Abstract:
A resilient flexure has one end attached within a cavity in a rigid member, and the other end attached within a cavity in a structural mode. The cavities are shaped to limit the flexure&#39;s bend radius. The member and the node have mating surfaces that abut to constrain the amount of rotation of the member. The aforementioned elements are shaped to prevent the strain in the flexure from exceeding its design limit when the joint is at its maximum angular deflection and the attached member is fully collapsed.

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 

   BACKGROUND OF THE INVENTION 
   The present invention is related to a resilient monolithic joint for collapsing a structure to a reduced volume for storage, and subsequently restoring the structure to its useful configuration without requiring the application of an external force. More particularly, the present invention is a joint comprised of a piece of resilient, deformable material attached at one end to a rigid member and at its other end to a structural node. The material can be deformed when it is desired to collapse the member and, when it is desired to deploy the member, will return to its original shape in the absence of the application of an external force. 
   It is ofttimes necessary to transport a structure that occupies considerable volume. Where space on the vehicle being used to transport the structure is at a premium, e.g., a launch vehicle for reaching a space station, it is desirable to collapse the structure to occupy a considerably less volume and subsequently deploy the members to re-form the original structure without undue difficulty or requiring tools that would also occupy space as well as add mass. 
   One approach is to construct a joint of two parts where one part rotates relative to the other by means of sliding contact, for example, a ball and socket or a pin and clevis. The two parts require a clearance between them to allow for the desired relative rotation. The inherent problem is that, for a deployable structure using a plurality of such joints, clearance between each pair of joint parts is cumulative. This creates the problem known as “dead band,” where movement at one end of a structure is not communicated to the other end until the intervening clearances are taken up. Where structural tolerances are small, “dead band” is a significant problem. 
   Furthermore, such joints require the application of force to deploy the collapsed members and re-form the original structure, i.e., at least as much force as was required to originally collapse each member. Deployment may also require the use of tools. For terrestrial applications, the foregoing may be considered as inconveniences; however, where the deployment is to be extraterrestrial, both of the foregoing present serious drawbacks. 
   In view of the aforementioned problems with two-piece joints, a monolithic joint comprised of a compliant material has been used. An example of such a joint is shown in U.S. Pat. No. 4,432,609. A further refinement is to use a joint material that is resilient and returns to its original shape without requiring the application of an external force. Examples of this approach are shown in U.S. Pat. Nos. 3,386,128; 5,196,857; 6,175,989 and 6,772,479. However, both such joints fail to ensure that the maximum design strain of the joint material is not exceeded when the attached member is rotated to an extreme position. This shortcoming could cause the joint to fail. 
   There a need in the art for a joint that avoids the “dead band” problem inherent to two-piece joints, as well as overcomes the shortcoming of monolithic joints in failing to ensure that the strain design limit of the joint material is not exceeded. The present invention is a monolithic joint that, by its intrinsic nature, avoids the “dead band” problem, while ensuring that the strain of the joint material does not exceed its design limit. Furthermore, the work expended to bend the joint material is stored and subsequently used to restore the joint to its neutral position without requiring the application of an external force. The present invention thus fulfills the aforementioned needs in the art. 
   SUMMARY OF THE INVENTION 
   Briefly, the present invention is comprised of a monolithic joint that allows a rigid, structurally efficient member to be rotatably collapsed and then subsequently deployed, without requiring the application of an external force, into its original configuration. It is thus suitable for both terrestrial as well as extraterrestrial applications. A flexure comprised of a less structurally efficient, resilient material has one end attached to a cavity in the member, while its other end is inserted into a cavity in a structural node. Both cavities are shaped to limit the flexure&#39;s bend radius. In addition, the member and the node have mating surfaces that abut to also constrain the amount of rotation. 
   The combination of these two design elements prevents the strain in the flexure from exceeding its design limit when the joint is at its maximum angular deflection and the attached member is fully collapsed. The joint of the present invention displays strength-stability and stiffness properties comparable to those of a kinematically equivalent, sliding contact mechanism, but without the “dead band” problem. 
   Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, and illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of two nested joints of the present invention, with the joints unbent and the respective attached rigid members in their deployed configuration. 
       FIG. 2  is a perspective view of four joints of the present invention, with the joints unbent and the respective attached members in their deployed configuration. 
       FIG. 3  is a cross-sectional view of the two joints shown in  FIG. 1  with the joints bent to their maximum extent and the respective attached members in their collapsed configuration. 
       FIG. 4  is a perspective view of the four joints of the present invention shown in  FIG. 2 , with the joints bent to their maximum extent and the respective attached members in their collapsed configuration. 
       FIG. 5  is a perspective view of a rectilinear flexure used in the joint of the present invention, which is also shown in section in  FIGS. 1 and 3 . 
       FIG. 6  is a perspective view of an arcuate flexure that may be used in the joint of the present invention. 
   

   DETAILED DESCRIPTION 
   Turning to the drawings,  FIG. 1  illustrates flexure joints  11  and  13  of the present invention. Joint  11  is comprised of flexure  15 , structural node  17 , and structural connector  19 . Node  17  is attached atop nonarticulating rigid member  20 . Node  17  includes cavity  21  and connector  19  includes cavity  23 . Flexure  15  is attached at its two ends, respectively, to base region  25  of cavity  21  and base region  27  of cavity  23 . Cavity  21  includes curved surface  29  having radius of curvature R 1 . Cavity  23  includes curved surface  31  also having radius of curvature R 1 . Cavity  21  also includes base  33  and planar, parallel lateral sides, with only side  34  being shown. Cavity  23  also includes planar, parallel lateral sides, with only side  36  being shown. Node  17  and connector  19  include mating surfaces  37 . Member  39  is fixedly attached to connector  19 . 
   Joint  13  is comprised of flexure  41 , node  17 , and connector  45 . Node  17  also includes cavity  47 , and connector  45  includes cavity  49 . Flexure  41  is attached at its two ends, respectively, to base region  51  of cavity  47  and base region  53  of cavity  49 . Cavity  47  includes curved surface  55  having radius of curvature R 2 , base  57 , and planar, parallel lateral sides, with only side  59  being shown. Cavity  49  includes curved surface  61  also having radius of curvature R 2 , as well as parallel lateral sides, with only side  63  being shown. Node  17  and connector  45  include mating surfaces  65 . Member  67  is fixedly attached to connector  45 . 
   Flexures  15  and  41  are composed of a resilient material such that after each is bent or otherwise deformed from its unstrained or neutral shape, i.e., the flat shape shown in  FIG. 1 , each of them stores as potential energy the work expended to deform them, and thus tends to return to its undeformed, neutral shape. Such resilient materials include spring steel, Copper-Beryllium alloy, unreinforced plastic, polymer fiber reinforced plastic, fiber glass reinforced plastic, carbon fiber reinforced plastic, and various shape memory alloys. The aforementioned materials are well known to those skilled in the mechanical and material arts, and any such material may be used depending upon the desired modulus and strain-to-failure properties, as will become readily apparent from the following discussion. 
   Near equiatomic Nickel-Titanium is an example of a shape memory alloy that may be used to form flexures  15  and  41 . The foregoing alloy, in addition to creating a restoring moment to enable self-deployment, permits the recovery of strains greater than the strain recovery for non-phase changing materials. Moreover, near equiatomic Nickel-Titanium can affect the recovery rate of a single flexure or sequence the strain release for a set of flexures by means of either passive or active manipulation of the alloy&#39;s phase. 
   More particularly, near equiatomic Nickel-Titanium is capable of a solid state phase transformation between a high and low temperature phase where the latent energy of the transformation is either an addition or subtraction of thermal and/or mechanical energy to or from the alloy. The addition of mechanical energy alone can induce a transformation from the high to the low temperature phase, whereupon the alloy will exhibit a phenomenon known in the art as superelasticity. When in a superelastic state or a thermally and mechanically induced low-temperature state, the alloy can be deformed to a maximum recoverable strain higher than non-phase changing materials, and thus is more compliant. This response is desirable for the present invention because a greater maximum strain would permit flexure  15  to achieve a smaller bend radius for a given cross-section, and thus allow joint  11  to be more compact while having a lower mass. 
   Furthermore, the phase of near equiatomic Nickel-Titanium may be manipulated to retard the strain release of flexure  15 , i.e., decrease the rate of its return to its neutral shape to a rate less than that of a flexure composed of a non-phase changing material, as well as coordinate the time when the strain release commences relative to other joints, to provide a degree of control over the deployment of member  39  that is not possible with flexures fabricated from non-phase changing materials. For example, phase manipulation may be used to sequence the respective strain release from a set of flexures, and thereby sequence their respective deployments. When the latent energy of the transformation is obtained from the surrounding environment, e.g., from solar radiation, or transferred to the surrounding environment, e.g., by conduction, radiation, or convection, the manipulation is considered passive. If this energy is obtained from, or transferred to, ancillary mechanical or thermal actuation systems, the manipulation is considered active. 
     FIGS. 1 and 2  show members  39  and  67  in their deployed positions.  FIG. 2  also shows deployed members  69  and  71 . To collapse member  39  to facilitate storage and transportation, an external normal force F 1  is applied to it. When the counterclockwise moment about joint  11  created by force F 1  exceeds the restoring moment of flexure  15 , flexure  15  bends and member  39  rotates counterclockwise. The application of a normal force F 2  that exceeds the restoring moment of flexure  41  similarly causes flexure  41  to bend and member  67  to rotate clockwise about joint  13 . 
   As shown in  FIG. 3 , mating surfaces  37  abut when member  39  is rotated to its fully collapsed position. This abutment limits the maximum rotation of member  39  to an angle α of 90° and, in combination with the radius of curvature R 1  of cavity surfaces  29  and  31 , limits the maximum strain realized in flexure  15 . The radius of curvature R 1  should be adjusted in view of the material used to fabricate flexure  15  to ensure that the design strain limit of flexure  15  is not exceeded. 
   Although surfaces  29  and  31  are described as being curved with a constant radius of curvature R 1 , the aforementioned surfaces may, in the alternative, be elliptical or arcuate, in order to provide the desired strain profile for flexure  15  as it bends. 
   When member  39  is in its fully collapsed position, i.e., at an angle α of 90°, the work expended to rotate member  39  to this position is stored in flexure  15 . While member  39  is in its collapsed position, flexure  15  is applying a restorative moment tending to rotate member  39  back to its deployed position. Thus, to maintain member  39  in its collapsed configuration, a fastening means (not shown) well known to those skilled in the mechanical arts, e.g., a fastener or launch lock, restrains it. In essence, the fastening means serves to apply a normal force F 1  to member  39  sufficient to overcome the restorative moment of flexure  15 . Upon release or disengagement of the fastening means, the restraining normal force F 1  is removed and the restorative moment stored in flexure  15  causes member  39  to return to its deployed position, i.e., the neutral position shown in  FIG. 1 , without the aid of an external force. 
   The corresponding elements of joint  13  cooperate in the same manner as described with respect to the elements of joint  11  in changing the deployed position of member  67  shown in  FIGS. 1 and 2  to the collapsed configuration shown in  FIGS. 3 and 4 , and will not be repeated for the sake of brevity. However, it is noteworthy that the shape of mating surfaces  65  is different than the shape of mating surfaces  37  due to the different locations of joints  11  and  13  on node  17 . 
   Flexures  15  and  41  are nested in node  17  to provide for a more compact profile when the structure is in its collapsed configuration than would be the case without such nesting. More particularly, bases  33  and  57  are separated by a nesting distance d. The width of the profile comprised of node  17  together with joints  11  and  13  decreases as the nesting distance d is increased. 
     FIG. 4  shows members  39 ,  67 ,  69  and  71  in their collapsed positions. Members  69  and  71  are collapsible by means of joints  73  and  75 , respectively, which have corresponding elements cooperating in the manner previously described in detail with respect to joint  11  and member  39 . 
     FIG. 5  is a perspective view of flexure  15 , and shows that flexure  15  has a rectilinear cross-section. Also shown is end  77 , which is attached to base region  27  of cavity  23  in connector  19 . Alternatively, a joint of the present invention may incorporate arcuate flexure  79 , a perspective view of which is shown in  FIG. 6 . Flexure  79  has an arcuate cross-section, which provides a restorative moment greater than that of a rectilinear flexure, such as flexure  15 , having a similar cross-section area. Flexure  79  would thus be more stable than flexure  15  when the joint is in its deployed configuration. If joint  11  were to incorporate flexure  79 , end  81  would be attached to base region  27 . 
   It is to be understood that the preceding is merely a detailed description of an embodiment of this invention, and that numerous changes to the disclosed embodiment can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.