GB1580958A - Push button switch - Google Patents

Description

(54) PUSH BUTTON SWITCH (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London W.C.2. England 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: This invention relates to a push-button switch in which a so-called collapse action is provided. Such switches are used, inter alia, in push button keyblocks for telephone subscriber's instruments.

The type of force-travel displacement known as collapse action, which is often needed for push-button switches, ensures that once the depression of the button has reached a certain point, the force needed to complete the travel is reduced significantly, which ensures a follow-through to complete the depression. It also ensures that, due to this follow-through, the risk that there will be a very short dwell time for the operate button is reduced. During the push-button operating stroke a relatively high force is reached early in the stroke, this being followed by a rapid decrease to a minimum, whereafter further depression gives a gradual increase in force to complete the follow-through. A collapse action has the advantage that it gives the user a positive tactile feedback, confirming that the button has been properly operated.

An object of the invention is to provide a push-button switch with a collapse action, which is inexpensive.

According to the invention there is provided a push-button switch in which the movable element is a strip of a resilient sheet material which is bowed between its ends, in which the strip is formed as part of a larger area of the resilient sheet material by two slits therein, in which the strip is maintained bowed by clamping members which clamp the areas of the material outside the slits, the clamping members being so shaped that the material and the strip are held bowed, in which the push-button shank acts on a region of the strip between one of its ends and the crest of the bowed portion, there being an aperture in one of said clamping members through which the shank acts on the strip, in which electrical circuit means to be operated are on the opposite side of the strip from the push-button shank, and in which when the push-button is depressed the strip moves with a collapse action to a position in which it operates the electrical circuit means.

Note that although the circuit means to be controlled are usually electrical contacts this will not always be the case. Hence the term electrical circuit means as used in the preceding paragraph embraces any means whereby the user can directly or indirectly control an associated electrical circuit. One possible arrangement not using electromechanical circuits is an electro-optical coupler whose light beams can be interrupted or an interruption removed by movement of the bowed strip, e.g. by a tab bent from the strip which, dependent on the formation of the strip either does or does not interrupt the beam.

Embodiments of the invention will now be described with reference to the drawings accompanying the Provisional Specification, in which, Fig. 1 is an end-on view of clamp members with resilient material therebetween.

Fig. 2 is an exploded view of a switch embodying the invention, but without its push-button.

Fig. 3 shows schematically the positions of a push-button shank in a switch embodying the invention.

Fig. 4 is an exploded view of a pushbutton keyblock for telephone application, using switches embodying the invention.

Fig. 5 is an end-on view of one contact mechanism of the keyblock of Fig. 4, used to explain its operation.

In producing the strip forming the movable element of the switch, two parallel slits 1, 2, Fig. 2 are cut in a sheet or membrane 3 of a resilient material, and the material bowed by being clamped between two clamp members 4, 5, Figs. 1 or 2. As can be seen the clamping faces are curved substantially sinusoidally, so that the sheet 3 assumes the configuration shown in the exploded view of Fig. 2.

The two slots 1 and 2 define a strip 6, whose two ends are fixed between the members 4 and 5, and the clamping members are cut away above and below this strip.

Hence the strip can move independently of the portion of the sheet on either side of it.

To actuate the switch the strip 6 is deflected by a push-button whose shank is off-centre from the crest or summit of the strip 6, typically about one third of the distance between two of its troughs. We now refer to Fig. 3, which shows three stages of the operation of the device. In Fig. 3 (a) we see the rest position with the button 7 above the strip 6, and a bottom plate 8 which in the telephone set example is a printed circuit board. As the button is depressed, see Fig. 3 (b) the strip deforms and presents a rapidly increasing force to the motion of the button.

A point is reached in this deformity at which the material of the strip which has been forced upwards and to the right suddenly inverts and the force which it exerts on the button falls sharply. This portion is then constricted by the bottom plate 8, and further movement of the button deforms the rest of the strip with a slowly increasing force until the button reaches the preset stop. During the completion of the button travel the portion of the strip remains in contact with the bottom plate to cause the desired controlling action.

On release of the button, the strip 6 returns to the rest position and urges the button to its rest position e.g. a helical spring on the shank of the button 7 and co-operating with fixed stops.

In the arrangement described above, the slits 1, 2 are parallel; however these slits may be differently shaped. Thus by shaping them in various ways the switch action can be persuaded to commence at a desired point and to progress in a defined way. This has not been investigated in detail, but investigation of the relations between slit configuration and switch action should be susceptible of relatively uncomplicated experimental investigation.

Switches operating on the principles described herein embody a degree of hysteresis in their action, as will be seen from a study of Fig. 3, and also of Fig. 5, which will be referred to in more detail later.

Fig. 4 is an exploded schematic of a telephone keyblock using switches such as described above, shown face downwards, a typical push button key being shown at 10.

This key has an operating wedge, as can be seen, which acts on the bowed portion of the strip. The keys are held in the rectangular hole in a base moulding 11 which, with another moulding 12 provides the clamping members. For each row of key positions those clamping members provi e one sine wave or other shape cycle. The switch operating strips are cut from a single sheet 13, which is a thin flexible sheet whose characteristics are such that the desired collapse action will be provided.

The assembly is completed by a baseboard 14, which is a printed circuit board bearing contacts and appropriate interconnecting strips. The board may be extended to carry other circuit elements.

The base-board 14 and other members are staked together via the holes such as 15 and stakings such as 16.

We now refer to Fig. 5 which shows one contact mechanism in three states, (a) normal, (b) half operated and (c) fully operated. The operating force is applied at 20 to the bowed strip which can be a flexible printed circuit carrier with contacts on it which co-operate with contacts on the base board as indicated at 21.

In the switches described above the configuration of the clamps is sinusoidal, but the clamping faces can have other configurations, as long as the surrounding matenal is force-shortened relative to the middle active limb. However, the sinusoidal shape shown is convenient from the manufacturing aspect.

The angle at which the leaf spring leaves the clamps in an inward" direction -- see Fig. 5 - is important as it is this angled exit which supplies the restoring mechanism.

Without this angled exit there is a risk that the leaf would be bistable, in which case it would tend to stay locked in the operated position. This would make a restoring spring such as referred to above essential in a normal, e.g. telephone subscriber's set, application of push-button switches.

The angle at the other end of the leaf the left-hand end in Fig. 5 - is less critical and is in fact nominally neutral. The angles at which these leaf spring portions leave the clamps can be readily selected on an experimental basis. With suitably selected angles, the need for a restoring spring may be eliminated.

The switching means used may take various forms. Thus, if the membrane is metallic, it may itself be used as one contact, the other being a pad on a printed circuit board, which also provides the fixed bottom surface. Twin contacts may be provided by partially slitting the tongue as shown in Fig.

4, to give two contact areas with a degree of independence. The membrane may be made of a suitable plastics material with electrically conducting tracks and pads. In addition to the features first mentioned, this allows each pair of switching contacts to be electrically isolated when used in a Keyblock array.

Either type of membrane may be used to obtain a change in electrical capacitance as the membrane approaches the fixed bottom surface, which again may be a printed circuit board. An electrical impulse may be generated by making the membrane from a piezo-electric material which will produce a voltage peak when the wave inverts.

Finally the arrangement may be used solely to provide collapse action, the switching means being entirely separate.

We now discuss the hysteresis action and its relevance to the operation of the pushbutton switches described herein.

There are two separate aspects to any collapse-action switch operation. The first of these is the collapse action itself, which, as mentioned above gives tactile "feel". However, this cannot of itself ensure a full positive follow-through action because if the non-hysteretic toggle spring effect fully takes over, the essential restoring face would not be preset. Thus we also need a hysteresis or lost-motion action to ensure that once the "contact" has closed the button has to be positively withdrawn by some distance before the "contact" re-opens.

A "perfect" action is possible by introducing a hysteresis effect into the collapse action so that the force displacement characteristic is different in the operate and release directions. This way the operate characteristic curve can cross the zero force line between the two limits of travel, and the return curve can take many shapes as long as it remains above the zero force axis. The combination of the hysteresis requirement with the tactile button mechanism gives an excellent but economical key action.

Novel features and Advantages.

(1) Collapse action provided by a single membrane which is initially flat, and is corrugated by clamping between two suitably shaped plates.

(2) When used in an array of switches the invention provides collapse action on all switches independently with a few pieceparts and a simple construction.

(3) The flexing membrane is flat and does not require any forming or moulding.

(4) The membrane is held in the required shape by the clamping plates which may be plastics mouldings. This will provide more consistent and accurate characteristics than methods which rely on permanent forming of the membrane.

(5) When the wave in the membrane inverts it does so suddenly with a snap action, which is ideal for a switch action as slow operation is thus impossible.

(6) The timing of switching action and the collapse action is inherent in the design and is ideal since the switching coincides with the minimum of the force displacement characteristic.

WHAT WE CLAIM IS: 1. A push-button switch in which the movable element is a strip of resilient sheet material which is bowed between its ends, in which the strip is formed as part of a larger area of the resilient sheet material by two slits therein, in which the strip is maintained bowed by clamping members which clamp the area of the material outside the slits, the clamping members being so shaped that the material and the strip are held bowed, in which the push-button shank acts on a region of the strip between one of its ends and the crest of the bowed portion, there being an aperture in one of said clamping members through which the shank acts on the strip, in which electrical circuit means to be operated are on the opposite side of the strip from the push-button shank, and in which when the push-button is depressed the strip moves with a collapse action to a position in which it operates the electrical circuit means.

2. A switch as claimed in claim 1, and in which the two slits which define the movable strip are substantially parallel.

3. A switch as claimed in claim 1 or 2, and in which the clamping members are so shaped that the bowing is of a substantially sinusoidal configuration.

4. A switch as claimed in claim 1, 2 or 3, in which the movable portion of the resilient sheet material leaves the clamping members at the end thereof opposite from that near which the shank acts at an angle which determines the restoration characteristics of the switch.

5. A switch as claimed in any one of claims 1 to 4, and in which the electrical circuit means operated by the movable strip is an electrical contact set.

6. A switch as claimed in claim 5, and in which the contact set includes two stationary contacts which are bridged by a contact on the movable strip when the switch has been operated.

7. A switch as claimed in claim 5 or 6, and in which the movable strip acts as one of the switch contacts.

8. A push-button key block, such as used in a telephone subscriber's instrument, in which each of the push-buttons is a pushbutton switch as claimed in any one of

Claims (1)

1. claims 1 to 7.

   9. A key block as claimed in claim 8, and in which the resilient material from which the movable strips are made is a single sheet of the resilient material having a movable strip, or a pair of movable strips, per push-button.

   10. A key block as claimed in claim 9, in which the clamping members consist of two generally rectangular members with curved portions associated with each push-button.

   11. A push-button switch substantially as described with reference to Fig. 1 to 3 of the drawings.

   12. A push-button key block substantially as described with reference to Figs. 4 and 5 of the drawings.