Device for heating and / or cooling metal object

Abstract

To provide a device for heating and / or cooling a metal object, particularly for heating and / or cooling a container, which is capable of measuring the temperature of a metal object more accurately than the prior art.SOLUTION: A device for heating a metal object, such as a container having a metal part, includes a stationary part for supporting the metal object, heating means and a measurement sensor for estimating the temperature of the metal object, which includes at least one measurement coil and an electronic device. The measurement coil is a part of an electric circuit. The electronic device is connected to the electric circuit, supplies an alternating voltage at a predetermined frequency between 100 kHz and 500 kHz, and measures a resistance of the electric circuit thus supplied.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a device for heating and / or cooling metal objects, in particular a device for heating and / or cooling a container having at least one part made of a metallic material.

[0002] The invention was initially developed in the context of an induction cooktop for heating a vessel having a bottom part made of a ferromagnetic material that can be heated by electromagnetic induction. Therefore, in the following, reference is mainly made to the application in which the device is an induction cooktop. However, the invention can be applied equally to vessels having at least one metal part, as well as to any device configured to heat and / or cool other metal objects, regardless of whether the metal object or vessel is an integral part of the device. This is advantageously applied when the metal part is made of a ferromagnetic material. [Background technology]

[0003] Induction cooktops are becoming more and more popular on the market, as they offer many advantages over traditional cooktops based on heating by combustion: Indeed, they do not require a flame, do not release combustion products into the environment of use, and do not have the risks associated with gas leaks, and are generally more efficient, allowing for faster and more uniform cooking.

[0004] As is known, an induction cooktop comprises a stationary surface, usually made of a glass-ceramic material, and a heating coil. The heating coil is arranged below the stationary surface and is connected to a power supply. The power supply supplies the heating coil with an alternating voltage, usually having a frequency between 20 kHz and 70 kHz, so that the heating coil generates a variable electromagnetic field. The electromagnetic field generates an induced current in the ferromagnetic bottom part of the container, which generates heat by the Joule effect, thereby heating the container and the food contained therein.

[0005] The widespread popularity of induction cooktops has brought about new needs, such as the need to accurately determine the temperature of a container located on the stationary surface of the cooktop.

[0006] Indeed, knowing the temperature of the container, and thus the approximate temperature of the food contained in it, makes it possible to optimize the cooking of the food (taking into account the food that must be cooked at a predetermined temperature) and to protect the container itself (whether it is a pot, frying pan or any other type of container) to avoid its damage due to excessive temperatures during the cooking of the food, thus making it possible to manage energy consumption as best as possible.

[0007] The same need to know the temperature of a container, and thus the approximate temperature of the food contained therein, also exists in other devices intended to heat or cool containers, in particular devices for preparing food.

[0008] Similarly, the need to know the temperature exists when heating and / or cooling other metal objects.

[0009] The problem of determining the temperature of a container and therefore the temperature of the food contained in it has already been dealt with, and in the prior art it is possible to find induction cooktops which measure the temperature of the container.

[0010] In some of these induction cooktops, the temperature of the container is measured by means of a heating coil. For example, this type of induction cooktop is described in EP 1 378 807. In this patent application, the heating coil is designed not only to heat the bottom part of the container, but also as a temperature sensor to determine the temperature of the bottom part of the container. For that purpose, the voltage supplied to the heating coil for heating the container may be interrupted during the temperature detection, and therefore the heating coil may be time-controlled.

[0011] Induction cooktops of this type have considerable drawbacks: Firstly, they have a complex design, in fact, the dual function of the heating coil must be taken into account during the design process.

[0012] In other induction cooktops, the temperature measurement of the container is carried out using a measuring coil separate from the heating coil. In this way, heating in a uniform manner and without discontinuities can be carried out. As described in WO200803872, the induction cooktop comprises a sensor for measuring the temperature of the container, which comprises a circuit with a measuring coil. The measuring coil is located in the center of the heating coil and is supplied with a square wave whose frequency corresponds to the resonant frequency of the circuit. Summary of the Invention [Problem to be solved by the invention]

[0013] However, in these induction cooktops the temperature measurement is not precise. In this context, the technical objective forming the basis of the present invention is to create a device for heating and / or cooling metal objects, in particular for heating and / or cooling containers, which at least partially overcomes the above mentioned drawbacks.

[0014] In particular, the technical object of the present invention is to create a device for heating and / or cooling metal objects, in particular a device for heating and / or cooling a container, which makes it possible to measure the temperature of the metal object more accurately than in the prior art.

[0015] Furthermore, a technical object of the present invention is to create an induction cooktop that at least partially overcomes the above-mentioned drawbacks by advantageously making it possible to measure the temperature of a container placed thereon more accurately than in the prior art. [Means for solving the problem]

[0016] The technical object and intent set forth herein is substantially achieved by a device for heating and / or cooling a metal object, in particular for heating and / or cooling a container (having at least one part made of a metallic material), in accordance with the subject matter defined in independent claim 1. Particular embodiments of the invention are defined in the corresponding dependent claims.

[0017] Further features and advantages of the present invention will become more apparent from the detailed description of some preferred non-limiting embodiments of a device for heating and / or cooling a metal object, illustrated with reference to the accompanying drawings, in which the device consists of an induction cooktop and the metal object is a container having at least one portion made of a metal material. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic side view, with parts cut away to more clearly illustrate the remainder, of a first embodiment of an induction cooktop according to the invention, with a container thereon; [Diagram 2] FIG. 2 is a schematic top view of some elements of the first embodiment of the induction cooktop of FIG. 1, shown without the container. [Diagram 3] FIG. 2 is a schematic side view, with parts cut away to more clearly illustrate the remaining parts, of a second embodiment of an induction cooktop according to the invention, with a container thereon; [Figure 4] FIG. 4 is a schematic top view of some elements of the second embodiment of the induction cooktop of FIG. 3, shown without the container. [Diagram 5] FIG. 2 is a schematic top view of multiple measurement coils connected in series with each other. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] It will be understood that the following description of a metal object consisting of a container having metal portions also applies, with appropriate adjustments as necessary, to other metal objects which the device is intended to heat and / or cool.

[0020] For the purposes of the present invention, the term "container" refers to any container advantageously suitable for receiving a substance to be heated (e.g. a food substance), such as, for example, a pot or a frying pan. As mentioned above, the present invention applies when the container has at least one part made of a metallic material (hereinafter more simply referred to as "metal part"). Preferably, the metal part is the bottom part of the container. Moreover, advantageously, in some embodiments, the metal part is made of a ferromagnetic material.

[0021] Generally, the device according to the invention comprises a stationary part configured to support the container 2 in use, and temperature changing means arranged to act on the container 2 arranged on the stationary part in order to change its temperature (and therefore the temperature of what is contained therein). Depending on the embodiment, the temperature changing means can be of any type and can be configured either for simply heating the container 2 or for simply cooling the container 2, or for both heating and cooling the container 2 (depending on the requirements of use). In some preferred embodiments, the temperature changing means are induction heating means. All these types of temperature changing means are known to experts in the field and will not be described in detail here.

[0022] With reference to said figures, the reference number 1 indicates as a whole a device according to the invention consisting of an induction cooktop (and therefore a device in which the temperature changing means are only arranged to heat a container 2).

[0023] In the case of a device consisting of an induction cooktop, the container 2 is not always part of the device itself, but in contrast in other embodiments the container 2 is part of the device and it is also possible that the device according to the invention comprises several containers 2 which are interchangeable and positionable in a stationary part.

[0024] The induction cooktop 1, like any other device equipped with induction heating means, has a bottom part 3 (metal part) made of ferromagnetic material and is therefore configured to heat a container 2 that can be heated by electromagnetic induction.

[0025] Like prior art induction cooktops, the induction cooktop 1 according to the invention firstly comprises a stationary surface 4 which forms the stationary part of the device and is adapted to support the container 2 and which is advantageously made of a glass-ceramic material.

[0026] The induction cooktop 1 also comprises heating means 5 forming the temperature variation means and comprising a heating coil 6 (seen in figures 1 and 3) located below the stationary surface 4 and operatively connected to and operable by a power supply. The heating coil 6 is configured to generate an electromagnetic field in the bottom part 3 of the container 2 located on the stationary surface 4. In an induction cooktop, the power supply usually consists of a mains network, and the induction cooktop 1 also comprises an inverter connectable to its input to the mains and to its output to the heating coil 6. The inverter converts the input voltage (for example 230V, 50Hz) into an alternating voltage required to generate a time-variable electromagnetic field (advantageously having a frequency between 20kHz and 70kHz) above the stationary surface 4. However, it is emphasized that the way in which the heating coil 6 works and the way in which heating by electromagnetic induction occurs will not be described in detail below, since it is known to those skilled in the art.

[0027] In known manner, the induction cooktop 1 may comprise several coaxial heating coils 6 which may be operated independently and / or simultaneously, so as to allow simultaneous heating of several containers 2, or several independent heating coils 6 arranged at different positions relative to the stationary surface 4, in the latter case what is explained below for a single heating coil 6 may be considered to apply also to each of them. A similar solution may also be adopted for any other device equipped with induction heating means for heating containers 2 arranged on a stationary part.

[0028] Advantageously, the heating means 5 also comprise a number of ferrites 7 located below the heating coil 6 and configured in a known manner to focus the electromagnetic field on the bottom part 3 (or more generally on the metal part) of the container 2. However, the invention is not limited by the number or position of the ferrites 7. In the illustrated embodiment, for example, the heating means 5 comprises eight ferrites 7 extending radially with respect to the central axis of the heating coil 6 and positioned so as to be equidistant from one another (in particular, in the two illustrated embodiments, each ferrite 7 is spaced apart from the adjacent ferrite 7 by an angle of 45°). In contrast, in other embodiments, the number and / or position of the ferrites 7 may be different.

[0029] According to an innovative aspect of the invention, the device also comprises a measurement sensor 8 for estimating, in use, the temperature of a metal part of the container 2 arranged on the stationary part 4. The measurement sensor 8 comprises at least one measurement coil 9, separate and independent of the temperature changing means, and an electronic device 10.

[0030] If the device is an induction cooktop 1 , the measurement sensor 8 can be used to estimate the temperature of the bottom part 3 of a container 2 placed on a stationary surface 4 above the heating coil 6 .

[0031] At least one measuring coil 9 is part of the electric circuit. Furthermore, the at least one measuring coil 9 comprises a number of turns. The measuring coil 9 is positioned so as to be magnetically coupled to a metallic part of the container 2 arranged on the stationary part.

[0032] When the device consists of an induction cooktop 1, the measuring coil 9 is separated from the heating coil 6 so that it is independent of the temperature changing means, and furthermore, in the case of the induction cooktop 1, the measuring coil 9 is sandwiched between the heating coil 6 and the stationary surface 4 so that the measuring coil 9 is magnetically coupled to the metal part of the container 2.

[0033] The electronic device 10 is operatively connected to at least one measuring coil 9. In particular, according to a main innovative aspect of the present invention, the electronic device 10 is connected to an electric circuit of which the at least one measuring coil 9 is a part, supplies an AC voltage at a predetermined frequency between 100 kHz and 500 kHz and measures the resistance and / or inductance of the electric circuit during said supply (i.e. the apparent resistance and / or inductance of the electric circuit at the predetermined frequency of said AC voltage). According to the present invention, the predetermined frequency of the supplied AC voltage is fixed, predetermined and independent of the temperature of the vessel 2, as is the frequency of the resistance and / or inductance measurements. In particular, the predetermined frequency is different from the resonant frequency of the electric circuit.

[0034] Indeed, according to the invention, the resistance and / or inductance values ​​measured at the end of the electric circuit of which the at least one measuring coil 9 is part are used / utilised to estimate the temperature of the vessel 2 located in its stationary part (more specifically in its metallic part). Indeed, as is known, the resistance (R) and inductance (L) measurable at the end of a power coil through which an alternating current flows depends on various factors, including the presence in the vicinity of metallic or ferromagnetic objects that may interact with the magnetic field generated by the coil itself and thus generate induced currents.

[0035] This also applies to the at least one measuring coil 9 provided by the invention, which, during normal use of the device, is at least magnetically coupled to the metallic parts of the container 2 and generates an induced current therein, so that, for example, the resistance measurable at the end of an electric circuit is determined not only by the (small) Joule effect losses in the material of which the at least one measuring coil 9 is made, but also by the Joule effect losses which occur in the container 2 as a result of the circulation of the induced current (especially in the case of ferromagnetic metallic parts).

[0036] It is also known that the resistivity of a metal or ferromagnetic material increases with increasing temperature, so that an increase (or decrease) in the temperature of the container 2 will increase (or decrease) the resistance measurable at the end of the electrical circuit. The inductance measurable at the end of the electrical circuit will also change with changing temperature.

[0037] Electrically, in particular in the case where the measurement sensor 8 comprises several measurement coils 9, each measurement coil 9 has a respective resistance R b and inductance L b and therefore the respective impedances Z b =R b +jX b , reactance X b L b Function of (X b =ωL b , ω represents the angular frequency of the electrical circuit). Considering the entire electrical circuit connected to the electronic device 10, it can be characterized as having a R b (real part) and L b (related to the imaginary part), and therefore Z b The equivalent impedance Z whose total value depends on each value of eq =R eq +jX eq However, as noted above, the resistance R measured by the electronic device 10 mis The equivalent resistance of the electric circuit is R eq is the temperature T of container 2 rec The resistance R of the container 2 coupled to the measuring coil 9 depends on rec When the unloaded behavior of the electric circuit at different temperatures is known, the resistance R of the container 2 coupled to the measuring coil 9 depends on rec It is possible to determine at any time the resistance R rec By knowing the temperature T of container 2, rec (in particular the temperature of the bottom part 3 of the container 2). misand the measured impedance Z mis The equivalent inductance L of the electric circuit that can correspond to each of eq and the equivalent impedance Z of the electric circuit eq are the temperature T of container 2, respectively. rec The inductance L of the container 2 coupled to the measurement coil 9 depends on rec and the impedance of container 2, Z rec When the unloaded behavior of the electric circuit at different temperatures is known, the inductance L of the container 2 coupled to the measuring coil 9 depends on rec or the impedance Z of container 2 rec As a result, the inductance L of the container 2 can be determined rec or the impedance Z of container 2 rec By knowing each of these, the temperature T of container 2 can be calculated. rec It may be possible to estimate the temperature of the metal parts of the container 2 to which the measuring coil 9 is coupled (in particular the temperature of the metal parts of the container 2 to which the measuring coil 9 is coupled). How this can be done is known to those skilled in the art and will not be described in further detail. However, it is noted, purely by way of example, that a method for measuring the impedance of an electric circuit at the frequency of interest which, with suitable adjustments, may be used in the context of the present invention is described in the article "An LMS Impedance Bridge" by George R. Steber, WB9LVI;QEX, September / October 2005, p. 41-47.

[0038] Advantageously, the electronic device 10 is connectable to an electronic processing unit configured to receive data of the measured resistance and / or inductance of the electric circuit and to process such data in such a way as to estimate the temperature of the metal parts of the container 2. Therefore, in the following, when "temperature measurement" is mentioned, it means a temperature measurement inferred from the above-mentioned resistance and / or inductance measurements. The electronic processing unit may itself be part of the device or may be separate therefrom, and furthermore, the electronic device 10 and the electronic processing unit may comprise a single device.

[0039] Similar to the situation in other prior art devices, the temperature estimated using the measurement sensor 8 can then be used by the device itself to automatically adjust the operation of the temperature changing means, for example based on a program pre-set by the user (based on the estimated temperature value the device is able to automatically adjust the operation of the temperature changing means to increase, keep stable or decrease the temperature).

[0040] In some embodiments, not shown in the attached figures, the at least one measuring coil 9 consists of a single measuring coil 9. Preferably, in embodiments in which the at least one measuring coil 9 consists of a single measuring coil 9, in plan view each turn of the measuring coil 9 has a circular shape, preferably with a diameter between 40 mm and 120 mm, advantageously between 50 mm and 90 mm.

[0041] According to a second innovative aspect of the invention, in contrast, the at least one measuring coil 9 preferably comprises a plurality of measuring coils 9, advantageously electrically connected to one another in series and / or in parallel, embodiments of which are explained in more detail below.

[0042] In some embodiments, the measurement coil 9 or the measurement coil(s) 9 are off-centre with respect to the temperature varying means.

[0043] In contrast, in some embodiments, the measurement coils 9 are distributed around a central axis, i.e. the central axis of the heating coil 6 in the case of the induction cooktop 1. Furthermore, in preferred embodiments, the measurement coils 9 are arranged at the vertices of a polygon, preferably a regular polygon centered on the central axis. For example, in the embodiment shown in Figures 2 and 4, the measurement coils 9 consist of four measurement coils 9 distributed at the vertices of a square. An embodiment in which the measurement coils 9 are arranged around a central axis advantageously makes it possible to obtain very precise temperature measurements.

[0044] In embodiments in which the heating means 5 comprises ferrites 7, each measurement coil 9 may be positioned in plan view such that it is offset relative to the ferrites 7 (i.e. in such a way that it does not overlap any ferrites 7). For example, each measurement coil 9 may be positioned between an adjacent pair of ferrites 7, as is evident from Figures 2 and 4, in which the heating means 5 comprises eight ferrites 7 and the measurement sensor 8 comprises four measurement coils 9.

[0045] It is emphasized that in both of the embodiments shown in the figures, the measurement coils 9 are four. However, other embodiments not shown are also possible, in which the number of measurement coils 9 is different from four and / or the position of the measurement coils 9 is different (whether the heating means 5 comprises ferrite 7, the heating means 5 does not comprise ferrite 7 or the temperature variation means takes a different form). For example, in some embodiments there are eight measurement coils 9 located at the vertices of an octagon (again, in case the heating means 5 comprises ferrite 7, the measurement coils 9 are advantageously located, in plan view, between two adjacent ferrites 7).

[0046] Advantageously, each measuring coil 9 comprises a number of turns. Preferably, each measuring coil 9 comprises a number of turns equal to or different from the number of turns of each other measuring coil 9, advantageously between 5 and 50 turns (advantageously, even in embodiments where there is a single measuring coil 9, it comprises a number of turns that is between 5 and 50 turns). For example, in some embodiments, the measuring coils 9 all have the same number of turns. In contrast, in other embodiments, the measuring coils 9 all have different numbers of turns. In contrast, in still other embodiments, some measuring coils 9 have the same number of turns that differs from the number of turns of one or more of the other measuring coils 9. Indeed, the invention is not limited by the number of turns of the measuring coils 9.

[0047] Furthermore, the turns of the different measurement coils 9 may have the same or different shapes. For example, in some embodiments, in plan view, the turns of the measurement coil 9 have a circular shape (as shown in FIG. 2). In other embodiments, in plan view, the turns of the measurement coil 9 have a plectrum shape (as shown in FIG. 4). In contrast, in still other embodiments, in plan view, the turns of the measurement coil 9 have an elliptical or boomerang shape (not shown). Also possible are embodiments in which the turns of the different measurement coils 9 have different shapes in plan view (for example, one measurement coil 9 has a cross section that is circular and another measurement coil 9 has a cross section that is plectrum shaped) or shapes different from those described.

[0048] As can be seen from the attached figures, the plectrum shape allows the measurement coil 9 to better adapt to the presence of ferrite 7 compared to shapes other than the plectrum, such as for example a circular shape.

[0049] Moreover, even the winding direction of the measuring coils 9 is not limiting in the present invention. As is known, the expression "winding direction" means the direction in which the coils are wound with respect to the extension axis of the measuring coils 9. In particular, in some embodiments the measuring coils 9 are wound in the same direction, whereas in other embodiments, it is possible to distinguish among the measuring coils 9 one or more first measuring coils 9 wound in a first direction and one or more second measuring coils 9 wound in a second direction opposite to the first direction. As is known, in the case of measuring coils 9 wound in opposite directions, if a current flows in the same direction in both measuring coils 9, the electromagnetic field generated by one of the first measuring coils 9 and the electromagnetic field generated by one of the second measuring coils 9 will be opposite to each other.

[0050] The turns of each first measuring coil 9 surround, in plan view, a respective first region, and the turns of each second measuring coil 9 surround, in plan view, a respective second region. In some cases, each first region and each second region has a mutually different amplitude. For example, it is possible either that all of the first regions have mutually equal amplitudes, that all of the second regions have mutually equal amplitudes, and that the amplitude of the first region differs from the amplitude of the second region, or that at least one first region has a mutually different amplitude from another first region, and / or that at least one second region has a mutually different amplitude from another second region. It is emphasized that these descriptions are merely examples of possible embodiments of the invention, and therefore they are not to be considered as limiting the invention.

[0051] Furthermore, with reference to embodiments in which the first winding direction of the first measuring coil 9 is opposite to the second winding direction of the second measuring coil 9, advantageously the sum of all first areas differs from the sum of the second areas by no more than 20%, preferably no more than 10%.

[0052] Advantageously, this type of embodiment makes it possible to significantly reduce the influence of the voltage induced in the measuring coil 9 by the electromagnetic field generated by the heating coil 6 and makes it possible to make the resistance and / or inductance measurement less sensitive to small defects in the bottom part 3 of the container 2 (a surface that is not perfectly flat, a lack of homogeneity in the material, the presence of small depressions, etc.).

[0053] In some embodiments in which the device comprises induction heating means (not shown), in plan view, each measurement coil 9 overlaps a heating coil 6. Indeed, this kind of configuration makes it possible, as is known, to advantageously reduce the influence of the heating coil 6 on the measurement, since the electromagnetic field in the immediate vicinity of the conductor is very low (at least compared to the central zone of the heating coil 6).

[0054] Finally, the Applicant proposes a measurement sensor 8 for estimating, during use, the temperature of any metal object, in particular a metal part of a container 2, said measurement sensor 8 comprising at least one measurement coil 9 and an electronic device 10, At least one measuring coil 9 is part of the electric circuit and comprises a number of turns; The electronic device 10 is connected to an electric circuit, and in use supplies an AC voltage of a predetermined frequency of 100 kHz to 500 kHz to the electric circuit, and in use measures the resistance and / or inductance of the electric circuit supplied with the predetermined frequency of 100 kHz to 500 kHz. We reserve the right to protect the measurement sensor 8 independently, either alone or by filing divisional patent applications.

[0055] Optionally, the measurement sensor 8 may also comprise one or more of the features described above in relation to the measurement sensor 8 being part of the device according to the invention.

[0056] The present invention provides important advantages.

[0057] Thanks to this invention it is now possible to make a device for heating and / or cooling metal objects, which allows a more accurate measurement of the temperature of the metal object than in the prior art.

[0058] In particular, thanks to the invention it has been possible to create an induction cooktop which makes it possible to measure the temperature of a pan more accurately than prior art induction cooktops.

[0059] More specifically, in the case of an induction cooktop, the presence of several measuring coils allows a more accurate measurement of the temperature of the metal object compared to the prior art, where only one measuring coil is located in the center of the heating coil. Indeed, tests carried out by the applicant have shown that the presence of a single measuring coil only allows a local measurement (and is therefore very sensitive to variations in the material and shape of the bottom of the container). For example, if the bottom of the container is not perfectly flat or has a depression only 2 mm deep, this will cause variations in the measurement and therefore very large errors.

[0060] Similar advantages apply to devices intended for heating and / or cooling other types of metallic objects.

[0061] Finally, it should be noted that the invention is relatively easy to manufacture and not too expensive to implement.

[0062] The invention described above may be susceptible to modification and adaptation in several ways without thereby departing from the scope of the inventive concept.

[0063] All the details may be replaced by other elements which are technically equivalent; likewise the materials used and the shapes and dimensions of the various components may be varied according to requirements.

Claims

1. A device for heating and / or cooling a metal object, such as a container 2 having a metal part 3 made of a metallic material, said device 1 comprising: a stationary part 4 configured to support said metal object in use; temperature changing means arranged to act on said metal object arranged on said stationary part 4; - a measurement sensor 8 for estimating a temperature during use of the metal part of the metal object, said measurement sensor 8 comprising at least one measurement coil 9 and an electronic device 10; Equipped with said at least one measuring coil 9 being part of an electric circuit and comprising a number of turns; the electronic device 10 is connected to the electrical circuit and, in use, supplies an AC voltage at a predetermined frequency between 100 kHz and 500 kHz, and, in use, measures the resistance and / or inductance of the electrical circuit supplied with the predetermined frequency between 100 kHz and 500 kHz. A device for heating and / or cooling metal objects.

2. The device according to claim 1 , wherein the at least one measuring coil (9) consists of a single measuring coil (9).

3. The device according to claim 1 , wherein the at least one measuring coil (9) comprises a plurality of measuring coils (9) electrically connected to one another.

4. 4. The device according to claim 3, wherein the measuring coils (9) are distributed around a central axis of the heating coil (6).

5. 5. The device according to claim 4, wherein the measurement coils (9) are arranged at the vertices of a polygon, preferably a regular polygon centered on the central axis.

6. The device according to claim 5 , wherein the plurality of measuring coils (9) consists of four measuring coils (9) arranged at the vertices of a square.

7. 7. A device according to claim 3, wherein each measuring coil (9) comprises one or more turns, and in plan view, each turn of said measuring coil (9) has a circular shape with a diameter of 40 mm to 120 mm, preferably 50 mm to 90 mm.

8. 8. The device according to claim 3, wherein each measurement coil 9 of the plurality of measurement coils 9 has a number of turns equal to or different from the number of turns of another measurement coil 9 of the plurality of measurement coils 9, said number of turns being between 5 and 50.

9. 9. The device according to claim 3, wherein all measuring coils 9 of the plurality of measuring coils 9 are wound in the same direction.

10. 9. The device according to claim 3, wherein, of the plurality of measuring coils 9, one or more first measuring coils 9 wound in a first direction and one or more second measuring coils 9 wound in a second direction opposite to the first direction can be distinguished.

11. 11. The device according to claim 10, wherein the turns of each of the first measurement coils 9 surround, in plan view, a respective first region and the turns of each of the second measurement coils 9 surround, in plan view, a respective second region, and wherein each of the first regions and each of the second regions have amplitudes equal to or different from each other.

12. 12. A device according to claim 10 or 11, wherein the sum of the first areas differs from the sum of the second areas by no more than 20%, preferably no more than 10%.

13. 13. The device according to claim 3, wherein in plan view, each turn of each measuring coil 9 of the plurality of measuring coils 9 is circular, plectrum-shaped, elliptical or boomerang-shaped.

14. The device according to claim 3 , wherein the measurement coils are electrically connected in parallel and / or in series with each other.

15. A device according to any one of claims 1 to 14, wherein the electronic device 10 is connectable to an electronic processing unit configured to receive data relating to the resistance and / or inductance of the electric circuit measured by the electronic device 10 and to process such data to estimate the temperature of the metal object.

16. 16. A device according to any preceding claim, wherein the electronic device (10) is configured, in use, to supply power to the electrical circuit at a predetermined frequency different to a resonant frequency of the electrical circuit.

17. said device is an induction cooktop for heating a vessel 2 comprising a metal part made of ferromagnetic material, The stationary portion is a stationary surface 4, said temperature varying means being heating means 5 arranged below said stationary surface 4 and comprising a heating coil 6 operably connected to and powerable by a power supply to generate an electromagnetic field; said at least one measuring coil 9 is distinct from said heating coil 6 and is sandwiched between said heating coil 6 and said stationary surface 4; A device according to any one of claims 1 to 16.

18. The device according to claim 17 when dependent on claim 3, wherein the heating means 5 also comprises a plurality of ferrites 7 located below the heating coil 6, and in plan view, each measuring coil 9 of the plurality of measuring coils 9 is positioned offset relative to the ferrites 7, preferably between two adjacent ferrites 7.

19. A device according to claim 17 or 18 when dependent on claim 3, wherein each measuring coil 9 of the plurality of measuring coils 9 overlaps, in plan view, with a heating coil 6.

20. The device of claim 1 , further comprising the metal object.

21. 21. The device according to any one of claims 1 to 20, wherein the metal object is the container 2 with a metal part, the metal part of the container 2 being a bottom part 3 and / or made of a ferromagnetic material.

22. 22. A device according to any one of claims 1 to 21, wherein the electronic device (10) is connected to the electrical circuit and, in use, measures only the resistance of the electrical circuit.

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