Patent Grant
8338761
1. Field of the Invention
The present invention relates to the field of microwave heating. In particular, the present invention relates to a microwave heating device comprising a feeding structure enabling the device to operate in substance independently of the load to be heated.
2. Description of the Related Art
The art of microwave heating involves feeding of microwave energy into a cavity. When heating a load in the form of e.g. food by means of a microwave heating device, there are a number of aspects which have to be considered. Most of these aspects are well-known to those skilled in the art and include, for instance, the desire to obtain uniform heating of the food at the same time as a maximum amount of available microwave power is absorbed in the food to achieve a satisfactory degree of efficiency. In particular, the operation of the microwave heating device is preferably independent of, or at least very little sensitive to, the nature of the load to be heated.
In European patent EPO478053, a microwave heating device in the form of a microwave oven cavity being supplied with microwaves via an upper and a lower feed opening in a side wall of the oven cavity is disclosed. The supply is made via a resonant waveguide device having a Q-value which is higher than the Q-value/s of the loaded cavity. The waveguide is so dimensioned that a resonance condition is established in the waveguide device. The resonance condition gives a phase lock of the microwaves at the respective feed openings, where the phase lock preferably is in synchronism with the desired cavity mode/s.
The present invention provides a microwave heating device with reduced dependency on the nature of the load to be heated and/or to alleviate limitations in terms of flexibility with regard to the feeding of the microwaves.
According to an aspect of the present invention, a microwave heating device is provided. The microwave heating device comprises a cavity arranged to receive a load to be heated and a feeding structure for feeding microwaves into the cavity. The feeding structure comprises a transmission line for transmitting microwave energy generated by a microwave source and a resonator arranged at the junction between the transmission line and the cavity for operating as a feeding port of the cavity. The dielectric constant of the material constituting the interior of the resonator and the dimensions of the resonator are selected such that a resonance condition is established in the resonator for the microwaves generated by the source and impedance matching is established between the transmission line, the resonator and the cavity.
A resonator may be arranged at the junction between the transmission line and the cavity for operating as a feeding port in order to achieve a stable field pattern in the cavity. Advantageously, an adequate and stable matching is also provided. The dielectric constant of the material constituting the interior of the resonator and the dimensions of the resonator are selected such that a resonance condition is established in the resonator for the microwaves generated by the source and impedance matching is established between the transmission line, the resonator and the cavity. In this way, a resonator having a high Q-value, in particular higher than the Q-value/s of a loaded cavity, is provided at the junction between the transmission line and the cavity. The present invention provides a microwave heating device which is in substance independent of, or at least very little sensitive to, the load (or nature of the load) arranged in the cavity. In particular, the microwave heating device is very little sensitive to load variation.
Further, as compared to e.g. a cavity fed via a regularly sized aperture without any resonator (i.e., an air-filled waveguide connected to the cavity), the present invention provides a more stable heating device is provided. The heating device may be operated at a stable frequency in substance independently of (or at least less dependent of) the load arranged in the cavity.
Further, because of transmitting properties, the use of a resonator facilitates the impedance matching between the transmission line and the cavity.
The present invention further provides a microwave heating device having a feeding aperture (or feeding port) of smaller dimensions than conventional feeding apertures, thereby resulting in feeding of a “cleaner” mode, i.e. preferably a single mode, in the cavity. For example, the present invention enables the reduction of the feeding aperture from the standard size of minimum 61 mm (the normal size being approximately 80-90 mm) to about 6-20 mm.
Further, to ensure feeding of a single mode in the cavity, as the design of the resonator determines its transmitting properties, the cavity may be designed in accordance with the design of the resonator to support a mode corresponding to the frequency at which the microwaves are fed into the cavity.
According to an embodiment, the material constituting the interior of the resonator has a dielectric constant greater than that of the material constituting the interior of the transmission line and the cross-sectional dimension of the resonator is selected such that it is smaller than that of the transmission line. As will be illustrated in more detail in the following, the size of the resonator, i.e. the size of the feeding port, is scaled down with the square root of the dielectric constant (√{square root over (∈)}) of the material constituting the interior of the resonator.
For example, the dielectric material constituting the interior of the resonator may be a ceramic, such as e.g. aluminum dioxide (Al2O3), titanium dioxide (TiO2) and different titanates e.g. magnesium titanate (MgTiO3) and calcium titanate (CaTiO3). Advantageously, the dielectric constant (∈) is comprised in the range of 3-150 and is preferably higher than 10.
Optionally, the resonator may be coated with a metal, which is particularly advantageous if the constant of the dielectric material is relatively low, for instance in the order of 10, for avoiding, or at least reducing, microwave leakage from the resonator. However, if the dielectric constant is relatively high, for instance in the order of 80-90 (such as for example TiO2), a metal coating is not necessary.
According to another embodiment, the microwave source is a solid-state microwave generator comprising semiconductor elements. The advantages of a solid-state microwave generator comprise the possibility of controlling the frequency of the generated microwaves, controlling the output power of the generator and an inherent narrow-band spectrum.
It will be appreciated that the transmission line may be a standard one such as, e.g., a waveguide, a coaxial cable or a strip line.
The resonator is an elongated piece of dielectric material having the same type of cross-sectional shape as that of the transmission line. For example, the resonator and the transmission line may have a cylindrical or rectangular cross-section. However, the resonator typically has smaller dimensions.
According to an embodiment, the microwave heating device may further comprise at least one additional feeding structure and microwave source, such as any of the feeding structures and microwave sources defined above, for feeding microwaves in the cavity via an additional resonator. In addition to the microwave heating device having low sensitivity to the nature of the load, this embodiment provides a cavity fed from two apertures (or feeding ports) with a reduced crosstalk compared to other microwave heating devices.
The microwave sources are respectively operated at different frequencies. In the case of a microwave heating device comprising two feeding structures, the cavity of the microwave heating device is excited with two different frequencies via two feeding ports, respectively. Operating the microwaves sources at different frequencies is particularly advantageous for reducing crosstalk. For example, in the case of a cavity comprising, e.g., two feeding structures, a first feeding structure comprises a first resonator configured to transmit microwaves at a well-defined first frequency F1 while the second feeding structure comprises a second resonator configured to transmit microwaves at a well-defined second frequency F2. The second resonator is somewhat configured to block, or at least strongly limit, the transmission through itself of the microwaves fed into the cavity from the first feeding port. This reduces significantly crosstalk between the two feeding ports. In addition, it will also in substance prevent transmission of unwanted frequencies, harmonics and sub-harmonics, i.e. electromagnetic compatibility (EMC).
Although the above example is described with a cavity comprising two feeding structures or resonators, it will be understood that the same principle applies for, and the same advantage with respect to the reduction of cross-talk may be obtained with, a cavity comprising more than two feeding structures.
In the case of a microwave heating device comprising two feeding ports, the feeding ports may be orthogonally arranged at the walls of the cavity. Particularly if the microwaves transmitted from the two feeding ports have the same frequency. In general, for more than one feeding structure, the location of the feeding ports at the walls of the cavity may be optimized to achieve a uniform heating pattern.
Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.
The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, in which:
All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested.
As an introduction to the concept of the present invention,
Using the coordinate system (x, y, z) represented in
In the structure 1 described with reference to
As an example, the resonant waveguide 20 is assumed to be a waveguide filled with Aluminum Oxide, Al2O3, whose dielectric constant (∈) is assumed to be equal to 9. The resonant waveguide or ceramic-filled waveguide 20 is further assumed to be coated with metal in order to avoid, or at least minimize, microwave leakage. It is noted that if the dielectric constant was significantly higher, it would not be necessary to assume the presence of a metal coating as the energy leakage would be strongly evanescent.
The dimensions of the waveguide 20 are chosen to provide resonance conditions, i.e. to form a resonator 20. For minimizing reflection at the junction between the two air-filled transmission lines, the impedances need to be matched (i.e., sufficiently close). The equation for the characteristic impedance Z0 for a propagating mode in a waveguide is expressed as:
where η is the impedance for free space (equal to 120 π), fc is the cut-off frequency for the propagating mode in the waveguide, f is the frequency of operation and f is larger than fc (f>fc) if the mode propagates.
In view of equation 1, it is preferred to accomplish the same, or at least almost the same, cut-off frequencies in all three waveguides, thereby providing a junction with very low reflection. For obtaining the same cut-off frequencies, the width of the resonant waveguide needs to be scaled with the square root of its dielectric constant √{square root over (∈)} in comparison with the width of the air-filled waveguide. In the present example, assuming an air-filled waveguide having a width of 80 mm, the width of the resonant waveguide (or resonant body) is equal to approximately 26.67 mm
when Al2O3 (∈=9) is used as the dielectric material inside the resonator.
In the present example, where both ends of the structure 1 are open, the length of the resonant waveguide cannot be directly selected to be a whole number of half-wavelength to accomplish resonance (at a specific frequency) in the resonant waveguide 20. Instead, e.g. in the case of the TE102 mode, the length needs to be larger than one wavelength. This is the necessary condition to have resonance in a resonator completely enclosed by metal. The length of the resonator is, in this case for the TE102 mode, selected to be 38.5 mm and the height is arbitrarily selected to be 10 mm, thereby resulting in a resonance close to the center of the ISM band 2.4-2.5 GHz.
The above example illustrates the concept of the present invention using a waveguide structure 1 comprising two air-filled transmission lines and a resonant waveguide. In the microwave heating device of the present invention, the second transmission line corresponds to a cavity, and the first transmission line and the resonant waveguide correspond to the feeding structure for feeding microwaves into the cavity.
With reference to
The microwave oven 300 comprises a cavity 350 defined by an enclosing surface. One of the side walls of the cavity 350 may be equipped with a door (not shown) for enabling the introduction of a load, e.g. a food item, in the cavity 350.
The microwave oven 300 comprises a feeding structure 325 for feeding microwaves into the cavity 350 via a single feeding aperture 320a. The feeding structure comprises a transmission line 330 for transmitting microwave energy generated by a microwave source 310. The feeding structure further comprises a resonator 320 arranged at the junction between the transmission line 330 and the cavity 350 for operating as a single feeding port 320a of the cavity.
Although the microwave oven 300 described with reference to
The microwave oven 300 further comprises a microwave source 310 connected to the feeding port 320a of the cavity 350 by means of the transmission line or waveguide 330 and the resonator 320.
Although the resonator 320 is considered to constitute the feeding port of the cavity, it is understood that the face or end 320a of the resonator body 320 adjacent to the wall of the cavity corresponds to the feeding port. In the following, when referring to the feeding port, reference will be made to either the face 320a of the resonator 320 or the resonator 320, interchangeably.
According to an embodiment, the resonator is an elongated piece of dielectric material, extending along the direction of propagation (axis x), and preferably having the same type of cross-sectional shape as the transmission line 330 (e.g. rectangular, circular, etc.).
The dielectric constant of the material constituting the interior of the resonator 320 and the dimensions of the resonator 320 are selected such that a resonance condition is established in the resonator 320 for the microwaves generated by the source 310 and impedance matching is established between the transmission line 330, the resonator 320 and the cavity 350 in accordance with, e.g., the design rules described with reference to
In particular, referring to
Further, the microwave oven may comprise a switch (not shown) associated with the feeding port 320 and arranged in the transmission line 330 for stopping the feeding from the feeding port 320.
According to an embodiment, the resonator is advantageously designed to be full-wave resonant, i.e. resonant for one wavelength, thereby giving a mode index of 2 in the length dimension (i.e. along the x-direction).
According to an embodiment, the microwave source 310 is a solid-state based microwave generator comprising, for instance, silicon carbide (SiC) or gallium nitride (GaN) components. Other semiconductor components may also be adapted to constitute the microwave source 310. In addition to the possibility of controlling the frequency of the generated microwaves, the advantages of a solid-state based microwave generator comprise the possibility of controlling the output power level of the generator and an inherent narrow-band feature. The frequencies of the microwaves that are emitted from a solid-state based generator usually constitute a narrow range of frequencies such as 2.4 to 2.5 GHz. However, the present invention is not limited to such a range of frequencies and the solid-state based microwave source 310 could be adapted to emit in a range centered at 915 MHz, for instance 875-955 MHz, or any other suitable range of frequency (or bandwidth). The present invention is for instance applicable for standard sources having mid-band frequencies of 915 MHz, 2450 MHz, 5800 MHz and 22.125 GHz. Alternatively, the microwave source 310 may be a frequency-controllable magnetron such as that disclosed in document GB2425415.
In general, the number and/or type of available mode fields in a cavity are determined by the design of the cavity. The design of the cavity comprises the physical dimensions of the cavity and the location of the feeding port in the cavity. The dimensions of the cavity are generally denoted by the reference signs h, d and w for the height, depth and width, respectively, in
Referring to the design rules described with reference to
In addition, the tuning may be accomplished via local impedance adjustments, e.g., by introduction of a tuning element (such as a capacitive post) arranged in the transmission line or in the cavity, adjacent to the resonator.
In the present example, the cavity is designed to have a width of 232 mm, a depth of 232 mm and a height of 111 mm. The feeding port 320 may be arranged at, in principle, any walls of the cavity. However, there is generally an optimized location of the feeding port for a predefined mode. In the present example, the feeding port 320a is located in the upper part of a side wall of the cavity, on the right hand-side in the cavity 300 shown in
With reference to
For local impedance adjustment, the microwave heating device 300 may further comprise a tuning element (not shown) arranged in the transmission line 330 or in the cavity 350, adjacent to the resonator 320.
With reference to
The microwave heating device 500 is similar to the microwave heating device 300 described with reference to
In such a configuration, microwaves at a first frequency can be fed into the cavity 550 using the first feeding port or resonator 520 while microwaves at a second frequency can be fed into the cavity 550 using the second feeding port or resonator 520′.
It will be appreciated that the additional feeding structure 525′ and additional microwave source 510′ may be characterized in a similar manner as, and/or may comprise the same further features as, the feeding structure 325 and microwave source 310 described in the above with reference to
Referring to
In the present example, the cavity is designed to have a width of 261 mm, a depth of 340 mm and a height of 170 mm. The second feeding port 520′ is arranged at the center of the ceiling wall of the cavity (x=w/2; y=d/2; z=h). The resonant dielectric bodies 520 and 520′ are made of Al2O3 (c=9) and have substantially equal width and height, 26.67 mm and 10 mm, respectively. However, the length of the resonator differs, wherein the first resonator 520 has a length of 40.5 mm while the second resonator 520′ has a length of 38.0 mm.
The microwave heating device 500 is advantageous in that it comprises a double fed cavity 550 in which crosstalk between the two feeding ports is reduced as compared to a conventional double fed cavity. The lowering of the crosstalk obtained with the use of ceramic resonators as compared to the use of regularly-sized, air-filled waveguides will now be illustrated with reference to
The definition of the curves S11, S12, S21 and S22 given above will be the same in the following.
A simulation was performed for a microwave heating device 800 identical to the microwave heating device 500 described with reference to
As the resonators were removed, an adjustment of the impedance in the feeding structure (junction between the transmission lines 830 and 830′ and the cavity 850) was realized to obtain a similar impedance matching as the matching obtained for the microwave heating device 500 described with reference to
Thus, even with a similar impedance matching as the standard microwave heating device 800 using regularly sized, air-filled feeding ports such as described with reference to
In addition to the reduction of crosstalk, the double feeding at different frequencies of the cavity of the microwave device is advantageous in that it enables a number of possible regulations of the microwave heating device and, in particular, optimization of the heating pattern in the cavity. For example, still in the case of a cavity with two feeding ports, the two resonators may be configured to excite modes resulting in complementary heating patterns in the cavity, thereby providing uniform heating in the cavity. If the first resonator is configured to transmit microwaves at a first frequency resulting in a first heating pattern (or first mode) with hot and cold spots at specific locations in the cavity, the second resonator may be configured to transmit microwaves at a second frequency such that the presence of hot and cold spots in the first heating pattern is compensated by the second heating pattern (or second mode) obtained by the second resonator (or second feeding port). In other words, the effect of the presence of hot and cold spots in a first mode field, i.e. the presence of hot and cold spots in the cavity, may be eliminated, or at least reduced, by the heating pattern of a second mode field thanks to an adequate configuration of the feeding ports (resonators).
In the present invention, as each of the feeding structures is connected to a microwave energy source, simultaneous feeding of microwaves at different frequencies is possible. However, depending on the application, e.g. for a specific type of load or a specific cooking program (or function), it is also possible to operate the microwaves sources such that feeding of the microwaves into the cavity switches between the two feeding ports. Such flexibility in feeding microwaves into the cavity allows for a controlled regulation accounting for e.g. change in the load (change in geometry, weight or state) during heating.
In order to implement such type of regulation, the microwave heating device 500 may further comprise a control unit 580 connected to the microwave sources 510 and 510′ of the microwave heating device for controlling these sources, such as, e.g., their respective output powers. The control unit 580 may obtain information about the load and conditions in the cavity, by means of sensors (not shown) arranged in the cavity and connected to the control unit 580. The control unit 580 may further be configured to control, during an operation cycle, the frequency of operation of the sources and their respective time of operation during the cycle.
While specific embodiments have been described, the skilled person will understand that various modifications and alterations are conceivable within the scope as defined in the appended claims.
For example, although a cavity having a rectangular cross-section has been described in the application, it is also envisaged to implement the present invention in a cavity having a geometry describable in any orthogonal curve-linear coordinate system, e.g. a cavity having circular cross-section.
Further, although a cavity comprising only two feeding structures has been described to illustrate the reduction of crosstalk, a cavity comprising more than two feeding ports can be envisaged.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09155733 | Mar 2009 | EP | regional |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4776918 | Otsubo et al. | Oct 1988 | A |
| 5134965 | Tokuda et al. | Aug 1992 | A |
| 6158383 | Watanabe et al. | Dec 2000 | A |
| 20030106891 | Fagrell et al. | Jun 2003 | A1 |
| 20090295509 | Master et al. | Dec 2009 | A1 |
| Number | Date | Country |
|---|---|---|
| 0478053 | Apr 1992 | EP |
| Number | Date | Country | |
|---|---|---|---|
| 20100237067 A1 | Sep 2010 | US |
