Abstract:
A tension-compression base structure including tension elements (such as flexible cables or ropes) and compression elements (such as rigid legs) is provided with a slidably adjustable path for the tension elements around or within the compression elements, thereby enabling a degree of adaptability to support surfaces that may not be ideally flat, such as on outdoor terrain. Such a tension-compression base may be used to support a platform, a stool, or an item of equipment that may be desirably held in a preferred orientation irrespective of terrain irregularities, and as furniture may be configured to rock with a user&#39;s body as a form of active seating.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to base structures capable of supporting a mass over a support surface, and more particularly to adaptive tension-compression base structures capable of holding a preferred orientation over irregular terrain. 
       BACKGROUND OF THE INVENTION 
       [0002]    The term “tensegrity” was originally coined by futurist designer and inventor R. Buckminster Fuller in the 1960s to describe systems of tension elements (e.g., ropes, cables, or cords) and compression elements (e.g. bars, rods, tubes, or other rigid strut-type components) held in a state of static pre-stressed equilibrium to define a three-dimensional frame structure, wherein the compression elements generally do not touch each other. The pulling forces applied by the tension elements are resisted by the rigid compression elements, and a tensegrity system remains stable even against externally applied forces. The word “tensegrity” itself combines “tension” and “structural integrity.” Fuller&#39;s U.S. Pat. No. 3,063,521 (filed in 1959 and issued in 1962) covers various basic tensegrity concepts, and Fuller and others have patented many variations since then. 
         [0003]    When properly designed and constructed, tensegrity structures have proven to be robust and durable. Pioneering sculptor Kenneth Snelson&#39;s well-known “Needle Tower” sculpture, constructed of metal tubes and wire, has stood outdoors at the Hirshhorn Museum in Washington, D.C. for decades. Tensegrity structures can be suitable for furniture as well. A line of tensegrity sitting stools named after Snelson is offered by designer Sam Weller (samweller.co.uk). 
         [0004]    The tensegrity concept has been well developed and used frequently in the decades since the 1960s (and even before then, as some structures—including the London Skylon tower dating from 1951—employed some tensegrity principles even before the term was coined). Tensegrity is capable of enabling lightweight but robust structures combined with artful design; there are many designs for furniture, bridges, buildings, sports stadiums, toys, and other structures—large and small—that employ tensegrity principles. 
         [0005]    But for all their benefits, most tensegrity structures remain rigid and poorly adapted to use upon irregular surfaces. The pre-stressed balance between tension and compression provides little freedom for movement. Because of this, tensegrity furniture is not often suitable for use outdoors. The Snelson stools referenced above, for example, remain flat and balanced only on a flat floor; on an inclined surface the entire structure including the seating surface will also be inclined and vulnerable to tipping over, and on an irregular surface the legs of the stool will wobble. This, unfortunately, also holds true for many other pieces of tensegrity furniture. 
         [0006]    Accordingly, there is a need for an adaptive tension-compression structure based on tensegrity principles but more capable of being used on inclined and irregular surfaces. Such a structure would be easily adjustable to various support surfaces and yet remain strong and stable as furniture or as a base for equipment or other objects. 
       SUMMARY OF THE INVENTION 
       [0007]    An adaptive tension-compression structure according to Applicant&#39;s invention addresses some of the shortcomings of prior tensegrity structures described above. 
         [0000]    Like many of these prior structures, the basic form of a tension-compression structure according to the invention is a tensegrity prism—in its simplest form, three compression elements held together with tension elements in a shape that resembles a twisted triangular prism. However, the present tension-compression structure includes a slidably adjustable path for the tension elements holding the compression elements together. The sliding adjustability of the tension elements provides additional compliance (in the circumferential direction) for the structure while maintaining its basic geometry, thereby providing a stable base for furniture or any other suitable structure—including but not limited to equipment that may be desirably held in a preferred orientation over a variable or irregular surface. 
         [0008]    A tension-compression structure according to the invention may also be employed as a base for “active seating” furniture on a level surface or an irregular surface. The slidably adjustable tension elements allows a structure according to the invention to rock somewhat, subject to the user&#39;s control, encouraging some muscle activity to keep the structure (such as a stool) in a preferred position. 
         [0009]    In an embodiment of a tension-compression structure according to the invention, the compression elements are hollow tubes that serve as pathways for the tension elements. However, in an alternative embodiment, the tension elements may also be provided with pathways adjacent to the compression elements. 
         [0010]    In several possible embodiments of the invention the tension-compression structure is collapsible for storage or transportation. In one embodiment, one of the compression elements is axially collapsible via a spring-biased detent (or another suitable locking mechanism), thereby releasing the pre-stress on the system and enabling the structure to be broken down into a bundle of parallel legs (with some flexible parts folded therein). In another embodiment, a connector integrated into a tension element may be disconnected, thereby once again enabling the structure to be collapsed into a bundle. 
         [0011]    With the present tension-compression structure in an uncollapsed state, when weight is applied, constraints applied by an upper platform and a lower set of tension segments prevent the structure from losing its integrity, while the sliding tensegrity tension elements accommodate surface irregularities until a stable position is reached. 
         [0012]    Accordingly, then, a number of disadvantages of other known tensegrity support structures—particularly their shortcomings on irregular terrain and their inability to be collapsed for storage or portability—are addressed by the tension-compression structures of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other objects, features, and advantages of the invention will become apparent from the detailed description below and the accompanying drawings, in which: 
           [0014]      FIG. 1  is a perspective view of a base structure according to the invention capable of serving as a stool, including three compression elements serving as legs and an upper surface seating platform; 
           [0015]      FIG. 2  is a side view of the base structure of  FIG. 1 ; 
           [0016]      FIG. 3  is a bottom view of the base structure of  FIG. 1 ; 
           [0017]      FIG. 4  is a perspective view of a base structure according to the invention with one leg situated atop a support surface irregularity; 
           [0018]      FIG. 5  is a side view of the base structure of  FIG. 4 ; 
           [0019]      FIG. 6  illustrates a base structure according to the invention provided with a telescoping leg enabling the structure to be collapsed for storage or transportation; 
           [0020]      FIG. 7  is the base structure of  FIG. 6  in its collapsed state; 
           [0021]      FIG. 8  illustrates a base structure according to the invention provided with a disconnectable tension element enabling the structure to be collapsed for storage or transportation; 
           [0022]      FIG. 9  is the base structure of  FIG. 8  in its collapsed state; and 
           [0023]      FIG. 10  is a depiction of a base structure according to the invention as seen from below, with arrows indicating a path of slidably adjustable tension elements. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The invention is described below, with reference to detailed illustrative embodiments. It will be apparent that a tension-compression base structure according to the invention may be embodied in a wide variety of forms. Consequently, the specific structural and functional details disclosed herein are representative and do not limit the scope of the invention. 
         [0025]      FIG. 1  illustrates a tension-compression base structure  110  according to the invention configured as a stool. There are three compression elements  112 ,  114 , and  116  illustrated and serving as legs; they are arranged generally as a tensegrity prism—i.e., as illustrated, a triangular prism wherein the top and bottom triangles are rotated with respect to each other. This arrangement, with tension elements  118  stretched between a first (upper) end  120 ,  122 , and  124  of each of the legs and a second (lower) end  126 ,  128 , and  130  of one of the adjacent legs, is a stable tensegrity configuration. Although  FIG. 1  shows three compression elements or legs  112 ,  114 , and  116 , it should be noted that tensegrity configurations are possible having other numbers of compression elements. It should further be noted that the arrangement of the legs might not be strictly prismatic, as the term is generally understood; the leg spacing at the upper ends may differ from the spacing at the lower ends. 
         [0026]    The stool  110  of  FIG. 1  includes a platform  132  at the upper end. As illustrated, the platform may be essentially rigid, or in an embodiment of the invention it may be soft and compliant (and, for example, made from a suitable fabric). This platform  132  serves as a seat for the stool  110 . Regardless of whether the platform  132  is rigid or flexible, the platform also maintains the upper ends  120 ,  122 , and  124  of the legs  112 ,  114 , and  116  in a desired maximum spacing, which in the case of the illustrated stool  110  is essentially an equilateral triangle (though in alternative embodiments of the invention, the triangle need not be equilateral). The platform prevents the upper ends  120 ,  122 , and  124  of the legs  112 ,  114 , and  116  from moving outward more than the size of the platform  132  accommodates. Accordingly, if the platform  132  is rigid, the legs  112 ,  114 , and  116  may be pivotably attached at or near a periphery of the platform  132  in a triangular configuration. If the platform  132  is soft and flexible, pockets may be formed near the edge of the platform  132  to accommodate the upper ends  120 ,  122 , and  124  of the legs  112 ,  114 , and  116 , or the legs may be flexibly fastened to the platform  132  in another suitable manner. In an embodiment of the invention the pockets are spaced relatively equally around the perimeter of the platform, but they need not be evenly spaced. 
         [0027]    In the illustrated embodiment, the compression elements, or legs  112 ,  114 , and  116 , are fabricated from tubes of a suitably strong metal, such as steel, though other materials may of course be used. This tubular construction enables a lightweight structure. Feet may be provided at the lower ends of the legs to provide good frictional contact with the terrain; such feet may optionally be pivoting. 
         [0028]    When tubes are used as the compression elements  112 ,  114 , and  116 , the tension elements  118  extending between the upper portions of each leg and the lower portion of adjacent legs may be formed from a single loop of rope or cable; this configuration will be discussed further in connection with  FIG. 10  below. In this disclosed single loop configuration, the tension elements  118  extend externally between an upper portion  120  of a first leg  112  and a lower portion  126  of a second leg  114 , then internally within the tube-shaped second leg  114  from its lower portion  126  to its upper portion  122 , then externally between the upper portion  122  of the second leg  114  to a lower portion  128  of a third leg  116 , then internally within the third leg  116 , then externally from a upper portion  124  of the third leg  116  to a lower portion  130  of the first leg  112 , then internally within the first leg  112  to the upper portion  120  thereof to complete the loop. In an embodiment of the invention, the tension elements  118  need not be positioned inside the legs; but rather, are guided adjacent to the legs by eyes, pulleys, or other suitable structures. In such a case, the legs may be made from any suitably rigid material, such as metal, wood, or some plastics or composites. 
         [0029]    The tension-compression structure disclosed herein further includes a lower end constraint  134 , which as illustrated includes a plurality of tension segments  136  (preferably flexible) fixably attached to the lower portion  126 ,  128 , and  130  of each of the legs  112 ,  114 , and  116  and joined at a junction  138  at a midpoint. Other embodiments of constraints are possible here; the lower end constraint  134  might take a triangular configuration (like the platform  132 ) or may be rigid in nature. 
         [0030]      FIGS. 2 and 3  illustrate the tension-compression structure of  FIG. 1 , but in side view and bottom view, respectively. 
         [0031]      FIG. 4  illustrates a stool  110  according to the invention placed upon a terrain irregularity, which is represented in  FIG. 4  by a raised block  410 . As illustrated, the stool  110  has been adjusted to accommodate the irregularity as enabled by the invention. 
         [0032]    As shown in  FIG. 4 , the legs  112 ,  114 , and  116  of a stool  110  according to the invention are constrained somewhat by the upper platform  132  and the lower constraint segments  134 , but otherwise are essentially free to adjustably slide along the tensegrity tension elements  118  to accommodate irregular terrain (subject, of course, to a desirable level of friction between the legs  112 ,  114 , and  116  and the tension elements  118  which tends to avoid excessive and undesirable adjustments as weight is placed on the stool  110 ). 
         [0033]    When weight is applied, the legs  112 ,  114 , and  116  of a tension-compression structure  110  according to the invention move outward at both top and bottom against the constraints (the upper platform  132  and the lower constraint segments  134 ), and individually move down and/or outward to meet the terrain. Once the legs  112 ,  114 , and  116  are in position, the upper platform  132  can be adjusted to suit the user&#39;s needs or comfort. 
         [0034]    The irregularity shown in  FIG. 4  is exaggerated for purposes of illustration. Although a tension-compression base structure  110  according to the invention can accommodate this movement and more, it is to be expected that some instability may result as the degree of irregularity begins to exceed the capacity of the structure to mo 110   ve  to accommodate it; the center of gravity of weight placed upon the platform  132  will at some point move outside of the region bounded by the legs  112 ,  114 , and  116  and the structure  110  may then be subject to tipping. Accordingly, a structure according to the invention is suitable for moderately irregular terrain. 
         [0035]    An article of furniture according to the invention, such as a stool, may also be employed for “active seating” on any suitable surface. The sliding adjustability of the tension elements in a structure according to the invention allows the structure to move and comply with shifts in a user&#39;s position or center of gravity, thereby encouraging some continuous use of the user&#39;s core muscles to maintain a desired position. Some users may find this desirable, particularly in an office setting or other circumstance that would otherwise be primarily sedentary. 
         [0036]      FIG. 5  illustrates the tension-compression structure of  FIG. 4 , but in side view for an enhanced understanding of the structure and the relationship among the parts. 
         [0037]      FIG. 6  illustrates a base structure  610  according to the invention provided with a telescoping leg  614 . The telescoping leg  614  enables the structure  610  to be collapsed for portability. 
         [0038]    As illustrated, one of the compression elements or legs  612 ,  614 , and  616 , is a telescoping leg  614  formed from two segments, an upper segment and a  640  lower segment  642 . The upper segment  640  is slightly smaller in diameter than the lower segment  642 , and hence, the upper segment  640  is capable of sliding axially into and out of the lower segment  642 . To maintain the telescoping leg in its fully extended position, the upper segment is provided with a spring-biased pushbutton protrusion  644  at the lower end of the upper segment  640 , configured to lock with a mating hole at an upper end of the lower segment  642 . When the two segments  640  and  642  are so engaged, the protrusion extends through the hole in the lower segment and the two segments are kept in an extended configuration. To collapse the telescoping leg  614 , the pushbutton  644  is depressed to disengage it from the hole, and the two segments  640  and  642  may then be slid together. This releases the tension on the tension elements  618 , and allows the stool  610  of  FIG. 6  to be collapsed into a bundle  710  ( FIG. 7 ). In such a configuration, the upper platform  632  is desirably flexible, and is able to bend, fold, or otherwise conform with the collapsed bundle of legs. And the tension elements  618 , being flexible, are also able to move and comply with the collapsed state  710  of the structure  610 . Although a pushbutton  644  detent is described in some detail herein, it will be readily recognized that other locking structures are possible, such as twist locks and flip locks, as well as yet other alternatives that will be understood by a person of ordinary skill in mechanical design. 
         [0039]      FIG. 7  is the base structure  610  of  FIG. 6  in its collapsed state  710 . In its collapsed state  710 , a tension-compression structure according to the invention can be kept in a tube-shaped container or convenient shoulder bag, or strapped to a pack for easy transport. It should be noted that the tension elements  618  shown in  FIG. 7  are illustrated in one possible slack state; the tension elements  618  can be arranged in other ways, tucked, wrapped, or folded around the compression elements  612 ,  614 , and  616  and other portions of the structure  710  as desired. 
         [0040]      FIG. 8  illustrates a base structure  810  according to the invention provided with a disconnectable tension element  818 . Disconnecting this tension element  818  releases the tension and enables the structure  810  to be collapsed into a bundle  910  ( FIG. 9 ) for storage or transportation. 
         [0041]    The disconnectable tension element, in an embodiment of the invention, may take the form of a carabiner (on one end of the tension element  818 ) and loop (on the other end), or alternatively one of many different kinds of release mechanisms, including but not limited to plastic quick-disconnect clips, magnetic mechanisms, hooks, and many other possibilities. In an embodiment of the invention, the tension element  818  is not fully disconnected to collapse the structure, but is loosened sufficiently to allow the legs  812 ,  814 , and  816  to move into a collapsed bundle  910  ( FIG. 9 ); in this configuration, a strap buckle, turnbuckle, or lever apparatus may be used, and other options will be recognized by a practitioner of ordinary skill in the art. 
         [0042]      FIG. 9  is the base structure  810  of  FIG. 8  in its collapsed state  910 . As with the embodiment of  FIGS. 6-7 , the structure resembles a bundle and can be easily stored or transported. As with  FIG. 7 , the tension elements  818  are shown in one possible collapsed configuration, and can be arranged in other ways as well. 
         [0043]      FIG. 10  illustrates the path taken by the tension elements  118  in a base structure  110  according to the invention via arrows. 
         [0044]    As discussed above in connection with  FIG. 1 , an embodiment of the invention includes tension elements  118  formed from a single loop of material, either as a closed loop or as an openable loop (as shown, for example, in  FIGS. 8-9 ). The arrows in  FIG. 10  illustrate an exemplary pathway for such a single closed or openable loop, inside each of the legs and connecting adjacent legs as described above. Although the arrows of  FIG. 10  are shown as having a directionality, this is solely to enable an understanding and to more easily trace the entire path—it should be recognized that the tension elements are capable of sliding adjustability in both directions, not just in the direction represented by the arrows. 
         [0045]    As shown in  FIG. 10 , starting somewhat arbitrarily at the upper end  120  of the first leg  112 , the tension element  118  traverses the structure  110  externally along a first arrow  1010  to the lower end  126  of the second leg  114 , then internally within the second leg along second and third arrows  1012  and  1014  to the upper end  122  of the second leg  114 , then externally again along a fourth arrow  1016  to the lower end  128  of the third leg  116 , then internally again along fifth and sixth arrows  1018  and  1020  to the upper end  124  of the third leg  116 , then externally again along a seventh arrow  1022  to the lower end  130  of the first leg  120 , then internally again along eighth and ninth arrows  1024  and  1026  to the upper end  120  of the first leg  112 , closing the loop. 
         [0046]    As noted above, the tension elements traverse the structure inside each of the legs  112 ,  114 , and  116  and outside the legs (and between adjacent legs). As the tension elements transition between inside and outside of the legs, it is considered advantageous to provide a smooth and non-abrasive surface for them to slide over. Accordingly, the legs may be provided with saddle structures  1030 , either internal to the legs or at the openings where the tension elements enter or exit the tubular legs. Such saddle structures  1030  provide the ability for the tension elements  118  and compression elements  112 ,  114 , and  116  to move with respect to each other, while ensuring the tension elements  118  do not tend to fray over the course of time. The saddle structures  1030  also provide sufficient friction to ensure the tension-compression structure remains in a desired orientation and position without undue adjustment while it is being used. A practitioner of ordinary skill will recognize, of course, that the saddle structures may be replaced with pulleys, wheels, or simply flanged entry/exit holes or lips as desired; there are many other possibilities. 
         [0047]      FIG. 10  shows the legs  112 ,  114 , and  116  of the tension-compression structure held together at the bottom via a central junction  138  among three flexible constraint segments  136 , as discussed above with reference to  FIG. 1 . It should be noted that an alternative arrangement uses constraint segments to connect the legs in a triangular configuration without a central junction; neither this path nor the configuration that includes a central junction is adapted for sliding adjustment, as it is intended to constrain the lower portions of the legs. Other configurations for a lower constraint may be apparent to a practitioner of ordinary skill, and shall be deemed to be within the scope of the invention. 
         [0048]    It should be observed that while the present invention has been described as primarily a sitting stool, a tension-compression structure according to the invention may be used for numerous other applications other than stools. Other types of furniture (including broader chairs, tables, etc.), tables and other platforms, and equipment bases (substituting for a tripod, for example) may also be made according to the invention. 
         [0049]    It should be observed that while the foregoing detailed description of various embodiments of the present invention is set forth in some detail, the invention is not limited to those details and a tension-compression structure made according to the invention can differ from the disclosed embodiments in numerous ways. In particular, it will be appreciated that embodiments of the present invention may be employed in differing applications and may be configured in various manners that may depart in some details from the exemplary details set forth above. It should be further noted that functional distinctions are made above for purposes of explanation and clarity; specific structural distinctions in an apparatus according to the invention may not be drawn along the same boundaries. Hence, the appropriate scope hereof is deemed to be in accordance with the claims as set forth below.