can be adjusted in the case of water at the boiling point.

It can be provided that an operator can interrupt the holding process and resume it later or, in the meantime, can select other, power-controlled area power densities. For example, a corresponding operating action on a control element can be used to return to an area power density after a few minutes, which was previously set with stopping during a stopping process.

These and other features emerge from the claims and also from the description and the drawings, the individual features being implemented individually or in groups in the form of sub-combinations in one embodiment of the invention and in other fields, and being advantageous and capable of being protected by themselves Can represent versions for which protection is claimed here. The division of the application into individual sections and sub-headings does not limit the general validity of the statements made under these.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1

eine sehr schematische Darstellung eines Kochfelds, mit dem das erfindungsgemäße Verfahren durchgeführt werden kann,

Fig. 2

ein möglicher Funktionsablauf zur Darstellung des erfindungsgemäßen Verfahrens und

Fig. 3 und 4

verschiedene Verläufe für Temperatur und Flächenleistungszufuhr bei verschiedenen Beheizungsvorgängen bzw. Zuständen am Kochfeld entsprechend Fig. 1.

Exemplary embodiments of the invention are shown schematically in the drawings and are explained in more detail below. The drawings show:

Fig. 1

2 shows a very schematic representation of a hob with which the method according to the invention can be carried out,

Fig. 2

a possible functional sequence for representing the method according to the invention and

3 and 4

Different curves for temperature and area power supply for different heating processes or conditions on the cooktop accordingly Fig. 1 .

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the Fig. 1 a hob 11 is shown very schematically as an induction hob, which is designed to carry out the method according to the invention. The hob 11 has a hob plate 12 and an induction coil 14 arranged underneath. Power electronics 16 for the induction coil 14 are controlled by a controller 17 for setting a power supply or area power supply. The controller 17 is also connected to an operating element 18 of the hob 11, shown here with a capacitive sensor element under the hob plate 12.

The induction coil 14 defines, so to speak, a hotplate 20 on the hob 11, on which a cooking vessel 22 is placed. This is shown here more as a saucepan, which can also be fried in a saucepan. Alternatively, it can of course also be a significantly higher saucepan or a significantly lower pan. Possibilities for addition into the cooking vessel 22 are also shown. On the right is shown a piece of meat 24 which is possibly to be fried in the cooking vessel. The addition of water 25 into the cooking vessel 22 with a vessel 26 is shown on the left.

Instead of a single induction coil 14, depending on the size of the cooking vessel 22, a hotplate 20 can also be formed by a plurality of induction coils, for example two to four or even more. Such induction coils are for example in the EP 2945463 A1 and the WO 2009/016124 A1 disclosed. Several of these induction coils are then operated like a single common induction coil, advantageously with one same area power density for the bottom of the cooking vessel 22, so that they can be viewed here as a single induction coil. For the aforementioned temperature control, all induction coils of a hotplate are then considered and not just a single induction coil.

The controller 17 can be connected to the power electronics 16 and the induction coil 14 in accordance with the aforementioned EP 2330866 A2 recognize a change in temperature from the operating parameters of the induction coil 14. For the details, expressly refer to this EP 2330866 A2 Referred.

In the functional diagram in Fig. 2 is shown schematically how the method according to the invention can proceed. When the cooking vessel 22 begins to be set up with unknown content therein on the hotplate 20 and heating operation begins, the power supply or surface power supply at the induction coil 14 is already detected by the control 17 by means of the power electronics 16. The area power supply can be calculated from a geometric size of the induction coil 14 known to the controller 17 from a power supply flowing through the power electronics 16. If the holding mentioned is then activated at a certain point in time as a function activation, an attempt must be made to differentiate the state at that point in time depending on the process at the boiling point of water on the one hand and process at a different temperature on the other hand, that is to say a characterization. This leads to the case analysis.

If the function of holding is activated due to the presence of a state in which a largely constant temperature can be detected on the cooking vessel 22 without much being regulated, it can be concluded in the characterization that a process is present at the boiling point of water. For this purpose, the controller 17 can also evaluate various additional factors, for example, which are not shown here, such as, for example, the amount of the current area power supply. In order to keep a process at the boiling point of water, i.e. to bring water to a boil and keep it boiling, an area power supply between 0.5 W / cm ² and 6 W / cm ² is usually required. If the current area power supply is significantly above or below this, an error may have occurred and holding may then no longer be activated. However, if such a plausibility check also reveals that a process at the boiling point can be present, then there is a state with a constant evaporation rate, namely the boiling of the water. The further steps are explained in more detail below.

However, if the characterization and the case analysis show that there is no process at the boiling point of water, but a so-called temperature control process, because the temperature control must intervene to compensate for slightly fluctuating temperatures, a temperature controller will start operating after activation of the hold. This means that the controller 17 then just tries to regulate the power supply or area power supply by means of the power electronics 16 in such a way that the temperature prevailing at the time when the function of the holding function is activated is maintained. Temperature deviations are therefore corrected. In both cases, this can then be continued for a long time or indefinitely as a held state. Certain maximum durations can be provided as a safety function, after which the method is ended, since after all a type of automatic cooking program is running and an operator could possibly forget that the hob 11 is switched on. For example, after 30, 60 or 90 minutes, the area power supply can be significantly reduced, for example to 10% to 30% or 50%. Alternatively, the area power supply can be switched off completely after this time. Before reducing or switching off, an operator can be made aware of this optically and / or acoustically, but this need not necessarily be the case.

In Fig. 3 the behavior over time for the temperature T on the left y-axis and the area power supply P on the right y-axis are shown for the first case, the area power supply P in particular not being shown linearly. The temperature T rises, and does so relatively slowly, because water is heated in the cooking vessel 22 and thus a lot of energy has to be introduced for a temperature increase. The water boils in the cooking vessel 22 at a temperature of 100 ° C., whereupon the temperature T becomes constant. At a certain point in time t *, the hold is activated, that is to say when the operator is of the opinion that exactly this state should be continued with boiling water and also with this degree of boiling. From then on, the temperature T remains constant. An area power supply may initially have been somewhat higher, as shown by the thick line, for example 10 W / cm ² . Then an operator may have reduced it slightly before the time t *, for example because the water in the cooking vessel 22 has boiled too much, for example to 4 W / cm ² . If a desired cooking pattern has been set for the second somewhat lower area power supply, the hold is activated. Further cooking takes place with the area power supply at time t *. This is also in the Fig. 3 shown.

If the sudden drop in temperature mentioned at the outset occurs, here for example to a temperature of about 60 ° C., the temperature T goes down and the area power supply is maintained for the time being. Since the controller 17 then sees that the temperature T rises only slowly, it is clear that a larger amount of additional food to be cooked, in particular additional water 25 according to Fig. 1 , has been introduced into the cooking vessel 22. Then it can either continue to be heated with the area power supply P as at time t * until the water in the cooking vessel 22 boils again and the temperature T = 100 ° C. is reached again with a cooking pattern that largely differs from the previous one from time t * will have approximated. This constant area power supply is shown at 4 W / cm ² . Alternatively, the area power supply can be increased at least until a constant temperature T has been determined again, for example increased to the area power supply used at the beginning of the heating, here 10 W / cm ² . This is shown in dashed lines. If a constant temperature T is then determined, it is possible to switch back to the previous area power supply at time t *. The brief increase in the area power supply then serves to reach the temperature T = 100 ° C more quickly. This is in the Fig. 2 Shown at the bottom right with the case of cooling as a sudden drop in temperature and reheating until the boiling point has been reached again.

If the control 17 determines that a signal drop occurs suddenly and possibly even stepwise, for example within a few seconds, then it can be concluded that the cooking vessel 22 has been moved on the hob 11, for example by 0.5 cm to 3 cm. Alternatively, the cooking vessel may also have been briefly removed from the hotplate 20 and then set up again. In this case, the controller 17 can advantageously maintain the area power supply from the time t * and does not need a brief increase.

In Fig. 4 it is shown how the curves for the temperature T and the area power supply P over time appear when the meat 24 is to be roasted in the cooking vessel 22. With a typically high area power supply, an operator will heat the cooking vessel 22 strongly if searing of steak, for example, is desired. There is probably only a little oil or fat in a pan as a cooking vessel 22, so it does not have to be heated very much. The temperature T rises fairly steadily. At time t ', a temperature is reached which is considered good and sufficient by an operator for the desired roasting of steak, usually somewhat above 220 ° C. The stop is therefore operated here at time t '. At this point in time, the control 17 via the power electronics 16 still detects a change in the temperature of the cooking vessel 22 so she knows that no process can take place at the boiling point of water, as previously explained. For this reason, a temperature control is set at this point in time after the case analysis and the temperature from time t 'is kept constant from now on. Even if at first glance the process looks very much like one Fig. 3 with the constant area power supply of the first case, the cause is different. In the Fig. 3 the temperature is kept at 100 ° C by boiling water in the cooking vessel 22, as long as no quenching or the like. he follows. In the Fig. 4 a first temperature control to the value determined at time t 'is actually carried out.

If a sudden drop in temperature is detected at time t ", the temperature control which has already been carried out tries to compensate for this and to regulate back to the temperature from time t 'as quickly as possible. During the beginning of the heating, a very high or possibly also the maximum If the area power density has been chosen, for example 7 W / cm ² , then after t 'a lower area power density has been used, which has been chosen to maintain this temperature, for example 3 W / cm ^2. The sudden drop in temperature at the time To compensate t ", the area power density can be increased again and, in particular, set to a maximum again. As soon as the sudden drop in temperature is corrected again and the temperature is reached again at time t ', the temperature control will also reduce the area power density again, as shown here. The control behavior of the temperature controller can, for example, as shown here, be designed as a two-point controller. In an advantageous embodiment, however, a continuous controller is used which adjusts the power specification in proportion to the temperature deviation from the controller setpoint, or even additionally as a function of its derivative and / or integral. Such controllers, for example P, PI, PD or PID controllers, are known to the person skilled in the art.

If the temperature control or the controller 17 determines that the sudden temperature drop has occurred to a significantly lower temperature than that at the time t ', and a temperature rise may take place very quickly, for example within 15 seconds, then a process of the aforementioned quenching can take place of a seared meat or steak. This is shown by the dotted temperature curve. A certain amount of liquid is therefore added to the seared meat. Then the operation of the controller 17 changes as shown in FIG Fig. 2 also shows from the case of constant temperature control to the case of constant area power density. Usually after roasting of seared meat, in order for example, to make a sauce, which made it simmer or simmer very easily. However, it should certainly not boil fizzy. Therefore, a temperature of T = 100 ° C cannot be exceeded after reheating, the liquid introduced prevents this. Now you should switch to a constant evaporation rate or a constant area power density. However, this is actually not known to the controller 17, since the area power density at the time t 'was too high and led to a temperature of 220 ° C. or kept it. Here, after reaching a constant temperature, in the present case namely of approximately 100 ° C., a change can be made to a freely selected fixed value for the area power supply. This can be between the 0.5 W / cm ² and 5 W / cm ² mentioned at the outset, for example 2 W / cm ² or 3 W / cm ² , shown here as dotted with 2 W / cm ² . Here, the controller 17 can also include how large the area power density was at the time t ′, in order to be able to roughly estimate whether an operation takes place at rather high temperatures or rather lower temperatures. The initial slope of the temperature after the time t "can also be taken into account.

Finally, in the Fig. 2 also shown that, starting from a case of a constant area power density, the water in the cooking vessel 22 is boiled, that is, there is a case of dry cooking. If the temperature then begins to rise again, a safety shutdown can intervene in order to avoid damage or burning of remaining food or food in the cooking vessel 22.

In the second case of regulating to a constant temperature, this case cannot be recognized so easily, since regulating to a constant temperature takes place. However, it can be recognized whether a lower or significantly lower area power density is required to reach the constant temperature from a certain point in time. This could also be recognized as a case of dry cooking with a resulting safety shutdown.

Claims (14)

1. Method for operating a cooking hob (11) for maintaining a state, which exists at the time of activation of the maintaining operation, at a cooking point (20) of the cooking hob (11) with a cooking vessel (22) on it, comprising the steps of:

   - a cooking vessel (22) being placed onto a cooking point (20) of the hob (11) and being heated by the cooking point (20) or an inductive heating device (14) of the cooking point according to requirements,

   - detecting a change in temperature of the cooking vessel (22) as a change in state,

   - detecting the heating process of the cooking vessel (22) and evaluating of the supplied power and/or a temperature of the cooking vessel and/or the profile thereof with respect to time,

   - activating the maintaining operation by an operator for maintaining the state, which is existing at this time, at the cooking point (20) with the cooking vessel (22) placed on it,

   characterized by the further steps of:

   - differentiating the state at the cooking point (20) existing at this time firstly into a process at the boiling point of water and secondly into a process which is different therefrom or into a process which takes place at a different temperature without a phase transition of water,

   - wherein, in the case of a decision for of a process at the boiling point of water, the power supply at this time is then kept largely constant or a customary power supply for continued boiling is set,

   - wherein, in the case of a decision for of a process not at the boiling point of water, the process is regulated at a constant temperature of the cooking vessel (22) by adapting the power supply.
2. Method according to claim 1, characterized by determining a variable of the placed-on cooking vessel (22), on the basis of a variable, which is known in the hob (11), of the cooking point (20) or the heating device (14) thereof which is operated for the cooking vessel (22).
3. Method according to claim 1 or 2, characterized in that the hob is an induction hob (11) with an inductively heated heating device (14), wherein a change in temperature of the cooking vessel (22) is detected from operating parameters for the inductively heated heating device (14).
4. Method according to any of the preceding claims, characterized in that, in the event of a sudden drop in temperature after activating of the maintaining function, the temperature is again brought to the previous temperature before the sudden drop in temperature and the time period taken until the temperature is again at the previous temperature before the sudden drop in temperature or until the change in temperature is compensated for again is detected.
5. Method according to claim 4, characterized in that, directly after detection of the sudden drop in temperature, the previously used control variable temperature or power supply is continuously used until compensation.
6. Method according to claim 4 or 5, characterized in that the sudden drop in temperature in the case of a time period of less than 10 seconds until compensation is evaluated as the introduction of a product (24) to be fried into the cooking vessel (22), wherein the cooking vessel then continues to be heated with the previous temperature or the temperature which has been reached again being maintained.
7. Method according to claim 4 or 5, characterized in that a sudden, sharp drop in temperature, preferably with subsequent temperature limiting, is evaluated as the introduction of water (25) into the cooking vessel (22), wherein the cooking vessel then continues to be heated with the previous power supply or power, or is heated at a customary power for continued boiling of water.
8. Method according to any of the claims 4 to 7, characterized in that, in the event of a sudden change in signal or change in temperature with a change time of less than 5 seconds, a displacement of the cooking vessel (22) is identified.
9. Method according to claim 8, characterized in that, when a displacement of the cooking vessel is identified, the signal deviation which is caused by the displacement and not by an actual change in temperature, is not considered to be a control deviation.
10. Method according to any of the preceding claims, characterized in that, in the case of a process with a temperature at the boiling point of water having been identified, the heating device (14) is supplied with a constant power of between 0.5 W/cm² and 7 W/cm².
11. Method according to any of the preceding claims, characterized in that, in processes at the boiling point, boiling-dry is identified when water no longer covers the pot base and, as a result, the said pot base is warmer than when it is covered with water, and this is suitably signaled to an operator and/or the power output is reduced or stopped.
12. Method according to any of the preceding claims, characterized in that, during the maintaining process, an operator has the option of adapting or finely adjusting the actual maintaining level once again, wherein, when this fine adaptation is performed, the setpoint temperature is adapted, in the event of a temperature control operation, and/or the set power density per unit area is adapted, in the case of water at the boiling point.
13. Method according to any of the preceding claims, characterized in that an operator can interrupt the maintaining process and can later restart the said maintaining process or can select other, power-controlled power densities in the meantime.
14. Method according to any of the preceding claims, characterized in that the measurement variable which is correlated to the cooking vessel temperature is the period duration of the resonant circuit of this cooking point and/or another variable is derived therefrom.

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