Stretched membrane heliostat with integral bladder

A stretched membrane heliostat having a membrane mounted on a generally circular frame, there being a double-walled portion of the membrane that extends in a circle near the periphery of the membrane to form a bladder that is inflatable to tension the membrane.

BACKGROUND 
This invention relates to apparatus for tensioning a membrane of sheet 
material over a support frame, the stretched membrane being used to 
support a solar reflecting surface. In the manufacture of heliostats that 
use a taut membrane held over a support frame it is necessary to generate 
and maintain a uniform tensioning force in the membrane so as to provide a 
smooth planar surface for high efficiency solar reflection. Problems 
initially arise with respect to how to pull the membrane to the high 
tension levels which can be in the 50 to 100 pounds-per-inch range. 
Conventional methods of installing a tensioned membrane over a support 
frame required either the membrane alone or the frame alone being loaded 
during the attachment of membrane to frame. This necessitated the loaded 
element to be overloaded to achieve the desired final tension. To achieve 
these extra high loads required investment in a substantial structural 
fixture. Another area of concern lies with maintaining the stretched 
membrane at the desirable level of tension. For example, rivets and screws 
have a significant slippage and will relieve a large portion of the 
preload, bond materials may creep with time, and welding is difficult 
since the material is under load. In addition, ambient temperature changes 
will cause significant tension changes in the membrane, particularly when 
materials with different coefficients of expansion are used in support 
frame and membrane. Another problem with the prior art stems from support 
frame manufacturing errors and the existence of in-plane frame deflections 
which result in non-uniform membrane strain. In addition, former heliostat 
designs do not lend themselves to the convenience of attachment of 
membrane to frame in the field. 
SUMMARY OF INVENTION 
In view of the foregoing it is an object of the present invention to 
provide an apparatus for tensioning a membrane which does so by preloading 
the membrane and its support frame simultaneously. 
Another object of the invention is to provide a solar reflector having the 
capability to vary the tensioning forces in the reflective surface 
supporting membrane as necessary so as to maintain the membrane at the 
most desirable tension load. It is a further object of the invention to 
provide a membrane tensioning apparatus that avoids the build-up of 
non-uniform strain in the membrane so as to achieve a uniformly loaded, 
smooth planar surface. 
A still further object of the invention is to provide a heliostat which is 
suitable for in-the-field connection of membrane to frame. 
Other and further objectives of the invention will become apparent upon 
referring to the brief summary below, the detailed description thereafter 
following, and the drawings annexed hereto. 
Accordingly, the present invention provides an apparatus for tensioning a 
membrane on a support frame while simultaneously loading both support 
frame and membrane. The frame and membrane are preferrably circular in 
planform. There is a peripheral portion of the membrane that is 
double-walled and which extends in a circle. This double-walled portion 
and the interwall space define a fluid-inflatable bladder. 
Circumferentially extending portions of the membrane that are radially 
outward of the bladder attach to the support frame. Pressurization of the 
bladder will simultaneously induce compressive forces on the frame and 
uniform tensioning forces in the membrane. In one embodiment of the 
invention the bladder is pressurized by the liquid phase of a hardenable 
substance which is allowed to solidify to permanently hold the membrane at 
a desired tension. In another embodiment of the invention which permits 
the use of a frame that exhibits a low-torsional stiffness prior to 
membrane attachment, there are two membranes--one membrane engaging the 
top edge of the frame and the other engaging the bottom edge. 
It is noted that portions of this invention were made with government 
support under DOE Contract No. EG-77-C-01-4042, SERI Sub IP-2-02060.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIGS. 1 and 2 show a stretched membrane heliostat 11 that includes a steel 
support frame 13. The cross-section of frame 13 is a rectangular tube for 
which the height, width and thickness may be selected to provide the 
required torsional stiffness and out-of-plane bending stiffness. A 
circular frame 13 is preferred since it is the most efficient structure 
for loading. A circular membrane 15 comprising thins sheet (0.012 in.) 
steel, and having an integral bladder 17 formed from double wall portions 
19 and 21 is affixed to the upper wall 23 of frame 13 by conventional 
means such as welding, rivoting or bonding with high strength adhesive. In 
one embodiment of the invention the membrane is releasably attached to 
frame 13 by a system of circumferentially extending bolts and nuts. This 
allows quick assembly and disassembly of membrane 15 in the field and 
elsewhere. The most efficient membrane material is one with the highest 
strength to weight ratio that can be obtained in the 0.005 in. to 0.012 
in. thickness range. The bladder 17 is formed when the annular strip 21 is 
welded or bonded to the lower surface of membrane 15, opposite the portion 
19. The welding or bonding provide hermetic seams. 
In another embodiment of the invention, shown in FIG. 3, a support frame 
27, having a triangular cross-sectional figuration, is used. Here, the 
membrane 15a is affixed by conventional means to the inner tab portion 29 
of frame 27. The wall 19 holds a fluid valve (not shown) of conventional 
design to which means for pressurizing bladder 17 either hydraulically or 
pneumatically is connected. In order to minimize weight of pneumatic 
system is preferred. 
Still another embodiment is shown in FIG. 8. Here the frame 28 is of a 
tubular construction, and circular in planform as are the other preferred 
embodiments. The membrane 15(b) of similar construction to membrane 15, is 
attached by conventional means to the circumferentially extending inner 
tab 30. 
When the invention is to be assembled in the configuration shown in FIGS. 1 
and 2, the membrane 15, with bladder 17 unpressurized, is aligned with 
frame 13 and affixed thereto by means previously described. Bladder 17 is 
then gradually pressurized until the resulting tension in membrane 15 
reaches the desirable level. This loading procedure will provide for a 
self-centering tendency of the membrane to an equilibrium position which 
will tend to uniformly strain the membrane. It will also have a 
stabilizing effect on any in-plane deflections and manufacturing errors in 
the support frame. 
In a further embodiment of the invention, the bladder 17 is pressured to a 
desired level with a liquid mixture which will solidify into a rigid mass 
after curing. A silicone rubber would be suitable for this purpose. This 
rigidizing of the membrane provides for a stabilizing effect on the 
heliostat. 
The double membrane embodiment shown in FIGS. 4 and 5 has a top membrane 
15b and a bottom membrane 15c which include the major metallic portions 31 
and 33 which are interconnected by way of a connective woven material 35 
which extends from upper zippers 39 around frame 47 and to the lower 
zipper 41. The zippers are of conventional design. The woven material 35 
is of a substance suitable for outdoor exposure such as an ultraviolet 
stabilized polypropylene, however glass cloth, rayons and other high 
strength fabrics may be used. Wire fabric is also suitable. The strands of 
material 35 are arranged in a bias pattern which is symmetrical with 
respect to the radial direction at all locations around frame 47. Inner 
bladders 43 and 45 shown in FIG. 5 are composed of an elastomer such as 
Butyl rubber. This double membrane embodiment permits the use of frames 
having low torsional stiffness such as the frame 47 which is a ring formed 
from flat bar stock. Such a frame is more easily formed into a circle than 
frames of high torsional stiffness such as the frame 1 shown in FIG. 2 
which has a closed, hollow cross-sectional configuration. Frames of "I" 
beam and channel cross-sectional configurations are other suitable 
constructions having a relatively low torsional stiffness. 
In this regard FIG. 6 shows a double membrane embodiment using a frame 54 
formed from channel bar stock and FIG. 7 illustrates the use of an I-beam 
frame. Both of these examples of frames support a double membrane 
structure similar to that shown in FIG. 5. 
When the double membranes are attached to frame 47 and tensioned the 
resultant assembly will have the required torsional stiffness and a high 
degree of frame flatness is attainable. A preferrable method of assembly 
includes unzipping and detaching portions 31 and 33 and then positioning 
the connective material 35 around frame 47. The portions 31 and 33 may 
then be attached via zippers 39 and 41 and the assembly set upon a fixture 
(not shown) that provides a flat planar aligning surface for frame 47. 
Bladders 43 and 45 are partially pressurized to provide some membrane 
tension and the frame 47 is pushed into a flat position against the 
aligning fixture surface. During this step the connective material 35 can 
slip relative to frame 47 are required to allow the frame 47 to freely 
twist into a flat plane. Bladders 43 and 45 may then be further 
pressurized to tension membrane portions 31 and 33 to the desirable level 
and the material 35 affixed to the frame by means of fasteners 51. This 
assembly method ensures that a relatively imperfect and low cost frame may 
be formed into a flat assembly of high precision. 
While there has been described particular embodiment of the invention, it 
will be obvious to those skilled in the art that various changes and 
modifications may be made therein without departing from the invention 
and, therefore, it is aimed to cover all such changes and modifications as 
fall within the true spirit and scope of the invention.