GB1587408A

GB1587408A - Keypads

Info

Publication number: GB1587408A
Application number: GB3863176A
Authority: GB
Original assignee: Post Office
Current assignee: Post Office
Priority date: 1977-09-14
Filing date: 1977-09-14
Publication date: 1981-04-01

Description

(54) IMPROVEMENTS IN OR RELATING TO KEYPADS (71) We, THE POST OFFICE, a British corporation established by statute of 23 Howland Street, London, W1P 6HQ do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to electronic touch keypads.

In one type of switch used with touch keypads employing no-travel keys, the switch is actuated by the proximity of a finger to a sensor element of the switch. A proprietory example of this type of switch is a twelve function switch called "Electronic touch keypad" manufactured by Fotherby-Willis.

This type of switch operates on the principle of capacitative coupling with the human body.

When a human finger is brought within a certain distance, referred to in this specification as the maximum actuation distance, of a sensor element of the switch, the body capacitance alters the capacitance in a tuned circuit in the switch, and the altered response of this tuned circuit to an oscillator causes the switch to operate. Other methods of operation are, of course, possible. This type of switch is frequently arranged in the form of a matrix and the sensor elements, the switch matrix and associated circuitry are mounted on a printed circuit board. This type of switch can of course be actuted through a front panel of Perspex (RTM) or other insulative material. The present invention particularly relates to a finger plate for use with this type of keypad, and the combination of switches and keypad.

According to a first aspect of the invention there is provided a switch assembly, comprising a plurality of switches operated by capacitative coupling between a sensor element of each switch and the human body when a finger of the human body is placed within a maximum actuation distance of said sensor element, each said sensor element being disposed adjacent a first flat surface of a flat insulative finger plate provided in a second surface thereof with a plurality of recesses, each recess being so dimensioned that a human finger may be inserted therein and being associated with a respective sensor element, so that said second surface is more than said maximum actuation distance from any sensor element and the bottom of each recess is within the maximum actuation distance of the associated sensor element.

According to a 'second aspect of the invention there is provided a finger plate for use in a switch assembly according to the first aspect, said finger plate comprising a flat sheet of insulative material having a first flat surface and a second surface, the second surface having a rectangular array of recesses, each recess being so dimensioned that a human finger may be inserted therein, such that when said first surface is adjacent said plurality of switches said second surface is separated from any sensor element by more than said maximum actuation distance, and the bottom of each recess is within said maximum actuation distance of a respective sensor elemerit, and such that only one of said plurality of switches is activated when a finger of the human body is inserted into the recess relevant to that switch so as to penetrate below said maximum actuation distance.

Preferably the recesses are in the form of spherical caps and may have a spherical radius of i2mm and a chordal radius of intersection with the surface of 16mm.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a plan of a finger plate according to the invention.

Figure 2 shows a section along line AA of Figure 1.

In a touch keypad employing no-travel keys of the type described in the introduction to this specification, the close space between the keys considerably increases the danger of operating two switches at once, particularly when a finger touches the chosen key at a shallow angle. The present invention seeks to obviate this risk. This is done by placing a finger plate of the type illustrated in Figures 1 and 2 over the top of the switch matrix.

This finger plate has to be made of an insulative material; It may with advantage be made of Perspex (RTM). The thickness of the finger plate between the rear surface 1 and the front surface 2 is greater than the maximum actuation distance of the switches.

For the Fotherby-Willis twelve function "Electronic touch keypad" mentioned above the maximum actuation distance is 6mm. The thickness of the finger plate for use with this particular switching matrix is therefore chosen to be 6.35mm. This means that anyobdy placing their hand across the entire finger plate will not actuate any of the switches, since no part of the hand will be within the maximum actuation distance of the switches. The finger plate has a number of recesses 3 formed in the surface 2 each recess associated with one switch. The distance of the bottom of these recesses from surface 1 is less than the maximum actuation distance of the switches; in this particular case this distance is chosen to be about 3mm.Thus when the finger plate is positioned over the switching array with the sensor elements touching the surface 1 the only way a switch can be actuated is when a finger is actually disposed within one of the recesses 3. With this type of finger plate it is virtually impossible to operate two keys simultaneously with one finger. The recesses 3 are in the form of spherical caps, which can be cut with a 24mm bullnose cutter.The radius of the sphere is 12mm, and the radius of the chordal circle of intersection with surface 2 is 16mm, the precise depth of the recess is then 12(12282)1'2= l2-801'3.055mm. It will be realised of course that finger plates of the type shown in Figure 1 and Figure 2 can be located either in contact with the switching matrix with which they are to be used, or a short disance above the surface of the switching matrix.

If this is done surface 2 must be arranged so that the distance between the switches in the switching matrix and surface 2 is greater than the maximum actuation distance, and the distance between the bottom of the recesses 3 and the surface of the switching matrix is less than the maximum actuation distance. Furthermore the individual capacitative coupled switches could be set into the plastics sheet in recesses in surface 1, when the same criteria apply to the sheet dimensions as previously described i.e. the distance of surface 2 from the sensor element of the switch must be greater than the maximum actuation distance, and the distance of the base of recesses 3 from the sensor element of the switch must be less than the maximum actuation distance. The finger plate may also be made of a double sheet of material, and key designations may be inscribed on the bottom sheet.

The area of a finger plate designed to operate with a matrix array of switches, and the distribution of recesses 3 on the surface of the finger plate, is determined by the structure of the matrix switch array. For the particular example given, the dimensions are those shown in Figure 1, all dimensions being in millimetres.

WHAT WE CLAIM LS:- 1. A switch assembly, comprising a plurality of switches operated by capacitative coupling between a sensor element of each switch and the human body when a finger of the human body is placed within a maximum actuation distance of said sensor element, each said sensor element being disposed adjacent a first flat surface of a flat insulative finger plate provided in a second surface thereof with a plurality of recesses, each recess being so dimensioned that a human finger may be inserted therein and being associated with a respective sensor element, so that said second surface is more than said maximum actuation distance from any sensor element and the bottom of each recess is within the maximum actuation distance of the associated sensor element.

2. A switch assembly as claimed in claim 1 in which said recesses have the form of caps of spheres.

3. A switch assembly as claimed in claim 2 wherein the radius of said sphere is substantially 12mm.

4. A switch assembly as claimed in claim 2 or 3 in which the radius of the chordal circle of intersection of said sphere and surface is substantially 16mm.

5. A finger plate for use in a switch assembly as claimed in claim 1 said finger plate comprising a flat sheet of insulative material having a first flat surface and a second surface, the second surface having a rectangular array of recesses, each recess being so dimensioned that a human finger may be inserted therein, such that when said first sur face is adjacent said plurality of switches said second surface is separated from any sensor element by more than said maximum actua tion distance, and the bottom of each recess is within said maximum actuation distance of a respective sensor element, and such that only one of said plurality of switches is activated when a finger of the human body is inserted into the recess relevant to that switch so as to penetrate below said maxi mum actuation distance.

6. A finger plate as claimed in claim 5 wherein said recesses have the form of caps of spheres.

7. A finger plate as claimed in claim 6 wherein the radius of said spheres is substantially 12mm.

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Claims

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1 and 2 over the top of the switch matrix.

2. A switch assembly as claimed in claim

1 in which said recesses have the form of caps of spheres.

8. A finger plate as claimed in claim 6

or 7 in which the radius of the chordal circle of intersection of said sphere and said surface is substantially 16mm.

9. A finger plate substantially as hereinbefore described and with reference to Figures 1 and 2.