GB2222029A - Pressure sensitive potentiometer - Google Patents

Abstract

A potentiometer comprises: a track (4) comprising electrically resistive material; and a track (1) of electrically conductive material. The track of electrically conductive material overlies the track comprising electrically resistive material and is carried on a substrate (2). The track of resistive material is carried on a substrate (5) spaced from the flexible substrate by a spacer (3). By pressing the flexible substrate, the tracks may be brought into contact at a chosen location along the track comprising resistive material. <IMAGE>

Description

A POTENTIOMETER This invention relates to a potentiometer.

The general concept of a membrane switch is well known, with two contacts of a "normally open" switch implemented as two layers of insulating flexible substrate, each printed with a pattern of conductive ink and held a small distance apart by a spacer element containing a void to form an air gap between the two switch layers. The switch contacts are closed when an external force (usually finger pressure) is applied to one of the substrates above the spacer void causing it to flex into the void until the two printed conductive patterns meet.

The disadvantages of this arrangement when compared with electromechanical switches are twofold: a lower current switching capacity is achieved; and the switching configuration is limited to a "momentary" type only. Modern digital electronic techniques, however, easily allow these disadvantages to be overcome and the inherent advantages of a flat, easyclean, ingress-proof switching panel have caused the displacement of electromechanical switches in many new equipment designs. When a variable control input is required, the robust nature and aesthetic appeal of these panels is compromised when a conventional potentiometer is required to be fitted. A neater alternative is to accept the variable control input electronically, offering the user up/down entry keys.

Whilst suitable in some applications, it can be tedious or simply too slow if a rapid variation of the control is required.

According to the present invention, there is provided a potentiometer comprising: a track comprising electrically resistive material; and a track of electrically conductive material, one of the tracks at least partially overlying the other and one of them being carried on a flexible substrate whereby as a result of pressing this substrate the tracks may be brought into contact at a chosen location along the track comprising electrically resistive material.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a section through an example of a potentiometer according to the invention; Figs. 2, 3 and 4 show respectively in plan view a resistance element on its substrate, a spacer and a wiper of the potentiometer; Fig. 5 shows the electrical equivalent of the potentiometer; Fig. 6 is a section through another example of a potentiometer according to the invention, as an exploded view for the sake of clarity.

Fig. 7 is a plan view of part of an alternative form of resistance element for the potentiometer of Figs. 1,2,3 and 4 or Fig. 6 Fig. 1 depicts a section, viewed from one end, of a potentiometer comprising an electrically conductive wiper contact 1 on a wiper substrate 2; a spacer element 3; and a resistance element 4 in the form of a track of electrically resistive material (such as printed carbon ink) on a substrate 5.

Figs. 2, 3 and 4 show a plan view of each of the three essential components namely: the resistance element 4 with substrate 5 (the element 4 being between end terminals 6 and 7 of conductors 8 and 9 respectively, all carried on the substrate 5); the spacer element 3; and the wiper contact 1 on its substrate 2. Operation is effected by applying a force in the direction shown by arrow A in Fig. 1. Pressure is brought to bear (usually finger-pressure) on the wiper contact area of substrate 2 so that the flexible wiper substrate 2 deflects into the space in the airgap channel until the wiper contact 1 is forced into contact with the resistance element 4 below (the substrate 5 carrying the resistance element 4 not flexing as it is either made of a rigid material or mounted on to a rigid surface).At this point, the device can be seen to mimic a true potentiometer, and can be represented by the electrical symbol of Fig. 5.

A potential difference could now be applied between the two ends of the resistance element 4 via conductors 8 and 9, and any proportion of this (with respect to one end) obtained at the wiper contact 1 by applying the pressure at any point along the channel either in a continuous sweeping action or an intermittent application of pressure.

However, unlike a conventional potentiometer, the wiper contact 1 does not stay permanently in contact with the resistance element 4. When the pressure is removed, the wiper substrate 2 deflects back to its original flat position, and the wiper contact 1 becomes "open circuit". With modern microprocessor technology this is not an insurmountable problem, but it would certainly be more useful if there were means of indicating when the device is in active operation.

An embodiment of the invention to achieve this will now be described, this representing a modified version of Fig. 1 incorporating shielding to guard against electromagnetic interference, and a switch contact which, when closed, indicates that the device is in active operation and a valid potentiometer signal is obtainable.

Fig. 6 shows an end sectional view of the device arranged (as in the earlier case) as a simple linear channel. Starting at the top of the assembly, layer 10 is a top graphic layer, and consists of a transparent (or translucent) polymer film, printed on the underside with a graphic design which is visible from above; layer 11 is an adhesive which bonds layer 10 to layer 12; and layer 12 is a flexible wiper substrate consisting typically of a polymer film such as polyester, which is printed on its upper surface with a grid pattern 13 of conductive ink (which is terminated for connection to "ground") to form a shield against electromagnetic interference, and printed on its lower surface with a conductive ink track which forms the wiper contact 14.

Layer 15 is a spacer consisting of a polymer film coated with adhesive on both sides. It performs the dual function of forming the air-gap channel between the resistance element and the wiper contact and bonding together layers 12 and 16 of the assembly.

Layer 16 is a flexible resistance element substrate consisting of a material similar to that used in layer 11, but carrying on its upper surface a printed resistive carbon ink track and printed conductive terminations (to provide resistance element 17), and on its lower surface a printed conductive track 18 forming one contact of an "activity" switch.

Layer 19 is another spacer, of similar form to layer 15, forming an air-gap channel for the "activity" switch, and bonding together layers 16 and 20 of the assembly. Layer 20 is a substrate which carries on its upper surface a conductive track 21 which forms the lower half of the activity switch.

Layers 22 and 23 are an adhesive layer (as layer 11) and a rigid backing plate layer respectively, and are needed only if the layer 20 is not a rigid one.

Operation is effected as with the Fig. 1 embodiment, with (for example) finger pressure being brought to bear on the top surface in the direction of arrow B.

This causes a deflection in layers 10, 11 and 12, and contact is made between the wiper contact 14 and the resistance element 17. With layer 12 now bearing directly upon it, layer 16 now deflects down through the air-gap channel of layer 19 until track 18 meets track 21 on layer 20 (which does not deflect) to close the activity switch. Thus, the wiper contact 14 and the resistance element 17 of the potentiometer are always in contact (and therefore giving a valid reading) when the activity switch is closed.

A total of six connections is required for the device: one for the screen grid 13 (to be "grounded"); one for the wiper contact 14; two (one for each end) for the resistance element 17; and two for the activity switch. Physical connection would normally be achieved by bringing all six conductive tracks to one common connection "tail" which may then be inserted into a suitable connector, or an insulation displacement type of connector (which pierces and crimps to the tail itself) could be attached.

Whilst no specific mention of scale has been made, it is anticipated that the overall thickness of layers 10 to 22 in Fig. 6 would be no more than 2 to 3 millimetres. The width of each of the air-gap channels would be of the order of 10 to 15 millimetres. The length of the channel is virtually without limit, and the shape need not be straight - indeed it could be virtually any shape within a two dimensional panel.

For a potentiometer according to Figs. 1,2,3 and 4 or Fig. 6 with a long length of resistance element 4 or resistance element 17, it can be difficult to achieve a properly linear relationship of resistance with length, for example if resistive carbon ink is printed on to the respective substrate to provide the resistance element. This can be substantially overcome if the resistance element is provided by a plurality of discrete resistive portions, each of them being short relative to the length of the resistance element, the portions being joined in series by conductive portions.

An example of this is shown in plan view in Fig. 7, in which a resistance element for use as element 4 or 17 is in the shape of a "square wave" track, and comprises a plurality of short conductive portions 24 (for example of silver) at right angles to the longitudinal axis of the element and a plurality of generally Ushaped resistive portions 25 (for example of printed resistive carbon ink), each of which portions joins by its limbs a respective pair of adjacent portions 24 and the portions 25 being on alternate sides of the longitudinal axis of the resistance element, so that the portions 25 are connected in series via the portions 24 (which underlie the wiper contact 1 or 14).

By way of example, the width of the track at each of portions 24 and the width of the track at each of portions 25 could be 0.75 millimetres and the distance between each of portions 24 and the distance between the limbs of each of portions 25 could also be 0.75 millimetres.

Claims (5)

1. A potentiometer comprising: a track comprising electrically resistive material; and a track of electrically conductive material, one of the tracks at least partially overlying the other and one of them being carried on a flexible substrate whereby as a result of pressing this substrate the tracks may be brought into contact at a chosen location along the track comprising electrically resistive material.

2. A potentiometer according to claim 1, wherein the track of electrically conductive material is carried on the flexible substrate.

3. A potentiometer according to claim 1 or 2, wherein the track of electrically conductive material at least partially overlies the track comprising electrically resistive material.

4. A potentiometer according to any preceding claim, in combination with a switch comprising first and second further tracks of electrically conductive material at least one of which is carried on a further flexible substrate, the arrangement being such that if the first substrate has been pressed to bring the first-mentioned tracks into contact, the first and second tracks are in contact with each other.

5. A potentiometer substantially as herein described with reference to Figs. 1,2,3 and 4; or Figs. 1,2,3,4 and 7; or Fig. 6; or Figs. 6 and 7 of the accompanying drawings.