WO2002034013A1 - Device for heating a material by microwave application - Google Patents

Abstract

The invention concerns a microwave heating device designed for use in an oven for cooking food. Said device comprises a microwave generator (1) and means for diffusing (2) said energy in a working zone (3). The invention is characterised in that at least a patch antenna (10) adapted to sense the microwaves is arranged in the working zone. Said antenna is connected to an electromagnetic wave propagating line (11) of length L to ensure propagation of the microwaves sensed by the antenna, and whereof the end (12) is adapted to reflect part of said waves towards the antenna. The antenna (10) then retransmits said reflected waves into the chamber (26) with a specific phase shift determined by the length L of the propagation line.

Description

 DEVICE FOR HEATING A MATERIAL BY APPLYING

MICROWAVE

The present invention relates to a device for heating a material by applying microwave energy comprising a microwave generator and means for diffusing this energy in a working area. The invention relates in particular, but not exclusively, to household microwave ovens intended for cooking food.

The devices for heating a material by applying microwave energy come up against the problem of the distribution of the intensity of the microwave energy in the working area. In fact, it is difficult to obtain a homogeneous distribution of the microwave energy. In addition, the microwave reflections cause the presence of standing waves with areas where the intensity of the microwave energy is low, and therefore where the heating is insignificant. A first solution used in microwave ovens is to place a wave stirrer called "stirrer" in the microwave field. A conventional stirrer has a rotating propeller, the blades of which stir and randomly reflect the waves in the oven cavity. Another solution commonly used these days is to place the food on a turntable so that it successively crosses the areas of low and high microwave energy. These solutions are not fully satisfactory, especially in the central region of the cavity where sufficient energy is not obtained. Thus, it is practically impossible to make a pizza without using a special accessory allowing a better distribution of the waves. These solutions also do not make it possible to modify the distribution of the micro- waves depending on the type of food or during the cooking process. In addition, the motor and the drive mechanism of a turntable entail a significant additional manufacturing cost.

The object of the present invention is to remedy these drawbacks by controlling the distribution of microwave energy in the work area and by controlling the distribution of standing waves.

According to the invention, at least one antenna of the plate type suitable for picking up microwaves is placed in the working area, said antenna being connected to a line of propagation of electromagnetic waves of length L so as to ensure propagation microwaves picked up by the antenna, the end of said propagation line being adapted to reflect at least part of these waves towards the antenna.

The antenna then re-emits the reflected microwaves at the end of the propagation line with a determined phase shift relative to the incident microwaves. This phase shift, determined by the length L of the propagation line, can be chosen so as to obtain a constructive superposition of the incident waves and the re-emitted waves. Thanks to this arrangement, the microwave energy can be intensified in a chosen region of the working area, in particular in order to obtain a more homogeneous general distribution of the microwaves.

According to another particularly advantageous characteristic of the invention, the length of the propagation line is variable between the length L and a length L + ΔL thanks to at least one actuator modifying the position of the end of the propagation line. This arrangement makes it possible to modify the intensity of the energy microwave in a given region based on heating needs. For example, in the case of cooking a food, the length of the propagation line L is modified according to the nature of the food, its shape, its position in the working area or the cooking progress. This arrangement also makes it possible to vary the position of the standing waves during the heating operation.

Other characteristics and advantages of the invention will emerge from the description which follows, given by way of nonlimiting example, with reference to the appended drawings in which:

- Figure 1 is a vertical sectional view of an oven comprising a heating device by applying microwave energy produced according to the invention;

- Figures 2a and 2b are top views of two forms of plate antenna implemented to realize the invention;

- Figure 3 shows a graph of a temperature rise reading of a 25-bin tray placed in a microwave oven of the prior art. - Figure 4 shows the same reading graph made in the previous oven to which the device of the invention has been added.

- Figure 5 illustrates another embodiment of a heating device in which there is shown schematically a perspective view of a hearth of a cooking chamber comprising an array of plate type antennas.

As shown in FIG. 1, the device for heating a material by applying microwave energy comprises a microwave generator 1 and means 2 for diffusing this energy in a zone of work 3. The microwave generator 1 is a magnetron whose antenna 4 radiates in a waveguide 5 which makes it possible to diffuse the microwave energy in the work area 3 via an opening 6. The magnetron 1 is connected to a current supply 7 of known type. Of course, it is possible to use other types of microwave generator, and in particular solid state generators, without departing from the scope of the invention. To make a household appliance for cooking food, a microwave generator is used at 2.45 GHz frequency, but the invention can be implemented with generators working at other frequencies and in particular at 915 MHz for industrial applications.

According to the invention, at least one antenna 10 of the plate type suitable for picking up microwaves is placed in the working area 3, said antenna being connected to a line of propagation of electromagnetic waves 11 of length L so as to ensuring the propagation of the microwaves picked up by the antenna, the end 12 of said propagation line being adapted to reflect at least part of these waves towards the antenna 10.

Plate antennas, called "patch antenna" in English, are well known. They generally comprise one or more substantially flat radiating plates 13 of conductive material arranged parallel to a conductive surface 14 connected to ground, called ground plane. The insulation between the radiating plate, forming the antenna proper, and the ground plane can be achieved by a dielectric material or air. Most often the radiating plate is rectangular, but as we will see later we can use other shapes. The dimensions of the radiating plate must be adapted to capture the microwaves emitted in the work area. For this, the longitudinal and transverse dimensions of the plate are preferably between λ / 4 and λ, λ being the wavelength of the microwaves diffused in the working area. Naturally, this antenna adapted to pick up microwaves can also emit microwaves at the same frequency.

The electromagnetic propagation line 11 generally comprises a conductor 15 disposed near a surface 16 connected to ground. The insulation between the two can be achieved by dielectric material or air.

The end 12 of the propagation line is adapted to ensure good reflection of the electromagnetic waves. For this, the end 12 can be left free in the air. But preferably the end is in contact with a surface 17 perpendicular to the line of propagation and connected to ground. This gives a reflection of a large part of the electromagnetic waves towards the antenna. This reflection is different from that which occurs on the wall 20 of the enclosure. In fact, in the case of the wall of the enclosure, the absence of a conductor implies that the reflected electric field is in phase opposition with the incident electric field, which results in a significant loss of energy.

The antenna 10 is connected to the propagation line 11 by providing a contact point 21 between the radiating plate 13 and the conductor 15 of the propagation line. Its position is chosen so as to ensure good transmission to the propagation line 11 of the electromagnetic waves picked up by the antenna 10. Preferably the contact point 21 is located on the underside of the radiating plate 13 between its center and its periphery, as best seen in Figures 2a and 2b.

The antenna 10 thus placed in the work area picks up the incident microwaves arriving at its surface and transmits them to the propagation line. At the end 12 of the propagation line the waves are reflected towards the antenna. The antenna 10 then re-emits these waves in the work area 3 with a certain phase shift ΔΦ with respect to the incident waves given by the following formula:

ΔΦ = (4π / λ) x L

Where, λ corresponds to the wavelength of the microwaves scattered in the working area and L corresponds to the length of the propagation line 11, i.e. the length of the conductor 15 from its point of contact with the plate radiating 13 to the end 12 of the line. The choice of the length L therefore determines the phase shift ΔΦ between the incident wave and the re-emitted wave. If one chooses L so that the incident waves and the re-emitted waves are in phase opposition one obtains a destructive superposition of the waves and therefore a reduction of the microwave energy in the region of the antenna. On the other hand if one chooses L so as to obtain a constructive superposition of the incident waves and the retransmitted waves, one obtains an intensification of the microwave energy in the region of the antenna, therefore a higher capacity of heating of the material. Of course all the choices of the length L between these two situations are possible.

Advantageously, the shape of the plate antenna 10 has a shape suitable for emitting a circularly polarized field. There are various forms plate antennas to obtain such a field. These antennas are generally characterized by substantially equal longitudinal and transverse dimensions, as shown in Figures 2a and 2b showing two forms of circularly polarized antenna 10,10 '. The first form of antenna 10 shown in FIG. 2a is square with two opposite cut corners 22,23. The second antenna shape 10 'shown in Figure 2b is circular with two rectangular notches 22', 23 'diametrically opposite. The field created by this type of antenna performs a rotational movement and allows good distribution of microwave energy in the work area.

Thanks to such a device, the distribution of the microwave energy in the working area can be modified in a safe manner, either by intensifying this energy or by decreasing it. This invention can be used for many microwave heating devices and in particular for household appliances for cooking food such as ovens.

To make such an oven, as shown diagrammatically in FIG. 1, the working area 3 is delimited by a microwave-tight cooking enclosure 26, the wall 20 of which is connected to ground comprises at least one vault 27 and a hearth 28, and by a door, not shown, for introducing food. Said enclosure 26, the microwave generator 1 and programming means 29 for the operation of the generator 1 are housed in a casing 30 forming the housing of the oven.

For the programming means 29, it is possible to use all the devices known for this type of household appliance. It can be a simple timer or a more complex programming system controlling the power and operating cycles of the microwave generator according to the type and condition of the food to be cooked. In this case the programming means 29 are connected to keys, not shown, making it possible to select the appropriate program.

Preferably, the antenna 10 is separated from the food to be cooked by a protection 31 transparent to microwaves. This avoids any projections on the antenna 10 which could damage it or modify its characteristics. Currently, the protection 31 is produced by a glass plate extending parallel to the sole 28. Of course, the invention does not exclude the use of a turntable disposed above the protection 31.

As shown in FIG. 1, part 14 of the wall of the enclosure 26 is used to produce the ground plane of the antenna 10, this makes it possible to minimize the space requirement of the antenna 10 in the cooking enclosure and reduce manufacturing costs.

The insulation between the radiating plate 13 and the ground plane 14 is provided by an air space which allows to withstand significant powers and which guarantees a better longevity than a dielectric material.

The radiating plate 13 of the antenna can be kept at a distance d from the part 14 of the wall of the enclosure 26 by insulating supports. However, in the preferred embodiment, a single conductive support 32 is used which extends from an electromagnetically neutral point 33 of the antenna, to the part 14 of the wall. This electromagnetically neutral point generally corresponds to the geometrical center of the antenna as shown by the references 33 and 33 ′ in FIGS. 2a and 2b. Thus the mounting of the antenna is simplified.

The length L of the propagation line 11 is calculated so that the superposition of the microwaves scattered in the enclosure and the microwaves re-emitted by the antenna achieves a homogeneous distribution of the microwaves in the enclosure 26.

According to a particularly advantageous characteristic of the invention, the length of the propagation line 11 is variable between the length L and a length L + ΔL thanks to at least one actuator 35 modifying the position of the end 12 of the propagation line . As can be seen in FIG. 1, the end 12 of the line can pass from position 36 to position 37 shown in dotted lines. Thanks to this arrangement, different distributions of microwave energy are obtained in the enclosure.

Advantageously, the actuator 35 is connected to the programming means 29 which comprise control means 39 for said actuator and storage means 38 for the different lengths of the propagation line adapted to the progress of the different cooking programs.

For example, to defrost you may want a higher intensity of microwave energy in the center of the food, while to bake a cake it is preferable to have a homogeneous distribution of microwave energy. The distribution of microwave energy can also be changed depending on the shape or position of the food. Depending on the program selected by the user, the programming means 29 select from the stored lengths the length L suitable for this program. This value is then transmitted by the control means 39 as a setpoint to the actuator 35. The programming means 29 can also vary the length of the propagation line 11 cyclically during the heating operation. This arrangement allows to move the standing waves that appear in the enclosure and we can then do without the use of a turntable. Of course, all the algorithms for controlling the variation in the length of the propagation line are possible.

The variation in length ΔL necessary to cover all the desired microwave energy repair configurations is less than or equal to a quarter of the wavelength λ, or 3 cm for a frequency of

2.45 GHz of the microwaves diffused in the enclosure.

In the preferred embodiment, the propagation line 11 is a coaxial line of circular cross section, using air as an insulator between its conductor 15 and a peripheral tubular shield formed from at least the surface 16. The end 12 of the line is produced by an annular conductive piston 41 occupying the space between the central conductor 15 and the shield 16 of the line. This piston 41 is movable along the coaxial line 11 by means of the linear actuator 35. The front face 17 of the piston 41 thus arranged in the propagation line 11 forms a surface connected to ground against which a substantial portion of the electromagnetic wave is reflected and which constitutes the end of the line. This device is particularly interesting because it makes it possible to continuously vary the length of the line of spread and it is relatively simple to implement. But of course one can use other means, for example one can use a telescopic coaxial line or a means of connection of the line of length L to a portion of line of length ΔL.

The antenna 10 can be arranged in any region of the enclosure 26 where an electromagnetic field prevails. However, it is preferably arranged in the central region of the sole 28. In fact, as can be seen in FIG. 3, the microwave energy is generally weak above this region A / B _/ C _/ D _/ EjF with a prior art microwave oven. By arranging an antenna 10 according to the invention in the central region of the sole 28, a more intense heating energy is obtained in the region A, B, C, D, E, F as can be seen in FIG. 4.

According to another embodiment of the invention, represented in FIG. 4, the cooking enclosure 26 comprises an array 50 of n plate antennas Al, A2, ..., An respectively connected to propagation lines 51 , 52, ..., 65 of lengths L1, L2, ..., l> n. In the case shown, this network consists of 25 antennas arranged in a square. This network 50 can be arranged parallel to all sides of the wall 20 of the cooking enclosure 26. The different lengths L1, L2, ..., Ln of the propagation lines are calculated so as to obtain a constructive superposition of the micro- waves re-emitted at a point 49 of the enclosure, called focal point, where a particularly high microwave energy is obtained. Preferably, this array of antennas can constitute the bottom of an oven and is protected for example by a screen transparent to microwaves. These antennas A1, A2, ..., An can be connected to propagation lines of variable length, as previously described. Advantageously, the lengths L1, L2, ..., Ln of the propagation lines 51, 52, ..., 65 can vary thanks to actuators 71, 72, ..., 85 modifying the position of their ends, by so as to move the focus point 49 during the cooking operation. Thanks to this arrangement, it is possible to sweep the volume of the food and thus ensure uniform cooking.

Claims

1. A device for heating a material by applying microwave energy, comprising a microwave generator (1) and means (2) for diffusing this energy into a work area (3), characterized in that '' at least one antenna (10) of the plate type suitable for picking up microwaves is placed in the working area (3), said antenna being connected to a line of propagation of electromagnetic waves (11) of length L of so as to ensure the propagation of the microwaves picked up by the antenna, the end (12) of said propagation line being adapted to reflect at least part of these waves towards the antenna (10). 2. Heating device by applying microwave energy according to claim 1, characterized in that the plate type antenna has a shape adapted to emit an electromagnetic field with circular polarization. 3. Heating device by application of microwave energy according to claim 1 or 2, characterized in that the end (12) of the propagation line (11) is in contact with a surface (17) connected to the mass. 4. Heating device by applying microwave energy according to any one of claims 1 to 3, characterized in that the working area is delimited by a microwave-tight cooking enclosure (26), the wall of which (20) connected to ground comprises at least one vault (27) and a sole (28), and by a food introduction door, said enclosure (26), the microwave generator (1) and means of programming (29) of the generator operation being housed in a casing (30) forming the housing of an oven. 5. A device for heating by applying microwave energy according to claim 4, characterized in that the antenna (10) is separated from the food to be cooked by a protection (31) transparent to microwaves. 6. Heating device by application of microwave energy according to claim 4 or 5, characterized in that the ground plane of the antenna (10) consists of a part (14) of the wall of the enclosure (26). 7. Heating device by application of microwave energy according to any one of claims 4 to 6, characterized in that the radiating plate (13) of the antenna (10) is maintained at a distance d from the wall of the enclosure (26) by a conductive support (32) extending from a point ( 33) electromagnetically neutral from the antenna to the wall. 8. Heating device by applying microwave energy according to any one of claims 4 to r characterized in that the length L of the propagation line is calculated so that the superposition of the microwaves scattered in the enclosure (26) and microwaves re-emitted by the antenna (10) achieves a homogeneous distribution of the microwaves in the enclosure. 9. Heating device by application of microwave energy according to any one of claims 4 to characterized in that the length of the propagation line is variable between the length L and a length L + ΔL thanks to at least one actuator (35) modifying the position of the end (12) of the propagation line (11) . 10. A device for heating by application of microwave energy according to claim 9, characterized in that the actuator (35) is connected to the programming means (30) which comprise control means (39) of said actuator and storage means (38) of the different lengths of the propagation line adapted to the progress of the different cooking programs. 11. Heating device by application of microwave energy according to claim 9 or 10, characterized in that the variation ΔL of the length of the propagation line is less than or equal to a quarter of the wavelength of the microphones. -waves broadcast in the working enclosure. 12. Heating device by application of microwave energy according to any one of claims 9 to 11, characterized in that the line of propagation of electromagnetic waves (11) is a coaxial line whose end (12) is produced by an annular conductive piston (41) occupying the space between its central conductor (15) and its peripheral shielding (16) and movable along the coaxial line by means of a linear actuator (35). 13. Heating device by application of microwave energy according to any one of claims 4 to 12 characterized in that the antenna is arranged in the central region of the sole (28). 14. A device for heating by application of microwave energy according to any one of claims 4 to 12, characterized in that the cooking enclosure (26) comprises an array (50) of n plate antennas (Al, A2, ..., An) respectively connected to propagation lines (51, 52, ..., 65) of lengths L1, L2, ..., Ln, these lengths being calculated so as to obtain a constructive superposition of the microwave re-emitted at a point (49) of the enclosure called focal point. 15. Heating device by application of microwave energy according to claim 14, characterized in that the lengths L1, L2, ..., Ln, of the propagation lines (51, 52, ..., 65) can vary by means of actuators (71, 72, ..., 85) modifying the position of their ends, so as to move said focal point (49) during the cooking operation.