Abstract:
A thin film reflector with a compliant border has been developed. The thin film reflector includes a reflective membrane and a support apparatus that applies a tensile force to the reflective membrane. A compliant border region is located between the reflective membrane and the support apparatus to distribute the tensile forces applied by the support apparatus evenly across the reflective membrane.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates generally to thin film reflectors. More specifically, the invention relates to a compliant border for thin film reflectors. 
         [0003]    2. Background Art 
         [0004]    Thin planar membranes are typically thin polymer films with, a reflective metal coating. These membranes are used in space flight and orbital applications to reflect or concentrate solar energy. A reflective membrane may be used to protect orbital structures and equipment such as satellites from direct exposure to solar radiant flux. Alternatively, a reflective membrane may be used to concentrate solar energy on equipment such as a solar panel that powers a satellite. 
         [0005]    When reflective membranes are deployed, they are typically held in place by application of a tensile or “stretching” force that is applied by attaching cables to the comers of the membrane.  FIG. 1  shows an example of a prior art corner support  10  In this example, tire four corners of the rectangular shaped membrane  12  are attached to support cables  14 . A tensile force  16  is applied to each cable  14  and the membrane  12  extends and holds its shape. The cables attempt to uniformly distribute the load across the membrane. In order to avoid wrinkles, the shape and elastic properties of the membrane must be precisely matched with the shape and elastic properties of the cables. Without a precise match, the membrane develops inherent wrinkles  18  called “Poisson wrinkles” on its surface. These wrinkles  18  have a negative impact on the reflective performance of the membrane.  FIG. 2  shows another example of a prior art border support  20 . In this example, the membrane  22  is held in place with catenary-shaped support cables  24 . As with the previous example, a tensile force  26  is applied to each support cable  24  to hold the shape of the membrane. Poisson wrinkles  28  are also inherent in the membrane of this example. 
         [0006]    In order to minimized the presence of wrinkles, the shape and elastic properties of the cables must be precisely matched to the shape and properties of the membrane. However, the precise matching is difficult. Consequently, a compliant border of a reflective thin film membrane that reduces the presence of wrinkles is needed. 
       SUMMARY OF INVENTION 
       [0007]    In some aspects, the invention relates to a thin film reflector, comprising: a reflective membrane; a support apparatus that applies a tensile force to the reflective membrane; and a compliant border between the reflective membrane and the support apparatus. 
         [0008]    In other aspects, the invention relates to a thin film reflector, comprising: a reflective membrane; a support apparatus that applies a tensile force to the reflective membrane; and means for reducing wrinkles in the reflective membrane by utilizing a compliant border between the reflective membrane and the support apparatus, 
         [0009]    In other aspects, the invention relates to a thin planar membrane apparatus, comprising: a thin-film membrane; a support apparatus that applies a tensile force to the thin-flim membrane; and a compliant border between the thin-film membrane and the support apparatus. 
         [0010]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    It should be noted that identical features in different drawings are shown with the same reference numeral. 
           [0012]      FIG. 1  shows a view of a prior art corner support for a reflective membrane. 
           [0013]      FIG. 2  show a view of a prior art border support for a reflective membrane. 
           [0014]      FIG. 3  shows a view of a compliant border support for a reflective membrane in accordance with one embodiment of the present invention. 
           [0015]      FIGS. 4   a - 4   c  show detailed views of a compliant border support for a reflective membrane in accordance with one embodiment of the present invention. 
           [0016]      FIGS. 5   a - 5   c  show detailed views of membranes of a compliant border support for a reflective membrane in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    A compliant border for thin film reflectors has been developed.  FIG. 3  shows an example of a compliant border support  30  for a reflective membrane. The membrane is typically of a light weight; thin polymer that is rendered reflective by coating it with, metallized evaporative coating. The membrane typically has a thickness of 1.0-127 microns. In this embodiment, the membrane  32  is supported by parabolic or circular shaped border cables  34  with an applied tensile force  36 . A compliant interface  38  is located between the membrane  32  and the cables  34 . The interface  38  greatly reduces to precision required match the shape and elastic properties of both the cables and the membrane. This has the effect of eliminating wrinkles on the membrane  32 . The lack of Poisson wrinkles indicates a biaxial tensile stress in the membrane  32 . 
         [0018]      FIG. 4   a  shows a detailed view of the membrane  32 , cable  34  and compliant interface  38  shown in  FIG. 3 ,  FIG. 4   b  shows an alternative embodiment of the present invention. In this example, the compliant interface  40  is a separate surface of an elastomer membrane or other elastic type material. This interface  40  is separated from the membrane  42  by a distinct edge  41 . In this embodiment, the interface  40  absorbs and distributes the tensile forces applied by the cables so that Poisson wrinkles do not appear on the membrane  42 .  FIG. 4   c  shows another embodiment of the present invention. In this example, the cable  44  is isolated from the membrane  46  by a series of cords  48  running perpendicular to the membrane edge. The cords absorb and distribute the shear strain applied by the cables so that Poisson wrinkles do not appear on the membrane  46 . 
         [0019]      FIGS. 5   a  shows another embodiment of the present invention. In this example the cable  52  is isolated from the membrane  50  by a compliant interface  54  of slits in a membrane surface. The slits are oriented perpendicular to the edge of the membrane  50 . The slits absorb and distribute the tensile forces applied by the cables so that Poisson wrinkles do not appear on the membrane  46 . 
         [0020]      FIGS. 5   b  and  5   c  show another embodiment of the present invention. In this example the cable  56  is isolated from the membrane  58  by a compliant interface  60  of indentions in a membrane surface.  FIG. 5   c  shows a cross-sectional view of the compliant interface  60 . The indentions are oriented perpendicular to the edge of the membrane. As with corresponding elements of other embodiments of the present invention, the indentions absorb and distribute the shear strain applied by the cables so that Poisson wrinkles do not appear on the membrane  58 . The indentions of the interface are fanned in the membrane surface by a thermal forming process that uses the application of heat and a vacuum. The heated material of the membrane flows into a forming mandrel that forms the shape of the indentions. 
         [0021]    While the previous examples have been used with a reflective membrane, it should be understood that the present invention may also be used with a non-reflective membrane. Non-reflective materials may include both clear and opaque materials. For example these materials could be a protective film that is used to protective the underlying surface from thermal exposure. The use of a compliant border serves to more evenly distribute the stress load across the membrane and consequently lengthen its life span. 
         [0022]    In some other embodiments the reflective membrane could be used to reflect or concentrate solar radiant flux. The membrane could be used on satellites, solar sails, or other space applications. Additionally, other mechanisms besides cables could be used to apply the tensile force to the membrane such as fixed points or a support frame. While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which, do not depart from the scope of the invention as disclosed here. Accordingly, the scope of the invention should be limited only by the attached claims.