Abstract:
A flexible force or pressure sensing mat includes a first sheet of electrically conductive first paths, a second sheet of electrically conductive second paths, and a sensing layer positioned between the first and second sheets. The first and second conductive paths are oriented transversely to each other, and the locations of their intersections define individual sensing areas or sensors. The sensing layer is made from materials that have first and second electrical characteristics—such as capacitance and resistance—that vary in response to physical forces exerted thereon. A controller repetitively measures the multiple electrical characteristics of each sensor in order to produce a near real time pressure distribution map of the forces sensed by the mat. The mat can be used on a patient support surface—such as a bed, cot, stretcher, recliner, operating table, etc.—to monitor and help reduce the likelihood of a patient developing pressure ulcers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 13/631,981 filed Sep. 29, 2012 by applicants Geoffrey Taylor et al., entitled FLEXIBLE PIEZOCAPACITIVE AND PIEZORESISTIVE FORCE AND PRESSURE SENSORS. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to sensors for measuring forces or pressures exerted thereon, and more particularly to a sensing mat that can be used to monitor interface pressures between a person and a surface. 
         [0003]    Force sensing mats may be used to detect interface pressures between a patient and a surface on which he or she is lying or sitting. In a healthcare setting, the surface may be the top surface of a patient support device—such as a hospital bed, stretcher, cot, chair, or the like—or it may be another type of surface. Knowing these interface pressures can be useful for helping to prevent and/or treat pressure sores, as well as for other purposes. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides a flexible, dual sensor force detecting mat or array that is adapted to detect interface forces exerted between a person and a support surface, as well as between any other forces that may be exerted on the force detecting mat or array. The mat or array utilizes a combination of at least two different sensing technologies that, in combination, provide more information that a single sensing technology. Such additional information can be useful for a variety of purposes, such as, but not limited to, improving the dynamic range of the forces that are able to be sensed by the sensing mat. In one embodiment, the two different sensing technologies include piezoresistive sensors and piezocapacitive sensors. 
         [0005]    According to a first embodiment, a flexible force sensing mat is provided that includes a first sheet, a second sheet, a layer of sensing material, and a controller. The first sheet has a plurality of first conductive paths supported thereon and the second sheet has a plurality of second conductive paths supported thereon. The layer of sensing material is positioned in contact with, and between, the conductive paths on the first and second sheets. The layer of sensing material has first and second electrical characteristics that vary in response to physical forces exerted thereon. The second plurality of conductive paths on the second sheet are oriented transverse to the plurality of first conductive paths on the first sheet. The controller is adapted to detect changes in both the first and second electrical characteristics when force is applied to the force sensing mat. 
         [0006]    According to a second embodiment, a flexible force sensing mat is provided that includes first, second, and third sheets, and first and second layers of sensing material. The first sheet includes a plurality of first conductive paths supported thereon. The first layer of sensing material is positioned in contact with the first conductive paths and has a first electrical characteristic that varies in response to physical forces exerted thereon. The second sheet includes a plurality of second conductive paths supported thereon. The second sheet is positioned in contact with the first layer of sensing material on a side of the layer of sensing material opposite the first sheet. The second layer of sensing material is in contact with the plurality of second conductive paths and has a second electrical characteristic that varies in response to physical forces exerted thereon. The second electrical characteristic is different from the first electrical characteristic. The third sheet includes a plurality of third conductive paths supported thereon and the third sheet is positioned in contact with the second layer of sensing material on a side of the second layer of sensing material opposite the second sheet. 
         [0007]    According to other embodiments, the first sheet, second sheet, and layer or layers of sensing material are elastically stretchable in at least two co-planar and orthogonal directions. The first, second, and/or third sheets may be made of nylon. The conductive paths may be defined by metal plated to the respective sheets. 
         [0008]    In some embodiments, the first electrical characteristic is capacitance and the second electrical characteristic is resistance. The detection of the first and second electrical characteristics may be accomplished by feeding first and second signals to the conductive paths wherein the first and second signals have different frequencies. 
         [0009]    In the embodiments where a single layer of sensing material has both the first and second electrical characteristics, the sensing material may include carbon black and glycerin mixed together. The carbon black and glycerin may be supported in a foam pad positioned between the first and second sheets. The glycerin acts as a liquid dielectric which holds in suspension the carbon black, which acts as a piezoresistive substance. 
         [0010]    Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited to the details of operation or to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention may be implemented in various other embodiments and is capable of being practiced or being carried out in alternative ways not expressly disclosed herein. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. Further, enumeration may be used in the description of various embodiments. Unless otherwise expressly stated, the use of enumeration should not be construed as limiting the invention to any specific order or number of components. Nor should the use of enumeration be construed as excluding from the scope of the invention any additional steps or components that might be combined with or into the enumerated steps or components. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is an exploded perspective view of a first embodiment of a force sensor having a piezocapacitive layer according to one aspect of the invention; 
           [0012]      FIG. 2  is a vertical sectional view of the sensor of  FIG. 1 ; 
           [0013]      FIG. 3  is an exploded perspective view of a second embodiment of a force sensor in which a central foam dielectric pad thereof is perforated; 
           [0014]      FIG. 4  is a vertical sectional view of the sensor of  FIG. 3 ; 
           [0015]      FIG. 5  is an exploded perspective view of a third embodiment of a force sensor having a piezocapacitive layer and a separate piezoresistive layer; 
           [0016]      FIG. 6  is a vertical sectional view of the sensor of  FIG. 5 ; 
           [0017]      FIG. 7  is an exploded perspective view of a fourth embodiment of a force sensor having a combined layer of piezocapacitive and piezoresistive material; 
           [0018]      FIG. 8  is a vertical sectional view of the sensor of  FIG. 7 ; 
           [0019]      FIG. 9  is a schematic diagram of an apparatus useable both for determining transfer functions of the sensors shown in  FIGS. 1-8 , and for measuring pressure exerted on the sensors; 
           [0020]      FIG. 10A  is a graph showing capacitance versus applied pressure for the first sensor embodiment of  FIGS. 1 and 2 ; 
           [0021]      FIG. 10B  is a graph showing capacitance of the first sensor embodiment of  FIGS. 1 and 2  plotted as a function of increasing and decreasing pressure applied to the sensor, using the test circuitry shown in  FIG. 9 ; 
           [0022]      FIG. 11A  is a graph showing capacitance versus applied pressure for the second sensor embodiment of  FIGS. 3 and 4 ; 
           [0023]      FIG. 11B  is a graph showing capacitance versus increasing and decreasing pressures for the second sensor embodiment of  FIGS. 3 and 4 , using the test circuitry shown in  FIG. 9 ; 
           [0024]      FIG. 12  is a graph showing capacitance versus applied pressure for a first variation of the second sensor embodiment of  FIGS. 3 and 4 , filled with glycerin; 
           [0025]      FIG. 13  is a graph showing capacitance versus pressure for a second variation of the second sensor embodiment of  FIGS. 3 and 4 , filled with both glycerin and iodine; 
           [0026]      FIG. 14  is a graph showing capacitance versus applied pressure for the third sensor embodiment of  FIGS. 5 and 6 ; 
           [0027]      FIG. 15  is a graph showing conductance versus applied pressure for the third sensor embodiment of  FIGS. 5 and 6 ; 
           [0028]      FIG. 16  is a graph showing capacitance versus applied pressure for the fourth sensor embodiment of  FIGS. 7 and 8 ; 
           [0029]      FIG. 17  is a graph showing conductance versus applied pressure for the fourth sensor embodiment of  FIGS. 7 and 8 ; 
           [0030]      FIG. 18A  is a graph showing susceptance plotted as a function of applied pressure for the fourth sensor embodiment of  FIGS. 7 and 8 ; 
           [0031]      FIG. 18B  is a graph showing conductance plotted as a function of applied pressure for the fourth sensor embodiment of  FIGS. 7 and 8 ; 
           [0032]      FIG. 19  is a variation of  FIG. 18 , in which the product of conductance and capacitance for the fourth sensor embodiment of  FIGS. 7 and 8  is plotted; 
           [0033]      FIG. 20  is a graph showing a plot of capacitance versus applied pressure for a modified configuration of the third sensor embodiment of  FIGS. 5 and 6 ; 
           [0034]      FIG. 21  is a graph similar to that of  FIG. 20 , but with the sensors of  FIGS. 5 and 6  modified by insertion of a 10,000 ohm resistor in series with the piezoresistive layer of the sensor; 
           [0035]      FIG. 22  is an exploded view of a modification of the fourth sensor embodiment of  FIG. 7 ; 
           [0036]      FIG. 23  is a vertical sectional view of the sensor of  FIG. 22 ; 
           [0037]      FIG. 24  is a graph showing capacitance versus applied pressure for the sensor of  FIGS. 22 and 23 , measured at 30KHz; 
           [0038]      FIG. 25  is an expanded scale version of  FIG. 24  showing capacitance versus pressure for a smaller range of pressures; 
           [0039]      FIG. 26  is a plot of conductance versus increasing and decreasing pressure on the modified fourth sensor embodiment of  FIGS. 22 and 23 ; 
           [0040]      FIG. 27  is a plot of the multiplicative product of conductance and capacitance versus increasing and decreasing pressures on the modified fourth sensor embodiment of  FIGS. 22 and 23 ; 
           [0041]      FIG. 28  is an exploded perspective view of one embodiment of a pressure sensing mat; and 
           [0042]      FIG. 29  is a plan view of the pressure sensing mat of  FIG. 28  shown with a top cover and upper conductive layer removed. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0043]    According to various aspects of the present invention, an individual pressure sensor, or an array of pressure sensors incorporated into a mat, are provided that can be used to measure forces or pressures exerted on individual areas of an object, such as a human body supported by a bed, a chair, a cot, a stretcher, an operating table, or another object. In some embodiments, the pressure sensors, or pressure sensing mats, are designed such that the electrical capacitance of the sensor varies in a repeatable fashion as function of force or pressure applied to the sensor, a property which is referred to as piezocapacitance. This property enables the sensors to measure force or pressure exerted on the sensor by applying an alternating voltage or current to terminals of the sensor and measuring the output current or voltage of the sensor, which varies with force or pressure according to a transfer function that has been previously obtained for the sensor by a calibration procedure in which the impedance of the sensor is measured and recorded for a sequence of known calibrating forces or pressures applied to the sensor. 
         [0044]    When the individual sensors are configured into an array of individual sensors, a pressure map can be easily obtained that graphically, or numerically, defines the distribution of pressures exerted on a human body by the object on which the body is supported. 
         [0045]    The force sensors, in some embodiments, include both a piezoresistive characteristic and a piezocapacitive characteristic, thus resulting in sensors in which both the DC conductance, as well as the electrical capacitance, vary as a function of applied forces or pressures. The combined variation of conductance and capacitance affords increased versatility to the hybrid sensors, including wider dynamic ranges. 
       EXAMPLE 1 
       [0046]    Referring first to  FIGS. 1 and 2 , a first embodiment of a piezocapacitive pressure sensor  50  according to one aspect of the present invention is shown that includes a flexible conductive sheet  51  which functions as the first conductive plate of a capacitor. Base conductive sheet  51  is made of a thin, flexible, elastically stretchable fabric which is electrically conductive. In an example embodiment of sensor  50 , base conductive sheet consists of a two inch square piece of a elastically stretchable woven electrical conductive fabric made of silver plated nylon threads, having a thickness of 0.4 mm, a weight per unit/area of 4.3 oz. per square yard, and a surface resistance of about 0.5 ohms per square. Such fabric is available from LESS EMF Corporation, 809 Madison Ave., Albany, N.Y. 12208 as catalog or part number A321. 
         [0047]    As shown in  FIG. 1 , base conductor sheet  51  has a front laterally disposed edge  52 , a parallel rear laterally disposed edge  53 , and left and right parallel fore-and-aft disposed edges  54 ,  55 . Base conductor sheet  51  has an integral rectangular-shaped conductive fabric connector tab  56  which is coplanar with and protrudes perpendicularly outwards from a corner part of the square conductor sheet. Thus, as shown in  FIG. 1 , base connector tab  56  has a front laterally disposed edge  57  which is a collinear extension of front edge  52  of base conductor sheet  51 , an outer fore-and-aft disposed edge  58  parallel to and offset laterally to the right of right-hand edge  55  of the base conductor sheet  51 , and a laterally inwardly extending rear edge  59  which is parallel to front edge  57 . 
         [0048]    Sensor  50  further includes a dielectric pad or core  60  which is supported on the upper surface  61  of base conductor sheet  51  ( FIGS. 1 and 2 ). As shown in the figures, dielectric pad  60  has approximately the same outline shape and size, e.g., a two inch square, as base conductor sheet  51 , so that the dielectric pad  60  seats congruently of the base conductor sheet. In an example embodiment of sensor  50 , dielectric pad  60  is made of a 2-inch square piece of elastically deformable polyurethane open-cell foam having a thickness of about 0.025 inch. The dielectric pad  60  is cut from a piece of open cell polyurethane foam having a density of about 28.52 kg/cubic meter, obtainable from Burnett and Company, Foam Division, 2112 Montevideo Road, Jussea, Md. 20604, stock number SBZJJ. The dielectric pad  60  has a dielectric constant or relative permittivity of about 4. 
         [0049]    Referring to  FIG. 1 , it may be seen that piezocapacitive sensor  50  has an upper flexible conductive sheet  71  which functions as the second plate of a capacitor. Outer flexible conductive sheet  71  may be substantially identical in construction to base conductive sheet  51 . However, as shown in  FIG. 1 , upper flexible conductive sheet  71  is flipped over and rotated 90 degrees relative to base conductive sheet, so that a connector tab  76  of the upper conductive sheet  71  extends forward from the front edge of sensor  50 , so that it does not overlie the rightwardly extending connector tab  56  of base conductive sheet  51 . 
         [0050]    As shown in  FIGS. 1 and 2 , upper conductive sheet  71  has a lower flat surface  77  which contacts upper flat surface  78  of dielectric pad  60 , and dielectric pad  60  has a lower surface  79  which contacts upper surface  61  of base conductive sheet  51 . As shown in  FIG. 2 , the sandwiched relationship between base conductive sheet  51 , dielectric pad  60 , and upper conductive sheet  71  is maintained by encapsulating these three elements in a flat flexible envelope  80 , thereby restraining these elements from relative movement with respect to each other. 
         [0051]    As shown in  FIGS. 1 and 2 , encapsulating envelope  80  includes upper and lower flexible liquid impervious polyurethane cover sheets  81 ,  82  made of 2-mil thick polyurethane film. Upper and lower encapsulating polyurethane sheets  81 ,  82  have a square shape, and are slightly larger in area than upper conductive sheet  71 , dielectric pad  60  and base conductive sheet  51 . This size relationship enables front, rear, left and right outer peripheral edges  91 ,  92 ,  93 ,  94  of upper encapsulating cover sheet  81  to be sealingly joined by adhesive, ultrasonic bonding, or other suitable techniques to corresponding outer peripheral edges  101 ,  102 ,  103 ,  104  of lower encapsulating cover sheet  82 . 
         [0052]      FIG. 9  is a schematic diagram of a pressure measurement apparatus  110  according to another aspect of the present invention, showing how the sensor of  FIGS. 1 and 2  can be connected to test circuitry to measure how its capacitance, conductance, or admittance changes in response to external pressure exerted on the sensor. From these measurements the transfer function of the sensor may be plotted. The apparatus of  FIG. 9  is also useable to measure forces or pressures exerted on sensors  50  for which the impedance-versus-force transfer function has been previously determined. 
         [0053]    As shown in  FIG. 9 , apparatus  110  includes a selectable frequency signal generator  111  which outputs a sinusoidal current that is adjustable to a selectable voltage V1, measured by a voltmeter  112 . Signal generator  111  has connected in series with output terminal  113  thereof a variable voltage DC power supply  114  which outputs a voltage selectable between zero and a predetermined maximum value. The output terminal  115  of DC power supply  114  is connected to one terminal, e.g., upper connector tab  76  of sensor  50 . The opposite terminal, e.g. lower connector tab  56  of the sensor  50  is connected through a current sampling resistor  116  to the lower or ground output terminal  117  of signal generator  111 . 
         [0054]    Current flowing through sensor  50  in response to a DC voltage, an AC voltage, or a combination of both AC and DC voltages applied to terminals  76 - 56  of the sensor, is measured by measuring the voltage drop V2 across resistor  116  using a voltmeter  118 . Thus the DC conductance of sensor  50  may be measured by applying a DC voltage or low-frequency AC signal to the sensor. The AC conductance or susceptance, which is proportional to capacitance, may be measured by applying a higher frequency test voltage to the terminals of sensor  50 , or, alternatively, by substituting a capacitance meter or bridge for the signal generator  111 . 
         [0055]      FIG. 10A  is a graph showing the variation of capacitance of sensor  50  of  FIGS. 1 and 2  as a function of increasing applied pressure, as measured by a capacitance meter. 
         [0056]    Referring to  FIG. 9 , the total impedance seen by signal generator  111  is: 
         [0000]        Z 1= R 1 2   +Zs   2    
         [0000]      where 
         [0000]    
       
      
       Zs=Rs 
       2 
       +Xs 
       2  
      
     
         [0057]    and Rs is the resistive component of sensor impedance Zs, and Xs is the capacitive reactive component of sensor impedance Zs. In other words, Xs=½πrfCs where f is the signal generator frequency and Cs is the capacitance of the sensor. The current Is through sampling resistor R 1  and sensor  50  is: 
         [0058]    Is=V1/Z1, and for R 1  selected to be much smaller than Zs, Is=V1/Zs. 
         [0059]    For the capacitance component of sensor impedance, Zs, Is=V1×Bs, where Bs is the susceptance of the capacitive component of the sensor, 
         [0060]    Bs=2πfCs. 
         [0061]    For the resistive component of sensor impedance Zs, Is=V1/Gs, where Gs is the DC conductance of the sensor. 
         [0062]    Referring to  FIG. 9 , 
         [0063]    V2=Is R 1 =V1(2π)fCsR 1  for the capacitance component of a sensor, and 
         [0064]    V2=IsR 1 =V1GsR 1  for the resistive component. Thus, the capacitance of a sensor may be calculated from the equation: Cs=(V2/V1)(2πrfR 1 ), or 
         [0065]    Cs=k1(V2/V1); for f=30 KHz and R 1 =1,000 ohm, 
         [0066]    k1=5.305×10 −9  farads=5.305 nanofarads, and for V1=9 volts, Cs=kc×V2=0.5895 of/volt. 
         [0067]    For the resistance component of sensor  50 , Gs=V2/V1, R 1 =1,000 ohms, V1=9 volts; 
         [0068]    Gs=kg (V2)=0.1111 millimhos/volt. 
         [0069]      FIG. 10B  is a graph which plots the transfer function of the sensor  50  of  FIGS. 1 and 2 . 
       EXAMPLE 2 
     Perforated Pad 
       [0070]      FIGS. 3 and 4  illustrate a modified sensor  120  according to another embodiment that has been modified from the sensor  50  shown in  FIGS. 1 and 2 . Modified sensor  120  is substantially similar in construction and function to sensor  50 , with the primary difference being that the central dielectric pad  130  of sensor  120  contains perforations. In an example embodiment of sensor  120 , central dielectric pad  130  has an array of circular holes  131  defined through the thickness dimension of the pad and spread over the entire area of pad  130 . Each hole has a diameter of ½ inch and is spaced apart by ¼ inch from adjacent holes. The holes  131  occupy about fifty percent of the surface area of the pads. 
         [0071]      FIG. 11  A is a plot of capacitance versus applied pressure for sensor  120  shown in  FIGS. 3 and 4 . 
         [0072]      FIG. 11  B shows the variation of voltage ratios V2/V1 proportional to capacitance of sensor  120  as a function of increasing (up) and decreasing (down) external pressure exerted on the sensor, i.e., the graphical representation of the transfer function of the sensor. 
       EXAMPLE 3  
     Perforated Pad Saturated with Glycerin 
       [0073]      FIG. 12A  shows the variation of capacitance versus external pressure for a first variation  120 A of the sensor  120  (Example 3) of  FIGS. 3 and 4 , in which the central perforated dielectric pad  130  thereof has a weight of about 1 gram and is saturated with 2 grams of glycerin. 
       EXAMPLE 4  
     Perforated Pad Saturated with Glycerin Doped with Iodine 
       [0074]      FIG. 13A  shows the variation of capacitance versus external force or pressure for a second variation  120   b  of sensor  120  (Example 4) shown in  FIGS. 3 and 4 , in which the central perforated dielectric pad  130  thereof is saturated with 1 gram of glycerin doped with 1 gram of a 2.5% solution of iodine in isopropyl alcohol. 
       EXAMPLE 5  
     Hybrid Piezocapacitive-Piezoresistive 
       [0075]      FIGS. 5 and 6  illustrate an embodiment of a piezocapacitive-piezoresistive sensor  240  according to another aspect of the present invention. Sensor  240  has separate pressure sensing layers. 
         [0076]    As shown in  FIGS. 5 and 6 , hybrid or composite piezocapacitive-piezoresistive sensor  240  includes a first pressure sensing layer consisting of a piezocapacitive section  250  which is substantially identical in construction and function to the modified sensor  120  shown in  FIGS. 3 and 4 . Piezocapacitance section  250  is similar to the first variation  120 A of sensor  120  described above in which a central perforated foam dielectric pad  260  thereof is saturated with 2 grams of glycerin. 
         [0077]    Referring to  FIGS. 5 and 6 , it may be seen that hybrid sensor  240  includes a second pressure sensing layer consisting of a piezoresistive section  280  which is positioned below piezocapacitive sensor section  250 . However, the location of piezoresistive section  280  relative to piezocapacitive section  250  is not critical, and may optionally be positioned above the piezocapacitive section. 
         [0078]    As shown in  FIGS. 5 and 6 , piezoresistive section  280  of hybrid sensor  240  has a laminated construction which is similar to that of piezocapacitive section  250 . Thus, piezoresistive section  280  has a base conductive sheet  291  which consists of a thin, square sheet of conductive stretchy fabric which is substantially identical to upper conductive sheet  71  of sensor  50 , and upper conductive sheet  271  of piezocapacitive section  250  of hybrid sensor  240 . Piezoresistive section  280  includes an upper conductive sheet  311  which is substantially identical to base conductive sheet  251  of piezocapacitive sensor  250 . Upper conductive sheet  311  of piezoresistive section  280  and base conductive sheet  251  of piezocapacitive section  250  comprise a single element  251 - 311 . 
         [0079]    Referring still to  FIGS. 5 and 6 , it may be seen that piezoresistive section  290  of hybrid sensor  240  includes a piezoresistive pad or core  390  which is supported on the upper surface  301  of base conductor sheet  291 . As shown in the figures, piezoresistive pad  390  has the same outline shape and size, e.g., a two inch square, as base conductor sheet  291 . In an example embodiment of sensor  240 , piezoresistive pad  390  consists of a two inch square piece of type S8ZJJ polyurethane foam having a thickness of about 0.025 inch. The pad  390  is cut from a piece of unperforated foam which is impregnated with 2 grams of carbon lamp black having a particle size range of about 20 nm to about 40 nm. 
         [0080]      FIG. 14  show the variation of voltage ratios V2/V1 measured at 30 KHz, proportional to admittance and hence capacitance, for increasing and decreasing pressure exerted on piezocapacitive section  150  of hybrid sensor  240 , Example 5. 
         [0081]      FIG. 15  show the variation of conductance measured at 10 KHz versus external pressure exerted on the piezocapacitive section  250  and piezoresistive section  280  of hybrid sensor  240 , Example 5, measured for increasing and decreasing pressures. 
       EXAMPLE 6  
     Leaky Dielectric 
       [0082]      FIGS. 7 and 8  illustrate another embodiment  350  of a hybrid pressure sensor according to an aspect of the present invention. The embodiment  350  shown in  FIGS. 7 and 8  is structurally similar to the embodiment  120  shown in  FIGS. 3 and 4  and described above. However, embodiment  350  utilizes in place of the foam dielectric pad  130  a “leaky dielectric” pad  360 . 
         [0083]    In an example embodiment of the leaky dielectric piezocapacitive-piezoresistive pressure sensor  350 , upper conductive sheet  341  and lower conductive sheet  321  are substantially identical to upper and lower conductive sheets  141 ,  121 , respectively, of the embodiment  120  of a piezocapacitive sensor shown in  FIGS. 3 and 4  and described above. The central dielectric pad  360  of sensor  350  has a composition and construction similar to that of un-perforated open-cell dielectric pad  60  of example 1 made of polyurethane foam shown in  FIGS. 1 and 2 . However, central dielectric pad  360  is treated to give it a piezoresistive characteristic in addition to a piezocapacitive characteristic by thoroughly mixing carbon black particles, of the type described above for example 5, with glycerin, and kneading the mixture of glycerin and carbon black particles into the foam pad. 
         [0084]      FIG. 16  shows the variation of capacitance of leaky dielectric sensor  350  as a function of external pressure exerted on the sensor, measured at a relatively high frequency of 30KHz and thus displaying the capacitive part of the sensor transfer function 
         [0085]      FIG. 17  shows the variation of the conductance of leaky dielectric sensor  350  as a function of external pressure exerted on the sensor, measured at a relatively low frequency of 3 Hz and thus displaying the resistive part of the sensor transfer function. 
         [0086]      FIG. 18  shows the variation of both capacitance and conductance of the leaky dielectric sensor  350  as a function of external pressure exerted on the sensor. 
         [0087]      FIG. 19  shows the variation of the product of susceptance and conductance versus pressure transfer functions of the leaky dielectric sensor  350  as a function of external pressure exerted on the sensor. As may be seen by comparing  FIG. 19  to  FIG. 18 , the product transfer function is substantially more linear and has substantially less hysteresis than either of the individual conductance or susceptance transfer functions. 
       EXAMPLE 7  
     Modification of Example 5, Hybrid Sensor with Paralleled Sections 
       [0088]      FIG. 20  shows the variation of capacitance and conductance of a variation  240 A of the sensor shown in  FIGS. 5 and 6 , in which the piezoresistive and piezocapacitive layers are electrically paralleled by connecting together their outer terminals  226 ,  296 , as shown in hybrid configuration 2 of  FIG. 9 , to configure the sensor as a two terminal device, as a function of applied pressure for test frequencies of 30KHz and 3KHz. 
       EXAMPLE 8  
     Modification of Example 7 with Series Resistance 
       [0089]      FIG. 21  shows the variation of capacitance and conductance with pressure for a modification  240 B of the parallel two-terminal sensor configuration  240 A, in which a 10,000 ohm resister is inserted in series with the sensor. As may be seen by comparing  FIG. 21  with  FIG. 20 , the voltage versus pressure transfer function with a 10,000 ohm series resistance is substantially more linear and has substantially less hysteresis than the transfer function without a series resistance. Optionally a numerical value of a resistance such as 10,000 ohms may be inserted computationally in series in place of an actual resistance. 
       EXAMPLE 9 
       [0090]      FIGS. 22 and 23  illustrate a simplified modification  450  of the leaky sensor  350  shown in  FIGS. 7 and 8  and described above, in which the outer protective envelope is eliminated. 
         [0091]      FIG. 24  is a graph showing the variation of capacitance of the simplified leaky sensor  450  as a function of increasing and decreasing pressures exerted on the sensor. 
         [0092]      FIG. 25  is an expanded scale version of  FIG. 24  showing capacitance versus pressure on sensor  450  for a smaller range of pressures. 
         [0093]      FIG. 26  is a plot of conductance versus increasing and decreasing pressures on the simplified leaky sensor  450 . 
         [0094]      FIG. 27  is a plot of the product of conductance times capacitance versus increasing and decreasing pressures on the simplified leaky sensor  450 . 
       Pressure Sensing Mat 
       [0095]      FIG. 28  shows one embodiment of a pressure sensing mat  20  according to another aspect of the present invention. Pressure sensing mat  20  is adapted for being positioned between a patient and a support surface on which a patient is positioned in order to detect the interface pressures between the patient and the patient support surface. Thus, for example, pressure sensing mat  20  may be used on the seat of a wheelchair, or on the top of a bed, stretcher, cot, operating table, or any type of furniture which a patient might lie or sit on (e.g. a recliner). When so used, mat  20  will customarily lie on top of the cushion, mattress, or other soft structure which is provided on the support surface. However, it is also possible for mat  20  to be integrated into the cushion, mattress, or other soft structure. However constructed, mat  20  detects a distribution of interface pressure between the support structure and those portions of the patient&#39;s body that are in contact with the support structure. This information can be used to help reduce any interface pressures that exceed a desired level, and thereby reduce the likelihood of bed sores developing. 
         [0096]    In one embodiment, the outputs from pressure sensing mat  20  are used to control the inflation and deflation of one or more air bladders, or other inflatable structures, that are contained within the cushion, mattress, or other soft structure. The outputs are used to adjust the fluid pressures within the bladders so as to reduce the interface pressures in those areas where the interface pressure between the patient and the bladder(s) are relatively high. This helps spread the interface forces between the patient and the support surface over a greater area, thereby reducing the interface pressures and the likelihood of developing pressure sores. One manner in which a pressures sensing mat, such as pressure sensing mat  20 , can be used to automatically adjust fluid pressures inside of an inflatable support structure is disclosed in U.S. patent application Ser. No. 12/075,937 filed on 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. 
         [0097]    As illustrated in more detail, pressure sensing mat  20  includes a top cover  22 , a bottom cover  24 , an upper conductive layer  26 , a lower conductive layer  28 , and a central sensing layer  30 . Top and bottom covers  22  and  24 , respectively, made be made of the same material as cover sheets  81  and  82 , described previously, or they may be made of other materials. In some embodiments, top cover  22  and bottom cover  24  are made from a waterproof material that is elastically stretchable. Such materials are available from Eastex Products of Holbrooke, Mass., or Dartex Coatings of Nottingham, United Kingdom. Top cover  22  and bottom cover  24  are sealed together about their periphery to thereby envelope layers  26 ,  28 , and  30 . Electrically conductive leads, however, pierce this seal in order to provide electrical communication to the conductive layers  26  and  28 , as well as the sensing layer  30 , as will be discussed in greater detail below. 
         [0098]    In one embodiment, sensing layer  30  is a leaky dielectric pad that is the same as leaky dielectric pad  360  described above. Upper conductive layer  26  includes a plurality of nonconductive columns  32  that are alternately separated by a plurality of conductive columns  34 . Lower conductive layer  28  includes a plurality of nonconductive rows  36  that are alternately separated by a plurality of conductive rows  38 . The manner in which upper and lower conductive sheets  26  and  28  are constructed is described in more detail in U.S. patent application Ser. No. 13/644,961 filed Oct. 4, 2012 by applicant Geoffrey Taylor and entitled PRESSURE SENSING MAT, the complete disclosure of which is also hereby incorporated herein by reference. When so constructed, upper and lower conductive sheets  26  and  28  are elastically stretchable and capable of carrying electrical signals along their respective conductive columns  34  and conductive rows  38 . 
         [0099]      FIG. 29  illustrates a plan view of pressure sensing mat  20  shown with top cover  22  and upper conductive layer  26  removed, wherein the electrical components and circuitry used to read and process the electrical outputs at each intersection of the conductive columns  34  with conductive rows  38 . Each such intersection defines, in essence, an individual pressure sensor. The product of the number of conductive columns  34  and conductive rows  38  therefore defines how many individual pressure sensors pressure sensing mat  20  is capable of having. When pressure is exerted by a patient on sensing mat  20 , this is detected by the change in the corresponding electrical characteristics of the sensors in the area where the pressure changed. As was described above, the magnitude of external pressures exerted on each of the sensors can be accurately determined by measuring the conductance of each sensor using an applied DC voltage or a low frequency alternating current test signal having a frequency of, for example, 1 Hz to 30 Hz, while the capacitance of each individual sensor can be measured by applying a higher frequency alternating current test voltage or current of, for example, 30KHz. Moreover, combinations of DC or low frequency test voltages or currents may be applied to each sensor simultaneously or sequentially with higher frequency test voltages or currents to determine the interface pressure exerted on the sensor. 
         [0100]    As shown in  FIG. 29 , a controller  40  carries out the electrical processing necessary to read the susceptance and conductance of each individual sensor. Controller  40  is in electrical communication with a pair of communication links  44 . Each communication link  44  communicatively couples controller  40  with a preprocessing circuit boards  46 . Such links may utilize any suitable form of communication, such as a serial connection, a parallel connection, or another type of connection. In one embodiment, the communication links  44  follow the I squared C protocol. Other protocols, such as, but not limited to, CAN, LIN, and others may be used. 
         [0101]    Each preprocessing circuit board  42  is in electrical communication with a plurality of wires or conductors  46 . Wires or conductors  46  are each in electrical communication with an individual one of conductive columns  34  or of conductive rows  38 . Controller  40  communicates with preprocessing circuit boards  42  to send signals to individual ones of the sensors defined in mat  20  and monitor the response to those signals. Controller  40  accomplishes this by picking the specific row conductor  38  and specific column conductor  34  whose intersection defines the sensor desired to be read. Controller  40  then uses the preprocessing circuit boards  42  to measure the susceptance and conductance at that chosen sensor. These readings are stored in a memory accessible to controller  40 , which may either be contained within controller  40 , or which may be in communication with controller  40  via a cable  48 . Cable  48  includes a connector  49  that enables it to be connected to an appropriate consumer of the information generated by controller  40 . In one embodiment connector  49  is a USB connector. Other types of connectors may be used. 
         [0102]    In some embodiments, as was noted previously, the consumer of the data generated by controller  40  may be a mattress. In other embodiments, connector  49  is plugged into a personal computer, laptop computer, or tablet computer, and the data generated by controller  40  is able to be stored and/or further processed by the attached computer. Regardless of the consumer, controller  40  is configured to monitor the capacitance and susceptance of each individual sensor multiple times a second. The data from the results of these measurements can be used to create a graphical display of patient interface pressures that are spatially distributed over the area of the pressure mat  20 . 
         [0103]    In one embodiment, controller  40  is enveloped within top and bottom covers  22  and  24  so that controller  40  is not visible to a user of mat  20 . Further, both circuit boards  42  may be each less than half an inch thick (such as, for example, 2 millimeters), and each may take up less than a square inch of surface area. Such dimensions help to ensure that a patient will not likely be able to feel these circuit boards within mat  20 , and thus will not be discomforted by them. This is especially true if the circuit boards  42  and controller  40  are positioned along the edges of the sensing mat  30 . Controller  40  may be positioned in a corner at a foot end of the sensing mat  20  in order to reduce the likelihood of it being felt by a patient. 
         [0104]    Still further, in some embodiments, controller  40  and preprocessing boards  42  are manufactured from flexible electronics, commonly known as flex circuits. Such flexible electronics are mounted to a flexible plastic substrate, such as, but not limited to, a polyimide, a polyether ether ketone (PEEK), or a conductive polyester film. By using flexible electronics, controller  40  and preprocessing boards  42  are able to physically bend, thereby helping to protect them against breakage and also reducing any discomfort they might otherwise cause to a patient. 
         [0105]    Controller  40  may be a conventional commercially available microcontroller, microprocessor, or other programmable device, that is programmed to carry out the functions described herein. Controller  40  includes, in some embodiments, the circuitry of  FIG. 9 , as well as any additional components necessary for reading the voltages, resistance, and other electrical characteristics described with respect to  FIG. 9 . When pressure sensing mat  20  utilizes a single leaky dielectric layer  30  that is the same as dielectric pad  360 , controller  40  is configured to implement the same functions as the test circuitry of  FIG. 9  that corresponds to the “piezocapacitive and leaky test configuration,” rather than the “piezocapacitive and piezoresistive hybrid configurations” (either #1 or #2). 
         [0106]    However, it will be understood by those skilled in the art that pressure sensing mat  20  can be modified to include a separate piezocapacitive layer and a separate piezoresistive layer. When such separate layers are included, an additional conductive layer having either conductive rows  38  or conductive columns  34  is added to the mat. Such a mat will therefore include, inside covers  22  and  24 , a bottom conductive layer (either rows or columns), a piezocapacitive layer on top of the bottom conductive layer, a middle conductive layer (either rows or columns, but opposite of the bottom conductive layer), a piezoresistive layer on top of the middle conductive layer, and a top conductive layer (either rows or columns, but the same as bottom conductive layer). Of course, the position of the piezoresistive and piezocapacitive layers can be reversed, if desired. 
         [0107]    In another alternative embodiment (not shown), instead of utilizing a single sensing layer  34  made of the same material as dielectric pad  360 , pressure sensing mat  20  includes a sensing layer  34  made of a plurality of individual squares (or other shapes) of dielectric pads  360 , wherein each square is positioned at an intersection of a row conductor  38  and a column conductor  34 . Each square dielectric pad  360  is spaced apart from its neighboring dielectric pads  360  so that the pads are electrically isolated from each other. Such spacing may be filled by any suitable electrically insulating material that is flexible, and in some embodiments, elastically stretchable. Alternatively, the spacing may be left empty, and each individual pad  360  may be fixed in position by alternative means, such as by adhesive, stitching, or other means. Regardless of the manner of affixing pads  360  in position, only the dielectric pad  360  corresponding to an individual sensor will be in electrical series between the conductive row and conductive column corresponding to that sensor. This contrasts with the embodiment of mat  20  shown in  FIG. 28  where the entire sensing layer  34  is effectively in electrical series between the corresponding conductive row and conductive column. 
         [0108]    When mat  20  is made of an array of hybrid sensors that vary in capacitance and conductance in response to external pressures exerted on the sensors, the mat provide significant advantages over pressure sensing mats that vary with respect to only capacitance, or with respect to only conductance. For example, response time to pressure impulses and linear dynamic range regions, among other parameters, vary differently for the conductive and capacitive sections of the hybrid sensors. These variations enable adaptive optimization of sensor accuracy, repeatability, and response time by judicious choices of combinations of the frequencies of voltages or currents used to sample the individual sensors. Further, the sensing of dual electrical properties (e.g. capacitance and resistance) enables flexible pressure sensing mats to be constructed that have a wider dynamic range than mats that measure only a single electrical property. 
         [0109]    The above description is that of several embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. This disclosure is presented for illustrative purposes and should not be interpreted as an exhaustive description of all embodiments of the invention or to limit the scope of the claims to the specific elements illustrated or described in connection with these embodiments. For example, and without limitation, any individual element(s) of the described invention may be replaced by alternative elements that provide substantially similar functionality or otherwise provide adequate operation. This includes, for example, presently known alternative elements, such as those that might be currently known to one skilled in the art, and alternative elements that may be developed in the future, such as those that one skilled in the art might, upon development, recognize as an alternative. Further, the disclosed embodiments include a plurality of features that are described in concert and that might cooperatively provide a collection of benefits. The present invention is not limited to only those embodiments that include all of these features or that provide all of the stated benefits, except to the extent otherwise expressly set forth in the issued claims. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construe