Method for temperature determination

Abstract

To determine the temperature of boiling water in an induction hob including induction heating coils which can be individually driven and which, in a common heating mode, form a cooking point for a cooking vessel containing water, the cooking vessel is positioned over at least two induction heating coils. The induction heating coils are operated in the heating mode to bring the water in the cooking vessel to boil and each induction heating coil heats that region of the cooking vessel base arranged above it. During the heating mode, the oscillation response at each induction heating coil is used to detect whether temperature of the region of the cooking vessel base above this induction heating coil increases. The induction heating coils are operated in the heating mode until one induction heating coil detects that the temperature gradient of the cooking vessel base above the induction heating coil has reached zero.

Claims

1. A method for determination of a temperature in an induction hob, said induction hob comprising a plurality of induction heating coils, wherein said induction heating coils can be individually driven and, in a common heating mode, form a cooking point for a cooking vessel containing water, wherein said method comprises the following steps: positioning a cooking vessel with a cooking vessel bottom containing water such that said cooking vessel covers at least two said induction heating coils by way of said cooking vessel base; operating said induction heating coils in said common heating mode in order to bring said water in said cooking vessel to boil, this intending to be detected as temperature determination; during said common heating mode, heating, by said induction heating coils, a region of said cooking vessel base being arranged above said induction heating coil; during said common heating mode, detecting, by at least one said induction heating coil, an oscillation response indicating whether a temperature of said region of said cooking vessel base above said induction heating coil changes or increases with a temperature gradient; operating said induction heating coils in said common heating mode at least until one said induction heating coil detects that said temperature gradient of said cooking vessel base above said induction heating coil is approaching zero or has reached zero; determining at least one of said induction heating coils to be a measuring coil; and operating said measuring coil in a measuring mode and no longer in said common heating mode, with said measuring coil, in said measuring mode with a measuring power of up to a maximum of 50% of a maximum power of said induction heating coil, transmitting energy into said cooking vessel base for a short time and then detecting a fed-back oscillation response, with a time profile of said oscillation response being evaluated after several coupling-in operations of said measuring power, with said water in said cooking vessel being determined to be boiling in an event that said gradient of said time profile is approaching zero or has reached zero.

2. The method according to claim 1, wherein an induction heating coil which first has a temperature gradient reaching zero during said common heating mode is determined to be said measuring coil.

3. The method according to claim 1, wherein an induction heating coil which has a lowest power input into said cooking vessel is determined to be said measuring coil.

4. The method according to claim 1, wherein an induction heating coil which has a lowest degree of coverage by said cooking vessel is determined to be said measuring coil.

5. The method according to claim 1, wherein each of said induction heating coils are operated in said common heating mode at least until said temperature gradient of said cooking vessel base which is located above each of said induction heating coils has reached zero.

6. The method according to claim 1, wherein said measuring coil transmits energy into said cooking vessel base in said measuring mode with said measuring power for half a cycle, and then detects a fed-back oscillation response.

7. The method according to claim 1, wherein, after said first induction heating coil has or detects a temperature gradient which has reached zero, said heating mode of each of said induction heating coils, which operate in said common heating mode for said cooking vessel, is continued for at least 10 seconds at a constant power, with said previously determined measuring coil being operated in said measuring mode after said time elapses.

8. The method according to claim 7, wherein said heating mode of each of said induction heating coils, which operate in said heating mode for said cooking vessel, is continued for at least 30 seconds at a constant power.

9. The method according to claim 1, wherein, after each of said induction heating coils of said cooking point have a temperature gradient which has reached zero or have detected a temperature gradient which has reached zero, said common heating mode of each of said induction heating coils, which operate in said common heating mode for said cooking vessel, is continued for at least 10 seconds at a constant power.

10. The method according to claim 1, wherein, on a basis of values, which are stored in a memory, for a level of a total added power input of each of said induction heating coils, which are operated jointly as said cooking point in said common heating mode for a cooking vessel, into said cooking vessel and on a basis of a time until said temperature gradient of said first induction heating coil or said temperature gradient of said last induction heating coil has reached zero, a time for which said common heating mode is continued after said temperature gradient of said first induction heating coil or said last induction heating coil has reached zero up to a time at which one of said induction heating coils is operated as a measuring coil is determined.

11. The method according to claim 1, wherein, after a considerable reduction in said power at said measuring coil during said temperature determination by said measuring coil, a profile of a water temperature of water in said cooking vessel is set equal to a profile of said cycle duration at said measuring coil.

12. The method according to claim 1, wherein, after it is identified that said water in said cooking vessel is boiling, said power, of said induction heating coils or of said cooking point is reduced in order to prevent said water from boiling over.

13. The method according to claim 12, wherein said power of said induction heating coils or of said cooking point is reduced by at least 50%.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

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

(2) FIG. 1 shows a schematic view of an arrangement of a plurality of induction heating coils of an induction hob with a cooking vessel positioned on it;

(3) FIG. 2 shows a schematic side view of heating of the cooking vessel from FIG. 1 with the induction heating coils situated beneath it, wherein two induction heating coils operate in the heating mode, together with water currents which are produced;

(4) FIG. 3 shows a modification to the illustration from FIG. 2, wherein one induction heating coil operates in the heating mode and one operates in the measuring mode, together with water currents which are produced; and

(5) FIG. 4 shows an illustration of profiles both of the water temperature at two points in the cooking vessel and also of signals of an induction heating coil firstly in the heating mode and secondly in the measuring mode.

DETAILED DESCRIPTION

(6) FIG. 1 schematically shows how a large number of individual induction heating coils 13, here with a round shape, can be provided in an induction hob 11. This is known from the abovementioned document EP 1463383 B1. A cooking vessel 15 is positioned on the hob, specifically in such a way that it covers more than 50% of four induction heating coils 13a to 13d. The induction heating coils 13b and 13d are completely covered, and approximately 70% to 80% of the induction heating coils 13a and 13c is covered. Induction heating coils to the left and to the right of the induction heating coils 13d are also covered to a slight extent. However, this degree of coverage is so slight that this is identified and the induction heating coils are definitively not used as cooking point for the cooking vessel 15 in the heating mode.

(7) The side view in FIG. 2 of the induction hob 11 according to the invention comprising a hob plate 12 shows how the two induction heating coils 13a and 13b are situated beneath the cooking vessel 15 and, respectively, how the cooking vessel is positioned on the hob plate 12 above the induction heating coils. The induction heating coils 13c and 13d are not shown in the figure, but the same substantially applies to them. The cooking vessel 15 has a cooking vessel base 16 which is suitable for inductive heating and usually has a thickness of a few millimeters, for example 4 mm to 10 mm. A cooking vessel base 16 of this kind is generally of multi-layered design with a topmost layer which is composed of the same material as the side wall of the cooking vessel 15 and is usually produced by deep-drawing, that is to say with an integral material transition. A heat-distributing layer which is composed of copper and has a thickness of a few millimeters is often arranged beneath the topmost layer. A thin layer of stainless steel, which is likewise suitable for inductive heating, can in turn be provided beneath the heat-distributing layer. The thickness of the thin layer can be 1 mm to 2 mm at most. At the same time, this is approximately the maximum penetration depth of inductive fields, which will be explained further below.

(8) The induction heating coils 13a and 13b are connected to a controller 19 of the induction hob 11 and are supplied with power in a manner driven by means of the controller, usually by means of a power section, not illustrated here, or corresponding resonant circuit arrangements.

(9) Thin arrows each show a power input 21a and 21b from each of the induction heating coils 13a and 13b into the cooking vessel 15 or into the cooking vessel base 16. This is known to a person skilled in the art and therefore does not need to be discussed in further detail. As mentioned above, the penetration depths of the power input 21 is less than 2 mm, advantageously less than 1 mm. The heat which is produced is distributed from this lowermost layer of the cooking vessel base 16 upwards through the further structure of the cooking vessel base 16, under certain circumstances with a corresponding transverse distribution. At the top face of the cooking vessel base 16, heat is transferred to water 17 which is located above the cooking vessel base in the cooking vessel 15. Owing to the heat which is introduced, this heated-up water rises, this being indicated by the wide arrows. It goes without saying that the water currents 23a and 23b, here also further illustrated by further water currents 23, are thoroughly mixed.

(10) FIG. 4 shows a graph, which is to be schematically understood, with a thick solid line indicating the temperature T.sub.W of the water 17 in the cooking vessel 15 as a kind of average temperature, that is to say not only measured at individual discrete points but rather as an average at a large number of points. In particular, the temperature can also be a temperature at the water surface, where the temperature of the water 17 is usually the lowest during boiling.

(11) The thick dashed line illustrates the temperature of the water above the left-hand-side induction heating coil 21a close to the cooking vessel base 16. The water 17 will be the hottest and boil the quickest here. The value of 100 C. is also indicated for the temperature of the water 17. The profile levels are approximately to scale in relation to one another in the case of the water temperatures illustrated by thick lines.

(12) The thin solid line illustrates the measurement value cited in the introductory part or the cycle signal of that induction heating coil 13b which is used as a measuring coil in the measuring mode. The dashed thin line illustrates the cycle signal of the induction heating coil 13a which is operated in the heating mode. The magnitude of these two cycle signals must not differ from one another in absolute terms, this difference being illustrated here only for reasons of clarity in order to better show their relative profiles. In particular, the cycle signals can be largely congruent, primarily at the start.

(13) In order to carry out the method according to the invention, after the cooking vessel 15 is placed onto the induction hob 11 and, respectively, over the induction heating coils 13, the controller 19 detects, in a known manner, which of the induction heating coils are actually covered and the extent to which the induction heating coils are covered or the degree of coverage of the induction heating coils. In the case of the induction heating coils 13 of the configuration in FIG. 1, the abovementioned induction heating coils 13a to 13d are sufficiently covered. If an operator has now selected a power level for the operation of the induction hob 11 with which the water 17 in the cooking vessel 15 is intended to be brought to boil as rapidly as possible, the heating mode of the four induction heating coils 13a to 13d starts. In this case, the four induction heating coils form a joint cooking point. The four induction heating coils can be operated at the maximum power, in particular a boost power which is known for induction heating coils. This is illustrated in FIG. 2, the induction heating coils 13a and 13b generate a power input 21a and 21b in the cooking vessel base 16, in particular in the lowermost layer of the cooking vessel base. The inductively generated heat spreads upwards and enters the water 17 at the top face of the cooking vessel base 16 or is transferred there. This produces water currents 23, in particular powerful water currents 23a and 23b which rise from the top face of the cooking vessel base 16.

(14) According to a first variant of the method, the induction heating coil 13b can now be determined to be a measuring coil since it has the lowest identifiable degree of coverage by the cooking vessel 15 or the cooking vessel base 16. This determination can be performed even when the measuring coil 13b is also operated together with the others in the heating mode as a cooking point. As an alternative, the cycle signal, which is illustrated using a dashed line in FIG. 4 and which will run relatively uniformly for most of the induction heating coils at the beginning, can be evaluated for each induction heating coil 13. Then, that induction heating coil in which the gradient first reaches approximately zero can be determined to be a measuring coil and change over to the measuring mode. In a yet further refinement of the invention, that induction heating coil in which this profile becomes constant or has a gradient of zero last in comparison to the other induction heating coils can be used as a measuring coil in the measuring mode.

(15) In the exemplary embodiment described here, this situation of the gradient having reached zero last applies to induction heating coil 13b. This means that the temperature is higher or was already high earlier above all of the other induction heating coils 13 of the cooking point.

(16) At the same time, FIG. 4 shows how the water temperature, illustrated using a dashed line, likewise reaches the illustrated maximum value of 100 C. as water temperature at the time at which the increase in the cycle signal of one of the induction heating coils reaches zero. In particular, this is the temperature of the water just above the cooking vessel base 16 over precisely the induction heating coil with the profile, illustrated using a dashed line, of the cycle signal. Owing to the water temperature, which no longer increases, at 100 C., the cooking vessel base 16 can no longer be further heated in this region, and therefore the cycle signal at the induction heating coil no longer increases further either. The thick solid line as temperature T.sub.W of the water 17 in the cooking vessel 15 increases approximately constantly after a short delay at the start. Owing to the changeover of an induction heating coil as a measuring coil, the introduced power is reduced and the slope then becomes flatter.

(17) The induction heating coil 13b which is now operated in the measuring mode as a measuring coil with the measuring power has the solid profile with the thin line. The measuring power is, for example, 5% of the maximum power. The profile of the cycle signal at the measuring coil 13b also shows that, after the changeover to the measuring mode, this measuring coil transmits virtually no more energy into the cooking vessel base and therefore does not attempt to heat the cooking vessel base any further. Since the water 17 which is located in the cooking vessel 15 is still not at 100 C. overall, that is to say is not yet all boiling, but rather is only at 80 C. to 90 C. for example, this relatively cooler water drops back down to this region of the cooking vessel base and cools it down to less than 100 C. Therefore, the cooking vessel base is cooled in comparison to the previous heating mode of the measuring coil 13b. This can be identified by the illustrated drop in the cycle signal of the measuring coil. After a certain time, for example 10 seconds to 30 seconds, this region of the cooking vessel base is at the temperature of the relatively cooler water which is flowing down, and therefore the cycle signal of the measuring coil also runs virtually identically to the water temperature. For reasons of better understanding, this is illustrated jointly and, respectively, congruently here, but this does not have to be the case.

(18) At the same time, it can be seen how the temperature, illustrated using a dashed line, of the water remains at 100 C., for example, above the induction heating coil 13a according to FIGS. 2 and 3 which continues to operate in the heating mode. The temperature cannot become any higher, and finally energy is further input by the heating coil. Therefore, the temperature remains, as it were, at the upper limit.

(19) The states in the cooking vessel 15 during this period of time are shown in FIG. 3. The induction heating coil 13a in the heating mode continues to effect the power input 21a into the cooking vessel base 16 above it, this generating the powerful water current 23a. This water current circulates as it were and causes water 17 which is located in the upper region to move downwards and strike that region of the cooking vessel base 16 which is situated above the measuring coil 13b in the form of water current 23, which is illustrated by thin arrows. By changing the mode of the induction heating coil 13b from the heating mode to the measuring mode, in which the induction heating coil then couples almost no more power into the cooking vessel base, almost 25% of the heating power is lost anyway. Since the aim of the method according to the invention is substantially only for the situation of thorough boiling of the water to be achieved and not for accurate temperature measurement to take place at any temperature below that, empirical values, which can be stored in the controller 19 as explained above, can be used to further determine a certain continued run time for the induction heating coil 13b in the heating mode, the water in the cooking vessel 15 still not being completely thoroughly boiled after the continued run time has elapsed.

(20) After a certain time, owing to the continued power input of the other three induction heating coils which advantageously takes place at the same or maximum power, the total or average temperature of all of the water reaches approximately 100 C., in particular after sufficient thorough mixing of the water which is heated by the cooking vessel base 16 above the heating coils with the rest of the water. If, then, in the right-hand region in FIG. 4, the thin and solid cycle signal of the measuring coil again has the gradient zero or becomes constant, all of the water 17 in the cooking vessel 15 is boiling. This also applies for the temperature T.sub.W of the water.

(21) In the case of the water currents 23a and 23b illustrated by thick arrows in FIG. 2, it should be noted that sometimes large or even very large steam bubbles are also formed here, the steam bubbles rising upward. The steam bubbles also effect a large amount of the self-mixing of the water 17 in the cooking vessel 15.

(22) On the basis of the description relating to FIGS. 1 to 3 and on the basis of the profiles in FIG. 4, it is also possible to easily imagine, as explained in the introductory part, how the heating mode of all of the induction heating coils, in particular also of the induction heating coil which is determined to be a subsequent measuring coil, is continued for a certain time after a constant cycle signal is reached by the measuring coil. The graph in FIG. 4 shows that it lasts for a certain time, for example 10 seconds to 40 seconds, after boiling of the water just above the cooking vessel base until all of the water in the cooking vessel is boiling.