Device for heating and / or cooling metal objects

Abstract

The invention relates to a device for heating and / or cooling metal objects. An apparatus (1) for heating a metal object, such as a container (2) having a metal portion, the apparatus (1) comprising: a support portion for supporting the metal object; a heating device (5); a measurement sensor (8) for estimating the temperature of the metal object, the measurement sensor (8) comprising at least one measurement coil (9) and an electronic device (10). The measuring coil (9) is part of the circuit. An electronic device (10) is connected to the circuit in order to provide the circuit with an alternating voltage at a predetermined frequency between 100 kHz and 500 kHz and to measure the resistance of the circuit powered in this way.

Description

Technical Field

[0001] The present invention relates to a device for heating and / or cooling a metal object. The present invention particularly relates to a device for heating and / or cooling a container having at least one portion made of a metal material. Background Art

[0002] The invention was originally proposed with reference to an induction cooker for heating a container having a bottom made of a ferromagnetic material that can be heated by electromagnetic induction. Thus, in the following, reference will mainly be made to applications in which the device is an induction cooker. However, the invention can be similarly applied to any device configured to heat and / or cool a container having at least one metal part as well as any other metal object, regardless of whether the metal object or the container is an integral part of the device. The invention can be advantageously applied if the metal part is made of a ferromagnetic material.

[0003] Induction cookers are becoming more and more common on the market, as they offer many advantages over traditional stoves based on combustion heating. In fact, induction cookers do not require flames, do not emit combustion products into the environment in which they are used, and do not have the risks associated with gas leaks. In addition, they are generally more efficient, with faster and more even cooking results.

[0004] As is known, an induction cooker comprises a support surface, usually made of glass-ceramic material, and a heating coil. The heating coil is placed under the support surface and is connected to a power source. The power source supplies an alternating voltage, usually with a frequency between 20kHz and 70kHz, to the heating coil, whereby the heating coil generates a variable electromagnetic field. This electromagnetic field induces a current in the ferromagnetic bottom of the container, generating heat due to the Joule effect, thereby heating the container and the food contained therein.

[0005] The widespread popularity of induction cookers has brought about new requirements, such as the requirement to accurately determine the temperature of a container located on the support surface of the induction cooker.

[0006] In fact, knowing the temperature of the container, and therefore roughly the temperature of the food it contains, allows optimizing the cooking of the food (taking into account the food that must be cooked at a predetermined temperature), protecting the container itself (whether it is a pot, frying pan or other type of container), avoiding damage to the container due to excessive temperatures during the cooking of the food, and managing energy consumption as well as possible.

[0007] A similar need to know the temperature of a container and thus roughly the temperature of the food contained in the container also exists in other apparatus for heating or cooling containers, in particular apparatus for preparing food.

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

[0009] The problem of determining the temperature of a container and thus of the temperature of the food contained in the container has already been solved, and in the prior art even induction cookers can be found which measure the temperature of a container.

[0010] In some of these induction cookers, the temperature of the container is measured using a heating coil. For example, this type of induction cooker is described in European patent application EP 1378807. In this patent application, the heating coil is not only designed to heat the bottom of the container, but also designed as a temperature sensor to determine the temperature of the bottom of the container. For this purpose, during the temperature detection, the voltage supplied to the heating coil to heat the container is also interrupted, so that the heating coil is controlled in a timed manner.

[0011] This type of induction cooker has considerable disadvantages. First of all, their design is complex: in fact, the dual function of the heating coil must be taken into account during the design process.

[0012] In other induction cookers, a measuring coil different from the heating coil is used to measure the temperature of the container instead. In this way, the heating can be carried out in a uniform manner without interruption. As described in international patent application WO 200803872, the induction cooker comprises a sensor for measuring the temperature of the container, the sensor having an electrical circuit including a measuring coil. The measuring coil is located in the center of the heating coil and is provided with a square wave having a frequency corresponding to the resonant frequency of the electrical circuit.

[0013] However, in these induction cookers, the temperature is not measured precisely. Summary of the invention

[0014] In this context, the technical aim underlying 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.

[0015] In particular, the technical purpose 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 allows measuring the temperature of the metal objects more accurately than in the prior art.

[0016] Furthermore, the technical purpose of the present invention is to create an induction cooker that at least partially overcomes the above-mentioned drawbacks by advantageously allowing a more precise measurement of the temperature of a container placed thereon than in the prior art.

[0017] The indicated technical aim and objects are substantially achieved by a device for heating and / or cooling metal objects, in particular for heating and / or cooling containers having at least one portion made of metal material, as defined in independent claim 1. Specific embodiments of the invention are defined in the corresponding dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Other characteristics and advantages of the invention will become more apparent from the detailed description of several 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 cooker, the metal object being a container having at least one portion made of metal material, in which:

[0019] - Figure 1 is a schematic side view of a first embodiment of an induction cooker according to the present invention with a container thereon, with some parts cut away to better illustrate other parts;

[0020] - Figure 2 Does not contain container Figure 1 A schematic top view of several components of a first embodiment of an induction cooker;

[0021] - Figure 3 is a schematic side view of a second embodiment of an induction cooker according to the present invention with a container thereon, with some parts cut away to better illustrate other parts;

[0022] - Figure 4 Does not contain container Figure 3 A schematic top view of several components of a second embodiment of an induction cooker;

[0023] - Figure 5 Schematic top view of a plurality of measuring coils connected in series with each other. DETAILED DESCRIPTION

[0024] The following description concerning a metal object consisting of a container having metal parts is also to be understood to apply, with appropriate adjustments where necessary, to any other metal object which the apparatus is intended to heat and / or cool.

[0025] For the purposes of the present invention, the term "container" means any container, such as a pot or a frying pan, which is advantageously suitable for receiving a substance to be heated (e.g. a food substance). As mentioned above, the present invention is applicable to the case where the container has at least one portion made of a metallic material (hereinafter referred to as "metal portion" for short). Preferably, the metal portion is the bottom of the container. Furthermore, advantageously, in some embodiments, the metal portion consists of a ferromagnetic material.

[0026] Typically, the apparatus according to the invention comprises a support portion configured to support a container 2 during use, and a temperature variation device configured to act on the container 2 placed on the support portion in order to change its temperature (and therefore the temperature of the contents therein). Depending on the embodiment, the temperature variation device may be of any type and may be configured to heat the container 2 only, or to cool the container 2 only, or to both heat and cool the container 2 (depending on the use requirements). In some preferred embodiments, the temperature variation device is an induction heating device. All these types of temperature variation devices are known to those skilled in the art and will therefore not be described in detail herein.

[0027] With reference to the above-mentioned figures, reference numeral 1 denotes as a whole a device consisting of an induction cooker according to the invention (wherein the temperature variation device is then configured only as a device for heating a container 2 ).

[0028] Although in the case of an apparatus consisting of an induction cooker, the container 2 is usually not part of the apparatus itself, in other embodiments, the container 2 is part of the apparatus; the apparatus according to the invention may also comprise a plurality of containers 2 which are interchangeable and can be placed on a support part.

[0029] The induction cooker 1 and any other device comprising an induction heating device is configured to heat a container 2 having a bottom 3 (metal part) made of a ferromagnetic material, which can then be heated by electromagnetic induction.

[0030] Similar to the induction cookers of the prior art, the induction cooker 1 according to the invention firstly comprises a support surface 4 which forms a support part of the device, which is configured to support the container 2 and which is advantageously made of a glass ceramic material.

[0031] The induction cooker 1 further comprises a heating device 5, which forms a temperature variation device and comprises a heating coil 6 (in Figure 1 and Figure 3 ), the heating coil 6 is placed below the support surface 4 and is operationally connectable to and powered by a power source. The heating coil 6 is configured to generate an electromagnetic field at the bottom 3 of the container 2 placed on the support surface 4. As is usually the case with induction cookers, the power source usually consists of a building mains network, and the induction cooker 1 also includes an inverter that can be connected to the mains at its input and to the heating coil 6 at its output. The inverter converts an input voltage (for example an input voltage of 230V and 50Hz) into an alternating voltage (advantageously with a frequency between 20kHz and 70kHz) required to generate an electromagnetic field that is variable over time above the support surface 4. However, it should be emphasized that the working mode of the heating coil 6 and the way in which electromagnetic induction heating occurs will not be described in further detail below, as they are known to those skilled in the art.

[0032] In a known manner, the induction cooker 1 may comprise a plurality of coaxial heating coils 6 which may be activated independently and / or simultaneously, or a plurality of independent heating coils 6 placed at different positions relative to the support surface 4, so as to allow simultaneous heating of a plurality of containers 2; in the latter case, the following description for a single heating coil 6 may also be considered applicable to each of them. Similar solutions may also be adopted for any other device comprising an induction heating device for heating a container 2 placed on a support portion.

[0033] Advantageously, the heating device 5 further comprises a plurality of ferrites 7, which are placed below the heating coil 6 and are configured in a known manner to concentrate the electromagnetic field on the bottom 3 of the container 2 (or more generally on the metal part). However, the invention is not limited by the number or positioning of the ferrites 7. For example, in the embodiment shown in the diagram, the heating device 5 comprises 8 ferrites 7, which extend radially relative to the central axis of the heating coil 6 and are placed equidistantly from each other (in particular, in the two examples shown, each ferrite is spaced apart from the adjacent ferrite at an angle of 45°). In contrast, in other embodiments, there may be a different number of ferrites 7, and / or the ferrites 7 may be placed differently.

[0034] According to an innovative aspect of the invention, the device further comprises a measuring sensor 8 for estimating, in use, the temperature of a metal part of a container 2 placed on the support 4. The measuring sensor 8 comprises at least one measuring coil 9 and electronics 10 distinct and independent from the temperature variation means.

[0035] In the case where the device is an induction cooker 1 , the measuring sensor 8 can be used to estimate the temperature of the bottom 3 of the container 2 placed on the support surface 4 above the heating coil 6 .

[0036] The at least one measuring coil 9 is part of an electrical circuit. Furthermore, the at least one measuring coil 9 comprises a plurality of turns. The measuring coil 9 is placed in such a way as to magnetically couple with a metal part of the container 2 placed on the support part.

[0037] If the device consists of an induction cooker 1, in order to make the measuring coil 9 different from and independent of the temperature variation device, the measuring coil 9 is different from the heating coil 6; moreover, in order to make the measuring coil 9 magnetically coupled to the metal part of the container 2, in the case of the induction cooker 1, the measuring coil 9 is placed between the heating coil 6 and the support surface 4.

[0038] The electronic device 10 is operatively connected to the at least one measuring coil 9. In particular, according to a main innovative aspect of the invention, the electronic device 10 is connected to a circuit of which the at least one measuring coil 9 is a part, so as to supply said circuit with an alternating voltage of a predetermined frequency between 100 kHz and 500 kHz and to measure the resistance and / or inductance of the circuit during this supply (i.e. the apparent resistance and / or inductance of the circuit at the predetermined frequency of this alternating voltage). According to the invention, the predetermined frequency of the supply alternating voltage, as well as the predetermined frequency of the resistance and / or inductance measurement, are fixed and predefined and do not depend on the temperature of the container 2. In particular, the predetermined frequency is different from the resonant frequency of the circuit.

[0039] In fact, according to the invention, the resistance and / or inductance values ​​measured at the ends of the circuit of which said at least one measuring coil 9 is a part are used / can be used to estimate the temperature of the container 2 placed on the support portion, more specifically of the metal portion thereof. In fact, as is well known, the resistance (R) and inductance (L) measurable at the ends of an energized coil in which an alternating current circulates depend on various factors, including the presence of metal or ferromagnetic materials in the vicinity, which can interact with the magnetic field generated by the coil itself and can thus generate induced currents therein.

[0040] This applies to the at least one measuring coil 9 provided by the invention, which, during normal use of the device, is magnetically coupled to at least the metal parts of the container 2 and circulates the induced current therein. For example, the resistance measurable across the circuit is then determined not only by the (small) Joule effect losses in the material constituting the at least one measuring coil 9, but also (most importantly, in the case of ferromagnetic metal parts) by the Joule effect losses arising in the container 2 as a result of the circulation of the induced current.

[0041] It is also known that the resistivity of metal or ferromagnetic materials increases with increasing temperature, so an increase (or decrease) in the temperature of the container 2 will result in an increase (or decrease) in the resistance measurable across the circuit. The inductance measurable across the circuit also changes with temperature.

[0042] Electrically, with particular reference to the case where the measuring sensor 8 comprises a plurality of measuring coils 9, each measuring coil 9 can be characterized as having a corresponding resistance R b and the corresponding inductance L b , so the corresponding impedance Z b =R b +jX b , reactance X b YesL b Function (X b =ωL b, where ω represents the angular frequency of the circuit). Considering the circuit connected to the electronic device 10 as a whole, it consists of an equivalent impedance Z eq =R eq +jX eq Characterization, equivalent impedance Z eq =R eq +jX eq The value depends on the R of the different measuring coils 9 that are part of the circuit b (real part) and L b The corresponding value of (associated with the imaginary part) thus depends on Z b However, as mentioned above, the equivalent resistance R of the circuit eq , that is, the resistance R measured by the electronic device 10 mis , depends on the resistance R of the container 2 coupled to the measuring coil 9 rec , the resistance of container 2 is R rec Depends on the temperature T of container 2 rec When the no-load behavior of the circuit at different temperatures is known, the resistance R of the container 2 coupled to the measuring coil 9 can be determined at any time. rec Conversely, knowing the resistance R rec This allows the temperature T of the container 2 (particularly the bottom 3 of the container 2) to be estimated. rec Similarly, the inductance L measured by the electronic device 10 is mis and impedance Z mis The equivalent inductance L of the corresponding circuit eq And the equivalent impedance Z of the circuit eq respectively depends on the inductance L of the container 2 coupled to the measuring coil 9 rec and the impedance Z of container 2 rec , the inductance L of container 2 rec and the impedance Z of container 2 rec Depends on the temperature T of container 2 rec When the no-load behavior of the circuit at different temperatures is known, the inductance L of the container 2 coupled to the measuring coil 9 can be determined at any time. rec or the impedance Z of container 2 rec Conversely, we can find out the inductance L of container 2. rec or the impedance Z of container 2 rec This makes it possible to estimate the temperature T of the container 2 (in particular the metal part of the container 2 to which the measuring coil 9 is coupled). recThe manner in which this can be done is known to those skilled in the art and will not be described in further detail. However, by way of example only, it should be noted that a method for measuring the impedance of a circuit at a frequency of interest is described in the article "An LMSImpedance Bridge" by George R. Steber; WB9LVI; QEX, September / October 2005, pages 41-47, which method can be used in the context of the present invention with appropriate adjustments.

[0043] Advantageously, the electronic device 10 can be connected to an electronic processing unit configured to receive data on the measured resistance and / or measured inductance of the circuit and to process these data in order to estimate the temperature of the metal part of the container 2. Therefore, hereinafter, when referring to "temperature measurement", we mean the temperature measurement result estimated from the above-mentioned resistance and / or inductance measurement results. The electronic processing unit itself can be part of the device or independent of the device; moreover, the electronic device 10 and the electronic processing unit can consist of a single device.

[0044] Similar to the situation in other prior art devices, the temperature estimated using the measuring sensor 8 can then be used by the device itself to automatically adjust the operation of the temperature change device, for example based on a user preset program (based on the estimated temperature value, the device will be able to automatically adjust the operation of the temperature change device to increase, maintain stability or decrease the temperature).

[0045] In some embodiments not shown in the drawings, the at least one measuring coil 9 consists of a single measuring coil 9. Preferably, in the embodiments in which the at least one measuring coil 9 consists of a single measuring coil 9, each turn of the measuring coil 9 has a circular shape in plan view, with a diameter preferably between 40 mm and 120 mm, advantageously between 50 mm and 90 mm.

[0046] In contrast, according to the second innovative aspect of the present invention, preferably, the at least one measuring coil 9 comprises a plurality of measuring coils 9 electrically connected to each other, advantageously in series and / or in parallel. These embodiments will be described in more detail below.

[0047] In some embodiments, the one or more measuring coils 9 are offset from the center relative to the temperature variation device.

[0048] In contrast, in some embodiments, the measuring coils 9 are distributed around the central axis, in the case of the induction cooker 1, around the central axis of the heating coil 6. Furthermore, in a preferred embodiment, the measuring coils 9 are placed at the vertices of a polygon, preferably a regular polygon centered on the central axis. Figure 2 and Figure 4In the embodiment shown in , the measuring coils 9 consist of four measuring coils 9 placed at the vertices of a square. Embodiments in which the measuring coils 9 are distributed around a central axis advantageously allow very precise temperature measurements to be obtained.

[0049] In the embodiment in which the heating device 5 comprises ferrites 7, each measuring coil 9 can be placed in a way offset relative to the ferrites 7 in plan view (i.e. in a way that they do not overlap with any ferrite 7). Figure 2 and Figure 4 As can be clearly seen in the figure, the heating device 5 includes 8 ferrites 7 and the measuring sensor 8 includes 4 measuring coils 9 , each of which can be placed between a pair of adjacent ferrites 7 .

[0050] It should be emphasized that in the two embodiments shown in the figures, there are 4 measuring coils 9. However, other embodiments not shown are also possible, in which the number of measuring coils 9 is different from 4 and / or in which the measuring coils 9 are placed differently (regardless of whether the heating device 5 comprises ferrites 7, or the heating device 5 does not comprise ferrites 7, or the temperature variation device takes a different form). For example, in some embodiments, there are 8 measuring coils 9 placed at the vertices of an octagon (also in this case, if the heating device 5 comprises ferrites 7, the measuring coils 9 are advantageously placed between two adjacent ferrites 7 in plan view).

[0051] Advantageously, each measuring coil 9 comprises a plurality of turns. Preferably, each measuring coil 9 comprises a number of turns that is equal to or different from the number of turns of each other measuring coil 9 and is advantageously between 5 and 50 turns (advantageously, even in embodiments where there is a single measuring coil 9, it comprises a number of turns between 5 and 50). 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 a different number of turns. In contrast, in still other embodiments, some measuring coils 9 have the same number of turns that is different from the number of turns of one or more other measuring coils 9. In fact, the present invention is not limited by the number of turns of the measuring coils 9.

[0052] Furthermore, the turns of different measuring coils 9 may have the same or different shapes from each other. For example, in some embodiments, the turns of the measuring coils 9 have a circular shape (e.g. Figure 2 In other embodiments, the turns of the measuring coil 9 have a paddle shape in plan view (as shown in Figure 4). In contrast, in other embodiments, in plan view, the turns of the measuring coil 9 have an elliptical shape or a boomerang shape (not shown in the figure). In plan view, the turns of different measuring coils 9 have different shapes from each other (for example, some measuring coils 9 have a circular cross section, while other measuring coils 9 have a paddle-shaped cross section), or have a shape different from the described shape.

[0053] As can be seen from the figures, the paddle shape allows the measuring coil 9 to adapt better to the presence of the ferrite 7 than a shape other than a paddle, for example a circular shape.

[0054] Furthermore, even the winding direction of the measuring coil 9 is not restrictive for the present invention. As is well known, the expression "winding direction" refers to the way the coil is wound relative to the extension axis of the measuring coil 9. In particular, in some embodiments, the measuring coils 9 are wound in the same direction, whereas in other embodiments, 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 can be identified. As is well known, in the case of the measuring coils 9 wound in opposite directions to each other, if current flows through the above-mentioned two measuring coils 9 in the same direction, 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 are opposite to each other.

[0055] In plan view, the turns of each first measuring coil 9 enclose a corresponding first area, and in plan view, the turns of each second measuring coil 9 enclose a corresponding second area. In some cases, each first area and each second area have different amplitudes from each other. For example, it is possible that all first areas have equal amplitudes from each other, all second areas have the same amplitudes from each other, and the amplitudes of the first areas are different from the amplitudes of the second areas, or that at least one first area has a different amplitude from another first area and / or at least one second area has a different amplitude from another second area. It should be emphasized that what is described are only examples of possible embodiments of the invention, and therefore, they should not be considered as limiting the invention.

[0056] Furthermore, with regard to the embodiment 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 by no more than 10%.

[0057] Advantageously, this type of embodiment allows 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 to make the measurement of the resistance and / or inductance less sensitive to small defects in the bottom 3 of the container 2 (surface not completely flat, material inhomogeneities, presence of small grooves, etc.).

[0058] In some embodiments in which the device comprises induction heating means (not shown in the figures), each measuring coil 9 overlaps, in plan view, with a heating coil 6. In fact, this type of configuration advantageously allows reducing the influence of the heating coil 6 on the measurement, since it is known that the electromagnetic fields in the immediate vicinity of the conductor are very low (at least compared to the central zone of the heating coil 6).

[0059] Finally, the Applicant reserves the right to independently protect, even by filing a divisional patent application, only a measuring sensor 8 for estimating the temperature of any metallic object, in particular a metallic part of a container 2, in use, wherein the measuring sensor 8 comprises at least one measuring coil 9 and an electronic device 10, and wherein:

[0060] The at least one measuring coil 9 is part of an electrical circuit and comprises a plurality of turns; and

[0061] The electronic device 10 is connected to the circuit to provide the circuit with an alternating voltage of a predetermined frequency between 100kHz and 500kHz in use, and to measure the resistance and / or inductance of the circuit powered at the predetermined frequency between 100kHz and 500kHz in use.

[0062] Optionally, the measurement sensor 8 may also comprise one or more of the features described above in relation to the measurement sensor 8 as part of the device according to the invention.

[0063] The invention brings important advantages.

[0064] Thanks to the invention, it is possible to produce a device for heating and / or cooling a metal object which makes it possible to measure the temperature of the metal object more accurately than in the prior art.

[0065] In particular, thanks to the invention, it is possible to produce an induction cooker that can measure the temperature of a container more accurately than prior art induction cookers.

[0066] In more detail, in the case of an induction cooker, the presence of a plurality of measuring coils allows a more precise measurement of the temperature of a metal object than in the prior art in which there is only a single measuring coil placed in the center of the heating coil. In fact, tests carried out by the applicant have shown that the solution in which there is a single measuring coil allows only a local measurement (which is then very sensitive to variations in the material of the container bottom and its shape): for example, a container bottom that is not completely flat or has grooves (even if only 2 mm deep) leads to variations in the measurement result, the resulting error being too high.

[0067] Similar advantages apply in the case of devices for heating and / or cooling other types of metal objects.

[0068] Finally, it should be noted that the present invention is relatively easy to produce and that even the costs associated with implementing the invention are not very high.

[0069] The invention described above can be modified and adapted in many ways without departing from the scope of the inventive concept.

[0070] All the details may be replaced by other technically equivalent elements, the materials used as well as the shape and dimensions of the various components may vary as required.

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 metal material, the device (1) comprising: a support portion (4), the support portion (4) being configured to support the metal object in use; a temperature variation device, the temperature variation device being configured to act on the metal object placed on the support portion (4); a measuring sensor (8) for estimating, in use, the temperature of the metal part of the metal object, the measuring sensor (8) comprising at least one measuring coil (9) and electronics (10); in: The at least one measuring coil (9) is part of an electrical circuit and comprises a plurality of turns; and The electronic device (10) is connected to the circuit to provide the circuit with an alternating voltage of a predetermined frequency between 100kHz and 500kHz in use, and to measure the resistance and / or inductance of the circuit powered at the predetermined frequency between 100kHz and 500kHz in use.

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) which are electrically connected to each other.

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

5. Device according to claim 4, wherein the measuring coils (9) are placed 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) placed at the vertices of a square.

7. An apparatus according to any one of claims 3 to 6, wherein each measuring coil (9) comprises one or more turns, and wherein in a plan view, each turn of the measuring coil (9) has a circular shape, the diameter of which is between 40 mm and 120 mm, preferably between 50 mm and 90 mm.

8. The device according to any one of claims 3 to 6, wherein each measuring coil (9) of the plurality of measuring coils (9) comprises a number of turns which is equal to or different from the number of turns of another measuring coil (9) of the plurality of measuring coils (9), the number of turns being between 5 and 50.

9. The device according to any one of claims 3 to 6, wherein all measuring coils (9) of the plurality of measuring coils (9) are wound in the same direction.

10. The device according to claim 3, wherein within 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 identified.

11. The device according to claim 10, wherein in a plan view, the turns of each first measuring coil (9) surround a corresponding first area, and in a plan view, the turns of each second measuring coil (9) surround a corresponding second area, and wherein each first area and each second area have a size that is equal to or different from each other.

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

13. The device according to any one of claims 3 to 6, wherein in a plan view, each turn of each measuring coil (9) of the plurality of measuring coils (9) has a circular shape, a paddle shape, an elliptical shape or a boomerang shape.

14. The device according to claim 3, wherein the measuring coils (9) are electrically connected to one another in parallel and / or in series.

15. A device according to any one of claims 1 to 6, wherein the electronic device (10) is connectable to an electronic processing unit, which is configured to receive data about the resistance and / or inductance of the circuit measured by the electronic device (10) and to process these data to estimate the temperature of the metal object.

16. A device according to any one of claims 1 to 6, wherein the electronic device (10) is arranged to power the circuit in use at a predetermined frequency different from a resonant frequency of the circuit.

17. The apparatus of claim 1, wherein: The device is an induction cooker for heating a container (2), the container (2) comprising a metal part made of ferromagnetic material; The supporting portion is a supporting surface (4); The temperature variation device is a heating device (5) comprising a heating coil (6), the heating coil (6) being placed below the support surface (4) and being operatively connectable to and powered by a power source to generate an electromagnetic field; The at least one measuring coil (9) is different from the heating coil (6) and is arranged between the heating coil (6) and the support surface (4).

18. An apparatus according to claim 17 as cited in claim 3, wherein the heating device (5) further comprises a plurality of ferrites (7) placed below the heating coil (6), and wherein in a plan view, each of the plurality of measuring coils (9) is placed offset relative to the ferrite (7), preferably between two adjacent ferrites (7).

19. The device according to claim 17 as dependent on claim 3, wherein in a plan view, each measuring coil (9) of the plurality of measuring coils (9) overlaps the heating coil (6).

20. The apparatus according to any one of claims 1 to 6, wherein the apparatus further comprises the metal object.

21. Apparatus according to any one of claims 1 to 6, wherein the metal object is a container (2) comprising a metal part, and wherein the metal part of the container (2) is a bottom (3) and / or is made of a ferromagnetic material.

22. Apparatus according to any one of claims 1 to 6, wherein the electronic device (10) is connected to the circuit to measure, in use, only the resistance of the circuit.

Images