Technical Background A microwave heater for domestic use, such as a mi- crowave oven, comprises a large amount of electronics for controlling its. different functions. Some of these func- tions are controlled by whether an associated door is open or shut. A typical example of a function which is controlled by the position of the door is feeding of mi- crowaves to the microwave cavity of the oven or heater.

Such feeding should only be possible when the door is in the shut position which seals the microwave cavity. For reasons of safety, one or more switches are used in most cases, which are actuated mechanically when the door is opened or shut.

In the following, the expression"microwave oven" will often be used, although a microwave-based heater in the wider sense of the word is related to. The expression "microwave oven"should thus be understood as a heating device, preferably for domestic use, which causes heating of a load by means of microwaves, the load being placed in a microwave cavity.

Summary of the Invention An overall object of the present invention is to im- prove dependability and life of a microwave-based heater for domestic use, such as microwave ovens and similar ap- paratuses.

The object mentioned above is achieved by means of a microwave oven, an arrangement or a method of the type defined in the appended claims.

The present invention is based on the understanding that an oven and its components can be exposed to great impulsive forces when shutting a door of the oven-up to 200 G has been measured. As regards some of the elec- tronic components available in the oven, comprising for example said switches, such forces may exceed the pro- posed limit values. There is thus a risk of malfunction or breakdown.

According to the present invention, in line with this fundamental understanding, a microwave heater is thus provided, for which the effects of the impulsive force on switches and electronics when shutting the door are essentially eliminated, or at least considerably re- duced.

According to the present invention, a method is fur- ther provided for controlling the operation of a micro- wave oven in accordance with the position of the door of the microwave oven, the effects of impulsive force on switches and electronics when shutting the door being es- sentially eliminated by the method.

In its widest sense, the present invention thus com- prises a microwave oven in which at least one operational switch is resiliently mounted in the oven, possible shocks being absorbed so as not to propagate into the switch. Consequently, the switch is protected in an effi- cient manner from impulsive forces.

According to the invention, the resilient suspension of one or more operational switches may advantageously be provided by mounting these on a circuit board which is

resiliently suspended. Preferably, the switches are sol- dered directly on the circuit board. Yet another advan- tage of the present invention is thus that it is possible to avoid cables and cable lugs which are required accord- ing to prior-art technique for connecting the switches to the circuit boards. Due to the resilient suspension, a considerable reduction of forces on the fixing points (e. g. the soldering points) between the switches and the circuit board is obtained, as well as a reduction of im- pulsive forces on the circuit board itself and components mounted on the circuit board.

It has been found that the resilient suspension of the switch can lead to uncertainty when determining the switch-controlling position of the door in relation to the access opening of the microwave cavity or a frame structure which defines said access opening. According to a further development of the invention, it is thus possi- ble to compensate for a possible displacement of the switch, being caused by said resilient suspension, in re- lation to the frame structure of the microwave cavity.

According to one aspect of the present invention, an arrangement in a microwave oven is provided, as well as a microwave oven provided with such an arrangement which allows elimination of impulsive force on switches and electronics, while the accuracy is maintained when de- tecting the position of the door of the microwave oven in relation to the cavity.

According to one aspect of the invention, a method is provided for controlling the operation of a microwave oven, the motion of the oven door belonging to the micro- wave oven to and from the shut position being used to ac- tuate at least one operational switch by the terminal phase of the motion of the oven door to the shut position and the starting phase of the motion of the door from the shut position being converted to a switch-actuating ro- tary motion. At least when shutting the door said switch is further allowed to move resiliently, while the actua-

tion position of the switch-actuating rotary motion in relation to said switch is at least essentially main- tained so that impulsive forces on said switch, which are caused by the shutting of the door, are reduced while maintaining an unaffected relation between the actuation of the switch and the motion of the door.

According to another aspect of the invention, an ar- rangement is provided which reduces impulsive forces on switches, and preferably also electronic components con- nected therewith, by being resiliently suspended in the microwave oven. A means for compensating for a possible displacement of one or more switches in relation to the frame structure of the microwave cavity is preferably ar- ranged in said arrangement.

In a preferred embodiment of the arrangement accord- ing to the invention, a first axis of rotation is in- cluded, the extension of which is essentially orthogonal to the expected direction of the impulsive forces. Fur- thermore, said first axis of rotation, in the plane per- pendicular to its extension, is non-displaceable in rela- tion to the frame structure of the microwave cavity.

There are actuating means on the oven door which, when the oven door is moved to and from the shut position, are adapted to convert the motion of the door to a rotary mo- tion of a first rotating means round the first axis of rotation. Moreover, a second axis of rotation is in- cluded, the extension of which is also essentially or- thogonal to the expected direction of the impulsive forces and which is, in the plane perpendicular to its extension, non-displaceable in relation to the switches.

The second axis of rotation guarantees a switch-actuating rotary motion of a second rotating means which in an ac- tuation position that is predetermined for each switch actuates the respective switches. The arrangement is de- signed in such a manner that a displacement between the first axis of rotation and the second axis of rotation in a plane perpendicular to the extension of the axes is al-

lowed. It should thus be understood that the directions of axis of the axes are essentially parallel. A displace- ment-compensating means converts a rotation round the first axis of rotation to a rotation of essentially the same magnitude round the second axis of rotation.

The motion of the oven door thus causes a rotary Ro- tion of the first rotating means round the first axis of rotation. The angle of rotation of the rotation round the first axis of rotation is directly related to the posi- tion of the oven door in relation to the frame structure of the microwave cavity. The rotation of the first means of rotation round the first axis of rotation is trans- ferred into a rotation of the same magnitude of the sec- ond rotating means, round the second axis of rotation, the angle of rotation of which thus is also related to the position of the oven door. The respective switches are actuated at an angle of rotation, which is predeter- mined for each switch, of the rotary motion round the second axis of rotation. Although the second axis of ro- tation is displaced due to the above-mentioned resilient suspension of the switches, in relation to the first axis of rotation, the switches will thus be actuated in the intended position of the oven door.

Said transfer of the rotary motion from the first axis of rotation to the second axis of rotation is pref- erably provided by means of a crossbar interface. In this context, the term"crossbar interface"should be under- stood as a connection between two rotary motions, wherein a parallel displacement, within determined limits, is al- lowed between the axes of rotation, while an exact trans- fer of the rotary motion from the first axis of rotation to the second axis of rotation is maintained.

In a preferred embodiment, the crossbar interface comprises a plate between a first and a second rotatingly mounted element, which then constitute the above- mentioned rotating means. In the plate, on a first side thereof, a first sliding groove is arranged, with which

the first element is engaged in order to be able to move in relation to the crossbar interface only along a first direction essentially perpendicular to the axis of rota- tion. Moreover, in the plate, on its other side opposite to said first side, there is a second sliding groove with which the second element is engaged in order to be able to move in relation to the crossbar interface only along a second direction essentially perpendicular to the axis of rotation. Besides, said first and said second direc- tion are non-parallel, a displacement of the first ele- ment in relation to the second element being allowed in a plane which is perpendicular to the axes of rotation.

However, due to the connection, the angle of rotation of the rotation of the first element round the first axis will be transferred to an angle of rotation of the same magnitude of the second element round the second axis.

Thus, the actuation position of the switch-actuating ro- tary motion of the second element in relation to the switches will be essentially unaffected by the resilience of the switches in relation to the frame structure of the microwave cavity.

Brief Description of the Drawings In the following, the invention will be further il- lustrated by virtue of a preferred embodiment. In the de- tailed description below, reference will be made to the accompanying drawings, in which Fig. 1 schematically shows a microwave oven for do- mestic use, in which the present invention is applied, Fig. 2 schematically shows a circuit board holder and a circuit board for resilient mounting in accordance with the present invention, and Fig. 3 schematically shows an exploded view of the preferred embodiment of a switch arrangement according to the present invention.

Description of a Preferred Embodiment Fig. 1 schematically shows a microwave oven 10 for domestic use. The microwave oven comprises an external casing 11 and a frame structure with folded lateral sides or a front plate 12, against which a door 14. is sealed in the shut position. Moreover, in the oven 10 there is a microwave cavity 13, in which food which is to be heated or defrosted is to be placed. Access to the cavity is al- lowed via an opening in the front of the oven 10, the opening being defined by the frame structure 12 and being sealed by means of the door 14 before microwaves are fed to the cavity. One or more locking hooks 15 are mounted on said door, which are arranged to engage with locking devices 16 located in the oven. These locking devices comprise at least one switch arrangement which ensures that microwave feeding to the cavity is only allowed when the locking hooks 15 are in the intended engagement with the locking devices 16.

Apart from the above-mentioned parts, the oven 10 comprises such parts as are usually found in microwave ovens, such as ventilation means 17 for transferring heat out of the cavity, a display 18, a panel 19 for setting the cooking time, push buttons 20 for controlling the oven, start and stop buttons 21, and a plug 22 for con- necting the oven 10 to the mains.

In the microwave oven 10 there is control electron- ics, parts of which, especially the electronics which is actuated by the position of the oven door 14 in relation to the microwave cavity, are mounted on a circuit board.

The position of the oven door is detected by means of one or more switches which, in their turn, actuate the elec- tronics depending on the position of the door. In the preferred embodiment of the present invention, three op- erational switches are used. According to the invention, these switches are at least to some extent resiliently suspended for the purpose of reducing the effects of im-

pulsive forces that may appear when shutting the door 14 against the front plate 12 of the microwave oven.

In the preferred embodiment of the present inven- tion, the switches are soldered directly on a circuit board 25 which, in its turn, is resiliently mounted in a circuit board holder 26. The holder 26 in itself is fix- edly mounted in relation to the front plate 12 of the oven. A schematic picture of a circuit board and a holder in which the circuit board may be resiliently mounted is shown in Fig. 2.

The circuit board holder 26 which is shown in Fig. 2 comprises springs 27 by means of which the circuit board 25 may resiliently snap into the correct position.

According to the preferred embodiment of the present invention, actuation of the switches is effected by the position of the locking hook 15 mounted in the door 14 being converted to a rotation of a lock arm 31 which is mounted in a non-displaceable manner in relation to the front plate 12 of the microwave oven. The lock arm is mounted in a bearing or an attachment 28. in the circuit board holder 26, the locking hook 15, when shutting the door 14, being inserted into an opening 29 in the circuit board holder in order to engage with the lock arm 31. The rotation of the lock arm 31 is at the same time trans- ferred to a rotation of a switch-actuating cam wheel 32 which is mounted in a non-displaceable manner in relation to the switches 33a, 33b, 33c, and which, by its rotation in an actuation position which is predetermined for each switch 33, actuates a breaker of the respective switches.

Due to the resilient suspension of the switches 33, and thus of the cam wheel 32, the position of the cam wheel 32 in relation to the lock arm 31, and thus to the front plate 12 of the microwave oven, will, however, be variable within predetermined limits. According to the invention, the switch arrangement therefore comprises a means for compensating 34 for a resilient displacement of the switches 33 in relation to the lock arm 31.

The preferred embodiment of a displacement- compensating switch arrangement in accordance with the present invention is shown in Fig. 3.

The means for compensating for a resilient displace- ment of the switches 33 in relation to the lock arm 31, and thus in relation to the front plate 12 of the oven, comprises in the preferred embodiment a crossbar inter- face 34. The crossbar interface 34 comprises a plate with a first side and a second side, the connection on its first side being provided with a first groove 35, and on its second side with a second groove 36. The first side rests against the lock arm 31, and the crossbar interface 34 can, by means of said first groove 35, slide in a first direction in relation to the lock arm 31. The sec- ond side of the crossbar interface 34 rests against the cam wheel 32 which, by means of said second groove 36, can slide in a second direction in relation to the cross- bar. interface 34. The first direction and the second di- rection are not parallel to one another, which means that a rotation unambiguously is transferred between the lock arm 31 and the cam wheel 32 at the same time as a sliding motion, within certain limits, is allowed between the lock arm 31 and the cam wheel 32 in a plane perpendicular to the axes of rotation.

A description in more detail of the preferred em- bodiment of the present invention will follow with refer- ence to Fig. 3.

The switch arrangement comprises a switch holder 37, in which a first 33a, a second 33b and a third 33c switch are fixedly mounted. Said switches thus have a fixed lo- cation and a fixed position in relation to the switch holder 37. In the switch holder 37, there is a recess 38 which in the preferred case is essentially a quarter of a circle. In the centre of said recess 38, a pivot 39 is arranged which defines the axis of rotation round which the switch-actuating motion of rotation occurs. The cam wheel 32 is further arranged on the pivot 39 and com-

prises the switch-actuating means which is rotatable round said axis of rotation. In an actuation position of the rotation of the cam wheel 32, the actuation position being predetermined for the first 33a and the second 33b switch, one or more breakers of the respective switches are actuated. It will be understood that the cam wheel\' 32, the switch holder 37 and the switches 33a, 33b, 33c are non-displaceable in relation to one another.

The rotary motion of the cam wheel 32 round its axis of rotation is transferred from the lock arm 31 which is rotatingly and non-displaceably arranged in the bearing 28 in relation to the frame structure 12 of the microwave cavity. The lock arm 31 is arranged to engage with one of the locking hooks 15 mounted on the door 14, whereby the motion of the door in connection with the terminal phase of its shutting and the starting phase of its opening is transferred to a rotary motion of the lock arm 31. The rotation of the lock arm 31 occurs round an axis of rota- tion which is defined by the holder or bearing 28 and which is parallel to, but displaceable in relation to, the axis of rotation of the cam wheel 32. In order to transfer a rotary motion from the lock arm 31 to the cam wheel 32 with a simultaneous compensation for a possible resilient displacement between them, a crossbar interface 34 is arranged between the lock arm 31 and the cam wheel 32.

In the preferred embodiment, the crossbar interface 34 comprises a plate which on a first side is provided with a first sliding groove 35 and on a second side is provided with a second sliding groove 36. The lock arm 31 is, in the normal use of the device, engaged with said first sliding groove 35 for the purpose of allowing a displacement of the lock arm 31 in relation to the cross- bar interface 34 only in a direction along the first sliding groove 35.

Furthermore, the cam wheel 32 is, when the device is used normally, engaged with said second sliding groove 36

for the purpose of allowing a displacement of the cam wheel 32 in relation to the crossbar interface 34 only in a direction along said second sliding groove 36. Conse- quently, a resilient displacement is allowed of the lock arm 31 in relation to the cam wheel 32 in a plane which is essentially perpendicular to said axes of rotation, and thus also a resilient displacement of the switches 33, and the circuit board 25 on which these may be mounted, in relation to the frame structure 12 of the mi- crowave cavity and its sealing door 14. At the same time an exact transfer of a rotary motion of the lock arm 31 is ensured round its axis of rotation to a rotary motion of the same magnitude of the cam wheel. 32 round its axis of rotation. On the whole, independent of such a dis- placement, the switches 33a, 33b will thus be actuated in a predetermined actuation position of the cam wheel 32, the actuation position thus only being dependent on the position of said door 14 in relation to the frame struc- ture 12 of the microwave cavity.

The arrangement also comprises a third switch 33c.

The third switch is not actuated by the cam wheel 32, but by a resilient pin 40 which is pressed in by the door 14 when shutting against the frame structure 12 of the mi- crowave cavity. As regards this switch, the accuracy with reference to the position of the door 14 will be lower than for the first 33a and the second 33b switch. How- ever, the third switch 33c preferably controls non- critical processes, such as activation of illumination.

Fig. 3 also shows a section of the circuit board 25 on which the switches 33a, 33b, 33c are mounted.

Although the invention has been described above by virtue of a preferred embodiment, which is shown in the accompanying drawings, it should be understood that a plurality of other embodiments are possible within the scope of the definition of the invention as stated in the claims.