Patent Publication Number: US-7594442-B2

Title: Resistance varying sensor using electrically conductive coated materials

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/726,545, titled “Pressure Responsive Article,” filed on Oct. 14, 2005, the contents of which are hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates generally to pressure responsive electrical sensors, and more particularly, concerns pressure responsive articles which may serve as an electrical pressure sensor and/or a pressure responsive switch. 
     BACKGROUND 
     Pressure responsive electrical switches are known in the art. One known switch includes a deformable insulating material, such as silicone rubber, in which electrically conductive metal particles are suspended. The particles are dispersed in the body so that the body is essentially non-conductive when not compressed. However, when the body is compressed, particles are brought into engagement, increasing the conductivity of the body. Thus, if the body is provided between two electrical terminals, it will serve as an open switch (non-conductive) in the absence of a compressive force and a closed switch (conductive) when a compressive force is applied. 
     Despite prior efforts to provide such pressure responsive electrical sensors, known sensors exhibit a number of shortcomings. For example, the sensors are limited in the range of pressures they will detect. Furthermore, when applied to a fabric article, such as a pillow, they suffer from the disadvantage that the sensor may be felt in the article. Additionally, the sensors do not increase in measured resistance when compressed. 
     SUMMARY 
     In accordance with one embodiment, a sensor includes a fibrous region contacting at least two electrodes. The fibrous region has an electrically conductive coating. The fibrous region has a resistance and when the fibrous region is compressed, the resistance changes. 
     In another embodiment, a sensor includes a compressible material having a resistance and two electrodes. Compression of the compressible material causes the resistance of the compressible material to increase. Additionally, the electrodes are in electrical communication with the compressible material when the compressible material is compressed. 
     In another embodiment, a sensor includes a compressible material having a resistance and two electrodes printed on a substrate. When the compressible material is compressed, the resistance of the compressible material changes. The electrodes are in electrical communication with the compressible material. 
     In another embodiment, a method of making a pressure responsive sensor includes providing a compressible material, applying a conductive coating to the compressible material to provide a coated compressible material having a resistance, and placing the coated compressible material in contact with at least two electrodes, wherein when the compressible material is compressed, the resistance changes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing brief description, as well as further objects, features and advantages of the present invention will be understood more completely from the following detailed description of presently preferred embodiments, with reference being had to the accompanying drawings in which: 
         FIG. 1  is a side-sectional view taken through a cushion  10  of an embodiment. 
         FIG. 2  is a top sectional view of the cushion with the fiberfill material removed. 
         FIG. 2A  is a top plan view of a cushion surface having electrodes printed thereon. 
         FIG. 3  is an electrical schematic diagram illustrating how the resistance of fiberfill material  18  may be determined. 
         FIG. 4  is a sectional side view, similar to  FIG. 1 , showing the cushion  10  in a compressed state. 
         FIG. 5  is an electrical schematic diagram of sensing circuitry for a pressure sensor. 
         FIG. 6  is a top sectional view of a comforter embodiment. 
         FIG. 7  is a side cross-sectional view of a pressure-sensitive switch according to an embodiment. 
         FIG. 8  is a cross-sectional view of the pressure-sensitive switch of  FIG. 7  taken along line  8 - 8 . 
         FIG. 9A  is a plan view of a circuit layout for switch electrodes for the embodiment of  FIG. 7 . 
         FIG. 9B  is a plan view of a circuit layout including illumination elements and switch electrodes for the embodiment of  FIG. 7 . 
         FIG. 10  is an exploded perspective view of the switch of  FIG. 7 . 
         FIG. 11  is a perspective view of the switch of  FIG. 7 . 
         FIG. 12  is a cross-sectional view of a switch of the embodiment in  FIG. 7  integrated into an automotive headliner, the switch being in a non-pressed state. 
         FIG. 13  is a cross-sectional view of a switch of the embodiment in  FIG. 7  integrated into an automotive headliner, the switch being in a pressed state. 
         FIG. 14  shows a manufacturing process for coating a compressible material with a conductive ink to make the compressible material conductive. 
         FIG. 15  shows a manufacturing process for making an inherently compressible conductive material where the material itself is conductive. 
         FIG. 16  shows a manufacturing process for making a conductive impregnated compressible material arrangement where a conductive material is added to a non-conductive compressible material. 
         FIG. 17  shows a surface of a conductive coated foam in a relaxed condition. 
         FIG. 18  shows a surface of a conductive coated foam in a distorted condition. 
     
    
    
     DETAILED DESCRIPTION 
     Set forth below are various embodiments of pressure responsive sensors and switches. The sensor and switch embodiments are illustrated by depicting their use in a variety of applications. However, it is understood that the switches, switching elements, and switching circuits described herein are not to be limited to the particular embodiments described. For example, the disclosed switches may be useful for automotive occupancy sensors, bed and bedding sensors, pillow sensors etc. Nevertheless, the switches, switching circuits and switching elements may be used in a variety of applications other than those that are specifically described, including but not limited to a wide variety of pressure switches, strain gauges, weight sensors, automotive switches, consumer electronics (e.g., cameras and music players), or controls generally. 
     The switch embodiments described herein comprise conductive compressible materials. In general, compressible materials may include, but are not limited to, fibers, fiber-fill, tufted material, textiles, foams, foam rubber, or sponge material. As discussed in detail below, the compressible conductive materials described herein may be manufactured by a variety of processes. In one embodiment, they are coated with or impregnated with a conductive coating such as an ink. In another embodiment, the compressible conductive materials are inherently conductive. Another embodiment the compressible material is made conductive by the addition of conductive fillers. 
     Foams are polymeric materials with a dispersed gas or air phase contained in them. The dispersed phase is contained in a number of cells defined within the foam. Foams may be characterized as open-cell or closed-cell foams. In an open-cell foam, the cells are connected to and in fluid communication with one another, whereas in a closed-cell foam, the cells are isolated from and not in fluid communication with one another. Examples of foams which may be used in the compressive conductive materials described herein include but are not limited to urethane, ethylene vinyl acetate, expanded polyethylene, polyurethane, polytetrafluoroethylene, polypropylene, polyvinylidene fluoride, vinyl acetate, polyvinyl acetate, polychloroprene, polystyrene, linear low density polyethylene, polyolefin, polyether, and nitrocellulose ester foams. Sponge materials may include, for example, natural sponge, synthetic sponge, and sponge rubber. 
     In another embodiment, the compressible conductive materials comprise a fiberfill. Fiberfills generally include but are not limited to textured yarn, quilt batting, polyethylene terephthalate (PET), organic cotton, foam, broadcloth, nylon, heirloom, yarn, polyfil, cotton, filament, glass, cardboard, and fibermesh fiberfills. Some examples of commercially available fiberfills include Flojet-15® (an ePTFE Fiber available from Albany International of Albany, N.Y.), Telar® (a continuous fine denier filament from Fiber Technology Corp. of Lorton, Va.), Dacron® (a polyester fiber from Invista of Wichita, Kans.), Thinsulate® (a micro fiber blend polyolefin &amp; polyester from 3M of St. Paul, Minn.), and Kodel® two ounce (2 oz.) fiberfill (from Eastman Kodak Co. of Rochester, N.Y.). 
     As discussed herein, fiberfill is a filling material used for cushions, pillows, bedding, apparel, and other applications. The bulk of the fiberfill includes fibers loosely arranged (e.g., loose, unordered, tufted, or not generally bonded together). The individual fibers may comprise monofilaments or intertwined filaments. When inserted into the interior of an article such as a pillow, comforter, cushion, etc., the fibers provide support and form. Voids or interstices between individual fibers allow the mass to be compressed (i.e., reduced in volume) when a compressive force is applied. The fibers preferably have a stiffness that enables the fiberfill mass to return to a relaxed volume when the compressive force is removed. 
     Fiberfills generally differ from woven or non-woven textiles in their form and compressibility. A fiberfill is generally more compressible than a textile. In the embodiments described herein, the fiberfills have an uncompressed to fully compressed volume ratio of at least about 1.5:1. However, a ratio of about 5:1 is preferred. Moreover, a woven or non-woven textile has a highly ordered appearance to the fibers that make up the textile. While both foams and fiberfills are compressible, fiberfills lack the integral structure and generally defined shape of foams. 
     As mentioned above, certain of the pressure responsive sensors described herein comprise a conductive coating. A conductive coating typically comprises a resin, a vehicle, and a conductive material. Conductive materials include, but are not limited to conductive granules of silver, copper, zinc, nickel, aluminum, stainless steel, graphite, iron, carbon, carbon nanotubes, conductive polymers, and nano versions of silver, gold, platinum, and/or palladium. The resins may be any type of resins typically used for surface coatings, such as acrylamide, acrylics, phenolics, bisphenol A type epoxy, shellac, carboxymethylcellulose, cellulose acetate butyrate, cellulosics, chlorinated polyether, chlorinated rubber, epoxy esters, ethylene vinyl acetate copolymers, maleics, melamine, natural resins, nitrocellulose solutions, isocyanates, hydrogenated resin, polyamide, polycarbonate, rosins, polyesters, polyethylene, polyolefins, polypropylene, polystyrene, polyurethane, polyvinyl acetate, silicone, vinyls and water thinned resins. The selected resins may be either water soluble or soluble in an organic solvent-based system. Alternatively, the resin may be dispersible in a suitable liquid, rather than truly soluble therein. A liquid dispersion medium may be used in which the resin is dispersed, but in which other materials are truly dissolved. The resin may be used with or without crosslinking. If crosslinking is desired, it may be obtained by using a crosslinking agent or by application of heat or radiation (e.g., infrared, electron beam (EB), or ultraviolet radiation (UV)) to the composition. 
     As indicated above, the resin may be dissolved or dispersed in various liquids that serve as a vehicle for carrying the resin to facilitate its application to fiber or foam. The vehicle may be water based, water miscible, water dispersible, or two-part. The vehicle may also be solvent based, plastisol based, etc. Specific conductive compositions that are suitable for use in the embodiments described below are those described in U.S. Pat. Nos. 5,626,948 and 5,455,749, the entire contents of which are incorporated by reference herein. 
     As explained below with respect to  FIG. 14 , an example of a conductive coating (a silver-based polymer thick film ink) suitable for coating the foams and fiberfills described herein is Electrodag 820B (available from Acheson Colloids of Port Huron, Mich.) and is known to adhere to polyester (which is a preferred fiberfill material). Where the coating is to be applied in a dipping or spraying operation, it typically includes suitable amounts of solvent (thinner) to ensure that it is fully distributed in the desired areas of the compressible material. Fast drying thinners are especially preferred for dipping and spraying operations. Another exemplary conductive coating comprises about 30 percent to about 60 percent of a urethane dispersion, about 30 percent to about 60 percent silver powder, about one (1) percent defoamer, and about 20 percent to about 30 percent silver flakes (all percentages by weight). A preferred example of a washable, water-based silver ink coating comprises about 29.8 percent of a Zeneca R972 urethane dispersion, about one (1) percent of a C. J. Patterson, Patcoat 841 Defoamer, about 45.2 percent of IIRP Metals D3 Silver powder, and about 24 percent of Techniks 135 silver flakes (all percentages by weight). A coating is prepared for application by mixing about one (1) part conductive coating with about three (3) parts water for the water-based solution. Water is added until the viscosity is about one thousand centipoise (1000 cp) or less. This allows the coating to flow around and through the fibers as well as allowing for spraying. 
     With respect to embodiments including a compressible material that is inherently conductive, the term “inherently conductive” is used to indicate that the material itself (e.g., the foam or fiberfill) is conductive, as opposed to a material which is formed first and then treated or coated with a conductive constituent. Inherently conductive materials include a conductive component that is incorporated during the process of making or forming the material. In certain embodiments, they also include fibers or foams whose chemical composition and/or structure imparts electrical conductivity. 
     A compressible material impregnated with a conductive filler is a fiberfill, foam, or sponge that further includes a conductive powder or small conductive fibers that are added to a fiberfill, foam, or sponge to make the filler conductive. A conductive component is added to a non-conductive compressible material. The non-conductive fiberfill, foam, or sponge is already produced and the conductive components are then deposited within. The conductive components may be inherently conductive fibers, fibers coated with a conductive substance, metal fibers, metal powder, carbon powder, carbon fibers, etc. 
     Turning now to the drawings,  FIG. 1  is a side-sectional view taken through a cushion  10 . The cushion has a filling  12  which may be foam or rubber, down, or any other conventional filling material. The cushion is enclosed within a cover  14 , which may be fabric. The cushion has an interior space  16  which is open at the top and bottom and is filled with a conductive compressive material, which in the embodiment of  FIG. 1  is fiberfill  18 . Interior space  16  is broadly a pouch (e.g., a container) that holds fiberfill  18 . As described in greater detail below, fiberfill  18  may be inherently conductive, or it may comprise a conductive coating or filler. 
       FIG. 2  is a top sectional view of the cushion  10  with the conductive fiberfill material  18  removed. Because the fiberfill material  18  is removed, the inner surface of the bottom portion of cover  14  is visible in the view of  FIG. 2 . As may be seen, the interior surface of cover  14  is formed with a pair of electrically conductive traces  20 ,  20  which face into the space  16 . Preferably, conductive traces  20 ,  20  are formed by introducing them on cover  14 , as by printing, and both traces  20 ,  20  are formed on the same surface of cover  14  in a single printing operation. In another embodiment, the top and bottom portions of cover  14  each include a single trace  20 . Assembly includes sewing the edges of covers  14  together with each other and stuffing fiberfill material  18  between each of covers  14 . The resistance of fiberfill material  18  is measured between the two opposing single traces  20 . However, conductive traces  20   a ,  20   b  may also be printed on opposing covers (e.g., on different covers  14  and on opposite side of fiberfill material  18 ). Conductive traces  20   a ,  20   b  (as shown in  FIG. 2A  below) may also both be printed on opposite covers  14 . That is to say, each side of fiberfill  18  may be in contact with a set of conductive traces  20   a ,  20   b.    
     Printing processes that may be used with respect to the embodiments described herein include, but are not limited to, screen printing, off-set printing, gravure printing, flexographic printing, pad printing, intaglio printing, letter press printing, ink jet printing, and bubble jet printing. The ink is preferably selected to be compatible with the substrate and the printing process. Depending on the process selected, relatively high viscosity pastes may be used, as well as liquid inks having a viscosity of five thousand centipoise (5000 cp) (Brookfield viscosity) or less. High viscosity pastes are well-suited for screen printing processes while lower viscosity inks are better suited for processes such as gravure and flexographic printing. Depending on the specific printing process and substrate, shear thinning ink such as pseudoplastic or thixotropic inks may be used, as well as dilatent or shear thickening inks. 
     Conductive traces  20 ,  20 , as mentioned above, are preferably formed on cover  14  by printing methods. In selecting a printing method and conductive ink, the substrate material should be taken into account. Cover  14  may comprise a wide variety of substrate materials, including plastics, spun and non-spun fabrics, woven fabrics, non-woven fabrics, knit fabrics, foams or combinations thereof. Cover  14  may also comprise natural and synthetic materials or fibers, and water proof and non water proof materials. 
     Each of snap connectors  21 ,  21  (of a type which is commonly found in garments) is provided on the surface of article  10  and is electrically connected to traces  20 ,  20 , respectively. Although not shown, it will be understood that the opposing cover  14  also includes conductive traces  20 ,  20 , which are electrically connected to the other of snap connectors  21 ,  21 . Snap connectors  21 ,  21 , connect to separate electrical wires  22 ,  22  and provide electrical connections to cushion  10  and traces  20 ,  20  at opposite ends of space  16 . 
       FIG. 2A  shows an alternative and preferred embodiment of cushion  10  of  FIG. 1 , wherein electrode traces  20   a ,  20   b  are disposed in a non-opposing relationship to one another on cover  14 . In the embodiment of  FIG. 2A , traces  20   a ,  20   b  are disposed on the same surface of cover  14 . By placing both traces  20   a ,  20   b  on the same substrate, i.e. cover  14 , the number of printing operations is reduced. Moreover, by placing them in a non-opposing relationship, traces  20   a ,  20   b  may both be located on a side of cushion  14  which is away from the user (e.g., traces  20   a ,  20   b  are on a side of article  10  opposite from the user) or faces away from the user. For example, traces  20   a ,  20   b  may be located on the bottom side of an automotive seat. Thus, the user would not be able to detect the presence of traces  20   a ,  20   b.    
     Conductive fiberfill material  18  has an uncompressed or relaxed state and a compressed state. In its uncompressed state, conductive fiberfill material  18  has a resistance that substantially prevents electric current from flowing from trace  20   a  to trace  20   b , e.g. about ten meg-ohms (10 MΩ). As conductive fiberfill material  18  is compressed, traces  20   a ,  20   b  remain spaced apart at a substantially fixed distance. The resistance of conductive fiberfill material  18  decreases, causing an electric current to flow from trace  20   a , through fiberfill material  18 , and to trace  20   b  with a fiberfill resistance detectably lower than when in a relaxed state, e.g. about ten kilo-ohms (10 kΩ). In an alternative embodiment, fiberfill material  18  has an uncompressed state resistance of five meg-ohms (5 MΩ), an intermediate compressed state (e.g., one-half of full compression) resistance of two and one half meg-ohms (2.5 MΩ), and a fully compressed resistance of one kilo-ohm (1 kΩ). 
     As explained below in detail with respect to  FIG. 5 , the resistance of conductive fiberfill material  18  may be used to trigger a signal when a predetermined resistance is reached (e.g., due to compression of conductive fiberfill material  18 ). A trigger threshold may be adjusted by determining a value for an input resistor that is in series with conductive fiberfill material  18 . For example, the input resistor value may be predetermined through experimentation or calculation where the trigger threshold is at half compression of conductive fiberfill material  18 . The input resistor value may also be determined based on the characteristics of conductive fiberfill material  18 , such as whether silver or carbon particles are used to provide conductance, or based on density of fiberfill is used. Moreover, the input resistor may be chosen based on environmental conditions such as the presence (or possibility of presence) of moisture or humidity. 
     Alternatively, conductive fiberfill material  18  may have a mid-range resistance in a relaxed state such as about fifty kilo-ohms (50 kΩ). When compressed, the resistance may reduce to a level such as about 10 kilo-ohms (10 kΩ). The resistance ranges provided are not limiting and are merely exemplary. It should be appreciated that the useful range of resistances from a relaxed state and compressed state is dependant upon the environment as well as the capability of the resistance measuring circuitry employed. In another embodiment, when using a silver-based conductive constituent, fiberfill material  18  has a resistance of about one milliohm (1 mΩ) when compressed. 
     The pressure responsive sensor embodiments described herein may be used to indicate a switching event based on the resistance of the sensor.  FIG. 3  is an electrical schematic diagram illustrating how the resistance of fiberfill material  18  may be determined in accordance with an embodiment. A reference voltage E r  is in series circuit with a reference resistor R r . A voltage sensor  30 , such as a voltmeter, is connected across resistor R r  to sense the voltage E o  across it and the resistance R s  of the fiberfill material  18  can be determined from equation 1.
 
 R   s   =[E   r   /E   o 31 1]× R   r    (1)
 
     Where: 
     R s  is the resistance of the fiberfill material  18 , 
     E r  is the reference voltage, 
     E o  is the voltage across reference resistor R r , and 
     R r  is the value of the reference resistor. 
       FIG. 4  is a sectional side view, similar to  FIG. 1 , showing the cushion  10  in a compressed state. In the embodiment of  FIG. 4 , the compressible conductive material comprises a conductive fiberfill material  18 . As indicated above, fiberfill  18  may be inherently conductive, may include a conductive coating, or may include a conductive filler. 
     In its expanded state (see  FIG. 1 ), the fibers of conductive fiberfill material  18  will be spaced apart from one another at a maximum spacing (i.e., the maximum spacing is determined by the maximum expansion of fiberfill  18  or the volume of the interior space  16  along with the amount of fiberfill  18  packed therein). Thus, in a relaxed state, cushion  10  will assume a minimally conductive condition in which the resistance R s  is maximized. Compressing cushion  10  (compare  FIG. 4  and  FIG. 1 ) will cause the fibers of conductive fiberfill material  18  into closer proximity, increasing the conductivity of conductive fiberfill material  18  and decreasing the resistance R s . 
     As mentioned above, in one embodiment a conductive fiberfill is provided by applying a coating to an otherwise non-conductive fiberfill material. In this embodiment, the coating covers the outside surface of the fiber strands. The resin component of the coating adheres to the surface of the fibers, binding the conductive particles (e.g., silver, nickel, carbon) to the fibers. Due to the contact between adjacent fibers, one or more continuous conductive paths are defined through or around the fiberfill mass. Because the fiberfill necessarily includes voids, the fiberfill mass is generally less conductive in its relaxed or uncompressed state than in a compressed state. When the fiberfill mass, such as conductive fiberfill material  18 , is compressed the voids are reduced in size, causing more coated fibers to touch each other. The increased fiber-to-fiber contact increases the number of conductive paths through and around the fiberfill mass, causing it to become more conductive (and less resistive) overall. The resistance is measured at electrodes or conductive traces (see conductive traces  20   a ,  20   b  of  FIG. 2A ) that contact conductive fiberfill material  18 , typically at an exterior surface of conductive fiberfill material  18 . 
     It should be appreciated that the change in resistance R s  can be used to perform a useful function. For example, if cushion  10  were a pillow with a sleeping user resting his head on it, the increased resistance when the user lifts his head could be used to turn on a light in his bedroom. Similarly, if cushion  10  or another conductive compressive article were provided in the seat of a motor vehicle, it could be used to detect the presence and/or the weight of the driver and use that information to adjust vehicle settings (e.g., the seat or mirror positions) based on the presence or weight of the driver. 
     One of the advantages of using a compressible conductive material, such as conductive fiberfill  18 , as a pressure responsive sensor is that, owing to the use of conductive coatings, fillers or inherently conductive fiberfill, the sensor is substantially undetectable. A user resting his head on the cushion  10  or sitting on it could not detect the presence of a pressure responsive sensor. Similarly, the user could not detect the presence of the sensor by handling cushion  10  if the wire leads  22 ,  22  were hidden or if a wireless connection were provided. Moreover, by providing resistance sensing electrodes as printed traces the possibility of detection is minimized. The presence of the sensor apparatus is even less detectable if the electrodes are disposed on a surface facing away from the user, as illustrated in the embodiment of  FIG. 2A . 
     The pressure responsive articles described herein can be used with a variety of switching and sensing circuitry. As shown in the electrical schematic diagram of  FIG. 5 , sensor  30  provides an output voltage to a voltage comparator  35  which compares the outputs of voltage sensor  30  with a reference value X. Should the output of voltage sensor  30  have a certain relationship to the reference value X (above or below, depending upon the application), comparator  35  would trigger an output  40 . In one embodiment, output  40  may be an infrared transmitter, the signal of which could be sensed remotely. In another embodiment, the output  40  is a discrete output that may turn on a light or signal an electronic control unit (ECU) such as in a vehicle. 
     For vehicle safety in particular, a vehicle seat detector may use a single pressure sensor, multiple pressure sensors, or a sensing matrix for determining occupant position (e.g., leaning to the front, side, or back) and weight. An ECU may take information from the pressure sensors directly by reading a resistance value, or indirectly from a vehicle communication message to provide more refined control based on the occupant information (e.g., occupant position, size, and weight). In an embodiment, the pressure sensor or sensors may be used to tailor the operation of safety systems including, but not limited to, front airbags, side airbags, air curtains, seat belt pretensioners, pyrotechnic tensioners, pyrotechnic crash pulse adjustment, various precrash devices including, but not limited to, such as brakes and steering, data recorders (e.g. a black box), and various post crash devices including, but not limited to, information transmitters, seat and belt adjusters, steering wheel and seat position adjusters, etc. 
     With respect to airbags, a determination as to whether an airbag should be fired may be made based on the occupant&#39;s weight and position, as well as an indication that there is no occupant for a particular seat. Moreover, the airbag deployment parameters may be tuned for the particular occupant detected. In one embodiment, the rate of fill for an airbag may be adjusted using occupant information. In another embodiment, the shape of the airbag in deployment may be adjusted using occupant information. 
     In an especially preferred embodiment, the compressible conductive materials described herein comprise a fiberfill material. An advantage of using a fiberfill material as a conductive compressible fiberfill material  18  in a pressure responsive sensor is that an extremely large range of pressures may be sensed. In comparison to sensors which are made of a solid, somewhat compressible material, tufted materials experience a much larger change in volume, from a fluffy, tufted state to a tightly compacted state. As mentioned previously, the fiberfills used in the embodiments described herein generally have an uncompressed to fully compressed volume ratio of at least about 1.5:1, with a ratio of at least about 5:1 being preferred. As a result, tufted materials can exhibit a much larger range of values for resistance R s . 
     For example, it would not be unusual for the fiberfill material comprising conductive compressible fiberfill material  18  to exhibit a resistance range with a ratio of about 10,000:1. The corresponding range of pressures would be determined by the compression characteristics of cushioning material  12  contained in cushion  10 . Thus, it should be possible to sense a wide range of pressures in different pressure classes by selecting an appropriate material. Pressure sensing may be used, for example, to detect the mass of a vehicle occupant and enhance safety equipment (e.g., adjusting airbag deployment parameters using the seat occupant&#39;s mass). 
     In another embodiment, cushion  10  may include multiple layers of fiberfill. Each layer may have the same or different electrical properties as well as compressive or resilient properties. That is to say, each layer may exhibit a different resistance value when compressed. The resistance values may correspond to different weights applied. For example, a fifty pound (50 lb) weight may trigger the first layer, whereas a five hundred pounds (500 lb) weight may trigger the third layer. Moreover, the force required to compress each layer to a predetermined resistance may be different depending upon the type of compressible material used (e.g., foam or fiberfill), the characteristics of the compressible material (e.g., density), or the conductive constituent (e.g., silver or carbon). In this way, detection of weight and position are tuned for an application. 
     In addition to weight sensing, the multi-tiered embodiments may be used to determine the breadth (e.g., the seating surface area)of a passenger. For example, the seat may be subdivided laterally into three regions. The center region is most likely to be subject to the most significant weight. The side regions are subject to more or less weight depending upon the width or breadth of the passenger or the seated position (e.g., the passenger is seated as shifted left or shifted right). In this way, both the weight and the breadth of the passenger can be estimated to tailor the operation of safety systems. 
     When used in sensitive or demanding roles (e.g., automotive applications), the pressure sensors preferably include individual calibration to determine their switching points. Such calibration is typically done when the pressure sensors are manufactured, but may also be done at various stages of vehicle assembly or post-production. The calibrations may include a change to the input resistor value in series with the conductive compressible fiberfill material  18 , or the calibration may be an entry or entries into a lookup table. Moreover, the lookup table may include a plurality of calibrations for a number of pressures applied to the pressure sensor where a characteristic curve is developed such that values between the calibration points may be interpolated. 
     It should also be appreciated that the previously described fabric article or cushion  10  can be washed and handled in essentially the same manner as a conventional fabric article without diminishing or otherwise effecting the operation of the pressure responsive sensor. 
     In one embodiment, compressible conductive material such as fiberfill  18  is formed by applying an electrically conductive coating to an otherwise non-conductive compressible material. As mentioned previously, the coating includes a resin component having conductive particles dispersed in it. The resin adheres to the compressible material, thereby binding the conductive particles to it. In one exemplary embodiment, a conductive coating known as Electrodag 820B (available from Acheson Colloids of Port Huron, Mich.) is diluted with a fast-drying thinner that is appropriate for the resin system, in a ratio of about one (1) part coating to about three (3) parts thinner. The fiberfill is permitted to soak in the mixture for about five (5) to about ten (10) minutes and is then post-cured, preferably by thermal curing. However, depending on the resin, other curing processes such as chemical curing, electron beam, or ultraviolet (UV) curing may be used. After it is cured, the now conductive fiberfill is introduced without appreciable compression into space  16  of cushion  10 . For a more detailed description of an exemplary coating process, see  FIG. 14 . 
     The pressure responsive sensors described herein may be incorporated into a number of different articles. As illustrated in  FIG. 6 , in one embodiment pressure responsive sensors may be incorporated into a comforter to perform a variety of sensing and/or switching tasks.  FIG. 6  is a top sectional view of a comforter  50 . The padding  51  of comforter  50  is provided with three cut-out spaces  56 ,  56 ,  56  into which are fitted three cushions  10 - 1 ,  10 - 2 , and  10 - 3 . Cushions  10 - 1 ,  10 - 2 , and  10 - 3  comprise a covering and contain a compressible conductive material of the type already described. In an exemplary embodiment, cushions  10 - 1 ,  10 - 2 , and  10 - 3  have covers (not shown) with empty interiors. The covers define pouches for containing fiberfill  18 . Thus, the covers act as retaining structures that impart structural definition and integrity to the fiberfill  18  contained in them. 
     The covers (not shown) of cushions  10 - 1 ,  10 - 2 , and  10 - 3  is preferably constructed of a textile that is pleasant to a user. The interior of the outer casing may be printed, sprayed, or otherwise coated with a moisture/liquid-resistant barrier or moisture/liquid-impervious lamination, coating, or cover, to avoid water penetration into the inner space where fiberfill  18  is contained. It is especially preferred to provide an electrically insulative covering that will substantially prevent liquids from contacting fiberfill  18  so that the performance of fiberfill  18  is maintained. The fiberfill performance characteristics reduced by moisture or water intrusion include the electrical properties, as well as the ability to return to a relaxed volume when a compressive force is removed. For example, a spill may occur where comforter  50  is used near liquids. Where the outer casing of cushions  10 - 1 ,  10 - 2 , and  10 - 3  are liquid resistant, the liquid will not substantially penetrate cushions  10 - 1 ,  10 - 2 , and  10 - 3  and damage fiberfill  18 . In another example, a pouch containing fiberfill may be washable because the pouch is made liquid-proof by lamination or coating. In this case, a switch including a conductive compressible fiberfill is intended to be used in harsh environments, including washing, and will operate under those conditions. Although a variety of known moisture barriers may be used, an exemplary moisture barrier film is the polyurethane film sold as Product No. 3220 by the Bemis Company of Shirley, Mass. 
     Cushions  10 - 1 ,  10 - 2 , and  10 - 3  are connected to a connection pod  52  which is provided with snap connectors  54  (not shown) to permit connection of a harness (not shown) connecting pod  52  to a control box (not shown). With comforter  50 , a user may actuate any of the cushions  10 - 1 ,  10 - 2 ,  10 - 3  through the use of pressure, as already explained. Through programming of the control box, the different cushions may provide alternative actuation points, or they may actuate different functions. For examples, cushions  10 - 1 ,  10 - 2  and  10 - 2  could be used to turn lights on or off or to turn a television set or entertainment center on or off. 
     Referring now to  FIGS. 7 and 8 ,  FIG. 7  is a side cross-sectional view of a pressure-sensitive switch  80  according to an embodiment. In  FIG. 7 , switch  80  is in an uncompressed or relaxed state.  FIG. 8  is a cross-sectional view of the pressure-sensitive switch of  FIG. 7  taken along line  8 - 8 . Switch  80  includes a domed pushbutton  82 , a compressible material  90 , a body  100 , a back cover  110 , and a sensing circuit  120 . Pushbutton  82  is the mechanical input for switch  80  and is intended to be depressed by a user or other means (e.g., a lever, or an interference). Pushbutton  80  further includes a hollow generally cylindrical portion  86  and a compression surface  84 . 
     Body  100  is a rigid material, such as plastic, and may be transparent or translucent (explained in detail below with respect to  FIG. 10 ) so as to transmit light, as well as to provide structural stability to switch  80 . Body  100  further includes a retainer rim  102  and a lip  104 . Compressible material  90  is a resilient material and is preferably a resilient foam of the type described previously. 
     Compressible material  90  is also preferably conductive. In the embodiment of  FIGS. 7-8 , switch  80  is a finger-actuated switch which comprises a conductive foam. The conductive foam may be inherently conductive, or it may be a non-conductive foam having a conductive surface coating. It may also include a conductive filler. Where compressible material  90  is inherently conductive or has a conductive filler, the sensing electronics (not shown) detect a button press when the resistance of compressible material  90  decreases. Where compressible material  90  is a non-conductive foam having a conductive surface coating, the sensing electronics (not shown) detects a button press when the resistance of compressible material  90  increases. 
     In one exemplary embodiment, compressible material  90  comprises an inherently conductive foam such as low density conductive flexible polyurethane foam (available from Conductive Plastics Co. of Media, Pa.). In another exemplary embodiment, compressible material  90  is a foam that is impregnated with a silver-based ink coating of the type described in U.S. Pat. No. 5,636,948 and also described above. In yet another embodiment, compressible material  90  is a non-conductive foam that has a conductive surface coating. 
     Sensing circuit  120  includes at least two electrodes and is explained in detail below with respect to  FIGS. 9A and 9B . Back cover  110  is a snap-on cover to hold together switch  80  and includes snaps  112 . When assembled, back cover  110  is pressed towards body  110  and snaps  112  resiliently deflect in a radially outward direction and engage lip  104  to hold switch  80  together. Further, sensing circuit  120  and back cover  110  provide a rigid surface for compressible material  90  to be compressed against when pushbutton  82  is pressed. The resiliency of compressible material  90  provides in part a return feature (e.g., movement back of pushbutton  82  after released) of both compressible material  90  and pushbutton  82 . In addition, the dimensions and materials of construction of pushbutton  82  and compressible material  90  are preferably selected to ensure that it substantially returns to its normal state and height after being released from a compressed state. For example, pushbutton  82  may be made from rubber, or a rubber-like material. Moreover, pushbutton  82  preferably exhibits minimal hysteresis. 
       FIG. 9A  is a plan view of an exemplary circuit layout  200  of sensing circuit  120  for the embodiment of  FIG. 7 . Circuit layout  200  includes a substrate  202 , a first contact  204 , a second contact  206 , a first electrode pattern  210  and a second electrode pattern  212 . First contact  204  and second contact  206  interface switch electronics (explained in detail with respect to  FIG. 5 ). As shown in  FIG. 7  (see sensing circuit  120 ), substrate  202  is placed between back cover  110  and body  100  and electrode patterns  210 ,  212  are located so as to contact compressible material  90 . First electrode pattern  210  and a second electrode pattern  212  are printed on the same surface of substrate  202  and are printed about fifteen millimeters (15 mm) wide and are formed from an ink comprising about 30 percent to about 60 percent of a urethane dispersion, about 30 percent to about 60 percent silver powder, about one (1) percent defoamer, and about 20 percent to about 30 percent silver flakes (all percentages by weight). A preferred example of a washable, water-based silver ink comprises about 29.8 percent of a Zeneca R972 urethane dispersion, about one (1) percent of a C. J. Patterson, Patcoat 841 Defoamer, about 45.2 percent of IIRP Metals D3 Silver powder, and about 24 percent of Techniks 135 silver flakes (all percentages by weight). Electrode patterns  210 ,  212  are preferably applied to substrate  202  by a printing process of the type described above. 
     In an alternative embodiment, electrode patterns  210 ,  212  may also be a carbon-based ink about ten millimeters (10 mm) wide. The carbon-based ink may be an ink comprising from about 30 percent to about 60 percent of a carbon dispersion, from about 30 percent to 60 percent of a urethane dispersion, from about one-half (0.5) percent to about two (2) percent of a thickener flow additive, and from about five (5) percent to about 9 percent of a humectant (all percentages by weight). A preferred embodiment of a washable, carbon-based semi-conductive ink comprises about 49 percent CDI 14644 carbon dispersion, about 42.25 percent Zeneca R-972 Urethane dispersion, about one (1) percent RM-8W Rohm &amp; Haas flow thickener, and about 7.75 percent diethylene glycol humectant (all percentages by weight). 
     Additionally, electrode patterns  210 ,  212  may be copper-based ink such as a copper water based ink, product number 599-Z1240 (available from Spraylat Corporation of Mount Vernon, N.Y.). However, copper-based ink may be less desirable for an environment where corrosion and/or oxidation may reduce the performance of electrode patterns  210 ,  212 . 
     In general, the printed conductive ink compositions for electrodes (e.g. electrode traces  20   a ,  20   b  of  FIGS. 2 ,  2 A, electrode patterns  210 ,  212  of  FIG. 9A , and contacts  222 ,  224 ,  226 ,  228  of  FIG. 9B ) may comprise electrically conductive liquids, inks, pastes, powders and/or granules. The conductive inks described herein are similar to the coatings describe above. However, the inks generally include a lesser component, or lack thereof, of the vehicle (e.g., thinner). Generally, the consistency of the ink is tuned for a printing process whereas the coating is formulated to provide a penetrating liquid. As indicated above, the resin may be dissolved or dispersed in various liquids that serve as a vehicle for carrying the resin to facilitate its application to a substrate (e.g., by a printing process). The vehicle may be water based, water miscible, or water dispersible. It may also be solvent based, plastisol based, or two-part based, etc. Specific conductive compositions that are suitable for use in the embodiments described below are those described in U.S. Pat. Nos. 5,626,948 and 5,455,749. 
       FIG. 9B  is a plan view of an alternative circuit layout  220 , including illumination elements and switch electrodes for sensing circuit  120  of the embodiment of  FIG. 7 . In addition to the elements of  FIG. 9A , layout  220  further includes cathode contacts  222 ,  226 , anode contacts  224 ,  228 , and light emitting diodes (LEDs)  230   a ,  230   b ,  230   c ,  230   d . When switch electronics are connected to contacts  204 ,  206 , a lamp driver circuit may further turn on (i.e., activate) LEDs  230   a - 230   d  by way of providing a positive voltage to anodes  224 ,  228 , and grounding cathodes  222 ,  226 . By detecting the state of switch  80 , the driving circuits may turn LEDs  230   a - 230   d  on and off in response to the switched state (e.g., on or off). 
       FIG. 10  is an exploded perspective view of the switch of  FIG. 7 . In this embodiment, body  100  includes illumination recesses  240   a ,  240   b ,  240   c ,  240   d  that are located so as to mate with LEDs  230   a - 230   d  of  FIG. 9B . Moreover, body  100  is transparent (e.g., clear) and will guide the light (i.e., behave as a transmission channel for the light) from LEDs  230   a - 230   d  throughout. During assembly, pushbutton  82  is placed in body  100 . Compressible material  90  is placed within pushbutton  82 , and sensing circuit  120  (not shown in  FIG. 10 ) is placed between back cover  110  and body  100 . 
       FIG. 11  is a perspective view of the switch of  FIG. 7  in an assembled state. Domed pushbutton  82  protrudes above retainer rim  102 . Body  100  is generally cylindrical and extends down to lip  104  (see  FIG. 7 ). Snap  112  attaches back cover  110  to body  100 . As shown, illumination recesses  240   a  and  240   b  accept LEDS  230   a  and  230   b  (not shown). 
       FIG. 12  is a cross-sectional view of a switch of the embodiment in  FIG. 7  integrated into an automotive headliner, the switch being in a non-pressed state  300 . Although shown and described herein with respect to an automotive environment, the switches described herein are applicable to a variety of other environments and may be used in any location where movement of a compressible material  90  is used to indicate a switching event. 
     The headliner of  FIG. 12  includes a foam layer  122  and a thin fabric layer  124 . Switch  80  is placed in a cylindrical space  302  provided through foam layer  122 . Retainer rim  102  is preferably substantially flush with the interface of foam layer  122  and fabric layer  124 . Because domed pushbutton  82  protrudes beyond retainer rim  102 , a bulged portion  126  of fabric layer  124  is present. In an automotive headliner embodiment, back cover  110  is typically placed against a rigid surface such as the inside surface of the vehicle roof. Thus, when a user presses pushbutton  82 , compressible material  90  is compressed rather than moving switch  80  as a whole. 
       FIG. 13  is a cross-sectional view of a switch of the embodiment in  FIG. 7  integrated into an automotive headliner, the switch being in a pressed state  400 . Here, a force  130  presses pushbutton  82  within switch  80  and compresses compressible material  90 . Further, pushbutton  82  ensures that compressible material  90  is in pressing contact with electrode patterns  210 ,  212  (see  FIG. 9A ). Sensing electronics are connected to first contact  204  and second contact  206  measure the resistance of compressible material  90 . 
     As indicated above, in certain embodiments of the pressure sensitive sensors described herein, the sensor resistance increases upon compression, while in other embodiments it decreases upon compression. In one embodiment, compressible material  90  is a foam material with a conductive surface coating. The conductive surface coating does not penetrate through the thickness of compressible material  90 , except perhaps to a small penetration depth. Thus, the conductive path for electrical current flow is substantially along the surface of compressible material  90 . As a result of the surface coated configuration, the resistance between electrode patterns  210 ,  212  will increase upon compression of compressible material  90 , as explained below in detail with respect to  FIG. 18 . Alternatively, when compressible material  90  is, for example, an inherently conductive foam or a foam with a coating that is distributed throughout the thickness of compressible material  90 , the resistance between electrode patterns  210 ,  212  will decrease upon compression of compressible material  90 . 
     Alternatively, where switch  80  includes LEDs  230   a - 230   d , the sensing electronics may switch the LEDs on and off. In that case, body  100  (being a light guide and receiving the light output of LEDs  230   a - 230   d ) will guide the light to a visible portion at rim  102  (see  FIG. 7 ) that a user may see. Moreover, where switch  80  is embodied in an automotive headliner application, the light shines through fabric layer  124  and is visible by a passenger. 
     Switching electronics (see  FIG. 5 ) generally detect a switching event and act accordingly (e.g., turning on a light, setting or resetting an output, adjusting volume, or sending a network message in a vehicle). As shown in  FIG. 13 , when fully pressed, pushbutton  82  does not travel so far within switch  80  as to cause bulged portion  126  (of  FIG. 12 ) to stretch in an opposite manner. Thus, fully pressed bulged portion  126  (of  FIG. 12 ) becomes generally flat portion  132 . 
     In manufacturing the compressible material, e.g. filling  12  (see  FIGS. 1-6 ) or compressible material  90  (see  FIGS. 7-13 ), the compressible material may contain a variety of materials and may be manufactured in a variety of ways as described by the manufacturing processes of  FIGS. 14-16 . 
     In the embodiment of  FIG. 14 , a non-conductive compressible material is coated with a conductive coating to provide a compressible conductive material. In the embodiment of  FIG. 15 , a method of making an inherently compressible conductive material is described. In the embodiment of  FIG. 16 , a method of impregnating a compressible material with a conductive constituent such as conductive filler is described. 
       FIG. 14  shows a manufacturing process  1000  for coating a compressible material with a conductive ink to make the compressible material conductive. Manufacturing process  1000  is used for coating a non-conductive fiberfill material. The manufacturing process  1000  is also used for coating non-conductive foam with a conductive material. In contrast to the other methods disclosed herein for compressible materials, here the material is coated with a conductive substance rather than being inherently conductive or being made conductive by adding a conductive filler. The coating process begins at step  1010  where a compressible material, such as fiberfill is selected. In one exemplary embodiment, the fiberfill is a polyester fiberfill such as Dacron® (a polyester fiber from Invista of Wichita, Kans.). In another exemplary embodiment, the fiberfill is Kodel® two ounce (2 oz.) fiberfill (from Eastman Kodak Co. of Rochester, N.Y.). The process continues at step  1012 . 
     At step  1012 , a coating is selected which will adhere to the fiberfill material or foam. In one preferred embodiment, a conductive coating known as Electrodag 820B (available from Acheson Colloids of Port Huron, Mich.) is selected. Electrodag 820B is particularly well suited for use with polyester fiberfills because of its adhesion to them. Moreover, a suitable fast-drying thinner is also selected that is compatible with the conductive coating and the filler material. The process continues at step  1014 . 
     At step  1014 , the coating is prepared for application by mixing about one (1) part conductive coating with about three (3) parts thinner. Thinner is added until the viscosity is about one thousand centipoise (1000 cp) or less. This allows the coating to flow around and through the fibers. When high density foam is used as the compressible material, the coating flows over the surface of the foam structure, without substantially penetrating it. When low-density foam is used, the coating flows through the interstices of the foam structure. The use of fast-drying thinners reduces manufacturing time. The process continues at step  1016 . 
     At step  1016 , the compressible material is soaked (e.g., dipped or submerged within the prepared coating liquor) for approximately five (5) to approximately ten (10) minutes. By submerging fiberfill within the coating liquor, the coating substantially coats all fibers through the mass. Alternatively, the conductive coating is sprayed on the compressible material. For example, in one embodiment a carbon-based coating is sprayed on the fiberfill or foam/sponge material. Spraying the coating liquor reduces costs because it allows for a more focused application of the coating at areas adjacent to the traces or electrodes. However, spraying (as opposed to dipping) will typically reduce the sensitivity (e.g., the range of resistance change) of the pressure switch because only the conductive portion of the compressible material is responsive to compression. 
     When a non-porous sponge or foam is used, the density of the material and lack of passageways therethrough will keep the coating liquor from penetrating. Thus, the non-porous sponge or foam will have a surface coating only. This is different from the fiberfill that provides a surface coating to each fiber, but all the way through the fiber mass. The process continues at step  1018 . 
     At step  1018 , the coated compressible material is removed from the coating mixture and is allowed to dry. The fast-drying thinner quickly evaporates and allows the compressible material to air-dry. The process then ends. Following production of the coated compressible material, the material may be used in the assembly of switches described herein. 
       FIG. 15  shows a manufacturing process  1100  for making an inherently compressible conductive material  90 . In general, a conductive material is added to a masterbatch and combined with the raw polymer before forming it into the desired structure (e.g., foam or fiberfill). The introduction of a conductive component into the masterbatch avoids the need for adding conductive coatings or interstitial fillers after the foam or fiberfill is made. The conductive components (e.g., silver powder) allow the fiber or foam itself to be conductive rather than requiring a coating or post-addition of an interstitial filler. Process  1100  begins at step  1102  where a masterbatch is made. A masterbatch is typically a concentrated mixture of additives (such as a conductive additive or filler) as well as pigments that are mixed with a resin for encapsulation. The mixture is then typically granularized allowing for transport, measuring, and addition to a raw polymer in a manufacturing process (e.g., spinning or foaming). The process then proceeds to step  1104 . 
     At step  1104 , a conductive additive, such as carbon or silver, is added to the masterbatch and is mixed thoroughly therethrough. For example, conductive additives include, but are not limited to, conductive powders or flakes, or metal fragments or filaments. More particularly, examples of conductive powders include silver powder and carbon powder. The process then proceeds to step  1106 . 
     At step  1106 , the masterbatch is combined with the raw polymer used to form the compressible material, which is then formed into fiber or foam, for example, by spinning or foaming, respectively. Spinning involves twisting separate filaments together to form a multi-filament fiber. The resulting fiber is typically much stronger than the individual filaments. Alternatively, a monofilament fiberfill comprising single fibers may be formed by feeding the melted &amp; mixed raw polymer and masterbatch through a small hole. Foaming may be performed by a variety of methods. For example, polyurethane foam may be manufactured by adding volatile agents (e.g., blowing agents) to the mixture. In another example, foam may be produced by adding water to one of the liquid precursors of polyurethane before mixing. A reaction produces carbon dioxide bubbles that, along with the polyurethane, form a solid foam when polymerization is complete. The process then ends. 
       FIG. 16  shows a manufacturing process  1200  for making a compressible material impregnated with a conductive filler. Typically, a conductive powder or small conductive fibers are added to a fibrous material or open-cell foam to make it conductive. Alternatively, a carbon suffusion process may be used to chemically saturate the fiber. The carbon particles are introduced to the fiber and become part of the structure. The carbon particles, being conductive, make the fibers conductive proximate their outer surface. A conductive fiber formed from a carbon suffusion process may be purchased as Resistat® or Sanstat® available from Shakespeare Company, LLC of Columbia, S.C. The process  1200  begins at step  1202  where a non-conductive fiber is selected and added to a mixer. The process then proceeds to step  1204 . 
     At step  1204 , a conductive component is added to a non-conductive compressible material. The non-conductive fiber or foam are already produced and are mixed thoroughly with the conductive components. The conductive components may be inherently conductive fibers, fibers coated with a conductive substance, metal fibers, metal powder, carbon powder, carbon fibers, etc. The process then ends. 
     As mentioned previously, in certain embodiments of the pressure responsive sensors described herein, compression of the sensor causes resistance to increase. An embodiment of this type of sensor is illustrated in FIG.  17 . Referring to the figure, a coated foam  500  is shown in a relaxed condition. Foam  502  has a conductive coating that is applied to its surface. The coating does not penetrate through the thickness of foam  500 , except to a small penetration depth from the surface. In one embodiment, the coating thickness is about two microns (2μ). The coating is preferably applied such that when a compressive force is applied to coated foam  500 , discontinuities or cracks in the coated surface occur, thereby reducing the conductive surface area and increasing the resistance of coated foam  500 . In contrast, foams that comprise a conductive coating through their thickness will experience a decrease in resistance on compression because more of the conductive particles dispersed in the coating are brought into contact with one another. Referring again to  FIG. 17 , in a relaxed condition, e.g. non-compressed and non-stretched, foam  502  has a coating  504  and has first surface dimensions  510  and  512 . 
     A resistance of coating  504  is determined primarily by the materials of the conductive coating and the thickness of the coating. The resistance, in this example, is measured from measurement points  520  and  522 . Although coating  504  includes voids  506  and cracks  508 , the coating nonetheless presents a resistance at measurement points  520  and  522  because there is a current path between them in the non-broken regions of coating  504 . In an embodiment, the resistance of coating  504  at rest (e.g., the relaxed condition) is about five kilo-ohms (5 kΩ). In addition, coating  504  may be applied so that there are substantially no cracks  508  or voids  506  when foam  502  is in the relaxed state. Where a silver-based conductive coating is used (or other highly conductive element) the resistance at rest is about 1 milliohm (1 mΩ). 
       FIG. 18  shows conductive coated foam  500  in a distorted condition. By distorting or stretching foam  502 , the coating surface dimensions are increased and foam  502  and coating  504  have a second surface dimension  510   a  and second surface dimension  512   a  that are both greater than first dimensions  510  and  512 . A force applied to foam  500  may cause the distortion of the surface. Such distortion may be caused, for example, by a compressive force applied to a foam (see force  302  of  FIG. 13 ). The force causes the foam to compress along the direction of the force, but the surface coating  504  would stretch or distort due to the lateral expansion of the foam. In another example, surface coating  504  may be applied to a pillow casing. In this example, a localized force (e.g., a user&#39;s head resting on the pillow) would cause a local distortion of the pillow casing. At least where the force interacts with the pillow casing, coating  504  will be distorted. 
     As a result of coating  504  being distorted, e.g. stretched over a larger area, the voids  506  and cracks  508  are stretched to larger proportions. In other words, by the distortion or stretching of coating  504 , the voids become larger and the cracks become separated. The resulting increase of discontinuities in coating  504 , the thinning of coating  504 , and increased distance between measurement points  520  and  522  yield an overall increase in resistance. In this way, a circuit connected at measurement points  520  and  522  is able to detect a distortion of coating  504  and may interpret the distortion as pressure applied to foam  502  that stretches or compresses coating  504 . In an embodiment, when the surface area of coating  504  is doubled in the distorted configuration, the resistance is about one meg-ohms (1 MΩ) between measurement points  520  and  522 , as opposed to about five kilo-ohms (5 kΩ) between measurement points  520  and  522  in the relaxed configuration. In an embodiment where a carbon ink is used for coating  504 , the relaxed resistance is about fifty ohms (50Ω). Alternatively, where a silver ink is used, the relaxed resistance is about 1 milliohm (1 mΩ). Measurement points  520  and  522  may, in an embodiment, comprise silver traces or pads printed upon or under coating  504 . With the use of external circuitry, such as that described in  FIG. 5 , the resistance of coating  504  may be used to determine the amount of distortion present. 
     In another embodiment, a coating  504  may comprise two-layers, including a carbon lower layer and a silver upper layer. The lower carbon layer, as compared with the upper silver layer, provides conductivity over a wide range of distortion. In comparison, the silver upper layer may become an open circuit (e.g., having substantially infinite resistance) when highly distorted. In this way, a measurement of the distortion of coating  504  is improved because an open-circuit will not occur over the entire coating  504  unless the lower carbon layer is distorted beyond its limits. When distortion occurs such that the upper silver layer is an open circuit, the lower carbon layer remains conducting and thus, provides an indication to a circuit that the pressure sensor is still at least connected. 
     In another embodiment, coating  504  may be a trace printed on a surface rather than a coating that substantially covers a surface. In yet another embodiment, coating  504  may be a multi-layer trace that preferably includes a carbon lower layer and a silver upper layer. 
     The present invention has been particularly shown and described with reference to the foregoing embodiments, which are merely illustrative of the best modes for carrying out the invention. It should be understood by those skilled in the art that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention without departing from the spirit and scope of the invention as defined in the following claims. The embodiments should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. Moreover, the foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application. 
     With regard to the processes, methods, heuristics, etc. described herein, it should be understood that although the steps of such processes, etc. have been described as occurring according to a certain ordered sequence, such processes could be practiced with the described steps performed in an order other than the order described herein. It further should be understood that certain steps could be performed simultaneously, that other steps could be added, or that certain steps described herein could be omitted. In other words, the descriptions of processes described herein are provided for illustrating certain embodiments and should in no way be construed to limit the claimed invention. 
     Accordingly, it is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments and applications other than the examples provided would be apparent to those of skill in the art upon reading the above description. The scope of the invention should be determined, not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. It is anticipated and intended that future developments will occur in the arts discussed herein, and that the disclosed systems and methods will be incorporated into such future embodiments. In sum, it should be understood that the invention is capable of modification and variation and is limited only by the following claims. 
     All terms used in the claims are intended to be given their broadest reasonable constructions and their ordinary meanings as understood by those skilled in the art unless an explicit indication to the contrary is made herein. In particular, use of the singular articles such as “a,” “the,” “said,” etc. should be read to recite one or more of the indicated elements unless a claim recites an explicit limitation to the contrary.