Abstract:
A friction damper with a housing including a cavity. The damper includes a first member disposed in the cavity and movable in the cavity; a second member disposed in the cavity; an intermediate member between the first and second members, the first member being in frictional engagement with the intermediate member; and at least one magnetic field generator mounted to magnetically couple the first and second members thereby maintaining the first member in frictional engagement with the intermediate member and wherein the first member is movable against the intermediate member to generate a damping force.

Description:
This application is a divisional of U.S. patent application Ser. No. 09/737,889, filed Dec. 15, 2000, which is now U.S. Pat. No. 7,040,467. 

   FIELD OF INVENTION 
   The invention relates to a passive damper, and more particularly the invention relates to a friction damper where the normal force of the friction is provided by magnetic attraction between first and second damping members. 
   BACKGROUND OF INVENTION 
   Friction dampers generally apply a frictional force to a moveable member to dissipate translational or rotational energy of the member to produce acceptable member movement. 
   Prior art frictional dampers are typically comprised of surface effect dampers of the type described in U.S. Pat. No. 5,257,680 to Corcoran et al., and U.S. Pat. No. 4,957,279 to Thorn. Surface effect dampers operate by dissipating translational or rotational energy by working an elastomeric element to convert kinetic energy to heat. Such conventional dampers are generally comprised of a housing with an inner wall, and an elastomeric member movable through the housing. Interference between the inner wall and the elastomeric member produces the friction damping. 
   Additionally, frictional damping may be supplied to a movable member by a friction damper that utilizes a controllable fluid to precisely control the supplied damping force. Such devices are well known in the art as magnetorheological (MR) fluid devices and examples of MR devices can be found in commonly assigned U.S. Pat. No. 5,284,330 to Carlson et al.; and U.S. Pat. No. 5,277,281 also to Carlson et al. MR devices may be of the rotary or linear acting variety and such dampers employ a controllable MR fluid comprised of fine soft-magnetic particles disbursed within a liquid carrier. MR fluids exhibit a “thickening” behavior (a rheology change) sometimes referred to as an apparent viscosity change upon being exposed to a magnetic field of sufficient strength. The higher the magnetic field strength exposed to the MR fluid, the higher the damping force that can be achieved with a particular MR device. Although effective in providing damping in a large number of applications, conventional surface effect and MR friction dampers have a number of shortcomings. First, prior art dampers are sensitive to temperature changes and thermal expansion. When the prior art dampers are subjected to significant temperature increases or decreases the viscosity of the MR fluid may be affected and the change in fluid viscosity may in turn affect the supplied damping force. Such temperature changes can also affect the properties of the elastomer damping element and can cause the elastomeric damping element to contract or expand and experience dimensional changes. Changes to the damping element dimensions or properties will change the damping forces supplied by the surface effect friction damper. 
   Surface effect damping is provided by a carefully calculated interference, between the housing and elastomer element. In MR devices effective damping is ensured by maintaining a precisely defined gap dimension between the housing and piston member. The MR fluid flows through the defined gap. As a result of the foregoing, prior art dampers are very sensitive to dimensional tolerancing and tolerances must be tightly maintained in order for prior art friction dampers to provide effective damping forces. However, overtime, through repetitive use of the dampers, the critical tolerances between moving damper components are frequently lost and the deviations in the part tolerances negatively affects the forces provided by the friction damper. Finally, prior art friction dampers can be difficult to assemble and only a specific range of materials are acceptable for use in such prior art friction dampers. 
   The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative friction damper is provided including features more fully disclosed hereinafter. 
   SUMMARY OF THE INVENTION 
   This is accomplished by the present invention that provides a friction damper that provides effective damping forces without sensitivity to temperature changes or tolerances between component parts; is easily assembled and may incorporate components made from a variety of materials. 
   In one aspect of the present invention this is accomplished by the friction damper of the present invention. The damper comprises a housing including a cavity formed therein; a first member disposed in said cavity and movable in said cavity; a second member disposed in said cavity; an intermediate member between the first and second members, the first member being in frictional engagement with the intermediate member; and at least one magnetic field generator mounted to magnetically couple the first and second members thereby maintaining the first member in frictional engagement with the intermediate member and wherein the first member is movable against the intermediate member to generate a damping force. 
   The second member may be movable with the first member. Before the first member is displaced, the first and second members are aligned. When the first member is initially displaced, the second member lags behind the first member by a distance, and then is drawn towards the first member so that the first and second members are substantially aligned. Alternatively, the second member may be fixed. In both instances the first and second members are coupled magnetically and as a result, stiction between moveable damper members is eliminated by the damper of the present invention which provides for a smooth onset of damping force. 
   Additionally, in an alternate embodiment the first and second members and intermediate members may not be located in housing. In such an embodiment the ends of the intermediate member are fixed. 
   The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of the friction damper of the present invention with the housing endcap removed; 
       FIG. 2  is a longitudinal sectional view of the damper of  FIG. 1 ; 
       FIG. 3  is a lateral sectional view taken along line  3 - 3  of  FIG. 1 ; 
       FIG. 4  is the longitudinal section view of  FIG. 1  illustrating the alignment of the first and second members and intermediate member before the first member is moved; 
       FIG. 5  is the longitudinal section view of  FIG. 4  after the first member is displaced; 
       FIG. 6  is a plot of Force versus Displacement for a prior art friction damper; 
       FIG. 7  is a plot of Force versus Displacement for the friction damper of the present invention; 
       FIG. 8  is a longitudinal section view of a second embodiment friction damper of the present invention; 
       FIG. 9  is a lateral sectional view taken along line  9 - 9  of  FIG. 8 ; 
       FIG. 10  is a longitudinal section view of a third embodiment friction damper of the present invention; 
       FIG. 11  is a longitudinal sectional view of the first member, second member, intermediate member and bearing layer of a fourth embodiment of the friction damper of the present invention; and 
       FIG. 12  is a longitudinal sectional view of an alternate embodiment of the present invention with the first and second members and intermediate member unenclosed by a housing with the ends of the intermediate member fixed. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Now turning to the drawing figures wherein like parts are referred to by the same numbers in the several views,  FIGS. 1-5  disclose a first embodiment friction damper  10 . 
   Friction damper  10  includes an elongate, tubular housing  12  with housing wall  14  that defines an inner housing surface  16  and housing cavity  17 . The housing is most preferably made of a non-metallic material such as plastic and although the housing is shown and described as being tubular with a circular cross-section, it should be understood that the housing may have any suitable cross section such as a rectangular or square configuration for example. 
   The housing includes first and second housing ends  18  and  20  respectively and the ends are closed by respective first and second end caps  22  and  24 . At least one of the end caps is removably attached to its respective housing end. For purposes of describing the preferred embodiment of the present invention, end cap  24  is fixed to second housing end  20  and end cap  22  is removably attached to first housing end  18  by a threaded connection, interference fit or other conventional removable attachment means. End cap  22  includes hole  26  through which shaft  28  extends outwardly from the housing cavity so that the shaft end  30  may be connected to a movable component of a toy, haptic device, automobile door or appliance such as a washing machine, for example. Bracket  32  is made integral with end cap  24  and the bracket serves as a means for anchoring the second housing end during operation of the damper  10 . 
   Longitudinally extending opposed slots  34  and  36  are provided along the inner surface of housing wall  14  and the slots open at the housing ends  18  and  20 . Slots  34  and  36  are adapted to receive the longitudinal edges of an intermediate member  40 . As the description proceeds the intermediate member may also be referred to as an intermediate friction layer. With the longitudinal edges of member  40  located in slots  34  and  36  member  40  extends diametrically across the cavity  17  to divide the larger cavity into smaller first and second cavities  17   a  and  17   b  respectively. See  FIGS. 1 and 3 . The intermediate friction layer  40  may be made from a variety of non-magnetic materials such as polyethylene or other plastics, stainless steel, brass, laminates or composite materials used in brake pads for automotive applications. 
   The friction damper  10  of the present invention is insensitive to wear and dimensional tolerances. Variations in the thickness of the friction layer  40  due to wear or loose manufacturing tolerances have little effect on damper performance because the normal frictional force loading of the first and second frictional elements  50  and  60  is not determined dimensionally as it is in other friction dampers such as surface effect dampers for example. Thus, even if a large portion of the intermediate member  40  were to be worn away over time, the normal loading force between the elements  50  and  60  would not be affected. 
   The first member  50  is located in cavity  17   a  and is connected to one end of shaft  28  in a conventional manner. The shaft is connected to the nonmagnetic seat  52  of member  50  and the seat in turn supports a pair of permanent magnets  54   a ,  54   b  that are located side-by-side in the seat. The seat may be made of any suitable nonmagnetic material such as a plastic or aluminum for example. As shown in  FIGS. 2 and 3 , when the member  50  is located in cavity  17   a , the seat is at all times located on and in frictional engagement with the intermediate member  40  and the magnets  54   a ,  54   b  are prevented from becoming unseated by a metallic keeper plate  56  that is attracted to the magnets. In addition to preventing magnet displacement, the plate  56  closes magnetic circuit  70  to produce a reliable and consistent magnetic field. As the shaft  28  is moved axially in either of the directions identified by arrows  100  and  102 , the member  50  is moved in the same axial direction and the seat and magnets are maintained in frictional engagement with the member  40 . 
   The second member  60  is located in cavity  17   b  and is magnetically coupled to member  50 . The magnetic coupling is represented by the dashed font representation of magnetic field  70 . Like first member  50 , second member  60  includes side-by-side magnets  64   a ,  64   b  that are supported in seat  62 . A keeper plate  66  covers the magnets along one side of the seat. As shown in  FIG. 2  the magnets  54   a ,  54   b  and  64   a ,  64   b  are supported in their respective seats so that their north and south poles are oriented in the manner required to produce the coupling magnetic field  70 . The keeper plates  56  and  66  complete the magnetic coupling circuit. In this way, the members  50  and  60  are mutually attracted through the intermediate member  40 . When member  60  is located in cavity  17   b , seat  62  is at all times in frictional engagement with member  40 . As will be described in greater detail hereinbelow, second member  60  moves in the general directions identified by arrows  100  and  102 , and follows the movement of first member  50 . 
   It should be understood that the friction damper of the present invention does not need to be in the specific horizontal orientation disclosed for the damper to provide effective damping. Although in  FIG. 3  cavity  17   a  is shown as an upper cavity and cavity  17   b  is shown as a lower cavity, the orientations of cavities  17   a  and  17   b  could be reversed with the cavity  17   b  as the upper cavity and cavity  17   a  as the lower cavity or the housing could be rotated to any angle from the position of  FIG. 3 . 
   Operation of friction damper  10  will now be described. For simplicity as the description proceeds the operation of damper  10  will be described in terms of movement of member  60  in direction  100 . However it should be understood that the damper  10  operates in the same way if moved in direction  102  shown in  FIG. 2 . 
   After the damper is assembled, end  24  is fixed via bracket and shaft end  30  is connected to a movable object of interest. When the damper is assembled the first and second members  50  and  60  are magnetically coupled and are maintained against the surface of member  40  by magnetic field  70 . 
   Turning now to  FIGS. 4 and 5 , the second member  60  tends to align with the first member  50  due to the magnetic attraction between magnets  54   a ,  54   b  and  64   a ,  64   b  as shown in  FIG. 4 . As the first member moves axially along intermediate member  40  in direction  100 , the member  60  tends to lag behind or become partially decoupled from the first member  50 . See  FIG. 5 . The lag of member  60  is represented by distance X in  FIG. 5 . This is because the members  50  and  60  are effectively coupled by a magnetic spring. Although the shaft  28  is directly coupled to the first member, the second member  60  is effectively coupled to the magnetic spring. Thus before the second member begins to follow the movement of the first member the magnetic spring must first be displaced sufficiently to provide enough force to overcome the friction of the first element. Once the second member overcomes the friction of the first element, the second member  60  is rapidly drawn back towards the first member until the first and second members are substantially aligned. As the first member continues to be displaced, the second member lags behind the first member by a relatively small lag distance Y. See  FIG. 4 . 
   This partial decoupling of the first and second members provides a significant advantage in terms of the “feel” of the damper. The partial decoupling ameliorates much of the stick-slip normally associated with a friction damper. Such stick-slip is referred to by those skilled in the art as stiction. The effects of stiction in prior art friction dampers are illustrated graphically in  FIG. 6 . The damping force supplied by a prior art damper initially is high as a result of the resistance to movement because of the presence of stiction and then once the damping element overcomes the forces of stiction at point A of  FIG. 6 , the damper force is lowered to its operating level.  FIG. 7  graphically illustrates the damper of the present invention that provides a smooth, stiction-free increase in force until the damper force reaches its working level. See the point identified as B in  FIG. 7 . 
   A second embodiment friction damper  200  is illustrated in  FIGS. 8 and 9 . The second embodiment friction damper  200  comprises all of the elements comprising friction damper  10  except for the movable second member  60 . The second embodiment friction damper includes a stationary second member  205  that spans the longitudinal dimension of the housing. The second member is located in the chamber with the lower member edges located supported on the inner surface of the housing wall. See  FIG. 9 . 
   A third embodiment friction damper  300  is illustrated in  FIG. 10 . The third embodiment friction damper includes all of the elements of the second embodiment friction damper  200  except for an intermediate member  210  that extends the longitudinal length of the housing. Rather, in the third embodiment friction damper  300 , the intermediate member is attached to the underside of the first seat  52  to be moveable with the seat as member  50  is moved relative to the second member. The member  210  is attached by an adhesive or other conventional means. In an alternate embodiment, the friction damper could include both the friction layer  210  along the underside of the seat  52  and intermediate member  40  along the complete length of the second member  205 . 
   A fourth embodiment friction damper  400  is illustrated in  FIG. 11 . The friction damper includes the features of the second embodiment friction damper  200  and also includes a bearing member  405  that is made of a rubber or other resilient material, and the member  405  is fixed to the underside of the seat  52  in a conventional manner. The bearing member includes a plurality of spaced apart semispherical ribs  410  that extend laterally in a parallel manner across the member. The ribs remain in frictional engagement with the intermediate member during operation of the damper  400 . 
   In the second, third and fourth embodiments, the first member  50  is magnetically coupled with the second member  205 . The second member is a soft magnetic layer such as iron or steel. In friction dampers  200 ,  300  and  400  as the first member is moved longitudinally, the smooth stiction free behavior of  FIG. 7  is achieved with a smooth increase in the force supplied by the dampers  200 ,  300  and  400 . 
   In the first through fourth embodiments  100 ,  200 ,  300  and  400  the first and second members  50  and  60  and intermediate friction layer  40  may be unenclosed by housing  12 . Such a configuration is shown in  FIG. 12 . In such an alternate configuration, the ends of intermediate member  40  are maintained fixed by attachment members  500  and  502  which may be any suitable attachment member such as a bracket or the like. An alternative to the fixed member ends includes fixing portions of the longitudinal edges of the member  40  in such a manner that does not interfere with the displacement of members  50  and  60 . 
   While I have illustrated and described a preferred embodiment of my invention, it is understood that this is capable of modification, and therefore do not wish to be limited to the precise details set forth, but desire to avail myself of such changes and alterations as fall within the purview of the following claims.