Depressible contactless key for generating electric signals in an electronic keyboard

This invention relates to an elastomeric material which is conductive under pressure and suitable for use in keyboards of data processing equipment.

Elastomeric materials (rubbers) are known which are rendered conductive by adding materials such as carbon black or metal powders is the mix or composition. Some compositions and processes for poducing conductive rubbers are given in the book: Conductive Rubbers as Plastics, by R. H. Norman - Elsevier, Amsterdam, London, New York, 100.

Elements of conductive rubber are used as contact materials in known types of keyboard.

For application in keyboards, piezoconductive rubbers are more interesting, that is those rubbers which become conductive under the effect of applied pressure, because they allow keyboards without contacts exposed to oxidation to be obtained, inasmuch as contact takes place within the piezoconductive material.

piezoconductive elastomers are used for producing fixed contact. In the known materials the piezoconductive effect disappears after a few thousand actuations of the elastomeric element, for which reason these materials are not suitable for use in keyboards.

The ject of the invention is to provide an elastomer which becomes conductive under the effect of pressure and preserves this characteric for a number of operations of the order of at least some hundreds of nousands.

This prolem has been solved by means of the piezoconductive elastomerionterial according to the invention, as claimed in claim 1.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings in which:

• Fig. 1 is a front view of an apparatus for preparing an elastomer according to the invention;
• Fig. 2 shows a key employing the piezoc:nductive elastomer according to the invention;
• Fig. 3 shows a detail of a keyboard employing the elastomer according to the invention;
• Fig. 4 is a graph relating to the key of Fig. 2;
• Fig. 5 is a diagram of a measuring cirotit used for the graph of Fig. 4.

Fig. 2 shows a key 10 which uses a path 11 of piezoconductive elastomer according to the invention and can be employed to replace a normal contact-type key as an input device for a data processing apparatus, with an interface towards semiconductor electronic circuits of high input impedance.

assume a low resistance when this pressure is exceeded, and then to reacquire the insulating properties on release of the key, presenting a certain hysteresis, but with a negligible delay. It is moreover essential that the patch preserves these characteristics for at least 100,000 operations of the same key, with a contact resistance always below 10,000 ohms.

According to the known literature (B. E. Spingett: Conductivity of a system of metallic particles dispersed in an insulating medium - J. A. Phys., Vol. 44, No. 6, June 73, pp. 2925 - 26, and C. H. Kuist: Anisotropic conduction in elastomeric composites - Proc. 7 Am. Conn. Symposium, June 1974, pp. 203 - 209), on varying the percentage of metal powder in a matrix of insulating elastomer the conductivity of the whole shows a distinct transition from insulating material to conductive material for a well-defined percentage by volume (V ) of metal powder close to 0.2 and dependent to a certain extent on the grain size and on the shape of the metal granules, presenting piezoconductive characteristics for percentages by volume of metal powders a little lower than V . m

In particular in the article by C. H. Kuist, percentages of metal (nickel) powder by volume ranging between 0.08 and 0.18 are suggested for the piezoconductive rubbers.

It is known that the application of a magnetic field during the polymerization of rigid epoxy plastics materials containing metal powders (see the said book Conductive Rubbers and Plastics, page 82) brings a considerable increase in the conductivity of the whole in the direction of the magnetic field; it could therefore be expected from this that also with elastomers the application of a magnetic field during the polymerization would lead to a lowering of the transition percentage V _m between the piezoconductive condition and the conductive condition in the preferred direction established by the magnetic field.

Surprisingly, it has been found that on dispersing nickel powders in a matrix of elastomeric binder, for example of the type Silastic E manufactured by Dow Corning, and maintaining the composition under the effect of a magnetic field during the polymerization of the binder, the rubber proves to be insulating in the absence of pressure even with percentages of powder between 15 and 27%, which greatly exceed the indicated limit values. On the other hand, the endurance of the piezoconductive characteristics improves decisively and passes from a few thousand operations of the key to several hundreds of thousands before irregularities of operation not tolerated by noraml electronic utilization circuits are encountered.

It has also been found that on exceeding these percentages, the riezoconductive characteristics become worse again and that the optimum value is found for percentages around 21%.

The preferred metal material is a nickel powder consisting of spnerical grains and having the maximum hardness compatible with preservation of the magnetic characteristics. More particularly, good results have been obtained with a nickel powder known by the trade name of Alloy 79 GS, supplied by the Baudier Company, of Liancourt (France).

The powder consists of 93.94% of nickel, 3.5% of silicon, 1% of tron, 1.6% of boron and 0.05% of carbon and has a Rockwell C hardness of 18-22; the spherical granules have a diameter between 100 and 150µ.

A preferred composition of the piezoconductive elastomer is constituted by:

• Silastic E silicone rubber from Dupont: by weight: 30 parts;
• 79 GS nickel powder from Baudier: 70 parts;
• Silastic E hardener from Dupont: 3 parts.

Preparation of the piezoconductive elastomer requires careful mixing of the powder and silicone rubber, addition of the hardener, a first degassing of the mix under vacuum and casting in the mould followed by a second degassing under vacuum and introduction into the magnetizer, which applies a magnetic field with a direction perpendicular to the faces of the sheet during the polymerization of the binder.

The thickness of the sheet or film of piezoconductive elastomer may vary from 0.4 to 0.8 mm, the preferred thickness being 0.6 mm.

The intensity of the magnetic field during the polymerization is not critical, provided that the field reaches an intensity of at least 500 oersteds. Above this value no appreciable variations are found in the results.

The apparatus used for preparing the elastomer is shown diagrammatically in Fig. 1, in which a mould 31 of non-magnetic material, in which the elastomer mix 36 is cast, is between two pole pieces 32 and 33 a magnetizer 37 which are interconnected by an external magnetic circuit (not shown). The magnetic flux is maintained throughout the time of polymerization of the elastomer by the current flowing in two windings 34 and 35. During the polymerization, the elastomer is kept at room temperature. Under these conditions, complete polymerization requires about 18 hours.

The time required for preparing the piezoconductive elastomeric material can be reduced to 10 minutes, still in a magnetic field, if the temperature of the mould 31 is brought to 100°_C.

All the samples tested have exceeded the prescribed minimum life of 100,000 operations, with peaks of more than 1,000,000 operaticns.

Fig. 5 shows a simple circuit used for detecting the characteristics of the key of Fig. 2, comprising a DC voltage generator 41 producing 5 V and a 50 KΩ limiting resistor 42 in series with the key 10.

Fig. 4 is a graph of the voltage drop detected across the terminals 43 and 44 of the resistor 42 as a function of the force F applied to the key. The phenomenon of hysteresis between actuation and release of the key is obvious from the graph.

An alternative embodiment of a keyboard employing the piezoconductive rubber according to the invention is shown in Fig. 3.

In.the keyboard 23, a single sheet 24 of piezoconductive material produced in a magnetic field in accordance with the invention is used, instead of individual patches of piezoconductive elastomer as in the key of Fig. 2. The sheet 24 is stuck by means of a conductive adhesive to islands 26 of a printed circuit board 25 which constitute one of the contact terminals of the keys 27.

A second, flexible, printed circuit board 28 is stuck by means of conductive adhesive on top of the sheet 24 of piezoconductive material at islands 29 in line with the islands 26. The islands 26 and 29 constitute contact terminals of the keys 27..

Springs 30, in combination with the movement of buttons 22, transmit the force applied to the buttons 22 to the piezoconductive sheet 24, causing locally the formation of passages of relatively low resistivity between the islands 26 and 29, while the material of the piezoconductive sheet 24 interposed between the keys 27 and not subjected to pressure maintains its insulating properties. The solution of Fig. 3, which is functionally equivalent to the modular solution of Fig. 2, is more convenient for producing keyboards with a large number of keys inasmuch as it drastically reduces the number of parts.

With this solution, it is possible to construct keys which simultaneously close a plurality of independent circuits by using a plurality of separate and insulated patches of piezoconductive material according to the invention or acting simultaneously on different points of the same sheet of piezoconductive material by means of actuating elements insulated from one another.

It is possible to make numerous variations in the solutions exemplified here as regards the type of elastomer and of magnetic conductive material, the form of the key, the level of modularity, that is the manner of grouping the keys, the actuating mechanism, and the production of contact between the piezoconductive material and the encoding circuit, without departing from the scope of the invention as claimed.