Pushbutton Variable Capacitor

Abstract

A pushbutton key unit of a data input device for, say, a computer, comprising a push rod with a pull-back spring and a mechanical motion-to-electric signal converter associated with said push rod and having the form of a variable capacitor. The pushbutton key is simple in manufacture and reliable in operation.

Description

The present invention relates in general to data input devices of, for example, computers, and more particularly is concerned with pushbutton key units of these devices. The invention can also find application in consoles of diverse automatic systems, medical equipment, communications and the like.

Known in the art are pushbutton key units for data input devices, which comprise a shifting push rod with a pull-back spring mounted in a frame and a mechanical motion-to-electric signal converter associated with said rod.

This known signal converter is in the form of a winding on a toroidal core installed in the frame and affected by the field of a permanent magnet mounted on the push rod. Any shift of the push rod results in a corresponding field strength change which causes a certain change of the winding inductance and, hence, of the output signal.

Because of the relatively great number of mechanical parts, inductances as well as precise subassemblies, requiring individual adjustment, the known pushbutton key units are difficult and costly to manufacture and insufficiently reliable.

Accordingly, an object of the present invention is to overcome the disadvantages associated with the foregoing types of devices.

A more specific object of the present invention is to provide a pushbutton key unit for a data input device, which is simple in design and reliable in operation as compared with the known pushbutton key units.

These and other objects are achieved by providing a pushbutton key unit for a data input device of, say, a computer incorporating a frame which holds a shifting push rod with a pull-back spring linked with a mechanical motion-to-electric signal converter, wherein said signal converter, according to the present invention, is a capacitor at least one isolated plate of which is mounted on said push rod and at least two other plates are fixed on the frame through an insulating packing.

In accordance with one form of the invention, said packing isolating the plate of said capacitor from said push rod serves as a shock absorber.

It is also expedient to place the fixed plates of said capacitor in a plane perpendicular to the shift line of said push rod.

The fixed plates can also be divided into two groups placed in two planes which are perpendicular to the shift line of said push rod and which lie on the opposite sides of the plate attached to said push rod.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 illustrates a possible arrangement of a pushbutton key unit of a data input device, according to the invention, and

FIG. 2 is another possible arrangement of a pushbutton key unit of a data input device, according to the invention.

The pushbutton key unit incorporates a frame 1 (FIG. 1) holding a push rod 2 with a pull-back spring 3, whereas a mechanical motion-to-electric signal converter is constructed in the form of a capacitor whose moving plate 4 is attached to the push rod 2 and whose fixed plates 5 are located in a plane perpendicular to the shift line of the push rod 2 and fixedly mounted through an insulating packing 6 on the frame 1. The plate 4 is attached to the push rod 2 through a shock-absorbing packing 7 which also serves as an insulator. The plate 4 is coated on its side opposite the push rod 2 with a dielectric layer 8.

In another embodiment of the present invention (FIG. 2) the number of the plates 4 and 5 in the pushbutton key unit is doubled. The fixed plates 5 are divided into two groups placed in two planes, each of which is perpendicular to the shift line of the push rod, and are fixedly mounted to the frame 1 through insulating packings 6. Each of the moving plates 4 is associated through an insulating packing 7 with the push rod 2, the opposite sides of the plate 4 being coated with a dielectric layer 8.

The keyboard of the data input device consists of a set of separate pushbutton key units. Therefore all of the fixed plates 5 can be mounted on a common insulating packing 6, such as a printed board.

One of the fixed plates 5 of each pushbutton key unit must be connected to an external source (not shown in the drawing) of an alternative, say, pulse voltage and the other fixed plate 5 must be coupled with the input of an amplifier connected with associated electronics of the data input device (said amplifier and associated electronics are not shown in the drawing).

The pushbutton key unit functions in the following way.

When the push rod 2 is in its initial position, capacitance C.sub.min between the fixed plates 5 is at its lowest values and approximately equals:

C.sub.min .congruent.4.43.times.10.sup.-.sup.2 (S/d.sub.1),

where

S is the area of the fixed plate 5,

d.sub.1 is the maximum space between the planes of the fixed plate 5 and the moving plate 4.

When S=0.5 sq. cm and d.sub.1 =0.5 cm, C.sub.max .congruent.0.2 pF.

When the push rod 2 is pressed the value of capacitance C.sub.max between the fixed plates is determined by the equation:

C.sub.max .congruent.4.43.times.10.sup.-.sup.2 (.epsilon.S/d.sub.2),

where

.epsilon. is the dielectric coefficient of the dielectric layer 8,

d.sub.2 is the thickness of the dielectric layer 8.

When .epsilon.=5 and d.sub.2 =0.005 cm, C.sub.max =22 pF. In this case capacitance selection ratio (C.sub.max /C.sub.min) is equal to 110.

Where said selection ratio is insufficient, the pushbutton key unit shown in FIG. 2 is preferable. This key unit incorporates two capacitors, 9 and 10. The value of the first is highest when the push rod 2 is pressed to the utmost, and the second gains its full value when the push rod 2 is in its initial position, that is, unpressed. Suitable interconnection of the fixed plates 5 of the capacitors 9 and 10 makes it possible to substantially enhance the selection ratio C.sub.max /C.sub.min.

The herein proposed pushbutton key unit of a data input device is of simple construction, reliable in operation, has low overall dimensions and weight and can resist shock and vibration.

The pushbutton key unit according to the invention is operable in a wide temperature range.

The simplicity of the pushbutton key unit of the present invention insures a high-yield volume production of the device in highly automated production lines. Besides, the construction of data input devices comprising a number of keys can be further simplified by using several common printed boards carrying the capacitor fixed plates of all the key units.

Claims

We claim:

1. A pushbutton key unit of a data input device for, say, a computer, comprising a frame, a push rod mounted in said frame and capable of moving axially relative to said frame, a pull-back spring for said push rod, a mechanical motion-to-electric signal converter associated with said push rod and comprising a capacitor, at least one isolated substantially rigid plate of said capacitor being mounted on and movable with said push rod and at least two other substantially rigid plates of said capacitor being fixed on said frame through insulating packings and connected to information output means for the data input device.

2. A pushbutton key unit as claimed in claim 1, in which said packing isolating said capacitor plate from said push rod comprises a shock-absorbing material.

3. A pushbutton key unit as claimed in claim 1, in which said fixed plates lie in a single common plane which is perpendicular to the shift line of said push rod.

4. A pushbutton key unit as claimed in claim 1, in which said fixed plates are divided into two groups which are placed in two planes perpendicular to the shift line of said push rod and lie on the opposite sides of said isolated plate attached to said push rod.