Abstract:
A container with an inside surface and a mass mounted for oscillation in the container with a pair of bellows in the container each having a bias spring therein and a removable end to expose the interior of the bellows to exchange the spring for easy tuning of the damping characteristics and a plurality of balls, one each positioned in a plurality of troughs around the periphery of the mass proximate the ends there to bear against the inside surface so as to provide low friction oscillation of the mass in the container.

Description:
This is a divisional of application Ser. No. 08/591,922 filed Jan. 25, 1996, now U.S. Pat. No. 5,873,438. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to tuned mass damping devices and more particularly to such dampers which may find use in reducing the periodic motion of elongated structures such as booms. The invention may have particular utility with booms mounted on satellites to hold measuring equipment the accuracy of which may be reduced due to the sway of the boom resulting from disturbances such as thermal distortion shock caused by, for example, transient thermal distortions of solar panels. 
     2. Description of the Prior Art 
     In the prior art, tuned mass dampers for reducing sway are known. Such dampers usually contain a mass mounted for movement in a container of fluid or a magnetic field and positioned by a spring. By proper selection of the mass and spring, the mass will have the same natural frequency as the boom, or other device to which the damper is mounted, so that when the boom experiences shock and begins to sway in a direction, the mass begins to vibrate or oscillate in the same direction and at substantially the same frequency. However, since the boom is an input to the damper, the damper vibrates 180 degrees out of phase with the boom, which motion tends to cancel the boom motion. Since the boom is now vibrating at an off-resonant frequency and the damper has absorbed a substantial portion of its energy, the boom displacement is much smaller and is effectively damped out by the fluid or by the magnet in the damper. Such dampers are satisfactory for high frequency vibrations but because frequency is proportional to the ratio between the square root of the spring constant to the mass, at low frequencies e.g. 1.5 hertz, the mass becomes too large for the spring and cannot be effectively supported. The result is that the mass begins to sway and move in directions other than that required for proper damping. 
     BRIEF DESCRIPTION OF THE PRESENT INVENTION 
     The present invention overcomes the problems in the prior art by providing a damper with a mass which is constrained to move in the desired direction. By making the mass cylindrical and positioning it within a housing closely adjacent the mass, motion in the fluid container in only the desired direction is permitted. The fluid may be varied to make the vibration tunable and, in fact, the present invention provides for tunable damping without having to change the fluid. The tuning of the damping is accomplished by providing a pair of bellows with changeable internal springs therein to change the volumetric stiffness of the bellows and thus provide different characteristics to the fluid expansion and contraction in the chambers surrounding the mass. Thus, in testing the damping characteristics for a particular use, only the springs internal to the bellows need be changed for fine tuning. In the event that the sliding friction between the mass and the adjacent container housing is too large, a nearly frictionless motion is provided by using a linear bearing with, for example, circulating balls. A specific improvement to the ball bearing mounting is shown in the present invention by the use of a plurality of linear troughs in the mass each of which entraps a single ball so that there is no sliding friction between the mass and the walls or between adjacent balls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an example of the basic tuned damper of the present invention; and, 
     FIG. 2 shows a second embodiment of the present invention incorporating both the fine tuning of damping and reduction of friction with motion of the mass. 
     FIG. 3 a  shows a ball of the present invention in a semicircular groove. 
     FIG. 3 b  shows a ball of the present invention in a “V” shaped groove. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1 a damper  8  is shown comprising a hollow moveable mass  10 , slideably mounted in a cylindrical container  12  having a first end piece  14  fastened to cylinder  12  by conventional means, such as bolts, not shown and sealed to prevent fluid loss by a grommet  16 . A second end piece  18  is fastened at a second end in a recess  20  of cylinder  12  by conventional means such as bolts, also not shown. The cylinder  12  and end pieces  14  and  18  form a chamber  22  within which mass  10  may move back and forth. 
     A spring  30  of predetermined stiffness is fastened at one end thereof to a protrusion  32  of end piece  14  and at the other end thereof to a recess  34  in mass  10  so that mass  10  will be positioned by spring  30  until subjected to a force allowing mass  10  to oscillate, only horizontally, back and forth in chamber  22  at a frequency determined by the size of mass  10  and stiffness of spring  30 . 
     The first end piece  14  has a filling port  36  therethrough which allows the introduction of a damping fluid, shown by arrow  38 , into the chamber  22 . After filling, port  36  is sealed in conventional manner. 
     At the second end of cylinder  12 , in recess  20 , a thermal expansion bellows  40  is connected at one end thereof to a protrusion  42  in end piece  18  and at the other end thereof to a sealing member  44 . End piece  18  has a small opening  48  therethrough connecting the interior of bellows  40  to chamber  22 . This allows transfer of fluid from chamber  22  to the interior of bellows  40  to accommodate expansion and contraction of the fluid under modest temperature variations. 
     In one application of the apparatus of FIG. 1, the damper may be used to compensate for unwanted vibrations of, for example, a boom shown in FIG. 1 by reference numeral  50 . The unwanted oscillations will be transverse to the length of the boom and accordingly it is desired that the mass  10  move in the same direction, i.e. from right to left in FIG.  1 . Accordingly, the damper  8  is shown mounted to boom  59  horizontally as indicated by dashed lines  52  and  54  and, as explained above, will vibrate 180 degrees out of phase with the boom to help cancel the boom motion. 
     For many applications, the apparatus of FIG. 1 will perform satisfactorily, but for some high accuracy or specialized uses, there may be inaccuracies or unnecessary costs associated with the FIG. 1 damper. For example, in order to provide the exactly correct amount of damping, the damping fluid  38  in chamber  20  is first chosen to have a viscosity which is believed to provide the best absorption of energy from the oscillating system and provide the desired amount of damping for the specific intended use. The boom and the damper are then tested to check the damping characteristics and, if they are not right, the fluid has to be drained and new fluid with different viscosity inserted for a re-test. This process is repeated until the desired damping characteristics of the system are obtained. Such a procedure is quite costly and time consuming and adds considerable cost to the damper. 
     Another difficulty with the FIG. 1 damper is a result of mass  10  sliding in chamber  22  because too much friction may be involved for optimum damping effectiveness. Conventional linear bearings may be used to reduce the friction and in some cases may be sufficient. However, even using conventional linear bearings between mass  10  and the interior of cylinder  12  there may be too much friction because of contact between the balls. These problems are overcome with the improvements of FIG.  2 . 
     In FIG. 2, a damper  108  (which may also be attached to a boom as in FIG. 1 but not shown in FIG. 2 for simplicity) is shown comprising a moveable mass  110 , slideably mounted in a cylindrical container  112  having a first cylindrical end piece  114  fastened to the right end of cylinder  112  by conventional means, not shown. A spring  116  has a first end fastened in a recess  118  of mass  110  and a second end fastened to end piece  114  so that mass  110  is positioned thereby. Mass  110  is shown having an orifice  120  extending between its left and right sides in FIG. 2 so as to permit the passage of the damping fluid therethrough. The damping fluid may be inserted in the cylindrical container  112  in a manner similar to that shown in FIG.  1 . As was the case in FIG. 1, the mass  110  and the spring  116  are chosen to have the frequency of oscillation matching the particular use to which it is to be put e.g. the frequency of the boom to which it will be mounted. 
     A plurality of troughs  122 ,  124 ,  126  and  128  are shown in the outer edge of mass  110  and are cross-sectionally shaped to constrain the movement of balls such as  132 ,  134 ,  136  and  138  in all but the desired direction, horizontally in FIG.  2 . For example, the grooves may be of slightly greater diameter than the balls as is shown in FIG. 3 a  where a semicircular groove  122   a  supports the ball  132   a , or, as shown in FIG. 3 b , may be a “V” shaped groove  122   b  supporting a ball  132   b . In either case, the ball is constrained for motion only into and out of the plane of the paper. The plurality of balls  132 ,  134 ,  136  and  138  in the troughs  122 ,  124 ,  126  and  128  respectively engage the inner surface of cylinder  112  and provide rolling motion for mass  110 . The lengths of the troughs are made to accommodate the amount of motion expected of mass  110  oscillating back and forth in use. In the event that the mass  110  moves more than expected, the balls (although moving less distance than the mass) may nevertheless reach the ends of the trough where they may encounter greater friction due to the worming effect and/or tolerance errors. However, the device is completely self centering so that when the motion decreases to the expected limits, the balls will move to the center and at rest assume the position shown in FIG.  2 . This feature assures the device will remove the maximum amount of energy from the system by minimizing mass friction. There should be at least two troughs around the diameter of mass and preferably three or more to prevent any contact between the outer surface of mass  110  and the inner surface of cylinder  112 . Using the balls eliminates the sliding friction between the mass  110  and the cylinder  112  and since a single ball is used, there is no friction between balls. Thus the possible excessive friction of the FIG. 1 damper has been avoided. 
     Cylindrical end piece  114  has an abutment  140  and a first cylindrical end member  142  is seated thereon. Cylindrical end member  142  has an inwardly extending ledge  144  and a removable end cap  146  with a hole  148  extending centrally therethrough. End cap  146  is mounted against ledge  144 . A first bellows  150  has a right end which is fixed to the ledge  144  and extends to the left towards the interior of cylindrical container  112 . The left end of bellows  150  is sealed to a circular plate  152  which has a central rod  154  extending back to the right so as to be guided in the hole  148 . A spring  156  is positioned in the interior of bellows  150  between the circular plate  152  and the end cap  146  and provides additional volumetric stiffness to the bellows  150 . 
     The left end of damper  108  in FIG. 2 is similar to the right end. A second cylindrical end piece  164  is fastened to the left end of cylindrical container  112  by conventional means, not shown. End piece  164  has an abutment  166  and a second cylindrical end member  168  is seated thereon. Cylindrical end member  168  has an inwardly extending ledge  170  and a removable end cap  172  with a hole  174  extending therethrough. End cap  172  is mounted against ledge  170 . A second bellows  180  has a left end which is fixed to the ledge  170  and extends to the right towards the interior of cylindrical container  112 . The right end of bellows  180  is sealed to a circular plate  182  which has a central rod  184  extending back to the left so as to be guided in the hole  174 . A spring  186  is positioned in the interior of bellows  180  between the circular plate  182  and the end cap  172  and provides additional volumetric stiffness to the bellows  180 . 
     It is seen that as the mass  110  moves to the right and left in FIG. 2, the fluid pushes against circular plates  152  and  182  to collapse bellows  150  or  180  against the force supplied by spring  156  or  186 . The amount of damping that this provides to the system is controlled in part by the stiffness of the springs  156  and  186  so all that is needed to change or fine tune the damping effect, is to remove the end caps  146  and  172  and replace springs  156  and  186  with springs having different stiffness. Thus, testing of the damper is considerably easier, much less time consuming and less expensive than the draining and replacement of the fluid as in FIG.  1 . 
     It is thus seen that we have provided a damper that is constrained to move only in the desired direction for proper damping at low frequencies. We have also provided a damper that is easily fine tuned and has a minimum of friction between the moving mass and the container. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, in the event that it is desired to accommodate the damper to extreme forces, as, for example, the force exerted on the equipment upon take off from a launching site, additional springs located near the interior portions of end pieces  114  and  164  may be employed to provide a soft stop for mass  110 . Also, if reduced rolling friction is desired but extreme accuracy is not required, the first and second bellows may be omitted and a single temperature compensating bellows such as shown in FIG. 1 employed. Furthermore, when the oscillations to be damped may occur in more than one plane, two dampers mounted on the member at right angles to each other may be employed.