CONDUCTIVE PRESSURE SENSITIVE TEXTILE

Abstract: A fabric including within its construction a first elongated electrical conductor crossed by a second elongated electrical conductor, the conductors being normally biased apart at a crossover point of said fibres with an air gap between them, whereby application of pressure in a direction substantially normal to a plane of the fabric causes the conductors to make contact. The fabric may be woven, knitted, non-woven or plaited. The fabric can be used as a pressure sensor, switch or other sensor.

Description: CONDUCTIVE PRESSURE SENSITIVE TEXTILE The present invention relates to methods of constructing one or more pressure activated electrical switches or sensors in fabric, in the preferred embodiment as integral elements of a single fabric sheet. Electrically conductive fabric sheets are known in the art and are described, for example in the applicant's earlier British patent application 2,339,495. The known conductive fabric sheets typically comprise two conductive layers separated by an insulating layer which can be bridged upon application of pressure on the conductive layers. Although such fabric assemblies can function well, there are inevitable drawbacks with having to have three or more fabric layers, including additional cost, fabric thickness, need to maintain alignment between the various layers, movement of the layers during use and so on. The present invention seeks to provide an improved conductive textile. According to an aspect of the present invention, there is provided a fabric as specified in claim 1. The preferred embodiment provides a woven, knitted, non- woven or plaited fabric including in its woven, knitted, non- woven or plaited construction a first elongated electrical conductor crossed by a second elongated electrical conductor, the conductors being normally biased apart at the crossover point with an air gap between them whereby the application of pressure normal to the plane of the fabric causes the conductors to make contact. Preferably, the fabric includes a plurality of spaced first conductors and/or a plurality of spaced second conductors thereby forming a plurality of said crossover points. The conductors may comprise electrically conductive filaments or fibres. Advantageously, the fabric is a woven fabric; the warp of which may include at least one said first electrical conductor and the weft may include at least one said second electrical conductor. A number of means may be employed, separately or in combination, to bias the conductors apart at the crossover points; in one preferred embodiment this being achieved by including insulating fibres or filaments in the fabric. For example, the biasing apart may be effected by employing, as at least one of the electrical conductors, an electrical conductor having insulating filament or fibre wound round it to leave the surface of the conductor exposed at the crossover point. In another example, the biasing apart is effected by twisting at least one of the electrical conductors together with insulating filament or fibre. Alternatively, the biasing apart may be effected by employing, as at least one of the electrical conductors, an electrical conductor which is supported on and between deformable protuberances of an insulating filament or fibre. In another embodiment, the biasing apart may be effected by including in the weave warp and/or weft floats over more than one yarn. It is preferred that the electrical conductors have an electrical property which is proportional to or reproducible from the length of the conductor. The length of a conductor or plurality of connecting conductors may then be determined from measurement of that property. Advantageously, the electrical property is resistance. For some applications, it will be advantageous for the fabric to have at least one set of spaced electrical conductors, at least some of said set being electrically connected together to form at least one bus bar. Where said set of spaced electrical conductors comprise electrically conductive filaments or fibres in the warp or weft of a woven construction, electrical connection between conductors of that set may be provided by one or more electrically conducting filaments or fibres in the weft or warp, respectively. Alternatively, said electrical connection may be effected after the weaving process. In a preferred embodiment, there is provided a fabric including a plurality of weft fibres and a plurality or warp fibres, first and second conductive fibres within the weft and warp fibres and at least one insulating fibre within the weft and/or warp fibres, the insulating fibre acting to bias apart said first and second conductive fibres so as to provide space therebetween. The fabric may include a plurality of insulating fibres within one of the weft and warp fibres, which insulating fibres provide a bridge for a conductive fibre in the other of the weft and warp fibres, such that said conductive fibre floats over one or more conductive fibres in the one of the weft and warp fibres. In another embodiment, one or more insulating fibres is provided around at least one of the conductive fibres, for example helically disposed therearound. Alternatively, one or more conductive fibres could be provided around at least one insulating fibre, with the insulating fibre including portions, for example projections, extending beyond the perimeter of the conductive fibre or fibres. The insulating fibre can thus provide the spacing means for spacing the conductor from other conductors within the fabric layer. It will be apparent that the invention can provide a conductive textile for a pressure sensor or switch or other conductive device within a single layer of fabric. This can obviate the problems discussed above. In addition, it is possible to reduce the edge effect (non-linearity of resistance relative to position) which is intrinsic to three-layer structures and which must be corrected for to provide accurate measurements. Moreover, it is possible to have significantly higher resolution, possibly ten times or more, relative to the three layer device; the resolution being dependent upon weaving techniques and fibre dimensions. With the preferred embodiments, it is possible to provide for contact of the conductive fibres upon the application of a specific pressure or pressures to the fabric and this can be determined by the size of the air gap, the tension of the weave, the deformability of the conductors and the compressibility of the insulators. Moreover, it is possible to provide a range of pressure sensitivities within a single fabric structure. For example, with the embodiment of floating conductor (described with reference to Figure 3 below) different pressure sensitivities can be provided with a plurality of bridges having a different number of conductors below the bridges and/or different insulating fibres, such as different thicknesses or compressibilities. Similar effects can be envisaged with respect to the other embodiments of fibre disclosed herein. As an alternative, there can be provided two or more layers of the described fabric, having the same or different structures. According to another aspect of the present invention, there is provided a fibre including a conductive yam around which is wrapped at least one insulating yam. Preferably, there are provided two or more insulating yams helically wound around the conductive yam. According to another aspect of the present invention, there is provided a fibre including an insulating yam around which is wrapped at least one conductive yam, the insulating yam including portions extending beyond the conductive yam or yams. Preferably, there are provided two or more conductive yams helically wound around the insulating yam. The projecting portions could be strands of fibre, protrusions and the like. It is possible with the present invention to provide an electrically conductive textile having the features described in British patent application 2,339,495 with only a single layer of fabric. The preferred embodiments of fabric can be significantly cheaper to produce than the structure described in British patent application 2,339,495. Various embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a grid arrangement of elongate conductors; Figure 2 depicts the effects of applied pressure on a crossover between two conductors; Figure 3 is a perspective view of an embodiment of fabric with floating conductors; Figure 4 shows the operation of the fabric of Figure 3; Figure 5 shows various views of an embodiment of yam; Figure 6 shows various views of another embodiment of yam; Figures 7a to 7c show various embodiments of conductive and insulating yams; Figure 8 shows another embodiment of composite yam; Figure 9 shows variations of the embodiment of yam with floating conductors; Figure 10 is a schematic diagram of an embodiment of woven bus bars; Figure 11 shows an example of technical specification of weave structure; and Figure 12 shows an example of individually addressable multiplexed switches within a woven fabric construction. Referring to the Figures, in the embodiment of Figure 1, the piece of fabric preferably comprises at least two sets of elongate electrical conductors. Typically, the conductors in each set are arranged in parallel relative to one another and one set of conductors is arranged perpendicular relative to the other set to form an arbitrarily spaced grid, as shown in Figure 1. The elongated electrical conductors are typically mono-filament or multi- filament conductive fibres, while the remainder of the piece of fabric is composed of insulating fibres. Where any two conductors cross over one another, the construction of the fabric and/or the conductive fibres maintains their physical separation, as shown in the cross-sectional view of two conductors in Figure 2(a). When pressure is applied normal to the plane of the fabric, the conductive fibres are caused to deflect and make electrical contact, as in Figure 2(b). Thus, each crossover point constitutes a momentary contact electrical switch, which will maintain contact while the applied pressure exceeds a threshold. The threshold pressure can be predetermined and controlled at manufacture. The switches also exhibit an analogue switching region, as the area of contact shared by the two conductors varies according to the applied pressure, until a maximum contact area is achieved, as shown in Figure 2(c). The manufacturing variables of the piece of fabric can be controlled such that, in use, the switches operate predominantly within this analogue region, demarcated by the dashed lines in Figure 2(d). If this area of contact is measured through some electrical property, for instance resistance, the crossovers can constitute pressure sensors. Although the piece of fabric can be of knitted or felted construction, it is envisaged that the primary application of this technology will be to woven fabric structures. In this latter case, the two sets of conductive fibres can constitute warp and weft yams, respectively, with insulating yams composing the remainder of the piece of fabric and acting to space apart the individual conductive ya s of each set. A typical example of a woven piece of fabric, incorporating two crossover points, is shown in Figure 3. Separation Techniques A number of techniques can be used for maintaining a degree of physical separation between two conductive fibres at a crossover point. These techniques include the use of weave structures with floated yams and composite conductive/insulating yams. The different techniques may be used together, allowing, for example, a piece of fabric that incorporates both conductive cored composite yam and a weave structure with floats. Separation technique - Weaving with floats over one or more yams The first described separation technique is the use of a weave structure with floats, a term applied to a portion of weft yam that passes over or under more than one warp yam or vice- versa. To achieve separation of the two conductive yams at a crossover, typically, the weft conductive yam is floated over the warp conductive yam and one or more insulating warp yams to either side, as is shown in Figure 3. As a result, the two conductive yams share little or no physical contact area, as shown in the cross-sectional view, longitudinal to the weft, of Figure 4(a). If the conductive warp yam is of smaller diameter than the surrounding insulating warp yams, their physical separation can be effected, as shown in Figure 4(b). When pressure is applied normal to the plane of the fabric, the yams and surrounding fabric deflect, and the two conductors make electrical contact, as in Figure 4(c). Increasing applied pressure increases the area of contact, as in Figure 2(c). The yams must exhibit sufficient elasticity to recover from the deflection upon removal of the applied pressure, and thus return to their separated positions, breaking the electrical contact. Separation technique - Conductive cored yam encircled with displaceable insulator Another separation technique involves using a specific composite constmction for the conductive yams. In this composite yam, a conductive mono-filament or multi-filament core yam is twisted, braided, spun, plaited, co-moulded, coated, sleeved or otherwise partially encircled by insulating material, as shown in Figure 5(a). When a crossover point between two conductive yams, at least one of which is of this nature, is not subject to pressure, the insulating material is interposed between the conductors, as in Figure 5(b), ensuring physical separation. However, when subjected to pressure normal to the plane of the fabric, the encircling insulating material can twist, compress, move aside or otherwise deflect to allow electrical contact between the core conductor yams, as Figure 5(c) shows. Upon removal of the applied pressure, the insulating material springs back into position and/or shape between the conductors to break (open) the electrical contact. The geometry of the composite yam and the compressibility, stiffness and surface textures of its constituent yams contribute to determining the pressure threshold of a crossover point and can readily be determined by experiment. Composite yams of this type may be used to construct plain weave crossover points, without the float structures described above. Separation technique - Compressible, insulating cored yam encircled with conductor Another separation technique involves another type of composite construction for the conductive yams. In this composite yam, which is a reverse case of the yam detailed above, an insulating mono-filament or multi-filament core yam is twisted, spun, braided, plaited, co-extruded, coated, sleeved or otherwise partially encircled by conductive yams or material. Additionally or alternatively, a conductive core may be co-extraded with an insulating coating and then subjected to post production processing to selectively expose areas of the conductive core. The conductive yams are partially embedded into the insulating core yam, such that the compressible, yielding surface of the core yarn stands proud of the conductive yams, as shown in Figure 6(a). Alternatively, but to the same end, thin conductive yams may be twisted or spun with larger insulating yams such that the insulating yams stand proud of the conductive yams. When a crossover point between two conductive yams, at least one of which is of this nature, is not subject to pressure, the insulating material that stands proud of the conductive yams ensures physical separation of the conductors, as Figure 6(b). However, when subject to pressure normal to the plane of the fabric, the insulating material can compress to allow electrical contact between the embedded conductor yams, as shown in Figure 6(c). Upon removal of the applied pressure, the insulating material springs back into position to hold the conductors apart and break the electrical contact. The geometry of the composite yam and the compressibility, stiffness and surface textures of its constituent yams contribute to determining the pressure threshold of a crossover point and can be readily determined by experiment. Composite yams of this type may be used to construct plain weave crossover points, without the float structures described above. Separation technique - Conductive Cored Yam Encircled with Displaceable Insulator Referring to Figures 7(a) to 7(c), there are shown various embodiments of yam with both insulator and conductor. In Figure 7(a) there is a core yam substantially circular in cross- section which can be insulating or conductive as desired. Spun, braided or twisted around the core there are larger diameter insulating yams and smaller diameter conductive yams. As can be seen in the Figures, when no pressure is applied to the yam, the conductive fibres remain spaced from the other conductor(s). However, upon application of a compressing force above the threshold, the insulating yams are compressed and/or moved to allow contact of the conductive yams on the conductive base (which may be another composite yam of this type). In Figure 7(b) there is simply a conductive core having coated thereon or extruded therewith one or more insulating ribs, preferably in a helical arrangement. As can be seen, when no pressure is applied, the conductive core remains spaced from any conductive base upon which the composite is placed (the base may be the another composite structure such as this). However, upon application of a compressive force, there is compression of the insulating rib(s) to allow electrical contact. In Figure 7(c) a deformable conductive core has formed therewith an insulating sleeve from which sections are then removed to leave grooves with conductive troughs. Compression of the structure will cause deformation of the grooves such that a conductive substrate, which may for example be a plate or fibre-like conductor, will make electrical contact with the conductive core. It is not necessary for any part of the conductive core to be removed to create the groove, merely to enough insulator to be removed to allow access to the core. Separation technique - Self-Separating Sensory Composite Nam In Figure 8 there is shown an embodiment of composite yam having a core around which there is braided a conductive/insulating yam with floating conductors, which enables the detection of pressure applied at a point along the length of the structure. Parameters Controlling Actuation Pressure A number of controllable manufacturing parameters determine the actuation pressure of a crossover point between two conductors in a woven piece of fabric. a) Relative diameters of conductive and insulating yams As discussed above, if the conductive yams in the weave are of a smaller diameter or cross-section than the insulating yams, the conductive yams at a crossover point are separated by a greater distance. The conductive yams must be deflected further in order to make contact, thus requiring a greater actuation pressure. b) Propensity of conductive yam to make electrical contact A number of variables contribute to the propensity of a conductive yam to make mechanical electrical contact. Conductive yams with very smooth and/or hard surfaces tend to smaller areas of contact than fibrous and/or compressible yams when contacted together under similar pressures. Mono-filament conductors of circular cross-section similarly offer less contact area than prism shaped or multi-filament yams. Specifics of the composite yams are described above. c) Fabric stiffness The actuation pressure required to deflect the conductors at a crossover and make electrical contact is directly governed by the stiffness of the conductive and surrounding insulating yams, and the general stiffness of the fabric, which in turn is governed by the weave structures used, the yam spacing and the level of weft compacting, or beat, used. Stiffer fabric requires a greater force for a given deflection and will therefore result in crossovers of greater actuation pressure. d) Number of adjacent conductive yams If multiple adjacent conductive yams are used instead of a single warp or weft conductive yam, as in Figure 9(a), the actuation pressure is reduced. Wider conductors with a greater number of adjacent yams, as shown in Figure 9(b), both offer a larger contact area at a crossover point and require less angular deflection of the yams, and thus less pressure, to make contact. e) Number of yams floated If a conductive weft yam is floated over a minimum number of warp yams to ensure separation at a crossover point, as shown in Figure 9(a), the actuation pressure is correspondingly lesser than if the conductive weft is floated over a larger number of adjacent warp yams, as shown in Figure 9(c). Implications to note on actuation pressures Controlling the aforementioned manufacturing parameters allows crossover points with predetermined actuation pressures to be woven into a piece of fabric. The threshold pressures for both electrical contact to be made and maximal contact to be achieved can be determined independently. Crossover points with different pressure thresholds may be incorporated into a single piece of fabric. This enables the constmction of, for instance, a group of neighbouring crossover points that make contact consecutively with increasing pressure and together constitute a quantised pressure sensor. Another implication of controlling the parameters at a crossover point is that the two conductive yams may be woven to be in permanent electrical contact, regardless of applied pressure. Principally, this may be achieved through the use of a plain weave stracture at the crossover point, where the conductive weft is not floated over any additional warps, but instead shares a large, permanent contact area with the conductive warp yam. This allows, for instance, the woven construction of bus-bars, discussed herein. Conversely, if the actuation pressure threshold of a crossover point is made very large, the two conductive yams may be woven such that they never make electrical contact under typical operating conditions. This allows two conductors to pass over one another and remain electrically independent. This facility to design crossover points that make or fail to make contact within a grid of conductors allows the routing of current throughout the piece of fabric akin to the tracks of a printed circuit board. Addressing the Matrix of Crossovers Each crossover point between two conductors may be treated as an independent switch, with the array of crossovers constituting a row-column addressed matrix, similar to the majority of existing keyboards. In order to achieve this, each conductive yam must be individually connected to a suitable circuit for scanning the matrix. Making this number of connections to the piece of fabric can prove inconvenient. Alternatively, a scheme which requires far fewer connections to the piece of fabric is to address the matrix of crossovers through electrical bus-bars, as shown in Figure 10. These bus-bars each serve to interconnect the conductors of one set. The number of connections to the piece of fabric does not scale with the number of crossovers. The bus-bars may be sewn, embroidered, printed, adhered, mechanically clamped or crimped to the piece of fabric in order to make electrical contact with the matrix of conductors. Most attractively, they can also be of woven construction, integral to the piece of fabric in a similar manner to the matrix. A typical arrangement is also shown in Figure 10. Some reproducible electrical characteristic, for example resistivity, can be measured to ascertain the length of a conductor and/or bus-bar. The position of a "closed switch" at a crossover in the matrix can be deduced from these measurements. For example, first assume that the conductive yams of the matrix exhibit a linear resistivity, and that connections are made to three perfectly conductive bus-bars as shown in Figure 10. If the switch at crossover point D is closed, the resistance RAB measured from bus-bar A to bus-bar B is given by: RAB=K(X+Y) where K is a constant determined by the absolute lengths, cross-sectional areas and resistivities of the conductive yarns, and distances X and Y are the orthogonal vector components of point D, where 0<=(X,Y)<=1. Similarly, the resistance measured from bus-bar B to bus-bar C is given by: RBC = K(Y+1-X). Substituting gives: X=[((RAB)/K)-((RBC)/K)+l]/2 and: Y=[((RAB)/K) + ((RBC)/K)-l]/2. A typical example This section details an example of weaving instructions for constructing a typical piece of fabric. A piece of fabric of arbitrary size may be reproduced from these specifications, although the repeat for a 250 mm width has been included. The crossover points are evenly spaced in a grid some 8.5 mm apart. Using the specified yarns and weave structures, the pressure threshold of the crossover points is roughly 80 kiloPascals, equivalent to 4 Newton force on a typical fingertip area of 50 square millimetres. The specifications also incorporate two bus-bars in the warp yams, at either side of the piece of fabric. The warp has been designed with two selvedge edges consisting of a twisted multi- filament yam, BASF F901 G004, 8 warp threads at either edge of the warp on shafts 1-4, shown diagrammatically in Figure 10(a). The warp continues to use a 100% cotton 2/18' s yam set at 24 ends per inch. This is interspersed with conductive mono-filament type BASF F901 A013 every 8 warp threads on shafts 8, 16 and 24. The lifting sequence/ peg plan determines the order in which the shafts are moved to lift or leave the warp threads. A weft thread of the same cotton is passed through the shed of lifted warp threads, as in the peg plan of Figure 10(b) and substituted with the conductive mono-filament F901 A013 on every 6th pick. This determines the weft thread floats over the conductive warp threads. Individually Addressable Multiplexed Switches Within a Woven Fabric Construction Figure 12 shows an embodiment of individually addressable multiplexed switches which can be formed form any of the embodiments described above. As can be seen, a grid of conductor crossover points are produced, by any of the above-described methods, and two bus bars provided with the permanent electrical connections as shown in the Figure. The switches provide, when closed, the closed circuits as shown in the example matrix configurations. More specifically, when each input line D* is connected to a positive potential in turn, the three resulting 3 -bit patterns produced at the outputs Ql, Q2, Q3 uniquely identify a closed switch within the matrix of crossovers. Connecting the matrix to the inputs Dl, D2 and D3 and outputs Ql, Q2 and Q3 according to a binary code allows more graceful response to multiple closed switches therein.

Claims: CLAIMS 1. A fabric including within its construction a first elongated electrical conductor crossed by a second elongated electrical conductor, said conductors being normally biased apart at a crossover point of said fibres with an air gap between them, whereby application of pressure in a direction substantially normal to a plane of the fabric causes the conductors to make contact. 2. A fabric according to claim 1 , including a plurality of spaced first conductors and/or a plurality of spaced second conductors, forming a plurality of said crossover points. 3. A fabric according to claim 1 or 2, wherein the conductors comprise electrically conductive filaments or fibres. 4. A fabric according to any one of claims 1 to 3, which is woven, knitted, non- woven or plaited. 5. A fabric according to claim 4, including warp and weft filaments, wherein the warp filaments include said first electrical conductor or conductors and the weft filaments include said second electrical conductor or conductors. 6. A fabric according to any preceding claim, including insulating fibres or filaments which bias the first and second electrical conductors apart at the crossover point. 7. A fabric according to claim 6, wherein said biasing apart is effected by locating an electrical conductor of relatively smaller cross-section between insulating filaments or fibres of relatively larger cross-section. 8. A fabric according to claim 6, wherein the weave includes warp and/or weft floats over more than one yam to effect the biasing apart of first and second electrical conductors at the crossover point. 9. A fabric according to claim 6, wherein said biasing apart is effected by employing, as at least one of the electrical conductors, an electrical conductor including insulating filament or fibre wound around it to leave the surface of the conductor exposed at the crossover point. 10. A fabric according to claim 6, wherein said biasing apart is effected by twisting at least one of the electrical conductors together with insulating filament or fibre. 11. A fabric according to claim 6, wherein said biasing apart is effected by employing, as at least one of the electrical conductors, an electrical conductor which is supported on and between deformable protuberances of an insulating filament or fibre. 12. A fabric according to any preceding claim, wherein the electrical conductors have an electrical property which is proportional to the length of the conductor, whereby the length of a conductor or plurality of connecting conductors can be determined from measurement of that property. 13. A fabric according to claim 12, wherein the electrical property is electrical resistance. 14. A fabric according to any preceding claim, including at least one set of spaced electrical conductors, at least some of said set being electrically connected together to form at least one bus bar. 15. A fabric according to claim 14, wherein said set of spaced electrical conductors comprises electrically conductive filaments or fibres in the warp or weft of a woven construction and electrical connection between conductors of that set is provided by one or more electrically conducting filaments or fibres in the weft or warp, respectively. 16. A fabric according to claim 14, wherein set of spaced electrical conductors comprises electrically conductive filaments or fibres in the waφ or weft of a woven construction and said electrical connection is effected after the weaving process. 17. A fibre including an insulating yam and a conductive yam, the insulating yam including portions extending beyond the conductive yam. 18. A fibre according to claim 17, wherein there are provided two or more conductive yams helically wound around the insulating yam.

Inventors: PETA-THOMPSON ASHA, SWALLOW STANLEY SHIGEZO, PETA-THOMPSON, ASHA, SWALLOW, STANLEY, SHIGEZO
Applicants: PETA THOMPSON ASHA, SWALLOW STANLEY SHIGEZO, UNIV BRUNEL, BRUNEL UNIVERSITY, PETA-THOMPSON, ASHA, SWALLOW, STANLEY, SHIGEZO
Application year: 2001
Main classification: G06K11
IPC codes: D02G3, D03D15, G01L1, G06F3, H01H1, H01H13