Abstract:
A damper containing magneto-rheological (“MR”) fluid and a plunger lies between and is mechanically coupled both to a fixture and a vibration source. The plunger has a head immersed in the MR fluid. Annular magnets circumscribe the damper and produce a magnetic field surrounding the damper. A tubular shielding sleeve composed of magnetically impermeable material surrounds a portion of the damper. The sleeve is mechanically coupled both to the fixture and the vibration source by springs, and can translate relative to the damper to affect the strength of the magnetic field acting on the MR fluid surrounding the plunger head. The sleeve oscillates responsive to vibration of the vibration source and controls the viscosity of the MR fluid surrounding the plunger head in proportion to the amplitude of the sleeve&#39;s oscillation. The resonant frequency of the sleeve is adjusted to approximate the fundamental resonance of the fixture.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of Ser. No. 11/251,008 U.S. Pat. No. 7,445,094 filed on Oct. 11, 2005, and claims the benefit of the foregoing filing date. 

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

   FIELD OF THE INVENTION 
   The present invention is generally related to the field of vibration isolation. More particularly, the present invention is a passive system that uses magneto-rheological fluid to reduce vibration transmission over a broad frequency spectrum. 
   BACKGROUND OF THE INVENTION 
   A problem often faced in the field of engineering is vibration isolation. Many current applications use devices to isolate a sensitive component from a vibrating environment, or to reduce the transmission of vibration from a vibrating component into its surroundings. Currently, there are numerous ways of accomplishing vibration isolation, including passive apparatus, such as spring and damper systems, and active devices, which utilize actuation to achieve isolation and can be adapted to comply with environmental parameters and flight conditions. The drawbacks to active devices are that they require additional power sources, and are typically more complex and less robust than passive mechanisms. For many applications, passive systems are preferred because they are simpler and self-contained. U.S. Pat. No. 6,135,390 issued to Sciulli et al., teaches a passive mechanism for isolating spacecraft using titanium flexures to act as soft springs. U.S. Pat. Nos. 5,947,240 and 5,803,213 issued to Davis et al., disclose a system of passive dampers in a closed geometric shape for use in load vibration isolation. 
   A fairly recent active material development is magneto-rheological (“MR”) fluid. MR fluids are comprised of micron sized, magnetically-polarized particles suspended in a carrier fluid. When activated by a magnetic field, the particles align along magnetic field lines and change the material&#39;s flow characteristics, such as its viscosity and bulk modulus. By varying the magnetic field flux acting on the MR fluid, the viscous damping may be modulated. U.S. Pat. No. 5,683,615 issued to Munoz describes the behavior and different chemical compositions of MR fluids. 
   Most prior vibration isolation devices using MR fluids have employed an active control system to regulate the magnetic field actuating the MR fluid. For example, U.S. Pat. Nos. 6,082,719; 6,196,529; and 6,196,528 issued to Shtarkman et al., disclose a spacecraft antenna vibration control damper. The vibration of the spacecraft antenna during maneuvers is sensed by an external sensor, which in turn activates the magnetic field on the MR fluid through a controller and a power supply. 
   The few inventions that have utilized a partially passive system for MR fluid damping are mostly for motor vehicle applications and have some aspect of active isolation. For example, U.S. Pat. No. 5,632,361 issued to Wulff et al., involves a passive MR damper that includes a constant magnetic field created by a permanent magnet in a piston, while also having a variable magnetic field provided by an electric coil. The constant magnetic field provides a “pre-stress” on the MR fluid. It is not completely passive in that it requires active control for the main isolation with the electric coil. The passive permanent magnet is typically for backup and “pre-stress.” 
   There is a need in the art for a passive system that makes beneficial use of the advantages inherent to magneto-rheological fluid to obtain vibration isolation over a broad frequency spectrum. The present invention fulfills this need in the art. 
   SUMMARY OF THE INVENTION 
   The present invention is a passive vibration isolation system that uses a damper containing magneto-rheological (“MR”) fluid in conjunction with elements that provide a varying magnetic field to provide frequency dependent isolation to hardware fixtures subject to vibration. The varying magnetic field increases the viscosity of the MR fluid, thus increasing the damping by interaction between the MR fluid and a plunger as the isolation system is vibrated. 
   In one embodiment, a tuned oscillator including a permanent magnet mounted on an adjustable spring oscillates within a primary wire coil at a designed frequency to induce an electric current that, in turn, creates a magnetic field in a secondary wire coil surrounding an MR fluid-filled damper. The magnetic field created by the secondary wire coil actuates the MR fluid and changes the viscosity and damping characteristics of the damper. 
   In another embodiment, a shielding sleeve, composed of magnetically impermeable material, is mounted on springs. A damper containing MR fluid is located within the hollow sleeve such that the sleeve regulates the magnetic field acting on the MR fluid by oscillating relative to the damper. The shielding sleeve oscillates when excited by a vibration source. The damping of the MR fluid increases at the resonance frequency of the sleeve, where sleeve displacements are large, and the damping is minimized at other frequencies when the sleeve displacement is small. 
   The resonance frequency of the damping elements of the invention are adjusted to match the resonance frequency of the system being isolated. This reduces vibration transmission at the resonance frequency of the system being isolated, while also providing low vibration transmission above the aforementioned resonance frequency. 
   Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, and illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically illustrates a vibration damping system of the present invention using primary and secondary wire coils to generate a magnetic field to regulate the viscosity of MR fluid contained in an MR fluid damper. 
       FIG. 2  is a graph illustrating the transmission ratios of the single degree of freedom isolation system shown in  FIG. 1 , for dampers having different fixed damping ratios, ζ, and an MR fluid damper of the present invention. 
       FIG. 3  is a graph of the damping ratio, ζ, plotted against the normalized frequency of the MR fluid damper, for two frequency bandwidths. 
       FIG. 4  is a schematic drawing of an embodiment of the present invention wherein a magnetically-impermeable shielding sleeve partially enclosing an MR fluid damper is oscillated by a vibration source to vary the magnetic field affecting the MR fluid contained in the damper, and thereby affect the viscosity of the MR fluid. 
   

   DETAILED DESCRIPTION 
   The present invention utilizes the properties of MR fluids to provide a passive and adjustable vibration isolation system. Referring to  FIG. 1 , passive damping system  10  of the present invention is schematically illustrated wherein MR fluid damper  12  is shown positioned in parallel with suspension spring  14 , with both elements being located in between and connected to top fixture  15  and bottom fixture  16 . When not at rest, bottom fixture  16  vibrates at a frequency of ω. Damper  12  is rigidly connected to bottom fixture  16  by rigid rod  17 . Damper  12  is slideably connected to top fixture  15  by plunger  18 . Plunger  18  is comprised of rigid stem  19  and head  20 . Damper  12  is comprised of a cylindrical housing that contains plunger head  20  and MR fluid  21 . 
   Primary solenoid coil  22  circumscribes permanent magnet  24 . Magnet  24  is mounted on an end of adjustable spring  26 , such as a cantilever or coil spring. The other end of spring  26  is attached to bottom fixture  16 . The stiffness of spring  26 , k a , is adjustable. Magnet  24  is constrained to only oscillate along one axis, i.e., vertically, which is collinear with the primary oscillation of bottom fixture  16 . Primary solenoid coil  22  is fixed relative to magnet  24  and bottom fixture  16  (its supporting structure is not shown), so that coil  22  remains stationary relative to the aforementioned elements when bottom fixture  16  vibrates at a frequency ω and magnet  24  consequently oscillates. Secondary solenoid coil  28  circumscribes damper  12 , and is rigidly attached to damper  12  so that it translates with damper  12 . Alternatively, coil  28  may be fixed in position relative to damper  12 , so that coil  28  remains stationary with respect to damper  12  when damper  12  moves in response to the vibration of bottom fixture  16 . Wires  30  electrically connect primary solenoid coil  22  to secondary solenoid coil  28 . 
   The resonance frequency, ω o , of the system comprised of spring  26  and connected magnet  24 , is determined by the square root of the ratio of the stiffness of spring  26 , k a , and the mass, m a , of magnet  24 , as shown by the following equation: 
                   ω   o     =           k   a       m   a         .             (   1   )               
It follows that the resonance frequency, ω o , can be adjusted by appropriately adjusting the variable stiffness, k a , of spring  26 . The significance of this feature will become apparent from the following discussion.
 
   When subjected to vibration from bottom fixture  16 , magnet  24  oscillates freely within and relative to stationary primary solenoid coil  22 , resulting in a changing magnetic flux through primary solenoid coil  22 . This induces a flow of electrical current, i(t), through wires  30  and consequently through secondary solenoid coil  28 . 
   The induced current, i(t), flowing through secondary solenoid coil  28  creates a magnetic field having a strength, β, acting on MR fluid  21 . The magnetic field strength, β, is dependent on the induced current, i(t), the number of coils per length in the secondary solenoid coil  28 , n, and the permeability constant, μ o , in accordance with the following equation:
 
β=μ o   ni ( t ).  (2)
 
   The damping of damper  12  increases in proportion to the viscosity of enclosed MR fluid  21 , and is also a function of the size and geometry of plunger head  20 , and the inner diameter of MR fluid damper  12 . The viscosity of MR fluid  21  and, concomitantly, the viscous damping of damper  12  and isolation system  10  increase in proportion to the magnetic field strength, β, acting on MR fluid  21 . 
   The amount of current i(t) induced by primary solenoid coil  22  depends on the motion of magnet  24 , which, in turn, depends on its resonance frequency and the vertical motion of bottom fixture  16 . Since system  10  is entirely passive, it is relatively simple and robust, and can be used in a variety of applications where power from an external source is either limited or nonexistent. 
   In  FIG. 1 , MR fluid damper  12 , primary solenoid coil  22 , magnet  24 , and adjustable spring  26 , are shown in parallel with suspension spring  14 . Alternatively, more than one spring similar to suspension spring  14  and more than one MR fluid damper  12  could be deployed between top fixture  15  and bottom fixture  16 , and the foregoing elements could be deployed in combinations of parallel and series configurations, depending on the mass of top fixture  15  and the magnitude and frequency of the vibration of bottom fixture  16 . 
   If a high frequency of vibration, ω, is expected, rod  17  could be comprised of two sections, with each section being respectively attached to the ends of a stiff spring (not shown), to attenuate the low-amplitude vibration that would otherwise be transmitted to top fixture  15 . 
   For isolation systems of the type schematically shown in  FIG. 1 , isolation of top fixture  15  typically increases as the vibration frequency, ω, of bottom fixture  16  increases above √{square root over (2)} times the resonance frequency, ω n , of the system comprised of suspension spring  14  and top fixture  15 ; which is defined as the square root of the ratio of the stiffness of suspension spring  14 , k p , to the mass of top fixture  15 , m. 
   
     
       
         
           
             
               
                 
                   ω 
                   n 
                 
                 = 
                 
                   
                     
                       
                         k 
                         p 
                       
                       m 
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   The transmission ratio quantifies the reduction of the motion transmitted from bottom fixture  16  to top fixture  15 , mathematically defined as: 
                   Transmission   ⁢             ⁢             ⁢   Ratio     =         1   +       (     2   ⁢   ζ   ⁢           ⁢   r     )     2             (     1   -     r   2       )     2     +       (     2   ⁢   ζ   ⁢           ⁢   r     )     2                   (   4   )               
where: ζ is the damping ratio of MR fluid damper  12 ; and
 
   r is the normalized frequency given by the equation 
   
     
       
         
           
             
               
                 r 
                 = 
                 
                   
                     ω 
                     
                       ω 
                       n 
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
     FIG. 2  is a graph of the transmission ratio of system  10  plotted as a function of normalized frequency of bottom fixture  16 , ω/ω n , for ω o =ω n , assuming that MR fluid damper  12  has the variable damping ratio, ζ, shown by the solid line (narrow frequency bandwidth) in  FIG. 3 ; and for MR fluid damper  12  having the fixed damping ratios of ζ=0.8, 0.25, and 0.05. For ω&gt;√{square root over (2)}ω n  (ω/ω n &gt;√{square root over (2)}), the transmission ratio is primarily determined by the damping ratio, ζ, of MR fluid damper  12 , which is proportional its viscous damping. More particularly, the transmission ratio of isolation system  10  decreases with decreasing damping of MR fluid damper  12  for vibration frequencies, ω, above √{square root over (2)}ω n . At ω=ω n , however, the transmission ratio is greater than one and can become very large if the damping is small, e.g., ζ=0.05, which is obviously undesirable. For ω approaching ω n  (with ω n =ω o ), the damping ratio of MR fluid damper  12  increases from 0.05 to 0.25. The result of this transition, as shown in  FIG. 2 , is reduced transmission for ω below √{square root over (2)}ω n  (ω/ω n &lt;√{square root over (2)}) in comparison to ζ=0.05, as well as reduced transmission for ω above √{square root over (2)}ω n  (ω/ω n &gt;√{square root over (2)}) in comparison to ζ=0.25. 
     FIG. 2  demonstrates that if the resonance frequency, ω o , of the system comprised of magnet  24  and spring  26  is set (by appropriately adjusting the stiffness of adjustable spring  26 ) proximate to the resonance, ω n , of the system comprised of suspension spring  14  and fixture  15  (as defined by equation 3), then MR fluid damper  12  will exhibit high damping for ω proximate to ω n , and lower damping for ω above √{square root over (2)}ω n . System  10  thus provides for low vibration transmission over the entire frequency bandwidth of the vibration of bottom fixture  16 . 
   If the damping provided by MR fluid damper  12  for ω near ω o  (for ω o =ω n ) is larger than the 0.25 maximum shown in  FIG. 3 , then the transmission ratio at resonance (ω=ω n ) shown in  FIG. 2  would further decrease. Moreover, referring to the damping ratio profile for MR fluid damper  12  shown in  FIG. 3 , if the relatively narrow frequency bandwidth shown by the solid line is replaced with the broad frequency bandwidth denoted by the dashed line, the lobes on either side of ω=ω n  would be reduced in magnitude. The system responses characterized by  FIGS. 2 and 3  illustrate examples of elements that could be used in conjunction with the present invention and are presented to facilitate understanding; they are not intended to limit or restrict the scope invention. 
     FIG. 4  is a schematic drawing of passive damping system  50 , another embodiment of the present invention including annular permanent magnets  52  and MR fluid damper  54 , which contains MR fluid  56 . Annular permanent magnets  52  comprise means for producing a magnetic field affecting MR fluid  56 . MR fluid damper  54  is positioned in parallel with suspension spring  58 , with both elements being located in between and connected to top fixture  60  and bottom fixture  62 . 
   Damper  54  is rigidly connected to bottom fixture  62  by rigid rod  64 , and slideably connected to top fixture  60  by plunger  66 . Plunger  66  is comprised of stem  68  and head  70 . Annular permanent magnets  52  circumscribe MR fluid damper  54 . Tubular magnetic shielding sleeve  76  is open on both ends, is comprised of magnetically impermeable material, and shields the area of MR damper  54  near plunger head  70  from the magnetic field of magnets  52  when system  50  is at rest, i.e., when top fixture  60  and bottom fixture  62  are at rest. The interposition of shielding sleeve  76  reduces the strength of the magnetic field acting on MR fluid  56  around the plunger head  70 . 
   Shielding sleeve  76  is suspended between top fixture  60  and bottom fixture  62  by springs  78 , which are attached, respectively, to top fixture  60  and bottom fixture  62 . Springs  78  comprise means for oscillating shielding sleeve  76  relative to damper  54 . Springs  78  have a total spring rate (stiffness) of k s . The resonance frequency of shielding sleeve  76 , ω 1 , is determined by the square root of the ratio of the total spring stiffness, k s , to the mass of shielding sleeve  76 , m s , as mathematically stated by the following equation: 
   
     
       
         
           
             
               
                 
                   ω 
                   1 
                 
                 = 
                 
                   
                     
                       
                         k 
                         s 
                       
                       
                         m 
                         s 
                       
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 6 
                 ) 
               
             
           
         
       
     
   
   When the top fixture  60 , or bottom fixture  62 , vertically vibrates upon being excited by an external source (not shown), shielding sleeve  76  oscillates on springs  78 . Suspension spring  58  has a spring stiffness k p . If the non-vibrating fixture has a mass m d , then the system comprised of suspension spring  58  and the non-vibrating fixture will have a fundamental resonance of ω 2  mathematically expressed as 
   
     
       
         
           
             
               
                 
                   ω 
                   2 
                 
                 = 
                 
                   
                     
                       
                         k 
                         p 
                       
                       
                         m 
                         d 
                       
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   By setting ω 1  proximate to ω 2  shielding sleeve  76  will exhibit large displacements at resonance, ω 1 , and allow MR fluid  56  around plunger head  70  to be exposed to a stronger magnetic field than when system  50  is at rest. The varying magnetic field will increase the viscosity of MR fluid  56 , thus increasing the damping from interaction between MR fluid  56  and plunger head  70  as the non-vibrating fixture vibrates at or near its resonance, ω 2 . Therefore, system  50  should provide the same isolation advantages as discussed in conjunction with system  10  and graphically illustrated in  FIG. 2 . System  50 , however, differs from system  10  in that it uses permanent magnets  52  and oscillating shielding sleeve  76 , rather than an induced electrical current to produce the varying magnetic field causing MR fluid  56  to react and become more viscous near plunger head  70 . 
   It is to be understood that the preceding is merely a detailed description of several embodiments of this invention and that numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.