Abstract:
Negative-stiffness-producing mechanisms can be incorporated with structural devices that are used on spacecraft that provide thermal coupling between a vibrating source and a vibration-sensitive object. Negative-stiffness-producing mechanisms can be associated with a flexible conductive link (FCL) or “thermal strap” or “cold strap” to reduce the positive stiffness of the FCL. The negative-stiffness-producing mechanisms can be loaded so as to create negative stiffness that will reduce or negate the natural positive stiffness inherent with the FCL. The FCL will still be able to provide maximum thermal conductance while achieving low or near-zero stiffness to maximize structural decoupling.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. application Ser. No. 13/587,207, filed on Aug. 16, 2012 which is incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to improved designs of devices used on spacecraft and commonly referred to as thermal straps or cold straps or flexible conductive links (FCLs) for providing thermal (conductive) coupling and structural decoupling between cryogenic components such as a vibrating cooling source and a motion-sensitive element or focal plane array (FPA) having highly critical alignment requirements. The present invention provides means for reducing the stiffness of thermal straps through the use of negative-stiffness mechanisms thereby improving their structural decoupling. In the subsequent discussions, the terms thermal strap, FCL and cold strap are used interchangeably. Also, the combination of negative-stiffness mechanisms with a thermal strap or an FCL or a cold strap will be referred to as a “negative-stiffness thermal strap (NS thermal strap)” or a “negative-stiffness FCL (NSFCL)” or a “negative-stiffness cold strap (NS cold strap).” 
         [0003]    A critical tradeoff in the design of the thermal strap is maximizing the thermal conductance, which improves the overall performance of the thermal strap, and maximizing the structural decoupling which requires minimizing the stiffness. These design factors present conflicting design goals to the spacecraft engineer. It would therefore be beneficial if a thermal strap or other coupling device could attain maximum thermal conductance while at the same time maximizing structural decoupling in order to effectively isolate vibrations from the motion-sensitive equipment. My previous thermal strap invention, Improved Thermal Straps for Spacecraft, U.S. application Ser. No.  13 / 587 , 207 , filed on Aug.  16 ,  2012 , solves these and other needs. 
         [0004]    In my previous thermal strap invention, it was shown that negative-stiffness mechanisms could improve the structural decoupling of a thermal strap or could improve the thermal conductance, or could improve both the structural decoupling and the thermal conductance. In that invention two thermal straps were used in series. The first thermal strap was combined with negative-stiffness mechanisms that removed much or all of the stiffness of the first thermal strap in the axial direction and in directions transverse to the axial direction. The first thermal strap was relatively stiff in tilt, or rotation about any transverse axis. The second thermal strap provided low tilt stiffness and structural decoupling in tilt but did not have the benefit of negative-stiffness mechanisms. However, with the higher thermal conductance that can be achieved in the first thermal strap for the same or lower axial and transverse stiffnesses, the thermal conductance of the second thermal strap can be made lower to allow for a lower tilt stiffness so that the combined thermal straps will provide improved thermal coupling or improved structural decoupling, or both improved thermal coupling and structural decoupling compared with conventional thermal straps. 
         [0005]    My prior thermal strap invention relied on mechanisms which can apply negative stiffness to an elastic structure having positive stiffness in order to cancel, or nearly cancel the positive stiffness of the structure. These previous inventions utilized negative-stiffness mechanisms to provide vibration isolation systems capable of supporting an object having weight (an object with mass in a gravitational field) and providing low stiffness and low natural frequencies in both the vertical (gravity) direction and in the lateral or horizontal directions. The low horizontal stiffness and low horizontal natural frequencies were achieved by using the weight of the object to load vertically oriented beam-columns close to their critical buckling loads (the loads at which their lateral stiffness becomes zero). This approach made use of the “beam-column” effect, which refers to the reduction in the bending stiffness of a beam when it is loaded in compression to make the beam behave as a beam-column. It can be shown that the beam-column effect in a vertically oriented beam-column is equivalent to a horizontal spring and a negative-stiffness mechanism, and the magnitude of the negative stiffness increases with an increase in the weight load. The low vertical stiffness and low vertical natural frequency was achieved by using a support spring connected to a negative-stiffness mechanism in the form of horizontally oriented beam-columns which are spring loaded in compression so that the negative stiffness removes much of the stiffness of the support spring and the stiffness of the beam-columns. These vibration isolation systems are used to isolate vibration-sensitive objects from the vertical and horizontal vibrations of a vibrating support, i.e., to reduce the magnitude of the vibrations transmitted from the vibrating support to the object. 
         [0006]    These prior vibration isolation systems are described in U.S. Pat. No. 5,530,157, entitled “Vibration Isolation System” issued May 10, 1994, U.S. Pat. No. 5,370,352, entitled “Damped Vibration System” issued Dec. 6, 1994, U.S. Pat. No. 5,178,357, entitled “Vibration Isolation System” issued Jan. 12, 1993, U.S. Pat. No. 5,549,270, entitled “Vibration Isolation System” issued Aug. 27, 1996, U.S. Pat. No. 5,669,594, entitled “Vibration Isolation System” issued Sep. 23, 1997, U.S. Pat. No. 5,833,204, entitled “Radial Flexures, Beam-Columns and Tilt Isolation for a Vibration Isolation System issued Nov. 10, 1998, which are all hereby incorporated by reference in this present application. These vibration isolators exhibit low stiffness, and low fundamental resonant frequencies, high damping to limit resonant responses of the composite system, effective isolation at the higher frequencies, and can provide high isolator internal resonant frequencies. 
         [0007]    It would therefore be beneficial if a thermal strap or other coupling device could attain maximum thermal conductance while at the same time maximizing structural decoupling in order to effectively isolate vibrations from the motion-sensitive equipment. It also would be beneficial if tilt stiffness associated with the thermal strap could be reduced through application of negative stiffness for tilt, thereby improving the thermal coupling, It would also be beneficial if means could be shown for reducing parasitic heat transfer in the thermal strap. The present invention solves these and other needs. 
       SUMMARY OF THE INVENTION 
       [0008]    My present invention provides improved means for reducing the vibrations or forces transmitted from a vibrating source on the spacecraft to a vibration-sensitive object on the spacecraft through a connection that has its stiffness reduced through the use of negative-stiffness mechanisms, and in particular, a thermal strap or cold strap or FCL between a vibrating cooling source such as a cryocooler and a motion-sensitive element such as an infrared (IR) detector or focal plane array (FPA) having highly critical alignment requirements. 
         [0009]    My present invention provides a negative-stiffness mechanism that can remove axial stiffness, transverse stiffness and tilt stiffness from a thermal strap and provides improved thermal coupling and structural decoupling. By providing negative stiffness for tilt that was not shown in my previous invention, the present invention provides improved structural decoupling, thereby improving the thermal coupling, Means are also shown for reducing parasitic heat transfer. 
         [0010]    One aspect of the present invention is a negative-stiffness mechanism that consists of two negative-stiffness mechanisms connected in series. A first negative-stiffness mechanism removes transverse stiffness from the thermal strap and includes axially-compressed axially oriented beam columns that connect to a base structure and an intermediate structure. The base structure connects to the vibrating cooling source such as the cryocooler. Another set of structural members also connects to the base structure and support compression springs that provide the axial compressive force on the beam-columns. The beam-columns are compressed beyond their critical buckling loads and thereby provide negative stiffness for translation of the intermediate structure in any transverse direction. 
         [0011]    The intermediate structure supports a negative-stiffness mechanism that removes axial and tilt stiffness from the thermal strap. It consists of three axial-negative-stiffness-producing mechanisms that are radially spaced from the axial axis that passes through the center of the thermal strap and are circumferentially spaced at 120°. The axial-negative-stiffness-producing mechanisms consist of axially-compressed circumferentially-oriented flexures that are connected at their ends to spaced pairs of axially oriented flexures that are supported on the intermediate structure. The axially oriented flexures are deflected during assembly in order to provide the compressive force on the circumferentially oriented flexures. The centers of the circumferentially oriented flexures are connected to an end structure (or a portion of the payload structure) that connects to the cooled vibration sensitive element such as the FPA. The structural connections between the circumferentially oriented flexures and the end structure are thermally insulated in order to reduce parasitic heat transfer into the end structure and the FPA. The three axially compressed circumferentially oriented flexures are compressed beyond their critical buckling loads and thereby provide axial negative stiffness for axial translation of the end structure. Because of their radial spacing from the center axial axis they also provide tilt negative stiffness for rotation of the end structure in any tilt direction. 
         [0012]    The base structure has an inner section and an outer section that are thermally insulated from each other to reduce parasitic heat transfer. The inner section connects to the cold tip of the cryocooler and the outer section connects to the body of the cryocooler. One end of the thermal strap also connects to the inner section of the base structure, and the cold tip of the cryocooler, and the other end connects to the end structure and the FPA. The structure forming the various negative-stiffness-producing mechanisms is connected to the outer section of the base structure in order to thermally insulate these structures from the cold tip of the cryocooler and the composite thermal strap. 
         [0013]    Because the three axial-negative-stiffness-producing mechanisms are relatively stiff for translation in any transverse direction, the transverse negative stiffness produced by the axially compressed axially oriented beam columns produces nearly the same transverse negative stiffness for translation of the end structure in any transverse direction, thereby removing stiffness from the thermal strap in any transverse direction. Also, because the beam columns between the base structure and the intermediate structure are very stiff axially, the axial-and-tilt-negative-stiffness-producing  mechanisms effectively remove axial and tilt stiffness from the thermal strap. The combined negative stiffness mechanisms therefore remove axial, transverse and tilt stiffness from the thermal strap. 
         [0014]    All in all, the present invention provides an improved stiffness reducing system that will reduce the transmission of vibrations or forces from a vibrating source on the spacecraft to a vibration-sensitive object on the spacecraft through a connection such as a thermal strap that has its stiffness reduced through the use of negative-stiffness mechanisms. This reduction in stiffness can be performed with little or no reduction in the thermal coupling. This system can also provide better thermal coupling without reducing the structural decoupling as well as better thermal coupling and better structural decoupling compared with conventional thermal straps. Other features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a perspective view of one embodiment of a negative-stiffness thermal strap made in accordance with the present invention; 
           [0016]      FIG. 2  is a blown up view showing the various components which form the negative-stiffness thermal strap of  FIG. 1 ; 
           [0017]      FIG. 3  is a cross sectional view of the negative-stiffness thermal strap shown in  FIG. 1 ; 
           [0018]      FIG. 4  is another side elevational view of the negative-stiffness thermal strap shown in  FIG. 1 ; 
           [0019]      FIG. 5  is another side elevational view of the negative-stiffness thermal strap shown in  FIG. 1 ; 
           [0020]      FIG. 6  is a perspective view of another embodiment of a negative-stiffness thermal strap made in accordance with the present invention; 
           [0021]      FIG. 7  is a blown up view showing the various components which form the negative-stiffness thermal strap of  FIG. 6 ; 
           [0022]      FIG. 8  is a cross sectional view of the negative-stiffness thermal strap shown in  FIG. 6 ; 
           [0023]      FIG. 9  is another side elevational view of the negative-stiffness thermal strap shown in  FIG. 6 ; 
           [0024]      FIG. 10  is another side elevational view of the negative-stiffness thermal strap shown in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    As is shown in the drawings for purposes of illustration, the present invention is embodied in a stiffness reducing system that reduces vibrations or forces transmitted from a vibrating source on the spacecraft to a vibration-sensitive object on the spacecraft through a connection that has its stiffness reduced through the use of negative-stiffness mechanisms. As the present invention is described in detail as applied to particular negative-stiffness thermal straps or NSFCLs shown in  FIGS. 1-10 , those skilled in the art will appreciate that these systems can be used with other structural components used to couple vibration-sensitive objects with a vibrating source on a spacecraft. 
         [0026]      FIGS. 1-5  show one embodiment of a composite negative-stiffness thermal strap or NSFCL  10  made in accordance with the present invention. The NSFCL  10  is designed to reduce the transmission of omnidirectional vibrations between a vibration sensitive object and the vibrating source. A axial-tilt negative-stiffness-producing mechanism  12  and a transverse negative-stiffness-producing mechanism  14  are connected in series and are coupled with conventional FCLs to reduce their stiffness and improve their structural decoupling. As can be seen in  FIGS. 1-5 , the composite NSFCL  10  is made from a number of individual FCLs  16 , each having a first end  18  and second end  20 . Each of the first ends  18  of the FCLs are connected to a base structure  22  having an inner portion  23  and each of the second ends  20  are connected to an end structure  24 . For example, the inner portion  23  of the base structure  22  could be connected to a vibrating cooling source such as the cryocooler. The end structure  24  could be attached to, for example, a motion-sensitive element such as an infrared (IR) detector or focal plane array (FPA) having highly critical alignment requirements. These individual FCLs  16  are operatively connected with the axial-tilt negative-stiffness-producing mechanism  12  and the transverse negative-stiffness-producing mechanism  14  and are also thermally insulted from the straps as well, as will be explained below. The combined negative stiffness mechanisms  12  and  14  therefore remove axial, transverse and tilt stiffness from the FCL. 
         [0027]    The base structure  22  further includes an outer portion  25  which is designed for attachment to the negative-stiffness-producing mechanisms  12  and  14  and the supporting structures associated with these mechanisms  12  and  14 . The base structure  22  can be designed to minimize parasitic heat transfer between the outer portion  25  and the inner portion  23  which can be kinematically connected to provide flexibility to cope with differential thermal expansions between the warmer outer portion  25  and cooler inner portion  23  and provide sufficient stiffness so that the negative-stiffness mechanisms operate properly. In that regard, the inner portion  23  and outer portion  25  of the base structure  22  must be structurally connected together to act as a single vibrating structure, yet must remain thermally isolated from each other to minimize parasitic heat transfer. The outer portion  25  can be made from a thermally isolating material which is sufficiently strong support the components mounted thereto while creating thermal isolation between the mounted components and the components attached to the inner portion  23  of the base structure  22 . 
         [0028]    The transverse negative-stiffness mechanism  14  is designed to remove transverse stiffness from the composite NSFCL  10  and includes axially-compressed axially oriented beam columns  26  that connect to the base structure  22  and an intermediate structure  28 . A set of structural members, referred to as compression spring supports  30 , is also connected to the outer portion  25  of the base structure  22 . Each beam column  26  is associated with a support compression spring  32  that provides an axial compressive force on the beam-column  26 . Each of the beam-columns  26  are compressed beyond their critical buckling loads and thereby provide negative stiffness for translation of the intermediate structure  28  in any transverse direction. 
         [0029]    The intermediate structure  28  supports the axial-tilt negative-stiffness mechanisms  12  that remove the axial and tilt stiffness from the NSFCL. As is shown in  FIGS. 1-5 , which shows one particular embodiment of the invention, three axial-tilt negative-stiffness-producing mechanisms  12  are radially spaced from the axial axis that passes through the center of the NSFCL and are circumferentially spaced at 120°. Each individual axial-tilt negative-stiffness-producing mechanism  12  consist of at least one axially-compressed circumferentially-oriented flexure  34  that is connected at its ends with end blocks  36 ,  38  to a pair of spaced axially oriented end flexures  40  that are supported on the intermediate structure  28 . Each axially oriented end flexure  40  is deflected during assembly in order to provide the compressive force on its associated circumferentially oriented flexure  34 . The center of each circumferentially-oriented flexure  34  is connected to a thermally insulated structural connector  44  that is connected to the end structure  24  (or the payload structure) that connects to the cooled vibration sensitive element such as the FPA, thereby reducing parasitic heat transfer into the end structure and the FPA. The three axially-compressed circumferentially oriented flexures  34  are compressed beyond their critical buckling loads and thereby provide axial negative stiffness for axial translation of the end structure  24 . Because of their radial spacing from the center axial axis, these axially-compressed circumferentially oriented flexures  34  also provide tilt negative stiffness for rotation of the end structure  24  in any tilt direction. 
         [0030]    The inner portion  23  of the base structure  22  and each compression spring support  30  are thermally insulated from each other to reduce parasitic heat transfer by mounting each of the compression spring supports  30  on the outer portion  25  of the base structure  22 . The axially oriented beam-columns  26  of the transverse negative-stiffness mechanism  14  are in the form of thin cylindrical rods. Each beam-column  26  includes a first end  48  which is press fitted into the outer portion  25  of the base structure  22 . The second end  50  of each beam column  26  is attached (via a press fitting) to the intermediate structure  28  (see  FIG. 3 ). The compression spring  32  includes a first end  52  which sits within a recess  54  formed on the intermediate structure  28 . The other end  56  of the compression spring  32  is placed within a recess  58  formed on the upper plate  60  of the compression spring support  30 . Each compression spring  32  applies a compressive force on its associated beam column  26 . Each beam column  34  may have a notched region  62  located near each end  48 ,  50  which can provide design versatility for the beam columns. During assembly, the beam-columns  26  are compressed beyond their critical buckling loads and thereby provide negative stiffness for translation of the intermediate structure  28  in any transverse direction. This negative stiffness removes positive stiffness from the FCLs  16  as well as the positive stiffness of the compression spring  32 . 
         [0031]    Referring now to  FIGS. 6-10 , another embodiment of a composite negative-stiffness thermal strap or NSFCL  110  is disclosed. This NSFCL  110  utilizes the same basic components used in conjunction with the embodiment of  FIGS. 1-5 . However, in this embodiment, intermediate structure extensions  112  are utilized in conjunction with the compression spring supports  30 . These intermediate structure extensions  112  allow for a reduction in the height of the unit for the same lengths of the beam-columns  26  and the end flexures  40 . As can best be seen in  FIG. 8 , the first end  48  of each beam column  26  is press fitted into the outer portion  25  of the base structure  22 . However, the second end  50  of each beam column  26  is not placed directly into the intermediate structure  28 , as is shown in the previous embodiment, but rather, is press fitted into a top plate  114  formed on the intermediate structure support  112 . As can be seen in the figures, each intermediate structure extension  112  is mounted to the intermediate structure  28  and extends upward to its top plate  114  which receives the end  50  of the beam column  26 . The compression spring  32 , in turn, is placed between the upper plate  60  of the compression spring support  30  and the top plate  114  of the intermediate structure extension  112 . Notches  116  are formed in the intermediate structure  28  to allow for the beam-columns  26  The compression springs  32  produce the same compressive force that will be placed on each beam column  26 . During assembly, the beam-columns  26  are compressed beyond their critical buckling loads and thereby provide negative stiffness for translation of the intermediate structure  28  in any transverse direction. 
         [0032]    The FCLs that can be used in accordance with the present invention include the conventional FCLs disclosed herein along with still other FCLs. For example, standard foil FCLs, such as the ones shown in my previous thermal strap invention, Improved Thermal Straps for Spacecraft, U.S. application Ser. No. 13/587,207, filed on Aug. 16, 2012, could be utilized as well. While the embodiments disclosed herein show the use of three FCLs to create a composite NSFCL unit, it should be appreciated that more of even less FCLs could be utilized. It should be appreciated that the number, type and arrangement of FCLs can be varied without departing from the spirit and scope of the present invention. Also, it should be appreciated that the size, shape and makeup of the FCL used in accordance with the present invention is not limited to the particular FCLs depicted in  FIGS. 1-10 , but could include any one of a number of FCLs. Additionally, the embodiments disclosed herein utilize three beam-columns to support the intermediate structure  28 . It should be noted that more or less beam-columns could be utilized to support the intermediate structure without departing from the spirit and scope of the present invention. 
         [0033]    The FCLs can be made from materials having high thermal conductivity, such as pure aluminum and pure copper. The inner portion  23  of the base structure  22  could be made from materials with very high thermal conduction materials such as pure copper or pure aluminum. The outer portion  25  of the base structure  22  and the thermally insulated structural connector  44  can be made from high strength-to-weight structural metal alloys or structural materials having low thermal conductivities. Similarly, the components in the negative-stiffness mechanisms could be made from high strength-to-weight structural metal alloys such as aluminum and titanium alloys or structural materials having low thermal conductivities. Other structural materials having suitable strength, elastic, thermal and mass properties can also be used. 
         [0034]    While one particular form of the invention has been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except by the attached claims.