CA1189168A

CA1189168A - Heating appliance

Info

Publication number: CA1189168A
Application number: CA000397020A
Authority: CA
Inventors: Yohzoh Ishimura
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1981-03-02
Filing date: 1982-02-25
Publication date: 1985-06-18
Also published as: DE3274757D1; JPS57144826A; AU547740B2; EP0073250A1; EP0073250B1; EP0073250A4; JPH0160739B2; WO1982003113A1; AU8142682A

Abstract

ABSTRACT

TITLE: Heating appliance This invention relates to a fail-safe device for a heating appliance such as an electric oven or a micro-wave oven, and more particularly to such a device for one having an electronic controller including a microcomputer (13) for controlling a heating device such as a high fre-quency generator (31) and a keyboard (10) as an input de-vice for the controller. By bonding an electrically con-ductive metallic thin sheet (17) to a constitutional component of the keyboard on an operational panel (2) at the front of a main body (1) especially a component dis-posed closer to the actuation surface than input keys such as cook keys (4) and (5) and grounding the sheet (17) via the main body (1), the appliance is prevented from self-triggering, causing abnormal heating due to in-coming noise, static electricity, etc. and, thus, falling into a dangerous condition.

Description

SPECIFIC.ATION

TITLE. Heating appliance TECHNICAL FIEL~
This invention relates to an operational panel struc-ture as the input unit of an electronic controller includ-ing a microcompu-ter LSI chip or the like for controlling the heat source of a heati.ng appliance such as an electric oven or microwave oven.

BACKGROUND ART
The conventional heating appliance of the above de~
scribed type is in the constant danger that it could be self-triggered due to incoming noise, a surge of lighten-ing, noi.se static electricity, etc. and-become operative against the user's will.
The most dangerous aspect of any heating appliance of the above type is that it would start heating inadvert-ently even in the absence of a heating start instructionto a controller. Under these circumstances, the tempera-tures of the appliance body and the door rise drastically and the user would burn himself on -those portions or the appliance itself would catch a fire and ignite surround-ing in~Lammables such as a curtain~
In addition, unloaded heati.ng in a microwave ovenresults in increasing the quantity of microwave radiations 9~

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leaking ~rom the periphery of the door and shortelling -the li~e of power unit components such as a magnetron.
Fig. 1 is a perspective view showing the appearance o~ a heatin~ appliance according to an embodiment of the presen-t invention; Fiy. 2 is a cross sectional sicle view of the appliance; Fig. 3 is a cross sectional view o~
part of the appliance taken on the line Y~Y'; Fig. 4 is a circuit diagram of a control circuit o~ the appliance;
Figs. 5 and 6 are cross sectional views of part of a heating appliance according to another embodiment of the present invention; and Fig. 7 is a cross sectional view of part of an operational panel in a conventional heating appliance.
With the recent remarkable advance in the electronic controller technology using microcomputers, microcomputer-aided appliances are in widespread use. However, the microcomputers have the disadvantage that they are very susceptible to static electricity and surge current. A
conventional heating appliance as shown in Fig. 7 includes in an operational panel 2 at -the front of its main body 1 an ornamental plate 3, cook keys 4 and 5, cook ke~
switches 6 and 7 and a board A9 carrying insulating spac-ers 8 and the cook key switches 6 and 7, and the ornamental plate 3, the cook keys 4 and 5, the cook key switches 6 and 7, -the insulating spacers a and the board A9 together constitute a keyboard 10. A metallic bac]c plate 11 is disposed in contact with the board A9 and electrically - 2a -conneeted to a chassis 12. Another board B 15 is provicled which earrles a microeomputer 13 responsive -to signals from input keys sueh as the eook keys 4 ancl 5, elec-tronic components 14 and the like. When the conventional heat-ing applianc`e is manua~ly actuated, static charge on the operator's body may be conveyed to the reinforcin(~ back plate 11 so that the baek plate 11 serves as an an-tenna to feed curren-t to the coolc switehes 6 and 7 and so forth in the keyboard 10 and send an error signal -to the micro-eompu-ter 13, the elee-tronie eomponen-ts 14 and the like on the board B 15. Such error signal would lead to a danger-ous situation such as destruc-tion of the microcomputer 13, electronic ~omponents lfi and so forth and an over-heated condition in a variety of parts in the main body 1. While, of course, safety is usually taken into con-sideration in the design of an e:Lectronic circuit includ-ing the microcomputer 13 and a software con-tained in -the microcomputer 13, the microcompu-ter 13 would perform faulty operations or be destroyed and display tubes would provide an erroneous display in the case of an appliance having such display tubes. Furthermore, the appliance would oscillate inadvertently. A more reliable fail-safe device is an important development objective.

DISCLOSURE OF THE INVENTION
Accordingly, it is an ob~ect of the present invention to provide a heating appliance wherein an electronic con~
troller including a microcomputer LSI chip and the li]ce for controlling a heat source of the appliance is pre-vented from conducting faulty operation due to static electricity charged on the operator's body or surge cur rent by providing an electrically conductive, metallic thin sheet in tight contact with a component or components constituting a keyboard mounted on a front operational panel as an input device for the controller, the metal]ic sheet being earthed via an appliance main body or chassis.

BEST MODE FOR CARRYING OUT THE INVENTION
Referring -to Figs. 1 to 4, there is illustrated a heating appliance constructed according -to an embodiment of the presen-t inventlon, wherei.n a metallic thin shee-t 17 is bonded between a~ insu:Lating sheet 16-and an ornamental plate 3 which are componen-ts of a keyboard 10 and is provided wi-th a grounding conductor 18 ex-tending from its one end and leading -to the ground via a chassis 12.
In Figs. 1 and 2, a door 19 is provided to freely open across a frorlt opening in the main body 1~ A heat-ing chamber 20 is defined within the main body 1. An operational panel 2 is disposed at the front of the main body 1 and on one side of the door, 19 which .1!
'.~ '' panel is made of an insulating material such as plastic and environed by~ an operational panel f:ramework 21. As seen in Fig. 3, -the keyboard 10 is disposed at the back of the ope.ra-tional panel 2 to extend o~er an aperture 22.
5 Within keyboard 10 there are provided a plurality of in put keys comprised of conductive material typicall.y carbon peast, that is, cook keys 4 and 5 and a cook start key 23. The above-mentioned keyboaxd 10 comprises said orna-mental plate 3 which is exposed to the operator's fingers, the electrically conductive me-tallic sheet 17 typically made of aluminum foil or stainless steel foil, pre:Eerably a material of high tensile strength, and adhesively se-cured between the ornamental plate 3 and the insulating sheet 16, the insulating sheet 16, the plurality of input keys bonded to the insulating sheet 16, a plastic board A9 carrying switches 6 and 7 disposed in juxtaposition with said input keys~ for example, cook keys 4 and 5, and an insulating spacer 8 for spacing the board A9 from the insulating sheet 16. The grounding conductor 18 extend-ing from the one end of the metallic thin sheet 17 as anintegral unit is bent around the board A9 and electrical-ly connected to the board A9 and a stepped portion 24 of a me-tallic back plate 11 disposed behind the board A9 and leading to the chassis 12 in the main body 1.
The shape of said electrically conductive metallic sheet 17 will be described in further detail. I-t is de-sirable from safe points oE view tha-t the bonding area ~"'`'" ~ =~

oE the sheet 17 be greater -than the area of -the aper-ture ~2 in the opera-~ional panel 21 as seen in Fig. 3. In other words, the width of the sheet is larger than Ql in Fig. 3 and the ver-tical leng-th thereof is larger -than Q2 in Fig. 1. It is necessary that the sheet 17 be positioned closer to the operation surface than the cook keys 4 and 5. These area and position relationships obvia-te the possibility oE faulty operation because the coo]c keys 4 and 5 are e]ectrically isolated from the exposed surface of the sheet 17 even when part of the operator's body comes into contact with the operational panel framework 21 or the ornamental plate 3 in actuating the keyboard manually from the front.
In cases where the operational panel framework 21 is not used, safety can be guaranteed by bonding the sheet 17 having an area greater than the area of the board A5 in the above specified position.
Although in the embodiment of Fig. 3 the sheet is sandwiched between the stepped portion 24 of the back plate 11 and the board A9, it is obvious that it may be interposed directly between the back plate 11 and the board without using the stepped portion.
Further, provided that as in the above embodiment, the ornamental plate 3 and the insulating sheet 16 are substantially equal in size and the sheet 17 is equal in size to these components as well, the sheet may be bonded more easily and lends itself better to mass ~, S',~ ' i S

production.
Unlike the~above illus-trated embodimen-t, the sheet 17 may be bonded betwen the insulating sheet 16 and the cook keys 4 and 5.
The reference numeral 25 represents a pad on which wires connected to the switches 6 and 7 associated with the cook keys 4 and 5 are concentrated, and lead wires 26 leading from the pad 25 are connected to the electron-ic control device including the microcomputer 13~ elec-tronic components 14 and so forth mounted on a board s 15.
When a food is put in the heating chamber 20 in the main body 1 of the above described and illustrated heat-ing appliance and the door 19 is closed, the door switches 27 and 28 shown in Fig. 4 are closed. Then, as the cook key 4 or 5 and the cook start key 23 in the keyboard 10 are actuated, the microcomputer 13 operates so that a contact 30 of a relay 29 is closed and a high frequency generator 31 oscillates to generate high frequency waves.
It is evident from the foregoing description con-cerning the embodiment that, even if the keyboard 10 is actuated with the finger of the operator carrying static electri.city, any error signal is never conveyed to the microcomputer 13, the electronic components 14 and so forth mounted on the board B 15 by way of the operational panel framewor:k 21 because the operational panel frame-work 21 .is made of an electrically insulating material.
In addition, i:n the event that static electricity is ~ , F = ~ =~_~

~ 8 --discharged frorn the opera-tor's body, an error siynal i.s not fed to the ~icrocomputer 13, -the electronic components 14 and so forth on the board B 15 and the appliance is free from any safety problern because the groundlng con-ductor 18 ex-tending from the metallic shee-t 17 made of the aluminum or stainless foil and constituting the key-board 10 is in electric contact with the back plate 11 grounded via the chassis 12 in the main body 1.
It is further possible to reduce the overall thick-ness of the keyboard 10 because the grounding conductor 18 extending from the sheet 17 and so forth is a metallic sheet such as an aluminum foil or a stainless steel sheet rather than a conductive rubber member or the like. The respective sheets constituting the keyboard 10, for exam ple, the sheet 17 and the insulating sheet 16~ are more desirable for mass production than those made of rubber because the former may be roll-shaped and can be easily printed and bonded. This of course leads to low profile and compactness of the operational panel region.
The operator may depress the input keys on the key-board 10 almost as lightly as in the case where the key-board is not provided with the sheet 17 and more lightly than in the case o a rubber-made keyboard, as if the keyboard were not provided with the sheet 17. Thus, the ease of operati.on is not affected in the least.
It is generally known that static electricity which may be charged on the human body is as high as of the order of abou-t 10 to 15 KV, -thouyh it depends on weather conditions~ the ~o-tential at the human body with respect to the ground, kinds of clothes, etc. While an aluminum deposited film does not perform its duty for this reason, 5 the thin metallic sheet made of aluminum foil or stainless steel foi] as in the embodiment achieves its purpose suc~
cessfully even in the presence of such high voltage.
In the case where an electrically conductive metal layer is vacuum~deposited on each of the insulating sheet 10 16 and the ornamental plate 3, the problem occurs that the deposited conductive layer is strained OIl repeated operation of the input keys in the keyboard 10 and is eventually cracked or broken, thus failing to ensure satis-factory conductivity. However, the sheet 17 of metal foil 15 has a high tensile strength and is thus excellent in du-rability and shock resistance.
Moreover, in the event that the ornamental plate 3 is damaged for any reason, the provision of the sheet 17 prevents low voltage electric shocks in advance and as-20 sures a greater degree of safety~
The above described advantages are also available in an alternative embodiment as illustrated in Fig. 5, where-in a grounding conductor 18 extending from one end of the sheet 17 lies sandwiched between a rib 32 of the opera 25 tional panel 2 and a bent portion 33 of the back plate 11 and held in electric contact with the back plate 11 for grounding purposes. This structure is easier to assemble ~ ~.R ~
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and more sui-table for mass production than the e~bodiment shown in Fig. 3.~
As seen in Fig. 6, the grounding conductor 18 of the sheet 17 may be forced against the back pla-te ll and held in electric contact wi-th the back plate ll for grounding purposes by means of a resilient leaf spring 34 spot-welded to the back plate 11, in whlch case the above de-scribed advantages may be similarly expec-ted. With -this arrangement, the leaf spring 34 abso:rbs erros in assembl-ing the keyboard so that the grounding conductor 18 issecured more tightly in electric contact with the back plate ll.

INDUSTRIAI, APPLICABILITY
As described hereinbefore, the heating appliance embodying the present invention is such that the elec-trically conductive me-tallic thin sheet is dispoed at the keyboard of the operational panel actuatable by the human body or the like and is grounded via the chassis or a portion of the main body. This arrangement pre-vents a faulty operation of the electronic contro].ler including the microcomputer due to the static electricity accumulated on the human body or the like or a suxge cur-rent and assures an increased degree of safety.

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Claims

CLAIMS:

1. A heating appliance comprising a heating chamber de-fined in its main body, a heating means for heating the interior of said heating chamber, a controller including a microcomputer for controlling said heating means, an operational panel disposed at the front of said main body and made of electrically insulating material and a key-board disposed on said operational panel and having a plurality of input keys, characterized in that an elec-trically conductive metallic thin sheet is interposed between an ornamental plate and an insulating sheet con-stituting said keyboard or between said insulating sheet and said input keys and grounded via a portion of said main body.

2. A heating appliance according to Claim 1 wherein said electrically conductive metallic thin sheet is inte-grally provided with a grounding conductor for electric connection to a chassis of said main body.

3. A heating appliance according to Claim 2 wherein one end of said grounding conductor of said sheet is forced against a board A for said keyboard by means of a back plate and said back plate is grounded via the chassis of said main body.

4. A heating appliance according to Claim 3 wherein said grounding conductor is sandwiched between said back plate and a rib of said operational panel.

5. A heating appliance according to Claim 3 wherein said grounding conductor is forced against said operational panel by means of a leaf spring secured to said back plate.