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RELATED APPLICATIONS 
     This application is based on Provisional Application Ser. No. 60/314,744 filed on Aug. 24, 2001, entitled “Retractable Structures Comprised of Interlinked Panels.” 
    
    
     BACKGROUND OF THE INVENTION 
     Structures that transform in size or shape have numerous applications in many fields. My prior patent, U.S. Pat. No. 5,024,031, hereby incorporated by reference as if fully disclosed herein, teaches methods for constructing transformable truss-structures in a variety of shapes. The teachings therein have been used to build structures for diverse applications including architectural uses, public exhibits and educational toys. 
     One particular embodiment disclosed in U.S. Pat. No. 5,024,031 is a retractable structure that expands from a compact ring of links to form a self-supporting structural dome. In its most basic embodiment, such a structure is made entirely of a latticework of links. The structure would be comprised of structural elements only, and the structural dome, when extended, retains a somewhat skeletal appearance. As an extension of that embodiment, a method was also disclosed to incorporate panel elements that may be attached to the outward side of structure, thereby creating a substantially smooth, continuous covering that covers the entire dome in its extended position (See FIGS. 28-33 of U.S. Patent No. 5,024,031). 
     Such an arrangement can be improved upon. Since the panel elements are separate pieces from the structural member themselves, they add additional elements that may negatively affect the structural integrity of the structure. Additional elements would also raise the cost to build and maintain such a structure. A further concern is that when building a large structure, the panel elements may catch the wind and cause them to be dislodged from the structural elements. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, a retractable structure is presented that incorporates an additional useful feature. I have discovered a way to construct such retractable structures whereby the links are themselves panel elements. Thus, the structural members themselves form a continuous surface, leading to a more economical, structurally sound and cleaner design. 
     Such links can be assembled to form planer and three-dimensional structures. In their planar embodiments, retractable structures according to this invention may be comprised exclusively of panels hinged together. In their three-dimensional embodiments, whether conical, hemispherical or other shapes, panels are connected to one another via small hub elements. 
     This discovery represents a significant improvement over the earlier invention, and offers the promise of building of practical architectural structures with retractable features. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a typical panel-link with three pivot points. 
     FIG. 2 shows a tongs linkage comprised of eight separate panel-links in its extended position, forming a wedge-shaped surface. 
     FIG. 3 shows the same linkage in a partially retracted position, its edges lying along the lines of a similar angle to that in FIG.  1 . 
     FIG. 4 shows the same linkage in a fully retracted position, its edges lying along the lines of a similar angle to that in FIG.  1 . 
     FIGS. 5,  6 , and  7  show perspective views of the same linkage in its extended, partially retracted and fully retracted positions respectively. 
     FIG. 8 shows a structural arch to which twelve extended tongs-linkages have been attached, said linkages also being joined to one another, to form a semi-circular planar surface. 
     FIG. 9 shows the same structure wherein all twelve tongs-linkages have been fully retracted into the structural arch, thereby providing a large semi-circular opening. 
     FIG. 10 shows a perspective view of the planar arch structure in its extended position. 
     FIGS. 11 and 12 show perspective views of the planar arch structure in its partially retracted and fully retracted positions respectively. 
     FIG. 13 shows two planar tongs-linkages about to be joined to one another by six hub elements. 
     FIG. 14 shows same two linkages joined together by the hub elements. 
     FIG. 15 shows a sectional view revealing an angled relationship between the two linkages. 
     FIG. 16 shows a perspective view of a retractable conical structure comprised of twenty planar tongs-linkages, shown here in its extended position, each linkage being joined to one another by hub elements, thereby forming a complete ring. 
     FIG. 17 shows the same structure in elevation view. 
     FIGS. 18 and 19 show perspective views of the same structure in its partially retracted and fully retracted positions. 
     FIG. 20 shows a plan view of a curved tongs-linkage in its extended position. 
     FIG. 21 shows an elevation view of the same linkage. 
     FIGS. 22,  23  and  24  show perspective views of the curved tongs-linkage in its extended, partially retracted and fully retracted states respectively. 
     FIG. 25 shows a perspective view of a retractable domed structure in its extended position. 
     FIG. 26 shows an elevation view of the domed structure. 
     FIGS. 27 and 28 show the same retractable dome in its partially retracted and fully retracted positions respectively. 
     FIG. 29 shows a panel-link that is constructed of a frame with a sheet material to provide an infill, along with a linear link. 
     FIG. 30 shows a tongs linkage in its extended position, whereby the linkage is comprised of four panel-links and four linear links. 
     FIG. 31 shows the tongs linkage in a partially retracted position, the slide elements having moved along the supporting structure. 
     FIG. 32 shows the tongs linkage in its fully retracted position so that it is fully retracted to within the supporting structure. 
     FIG. 33 shows a semi-circular retractable structure consisting of twelve tongs linkages. 
     FIG. 34 shows a stationary supporting arch and a supporting track. 
     FIG. 35 shows the retractable structure and supporting structure together. 
     FIGS. 36 and 37 show the structure in its partially extended and fully retracted position respectively. 
     FIG. 38 shows an elevation view of the same retractable structure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 is shown a typical panel-link element  8  which has a polygonal profile. Panel link element  8  has one central pivot  7  and two end-pivots  5  and  9 . A single panel-link element is the most basic element in the structure. Two panel-link elements can be pivotally connected to each other by their central pivots to form a link-pair. 
     A plurality of these link-pairs can be pivotally connected to each other by their end-pivots to form a structure that can extend like a pair of extendable tongs (hence, a “tongs linkage”). FIG. 2 shows such a tongs linkage  20  which is comprised of eight panel-links  2 ,  4 ,  6 ,  8 ,  10 ,  12 ,  14  and  16 . Links  6  and  8  are pivotally joined together via pivot  7  to form a link-pair. Similarly links  2  and  4  are joined, as are links  10  and  12 , as are links  14  and  16 . Each link-pair is itself joined to a neighboring pair via two pivots. Linkage  20  is shown in an extended position whereby a triangle-shaped surface is formed. A line  22  passes through one end-pivot each of all of the panel-links. A second line  21  passes through the other end-pivot of the panel-links. 
     In FIG. 3 linkage  20  is shown in a partially retracted position. A line  23  passes through one end-pivot each of the eight panel-links and a second line  24  passes through the other end-pivot. The angle formed between lines  23  and  24  is identical to the angle formed between lines  21  and  22 . 
     In FIG. 4 linkage  20  is shown in a fully retracted position. A line  25  passes through one end-pivot each of the eight panel-links and a second line  26  passes through the other end-pivot. The angle formed between lines  25  and  26  is identical to the angle formed between lines  21  and  22 . 
     FIG. 5 shows a perspective view of linkage  20  in an extended position. Link-pair  14 , 16  lies in an offset plane from link-pair  10 , 12  which itself lies in an offset plane from link-pair  6 , 8 . This last link-pair is itself offset from link-pair  2 , 4 . FIG. 6 shows a perspective view of linkage  20  in a partially retracted position. The offsets between adjacent link-pairs allow them to slide over one another without interference. FIG. 7 shows linkage  20  in its fully retracted position whereby all of the panel-links are stacked over one another. 
     FIG. 8 shows a semi-circular retractable structure  60  which is comprised of 12 tongs-linkages  20 , 30 , 32 , 34 , 36 , 38 , 40 , 42 , 44 , 46 , 48  and  50  each one of which is pivotally joined to its neighboring linkage via those end pivots which lie in along a line. Shown in its extended position, structure  60  forms a semi-circular solid wall. Structure  60  is further comprised of a stationary arch  52  which supports the linkages. Linkage  20  is attached to arch  52  via two sliding connections  27  and  28 . Similarly all of the remaining linkages are attached to arch  52  by sliding connections. Structure  60  is further comprised of a linear track  60  which supports tongs linkages  20  and  50  along their unattached edges. FIG. 9 shows structure  60  in its fully retracted position whereby the linkages have retracted to overlap arch  52  thereby providing a semi-circular opening. FIGS. 10,  11  and  12  shows perspective views of structure  60  in its extended, partially retracted and fully retracted positions respectively. 
     FIG. 13 shows two tongs-linkages  70  and  71  in proximity to five hub elements  72 ,  73 ,  74 ,  75  and  76 . FIG. 14 shows linkages  70  and  71  joined to one another via those same five hub elements. FIG. 15 shows a sectional view of the joined tongs-linkages revealing an angular relationship between them, said angle being formed by the hub elements. FIG. 16 shows a retractable structure  80  having a conical form which is comprised of twenty tongs linkages  70 ,  71  and  81  through  98  (consecutively). Each tongs linkage is connected to two of its neighboring linkages via adjoining hub elements. For example linkage  70  is joined to linkage  71  via hub elements  72 , 73 , 74 , 75  and  76 . Structure  80  is further comprised of a base support  79 . Each tongs linkage is joined to base support  79  via two sliding connections. 
     FIG. 17 shows an elevation view of structure  80 . FIG. 18 shows a perspective view of structure  80  in a partially retracted position. FIG. 19 shows structure  80  in a fully retracted position. 
     FIG. 20 shows a tongs linkage  100  which is comprised of twelve panel-links and  14  hub elements. Linkage  100  is shown in its extended position forming a continuous triangular shaped surface. Link-pair  121 , 122  is joined to link-pair  123 , 124  via hub elements  102 , 112 . Link-pair  123 , 124  is joined in turn to link-pair  125 , 126  via hub elements  103 , 113 . Similarly each successive link-pair is joined to its neighboring pair by a pair of hub elements. Linkage  100  is shown in elevation view in FIG.  21 . It may be seen to have a curved profile, this curvature being introduced by the various hub elements. FIG. 22 shows a perspective view of linkage  100  in its extended position. The edges of the panel-links may be seen to lie in planes  140  and  141 . Likewise, the center-planes of the hub elements lie in planes  140  and  141 . 
     FIG. 23 shows linkage  100  in a partially retracted position. The center-planes of the hub elements lie in planes  142  and  143 . The angle formed between planes  142 , 143  is identical to the angle formed between  140 ,  141 . FIG. 24 shows linkage  100  in its fully retracted position. The center-planes of the hub elements lie in planes  144  and  145 . The angle formed between planes  144 , 145  is identical to the angle formed between  140 , 141 . FIG. 25 shows a retractable structure  110 , which is a dome, in its fully extended position. Structure  110  is comprised of  24  linkages which are similar to linkage  100 , each linkage being connected to its neighbor by adjoining hub elements. Structure  110  is supported by a base  115  to which each of the  24  linkages are attached by sliding connections. 
     FIG. 26 shows structure  110  in elevation view. FIGS. 27 and 28 show structure  110  in its partially retracted and fully retracted positions respectively. 
     FIG. 29 shows panel-link  123  that is constructed of framing elements  140  that connect the three pivots, and border the polygonal profile. Link  123  is further comprised of an infill  141  which is a sheet material attached to frame  140 . Also shown in FIG. 29 is a linear link  124  which has three pivots. 
     FIG. 30 shows a tongs linkage in its extended position, whereby the linkage is comprised of four panel-links and four linear links. Link-pairs are made up of one panel-link and one linear link respectively. For example panel-link  123  is joined to linear link  124  by their central pivots. In its extended position the four panel-links form a triangular-shaped surface that is one layer thick. The linear links serve to synchronize the motion of the linkage, but do not provide covering themselves. Also shown in FIG. 30 is a stationary supporting structure  134  to which two links  127  and  128  are joined via slide elements  130  and  132  respectively. 
     FIG. 31 shows the tongs linkage in a partially retracted position, the slide elements having moved along the supporting structure. FIG. 32 shows the tongs linkage in its fully retracted position so that it is fully retracted to within the supporting structure. FIG. 33 shows a retractable structure  100  consisting of twelve tongs linkages which are attached to one another by their end pivots. Structure  200  is shown in its extended position whereby a continuous surface is formed having a semi-circular profile. 
     FIG. 34 shows a stationary supporting arch  220  and a supporting track  210 . FIG. 35 shows the retractable structure  200  attached to supporting structure  220  by a series of slide connections. Track  210  supports the edges of those linkages that lie on the border of the semi-circle. 
     FIGS. 36 and 37 show the structure  200  in its partially extended and fully retracted position respectively. Finally, FIG. 38 shows an elevation view of the retractable structure  200 . 
     It will be appreciated that the instant specification, drawings and claims set forth by way of illustration and not limitation, and that various modification and changes may be made without departing from the spirit and scope of the present invention.

Summary:
This invention discloses tongs-linkage, which, in its extended position, provides an essentially triangular-shaped surface, whereby the links of the tongs-linkage are themselves the panels that form the surface. Such assemblies may be planar, or, by use of intermediate hub elements, may form a surface with curvature. As such an assembly is compressed, the panel-links slide over one another, compressing down to a compact stack. Such tongs-linkages may be joined to similar linkages by pivots lying along their respective edges thereby forming extended structural surfaces. Surfaces that are planar, cone-shaped and doubly-curved surfaces of revolution are disclosed. In each case when the structure is retracted it compresses down to a compact linear element or ring.