Microwave oven

Abstract: A microwave oven comprises an oven cavity, a microwave radiation source for feeding microwaves to the cavity, and an oven door with a window. Mist formation on the inside of the window is prevented by an air stream directed thereto. A shutter device, which comprises a shutter housing (10) and a shutter plate (11) that is pivotable about a pivot axis (25), is arranged for shutting off the air stream. The shutter plate (11) is pivoted by an operating means (13), which produces a movement in line with the pivot axis, and a motion converter (12), which converts this movement to a pivotal movement of the shutter plate (11).

Description: FIELD OF THE INVENTION This invention relates to a microwave oven comprising an oven cavity, a microwave radiation source for feeding microwaves to the cavity, and an oven door for closing the cavity, at least part of the oven door forming a window for optical inspection of food placed in the cavity, and mist formation on the inside of the window facing the cavity being prevented by an air stream, which can be shut off by means of a shutter device having an adjustable shutter plate, which is pivotable about a pivot axis between an open position, in which the air stream is allowed to pass, and a closed position, in which the air stream is shut off. As a rule, the door of a microwave oven of the above type is provided with a window enabling the user to see the food being cooked. When cooked, food emits water vapour, which tends to condense when coming into contact with the less hot surfaces of the oven, including the window in the oven door. As a result, mist will form on the inside of the window, thus obstructing or preventing observation of the food in the oven cavity. In order to obviate mist formation, an air stream is directed towards the inside of the window. For instance, the air stream can be supplied through a perforation in one side wall of the cavity adjacent to the oven door. After absorbing the mist on the inside of the window, the air stream is drawn off through a corresponding perforation in the opposing side wall of the cavity. The air stream may, for example, be generated by drawing off part of the cooling air used for cooling a transformer in the power unit of the microwave oven, as well as the magnetron serving as microwave radiation source. Modern microwave ovens further have access to a hot-air function, which means that a flow of hot air is produced in the cavity in order to heat the food. The hot-air function may be used either separately or in combination with microwave heating. Hot-air heating is typically used when baking buns. If hot-air heating is to yield good results, the flow of hot air has to be evenly distributed in the cavity. The above-mentioned air stream used for obviating mist formation on the window of the oven door disturbs the flow of hot air in the oven and impairs the cooking results in those parts of the oven cavity where the air stream passes, for instance such that the buns located in these parts of the cavity are not done. The disturbance caused is due to the air stream having a lower temperature than the flow of hot air. In order to avoid any such disturbance, the air stream has to be shut off when the hot-air function of the oven is used. DESCRIPTION OF THE PRIOR ART The prior art encompasses the use of a shutter device having a shutter plate that can be opened and closed in order to shut off the air stream mentioned above. There are various known mechanisms for operating the shutter plate. Thus, the shutter plate can be manually operated via a control provided at the front of the oven, the movement of the control being transferred to the shutter plate via levers and link arms. A return spring serves to return the shutter plate to the open position. This mechanism has the disadvantage of comprising a large number of components that require many manufacturing steps and, hence, are expensive to make. Also, there is a great deal of noise when the shutter plate is adjusted. Generally speaking, manual adjustment of the shutter plate by means of such a mechanism must be regarded as a technical stage that has passed, considering that today's microwave ovens are highly automated. In another prior-art mechanism, use is made of an electric motor, which opens and closes the shutter with the aid of a step-down gear and a cam curve. The electric motor is controlled with the aid of limit switches further increasing the costs of the mechanism. In yet another prior-art mechanism, use is made of a pull-type electromagnet which, via a lever, acts on the shutter, which is returned to its initial position by a return spring. In order to avoid overheating of the electric coil forming part of the mechanism, there have to be provided components for regulating the operating current. This type of construction is distinguished by the loud bang produced when the shutter plate is opened or closed. A common denominator of the prior-art mechanisms for operating the shutter plate is that they are fairly bulky and thus more or less unsuitable for today's compact oven designs. SUMMARY OF THE INVENTION The object of the invention is to provide a shutter device having an operating mechanism that does not suffer from the drawbacks of the prior art. According to the invention, this object is achieved by a microwave oven as defined in claim 1 which is of the type mentioned by way of introduction and is characterised in that the shutter device comprises, for adjusting the shutter plate, an electrically-controllable operating means having a motion means, which is located in the extension of the pivot axis and which, upon actuation of the operating means, is displaced in the direction of the pivot axis; and a mechanical motion converter for converting the axial movement of the motion means to a pivotal movement for pivoting the shutter plate, the motion converter having an inlet engaging the motion means so as to take up the axial movement thereof, as well as an outlet engaging the shutter plate for transmitting the pivotal movement. The combination of an operating force in the direction of the pivot axis and the above motion converter enables a production-friendly motion mechanism having a compact design and few component parts and being suited for automatic adjustment of the shutter plate. A special object of the invention is to provide adjustment of the shutter plate with no noise coming either from the motion mechanism itself or from the shutter plate when passing to the open or the closed position. According to the invention, this object is achieved by a preferred embodiment of the microwave oven, which is characterised in that the operating means comprises a component having a heat-sensitive body and an electric heating element, said body being heated and expanded upon the supply of an actuating signal and causing the displacement of the motion means, thereby producing a closing movement of the shutter plate, whereas an opening movement of the shutter plate is produced when the actuating signal ceases and said body contracts. Such a thermal operating means is essentially soundless. At the same time, the opening and closing movements of the shutter plate become fairly slow, so that as little noise as possible is produced at the end positions of the shutter plate. This operating means further has the advantages of a very compact design, a high reliability as well as a good precision in the opening and closing movements. The fact that the thermal operating means is fairly slow is of no practical importance in view of the cooking times involved when using the hot-air function of the oven. Further embodiments of the microwave oven according to the invention are stated in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS A non-restricting, preferred embodiment of the invention will be described below with reference to the accompanying drawings, in which Fig. 1 is an exploded view showing the component parts of the microwave oven that are relevant to the invention, Fig. 2 is a side view of the shutter device, Fig. 3 is a side view of the shutter plate, Fig. 4 is a side view of the electric operating means, Fig. 5 is a side view of the motion converter, and Fig. 6 is a cross-section of the motion converter taken along line A-A in Fig. 5. In the exploded view of Fig. 1 showing a microwave oven according to the invention, the outer casing, the control panel and the door of the oven have been removed. Thus, Fig. 1 shows the oven components that are relevant to the invention. The oven cavity and the associated parts of the oven chassis are designated 1. The cavity opening, which is closed by the oven door (not shown) when the oven is being used, is designated 2. Reference numeral 3 designates a rotary bottom plate provided at the bottom of the cavity. Further, reference numeral 4 designates a waveguide for feeding microwaves to the oven cavity. A magnetron 5 serves as the microwave radiation source of the oven, and its underside carries an aerial which, when the magnetron is mounted on the waveguide 4, is introduced into the marked hole therein. Reference numeral 6 designates the power unit of the oven, which includes a transformer. Further, a cooling-fan device 7 is provided for cooling the magnetron 5 and the transformer. The cooling air is drawn in through the hole 7' and is discharged essentially horizontally at 7" on the fan device. A first air interceptor 8 is arranged to intercept part of the air flow from the fan device and direct it towards the magnetron 5 and into a second air interceptor 9, which serves to intercept part of the flow of cooling air and form the above-mentioned air stream for obviating any mist on the window of the oven door. This part of the air flow is, by the air interceptor 9, directed towards the shutter device of the microwave oven. The shutter device consists of a shutter housing 10, a shutter plate 11, a motion converter 12 and an electrically-controllable operating means 13. The shutter housing 10 is intended to be mounted over perforations 14 in the side wall of the cavity. Through the perforations 14, there arises an air stream directed towards the window (not shown) of the oven door. In the opposing side wall of the cavity, there are corresponding outlet perforations 15 for the air stream. Fig. 2 is a side view of the shutter device comprising the shutter housing 10, the shutter plate 11, the motion converter 12 and the electric operating means 13. The shutter plate 11 is shown in closed position, in which it closes the opening of the shutter housing. Further, the shutter housing is provided with a circumferential flange 16 to be applied against the cavity wall. The shutter plate 11 has an upper pivot pin 17 and a lower pivot pin 18 (see Fig. 3). The pivot pin 17 is equipped with a flange 19, and the pivot pin 18 is equipped with two flanges 20, 21, which are spaced apart by an intermediate bearing member 22. The pivot pin 17 is inserted in a bearing hole 23 (see Fig. 2) in the bearing housing, while the bearing member 22 rests in an open bearing groove 24 in the shutter housing. The motion converter 12 is slidably mounted in the shutter housing so as to be displaceable in the direction of the pivot axis 25. As shown in more detail in Fig. 5 and the cross-section of Fig. 6, the motion converter is in the form of an elongate cylinder having two diametrically longitudinal raised portions 26, 27. The cylindrical body has an internal hollow cylinder, which has two opposed helical grooves 28, 29 (indicated by dashed lines in Fig. 5). At the bottom, the helical grooves 28, 29 end in a transversal hole 30 through the cylindrical body. The cylindrical element of the motion converter 12 rests in a correspondingly-shaped slide-bearing member in the shutter housing, while the raised portions 26, 27 rest on adjoining, straight bearing surfaces. A bridge element with corresponding bearing surfaces (not shown in Fig. 2) forms a slide bearing for the motion converter, whose surrounding bearing surface has a shape that corresponds to the outline of the cross-section of Fig. 6. Owing to this bearing arrangement, the motion converter can be displaced in relation to the shutter housing in the direction of the pivot axis 25, while any pivotal movement of the motion converter in relation to the shutter housing is prevented by the raised ortions 26, 27 and the corresponding guide grooves in the shutter housing. In addition to the cylindrical body with the helical grooves 28, 29, the motion converter comprises a groove-following means having a shaft member, whose centre coincides with the pivot axis 25. In the illustrated embodiment, the shaft member and the groove-following means are formed of, respectively, the pivot pin 18 and a T-shaped end portion 31 thereon (see Fig. 3). The arms of the T-shaped end portion 31 are inserted in the respective helical grooves when the motion converter 12 is placed in the above-mentioned slide bearing. When the motion converter 12 is moved upwards or downwards, as seen in the drawing, the T-shaped end portion 31 follows the helical grooves 28, 29 and thus causes the shutter plate 11 to pivot. The shutter housing 10 is provided with a fastening device 32 for the operating means 13. As appears from Fig. 4, the operating means has fastening lugs 33, 34 enabling it to be screwed into corresponding screw holes 35, 36 in the fastening device 32. Further, the operating means comprises a cylindrical motion means 37, which is equipped with an upper and a lower flange 38 and 39, respectively. The motion converter 12 has hook-equipped, flexible gripping arms 37a, 38a, as well as a centre hole 40. When the motion converter 12 is mounted, the gripping arms 37a, 38a are pressed over the flange 38, such that their hooks are brought into engagement with the underside of the flange. At the same time, the motion means 37 is inserted in the centre hole 40 to engagement therewith. The distance between the points of attachment of the gripping arms 37a, 38a on the motion converter is smaller than the diameter of the flange 38, such that the gripping arms are directed obliquely outwards when engaging the flange 38. This ensures the capacity of the gripping arms to take up tractive forces, and hence ensures the closing movement of the shutter plate, also at temperatures at which the material of the gripping arms softens somewhat. Moreover, the operating means 13 includes a component marketed under the tradename of "Termoactuator". This component comprises a wax body, which expands when heated, and a heating element, which is actuated by the supply of mains voltage via connectors 41. When the wax body is heated, the motion means of the component at issue is displaced approximately 5 mm. By a suitable choice of the pitch of the helical grooves 28, 29, this movement is converted to the aimed-at pivoting angle of the shutter plate 11. Conveniently, the helical grooves 28, 29 are so designed that there is a certain play between the grooves and the T-shaped end portion 31 at the end positions of the shutter plate. However, the shutter plate 11 is equipped with a circumferential, upright edge 42 close to its edge sides. This edge is indicated by a dashed line in Fig. 3. The edge 42 is introduced in the opening of the shutter housing when the plate is closed and there generates a flow resistance to air trying to pass the closed plate. As a result, the air generates a compressive force that is applied against the shutter plate, which contributes to seal the plate without the use of any special seals. Alternatively, the shutter plate may be sealed by replacing the pivot pins 17, 18 with a shaft that is movable in relation to the plate blade and is biased against the blade in customary fashion. By adjusting the pivoting angle of the shutter plate, the latter can be maintained pressed against the shutter housing by the bias when in its closed position. When the oven is in operation, the shutter device operates as follows. When the hot-air function of the microwave oven is selected, the control unit of the oven sees to it that mains voltage is automatically supplied to the "Termoactuator". The wax body of the "Termoactuator" is heated, and the motion means 37 is displaced upwards, as seen in the drawing, as is the motion converter 12 while sliding in the slide bearing mentioned above. The T-shaped end portion 31 follows the helical grooves 28, 29 and thus produces a closing movement of the shutter plate 11. When the hot-air function of the oven is shut off, the actuating signal ceases, the wax body cools and contracts, and the motion means 37 is displaced downwards, as seen in the drawing, entraining the motion converter 12 via the gripping arms 37a, 38a, and thus producing an opening movement of the shutter plate 11. As appears from the drawings, all the means and devices used for opening and closing the plate are centred with respect to the pivot axis 25. Both the shutter housing and the shutter plate are preferably made of injection-moulded plastics, which enables good dimensional accuracy. All in all, this means that the construction is favourably loaded, which naturally contributes to the good reliability of the construction. From the point of view of production, the shutter device according to the invention is advantageous in that it can be mounted as a prefabricated module containing all the component parts involved.

Claims: A microwave oven comprising an oven cavity (1), a microwave radiation source (5) for feeding microwaves to the cavity, and an oven door for closing the cavity, at least part of the oven door forming a window for optical inspection of food placed in the cavity, and mist formation on the inside of the window facing the cavity being prevented by an air stream, which can be shut off by means of a shutter device (10, 11, 12, 13) having an adjustable shutter plate (11), which is pivotable about a pivot axis (25) between an open position, in which the air stream is allowed to pass, and a closed position, in which the air stream is shut off, characterised in that the shutter device comprises, for adjusting the shutter plate (11), an electrically-controllable operating means (13) having a motion means (37), which is located in the extension of the pivot axis and which, upon actuation of the operating means, is displaced in the direction of the pivot axis, and a mechanical motion converter (12, 31) for converting the axial movement of the motion means (37) to a pivotal movement for pivoting the shutter plate (11), the motion converter (12, 31) having an inlet engaging the motion means (37) so as to take up the axial movement thereof, as well as an outlet engaging the shutter plate (11) for transmitting the pivotal movement. A microwave oven as claimed in claim 1, characterised in that the motion converter (12, 31) comprises a screw device having a first and a second element for producing the motion conversion, wherein the first element (12) is provided with a hollow cylinder, whose axis coincides with the pivot axis (25) and whose surface is formed with a helical groove (28, 29), and the second element is provided with a shaft member (18), whose centre coincides with the pivot axis, and a groove-following means (31), which is disposed on the shaft member and adapted to engage the helical groove so as to follow it during axial displacement of the motion means (37). A microwave oven as claimed in claim 2, characterised in that the surface of the hollow cylinder is formed with two diametrically opposed helical grooves (28, 29), and the groove-following means consists of a T-shaped end portion (31) on the shaft member, the respective arms of the T-shaped end portion engaging the opposed helical grooves. A microwave oven as claimed in claim 3, characterised in that the hollow-cylinder-equipped first element (12) forms the inlet of the motion converter and engages the motion means (37), wherein the shaft member with the T-shaped end portion (31) forms part of a pivot pin (18), which is fixedly mounted on the shutter plate (11) and which, along with another pivot pin (17), is part of the suspension of the shutter plate in the shutter device and forms said pivot axis. A microwave oven as claimed in claim 4, characterised in that the pivot pins (17, 18) form part of a suspension shaft, which is mounted on the shutter plate (11) with torsional bias in the closing direction of the plate, and the path of movement of the motion means (37) and the pitch of the helical grooves (28, 29) are adapted to produce such an angle of rotation of the suspension shaft that the shutter plate (11) is maintained under engagement pressure in the closed position owing to said bias. A microwave oven as claimed in any one of claims 2, 3, 4 and 5, characterised in that the hollow-cylinder-equipped first element (12) engages the motion means (37), wherein the hollow-cylinder element on its outside is provided with at least one longitudinal, raised portion (26; 27) in the direction of the pivot axis, and the shutter device is formed with a guide groove in which said raised portion runs when the motion means, and hence the hollow cylinder, is displaced, such that torsional forces acting on the hollow cylinder are taken up by the guide groove and any torsional stress on the motion means is eliminated. A microwave oven as claimed in any one of claims 2, 3, 4, 5 and 6, characterised in that the shaft member (37) of the motion means is provided with a circumferential flange (38), wherein the hollow-cylinder-equipped first element is provided with at least two gripping arms (37a, 38a), which are diametrically located in relation to the pivot axis and are flexible transversely of this axis, as well as one contact surface for engaging an end portion of the shaft member, each gripping arm is provided with an engagement hook facing the flange, and the gripping arms are so mounted in the hollow cylinder as to be directed obliquely outwards away from the pivot axis when the hooks engage the flange. A microwave oven as claimed in any one of the preceding claims, characterised in that the shutter device comprises a shutter opening that is closable by the shutter plate (11) in its closed position, wherein the shutter plate has, on the side facing the opening, an upright sealing edge (42) arranged round the plate close to its periphery. A microwave oven as claimed in any one of claims 6, 7 and 8, characterised in that the shutter device comprises a shutter housing (10) having bearings (23, 24) for the pivot pins (17, 18) of the shutter plate (11), a fastening device for the operating means (13), and said guide groove for the raised portion of the hollow-cylinder element, and the shutter housing and the shutter plate are made of injection-moulded plastics. A microwave oven as claimed in any one of the preceding claims, characterised in that the operating means comprises a component having a heat-sensitive body and an electric heating element, said body being heated and expanded upon the supply of an actuating signal and causing the displacement of the motion means (37), thereby producing a closing movement of the shutter plate, whereas an opening movement of the shutter plate is produced when the actuating signal ceases and said body contracts. A microwave oven as claimed in any one of the preceding claims, characterised in that the oven comprises a control unit for controlling selectable cooking programs and heating modes, which are selected via a control panel arranged on the oven, and wherein such a selectable heating mode involves the use of a hot-air function, wherein the control unit is adapted to automatically supply an actuating signal to the operating means so as to close the shutter plate when the hot-air function of the oven is selected.

Inventors: OVE FREDRIKSSON, OVE, FREDRIKSSON, Ove, Fredriksson, c/o Whirlpool Europe s.r.l.
Applicants: WHIRLPOOL EUROP, WHIRLPOOL EUROPE B.V.
Application year: 1995
Main classification: H05B6
IPC codes: H05B6