COLLAPSIBLE STRUCTURE

Abstract: A deployable truss structure desirable for space applications is disclosed. The structure is comprised of a plurality of frame units joined together, each frame unit comprised of rod members that form a triangular frame. Four such frame units, together with four hinged batten members, define a cube. The frame units collapse on each other during the transition to the stowed condition. Such a construction eliminates member strain during deployment, giving greater freedom in selecting member size.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to truss structures and, more particularly, to collapsible truss structures desirable for space applications. 2. Description of the Related Art Truss structures are desirable in space applications because they offer high strength and/or stiffness and low mass. External loads on trusses are reacted to by the truss members in pure tension or columnar compression. Such highly directed loading allows the use of uni-directional filamentary composite tubes as members, which have very high ratios of material stiffness-to-mass. The attendant truss property of repeating symmetry makes efficient packaging possible. The evolution of deployable space trusses has been toward greater performance in terms of high stiffness and low mass. For example, a deployable lattice column is described in U.S. Patent No. 3,486,279 to James E. Webb. Figure 7 thereof discloses a truss structure in which the longerons are comprised of continuous flexible rods. Thus, there are no hinges along the longerons and the full length of the longeron is utilized in terms of resisting collapse of the column, since the longerons maintain a tendency to spring back to a straight shape. The absence of longeron hinges results in a structure of low mass. The column, however, is limited in cross section because of strain in the packaged or collapsed condition. Hinging along the longerons would allow greater freedom in longeron member size, but hinge compliance would degrade member stiffness. Similar comments apply to U.S. Patent No. 4,532,742 to Koryo Miura, which discloses a similar structure employing continuous, flexible rods as longerons but adds spacers 2 along the longitudinal axis of the structure. The development of single-degree-of-freedom hinge technology has significantly reduced hinge compliance, and articulated structures have become viable. The structure disclosed in U.S. Patent No. 4,480,415 to Peter Truss, in which the longerons are arranged so as to form a triangular cross-section, includes longeron members that are formed from pairs of foldable rod members. Pressure directed at a longeron along the triangular face collapses the longeron. Disclosed in Figure 2 of the above-referenced patent to Webb is a deployable column in which the longerons do not fold, but in which the diagonals do collapse. Although single-degree-of-freedom hinges reduce hinge compliance, such hinges also introduce a problem: in general, packaging a truss structure using such hinges involves member strain during the transition from fully deployed to fully collapsed, even though either extreme is unstrained. As a result, force is required for deployment which depends on the degree of strain, and member sizing is limited to that which can survive the strain. Thus, there is a continuing need for improvements in truss structures for space applications that overcome the problems mentioned above. US-A-4599832 to Benton et al. describes extendable structures which can be collapsed to a shorter length or extended to a longer length. The structures described by Benton et al., e.g. as shown in Figure 1 of the reference, include four rigid members 19-22 connected at their end points to form a square. Longeron elements 30, 32, 34 and 36 each extend from a respective corner of the square. Four buckling springs 75-78 are connected at their ends to the longeron elements 30, 32, 34 and 36. A second set of four longeron elements 29, 31, 33 and 35 extend from the connections of the buckling springs 75-78 with the longeron elements 30, 32, 34 and 36. Four rigid elements 15-18 are connected at their end points to form a square and are connected, at the corners of the square, to the longeron elements 29, 31, 33 and 35. The buckling springs 75-78 are slightly bowed outward from the structure 50 as to exert an outwardly directed force at the joints of the longeron elements and the buckling springs. Since these joints are continuously urged outward, chains or cables 85-88 must be connected between each joint being urged outward, and a joint arranged at a diagonal relationship thereto. Figure 2 of the Benton et al. reference shows a structure in a partially collapsed state and Figure 3 shows a structure in a fully collapsed state. As apparent from each of these figures, buckling springs 75-78 are bent or buckled to a great extent while the structure is being collapsed and while the structure is in a collapsed state. Referring to Figure 1, these buckling springs are partially buckled or bent even when the structure is in a fully deployed state. In this regard, buckling springs 75-78 are always under some tension when the structure is in a collapsed state. This tension is transferred to the ends of the buckling springs and, therefore to the joints between the buckling springs and the longeron elements. Such a continuous tension on the buckling springs and joints can cause these elements to wear out and/or break. SUMMARY OF THE INVENTION The present invention overcomes the above problems, and others, by providing a structural unit in which the longerons do not fold, the diagonals do not fold, and there is no deployment strain on the members. The non-folding longerons and diagonals minimize the number of hinges required so that compliance, as well as weight, is reduced. The lack of deployment strain allows the use of members of any stiffness and size. Strain that occurs during articulation of a hinged structure arises because of one or both of the following effects: (a) two hinges, which are associated because an articulating member connects them, change in angular orientation with respect to each other; (b) one of the two associated hinges moves along the axis of the other. In other words, the respective planes in which two hinges rotate change in terms of relative angle, or one hinge moves along the axis of the other, notwithstanding the presence of a connecting rod member. In the present invention, neither of these phenomena occur; that is, hinges which are associated do not change with respect to each other in position or orientation. The present invention may be used to construct a truss structure especially suited to space applications. The basic "building block" of the present invention consists of three rigid members that form a frame unit in the shape of a right triangle, the members being a longeron, a diagonal, and a batten. This frame unit does not change shape during packaging or during the transition from stowed to deployed condition and is connected to three other identical frames by connecting the rigid batten members end-to-end in a quadrilateral. The batten members so connected form a planar surface. The longeron and diagonal members are connected to their associated batten by hinges and pivot about an axis parallel to the batten member. The present invention provides a truss structure as defined in the introductory part of claim 1 characterised in that said members of each frame unit further comprise four non-folding diagonals and four folding battens, each diagonal being connected to the free end of an associated longeron and to the free end of the rigid batten associated with that associated longeron, each diagonal being pivotally connected at the rigid batten such that the diagonal rotates about the same axis as the longeron, and having two trough portions located opposite and adjacent to each other at approximately the midpoint of the diagonal, each of the folding battens being hinged at its mid point about an axis perpendicular to the face of the quadrilateral and pivotally connected at its ends to the junction of a diagonal and longeron such that the folding battens pivot during the transition between collapsed and deployed positions of the truss structure. The folding battens combine with the longeron, diagonal and rigid battens in the deployed state to form a quadrilateral face of the structure. During the transition between the stowed and deployed conditions, the folding batten axis of rotation remains perpendicular to the rigid batten plane, in keeping with the lack of member strain which is characteristic of the present invention. A truss beam structure may be assembled using a plurality of such structural units connected together such that the longerons extend along the longitudinal axis of the beam structure, while the battens extend perpendicular to the longerons and the diagonals brace the longerons and battens. Thus, the present invention comprises a structure in which the load-carrying members (longerons and diagonals) do not fold, but yet may be stowed into a compact package. Additionally, deployment does not cause member strain, allowing more freedom in choosing member size and composition and providing a rigid, low-mass structure. The present invention is able to combine rigid structural elements with the articulation necessary to compactly collapse the structure through the use of the unique hinge clusters provided in the preferred embodiment of the invention and which allow the structure to collapse into a compact size and still utilise members that do not fold. The hinge clusters include corner joints and frame member end pieces that allow the various frame members to meet at the desired angles and still rotate between collapsed and deployed conditions with a single degree of freedom. The features that are believed to be characteristic of the present invention, both as to its organisation and method of operation, together with further objectives and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which a presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 shows a perspective view of a column structure in accordance with a preferred embodiment of he present invention from a fully deployed state through mid-deployment stages to a fully collapsed state. FIGURE 2 shows a perspective view of a base hinge cluster used to join various structural members. FIGURE 3 shows a perspective view of a mid-structure hinge cluster used to join various structural members. FIGURE 4 shows further detail of the diagonal members of FIGURE 1(a). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIGURE 1 shows a deployable truss structure constructed in accordance with the present invention comprised of two frame sections 101 and 102. FIGURE 1(a) shows the beam structure in its fully deployed state, while FIGURES 1(b) and 1(c) show the structure through mid-deployment stages to a fully collapsed state shown in FIGURE 1(d). The two sections 101 and 102 are mirror images of each other; section 101 will be described in detail. The section 101 has a quadrilateral cross section and has four longerons 12, 14, 16, and 18 extending along its longitudinal axis. The longerons define four faces of the section 101. Longerons 12 and 18 are connected to either end of a batten member 20. The other longerons are similarly connected with batten members 22, 24, and 26, all of which are perpendicular to the longeron members. A diagonal member 28 braces longeron member 12 and batten member 20 by extending between the ends of the longeron 12 and batten 20 that are not joined. Similarly, diagonal member 30 extends between longeron member 14 and batten member 22, diagonal member 32 extends between longeron 16 and batten 24, and diagonal 34 extends between longeron 18 and batten 26. Thus, each face of the section 101 is bisected by a diagonal. FIGURE 1 shows one such face partially shaded to aid in visualizing the stages of deployment. Completing the frame section are four folding batten members 36, 38, 40, and 42, extending between the longeron members and located on each quadrilateral face opposite batten members 20, 22, 24, and 26, respectively. The structure defined thus far is the smallest structural unit that may be used to construct a column in accordance with the preferred embodiment of the present invention. FIGURE 1 shows a short column constructed of two structural units 101, 102. It is readily apparent that more than two such units 101, 102 could be used to construct a longer beam structure. Thus, longeron 12 is followed along the longitudinal axis of the structure by longeron member 112. Likewise, longeron member 14 is associated with longeron member 114, longeron member 16 is associated with longeron member 116, and longeron member 18 is associated with longeron member 118. Furthermore, the longeron members 112 through 118 are joined together by batten members 120, 122, 124, and 126, respectively. The two structural units 101, 102 "share" folding batten members 36, 38, 40, and 42. In the preferred embodiment, the diagonal members of one structural unit are located as "mirror images" of diagonal members of an associated structural unit. Thus, diagonal member 128 is located on the same quadrilateral face as diagonal member 28, and is joined at the folding batten member 36 at the same end as diagonal member 28. Likewise, diagonal member 130 is associated with diagonal member 30, diagonal member 132 is associated with diagonal member 32, and diagonal member 134 is associated with diagonal member 34. Other structural units could be added onto the structure 10 in order to create a columnar structure of much greater extent than that illustrated. FIGURE 2 shows a base hinge cluster that is used at the corners of the structure where the rigid batten members meet diagonal and longeron members. The hinge cluster includes a corner joint, which is provided with a plurality of flat projections. The ends of the longerons and diagonals are hinged with the flat projections. Each of the flat projections has a hole, as does the end of the frame member to be joined. The holes in the corner joint and the end of the frame member are aligned and a pin is inserted through the hole. In this way, the respective frame members are free to rotate relative to the hinge joint in the desired manner. Each of the hinge clusters used in the present invention is configured so as to allow a single degree of freedom during rotation. For example, the diagonals rotate about an axis parallel to the batten members, while their longitudinal axis is inclined to the rotational axis. Therefore, the diagonal end that is hinged with the hinge joint is provided with a dog-leg bend, as shown in FIGURE 2 and FIGURE 3. The rigid batten member is also joined to the corner joint, adjacent the flat projections. As illustrated in FIGURE 2, the base hinge cluster only allows the longeron and diagonal member to rotate. The rigid batten members are fixed in position and do not rotate. FIGURE 3 shows a mid-structure hinge cluster at which longerons, diagonals, and folding batten members are joined. This hinge cluster is constructed similarly to the hinge cluster of FIGURE 2, expect that all frame members joined by the mid-structure hinge cluster rotate relatively to the hinge cluster. Once again, all rotation is through a single degree of freedom. FIGURE 3 shows a hinge joint 200 at which various members are pivotally connected. Joint 200 connects longeron members 12 and 112 to each other, joins folding batten members 36 and 38 together (forming a corner of the structure) and also connects associated diagonal members 28 and 128. During the transition from the deployed state to the collapsed state of the column structure, folding batten members 36 and 38 will hinge outward at their midpoint hinges, away from the column structure, longeron members 12, 112 will pivot toward each other as illustrated in FIGURE 1(b), and diagonal members 28 and 128 will follow accordingly. FIGURE 4(a) shows a detail of a diagonal member 220 illustrating the configuration at the midpoint of the diagonal member with cut-out or trough portions 222 and 224 located on opposite sides of the diagonal member. This configuration is necessary because, upon collapsing the structure 10, the diagonal members will come to rest in a substantially coplanar fashion. When a diagonal member 220 is laid on top of a similar diagonal member and perpendicular thereto with the trough portions facing each other, the two diagonal members will be substantially coplanar. In this way, diagonal member 220 can cross two other similar diagonal members in a perpendicular manner while still lying coplanar therewith. Thus, four similar diagonal members can cross perpendicularly to each other while lying flat. FIGURE 4(b) illustrates an arrangement of four diagonal members 220, 230, 240, 250 resting in interleaved, substantially coplanar fashion following collapse of the structure. Although a particular embodiment of the present invention has been described and illustrated herein, modifications and variations may be readily apparent to those skilled in the art. For example, although the batten members and longerons are shown as being of equal length, such a configuration is not a requirement of the invention. It is also contemplated that certain durable, high strength materials may be used in producing a truss structure in accordance with the present invention. For example, the hinges and joints may be constructed of stainless steel, and the frame members may be constructed of graphite epoxy composite tubing. Accordingly, the scope of the present invention should be considered in terms of the following claims, which are not to be limited to the details of the embodiment described and illustrated herein.

Claims: A truss structure movable between a collapsed position and a deployed position, having: at least one frame unit (101), each frame unit (101) comprising frame members including four rigid, elongate battens (20, 22, 24 and 26), and four non-folding longerons (12, 14, 16 and 18), the four rigid battens being connected at their end points so as to form a quadrilateral, each longeron (12, 14, 16 and 18) being pivotally connected at a first end to an associated rigid batten (20, 22, 24 and 26) at each corner of the quadrilateral such that each longeron (12, 14, 16 and 18) pivots about an axis parallel to the longitudinal axis of the associated rigid batten (20, 22, 24 and 26) and such that, in the deployed position, each longeron (12, 14, 16 and 18) is perpendicular to the plane defined by the quadrilateral, characterised in that said frame members of each frame unit further comprise four non-folding diagonals (28, 30, 32 and 34) and four folding battens (36, 38, 40 and 42), each diagonal (28, 30, 32 and 34) being connected to the free end of an associated longeron (12, 14, 16 and 18) and to the free end of the rigid batten (20, 22, 24 and 26) associated with said associated longeron (12, 14, 16 and 18), each diagonal (28, 30, 32 and 34) being pivotally connected at the rigid batten (20, 22, 24, 26) such that the diagonal (28, 30, 32 and 34) rotates about the same axis as the longeron (12, 14, 16 and 18), and having two trough portions (222 and 224) located opposite and adjacent to each other at approximately the midpoint of the diagonal (28, 30, 32 and 34), each of the folding battens (36, 38, 40 and 42) being hinged at its midpoint about an axis perpendicular to the face of the quadrilateral and pivotally connected at its ends to the junction of a diagonal (28, 30, 32 and 34) and longeron (12, 14, 16 and 18) such that the folding battens (36, 38, 40 and 42) pivot during the transition between collapsed and deployed positions of the truss structure. The truss structure of Claim 1, further comprising a joint joining two battens (20) and (22), a longeron (12) and a diagonal (30); wherein the longeron (12) and diagonal (30) are joined to the joint by single degree of freedom hinges (200) such that the longeron (12) pivots in a plane perpendicular to the plane defined by the battens (20) and (22) and the diagonal (30) pivots about an axis parallel to the plane defined by the battens (20) and (22). The truss structure of Claim 1, wherein the frame members (12), (14), (16), (18), (20), (22), (24), (26), (28), (30), (32), (34), (36), (38), (40) and (42) are pivotally connected by hinges (200) and the hinges are constructed as single-degree-of-freedom hinges that are limited to movement in a single plane. The truss structure of Claim 1, wherein the rigid battens (20, 22, 24 and 26); folding battens (36, 38, 40 and 42) and longerons (12, 14, 16 and 18) are of equal length. The truss structure of Claim 1 wherein the frame members (12), (14), (16), (18), (20), (22), (24), (26), (28), (30), (32), (34), (36), (38), (40) and (42) are constructed of graphite/epoxy tubes. The truss structure of Claim 1, further comprising a joint joining two battens (36), (38), two longerons (12) and (112) and two diagonals (28) and (128) by single degree of freedom hinges (200) such that the longerons (12) and (112) pivot in a plane perpendicular to the plane defined by the battens (36) and (38) and each diagonal (28) and (128) pivots about an axis parallel to the plane defined by the battens (36) and (38).

Inventors: ADAMS, LOUIS R
Applicants: ASTRO AEROSPACE CORPORATION
Application year: 1988
Main classification: B64G9
IPC codes: E04H12, E04B1, B64G1, B64G99