Abstract:
An adaptive cushion for reducing pressure on body parts of a person positioned on a chair or bed includes an overlay cushion having a plurality of individual air bladder cells, each having thereon a force sensor. The cushion includes a controller for inflating and deflating individual air bladder cells to air pressures that tend to reduce the interface pressures sensed by the force sensors. A pressure reduction method includes varying the inflation pressure in a first air bladder cell while measuring the sum of the interface pressures exerted on all or a plurality of the air bladder cells, re-pressurizing the first cell to that air pressure for which a minimum total interface pressure was obtained, repeating this process for the remaining air bladder cells.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. patent application Ser. No. 12/075,937 filed Mar. 15, 2008 by applicant Geoffrey Taylor and entitled ADAPTIVE CUSHION METHOD AND APPARATUS FOR MINIMIZING FORCE CONCENTRATIONS ON A HUMAN BODY, the complete disclosure of which is hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates to methods, articles and apparatus for comfortably supporting a seated or recumbent human body. More particularly, the invention relates to a method and apparatus for minimizing concentration of forces on supported body parts using an adaptive cushion having a matrix of air bladder cells which are dynamically pressurized in response to measurements of body forces exerted on sensors overlying the cells. 
     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. High pressure alone is generally not sufficient to deleteriously affect tissue. Deep-sea divers for example, are subjected to high, but evenly distributed normal forces and do not suffer from tissue damage. If, however, there is a sufficiently large external pressure gradient on a body part, resulting from, for example, a low-pressure area adjacent to a high-pressure area, internal body fluids can migrate to the area of lower pressure. Tangential or shear forces exerted externally on a body part can also collapse internal capillaries and blood vessels by distorting them along their longitudinal axes. It is therefore extremely important to know both the surface force gradient (pressure gradient) and the externally applied shear force exerted on tissue, because it is the combination of these factors that leads to tissue strain and subsequent tissue death. Thus, even relatively small external shear and normal forces, which may be independent of one another, can combine to produce damagingly large shear stresses on internal tissue. 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. 
     There are available a variety of pressure/force sensors, shear sensors and sensor arrays which are useable for measuring normal and shear forces exerted on human tissues. For example, the present inventor&#39;s U.S. Pat. No. 5,751,973, Nov. 5, 1996, Multi-Directional Piezoresistive Shear And Normal Force Sensors For Hospital Mattresses And Seat Cushions discloses thin, planar sensors for measuring reaction forces exerted by mattresses or chair pads on the body of a recumbent or seated patient. One embodiment of the invention disclosed in the specification of the ‘973 patent includes a sensor comprised of a two-dimensional array of isolated sensor element pads, each consisting of a thin, flat layer formed of a non-conductive elastomeric polymer matrix filled with electrically conductive particles.’ A matrix of upper and lower conductive elements in electrical contact with upper and lower sides of each sensor pad enables separate measurements to be made of the electrical resistance of each pad. Pressure exerted on each pad, e.g., in response to a normal force exerted on the sensor matrix by a person&#39;s body, reduces the thickness of the sensor pad, and therefore its electrical resistance by a bulk or volume piezoresistive effect. 
     The present inventor also disclosed a novel method and apparatus for measuring pressures exerted on human feet or horses&#39; hooves in U.S. Pat. No. 6,216,545, Apr. 17, 2001, Piezoresistive Foot Pressure Measurement. The novel apparatus disclosed in the “545 patent includes a rectangular array of piezoresistive force sensor elements encapsulated in a thin, flexible polymer package. Each sensor element includes a polymer fabric mesh impregnated with conductive particles suspended in an elastomeric matrix such as silicone rubber. The piezoresistive mesh layer is sandwiched between an array of row and column conductor strip laminations, preferably made of a nylon mesh impregnated with printed metallic paths. Each region of piezoresistive material sandwiched between a row conductor and column conductor comprises an individually addressable normal force or pressure sensor in a rectangular array of sensors, the resistance of which varies inversely in a pre-determined way as a function of pressure exerted on the sensors, and thus enabling the force or pressure. distribution exerted by an object contacting the array to be mapped. 
     In U.S. Pat. No. 6,543,299, Apr. 8, 2003, Pressure Measurement Sensor With Piezoresistive Thread Lattice, the present inventor disclosed a transducer sensor array for measuring forces or pressures exerted on a surface, the array including a fabric-like, two-dimensional lattice of individual force or pressure sensor transducer elements comprising intersecting regions of pairs of elongated, flexible threads, each consisting of a central electrically conductive wire core covered by a layer of piezoresistive material which has an electrical resistivity that varies inversely with pressure exerted on the material. 
     In U.S. Pat. No. 7,201,063, Apr. 10, 2007, Normal Force Gradient/Shear Force Sensors And Method Of Measuring Internal Biological Tissue Stress, the present inventor disclosed a normal force gradient/shear force sensor device and measurement method for measuring internal stresses in tissues of a person supported by a chair or bed. The device includes a planar matrix array of peripheral normal force sensors radially spaced from central shear force sensors, each including an electrically conductive disk located within a circular opening bordered by circumferentially spaced apart electrodes. The disk and electrodes are located between upper and lower cover sheets made of a stretchable material such as polyurethane, one cover sheet being adhered to the disk and the other sheet being adhered to a support sheet for the electrodes. Motion between the cover sheets in response to shear forces exerted on the array causes the disk to press more or less tightly against the electrodes, thus varying electrical conductance between the disk and electrodes proportionally to the magnitude and direction of the shear force. Each normal force sensor includes an electrically conductive film pressed between row and column conductors. Measurements of conductance values of pairs of sensor, which vary proportionally to normal forces exerted on the sensor, are used to calculate a gradient vector of normal forces exerted by a body part on the sensor array, which is combined with the shear force vectors in an algorithm to calculate internal reaction shear forces, e.g., on flesh near a bony prominence. 
     The first group of the present inventor&#39;s patents identified above disclosed shear and normal force sensors and arrays which are useful in producing maps of normal and shear forces exerted at discrete points on a surface, such as a human body part, by an object such as the supporting surface of a chair or bed. The last of the present inventor&#39;s patents identified above provided an effective means for measuring shear forces and stresses on human tissue which is located some distance below the surface of the skin. 
     In U.S. Pat. No. 6,721,980, Force Optimization Surface Apparatus And Method, the present inventor and co-inventors disclosed an apparatus including a mattress which included a plurality of laterally disposed, tubular sausage-shaped air bladders, each having thereon an individual force sensor. The apparatus included a mechanism for individually inflating each of the air bladders, monitoring the pressure in each individual bladder while a person was lying on the mattress monitoring the force exerted on that particular bladder, adjusting the pressure of that individual bladder for the purpose of minimizing force exerted by that particular bladder on the person&#39;s body, and repeating the foregoing steps for each bladder cell in turn. 
     The method described in U.S. Pat. No. 6,721,980 of measuring force exerted by a person&#39;s body on a single individual air bladder cell while adjusting the inflation pressure in that cell may be suitable for single air bladder systems, and for those conditions in which the body of a supported patient freely conforms to the support surface. However, for the more frequently encountered cases in which portions of a patient&#39;s body are straddled between and supported by adjacent air bladder cells, the force measured on a particular bladder whose air pressure is bing adjusted may be minimal for a particular inflated pressure. But the pressure which may minimize force exerted on a particular air bladder cell will in general not be the optimum pressure for minimum total force concentrations on a person&#39;s body. This is because while the force exerted on a particular air bladder cell may be minimized, forces exerted on air bladder cells adjacent to the air bladder cell in which the pressure is being varied may be substantially increased because the load weight is shifted to the adjacent cells. 
     A similar limitation of the prior art methods and apparatus occurs when a portion of a patient&#39;s body is supported in a cantilevered manner from one or more adjacent air bladder cells while pressure is varied in a particular air bladder cell. Again in that case, load forces are transferred to adjacent air bladder cells. Accordingly; it would be desirable to provide a method and apparatus which accounted for all forces exerted on all air bladder cells while varying pressure in any individual cell The present invention was conceived of to provide a method and apparatus for minimizing body force concentrations on parts of a human body supported by a chair or bed cushion, which includes measuring forces exerted on body parts. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide an adaptive cushion method and apparatus for minimizing reaction forces exerted by a bed, chair or other such object on body parts of a person lying or seated on the object. 
     Another object of the invention is to provide an adaptive cushion method and apparatus which includes an overlay cushion for placement on a bed mattress or chair, the cushion including a matrix of individually pressurizable air bladder cells and an array of surface force sensor transducers which includes an individual sensor vertically aligned with each air bladder cell, and an electronic control system for receiving force sensor signals and dynamically varying inflation pressures applied to individual air bladder cells to inflate or deflate the individual cells to pressures calculated by a control system algorithm to minimize force concentrations on parts of a body supported by the cushion. 
     Another object of the invention is to provide stretchable surface force transducers which are conformable to protuberances of a human body. 
     Another object of the invention is to provide stretchable surface force sensors which have an asymmetric, diode-like current-versus-voltage transfer function. 
     Another object of the invention is to provide a matrix array of stretchable surface force sensor transducers which have a non-bilateral current-versus-voltage transfer functions, thus minimizing cross-talk ambiguities occurring during X-Y addressing of individual sensors to map forces exerted on the array. 
     Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification, drawings and claims. 
     It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, I do not intend that the scope of my exclusive rights and privileges in the invention be limited to details of the embodiments described. I do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the present invention comprehends a method and apparatus for minimizing high concentrations of reaction forces exerted by a chair, bed or other such object on protruding parts of the body of a person seated or lying on the object. A body force minimization apparatus according to the present invention includes an adaptive cushion for placement on a mattress or chair, the cushion having a matrix of air bladder cells which are individually pressurizable by means of an air compressor and valves to variable pressures. 
     In a typical embodiment of the adaptive cushion suitable for use on bed, the air bladder cells may be arranged in a 6×2, X-Y rectangular grid, thus dividing the cushion into left and right columns, each having 6 longitudinally spaced apart zones running in the long, head-to-feet direction of the bed. 
     The adaptive cushion apparatus according to the present invention also includes a flexible, stretchable planar array of force sensor transducers of novel construction, which is preferably positioned on the upper surface of the cushion, the array having at least one sensor in vertical alignment with each air bladder cell of the cushion. 
     The sensor array according to the present invention includes stretchable fabric row and column conductors which have sandwiched between inner facing conductive surfaces thereof a stretchable fabric sheet coated with a piezoresistive material. Thus constructed, the planar sensor array is elastically deformable in response to forces exerted on the array by the weight of a human body supported on the upper surface of the sensor array overlying the air bladder cells. Preferably, the sensor array placed on the upper surfaces of the air bladder cells and maintained in that position by a form-fitting, waterproof, contour sheet. The fabric matrices for both row and column conductors, as well as the central piezoresistive layer, are all made of a material which is elastically deformable in any direction within the plane of the material. In a preferred embodiment, the fabric matrices or the row conductor sheet and column conductor sheet are plated with a copper base coat and nickle cover coat. The central piezoresistive sheet consists of a synthetic fabric matrix coated with piezoresistive coating. The sensor array also has an upper cover sheet which is made of a fabric such as Lycra which has a two-way stretch characteristic, i.e., is elastically stretchable in orthogonal directions. 
     An adaptive cushion apparatus according to the present invention includes an electro-pneumatic controller which is effective in alternately pressurizing and venting individual air bladder cells to control pressures, in respect to forces exerted by a human body on individual sensors aligned with the air bladder cells. The electro-pneumatic controller includes an electronic control system for applying a voltage or current individually to each force sensor and measuring the resultant current or voltage to thereby determine electrical resistance of the sensor, which is inversely proportional to the force or pressure exerted on the sensor, by for example, a person seated or lying on the cushion covered by the sensor array. 
     The electronic control system also includes a computer which receives as inputs electrical signals from individual sensors representative of their resistance, and hence forces or pressures exerted on the upper surface of each sensor. 
     The body force minimization apparatus according to the present invention also includes a pneumatic system which has a source of pressurized air, such as a compressor, for inputting pressurized air through a manifold and individually controllable inlet selector valves to each individual air bladder cell. The apparatus also includes an air pressure transducer for monitoring the air pressure within a selected cell, and outputting to the computer an electrical signal representative of the measured pressure. 
     Each air bladder cell inlet valve is electrically operable and has a first, open position in which air from an outlet port of the manifold is conducted to a selected air bladder cell to inflate it to a desired set pressure, and a second, closed position effective in maintaining a desired set pressure within the cell. 
     The pneumatic system also includes a vent valve coupled to the inlet port of the manifold. With the vent valve and a selected air bladder cell value in a second, open position, pressurized air from a selected air bladder cell is vented to the atmosphere through a exhaust port of the vent valve to reduce the pressure in the individual air bladder cell to a lower controllable value. Each valve is electrically connected to an output control port of the computer; and operably controllable by signals on the output control port. 
     The present invention also includes a method for electronically controlling the body force minimization apparatus. The method includes an algorithm implemented in the control system computer. That algorithm receives as inputs force measurements from individual air bladder cells, and outputs command signals which individually adjust the air pressure in each air bladder cell to values which are effective in minimizing force concentrations on body parts supported by the cushion. 
     According to the algorithm, each of the air bladder cells is inflated to predetermined upper set pressures, which may be the same or different for different cells, prior to a person&#39;s lying or sitting on the cushion. Next, a person is positioned on the cushion, while forces exerted by the person&#39;s body on each sensor are initially monitored by computer controlled measurement of the electrical resistance of each force sensor. A first, “zone-one” air bladder cell is then deflated under computer control to a predetermined lower set pressure. Although zone-one may correspond to any individual air bladder cell, such as the upper left-hand corner cell value in a 6-row by 2-column of air cells for use on a bed, a preferred mode of operation is to choose as zone-one the cell on which the highest body force was measured during the initial monitoring process. 
     During the step of deflating the first, zone-one air bladder cell, which is done in a slowly varying, ramped fashion, the forces exerted on each of the cells including the zone-one cell are measured, and the sum and optionally the average of those forces calculated by the computer. At the end of the downwardly ramped deflation step, the air pressure corresponding to the lowest sum and average of all force sensor readings is noted. The zone-one cell is then re-inflated to that pressure corresponding to the lowest sum and average force sensor readings, to complete the cycle for zone-one. 
     The pressure-ramping cycle described above for the first zone, i.e., zone-one, is repeated in turn for each remaining zone of the air bladder cell cushion. Preferably, the sequence of zone deflation, re-inflation pressure-ramping cycles corresponds to successively smaller force concentrations. In other words, zone-one is chosen as the zone at which the highest surface body force was measured, zone-two would correspond to that zone having the second highest body force measurement, etc. 
     After the pressure-ramping cycle has been completed for each of the zones of the adaptive cushion, those steps are repeated for all of the zones, but using a reduced range of pressure, i.e., lower upper set pressures and higher lower set pressures. The sequence is then repeated again until the successively smaller adjustments in force measurements fall below a predetermined threshold level, at which time the cyclical operation of the system reverts to a passive state. 
     In the passive state, the computer monitors each of the force sensor outputs. Restoration of the control system to active cyclical operation is initiated by a significant change of any force measurement above a predetermined threshold in response, for example, to patient movements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partly diagrammatic perspective view of a body support cushion apparatus with adaptive body force concentration minimization according to the present intention. 
         FIG. 2A  is a fragmentary upper perspective view of the apparatus of  FIG. 1 , 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. 2B  is a fragmentary view of the mattress overlay of  FIG. 2A , showing an individual air cell thereof. 
         FIG. 3  is a diagrammatic side elevation view of the apparatus of  FIGS. 1 and 2 , showing certain bladder cells thereof deflated to reduce support forces exerted on parts of a human body supported by the mattress overlay. 
         FIG. 4  is a vertical sectional view of the mattress of  FIG. 2 , taken in the direction of line  4 - 4 . 
         FIG. 5  is a fragmentary exploded perspective view of the mattress of  FIG. 1 , showing elements of a force sensor arrangement thereof. 
         FIG. 6  is a diagrammatic view showing a preferred relationship between the dimensions of adjacent air bladder cells and the width of an insulating strip between conductors of sensors on the cells. 
         FIG. 7  is an electrical resistance-vs.-normal force diagram for the sensors of  FIG. 5 . 
         FIG. 8  is a partly schematic view of a preferred modification of sensor elements of the array of  FIG. 1 , which includes a diode junction. 
         FIG. 9  is a current-vs-voltage (I-V) diagram for the sensor elements of  FIG. 8 . 
         FIG. 10A  is a schematic diagram showing a six row by two column matrix of the sensors of  FIG. 5 . 
         FIG. 10B  is a view similar to that of  FIG. 10A , but showing sensors modified to include a diode junction. 
         FIG. 11  is a block diagram of electro-pneumatic controller elements of the apparatus of  FIG. 1 . 
         FIG. 12  is a simplified perspective view of the electro-pneumatic controller of  FIG. 11 . 
         FIG. 13  is a flow chart showing operation of the apparatus of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1-11  illustrate various aspects of a method and apparatus for minimizing body force concentrations on a human body using an adaptive cushion according to the present invention. The example embodiment of the invention depicted in  FIGS. 1 and 3 , 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. 1 and 2 , an adaptive cushion apparatus  20  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  21 . Cushion  21  has an appropriate size and shape to fit conformally on top of a standard size hospital bed. Thus, an example embodiment of cushion  21  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  23  are sealingly joined at edges thereof to form a hermetically sealed body which has a hollow interior space  22 A. 
     As shown in  FIG. 2 , mattress overlay cushion  21  is constructed as a rectangular, two-column by six-row array of 12 individual inflatable air bladder cells  22 . Each air bladder cell  22  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. 1 and 2 , bladders  22  are arranged in left and right columns, each having 6 longitudinally spaced apart, laterally disposed, laterally elongated bladders. As shown in  FIGS. 2 and 4 , each air bladder cell has a flat base panel  23 , left and right end panels  24 ,  25 , head and toe or front and rear panels  26 ,  27 , and an upper panel  28 . The bladders  22  are preferably made of a thin sheet of a flexible, preferably elastomeric material such as neoprene rubber or polyurethane, having a thickness of about 0.014 inch. Optionally, each air bladder cell 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  22  of mattress cushion overly  21  are not believed to be critical. However, it is believed preferable to arrange mattress overlay  21  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. 1 ,  2 A and  3 , mattress overlay cushion  21  has a left-hand column of six air bladder cells  22 L1- 22 L6, and a right-hand column of six cells  21 R1- 21 R6. 
     As shown in  FIGS. 4 and 6 , the bladders are stacked closely together in both front and rear and side by side directions, with minimum longitudinal and lateral spacings  29 ,  30 , respectively, that are preferably vanishingly small so that adjacent bladder cells physically contact each other. 
     As indicated in  FIGS. 1 and 2 , each bladder cell  22  is provided with a tubular air inlet port  31  which protrudes through a side wall, e.g., a left or right side wall  24  or  25 , and communicates with a hollow interior space  22 A within the bladder. Air admitted into or exhausted from hollow interior space  22 A through port  31  of an air bladder cell  22  enables the cell to be inflated or deflated to a selected pressure. 
     Although the shape of each air bladder cell  22  of-cushion  21  shown in  FIGS. 1 and 2  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  22  of cushion  21  may be parts of a unitary structure with a common base panel  23  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  22  are separate bodies or upper inflatable shell-like portions protruding upwardly from a common base, air inlet/exhaust port tubes  31  of each air bladder cell  22 , or selected air bladder cells  22 , may be located in the base panel  23  of the cell and protrude downwardly from the cell, rather than being located in a side wall and protruding outwardly, as shown in  FIGS. 1 and 2 . 
     As shown in  FIGS. 1 ,  2  and  5 , body force minimization apparatus  20  includes a force sensor array  32  which has a matrix of individual force sensors  33 , with at least one sensor positioned on the upper surface  34  of each air bladder cell  22 . As will be explained in detail below, each force sensor  33  is 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  21 . In a preferred embodiment, force sensor array  32  is maintained in position on the upper surfaces of air bladder cells  22  by a water-proof, form-fitting contour fabric sheet  21 A which fits tightly and removably over cushion  21 , as shown in  FIG. 3 . 
     Referring to  FIG. 1 , it may be seen that body force minimization apparatus  20  includes an electronic control module  35 . As will be explained in detail below, electronic control module  35  includes sensor interface circuitry  36  for electrical interconnection to sensors  33 . Electronic control module  35  also includes a computer  37  which is interconnected with sensor interface circuitry  36 . Computer  37  is programmed to receive input signals from sensor interface circuitry  36 , measure the resistance of individual sensors  33  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  22  which are calculated to minimize force concentrations on the cells. 
     In a preferred embodiment of apparatus  20 , measurement of the resistance of each sensor  33  is facilitated by arranging the sensors into a matrix array of rows and columns. With this arrangement, individual resistances of a 6×2 array  32  of sensors  33  may be measured using 6 row interface conductors  35  and a 2 column interface conductors  39 , as shown in  FIG. 1 . 
     To avoid cross talk between measurements of individual sensors  33 , 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  33  which do not have a diode-like characteristic, the force sensor array  32  can be partitioned into 12 separate rectangular sensors  33  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. 1 , body force minimization apparatus  20  includes an air pump or compressor  40  for providing pressurized air to the input port  42  of a selector valve manifold  41 . Selector valve manifold  41  has 12 outlet ports  43 A, each connected through a valve  43  to a separate air bladder cell inlet port  31 . As will be explained in detail below, the compressor  40 , selector valve manifold  41  and valves  43  are operably interconnected to computer  37  and an air pressure transducer  44 . Pressure transducer  44  outputs an electrical signal proportional to pressure, which is input to computer  31 . This arrangement enables the inflation pressure of each air bladder cell  22  to be individually measured and varied under control of the computer  37 . 
       FIGS. 2 ,  4  and  5  illustrate details of the construction of force sensor array  32 . As shown in those figures, sensor array  32  includes an upper cover sheet  45  made of a thin flexible, elastically stretchable material. In an example embodiment of sensor array  32  fabricated by the present inventor, cover sheet  45  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. 247579, obtained from the Milliken &amp; Company, P.O. Box 1926, Spartanburg, S.C. 29304. 
     Referring to  FIG. 5 , sensor array  32  includes an upper, column conductor sheet  46  which is fixed to lower surface  47  of upper flexible cover sheet  45 , by flexible adhesive strips  47  made of 3M transfer tape  950 , or a flexible adhesive such as Lepage&#39;s latex contact adhesive. Column conductor sheet  46  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  46  is electroless plated with a base coating of copper, followed by an outer coating of nickle. The metallic coatings completely impregnate the surfaces of fibers adjacent to interstices of the mesh fabric, as well as the upper and lower surfaces  47   48  of the conductor sheet  46 , thus forming electrically conductive paths between the upper and lower surfaces  47  and  48 . 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  32 , upper conductive sheet  46  was fabricated from the Woven Silver, Catalog #A151 material described above. The surface resistivity of upper and lower surfaces  47 ,  48  of that material was about 1 ohm per square or less, and the inter-layer resistance between upper and lower surfaces  47 ,  48  was about 50 ohms per square. 
     In a preferred embodiment of sensor array  32  according to the present invention, individual conductive pads, or rows or columns of conductors; are formed by etching metal-free channels vertically through conductor sheet  46 , from the top of upper conductive surface  47 , all the way to the bottom of lower conductive surface  48 . Thus, as shown in  FIG. 5 , narrow longitudinally disposed straight channels  49  are etched through upper column conductor sheet  46 . This construction results in the formation of two adjacent, relatively wide, longitudinally elongated left and right planar column electrodes  50 ,  51 . The adjacent left and right column electrodes are separated by a relatively thin channel  49 , thus electrically isolating the adjacent column electrodes from each other. 
     According to the present invention, insulating channels  49  are etched through upper conductor sheet  46  to form column electrodes  50  and  51  by the following novel process. 
     First, to prevent capillary wicking and resultant wetting of a subsequently applied etchant solution to fabric conductor sheet  46 , the sheet is, pre-processed by treating it with a hydrophobic substance such as PTFE. The treatment is preferably made by spraying the conductor fabric sheet  46  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. Preferably, areas of fabric conductor sheet  46  which are to have insulating channels  49  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  46  to make it hydrophobic, sheets of masking tape are adhered tightly to both upper and lower surfaces  47 ,  48  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  49  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  49  have been stripped from conductor sheet  46 , the conductive metal coatings of the fabric sheet aligned with the channels is chemically etched away. A preferred 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  47 ,  48  of conductor sheet  46 , over channels  49 . The etchant gel is allowed to reside on channels  49  for approximately 1 hour, at room temperature, during which time the nickel and copper plating of the fabric matrix of conductor sheet  46 , in vertical alignment with channels  49 , is completely removed, thus making the channels electrically insulating. This process separates the conductor sheet into left and right column electrodes  50 ,  51 , respectively. 
     The etching process which forms insulating channel  49  is completed by rinsing the etchant gel from upper and lower surfaces  47 ,  48  of conductor sheet  46 , followed by removal of the masking tape from the upper and lower surfaces. 
     Referring still to  FIG. 5 , it may be seen that sensor array  32  includes a thin piezoresistive sheet  52  which has on an upper surface  53 , that is in intimate contact with lower surfaces of left and right column electrodes  50 ,  51 . Piezoresistive sheet  52  also has a lower surface  54  which is in intimate electrical contact with the upper surfaces of row electrodes  55  on a lower row conductor sheet  56 . Lower, row conductor sheet  56  has a construction exactly similar to that of upper, column conductor sheet  46 . Thus, lower row conductor sheet  56  has upper and lower conductive surfaces  57 ,  58 , and narrow, laterally disposed insulating channels  59  which are positioned between and define row electrodes.  61 ,  62 ,  63 ,  64 ,  65 ,  66 . 
     The function of piezoresistive sheet  52  of sensor array  32  is to form a conductive path between column and row electrodes, e.g., left-hand column electrode  50  and rear row electrode  61 , 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  32 , piezoresistive sheet  52  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, 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 as 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 Power: 
     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  52  is submerged in the piezoresistive coating mixture. Excess material is rolled off and the sheet is hung and allowed to air dry. 
       FIG. 6  illustrates calculation of a minimum spacing S between adjacent air bladder cells  22 , and a minimum width of non-conductive strip  49  between adjacent conductors of sensor array  32 . 
     Referring to  FIG. 6 , as a patient sinks into a deflating bladder  22 , the upper force sensor layer  33  is drawn down and away from the bladder over which it was initially positioned. If the non-conductive strip  49  is too narrow, there is a possibility that the conductive portion will overlay the deflating bladder 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  49  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 (C1 and C2) as shown in  FIG. 6 , the width S of non-conductive strip  49  should be made equal to or greater than (C1+C2−the width of the bladder) to prevent forces being misread as coming from a neighboring cell. 
       FIG. 7  illustrates the electrical resistance of a one-inch square force sensor  33  using a piezoresistive sheet  52  having the formulation listed for example I above, and fabricated as described above, as a function of normal force or pressure exerted on the upper surface of cover sheet  45  of sensor array  32 . As shown in  FIG. 7 , the resistance varies inversely as a function of normal force. 
     As shown in  FIGS. 1 and 5 , left and right column electrodes  50  and  51 , in vertical alignment with row electrodes  61 ,  62 ,  63 ,  63 ,  65 ,  66 , of 12 form with piezoresistive layer sheet  52  between the column and row electrodes a 2×6 rectangular matrix array of 12 force sensors  33 . 
     Optionally, the upper and lower electrodes for each sensor  33  could be segmented into electrically isolated rectangular pads by etching channels  49 ,  59  through both upper conductive sheet  46  and lower conductive sheet  56 . This arrangement would require a separate pair of lead-out conductors for each of the 12 sensors, i.e., a total of 24 leads. 
     Preferably, as shown in  FIGS. 1 and 5 , sensor array is arranged into rows and columns, thus requiring only 8 lead-out conductors. However, as shown in  FIG. 10A , if matrix addressing of sensor array  32  is used to measure the resistance of individual sensors  33  to thereby determine normal forces exerted on the sensors, there is a substantial cross-talk between the resistance on an addressed sensor  33  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  33  to give them a diode-like characteristic. As may be confirmed by referring to  FIG. 10B , the cross-talk between sensors  33  which have a non-bilateral, polarity-sensitive transfer function, mitigates the cross-talk problem present in the matrix of symmetrically conductive sensors  33  shown in  FIG. 10A . 
     Sensors  33  are modified to have a diode-like characteristic by modifying the preparation of piezoresistive layer sheet  52 , as follows: First, a piezoresistive layer sheet  52  is prepared by the process described above. Then, either the upper surface  69  or the lower surface  70  of the piezoresistive coating  67  of Piezoresistive sheet  52  is modified to form thereon a P-N, semiconductor-type junction. 
     Modification of piezoresistive coating  67  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  69  or lower surface  70  of piezoresistive coating  68  on piezoresistive sheet  52 . 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  68  functions as an N-type semiconductor of P-N junction diode. 
       FIG. 8  illustrates a sensor  33  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. 9  shows a typical I-V curve for sensor  33  of  FIG. 8 . 
     As stated above, the advantage of modifying sensors  33  by adding a semi-conductive layer that acts like a diode is that it reduces cross talk between sensors. As is shown in  FIG. 10A , 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. 10A , 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. 10A , which in turn would result in the following incorrect value of resistance:
 
 R   x1   Y   1   =R   11 //( R   12   +[R   22   ]+R   21 ) 1   R   x1   Y   1   =R   11 ( R   12   +[R   22   ]+R   21 )/( R   11   +R   12   +[R   22   ]+R   21 ) 1  
 
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  33  were each lower, e.g., 1 ohm, because of greater forces concentrated on those sensors  33 , 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  33 , as shown in  FIG. 10B , 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. 10A and 10B . 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   x1y1 =10(1+∞+1)/(10+1+∞+1)=10 ohms, the correct value.
 
     Thus, modifying each sensor  33  element 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  33 . This enables the correct value of electrical resistance of each sensor element  33  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. 
     The above-described components of force minimization apparatus  20  according to the present invention 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 of the present invention. An understanding of this method and apparatus may be facilitated by referring to  FIG. 11 , which is a block diagram of an electro-pneumatic controller system components  20 A of apparatus  20 , in conjunction with the diagrammatic view of the apparatus shown in  FIG. 1 , and the perspective view shown in  FIG. 5 . 
     Referring to  FIG. 11 , it may be seen that electro-pneumatic controller apparatus  20 A includes a computer  37  which is bidirectionally coupled to force sensor array  32  through force sensor interface module  36 . The sensor interface module  36  includes a Digital-to-Analog Converter (DAC)  71  for generating in response to control signals from computer  37  test voltages or currents which are directed to matrix-addressed individual force sensors  33 . 
     Individual force sensors  33  are addressed by connecting one terminal of a current or voltage source controlled by DAC  71  to a selected one of X-row conductors 1-6 by an X multiplexer  72 , and connecting the other terminal of the source to a selected one of Y-column conductors 1 or 2 by a Y multiplexer  73 . Sensor interface module  37  also included an Analog-to-Digital Converter (ADC)  74  which measures the voltage drop or current through a sensor  33  resulting from application of a test current or voltage, and inputs the measured value to computer  37 . Using predetermined scale factors, computer  37  calculates the instantaneous value of electrical resistance of a-selected addressed sensor  33 , and from that resistance value, a corresponding normal force instantaneously exerted on the addressed sensor. 
     In response to control signals cyclically issued by computer  37 , X multiplexer  72  and Y multiplexer  73  are used to cyclically measure the resistance of each force sensor element  33 , at a relatively rapid rate of, for example, 3,000 samples per second, enabling computer  37  to calculate the force exerted on each force sensor  33  at that sampling rate. 
     Referring still to  FIG. 11 , apparatus  20  includes a pressure control module  75  for dynamically controlling the air pressure in each individual air bladder cell  22 , in response to command signals issued by computer  37 , based ‘upon values of force measured by sensor array  32  and an algorithm programmed in the computer. As shown in  FIG. 11 , pressure control module  75  is operably interconnected to air compressor  40  and air pressure transducer  44  at output port  76  of the compressor to pressurize air in the outlet port to a value controllable by computer  37 . 
     Outlet port  76  of compressor  40  is coupled to inlet port  42  of a 12-outlet port manifold  41 . In response to electrical control signals issued by computer  37  and routed through pressure control module  75 , each of 12 individual air bladder cell inlet selector valves  43  connected to separate outlet ports  43 A of manifold  41  is individually controllable. 
     In a first, open position of a selector valve  43 , the air inlet port  31  of a selected air bladder cell  22  is pressurized to a pressure measured by transducer  44  to a predetermined value, by turning on compressor  40 , to thereby inflate the cell to a desired pressure. Alternatively, with compressor  40  in an off-mode, a vent valve  77  coupled to the input port  42  of manifold  41  may be opened to deflate an air bladder cell  22  to a lower pressure value by exhausting air to the atmosphere. 
     After a selected one of the 12 selector valves  43  has been opened in response to a command signal from computer  37  for a time period sufficient to inflate a selected air bladder cell  22  to a predetermined pressure, an electrical signal output by pressure transducer  44 , which is proportional to the pressure in that cell and input to computer  37 , results in the computer outputting a closure command signal to the valve and a shut-off command signal to compressor  40 . 
     When a selected selector valve  43  and vent valve  77  have been opened in response to command signals from computer  37  to deflate a selected air bladder cell  22  to a lower predetermined pressure, an electrical signal from pressure transducer  44  input to computer  37  results in an electrical closure command signal being output from the computer. That command signal closes vent valve  77  and the open selector valve  43 , 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  22  is sequentially adjustable by sending a command signal to a selector valve  43  to open that valve, and operating compressor  40  and/or vent valve  77  to inflate or deflate the air bladder cell to a predetermined pressure. 
       FIG. 12  is a simplified perspective view of a preferred embodiment of an enclosure for electro-pneumatic apparatus  20 A shown in  FIG. 11  and described above. As shown in  FIGS. 11 and 12 , electro-pneumatic controller  20 A includes an operator interface module  78 . Operator interface module includes manual controls, including a multi-function, on/off, mode control switch and button  79 , up and down data entry slewing buttons  80 ,  81 , and a digital display  82 . Display  82  is controllable by switch  99  to selectively display air pressure within and force on selectable air bladder cells  22 , and the sum and average of all forces exerted on sensors  33 . 
     As shown in  FIG. 12 , electro-pneumatic controller  20 A is preferably contained in a box-like enclosure  83  which has protruding from a rear panel  84  thereof an L-bracket  85  for suspending the enclosure from a side board or end board of a bed. Enclosure  83  of electro-pneumatic controller  20 A also includes a tubular member  86  for interfacing air hoses  87  with air bladder cells  22 , row and column conductors  88 ,  89 , to sensors  33  of sensor array  32 , and an electrical power cord  90  to a source of electrical power for powering the components of apparatus  20 A. 
     Force Minimization Algorithm 
     The force minimization apparatus described above is made up of a multiplicity of air bladder cells  22 . Each cell  22  has on its upper surface a separate force sensor  33 . An air pressure transducer  44  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  21  and the air bladder cell. Each piezoresistive force sensor  33  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  22 , the maximum force corresponding to the lowest resistance path across any part of each sensor. 
     As shown in  FIG. 3 , each air bladder cell  22  supports a different longitudinal zone of the user such as the head, hips or heels. The compressor  40  and selector valves  43  controlling the air pressure in each zone are controlled by force sensors  33  and pressure measurements made by pressure transducer  44 , using a novel algorithm implemented in computer  37 . 
     There can be a minimum of one zone using one air bladder cell  33 , and up to N zones using n air bladder cells, wherein each zone has a force sensor  33  to measure the maximum force on that air bladder cell, the pressure transducer  44  being used to measure the air pressure in that air bladder cell. The control algorithm is one of continuous iteration wherein the force sensors  33  determine the peak force on the patient&#39;s body, and the pressure transducer  44  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  21 . 
     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. Set: Pset, start, close vent valve 
     1A. Set: i=1 
     2. Select zone i by opening selector valve i 
     3. Turn the compressor on. 
     4. Measure the air pressure in the air bladder cell in zone i 
     5. Pressurize the zone i air bladder cell to Pset 
     6. Increment i by 1 and repeat steps 2-5 until i=N 
     7. Set: i=1 and select zone i 
     8. Obtain the force sensor readings for all zones. 
     9. Open Vent valve. 
     10. Deflate the zone i 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 Pset. 
     11. Measure forces on all air bladder cells as the single, zone i 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 i 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 i 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 i selector valve. Maintain the pressure in zone i 
     15. Increment i by 1 
     16. Repeat steps 8 thru 15 until i=N. 
     17. Reduce Pset. 
     18. Repeat Steps 1A thru 16 (i.e., with a reduced Pset). 
     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. 13  is a flow chart showing the operation of apparatus  20  utilizing the algorithm described above. Table 1 lists appropriate lower and upper initial set pressures for bladders  22 , as a function of the weight of a patient or other person supported by overlay cushion  21  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  
                 mm Hg 
                 12.18 ± 2  
                 mm Hg 
               
               
                 120-164 Pounds 
                 6″ ± 0.7:  
                 H 2 O 
                 8″ ± 0.7: 
                 H 2 O 
               
               
                   
                 11.25 ± 2  
                 mm Hg 
                 15 ± 2  
                 mm Hg 
               
               
                 165-199 Pounds 
                 8″ ± 0.7:  
                 H 2 O 
                 10″ ± 0.7:  
                 H 2 O 
               
               
                   
                 15 ± 2  
                 mm Hg 
                 18.75 ± 2  
                 mm Hg 
               
               
                 200-250 Pounds 
                 10 ± 0.7:  
                 H 2 O 
                 12″ ± 0.7:  
                 H 2 O 
               
               
                   
                 18.75 ± 2  
                 mm Hg 
                 22.49 ± 2  
                 mm Hg 
               
             
          
           
               
                 Maximum Pressure 
                   
                 26″ ± 0.7:  
                 H 2 O 
               
               
                   
                   
                 48.74 ± 4  
                 mm Hg 
               
               
                   
               
             
          
         
       
     
     In a variation of the method and apparatus according to the present invention and described above, after the pressures in each air bladder cell have been optimized for minimum force concentration, inlet tubes  31  could be permanently sealed, and the adaptive cushion  21  permanently disconnected from pressure control module  75 . This variation would also enable the custom fabrication of cushions  21  using air bladder cells  22 , for customizing chair cushions to minimize force concentrations on a particular individual. Similarly, the variation of the method and apparatus according to the present invention could be used to customize saddle cushions or car seats.