Patent Application
20110298687
The present invention relates to lightweight antenna arrays and more particularly to an attachment mechanism for attaching a lightweight antenna array to a structure.
Antenna structures have been developed to provide light-weight antenna arrays including “active” or “phased” array antennas. However in many cases the lightweight materials and carefully calibrated electrical elements produce a delicate antenna structure. At the same time, these lightweight antenna structures may be deployed on platforms that are exposed to various thermal and structural loads and possibly harsh environmental conditions.
For example, one application for lightweight antenna structures is high-altitude surveillance, such as high-altitude balloons. During the balloon's flight, temperature conditions through the atmosphere may change considerably, causing the balloon material to expand or contract. The material of the balloon itself differs from the antenna structure and may have a different coefficient of thermal expansion. Due to the mismatch in thermal expansion between the balloon platform and its antenna payload, the balloon may expand more or less than the antenna structure, thereby stressing the joint or bond between the balloon and the antenna. These “thermal” stresses due to differential thermal expansion can cause failure of the joint or the antenna structure itself, or cause other problems such as warping or mis-alignment of the antenna structure. Prior attachment mechanisms include direct adhesive bonding, mechanical joints, and lanyards. Adhesively bonding the panel directly to the balloon material does not account for the thermal mismatch between the materials, or variations in the two surfaces (such as surface features on the panel, or curvature of the balloon). Rigid mechanical joints at the corners of the panels can lead to structural failure at the corners. Lanyards, loops, and other similar attachments may not be precise enough for alignment of the antenna array, and the antenna panels may bend, swing, or move out of place. These attachment structures can also add significant weight to the system.
Accordingly there is still a need for an attachment mechanism for attaching lightweight antenna structures to a platform exposed to various thermal and/or other stresses.
The present invention relates to lightweight antenna arrays and more particularly to an attachment mechanism for attaching a lightweight antenna array to a structure with a different coefficient of thermal expansion. In one embodiment, an antenna system includes an array of antenna panels that are mounted to a platform structure, such as a high-altitude balloon. An attachment mechanism is provided to mount the antenna panels to the platform, providing a fixed structural mount while insulating the panels from the mismatch in thermal expansion. In one embodiment, the attachment mechanism comprises a support structure between the panels and the platform. The support structure includes a plurality of spacer elements that separate the antenna panels from the platform. The spacer elements are made of a stiff foam material and are adhered at one end to the platform and at the opposite end to an antenna panel. The spacer elements are located and dimensioned according to the thermal and structural properties of the antenna panels and the platform, in order to provide a strong structural mount for the panels while also spacing the panels away from the platform, thereby providing flexibility for the mismatch in thermal expansion between the two structures.
In one embodiment, an antenna structure includes a platform having a first coefficient of thermal expansion; an antenna panel having a second coefficient of thermal expansion different from the first coefficient, and having first and second opposite ends; and a support structure mounting the panel to the platform. The support structure includes a first spacer element with a first height at the first end of the panel, and a second spacer element with a second height less than the first height between the first and second ends of the panel; a first adhesive layer adhering each spacer element to the platform; and a second adhesive layer adhering each spacer element to the antenna panel. A yield strength of the adhesive layers is less than a yield strength of the spacer elements.
In one embodiment, an antenna structure includes a platform having a curved surface; an array of antenna panels; and first and second blocks mounting each panel to the curved surface. The first and second blocks have first and second heights, respectively, that are different from each other. Each block is adhered to the curved surface, and the blocks comprise a foam material. Each block is approximately 0.5 inches in width, and the blocks are spaced apart from each other by approximately 2-5 inches.
In one embodiment, a method of mounting an antenna panel to a platform includes providing a platform having a first coefficient of thermal expansion; providing an antenna panel having a second coefficient of thermal expansion different from the first coefficient, and having first and second opposite ends; and mounting the panel to the platform. Mounting the panel to the platform includes providing a first spacer element with a first height at the first end of the panel; providing a second spacer element with a second height less than the first height between the first and second ends of the panel; adhering each spacer element to the platform with a first adhesive layer; and adhering each spacer element to the antenna panel with a second adhesive layer. A yield strength of the adhesive layers is less than a yield strength of the spacer elements.
The present invention relates to lightweight antenna arrays and more particularly to an attachment mechanism for attaching a lightweight antenna array to a structure with a different coefficient of thermal expansion. In one embodiment, an antenna system includes an array of antenna panels that are mounted to a platform structure, such as a high-altitude balloon. The antenna panels have a higher coefficient of thermal expansion than does the platform structure, meaning that the material of the panels expands more with temperature than does the platform. Despite this mismatch in thermal expansion between the two structures, the antenna panels need to be firmly mounted to the platform in order to be properly oriented and aligned with each other.
According to an embodiment of the invention, an attachment mechanism is provided to mount the antenna panels to the platform, providing a fixed structural mount while insulating the panels from this mismatch in thermal expansion. The attachment mechanism acts as a buffer for thermal stresses. In one embodiment, the attachment mechanism comprises a support structure between the panels and the platform. The support structure includes a plurality of spacer elements that separate the antenna panels from the platform. The spacer elements are made of a stiff foam material and are adhered at one end to the platform and at the opposite end to an antenna panel. The spacer elements are located and dimensioned according to the thermal and structural properties of the antenna panels and the platform, in order to provide a strong structural mount for the panels while also spacing the panels away from the platform, thereby providing flexibility for the mismatch in thermal expansion between the two structures. The spacer elements provide a flexible link between the antenna panel and the platform.
An antenna system 10 according to an embodiment of the invention is shown in
In one embodiment, the platform 12 is a high-altitude balloon made of a material such as a polymer film, or laminated layers of high-strength fiber material such as Dyneema® (DSM Dyneema LLC, Stanley, N.C.). The material may be very thin, for example 0.004 inches. This material has a first coefficient of thermal expansion, which indicates the extent to which the material expands with temperature. In one embodiment the coefficient of thermal expansion is approximately −8 ppm/° C. (where ppm is parts per million). That is, the coefficient is negative, meaning that the material actually contracts with increasing temperature. This can cause a large mismatch in linear movement between the platform material and the antenna panels.
In one embodiment, the panels 16 of the antenna array 14 are panels of active or “phased” antenna elements. The entire array 14 includes many panels 16 arranged together, spaced apart from each other by a small distance δ. In one embodiment, δ is approximately 1 inch. In one embodiment each panel is approximately 1 square meter in size. The panels 16 cooperate together to form the aperture of the antenna array. The active antenna elements on the individual panels 16 and the panels themselves are spaced and aligned with each other precisely in order to enable the antenna elements to cooperate together to send and receive signals. In one embodiment, the panels 16 are made up of layers of thin sheets adhered together, such as thin films of liquid crystal polymer (LCP). These thin films are corrugated and adhered together, and may have circuits or other components printed on them. This material has a second coefficient of thermal expansion that is higher than the first coefficient of the balloon material. That is, the panels 16 expand more with increasing temperature than the balloon material expands. In one embodiment, the panel has a coefficient of thermal expansion of approximately 17 ppm/° C. This coefficient is positive, meaning that the panels expand with increasing temperature.
The panels themselves are substantially rigid. In one embodiment, the panels are light-weight antenna panels that are rigid and relatively fragile, such as active electronically scanned array (AESA) panels. These panels have a delicate structure of electrical components and layers of light-weight material. In one embodiment, the panels have a thin film folding structure, including spaced apart sheets and layers acting as a support structure (as indicated by dotted lines in
In the embodiment shown in
In one embodiment, the spacer elements 22 are discrete blocks spaced apart from each other, such as, for example, the cylindrical blocks 24 shown in
At the same time that the spacer elements 22 fix the panels 16 in place, the spacer elements 22 also provide flexibility, enabling the platform 12 to expand and contract without transmitting this movement directly to the panels 16. The spacer elements 22 lift the panels 16 away from the surface 20 of the platform so that the panels 16 do not actually contact the surface 20. When the platform 12 contracts, the spacer elements 22 absorb some of this movement (strain) without transmitting it to the panels 16. The space between each spacer element 22 also enables the platform 12 to expand or contract without directly affecting the lifted panels 16. When the platform 12 contracts, the spacer elements 22 are stressed, as they adjust between the two mismatched structures 12, 16. However the spacer elements 22 and the adhesive layers (described below) are selected such that these elements can withstand the stress from the thermal mismatch, thereby acting as a buffer between the platform 12 and the panels 16 and insulating the panels from the thermal mismatch.
In one embodiment, the spacer elements 22 are made of a lightweight cellular material, such as a foam material. In one embodiment the material is a rigid, low-density foam, such as polymethacrylimide. This material is light-weight (low density) and stiff, providing a high strength-to-weight ratio.
In one embodiment, the foam material is Rohacell® (Evonik Industries, Darmstadt, Germany), a shear- and pressure-resistant, light-weight foam structure. In particular, Rohacell® P190 was found during testing to provide a sufficient stiffness for supporting the panels 16, while also being able to adjust to expansion and contraction without fracturing. In another embodiment, the material is Rohacell® 200WF.
In one embodiment, the spacer elements are secured to the platform and to the panels by adhesive, as shown for example in
The blocks 124a-c are adhered to both the panel 116 and the platform 112. The blocks each have a top surface 126 and a bottom surface 128. In the embodiment of
In one embodiment, the coefficients of thermal expansion of the various materials are listed, from highest to lowest, as follows: the adhesive layers 130, 132, the antenna panels, and the platform material.
In one embodiment, the foam material of the blocks 124a-c has a higher yield strength than the adhesive of the layers 130, 132. As a result, the adhesive reaches its yield strength before the foam does, and the adhesive begins to yield. Its elastic modulus is effectively reduced, and the material becomes less stiff. The adhesive is then able to absorb the strain due to the differential expansion of the platform and panels during thermal loading. In one embodiment, the adhesive has a high elongation (such as, for example, above 300%, such as approximately 400%), which enables the adhesive to absorb the strain without failing. The adhesive is a flexible bonding adhesive that remains flexible at low temperature, so that it deforms and adjusts to accommodate movement of the platform, blocks, and panels relative to each other. The adhesive also acts as a damping mechanism, to protect the array from vibrations, and an electrical insulator.
In one embodiment, the adhesive is initially stiffer than the foam, but has a lower yield strength. Once exposed to high strains, the adhesive yields and becomes less stiff. This reduces stress on the foam and allows high strains to be absorbed by the adhesive without failure of the joint. The yield strength of the various materials is the stress at which the material begins to deform plastically, and can be determined through tensile testing (measuring stress and strain as a sample of the material is pulled until it yields or breaks).
In one embodiment, the surface 20 of the platform 12 is curved, as shown in
Referring to
Another embodiment of the invention is shown in
The location and spacing of the blocks 224a-c will now be described with reference to
Additionally, extra supporting blocks may be provided near, but not directly on, sensitive electrical components in order to provide support for these components and prevent them from sagging and bending. For example, in
Referring again to
The spacing of the blocks 124, 224 also has to account for the desired flexibility of the support structure, to accommodate the thermal mismatch of the panels and the platform, as described above. Thus, if too many blocks are provided, too close to each other, then the expansion or contraction of the platform may be transmitted to the panel. If the blocks are spaced apart, the open space between the blocks provides clearance through which the platform can move without directly affecting the panel. When the size and spacing of the blocks is determined for each panel, the quantity of blocks can be determined, based on the number of panels that make up the entire array.
Tests were conducted to determine an optimal spacing between the spacer element to maintain flexibility and support. Tests were also conducted to determine an optimal spacing based on stresses in the antenna array and the adhesive layers from deflections and vibrations that could be expected in the structure. In one embodiment, the spacing between the spacer elements was approximately 5 inches, and in another embodiment approximately 2 inches. In another embodiment, the spacing was between approximately 2-5 inches. Tests showed that this spacing provides sufficient support for the array, does not interfere with radio frequency signals, and provides flexibility for relative thermal expansion.
The dimensions of the blocks themselves were also tested to determine a size that provided both a flexible spacing away from the platform as well as a rigid and fixed mount to the platform. In one embodiment, the end blocks 124a, 224a, 124c, 224c are approximately twice as tall in height as they are in diameter, such as 1 inch in height H1 and ½ inch in diameter, and the central block 124b, 224b is approximately the same in height and diameter, such as ½ inch in height H2 and ½ inch in diameter. In another embodiment, the end blocks are approximately ¾ inch in height, and the central block is approximately ½ inch in height, with both blocks having a ½ inch diameter. Of course, the blocks need not be cylindrical, and in other embodiments they have other cross-sections with a width of approximately ½ inch. When the surface 120, 220 is curved, the central block can be made shorter than the end blocks, to accommodate the curved shape of the platform (as shown in
By providing individual, discrete points of attachment for each panel to attach the panel to the platform, the support structure 18, 118, 218 enables each panel to remain flat and level, even while the platform surface 20, 120, 220 is curved. The spacing δ between each panel also enables each panel to sit at a slightly different angle relative to its neighboring panels, to follow the curve of the platform. Optionally, the bottom surface 228 (see
In one embodiment, the platform 12, 112, 212 is a large, cylindrical, inflated structure, with a radius of approximately 30 m. In one embodiment, the platform 12, 112, 212 is positioned inside a larger balloon, which is deployed at high altitude for surveillance. The balloon operates at an altitude of approximately 65,000 to 80,000 feet. During the balloon's flight, the ambient atmospheric temperature can vary from approximately 25° C. to approximately −80° C. The panels are adhered to the inner cylindrical inflated structure by a support structure that includes foam spacers that lift the panels away from the curved surface while securely fixing them to it. The outer, larger balloon surrounds the inner balloon with the mounted antenna panels, protecting the panels from wind and other environmental elements. The panels are mounted around the circumference of the cylindrical platform structure, so that the antenna points in all directions. In one embodiment, the antenna panels form an active electronically scanned array.
In one embodiment, the surface 20, 120, 220 is an outer-facing surface of the platform 12, 112, 212, such as the exterior surface of a balloon. In another embodiment, the surface is an inner-facing surface of the platform, such as the interior surface of a balloon.
In one embodiment, the materials used in the antenna system are identified as follows and have the following material properties (with two spacer materials identified as options):
In an embodiment, the elastic modulus of the spacer element is above approximately 0.01 Msi, and in another embodiment between approximately 0.03-0.06 Msi, and in another embodiment approximately 0.05 Msi. In one embodiment, the density of the spacer element is above approximately 0.001 lb/in3, and in another embodiment between approximately 0.005-0.01 lb/in3, and in another embodiment approximately 0.007 lb/in3. In one embodiment, the ultimate strength of the spacer element is above approximately 0.2 ksi, and in another embodiment between approximately 0.5-1.5 ksi, and in another embodiment between approximately 0.9-1.2 ksi, and in another embodiment approximately 1.0 ksi.
In one embodiment, the yield strength of the foam is greater than the yield strength of the adhesive.
Although the present invention has been described and illustrated in respect to exemplary embodiments, it is to be understood that it is not to be so limited, and changes and modifications may be made therein which are within the full intended scope of this invention as hereinafter claimed. For example, the antenna panels may be attached to a structure other than a high-altitude surveillance balloon, and the platform need not be inflatable.
This invention was made with Government support under Contract No. FA8750-06-C-0048 awarded by the Defense Advanced Research Projects Agency. The United States Government may have certain rights to this invention.
