Abstract:
A transducer sensor array for measuring forces or pressures exerted on a surface includes a lattice of individual force or pressure sensor transducer elements comprising intersecting regions of pairs of elongated, flexible threads, each consisting of a central electrically conductive wire core covered by a layer of piezoresistive material having an electrical resistivity which varies inversely with pressure exerted on the material. The threads are arranged into two parallel planar sets, one set forming parallel spaced apart rows and the other set forming parallel spaced apart columns angled with respect to the rows. Row and column piezoresistive threads are retained in physical contact with one another at cross-over intersection points forming a lattice of piezoresistive junctions comprising individual force sensing elements, either by being bonded between a pair of thin, flexible, upper and lower laminating sheets, or by being interwoven to form a fabric mesh. In either case, the sensor array formed by the piezoresistive threads has a highly flexible, fabric-like characteristic which enables the array to readily conform to irregularly curved object surfaces. External normal forces or pressures exerted on an upper surface of the array placed on an object surface causes the electrical resistance of piezoresistive junctions which are compressed in response to the external forces to vary in a predetermined way, enabling a two-dimensional plot of electrical resistance values of each junction node to be made, thus enabling a two-dimensional matrix or map of pressure values exerted on each point of a surface, overlain by the sensor array to be generated. In one embodiment, the piezoresistive layer covering each piezoresistive thread consists of an electrically conductive elastomer, such as silicone rubber impregnated with carbon particles. In another embodiment, the piezoresistive threads consist of a wire core spirally wrapped with a plurality of thin polymer filaments that have a relatively poor electrical conductivity, such as nylon fibers which have carburized outer surfaces.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    A. Field of the Invention  
           [0002]    The present invention relates to transducers, apparatus and methods used for measuring forces exerted on a surface. More particularly, the invention relates to a novel force sensor array for pressure measurement and mapping which includes a fabric-like lattice of piezoresistive threads that are arranged in rows and columns, intersecting pairs of which conductively contact one another to form individual piezoresistive force sensing elements.  
           [0003]    B. Description of Background Art  
           [0004]    There are a wide variety of situations in which it would be desirable to measure normal forces exerted at discrete points on a surface, using such measurements, for example, to produce a map of pressures exerted on various portions of the surface by an object. Examples of such applications for utilizing pressure maps are given in the present inventor&#39;s U.S. Pat. No. 5,571,973, Nov. 5, 1996 , Multi-Directional Piezoresistive Shear And Normal Force Sensors For Hospital Mattresses And Seat Cushions . In that patent, the present inventor disclosed thin, planar sensors for measuring reaction forces exerted by mattresses or chair pads on the body of a recumbent or seated patient. One embodiment of the invention disclosed in the specification of the &#39;973 patent includes a sensor comprised of a two-dimensional array of isolated sensor element pads, each consisting of a thin, flat layer formed of a non-conductive elastomeric polymer matrix filled with electrically conductive particles. A matrix of upper and lower conductive elements in electrical contact with upper and lower sides of each sensor pad enables separate measurements to be made of the electrical resistance of each pad. Pressure exerted on each pad, e.g., in response to a normal force exerted on the sensor matrix by a person&#39;s body, reduces the thickness of the sensor pad, and therefore its electrical resistance, by a bulk or volume piezoresistive effect.  
           [0005]    The present inventor also disclosed a novel method and apparatus for measuring pressures exerted on human feet or horses&#39; hooves in U.S. Pat. No. 6,216,545, Apr. 17, 2001, Piezoresistive Foot Pressure Measurement. The novel apparatus disclosed in the &#39;545 patent includes a rectangular array of piezoresistive force sensor elements encapsulated in a thin, flexible polymer package. Each sensor element includes a polymer fabric mesh impregnated with conductive particles suspended in an elastomeric matrix such as silicone rubber. The piezoresistive mesh layer is sandwiched between an array of row and column conductor strip laminations, preferably made of a nylon mesh impregnated with printed metallic paths. Each region of piezoresistive material sandwiched between a row conductor and column conductor comprises an individually addressable normal force or pressure sensor in a rectangular array of sensors, the resistance of which varies inversely in a predetermined way as a function of pressure exerted on it, and thus enabling the force or pressure distribution exerted by an object contacting the array to be mapped.  
           [0006]    The present invention was conceived of to provide a pressure or force sensor array of simplified construction and few components, which has the form of highly flexible fabric-like lattice which facilitates conforming the sensor array to compoundly curved surfaces, thereby facilitating mapping force or pressure distributions on such surfaces.  
         OBJECTS OF THE INVENTION  
         [0007]    An object of the present invention is to provide a two-dimensional sensor array for measuring forces exerted by an object on a surface at discrete points in a two-dimensional lattice, thereby enabling the distribution of pressure exerted by an object on a surface on which the sensor array is placed to be mapped.  
           [0008]    Another object of the invention is to provide a two-dimensional pressure mapping sensor array which employs a lattice of elongated, intersecting piezoresistive threads.  
           [0009]    Another object of the invention is to provide a two-dimensional pressure sensor array which includes a lattice of piezoresistive threads, intersecting regions of which form individual piezoresistive sensor elements.  
           [0010]    Another object of the invention is to provide a piezoresistive force sensor array for measuring forces exerted by an object on discrete points of a surface on which the array is placed, thereby enabling mapping of pressure exerted on the surface by the object.  
           [0011]    Another object of the invention is to provide a piezoresistive force sensor array which includes a lattice of intersecting piezoresistive threads, each thread having an electrically conductive core clad with a piezoresistive material, contacting regions of which piezoresistive threads form individual piezoresistive sensor elements, the electrical resistance of which varies inversely with externally applied pressure urging the intersecting threads together.  
           [0012]    Another object of the invention is to provide a pressure sensor array having the form of a flexible fabric lattice which is readily conformable to a curved surface, the fabric lattice comprising intersecting piezoresistive resistive threads each consisting of a flexible wire core clad with a resiliently deformable piezoresistive material.  
           [0013]    Another object of the invention is to provide a two-dimensional pressure sensor array for measuring the magnitude and location of forces exerted by an object on a surface on which the array is placed, the array comprising a fabric lattice made of intersecting piezoresistive threads, each of which includes a core made of a flexible electrically conductive wire clad with a piezoresistive elastomeric material.  
           [0014]    Another object of the invention is to provide a piezoresistive pressure sensor array which comprises a fabric lattice made of interwoven warp and woof threads, each of which includes a flexible electrically conductive wire core wrapped with semi-conductive filaments made of a flexible fibrous material such as surface-carburized nylon fibers, intersecting regions of the warp and woof filaments forming a two-dimensional array of piezoresistive pressure sensor elements in which the surface electrical contact resistance between warp and woof strands is inversely proportional to an external force urging the intersecting strands together.  
           [0015]    Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification, drawings and claims.  
           [0016]    It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, I do not intend that the scope of my exclusive rights and privileges in the invention be limited to details of the embodiments described. I do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims.  
         SUMMARY OF THE INVENTION  
         [0017]    Briefly stated, the present invention comprehends a two-dimensional array consisting of a lattice of individual force or pressure sensor elements comprising intersecting pairs of elongated, flexible strands or threads, each of which consists of a central electrically conductive wire core having a relatively low electrical resistivity, covered with a material having a relatively higher electrical resistivity. In one embodiment of the invention, each of a pair of flexible threads consists of a thin metal wire which is spirally wrapped with filaments made of nylon, the outer surface of each filament which has been carburized to make the surface electrically conductive. Preferably, the threads are arranged into two parallel planar sets; one set of threads forming parallel rows, and another set of threads forming parallel columns perpendicular to the rows. In one embodiment of the invention, row and column thread sets are arranged in two separate contacting planes, one on top of the other, to form a rectangular array. In another embodiment, the threads are interwoven into a mono-planar, rectangular array, or mesh. In both embodiments, a contacting region between each pair of crossed threads defines a lattice point and forms an electrical node of variable resistance. When the threads are urged together by a small force normal to the plane of the threads, a relatively small portion of the conductive filaments on the outer surfaces of the threads located at the node physically contact one another, thus resulting in a conductive path having a relatively large electrical resistance being formed at the node defined between the crossed threads. Increased pressure exerted on the planes of the threads forces larger portions of a greater percentage of the filaments into more intimate contact, thus decreasing the electrical resistance of the node by a surface piezoresistive effect, in a predetermined way as a function of applied pressure.  
           [0018]    In another embodiment of the invention, each of the pair of flexible threads of a sensor element consists of a thin metal wire which is clad with a resilient piezoresistive material, preferably an elastomeric piezoresistive material such as silicone rubber filled with carbon black or carbon fibers. When such threads are urged together by a force normal to a tangent contact plane in which a pair of crossed threads physically contact one another to form a resistive node, the size of the intersecting contact surface area increases, thus decreasing the electrical resistance of the intersection node by a surface piezoresistive effect. Moreover, compression of the elastomeric piezoresistive cladding material decreases electrical resistance of the node by a bulk or volume piezoresistive effect.  
           [0019]    In both embodiments of force sensor arrays using piezoresistive threads according to the present invention, electrical resistance at contacting intersections of threads decreases in a predetermined way with applied normal force, thus enabling quantitative measurements of that force by measuring the electrical resistance of the node, utilizing conventional means such as a wheatstone bridge circuit, in which a voltage is applied between a selected pair of row and column piezoresistive threads that intersect at the node.  
           [0020]    In a preferred embodiment of the present invention, piezoresistive threads of the type described above are woven into fabric to form a sensor array having the form of a two-dimensional, rectangular lattice, each intersecting region of a warp and woof thread thereof which defines a variable resistance node comprising an individual piezoresistive sensor element of an array of such sensor elements. Addressing warp and woof threads as row and column conductors, respectively, of the sensor array by conventional electronic means, enables a two-dimensional plot of electrical resistance values at each node to be made, thus enabling a two-dimensional matrix or map of pressure values exerted on each of the lattice points of the sensor array to be generated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is an upper plan view, on an enlarged scale, of a pressure measurement sensor array according to the present invention, comprising a non-woven fabric lattice of piezoresistive threads.  
         [0022]    [0022]FIG. 2 is a transverse sectional view of the sensor array of FIG. 1, taken along line  2 - 2  and on a further enlarged scale.  
         [0023]    [0023]FIG. 3 is an upper plan view of a variation of the sensor array shown in FIG. 1, in which piezoresistive threads thereof are interwoven.  
         [0024]    [0024]FIG. 4 is a fragmentary transverse sectional view of the sensor array of FIG. 1 or FIG. 3, on a further enlarged scale, in which FIG. 4A shows the disposition of a pair of piezoresistive threads intersecting to form a force sensing element, with no external force applied to the element. FIG. 4B shows the sensor element of FIG. 4A with a moderate external normal force applied thereto, and FIG. 4C shows the sensor element with a larger external force applied thereto.  
         [0025]    [0025]FIG. 5 is a fragmentary upper plan view on an enlarged scale of another embodiment of a piezoresistive pressure measurement sensor array according to the present invention, which utilizes piezoresistive threads that include a conductive wire core which is sheathed with a plurality of filaments of a material which has a lower electrical conductivity than the core.  
         [0026]    [0026]FIG. 6 is a fragmentary transverse sectional view of the sensor array of FIG. 5, on a further enlarged scale, in which FIG. 6A shows the disposition of a pair of piezoresistive threads intersecting to form a force sensing element, with no external force applied to the element. FIG. 6B shows the sensor element of FIG. 6A with a moderate external normal force applied thereto, and FIG. 6C shows the sensor element with a larger external force applied thereto.  
         [0027]    [0027]FIG. 7 is a graph showing electrical resistance plotted as a function of pressure exerted on sensor elements of the sensor array shown in FIGS.  1 - 4  or FIGS. 5 and 6.  
         [0028]    [0028]FIG. 8 is a partially diagrammatic block diagram showing the sensor array of FIGS.  1 - 4  or  5 - 6  interconnected with signal processing and display circuitry. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    [0029]FIGS. 1 and 2 illustrate a basic embodiment of a pressure measurement sensor array according to the present invention, which embodiment includes a non-woven fabric-like lattice of piezoresistive threads. As shown in FIGS. 1 and 2, a pressure measurement sensor array  20  according to the present invention includes at least one pair of elongated piezoresistive threads  21 R,  21 C which are disposed one on top the other. According to the invention, the longitudinal axes of each pair of piezoresistive threads  21 R,  21 C are inclined to one other at an angle, e.g., ninety degrees, to define a crossing point or lattice point. Thus, as shown in FIG. 1, threads  21 R,  21 C are arranged in rows and columns which are mutually perpendicular and spaced apart at equal intervals, thus forming a rectangular lattice  22  of crossing regions  23 M(a)(b), where (a) defines a row and (b) defines a column of the lattice such as  23 M(1)(1)) located at the upper left corner, or origin of the matrix, and  23 M(2)(1), at the intersection of piezoresistive threads  21 R 2  and  21 C 1  of row 2 and column 1, respectively, of the lattice.  
         [0030]    As shown in FIGS. 1 and 2, each row and column piezoresistive thread  21 R,  21 C is preferably of identical construction. Thus, each piezoresistive thread  21  includes an elongated, flexible core  24  made of a material having relatively high electrical conductivity. In an example embodiment of sensor array  20 , conductive core  24  consists of a length of 32 gauge stainless steel wire.  
         [0031]    According to the invention, the conductive core  24  of each piezoresistive thread  21  is clad with a layer  25  of material which has a different composition than that of conductive core  24 , the cladding layer material being selected to provide a surface and/or volume piezoresistive characteristic. In an example embodiment of piezoresistive threads  21  according to the present invention, cladding layer  25  consisted of an elongated tubular jacket coaxial with central conducting core  24 , and which was made from a conductive polymer. Preferably, the conductive polymer from which cladding layer  25  is made is an elastomer such as silicone rubber. The present inventor has found that a suitable material for piezoresistive cladding layer  25  is composed of about 50% milled carbon black having a grain size of 2-5 microns, which is thoroughly mixed with about 30% unpolymerized rubber, type BUNA N, and 20% ABS plastic resin/hardener, or silicone rubber (e.g., DOW CORNING RTV 732, obtainable from Neely Industries, 2704 West Pioneer Parkway, Arlington, Tex. 76013) and no hardener. Piezoresistive cladding layer  25  is formed by mixing the aforementioned components thoroughly into a thick slurry, extruding the slurry coaxially over a length of conductive core wire  24 , and allowing the cladding layer to air cure at room temperature, thereby forming a tubular jacket bonded to the core wire.  
         [0032]    The volume resistivity of cladding layer  25  of piezoresistive threads  21  can be adjusted to a desired value by varying the amount of carbon black added to the liquid rubber, and monitoring the resistance of the liquid mixture as these two components are being mixed together. The present inventor has found that a suitable range of volume resistivities for piezoresistive cladding layer  25  is about 50,000 ohm-cm to 100,000 ohm-cm for measurement of normal forces in the approximate range of 0-5 psi, and 100-300,000 ohm-cm for measurement of forces in the approximate range of 5-30 psi. In an example embodiment of sensor array  20  which was tested, each piezoresistive thread  21  consisted of a core  24  made of 28 gauge stainless steel clad with a coax)al layer  25  of silicone RTV 732, had a surface resistivity of 100 ohms/square, a volume resistivity of 100 ohm-cm, and a thickness of about 0.008 inch. Thus, each piezoresistive thread  21  had an outer diameter of about 0.075 inch.  
         [0033]    Referring now to FIG. 2 in addition to FIG. 1, it may be seen that row and column piezoresistive threads  21 R,  21 C are held in fixed relative positions defining a rectangular lattice by encapsulating the threads between lower and upper laminations consisting of thin sheets of a flexible polymer. In an example embodiment of sensor array  20 , lower and upper laminations  26  and  27  consisted of 0.002 inch thick sheets of polyurethane, which were adhered together at interstices  28  between row and column threads  21 R,  21 C by glue blobs  29  made of silicone RTV 732. As shown in FIG. 2, the aforementioned construction of sensor array  20 , using lower and upper laminating sheets  26 ,  27  enables row and column piezoresistive strands  21 R,  21 C to lie entirely in separate, parallel, contacting planes, such as an upper plane  30  and a lower contacting plane  31 . As shown in FIG. 2, column piezoresistive threads  21 C are in upper plane  30 , while row piezoresistive threads  21 R are in lower plane  31 . However, the sensor array  20  performs identically with column piezoresistive threads  21 C located in lower plane  31 , underlying upper plane  30 .  
         [0034]    [0034]FIG. 3 illustrates a variation  20 A of sensor array  20 , in which row and column piezoresistive threads  21 R ,  21 C are interwoven into a fabric mesh, rather than lying in separate planes. In this variation, piezoresistive threads  21 R,  21 C are held tightly together in a planar lattice, without requiring that either or both of row and column threads be adhered to or sandwiched between laminations. Thus, this variation dispenses with a requirement for lower and upper sheet laminations  26 ,  27 , as well as glue blobs  29 .  
         [0035]    [0035]FIG. 4 illustrates how the arrangement of row and column piezoresistive threads  21 R,  21 C, as shown in FIGS.  1 - 3 , forms individual force sensing elements  32  located at each crossing region or intersection  23 M(a)(b) between a row piezoresistive thread  21 R and a column piezoresistive thread  21 C. Thus, as shown in FIGS. 2 and 4, with no external force applied to sensor array  20 , each crossover region or lattice point  23  of the sensor array has thereat a piezoresistive row thread  21 R, the cladding layer  25 R of which has an outer longitudinally disposed cylindrical surface  33  that contacts an outer cylindrical surface  34  of cladding layer  25 C of a column thread  21 C at a contact area  35  in a plane tangent to the two contacting surfaces. Tangent contact area  35  provides electrically conductive continuity between row and column conductors  24 R,  24 C of piezoresistive threads  21 R,  21 C.  
         [0036]    With no external force applied to sensor array  20 , the electrical resistance of contact area  35  is relatively high since the volume resistivity of cladding layers  25  is relatively high, the surface resistivity of contacting surfaces  33  and  34  is relatively high, and the area of tangent contact area  35  is relatively small. However, as shown in FIGS. 4B and 4C, when a normal force N of increasing magnitude is applied to sensor array  20 , the electrical conductance of a tangent contact area  35  between a row and column piezoresistive thread pair  21 R,  21 C decreases, in a manner which will now be described.  
         [0037]    Referring first to FIGS. 2 and 4A, it may be seen that with no external force applied to sensor array  20 , tangent contact area  35  between a row thread  21 R and column thread  21 C is relatively small, since contacting cylindrical outer surfaces  33  and  34  of the cladding layers  25  of the threads which touch one another at contact area  35  both have circular cross sections. Under these circumstances, the small size of contact area  35  results in a relatively high electrical resistance. Moreover, with row and column threads  21 R,  21 C having undeformed circular cross sections, the radial distance through resistive cladding layers between row and column central conductive cores  24 R,  24 L is at a maximum, maximizing the total resistance measurable between the two central conductive wire cores.  
         [0038]    [0038]FIGS. 4B and 4C illustrate effects of external normal forces or pressures exerted on sensor array  20 . As shown in FIGS. 4B and 4C, sensor array  20  is placed with its lower surface  36  supported on a surface S and a force N exerted perpendicularly downwards on upper surface  37  of the array, resulting in a reaction force U being exerted upwardly by supporting surface S on lower surface  36  of the array. Since cladding layers  25 R,  25 C of row and column piezoresistive threads  21 R,  21 C are made of a resiliently deformable material, force pair N-U causes the circular cross sectional shapes of the cladding layers of the piezoresistive threads to deform from circles to ovals elongated in a plane perpendicular to the direction of force pair N-U, i.e., horizontally in FIG. 4. That deformation causes tangent contact area  35  to increase and the radial distance D between conductive wires  24 R,  24 L to decrease, both of which variations cause the electrical resistance R measurable between the conductive cores to decrease.  
         [0039]    For moderate values of normal force N, as shown in FIG. 4B, resilient deformation of piezoresistive threads  21 R,  21 C is relatively small, resulting in a relatively small reduction in electrical resistance R between the threads. Larger forces N exerted on sensor array  20  cause larger deformations of the piezoresistive threads, as shown in FIG. 4C, resulting in larger percentage reductions in resistance R. FIG. 7 illustrates in a general way the reduction in electrical resistance measurable between a pair of row and column piezoresistive threads  21 R,  21 C at an intersecting point  28 , as a function of normal force or pressure exerted on sensor array  20  at that point.  
         [0040]    [0040]FIGS. 5 and 6 illustrate another embodiment of a piezoresistive pressure measurement sensor array according to the present invention. That embodiment utilizes piezoresistive threads which each include a conductive wire core which is covered by a sheath made of a plurality of strands of a material which has a lower electrical conductivity than the core.  
         [0041]    As shown in FIGS. 5 and 6, a pressure measurement sensor array  40  according to the present invention includes at least one pair of elongated piezoresistive threads, including a row thread  42 R and column thread  41 C which are disposed one on top of the other. As shown in FIG. 5, the longitudinal axes of row and column threads  41 R,  41 C are angled with respect to one another, e.g., at ninety degrees, and are spaced apart at equal intervals to thereby form a rectangular lattice  42  of crossing regions  43 M(a)(b), where index letter (a) defines a row and index letter (b) defines a column of the lattice. For example, crossing region or lattice point  43 M(1)(1) is located at the upper left corner or origin of lattice  42 , and lattice pair  43 M(2)(1) is located at the intersection of piezoresistive threads  21 R 2  and  21 C 1  of row 2 and column 1, respectively, of the lattice.  
         [0042]    As shown in FIGS. 5 and 6, each row and column piezoresistive thread  41 R,  41 C is preferably of identical construction. Thus, each piezoresistive tread  41  includes an elongated, flexible core  48  made of a material having a relatively high electrical conductivity. In an example embodiment of sensor array  40 , the conductive core consists of a length of 40-gauge stainless steel wire.  
         [0043]    According to the invention, conductive core  44  of each piezoresistive thread  41  is sheathed with at least one and preferably a plurality of elongated flexible filaments, e.g., a pair  45 - 46 , made of a material which has a higher electrical resistivity than that of conductive wire core  44 . As shown in FIG. 5, filaments  45 ,  46  are each wrapped around each core  44 R,  44 C to form a tight helical spiral around the core. Preferably, each filament  45 ,  46  is wrapped parallel to and in contact with each adjacent filament, at the same pitch, thus forming a relatively void-free covering of core  44 . For ease of illustration, only two such filaments  45 ,  46  are shown wrapped around core. However, the preferred number of individual fibers  45  spirally wrapped around each core wire  44  is more than two, e.g., 20.  
         [0044]    As mentioned above, wrapping filaments  45 ,  46  are made of a material which has poorer electrical conductivity than conductive wire core  44 , in an example embodiment of piezoresistive threads  41  according to the present invention, filaments  45  consists of 20 strands of 0.001 inch diameter nylon having an electrically conductive surface, braided Into elongated sheaths having a diameter of about 0.025 inch and a linear resistivity of 30,000 ohms/foot.  
         [0045]    The example embodiment utilized nylon filaments which had outer surfaces that were carburized to make them electrically conductive, and were obtained from the Saunders Thread Company, Gastonia, N.C. 28054-0020. The carburized nylon filaments were formed into two bundles each consisting of 10 strands, and spiral wrapped in opposite directions in parallel, adjacent helices around a 40 gauge stainless steel core wire. Although this arrangement worked reasonably well, bare spots of core wire not covered by filaments on adjacent row and column threads could possibly contact one another and create a low resistance connection, or electrical short between a row and column thread pair. Accordingly, it was found (or believed to be desirable) to use at least 3 or preferably 4 different bundles of carburized filaments wrapped in adjacent spiral helixes around a stainless steel core wire. Each adjacent spiral helix is preferably wrapped in a direction opposite to its neighbor, e.g., clockwise, counter-clockwise, or right-handed, left-handed, etc. Also, it was found that using stainless steel core wires smaller than 40-gauge resulted in a sensor array lattice of greater flexibility. Preferably in this case, at least 2 and preferably 3 adjacent stainless steel wires of smaller diameter than 40 gauge were used to provide redundancy to help ensure operability of the sensor array even if a conductive core wire were broken.  
         [0046]    Spiral wrapped piezoresistive threads  41  may be held in fixed positions relative to one another to form a rectangular sensor lattice in the exact same manner as described above for sensor embodiment  20  employing clad conductive cores. Thus, piezoresistive threads  41  may be encapsulated in the manner as shown in FIG. 1, or interwoven into a fabric mesh in the manner shown in FIG. 3, both of which constructions are described above.  
         [0047]    [0047]FIG. 6 illustrates how the arrangement of row and column piezoresistive threads  41 R,  41 C forms individual sensing elements  52  located at crossing regions or intersections  43 M (a)(b) of a row piezoresistive thread  41 R with a column resistive thread  41 C. Thus, as shown in FIG. 6A, with no external force applied to sensor array  40 , each cross-over region or lattice point  43  of the sensor array has thereat at feast one of the resistive filaments  45 R,  46 R wrapped around a core  44  of a row piezoresistive thread  41 R in electrically conductive contact with at least one of the filaments  45 C,  46 C wrapped around a core  44 C of a column piezoresistive thread  41 C. The outer surfaces  53 ,  54  of filaments  45 R,  45 C contact each other at tangent contact regions  55  which provide electrically conductive continuity between row and column conductors  24 R,  24 C of piezoresistive threads  41 R,  41 C. With no external force applied to sensor array  40 , the electrical resistance of contact area  35  is relatively high, since each tangent contact area  55  is relatively small under these circumstances. However, as shown in FIGS. 6B and 6C, when a normal force of increasing magnitude is applied to sensor array  40 , the electrical conductance of tangent contact area  55  between row and column filaments  45 R,  46 R,  45 C,  46 C of row and column piezoresistive threads  41 R,  41 C decreases, as will now be described.  
         [0048]    Referring to FIGS. 5 and 6, it may be seen that with no external force applied to sensor array  40 , tangent contact area  55  between a row thread filament  45 R and column thread filament  45 C is relatively small. Under these circumstances, the small size of contact area  55  results in a relatively high electrical resistance R measurable by applying a voltage between a row conductor  44 R and column conductor  44 C.  
         [0049]    [0049]FIGS. 6B and 6C illustrate effects of external normal forces or pressures being exerted on sensor array  40 . As shown in FIGS. 6B and 6C, sensor array  40  is placed with its lower surface  56  supported on a surface S, and a force N exerted perpendicularly downwards on upper surface  57  of the array, resulting in a reaction force U being exerted upwardly by support surface S on lower surface  56  of the array. With no external force applied to sensor array  40 , conductive filaments  45 ,  46  of respective row and column filament bundles intermingle only slightly, resulting in a relatively high electrical resistance R between a row and column thread pair. However, as shown in FIGS. 6B and 6C, force pair N-U cause conductive filaments of row and column threads to intermingle more intimately, thereby causing the size of contact areas  55  between pairs of contacting filaments  45 R,  46 R,  45 C,  46 C to increase, thus decreasing surface resistance between the crossed filaments, and thereby causing the electrical resistance R measurable between conducting cores  44 R,  44 C to decrease.  
         [0050]    For moderate values of normal force N, as shown in FIG. 6B, increase in contact areas  55  between row and column filaments  45 R,  46 R,  45 C,  46 C is relatively small, resulting in a relatively small reduction in electrical resistance R between piezoresistive threads  41 R,  41 C Larger forces N exerted on sensor array  40  cause larger increases in the size of contact areas  55 , resulting in a larger percentage reduction in resistance R. FIG. 6 illustrates in a general way the reduction in electrical resistance R between a pair of row and column piezoresistive threads  41 R,  41 C at an intersecting lattice point  48 , as a function of normal force or pressure exerted on sensor array  40  at that point.  
         [0051]    [0051]FIG. 8 is a partially diagrammatic view showing a pressure measuring and mapping apparatus  60  according to the present invention. As shown in FIG. 8, apparatus  60  includes a pressure sensor array  20  or  40  comprised of a lattice array of force sensor elements  32  of the type described above, and associated signal processing and display circuitry  70 .  
         [0052]    As shown in FIG. 8, pressure measuring and mapping apparatus  60  according to the present invention includes an interface cable  71  connected at one end thereof by a connector  85  to a sensor array  20  or  40 . The other end of interface cable  71  is connected to an interface module  72  which provides means for applying electrical sampling signals between a selected column sensor thread  21 C or  41 C, and a selected row sensor thread  21 R,  41 R, to measure the electrical resistance R of a selected sensor element  32 . Resistance is measured by applying a known voltage across a sensor resistance element  32 , and measuring the resulting current, or applying a known current, and measuring the voltage drop across the element. Although a d.c. sampling signal can be used for measuring resistance of sensor elements  32 , preferably, an a.c. signal is used, to avoid potential polarizing effects on the sensor elements.  
         [0053]    Interface module  72  preferably contains a multiplexer  73 , which sequentially outputs a sequence of m×n signals, each signal being representative of the resistance value for a particular sensor element  32  at the intersection of the mth row thread with the nth column thread. Also in the preferred embodiment, an analog-to-digital converter (ADC)  74  is connected between an analog resistance measuring circuit  75  and multiplexer  73 , which is then of the digital variety, outputs a serial digital data signal on an RS232 port  76 . In the preferred embodiment, RS232 port  76  of interface module  72  is connected to serial data port  77  of a computer  78 .  
         [0054]    Computer  78  is used to control interface module  72 , directing the sequence of addressing sensors  32  in array  20  or  40 . Computer  78  also performs signal processing functions, using predetermined scaling factors to convert the resistance values of sensor elements  32  to digital values representing normal forces and pressures exerted on the sensors. In the preferred embodiment, a two dimensional matrix of digital numbers representing the pressures on each of the m×n sensors  30  in array  40  is utilized to produce area maps of those pressures, which are displayed on a monitor  79  and stored in digital memory if desired.