Abstract:
A force sensing array includes multiple layers of material that are arranged to define an elastically stretchable sensing sheet. The sensing sheet may be placed underneath a patient to detect interface forces or pressures between the patient and the support structure that the patient is positioned on. The force sensing array includes a plurality of force sensors. The force sensors are defined where a row conductor and a column conductor approach each other on opposite sides of a force sensing material, such as a piezoresistive material. In order to reduce electrical cross talk between the plurality of sensors, a semiconductive material is included adjacent the force sensing material to create a PN junction with the force sensing material. This PN junction acts as a diode, limiting current flow to essentially one direction, which, in turn, reduces cross talk between the multiple sensors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, and claims priority to, U.S. patent application Ser. No. 13/453,461 filed Apr. 23, 2012 by Geoffrey Taylor and entitled ELASTICALLY STRETCHABLE FABRIC FORCE SENSOR ARRAYS AND METHODS OF MAKING; which in turn is a continuation of U.S. patent application Ser. No. 12/380,845 filed Mar. 5, 2009 by Geoffrey Taylor, and entitled ELASTICALLY STRETCHABLE FABRIC FORCE SENSOR ARRAYS AND METHOD OF MAKING. This application is also a continuation-in-part of, and claims priority to, U.S. patent application Ser. No. 12/075,937 filed Mar. 15, 2008 by Geoffrey Taylor and entitled ADAPTIVE CUSHION METHOD AND APPARATUS FOR MINIMIZING FORCE CONCENTRATIONS ON A HUMAN BODY. 
    
    
     BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates to transducers or sensors used to measure forces or pressures exerted on a surface. 
     B. Description of Background Art 
     Whenever a human body is supported by an object such as a chair or bed, normal and shear forces produced in reaction to the weight of the individual are transmitted from the supporting surface through the skin, adipose tissues, muscles, etc. to the skeleton. The forces exerted on body parts by support surfaces, which are equal and opposite to body weight forces, can in some cases cause damage to tissues. Forces on body parts can compress internal blood vessels and occlude nutrients from the tissue, the product of the magnitude and duration of these forces determining whether tissue damage or morbidity will occur. The areas of the human body which are most at risk of developing tissue damage such as a pressure sore are: heel, ischial tuberosities, greater trochanter, occiput and sacrum. 
     Some prior art sensor arrays for sensing patient pressure have suffered from disadvantages. For example, with some prior art sensors arrays, if the array is used to measure pressures exerted on a human body by a very form-fitting, conformal wheelchair seat cushion or extremely low pressure bed mattress or cushion, the array will often interfere with the function of the cushion or bed support surface, and give erroneous force measurements which are used to map the way the bed or chair supports a person. Such errors result from a “hammocking” effect, in which a flexible but not drapable sensor array deployed between fixed support positions cannot conform precisely to the shape of a patient. This effect can occur for example, using sensor arrays that use wire core sensing elements which make the arrays essentially non-stretchable. The lack of conformability of a sensor array alters the way a cushion or bed supports a patient, and also frequently results in forces or pressures exerted on individual sensors in the array being larger than a patient would actually encounter in the absence of the sensor array. 
     Another situation in which existing force sensor arrays for measuring and mapping forces exerted on human body parts are less than satisfactory occurs when attempting to make such measurements in a non-obtrusive, non-interfering manner on body parts which have complex shapes such as the feet. 
     Still further, in some prior art sensor arrays, it can be difficult to measure the resistance of sensor elements in an array using matrix addressing of the sensor elements. The difficulty results from the fact that the electrical resistances of all the non-addressed sensor elements in an array shunts the resistance of each addressed sensor element, resulting in cross-talk inaccuracies in measurements of individual sensor element resistances. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the present invention comprehends novel pressure or force sensing transducers which include individual force sensing elements that are arranged in a planar array on or within a substrate consisting of a thin, flexible polymer sheet or a thin sheet of woven or non-woven fabric. 
     According to one embodiment of the invention, a flexible force sensing array is provided that includes an elastically stretchable sheet, a plurality of first conductive paths, a layer of sensing material, a layer of semiconductive material, and a plurality of second conductive paths. The first conductive paths are supported on the elastically stretchable sheet. The layer of sensing material is positioned in contact with the first conductive paths and the layer of sensing material has an electrical characteristic that varies in response to physical forces exerted on it. The layer of semiconductive material is positioned in contact with the layer of sensing material on a side of the layer of sensing material opposite the plurality of first conductive paths. The plurality of second conductive paths are positioned in contact with the layer of semiconductive material on a side of the layer of semiconductive material opposite the layer of sensing material. 
     According to another embodiment, a flexible force sensing array is provided that includes a first elastically stretchable sheet, a plurality of first conductive paths, an intermediate elastically stretchable sheet, a layer of semiconductive material, a second elastically stretchable sheet, and a plurality of second conductive paths. The plurality of first conductive paths are supported on the first elastically stretchable sheet and are parallel to each other. The intermediate elastically stretchable sheet is positioned in contact with the first elastically stretchable sheet and includes sensing material thereon that has an electrical characteristic that varies in response to applied physical forces. The layer of semiconductive material is positioned in contact with the intermediate elastically stretchable sheet to thereby form with the sensing material a PN junction. The plurality of second conductive paths are supported on the second elastically stretchable sheet and are in electrical contact with the semiconductive material. The second conductive paths are parallel to each other and transverse to the first conductive paths. 
     According to other embodiments, the sensing material is a piezoresistive material. The piezoresistive material may be supported by an elastically stretchable substrate. The elastically stretchable substrate may be made at least partially of nylon. The first and second elastically stretchable sheets may both be made from woven fabric. The woven fabric is nylon in one embodiment. 
     The semiconductive layer may be coated onto the layer of sensing material and include a metallic oxide. In some embodiments, the metallic oxide may include copper oxide. 
     A cover sheet may be included in some embodiments that is made from an elastically stretchable material. In some embodiments, the cover is a polyurethane or polyvinyl chloride. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partly broken away perspective view of a basic embodiment of a three-layer piezoresistive thread pressure sensor array according to one embodiment, which uses a pair of polymer film outer substrates and a central piezoresistive layer. 
         FIG. 2  is a vertical transverse sectional view or end view of the sensor array of  FIG. 1  taken in the direction  2 - 2 . 
         FIG. 3  is a partly broken-away, upper perspective view of a second, two-layer embodiment of a piezoresistive thread pressure sensor array, in which the central piezoresistive layer shown in the basic embodiment of  FIGS. 1 and 2  is replaced by a piezoresistive coating on conductive threads of the sensor array. 
         FIG. 4  is a vertical transverse sectional or end view of the sensor array of  FIG. 3 , taken in the direction  4 - 4 . 
         FIG. 5  is a fragmentary perspective view of a modification of the sensor array of  FIGS. 1 and 3  in which adjacent pairs of more closely packed row and column conductor threads are spatially and electrically isolated from each other by non-conductive threads. 
         FIG. 6A  is a fragmentary transverse sectional view of the sensor array of  FIGS. 1 and 2 , on a further enlarged scale, showing the disposition of crossed row and column conductive threads contacting a central piezoresistive layer to form force sensing elements, with no external force applied to the elements. 
         FIG. 6B  is a view similar to that of  FIG. 6A , but with a moderate normal force applied to the sensor elements. 
         FIG. 6C  shows the sensor elements with a larger external force applied thereto. 
         FIG. 7  is a graph showing electrical resistance plotted as a function of force or pressure exerted on sensor elements of the sensor arrays shown in  FIGS. 1 and 3 . 
         FIG. 8A  is a fragmentary transverse sectional view of the sensor array of  FIGS. 3 and 4  on a further enlarged scale, showing the disposition of row and column piezoresistive threads to form force sensing elements, with no external force applied to the array. 
         FIG. 8B  is a view similar to that of  FIG. 8A , but with a moderate normal force applied to the sensor elements. 
         FIG. 8C  shows the sensor element with a larger external force applied thereto. 
         FIG. 9  is a partly broken-away perspective view of a three-layer embodiment of a piezoresistive threads pressure sensor array, which uses a pair of fabric outer substrates and a central piezoresistive layer. 
         FIG. 10  is a fragmentary view of the sensor array of  FIG. 9  on an enlarged scale and showing a lower plan view of an upper horizontal row conductor part of the sensor array. 
         FIG. 11  is a fragmentary view of the sensor array of  FIG. 9 , on an enlarged scale and showing an upper plan view of a lower vertical column conductor part of the sensor array. 
         FIG. 12  is a vertical transverse sectional view, of the sensor array of  FIG. 9 , taken in the direction  12 - 12 . 
         FIG. 13A  is a partly broken-away, exploded upper perspective view of a fourth, two-layer piezoresistive thread pressure sensor array using fabric substrates in which the central piezoresistive layer of the embodiment shown in  FIG. 9  is replaced by a piezoresistive coating on conductive threads of the sensor array. 
         FIG. 13B  is a vertical transverse sectional view of the sensor array of  FIG. 13A , taken in the direction  13 B- 13 B. 
         FIG. 14  is a partly broken-away upper perspective view of a fifth, single layer embodiment of a piezoresistive thread pressure sensor array which has a single fabric substrate, in which both row and column piezoresistive threads are fastened to the same side of a single insulating substrate sheet. 
         FIG. 15  is an upper plan view of the sensor array of  FIG. 14 . 
         FIG. 16  is a vertical transverse sectional view of the sensor array of  FIG. 14 , taken in the direction  16 - 16 . 
         FIG. 17  is partly broken-away, exploded upper perspective view of a modification of the fabric substrate sensor arrays of  FIG. 9 ,  13  or  14  in which lower column conductive threads of the sensor array are disposed in a sinuous arrangement on the fabric lower substrate panel. 
         FIG. 18  is an upper perspective view of another modification of the single layer fabric substrate sensor array of  FIG. 14  in which both the row and column conductive threads are sinuously arranged and located on opposite sides of a piezoresistive substrate sheet. 
         FIG. 19  is an upper plan view of the sensor array of  FIG. 18 . 
         FIG. 20  is a lower plan view of the sensor array of  FIG. 18 . 
         FIG. 21  is a vertical transverse sectional view of the sensor array of  FIG. 19 . 
         FIG. 21A  is a fragmentary upper perspective view of a single layer fabric substrate sensor array in which both upper row and lower column piezoresistive threads are sinuously arranged and fastened to the same side of a single insulating substrate sheet. 
         FIG. 22A  is a schematic diagram showing the number of conductive lead-outs required to measure the resistance of individual sensor elements in a linear array. 
         FIG. 22B  shows sensor elements which do not have to be in a linear arrangement. 
         FIG. 23  is a schematic diagram showing a reduced number of lead-outs for matrix addressing an array of sensor elements arranged in a matrix array, including, but not limited to, the sensor array of  FIG. 39 . 
         FIG. 24  is a schematic diagram showing sensor elements of the array of  FIG. 23  modified to include a diode junction. 
         FIG. 25  is an upper perspective view of a force measuring sensor apparatus using two-layer sensor arrays of the type shown in  FIG. 5 . 
         FIG. 26  is a block diagram showing the sensor array of  FIGS. 1 and 3  interconnected with signal processing and display circuitry to comprise a force measurement system. 
         FIG. 27A  is a perspective view of a sock incorporating the sensory array of  FIG. 14-16  or  17 - 20 . 
         FIG. 27B  is a horizontal transverse sectional view of the sock of  FIG. 27A . 
         FIG. 28  is a typical electrical resistance-versus-normal force diagram of the sensors disclosed herein. 
         FIG. 29  is a partly schematic view of modifications of sensor elements of the arrays of  FIG. 1  and  FIG. 35 , in which sensor elements of the array have been modified to provide them with P-N, diode-type junctions. 
         FIG. 30  is a current-versus-voltage diagram for the sensor elements of  FIG. 27A . 
         FIG. 31  is an exploded perspective view of another embodiment of a force sensor array. 
         FIG. 32  is a perspective view of the sensor array of  FIG. 31 . 
         FIG. 33  is an exploded perspective view of components of another embodiment of a force sensor array. 
         FIG. 34  is a perspective view of the sensor array of  FIG. 33 . 
         FIG. 35  is a partly diagrammatic perspective view of a body support cushion apparatus with adaptive body force concentration minimization according to the present intention. 
         FIG. 36A  is a fragmentary upper perspective view of the apparatus of  FIG. 35 , showing a sensor array jacket of the apparatus removed from a mattress overlay cushion of the apparatus to thereby reveal individual air bladder cells of the mattress. 
         FIG. 36B  is a fragmentary view of the mattress overlay of  FIG. 36A , showing an individual air cell thereof. 
         FIG. 37  is a diagrammatic side elevation view of the apparatus of  FIGS. 35 and 36 , showing certain bladder cells thereof deflated to reduce support forces exerted on parts of a human body supported by the mattress overlay. 
         FIG. 38  is a vertical sectional view of the mattress of  FIG. 36 , taken in the direction of line  4 - 4 . 
         FIG. 39  is a fragmentary exploded perspective view of the mattress of  FIG. 35 , showing elements of a force sensor arrangement thereof. 
         FIG. 40  is a diagrammatic view showing an exemplary relationship between the dimensions of adjacent air bladder cells and the width of an insulating strip between conductors of sensors on the cells. 
         FIG. 41  is a block diagram of electro-pneumatic controller elements of the apparatus of  FIG. 35 . 
         FIG. 42  is a simplified perspective view of the electro-pneumatic controller of  FIG. 41 . 
         FIG. 43  is a flow chart showing operation of the apparatus of  FIG. 35 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIGS. 1-43  illustrate various aspects of elastically stretchable, conformable fabric force sensor arrays, and methods for making the arrays, according to the present invention. 
     Referring first to  FIGS. 1 and 2 , a first, basic, three-layer embodiment of a force sensor array is shown. 
     As shown in  FIGS. 1 and 2 , a three-layer force sensor array  30  includes a plurality m of elongated, straight thin conductive row threads  31 - 1  through  31 - m  and a plurality n of elongated, straight thin, conductive column threads  32 - 1  through  32 - n.    
     The electrically conductive row threads  31  and column threads  32  consist of an elastically stretchable monofilament or woven polymer core  31 C,  32 C, which has been treated to make the threads electrically conductive, as by silver plating the core to form coatings  31 P,  32 P on cores  31 C,  32 C, respectively. 
     One type of example embodiment of a sensor array  30  used row and column conductive threads  31 ,  32  made from silver plated nylon thread, 117/17 2 ply, catalog #A264, obtained from LESS EMF, 809 Madison Avenue, Albany, N.Y. 12208, USA. That conductive thread had a lineal resistivity of about 75 ohms per foot, and an elastic stretchability of about 1 percent, i.e., at least 10 times greater than that of a stainless steel wire of a similar diameter. 
     A second type of example embodiment of a sensor array uses row and column conductive threads made from silver plated stretchy nylon yarn, that plated yarn having the designation Shieldex, ®Lycra dtex 20, obtained from W. Zimmerman, GmbH &amp; Co. K6, Riederstrasse 7, D-88171, Weiter-Simmerberg, Germany. That conductive thread had a lineal resistivity of about 500 ohms per foot. The elastic stretchability of that conductive yarn is greater than 30 percent, i.e., at least 300 times greater than that of a stainless steel wire of a similar diameter. 
     As shown in  FIGS. 1 and 2 , a row threads  31  and column threads  32  lie in parallel planes but are inclined with respect to one another, such as at an angle of ninety-degrees. In the example embodiment  30 , row conductive threads  31  are fastened to the lower surface  34  on an upper substrate sheet  33 , and column conductive threads  32  are fastened to the upper surface  36  of a lower substrate sheet  35 . 
     As may be seen best by referring to  FIG. 2 , sensor array  30  includes a thin central lamination or sheet  37  made of a piezoresistive material. As shown in  FIG. 2 , opposed inner facing outer surfaces  38 ,  39  of row and column conductive threads tangentially contact upper and lower surfaces  40 ,  41 , respectively, of central piezoresistive sheet  37 . Thus, as shown in  FIGS. 1 and 2 , each crossing point or intersection of a row conductive thread  31  and a column conductive thread  32  forms a piezoresistive sensor element  48  which consists of a small portion of central piezoresistive sheet  37  that is electrically conductively contacted by a row conductive thread and a column conductive thread. 
     In example embodiments of sensor array  32 , piezoresistive sheet  37  was fabricated by coating a stretchy, i.e., elastically stretchable thin Lycra-like fabric sheet with a piezoresistive material. A suitable fabric sheet, which forms a matrix for supporting the piezoresistive material, was a fabric known by the trade name Platinum, Milliken, Style #247579, obtained from the manufacturer, Milliken &amp; Company, Spartenburg, S.C., USA. That fabric had a fiber content of 69 percent nylon and 31 percent Spandex, a thread count of about 88 threads per inch, and a thickness of 0.010 inch. 
     The piezoresistive material used to coat the fabric matrix is made as follows: 
     A solution of graphite, carbon powder, nickel powder and acrylic binder are mixed in proportions as required to obtain the desired resistance and piezoresistive properties. Silver coated nickel flake is used to achieve force response in the low force range of 0 to 1 psi, graphite is used for the mid range of 1 to 5 psi and Charcoal Lamp Black is used for high force range of 5 to 1000 psi. Following is a description of the substances which are constituents of the piezoresistive material: 
     Silver Coated Nickel Flake:
         Platelets approximately one micron thick and 5 microns in diameter.   Screen Analysis (−325 Mesh) 95%.   Apparent Density 2.8.   Microtrac d50/microns 12-17.   Available from: Novamet Specialty Products Corporation,
           681 Lawlins Road, Wyckoff, N.J. 07481   
               

     Graphite Powder:
         Synthetic graphite, AC-4722T   Available from: Anachemia Science
           4-214 De Baets Street   Winnipeg, MB R2J 3W6   
               

     Charcoal Lamp Black Powder:
         Anachemia Part number AC-2155   Available from: Anachemia Science
           4-214 De Baets Street   Winnipeg, MB R2J 3W6   
               

     Acrylic Binder:
         Staticide Acrylic High Performance Floor Finish   P/N 4000-1 Ph 8.4 to 9.0   Available from: Static Specialties Co. Ltd.
           1371-4 Church Street   Bohemia, N.Y. 11716   
               

     Following are examples of mixtures used to make piezoresistive materials having different sensitivities: 
     Example I for forces in the range of 0 to 30 psi:
         200 ml of acrylic binder   10 ml of nickel flake powder   10 ml of graphite powder   20 ml of carbon black       

     Example II for forces in the range of 0-100 psi
         200 ml of acrylic binder   5 ml of nickel flake powder   5 ml of graphite powder   30 ml of carbon black       

     Example III for forces in the range of 0-1000 psi
         200 ml of acrylic binder   1 ml of nickel flake powder   1 ml of graphite powder   40 ml of carbon black       

     The fabric matrix for piezoresistive sheet  37  is submerged in the piezoresistive coating mixture. Excess material is rolled off and the sheet is hung and allowed to air dry. 
     Upper and lower substrate sheets  33 ,  34  are made of a thin, flexible insulating material, such as 0.002 inch thick polyurethane or polyvinyl chloride (PVC). In one embodimenty, the substrate sheets  33 ,  34  are made of an elastomeric material which has a relatively high degree of elastic stretchability, so that sensor array  30  is readily stretchable and conformable to the surface of an irregularly-shaped object. It can be appreciated, however, that conductive threads  31 ,  32  should also be elastically stretchable to facilitate stretchability of sensor array  30 . This is because conductive threads  31 ,  32  are affixed to substrate sheet  33 ,  34 , respectively, by, for example, blobs of adhesive  42 , as shown in  FIG. 2 . Piezoresistive sheet  37  is also fixed to upper and lower substrate sheets  33 ,  34  by blobs of glue  42 . 
       FIGS. 6A-6C  illustrate how the arrangement of row and column conductive threads  31 ,  32 , in combination with central piezoresistive layer  37  of sensor array  30  shown in  FIGS. 1 and 2 , form individual force sensing elements  48 . Each force sensor element  48  is located at the cross-over or intersection point  49  of a row conductive thread, e.g.,  31 - 1 ,  31 - 2 , . . .  31 - m , with a column conductive thread, e.g.,  32 - 1 ,  32 - 2 , . . .  32 - n , for a MXN matrix of sensor elements. Thus, individual sensor elements may be identified by the nomenclature 48-XXX-YYY, where XXX denotes row number and YYY denotes column number. 
     As shown in  FIGS. 2 and 6A , with no external force applied to sensor array  30 , at each cross-over point  49  of a row conductive thread  31  and a column conductive thread  32  of sensor array  30 , there is an upper electrically conductive tangential contact region  43  between central piezoresistive layer  37  and the upper conductive row thread, and a lower electrically conductive tangential contact region  44  between the piezoresistive layer and the lower, column conductive thread. 
     With no external force applied to sensor array  30 , the electrical resistance between a row conductive thread  31  and column conductive thread  32 , which consists of the series resistance of upper contact region  43 , lower contact region  44 , and the effective resistance of piezoresistive material  45  of piezoresistive layer  37  between the upper and lower contact regions is relatively high. The relatively high resistance results from the fact that in this case, tangential contact regions  43  and  44  are relatively small, and the thickness of uncompressed piezoresistive volume  45  is at its maximum value. However, as shown in  FIGS. 6B and 6C , when sensor array  30  is placed on a supporting surface S and a normal force N of increasing magnitude is applied to upper surface  47  of the sensor array  30 , the electrical resistance between a row conductive thread  31  and a column conductive thread  32  decreases, as will now be described. 
     Referring still to  FIGS. 2 and 6A , it may be seen that with no external force applied to sensor array  30 , tangential contact regions  43 ,  44  between row and column conductive threads  31 ,  32  and central piezoresistive layer  37  are relatively small, since the threads have a circular outer cross-sectional shape, which tangentially contacts flat planar surfaces of the piezoresistive layer. Under these circumstances, the small sizes of contact regions  43 ,  44  results in relatively high electrical resistance between central piezoresistive layer  37  and row and column conductive threads  31 ,  32 . Moreover, with central piezoresistive layer  37  uncompressed, its thickness and hence resistance are at a maximum value. 
       FIGS. 6B and 6C  illustrate the effects of increasing external normal forces or pressures exerted on sensor array  30 . As shown in  FIGS. 6B and 6C , sensor array  30  is placed with its lower surface  46  supported on a surface S and a force N is exerted perpendicularly downwards on upper surface  47  of the array, resulting in a reaction force U being exerted upwardly by supporting surface S on lower surface  46  of the array. Since central piezoresistive layer  37  is resiliently deformable, the compressive force on it decreases the thickness T of the part of the layer between a row conductive thread  31  and a column conductive thread  32 . This reduction in path length through piezoresistive layer  37  between a row conductive thread  31  and a column conductive thread  32  causes the electrical resistance R between the threads to decrease in value. 
     For moderate values of normal force N, as shown in  FIG. 6B , resilient deformation of central piezoresistive layer  37  is relatively small, resulting in a relative small reduction in electrical resistance R between the threads. Larger forces N exerted on sensor array  30  cause a larger deformation of the central piezoresistive layer, as shown in  FIG. 6C , resulting in a larger percentage reduction in resistance R.  FIG. 7  illustrates in a general way the reduction in electrical resistance measurable between a row conductive thread  31  and a column conductive thread  32 , as a function of normal force or pressure exerted on array  30  at these points. 
       FIGS. 3 and 4  illustrate another embodiment  50  of a piezoresistive thread pressure sensor array in which the central piezoresistive layer shown in  FIGS. 1 and 2  and described above is replaced by a piezoresistive coating on either, or both, row conductive threads  51  and column conductive threads  52 . 
     Sensor array  50  is facially similar to sensor array  20  disclosed and shown in FIGS. 1 and 2 of U.S. Pat. No. 6,543,299, but differs from that sensor array in important ways. Thus, row and column piezoresistive threads  51 ,  52  of sensor array  50  are made of elastically stretchable polymer cores  51 C,  52 C which have been treated by silver plating the cores to form on the threads electrically conductive coatings  51 P,  52 P, respectively. The coatings on either or both cores  51 C,  52 C are clad with a layer  51 R,  52 R, respectively, of a material which has a piezoresistive characteristic. The piezoresistive material used to form cladding layers  51 R,  52 R on plated surfaces  51 P,  52 P of cores  51 C,  52 C, of piezoresistive conductive threads  51 ,  52  may have a composition similar to that described above for making piezoresistive sheet layer  37 . 
     A method for making piezoresistive sensor threads by cladding conductive threads with a layer of a piezoresistive material includes preparing a slurry of piezoresistive material having a composition described in examples 1, 2 and 3 above. A highly conductive polymer thread, such as silver plated nylon thread 117/17 2 ply, Cat#124 available from LESS EMF Inc., 804 Madison Avenue, Albany, N.Y. 12208, is then immersed in a container holding the slurry, for a period of about 10 seconds. The end of a thread which has been immersed is withdrawn from the container, and while it is still wet, drawn through a circular aperture through a scraper plate. 
     In an example embodiment, a conductive thread having a core diameter of 0.25 mm and wet-coated diameter in the range of about 0.4 mm to 0.5 mm was drawn through a #360 scraper having a diameter of 0.45 mm, thus resulting in a wet scraped diameter of about 0.45 mm. The scraped thread was then fed through a stream of air heated to a temperature of 70 degrees C. at a linear travel speed of 100 mm/minute for a period of 5 minutes, to thus form a solidified coating having a diameter of about 0.4 mm. 
     As shown in  FIGS. 3 and 4 , piezoresistive row and column threads  51 ,  52  are fastened to upper and lower substrate sheets  63 ,  65 , by suitable means such as adhesive blobs  74 . Substrate sheets  63 ,  64  are made of a thin, flexible material such as 0.003 inch thick elastomeric polyurethane or polyvinyl chloride (PVC) that has a relatively high degree of elasticity. 
     FIGS.  3  and  8 A- 8 C illustrate how the arrangement of row and column piezoresistive threads  51 ,  52  of sensor array  50  form individual force sensing elements  69 . In response to progressively larger compressive normal forces, piezoresistive cladding layers  51 R,  52 R on row and column conductive core threads  51 C,  52 C are progressively compressed into oval cross-sectional shapes of smaller diameter. Thus, as shown in  FIGS. 8A-8C , the electrical resistance of each sensor element  70  decreases in inverse proportion to applied pressure, as shown in  FIG. 7 . 
       FIG. 5  illustrates a modification  70  of the sensor arrays shown in  FIGS. 1 and 3  and described above. Modified sensor array  70  may alternatively employ the three-layer construction of sensor array  30  shown in  FIG. 1 , or the two-layer construction of sensor array  50  shown in  FIG. 3 . The modification consists of fabricating sensor array  70  with electrically insulating material between adjacent rows and/or columns of conductive threads. Thus, for example, the modification  70  of two-layer sensor  50  shown in  FIG. 3  includes elongated insulating threads  71 , made for example of 0.012 inch diameter polyester disposed between each pair of adjacent row conductive threads  51  and each pair of adjacent column conductive threads  52 . 
     The insulating threads  71  are secured in place by any suitable means, such as adhesively bonding the threads to substrate sheets  63 ,  65  (see  FIGS. 2 and 4 ). This constructing enables sensor array  70  to be substantially wrinkled or otherwise deformed to conform to an irregularly shaped surface, without the possibility of pairs adjacent row or column conductive threads  51  or  52  contacting one another to thus cause an electrical short circuit which would result in erroneous sensor element resistance measurements and force determinations. Optionally, insulation between adjacent pairs of row and column conductive threads could be applied by lightly spraying an aerosol insulation acrylic paint to hold the conductive threads in place. 
       FIGS. 9-12  illustrate a three-layer embodiment  80  of a piezoresistive thread force sensor array. Sensor array  80  is similar to the basic embodiment  30  of sensor array shown in  FIGS. 1-2  and described above. However, sensor array  80  uses upper and lower substrate sheets  83 ,  85  which are made of woven fabric rather than polymer films. This construction, in conjunction with the use of stretchy conductive row and column threads  81 ,  82  made of plated nylon or Lycra cores, results in a sensor array that is even more flexible, elastically stretchable and drapable than sensor array  30 . 
     As may be seen best by referring to  FIG. 10 , sensor array  80  includes a plurality of parallel, laterally spaced apart conductive row threads  81  which are fastened to the lower surface  84  of upper fabric substrate sheet  83 . The row conductive threads  81  are fastened to lower surface  84  of upper substrate sheet  83  by any suitable means. In one embodiment, as shown in  FIG. 10 , each row conductive thread  81  is fastened to a substrate sheet by sewing the thread to fabric substrate sheet  83  by a smaller diameter, non-conductive thread  90  arranged in an elongated zig-zag stitching pattern. In an example embodiment, threads  90  consisted of 0.005-0.010 inch diameter, 100% polyester woven thread. For greater strength required for sensor arrays used to measure larger forces, threads  90  may optionally be monofilaments. 
     In an example embodiment of a sensor array  80 , upper and lower substrate sheets  83 ,  85  were made from a light-weight, elastically stretchable fabric, both of the two following fabrics were tested and found suitable for substrate sheets  83 ,  85 . (1) Milliken “Mil/glass” brand, Style #247579, composed of 69% nylon, 31% spandex, and having a weight of 1.8 oz./sq. yd. (2) Milliken “Interlude” brand, product #247211, composed of 82% nylon, 18% Lycra, and having a weight of 3.2-3.4 oz. Per sq. yd. Both of the foregoing fabrics are available from Milliken &amp; Company, 23 Fiddler&#39;s Way, Lafayette, N.J. 07848. 
     As shown in  FIG. 11 , lower column conductive threads  82  are fastened to the upper surface  86  of lower fabric substrate sheet  85  by non-conductive threads  91  of the same type as non-conductive threads  90  and in the same zig-zag stitching manner. 
     As shown in  FIGS. 9 and 12 , three-layer fabric substrate sensor array  80  includes a central piezoresistive sheet  87 , which may have a composition and construction similar to that of central piezoresistive sheet  37  of sensor array  30  described above. 
     As may be seen best by referring to  FIG. 13B , upper, row piezoresistive threads  101  are attached to lower surface  114  of upper fabric substrate sheet  113  by insulating sewn threads  90  arranged in zig-zag stitches. Similarly, lower, column piezoresistive threads  102  are attached to the upper surface  116  of lower substrate sheet  115  by sewn threads  91  arranged in zig-zag stitches. 
       FIGS. 13A and 13B  illustrate another two-layer embodiment  100  of a piezoresistive thread force sensor array. Sensor array  100  is similar to sensor array  80 . However, in sensor array  100 , conductive row and column threads  81 ,  82  are replaced by piezoresistive threads  101 ,  102  which have the same characteristics as piezoresistive threads  51 ,  52  of the two-layer polymer film substrate sensor array  50  shown in  FIGS. 3 and 4  and described above. This construction eliminates the requirement for the central piezoresistive sheet  87  of three-layer fabric sensor array  80  described above. 
       FIGS. 14-16  illustrate a fifth, single layer embodiment  120  of a force sensor array in which row and column piezoresistive threads are attached to a single side of a single insulating fabric substrate sheet  127 . 
     As shown in  FIGS. 14-16 , single layer fabric force sensor array  120  has a single substrate sheet  127  which is made from a light-weight, elastically stretchable fabric. Both of the two following fabric were listed and found suitable for making substrate sheet  127 . (1) Milliken “Millglass” brand, Style #247579, composed of 69% nylon, 31% spandex, and having a weight of 1.8 oz./sq. yd., and (2) Milliken “Interlude” brand, product #247211, composed of 82% nylon, 18% Lycra, and having a weight of 3.2-3.4 oz. Per sq. yd. Both of the foregoing fabrics are available from Milliken &amp; Company. 
     A plurality of parallel, laterally spaced apart column piezoresistive threads  122  are fastened to the upper surface  130  of the substrate sheet. The column piezoresistive threads are made from silver-plated nylon thread, Catalog #A-264 obtained from LESS EMF, or from silver-plated stretchy nylon yarn, both of which are described in detail above in conjunction with the description of sensor array  30 . 
     In one embodiment of single fabric substrate sheet sensor array  120 , each column piezoresistive thread  122  is fastened to substrate sheet  127  by a smaller diameter, non-conductive thread  91  arranged in an elongated zig-zag stitching pattern. In an example embodiment, threads  91  consisted of 0.005-0.010 diameter, 100% polyester. 
     As shown in  FIGS. 14 ,  15  and  16 , sensor array  120  includes a plurality of parallel, laterally spaced apart piezoresistive row threads  121  which are also fastened to the upper surface  130  of substrate sheet  127 . As shown in  FIG. 16 , m row piezoresistive threads  121  are fastened to substrate sheet  127  by non-conductive threads  90  of the same type as threads  91  and in the same zig-zag stitching manner. 
     As shown in  FIG. 16 , opposed inner facing outer surface  128 ,  129  of row and column piezoresistive threads  121 ,  122  tangentially contact each other. Thus, as shown in  FIGS. 14-16 , each crossing of a row piezoresistive thread  121  with a column piezoresistive thread  122  forms a piezoresistive sensor element  138  which consists of a small portion of piezoresistive coatings of a row and column piezoresistive thread tangentially contacting one another. 
       FIG. 17  illustrates a modification of the force sensor arrays using fabric substrate sheets shown in  FIG. 9 ,  13  or  14  and described above. As shown in  FIG. 17 , a lower fabric substrate sheet  145  of modified force sensor array  140  has attached thereto lower, column conductive piezoresistive threads  142  which are sinuously curved with respect to parallel straight base lines between opposite ends of each thread, rather than lying directly on the base lines, as are the column conductive threads  82  of sensor array  80  shown in  FIG. 11 . With this arrangement, lower fabric substrate sheet  145  is even more readily elastically stretchable in directions parallel to the column thread base lines because longitudinally spaced apart points on the fabric substrate sheet are not constrained to be at maximum lengths by the less elastically stretchable conductive threads. Thus, the stretchability of the column substrate sheet  145  is limited only by its intrinsic stretchability since the arrangement of column conductive threads  142  allows them to conform readily to size of the substrate sheet by changing spacing between peaks and valleys of the sinuously curved conductive threads, i.e., altering the spatial wavelengths of the sinuous curves formed by threads. 
     Optionally, upper row piezoresistive threads  141  may also be sinuously arranged in the same manner as lower column piezoresistive threads shown in  FIG. 17 , to thus enhance elastic compliance, or stretchability, of sensor array  140  is in directions parallel to the row conductive threads as well as in directions parallel to the column piezoresistive threads. Also, either or both row and column conductive threads of three-layer sensor arrays such as those of the type shown in  FIG. 1  may be sinuously arranged to provide enhanced uniaxial or biaxial stretchability. 
       FIGS. 18-21  illustrate another modification  180  of the single fabric substrate sheet sensor array  120  of  FIG. 14 . Sensor array  180  has upper, row conductive threads  181  and lower, column conductive threads  182  which are both sinuously arranged on opposite sides of a fabric piezoresistive central substrate sheet  187 . This construction gives array  180  greater elasticity in directions parallel to the column conductive threads  182  as well as in directions parallel to row conductive threads  181 . 
       FIG. 21A  illustrates another modification  200 , which row and column piezoresistive threads  201 ,  202  are both sinuously arranged and attached to the upper surface  211  of an insulating substrate sheet  210 , in the manner shown in  FIG. 16 . 
       FIG. 22A  illustrates the number of conductive leads required to measure the resistance of individual elements of a linear array of sensor elements, to thus determine numerical values of force or pressure exerted on each sensor element. As shown in  FIG. 22A  a single common lead-out conductor C is connected to a linear array of intersecting lead-out conductors Li through Ln to form a plurality of sensor elements SI through Sn, by piezoresistive material at each intersection point. Thus, for a total of n sensors S, there are required a total R equal to n+1 lead-out conductors to measure the individual resistance of each sensor element SI through Sn and hence determine the forces F 1  through Fn exerted on each individual sensor element. 
       FIG. 22B  shows a plurality of sensor elements Sn+1, Sn+2, Sn+3 which are not necessarily arranged in a linear array, being located, for example, on individual finger tips. As shown in  FIG. 22B , n+1 lead-out conductors are also required for this configuration. 
       FIG. 7  illustrates the electrical resistance of a one-inch square piezoresistive force sensor element  48  using a piezoresistive sheet  37  having the formulation listed for an example sensor array  30  shown in  FIGS. 1 and 2 , and fabricated as described above, as a function of normal force or pressure exerted on the upper surface  47  of upper substrate sheet  33  of sensor array  30 . As shown in  FIG. 7 , the resistance varies inversely as a function of normal force. 
     As shown in  FIG. 1 , row conductive threads  31 - 1  through  31 - m , in vertical alignment with column conductive threads  32 - 1  through  32 - n  form with piezoresistive layer sheet  37  between the column and row conductive threads a m×n rectangular matrix array of m×n force elements  48 . 
     If upper and lower electrical connections to each sensor element  48  were electrically isolated from connections to each other sensor element, a separate pair of lead-out conductors for each of the sensors, would be required, i.e., a total of 2Qlead-out conductors for Q sensor elements or, if a single common electrode lead-out were employed as shown in  FIG. 22 , a total of Q+1 lead-outs would be required. 
     As shown in  FIG. 1 , sensor array  30  is arranged into a matrix of m rows and n columns, thus requiring only R=m×n lead-out conductors. However, as shown in  FIG. 23 , if matrix addressing of sensor array  30  is used to measure the resistance of individual sensors  48  to thereby determine normal forces exerted on the sensors, there is a substantial cross-talk between the resistance on an addressed sensor  48  and non-selected sensors because of parallel current paths to non-addressed sensors. To overcome this cross-talk problem, the present inventor has developed a method for modifying sensors  48  to give them a diode-like characteristic. As may be confirmed by referring to  FIG. 24 , the cross-talk between sensor elements  40  which have a non-bilateral, polarity-sensitive transfer function, mitigates the cross-talk problem present in the matrix of symmetrically conductive sensors  48  shown in  FIG. 23 . 
     Sensor elements  48  are modified to have a diode-like characteristic by modifying the preparation of piezoresistive layer sheet  37 , as follows: First, a piezoresistive layer sheet  37  is prepared by the process described above. Then, either the upper surface  40  or the lower surface  41  of the piezoresistive coating  37 A of piezoresistive sheet  37  is modified to form thereon a P-N, semiconductor-type junction. 
     Modification of piezoresistive coating  37 A to form a P-N junction is performed by first preparing a slurry which has the composition of one of the three example mixtures described above, but modified by the addition of 5 ml each of copper oxide (CuO) in the form of a fine powder of 50-micron size particles, and 5 ml of cuprous oxide (Cu 2 O) in the form of a fine powder of 50-micron size particles and thoroughly stir-mixing the foregoing ingredients. The resultant solution is then reduced using about 30 mg of solution of sodium borohydride, also known as sodium tetrahydroborate (NaBH 4 ) or ammonium phosphate, to form a solution having a pH of about 5.5. The solution is then coated onto the upper surface  40  or lower surface  41  of piezoresistive coating  37 B on piezoresistive sheet  37 . This coating process is performed using a roller coating process which results in about 0.5 ml of solution per square centimeters being applied. The surface coating is then allowed to air-dry at room temperature and a relative humidity of less than 20%, for 4 hours. After the coated surface has dried, it functions as a P-type semiconductor, while the uncoated side of coating  37 B functions as an N-type semiconductor of P-N junction diode. 
       FIG. 29  illustrates a sensor element  48  which has been prepared as described above to give the sensor a diode-like characteristic, and a circuit for obtaining the I-V (current versus voltage) transfer function of the sensor.  FIG. 30  shows a typical I-V curve for sensor elements  48  of  FIG. 29 . 
     As stated above, the advantage of modifying sensor elements  48  of sensor array  30  by adding a semi-conductive layer that acts like a diode is that it reduces cross talk between sensors. As is shown in  FIG. 23 , this cross-talk occurs because of the so-called “completing the square” phenomenon, in which three connections are made in a square matrix array of three non-addressed resistors that form the three corners of a square. Thus, any two connections in a vertical column and a third one in the same row function as either connection in an X-Y array of conductors. The resistor at the fourth corner of the square shows up as a phantom in parallel with an addressed resistor because the current can travel backwards through that resistor, and forward through the other resistors. Care and additional expense must be taken in the electronics to eliminate the contribution of this phantom. For example, if, as is shown in  FIG. 23 , a potential V is applied between row and column conductors X 1 Y 1 , to thereby determine the resistance of piezoresistive sensor resistance R 11 , reverse current flow through “phantom” resistor R 22  would cause the sum of resistances R 12 +R 22 +R 22  to shunt R 11 , resulting in the parallel current flow paths indicated by arrows in  FIG. 23 , which in turn would result in the following incorrect value of resistance: 
     R x1 y 1 =R 11 //(R 12 +[R 22 ]+R 21 ), R x1 Y 1 =R 11 (R 12 +[R 22 ]+R 21 )/(R 11 +R 12 +[R 22 ]+R 21 ), where brackets around a resistance value indicate current flow in a counterclockwise direction through that resistor, rather than clockwise, i.e., diagonally downwards towards the left. Thus, for example, if each of the four resistances listed above had a value of 10 ohms, the measured value of R 11  would be: 
     R 11 =10(10+10+10)/(10+10+10+10)=300/40=7.5 ohms, i.e., 25% below the actual value, 10 ohms, of R 11 . If the resistance values of R 12 , R 22  and R 21  of the three non-addressed piezoresistive sensor element  48  were each lower, e.g., 1 ohm, because of greater forces concentrated on those sensor elements  48 , the measured value of R 11  would be: 
     R 11 =10(1+1+1)/(10+1+1+1)=30/13=2.31 ohms, i.e., a value of about 77 percent below the actual value of R 11 . 
     On the other hand, by placing a diode in series with each piezoresistive sensor element  48 , as shown in  FIG. 24 , the electrical resistance of an element measured in a reverse, counterclockwise direction a test current flow through the sensor element, e.g., R 22 , would be for practical purposes arbitrarily large, or infinity compared to the clockwise forward paths of current through the other resistances shown in  FIGS. 23 and 24 . In this case, the measured resistance value for a 2×2 matrix of four resistances each having a value of 10 ohms would be: 
     R X1 Y 1 =10(1+∞+1)/(10+1+∞+1)=10 ohms, the correct value. Thus, modifying each sensor element  48  to include a p-n junction thereby give the sensor element a diode-like characteristic electrically isolates, i.e., prevents backward current flow, through each sensor element  48 . This enables the correct value of electrical resistance of each sensor element  48  and hence forces exerted thereon to be measured accurately R X1 y 1  using row and column matrix addressing rather than requiring a separate pair of conductors for each sensor element. 
       FIG. 25  illustrates a force measuring apparatus  150 . The apparatus  150  may use any of the types of sensor arrays described above, but in a particular example shown in  FIG. 25  uses a sensor array  70  of the type shown in  FIG. 5 . 
     As shown in  FIG. 25 , force measuring apparatus  150  used four sensor arrays  70 - 1 ,  70 - 2 ,  70 - 3  and  70 - 4 , each having a matrix of 16 row conductive threads by 16 column conductive threads. The four arrays are arranged in a square matrix, to thus form a composite sensor array  70 -C consisting of 32 rows×32 columns of conductive threads having formed at their intersection 32×32=1,024 sensor elements  88 . As shown in  FIG. 25 , each of the 32 row conductive thread lead-out wires and each of the 32 column conductive thread lead-outs is connected to a separate electrically conductive connector pin of a plurality of connector pins  154 - 1  through  154 - 64  of a pair of electrical interface connectors  153 - 1 ,  153 - 2 . 
       FIG. 26  illustrates a force measurement system  160  which utilizes the force sensor apparatus  150  described above. 
     As shown in  FIG. 26 , force measurement system  160  includes a computer  161  which is bidirectionally coupled to force sensor array  70  of force sensor apparatus  160  through a force sensor interface module  162 . The sensor interface module  162  includes a Digital-to analog Converter (DAC)  163  for generating in response to control signals from computer  161  test voltages or currents which are directed to matrix-addressed individual force sensors  88 . 
     As shown in  FIG. 26 , individual force sensor elements  88  are addressed by connecting one terminal of a current or voltage source controlled by DAC  163  to a selected one of X-row conductors  51 - 1 - 51 - m  by an X multiplexer  164 , and connecting the other terminal of the source to a selected one of Y-column conductors  52 - 1 - 52 - m  by a Y multiplexer  165 . Sensor interface module  162  also included an Analog-to-Digital Converter (ADC)  166  which measures the voltage drop or current through a sensor element  88  resulting from application of a test current or voltage, and inputs the measured value to computer  161 . Using predetermined scale factors, computer  161  calculates the instantaneous value of electrical resistance of a selected addressed sensor element  88 , and from that resistance value, a corresponding normal force instantaneously exerted on the addressed sensor. 
     In response to control signals cyclically issued by computer  161 , X multiplexer  164  and Y multiplexer  165  are used to cyclically measure the resistance of each force sensor element  88 , at a relatively rapid rate of, for example, 3,000 samples per second, enabling computer  161  to calculate the force exerted on each force sensor element  88  at that sampling rate. 
     Measurement system  160  includes an operator interface block  167  which enables values of force or pressures measured by sensor elements  88  to be displayed as numerical values and/or a graph or pressure/force map on the display screen of a computer monitor  168 , or outputted to a peripheral device such as a printer, or a network such as the internet, through an I/O block  169 . 
       FIGS. 27A and 27B  illustrate a sock  170  which includes one of the novel sensor arrays employing conductive threads which were described above, such as the single layer, fabric substrate piezoresistive thread sensor array shown in  FIG. 14-16  or  17 - 20 . 
     As shown in  FIG. 17 , sock  170  which includes a single layer fabric force sensor array  180  that is a modification of the planar force sensor array  120  shown in  FIGS. 14-16  and described above. The modification of force sensor array  120  to form force sensor array  180  may be best visualized by considering that the left and right side edges of the array  120  are brought upwards from the plane of the page to meet and form a hollow cylindrical tube. 
     Row conductor threads protruding  121  from the aligned edges of the array are then electrically conductively fastened to a first, row conductor ribbon cable  181 . Column conductive threads protruding from one edge of the rolled-up array are electrically conductively fastened to a second, column conductor ribbon cable  182 . Outer ends  183 ,  184  who protrude from an edge of array  120  are electrically connected to a resistance measuring circuit as shown in  FIG. 26  and described above. 
       FIGS. 31-34  illustrate modifications of fabric substrate force sensor arrays using conductive threads, in which the conductive threads are fixed to a fabric substrate sheet without the use of sewn stitching by adhesive applied directly to a conductive thread. Thus, a first, three-layer fabric sensor array  190  includes a plurality of parallel, spaced apart row conductive elastic threads  191  which are adhesively bonded to the lower surface  194  of an upper stretchable fabric substrate sheet  193  made of 3 mil thick polyester or either of the two Milliken fabrics described above. Sensor array  190  also includes a plurality of parallel spaced apart column conductive elastic threads  192  which are adhesively bonded to an upper surface  196  of a lower stretchable fabric substrate sheet  195 . A thin sheet of stretchable fabric prepared to give it a piezoresistive property in the manner described above comprises a central piezoresistive layer  197  which is positioned between row and column conductive threads  191 ,  192 . The foregoing three layers are then stacked on top of one another and dots of glue injected through the mesh openings of the fabric substrate of all three layers to adhere them together and thus form a completed sensor array  190 . 
     Sensor array  200 , shown in  FIG. 33 , utilizes a single substrate sheet  207 . Conductive row and column threads  191 ,  192 , separated by insulating threads  210 ,  211 , are adhered to upper surface  212  and lower surface  213  of sheet  207  by double-stick tape strips  213 ,  214 . 
       FIGS. 35-43  illustrate various aspects of a method and apparatus for minimizing body force concentrations on a human body using an adaptive cushion. The example embodiment depicted in  FIGS. 35 and 37  includes an adaptive cushion which is of an appropriate size and shape for use on a standard single or hospital bed. However, as will be clear from the ensuing description of that example embodiment, the size and shape of the adaptive cushion can be varied to suit different applications, such as for use on a fixed chair or wheel chair. 
     Referring first to  FIGS. 35 and 36A , an adaptive cushion apparatus  420  for minimum body force concentrations on a body of a person lying on a bed may be seen to include a longitudinally elongated, rectangular cushion overlay  421 . Cushion  421  has an appropriate size and shape to fit conformally on top of a standard size hospital bed. Thus, an example embodiment of cushion  421  had a laterally elongated, rectangular shape with a length of about 6 feet, a width of about 3 feet, and a thickness of about 4 inches. 
     The six panels of each air bladder cell  423  are sealingly joined at edges thereof to form a hermetically sealed body which has a hollow interior space  422 A. 
     As shown in  FIG. 36A , mattress overlay cushion  421  is constructed as a rectangular, two-column by six-row array of 12 individual inflatable air bladder cells  422 . Each air bladder cell  422  has a laterally elongated, rectangular shape, having a length of about 18 inches, a depth of about 17 inches, and a thickness of about 4 inches. As shown in  FIGS. 35 and 36 , bladders  422  are arranged in left and right columns, each having 6 longitudinally spaced apart, laterally disposed, laterally elongated bladders. As shown in  FIGS. 36 and 38 , each air bladder cell has a flat base panel  423 , left and right end panels  424 ,  425 , head and toe or front and rear panels  426 ,  427 , and an upper panel  428 . The bladders  422  are made of a thin sheet of a flexible, elastomeric material such as neoprene rubber or polyurethane, having a thickness of about 0.014 inch. The six panels of each air bladder cell  422  are sealingly joined at edges thereof to form a hermetically sealed body which has a hollow interior space  422 A. Optionally, each air bladder cell  422  may be fabricated from a tubular preform in which each end panel is sealingly joined to opposite transverse ends of the tubular preform. In either embodiment, adjacent panels of an individual air bladder cell are sealingly joined by a suitable method such as ultrasonic bonding, RF-welding or adhesive bonding. 
     The number, size, shape, relative positioning and spacing of air bladder cells  422  of mattress cushion overlay  421  are not believed to be critical. However, it is believed preferable to arrange mattress overlay  421  into symmetrically-shaped left and right columns each having at least five and preferably six longitudinal zones corresponding to major curvature of a longitudinally disposed medial section of a typical human body. Thus, as shown in  FIGS. 35 ,  36 A and  37 , mattress overlay cushion  421  has a left-hand column of six air bladder cells  422 L 1 - 422 L 6 , and a right-hand column of six cells  421 R 1 - 421 R 6 . 
     As shown in  FIGS. 38 and 40 , the bladders are stacked closely together in both front and rear and side by side directions, with minimum longitudinal and lateral spacings  429 ,  430 , respectively, that are vanishingly small so that adjacent bladder cells physically contact each other. 
     As indicated in  FIGS. 35 and 36 , each bladder cell  422  is provided with a tubular air inlet port  431  which protrudes through a side wall, e.g., a left or right side wall  424  or  425 , and communicates with a hollow interior space  422 A within the bladder. Air admitted into or exhausted from hollow interior space  422 A through port  431  of an air bladder cell  422  enables the cell to be inflated or deflated to a selected pressure. 
     Although the shape of each air bladder cell  422  of cushion  421  shown in  FIGS. 35 and 36  is that of a rectangular block, or parallelepiped, the air bladder cells may optionally have different shapes, such as convex hemispheres protruding upwards from the base of the cushion. Also, the array of air bladder cells  422  of cushion  421  may be parts of a unitary structure with a common base panel  423  which has individual rectangular-block shaped, hemispherical or hollow inflatable bodies of other shapes protruding upwardly from the common unitary base panel. 
     Whether individual air bladder cells  422  are separate bodies or upper inflatable shell-like portions protruding upwardly from a common base, air inlet/exhaust port tubes  431  of each air bladder cell  422 , or selected air bladder cells  422 , may be located in the base panel  423  of the cell and protrude downwardly from the cell, rather than being located in a side wall and protruding laterally outwards, as shown in  FIGS. 35 and 36A . 
     As shown in  FIGS. 35 ,  36  and  39 , body force minimization apparatus  420  includes a force sensor array  432  which has a matrix of individual force sensors  433 , with at least one sensor positioned on the upper surface  428  of each air bladder cell  422 . As will be explained in detail below, each force sensor  433  comprises a force sensitive transducer which has an electrical resistance that varies inversely with the magnitude of a normal, i.e., perpendicular force exerted on the sensor by an object such as the body of a person supported by overlay cushion  421 . In one embodiment, force sensor array  432  is maintained in position on the upper surfaces of air bladder cells  422  by a water-proof, form-fitting contour fabric sheet  421 A which fits tightly and removably over cushion  421 , as shown in  FIG. 37 . 
     Referring to  FIG. 35 , it may be seen that body force minimization apparatus  420  includes an electronic control module  435 . As will be explained in detail below, electronic control module  435  includes sensor interface circuitry  436  for electrical interconnection to sensors  433 . Electronic control module  435  also includes a computer  437  which is interconnected with sensor interface circuitry  436 . Computer  437  is programmed to receive input signals from sensor interface circuitry  436 , measure the resistance of individual sensors  433  and calculate therefrom the magnitude of forces exerted on each sensor, make calculations based on the force measurements, and issue command signals to control the pressure in individual air bladder cells  422  which are calculated using an algorithm to minimize force concentrations on the cells. 
     In one embodiment of apparatus  420 , measurement of the resistance of each sensor  433  is facilitated by arranging the sensors into a matrix array of rows and columns. With this arrangement, individual resistances of a 6×2 array  432  of sensors  433  may be measured using 6 row interface conductors and 2 column interface conductors  450 ,  451 , as shown in  FIG. 35 . 
     To avoid cross talk between measurements of individual sensors  433 , the aforementioned row-column addressing arrangement requires that each sensor have a non-bilateral, asymmetric current versus voltage characteristics, e.g., a diode-like impedance characteristic. As will be described in detail below, the present invention includes a novel sensor having the required diode-like characteristic. Alternatively, using force sensors  433  which do not have a diode-like characteristic, the force sensor array  432  can be partitioned into 12 separate rectangular sensors  433  each electrically isolated from one another, with a separate pair of interface conductors connected to upper and lower electrodes of each sensor. 
     As shown in  FIG. 35 , body force minimization apparatus  420  includes an air pump or compressor  440  for providing pressurized air to the input port  442  of a selector valve manifold  441 . Selector valve manifold  441  has 12 outlet ports  443 A, each connected through a valve  443  to a separate air bladder cell inlet port  431 . As will be explained in detail below, the compressor  440 , selector valve manifold  441  and valves  443  are operably interconnected to computer  437  and an air pressure measurement transducer  444 . Pressure transducer  444  outputs an electrical signal proportional to pressure, which is input to computer  437 . This arrangement enables the inflation pressure of each air bladder cell  422  to be individually measured and varied under control of the computer  437 . 
       FIGS. 36A ,  38  and  39  illustrate details of the construction of force sensor array  432 . As shown in those figures, sensor array  432  includes an upper cover sheet  445  made of a thin flexible, elastically stretchable material. In an example embodiment of sensor array  432  fabricated by the present inventor, cover sheet  445  was made of “two-way stretch” Lycra-like material which had a thickness of about 0.010 inch and a thread count of about 88 threads per inch. That material had the trade name Millglass Platinum, Style No. (24)7579, obtained from the Milliken &amp; Company, P.O. Box 1926, Spartanburg, S.C. 29304. 
     Referring to  FIG. 39 , sensor array  432  includes an upper, column conductor sheet  446  which is fixed to the lower surface of upper flexible cover sheet  445 , by flexible adhesive strips made of 3M transfer tape  950 , or a flexible adhesive such as Lepage&#39;s latex contact adhesive. Column conductor sheet  446  is made of a woven fabric matrix sheet composed of 92% nylon and 8% Dorlastan fibers, which give the sheet a flexible, two-way stretch elasticity. The fabric matrix sheet of conductor sheet  446  is electroless plated with a base coating of copper, followed by an outer coating of nickel. The metallic coatings completely impregnate the surfaces of fibers adjacent to interstices of the mesh fabric, as well as the upper and lower surfaces  447 ,  448  of the conductor sheet  446 , thus forming electrically conductive paths between the upper and lower surfaces  447  and  448 . The present inventor has found that a suitable conductive fabric for conductor sheet is a Woven Silver brand, Catalog #A251 available from Lessemb Company, 809 Madison Avenue, Albany, N.Y. 12208, USA. 
     In an example embodiment of sensor array  432 , upper conductive sheet  446  was fabricated from the Woven Silver, Catalog #A151 material described above. The surface resistivity of upper and lower surfaces  447 ,  448  of that material was about 1 ohm per square or less, and the inter-layer resistance between upper and lower surfaces  447 ,  448  was about 50 ohms per square. 
     In one embodiment of sensor array  432 , individual conductive pads, or rows or columns of conductors, are formed by etching metal-free channels vertically through conductor sheet  446 , from the top of upper conductive surface  447 , all the way to the bottom of lower conductive surface  448 . Thus, as shown in  FIG. 39 , narrow longitudinally disposed straight channels  449  are etched through upper column conductor sheet  446 . This construction results in the formation of two adjacent, relatively wide, longitudinally elongated left and right planar column electrodes  450 ,  451 . The adjacent left and right column electrodes are separated by a relatively thin channel  449 , thus electrically isolating the adjacent column electrodes from each other. 
     Insulating channels  449  are etched through upper conductor sheet  446  to form column electrodes  450  and  451  by the following novel process. 
     First, to prevent capillary wicking and resultant wetting of a subsequently applied etchant solution to fabric conductor sheet  446 , the sheet is pre-processed by treating it with a hydrophobic substance such as PTFE. The treatment can be made by spraying the conductor fabric sheet  446  with an aerosol containing a hydrophobic material such as PTFE. A suitable aerosol spray is marketed under the trade name Scotch Guard by the 3M Company, St. Paul, Minn. Areas of fabric conductor sheet  446  which are to have insulating channels  449  formed therein are masked from the hydrophobic treatment by adhering strips of masking tape which have the shape of the channels to the sheet before applying the hydrophobic material to the sheet. 
     Following the pre-processing of conductor sheet  446  to make it hydrophobic, sheets of masking tape are adhered tightly to both upper and lower surfaces  447 ,  448  of the conductor sheet, using a roller or press to insure that there are no voids between the masking tape and surfaces, which could allow etchant solution to contact the conductive surfaces. Next, strips of masking tape having the shape of insulating channels  449  are removed from the conductor sheet. Optionally, the strips of masking tape to be removed are preformed by die-cutting partially through larger sheets of masking tape. 
     After strips of masking tape corresponding to channels  449  have been stripped from conductor sheet  446 , the conductive metal coatings of the fabric sheet aligned with the channels is chemically etched away. One method of performing the chemical etching uses a concentrated solution of 10 mg ammonium phosphate in 30 ml of water. The ammonium phosphate solution is mixed with methyl cellulose solid powder, at a concentration of 10 percent methyl cellulose powder until a gel consistency is obtained. The etchant gel thus formed is then rollered onto the areas of upper and lower surfaces  447 ,  448  of conductor sheet  446 , over channels  449 . The etchant gel is allowed to reside on channels  449  for approximately 1 hour, at room temperature, during which time the nickel and copper plating of the fabric matrix of conductor sheet  446 , in vertical alignment with channels  449 , is completely removed, thus making the channels electrically insulating. This process separates the conductor sheet into left and right column electrodes  450 ,  451 , respectively. 
     The etching process which forms insulating channel  449  is completed by rinsing the etchant gel from upper and lower surfaces  447 ,  448  of conductor sheet  446 , followed by removal of the masking tape from the upper and lower surfaces. 
     Referring still to  FIG. 39 , it may be seen that sensor array  432  includes a thin piezoresistive sheet  452  which has on an upper surface  453 , that is in intimate contact with lower surfaces of left and right column electrodes  450 ,  451 . Piezoresistive sheet  452  also has a lower surface  454  which is in intimate electrical contact with the upper surfaces of row electrodes on a lower row conductor sheet  456 . Lower, row conductor sheet  456  has a construction exactly similar to that of upper, column conductor sheet  446 . Thus, lower row conductor sheet  456  has upper and lower conductive surfaces  457 ,  458 , and narrow, laterally disposed insulating channels  459  which are positioned between and define row electrodes  461 ,  462 ,  463 ,  464 ,  465 ,  466 . 
     The function of piezoresistive sheet  452  of sensor array  432  is to form a conductive path between column and row electrodes, e.g., left-hand column electrode  450  and rear row electrode  461 , the resistance of which path varies in a predetermined fashion as a function of normal force exerted on the sensor array. 
     In example embodiments of sensor array  432 , piezoresistive sheet  452  was fabricated by coating a stretchy, thin Lycra-like fabric sheet with a piezoresistive material. A suitable fabric sheet, which forms a matrix for supporting the piezoresistive material, was a fabric known by the trade name Platinum, Milliken, Style #247579, obtained from the manufacturer, Milliken &amp; Company, Spartanburg, S.C., USA. That fabric had a fiber content of 69 percent nylon and 31 percent Spandex, a thread count of about 88 threads per inch, and a thickness of 0.010 inch. The piezoresistive material used to coat the fabric matrix is made as follows: 
     A solution of graphite, carbon powder, nickel powder and acrylic binder are mixed in proportions as required to obtain the desired resistance and piezoresistive properties. Silver coated nickel flake is used to achieve force response in the low force range of 0 to 1 psi, graphite is used for the mid range of 1 to 5 psi and Charcoal Lamp Black is used for high force range of 5 to 1000 psi. Following is a description of the substances which are constituents of the piezoresistive material: 
     Silver Coated Nickel Flake:
         Platelets approximately one micron thick and 5 microns in diameter.   Screen Analysis (−325 Mesh) 95%.   Apparent Density 2.8.   Microtrac d50/microns 12-17.   Available from: Novamet Specialty Products Corporation,   681 Lawlins Road, Wyckoff, N.J. 07481       

     Graphite Powder:
         Synthetic graphite, AC-4722T   Available from: Anachemia Science   4-214 DeBaets Street   Winnipeg, MB R2J 3W6       

     Charcoal Lamp Black Powder:
         Anachemia Part number AC-2155   Available from: Anachemia Science   4-214 DeBaets Street   Winnipeg, MB R2J 3W6       

     Acrylic Binder:
         Staticide Acrylic High Performance Floor Finish   P/N 4000-1 Ph 8.4 to 9.0   Available from: Static Specialties Co. ltd.   1371-4 Church Street   Bohemia, N.Y. 11716       

     Following are examples of mixtures used to make piezoresistive materials having different sensitivities: 
     Example I for forces in the range of 0 to 30 psi:
         200 ml of acrylic binder   10 ml of nickel flake powder   10 ml of graphite powder   20 ml of carbon black       

     Example II for forces in the range of 0-100 psi
         200 ml of acrylic binder   5 ml of nickel flake powder   5 ml of graphite powder   30 ml of carbon black       

     Example III for forces in the range of 0-1000 psi
         200 ml of acrylic binder   1 ml of nickel flake powder   1 ml of graphite powder   40 ml of carbon black       

     The fabric matrix for piezoresistive sheet  452  is submerged in the piezoresistive coating mixture. Excess material is rolled off and the sheet is hung and allowed to air dry. 
       FIG. 40  illustrates calculation of a minimum spacing S between adjacent air bladder cells  422 , and a minimum width of non-conductive strip  449  between adjacent conductors of sensor array  432 . 
     Referring to  FIG. 40 , as a patient sinks into a deflating bladder  422 , the upper force sensor layer  433  is drawn down and away from the bladder over which it was initially positioned. If the non-conductive strip  449  is too narrow, there is a possibility that a conductor such as column conductor  450  overlying the deflating bladder will contact adjacent conductor  451  and, thus register forces that are not representative of the force over the bladder in which it was originally positioned. It is therefore necessary to make the non-conductive strip  449  wide enough to prevent this from happening. If we assume a simple situation wherein an air bladder cell is deflated until the center of the cell, then the force sensing layer is drawn down a distance equal to the diagonals (C 1  and C 2 ) as shown in  FIG. 40 , the width S of non-conductive strip  449  should be made equal to or greater than (C 1 +C 2 −the width of the bladder) to prevent forces being misread as coming from a neighboring cell. 
       FIG. 28  illustrates the electrical resistance of a one-inch square piezoresistive force sensor element  448  using a piezoresistive sheet  437  having the formulation listed for an example sensor array  432  shown in  FIGS. 35 and 36 , and fabricated as described above, as a function of normal force or pressure exerted on the upper surface  447  of upper substrate sheet  433  of sensor array  432 . As shown in  FIG. 28 , the resistance varies inversely as a function of normal force. 
     As shown in  FIGS. 35 and 39 , left and right column electrodes  450  and  451 , in vertical alignment with row electrodes  461 ,  462 ,  463 ,  464 ,  465 ,  466 , of 12 form with piezoresistive layer sheet  452  between the column and row electrodes a 2×6 rectangular matrix array of 12 force sensors  433 . 
     Optionally, the upper and lower electrodes for each sensor  433  could be segmented into electrically isolated rectangular pads by etching channels  449 ,  459  through both upper conductive sheet  446  and lower conductive sheet  456 . This arrangement would require a separate pair of lead-out conductors for each of the 12 sensors, i.e., a total of 24 leads. 
     As shown in  FIGS. 35 and 39 , sensor array is arranged into rows and columns, thus requiring only 8 lead-out conductors. However, as shown in  FIG. 23 , if matrix addressing of sensor array  432  is used to measure the resistance of individual sensors  433  to thereby determine normal forces exerted on the sensors, there is a substantial cross-talk between the resistance on an addressed sensor  433  and nonselected sensors because of parallel current paths to non-addressed sensors. To overcome this cross-talk problem, the present inventor has developed a method for modifying sensors  433  to give them a diode-like characteristic. As may be confirmed by referring to  FIG. 24 , the cross-talk between sensors  433  which have a non-bilateral, polarity-sensitive transfer function, mitigates the cross-talk problem present in the matrix of symmetrically conductive sensors  433  shown in  FIG. 23 . 
     Sensors  433  are modified to have a diode-like characteristic by modifying the preparation of piezoresistive layer sheet  452 , as follows: First, a piezoresistive layer sheet  452  is prepared by the process described above. Then, either the upper surface  469  or the lower surface  470  of the piezoresistive coating  467  of piezoresistive sheet  452  is modified to form thereon a P-N, semiconductor-type junction. 
     Modification of piezoresistive coating  467  to form a P-N junction is performed by first preparing a slurry which has the composition of one of the three example mixtures described above, but modified by the addition of 5 ml each of copper oxide (CuO) in the form of a fine powder of 50-micron size particles, and 5 ml of cuprous oxide (Cu 2 O) in the form of a fine powder of 50-micron size particles and thoroughly stir-mixing the foregoing ingredients. The resultant solution is then reduced using about 30 mg of solution of sodium borohydride, also known as sodium tetrahydroborate (NaBH4) or ammonium phosphate, to form a solution having a pH of about 5.5. The solution is then coated onto the upper surface  469  or lower surface  470  of piezoresistive coating  468  on piezoresistive sheet  452 . This coating process is performed using a roller coating process which results in about 0.5 ml of solution per square centimeters being applied. The surface coating is then allowed to air-dry at room temperature and a relative humidity of less than 20%, for 4 hours. After the coated surface has dried, it functions as a P-type semiconductor, while the uncoated side of coating  468  functions as an N-type semiconductor of P-N junction diode. 
       FIG. 29  illustrates a sensor  433  which has been prepared as described above to give the sensor a diode-like characteristic, and a circuit for obtaining the 1-V (current versus voltage) transfer function of the sensor.  FIG. 30  shows a typical 1-V curve for sensor  433  of  FIG. 29 . 
     As stated above, the advantage of modifying sensors  433  by adding a semi-conductive layer that acts like a diode is that it reduces cross talk between sensors. As is shown in  FIG. 23 , this cross-talk occurs because of the so-called “completing the square” phenomenon, in which three connections are made in a square matrix array of three non-addressed resistors that form the three corners of a square. Thus, any two connections in a vertical column and a third one in the same row function as either connection in an X-Y array of conductors. The resistor at the fourth corner of the square shows up as a phantom in parallel with an addressed resistor because the current can travel backwards through that resistor, and forward through the other resistors. Care and additional expense must be taken in the electronics to eliminate the contribution of this phantom. For example, if, as is shown in  FIG. 23 , a potential V is applied between row and column conductors X 1 Y 1 , to thereby determine the resistance of piezoresistive sensor resistance R 11 , reverse current flow through “phantom” resistor R 22  would cause the sum of resistances R 12 +R 22 +R 22  to shunt R 11 , resulting in the parallel current flow paths indicated by arrows in  FIG. 23 , which in turn would result in the following incorrect value of resistance: 
     R x1 y 1 =R 11 //(R 12 +[R 22 ]+R 21 ), R x1 Y 1 =R 11 (R 12 +[R 22 ]+R 21 )/(R 11 +R 12 +[R 22 ]+R 21 ), where brackets around a resistance value indicate current flow in a counterclockwise direction through that resistor, rather than clockwise, i.e., diagonally downwards towards the left. Thus, for example, if each of the four resistances listed above had a value of 10 ohms, the measured value of R 11  would be: 
     R 11 =10(10+10+10)/(10+10+10+10)=300/40=7.5 ohms, i.e., 25% below the actual value, 10 ohms, of R 11 . If the resistance values of R 12 , R 22  and R 21  of the three non-addressed piezoresistive sensors  433  were each lower, e.g., 1 ohm, because of greater forces concentrated on those sensors  433 , the measured value of R 11  would be: 
     R 11 =10(1+1+1)/(10+1+1+1)=30/13=2.31 ohms, i.e., a value of about 77 percent below the actual value of R 11 . 
     On the other hand, by placing a diode in series with each piezoresistive sensor element  433 , as shown in  FIG. 24 , the electrical resistance of an element measured in a reverse, counterclockwise direction a test current flow through the sensor element, e.g., R 22 , would be for practical purposes arbitrarily large, or infinity compared to the clockwise forward paths of current through the other resistances shown in  FIGS. 23 and 24 . In this case, the measured resistance value for a 2×2 matrix of four resistances each having a value of 10 ohms would be: 
     R x1 y 1 =10(1+∞+1)/(10+1+∞+1)=10 ohms, the correct value. 
     Thus, modifying each sensor  433  element to include a p-n junction thereby gives the sensor element a diode-like characteristic that electrically isolates, i.e., prevents backward current flow, through each sensor element  433 . This enables the correct value of electrical resistance R x1 y 1  of each sensor element  433  and hence forces exerted thereon to be measured accurately using row and column matrix addressing rather than requiring a separate pair of conductors for each sensor element. 
     The above-described components of force minimization apparatus  420  are interconnected to form a closed-loop servo control system. That system is effective in reducing body force concentrations using an algorithm according to the method described herein. An understanding of this method and apparatus may be facilitated by referring to  FIG. 41 , which is a block diagram of an electro-pneumatic controller system component  420 A of apparatus  420 , in conjunction with the diagrammatic view of the apparatus shown in  FIG. 35 , and the perspective view shown in  FIG. 39 . 
     Referring to  FIG. 41 , it may be seen that electro-pneumatic controller apparatus  420 A includes a computer  37  which is bidirectionally coupled to force sensor array  432  through force sensor interface module  436 . The sensor interface module  436  includes a Digital-to-Analog Converter (DAC)  471  for generating in response to control signals from computer  437  test voltages or currents which are directed to matrix addressed individual force sensors  433 . 
     Individual force sensors  433  are addressed by connecting one terminal of a current or voltage source controlled by DAC  471  to a selected one of X-row conductors  1 - 6  by an X multiplexer  472 , and connecting the other terminal of the source to a selected one of Y-column conductors  1  or  2  by a Y multiplexer  473 . Sensor interface module  437  also included an Analog-to-Digital Converter (ADC)  474  which measures the voltage drop or current through a sensor  433  resulting from application of a test current or voltage, and inputs the measured value to computer  437 . Using predetermined scale factors, computer  437  calculates the instantaneous value of electrical resistance of a selected addressed sensor  433 , and from that resistance value, a corresponding normal force instantaneously exerted on the addressed sensor. 
     In response to control signals cyclically issued by computer  437 , X multiplexer  472  and Y multiplexer  473  are used to cyclically measure the resistance of each force sensor element  433 , at a relatively rapid rate of, for example, 3,000 samples per second, enabling computer  437  to calculate the force exerted on each force sensor  433  at that sampling rate. 
     Referring still to  FIG. 41 , apparatus  420  includes a pressure control module  475  for dynamically controlling the air pressure in each individual air bladder cell  422 , in response to command signals issued by computer  437 , based upon values of force measured by sensor array  432  and an algorithm programmed in the computer. As shown in  FIG. 41 , pressure control module  475  is operably interconnected to air compressor  440  and air pressure transducer  444  at output port  476  of the compressor to pressurize air in the outlet port to a value controllable by computer  437 . 
     Outlet port  476  of compressor  440  is coupled to inlet port  442  of a 12-outlet port manifold  441 . In response to electrical control signals issued by computer  437  and routed through pressure control module  475 , each of 12 individual air bladder cell inlet selector valves  443  connected to separate outlet ports  443 A of manifold  441  is individually controllable. 
     In a first, open position of a selector valve  443 , the air inlet port  431  of a selected air bladder cell  422  is pressurized to a pressure measured by transducer  444  to a predetermined value, by turning on compressor  440 , to thereby inflate the cell to a desired pressure. Alternatively, with compressor  440  in an off-mode, a vent valve  477  coupled to the input port  442  of manifold  441  may be opened to deflate an air bladder cell  422  to a lower pressure value by exhausting air to the atmosphere. 
     After a selected one of the 12 selector valves  443  has been opened in response to a command signal from computer  437  for a time period sufficient to inflate a selected air bladder cell  422  to a predetermined pressure, an electrical signal output by pressure transducer  444 , which is proportional to the pressure in that cell and input to computer  437 , results in the computer outputting a closure command signal to the valve and a shut-off command signal to compressor  440 . 
     When vent valve  477  and a selected selector valve  443  have been opened in response to command signals from computer  437  to deflate a selected air bladder cell  422  to a lower predetermined pressure, an electrical signal from pressure transducer  444  input to computer  437  results in an electrical closure command signal being output from the computer. That command signal closes vent valve  477  and the open selector valve  443 , thereby maintaining the selected lower pressure in the selected air bladder cell. In an exactly analogous fashion, the air pressure in each other air bladder cell  422  is sequentially adjustable by sending a command signal to a selector valve  443  to open that valve, and operating compressor  440  and/or vent valve  477  to inflate or deflate the air bladder cell to a predetermined pressure. 
       FIG. 42  is a simplified perspective view of an embodiment of a housing for electro-pneumatic apparatus  420 A shown in  FIG. 41  and described above. As shown in  FIGS. 41 and 42 , electro-pneumatic controller  420 A includes an operator interface module  478 . Operator interface module  478  includes manual controls, including a multi-function, on/off, mode control switch and button  479 , up and down data entry slewing buttons  480 ,  481 , and a digital display  482 . Display  482  is controllable by switch  479  to selectively display air pressure within and force on selectable air bladder cells  422 , and the sum and average of all forces exerted on sensors  433 . 
     As shown in  FIG. 42 , electro-pneumatic controller  420 A is contained in a box-like housing  483  which has protruding from a rear panel  484  thereof an L-bracket  485  for suspending the housing from a side board or end board of a bed. Housing  483  of electro-pneumatic controller  420 A also includes a tubular member  486  for interfacing air hoses  487  with air bladder cells  422 , row and column conductors  488 ,  489 , to sensors  433  of sensor array  432 , and an electrical power cord  490  to a source of electrical power for powering the components of apparatus  420 A. 
     Force Minimization Algorithm 
     The force minimization apparatus described above is made up of a multiplicity of air  424  bladder cells  422 . Each cell  422  has on its upper surface a separate force sensor  433 . An air pressure transducer  444  is provided to measure the air pressure in each cell. Each force sensor is located in a potential contact region between a person lying on cushion  421  and the air bladder cell. Each piezoresistive force sensor  433  functions as a force sensitive transducer which has an electrical resistance that is inversely proportional to the maximum force exerted by a person&#39;s body on the air bladder cell  422 , the maximum force corresponding to the lowest resistance path across any part of each sensor. 
     As shown in  FIG. 37 , each air bladder cell  422  supports a different longitudinal zone of the user such as the head, hips or heels. The compressor  440  and selector valves  443  controlling the air pressure in each zone are controlled by sensors  433  and pressure measurements made by pressure transducer  444 , using a novel algorithm implemented in computer  437 . There can be a minimum of one zone using one air bladder cell  433 , and up to N zones using n air bladder cells, wherein each zone has a force sensor  433  to measure the maximum force on that air bladder cell, the pressure transducer  444  being used to measure the air pressure in that air bladder cell. The control algorithm is one of continuous iteration wherein the force sensors  433  determine the peak force on the patient&#39;s body, and the pressure transducer  444  measures the pressure at which the force occurs. At the end of a cycle sampling forces on all sensors, the bladder air pressure is restored to the pressure where the force was minimized for all zones. This process continues and the apparatus constantly hunts to find the optimal bladder pressures for each individual cell resulting in minimizing peak forces on a person supported by overlay cushion  421 . 
     Algorithm Description: 
     Given: 
     N Zones each containing one air bladder cell and numbered one to N 
     The air bladder cell of each zone is selectably connectable to an air pressure transducer to measure P# 
     Each air bladder cell is fitted with an individual force sensor capable of measuring the maximum force F# exerted on the surface of each cell. 
     A common compressor supplies air at pressures of up to 5 psi to selected individual air bladder cells of the zones. There is a normally closed vent valve for deflating a selected air bladder cell by exhausting air to the atmosphere through the vent valve. 
     There is a selector valve that selects which air bladder is being inflated with air or deflated by exhausting air to the atmosphere through the vent valve. 
     Algorithm Steps: 
     1. Pset::::Pset, start, close vent valve 
     2. Select zone i=1 by opening selector valve 1 
     3. Turn the compressor on. 
     4. Measure the air pressure in the air bladder cell in zone I 
     5. Pressurize the zone-one air bladder cell to a predetermined upper set pressure and close the selector valve value Pset. 
     6. Repeat for i+1 until i+1=N 
     7. Select Zone i=I 
     8. Obtain the force sensor readings for all zones. 
     9. Open Vent valve. 
     10. Deflate the zone-one air bladder cell to a predetermined minimum pressure and monitor all the force sensor readings on all air bladder cells. Maintain bladder pressures in all other air bladder cells at their upper set pressures. Measure forces on all air bladder cells as the single, zone-one air bladder is being deflated and compute the sum and optionally the average of all force sensor readings. 
     12. Store in computer memory the pressure reading of the zone-one air bladder cell at which the minimum sum and optionally the average of all force sensor readings occurs. 
     13. Restore the pressure in the zone one air bladder cell to the value where the minimum sum and average force sensor readings for all the force sensors was obtained. 
     14. Close the zone-one selector valve. Maintain the pressure in zone one. 
     15. Set: Count=i+1. 
     16. Repeat steps 2 thru 15 until Count=i+1=N. 
     17. Set: Pset=Pset, start−(Count*20%_(i.e., reduce the initial pressure in the zone one bladder). 
     18. Repeat Steps 2 thru 16 (i.e., with a reduced initial pressure). 
     Caveat 
     19. Constantly monitor all force sensors and if significant change (Delta F&gt;0.2*F#) is detected (patient moved) start over at Step  1 . 
       FIG. 43  is a flow chart showing the operation of apparatus  420  utilizing the algorithm described above. Table 1 lists appropriate lower and upper initial set pressures for bladders  422 , as a function of the weight of a patient or other person supported by overlay cushion  421  of the apparatus. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Patient Weight 
                 Minimum Pressures 
                 Start Pressure 
               
               
                   
                   
               
             
             
               
                   
                  75-119 Pounds 
                 5.5″ ± 0.7:H 2 O 
                 6.5″ ± 0.7:H 2 O 
               
               
                   
                   
                 10.31 ± 2 mmHg 
                 12.18 ± 2 mmHg 
               
               
                   
                 120-164 Pounds 
                 6″ ± 0.7:H 2 O 
                 8″ ± 0.7:H 2 O 
               
               
                   
                   
                 11.25 ± 2 mmHg 
                 15 ± 2 mmHg 
               
               
                   
                 165-199 Pounds 
                 8″ ± 0.7:H 2 O 
                 10″ ± 0.7:H 2 O 
               
               
                   
                   
                 15 ± 2 mmHg 
                 18.75 ± 2 mmHg 
               
               
                   
                 200-250 Pounds 
                 10″ ± 0.7:H 2 0 
                 12″ ± 0.7:H 2 O 
               
               
                   
                   
                 18.75 ± 2 mmHg 
                 22.49 ± 2 mmHg 
               
               
                   
                 Maximum Pressure 
                   
                 26″ ± 0.7:H 2 O 
               
               
                   
                   
                   
                 48.74 ± 4 mmHg 
               
               
                   
                   
               
             
          
         
       
     
     In a variation of the method and apparatus according described above, after the pressures in each air bladder cell  422  have been optimized for minimum force concentration, inlet tubes  431  may be permanently sealed, and the adaptive cushion  421  permanently disconnected from pressure control module  475 . This variation would also enable the custom fabrication of cushions  421  using air bladder cells  422 , for customizing chair cushions to minimize force concentrations on a particular individual. Similarly, the variation of the method and apparatus could be used to customize saddle cushions or car seats.