Abstract:
A tuned mass damper for attenuating vibrations in a selected plane of a mechanical structure, by magnetically induced eddy current damping. A moving damper mass is coupled to a vibrating mass through one or more flexures that confine relative movement of the damper mass substantially to the selected plane. A magnet structure is rigidly attached to either the vibrating mass or the damper mass, and a conductor plate is attached to the other of these masses. Movement of the damper mass relative to the vibrating mass induces eddy currents in the conductor plate and thereby generates a damping force that attenuates vibration in the selected plane.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to vibration damping mechanisms and, more particularly, to techniques for damping vibrations that may occur within a geometric plane of a mechanical structure. Mechanical structures of all kinds are subject to unwanted vibrations. Of particular interest are structures that are intended to provide an extremely stable platform for precision equipment, such as space-based telescopes. Vibrations may be due to related motors and other equipment. Without damping, vibrations may resonate with structural members and render the equipment inoperative or, at worst, may cause damage to the structure or its supported equipment.  
         [0002]     Ideally, damping should be achieved inertially without introducing a damper into a load path associated with the damped structure, which may distort a critical aspect of damped structure. The damping mechanism should preferably provide damping effectiveness in multiple directions, such as a single plane, should operate over a wide range of vibration amplitudes and, for use in space applications, should be operable over a wide range of temperatures, including cryogenic temperatures. The present invention meets and exceeds these requirements.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention resides in a planar vibration damping mechanism. Briefly, and in general terms, the present invention may be defined as a tuned mass damper for attenuating vibration of a mechanical structure in a selected plane, the damper comprising a frame for attachment to a vibrating mass; a movable damper mass; flexure means for connecting the movable damper mass to the frame to confine movement of the damper mass substantially to the selected plane; a magnet structure having at least two magnetic poles and arranged to generate a magnetic field across a gap in the magnet structure; and a conductor plate positioned for free movement within the gap in the magnet structure. The magnet structure is mechanically attached to either the frame or the damper mass, and the conductor plate is mechanically attached to the other of either the frame or the damper mass. Vibration of the frame is transferred to the damper mass through the flexure means, and is attenuated by generation of eddy currents and a resultant damping force on the conductor plate.  
         [0004]     Several different embodiments of the invention are possible. In one embodiment, the magnet structure is part of the damper mass and the conductor plate is attached to the frame, which of course is attached to the vibrating mass. In another embodiment, the magnet structure is attached to the frame and the conductor plate is attached to the damper mass. Ideally, the magnet structure comprises four magnetic pole pairs arranged in a symmetrical configuration, since use of fewer di-pole magnets does not provide equal damping force in both orthogonal directions in the selected plane.  
         [0005]     The flexure means in one group of embodiments comprises a plurality of elongated flexures extending in a parallel post-like arrangement from the frame to the magnet structure. Movement of the magnet structure mounted on these flexures is substantially confined to a plane perpendicular to the flexures. In a variant of this arrangement, the flexure means comprises a single elongated flexure extending from the frame to the magnet structure. Movement of the magnet structure relative to the frame is, for small excursions, substantially confined to a plane perpendicular to the single elongated flexure.  
         [0006]     In another embodiment of the invention, the flexure means comprises multiple L-shaped flexures arranged in a coplanar configuration. Each flexure is attached by one of its ends to the frame and by its other end to the damper mass. The flexures are designed to provide substantially identical stiffness properties with respect to both orthogonal directions in the coplanar arrangement and they function to confine movement of the damper mass to the selected plane.  
         [0007]     Another feature of the tuned mass damper is the inclusion of means to limit movement of the damper mass with respect to the frame. When elongated flexures are employed, this limiting means comprises multiple bumpers mounted inside the frame or on frame members extending about the flexures. In the embodiment employing L-shaped flexures, the means for limiting damping mass movement comprises multiple snubber assemblies. Each snubber assembly includes a pin affixed to either the frame or the damper mass, and a loosely fitting bushing fixed to the other one of the frame or the damper mass. The bushing is lined with a resilient material, which is contacted by the pin to limit lateral movement of the damper mass relative to the frame.  
         [0008]     It will be appreciated from the foregoing summary that the present invention represents a significant advance in the field of vibration damping of mechanical structures. In particular, the invention provides a tuned mass damper that attenuates selected vibration modes in all directions within a selected plane of a vibrating mass. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a diagrammatic side view of a first embodiment of the invention.  
         [0010]      FIG. 2  is a diagrammatic side view of a second embodiment of the invention.  
         [0011]      FIG. 3  is a diagrammatic side view of an embodiment of the invention similar to the embodiment of  FIG. 2 .  
         [0012]      FIG. 4  is a diagrammatic top view of the embodiment of  FIG. 3 .  
         [0013]      FIG. 5  is a diagrammatic side view of an alternate embodiment similar to that of  FIGS. 3 and 4 , but employing a single elongated flexure instead of multiple flexures to couple a damper mass to vibrating frame.  
         [0014]      FIG. 6  is a view similar to  FIG. 5 , but depicting an alternate bumper arrangement to limit movement of the damper mass.  
         [0015]      FIG. 7  is a fragmentary side view depicting yet another bumper arrangement for the single flexure embodiment of  FIGS. 5 and 6 .  
         [0016]      FIG. 8  is a diagrammatic top view of yet another embodiment of the invention, employing L-shaped flexures instead of elongated flexures.  
         [0017]      FIG. 9  is an enlarged cross-sectional view of a snubber assembly used in the  FIG. 8  embodiment to limit movement of the damper mass relative to the vibrating frame. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     As shown in the drawings for purposes of illustration, the present invention pertains to tuned mass damping mechanisms that provide for damping of mechanical vibrations in multiple directions in a single plane. In accordance with the invention, a moving damper mass is coupled to a vibrating mass through one or more flexures designed to permit movement of the damper mass in a selected plane. The one or more flexures are tuned to a known vibration mode of the vibrating mass. The damper further includes a mechanism to damp movement of the moving damper mass with respect to the vibrating mass, as will become clear from the following more detailed description of the several embodiments of the invention.  
         [0019]      FIGS. 1 and 2  are diagrammatic views depicting two principal embodiments of the invention. In  FIG. 1 , a vibrating mass of a structure is indicated by reference numeral  10 . The damping mechanism of the invention includes a frame member  12  rigidly attached to the vibrating mass  10 , and a movable, conductive damper mass  24  supported in the frame member  12  by a plurality of flexures  16  attached to an intermediate structure  14  and a second plurality of flexures  26 . Damping is provided by a permanent magnet  20  having upper and lower components  20 . 1  and  20 . 2 . In this embodiment, the magnet  20  is rigidly coupled to the frame member  12 , and therefore moves with the vibrating mass  10 . The magnet components  20 . 1  and  20 . 2  have a narrow gap between them, through which a magnetic field passes in opposite directions at different areas of the magnet components. Positioned within the gap is the conductor plate  24 . Movement of the vibrating mass  10  is, therefore, transferred through the flexures  16  and  26  to the movable damper mass  24 , The flexures  16  and  26  are the same length so that as they bend their vertical components of motion cancel, producing a purely in-plane motion.  
         [0020]     Following well known electromagnetic principles, movement of the conductive plate  24  through and perpendicular to the magnetic field set up between the magnet components  20 . 1  and  20 . 2  generates electrical current in the plate, proportional to the velocity and the magnetic flux density. If the magnetic flux passes through the plate  24  in opposite directions at spatially separated areas of the plate  24 , the induced currents are in opposite directions and form continuous loops in the plate. These loop currents are referred to as eddy currents. It is also well known that eddy currents generate a force that opposes the current-inducing force, and that the opposing force is in a direction that opposes the original conductor motion that resulted in the generation of current. That is to say, the force generated is a damping force. More specifically, the current, i, generated by movement of a conductor at velocity, v, through a magnetic field of flux density, B, is given by the proportionality:  
         i   ∝       v   ×   B     ρ       ,       
 
 where ρ is the resistivity of the conductor plate  24 . 
 
         [0021]     The force, F, generated by the induced currents is determined by integrating the product of current and magnetic flux density:  
       F   =         ∫   v     ⁢     i   ×   B       ⁢           ∝       ∫   v     ⁢         v   ×   B   ×   B     ρ     .             
 
         [0022]     In the embodiment depicted in  FIG. 1 , the magnets  20 . 1  and  20 . 2  are mechanically attached to the vibrating structure  10 , through the frame member  12 , and the conductor plate/moving mass  24  is suspended on the flexure arrangement  16 / 14 / 24 . The embodiment depicted in  FIG. 2  differs from that of  FIG. 1  in that the  FIG. 2  magnet structure  20  is the moving mass of the damper. A member  28  supports the magnet structure  20  and is suspended by the flexures  26  from the intermediate structure  14 . The conductor plate  24  in the  FIG. 2  embodiment is mechanically connected to vibrating mass  10 , through a connective structure that is omitted for clarity in the figure. Therefore, in the  FIG. 2  embodiment the magnet structure  20  is also the moving mass of the damper, and the conductor plate  24  is mechanically connected to the vibrating mass  10 . As in the  FIG. 1  embodiment, the flexures  16  and  26  are the same length so that as they bend their vertical components of motion cancel, producing a purely in-plane motion.  
         [0023]     A related embodiment of the invention is depicted in  FIGS. 3 and 4 . In this embodiment, the vibrating mass  10  (not shown in  FIGS. 3 and 4 ) is rigidly connected to a damper frame  30 , to which the conductor plate  24  is rigidly attached. Supported in the frame  30  is a magnet structure  20 ′. In this embodiment, the magnet structure includes four magnet pole pairs, indicated at  20 . 1 ′,  20 . 2 ′,  20 . 3 ′ and  20 . 4 ′ and arranged in a square configuration as indicated by the top view of  FIG. 4 . The magnet structure  20 ′ also includes upper and lower plates  32  and  34  of magnetically permeable material, and brackets  36  of non-magnetic material connecting the upper and lower plates. The magnet structure  20 ′, which also constitutes the moving damper mass, is supported in the frame  30  on multiple flexures  16 ′, which allow movement of the structure  20 ′ in the plane of the conductor plate  24 , thus transferring in-plane movement of the vibrating mass  10  to the moving damper mass. The frame  30  has multiple stops or bumpers  38  mounted inside the frame to limit movement of the magnet structure  20 ′ within the frame. Additional stops  40  may also be mounted beneath the lower plate  34  to limit any excessive downward excursion of the magnet structure  20 ′ in the event of axial buckling of the flexures  16 .  
         [0024]      FIGS. 5, 6  and  7  depict a variation of the embodiment of  FIGS. 3 and 4 , in which the multiple flexures  16 ′ are replaced by a single flexure  16 ′ that is centrally located to support the magnet structure  20 ′. In the configuration of  FIG. 5 , bumpers  38  are located on the inside walls of the frame  30 , in much the same way as in  FIG. 3 . In the configuration of  FIG. 6 , bumpers  42  are located on upwardly extending legs  44  beneath the structure  20 ′. The frame  30  in this embodiment need not extend around both sides of the magnet structure  20 ′, but simply includes a base member and an upwardly extending portion that attaches to the conductor plate  24 . In another variant of the bumper arrangement,  FIG. 7  depicts bumpers  46  in the form of curved leaf springs that extend up from the base of the frame  30 , adjacent to the single flexure  16 ′. A disadvantage of using a single flexure  16 ′ is that additional clearance is required for movement of the conductor plate  24  because motion of the moving damper mass follows a curved, rather than planar path. For small excursions, however, the damper mass moves substantially in the desired plane.  
         [0025]     The tuned mass damper of the invention is mounted to a structure whose vibrations are intended to be damped. Large vibrations may result in lateral or axial buckling of the flexures  16 , which is why the various bumpers  38 ,  40 ,  42  and  46  are required to limit lateral movement of the damper mass. The bumpers may be of suitable elastomeric material to reduce possible shock loads and ensure that the flexure stresses remain within acceptable limits. While motion of the damper mass remains within the limits set by the bumpers, i.e., within the “rattle space” set by the bumpers, the damper functions in a substantially linear manner. For potentially larger excursions of the damper mass, the damper functions in a non-linear manner, as an impact damper. The clearances in the damper are set by the deflections under gravity in which the device must be tested. This practically limits the device to have a resonance set at about 4-5 Hz or above. The damper can operate as a tuned device with relatively low damping designed to target a limited set of modes around the resonance of the device. Or by setting the device at a low frequency with high damping it can operate as an inertial damper to any modes having a frequency higher than the resonance of the device.  
         [0026]     Yet another embodiment of the invention is depicted in  FIG. 8 . In this embodiment, the conductor plate  24  is again mechanically connected to the vibrating mass  10  (not shown in this figure), through a generally rectangular frame  50 . A magnet structure  20 ′, which can be similar to the structure of  FIGS. 3 and 4 , with four magnetic pole pairs, is suspended in the frame  50  by four L-shaped flexures  52 . These flexures  52  allow movement in any direction within the plane of the conductor plate  24 . Each flexure  52  is designed to have the same stiffness in two orthogonal directions in a plane parallel to the conductor plate  24 , thus allowing relatively free movement of the magnet structure  20 ′ in this plane. The flexures  52  are relatively flexible in the common plane in which the flexures are positioned, but are stiff in any out-of-plane direction. Use of the L-shaped flexures  52  rather than four parallel, post-like flexures renders the entire structure much more compact. Multiple (three or four) cylindrical snubbers  54  are used to limit movement of the magnetic structure  20 ′ within its plane of movement. As depicted in  FIG. 9 , each snubber  54  includes a metal cylindrical bushing  56  attached to the magnet structure  20 ′, and a pin  58  attached to the frame  50  and positioned in the center of the metal bushing  56 . A rubber (or similar material) bushing  60  lines the metal bushing  56  and limits movement of the pin  58 .  
         [0027]     It will be appreciated from the foregoing that the present invention represents a significant advance in the field of vibration suppression using tuned mass dampers. In particular, the invention provides a tuned mass damper for suppression of vibration in multiple directions in a given plane of a vibrating mass. Multiple tuned mass dampers of this type may be employed to control anticipated vibrations in various members of a mechanical structure. It will also be appreciated that, although several embodiments of the invention have been depicted and described by way of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.