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 mechanism 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:
BACKGROUND OF THE INVENTION 
       [0001]    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. 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).” 
         [0002]    My 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. 
         [0003]    My previous 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. 
         [0004]    These above-described isolators provide excellent systems for isolating or reducing the transmission of vibratory motion between an object and the base by exhibiting low stiffness and low fundamental resonant frequencies and effective isolation at the higher frequencies while being capable of accommodating different weight loads without significantly degrading isolation system performance. 
         [0005]    Many spacecraft rely on devices, commonly referred to as thermal straps or flexible conductive links (FCLs) or cold straps for providing thermal (conductive) coupling and structural decoupling between cryogenic components such as a cryocooler cooling source and an infrared (IR) detector or focal plane array (FPA) having highly critical alignment requirements. 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. The present invention solves these and other needs. 
       SUMMARY OF THE INVENTION 
       [0006]    My present invention provides a 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, the 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. 
         [0007]    For example, in one aspect of the present invention, the negative-stiffness-producing mechanism can create negative stiffness that will reduce or negate the natural positive stiffness inherent with the FCL. In this manner, the FCL will still be able to provide maximum thermal conductance while achieving low or near-zero stiffness to maximize structural decoupling. Accordingly, the combination of the negative-stiffness-producing mechanism with the FCL offers a means for improving the performance of the FCL by improving its structural decoupling without decreasing its thermal conductance, or improving its thermal conductance without degrading its structural decoupling. 
         [0008]    In one aspect of the present invention, negative-stiffness mechanisms are coupled with an FCL producing a negative-stiffness FCL or NSFCL that is used to couple the vibration-sensitive object and the vibrating source in the spacecraft. The FCL consists of two FCLs in series, a first FCL and a second FCL. The first FCL is coupled with negative-stiffness mechanisms that remove much or all of the stiffness of the first FCL in an axial direction and in any transverse direction relative to the axial direction. This first FCL is relatively stiff in tilt, or rotation about any transverse axis, and is connected in series with a second FCL that provides thermal coupling and structural decoupling in the tilt directions. This second FCL can be relatively stiff in the axial direction and in the transverse directions since the low or zero axial and transverse stiffnesses of the first FCL effectively removes most or all of the axial and transverse vibrations or forces transmitted through the first FCL. Accordingly, the first FCL can be designed for higher thermal conductance so that the combined FCL, consisting of the first and second FCL in series, will provide effective thermal coupling, and the low or zero axial and transverse stiffness of the first FCL in combination with the low tilt stiffness of the second FCL will provide effective structural decoupling in the axial, transverse and tilt directions. The second FCL that provides the low tilt stiffness and structural decoupling for the tilt axes does not have the benefit of negative stiffness to reduce the tilt stiffness. However, with the higher thermal conductance that can be achieved in the first FCL for the same or lower axial and transverse stiffnesses, the thermal conductance of the second FCL can be made lower to allow for a lower tilt stiffness so that the combined FCLs will provide improved thermal coupling with the same structural decoupling or improved structural decoupling with the same thermal coupling, or both improved thermal coupling and structural decoupling compared with conventional FCL designs. 
         [0009]    In another aspect of the present invention, the first and the second FCLs can each be made with a plurality of FCLs. 
         [0010]    In one aspect of the present invention, the axial-negative-stiffness-producing mechanism utilizes compressed transversely-oriented flexures that cancel or nearly cancel the axial stiffness of the NSFCL. The inner ends of the flexures can be attached to a central hub that connects the first and second FCLs and provides effective thermal coupling between them. The transverse negative-stiffness-producing mechanism utilizes compressed axially oriented beam-columns that cancel or nearly cancel the transverse stiffness of the NSFCL. In this particular embodiment of the invention, the axially-oriented beam-columns are, in turn, attached to a base structure and a portion of the axial-negative-stiffness-producing mechanism. The base structure is attached to the vibrating cooling source and the NSFCL. 
         [0011]    All in all, the present invention provides a suitable 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 
         [0012]      FIG. 1A  is a perspective view of one embodiment of a negative-stiffness thermal strap made in accordance with the present invention; 
           [0013]      FIG. 1B  is a cross-sectional view taken along line  1 B- 1 B of  FIG. 1A ; 
           [0014]      FIG. 1C  is a cross-sectional view taken along line  1 C- 1 C of  FIG. 1A ; 
           [0015]      FIG. 2  is an elevational view of the negative-stiffness thermal strap shown in  FIG. 1 ; 
           [0016]      FIG. 3  is side elevational view of the negative-stiffness thermal strap shown in  FIG. 1 ; 
           [0017]      FIG. 4  is a perspective view of a different negative-stiffness thermal strap made in accordance with the present invention; 
           [0018]      FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 4 ; 
           [0019]      FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 4 ; 
           [0020]      FIG. 7  is an elevational view of the negative-stiffness thermal strap shown in  FIG. 4 ; 
           [0021]      FIG. 8  is side elevational view of the negative-stiffness thermal strap shown in  FIG. 4 ; 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    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. 1A-8 , 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. 
         [0023]      FIGS. 1A-3  show one embodiment of a negative-stiffness thermal strap or NSFCL  10  made in accordance with the present invention. Negative-stiffness mechanisms are coupled with conventional FCLs to reduce their stiffness and improve their structural decoupling. The FCL consists of two FCLs connected in series, a first FCL  11  and a second FCL  16 , that are used to connect a vibration-sensitive object  28  with a vibrating source (not shown). The thermal strap  10  is designed to reduce the transmission of omnidirectional vibrations between the vibration sensitive object  28  and the vibrating source. The first FCL  11  is made up from a number of individual FCLs  18  that are operatively coupled with an axial negative-stiffness mechanism  30  and a transverse negative-stiffness mechanism  14 . 
         [0024]    Each FCL  18  has a first end  20  and a second end  22 . Each first end  20  is connected to the inner portion  24  of a base structure  23  and each second end  22  is connected to a central hub  26 . The base structure  23  has an inner portion  24  and an outer portion  25  and is designed for attachment to the vibrating and cooling source. The inner portion  24  can be connected to the cold tip of a cryocooler (not shown) and the outer portion can connected to the body of the cryocooler. The base structure  23  can be designed to minimize parasitic heat transfer between the outer portion  25  and the inner portion  24  which can be kinematically connected to provide flexibility to cope with differential thermal expansions between the warmer outer portion  25  and cooler inner portion  24  and sufficient stiffness so that the negative-stiffness mechanisms operate properly. The center hub  26  is coupled to the vibration-sensitive object  28  with the second FCL  16 . As can be seen in these figures, the second FCL  16  is mounted to both the vibration-sensitive object  28  and the center hub  26 . Since the first FCL with the axial negative-stiffness mechanism  30  causes the center hub  26  to be quite stiff in tilt or rotations about any transverse axis, and since structural decoupling between the vibrating cold source and the vibration-sensitive object is desired for all three translations and all three rotations, the second FCL is quite flexible in tilt so as to provide the tilt structural decoupling. 
         [0025]    The axial negative-stiffness mechanism  30  includes transversely-oriented flexures  32  which can be compressed to create negative stiffness which will remove much or all of the axial stiffness associated with the FCLs  18 . The negative-stiffness-producing mechanism  30  operates in the same manner as the particular mechanisms disclosed in my previous patents, particularly, U.S. Pat. Nos. 5,669,594 and 5,833,204. 
         [0026]    The transverse negative-stiffness mechanism  14  comprises a number of axially oriented beam-columns  34  which are in the form of thin cylindrical rods. Each beam-column  34  includes a first end  36  secured to the base plate  24  and a second end  38  attached to a spring block  40  which forms a part of the negative-stiffness-producing mechanism  30 . In the embodiment of  FIGS. 1A-3 , there are two spring blocks  40  associated with the negative-stiffness-producing mechanism  30 . The transverse negative-stiffness mechanism  14  includes a loading mechanism  42  which provides a simple stiffness adjustment to the transverse negative-stiffness mechanism  14 . The loading mechanism  42  includes a pair of support rods  44  associated with each spring block  40 . Each support rod  44  includes an end  46  which extends into the outer base structure  25  and a free end  48  which extends through an opening (not shown) in the spring support  40 . Each end  48  of the support rod  44  is threaded so that a nut  50  can be used as a stop for supporting a mounting plate  52 . Another nut  54  located just above the mounting plate  52  maintains the mounting plate  52  secured to each support rod  44 . The loading mechanism  42  further includes a compression spring  56  placed between the mounting plate  52  and the spring block  40 . One end  58  of the compression spring  56  can be placed within a recess  60  found on the top surface of the spring block  40 . This recess  60  helps to prevent the compression spring  56  from moving laterally once loaded. The other end  62  of the compression spring  56  is in contact with a screw mechanism  64  associated with the mounting plate  52 . The screw mechanism  64  includes a turn screw  66  and an abutting structure  68  which contacts the end  62  of the compression spring  56 . The turn screw  66  can includes threads that engage threads cut into an opening in the mounting plate  52 . This turn screw  66  can be rotated to cause the abutting structure  68  to compress the compression spring  56  in order to develop a compressive force acting on the spring block  40 , which, in turn, is transferred to each of the beam-columns  34  associated with that particular spring block  40 . The turn screw  66  can be simply rotated to obtain the desired amount of compressive force needed to be applied to the beam-columns  34  in order to create the negative stiffness that will remove much or all of the transverse stiffness of the FCLs  18 . 
         [0027]    As can be best seen in  FIGS. 1B and 1C , four notched transversely-oriented flexures  32  are attached to the center hub  26  and the spring blocks  40  and are compressed using a tension bolt  70  and a pair of compression springs  72  which form a portion of the axial negative-stiffness-producing mechanism  30 . The tension bolt  70  is designed to extend through an opening  74  which extends through the center hub  26 . The free ends  76  of the bolt  70  are threaded and extend through openings  78  formed in each spring block  42 . Each compression spring  72  can be placed into a recessed cavity  82  formed in the spring block  42  (best seen in  FIG. 1B ) in order to hold the spring in place. A nut  84  and washer  86  at the threaded end  76  of the tension bolt  70  are used to squeeze each compression spring  72  against its respective spring block  40  to achieve a compressive force on each flexure  32 . 
         [0028]    Each nut  84  can be rotated accordingly to impart the needed compressive force to each of the flexures  32 . Each flexure  32  has a first end  88  and a second end  90  having a notch  91  machined or otherwise formed in close proximity to these first and second ends  88 ,  90 . Each flexure  32  is attached to the spring block  40  and center hub  26  using insulated fastening means  93 . Preloading of the flexures and fine tuning of the load to adjust the negative-stiffness effect are accomplished by simply turning each nut  84 , as may be needed. This arrangement of a tension bolt, die springs and fasteners is just one of a number of ways to load the flexures  32 . In this manner, the axial negative-stiffness mechanism can produce negative stiffness via the compressed flexures  32  which will remove much or all of the axial stiffness associated with the FCLs  18 . 
         [0029]    The FCL  16  can be made from a plurality of thin cylindrical rods  92  press-fit into end fittings  94  and  96  which are secured to the vibration-sensitive object  28  and center hub  26 . Alternatively, the FCL  16  can be made from FCL assemblies, such as those shown in  FIGS. 4-8 . 
         [0030]    As can be seen in  FIGS. 1A-3 , this particular embodiment utilizes transversely-oriented flexures  32  which are connected to negative-stiffness-producing mechanisms  30  and transverseley-oriented flexures  32 ′ which are free standing and not connected to a negative-stiffness-producing mechanism. Likewise, there are beam-columns  34  which are connected to loading mechanisms  42  and other sets of beam-columns  34 ′ which stand alone. These “free standing” flexures  32 ′ and beam-columns  34 ′ are utilized to provide additional lateral stability to the composite system. Alternatively, these free standing flexures  32 ′ and beam-columns  34 ′ could be attached to negative-stiffness-producing mechanisms, if desired. 
         [0031]    The negative-stiffness thermal strap  10  utilizes conventional FCLs  16  and  18  to provide a strong thermal link between the cooling source (the vibration source) and the vibration-sensitive object  28 , and additional structures that include the center hub  26 , the negative-stiffness mechanisms  30  and  14  and the base structures  24  and  25 . The center hub  26  and inner base structure  24  are part of the main thermal path between the cooling source and the vibration-sensitive object  28  and are designed to provide strong thermal coupling and sufficient stiffness and strength as well as minimum mass to cope with launch loads. In this embodiment of the invention the center hub  26  is thermally insulated from the negative-stiffness mechanisms  30  and  14  and the negative-stiffness mechanism  14  is thermally insulated from the inner base structure  24 . This is to minimize parasitic heat transfer from the negative stiffness mechanisms  30  and  14  and the outer base structure  25 . All the components of the negative-stiffness mechanisms  30  and  14  are designed for sufficient stiffness and strength so they operate properly and for minimum mass to cope with launch loads. The entire negative-stiffness thermal strap  10  is also designed so that its structural resonances avoid the primary vibration frequencies of the vibrating cooling source such as a cryocooler as well as harmonics associated with the primary vibrating frequencies. 
         [0032]    FCLs  16  and  18  are flexible yet thermally conductive so as to provide the simplest and most prevalent devices in cryogenic integration. They provide mechanical flexibility to cope with launch loads and/or differential thermal expansion stresses while still providing a strong thermal link. In the particular embodiment shown in  FIGS. 1A-3 , each FCL  18  is made with two rigid end pieces  100  and  102  and a number of small diameter wires  104 . An FCL also can be composed of multiple thin layers of foil, such as the one described below and depicted in  FIGS. 4-8 . Each of the end pieces  100  of the individual FCLs shown in  FIGS. 1A-3  is attached to the center hub  26  using suitable connectors. In this regard, the particular shape of the hub  26  could be made to accommodate the end pieces  100  of each FCL  18 , as is shown in  FIG. 1A . Likewise, the other end piece  102  of each FCL  18  is attached to the inner base structure  24 . In this regard, the inner base structure  24  could have a raised region  106  used as an abutment for attaching the end piece  102  thereto. This raised region  106  could be shaped to receive each end piece  102 . It should be appreciated that the attachment of these end pieces  100  and  102  are just one of a number of ways to attach the FCLs  18  in the negative-stiffness thermal strap  10 . There are also a number of ways the center hub  26  and the base structures  23  and  24  could be constructed. The center hub  26  and the inner base structure  24  could be plates with lightening holes made from very high thermal conduction materials such as pure copper or pure aluminum. The outer base structure  25  could be of a frame or truss construction made with high strength-to-weight structural metal alloys or structural materials having low thermal conductivities. Similarly, the components in the negative-stiffness mechanisms  16  and  30  could be made from high strength-to-weight structural metal alloys or structural materials having low thermal conductivities. 
         [0033]    Referring now to  FIGS. 4-8 , another embodiment of a negative-stiffness thermal strap  110  is disclosed. This negative-stiffness thermal strap  110  utilizes the same basic components used in conjunction with the embodiment of  FIGS. 1A-3 . However, in this embodiment, the FCLs  112  used with the negative-stiffness thermal strap is a foil-type FCL described above. As can be seen in  FIGS. 4-8 , the arrangement of the FCLs relative to each other and the negative-stiffness mechanisms  30  and  14  is somewhat different. For example, there is an FCL  112  directly located beneath each spring block  42  used in the negative-stiffness thermal strap  110 . FCLs are also located adjacent to each spring block  42  resulting in a total of eight FCLs used with this particular embodiment. This embodiment shows how a different number of FCLs can be used and arranged relative to the components of the negative-stiffness thermal strap. 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. 
         [0034]    The second FCL  16  used with this particular embodiment, that also provides tilt structural decoupling between the vibrating source and the vibration-sensitive object  28 , is also different from the FCL  16  used on the embodiment of  FIGS. 1A-3 . As can be best seen in  FIGS. 5-8 , the FCL  16  is made from three foil-type FCLs  120  with rigid end pieces  121  and  123  which connect the center hub  26  to the vibration-sensitive object  28 . End pieces  121  connect to the center hub  26  and end pieces  23  connect to the vibration-sensitive object  28 . Each FCL  120  has a substantial U shape and can be arranged in a circular pattern approximately 120° relative to each other. This construction makes the FCL  16  flexible in tilt in order to provide effective tilt structural decoupling between the vibrating source and the vibration-sensitive object  28  and also provide strong thermal coupling between the center hub  26  and the vibration-sensitive object  28 . 
         [0035]    The FCLs  112  used with the embodiment of  FIGS. 4-8  utilize two rigid end pieces  122  and  124  and multiple layers  126  of thin foil. The particular FCL  112  depicts a standard foil FCL which can be used in accordance with the present invention. 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. 1A-8 , but could include any one of a number of FCLs. Additionally, the embodiments disclosed herein utilize a pair of beam-columns to support one end of each spring block. Accordingly, four beam-columns are shown to support each spring block. It should be noted that more of less beam-columns could be utilized to support each spring block without departing from the spirit and scope of the present invention. 
         [0036]    The FCLs and elements in the main thermal path such as the center hub and the inner base structure can be made from materials having high thermal conductivity such as pure aluminum and pure copper, and the elements making up the negative-stiffness mechanisms can be made from high strength-to-weight structural materials such as aluminum and titanium alloys. Other structural materials having suitable strength, elastic, thermal and mass properties can also be used. 
         [0037]    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.