GB2136633A

GB2136633A - Membrane switch assembly

Info

Publication number: GB2136633A
Application number: GB08406940A
Authority: GB
Inventors: Alan David Matthewman; Rodney Dougal Lugton
Original assignee: BRODERNA DANIELSON
Current assignee: BRODERNA DANIELSON
Priority date: 1983-03-18
Filing date: 1984-03-16
Publication date: 1984-09-19
Also published as: GB8406940D0

Abstract

A membrane switch assembly comprises a membrane switch (6); and an actuator assembly mounted adjacent the membrane switch. The actuator assembly comprises a first portion, e.g. a compression spring (16), for engaging the switch (6), and a push button (3) cooperating with the compression spring (16) for engagement by a user. A resilient means, e.g. a further compression spring (13), is provided to urge the push button (3) towards a first position (shown in Figure 1A). The arrangement is such that initial movement of the push button (3) from the first position to a second position against the resilience of the compression spring (13) causes the compression spring (16) to actuate the membrane switch (6). The cooperation between the push button and the compression spring (16) enables the push button (3) to be moved from the second position to a third position while the membrane switch (6) remains in its actuated state. <IMAGE>

Description

SPECIFICATION Membrane switch assembly The invention relates to a membrane switch assembly comprising a membrane switch and an actuator assembly mounted adjacent the membrane switch.

Atypical membrane switch has a first surface, usually provided by a relatively rigid lamina, and a second surface, facing and spaced from the first surface, provided on a relatively flexible lamina, the first and second surfaces carrying electrical contacts which from part of an electrical circuit in use.

Membrane switches can be divided into two types, "push-to-make" in which the electrical contacts are normally spaced apart, and "push-to-break" in which the electrical contacts are normally in contact with one another. Actuation of the actuator assembly then breaks or makes the contacts respectively.

Membrane switches are finding increasing application in a wide variety of fields due to the comparatively simple nature of their construction, their cheapness, and their ease of actuation. Such switches are commonly utilised in apparatus which includes keyboards with actuators such as push buttons. One problem with such apparatus is that conventionally the push buttons move a significant distance in use and the flexible lamina of a typical membrane switch only moves through a very smail distance on actuation. Thus, it is difficult to substitute membrane switches into apparatus such as typewriters in which the typewriter keys are normally pushed through a distance of the order of three millimeters when the flexible iamina will only flex through a distance of 0.5 millimeters.Although a straight substitution could be made, it would be very difficult for an operator to detect such a small movement and this will also make it difficult for operators to become accustomed to the new "feel" of the keyboard.

In accordance with the present invention, a membrane switch assembly comprises a membrane switch; and an actuator assembly mounted adjacent the membrane switch; the actuator assembly comprising a first portion for engaging the switch, a second portion cooperating with the first portion for engagement by a user, and first resilient means urging the actuator assembly towards a first position, the arrangment being such that initial movement of the second actuator portion from the first position to a second position against the resilience of the first resilient means causes the first portion to actuate the membrane switch, the cooperation between the first and second portions enabling the second portion to be moved from the second position to a third position while the membrane switch remains in its actuated state.

With this invention, the second portion of the actuator assembly can travel through a greater distance than is essential for actuating the membrane switch. In this way, the second portion of the actuator assembly can appear to the user to be equivalent to the conventional push button although, in fact, a large amount of its movement is unnecessary.

Preferably, the first resilient means comprises a compression spring. In one example, the second portion is formed by part of the compression spring forming the first resilient means. This provides a particularly simple construction involving a minimum number of parts.

In a second example, the first portion of the actuator assembly is provided by second resilient means which engages and flexes a flexible lamina of the membrane switch during movement of the second portion of the actuator assembly from the first position to the second position. The second resilient means may comprise a compression spring and preferably the compression strength of the compression spring is greater than the flex strength of the flexible lamina.

One of the advantages of the second example and other examples in which the first portion is distinct from the first resilient means is that the strength of the first resilient means can be selected to give any desired resistance to movement of the second portion of the actuator assembly.

In a third example, the first resilient means may be formed by a portion of a flexible lamina of the membrane switch. For example the flexible lamina may be domed and engage the first portion to hold the actuator assembly in the first position. When the second portion is moved towards the second positon, the dome will flex to enable the membrane switch to make or break.

In the cases where compression springs are used for one or both of the first and second resilient means, the compression springs may have a substantially constant diameter with the addition of a centering rod extending through the compression springs and through aligned apertures in the membrane switch. Preferably, however, the or each compression spring is tapered so that self-centering is achieved. The tapering of the or each compression spring prevents the spring collapsing in use and, where the second resilient means comprises a compression spring and this tapers towards the membrane switch, enables pressure to be concentrated at a desired area on the flexible lamina of the membrane switch. Furthermore, the use of a tapered compression spring avoids the need to provide apertures in the membrane switch which can thus remain sealed.

Preferably, the actuator assembly is spaced from the membrane switch when the second portion is in the first position. This enables the amount of movement of the second portion of the actuator assembly before the switch is actuated to be closely controlled while relieving the flexible lamina of the membrane switch of constant pressure when the switch assembly is not being used.

The second portion of the actuator assembly may take a conventional form such as a button having a circular or square cross-section and conveniently includes a flange which retains the second portion in an aperture in a panel.

Conveniently, one or both of the first and second portions of the actuator assembly are formed by a plastics moulding which may be solid or hollow.

As has been indicated above, the switch assembly may be provided in a wide variety of equipment such as typewriters, data entry terminals, computers, calculators, telephones, cash terminals, security devices, and other apparatus incorporating keyboards and push button devices.

Some examples of switch assemblies in accordance with the present invention will now be described with reference to the accompanying diagrammatic drawings, in which: Figures 1A, 1B, and 1C illustrate a first example in different positions; and, Figures 2 to 4 are views similar to Figure 1A but illustrating second, third, and fourth examples respectively.

The drawings, which are not to scale, illustrate different membrane switch assemblies which may for example be used in conventional keyboards such as a typewriter keyboard. In all the examples pushto-make switches are described but the examples could be adapted fairly simply to include push-tobreak switches.

Figure 1A illustrates a portion of a keyboard panel 1 having a circular aperture 2 through which protrudes a moulded plastics push button 3 integrally formed with a flange 4. Suitable indicia are provided on an upper surface 5 of the push button 3 to indicate its purpose. In the case of a typewriter, the indicia may comprise one or more letters of the alphabet.

A conventional membrane switch 6 is positioned beneath the push button 3. The membrane switch comprises a flexible, plastics lamina 7 carrying electrical contacts 8, a spacer 9, and a relatively rigid plastics lamina 10 carrying further electrical contacts 11.Thelaminae7, 10 are bonded to the spacer 9 so that the electrical contacts, 8, 11 are seaied within a cavity 12 defined by the laminae 7, 10 and the spacer 9.

A compression spring 13 extends between an under surface 14 of the flange 4 of the push button 3 and an uper surface 15 of the flexible lamina 7.

Normally, the flange 4 is urged into engagement with the panel 1 by the compression spring 13. A second, tapered compression spring 16 is bonded under the centre of the push button 3 in alignment with the electrical contacts 8 mounted on the underside of the flexible lamina 7. In the position shown in Figure 1A, the compression spring 16 is spaced from the flexible lamina 7.

In operation, a user places his finger on the surface 5 of the push button 3 and depresses the push button against the resilience of the compression spring 13. This causes the compression spring 16 to engage the flexible lamina 7 and since the compression strength of the compression spring 16 is greater than the flex strength of the lamina 7, the flexible lamina 7 is flexed towards the lamina 10 until the electrical contacts 8 come into contact with the electrical contacts 11 (Figure 1 B). The dimensions of Figure 1A are not to scale and full flexure of the flexible lamina 7 will occur comparatively quickly before the push button 3 has moved through a significant distance.

The push button 3, however, can move further in a downward direction against the resilience of the compression spring 13 and the additional resilience of the compression spring 16 until it reaches a positon shown in Figure 1C. This over travel simulates the full travel characteristics of a conventional button operated switch. Of course, full travel of the button 3 is not necessary to cause actuation of the switch and by suitably spacing the compression spring 16 from the lamina 7 and by choosing a suitable compression strength for the spring 16, the amount of travel of the button 3 to cause actuation can be preselected. The effect of delaying compression of the compression spring 16 allows a "double pressure" feel to be obtained.

When the button 3 is released, it moves back to the position shown in Figure 1A under the influence initially of both the compression springs 13, 16 and for the last part of its movement (i.e. from the position shown in Figure 1 B) under the influence solely of the compression spring 13.

In a typical example, the compression spring 16 may be spaced 0.25 mm from the flexible lamina 7 in the position shown in Figure 1A, the push button 3 may require to be moved 0.5 mm to actuate the switch 6 and the full movement possible with the push button 3 may be 3 mm.

For clarity, the electrical circuit in which the electrical contacts 8, 11 are provided has been omitted.

Figures 2 to 4 illustrate further examples of membrane switch assemblies in accordance with the invention and the same reference numerals are used in these Figures as in Figure 1A for corresponding parts.

Figure 2 illustrates a plastics moulded push button 17 extending through the aperture 2 in the panel 1.

The push button 17 has a recess 18 of circular cross-section. A depending lug 19 of moulded plastics is fixed to an under surface 20 of the flange 4 via a cantilevered arm 21 with which it is integrally formed. The arm 21 may be joined to the flange 4 by a mechanical, welding, or adhesive technique. The arm 21 has a neck portion 22 of reduced thickness which acts as a resilient hinge enabling that part of the arm 21 adjacent the lug 19 to pivot towards the recess 18.

A compression spring 23 tapers from the upper surface 15 of the flexible lamina 7 towards the push button 17 and is received in an annular recess 24 formed partly by the arm 21 and partly by a plastics element 25 bonded to the under surface 20 of the flange 4. The recess 24 helps to centre the compression spring 23.

The membrane switch is spaced from the panel 1 by a conventional spacer 26.

In use, the push button 17 is depressed towards the membrane switch 6 against the resilience of the compression spring 23. The strength of the neck portion 22 is such that the lug 19 does not pivot during initial movement of the push button 17 so that on engaging the flexible lamina 7, it causes the lamina 7 to flex and to bring the electrical contacts 8, 11 into contact with one another. Once full flexure of the-flexible lamina 7 has occurred, the lug 19 will begin to pivot about the neck 22 in an anti-clockwise direction (as seen in Figure 2) on further movement of the push button 17 towards the membrane switch 6. During this movement, the lug 19 will be received in the recess 18 while the electrical contacts 8, 11 remain in contact with one another.The contacts 8, 11 will only cease contact when the push button 17 approaches its fully released position (as in the example shown in Figure 1A). The plastics element 25 can also act as a stop which engages the flexible lamina 7 to limit movement of the push button 17.

The example shown in Figure 3 is considerably simpler than that shown in Figure 2 in that only two components are required. A push button 27 of moulded plastics has an integral flange 28 including a downwardly facing annular recess 29. A compression spring 30 which tapers from the membrane switch 6 towards the push button 27 is received in the recess 29. The end of the compression spring 30 adjacent the membrane switch 6 is initially bent outwards as shown at 31 away from the centre of the compression spring 30 and then returned within the cross-section of the spring 30 to form a leaf spring 32 which engages the flexible lamina 7. In the position shown in Figure 3, the leaf spring 32 of the compression spring 30 rests on the flexible lamina 7 directly above the electrical contacts 8.In use, when the push button 27 is depressed, the compression spring 30 is compressed which causes the spiral of the compression spring to bear down upon the leaf spring 32 which causes the flexible lamina 7 to flex.

Once again, the spring 30 enables the push button 27 to continue to move after the membrane switch 6 has been actuated.

The example shown in Figure 4 includes a push button 27 similar to that shown in Figure 3 but in this case, a compression spring 33 of constant diameter extends between the flexible lamina 7 and the push button 27 to urge the push button 7 into engagement with the panel 1. A metal or plastics disc 34 is mounted in the compression spring 33 and includes a downwardly protruding, integral boss 35. In this case, when the push button 27 is depressed the spring 33 urges the disc 34 against the flexible lamina 37 causing the lamina 7 to flex. The position of the element 34 and the strength of the compression spring 33 is such that the switch 6 will be actuated before the push button 27 engages the element 34 thus enabling the push button 27 to continue to travel even after the switch 6 has been actuated.

Claims

1. A membrane switch assembly comprising a membrane switch; and an actuator assembly mounted adjacent the membrane switch, the actuator assembly comprising a first portion for engaging the switch, a second portion cooperating with the first portion for engagement by a user, and first resilient means urging the actuator assembly towards a first position, the arrangement being such that initial movement of the second actuator portion from the first position to a second position against the resilience of the first resilient means causes the first portion to actuate the membrane switch, the cooperation between the first and second portions enabling the second portion to be moved from the second position to a third position while the membrane switch remains in its actuated state.

2. An assembly according to claim 1, wherein the first portion is provided by second resilient means which engages and flexes a flexible lamina of the membrane switch during movement of the second portion of the actuator assembly from the first position to the second position.

3. An assembly according to claim 2, wherein the second resilient means comprises a compression spring.

4. An assembly according to claim 3, wherein the compression spring tapers towards the membrane switch.

5. An assembly according to claim 1, wherein the first portion comprises a depending lug connected to the second portion by a resilient hinge, the second portion including a recess to accommodate the lug during relative movement between the first and second portions.

6. An assembly according to claim 5, wherein the lug is connected to the second portion by a neck portion, the neck portion forming the resilient hinge.

7. An assembly according to claim 1, wherein the first portion is supported by the first resilient means, the first portion being spaced from the second portion when the second portion is in the first position.

8. An assembly according to any of the preceding claims, wherein the first resilient means comprises a compression spring.

9. An assembly according to claim 8, wherein the compression spring forming the first resilient means is received in a recess in the second portion of the actuator assembly.

10. An assembly according to claim 8 or claim 9, when dependant on claim 1, wherein the second portion is formed by part of the compression spring forming the first resilient means.

11. An assembly according to any of claims 8 to 10, wherein the compression spring forming the first resilient means tapers towards the second portion of the actuator assembly.

12. An assembly according to any of the preceding claims, wherein the first portion is spaced from the membrane switch when the second portion is in the first position.

13. A membrane switch assembly substantially as a hereinbefore described with reference to any of the examples shown in any of the accompanying drawings.