DE102016202234B3 - Method for selective heating of objects or groups of objects by high-frequency electromagnetic waves - Google Patents

Abstract

The present invention relates to a method of selectively heating objects or groups of objects of dielectric materials, wherein radiofrequency electromagnetic waves are coherently emitted through at least two antennas to selectively heat the objects. In the method, first several test heats are performed to determine the influence of each individual antenna and any combination of antennas on the heating of the objects. The data obtained from the test heats are evaluated to find a relationship between the antenna excitations and the distribution of the temperature rise. Subsequently, an optimization method for determining the antenna excitation is applied, with which the desired selective heating can be achieved. The antennas are finally excited for the selective heating of the objects with the determined excitation parameters for the emission of the electromagnetic waves. With the method, a better focusing power of the heat output is achieved than in non-coherent methods. The method does not require any elaborate simulations for determining the field distributions.

Description

Technical application

The present invention relates to a method for selectively heating objects or groups of objects consisting at least partially of lossy dielectric materials by high-frequency electromagnetic waves, in which the electromagnetic waves are emitted coherently via at least two antennas, so that the electromagnetic waves in a too Coherently overlay the heating object or a group of objects to be heated.

The use of high frequency electromagnetic radiation to heat dielectric materials has a long history and many uses. There are applications in the food industry, in the ceramics, rubber and plastics industries as well as in many specialized areas in the chemical industry.

In many of the above applications, a limitation or control of the RF power distribution (RF: RF) in the heating process is desirable or even required. Thus, in some cases, a uniform distribution of RF power is required, in other cases, such as when cooking different foods in the same cooking space, a targeted focusing or distribution of RF power is desired.

State of the art

For such a selective heating of objects or object groups, for example of food, there are different possibilities. A selective heating of an object is to be understood here as meaning that one or more subregions of the object or of an object group are heated by the electromagnetic radiation, while the remaining subregions are not affected as little as possible by the heating. A selective heating of an object group can also mean that only individual objects of the object group are heated in partial areas or completely.

From the US 2013/0284725 A1 is a method of heating food is known in which non-coherent electromagnetic radiation in the microwave range is used for heating. The microwave power is emitted over a range of frequencies over the individual antennas. In the method, the reflected power is detected during heating and evaluated. Resonances that appear at the reflected power are indicia of the absorption of microwave energy from different materials. As a result, the different foods are to be partially distinguished from one another and the power in this way selectively, so different levels, can be fed into different foods. However, with this method, no portions of a relatively homogeneous medium of the same material can be selectively heated. By using incoherent electromagnetic radiation, only power patterns and no field patterns can be superimposed. An incoherent overlay does not have particularly good focussing power.

Another possibility of a focused focusing or distribution of the RF power is the use of a coherent superposition of the radiation of multiple antennas. A coherent superposition requires a monofrequent, phase-controlled excitation of the individual antennas. For a selective heating, it is necessary to determine the field distributions that are generated by the individual antennas in the object or the object group. This is usually done by simulations. Once the field distributions are known, various optimization methods can be applied to achieve the desired power distribution in the object. For this purpose, suitable amplitudes and phases for the excitation of the individual antennas must then be selected. However, the exact determination of the fields that are generated by the individual antennas or radiation sources in the objects, very complex. The simulation also requires a very detailed CAD model of the geometry of the object to be heated. Furthermore, the dielectric properties of the materials of which the object is composed must be known accurately enough. Electromagnetic simulations are often complex and time-consuming in this context.

The JP 2008-66292 A describes a microwave oven with three antennas, via which an object in the oven is irradiated with microwaves. The frequency of the microwaves is modulated to optimize the absorption in the object. To minimize the reflections of electromagnetic radiation at the furnace chamber, a phase matching of the microwaves is performed.

The object of the present invention is to provide a method for selective heating of objects or groups of objects consisting at least partially of lossy dielectric materials, which allows a relatively accurate selective heating of both similar and differently composed objects and without the expense of Computer-aided modeling or simulation can be performed.

Presentation of the invention

The object is achieved by the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent claims or can be found in the following description and the embodiment.

In the proposed method, a coherent superposition of electromagnetic radiation is selected for the selective heating emitted by two or more antennas. Coherent here means that the antennas are operated at the same frequency and that the phase differences between the excitations of the individual antennas can be determined and controlled. Optionally, the amplitudes of the individual antennas can be determined and controlled. In the proposed method, first several test heats are performed on the object to be heated or the object group to be heated, in which parameters of the antenna excitation for the emission of the electromagnetic waves are varied to heat the object or the object group at different parameters. Among the parameters that are varied in the proposed method, at least one phase difference of the excitation counts in a simultaneous excitation of several of the antennas. In one embodiment of the method, all antennas are excited individually and in pairs during the test warm-ups, and the phase difference of the excitation in the pairwise excitation of the antennas is varied. By a pairwise excitation is to be understood here that - in some of the test warmings - only two of the antennas are excited simultaneously.

Instead of the object to be heated or the object group to be heated, a similar object or a similar object group can be used in the proposed method for the test warmings, which are used only for the test warming. By similar is meant that the object or the object group have the same geometric dimensions and the same material composition and material distribution as the object to be heated later or the object group to be heated later.

Before and after each individual test warming, the temperature of the object or group of objects is detected in order to obtain the temperature-dependent increase in temperature inside and / or at the surface caused by the respective test heating. Depending on the application, the temperature detection can take place, for example, with integrated temperature sensors or also with non-contact techniques such as magnetic resonance tomography. Preferably, only the surface temperature of the object or the object group is detected as temperature at least from one side, preferably from above. The data obtained from the test heats, i. H. In particular, the parameters used for each test warming and the location-dependent temperature rise thus obtained are evaluated in order to determine a quadratic dependence between the antenna excitations and the location-dependent temperature rise. Subsequently, an optimization method is then used to determine based on the determined dependence suitable parameters for the antenna excitation, with which a heating of the object or the object group is achieved, which comes as close as possible to a given selective heating. The selective heating is usually specified by a user who wants to heat the object or the object group selectively. After determining the appropriate parameters, the antennas are then excited to selectively heat the object to be heated or the object group to be heated with the determined parameters. The duration and amplitude of the excitation is chosen according to the desired temperatures and the desired rate of heating, which in turn can be specified by the user.

The proposed method thus consists of four steps. In the first step, a series of test heats of the object (s) to be heated - or of similarly similar objects - is carried out in order to influence the influence of each individual antenna as well as any antenna combination on the heating of the surface and / or the interior of the or to determine the objects. In the second step, the measurements taken during the test heats are evaluated by an algorithm for determining the quadratic dependence between the antenna excitations and the distribution of the temperature rise at the surface and / or inside the object or objects. In the third step, an optimization method for determining the antenna excitations, which best suits the reach the user desired selective heating. The last and fourth step then consists in the feeding of the antennas with high-frequency electromagnetic excitations whose phases and possibly also amplitudes meet the conditions determined in the preceding step. The feed takes place until the desired temperature profile, ie the desired final temperatures, is reached.

With the proposed method, corresponding objects can be selectively heated with higher accuracy than is possible with the non-coherent excitation according to the prior art. While in non-coherent methods only an additive superposition of performance patterns takes place, in the proposed method a linear superposition of the fields of the antennas takes place. This achieves a better focussing power of the heat output over non-coherent methods. This leads to more flexibility for the definition of heating zones and a higher power contrast. In particular, the method also allows the selective heating of homogeneous objects made of the same material. The method does not require a CAD model of the geometry of the objects to be heated and no complex simulation of the field distributions and is thus easier to perform. The effect of the field distributions is rather determined experimentally by the temperature measurements in the proposed method. This has the threefold advantage that the geometry and position of the object or objects need not be measured, that the dielectric properties of the object or objects need not be known, and that the high expenditure of time and the inaccuracies of simulation methods are avoided.

• (i) Das Verhältnis der Oberfläche des jeweiligen Objektes, deren Temperatur bei den Testerwärmungen erfasst wird, zum entsprechenden Objektvolumen ist relativ klein, so dass die Temperatur im Inneren der Objekte aus ihrer Oberflächentemperatur abgeleitet oder geschätzt werden kann.
• (ii) Der Hauptmechanismus der Erwärmung des Inneren des Objekts bzw. der Objekte ist die Wärmeübertragung von der Oberfläche in das Innere und die Wärmeübertragung zwischen dem zu erwärmenden und dem zu schonenden Teilbereich ist wesentlich langsamer. Dies trifft vor allem zu, wenn separate Objekte einer Objektgruppe selektiv erwärmt werden müssen.
• (iii) Das Ziel der Erwärmung betrifft nur die Oberfläche des bzw. der Objekte und die Temperatur im Inneren ist für die jeweilige Anwendung irrelevant.

With the proposed method, when surface temperature only is detected, the best results are obtained if at least one of the following three conditions applies to the objects or object groups to be heated:

• (i) The ratio of the surface of the respective object whose temperature is detected in the test heats to the corresponding object volume is relatively small, so that the temperature inside the objects can be derived or estimated from their surface temperature.
• (ii) The main mechanism of heating the interior of the object (s) is the transfer of heat from the surface to the interior, and the heat transfer between the portion to be heated and that to be protected is much slower. This is especially true when separate objects of a collection need to be selectively heated.
• (iii) The purpose of heating only affects the surface of the object (s) and the temperature inside is irrelevant to the particular application.

In addition to at least one of the above requirements, the dielectric properties of the object (s) to be heated should not be too dependent on their temperature or negligible.

In an advantageous embodiment of the method, if there are a number of N _A ≥ 2 antennas, at least the following N _A ² test warmings or test measurements are carried out. On the one hand, test warmings are carried out in each case with only one single antenna, wherein for each of the N _A antennas such a test warming is carried out at a constant excitation amplitude or alternatively with a non-constant amplitude profile, which is then recorded. On the other hand, a series of test measurements is carried out with pairwise excitation of antennas and a defined phase difference between the antennas. Hereby every possible (pairwise) antenna combination is measured. The amplitudes and phases of the antennas should in each case be constant. If this is not possible, both must be recorded again. The same number of test warmings are then performed with the same antenna combinations again at a different phase difference of the antenna excitations. This then gives a total of the N _A ² -Testerwärmungen or test measurements that are sufficient for the subsequent determination of the quadratic dependencies between the antenna excitations and the location-dependent temperature increase. As a phase difference in the pairwise excitation, for example, once a phase difference of 0 ° and once a phase difference of 90 ° can be selected. However, this is not mandatory. Other phase differences, which can also differ by less than 90 °, are possible.

The detection of the surface temperature of the object or the object group is preferably carried out with one or more infrared cameras having a resolution sufficient for the determination of the selective heating. For example, these infrared cameras can be mounted above the object or object group. From the infrared images of the surface (s) thus obtained, the temperature rise at each individual point of the detected surface can then be determined. In principle, of course, other techniques of detecting the surface temperature of the object or objects are possible.

For the determination of the quadratic dependence between the excitations of the antennas and the location-dependent temperature increase, the known equation for electromagnetic losses in dielectric materials can be used, in which the electromagnetic field strength E and the equivalent electrical conductivity σ of the dielectric material are received. Thus, the heating effect caused by the coherent combination of the radiation of the antennas in each volume element of the object or group of objects can be quantified.

In principle, different optimization methods can be used to determine the suitable parameters for a given selective heating. As a rule, the selective heating is predetermined by a user who specifies the subareas of the object or of the object group that are to be heated. If possible, the remaining areas of the object or of the objects should not be heated by the electromagnetic radiation. In a preferred embodiment of the method, an optimization method is therefore used to determine the appropriate parameters, which does not heat the ratio of average absorbed power density of the electromagnetic waves in the given selective subregions of the object or group of objects to the average absorbed power density in the remaining subregions are to be maximized, if necessary, while adhering to other constraints. Other constraints may include, for example, high uniformity of heating in the one or more selected areas and / or avoidance of hot spots or low hot spots in one or more areas to be conserved. Details of this optimization method are given in the later embodiment.

In one embodiment of the method, therefore, the object to be heated or the objects to be heated are first subjected to the test heats and selectively heated according to the parameters required for the selective heating. In applications in which multiple similar objects or groups of objects are to be heated in succession, it is not necessary to carry out new test measurements or test warmings on the object to be heated or the objects to be heated. Here, in a second embodiment, it is then sufficient to determine the parameters only once for the object or objects, and then to heat all other similar objects selectively using these parameters. Of course, this requires an identical geometric arrangement of the object or objects, as is usually the case in industrial processes.

In the proposed method, it is generally desirable to achieve the most uniform possible heating in the respectively selected subregions. This is not guaranteed by the optimization method used in every case. Therefore, in the selective heating in one embodiment of the method, the antenna excitation is interrupted periodically in order to achieve a more uniform temperature distribution in the sections affected by the selective heating by heat conduction.

Furthermore, it is also possible to repeat the steps carried out before the selective heating at different excitation frequencies - depending on the number of excitation frequencies - one or more times in order to obtain the parameters for different excitation frequencies. This can be advantageous, above all, in the case of a composition of the object or of the object group made of different materials, which absorb very differently at different frequencies. The use of different excitation frequencies can also lead to the improvement of the uniformity of the selective heating, even with relatively similar composite or homogeneous objects. In this way, the selective heating can then be carried out at the different frequencies in each case with the determined parameters.

The excitation frequencies used in the proposed method are preferably in the range between 1 MHz and 100 GHz. Particularly many possible applications are in the area of ISM bands (frequencies reserved for "Industrial, Scientific and Medical" purposes), preferably at frequency bands centered around 896 MHz, 915 MHz, 2450 MHz, 13.6 MHz and 27.2 MHz.

The proposed method can be used for selective heating in different technical fields. An example of this are canteen kitchens and the catering industry. Thus, with the proposed method, for example, in microwave or combi ovens, the food can be selectively heated, for example, to better take into account the properties of the individual ingredients, to better maintain the food value of the food, to accelerate the cooking process, the simultaneous preparation of different dishes or to enable new preparation processes. Another area of application is the heating of objects or object groups in the industry. Examples are the Food processing, for example pasteurization or drying processes, chemical engineering, for example microwave-assisted synthesis and processing of chemical substances or preparation processes for pharmaceutical products, material processing, for example polymerization, heating of liquid and solid bodies, inter alia in the ceramics and steel industry, or sintering , or even novel industrial processes such as the heating of silicon wafers, RF-assisted drilling of non-conductive materials by local microwave heating, deicing of missiles with microwaves, debindering and sintering of ceramics, etc. The method can also be in the environmental technology, for example, for refurbishment and Use decontamination of land or soil or for waste treatment. Of course, this is not an exhaustive list.

If necessary, the method can also be combined with non-coherent excitations by varying the operating frequency or the antenna phases or amplitudes over time. A further extension of the method can be realized by the alternation of periods of coherent irradiation, which differ from each other by different frequencies and / or phase and amplitude relationships.

Brief description of the drawings

The proposed method will be explained in more detail using an exemplary embodiment in conjunction with the drawings. Hereby show:

1 a schematic representation of a structure for the selective heating according to the proposed method;

2 a schematic representation of the implementation of a test heating with excitation of an antenna in the proposed method;

3 a schematic representation of the implementation of a test warming with pairwise excitation of two antennas in the proposed method;

4 an exemplary subdivision of the visible surface of the object to be heated in individual zones in the proposed method;

5 an exemplary selection of heating zones within the object of 4 ;

6 an exemplary subdivision of the visible surface of several objects into individual zones in the proposed method;

7 an exemplary selection of heating zones within the objects of 6 ; and

8th a schematic representation of the selective heating of an object according to the selection of 5 with the proposed method shows.

Ways to carry out the invention

In the following, the individual steps of the proposed method will be explained by way of example with reference to an embodiment.

For the implementation of the method corresponding hardware components are required, as they are highly schematic of the 1 to 3 can be seen. The method requires a multi-channel coherent source 3 high-frequency electromagnetic oscillations. The phases and optionally also the amplitudes of the individual channels of this source 3 must be controllable. There must also be an external or internal mechanism that controls the phases and, optionally, the amplitudes of the individual channels of the source 3 can measure and record. An example of an external mechanism is a network analyzer. The figures show a measuring device for this purpose 4 for the phases and amplitudes of the individual channels of the source 3 , The structure for carrying out the method requires at least two antennas 11 . 12 . 13 to the multi-channel source 3 can be connected. Furthermore, a temperature measuring device, for example an infrared camera 5 , which can measure and record the surface temperature of the object (s) to be heated.

1 shows an exemplary test setup with the multi-channel source 3 , to the three antennas 11 . 12 and 13 are connected. Between the antennas is the object to be heated 2 to recognize. The phases and possibly amplitudes of the individual channels of the source 3 be about the illustrated measuring device 4 recorded.

• – Anregung der Antenne i (i = 1, 2, ..., N_A) mit konstanter Amplitude über einen Zeitraum t_i. Kann die Amplitude nicht konstant gehalten werden, so wird der Amplitudenverlauf a_i(t) für 0 ≤ t ≤ t_i gemessen und aufgezeichnet. Diese Messphase benötigt N_A Testerwärmungen bzw. -messungen.

The first step in the proposed method is to perform the test heats. In the following it is assumed that N _A ≥ 2 antennas are used for the heating. Then, in the present example, the following test warmings or measurements must be carried out:

• - Excitation of the antenna i (i = 1, 2, ..., N _A ) with a constant amplitude over a period t _i . If the amplitude can not be kept constant, the amplitude curve a _i (t) for 0 ≦ t ≦ t _{i is} measured and recorded. This measurement phase requires N _A tester heating or measurements.

• – Gleichzeitige Anregung von Antenne i und j (i = 1, 2, ..., N_A, j = 1, 2, ..., N_A, i ≠ j) mit Amplituden a_i ^(ij) und a_j ^(ij) und Phasenunterschied ϕ₁ (z. B. ϕ₁ = 0°) über einen Zeitraum t₁ ^(ij). Wenn die Amplituden und Phasen nicht konstant oder unbekannt sind, wird deren Verlauf über diesen Zeitraum aufgezeichnet: a_i ^(ij)(t), a_j ^(ij)(t), ϕ₁(t). Diese Messphase benötigt ½-N_A(N_A – 1) Messungen bzw. Testerwärmungen.

2 shows schematically a test measurement for determining the temperature pattern or estimation of the field pattern, which is a single antenna in the object 2 causes. For this purpose, the antenna 11 excited with an amplitude a ₁ (t), which leads to a location-dependent increase in temperature at the surface of the object 2 leads. This effect is in 2 indicated schematically by the three black areas. This location-dependent temperature rise is with the infrared camera 5 captures the object 2 from the perspective 6 detected. The infrared camera 5 draws a picture before excitation of the antenna 11 and immediately after the interruption of the excitation, the temperature rise at the surface of the object 2 to investigate. In this test warming is thus only the antenna 11 operated. This scenario is performed for all antennas N _A. In the lower part of the figure, the amplitude curve a ₁ (t) can be seen, with which the antenna was excited, and by the measuring device 4 is recorded. If the antenna can be operated with known constant amplitude, the recording of the amplitude curve is not required.

• Simultaneous excitation of antenna i and j (i = 1, 2, ..., N _A , j = 1, 2, ..., N _A , i ≠ j) with amplitudes a _i ^(ij) and a _j ^{( ij)} and phase difference φ ₁ (eg, φ ₁ = 0 °) over a period t ₁ ^(ij) . If the amplitudes and phases are not constant or unknown, their history is recorded over this period: a _i ^(ij) (t), a _j ^(ij) (t), φ ₁ (t). This measurement phase requires ½-N _A (N _A -1) measurements or test warm-ups.

• – Gleichzeitige Anregung von Antenne i und j (i = 1, 2, ..., N_A, j = 1, 2, ..., N_A, i ≠ j) mit Amplituden b_i ^(ij) und b_j ^(ij) und Phasenunterschied ϕ₂ (z. B. ϕ₂ = 90°) über einen Zeitraum t₂ ^(ij). Wenn die Amplituden und Phasen nicht konstant gehalten werden können oder unbekannt sind, muss auch hier deren Verlauf über diesen Zeitraum aufgezeichnet werden: b_i ^(ij)(t), b_j ^(ij)(t), ϕ₂(t). Auch diese Messphase benötigt ½N_A(N_A – 1) Messungen.

3 schematically shows an example of one of the test heats with such a pairwise excitation of the antennas 12 and 13 , The location-dependent surface heating caused thereby is again due to the dark areas on the surface of the object 2 exemplified. In the lower part of the figure is the by the measuring device 4 recorded course of the amplitudes a ₂ ⁽²³⁾ (t) of the antenna 12 and a ₃ ⁽²³⁾ (t) of the antenna 13 and to recognize the course of the phase difference φ ₁ ⁽²³⁾ (t). Again, this record is only required if the corresponding quantities can not be kept constant or are unknown. With this test measurement, the temperature pattern can be determined or the field pattern can be estimated, which causes a combination of two specific antennas in the object.

• Simultaneous excitation of antenna i and j (i = 1, 2, ..., N _A , j = 1, 2, ..., N _A , i ≠ j) with amplitudes b _i ^(ij) and b _j ^{( ij)} and phase difference φ ₂ (eg, φ ₂ = 90 °) over a period t ₂ ^(ij) . If the amplitudes and phases can not be kept constant or are unknown, then their course over this period must also be recorded: b _i ^(ij) (t), b _j ^(ij) (t), φ ₂ (t). This measurement phase also requires ½N _A (N _A - 1) measurements.

Overall, therefore, N _A ² test measurements or test warm-ups must be performed. Before and after each of these test heats, an infrared image of the irradiated dielectric objects is taken. With the help of these infrared images, the temperature rise at each individual point of the visible surface of the objects is determined.

• – der einzelnen Antenne i: W_i ^(k)
• – der Antennen i und j mit Phasenunterschied ϕ₁: W_ij ^(k,l)
• – der Antennen i und j mit Phasenunterschied ϕ₂: W_ij ^(k,2)

The two-dimensional view of the objects captured with the infrared images is either automatically or manually subdivided into smaller zones, hereafter referred to as cells. This is in the 4 and 6 once for a single object 2 ( 4 ) or for a group of several objects 21 . 22 . 23 . 24 . 25 and 26 ( 6 ) shown schematically. These figures each show an exemplary subdivision of the objects into individual cells. The shape of the subdivision can look arbitrary, as long as it covers the entire surface of the objects and the individual cells do not share common intersections. In the following it is assumed that there are N _Q cells. In each cell k = 1, 2, ..., N _Q , the temperature rise values must be integrated after each of the measurements described above. This provides an estimate of the heat absorption in each individual cell. To estimate the exact heat that was absorbed, the mass and heat capacity of each cell would have to be known. However, since the goal of the method is to achieve a user-defined temperature increase, these values are irrelevant. The following nomenclature is introduced for the following calculations:
Heat absorption in cell k after stimulation

• The individual antenna i: W _i ^(k)
• - the antennas i and j with phase difference φ ₁ : W _ij ^{(k, l)}
• - the antennas i and j with phase difference φ ₂ : W _ij ^{(k, 2)}

The data obtained in the previous step of the test heats, that is the heat absorption in each cell and, if appropriate, the amplitude and phase response of the individual antennas, are used in the now starting second step of the method to quantify the heat impact of any combination of antenna excitations. It is assumed that all antennas are excited simultaneously with the same frequency, but each with a different amplitude and phase, which can each be time-dependent. Phase φ _i and amplitude a _{i of} the antenna i can be combined into a complex amplitude c _i (t) = a _i (t) · exp [jφ _i (t)]. The heat generated by the linear superposition of the fields of the antennas in cell k is:

Nomenclature:

• - c (t) is the gap vector of the complex excitations c ₁ (t), c ₂ (t), ..., c _NA (t)
• - T is the duration of the warming
• - σ (r) is the (equivalent) conductivity of the dielectric object
• - E _j (r) is the electric field that causes the antenna j at the observation point r.
• - (*) and ( ^H ) mean complex adjoint or
• complex transposition
• - Q _k is the complex matrix with matrix entries
  

The measurements in the test heats carried out in the first step determine the elements of each matrix Q _k . This is achieved by entering the measured values of the amplitude and phase profile of the antenna and the heat absorption for the N _A ² test measurements in the above formula. This results in N _A ² linear equations with N _A ² unknown elements for each matrix Q _k , which can thus be determined.

The test heats can also be carried out with other parameters, for example by different phase combinations with respective excitation of all antennas. In principle, any types of at least N _A ² test heats may be employed as long as a solvable system for the Q-matrix is generated. With a lower number of test heats, the Q matrix can not be uniquely calculated, and then an estimate is needed. This could be done, for example, by using iterative methods.

In the previous second step, the heating effect caused by the coherent combination of the radiation of the antennas was quantified. Following this, in the third step, an optimization algorithm is applied that determines the antenna excitations that produce the desired selective heating effect in the object (s).

For this purpose, the user defines one or more partial areas V _h of the object or objects to be heated. The 5 and 7 show an exemplary selection of heating zones based on the subdivisions of 4 and 6 , The selected zones or cells are indicated by hatching. The remaining area V _c should thus be spared from the heating. The heated area must be composed of all the heating zones or cells selected for this purpose.

For the determination of the suitable antenna excitations for this purpose, an algorithm is used in the present example, the ratio of the average power density in the selected To maximize the heating range to the average power density in the rest of the body, that is, to maximize the following quotient:

I _h and I _c represent the amount of indices of the heating cells that make up the heated or spared area. The individual elements of the complex matrix Q _k are known from the first step and can be used. The following matrices are named:

The sought excitation vector c is the eigenvector of the eigenvalue problem A · c = λB · c corresponding to the largest eigenvalue λ.

An alternative optimization algorithm, which takes into account not only the contrast, but also the uniformity in the area to be heated V _h and the undesired hotspots in the region V _{c to be} protected, may be as follows. For the area where the power must be focused (V _h ), the user selects a (scalable) upper limit (U) and lower limit (L) for the power density, which must not be exceeded or undershot. As a result, the objective of uniformity in the focused area is sought indirectly by the optimization algorithm, since the power density may vary only between the values L and U. In the area to be protected (V _c ), the user selects a further (scalable) upper limit (U '), which must not exceed the power density in this range. U must be larger than U '. This sets two other goals for the optimization algorithm simultaneously: the power density contrast (which is directly related to the temperature rise contrast) must be greater than L / U ', and the unwanted hotspots in the area to be conserved must have a power density less than U'.

For each heating cell with index k, this algorithm defines an N _A -dimensional complex vector F _k = Q _k ^1/2 c, where c is the N _A -dimensional complex vector of the antenna excitations and Q _k ^{1/2 is} the square root of Q Matrix of the cell with index k. The squared Euclidean norm || F _k || ^{2 of} this vector is equal to the power density in this cell when the antennas are excited with phases and amplitudes contained in vector c.

bzw.

In gleicher Weise wird in jeder Zelle des Bereichs V_h kontrolliert, ob||F_k|| < L^1/2. Falls ja, muss der Vektor nach der Formel

skaliert werden. In Zellen, in denen die Randbedingungen vom ursprünglichen Vektor F_k erfüllt werden, wird der Vektor gleichgelassen, d. h. F'_k = F_k.To find the vector c for which the upper and lower limits of the power density are met, the following iterative method can be used. One starts with an arbitrary start vector c ₀ (eg c ₀ = [1 1 1 ... 1] ^T ). The resulting power density in cell k is given by the formula || F _k || ² = C ₀ ^H Q _k c _{0 is} calculated. If the norm of the vector F _k exceeds the square root of the upper limit U or U '(depending on whether the cell k is in the area to be warmed up or conserved), the vector is scaled according to the following formula:

respectively.

Similarly, in each cell of the area V _{h, it is} checked that || F _k || <L ^1/2 . If so, the vector must be according to the formula

be scaled. In cells where the boundary conditions are met by the original vector F _k , the vector is left equal, ie F ' _k = F _k .

The previous procedure generates a new vector F _k 'that satisfies the constraints of the problem. Next, a search is made for a new excitation vector c ₁ which can produce a vector which can best approximate the vector F _k '. This means that one searches for the vector c ₁ , which is the error norm || Q 1/2 / kc ₁ - F _k || minimized. For example, this problem can be solved by the least squares method. The excitation vector c ₁ thus obtained is closer to the optimum excitation vector than the starting vector c ₀ . The same procedure is repeated with c ₁ as the start vector and a new vector c _{2 is} generated. By iterative repetition of the steps described above, a sequence of excitation vectors c ₁ , c ₂ , c ₃ , ..., c _n , ... is generated. The procedure is aborted when a convergence or cancellation criterion is reached. Ie. if one or more quality criteria of the field distribution have converged (eg, contrast, uniformity, hotspots) or if a maximum number of iterations is reached.

The antenna excitations (contained in vector c) determined in the third step determine the coherent feeding of the antennas at the same operating frequency in order to achieve the selective heating of the user-selected subregions. The user then selects only the desired final temperature of the areas to be heated and the speed of heating in the fourth step. The excitations are automatically scaled accordingly and the object or objects are selectively heated with the determined parameters of the antenna excitation.

If the signal source can not by itself keep the amplitudes and phase differences of the antennas constant, a control unit should be used which adapts the measured values to the setpoints from the third step. 8th shows an exemplary embodiment, in which additionally a control unit 7 is used, which controls (digital or analog) the amplitudes and phases of the signal source 3 on the basis of the measuring device 4 measured values and the setpoints, which were determined by the optimization algorithm of the third step. The figure shows here also the selective heating of the object 2 of the 4 with the determined parameters, the heated areas being indicated here again hatched.

Should the power density, which results from the optimization algorithm of the third step, be distributed too unevenly in the areas to be heated, then a periodic shutdown of the antenna operation can take place, the compensation of the temperature distribution by means of the effect of heat transfer inside the or the objects serves.

LIST OF REFERENCE NUMBERS

2

object

3

Multichannel coherent source of electromagnetic oscillations

4

Measuring device for measuring the phases and amplitudes

5

Infrared camera

6

Viewing angle of the infrared camera

7

additional control

11-13

antennas

21-26

Objects of a collection

Claims (13)

Method for selective heating of objects or object groups consisting at least partly of lossy dielectric materials by high-frequency electromagnetic waves, in which the high-frequency electromagnetic waves are transmitted via at least two antennas ( 11 . 12 . 13 ) are emitted coherently, so that the electromagnetic waves in an object to be heated ( 2 ) or a group of objects to be heated ( 21 - 26 coherently superimpose, wherein first - several test heats on the object to be heated ( 2 ), the object group to be heated ( 21 - 26 ) or in each case a similar object or a similar object group, in which parameters for an excitation of the antennas ( 11 . 12 . 13 ) are varied to radiate the electromagnetic waves to the object ( 2 ) or the object group ( 21 - 26 ) each at different parameters, the at least one phase difference of the excitation with simultaneous excitation of several of the antennas ( 11 . 12 . 13 ), - a temperature of the object before and after each test heating ( 2 ) or the object group ( 21 - 26 ) is detected to a location-dependent temperature rise in the object caused by the respective test heating ( 2 ) or the object group ( 21 - 26 ) or on the surface of the object ( 2 ) or the object group ( 21 - 26 ), - data obtained from the test warmings are evaluated to determine a dependence between the antenna excitations ( 11 . 12 . 13 ) and the location-dependent increase in temperature, and - an optimization method is used to determine suitable parameters for the excitation of the antennas on the basis of the determined dependence ( 11 . 12 . 13 ), with which a heating of the object ( 2 ) or the object group ( 21 - 26 ), which comes at least close to a given selective heating, and after the determination of the suitable parameters - the antennas ( 11 . 12 . 13 ) for selectively heating the object to be heated ( 2 ) or the object group to be heated ( 21 - 26 ) are excited with the determined parameters. Method according to Claim 1, characterized in that the antennas ( 11 . 12 . 13 ) are excited at least individually and in pairs at the test warming. A method according to claim 1 or 2, characterized in that at a number of N _A ≥ 2 antennas at least N _A ² test heats are performed. Method according to one of Claims 1 to 3, characterized in that, for a number of N _A ≥ 2 antennas, the following test heats are carried out: - excitation of only the antenna i (i = 1, 2, ..., N _A ) with a constant amplitude over a period of time t _i for all antennas; Simultaneous excitation of antenna i and j (i = 1, 2, ..., N _A , j = 1, 2, ..., N _A , i ≠ j) with amplitudes a _i ^(ij) and a _j ^{( ij)} and phase difference φ ₁ over a period t ₁ ^(ij) for all pairwise combinations of the antennas; and - Simultaneous excitation of antenna i and j (i = 1, 2, ..., N _A , j = 1, 2, ..., N _A , i ≠ j) with amplitudes b _i ^(ij) and b _j ^(ij) and phase difference φ ₂ ≠ φ ₁ over a period t ₂ ^(ij) for all paired combinations of the antennas. Method according to one of claims 1 to 4, characterized in that the surface temperature as the temperature of the object ( 2 ) or the object group ( 21 - 26 ) of at least one side with one or more infrared cameras ( 5 ) is detected. Method according to one of Claims 1 to 5, characterized in that the determination of the dependence between the excitations of the antennas ( 11 . 12 . 13 ) and the location-dependent temperature increase based on an equation for electromagnetic losses in dielectric materials. Method according to one of claims 1 to 6, characterized in that an optimization method is used to determine the appropriate parameters, which is a ratio of average absorbed power density of the electromagnetic waves in one or more predetermined selected areas of the object ( 2 ) or the object group ( 21 - 26 ) to the average absorbed power density in remaining areas of the object ( 2 ) or the object group ( 21 - 26 ), possibly while maintaining other constraints, maximized. A method according to claim 7, characterized in that as a further boundary condition (s) a high uniformity of heating in the one or more selected areas and / or avoidance of hotspots or low hotspots in one or more areas to be protected are specified. Method according to one of claims 1 to 8, characterized in that for a given temperature and / or heating speed required amplitudes of the excitation of the antennas ( 11 . 12 . 13 ) calculated from the data of the test warmings and the antennas ( 11 . 12 . 13 ) for selectively heating the object to be heated ( 2 ) or the object group to be heated ( 21 - 26 ) are excited with the calculated amplitudes. Method according to one of claims 1 to 9, characterized in that the excitation of the antennas ( 11 . 12 . 13 ) for selectively heating the object to be heated ( 2 ) or the object group to be heated ( 21 - 26 ) is interrupted periodically to achieve a more uniform temperature distribution in areas affected by the selective heating. Method according to one of claims 1 to 10, characterized in that the steps of the test warming up to the determination of the suitable parameters upon excitation of the antennas ( 11 . 12 . 13 ) are repeated at one or more different frequencies to allow selective heating of the object to be heated ( 2 ) or the object group to be heated ( 21 - 26 ) with different frequencies. Method according to one of claims 1 to 10 for the selective heating of objects ( 2 ) or object groups ( 21 - 26 ), especially of food, in catering. Method according to one of claims 1 to 10, for the selective heating of objects ( 2 ) or object groups ( 21 - 26 ) in the industry.

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