Abstract:
The present invention is directed to a method of and a device for adjusting the concavity of a springback antenna reflector. The method and device of the present invention can be used to adjust the concavity of the springback reflector prior to stowage within a satellite to correct actual or anticipated variations in the desired shape of reflector that are caused by storage of the reflector, fabrication of the reflector, thermal effects on the reflector, and moisture absorption by the material from which the reflector is fabricated. By adjusting the concavity of the reflector to correct the variations in the shape of the reflector, degradation of the performance of the reflector due to distortions in the shape of the reflector may be greatly reduced.

Description:
This invention was made with U.S. Government support under Contract No. NAS5-32900. The U.S. Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to spacecraft antenna reflectors and, more particularly, to a hub mounted bending beam for shape adjustment of springback reflectors. 
     2. Description of the Related Art 
     Spacecraft antenna reflectors are typically constructed as concave disks. Electrical specifications for the reflector dictate disk dimensions, specifically diameter and cross-sectional curvature. Spacecraft payload weight limits often constrain the reflector thickness to a level that renders the reflector vulnerable to dynamic forces associated with the spacecraft launch. Atmosphere drag and launch booster vibration may be particularly damaging to the reflector if the reflector is mounted in a typical operational configuration (i.e., on support collars on the external surface of the spacecraft) during launch. It is therefore desirable to store the reflectors in a confining envelope designed to protect the reflectors from launch stress. 
     The shape of the confining envelope requires temporary modification of the intrinsic antenna reflector shape to fit inside the envelope during launch. After launch, the reflectors are released from the envelope and returned to the original shape thereof on deployment. One approach for temporarily modifying the reflector shape is disclosed in Robinson, Simplified Spacecraft Antenna Reflector for Stowage and Confined Envelopes, U.S. Pat. No. 5,574,472, which is expressly incorporated in its entirety by reference herein. In the Robinson patent, a concave reflector fabricated from a flexible, semi-rigid material is deformed by application of a uniform force at diametrically opposed points at the periphery of the reflector. These forces cause the reflector to assume a shape similar to a taco shell which is maintained while the reflector is stowed. Upon deployment, the forces are removed from the reflector and the reflector reassumes its concave shape. 
     Deforming and stowing the reflector in this manner can cause distortion of the reflector from its desired shape. Additionally, other factors can cause distortion of the reflector from its desired shape. These factors include the predisposition of the reflector to fold on its own after fabrication, and thermal effects on and moisture absorption by the material from which the reflector is fabricated. The distorted shape ultimately results in the degradation of the performance of the reflector after the reflector is deployed and in use by the satellite. 
     Therefore, there is a need for an improved apparatus and method for adjusting the shape of springback reflectors to correct distortions caused by storage of the reflectors, fabrication of the reflectors, thermal effects and moisture absorption by the reflector material. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method of and a device for adjusting the concavity of a springback antenna reflector. The method and device of the present invention can be used to adjust the concavity of the springback reflector prior to stowage within a satellite to correct actual or anticipated variations in the desired shape of reflector that are caused by storage of the reflector, fabrication of the reflector, thermal effects on the reflector, and moisture absorption by the material from which the reflector is fabricated. By adjusting the concavity of the reflector to correct the variations in the shape of the reflector, degradation of the performance of the reflector due to distortions in the shape of the reflector may be greatly reduced. 
     According to one aspect of the present invention, a shape adjustment mechanism is provided for a concave antenna reflector fabricated from a resilient material and having a surface and a coupling member attached to the surface proximate the center of the reflector. The shape adjustment mechanism includes a first support member rigidly mounted on the coupling member, and a resilient member rigidly connected to the first support member. The resilient member has a proximal end that is connected to the first support member, and a free distal end that is offset from the surface of the reflector by a distance. The shape adjustment mechanism further includes a second support member that has a first end rigidly connected to the reflector and a second end proximate the distal end of the resilient member. The shape adjustment mechanism further includes an adjustment member coupled to the second support member and adapted to engage the distal end of the resilient member. When the adjustment member is moved longitudinally along the second support member, the adjustment member engages the distal end of the resilient member such that the distance between the distal end of the resilient member and the surface of the reflector is varied as the adjustment member moves toward or away from the reflector. 
     In one alternative embodiment of the present invention, the resilient member of the shape adjustment mechanism may be in the form of a leaf spring having an aperture proximate the distal end with the second end of the second support member passing through the aperture. In this embodiment, the shape adjustment member may engage the leaf spring in the area proximate the aperture in order to vary the distance between the distal end of the leaf spring and the surface of the reflector. In another alternative embodiment, the second support member includes external threads and the adjustment member is a pair of threaded nuts disposed on either side of the resilient member. The nuts move longitudinally along the second support member as the nuts are rotated and engage the resilient member in either direction to vary the distance between the distal end and the reflector. In yet another alternative embodiment, the adjustment mechanism is disposed on the concave side of the reflector. 
     According to another aspect of the present invention, an antenna reflector is provided that includes a concave dish fabricated from a resilient material and a coupling member attached to a surface of the dish proximate the center of the dish. The antenna reflector further includes a first support member rigidly mounted on the coupling member, and a resilient member rigidly connected to the first support member. The resilient member has a proximal end that is connected to the first support member, and a free distal end that is offset from the surface of the dish by a distance. The antenna reflector further includes a second support member that has a first end rigidly connected to the dish and a second end proximate the distal end of the resilient member. The antenna reflector further includes an adjustment member coupled to the second support member and adapted to engage the distal end of the resilient member. When the adjustment member is moved longitudinally along the second support member, the adjustment member engages the distal end of the resilient member such that the distance between the distal end of the resilient member and the surface of the dish is varied as the adjustment member moves toward or away from the dish. 
     In one alternative embodiment of the present invention, the resilient member of the antenna reflector may be in the form of a leaf spring having an aperture proximate the distal end with the second end of the second support member passing through the aperture. In this embodiment, the shape adjustment member may engage the leaf spring in the area proximate the aperture in order to vary the distance between the distal end of the leaf spring and the surface of the dish. In another alternative embodiment, the second support member includes external threads and the adjustment member includes a pair of threaded nuts disposed on either side of the resilient member. The nuts move longitudinally along the second support member as the nuts are rotated and engage the resilient member in either direction to vary the distance between the distal end and the dish. In yet another alternative embodiment, the adjustment mechanism is disposed on the concave side of the dish. 
     According to a still further aspect of the present invention, a method for adjusting a concave antenna reflector is provided for use with a reflector fabricated from a resilient material and having a surface and a coupling member attached to the surface proximate the center of the reflector. The method includes the steps of rigidly mounting a first support member on the coupling member and a second support member on the reflector. The method further includes the step of rigidly connecting a resilient member to the first support member. The resilient member has a proximal end rigidly connected to the first support member and a distal end disposed proximate the second support member. Configured in this manner, the distal end of the resilient member is separated from the surface of the reflector by a distance. The method further includes the step of changing the distance between the distal end of the resilient member and the surface of the reflector by moving an adjustment member longitudinally along the second support member. The adjustment member engages the distal end of the resilient member to move the distal end to one of increase and decrease the distance between the distal end and the surface. In alternative embodiments of the present invention, the surface of the reflector may be disposed on either the concave or convex side of the reflector. 
     The features and advantages of the invention will be apparent to those of ordinary skill in the art in view of the detailed description of the preferred embodiments, which is made with reference to the drawings, a brief description of which is provided below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is a simplified perspective view of an illustrative embodiment of a springback reflector in a manufactured configuration useful with the shape adjustment mechanism according to the present invention; 
     FIG.  1 ( b ) is a top view of the springback reflector of FIG.  1 ( a ); 
     FIG.  1 ( c ) is a side view of the springback reflector of FIG.  1 ( a ); 
     FIG.  2 ( a ) is a top view of the springback reflector of FIG.  1 ( a ) in a stowed configuration; 
     FIG.  2 ( b ) is a side view of the springback reflector of FIG.  1 ( a ) in a stowed configuration; 
     FIG.  3 ( a ) is a top view of the springback reflector of FIG.  1 ( a ) in a deployed configuration; 
     FIG.  3 ( b ) is a side view of the springback reflector of FIG.  1 ( a ) in a deployed configuration; 
     FIG. 4 is a perspective view of the hub portion of the springback reflector of FIG.  1 ( a ) including the adjustment mechanism according to the present invention; 
     FIG. 5 is a side elevation sectional view taken along line  5 — 5  of the hub portion and the adjustment mechanism of FIG. 4; 
     FIG. 6 is a side elevation sectional view taken along line  5 — 5  of the hub portion and the adjustment mechanism of FIG. 4 in a first adjusted position; and 
     FIG. 7 is a side elevation sectional view taken along line  5 — 5  of the hub portion and the adjustment mechanism of FIG. 4 in a first adjusted position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A springback antenna reflector is provided with elastic characteristics which allow the shape of the reflector to be redefined for stowage and returned to an original shape on deployment. FIG.  1 ( a ) is a simplified perspective diagram of an illustrative embodiment of the flexible thin-shell springback antenna reflector  10  in a manufactured configuration. 
     FIG.  1 ( b ) is a top view of the illustrative embodiment of the antenna reflector  10  in a manufactured configuration. FIG.  1 ( c ) is a side view of the illustrative embodiment of the antenna reflector  10  in a manufactured configuration. As shown in FIGS.  1 ( a )-( c ), in the illustrative embodiment, the reflector  10  is a parabolic shell having a coupling fixture  12  attached to the center thereof to which a support mast  14  is coupled. 
     The reflector  10  is constructed of a single thin, concave homogeneous sheet of flexible, semi-rigid material such as graphite-fiber reinforced plastic. The reflector  10  may be fabricated in a conventional manner, i.e., multi-layer lamination over a precision form of the correct shape. The dimensions of the reflector  10  may be determined in a conventional manner. The reflector may be made of conductive material or nonconductive material which is coated with conductive material. A design consideration of significant importance is that the reflector  10  be sufficiently flexible to be deformed into a stowage shape and deployed to a fully non-deformed state on deployment. This requires a construction in which the deformation strain on the reflector  10  is below the creep strain limit, that is, the force at which the reflector will not return to the original shape. 
     FIG.  2 ( a ) is a top view of the illustrative embodiment of the antenna reflector  10  in a stowed (deformed) configuration. FIG.  2 ( b ) is a side view having a substantially U-shaped cross-section of the illustrative embodiment of the antenna reflector  10  in the stowed configuration. FIG.  3 ( a ) is a top view of the illustrative embodiment of the antenna reflector  10  in a deployed configuration and FIG.  3 ( b ) is a side view of the illustrative embodiment of the antenna reflector  10  in the deployed position. 
     As illustrated in FIG.  2 ( a ), the reflector  10  is deformed by the application of a uniform force at diametrically opposed points  16  and  18  at the periphery of the reflector  10 . The reflector  10  may be maintained in the stowed configuration by a string  20  as shown in FIG.  2 ( a ), or by a container (not shown) in which the reflector  10  is stowed, e.g., the side rails of a space shuttle. If a string is used, it may be cut by pyrotechnic device  22 . In the alternative, a material may be chosen for the reflector  10  which allows the reflector  10  to be deformed at one temperature and maintained in the deformed state until deployed at another temperature. In short, the invention is not limited to the manner in which the reflector  10  is maintained in a deformed state and deployed. 
     The springback reflector obviates the disadvantages of a segmented design by providing a single-piece homogeneous reflector that can be fabricated using existing manufacturing processes, which can be deformed to fit into a protective launch envelope and returned to the desired shape upon deployment. No excess weight from cantilevers and motors is necessary, no motor control systems are required to perform stowage deformation or redeployment, and the lack of segmentation virtually eliminates possible catenation effects. The springback reflector allows the elimination of the manufacturing steps required for segmenting conventional reflectors, including costly cantilevers, ribs, and motor and control systems, and therefore allows significant cost savings. 
     Although the springback reflector is designed to return to the desired concave shape, the deformation and stowage of the reflector in the manner described above can cause distortion of the reflector from its desired shape. Additionally, other factors can cause distortion of the reflector from its desired shape. These factors include the predisposition of the reflector to fold on its own after fabrication, and thermal effects on and moisture absorption by the material from which the reflector is fabricated. The distorted shape ultimately results in the degradation of the performance of the reflector after the reflector is deployed and in use by the satellite. 
     In order to ensure that the springback reflector assumes the desired concave shape upon deployment, an adjustment mechanism according to the present invention is mounted on the hub portion of the reflector. The hub portion  30  of a reflector  10  implementing the present invention is shown in FIG.  4 . The hub portion  30  has a support panel  32  connected thereto at three equally spaced points in a manner that will be discussed in greater detail with reference to FIG.  5 . Referring to FIG. 4, the reflector  10  further includes three shape adjustments assemblies  40  connected to both the hub portion  30  and the support panel  32  proximate each of the points at which the support panel  32  is coupled to the hub portion  30 . 
     The support panel  32 , along with the coupling fixture  12  and the support mast  14 , provides the primary mechanical interface between the reflector  10  and the spacecraft (not shown). A receiving device, such as a feed horn (not shown), is mounted on the support panel  32  and is positioned at the desired focal point of the reflector  10 . The receiving device is electromechanically coupled to the coupling fixture  12  and the support mast  14  through an opening in the center of the reflector  10  and, in turn, connected to the spacecraft. Electromagnetic energy reflected by the reflector  10  is detected by the receiving device and passed through the coupling fixture  12  and mast  14  to the spacecraft for processing. 
     Referring to FIG. 5, the attachment mechanism for the support panel  32  and the shape adjustment mechanism  40  according to the present invention are shown in greater detail. The support panel  32  is mounted on the hub portion  30  at three points by monoball mounts  34  that are evenly spaced about the center of the reflector  10 . The monoball mounts  34  provide a moment-free connection which allows a slight rotation of the reflector  10  with respect to the support panel  32  when the reflector  10  is deformed into the stowed configuration and when the adjustment mechanisms  40  are manipulated to adjust the shape of the reflector  10 . 
     The adjustment mechanism  40  includes a first support member  42  that is rigidly mounted to the support panel  32  proximate one of the monoball mounts  34  and which extends upwardly away from the support panel  32  and reflector  10 . The adjustment mechanism  40  further includes a resilient member  44  in the form a leaf spring having a free distal end and a proximal end that is rigidly connected to the support member  48 , thereby forming a cantilever beam which extends outwardly from the first support member  42  beyond the outer edge of the support panel  32 . The resilient member  44  has an aperture  46  proximate the distal end and located beyond the outer edge of the support panel  32 . 
     The adjustment mechanism  40  further includes a second support member  48  having external threads and an outer diameter that is smaller than the inner diameter of the aperture  46 . The second support member  48  is rigidly connected at one end to the hub portion  30  and extends upwardly from the hub portion  30  in the same general direction as the first support member  42 . The free end of the second support member  48  passes through the aperture  46  of the resilient member  44 . Spherical adjusting nuts  50  engage the external threads of the second support member  48  and are located on either side of the aperture  46 . The spherical heads of the nuts  50  engage the resilient member  44  as the nuts  50  move longitudinally along the second support member  48  such that a force parallel to the longitudinal axis of the second support member  48  may be applied to the resilient member  44  without creating a moment at the distal end. In an alternative embodiment, the resilient member  44  may include a monoball mount at the aperture  46  that is engaged by nuts  50  with flat faces that are screwed on to the posts  48  on either side of the resilient member  44 . 
     Tuning of the reflector  10  is performed prior to stowing the reflector  10  in the spacecraft for launch. The geometry of the reflector  10  after assembly is measured using a well-known process, such as photogrametry. The information of the reflector geometry is used to determine the adjustments necessary to correct the distortions caused by effects such as stowing the reflector in a deformed position, the reflector&#39;s tendency to fold on its own, thermal effects, and the effects of moisture absorption. Once the necessary adjustments are determined, the shape adjustment mechanisms  40  are manipulated by moving the nuts  50  in the longitudinal direction along the second support member  48  to tune the reflector  10  to the desired shape. If the area of the reflector  10  proximate a given shape adjustment mechanism  40  requires increased concavity, the nuts  50  are rotated in the direction that moves the distal end of the resilient member  44  closer to the hub portion  30  of the reflector  10 . By forcing the end of the resilient member  44  toward the hub portion  30 , the resilient member  44  exerts a force in the upward direction as indicated by arrow  60  in FIG.  6 . The monoball mount  34  proximate the adjustment mechanism  40  allows the reflector  10  to rotate about the monoball mount  34  to increase the concavity of the reflector  10 . Additionally, the spherical heads of the nuts  50  ensure that the force  60  is exerted along the longitudinal axis of the second support member  48  without creating a moment on the resilient member  44  at the distal end. 
     If the concavity of the reflector  10  must be decreased to achieve the desired shape, the nuts  50  are rotated in the opposite direction to engage the distal end of the resilient member  44 , thereby forcing the distal end of the resilient member  44  away from the hub portion  30  as shown in FIG.  7 . As the end of the resilient member  44  is forced away from the hub portion  30 , the resilient member  44  exerts a force in the downward direction, as indicated by the arrow  70 , that tends to flatten the shape of the reflector  10 . After the calculated adjustments have been made, the geometry of the reflector  10  is measured again to determine if additional adjustments are necessary to tune the reflector  10  to the desired shape. 
     Although the adjustment mechanisms  40  as illustrated herein utilize the threaded nuts  50  on the second support member  48  to apply a force to the resilient member  44 , which is in the form of a leaf spring, other configurations for adjusting the distance between the reflector  10  and the resilient member  44  will be obvious to those of ordinary skill in the art. For example, instead of using threaded nuts on a support member with external threads, the adjustment mechanism could include sleeves that slide along the second support member  48  and engage the resilient member  44  to adjust the distance between the resilient member  44  and the reflector  10 . The sleeves could frictionally engage the second support member  48  with sufficient force to hold the sleeves in place against the force of the resilient member  44  or, alternatively, use set screws to hold the sleeves in place. Additionally, the second support member  48  could be disposed adjacent the resilient member  44  instead of passing through an aperture in the resilient member  44 , and include a nut, sleeve or other engagement member that engages the resilient member  44  such that a moment-free force may be applied to the resilient member  44 . Other configurations for varying the distance between the distal end of the resilient member  44  and the reflector  10  will be obvious to those of ordinary skill in the art and are contemplated by the inventors as having use with the adjustment mechanism according to the present invention. Moreover, the adjustment mechanisms  40  could be disposed on the convex side of the reflector  10  with the first support member  42  mounted on another rigid structural member, such as the coupling fixture  12 . 
     While the present invention has been described with reference to the specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions, and/or deletions may be made to the disclosed embodiment without departing from the spirit and scope of the invention.