CA1214238A

CA1214238A - Capacitive keyswitch

Info

Publication number: CA1214238A
Application number: CA000410276A
Authority: CA
Inventors: John R. Herron, Jr.
Original assignee: Illinois Tool Works Inc
Current assignee: Illinois Tool Works Inc
Priority date: 1981-08-28
Filing date: 1982-08-27
Publication date: 1986-11-18
Also published as: US4417294A; FR2512318A1; FR2512318B1; GB2105517B; NO822906L; JPS5873921A; IT8222970A0; GB2105517A; IT1205613B; DE3231533A1; BR8205018A

Abstract

CAPACITIVE KEYSWITCH

Abstract of the Disclosure A capacitive membrane keyboard is described which has an upper flexible membrane with a first pattern of interconnecting conductive areas deposited on it at each key station of the key-board. A second pattern of interconnecting conductive areas are deposited on a substrate below the first pattern. At each key station the conductive lands are aligned with each other. An electrically insulating spacer separates the pair of conductive lands at each key station when the keyswitch is unactuated. In one embodiment of the keyboard a dielectric film layer lays over all of the bottom conductive land areas and over the entire lower second circuit pattern.
In another embodiment, a dielectric layer is deposited on at least one of the conductive lands at each key station which consists of a polymer dielectric film or other suitable dielectric coating. In still a further embodiment, the lower and upper conductive areas may be made of an oxidizable metal, such as aluminum or tantalum, which has its surface oxidized in an oxidizing atmosphere to form a thin insulating dielectric layer on one or both of the lands of each key station. An elastomeric cushion may rest on top of the upper membrane so that when the actuator of the keyswitch is depressed it will cushion the force of the actuator so as to protect the upper membrane and may provide some return bias force.

Description

~2~2~8 CA tTIVE KEY SWITCH

Toledo of the Invention _ .
This invention relates to a capacitive membrane keyboard and more particularly to a capacitive membrane keyboard which has an upper Elbow membrane with a first pattern of interconnecting con-ductile areas deposited on it at each key station of the keyboard and a second pattern of interconnecting conductive areas on a substrate below the first pattern, the conductive areas being aligned with each other An electrically insulating spacer separates the pair of con-ductile Aurelius a each key station when the key switch is unactuated.

aye 3807 Background of the Invention Thin contact membrane key switches do not offer the extreme high reliability of other types of key switches, for example, such as the magnetic core key switches of the type shown in United States Patent No. 4,227,163, issued October 7, 1980 to Raymond Barnoski and assigned to Illinois Tool Works Inc. Contact membrane key switches, however, do offer sufficient reliability for many applications a an appreciably lower cost. One type of contact membrane keyboard is described in the article entitled "Touch Panel Switches Are Based On Membrane Switch Concept',' published in Computer Design, December 1979. In this type of key switch, two spaced-apart membrane layers coated with aligned conductive contact lands are separated in an aperture formed in a spacer. When the upper flexible membrane is depressed, contact is made between the upper and lower conductive lands to close the switch. Being a contact switch, however it does have the inherent deficiencies of all contact switches which include wear, the possibility of oxidation and corrosion of the conductive areas, etc.

While capacitive key switches are desirable because of their potentially greater reliability and longer life, such key switches as these have been designed in the past Snow have been appreciably more expensive than that of a simple contact membrane switch.
Another membrane contact switch is disclosed in United States Patent No. 3,676,607, issued July 11, 1972 to Donald H. Nash et at and assigned on its face to jell Telephone Laboratories, Case 3807 Incorporated. This keyboard employed field effect transistors, and it was recognized that actual contact between the conductive lands did not have to occur because the field effect transistors could be controlled by capacitance change if contaminants intervened between conductive upper and lower lands. However, the key switch of this patent was still designed to be basically a mechanical switch; and moreover, the design led to a split lower land area configuration of two lower lands which resulted in a capacitive key switch in which the available capacitive plate area is divided in half and relatively complicated detection circuitry was required.

Another capacitive membrane keyboard is described in an article entitled "Capacitive Membrane Keyboard Bars Contamination,"
which appeared in Electronic Products Magazine, June 15, 1981 issue.
This capacitive key switch is designed by Microswitch/Honeywell, and it employs an upper flexible membrane that carries a conductive land above a second membrane which has a second conductive land that is aligned with the upper conductive land. The two conductive lands are positioned in an aperture in a spacer which allows the upper member and land to be deflected towards the lower land until contact is made, thus the actual switching action is of the contact type. On the bottom of the lower membrane, however, -there is another conductive area. The conductive coatings on both sides of the lower membrane, therefore provide a fixed capacitance, while the movable upper membrane provides a variable capacitance that is in series with the fixed capacitance. Although theoretically the two facing lands at a key switch do not have to be brought into contact with each other, in practice, they must be in such a close proximity that the key switch will rely on actual contact to operate.

In the present invention, the simple technology of the membrane contact key switch may be utilized with the only change required being the provision of a dielectric layer between the upper and lower conductive lands which thereby provides a key switch with a high capacitance that has the simple construc~ionof a contact membrane key switch.

Accordingly, the invention comprehends a capacitive key switch comprising a first circuit support, an electrically insulating spacer, a dielectric structure, a second circuit support, and actuating means for actuating the key switch. The first circuit support is flexible and carries lo thereupon a first conductive circuit, which first conductive circuit includes a first conductive land. The second circuit support carries thereupon a second conductive circuit, which second conductive circuit includes a second conductive land. The spacer has an aperture there through and the first conductive land, the second conductive land, and the aperture are substantially in register, with the first conductive land and the second conductive land being in facing relation with respect to each other.
The dielectric structure is interposed between the first and second conductive lands. The actuating means comprises a plunger and a pad, the pad being interposed between the plunger and the first circuit support I

whereby application of a force urging the plunger toward the first circuit support urges the pad against the first circuit support and through the pad, urges the first circuit support toward the second circuit support, whereby capacitance between the first conductive land and the second conductive land is varied as the force is varied.

Description of the Drawings The present invention is described by reference to the drawings in which:
Fig. 1 is a cross-sectional view of a key switch of one embodiment of the keyboard in an unactuated condition; and Fig. 2 is a cross-sectional view of the key switch of Fig. 1 in an actuated condition.

Technical Description of the Invention The key switch of the present invention is illustrated by reference to the cross-sectional view of Figure 1. The key switch 10 consists of an actuating plunger 12 which is attached to a key cap (not shown) and may be retained in chassis 14 by a retaining washer 16 in a groove 18 of the plunger or by other conventional retaining methods. A bias spring 19 may be connected between the top surface of the chassis and the bottom of the rim 20 of the plunger to provide for return of the plunger after release of pressure on it. The pad 22 of elastomeric Case 3807 12'1 4~3~

material is disposed below the plunger 12, and it jests on the top surface of a flexible membrane 24 of a plastic film material, such as polyethylene, polypropylene, or other suitable material, such as MYLAR. The bottom surface of the flexible S membrane 24 carries a first relatively thin film 26 of conducting metal which forms an interconnecting circuit pattern including conductive land 28. A base 30 has a second thin film 32 of conducting metal on it which forms a second interconnecting circuit pattern including conductive land 36 that is aligned with the first land 28. A
relatively thick dielectric spacer 38, which may be a MYLAR
film, is interposed between the thin film 28, 36 which separate the conductive lands in the aperture 40.

In the illustrated embodiment of the present invention, a thin dielectric layer 34, such as any suitable plastic film material used for film capacitors, and which has a much thinner dimension than the spacer 38, is placed over the conducting film 32 and the land 36. The film 32 is shown as being deposited directly on the supporting base 30 although the film 32 could be deposited on a second plastic dielectric layer supported by the base instead of on the base itself. When the plunger 12 is depressed, the elastomeric pad 22 acts as a cushion which protects the upper membrane 24 and provides over travel. The pad 22 also provides a measure of return bias force when pressure on the plunger 12 is released. When the conductive lands 28 and 36 are in close proximity to each other, as shown in Fig. 2, due to the depression of the plunger 12, the thin dielectric separating layer 34 contacts both of the lands 28, 36 and this results in a relatively large capacitance value Case 3)~(~7 ~2~4~38 when the switch is in the actuated position, illustrated in Figure 2.

The key switch of the present invention can also achieve the desired high ratio of ON to OFF capacitance by replacing S the illustrated thin film 34 plastic layer with any other suitable dielectric material such as a conventional polymer dielectric film layer deposited on either, or both, of the facing surface lands 28 and 36. Another approach to providing a desired dielectric insulation layer would be to utilize a readily oxidizable metal for one or both of the lands 28, 36, for example, aluminum or tantalum which has its surface oxidized to provide a very thin insulating layer and a high dielectric constant which would enable manufacture of a still thinner key switch.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A capacitive keyswitch comprising a first circuit support, an electrically insulating spacer, a dielectric structure, a second circuit support, and actuating means for actuating the keyswitch; said first circuit support being flexible and carrying thereupon a first conductive circuit, said first conductive circuit including a first conductive land;
said second circuit support carrying thereupon a second conductive circuit, said second conductive circuit including a second conductive land; said spacer having an aperture therethrough; said first conductive land, said second conductive land, and said aperture being substantially in register with said first conductive land and said second conductive land being in facing relation with respect to each other; said dielectric structure being interposed between said first and second conductive land; said actuating means comprising a plunger and a pad; said pad being interposed between said plunger and said first circuit support whereby application of a force urging said plunger toward said first circuit support urges said pad against said first circuit support and through said pad, urges said first circuit support toward said second circuit support, whereby capacitance between said first conductive land and said second conductive land is varied as said force is varied.

2. A capacitive keyswitch as recited in claim 1 wherein said pad is elastomeric.

3. A capacitive keyswitch as recited in claim 1 wherein said dielectric structure is a thin dielectric film layer.