Stretched membrane device

[0001]

U.S. Patent Documents:
	2,300,251	1941	Flint
	2,952,189	1960	Pajes
	3,031,928	1962	Kopitko
	3,056,131	1962	Mcreary
	3.610.738	1971	Bochmann
	3,687,524	1972	Martinez
	3,757,479	1973	Martinez
	3,877,139	1975	Martinez
	3,880,500	1975	Kojabashian
	4,033,676	1977	Brantley, Jr. et al
	4,046,462	1977	Fletcher et al
	4,068,777	1978	Humphrey, et al.
	4,130,234	1978	Schmidt
	4,352,112	1982	Leonhardt, et al
	4,741,609	1988	Sallis
	4,987,826	1991	Deppert, et al.
	5,590,497	1997	Moore
	5,680,262	1997	Soliday, et al.
	5,813,830	1998	Smith, et al.
	5,990,851	1999	Henderson, et al.
	6,332,687	2001	Carreras, et al
	6,716,017	2004	Papadopoulas

[0002] Membranes, esp. polymeric membranes, provide an economical method of presenting large surfaces to electromagnetic, acoustic, or other energy, for the purpose of absorbing, reflecting, focusing, or other manipulation of this energy.

[0003] Flint (U.S. Pat. No. 2,300,251, 1941) describes fabricating a lens by using two transparent membranes with a clear fluid between. Focal adjustment is made by mechanically adjusting the frame, or by varying the fluid pressure between the membranes, or diaphragms.

[0004] Membranes have also enabled the design of inexpensive mirrors when a membrane is coated with a reflective coating, and then are stretched over a suitable frame.

[0005] Martinez (U.S. Pat. Nos. 3,687,524, 3,757,479, and 3,877,139 1972-1975) described a flat “glassless” mirror stretched over a sheet metal “pan”.

[0006] Additionally, the shaping of such stretched reflective membrane mirrors into a concave or convex lens shape enables the mirror surface to now become a reflective lens with an approximate spherical/parabolic shape useful for radiating, receiving, reflecting or focusing electromagnetic radiation or acoustic waves. A concave shape may be established by applying a vacuum within the sealed chamber formed by the membrane and it's supporting frame. A convex shape can be established by applying a positive pressure within the sealed chamber. For space-based applications where near vacuum ambient conditions exist, a second clear membrane may be placed over the reflective membrane, and positive pressure introduced between the two membranes to induce a concave shape into the reflective inner membrane.

[0007] Such designs have been suggested for use in solar concentrating dishes, radio antennas and also for imaging applications such as telescopes and holographic projection.

[0008] Prior art with reference to utilizing a stretched membrane material as a lens, or reflective lens include:

[0009] Pajes (U.S. Pat. No. 2,952,189, 1960) designed a drum which is evacuated to induce a concave shape.

[0010] Kopitko (U.S. Pat. No. 3,031,928, 1962) described a dual diaphragm system and a controlling method to maintain curvature.

[0011] Bochmann (U.S. Pat. No. 3,610,738, 1971) depicted an arrangement whereby a diaphragm was pulled by a cam to induce a vacuum, and hence concave shape.

[0012] Brantley, Jr. et al, (U.S. Pat. No. 4,033,676 1977) depicted two hoops separated by vertical strut members. The two membranes form a cavity inside the circumference, and membranes also form a pressure barrier around the strut members. The frame function is separate from the sealing pressure barrier.

[0013] Kojabashian (U.S. Pat. No. 3,880,500, 1975) described a frame design and a method utilizing two films that equalize the pneumatic forces on the supporting structure.

[0014] Soliday, et al. (U.S. Pat. No. 5,680,262 1997) utilizes a tubular frame of square cross section, and describes a tensioning method for the stretched membrane utilizing a plurality of pneumatic cylinders.

[0015] Carreras, et al (U.S. Pat. No. 6,332,687, 2001) describe utilizing a vacuum for primary deformation, and also using an outer mechanical ring and central plunger to achieve a more parabolic shape.

[0016] Each of these designs utilized some type of frame over which the reflective membrane is stretched:

[0017] Pajes (U.S. Pat. No. 2,952,189, 1960) and Bochmann (U.S. Pat. No. 3,610,738, 1971) utilized a drum design.

[0018] Kopitko (U.S. Pat. No. 3,031,928, 1962) utilized two parallel plates, with the film stretched at the midpoint.

[0019] One problem with previous designs is that although the reflective membrane affords an inexpensive method of presenting and manipulating large surfaces for the purpose of directing electromagnetic radiation or other energy, the framework for the membrane must be very strong to withstand the considerable pneumatic forces required to establish the membrane shape with minimal distortion. The primary aim of the frame is to establish a nearly perfect round shape with a circumferential raised portion that establishes a near perfect plane for the attachment and sealing of a non-porous membrane to the frame structure. An internal cavity is required to allow for the deformation of the membrane material. These past designs often utilized heavy and expensive frames.

[0020] Kopitko (U.S. Pat. No. 3,031,928, 1962), and Brantley, Jr. et al, (U.S. Pat. No. 4,033,676 1977), Kojabashian (U.S. Pat. No. 3,880,500,1975), Leonhardt, et al (U.S. Pat. No. 4,352,112 1982), and Soliday, et al. (U.S. Pat. No. 5,680,262 1997) all address this problem by utilizing two membranes with the vacuum or pressure within, thus equalizing much of the resulting forces. This has the disadvantage in that since both membranes stretch, the frame needs to be of considerable width to prevent the two membranes from touching in the middle when a vacuum is applied. Also, the sole structural element becomes the outer ring or drum, which can still oval and warp, distorting the desired focus or other manipulation of the directed electromagnetic or other radiation. The ovaling stress is non-uniform, and also tends to exacerbate the formation of wrinkles or waves in the membrane surface.

[0021] Inflatable systems eliminate much of the weight problem, and add a portability function, but tend to experience more distortion.

[0022] Mcreary (U.S. Pat. No. 3,056,131, 1962) circumvents the frame with an inflatable dish with reflective material on the back side, and transparent material at the front.

[0023] Wladimir von Maydell et al. (U.S. Pat. No. 3,326,624) describes an inflatable mirror for use in space-based applications.

[0024] One object of this invention that is believed to solve many of these problems is an improved frame design, utilizing a combination of two basic geometric shapes (a ring and a flat plane) which are mutually reinforced to arrive at a lightweight, rigid and true structure for supporting the stretched membrane.

[0025] Leonhardt, et al (U.S. Pat. No. 4,352,112 1982) describe utilizing a circumferential ring or rings with a vertical member to increase height in a drum-like arrangement. The ring or rings, however are described with two membrane surfaces forming the top and bottom walls of the structure.

[0026] Ring reinforcement has been applied in the past for a wide variety of unrelated applications, such as rolling a lip into the rim of a paper cup, a formed lip of a jar lid, and as with the molded lip on a container or with flying discs:

[0027] Schmidt (U.S. Pat. No. 4,130,234, 1978) and others discuss improved methods of rolling a lip surface into a cup design for greater strength. The goal here is to economically increase the strength of the rim.

[0028] Sassak (U.S. Pat. No. 5,116,275, 1992) discusses a flying toy with a reinforcing lip, but molded in.

[0029] Weiss (U.S. Pat. No. 5,366,403, 1994) discusses utilizing a reinforcing ring to convert a disposable plate into a flying toy.

[0030] Gilliam, et al. 6,761,283 2004 and others describe molding a lip, or “brow ridge” into a container and closure for same as a way of increasing the strength.

[0031] The primary function of the ring in this invention is unique—to provide mutual reinforcement along with a backplane element for the purpose of establishing a nearly perfect round shape with a circumferential raised portion that establishes a near perfect planar surface for the attachment and sealing of the membrane. Further, the application of negative or positive pressure inside the proposed structure causes a uniform warpage of the backplane element, increasing the strength of the structure without distortion of the manipulated electromagnetic, acoustic, or other energy.

[0032] Another object of this invention are methods of preventing wrinkles when a stretched membrane (esp. of polymeric material) is tensioned in multiple directions, as when it is stretched over a frame in a uniform manner, and then pneumatically deformed.

[0033] Sail battens have been utilized for minimizing wrinkling of sails, and also for shaping sails into an ideal aerodynamic form. Also similar to this invention, sail battens float on (are solely support by) the stiffened sail surface.

[0034] Mauney (U.S. Pat. No. 2,743,510 1956) describes an inflatable batten, and also one that forms an arc, but not a complete circle

[0035] Leonhardt, et al (U.S. Pat. No. 4,352,112 1982) describes prestressing a diaphragm in order to eliminate folds, and also discusses various reinforcing strips for the purpose of altering the stressed membrane configuration. The reinforcing ring depicted forms only an arc, and is not completely annular.

[0036] Henderson (U.S. Pat. No. 5,333,569 1994) describes an inflatable sail batten

[0037] Baird (U.S. Pat. No. 5,095,837 1992) describes a ram air inflatable form for a spinnaker sail, again with a curved surface, but not a completely round surface.

[0038] None of this prior art relates to preventing wrinkles in 360 degree round structure through the use of a circumferential batten.

[0039] Another object of this invention is to provide a means of uniformly tensioning the reflective membrane without inducing distortion via the application of heat to the circumference of the ring. Ambient temperature changes may affect the membrane material in a different way than the supporting structure, due to differences in thermal coefficient of expansion. The membrane may remain tightly stretched at one ambient temperature, but become loose when the ambient temperature is lowered, or vica versa. This problem increases as the size of the structure increases.

[0040] Martinez (U.S. Pat. Nos. 3,687,524, 3,757,479, and 3,877,139 1972-1975) described a method of tensioning a membrane mirror surface through the application of heat to shrink the film. This is a one-time manufacturing process, however, as opposed to a method of controlling the tension to accommodate changing ambient temperatures.

[0041] Sallis, (U.S. Pat. No. 4,741,609 1988) described a tensioning method for a membrane reflector utilizing an inflatable bladder, but no thermal component is mentioned.

[0042] Deppert, et al. (U.S. Pat. No. 4,987,826 1991) describes a dynamic thermal tensioning method for a piston rod-to-cylinder sealing ring, but utilizing a fluid filling medium.

[0043] Moore (U.S. Pat. No. 5,590,497 1997) discussed mechanical circumferential tensioning of prestress cables in a concrete tank.

[0044] Smith, et al. (U.S. Pat. No. 5,813,830 1998) utilizes springs for circumferential tensioning of a carbon seal containment barrier system inside a turbine engine.

[0045] Henderson, et al. (U.S. Pat. No. 5,990,851 1999) discusses circumferential tensioning of a space-based antennae, but not by thermal means.

[0046] Papadopoulas (U.S. Pat. No. 6,716,017 2004) discusses circumferential tensioning of an embossing roll, but not by thermal means.

[0047] The principle object of this invention is to provide an improved device for reflecting, radiating, or receiving electromagnetic radiation, acoustic waves, or other energy forms through the use of a membrane stretched across a lightweight, round, frame structure. The primary purpose of the structure is to establish a nearly perfect round shape with a circumferential raised portion that establishes a near perfect plane for the attachment of a membrane. The structure primarily consists of two basic geometric shapes—a flat semi rigid planar surface attached to a circumferential ring or stack of rings. The flat planar surface and circumferential ring(s) provide mutual reinforcement, and result in an internal cavity which may be utilized for clearance to permit the deformation of the said stretched membrane covering the cavity.

[0048] Further, the resulting shape becomes more rigid as vacuum or pressure is placed inside the sealed area between the structure and the reflective membrane. The primary concave or convex deformation is induced into the reflective membrane, but the semi-rigid backplane will also warp slightly and uniformly into a near-perfect concave or convex shape that is stronger and more rigid than the unstressed structure without causing significant distortion of the desired membrane manipulation. Further, the minimal concavity induced on a semi-rigid backplane minimizes the depth required of the supporting structure.

[0049] Another object of the invention comprises a unique method of preventing wrinkles that typically form in the periphery of the reflective membrane surface as it is deformed. Many planar materials, and especially polymeric materials, will develop large wrinkles or waves when tensioned into a concave or convex shape about a circular frame. These large wrinkles or waves scatter the object radiation, acoustic, or other energy away from the desired direction. This invention inhibits the propagation of these wrinkles or waves through the attachment of a flat, rigid “floating batten”. This stiffening batten has a circumference slightly less than that of the main structure, is solely supported by the membrane itself, and inhibits the wrinkles from propagating from the outside circumference inward.

[0050] Another object of the invention provides a unique method of thermally tensioning the stretched film membrane by applying heat circumferentially around the ring or rings. The ring heats up, expanding in a uniform manner, while the film remains at ambient temperature. Circumferential tensioning is uniform, minimizing the distortion of the desired manipulation of electromagnetic, acoustic or other energy. Method of circumferential heating may be via solar radiation, resistance heating, or other method of generating heat.

[0064] As shown in the drawings, the preferred embodiment in accordance with the present invention is an improved stretched membrane device for reflecting, radiating, or receiving electromagnetic radiation, acoustic waves, or other energy forms through the use of a membrane stretched across a lightweight, round, frame structure. The preferred embodiment, depicted in FIG. 1 comprises a round frame consisting of (1) a near perfectly round and flat semi rigid foamboard backplane element attached to (2) a stack of near perfectly round circumferential reinforcing ring elements. A reflective membrane element (3) consisting of a non-porous reflective material is stretched across the cavity formed by the structure, and attached to the circumferential ring, thereby forming one wall, and sealing said cavity. A vacuum (4) is placed into to cavity, causing a primary uniform concave deformation of the reflective membrane for the object of manipulating said forms of electromagnetic, acoustic, or other energy. The vacuum also induces a slight, uniform convex shape into the semi-rigid backplane material that increases the total strength of the frame structure with minimal distortion of the object manipulations of energy. An annular floating batten element (5) of slightly less diameter than the frame is attached to and solely supported by the reflective membrane, and prevents the propagation of large wrinkles or waves that form in the reflective membrane as it is tensioned by the application of vacuum. A circumferential heater (6) of solar or other source is provided to induce a uniform growth or shrinkage to the reinforcing ring, thereby evenly tensioning the reflective film, again with minimal distortion. Insulation (7) is applied over the heated rings for the purpose of retaining heat.

[0065] FIGS. 2-7 provide an understanding of the problems of fabricating a large supporting frame structure for the embodiment, and also the advantages of this improvement.

[0066] FIG. 2 depicts a typical ring element. The ring may constructed of any material, may be hollow or solid, and may be of any cross sectional area. FIG. 3 depicts an oval distortion typical of a ring when gravitational, mechanical, pneumatic or other forces are applied to it.

[0067] FIG. 4 depicts a round, flat planar surface which may be constructed of foamboard, cardboard, plywood, or honeycombed material with backing.

[0068] FIG. 5 depicts typical distortion, non-uniform warpage, or “potato chipping” that can occur from gravitational, mechanical, pneumatic or other forces are applied to it, and also by the absorption of moisture or other fluids into the material.

[0069] FIG. 6 depicts an assembly whereby a near perfectly round ring (2) as depicted in FIG. 2 is solidly attached to a near perfectly flat backplane surface (1) as depicted in FIG. 4. The circumferential rigidity imparted by the ring inhibits the distortion in the backplane element noted in FIG. 4, and the backplane material inhibits ovaling of the ring or rings element as shown in FIG. 2. The resulting assembly results in a nearly perfect round shape with a circumferential raised portion that establishes a near perfect plane (4) for the attachment and sealing of a membrane. An internal cavity (3) is formed by this assembly, which provides clearance for the uniform deformation of the said membrane.

[0070] FIG. 7 depicts an assembly with a stack of rings (4) with a means of attaching them to the backplane and to each other, providing additional depth to the cavity, and a non-porous membrane (1) attached and sealed to the circumferential plane established across the top, forming a sealed cavity into which a sub-ambient pressure (2) may be applied and held. Said sub-ambient pressure causes a primary uniform concave deformation in the membrane material, and also causes a slight but uniform convex deformation in the semi rigid backplane material (3), which increases the strength of the total assembly without inducing significant distortion to the membrane or to the object manipulation of the said electromagnetic, acoustic, or other energy.

[0071] FIG. 8 depicts an assembly with a stack of rings (4) providing additional depth to the cavity, and a non-porous membrane attached and sealed across the top, forming a sealed cavity into which a super-ambient pressure (1) may be applied and held. Said super-ambient pressure causes a primary uniform convex deformation in the membrane material (2), and also causes a slight but uniform concave deformation in the semi rigid backplane material (3), which increases the strength of the total assembly without inducing significant distortion to the membrane, or to the object manipulation of the said electromagnetic, acoustic, or other energy.

• • [0072] FIG. 9 depicts an assembly with a stack of rings (6) providing additional depth to the cavity, and two non-porous membranes attached and sealed across the top. Super-ambient pressure (3) may be applied and held between the two membranes, causing a convex manipulation of the outboard membrane (1), and a concave manipulation of the inboard membrane (2). The cavity between the inboard membrane and the backplane element is vented (4). Backplane element (5) remains flat in this instance. Such an arrangement would be advantageous in forming an optical lens by using two transmissive membranes. Another use would be for a space-based reflective lens with an outboard transmissive membrane, and a reflective inboard membrane.

[0073] FIG. 10 depicts large wrinkles or waves (1) that form in membrane films when tensioned in multiple directions, such as in FIG. 7, when a sub-ambient pressure is applied inside the sealed cavity. Many membrane materials display some small amount of elasticity when tensioned uniformly in one direction. When tension is simultaneously applied to said membrane in more than one direction, however, large wrinkles or waves form at the circumference of the membrane, and radiate inward. These waves or wrinkles display maximum amplitude (2) just inside the circumference, and decreasing amplitude as the deformation progresses toward the center. Said wrinkles or waves in the membrane material direct the object electromagnetic, acoustic, or other energy away from the desired manipulation.

[0074] FIG. 11 depicts a unique circular, flat, rigid annular batten (1) of slightly less diameter than the structure that it is firmly attached to, and solely supported by the reflective membrane (2). This floating annular batten inhibits the propagation of said waves or large wrinkles.

[0075] FIG. 12 depicts a unique method of thermally tensioning the stretched film membrane by applying heat circumferentially around the ring or rings. The ring(s) (1) heat up, expanding in a uniform manner, while the film (2) remains at ambient temperature. Circumferential tensioning minimizes deformation of the reflective membrane and also of the desired manipulation of electromagnetic, acoustic or other energy. FIG. 12 depicts a solar absorbing material (3) attached to the outer face of the reflective membrane. A conducting material (4) underneath carries the heat generated to the outer diameter of the rings. An insulating layer (5) is applied on top to prevent heat loss.

[0076] FIG. 13 depicts another method of applying circumferential heat via an electrical heating element. An electrical resistance heater element (1) is attached circumferentially to the ring(s), electricity is applied and controlled (2), and an insulating layer (3) is provided to prevent heat loss.

[0077] The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.