GB2193040A - Force location assembly - Google Patents

Abstract

A force location assembly comprises a flexible pad member (12) overlying a base member (1) and on which forces are applied in use by a writing instrument when the assembly is used in a signature verification system. A pair of superposed arrays (9, 11) each having substantially parallel electrical conductors are positioned between the base member (1) and the flexible pad member (12) with the conductors of one array transverse to the conductors of the other array, the arrays being biassed apart by means of a substantially incompressible fluid within a cavity (20). A spacer (10) is positioned between the arrays (9, 11), an electrical connection occurring between the conductors when a force is applied to the flexible pad member (12). An O-ring (4) extends around the base member (1) and a clamp (14) is forced over the base member (1) to clamp the flexible pad member (12) against the O-ring (4) so as to seal the cavity (20). <IMAGE>

Description

SPECIFICATION Force location assembly The invention relates to a force location assembly for use, for example, in signature verification systems.

A conventional force location assembly or pressure pad comprises a base member onto which an array of substantially parallel conductors is bonded; a separating member bonded onto the base member; and a superposed pad member to which a second array of substantially parallel conductors is bonded, the conductors of one array being transverse, usually at right angles, to the conductors of the other array. The assembly is connected by bonding the pad member to the separating member which is itself bonded to the base member. In some examples, the cavity defined between the conductor arrays and the separating member is filled with an incompressible liquid such as silicone.

One of the problems with these known assemblies is that they undergo considerable use and the pad member has only a limited lifetime. In the past, it has not been possible to replace this pad member since it is bonded to other parts of the assembly.

In accordance with the present invention, a force location assembly comprises a base member; a flexible pad member overlying the base member and on which forces are applied in use; a pair of superposed arrays of substantially parallel electrical conductors positioned between the base member and the flexible pad member and with the conductors of one pair transverse to the conductors of the other pair, the arrays being biassed apart; means for normally spacing the arrays apart to define a cavity therebetween whereby a force applied to the flexible pad member causes an electrical connection to occur between conductors of each array; a sealing member; and clamping means for clamping the flexible pad member to the base member with the sealing member sandwiched therebetween whereby after clamping the cavity is sealed.

The invention deals with the problem mentioned above by providing a sealing member and clamping means thus avoiding the need to bond the pad member to the spacing means.

This allows the pad member to be replaced if required and also reduces the number of bonding steps needed during manufacturing leading to an increase in the rate at which the assembly can be manufactured.

In some examples, the clamping means may be connected to the flexible pad member, for example forming part of the flexible pad member. Preferably, however, the clamping means comprises a separate clamping member which is removably secured to the base member.

The cavity may be filled with a gas or air under pressure or be filled with a chemically inert liquid. Examples of fluid filled assemblies are described in our copending British Patent Application No.8526113.

Each array of conductors may be mounted on a flexible support such as a plastics substrate, for example Kapton. In these arrangements, a sheet of electrically conductive material, for example, copper, is bonded onto the substrate and then etched using conventional masking and etching techniques to produce the conductors. Although such an arrangement is generally acceptable, it is important to obtain accurate positioning of the conductor arrays relatively to one another which is more difficult to achieve when each conductor is provided on a separate substrate.

Preferably, therefore, one of the arrays of conductors is printed onto the base member.

This not only increases the accuracy with which the remaining array can be positioned relatively to the one array but also reduces the cost of the assembly by dispensing completely with a separate member for the one array.

Any suitable technique may be used to print conductors on the base member. For example intaglio printing leads to very high accuracy which in turn leads to accuracy in determining the location of an applied force but in practice we believe that screen printing is a suitable technique.

In one method of providing an array of electrical conductors on a base member in accordance with the invention, the method comprises printing with electrically conductive inks a first conductor pattern on the base member; providing a first dielectric insulating layer having a number of connection apertures extending therethrough on the previously printed conductor array,; printing with electrically conductive inks a second conductor pattern on the first insulating layer to connect the connection apertures in a predetermined manner, interconnecting conductors being provided in the apertures electrically connected with respective ones of the conductors of the first and second patterns; and providing a second dielectric insulating layer on the second conductor pattern.

Typically, the base member will comprise a plastics member such as Kapton on which the various layers are built up. The first conductor pattern may comprise an array of substantially parallel conductors, the second conductor pattern being chosen so as to connect predetermined combinations of the parallel conductors, as required.

The interconnecting conductors may be provided prior to printing the second conductor pattern but preferably they are formed as the second conductor pattern is printed.

An example of an assembly and method in accordance with the present invention will now be described with reference to the accompany drawings, in which: Figure 1 is a cross-section through an example of a pressure pad according to the invention; Figure 2 is a plan of the pressure pad shown in Figure 1; Figure 3 illustrates schematically the arrangement of part of one of the conductor arrays of the Figure 1 example; Figure 4 illustrates a detailed example of a conductor array; Figure 5 illustrates the form of the first dielectric insulating layer positioned on the conductor array of Figure 4; and, Figure 6 illustrates the conductor pattern provided on the first dielectric insulating layer of Figure 5.

The pressure pad shown in Figures 1 and 2 comprises a rigid, rectangular base member or block 1 having four upwardly facing, chamfered edges 2, two of which are shown in Figure 1. Each chamfered face has a groove 3 in which is positioned an O-ring seal 4. The base member 1 includes a pair of internally screw-threaded channels 5, 6 closed by respective bolts 7, 8.

A flexi-circuit 9 is bonded to the base member 1. The flexi-circuit 9 comprises a plastics substrate (such as Kapton) having a thickness in the order of 0.002 inches and carrying an array of copper wires shown in more detail in Figure 3. As can be seen in Figures 1 and 2, the flexi-circuit 9 extends beyond the base member 1 and, in use, is connected to a connector which enables each of the conductor wires from the flexi-circuit to be connected to the remainder of position sensing apparatus (not shown) incorporating the pressure pad, for detecting signatures.

A rectangular separation member 10 rests on the flexi-circuit 9.

An upper flexi-circuit 11, similar to the flexicircuit 9, is bonded to the underside of a reinforced rubber cover member 12. The cover member 12 has a depending skirt 13 which overlaps the base member 1.

The cover member 12 is clamped to the base member 1 by means of a ring clamp 14 having a lower, tapering surface cooperating with the chamfer of the side faces of the base member 1.

As can be seen in Figure 1, the clamp 14 is forced down onto the base member 1 so as to clamp the skirt 13 and and the projecting ends of the flexi-circuits 9, 11 between the clamp 14 and the base member 1. The clamp 14 is secured to the base member 1 by bolts (not shown) extending through aligned apertures 16-19, in the clamp 14, skirt 13, flexicircuits 9,11 and base member 1 respectively.

It will be seen that the clamp 14 urges the flexi-circuit 9 and skirt 13 against the O-ring seal 4 thus sealing a cavity 20 defined between the flexi-circuits 9, 11 and the separation member 10. A flexible sealer may be used to ensure a leak free joint.

The cavity 20 is filled with a low viscosity silicone fluid to assist in maintaining the flexicircuits spaced apart. The cavity 20 is filled via one of the channels 5, 6, the other channel being opened to act as a vent. The fluid has a relatively high electrical resistivity so as to prevent inadvertent electrical connnection occurring between conductors of the two flexi-circuits in their normal spaced apart condition.

The arrangement of electrical conductors on the flexi-circuit 9 is shown in more detail in Figure 3. Only a portion of the array is shown in the drawing. The Array is divided into eleven blocks, three of which are shown in Figure 3. Each block contains sixteen parallel electrical conductors 21 extending from a respective coarse position conductor 22. In addition, each block includes another sixteen substantially parallel conductors 23 which are interleaved between respective pairs of the conductors 21. Corresponding conductors 23 from each block are connected to respective fine position conductors 24. Typically, the conductors 21, 23 are separated by 0.005 inches.

The flexi-circuit 11 is similar to the circuit 9 but only has three blocks of conductors 21, 23 connected in a similar manner to coarse and fine position conductors 22, 24 respectively. Larger or smaller areas can be constructed by varying the number of fine/coarse conductors.

When the pressure pad is used in a signature verification assembly, the blocks of the flexi-circuit 9 will extend in the general direction of signing while the blocks of the circuit 11 will extend transversely, usually at right angles, to the signing direction. Typically, the flexi-circuits 9, 11 are mounted so that the sets of conductors are spaced apart by about 0.004 inches.

In use, when a writing instrument is placed on the rubber cover member 12, the flexicircuit 11 is caused to flex towards the flexicircuit 9. Fluid displaced by the writing instrument helps to increase clearance between the flexi-circuits 9, 11 at other places and enables the use of close circuit clearances. The position of the writing instrument may be detected with applied loads of about 20 grams.

Once a portion of the flexi-circuit 11 has approached sufficiently closely to the flexi-circuit 9 an electrically conductive path will be formed between adjacent conductors of the two circuits. In practice, each of the conductors 22, 24 is connected via respective connectors (not shown) to a current source and suitable detection apparatus including a microcomputer. When an electrical path is formed between the two flexi-circuits, a small number of conductors 21, 23 of one flexi-circuit will connect with (usually contact) a similar number of conductors 21, 23 of the other flexi-circuit thereby permitting current to flow between re spective pairs of conductors. The microcomputer determines which of the coarse and fine conductors 22, 24 of each flexi-circuit are carrying a current so as to determine a matrix of contact points.This matrix is then analysed to determine the centre of pressure. This process is repeated at frequent intervals to build up a record of the movement of the writing instrument.

After the writing instrument is removed, the cover member 12 will return to the rest position shown in Figure 1.

It will be appreciated that since the different parts of the pressure pad (apart from the flexicircuit 9) are held together by means of the clamp 14, it is a relatively simple matter to replace the cover member 12, flexi-circuit 11, or the separation member 10 simply by disassembling the clamp 14.

In a modified form of the example shown in Figures 1 and 2, the lower flexi-circuit 9 is dispensed with by screen printing the circuit directly onto the base member 1. This is done in a series of steps during manufacture.

Initially, the array of parallel conductors is screen printed onto the base member 1 using a conventional screen printing technique and conventional conducting inks. In addition, various connecting conductors are also screen printed at the same time. An example of this layer is shown in Figure 4.

Next, a dielectric insulating layer such as Kapton is deposited on the conductor layer shown in Figure 4. The first dielectric insulating layer forms a mask which defines a number of circularly shaped areas, as shown in Figure 5. As can be seen in Figure 5 the circularly-shaped areas 25, 26, 27, 28, and 29 are in alignment with contact areas 30-34 respectively in the conductor layer shown in Figure 4.

A second conductor pattern is screen printed onto the first insulating layer so as to connect the apertures 25-29 in a predetermined manner. In addition, conductive inks will flow into the apertures thus providing an electrical connection between the conductors in the layer shown in Figure 4 and the conductors of the layer shown in Figure 6.

As can be seen in Figure 6, the second conductor pattern connects certain of the conductors in the layer shown in Figure 4.

Finally, a second dielectric insulating layer is laid down over the conductor pattern of Figure 6 to act as a protective layer.

The remainder of the construction of the pressure pad is otherwise the same as that shown in Figures 1 and 2.

Connection of the conductor patterns with remote devices is achieved by conductive adhesive or edge connectors.

Claims (8)

1. A force location assembly comprising a base member; a flexible pad member overlying the base member and on which forces are applied in use; a pair of superposed arrays of substantially parallel electrical conductors positioned between the base member and the flexible pad member and with the conductors of one pair transverse to the conductors of the other pair, the arrays being biassed apart; means for normally spacing the arrays apart to define a cavity therebetween whereby a force applied to the flexible pad member causes an electrical connection to occur between conductors of each array; a sealing member; and clamping means for clamping the flexible pad member to the base member with the sealing member sandwiched therebetween whereby after clamping the cavity is sealed.

2. An assembly according to claim 1, wherein the clamping means comprises a clamping member which is removably secured to the base member.

3. An assembly according to claim 1 or claim 2, wherein at least one of the arrays of conductors is mounted on a flexible support clamped to the base member by the clamping means.

4. An assembly according to any of the preceding claims, wherein the cavity contains a substantially incompressible fluid.

5. An assembly according to any of the preceding claims, wherein one of the arrays of conductors is printed onto the base member.

6. An assembly according to claim 5, wherein the array of conductors is screen printed onto the base member.

7. A force location assembly substantially as hereinbefore described with reference to the accompanying drawings.

8. A method of providing an array of electrical conductors on a base member, the method comprising printing with electrically conductive inks a first conductor pattern on the base member; providing a first dielectric insulating layer having a number of connection apertures extending therethrough on the previously printed conductor array; printing with electrically conductive inks a second conductor pattern on the first insulating layer to connect the connection apertures in a predetermined manner, interconnecting conductors being provided in the apertures electrically connected with respective ones of the conductors of the first and second patterns; and providing a second dielectric insulating layer on the second conductor pattern.