Abstract:
A cushion having a sitting surface is provided that includes a three dimensional system of rigid plastic levers used in combination with a foam cushioning material. Weight applied to the cushion is borne by the plastic levers. When sat on, the cushion conforms to the user&#39;s body and automatically shapes itself to uniformly distribute support pressure within each of four seat quadrants. As the body shifts in position, the cushion automatically reconfigures itself to maintain substantially uniform pressure distributions. Construction details and a method for low cost manufacture are presented.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to my copending provisional application, Ser. No. 60/072,072, filed Jan. 21, 1998. 
    
    
     SEQUENCE LISTING 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY 
     SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     This invention relates to support cushions, and in particular to a support cushion that includes a mechanical support system imbedded in foam cushioning material. The mechanical support cushion conforms to a user&#39;s body and maintains substantially uniform pressure distribution within each of a plurality of sections of the cushion. The invention as described herein relates to seat cushions used on chairs, and in particular on seats of wheelchairs. However, it will be appreciated that the support cushion can be readily adapted for use in any situation where it is desired to exert substantially uniform or balance pressures against a body resting thereon. For example, the support system can be used in beds, chairs back supports and the like. 
     In general, wheelchairs include a seat and backrest that are constructed from flexible material that easily folds when the wheelchair is collapsed. However, the flexible material of the seat and backrest does not provide optimum support or comfort for the occupant. Foam and air filled seat cushions and contoured universal backrests have been developed to provide improved support for individuals requiring special support, such as paraplegics or those having spinal deformities or muscular atrophy, but these seat cushions and backrests are difficult to adjust to accommodate specific needs and proportions of individual users and do not encourage proper control of body position. Users often over use their back muscles in efforts to maintain proper posture. Furthermore, wheelchair users and those confined to beds often become oversensitive to pressures against their skin and bodies so that it is desirable to develop a support cushion that exerts low pressures against a user&#39;s body and skin. 
     Support cushions have been developed that provide low pressures on a user&#39;s body. For example, U.S. Pat. No. 3,790,150, issued Feb. 5, 1974 to Lippert; U.S. Pat. No. 4,033,567, issued Jul. 5, 1977 to Lipfert; U.S. Pat. No. 4,283,864, issued Aug. 18, 1981 to Lipfert; and U.S. Pat. No. 4,484,778, issued Nov. 27, 1984 to Cousins et al., disclose various mechanical support systems that conform to the shape of the user&#39;s body and distribute pressure over user&#39;s body. However, such support cushions do not provide the necessary postural control and stability for persons such as active paraplegics and active wheelchair users. 
     Accordingly, it is desirable to develop a support cushion that not only conforms to the shape of the user&#39;s body, but also provides stabilizing forces that allow for postural control. Such a support cushion should provide a stable platform to permit improved control of body position and posture without overusing back muscles. Furthermore, such a support cushion must be durable and able to withstand extensive use for extended periods of time. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a support cushion and method for making the same that conforms to the shape of the user&#39;s body, distributes support pressure and provides a stable support surface for a user. 
     These and other objects will become apparent to those skilled in the art in light of the following disclosure and accompanying drawings. 
     In accordance with the invention, generally stated, a support cushion is provided that includes an outer foam shell having a cavity formed therein; foam filling disposed in the cavity of the outer shell; and a mechanical support structure disposed in the cavity and imbedded in the foam filing that distributes pressure exerted on the cushion by a user. The density of the foam shell preferably is greater than the density of the foam filing. In the preferred embodiment, the foam filing is a low density soft urethane foam. 
     Another aspect of this invention includes a method for making a support cushion, including the steps of: placing a preformed foam shell having a top panel and sidewalls extending downwardly therefrom in a form having dimensions slightly greater than the peripheral dimensions of the top panel; placing a mechanical support structure on the top panel of the foam shell in a cavity defined by the top panel and sidewalls; pouring a foam filing mixture in the cavity of the outer shell so that the mechanical support structure is imbedded in the foam filing after the foam filing cures; placing a bottom panel over the mechanical support before the foam filing cures; and removing the cushion from the form after the foam filing cures. 
     Other objects and features will be apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 is a cross-sectional view of the support cushion of the present invention; 
     FIG. 2 is a perspective view of a “H” lever system with free bearings; 
     FIG. 3 is a perspective view of a “H” lever system substantially equivalent to that shown in FIG. 2 except the system employs wider and thinner beams that permit twisting at their ends with negligible bending; 
     FIG. 4 is a schematic view of a three tiered lever support system, showing the levers adapting to support a user; 
     FIG. 5 is a perspective view of five “H-type” lever systems associated with the first and second tiers that are molded into a single unit with 16 “button” points of support; 
     FIG. 6 is a perspective view of a third tier support lever; 
     FIG. 7 is a perspective view of the first, second and third tier levers molded together with 64 button shaped points mounted on ends of lever arms associated with the first tier; 
     FIG. 8 is a photographic representation showing a complete assembly of the foam cover and mechanical support structure disposed within a form prior to pouring in the foam filing; 
     FIG. 9 is a photographic representation showing the foam filing mixture being poured into the form over the outer foam cover and on the mechanical support structure; 
     FIG. 10 is a perspective view of the support cushion; 
     FIG. 11 is a photographic representation showing a user seated on the support cushion, with Gaymar pressure measurement system used to measure sitting pressures; and 
     FIG. 12 is a schematic drawing of approximate pressure areas in mmHg as measured at selected points with Gaymar pressure measuring instruments; 
    
    
     Corresponding reference numerals will be used throughout the several figures of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description illustrates the invention by way of example and not by way of limitation. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what I presently believe is the best mode of carrying out the invention. 
     Referring now to the drawings, and in particular to FIGS. 1 and 10; a support cushion is shown, referred to generally by reference number  10 , that includes a mechanical support structure  12  imbedded in foam cushioning  14 . The foam cushioning  14  preferably is constructed from a first foam cushioning material  15  and a second foam cushioning material  16 . The first material  15  preferably is a medium density foam cover or shell. The shell  15  preferably is preformed, and includes a top panel  11  and sidewalls  13  extending downwardly from the outer periphery of the top panel so as to define a cavity  17  therebetween, with the top panel  11  and sidewalls  13  being integrally formed. The second material  16  preferably is a low density foam filing. As discussed below, the mechanical structure  12  is disposed in the cavity  17  of the outer shell  15 , and foam filing  16  then is poured in the cavity  17  of the foam shell  14  upon assembly of the cushion  10 . The foam cushion  14  supports the mechanical structure  12 . 
     The support structure  12  preferably includes a three tiered lever system  18 , having a first tier lever system  20  mounted on a second tier lever system  22  (FIGS.  5  and  7 ), and a third tier lever system  24  (FIGS. 6 and 7) on which the second tier lever system  22  rests (FIGS.  1  and  7 ). The support structure  12  also includes an array of button shaped support points  26  that are supported by ends  28  of lever arms  30  associated with the first tier system  20 . More specifically, the first tier lever system  20  includes a plurality of lever arms  30  arranged in groupings  34 , with each grouping  34  including a pair of lateral lever arms  30   a  connected at their midpoints by a transverse lever arm  30   b.  Lever arms  30   a  and  30   b  are referred to collectively as lever arms  30 . As shown in FIGS. 5 and 7, lever arms  30   a  and  30   b  of each grouping  34  are disposed in a “H” configuration, with each grouping  34  having four support points  26  attached to ends  28  of levers  30   a  (FIG.  5 ). 
     The second tier system  22  includes a plurality of lever arms  36 . Lever arms  36  are disposed in groupings  38 , and each grouping  38  includes a pair of lateral lever arms  36   a  connected at their midpoints by a transverse lever arm  36   b.  Lever arms  36   a  and  36   b  are referred to collectively as lever arms  36 . As shown in FIGS. 5 and 7, lever-arms  36   a  and  36   b  of each grouping  38  are disposed in a “H” configuration. As shown in FIG. 5, groupings  34  of the first tier system are secured to ends  40  of lever arms  36   a  such that each grouping  38  of the second tier system  22  has four groupings  34  of the first tier system  20  attached thereto. 
     The third tier system  24  includes a plurality of lever arms  44 . The lever arms  44  are arranged in groupings  46 , and each grouping  46  includes a pair of lateral lever arms  44   a  connected at their midpoints by a transverse lever arm  44   b.  Lever arms  44   a  and  44   b  are referred to collectively as lever arms  44 . As shown in FIGS. 6 and 7, lever arms  44   a  and  44   b  of each grouping  46  are disposed in a “H” shaped configuration. Ends  48  of the levers  44   a  preferably have a notch  50  extending downwardly as shown in FIG.  6 . The notch  50  is adapted to accommodate the middle lever  36   b  of the second tier groupings  34  so that each “H” shaped grouping  46  of the third tier supports four groupings  38  of the second tier  22  as shown in FIG.  7 . 
     To form a seat cushion such as can be placed on a wheelchair seat or other seat, four third tier groupings  46  are employed as shown in FIG.  8 . These four groupings  46  define four quadrants I, II, III and IV of the seat cushion. A support post  54  is secured to the middle lever  44   b  of each third tier  24  grouping  46  at a location corresponding to the fulcrum of that lever system. Thus, the support posts  54  define four distinct points of support for the seat cushion  10 . When four third tier groupings  46  are used, sixteen second tier  22  groupings  34 , sixty four first tier  20  groupings  34  and  256  support buttons  26  are employed in the mechanical support structure  12 . 
     The support cushion  10  provides substantially uniform pressure distribution and, at the same time, provides improved stability for users resting against the cushion  10 . When used as a seat cushion, the cushion  10  allows for better positioning of a user&#39;s pelvic area. The cushion  10  includes a support surface  60  that conforms to and supports the user&#39;s body. Body weight is carried by the three dimensional lever system  18  which conforms to the body and uniformly distributes tissue pressure within each of four seat quadrants I, II, III and IV. The mechanical support structure  12  is imbedded in plastic foam filing  16  which retains the levers  30   a,    30   b,    36   a,    36   b,    44   a,    44   b  in position and provides improved stability. 
     In use, the internal mechanical support structure  12  shapes itself to produce substantially uniform pressure and, without substantial elastic deformation, supports the user&#39;s weight. The soft plastic foam filing  16  in which the support structure  12  is imbedded is not subjected to any substantial deformation; thus, the foam cushioning  14  produces minimal force reaction and escapes wear experienced by conventional foam cushions. 
     The preferred embodiment of the wheelchair cushion is approximately 2 ¾ inches in thickness, 16 ½ inches in width and 16 ½ inches in depth. The cushion  10  carries body weight to four support points associated with the support posts  54 , with two support points associated with two posts  54  being, disposed under the user&#39;s pelvis and two support points associated with the other two posts  54  being disposed under the user&#39;s thighs. This “4-point” support provides a stable platform to permit good control of body position by users such as, for example, paraplegic wheelchair users who find that some conventional air or gel filled and foam cushions require them to overuse their back muscles to maintain posture. When wheelchair footrests are properly adjusted for thigh contact and the user is seated on the cushion  10 , pressure ranges from 50 to 65 mmHg under the buttocks and from 40 to 50 mmHg under the thighs for a person of average weight. 
     Overall deflection (compression of the cushion  10 ) is approximately zero. As the surface  60  of the cushion  10  conforms to the shape of the user&#39;s body, areas of the cushion  10  that move downward are accompanied by areas that move upward. Thus, the foam matrix cushion  10  is not subjected to overall compression. 
     The underside of the cushion in the preferred embodiment is layered with a panel  64 , preferably 13×13 inches of {fraction (3/16)} inch plywood. The four points of support associated with the four support posts  54  form an 8×8 inch square on this panel  64  which rests on a wheelchair seat or other seat. The four support points on which the body weight is concentrated provide a very stable platform that many disabled persons, such as active paraplegics, find helps in control of their wheelchairs and relieves back muscles from having to stay balanced on conventional air, gel or soft foam cushions. 
     The cushion  10  is constructed from relatively inexpensive components. The mechanical support structure  12  preferably is constructed from plastic moldings so that all moving, weight bearing parts are made of strong, durable plastic. The plastic moldings and foam cushion  14  can be manufactured quickly at relatively minimal cost. The cushion  10  also is very durable and has a long life. 
     The imbedded supports structure  12  of the cushion  10  is based upon a complex system of levers including the first tier lever system  20 , the second tier lever system  22  and the third tier lever system  24 . It is known that when a lever is supported at its center (its fulcrum) and a force is applied at one end, an exactly equal force must be applied at the other end if the lever is to be in equilibrium (not move). When three levers are assembled, as shown in FIG. 2, in the form of an “H” with fulcrums equidistant from their ends, all lever arms are equal and any forces (points of support) that exist on the ends of the arms must also be equal. Four such “H” configurations can be placed at the ends of another larger “H” so that a system of 16 points of support, all with identical forces must result when in equilibrium. Additional “H” structures can be added until a large number of support points exist. 
     FIG. 4 shows schematically how such a system supporting the human body might appear. A cushion 16 inches by 16 inches with 1 inch spacing of the support points would have 256 support points. This is a useful size and because 256 is equal to 2 raised to the 8th power, it leads to a convenient 3 dimensional array of “H” equivalents. 
     In the first prototype cushion, the “H” structures were created by simply laying round dowels on top of each other. These cushions, made of dowels and urethane foam, worked well but involved a great deal of hand labor. In order to make prototypes that would show how the cushions could be made in high production at low cost, a different approach to the provision of bearings as fulcrums for the levers was devised as shown in FIG.  3 . By using an integral unit composed of very thin beams, the desired “rocking” can be obtained through the low resistance to torsion (as compared to bending) of a very thin but wide beam. This approach allows a number of “H” elements to be integrated into a single part that can be made cheaply from molded plastic. The rounded buttons  26  preferably are ⅝ inch in diameter, and molded as part of the plastic lever systems  20 ,  22 ,  24  that form the sitting surface. 
     In the preferred embodiment of the cushion  10  as used for a wheelchair seat cushion, the foam outer layer  15  preferably is ½ inch in thickness, and lies between the body and the one inch array of 256 rounded support “button.” A non paraplegic person does not feel individual buttons but senses only a comfortable “soft” cushion (see FIG. 1) As discussed above, a square array of 256 button shaped support points  26  are mounted at the ends  28  of the first tier levers  30  on a one inch grid that results in a sitting surface  60  approximately 16×16 inches. The sitting surface is produced by an array of sixteen identical plastic units  70 , with each unit  70  including one second tier grouping  38 , four first tier groupings  34  and sixteen support buttons  26  (FIG.  5 ). Each plastic unit  70  is an integral unit that covers an area four inches by four inches and combines five “H” functions in a single molding (i.e., four first tier groupings  34  and one second tier grouping  38 ). Sixteen units  70  are required to cover the complete cushion seating area for a wheelchair seat. As discussed above, each second tier grouping  38  rests in one notch  54  of the third tier groupings  46  (see FIGS.  6  and  7 ). FIG. 7 shows one third tier grouping  46  united with four units  70  (assembly is inverted). Note that each of the units  70  can rock freely on the lever arms  44  of grouping  46  within the notch  54 . The assembly in FIG. 7 represents the structure found under one quadrant I, II, III or IV of one cushion  10 . 
     As discussed above, the support posts  54  and levers  30 ,  36 ,  44  preferably are molded plastic parts. This construction of the molded plastic parts allows for relatively easy and fast manufacturing and assembly of the cushion  10 . To assemble the cushion  10 , a preformed soft foam shell  15 , preferably ½ inch thick, is placed in a form or mold  73  having interior dimensions of 16.5×16.5×3 inches, as shown in FIG.  8 . This shell  15  becomes the outer surface of the finished cushion  10  (FIG.  10 ). Sixteen units  70  then are arranged on the foam, and four third tier groupings  46  are placed on the units  70  as shown in FIGS. 8 and 9. The liquid components that produce a low density soft urethane foam filing  16  are quickly mixed and poured into the form  73  (FIG.  9 ). The 13×13×{fraction (3/16)} inch plywood panel  64  is quickly placed over the four support posts  54  of the third tier groupings  46  to form a relatively stiff bottom that will rest on the wheelchair seat. The mixture for filing  16  quickly foams up and around all of the parts and sets relatively quickly. Once the foam filing  16  cures, the cushion  10  is removed from the form  73 . Excess foam is trimmed to form a smooth bottom and the cushion  10  assembly is finished (FIG.  10 ). Foam filing  16  used in the preferred embodiment is IPI International, Inc. ISOFORM® F-1474-026F-1474-026F Polyurethane Flexible Foam System. 
     Theoretically, it is expected that a lever support system as shown schematically in FIG. 3, would require equal forces on all support points and therefore equal pressure throughout the support surface. However, in practice, two sources of elasticity in the cushion  10  cause some variation in pressures that are measured. While the foam is very low in density and soft, it does have some resistance to deformation and, in addition, the plastic elements, while being very flexible, do elastically resist deformation to a certain extent. These two elastic resistances to deformation have some effect on the overall action of the cushion  10 . The main support pressure is obtained from the lever action that has been presented and tends toward that pressure that results from the weight divided by the area of support in contact with the body, but this pressure is altered somewhat by the effects of the elastic deformations described. Furthermore, the 4-point support system of the cushion  10  creates four separate support quadrants I, II, III and IV. The position of the body center of gravity, as affected by torso position and footrest position, changes the average pressures in each quadrant depending on how much of the body weight that quadrant is required to support. Thus, the cushion design of the present invention provides improved support for a wheelchair user, is relatively inexpensive to manufacture and is durable and reliable in operation. 
     Rudimentary measurements were performed on a female subject  78  using a pair of Gaymar pressure measuring devices  80 . FIG. 11 shows this experimental setup. FIG. 12 shows approximate pressure distributions that were measured (in mmHg) when she maintained an erect position with minimal floor support of her legs. 
     The support cushion  10  of the present invention also finds application in ergonomic chairs that provide improved comfort and health to persons who must sit for long times as, for example, when working at a computer. A seat incorporating the mechanical support structure  12  and foam cushioning  14  permits individual forward and backward rocking of each thigh and provides degrees of freedom that are not found in a conventional office chair. Rocking of the seat and thighs provides additional comfort, induces good sitting posture, especially when working without touching the back rest and may enhance the movement of fluids in the low back vertebrae. This degree of freedom can be produced by adding two additional levers to the design of the wheelchair cushion so that each of the two support points on either side are supported by levers that can rock at their midpoints. Thus, the 4-point support of the wheelchair cushion is changed to a 2-point support for an ergonomic chair seat. 
     In view of the above, it will be seen that the several objects and advantages of the present invention have been achieved and other advantageous results have been obtained. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.