Textile Pressure Sensor
Brief Description of the Figures:
Figure 1 shows a woven textile having electra-conductive yarns in warp and weft directions.
Figure 2 is a cross-section view of the textile similar to that shown in Figure 1.
Figure 3 is a cross-section of an alternative construction of textile, having yarns of different diameters and resilient compressible printed material.
Figure 4 is a cross-section showing an alternative construction.
Figure 5 shows a textile fabric having locally-applied separate sections of resilient compressible material Figure 6 shows electrical connections formed within a Figure 7 illustrates an application of a textile pressure sensor, beneath a pressure-bandage.
Background
Textile fabrics having conducting fibres or yarns are readily available, for example from Sefar AG of Switzerland, and these use a variety of conductive material such as carbon-coated nylon, or copper conductive yarns. Pressure sensing fabric structures are also available, for example from Eleksen Group Plc, UK; and these commonly involve multiple layers of conductive textile, coming into electrical contact on the application of a force. It is known in the prior art to construct such textile sensors from single sheets, having separate conductive elements that come into contact when pressed (though in practice, such contact-based single sheet fabric sensors are difficult to manufacture) . Some degree of variation of output can be achieved for example using Eleksen's five-layer sensor, and these are suitable for indicating an approximate amount of pressure, but such sensors are not able to provide accurate or repeatable measurements of pressure.
Summary of the Invention
A textile pressure sensor is provided that is breathable and flexible, producing more precise and repeatable measurements of locally applied forces than has previously been available in a textile sensor.
Such forces are measured at multiple locations on the textile surface, thus providing an indication of pressure applied.
The textile pressure sensor has multiple pressure sensing elements constructed within a single sheet of textile, including multiple resilient compressible junctions, electrically connected to sensing means; each junction comprising two overlapping electro-conductive yarns, separated by a gap; said gap including resilient compressible dielectric or alternatively electro-conductive material; such that an electrical property changes in relation to pressure applied to said junction.
The textile pressure sensor has applications in many fields, and is of particular use in medical pressure sensing, where the flexible and breathable nature of the sensor is desirable.
Description
The preferred embodiment and alternative constructions are described with reference to the figures: Figure 1 is a section of woven textile fabric 101, similar to a fabric marketed by Sefar AG of Switzerland, under the brand name PowerMatrix.
Electro-conducting fibres 102 and 103 are woven in the weft direction and include conductive yarn such as 105, continuously covered by a sleeve of insulating material, to be described in more detail later.
Similar conductive yarns 106 and 107 are woven in the warp direction. A number of other, non-conductive yarns such as 104 and 108 are also shown; for illustration purposes, these are shown alternating with conductive fibres in both warp and weft directions. Suitable textile fabrics may include various proportions of non-conductive to conductive yarns in either or both directions. For example, Sefar's PowerMatrix textile uses one conductive to five non-conductive yarns in each of warp and weft directions. Conductive and non-conductive yarns may have different diameters, an example of which will be described later. It is a requirement of the present invention that junctions are formed at the intersection of two overlapping adjacent electra-conductive yarns, and that the junction includes a gap between conductors. In woven textile 101, such junctions are formed between conductive yarns of warp and weft (for example within 102 and 107); and the gap between conductors is created by means of the insulating material that forms a sleeve around each conductor.
Woven fabrics of the type shown in Figure 1 have the advantage of providing rows and columns (in warp and weft respectively) of conductive fibres, forming multiple intersections at the points where a warp fibre overlaps a weft fibre. However, woven textiles provide limited stretch and flexibility compared with knitted textiles. In an alternative construction, a knitted textile fabric (not shown) having conductive yarns extending in one direction may be used; each conductive yarn formed into loop portions such that one conductive yarn overlaps an adjacent conductive yarn by means of a loop portion, thus providing two overlapping adjacent conductive yarns, separated by a gap.
Figure 2 is a cross-section view of textile 101, sectioned along warp yarn 106, at the junction with weft yarns 102 and 104. 104 is a conventional nylon monofilament yarn and carries no conductor. Yarn 102 is of a similar diameter to yarn lO4and comprises a copper conductor 105 at it's centre and insulating sleeve 201 formed from elastorneric material such as polyurethane or silicone, having a Shore Hardness of approximately 3OShoreA. Sleeve 201 thus forms an electrical insulator and has dielectric properties.
In addition, mechanically, sleeve 201 is resilient, readily deforming under applied force, but returning to substantially the original configuration on removal of the force. The mechanical properties such as Shore Hardness of sleeve 201 are selected according to the range of pressures or forces to be measured.
Yarn 106 similarly includes conductor 203 and dielectric insulating sleeve 202. Sleeve 202 may be of the same material or a different, harder material when compared with sleeve 201.
Thus, a mechanically resilient, compressible junction is provided between adjacent overlapping conductor 105 and conductor 203; the gap between conductors 203 and including resilient compressible dielectric material 202 and 201.
Figure 3 shows a cross section of an alternative fabric.
In practice weaving or knitting with yarns having a soft, resilient sleeve around conductors may be difficult, due to material handling issues in the loom. In such cases it is useful to use alternative means for providing a compressible resilient material in the gap between adjacent overlapping conductors.
Figure 3 shows a cross-section view along a conductive warp fibre 304 of a woven textile, at the junction with conductive (301) and non-conductive (307) weft fibres. Conductor 303 is coated with a substantially incompressible polyurethane coating such as a varnish, to form yarn 301. Similarly, conductor 304 is coated with a similar material to form yarn 302. If yarn 307 and other adjacent non-conducting yarns are of similar or smaller diameter than yarn 301, it is likely that yarn 301 will be substantially in mechanical contact with yarn 302 on output from manufacture. Since in this case both insulating sleeves are of material that does not compress significantly under the desired forces, it is necessary to create an additional gap 306 to that formed by the sleeve material. This may be achieved by several methods: In a first example, where the diameter of conducting yarns is the same as or greater than the diameter of non-conducting yarns, the textile may be heat-treated and set, to provide gaps between conducting yarns such as 301 and 302.
In a second alternative example, non-conducting yarns, such as 307 are of a larger diameter than conducting yarns such as 301, and this has the effect across a textile sheet of providing gaps between the smaller yarns.
Resilient compressible material 305 is applied locally (preferably by printing in un-cured form) so as to penetrate the gap 306. Once cured, material 305 forms a resilient compressible dielectric material in gap 306. Preferably, for the embodiment described later with reference to Figure 7, a Room-Temperature-Curing elastomeric Silicone with a hardness of between 3OShoeA and 7OShoreA is used.
Gap 306 between conductors 303 and 304 thus includes a first (less compressible) dielectric material, formed by the sleeves of conductive yarns 301 and 302, and a second (more compressible) resilient compressible dielectric material 305.
In a further alternative, material 305 is a Puff-Ink' material. Such materials are typically applied by printing and allowed to dry, then on the application of heat, a reaction forms internal bubbles, converting the material into a foam. In this case, a small gap may be created by the foaming action of the ink.
Figure 4 illustrates an alternative construction to Figure 3, where the gap 402 is provided by selectively removing a portion of the insulating sleeve. Yarn 401 differs from yarn 302 in that a portion of insulating sleeve has been removed around conductor 403, at the location of the junction with similarly stripped conductor 404. Portions can be selectively removed by laser searing at precise locations, for example at a junction.
Gap 402 thus includes an exposed portion 403 of conductive yarn 401, and a compressible resilient dielectric material 305 between conductors 404 and 403.
Figure 5 shows a woven or knitted textile fabric 501 having multiple force or pressure sensing regions such as 402, 502, 503 and 504, arranged across its surface in a pattern. Depending on the points t which force is to be measured, the pattern can be in the form of a grid, or otherwise. Each pressure-sensing region such as 502 must contain at least one junction between adjacent overlapping electro-conductive yarns.
However in some situations it is advantageous to include more than one such junction within each region. Similarly, areas outside of pressure-sensing I) regions may include unused conductive yarns.
Preferably, for reasons previously described, pressure-sensing regions such as 402, 502, 503 and 504 include resilient dielectric material applied after the textile construction process, and these are preferably positioned apart from each other, with intermediate spaces of open textile, facilitating breathability and flexibility of the textile sensor array as a whole.
Figure 6 illustrates a means of providing tracked electrical connections to a convenient connection point, in this example at the edge of the fabric.
205 Textile fabric 501 includes multiple pressure-sensing regions such as 601, 602 and 603. Regions 601 and 602 both coincide with a shared conductive warp yarn, and regions 601 and 603 coincide with a shared conductive weft yarn, and so on throughout the array of pressure 210 sensing regions. Connection 606 is formed by removal of the insulating sleeve at a junction between conductive warp and weft yarns and then applying a conductive material such as an epoxy resin having a high proportion of silver particles. Such conductive 215 pastes are commonly available, and may be applied by printing. Connection 604 thus provides an electrical connection between one conductor of junctions 602 and 601; and edge connector 606. Similarly, other junctions such as 605, by linking selected conductive 220 yarns, form electrical connections between terminals and the other conductor within a pressure-sensing junction, such as 601.
Conventional electronics are connected to terminals such as 601. In the preferred embodiment, capacitive 225 sensing is carried out by an AD7142 -Programmable Capacitance-to-Digital Converter with Environmental Compensation -manufactured by Analog Devices Inc. The capacitance between adjacent overlapping conductors, across the gap in each junction varies in 230 relation to the size of the gap. Thus, at a particular point at a junction, displacement of one conductor relative to another can be measured. In the preferred embodiment, using resilient means to return the conductors of a junction to substantially their 235 original positions on the removal of the force, the displacement is related to the force applied. In practice, a force is applied over an area including a junction and a surrounding area and hence the reading from a junction is indicative of a pressure applied 240 over its surrounding area. Where multiplejunctions are arranged in a pattern, pressure over the area covered by the pattern can be deduced.
An alternative embodiment measures resistance instead of capacitance: With reference to Figure 4, resilient 245 compressible dielectric material 305 is replaced with a resilient compressible conductive material, such material having the property of varying its electrical resistance in relation to its compression. For example -silicone rubber containing metallic 250 particles may be used, or material such as ZOFLEX 2-Part Liquid Conductive Rubber, available from Xilor Research LLC.
Figure 7 illustrates an application of the textile pressure sensor under a bandage. Compression bandages 255 such as 701 are used to treat various conditions including for example venous leg ulcers. In ideal conditions the bandage is applied and maintained so as to apply a pressure of 6OmmHg to the affected area.
The textile sensor described herein may be 260 manufactured at low cost in volume, using largely standard, automated processes such as printing and laser ablation. Thus the device allows for disposable, or semi-disposable use, particularly in high-value medical applications. A sensor including 265 an array of pressure-sending junctions, such as those shown in Figures 6 and 7, is connected to a circuit housed within unit 702. Unit 702 also includes a battery and output means; in the preferred example wireless telemetry to an analysis device, such as PDA 270 703. The sensor, assembled with unit 702 is included in the dressing 701 at an early stage of application, for example after the first dressing has been applied and a first turn of bandage has been loosely applied; but before subsequent, compression layers of bandage 275 have been applied. Sensor and unit 702 are thus held underneath compression bandage 701; and readings transmitted to the analysis device represent real-time or data-logged measurements from the textile pressure sensor.
280 In the preferred embodiment, analysis software is arranged to recognise rapid changes in measurements and may provide an alert. Such rapid changes may occur for example if the dressing shifts position, or if the sensor becomes wet, for example from seepage of 285 fluid from the wound. Alerts are also preferably provided when the measured pressure is outside of desired limits.