Abstract:
A telescoping boom mechanism for deploying an expandable structure, such a hoop-supported antenna, comprises a plurality of concentrically nestable tubular boom sections, that are mutually engageable with one another by way of roller assemblies and helical tracks formed thereon. The tubular boom sections are caused to telescopically expand or retract along a deployment axis by means of a rotational drive motor coupled to an outermost one of the tubular boom sections.

Description:
FIELD OF THE INVENTION 
   The present invention relates to expandable structure-deploying actuator mechanisms, such as those used for deploying spacecraft-stowed antenna structures and the like, such as a hoop supported antenna, and is particularly directed to a telescoping boom mechanism which, in its stowed configuration, comprises a plurality of concentrically nested tubular sections. These tubular sections are mutually engageable by way of helical tracks formed thereon, so that the tubular sections may expand telescopically along a deployment axis by means of a rotational drive actuator coupled to one of the tubular sections. 
   BACKGROUND OF THE INVENTION 
   One of the important characteristics desired of a space-deployable structure, such as, but not limited to, communication satellites and associated antenna structures that are transported aboard a space-launch vehicle, is that the structure be as lightweight and stowable in as compact as possible configuration. While a variety of deployment architectures have been proposed to date, telescoping boom designs are particularly attractive because of their highly ‘nested’ stowing capability. Currently, the most common telescoping boom deployment mechanisms are cable drive-based and lead screw-based mechanisms. 
   In the former mechanism, a cable, or series of cables, are routed between nesting boom sections in a manner that, when wound, the boom sections will tend to expand. Drawbacks to this approach include the necessity of a mechanism that insures that the spooling cable does not tangle, and the fact that the extension force is not along the axis of the boom, which results in a high risk of binding. In lead screw-based designs, a series of boom sections, each with its own nut element, are deployed through the sequenced engagement and disengagement of the nut elements. Such an approach requires a sequencing mechanism to engage the nuts in a precisely synchronized manner, in addition, the engaging nuts must be properly configured to prevent ‘cross-threading’. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, shortcomings of conventional telescoping boom designs, including those described above, are substantially mitigated by means of a ‘nested helix-based’ telescoping boom architecture. As will be described, the invention is configured of a plurality of concentrically nestable tubular boom sections, that are mutually engageable via helical tracks formed on their outer cylindrical surfaces and associated rollers formed on their inner surfaces. Except for the outermost or largest diameter boom section, and the innermost or smallest diameter boom section, each tubular boom section has both a helical track in the form of a helical ridge extending along its outer cylindrical surface and upper and lower roller assemblies at an interior surface portion thereof. The outermost tubular boom section contains interior roller assemblies but no exterior helical track; conversely, the innermost boom section contains no interior roller assemblies, but does contain an outer helical track. The interior roller assemblies of a relatively outer tubular boom section are arranged to tangentially engage opposite sides of the helical track of an adjacent, relatively interior tubular boom section. In response to rotation of the relatively outer tubular boom section and thereby its roller assemblies, the helix of the relatively interior tubular boom section translates that rotational force into an axial displacement of the relatively interior tubular boom section. 
   Pursuant to a non-limiting embodiment, the rotational force is supplied by way of an electrical motor, having its output shaft driving a spur gear that engages an internal ring gear affixed to the base end of the outermost tubular boom section. When the electric motor is energized to expand the boom, rotation of the outermost tubular boom section in a first direction causes its ‘lower’ interior roller assembly to be rotationally urged against the underside of the helical track on the outer cylindrical surface of the next-to-outermost tubular boom section. This action is translated by the helical track into outward linear displacement of the next-to-outermost tubular boom section along the common displacement axis. 
   As the lower roller assembly of the outermost tubular boom section continues to rotate, and thereby axially displace the next to outermost tubular boom section, the upper roller assembly eventually encounters a stop element in the upper edge of the helical track of the next to the outermost tubular boom section. This stop element prevents further axial displacement of the next-to-outermost tubular boom section, and causes the next-to-outermost tubular boom section to begin rotating in unison with the outermost tubular boom section. With the next-to-outermost tubular boom section rotating in unison with the outermost tubular boom section, the lower roller assembly of the next to outermost tubular boom section is urged against the underside of the helical track of the next interior tubular boom section, so as to cause linear displacement of that tubular boom section along the displacement axis. This roller assembly-based axial displacement of the respective boom sections continues until the motor is de-energized or until all of the boom sections have been fully deployed. Once the tubular boom sections have been deployed to their full telescopic extension, a cut-off switch is tripped to terminate further energization of the motor. 
   In order to retract the tubular boom sections into their nested configuration, the electrical drive to the DC motor is reversed from that used to expand the boom sections to their fully deployed condition. This operation of the motor causes the ‘upper’ roller assembly of the outermost tubular boom section to be urged against the top surface of the helical track of the next-to-outermost boom section, so as to effect downward or retracting linear displacement of the next-to-outermost tubular boom section along the common displacement axis. Namely, the retraction operation proceeds in the same manner as the expansion operation, except that it is now the upper roller assembly of a respective boom section that is acting upon the top surface of the helical track of an adjacent boom, so as to cause retraction of that adjacent boom. As a respective boom is retracted to its nested position it stops translating along the displacement axis and starts rotating so as to cause displacement of an adjacent, relatively radially interior boom section. In a complementary manner to the expansion operation, once all the boom sections have been fully retracted, an associated limit switch de-energizes the electric motor to terminate the retraction operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a reduced complexity diagrammatic illustration of the nested configuration of the telescoping boom in accordance with the present invention; 
       FIG. 2  is a partial perspective view of a respective tubular boom section of the telescoping boom of the invention; 
       FIG. 3  is a perspective view of the nesting of coaxially adjacent tubular boom sections of the telescoping boom of the present invention; 
       FIG. 4  is a diagrammatic side view of  FIG. 2  of a pair of interior roller assemblies adjacent to opposite surfaces of a helical track of a tubular boom section of the invention; 
       FIG. 5  is a partially exploded perspective view of a respective tubular boom section of the telescoping boom of the invention; 
       FIG. 6  is a partial cut-away view of a respective tubular boom section of the telescoping boom of the invention; 
       FIGS. 7 and 8  are respective partial cutaway perspective views of the electrical motor-based actuator mechanism for the telescopic boom of the invention; 
       FIG. 9  shows a stop element formed in a helical track of a respective tubular boom section of the telescoping boom of the invention; and 
       FIG. 10  is a schematic block diagram of the telescoping boom in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
   Attention is initially directed to  FIG. 1 , which is a reduced complexity diagrammatic illustration of the nested configuration of the telescoping boom in accordance with the present invention. As shown therein, the boom is comprised of a plurality of N generally tubular boom sections  10 - 1 ,  10 - 2 ,  10 - 3 , . . . ,  10 -N (four being shown in the illustrated example), that are concentric about a common displacement axis  12 , along which the tubular boom sections are mutually displaced in the course of the deployment of the boom. Except for the outermost (largest diameter) boom section  10 - 1 , and the innermost (smallest diameter) boom section  10 -N, each boom section, a partial perspective view of which is shown in  FIG. 2 , has both a helical track  14  in the form of a helical ridge extending along its outer cylindrical surface  15 , and a plurality of roller assemblies  16  and  17  at an interior surface portion  18  thereof. 
   As shown in  FIG. 1  and further depicted in the perspective nested view of  FIG. 3 , the outermost tubular boom section  10 - 1  contains no exterior helical track, but does contain a plurality of interior roller assemblies  16  and  17  at its interior surface portion  18 . In a complementary manner, the innermost boom section  10 -N contains no interior roller assemblies, but does contain a helical track  14 . As indicated by the arrows  19  in  FIG. 1 , and as depicted in the diagrammatic side view of  FIG. 4 , the plurality of interior roller assemblies  16 ,  17  of a relatively outer tubular boom section, such as those of the outermost tubular boom section  10 - 1 , are arranged to tangentially engage opposite sides of the helical track  14  of an adjacent, relatively interior tubular boom section, such as the tubular boom section  10 - 2 . As will be described below with reference to  FIG. 9 , the helical track of a respective tubular boom section includes a stop element (shown at  98  in  FIG. 9 ) that prevents further axial displacement of that tubular boom section relative to the adjacent tubular boom section whose roller assemblies are urged against its helical track. 
   The configuration of, and the manner in which the roller assemblies  16 ,  17  are attached to, a respective tubular boom section  10  are shown in  FIG. 3 , the partially exploded perspective view of  FIG. 5 , and the partial cut-away view of  FIG. 6 . In particular, the upper roller assembly  16  is shown as comprising a ring member  21  having a radial bore  23  that is sized to retain a ball bearing-supported roller or wheel element  25  that is press fit onto an axial support element  26 . A lock nut  27  is used to secure the roller bearing in a fixed position. The ring member  21  is bonded to a distal end portion  27  of the tubular boom section  10 . Adjacent to the upper roller assembly  16  is a lower roller assembly  17  shown as comprising a ring member  31  having a radial slot that is sized to retain a ball bearing supported roller or wheel element  35 , held by axial support element  36  while providing a limited amount of play to provide for adjustment of the lower roller element  17  relative to the upper roller assembly. A lock nut  37  is used to secure the lower roller in a fixed position. The lower roller assembly  17  is held in place by a pair of screws  41 ,  42  that pass through bores  51 ,  52  in the upper ring  21  and are screwed into threaded bores  61 ,  62  of the lower ring  31 . 
     FIGS. 7 and 8  are respective partial cutaway perspective views of the electrical motor-based actuator mechanism for the telescopic boom of the invention. As shown therein, a DC electric motor  80  is supported by a mounting bracket  82  at a base or terminal end portion of the boom assembly. Motor  80  has an output shaft  84  upon which is mounted a spur gear  86 . The spur gear  86  engages an internal ring gear  88 , which affixed to the base end  90  of the outermost tubular boom section  10 - 1 . Outermost tubular boom section  10 - 1  is rotationally supported by a dual ball bearing mount  92  to a support housing  94 . 
   It should be noted that the sequence of the deployment is not necessarily from outermost segment to innermost segment. The actual sequence of deployment will be determined by other forces, such as friction or the mechanical advantage provided by the slopes of the helical threads. In other words the section that takes the least amount of torque to rotate will translate first. However, for the purposes of providing a non-limiting example, it will be assumed that the forces acting on the various sections allow for deployment from the outermost section to the innermost section, without a loss in generality. Moreover, it should be pointed out that the structure being deployed is required to react torque through the innermost section. If the innermost tube is not held against rotation, the assembly will not translate. 
   When the electric motor  80  energized to expand the boom, the rotation of its output shaft  84  in a first rotational direction and the spur gear  86  affixed thereto causes rotation of the ring gear  88 . Since the ring gear  88  is solid with the base end of the outermost tubular boom section  10 - 1 , this operation of the motor causes rotation of the outermost tubular boom section  10 - 1  in the dual ball bearing mount  92 . As the outermost tubular boom section  10 - 1  rotates in the boom expanding direction, its lower roller assembly  17  is rotationally urged against the underside of the helical track  14  of the next-to-outermost tubular boom section  10 - 2 . This action of the lower roller assembly  17  against the underside of the helical track  14  of the next-to-outermost tubular boom section  10 - 2  is translated by the helical track into outward linear displacement of the next-to-outermost tubular boom section  10 - 2  along the common displacement axis  12 . 
   As shown in  FIG. 9 , as the lower roller assembly  17  of the outermost tubular boom section  10 - 1  continues to rotate, and cause axial displacement of the next to outermost tubular boom section  10 - 2 , the upper roller assembly  16  eventually encounters a stop element  98  in the upper edge of the helical track  14  of the next to the outermost tubular boom section  10 - 2 . As pointed out above, this stop element prevents further axial displacement of the next-to-outermost tubular boom section  10 - 2 , and causes the next-to-outermost tubular boom section  10 - 2  to begin rotating in unison with the outermost tubular boom section  10 - 1 . 
   With the next-to-outermost tubular boom section  10 - 2  rotating in unison with the outermost tubular boom section  10 - 1 , the lower roller assembly  17  of the next to outermost tubular boom section is urged against the underside of the helical track  14  of the next interior tubular boom section  10 - 3 , so as to cause linear displacement of that tubular boom section along the common displacement axis  12 . The roller assembly-based axial displacement of the respective boom sections described above continues until the motor is de-energized or until all of the boom sections have been fully deployed. Once the tubular boom sections have been deployed to their full telescopic extension, a cut-off switch shown at  100  in the schematic diagram of  FIG. 10  is tripped to terminate further energization of the motor. 
   In order to retract the tubular boom sections into their nested configuration shown in  FIGS. 1 and 2 , the electrical drive to the DC motor  80  is reversed from that used to expand the boom sections to their fully deployed condition, described above. This operation of the motor  80  causes the upper roller assembly  16  of the outermost tubular boom section  10 - 1  to be urged against the top surface of the helical track  14  of the next-to-outermost boom section  10 - 2 , so as to effect downward or retracting linear displacement of the next-to-outermost tubular boom section  10 - 2  along the common displacement axis  12 . Namely, the retraction operation proceeds in the same manner as the expansion operation, except that it is now the upper roller assembly  16  of a respective boom section that is acting upon the top surface of the helical track  14  of an adjacent boom, so as to cause retraction of that adjacent boom. Thus, as a respective boom is retracted to its nested position it stops translating along the displacement axis and starts rotating so as to cause displacement of an adjacent, relatively radially interior boom section. In a complementary manner to the expansion operation, once all the boom sections have been fully retracted, an associated limit switch de-energizes the electric motor to terminate the retraction operation. 
   As will be appreciated from the foregoing description, shortcomings of conventional telescoping boom architectures, including cable drive-based and lead screw-based mechanisms, referenced above, are effectively obviated by the nested helix-based telescoping boom mechanism of the invention. Using a reduced complexity spur gear—ring gear coupling arrangement, the helix based design of the invention enables the output drive of an electric motor to cause respective ones of a set of coaxial tubular boom sections to be linearly displaced when rotationally driven until all of the boom sections have been fully expanded/retracted. 
   While we have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art. We therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.