Abstract:
A vibration isolating system is disclosed. The vibration isolating system comprises a passive mechanical system comprising a damping assembly, a first resilient member coupled in series with the damping assembly, and a second resilient member coupled in parallel with the series combination of the damping assembly and the first resilient member. The vibration isolating system further comprises a support member coupled in series with the passive mechanical system, a viscoelastic mount coupled to the support member, and a motion limiter coupled to the support member such that the passive mechanical system transmits a force to the support member when the passive mechanical undergoes longitudinal displacement greater than a predetermined displacement.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments of the subject matter described herein relate generally to vibration isolators. More particularly, embodiments of the subject matter relate to two-stage, passive vibration isolators. 
       BACKGROUND 
       [0002]    Various systems and components operate in an environment where they are subject to vibrations from surrounding objects. Such systems and components typically exhibit improved performance and/or accuracy when the vibrations are reduced or removed. Some exemplary systems include satellite payloads, which can be subjected to both large-amplitude vibrations during the launch to orbit around the Earth, as well as small-amplitude vibrations at a different frequencies than the large amplitude vibration while operating in orbit. Such payloads can be sensitive to vibrations. They can therefore benefit from appropriate vibration isolation. Particularly, damping elements are included in the vibration isolation. One example of a passive damping and isolation system is the D-STRUT® isolation strut, manufactured by Honeywell, Inc. of Morristown, N.J. The D-STRUT® isolation strut is a three-parameter vibration isolation system that mechanically acts like a spring (KA) in parallel with a series spring (KB) and damper (CA) and is disclosed in U.S. Pat. No. 5,332,070 entitled “Three Parameter Viscous Damper and Isolator” by Davis et al. and U.S. Pat. No. 7,182,188 entitled “Isolator Using Externally Pressurized Sealing Bellows” by Ruebsamen et al. These patents are hereby incorporated by reference. 
         [0003]    Isolation systems are typically tuned for a specific vibration amplitude and resonant frequency. As previously described, some systems can experience different vibration amplitudes at different frequencies. It can be difficult for a single passive isolation system to isolate both types of vibration. Accordingly, vibration isolation systems, such as those for space satellite payloads, are typically tuned to a desired resonant frequency to optimally isolate one vibratory amplitude and frequency at the expense of effectiveness in isolating the other. Isolation systems are often designed or tuned to isolate small-amplitude on-orbit vibrations. Consequently, a satellite payload will frequently experience unmitigated large-amplitude vibrations during launch to orbit. As a result, the satellite payload is often reinforced with certain features and/or structures to survive large amplitude vibrations. The reinforcing features require additional volume in the payload area and impose additional energy costs during launch. Additionally, while useful during launch, once in orbit the reinforcing features or structures typically have no utility. 
         [0004]    It would be beneficial to design a single passive vibration isolator which can be tuned to reduce both small- and large-amplitude vibrations during launch and orbit of a satellite payload. Other systems besides satellite systems may similarly benefit from such vibration isolation. 
       BRIEF SUMMARY 
       [0005]    A vibration isolating system is disclosed. The vibration isolating system comprises a passive mechanical system comprising a damping assembly, itself comprising, a resilient housing having an inner surface defining a housing passage therethrough, a first bellows disposed within the housing passage, the first bellows spaced apart from the inner surface to define a first chamber having a first volume, a second bellows disposed within the housing passage, the second bellows spaced apart from the housing inner surface to define a second chamber having a second volume, a restrictive flow passage in fluid communication with the first and second chambers, and a piston coupled to at least the second bellows and disposed within the housing passage, the piston configured to receive a first force to thereby move the piston through the restrictive flow passage to increase the first volume and decrease the second volume. The passive mechanical system further comprises a first resilient member coupled in series with the damping assembly, and a second resilient member coupled in parallel with the series combination of the damping assembly and the first resilient member. The vibration isolating system further comprises a support member coupled in series with the passive mechanical system, a viscoelastic mount coupled to the support member, and a motion limiter coupled to the support member such that the passive mechanical system transmits a force to the support member when the passive mechanical undergoes longitudinal displacement greater than a predetermined displacement. 
         [0006]    Another vibration isolating system is disclosed. The vibration isolating system comprises a passive mechanical system, itself comprising a damping assembly comprising housing having a first end, a second end, an inner surface, and a passage defined by the inner surface extending between the first and second ends, a first bellows disposed within the passage and having a first bellows end, a second bellows end, and a first bellows outer surface, the first bellows end coupled to the first end, the second bellows end having a first bellows end surface, and the first bellows outer surface and inner surface of the housing defining a first chamber having a first volume, a second bellows disposed within the passage and having a third bellows end, a fourth bellows end, a second bellows inner surface, and a second bellows outer surface, the third bellows end coupled to the fourth bellows end, the second bellows inner surface defining a cavity therein, and the second bellows outer surface and inner surface of the housing defining a second chamber having a second volume, and a piston disposed within the passage, the piston having a shaft having a first section, a second section and a shaft outer surface, the first section at least partially disposed in the second chamber, the second section at least partially disposed outside of the second chamber and defining a flowpath with the inner surface of the housing, the flowpath in fluid communication between the first and second chambers, at least a portion of the shaft outer surface between the first and second sections coupled to the fourth bellows end, the piston configured to receive a first force to thereby move the piston through the restrictive flow passage to increase the first volume and decrease the second volume. The passive mechanical system further comprises a first resilient member coupled in series with the damping assembly, and a second resilient member coupled in parallel with the series combination of the damping assembly and the first resilient member. The vibration isolating system further comprises a support member coupled in series with the passive mechanical system, a viscoelastic mount coupled to the support member, and a motion limiter coupled to the support member such that the passive mechanical system transmits a force to the support member when the passive mechanical undergoes longitudinal displacement greater than a predetermined displacement. 
         [0007]    Another vibration isolating system is disclosed. The vibration isolating system comprises a passive mechanical system, itself comprising a damping assembly comprising a resilient housing having a first end, a second end, an inner surface, and a passage extending between the first and second ends, and an isolation assembly disposed in the passage and at least partially enclosed by the housing, the isolation assembly adapted to receive a force and to extend toward the first end of the housing in response to receiving the force. The passive mechanical system further comprises a first resilient member coupled in series with the damping assembly, and a second resilient member coupled in parallel with the series combination of the damping assembly and the first resilient member. The vibration isolating system further comprises a support member coupled in series with the passive mechanical system, a viscoelastic mount coupled to the support member, and a motion limiter coupled to the support member such that the passive mechanical system transmits force to the support member when the passive mechanical undergoes longitudinal displacement greater than a predetermined displacement. 
         [0008]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    A more complete understanding of the subject matter may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. 
           [0010]      FIG. 1  is a schematic of an exemplary system having two stage vibration isolators; 
           [0011]      FIG. 2  is a cross-sectional view of an exemplary isolator that may be implemented in the system of  FIG. 1 ; 
           [0012]      FIG. 3  is a detailed view of the cross-sectional view of an exemplary isolator that may be implemented in the system of  FIG. 1 ; 
           [0013]      FIG. 4  is a close up view of a section of the exemplary isolator of  FIG. 3 ; and 
           [0014]      FIG. 5  is a cross-sectional view of another exemplary isolator that may be implemented in the system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
         [0016]    “Coupled”—The following description refers to elements or nodes or features being “coupled” together. As used herein, unless expressly stated otherwise, “coupled” means that one element/node/feature is directly or indirectly joined to (or directly or indirectly communicates with) another element/node/feature, and not necessarily mechanically. Thus, although the schematic shown in  FIG. 4  depicts one exemplary arrangement of elements, additional intervening elements, devices, features, or components may be present in an embodiment of the depicted subject matter. 
         [0017]    “Adjust”—Some elements, components, and/or features are described as being adjustable or adjusted. As used herein, unless expressly stated otherwise, “adjust” means to position, modify, alter, or dispose an element or component or portion thereof as suitable to the circumstance and embodiment. In certain cases, the element or component, or portion thereof, can remain in an unchanged position, state, and/or condition as a result of adjustment, if appropriate or desirable for the embodiment under the circumstances. In some cases, the element or component can be altered, changed, or modified to a new position, state, and/or condition as a result of adjustment, if appropriate or desired. 
         [0018]    “Inhibit”—As used herein, inhibit is used to describe a reducing or minimizing effect. When a component or feature is described as inhibiting an action, motion, or condition it may completely prevent the result or outcome or future state completely. Additionally, “inhibit” can also refer to a reduction or lessening of the outcome, performance, and/or effect which might otherwise occur. Accordingly, when a component, element, or feature is referred to as inhibiting a result or state, it need not completely prevent or eliminate the result or state. 
         [0019]    In addition, certain terminology may also be used in the following description for the purpose of reference only, and thus are not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “side”, “interior”, and “exterior” describe the orientation and/or location of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0020]    A two stage vibration isolator can reduce large-amplitude vibrations at a first resonant frequency and small-amplitude vibrations at a second, lower resonant frequency. The two stage isolator can provide both damping and linear-elastic vibration isolation for both low- and high-amplitude vibrations. Additionally, a stop or motion limiter can be used to contact a piston and transmit the force to a viscoelastomeric member, which functions as a viscoelastic damper and isolator for high-amplitude vibrations. In this way, both high- and low-amplitude vibrations can be isolated with a single passive device. 
         [0021]      FIG. 1  illustrates an exemplary system having vibration isolation with damping and resilient members. The system  100  may be implemented in any one of numerous environments, such as in space, terrestrially, or under water. The system  100  includes a base  102 , a payload  104 , and at least one damping device or vibration isolation apparatus  106 . The base  102  is configured to provide a platform to which the payload  104  and vibration isolation apparatus  106  are coupled and may be any one of numerous application-appropriate devices. For example, in a space application, the base  102  can be a satellite, an arm of a satellite, a space station, or any one of numerous other conventionally-used space apparatus. The payload  104  is a device that preferably needs vibration damping and isolation to operate effectively. The payload  104  may be any one of numerous devices, such as, for example, a telescope or a camera. 
         [0022]    The vibration isolation apparatus  106  dampens and isolates vibration that may be experienced by the payload  104  and thus, is coupled between the payload  104  and the base  102 . Although a single vibration isolation apparatus  106  may be used, it may be preferable to employ more than one vibration isolation apparatus  106 . In one exemplary embodiment, three vibration isolation apparatus  106  are used in a tripod configuration to isolate vibration. In another exemplary embodiment, six vibration isolation apparatus  106  are implemented in a hexapod configuration to provide vibration isolation along six degrees of freedom. In other embodiments, two or more vibration isolation apparatuses  106  can be employed to isolate vibrations in any desired configuration. 
         [0023]    With reference now to  FIG. 2 , a diagram of an exemplary two stage vibration isolation apparatus  106  is provided. The vibration isolation apparatus  106  includes a pivot  108 , an isolation or damper assembly  110 , an outer resilient housing or segment  112 , an inner resilient housing or segment  113 , a support  114 , a viscoelastic mount  200 , internal stops  210 , and a mount plate  214 . The pivot  108  receives vibratory motion from the base  102  and couples the base  102  to the vibration isolation apparatus  106 . The outer resilient segment  112  operates along a parallel path to the series combination of the damper assembly  110  and inner resilient segment  113 , together damping and isolating the received vibratory motion. The damper assembly  110  travels within the outer resilient segment  112  up to the internal stops  210 . 
         [0024]    The inner and outer resilient segments  112 ,  113  can be metal or elastomeric components. Each preferably has a predetermined stiffness which can be varied depending on the frequencies expected for an application of the vibration isolation apparatus. Thus, the inner and outer resilient segments  112 ,  113  are not limited to particular shapes, and can incorporate other features, such as shielding or protective housing features, if desired. The resilient segments can be springs, having form and features typical of a spring, including linear stiffness, in some embodiments. The inner and outer resilient segments  112  can comprise additional elements and features without limitation, such as coatings, mounting or fastening attachments, and so on, as appropriate for the embodiment. 
         [0025]    The internal stops  210 , which are coupled to and stationary relative to the support  114 , inhibit extreme movement of the damper assembly  110 . Similarly, extreme movement of the outer resilient segment  112  is inhibited, as shown in  FIG. 2 . The amplitude of displacement of the damper assembly  110  permitted prior to contacting a stop can be adjusted by varying the position of the internal stops  210  within the outer resilient segment  112 . When displaced towards the left or right by a force from the pivot  108 , the damper assembly  110  contacts the internal stop  210  after traveling a maximum distance. Before traveling the maximum distance, as limited by the internal stops  210 , the damper assembly  110  can damp the vibration received, and maintain contact with the support  114  through the inner resilient segment  113 . After contacting the internal stops  210 , however, the damper assembly  110  has reduced damping performance, and the vibrations are additionally isolated by the resilience of the viscoelastic mount  200 . Accordingly, force from the pivot  108  in the form of vibratory motion is transmitted semi-rigidly to the viscoelastic mount  200  once the amplitude exceeds the amount permitted by the internal stops  210 . 
         [0026]    The viscoelastic mount  200  can be coupled to a mount plate  214 . The mount plate  214  can be a surface of the payload  104 , or coupled to the payload  104 . Accordingly, the vibration isolation apparatus  106  couples the payload  104  to the base  102  and inhibits vibratory motion from travelling from the base  102  to the payload  104 . The vibration isolation apparatus  106  provides different vibration isolation responses for those vibrations which are low-amplitude, such as those which do not cause the damper assembly  110  to contact the internal stops  210 , than for those which are high-amplitude, causing contact between the damper assembly  110  to contact the internal stops  210 . 
         [0027]    The damper assembly  110  is coupled to the pivot  108  via a shaft  116 . The outer resilient segment  112  can protect the damper assembly  110  from intrusion by foreign objects, as well as from impact damage and is configured to house the damper assembly  110  therein. The outer resilient segment  112 , as previously mentioned, can comprise a spring, or other linear or non-linear elastic element. As shown in  FIG. 3 , the support  114  can attach the vibration isolation apparatus  106  to the base  102  and may be either integrally formed as part of the outer resilient segment  112  or may be a separate piece coupled to the outer resilient segment  112 .  FIG. 3  illustrates a more detailed view of the assembly shown in  FIG. 2 , and references made to one can be viewed in the other. 
         [0028]    With reference to  FIG. 2 , the internal stop  210  preferably operates as, or cooperates with, a motion limiter  211 . The internal stop  210  can be present in any embodiment of the two stage vibration isolation apparatus  106 . Although shown as coupled to the outer resilient segment  112  along supports, in other embodiments, the internal stop  210  can be supported by different components, including the support  114 . Preferably, the internal stop  210  is positioned to inhibit motion of the damping element, here the damper assembly  110 , in a direction toward or away from the payload or other isolated component once the damper assembly  110  exceeds the desired motion limit. The internal stop  210  is stationary relative to the support  114 , and can be contacted by the motion limiter  211 . The motion limiter  211 , which can be integral with the first end plate  148 , as shown in  FIG. 4 , or a separate component, can at least partially surround the internal stop  210 , as shown. The motion limiter  211  can arrest movement of the damper assembly  110  by contacting the internal stop  210  in either direction, as shown. The amplitude of displacement permitted for the damper assembly  110  can be adjusted by adjusting the position of the internal stops  210 , the distance between the internal stops  210  and motion limiter  211 , or both. 
         [0029]    Thus, as shown in  FIG. 2 , the damper assembly  110  can move to either side when experiencing a force during vibratory excitement. The internal stop  210  as contacted by the motion limiter  211 , however, prevents motion of the damper assembly  110  past a certain amplitude. The amount of permitted displacement can be tuned or configured to permit greater amplitude toward one direction or other, if desired for the embodiment. In certain embodiments, the permitted displacement in either direction can be the same. 
         [0030]    Accordingly, the damper assembly  110 , via the motion limiter  211 , will contact a surface of the internal stop  210  and transmit force directly to the internal stop  210 . Because the internal stop  210  is coupled to the support  114 , the force will be transmitted directly to the viscoelastic mount  200 . The viscoelastic mount  200 , therefore, provides additional resilience and damping during vibrations which have amplitudes exceeding those permitted by the motion limiter  211  and internal stops  210 . 
         [0031]    In certain embodiments, the viscoelastic mount  200  can be present on the internal stop  210 , rather than near the support  214 . Thus, the internal stop  210  can be formed of a metal sufficiently strong to transmit loads from the damper assembly  110 , or it can include viscoelastic materials. The viscoelastic mount  200 , similarly, is preferably formed from a resilient material, such as rubber, silicone, and so on. The exact properties and composition of the viscoelastic mount  200  can vary between embodiments according to produce vibration isolating and damping characteristics desired for specific embodiments. Accordingly, when used herein, viscoelastic can include elastomers and viscoelastomers which exhibit both resilient and viscous properties, the exact degree of which can vary between embodiments. 
         [0032]      FIG. 4  illustrates a close-up view of the exemplary damper assembly  110  of  FIG. 3 . The damper assembly  110  includes an assembly housing  118 , a first bellows  120 , a second bellows  122 , a piston assembly  124 , fluid and, optionally, a temperature compensation device  126 . The assembly housing  118  is configured to operate with the other components of the damper assembly  110  to provide a fixed volume of space and to enclose and seal the fluid therein. The assembly housing  118  includes at least a tube  128  that has a first end  130 , a second end  132 , and an inner surface  134  that defines a passage  138  extending between the first and second ends  130 ,  132 . The assembly housing  118  also includes a longitudinal axis  142  along which the components in the passage  138  may travel. Preferably, the first end  130  includes an inlet  140 , the second end  132  includes an outlet  141 , and the tube  128  has no openings other than the inlet  140  and outlet  141 . However, in alternate embodiments, the tube  128  may be a single component having endwalls integrally formed or coupled to each of the first and second ends  130 ,  132 . 
         [0033]    The internal stop  210  is shown extending radially inward from the assembly housing  118 . The motion limiter  211  is shown partially surrounding the internal stop  210 , in an undisplaced position. As can be seen, any motion of the damper assembly  110  to the left or right which exceeds a certain amplitude will engage the motion limiter  211  with the internal stop  210 , altering the vibration response of the assembly. Thus, motion of the assembly housing  118  toward the left is arrested by the internal stop  210 , which can be positioned as desired for each embodiment. Similarly, motion of the assembly housing  118  toward the right also can be arrested by the internal stop  210  and motion limiter  211 . 
         [0034]    In one exemplary embodiment, such as illustrated in  FIG. 5 , the assembly housing  118  includes a damping plate  144  disposed in the middle thereof. The damping plate  144  is integrally formed or integrated as part of the assembly housing  118  and includes at least one duct  145  that extends through the damping plate  144 . The damping plate  144  can include a pipe  146  that extends axially outward from substantially the center of each side of the damping plate  144  along the longitudinal axis  142 . In such an embodiment, the ducts  145  also extend through the pipe  146 . 
         [0035]    Returning to  FIG. 4 , the first bellows  120  is disposed within the assembly housing  118  and is preferably configured to move along the longitudinal axis  142 . The first bellows  120  is coupled at one end to a first end plate  148  and at an opposite end to a second end plate  150  to thereby define first bellows interior cavity  152  therebetween. The first end plate  148  sealingly mates with the assembly housing first end  130  and couples the first bellows  120  thereto. The second end plate  150  couples to a support shaft  158  that is disposed within the first bellows interior cavity  152 . 
         [0036]    The support shaft  158  is configured to provide structural support for the first bellows  120  and guides the first bellows  120  along the longitudinal axis  142  during operation. The support shaft  158  may itself include a cavity  160  configured to receive other damper assembly  110  components therein. It will be appreciated that each of the first and second end plates  148 ,  150  include openings  162 ,  164  formed therein that are configured to accommodate components that may extend outside of the assembly housing  118 , such as the temperature compensation device  126 , shown in  FIG. 4 , the damping plate pipe  146 , illustrated in  FIG. 5 , or support shaft  158 . 
         [0037]    Similar to the first bellows  120 , the second bellows  122  is disposed within the assembly housing  118 , is coupled to a first and a second end plate  166 ,  168 , and is preferably configured to move along the longitudinal axis  142 . Although depicted in  FIG. 4  as being capable of traveling along the same axis  142  as the first bellows  120 , it will be appreciated that in other non-illustrated embodiments the second bellows  122  may move along any other suitable axis. The second bellows first end plate  166  sealingly mates with the assembly housing second end  132  and couples the second bellows  122  thereto. The second bellows second end plate  168  is coupled to the opposite end of the second bellows  122  and, together with the first end plate  166  and inner surface of the second bellows  122 , defines an interior cavity  170 . Just as above, each of the first and second end plates  166 ,  168  include openings  172 ,  174  formed therein that are configured to provide space for disposal of components that may extend outside of the assembly housing  118 , in this case, the piston assembly  124  or shaft  116  (shown in  FIG. 3 ). 
         [0038]    The piston assembly  124  is configured to operate with the first and second bellows  120 ,  122  to damp and isolate vibration received from the shaft  116  (shown in  FIG. 3 ), together with the viscoelastic mount  200 . The piston assembly  124  is disposed within the assembly housing  118  and is coupled between the first and second bellows  120 ,  122 . The piston assembly  124  includes a piston shaft  176  and piston flange  178 . The piston shaft  176  may be embodied as a single piece or multiple pieces (for example, shaft  176  and section  177 , as shown in  FIG. 4 ) and is aligned along the longitudinal axis  142  and is disposed in the second bellows interior cavity  170 . The piston shaft  176  has an end that is coupled to the shaft  116  and another end that is coupled to the piston flange  178 . In one exemplary embodiment, the piston shaft  176  extends through the second end plate opening  174  of the second bellows  122  and is coupled directly to the shaft  116 . It will be appreciated, however, that the piston shaft  176  may be coupled to the shaft  116  in any other suitable manner to receive vibratory motion therefrom. 
         [0039]    The piston shaft  176  includes a flowpath  180  extending at least partially therethrough for receiving fluid. In one exemplary embodiment, one section of the flowpath  180  has threaded walls that are configured to mate with a set screw. 
         [0040]    The piston flange  178  extends radially outward from the piston shaft  176  and may be either formed integrally as part of the piston shaft  176  or may be separately constructed and subsequently attached to the piston shaft  176 . The piston flange  178  includes an inner surface  184  and an outer surface  186 . The inner surface  184  is sealingly coupled to the second bellows second end plate  168 . The outer surface  186  may have any one of numerous configurations. However, in the embodiment shown in  FIG. 4 , the outer surface  186  is coupled to the first bellows  120  via another section of the piston  177 . As such, the outer surface  186  includes an extension  188  that mates with the piston section  177 . The piston section  177 , in turn, is coupled to the first bellows second end plate  150 . In another exemplary embodiment, the outer surface  186  is configured to couple to the first bellows  120  and the outer surface  186  is directly coupled to the first bellows second end plate  150 . 
         [0041]    As briefly mentioned previously, the damper assembly  110  components are preferably configured to operate together to sealingly enclose the fluid therein in a fixed volume of space. The volume of space is separated into subvolumes, each of which is disposed in a first chamber  192 , a second chamber  194 , and a restrictive flow passage  196 . The first chamber  192  is defined by a portion of the assembly housing inner surface  134  and an outer surface of the first bellows  120 , and the second chamber  194  is defined by another portion of the assembly housing inner surface  134  and an outer surface of the second bellows  122 . 
         [0042]    In one exemplary embodiment, a damping annulus  198 , defined by the piston flange  178  and assembly housing inner surface  134 , acts as the restrictive flow passage. In another exemplary embodiment, the restrictive flow passage  196  is defined by the ducts  145  that are formed in the damping plate pipe  146 , as shown in  FIG. 5 . Also in  FIG. 5 , the internal stop  212 , as surrounded by the motion limiter  211 , is present to arrest movement of the assembly housing  118  and transmit force to the viscoelastic mount  200 . The internal stop  212  can be positioned in a different location to adjust the amplitude of expansion of the first and second bellows  120 ,  122 . 
         [0043]    In still another embodiment, the restrictive flow passage  196  is defined by ducts  145  formed in the damping plate  144 . No matter the particular configuration, the first chamber  192 , second chamber  194 , and restrictive flow passage(s)  196  are filled with fluid. Thus, during the operation of the damper assembly  110 , when a force is exerted on the piston assembly  124 , fluid is pushed from the second chamber  194 , through the restrictive flow passage  196 , into the first chamber  192 . 
         [0044]    As illustrated in  FIG. 4 , the temperature compensation device  126  may be included in the damper assembly  110  to compensate for fluid expansion and/or contraction in response to temperature changes. The temperature compensation device  126  may have any one of numerous suitable configurations and may be disposed within the damper assembly  110  in any one of numerous manners. 
         [0045]    The above description relates to a two stage vibration isolator. During operation, the isolation strut is capable of transmitting fluid pressure from its moving piston to the sealed bellows outer surfaces, which can displace a certain amount prior to contacting an internal stop. Prior to contacting the internal stop, the vibration isolator can provide a well-damped response with tuned resonant frequency, suitable for low-amplitude vibrations. Once exceeding the permitted displacement of the isolation assembly, large-amplitude vibrations are isolated by the viscoelastic mount, as well as any residual stiffness and damping within the isolation assembly. Thus, the vibration isolator is capable of providing a first well-damped isolation response to low-amplitude vibrations at a first resonant frequency, and a second response for high-amplitude vibrations at a second resonant frequency. 
         [0046]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.