Abstract:
A cabinet for reducing the G-loading upon a delicate instrument produced by shock and vibratory forces. The cabinet includes an inner frame and an outer frame that are co-joined by a series of horizontal isolators and double acting isolator or shock absorber assemblies.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0001]     This invention was made with Government support under contract number NO0167-01-D-0063 awarded by Naval Surface Warfare Center Carderock Division. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to a cabinet for reducing the G-loading on sensitive instruments stored in the cabinet that are produced by shock or vibratory forces.  
       BACKGROUND OF THE INVENTION  
       [0003]     A shock and vibration isolation system is disclosed in U.S. Pat. No. 6,530,563 B1. The disclosed system includes a cabinet having an inner frame for supporting sensitive instruments that is mounted within an outer frame. Each frame is rectangularly shaped with the side walls of the inner frame being adjacent to and parallel with the side walls of the outer frame. Two opposed side walls of the inner frame are connected to the adjacent side walls of the outer frame by a series of wire rope isolators. The wire rope isolators are mounted so that each can slide freely in a vertical direction. A pair of double acting shock absorbers are also connected between each of the adjacent side walls of the inner and outer frames so that the shock absorbers can deflect in a vertical direction. The shock absorbers and the wire rope isolators combine to effectively attenuate shock and vibration forces moving along the vertical, horizontal and longitudinal axes of the system.  
         [0004]     As will become apparent from the disclosure below, the present invention represents a further improvement in the isolation cabinet disclosed in the above noted &#39;563 patent. The improvement is realized by relocating the wire rope or other horizontal isolators into positions where they can more effectively attenuate shock and vibratory forces moving in both the horizontal and longitudinal directions. This is accomplished by locating these horizontal isolators so that they will deflect in the same mode, whether the input is from the horizontal, longitudinal, or any combination of the two directions. This is an improvement over prior art systems because it allows the system to be mounted with no restrictions on orientation with respect to these directions. The isolator assemblies for attenuating shock and vibration in the vertical direction can be any double acting shock absorber, such as those referenced in the above noted &#39;563 patent, that is capable of supporting the inner cabinet weight and can include both mechanical and liquid spring units that work together to more effectively attenuate shock and vibratory forces acting in a vertical direction. The isolator assemblies are arranged to attenuate shock and vibratory forces to lower G-load levels acting upon the inner frame of the cabinet.  
       SUMMARY OF THE INVENTION  
       [0005]     It is therefore an object of the present invention to improve cabinets for protecting sensitive instruments against the harmful effects of shock and vibratory input forces.  
         [0006]     It is a further object of the present invention to lower the G-loads on sensitive instruments produced by relatively high shock and vibratory input forces.  
         [0007]     These and other objects of the present invention are attained by an isolation cabinet that includes an inner frame that is supported within an outer frame by a series of horizontal isolators and double acting shock absorber or isolator assemblies. The frames are generally rectangular shaped with the vertical corners of the inner frame being located adjacent to and parallel with the vertical corners of the outer frame. Each corner has a plate that extends vertically along the length of the frame and which is placed at a 45° angle with respect to the sides of the frame that form the corner. The horizontal isolators are mounted between the corner plates on slides so that they can move freely in a vertical direction. In one embodiment of the invention, double acting isolator assemblies each include a mechanical spring that acts in parallel with a liquid spring. The assemblies are mounted in pairs between adjacent sides of the frames so that the assemblies can deflect in a vertical direction.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     For a further understanding of these and objects of the present invention, reference will be made to the following Detailed Description which is to be read in conjunction with the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a perspective view of an isolator cabinet that embodies the teachings of the present invention for protecting sensitive instruments from high G-load produced by shock and vibratory forces;  
         [0010]      FIG. 2  is a side elevation of the cabinet of  FIG. 1  with some components removed for the sake of clarity;  
         [0011]      FIG. 3  is a sectional view taken along lines  3 - 3  in  FIG. 2 ;  
         [0012]      FIG. 4  is a enlarged view further illustrating the corer mounting arrangement of an isolator;  
         [0013]      FIG. 5  is an enlarged perspective view of an exemplary isolator unit utilized in the practice of the present invention;  
         [0014]      FIG. 6  is a side elevation illustrating an exemplary double acting isolator assembly utilized in the practice of the present invention;  
         [0015]      FIG. 6A  is an enlarged partial view in section showing an end section of the spring assembly;  
         [0016]      FIG. 6B  is an enlarged partial view showing the center section of the spring assembly;  
         [0017]      FIG. 6C  is an enlarged partial view showing a flanged cylinder for separating springs in the mechanical spring array;  
         [0018]      FIG. 7  is an enlarged partial top view illustrating the bottom portion of the isolator assembly;  
         [0019]      FIG. 8  is a section taken along lines  8 - 8  of  FIG. 7 ;  
         [0020]      FIG. 9  is a partial sectional view illustrating the mounting of a piston within the liquid spring used in the isolator assembly; and  
         [0021]      FIG. 10  is a schematic diagram showing the liquid springs control circuitry.  
     
    
     DETAILED DESCRIPTION  
       [0022]     With initial reference to  FIGS. 1-5 , the present invention will be described with reference to a cabinet generally depicted  10 , for protecting sensitive instruments, such as computers and the like, from high G-loads caused by shock or vibratory input forces. The cabinet  10  contains an outer frame  12  that is affixed to a main structure or ground and is thus exposed to seismic events. The cabinet  10  further includes an inner frame  13  that is suspended within the outer frame  12  by a plurality of wire rope isolators  15  and a series of isolator assemblies  17  that act in concert to reduce the G-loads acting upon the cabinet to levels such that a sensitive instrument  18  ( FIG. 1 ) that is stored in the inner frame  13  will not be harmed and will continue to operate in the event of a high cyclic input force.  
         [0023]     The inner and outer frames  12 ,  13  of the cabinet  10  are generally rectangular structures that share a common vertical axis so that the vertical comers of the inner frame are situated adjacent to those of the outer frame. As best illustrated in  FIG. 4 , a vertical plate  19  is located at each vertical corner of the inner frame  13  with the plate forming an angle of about 45° with the adjacent sides of the frame. Similarly, the adjacent vertically disposed corners of the outer frame  12  each contain a plate  20  that also forms an angle of about 45° with the adjacent sides of the outer frame. The adjacent plates  19 ,  20  are in parallel alignment with a gap separating the plates.  
         [0024]     With further reference to  FIG. 5 , each wire rope isolator  15  includes a pair of opposed blocks  21  and  22  with a wire rope  23  being threaded through the blocks and locked in place by crimping the block securely against each of the rope loops. Other means for locking the rope  23  to the blocks  21 ,  22 , such as set screws or the like, may also be employed. One of the blocks  22  is secured to a slide member  24  that is slidably contained within a guideway  25 . The opposite block  21  is secured to one of the corner plates which in this case is plate  19 , while the guideway  25  is affixed to an adjacent plate  20  so that the wire rope isolator  15  can move freely in a vertical direction within the gap separating the adjacent plates between the frames. In the assembly, the wire rope isolators  15  are mounted between the adjacent corners of the frames at the bottom and the top sections of the plates  19 ,  20 . However, the number of wire rope isolators in each gap may vary depending upon the specific application. A wire rope isolator suitable for use in the present embodiment of the invention is described in greater detail in U.S. Pat. No. 5,549,285, the disclosure of which is incorporated herein by reference. It should be noted herein that other horizontal isolators in lieu of the wire rope isolators, such as, for example elastomeric isolators, may also be employed in a similar manner and are intended to fall within the scope of the present invention.  
         [0025]     Four isolator assemblies  17  are also arranged to act between the inner and outer frames  12 ,  13  of the instrument cabinet  10 . Each assembly  17  includes a mechanical spring unit  31  and a fluid spring unit generally referenced  32  ( FIG. 6 ) that are vertically mounted in a side by side relationship between the two frames. The mechanical spring unit  31  is contained within a cylindrical sleeve  35  while the fluid spring unit  32  is contained within a cylindrical fluid tight housing  36 . The lower section of each housing is secured to a base  37  which in turn, is affixed to the lower part of one of the frames of the cabinet  10  by a first connector  38 . A piston rod  39  extends upwardly from the upper end of the fluid spring unit  32  in parallel alignment with an elongated linear arm  40  that passes upwardly from the upper end of the mechanical spring unit  31 . The piston rod of the fluid spring unit  32  and the linear arm of the mechanical spring unit  31  are tied together by a common yoke  42 . The yoke  42 , in turn, is attached to the other frame by a second connector  45 . As will be explained in greater detail below, the piston rod  39  and the linear arm  40  are forced to move together in unison as the shock and vibration isolator unit is stroked in a vertical direction.  
         [0026]     As noted above, the double acting mechanical spring unit  31  is contained within a tubular shell  35 . The linear arm  40  is slidably mounted in the central bore of the sleeve  65  to establish a close sliding fit between the sleeve and the arm. An array  67  of four compression springs are wound in series about the arm  40 . The spring array  67  resides within a recess  68  that is shared equally between the inner wall of the shell and the outer wall of the arm  40  when the assembly is not moved in either compression or tension. The array  67  includes a pair of outer ends comprising a compression side end spring  70  and a tension side end spring  71  which are spaced apart by two inner springs  72  and  73 . When in the neutral position, the compression side end spring  70  rests against one end shoulder  74  of the recess  68  and the tension side end spring  71  rests against the opposite shoulder  75  of the recess  68 . The springs are arranged to provide a range of preloads based on the dynamics of the system when the assembly is in the neutral or unstressed position.  
         [0027]     In this embodiment of the invention, the two side end springs  70  and  71  of the spring array  67  have the same spring rate as do the two inner springs  72  and  73 . The spring rate of the side end springs  70 ,  71  is typically higher than that of the inner springs  72 ,  73 . The preload of the inner springs  72  and  73  is much higher than the preload of the side end springs  70  and  71 . Each side end spring  70 ,  71  is separated from the adjacent inner springs  72 ,  73  by a flanged cylinder  76  that extends inwardly into a recess formed in the shell  35 . The flanged part of each cylinder  76  is arrested on a shoulder formed in the shell  35  which permits the cylinder  76  to move only toward the inner spring. The depth of penetration of each cylinder  76  is slightly less than the depth of the upper half of the recess which is formed by the shell, thus allowing the shell to move freely over the linear arm  40 . The two inner springs  72  and  73  are similarly separated by a center ring  77  ( FIG. 6B ).  
         [0028]     When the outer frame  12  of the cabinet  10  is exposed to a shock or vibratory load that is greater than the spring preload, the shell is initially driven upwardly over the linear arm  40  toward the inner frame  13 . As a result, the tension side end spring  71  is compressed between the flanged cylinder  76  and the shoulder of the recess  106  formed in the shell on the tension side of the recess. In this case, the tension side of the spring array  67  is on the right side of the isolator illustrated in  FIG. 6  and the compression side is on the left side of the isolator. At this time, the compression side end spring  70  remains in its initial preload position captured between the shoulder  106  formed in the upper half of the recess on the compression side of the system and the adjacent compression side flanged cylinder  76 .  
         [0029]     The tension side end spring  71 , having a higher spring rate than the inner springs  72  and  73 , is arranged so that it will resist the initial compressive load until the shell has been displaced a first distance toward the tension side of the assembly, whereupon the tension side spring is completely depressed. At this time, the inner springs  72  and  73 , which have a lower spring rate, take over the compressive load thereby storing addition energy toward the end of the compression stroke, but at the lower spring rate to considerably reduce the G forces transmitted to the inner frame  12  of the cabinet  10 .  
         [0030]     At the end of the compression cycle, the mechanical spring unit  31  will go into a tension mode of operation as the frames return to their original preloaded condition positions. As noted above, the mechanical spring unit  31  is a double acting unit and because the springs in the array  67  are arranged symmetrically about the center of the array, the assembly will respond in the same manner in both the compression and tension modes of operation. Accordingly at the beginning of the tension mode, the compression side end spring  70  will initially provide a stiff resistance to the rebound forces until such time as the end spring is fully compressed whereupon, the softer inner spring  72  and  73  stores the load energy to reduce the G forces acting upon the inner frame. Although the end springs in this example have a higher spring rate than the inner springs, the spring rate of the end springs may be made lower than that of the inner springs without departing from the teachings of the invention.  
         [0031]     The liquid spring unit  32  includes a cylindrical housing  36  that contains a central bore having three chambers of varying diameters. The larger diameter chamber  100  is located at the compression side of the housing  36  and is connected to the small diameter chamber  77  by an intermediate diameter chamber  78 . A piston  80  is slidably contained within the smaller diameter chamber  77  and is attached to piston rod  39 . The length of the small diameter chamber  77  is slightly greater than the stroke of the mechanical spring unit  31 , thus enabling the two spring assemblies to move together in unison to attenuate the vibratory G forces acting in both directions upon the system. The three chambers  77 ,  78 ,  100  are arranged so as to tune the natural frequency of the liquid spring far enough away from that of the inner frame  13  and equipment mass so that the two frequencies cannot combine to produce a deleterious effect upon the system.  
         [0032]     The function of the liquid spring unit  32  will be explained in greater detail with further reference to the diagram illustrated in  FIG. 10  and  FIGS. 7-9 . The large diameter chamber  100  on the compression side of the liquid spring housing is connected to an accumulator  82  by means of a manifold  83  that contains a compression side flow control circuit generally referenced  84  (see  FIG. 10 ). The control circuit  84  contains an orifice  85  that is adapted to orifice fluid from chamber  100  back to the accumulator  82  in the event the pressure in the chamber  100  exceeds a predetermined level during the compression cycle. A refill check valve  86  is placed in parallel over the control orifice  85  and is arranged to open when the fluid pressure in the accumulator  82  exceeds that in the large diameter chamber  100  which occurs when the liquid spring unit  32  changes from the compression mode of operation over to the tension mode of operation, the latter keeping the compression side of the bore filled with fluid during the tension cycle. A relief check valve  87  is also mounted in parallel with the control orifice  85  and the refill check valve  86  and is arranged to open in the event the isolator experiences an exceedingly high input force. Opening the relief valve releases the liquid spring unit  32  from the system and thus helps to reduce the adverse effect of the exceedingly high input load on the inner frame structure.  
         [0033]     The accumulator  82  is also connected to the smaller diameter chamber  77  by a second flow control circuit  88  that includes a flow control orifice  89 , a refill check valve  91  and a relief check valve  90 . During the tension cycle, the flow orifice  89  conveys fluid back from the small diameter chamber  77  to the accumulator  82  when the pressure behind the piston is greater than that in the accumulator. The refill check valve  91 , in turn, is arranged to open when the fluid pressure in the accumulator  82  exceeds the fluid pressure behind the piston so that fluid flow into the smaller chamber during the compression mode continues to fill the area behind the piston. The relief check valve  90  again is arranged to open in the event the G loading on the isolator exceeds a given limit, thereby completely releasing the liquid spring from the system.  
         [0034]     The valve components of the second flow control circuit  88  are mounted in a cartridge  92  that is located in a cavity  93  behind the smaller diameter chamber  77 . The cavity  93  is placed in fluid flow communication with the accumulator  82  by a flow line  95  and with the smaller chamber  77  of the liquid spring unit  32  by means of a conduit  96  ( FIG. 9 ). The piston rod  39  is arranged to move axially in the cartridge  92  and suitable seals are provided to prevent fluid flow passing between the cartridge and the piston rod.  
         [0035]     A pressure transducer  99  is mounted in the large diameter chamber  100  of the liquid spring unit  32  on the compression side of the piston  80  which measures the pressure in the chamber and transmits a signal indicative of the pressure to a signal conditioner  105 . A conditioned output signal is sent from the conditioner to a microprocessor  101  that contains a switching algorithm for controlling a control valve  102  through a control valve driver  104 . In response to the algorithm, the valve  102  is cycled to maintain a desired pressure on the compression side of the liquid spring unit  32  and thus limit the G loading on the inner frame  12  during the compression cycle.  
         [0036]     While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.