Method for operating an induction hob

Abstract

In a method for operating an induction hob including a controller and including a cooking point including an induction heating coil, a relationship between a cooking vessel temperature and a heating power of the induction heating coil is stored in the controller as area power which results in a constant cooking vessel temperature during long-term operation. By monitoring whether the cooking vessel temperature remains constant, increases or drops when a first relatively low heating power is set after a heating time at a high heating power, it is possible to set a target temperature, which corresponds to the first relatively low heating power, for frying processes.

Claims

1. A method for operating an induction hob for reaching a specific cooking vessel temperature in a defined manner, wherein said induction hob comprises a controller and a cooking point comprising at least one induction heating coil, said method comprising the steps of: inductively heating, by said induction heating coil, a cooking vessel being placed on said cooking point; providing a target temperature for said cooking vessel or an application which implies a specific target temperature being input into said controller of said induction hob before a heating process of said cooking vessel; heating said cooking vessel at a first relatively high heating power as area power for a first heating time at the beginning of the heating process; reducing a heating power of said induction heating coil as far as a first relatively low heating power, which would lead to said target temperature after a period of operation of 10 minutes to 30 minutes, after said first heating time; and performing a check to determine whether a temperature of said cooking vessel remains constant, increases or drops at said first relatively low heating power after a short checking time, wherein in a first case, in which said cooking vessel temperature remains constant and corresponds to said target temperature after said short checking time in an instance in which said cooking vessel is heated at said first relatively low heating power, said target temperature is deemed to have been achieved, and wherein in a further case, in which said cooking vessel temperature has not reached said target temperature after said short checking time by the said first relatively low heating power being set, a magnitude of said first relatively low heating power is enlarged by said controller in order to find a heating power which leads to a constant temperature during said short checking time.

2. The method according to claim 1, wherein, after a corresponding correlation of heating power and cooking vessel temperature has been found to a sufficiently accurate extent, said controller deems said heating process to be finished and indicates this to an operator or initiates further method steps.

3. The method according to claim 1, wherein: in a further case as a second case, in an instance in which said cooking vessel is heated at said first relatively low heating power, said cooking vessel temperature continues to increase after said short checking time, a cooking vessel temperature which lies below said target temperature is established and said cooking vessel is once again heated more strongly at an intermediate heating power for an intermediate heating time, and then, after said intermediate heating time, a check is once again made, by setting said relatively low heating power, to determine whether said cooking vessel temperature is still increasing or remains constant after a short checking time; and said first case of said target temperature having been reached applies in an instance in which said cooking vessel temperature remains the same.

4. The method according to claim 1, said target temperature lies between 200 C. and 250 C.

5. The method according to claim 3, wherein: in the case in which said cooking vessel temperature is still increasing after said intermediate heating time and after said short checking time, a cooking vessel temperature which lies below said target temperature is once again established and said cooking vessel is once again heated more strongly at an intermediate heating power for an intermediate heating time, and then, after said intermediate heating time, a check is once again made, by setting said relatively low heating power for a short checking time, to determine whether said cooking vessel temperature is still increasing or remains constant after said short checking time; and said first case of the target temperature having been reached applies in an instance in which said cooking vessel temperature remains constant.

6. The method according to claim 1, wherein in a further case as a third case, in an instance in which said cooking vessel is heated at said first relatively low heating power, said cooking vessel temperature drops after said short checking time and a cooking vessel temperature which lies above said target temperature is established.

7. The method according to claim 6, wherein: in said third case said cooking vessel is heated at an intermediate heating power of between 105% and 200% of said first relatively low heating power and a cooking temperature which is set at a constant value after said short checking time is checked and said cooking vessel temperature is determined from said check from a relationship, which is known in said controller, between cooking vessel temperature and heating power as area power; and on a basis of this, a heating power is again reduced to a heating power which would lead to said target temperature after a period of operation of 10 minutes to 30 minutes.

8. The method according to claim 7, wherein said intermediate heating power is greater than said first relatively low heating power.

9. The method according to claim 8, wherein said intermediate heating power is 10% to 100% greater than said first relatively low heating power.

10. The method according to claim 1, wherein said short checking time lasts from 1 second to 30 seconds.

11. The method according to claim 10, wherein said short checking time lasts from 5 seconds to 20 seconds.

12. The method according to claim 1, wherein said intermediate heating time lasts from 5 seconds to 60 seconds.

13. The method according to claim 1, wherein a heating power is reduced to a low heating power which corresponds to said target temperature and a check is made in an instance in which said cooking vessel temperature is constant and therefore corresponds to said target temperature.

14. The method according to claim 1, wherein: said cooking vessel is operated at a cooking point comprising one or more induction heating coils; and a power of said induction heating coils is taken into consideration jointly as area power or heating power.

15. The method according to claim 1, wherein a quantity of introduced energy or said heating power of said induction heating coil is monitored over time.

16. The method according to claim 1, wherein said first relatively high heating power is from 3 W/cm.sup.2 to 12 W/cm.sup.2.

17. The method according to claim 1, wherein said first relatively low heating power is from 0.3 W/cm.sup.2 to 2 W/cm.sup.2.

18. The method according to claim 1, wherein said intermediate heating power is from 1 W/cm.sup.2 to 12 W/cm.sup.2.

19. The method according to claim 1, wherein a cooking vessel size is ascertained by taking into account a degree of efficiency of said induction heating device due to coverage of said induction heating coil by said cooking vessel which has been placed on one of said cooking points.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Exemplary embodiments of the invention are schematically illustrated in the drawings and will be explained in greater detail in the text which follows. In the drawings:

(2) FIG. 1 shows a profile of the cooking vessel temperature, which is kept stable in the long term, as a function of an area power for a plurality of different cooking vessels;

(3) FIG. 2 shows a side view of an induction hob comprising an induction heating coil and a cooking vessel placed on the induction hob; and

(4) FIGS. 3 to 6 show different profiles of the cooking vessel temperature and the area power over time in various driving situations for empty cooking vessels, that is to say without the addition of a food.

DETAILED DESCRIPTION

(5) FIG. 1 shows how empirically ascertained values for four different cooking vessels indicate the relationship reflecting how the cooking vessel temperature which is reached or set in the long term depends on the corresponding area power. The figure shows that firstly the relationship is linear to some extent, that is to say can be determined by calculation very easily. Secondly, the temperatures at a specific area power differ from one another by only at most 30 C. to 35 C. Therefore, it is possible to relatively accurately determine which cooking vessel temperature is established at a cooking vessel after a specific relatively long period of operation, for example 10 minutes to 30 minutes, given a specific area power Q*/A.

(6) FIG. 2 shows an induction hob 11 comprising a hob plate 12 in which a cooking point 13 is formed. An induction heating coil 15 is arranged beneath the hob plate 12, the induction heating coil defining and also heating the cooking point 13. The cooking point could also consist of a plurality of induction heating coils, this playing no role in the invention. The induction heating coil 15 is supplied with power and driven by a controller 17, wherein the controller 17 can monitor the power which is fed into the induction heating coil 15. Furthermore, the controller 17 has a memory, not illustrated, in which a relationship between cooking vessel temperature and area power is stored, as it were in accordance with FIG. 1. In this case, it is possible for the calculated relationships to be stored when the temperature curves from FIG. 1 are approximately considered to be straight lines. As an alternative, temperature values for area power which increases in steps in each case can be stored with sufficiently good resolution.

(7) In an advanced refinement of the invention, it is possible for this to be stored in the controller 17 for a plurality of cooking vessels, so that the controller 17, as it were, knows precisely which of the four or even more curves from FIG. 1 is to be used in the respective case. As an alternative, specific parameters could also be input into the controller 17 by an operator or programmed into the controller externally, the specific parameters, independently of the specifically present cooking vessel, informing the controller 17 which cooking vessel is being used or which of the stored curves applies. Under certain circumstances, the controller 17 can then also identify the size range of a cooking vessel which is placed onto the cooking point 13 above it.

(8) It goes without saying that the surface area of the induction heating coil 15 is known. However, the area power is advantageously not based on the surface area of the induction heating coil 15, but rather on the surface area of the cooking vessel 19. In a suitable manner for the cooking point 13, the surface area or the base area of the cooking vessel 19 is moved in a relatively narrow region since suitable cooking vessels usually only have a variation in diameter of up to 3 cm within specific diameter classes. Cooking vessels which are considerably too large or considerably too small are rarely placed on a cooking point, and this could also be identified by the controller 17 and indicated to an operator as an error.

(9) FIG. 3 shows how heating is performed at time t=0 at a high heating power, here 7 W/cm.sup.2, which is constant. Heating lasts until time t1 as heating time, which can be predefined.

(10) A target temperature of 200 C. was input by a target person or else by an automatic controller or the like in advance. This temperature should be maintained at the cooking vessel 19, which is a pan in this case, in the long term. This temperature advantageously applies to the top side of the cooking vessel base, that is to say at the point where food, for example a steak which is to be fried, comes into contact with the cooking vessel 19. The topmost curve from FIG. 1 applies for the cooking vessel 19.

(11) After the heating time t1 elapses, the heating power is greatly reduced and set to 0.68 W/cm.sup.2. This corresponds to the topmost curve in FIG. 1 and the temperature of 200 C. is permanently maintained at this area power.

(12) FIG. 3 shows, in accordance with the first case, that the temperature T drops only slightly and then relatively rapidly, for example in 5 seconds to 20 or 30 seconds as adjustment time, becomes constant. Both the small temperature drop and also the constant temperature can be identified by an abovementioned method or in accordance with US 2011/120989 A1 or US 2013/087553 A1.

(13) Since the cooking vessel temperature now remains permanently constant at the area power of 0.68 W/cm.sup.2, this is fixed at 200 C. in accordance with FIG. 1 and can therefore be permanently maintained.

(14) In the next case in accordance with FIG. 4, heating is performed up to time t1 as heating time at a higher area power of 7 W/cm.sup.2, wherein the temperature T increases again. At time t1, the power is reduced to 0.68 W/cm.sup.2 in accordance with a target temperature of 200 C. which is also desired here. The controller 17 or the temperature detection means can now establish that the cooking vessel temperature continues to increase, albeit probably more weakly than before, at this area power which is now set. This therefore means that the cooking vessel temperature at time t2 still lies below the target temperature of 200 C. The time between t1 and t2 is the abovementioned checking time. Therefore, a considerably higher power, and in particular the previously set high power, of 7 W/cm.sup.2 is again set at time t2 which follows, for example, a few seconds to one or two minutes after time t1. The temperature T then increases strongly again. After a certain time as intermediate heating time between t2 and t3, for example a few seconds to one minute to three minutes, the power is again reduced to the power in accordance with the target temperature, that is to say to the first low heating power of 0.68 W/cm.sup.2 again. The temperature detection means now identifies that the cooking vessel temperature T first decreases to a certain extent and then, however, relatively rapidly, for example within one minute or even only a few seconds as adjustment time, exhibits only a small drop or becomes constant. Therefore, it is again the case that a constant cooking vessel temperature is reached at an area power of 0.68 W/cm.sup.2. This then has to be the target temperature 200 C. according to FIG. 1 or as described above in relation to FIG. 3. Renewed subsequent heating at the higher heating power was required in this case since the cooking vessel requires more energy than assumed by the controller in order to reach the specific temperature. The thermal capacity of the cooking vessel therefore differed from the value stored in the controller.

(15) The second time or intermediate heating time at a high heating power in FIG. 4 between t2 and t3 could also have a different area power than the heating time up to time t1 However, the heating processes should proceed relatively rapidly here, and therefore an at least high area power close to the maximum area power should be selected.

(16) The situation of overheating during the heating time is shown in FIG. 5. Here, heating is also performed at the high power of 7 W/cm.sup.2 at a desired target temperature of 200 C. for the heating time up to a time t1, whereupon the temperature T increases. Then, starting from time t1, heating is performed at the low area power of 0.68 W/cm.sup.2 for a checking time, that is to say for a few seconds to half a minute, in order to see whether the cooking vessel temperature becomes constant relatively rapidly here, which would be evaluated as the target temperature having been reached. However, the controller 17 establishes by means of the abovementioned temperature monitoring that the cooking vessel temperature also permanently falls after the checking time expires, even after one or two minutes as adjustment time. This means that a cooking vessel temperature considerably above the target temperature therefore prevails. The power can now be entirely switched off for a short time, for example 10 seconds to 30 seconds, in order to rapidly cool the cooking vessel down to the target temperature or close to the target temperature. Operation could then restart at the low heating power of 0.68 W/cm.sup.2 and, as shown by experience, the temperature would then become constant relatively rapidly and then even amount to the target temperature of 200 C.

(17) Alternatively, according to another possibility, an attempt is made to approximately determine the prevailing temperature. Therefore, a somewhat higher heating power than the intermediate heating power, specifically 0.8 W/cm.sup.2 here, is fed into the induction heating coil 15 for the intermediate heating time between t2 and t3. In the process, a constant temperature, which lies at approximately 230 C. according to FIG. 1, is established relatively rapidly. Therefore, the controller 17 knows that the temperature is still approximately 30 C. too high. The controller can then again, as described above, completely switch off the induction heating coil 15 for a short time, for example for 10 seconds to 30 seconds, for the purpose of somewhat more rapid cooling, wherein the low heating power is then set again for the purpose of reaching and maintaining the target temperature. As an alternative, the area power of 0.68 W/cm.sup.2, which corresponds to the target temperature, can be set starting from time t3, so that the cooking vessel temperature T drops to the target temperature somewhat more slowly, but which target temperature is then ultimately reached and maintained. Relatively rapid cooling can also be achieved by inserting the food which is to be cooked. The measurement value which corresponds to 200 C., and not the measurement value which corresponds to 230 C., is then advantageously used as the setpoint value for temperature regulation which follows the addition of food.

(18) FIG. 6 shows a further advantageous refinement of the method for reaching a specific cooking vessel temperature in a defined manner. If the constant steady-state temperature is not reached after a short period of time, irrespective of whether the signal is falling or increasing, no discrete power stages are subsequently approached between t2 and t3. Rather, a setpoint value T.sub.S of the temperature signal is ascertained after a fixed time, here at t2 at 230 C. The controller then adjusts the temperature signal to this setpoint value T.sub.S, for example using a proportional controller which can also have integral or differential components. Therefore, a constant temperature is reached relatively rapidly at t3, more rapidly than would be possible with discrete temperature stages. According to FIG. 1, an area power of 0.8 W/cm.sup.2 corresponds to a cooking vessel temperature of 230 C. Therefore, the cooking vessel temperature of 230 C. is maintained at this area power density. In this way, the corresponding correlation of power and constant temperature is found again, wherein the power which allows the temperature to be determined and therefore the temperature to be set is known. The specific cooking vessel temperature of 200 C. can now be approached on the basis of known relationships by power reduction starting from the known temperature, for example with the product to be cooked being inserted at the same time.

(19) Therefore, the temperature can be controlled and a specific temperature can be approached and maintained on a cooking vessel by way of the invention, without absolute temperature measurement and merely by relative temperature measurement, that is to say monitoring whether a temperature is increasing, dropping or is constant, and a known relationship between temperature and permanently set area power density.

(20) Furthermore, the invention makes use of the fact that, in a steady-state, that is to say a permanently prevailing state, a thermal resistance is connected in series with a parallel circuit as radiant heat resistance and convection heat resistance. The relationship which can be identified in FIG. 1 is the result.

(21) Therefore, the invention makes use of an energy balance in order to solve the problem presented at the outset. By seeking a steady state, that is to say a state without a change in the cooking vessel temperature, the inherent energy of the cooking vessel is kept constant. As a result, it is known that the energy which is introduced into the cooking vessel by the heater is entirely output again, be it by convection, thermal radiation or thermal conduction to the hob surface. However, the introduced energy can be measured by the heater. Since the relationship is known from FIG. 1, conclusions can be drawn about the absolute temperature by means of measurement of energy per unit time or power, given certain boundary conditions.