Microwave oven and method for controlling a microwave oven

Abstract

The invention refers to a 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), the waveguide (8) having a deformable portion (20), and an actuation device (24) comprising an actuator (26) connected to the deformable portion (20) for transmission of a deformation force to the deformable portion (20). The deformable portion (20) comprises a metallic surface that is closed with respect for microwaves.

Description

[0001] Specification

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

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

[0004] 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 suf ficiently free to move ) . The advantage is that microwaves have usually a signi ficantly deeper penetration into those materials compared to thermal radiation, especially by infrared wavelength . That enables relatively quick warming of food .

[0005] 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 , and are reflected by its walls and absorbed by the molecules of the food . The reflected microwaves within the chamber cause interference maxima ( also : "hot spots" ) where more energy is trans ferred to the molecules . That causes food to be warmed non- homogeneously . To reduce that ef fect , stirrers (mostly metal fans that reflect the microwaves di f ferently to the chamber ) and / or rotating plates for the food are used . Additionally, the amount of food placed within the chamber also influences the field distribution as well as the energy needed for being suf ficient for warming, especially since the impedance within 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 trans fer is not ef ficient .

[0006] The invention is based on the obj ect to enable for an improved microwave oven .

[0007] That obj ect is solved according to the invention by a 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 13 . Further expedient and optionally inventive per se developments are described in the dependent claims and the following speci fication .

[0008] The Microwave oven according to the invention comprises an oven cavity for receiving cooking goods , a magnetron for emitting microwaves , and a waveguide for guiding the microwaves from the magnetron to the oven cavity, the waveguide having a deformable portion . Further the microwave oven comprises an actuation device - which in turn comprises an actuator - connected to the deformable portion for transmission of a deformation force to the deformable portion, especially during or for an intended operation of the microwave oven . The actuation device is preferably configured to generate the deformation force based on an input signal ( or control value ; especially provided by a controller of the microwave oven) . The deformation of the deformable portion is preferably intended to adj ust an impedance to match to a load and configuration within the oven cavity, e . g . to a food ( cooking good) placed inside the oven cavity at a certain height . According to the invention the deformable portion comprises a metallic surface that is closed with respect for microwaves ( further referred to also as : "microwave tight" ) . In particular, the deformable portion comprises a conductive metallic surface and / or a conductive metallic net that guides microwave signals by reflecting them of f its inner walls through successive reflections .

[0009] The waveguide preferably comprises a tunnel leading from the magnetron to the oven cavity . That tunnel may be of a polygonal cross section or of an oval cross section - e . g . having an elliptical or circular shape . 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 .

[0010] In particular, the deformable portion comprises , or is , a flexible structure . The flexible structure has the metallic surface . For example , the deformable portion can be or comprise a metallic net . In particular, the deformable portion is a deformable metallic net , more in particular deformable by means of the deformation force . Alternatively or in addition, the deformable portion can be or comprise a flexible metal sheet .

[0011] Preferably, the deformable portion is a portion of the waveguide that is deformable , in particular in response to the transmission of the deformation force , by bending and / or reshaping the deformable portion . In particular the transmission of the deformation force results in a bending and / or a reshaping of the deformable portion .

[0012] In particular, the metallic surface of the deformable portion is variable , so that the surface and / or the shape of the deformable portion can be changed or altered, more in particular by means of the transmission of the deformation force applied by the actuator .

[0013] In particular, the deformable portion is a flexible wall transition portion or part between a first wall portion of the waveguide and a second wall portion of the waveguide arranged angularly to the first portion, for example arranged in an angle between 70 ° and 110 ° . The flexible wall transition portion or part defines a cross section of the waveguide . By means of the transmission of the deformation force applied by the actuator to the deformable portion, the shape of the flexible transition portion or part , and hence the cross section and / or the volume of the waveguide , is changed and / or altered .

[0014] The grid width of the metallic net is or can be such that it prevents microwaves from escaping the waveguide . In particular, the metallic net acts or can act like a Faraday cage in regard to blocking and / or reflecting electromagnetic fields , in particular microwave fields . In particular, the deformable portion reflects microwaves and / or is impermeable for microwaves .

[0015] More in particular, the deformable portion can be and / or consist of any of a metallic net , a wire mesh, a bendable or flexible sheet metal or a metal mesh . The deformable portion is a flexible portion, in particular a flexible portion or part in regard to its form or shape .

[0016] The deformable portion enables a flexible adj ustment of a cross section and / or of the volume of the waveguide .

[0017] The waveguide according to the invention comprises a deformable portion for flexibly and / or precisely adj usting a cross section and / or a volume , in particular inner volume , of the waveguide .

[0018] This flexible adj usting enables a precise matching of the impedance of the waveguide system in relation to the impedance of the oven cavity, and therefore an improved transmission of the microwave energy .

[0019] "deformable" in particular means that the shape , for example a cross section and / or a volume , of the deformable portion can be deformed, more in particular by means of the deformation force applied by the actuator to the deformable portion . In particular, "deformable" refers to the deformable portion being or comprising a flexible structure that can be deformed, for example a metallic net .

[0020] In particular, the deformation force is a force that deforms the deformable portion . For example , the deformation force is a force that deforms the metallic net . Preferably, by its transmission, the deformation force is applied to the deformable portion by means of the actuator . A deformation of the deformable portion is , in particular, an adj ustment of the shape of the deformable portion . In particular, this results in an adj ustment of a cross section and / or volume of the waveguide .

[0021] The transmission of the deformation force results in a deformation of the deformable portion . By means of the deformation, a cross section and / or volume of the deformable portion is changed . Therefore , also a cross section and / or volume of the waveguide is changed .

[0022] The transmission of the deformation force to the deformable portion having a flexible structure enables a precise adaption of the shape of the deformable portion and hence a precise adaption of the shape of the waveguide, for example of its cross section and / or volume. This enables a precise adaption of the impedance of the magnetron and the waveguide system to the impedance of the oven cavity. Hence, the invention enables a precise matching of the impedance of the magnetron and the waveguide system in relation to the impedance of the oven cavity.

[0023] 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 the invention which, in turn, enables impedance matching to a topical situation of the oven cavity, at least within a certain range .

[0024] According to an expedient embodiment, the deformable portion comprises a flexible structure that has the microwave tight metallic surface.

[0025] "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 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., a multiple times, than its surface area) . 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 .

[0026] 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 reali zed 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 .

[0027] In particular, the deformable portion comprises or is a variable metallic surface of the waveguide . According to an embodiment , the deformable portion comprises or is a metallic net . The metallic net can be used as a surface , especially variable surface , inside the waveguide . In particular, using a metallic net as a surface inside the waveguide can create a variable volume that enables adapting of the waveguide for a di f ferent cavity matching . Thus , according to the invention, the geometry of the waveguide is variable , by means of transmitting the deformation force to the deformable portion, and a variable geometry concept is obtained for the waveguide .

[0028] In particular, a variable surface allows to change the functional volume of the waveguide while keeping the electrical continuity in or over the entire inner surface of the waveguide . In particular, in relation to microwaves , the deformable portion, more in particular the variable metallic surface , is considered or behaves in the same manner as a solid metal .

[0029] The actuator can move the metallic net and / or the variable surface . In particular, the actuator, more in particular an active part of the actuator, for example a linear motor, can be arranged or put outside a housing . This allows or can allow a better temperature management . The motor for the actuator can be part of a stirrer system .

[0030] 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 reali zed 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 .

[0031] The net shape also enables cooling of the magnetron from the waveguide side when the meshes are used to letting a cooling airflow enter the inside of the waveguide . In that case , the microwave oven expediently comprises a generator for such cooling airflow that is configured to direct that airflow through the net shape into the tunnel of the waveguide .

[0032] 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 reali zed, 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 reali zed by riveting or the like . According to an expedient embodiment being alternative or even additional to the flexible structure , the deformable portion comprises a movable wall part that is part of a guide wall system of the waveguide . The guide wall system comprises , in particular, the above mentioned walls of the tunnel of the waveguide . Thus , the walls of the guide wall system make up the outer bounds for the microwaves that are to be transported ( guided) from the magnetron to the oven cavity . Within that embodiment the movable wall part may be , e . g . , a side wall or a part of a side wall of the waveguide ( especially its tunnel ) , preferably in the case of a polygonal cross section, that may be moved by means of the actuation device such that the area of the cross section of the tunnel is reduced or increased . Said wall part is preferably sti f f ( rigid) in that case .

[0033] According to an optional embodiment , the deformable portion comprises ( in addition to or as an alternative to the aforementioned flexible structure and / or the movable wall part ) a movable screening element . Such screening element is aligned within said tunnel of the waveguide . Especially, such screening element is reali zed by a ( especially polygonal ) rod (pillar ) or flap that is rotatably and / or laterally movable within the tunnel . Especially, the screening element is a so-called multi-mirror with respect to microwaves ( and in particular made of metal ) . Such a multi-mirror comprises several mirror surfaces with respect to microwaves that are aligned obliquely to each other . The flap may have a rather flat ( disc-like ) shape and may be used to more or less "open" or block the tunnel .

[0034] According to an expedient embodiment , the deformable portion and the actuation device - at least a force transmission part of the actuation device - are covered by a safety housing on a side opposite to an intended wave path of the microwaves. E.g., the tunnel of the waveguide is covered on its outside by the safety housing, at least around said regions. By that, microwave leakage to the outside may be prevented. Optionally, the actuation device is covered completely by the safety housing. Otherwise, the actuation device does not have to be covered completely by the safety housing. E.g., only a piston, a spindle or the like (resembling the force transmission part mentioned before) may be covered by the safety housing. In the latter case, a gap between the safety housing and the force transmission part has to be such small that microwaves may not pass that gap. The safety housing prevents leakage of microwaves at the deformable portion, especially in cases of a damaged deformable portion, and, thus, ensures security of the microwave oven.

[0035] 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 .

[0036] According to a development of the aforementioned embodiment , the flexible structure of the deformable portion is provided with a stabili zing substrate . That substrate may enable or at least improve a homogenous translation of the flexible structure over the width ( e . g . above-mentioned side of the cross section, especially its broad side ) of the waveguide or the deformable portion, respectively . Such substrate is , especially, a plastics film onto which the wired net or the punched foil is laminated or the printed net is printed to . The plastics film may provide for an elastic substrate and additionally for galvanic insulation of the flexible structure regarding the actuator . "Homogenous translation" is here and in the following understood in particular as (without or at least with only little ondulation or warping of the surface due to high flexibility of the flexible structure as well as due to a point-like contact point with the actuator .

[0037] In order to enable a deformation of the deformable portion being as homogenous as possible , the actuation device comprises multiple actuators ( i . e . , more than one ) for driving the flexible structure over the width of the waveguide , equally .

[0038] According to a preferred embodiment , the respective actuator is configured as an axial actuator . An axial actuator is typically an actuator which provides its driving force and driving movement along a longitudinal axis in contrast to usual ( gear ) drives that provide a rotational movement . The axial actuator may be a linear drive , a spindle drive , a piston drive or the like. The force transmission part in these cases are, e.g., the spindle, the piston etc.

[0039] According to a further preferred embodiment, the actuation device (especially, the respective actuator, in particular, the respective force transmission part) is electrically isolated from the flexible structure at a force transmission juncture between both. That ensures a microwave impermeable construction of the deformable portion and the actuation device.

[0040] According to yet a further preferred embodiment, the microwave oven comprises a controller. That controller is configured to control the actuation device in dependence on a load within the oven cavity and / or a topical (current) size of the volume of the oven cavity. By that, especially impedance matching of a microwave emission and guiding system (i.e., the magnetron and the waveguide) to the configuration and load within the oven cavity is enabled (and preferably performed) .

[0041] In particular, the controller is configured to gather information about the load, i.e. the kind and size of cooking goods (food, foodstuff) within the oven cavity, and / or the topical size of the volume of the oven cavity. The volume of the oven cavity is in general defined by a cavity housing having cavity walls (usually a bottom wall, a backside wall, a top wall, two side walls and a door) and may be adjusted by a "cavity divider". That cavity divider is typically a metal tray that may be inserted in different heights within the oven cavity and onto that food is placed during intended operation of the microwave oven . According to a development of the aforementioned embodiment , the microwave oven comprises an input interface for gathering information about the load within the oven cavity and / or the topical si ze of the volume of the oven cavity and for providing that information to the controller .

[0042] According to a first version of the aforementioned development , the input interface comprises a user interface for a user' s input of information regarding the si ze of the volume of the oven cavity and / or regarding the load, respectively . E . g . , the user interface comprises a touchscreen, knobs or the like which a user can use to input , into the controller, information about the food ( cooking goods ) placed into the oven cavity . In that case , the user may also set a use mode or recipe within the user interface . The user interface may also give further instructions to the user, optionally based on the recipe chosen, e . g . , to align the food in the tray on the lower / the middle / the higher level . Based on the information input into user interface , the controller prompts the actuator to move to predefined position to deform the deformable portion .

[0043] According to a second ( additional or alternative ) version of the aforementioned development , the input interface comprises a camera directed into the oven cavity . In that case , the controller is preferably configured to derive from the pictures taken ( e . g . , via a pattern recognition method) information about the food and / or the volume of the respective food item within the oven cavity .

[0044] Additionally or alternatively to the controlling of the actuation device in dependence on the load and / or the volume of the si ze of the oven cavity, the controller is expediently configured to control the actuation device in dependence on a recipe and / or a function chosen for usage of the microwave oven . The recipe may already include the kind of load ( cooking goods , food; however, not necessarily the volume or amount ) and additionally a roasting or cooking degree . The " function" may include operating schemes such as thawing, reheating and the like . E . g . , the controller is configured to control the actuation device to deform the deformable portion to a shape "A" in the case a cup of liquid is to be heated, to a shape "B" in the case of reheating a plate of food and to a shape "C" in the case of making popcorn . The respective " shapes" are preferably based on experimental data that has been embedded with the controller for respective recipes and functions , respectively, that may be chosen via the input interface by the user .

[0045] The information about the load is preferably information about the food to be cooked within the oven cavity . That information comprises preferably the kind (meat , vegetables , pastries , soup, beverage etc . ) and the volume and / or the weight of the food . The volume and the weight , respectively, provide information about the amount of food to be cooked . Especially an amount of water within the cooking goods ( food) and their volume have ef fects on the impedance of the oven cavity . Additional sensors may also be used for determination of information regarding the load . E . g . sensors for determination of the weight on the tray may be used . Alternatively, the weight may also be derived from the volume of the food and its kind, which in turn may be preferably recogni zed automatically by means of the camera .

[0046] According to a further expedient embodiment , as one of the sensors for deriving information about the load a so called (bi- ) directional coupler is used . Such a directional coupler may be used within high-frequency applications to measure the power (or another characteristic measure) of two waves that are directed in opposite directions. The directional coupler is preferably positioned at the waveguide such that a wave sensing part of it, especially an antenna, is enabled to sense at least an amount of the microwaves travelling within the waveguide. Also, the directional coupler is connected for signal transmission to the controller. Thus, by means of the directional coupler, the controller is configured to derive, as a (in particular combined) measure for the efficiency of the microwave oven's operation (that transports an information about the load) , the power of the waves directed towards the oven' s cavity as well as of the waves reflected backwards into the waveguide and towards the magnetron. The measure may, thus, indicate that too little microwave power is being absorbed (e.g., because the food has too little water) and, so, the impedance of the oven cavity does not match the impedance of the system realized by the magnetron and the waveguide. That measure is, e.g., a so-called standing wave ratio (SWR) . Thus, the usage of the directional coupler enables the controller to react to changes of the load within the oven' s cavity during operation of the microwave oven (i.e., cooking, thawing or the like) , in particular in real time.

[0047] Preferably, that predefined position is taken, by the controller, e.g. from a look-up-table stored within a memory unit assigned to the controller. The look-up-table is, according to a preferred embodiment, filled with information taken from experiments and / or simulations and provides a respective control value (that is used to move the deformable portion to its intended predefined position) for a combination of the load and the size of the volume of the oven cavity. A basis of the impedance matching is usually derived from a so called Rieke diagram which gives information about working points (especially depending on frequency and power) and parameter combinations that would lead to improper or even problematic operating states. The above-mentioned experiments are preferably conducted in view of the Rieke diagram to find configurations of the waveguide (i.e. deformations of the deformable portion) that would lead to proper placements of the working point for the current load and / or volume of the oven cavity within the Rieke diagram. Optionally, the power needed to perform a specific cooking scheme is taken from a user input (e.g., from a recipe chosen by the user) or which is suggested on the basis of the pattern recognition by means of the camera. The power output of the magnetron may also have an effect to the impedance and may therefore be taken into account, e.g. on the basis of the Rieke diagram.

[0048] The method, according to the invention, is intended and used for controlling a microwave oven, especially the microwave oven described herein. According to the method within a first step information (especially the information described above) about the load within the oven cavity and / or about the topical (current) size of the volume of the oven cavity is determined. As said information about the load, preferably, the information regarding the food etc. as described above is taken. As a second step, a control value for the actuation device is derived in dependence on said information. As a third step, the actuation device is controlled to move the deformable portion of the waveguide on the basis (i.e. by means) of the control value, especially to a predefined position.

[0049] 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 before , also .

[0050] According to a preferred embodiment of the method, the control value is derived from a lookup-table ( cf . above ) which contains a number of control values for certain loads and / or si zes of the volume of the oven cavity . Preferably, that lookup-table contains the control values for certain combinations of loads and si zes of the volume . Especially, those the control values are experimental data, based on the experiments described above .

[0051] Optionally, an arti ficial intelligence algorithm is used to derive the control value based on the information about load and si ze of the volume of the oven cavity . Such algorithm is preferably completed taught-in on the basis of the experimental data ( training data ) and configured to derive on the basis of that training data new control values for combinations of loads and si zes of the volume of the oven cavity which have not yet been tested .

[0052] According to a further preferred embodiment , the information about the load and / or about the topical si ze of the volume of the oven cavity is gathered by means of the camera and automated picture analysis . Especially, the above-mentioned pattern recognition method ( optionally Al based) is used to analyze the pictures taken and to derive the information needed therefrom . Alternatively, as described above , the information may also be derived from a manual input of the user into the mentioned user interface .

[0053] 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 .

[0054] In the following, an embodiment / examples of the invention is / are explained in more detail with reference to a drawing . Therein show :

[0055] Fig . 1 in a principal drawing schematically a microwave oven, and

[0056] Fig . 2 in a detail I I according to Fig . 1 schematically a magnetron and a waveguide of the microwave oven,

[0057] Fig . 3 , 4 in a view as in Fig . 2 schematically further embodiments of the waveguide ,

[0058] Fig . 5 in a top view the waveguide as in Fig . 4 ,

[0059] Fig . 6 in view as in Fig . 3 schematically a further embodiment of the waveguide ,

[0060] Fig . 7 in a schematic flowchart a method for controlling the microwave oven, and

[0061] Fig . 8 in view as in Fig . 2 schematically a further embodi- ment of the microwave oven .

[0062] Parts corresponding to each other are always provided with the same reference signs in all figures .

[0063] Fig . 1 shows a 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, 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 which is not displayed in detail. The microwave oven 1 further comprises a plate (or "tray 14") whereupon cooking goods ("food 16") may be placed within the oven cavity 4 at different heights (cf . dashed lines) .

[0064] The magnetron 6 is connected 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 19 and which serves to guide and transmit the microwaves to the oven cavity 4. The walls 19 are aligned to form the tunnel 18 with a (generally) rectangular cross section. The sum of the walls is also referred to as a guide wall system. In order to be able to adjust an impedance of the microwave emission and guiding system, i.e. of the magnetron 6 and the waveguide 8, the waveguide 8 comprises a deformable portion 20, s. Fig. 2.

[0065] The deformable portion 20 is in the embodiment of Fig. 2 made of a flexible structure, specifically a wired net 22. The wired net 22 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 19 and the wired net 22 the latter is welded to the adjacent metallic walls 19, thus causing a galvanic connection.

[0066] The deformable portion 20 may be deformed elastically by a deformation force (i.e. a certain value of that force) applied by an actuation device 24. The walls 19 are designed rigid with regard to that deformation force value. Specifically, the actuation device 24 comprises as an actuator a spindle drive 26, connected to the controller 10. A spindle 28 of the spindle drive 26 is coupled to the wired net 22. However, the connection is electrically isolated.

[0067] Additionally, at least a part of the waveguide 8 including the deformable portion 20 and a part of the actuation device 24 are housed by a safety housing 30. The safety housing 30 ensures that no microwave radiation leaks even when the wired net 22 is deformed or even damaged.

[0068] Furthermore, the actuation device 24 and the wired net 22 are designed such that a cross section of the waveguide 8 (taken normal to a central axis of the waveguide 8) stays of the same shape during deformation. In the present embodiment, the cross section of the waveguide 8 is rectangular. The deformation of the wired net 22, thus, causes only a size variation of the cross section (i.e., the rectangle gets bigger or smaller) and, thus of the volume of the waveguide 8, overall. For such homogenous deformation the wired net 22 is driven by several spindle drives 26 and / or has a stiffening component (e.g., a comparably stiff substrate, a stiffening beam or the like) that enables that the wired net 22 is not only deformed locally in the region around the contact between the wired net 22 and the spindle 28 but deformed, e.g. pushed into or pulled out of the tunnel 18, at the same amount over the whole width of the tunnel 18 (i.e. along the wall 19 that is normal to the plane of Fig. 2) .

[0069] The variation of the volume of the waveguide 8 enables for an impedance matching to the amount and type of food within the oven cavity 4 (which is referred to also as "load") and to the size of the volume of the oven cavity 4. For that, the controller 10 is configured to gather information about the amount and type of food within the oven cavity 4 from an input interface. The input interface may comprise one or both of a user interface 40 (e.g. a touch display) and a camera 42 directed into the oven cavity 4.

[0070] According to a method, described by means of Fig. 7, the controller 10 determines within a first step SI a size of the volume of the oven cavity 4. Since the tray 14 is made of metal, it restricts the volume of the oven cavity 4 with respect to microwaves in dependence on the height at which it is aligned within the oven cavity 4. The controller 10 derives within step SI at which height the tray 14 is aligned by means of the camera 42 (or alternatively from a user input into the user interface 40) . Within a second step S2, the controller 10 derives information about the load, i.e. the kind and amount of the food 16 placed on the tray 14 (cf . different sizes of food 16 in Fig. 1) . That may also be derived from a user input or automatically by means of a pattern recognition method applied to pictures taken with the camera 42. Optionally, the controller 10 may receive, e.g., by a respective input at the user interface 40, a recipe that should be cooked. From such recipe information about a cooking level (medium, rare, done etc.) and / or about a target power value may be derived.

[0071] Based on the information about the size of the volume and about the food 16, optionally further based on the target power value, the controller 10 derives within a third step S3 a control value for the actuation device 24 from a lookup-table stored within a storage of the controller 10. Within a fourth step S4 , the controller 10 outputs the control value to the actuation device 24 , especially to the spindle drive 26 , which causes the spindle drive 26 to move its spindle 28 to a predefined position corresponding to the control value . Thereby, the wired net 22 is deformed in order to enlarge or decrease the area of the cross section of the tunnel 18 .

[0072] The control values written into the lookup-table are experimental data which is determined from experiments with di f ferent foods 16 ( kind and / or amount ) on di f ferent heights of the tray 14 . The kind and amount of food 16 ( the latter determined preferably from the si ze of the food 16 by means of the pattern recognition method) is linked to an amount of water ( fluid) within the oven cavity 4 which, in turn, influences the impedance of the oven cavity 4 . The impedance of the oven cavity 4 with the food 16 inside , has influence on the working point of the magnetron 6 with regard to a respective Rieke diagram . E . g . , a bag of microwave popcorn is - regardless of its si ze - comparably similar to an empty oven cavity 4 whereas even a small amount of soup or beverage would lead to another location of the working point within the Rieke diagram .

[0073] Fig . 3 shows an alternative embodiment of the deformable portion 20 . Here , the waveguide 8 comprises several deformable portions 20 which are made up by wall parts 50 which are used instead of the respective wall 19 and which can be moved within the tunnel

[0074] 18 by means of respective spindle drives 26 . Also , one deformable portion 20 comprises a stub 52 that is aligned within a wall

[0075] 19 and may be moved through that wall 19 into or out of the tunnel 18 . Fig . 4 and 5 shows a further alternative embodiment of the deformable portion 20 . In that case , the deformable portion 20 comprises a multi-mirror pillar 54 which has the shape of a rhombus ( cf . Fig . 5 ) and which may be moved along the tunnel 18 by means of the spindle drive 26 - which is connected to the multi-mirror pillar 54 through a slit 56 . The position of the multi-mirror pillar 54 along the tunnel 18 also impacts on the impedance of the waveguide 8 .

[0076] Fig . 6 shows a still further embodiment of the deformable portion 20 . In that case , a wall section 58 is aligned movable along the tunnel 18 to open and close respective openings 60 for the microwaves to enter the oven cavity 4 .

[0077] Fig . 8 shows a further embodiment of the microwave oven 1 on the basis of the embodiment shown in Fig . 2 . The microwave oven 1 additionally comprises a directional coupler 70 . That directional coupler 70 in turn comprises an antenna (not shown in detail ) which is aligned within a longitudinal slit 72 within one of the walls 19 of the waveguide 8 . By means of that antenna and further circuitry (not shown in detail either ) , the directional coupler 70 enables the controller 10 to measure the power of the (micro- ) waves directed " forward" ( i . e . , from the magnetron 6 to the oven cavity 4 ) and the power of the waves reflected by the oven cavity 4 into the direction to the magnetron 6 ("backwards" ) . Those "backward waves" are mostly present when an amount of the " forward waves" is not absorbed by the food and reflected back to the Magnetron by the oven cavity 4 .

[0078] The directional coupler 70 is coupled to the controller 10 for signal ( data ) transmission . A relation between the power of the forward waves and the reflected "backward waves" ( also referred to as a "standing wave ratio") is being determined and taken by the controller 10 as a measure for the efficiency of the microwave heating within the oven cavity 4. The higher the power of the waves that are reflected, the less is the heating effect and probably also the higher is a risk for damaging the magnetron 6 (e.g., by overheating) . The controller 10 is configured to adjust, by controlling the actuating device 24, the deformable portion 20 in order to better match the impedance of the system consisting of the magnetron 6 and the waveguide 8 to the impedance of the oven cavity 4 (influenced by the size of the oven cavity 4, the volume and type of the food stuff 16 that is required to be heated) , such that the power of the backward waves is minimized and, consequently, the power of the forward waves is maximized. That impedance matching may be done, e.g., by means of a lookup table, wherein for distinct power relations, distinct shapes or degrees of deformation of the deformable portion 20 are laid down. The directional coupler 70 may also be implemented into the embodiments of Fig. 3-6.

[0079] The subject matter of the invention is not limited to the embodiment described above. Rather, further embodiments of the invention can be derived by the skilled person from the above description .

[0080] List of reference signs

[0081] 1 microwave oven

[0082] 2 housing

[0083] 4 oven cavity

[0084] 6 magnetron

[0085] 8 waveguide

[0086] 10 controller

[0087] 12 cavity housing

[0088] 14 tray

[0089] 16 food

[0090] 18 tunnel

[0091] 19 wall

[0092] 20 deformable portion

[0093] 22 wired net

[0094] 24 actuation device

[0095] 26 spindle drive

[0096] 28 spindle

[0097] 30 safety housing

[0098] 40 user interface

[0099] 42 camera

[0100] 50 wall parts

[0101] 52 stub

[0102] 54 multi-mirror pillar

[0103] 56 slit

[0104] 58 wall section

[0105] 60 opening

[0106] 70 directional coupler

[0107] 72 slit

[0108] 51 step

[0109] 52 step 53 step

[0110] 54 step

Claims

Claims1. 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) , the waveguide (8) having a deformable portion (20) , and- an actuation device (24) comprising an actuator (26) connected to the deformable portion (20) for transmission of a deformation force to the deformable portion (20) , wherein the deformable portion (20) comprises a metallic surface that is closed with respect for microwaves.

2. Microwave oven (1) according to claim 1, wherein the deformable portion (20) comprises a flexible structure that has the metallic surface, especially wherein the flexible structure is a metal and of a net shape with meshes small enough to be impermeable for microwaves.

3. Microwave oven (1) according to claim 1 or 2, wherein the deformable portion (20) comprises a movable wall part (50, 52, 58) that is part of a guide wall system of the waveguide ( 8 ) .

4. Microwave oven (1) according to one of claims 1 to 3, wherein the deformable portion (20) comprises a movable screening element (54) that is aligned within a tunnel (18) of the waveguide (8) .

5. Microwave oven (1) according to one of claims 1 to 4,27wherein the deformable portion (20) and the actuation device (24) are covered by a safety housing (30) on a side opposite to an intended wave path of the microwaves such as to prevent microwave leakage to the outside.

6. Microwave oven (1) according to one of claims 1 to 5, wherein the actuation device (24) and / or the deformable portion (20) 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 (20) and that only an area of the cross section is changed.

7. Microwave oven (1) according to claim 6, wherein the actuation device (24) comprises multiple actuators (26) for moving the flexible structure over the width of the waveguide (8) homogenously.

8. Microwave oven (1) according to one of claims 1 to 7, wherein the actuation device (24) is electrically isolated from the flexible structure (22) at a force transmission juncture between both.

9. Microwave oven (1) according to one of claims 1 to 8, comprising a controller (10) configured to control the actuation device (24) in dependence on a load within the oven cavity (4) and / or a topical size of the volume of the oven cavity (4) .

10. Microwave oven (1) according to claim 9, comprising an input interface (40) for gathering information about the load within the oven cavity (4) and / or the topical size of the volume of the oven cavity (4) .

11. Microwave oven (1) according to claim 10, wherein the input interface comprises a user interface (40) for a user's input of information regarding the size of the volume and the load, respectively, and / or wherein the input interface comprises a camera (42) directed into the oven cavity (4) .

12. Microwave oven (1) according to one of claims 9 to 11, comprising as a sensor for deriving information about the load; in particular wherein the sensor is a directional coupler (70) , preferably wherein the directional coupler (70) is positioned at the waveguide (8) such that a wave sensing part of it, especially an antenna, is enabled to sense at least an amount of the microwaves travelling within the waveguide (8) .

13. Method for controlling a microwave oven (1) according to one of claims 1 to 12, comprising the steps of determining information about the load (16) within the oven cavity (4) and / or about the topical size of the volume of the oven cavity (4) , deriving, in dependence on said information, a control value for the actuation device (24) , and controlling on the basis of the control value the actuation device (24) to move the deformable portion (20) of the waveguide (8) .

14. Method according to claim 13, wherein the control value is derived from a lookup-table which contains a number of control values for certain loads (16) and / or sizes of the volume of the oven cavity (4) , especially wherein the control values are experimental data.

15. Method according to claim 13 or 14, wherein the information about the load (16) and / or about the topical size of the volume of the oven cavity (4) is gathered by means of the camera (42) and automated picture analysis.

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