Household microwave oven and method for controlling a microwave oven

Abstract

The invention refers to a household microwave oven (1), comprising an oven cavity (4) for receiving cooking goods, a magnetron (6) for emitting microwaves, a waveguide (8) for guiding the microwaves from the magnetron (6) to the oven cavity (4), and a directional coupler (30) aligned at the waveguide (8) in a way to detect an amount of microwaves travelling through the waveguide (8) and to emit a coupler signal (C) based on the detection of the microwaves. Further, the microwave oven (1) comprises a controller (10) connected with the directional coupler (30) for signal transmission of the coupler signal (C) and indirectly with the magnetron (6) for operational control, wherein the controller (10) is configured to derive, during an operational state of the microwave oven (1), from the coupler signal (C) information about microwaves travelling through the waveguide (8) and to perform controlling of the operational state of the microwave oven (1) based on that information.

Description

[0001] Description

[0002] HOUSEHOLD MICROWAVE OVEN AND METHOD FOR CONTROLLING A MICROWAVE OVEN

[0003] The invention refers to a household microwave oven . Additionally, the invention refers to a method for controlling a microwave oven .

[0004] Household microwave oven applications are used for warming up or cooking food ( including also beverages ) . They are based upon dielectric warming of material by irradiating the material ( food) with microwaves . Such irradiation causes a torque or torsional moment to molecules having an electrical dipole moment ( and being also sufficiently free to move ) . The advantage is that microwaves have usually a significantly deeper penetration into those materials compared to thermal radiation, especially of infrared wavelength . That enables relatively quick warming of food .

[0005] Usually, for generating microwaves magnetrons are used as well as waveguides that guide the microwaves from the magnetron to a chamber wherein the food is to be placed . The microwaves enter the chamber ( also : "oven or cooking cavity" ) , which is made of metal , through one or several respective ports (wall openings ) and are reflected by its walls and absorbed by the molecules of the food, especially water molecules . The reflected microwaves within the chamber cause interference maxima ( also : "hot spots" ) where more energy is transferred to the molecules . That causes food to be warmed non- homogeneously . To reduce that effect , stirrers (mostly metal fans that reflect the microwaves differently to the chamber ) and / or rotating plates for the food are used . Additionally, the amount of food placed within the chamber also influences the energy needed or being sufficient for warming, especially since the impedance within ( or of ) the chamber is altered . However , the impedance of the magnetron and the waveguide system should match to the impedance of the chamber ( including the food placed therein) . Otherwise , the magnetron may overheat and / or energy transfer is not efficient . The invention is based on the problem to enable for an improved microwave oven .

[0006] That obj ect is solved according to the invention by a household microwave oven comprising the features of claim 1 . Additionally, that obj ect is solved according to the invention by a method for controlling a microwave oven according to the features of claim 15 . Further expedient and optionally inventive per se developments are described in the dependent claims and the following specification .

[0007] The household microwave oven according to the invention is configured, as a domestic appliance , for cooking food . The household microwave oven, "microwave oven" for short , comprises an oven cavity for receiving cooking goods ( foodstuff , e . g . a dish with different kinds of food forming a meal that should be reheated or a dish with one kind of food for cooking within the microwave oven, e . g . a casserole , a roast etc . ) . Further, the microwave oven comprises a magnetron for emitting microwaves and a waveguide for guiding ( directing or transmitting ) the microwaves from the magnetron to the oven cavity . Also , the microwave oven comprises a directional coupler aligned at the waveguide in a way to detect an amount of microwaves travelling through the waveguide and to emit a signal ("coupler signal" ) based on the detection of the microwaves , as well as a controller connected ( coupled) with the directional coupler for signal transmission of the coupler signal and being configured for operational control of the magnetron . The controller is configured to derive , during an operational state of the microwave oven, from the coupler signal information about microwaves travelling forward ( i . e . , towards the oven cavity) and / or backwards ( i . e . , from the oven cavity towards the magnetron ) through the waveguide and to perform controlling of the operational state of the microwave oven based on that information . "Operational state" of the microwave oven is to be understood here and in the following preferably as the state when the magnetron emits microwaves , purposely .

[0008] The controller being configured for operational control of the magnetron is to be understood, in particular, that the controller is connected with a power source driving the magnetron during the operational state and that the controller controls the power output of that power source to the magnetron . That power source is , preferably, a transformer or inverter .

[0009] Preferably, the controller is configured to control the operational state of the microwave oven in the form of a closed loop control . I . e . in particular , the controller uses the information as a feedback measure for the control actions taken before .

[0010] The waveguide preferably comprises a tunnel leading from the magnetron to the oven cavity . That tunnel may be of a polygonal (preferably a rectangular ) cross section or of an oval cross section - e . g . having an elliptical or circular shape . Often, the shape of the waveguide is referred to as a parallelepiped . The cross section, in this case , is preferably taken as being transverse to the course ( axis ) of the tunnel from the magnetron to the oven cavity . Preferably, walls of the waveguide forming that tunnel are made of metal .

[0011] The usage of the directional coupler for determining the information about microwaves travelling within the waveguide and the usage of that information for controlling the operational state make up for a very convenient and efficient operation of the microwave oven . That is because the power chosen by a user for an operational mode ( e . g . for heating a cup of milk, for reheating a meal etc . ) resembles the nominal power output by the magnetron . However, dependent on characteristics of the obj ect ( s ) to be heated, especially the cooking goods or the foodstuff , microwaves entering the oven cavity through the waveguide are usually not absorbed completely . Yet , the best efficiency would be if all microwaves are absorbed. Additionally, those microwaves not absorbed are usually reflected and enter the waveguide and travel backwards within the waveguide towards the magnetron ("backward microwaves") . Aside from less efficiency, those backward microwaves may cause a heat build-up at the magnetron which conveys a risk for damage to the magnetron. The microwaves travelling forward or backward are also a measure for the power delivered from the magnetron to the oven cavity: the higher the power delivered from the magnetron to the cavity (i.e. , the more "forward microwaves") the higher the energy the object (s) within the oven cavity are absorbing; otherwise, the backwards microwaves are a measure for the reflected, not absorbed power. Controlling the operation of the microwave based on the information of microwaves travelling at least in one direction within the waveguide, e.g., based on the backward microwaves, makes it possible to react to low efficiency and / or to avoid damage to the magnetron by means of changing operational parameters in good time.

[0012] According to a preferred embodiment, the directional coupler comprises an antenna for detecting the microwaves, wherein that antenna is aligned at a cut-out (i.e. an opening) within a wall of the waveguide.

[0013] Further, according to a preferred development, that cut-out is of a rectangular shape and aligned with its length axis parallel to a travelling direction (or longitudinal direction) of the microwaves. The cut-out may, thus, also be referred to as a longitudinal slit. Expediently, the cut-out is additionally aligned centered within the respective wall with respect to the width (taken square to the longitudinal direction) of the wall.

[0014] According to a further preferred development, the length of the antenna is one Nth of the wavelength of the microwaves, N being 1, 2 or 4. Expediently, the length of the antenna is one fourth of the wavelength (Lambda / 4) . That is a length typical for high frequency antennas to send and / or receive waves with a respective wavelength with sufficiently high efficiency. The cut-out preferably has a length corresponding to the length of the antenna. Especially, the cut-out is dimensioned such that an antenna carrier part of the directional coupler is aligned fittingly within the cut-out. Thus, the length of the cut-out would be the fourth of the wavelength plus an addition for the dimensions of the antenna carrier part, e.g. plus 1-2 mm.

[0015] A width of the cut-out, especially its maximum width, is in particular chosen depending on a distribution (or propagation) of an electromagnetic field over an internal surface of the waveguide. Such a distribution of the electromagnetic field (e.g. its current lines) is described e.g. in EP 0 545 873 A2. Towards the center of the (especially) rectangular waveguide (referring to a width of the waveguide) these lines are at least approximately parallel to the axis (longitudinal direction) of the waveguide (and of the microwaves travelling within) and therefore the width of the slot is preferably chosen at 4 to 6 mm, especially at 5mm, without causing leakage of the microwave field to the outside of the waveguide or interference with microwave propagation. The more the slot is positioned off center of the (especially rectangular) waveguide (or out of a region where the current lines are parallel) , the narrower the cut-out is chosen, expediently .

[0016] On the basis of the above dimensions, the antenna and, thus, the directional coupler are enabled to "sniff" (receive) a small percentage of the microwaves travelling within the waveguide.

[0017] According to an expedient embodiment, the directional coupler is realized as a PCB-component and comprises - especially additionally to the aforementioned antenna - a signal processing circuit that is configured at least to output the coupler signal such that it contains common (or in other words combined) information about microwaves that travel forward into the direction to the oven cavity (forward microwaves) and about the backward microwaves (i.e. the microwaves that travel backwards through the waveguide from the oven cavity to the magnetron) . Thus, the common (combined) information is bidirectional information with regard to the travelling direction of the microwaves within the waveguide . Especially, the aforementioned information about the backward microwaves is part of the common information .

[0018] Preferably, the directional coupler is already realized as a bidirectional coupler , which is advantageously the case with usual directional couplers . However , the directional coupler may also comprise two antennas ( i . e . two single-directional couplers integrated into one component ) for detection the forward and backward microwaves , respectively . Otherwise , within the light of the invention, the microwave oven may comprise two directional couplers configured for one-directional detection only, whereby those two directional couplers are aligned into opposite direction regarding the axis of the waveguide . Thus , one directional coupler would be used for detection of the forward microwaves and the other one for the backward microwaves .

[0019] Expediently, the magnetron is configured to output microwaves with a frequency of about ( i . e . + / - 100 MHz ) 2 , 45 GHz . The antenna preferably is adapted by the respective length to be sensitive for that frequency . E . g . , the length of the antenna ( or the antennas ) of the directional coupler is about 30 mm ( + / - 1-2 mm) .

[0020] Preferably, the coupler signal output by the directional coupler is an electric signal of 0-5 V and / or of 4-20 mA.

[0021] According to an expedient embodiment , the controller is configured to ( at least indirectly by means of controlling the power source ) control the magnetron in response to the information or the common information by means of modifying operational parameters such as an output power of the magnetron and / or radiation timing . Controlling the magnetron includes also to deactivate or shut down the magnetron and, thus , stop an operation of the microwave oven . According to a preferred embodiment , the controller is configured ( or : designed) to derive from the coupler signal ( and, thus , from the ( common ) information) a measure for the backward power of the microwaves travelling backwards and a measure for the forward power of the microwaves travelling forward through the waveguide , to determine a standing wave ratio on the basis of the forward and the backward power and to control the power source for the magnetron in response to the standing wave ratio . Generally, the respective measure for the power of the forward and backward microwaves may be a voltage value of the coupler signal , which optionally may be outputted over respective ports of the directional coupler . However , the processing circuit of the directional coupler may also be configured to determine the respective power value of the forward and the backward microwaves and output these as respective data within the coupler signal . From the respective measures , the controller computes the standing wave ratio : wherein

[0022] Umaxis the maximum voltage value measured at a distinct point of the waveguide ,

[0023] Umin is the minimum voltage value measured at a distance of the fourth of the wavelength,

[0024] Uvis the analogous voltage value measured for the forward wave ,

[0025] URis the analogous voltage value measured for the backward wave ,

[0026] Pv, R are the respective power values .

[0027] According to a further expedient embodiment , the waveguide comprises a deformable portion and the microwave oven comprises an actuation device connected to the deformable portion for transmission of a deformation force to the deformable portion ( i . e . for deforming the deformable portion) . The controller is in this case preferably configured to control the actuation device to deform the deformable portion in response to the information or the common information ( e . g . , the standing wave ratio ) . That deformation may be used alternatively or even additionally to changing the power and / or the radiation time (e.g., duration of radiation phases) as described before .

[0028] The deformable portion in connection with the actuation device enables a comparably flexible adjustment of the cross section and / or the volume of the waveguide. The size of the cross section (and, thus, taken in 3D the volume) of the waveguide have an impact on the impedance of the system realized by the magnetron and the waveguide. Thus, a waveguide having an - at least within a certain amount - adjustable impedance is realized by that aspect of the invention which, in turn, enables impedance matching to a topical situation of the oven cavity, at least within a certain range.

[0029] According to an expedient embodiment, the deformable portion comprises a flexible structure that has a microwave tight metallic surface.

[0030] "Flexible structure" is to be understood as a structure made of a material that is flexible with regard to the deformation force (i.e. a specific value of force) , especially that may be deformed elastically by the deformation force value applied. Preferably, the flexible structure and its material is chosen such that, within the intended deformation range, only elastic deformation would occur. Especially, such flexible structure is made of a corresponding sheet material (i.e. , the thickness of that sheet material is significantly thinner, e.g., multiple times, than its surface dimensions) . In contrast to such flexible structure, a wall (especially a tunnel wall) of the waveguide is considered as a rigid structure with no (or only a negligible amount of elastic) deformation in response to the application of the deformation force.

[0031] According to a preferred further development, the flexible structure is a metal and of a net shape with meshes small enough to be impermeable for microwaves. The net shape is realized especially in that a net made of metal wires ("wired net") is chosen. Alternatively, a printed "net", a punched metal foil and the like may also be chosen as the flexible structure. That configuration of the flexible structure enables that microwaves may be reflected at the flexible structure, i.e. , the flexible structure is similar to the walls of a common waveguide for microwave ovens.

[0032] As an alternative to the above net shape, the flexible structure may also be of a deformable metallic surface which in one case may be realized by a metallic foil or in another case by an accordion-like structure made by a number of metal plates connected by hinges that allow folding in a zig-zag manner.

[0033] Preferably, the deformable structure, at least its metallic surface, is galvanically contacted to the above-mentioned walls of the waveguide. Such galvanic contact may be realized, especially in the case of the flexible structure, preferably the metallic net, by a welding seam, e.g. a laser welding seam. Alternatively, the galvanic contact may also be realized by riveting or the like.

[0034] According to an optional embodiment, the actuation device and / or the deformable portion are configured such that a shape of a cross section of the waveguide stays at least generally the same during deformation of the deformable portion and that only the area of the cross section is changed. That means, during deformation of the deformable portion the cross section of the waveguide (i.e. its tunnel) , which is e.g. rectangular, stays of the same shape, e.g. stays rectangular, but changes in size, i.e. , gets bigger or smaller. In the case of the rectangle, e.g. , its height is varied by the deformation of the deformable portion. This may easily be realized with the aforementioned movable wall parts. In the case of the flexible structure, e.g. , the metallic net, that may be realized by providing said flexible structure at least over one side of the (in that case preferably rectangular) cross section of the waveguide. In the latter case, the deformation is preferably caused such that the flexible structure moves equally over the whole side. E.g. , the actuator is coupled by a stamp, bar or the like to the flexible structure that has at least a line contact over the whole length of the respective side of the cross section .

[0035] According to an optional embodiment , the controller is configured, in the context of a thawing mode , to determine during a course of the thawing mode' s operation whether a maximum of microwave absorption within the oven cavity has been reached and, when the maximum has been detected, to reduce the output power or to stop operation of the magnetron . Thus , in other words , the controller is configured to monitor the thawing process . The aforementioned maximum of microwave absorption is due to the transition of water between its aggregate states . At first , when the water within the cooking good ( s ) to be thawed is still frozen, absorption of microwaves is still low, thus leading to a high power of backward microwaves . When the water is melted its absorption is significantly higher than before , which leads to a low power of backward microwaves , especially to a standing wave ratio near 1 . When it comes to evaporating of the water , the absorption gets lower again, since the volume of the water within the cooking good ( s ) decreases . By monitoring the absorption of the microwaves within the oven cavity ( e . g . , by determining the ratio of backward microwaves to the forward microwaves ) and reducing the power or stopping the output of microwaves ( and / or deforming the deformable part ) , boiling or simmering of the cooking good ( s ) may be prevented and / or a more uniform heating of the food can be achieved . Optionally, a camera may be used for recognition of cooking goods within the oven cavity, especially for recognizing the food type (preferably by the controller ) . In particular, the controller is configured to recommend, based on such recognition, a recipe or cooking conditions ( especially based on specific ingredients of the recognizes food type ) and / or to take into account an expected impedance . For example , in that case , by means of the ratio of backward microwaves to the forward microwaves a different food "fingerprint" ( e . g . , a behavior other than expected, for example when the cooking goods have less water or the like ) may be recognized and, thus, be used by the controller to control the operational state of the magnetron.

[0036] According to an expedient embodiment, the controller is configured, in the context of a casserole (e.g. lasagna) cooking mode, to determine during a course of the casserole cooking mode's operation a slope of a course of microwave absorption within the oven cavity over time. Additionally, the controller is configured to, when the slope has reached a predetermined value, directly (i.e. promptly) or indirectly (e.g. , after waiting a predetermined duration of time) reduce the output power, to stop operation of the magnetron and / or to issue a notification to a user. During such a cooking process, water evaporates from the casserole and, therefore, the absorption of microwaves decreases. The slope will get lower after some time when a main part of the water has evaporated and the remaining water is in regions of the casserole where the way to the surface is long (thus, the water and / or the vapor is somewhat "trapped" within the casserole) or the water "reservoir" is so big that evaporation from it is at a constant level (e.g., within regions covered by sauce) . The predetermined duration of time may resemble an empirically chosen value that is needed for cooking the casserole thoroughly. That value may preferably be dependent on the size of the casserole and / or on a starting value of the absorption. The notification is preferably configured to inform the user about the ready state of the cooking good ( s ) .

[0037] According to a yet further expedient embodiment, the controller is configured to monitor, at least at the start of a cooking operation, whether the oven cavity is loaded with cooking goods by means of the information and / or the common information and if the oven cavity is not loaded with cooking goods to trigger a measure to avoid damage to the magnetron. E.g. , the controller is configured to derive from the information or the common information that the absorption of the microwaves is (at least near) zero. As the respective measure, the controller preferably shuts down (i.e., stops microwave radiation by) the magnetron, increases a cooling of the magnetron (e.g. by increasing a rotational frequency of a blower) and / or to issue a warning to the user.

[0038] According to a further optional embodiment, the controller is configured to compare the information, preferably the standing wave ratio, to a predetermined threshold, and if the information transgresses the threshold in a predetermined manner and for a predetermined amount of time (e.g., for several samples of a predetermined sample frequency) to trigger the action. Usually, the ideal value of the standing wave ration would be 1 (i.e. no reflection, all power generated is delivered to the load / food) . However, due to the definition of the standing wave ratio (cf. formula (1) ) , the value of the standing wave ratio is usually higher than 1. Thus, in particular if the standing wave ratio is higher than the predetermined threshold, the controller will conclude that the oven cavity is empty.

[0039] Furthermore, a sudden peak in the standing wave ratio's course may be taken as an indication of sparking within the cavity. According to an expedient embodiment, which resembles an independent invention, also, the controller is configured to monitor the coupler signal, especially the information or the common information about the microwaves travelling within the waveguide, respectively, preferably the above describes standing wave ratio for a peak. Preferably, the controller is configured to determine a width of such a peak and take only into account a peak that has quite narrow characteristics (i.e., short duration of time such as below 0,5 s to 3 s) compared to the duration of a prescribed duration of the operational state (especially a predetermined duration of a heating course) . Further, the controller is configured to interpret such a peak, especially such peak of the standing wave ratio, as an indication of a sparking event. Such indication of sparking is preferably taken (by the controller) as a trigger to control (especially modify) the operational state of the microwave oven. E.g., the controller is in that case configured to stop the magnetron or at least reduce the emitted power of the magnetron. The latter is achieved, e.g. , by respective control of the power source for the magnetron (preferably, by changing a clocking (on / off cycles) of the power source) and / or via deforming the optionally present deformable part) .

[0040] For a particularly reliable detection of above-mentioned peak of the coupler signal or the standing wave ratio, the controller is preferably configured to set a sample rate of the directional coupler or of a read out of the coupler signal at a sufficiently high value. Preferably, such sufficiently high value is at 10-20 Hz. Additionally or alternatively, the sample rate may also be adjustable over the duration of the cooking process. E.g. , at the start of the radiation (operational state of the magnetron, cooking process) the sample rate is chosen high (in particular at 10-20 Hz) to detect an empty oven cavity or abnormal load conditions and / or when standing wave ratio is over a certain threshold (i.e., when a probability of sparking due to less microwave absorption is high) and if the standing wave ratio is in a „good" range, where sparking is less probable, the sample rate is chosen low (e.g. at 1-5 Hz) . Since the ideal value is 1 (no reflection of microwaves backwards) and the SWR is by definition 1 or above, such a good range of the standing wave ratio is below 1,8 to 2 (i.e. , up to about 30-40 % backward power with regard to the forward power) . Such variable sample rate enables to preserve computational power or memory. Further, such sampling rates may be sufficient to prevent damage to the microwave oven, its components or a dish, since sparking would usually have to continue over few seconds (e.g. , two to five seconds) to cause damage.

[0041] According to further optional embodiments, the controller may be configured to make use of common techniques of data elaboration in order to recognize trends in the (preferably common) information, optionally including extrapolation of such information (data) . For another instance, the standing wave ratio, in particular its course over time, could comprise a sinusoid component because a stirrer or microwave turntable may cause periodical fluctuations in the impedance of the oven cavity . This sinusoid component is optionally filtered by the controller since the frequency of rotation of the stirrer or of the turntable is known . Further , that sinusoid component might be used to determine whether the stirrer or the turntable are still properly rotating or stopped due to a damage or wrong placement of the load . E . g . , the turntable may not be placed correctly upon its tracks , a food container may be too big ( e . g . a rectangular tray) so that it hinders the turntable to rotate correctly . Additionally or alternatively, the information about the backward microwaves could also be used by the controller to control the cooling of the magnetron ( cf . above ) . As has been described above , the magnetron will heat up during operation, whereby reflected power will add to the heating of the magnetron . Thus , the information about the backward microwaves ( in particular about the reflected power ) is expediently used to protect the magnetron and extend its working life .

[0042] The method, according to the invention, is intended and used for controlling a household microwave oven, especially the microwave oven described herein . The microwave oven comprises the oven cavity, the magnetron, the waveguide and the directional coupler . The method comprises the steps of deriving from the coupler signal the information about microwaves travelling backwards into the direction to the magnetron through the waveguide , and controlling the operational state of the microwave oven based on that information .

[0043] The microwave oven and the method for controlling that microwave oven share - in the present case - respective structural features as well as their advantages . Thus , the method steps of further developments of the invention may be derived from the steps and actions performed and taken by the controller as described herein . Accordingly, the controller is - within respective embodiments - configured to perform the method steps as described herein, also . The conj unction "and / or" is to be understood here and in the following in particular in such a way that the features linked by means of this conj unction can be formed both together and as alternatives to each other .

[0044] In the following, embodiments of the invention are explained in more detail with reference to a drawing . Therein show :

[0045] Fig . 1 in a principal drawing schematically a household microwave oven with a magnetron and a waveguide , and

[0046] Fig . 2 in a detailed perspective view schematically the magnetron and the waveguide ,

[0047] Fig . 3 in a top view schematically a detail of the waveguide ,

[0048] Fig . 4 in a sectional view IV-IV according to Fig . 3 the waveguide and a directional coupler aligned thereto ,

[0049] Fig . 5 in a schematic block diagram an example of the directional coupler , and

[0050] Fig . 6 , 7 in schematic diagrams of a microwave absorption over time a respective course of heating of different functions of the microwave oven .

[0051] Parts corresponding to each other are always provided with the same reference signs in all figures .

[0052] Fig . 1 shows a household microwave oven 1 in a schematic view . The microwave oven 1 comprises a housing 2 that houses several components of the microwave oven 1 . Some of these components are an oven cavity 4 ( also : cooking chamber ) , a magnetron 6 , a waveguide 8 and a controller 10 . The oven cavity 4 is housed by a cavity housing 12 enclosing the cavity 4 from five sides . A sixth side ( front side ) is left open and to be closed by a door 14 .

[0053] The magnetron 6 is connected indirectly via a power supply ( not depicted in detail ) to the controller 10 so that the controller 10 may control power supply to the magnetron 6 and, thus the emission of microwaves by the magnetron 6 . The magnetron 6 is also coupled to the waveguide 8 . The latter comprises a tunnel 18 which is delimited by metal walls 20 which serve to guide and transmit the microwaves to the oven cavity 4 . The walls 20 are aligned to form the tunnel 18 with a ( generally) rectangular cross section . The sum of the walls 20 is also referred to as a guide wall system. The microwaves enter the oven cavity 4 through respective ports 22 ( openings ) within the top wall of the cavity housing 14 .

[0054] Further , the microwave oven 1 comprises a sensor for measuring a power of the microwaves travelling from the magnetron 6 to the oven cavity 4 ("forward microwaves" , indicated by small solid line arrows at the ports 22 ) as well as a power of the microwaves reflected and travelling "backwards" through the waveguide 8 towards the magnetron 6 ("backward microwaves" , indicated by small dotted line arrows at the ports 22 ) .

[0055] That sensor is realized as a directional coupler 30 . The directional coupler 30 is connected to the controller 10 for signal transmission . The controller 10 is configured to perform a method for controlling the operation of the microwave oven 1 . The controller 10 hereby receives by means of an input interface ( not shown in detail , e . g . a user interface with a touchpad) a command for a distinct operation function ( e . g . thawing of a frozen meal , heating of a liquid, a cooking process etc . ; step SI ; cf . Fig . 8 ) . Based on that command the controller 10 controls the magnetron 6 ( especially the power supply) to emit microwaves according to the chosen operation function ( step

[0056] 52 ) , thus starting or activating an operational state of the microwave oven 1 . During that operational state of the microwave oven 1 ( i . e . , during microwave emission of the magnetron 6 ) , the controller 10 receives from the directional coupler 30 a "coupler signal C" ( step

[0057] 53 ) and derives therefrom an information about the forward and the backward microwaves ( step S4 ) . On the basis of this information, the controller 10 controls - especially as a closed loop control - the operational state of the microwave oven 1 (step S5) by means of outputting respective control signals, e.g. , to the magnetron 6.

[0058] In order to receive or sense the microwaves travelling within the waveguide 8, the directional coupler 30 is aligned in or over a cutout 32 within one of the walls 20 of the waveguide 8. Fig. 2 shows three possible positions for the cut-out 32 within a top wall 20. The cut-out 32 has dimensions that allow the directional coupler 30 to come into contact with the tunnel 18 of the waveguide 8 but also to avoid microwaves to exit the tunnel 18 via the cut-out 32.

[0059] The directional coupler 30 is in the present embodiment (cf. Fig. 4) realized as a PCB (printed circuit board) component comprising an antenna 34 and an electronics circuit 36 (also: "processing circuit") . The antenna 34 is realized as a metal track on the PCB and has a length LAof a quarter of the wavelength of the microwaves ("Lambda / 4") . The PCB (or the overall directional coupler 30) has a length LDwhich is slightly longer than the length LAof the antenna 34, in particular due to manufacturing reasons. E.g., the length LDis five to twenty percent longer than the length LA. The cut-out 32 has a length Lcsuch long that the directional coupler 30 may fit closely into the cut-out 32 (cf . Fig. 3, 4) . The same applies to the width W of the cut-out 32, which is for a microwave frequency of about 2,45 GHz around 5 mm. The length LAis about 30 mm. The directional coupler 30 is aligned within the cut-out 32 such that an offset V of the antenna 34 regarding the inside of the wall 20 is 2 mm or less. Preferably, the antenna 34 sits flush with the inside surface of the wall 20. Preferably, the cut-out 32 is aligned central or coaxial to a longitudinal or central axis 40 of the waveguide 8. Thus, the antenna 34 is also aligned parallel to the longitudinal axis 40. In the case that the cut-out 32 is offset to the side regarding the longitudinal axis 40, the width W is chosen smaller than 5 mm in order to avoid leakage of microwaves . In Fig . 5 a schematic block diagram of the directional coupler 30 is shown . The electronics circuit 36 comprises a diode 50 . The antenna 34 which detects the electromagnetic field within the tunnel 18 is connected to the diode 50 . Downstream of the diode 50 the electronics circuit 36 comprises a signal rectifier 52 . The signal rectifier 52 and the diode 50 make up for the high frequency signal handling . Downstream the signal rectifier 52 the electronics circuit 36 comprises a signal amplifier 54 and a signal conditioner 56 for conditioning the DC signal coming from the signal rectifier 52 and the signal amplifier 54 to a voltage of 0-5 V or 4-20 mA. Downstream the signal amplifier 54 is a signal output 58 for connection to the controller 10 .

[0060] The coupler signal C comprises information about the power of the forward and the backward microwaves , respectively . The controller 10 is configured to derive from the coupler signal C a standing wave ratio SWR ( step S4 , cf . formula ( 1 ) ) . The standing wave ratio SWR gives information about how much power of backward microwaves in relation to the power of the forward microwaves is present within the waveguide 8 . If the standing wave ratio SWR is 1 or near 1 , zero or only a low amount of backward microwaves are present . That indicates a high heating efficiency since all or almost all forward microwaves are absorbed within the oven cavity 4 by the load ( foodstuff or cooking goods ) placed there . A high standing wave ratio SWR indicates low absorption of microwaves .

[0061] In the latter case , the controller 10 controls the (power source of the ) magnetron 6 ( step S5 ) to adj ust the emitted power, the phase or also the frequency of the microwaves . The goal of these changes is to reduce the power of the backward microwaves and / or to increase absorption .

[0062] According to an optional embodiment , the waveguide 8 comprises a deformable wall portion 60 ( cf . dashed line in Fig . 1 ) . That deformable wall portion 60 is according to that example embodiment made of a flexible structure, specifically a wired net. The wired net has meshes with a size below a size that might be passed by the microwaves. For a "closed surface" with regard to the microwaves between the - preferably rigid and non-movable - walls 20 and the wired net the latter is welded to the adjacent metallic walls 20, thus causing a galvanic connection.

[0063] The deformable portion 60 may be deformed elastically by a deformation force (i.e. a certain value of that force) applied by an actuation device (not shown in detail) , e.g. an electrically actuated piston.

[0064] The walls 20 are designed rigid with regard to that deformation force value. By deforming the deformable portion 60 a cross section of the waveguide's 8 tunnel 18 is changed so that an impedance of the microwave emission and guiding system, i.e. of the magnetron 6 and the waveguide 8 changes. Thus, the controller 10 is enabled to match the impedance of the microwave emission and guiding system to the impedance of the oven cavity 4 which changes with different loads, i.e. with different cooking goods placed in the oven cavity 4.

[0065] As an example of an application of the microwave oven 1 and method described before, Fig. 6 shows a course of an absorption A of the microwaves over time t within the oven cavity 4 during the operation of the microwave oven 1 according to a thawing mode for a frozen meal. The absorption A may be derived by the controller 10 from the coupler signal C and / or from the standing wave ratio SWR.

[0066] At starting point S, the thawing mode starts and the magnetron 6 emits microwaves. Due to the still frozen meal (cooking goods) the water therein is still ice, which is transparent or at least nearly transparent to the microwaves. Thus, absorption A is quite low. With increasing melting of the water, the absorption A increases and causes heating of the cooking goods. However, for the thawing mode, low power is used in order to avoid boiling of the cooking goods. During the course of the thawing mode, the melted water heats up and at some point, starts to evaporate. Vapor in that state has a much higher specific volume and thus reduces absorption. So, with increasing evaporation a maximum or peak P will be reached during thawing. At that point, a risk to start boiling or cooking of the cooking goods gets higher. Therefore, the controller 10 stops the emission of microwaves or reduces the power (or deforms the deformable portion 60 for impedance matching) in order to avoid cooking of the cooking goods that should only thawed.

[0067] Fig. 7 shows as a further example of an application of the microwave oven 1 the course of the absorption A during a cooking mode for a casserole, here specifically a lasagna. Compared to the thawing mode, the power of the microwaves emitted by the magnetron 6 is significantly higher in order to cause cooking (boiling) of a sauce within the lasagna as well as melting of cheese. At the starting point S, the cooking mode starts and the magnetron 6 emits microwaves. The microwaves are absorbed by the lasagna and it begins to heat up. Thereby, water from the sauce and the cheese evaporates so that the absorption A decreases continuously, however, not with a linear curve. In particular, the slope of the curve gets lower as more and more water has evaporated. The controller 10 determines the slope of the curve, i.e. of the course of the absorption A over time t. The controller 10 further compares the slope with a predetermined threshold value and, when the threshold value is reached, starts a predetermined countdown during which the magnetron 6 is still activated. After the countdown is terminated, the controller 10 stops the emission of microwaves by the magnetron 6 and optionally issues a notification to a user of the microwave oven 1 that the lasagna is finished .

[0068] According to a further embodiment, the controller 10 monitors the power of the backward microwaves or the forward microwaves, preferably of a standing wave ratio computed on the basis of the power of the forward and backward microwaves (s. before, formula (1) ) . If a comparably narrow peak, e.g. over a duration of a maximum of 0,75 seconds, occurs, the controller 10 takes that as an indication for a sparking event. Such sparking may occur if the load is too low, e.g. , if the oven cavity 4 is empty, or a wrong tray is used at too high power. In that case, the controller 10 stops the emission of microwaves, reduces power to the magnetron 6 or deforms the deformable portion 60, in order to prevent further sparking and / or damage to the microwave oven 1.

[0069] The subject matter of the invention is not limited to the embodiments described above. Rather, further embodiments of the invention can be derived by the skilled person from the above description.

[0070] List of reference signs

[0071] 1 microwave oven

[0072] 2 housing

[0073] 4 oven cavity

[0074] 6 magnetron

[0075] 8 waveguide

[0076] 10 controller

[0077] 12 cavity housing

[0078] 14 door

[0079] 18 tunnel

[0080] 20 wall

[0081] 30 directional coupler

[0082] 32 cut-out

[0083] 34 antenna

[0084] 36 electronics circuit

[0085] 40 axis

[0086] 50 diode

[0087] 52 signal rectifier

[0088] 54 signal amplifier

[0089] 56 signal conditioner

[0090] 58 signal output

[0091] 60 deformable portion

[0092] 51 step

[0093] 52 step

[0094] 53 step

[0095] 54 step

[0096] 55 step

[0097] A absorption

[0098] C coupler signal

[0099] LA, C, D length

[0100] 5 starting point

[0101] P maximum t time

[0102] V offset

[0103] W width

Claims

Claims1. Household microwave oven (1) for cooking food, comprising- an oven cavity (4) for receiving cooking goods,- a magnetron (6) for emitting microwaves,- a waveguide (8) for guiding the microwaves from the magnetron(6) to the oven cavity (4) ,- a directional coupler (30) aligned at the waveguide (8) in a way to detect an amount of microwaves travelling through the waveguide (8) and to emit a coupler signal (C) based on the detection of the microwaves, and- a controller (10) connected with the directional coupler (30) for signal transmission of the coupler signal (C) and being configured for operational control of the magnetron (6) , wherein the controller (10) is configured to derive, during an operational state of the microwave oven (1) , from the coupler signal (C) information about microwaves travelling forward and / or backwards through the waveguide (8) and to perform controlling of the operational state of the microwave oven (1) based on that information .

2. Microwave oven (1) according to claim 1, wherein the directional coupler (30) comprises an antenna (34) for detecting the microwaves, wherein that antenna (34) is aligned at a cut-out (32) within a wall (20) of the waveguide (8) .

3. Microwave oven (1) according to claim 2, wherein the cut-out (32) is rectangular and aligned with its length axis parallel to a travelling direction of the microwaves.

4. Microwave oven (1) according to claim 2 or 3, wherein the length of the antenna (34) is one Nth of the wavelength of the microwaves, N being 1, 2 or 4 , wherein the cutout (32) has a length corresponding to the length of the antenna (34) , especially wherein an antenna carrier part of the24directional coupler (30) is aligned fittingly within the cut-out (32) .

5. Microwave oven (1) according to one of claims 1 to 4, wherein the directional coupler (30) is realized as a PCB- component and comprises a signal processing circuit (36) that is configured at least to output the coupler signal (C) such that it contains common information about microwaves that travel forward into the direction to the oven cavity (4) and about microwaves that travel backwards through the waveguide (8) .

6. Microwave oven (1) according to one of claims 1 to 5, wherein the controller (10) is configured to control the magnetron (6) in response to the information or the common information by means of changing an output power of the magnetron (6) and / or a timing of the radiation of the microwaves .

7. Microwave oven (1) according to one of claims 1 to 6, wherein the controller (10) is configured to derive from the coupler signal (C) a measure for the backward power of the microwaves travelling backwards and a measure for the forward power of the microwaves travelling forward through the waveguide (8) , to determine a standing wave ratio (SWR) on the basis of the forward and the backward power and to control at least a power source for the magnetron (6) in response to the standing wave ratio (SWR) .

8. Microwave oven (1) according to one of claims 1 to 7, wherein the waveguide (8) comprises a deformable portion (20) , wherein the microwave oven (1) comprises an actuation device (24) connected to the deformable portion (20) for transmission of a deformation force to the deformable portion (20) , and wherein the controller (10) is configured to control the actuation device (24) to deform the deformable portion (20) in response to the information or the common information.

9. Microwave oven (1) according to one of claims 1 to 7, wherein the controller (10) is configured, in the context of a thawing mode, to determine during a course of the thawing mode's operation whether a maximum of microwave absorption within the oven cavity (4) has been reached and, when the maximum has been detected, to reduce the output power or to stop operation of the magnetron ( 6 ) .

10. Microwave oven (1) according to one of claims 1 to 9, wherein the controller (10) is configured, in the context of a casserole cooking mode, to determine during a course of the casserole cooking mode's operation a slope of a course of microwave absorption within the oven cavity (4) over time and, when the slope has reached a predetermined value, to directly or indirectly reduce the output power, to stop operation of the magnetron (6) and / or to issue a notification to a user.

11. Microwave oven (1) according to one of claims 1 to 10, wherein the controller (10) is configured to monitor, at least at the start of a cooking operation, whether the oven cavity (4) is loaded with cooking goods by means of the information and / or the common information and if the oven cavity (4) is not loaded with cooking goods to trigger a measure to avoid damage to the magnetron ( 6 ) .

12. Microwave oven (1) according to claim 11, wherein the controller (10) is configured to compare the information, preferably the standing wave ratio, to a predetermined threshold, and if the information transgresses the threshold in a predetermined manner and for a predetermined amount of time to trigger the action.

13. Microwave oven (1) according to one of claims 1 to 12,wherein the controller (10) is configured to monitor the coupler signal (C) , especially the information or the common information about the microwaves travelling within the waveguide, respectively, preferably the standing wave ratio (SWR) for a peak, and to interpret such peak as an indication of a sparking event, preferably, wherein the controller is configured to take such indication as a trigger to control the operational state of the microwave oven (1) .

14. Microwave oven (1) according to claim 13, wherein the controller (10) is configured to set a sample rate of the directional coupler (30) or of a read out of the coupler signal (C) at a sufficiently high value, preferably at 10 to 20 Hz, and / or to decrease the sample rate over the duration of the operational state of the microwave oven (1) , starting at 10 to 20Hz .

15. Method for controlling a household microwave oven (1) , especially a microwave oven (1) according to one of claims 1 to 12, wherein the microwave oven (1) comprises- an oven cavity (4) for receiving cooking goods,- a magnetron (6) for emitting microwaves,- a waveguide (8) for guiding the microwaves from the magnetron(6) to the oven cavity (4) , and- a directional coupler (30) aligned at the waveguide (8) in a way to detect an amount of microwaves travelling through the waveguide (8) and to emit a coupler signal (C) based on the detection of the microwaves, wherein the method comprises the steps of+ deriving from the coupler signal (C) information about microwaves travelling forward and / or backwards into the direction to the magnetron (6) through the waveguide (8) , and + controlling the operational state of the microwave oven (1) based on that information.27

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