Abstract:
A magneto-rheological (“MR”) damper having a damper body tube containing an MR fluid. A piston assembly is disposed in the damper body tube and forms an annular flow gap between the piston assembly and the damper body tube. The piston assembly has a piston core containing ferrous material and an electromagnetic coil mounted on the piston core for generating a magnetic field. The damper further includes a ferromagnetic member positioned outside of the damper body tube substantially adjacent the piston assembly for providing at least a part of a magnetic flux return path for the magnetic field.

Description:
CROSS REFERENCE TO PENDING APPLICATIONS  
       [0001]    This application is also related to the following co-pending and commonly owned application which was filed on even date herewith by Ilya Lisenker: U.S. Ser. No. ______ entitled “MAGNETO-RHEOLOGICAL DAMPER WITH DUAL FLUX RING SPACER” and which is hereby incorporated by reference herein in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a magneto-rheological (MR) fluid damper, and more particularly, to a linearly-acting MR fluid damper suitable for vibration damping in a vehicle suspension system.  
         BACKGROUND OF THE INVENTION  
         [0003]    MR fluids are materials that respond to an applied magnetic field with a change in Theological behavior (i.e., change in formation and material flow characteristics). The flow characteristics of these non-Newtonian MR fluids change several orders of magnitude within milliseconds when subjected to a suitable magnetic field. In particular, magnetic particles noncolloidally suspended in fluid align in chain-like structures parallel to the applied magnetic field, changing the shear stress on adjacent shear surfaces.  
           [0004]    Devices such as controllable dampers benefit from the controllable shear stress of MR fluid. For example, linearly-acting MR fluid dampers are used in vehicle suspension systems as vibration dampers. At low levels of vehicle vibration, the MR fluid damper lightly damps the vibration, providing a more comfortable ride, by applying a low magnetic field or no magnetic field at all to the MR fluid. At high levels of vehicle vibration, the amount of damping can be selectively increased by applying a stronger magnetic field. The controllable damper lends itself to integration in vehicle suspension systems that respond to vehicle load, road surface condition, and driver preference by adjusting the suspension performance.  
           [0005]    MR fluid dampers are based on a piston assembly moving within a damper body tube providing a reservoir of MR fluid. As the piston assembly translates within the damper body tube, MR fluid is allowed to move around or through the piston assembly in a flow gap to the opposite portion of the damper body tube. A magnetic field passing across the flow gap changes the viscosity of the MR fluid in the flow gap. The flow gap thus provides shear surfaces to react to the viscosity of the MR fluid to provide damping.  
           [0006]    Increasing the damping performance of the MR fluid damper depends in part upon concentrating the magnetic field at the flow gap. To that end, conventionally, the piston assembly includes a generally cylindrical piston core having an annular recess holding a magnetic coil. The magnetic field from the coil is concentrated at the axially opposing flux pole pieces of a piston core at each end of the flow gap. A magnetic circuit is completed by a magnetic flux return path coupled to each flux pole piece.  
           [0007]    Efficiently concentrating the magnetic field at the flow gap requires, in part, an efficient magnetic flux return path. With some MR fluid damper designs, a “soft” magnetic material is used to encompass the piston assembly in order to conduct the magnetic field. Low carbon steel is an example of soft magnetic material. One beneficial feature of soft magnetic material is that it conducts magnetic flux better than “hard” magnetic material.  
           [0008]    Conventional MR fluid dampers utilizing soft magnetic material in the magnetic flux return path have various problems. For example, in some MR fluid dampers, a magnetic flux return path is provided by a damper body tube composed of a soft magnetic material such as a low carbon steel. The wall thickness of the damper body tube must be sufficient to avoid magnetic saturation at the higher damping levels. Magnetic saturation occurs when the required damping dictates a magnetic field that exceeds the maximum magnetic field that can be conducted by the wall of the damper tube body. Therefore, greater damping capacity requires a thicker damper tube body wall.  
           [0009]    In an MR fluid damper, the damping action occurs by forcing the MR fluid through a flow gap formed between the piston assembly and the wall of the damper body tube. Thus, for a given damper diameter, increasing the wall thickness of the damper body tube reduces the size, and hence, the damping capability, of the piston assembly. Further, the increased amount of steel in the thicker damper body tube increases manufacturing costs and damper weight.  
           [0010]    With other MR fluid damper designs, a magnetic flux return path is provided by a ferromagnetic flux ring surrounding the piston core. With these designs, a flow gap passes axially through, rather than around, the piston assembly. Consequently, a relatively thin-walled damper body tube may be made of a material that is not expected to contribute to the magnetic flux return path. Unfortunately, for a given diameter MR fluid damper relying upon a flux ring, the flow gap is moved inward toward the center of the damper body tube, thereby reducing the available shear surface area and hence, the damping capability. MR fluid dampers with flux rings require a structure to hold the flux ring about the piston core. These structures also block part of the available flow path, reducing damping capability. In addition, the cross-sectional area available for the piston core is reduced, decreasing the total amount of magnetic flux that can be conducted around the magnetic circuit, yet further reducing damping capability. As a compromise, some MR fluid dampers use a piston assembly with a thin flux ring, and the magnetic field return path relies on both the thin flux ring and the wall of the damper body tube. Consequently, thin flux ring MR fluid dampers also have problems as do dampers utilizing either a thick flux ring or no flux ring.  
           [0011]    Consequently, there is a need for an MR fluid damper with a magnetic field return path that does not saturate with higher damping requirements, does not unnecessarily limit the damping capacity and does not substantially increase the cost or weight of the MR fluid damper.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides an MR fluid damper with increased performance. The MR fluid damper of the present invention provides a desired magnetic flux return path without increasing the wall thickness of the damper body tube or changing the location of the flow gap. Thus, the desired magnetic flux return path is provided without adversely influencing the function of any other component of the MR fluid damper or diminishing its damping capacity.  
           [0013]    According to the principles of the present invention and in accordance with the described embodiment, the present invention provides a magneto-rheological (“MR”) damper having a damper body tube containing an MR fluid. A piston assembly is disposed in the damper body tube and forms an annular flow gap between the piston assembly and the damper body tube. The piston assembly has a piston core containing ferrous material and an electromagnetic coil mounted on the piston core for generating a magnetic field. The damper further includes a ferromagnetic member positioned outside of the damper body tube substantially adjacent the piston assembly for providing at least a part of a magnetic flux return path for the magnetic field. The use of a separate member to provide an additional (or parallel) magnetic flux return path permits increased damping performance without substantially increasing the cost or weight of the MR fluid damper.  
           [0014]    These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description of the invention given above and the detailed description of the embodiments given below, serve to explain the principles of the present invention.  
         [0016]    [0016]FIG. 1 is a cross section view of a magneto-rheological (MR) fluid damper.  
         [0017]    [0017]FIG. 2 is an enlarged cross section view of a portion of the MR fluid damper of FIG. 1 surrounding the piston assembly.  
         [0018]    [0018]FIG. 3 is an enlarged cross section view of an alternative bearing system for the MR fluid damper of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 1 illustrates a linearly-acting magneto-rheological (MR) fluid damper and in particular, a monotube gas-charged suspension strut  10 . In general, the strut  10  is designed for operation as a load-bearing and shock-absorbing device within a vehicle suspension system, and is connected between the sprung (body) and unsprung (wheel assembly) masses (not shown). The strut  10  comprises a housing  12  that includes a housing tube  14  with an open end  16  and a closed end  18 . The closed end  18  includes an opening  20 . A mounting bracket  22  near the closed end  18  is secured in position by a suitable means such as welding. The mounting bracket  22  has suitable openings  24  for connection to the unsprung mass of the vehicle at a location such as the steering knuckle (not illustrated). A spring seat  26  is also received on the housing tube  14  and is positioned as required by the particular application within which the strut  10  will operate. The spring seat  26  is fixed in position on the housing tube  14  by a suitable means such as welding.  
         [0020]    A piston assembly  28  is connected to a hollow piston rod  30  and is fixed in position within the housing tube  14 . The piston rod  30  extends through the opening  20 .  
         [0021]    The strut  10  further includes a damper body tube  32  that is slidingly received over the piston assembly  28 . The damper body tube  32  includes a first end  34  at an outboard position adapted to be connected to the sprung mass of the vehicle and includes a second end  36  at an inboard position. The second end  36  is supported about the piston rod  30  by a rod guide assembly  38  that is fixed in position within the damper body tube  32 . At maximum extension of the strut  10 , a rebound bumper  40  on the bottom of the piston assembly  28  is compressed against the rod guide assembly  38  to cushion the deceleration of the strut  10 . At maximum compression of the strut  10 , a bottom plate  42  at the second end  36  of damper body tube  32  is adapted to contact a jounce bumper  44  that comprises an elastomeric bushing that is positioned against the closed end  18  of housing tube  14  and about the piston rod  30 .  
         [0022]    The piston assembly  28  inside the damper body tube  32  includes a piston core  46  mounted on one end of piston rod  30  and formed of a ferromagnetic material. The piston assembly  28  further includes a magnet assembly  48  including a coil  50  mounted on piston core  46  to form flux pole pieces  52 ,  54  positioned on each axial end of the coil  50 . The coil  50  is connected to an electrical source (not shown) via an electrical connector  56  extending through piston rod  30 . The magnet assembly  48  also includes an annular flux ring  58  positioned around piston core  46  to form an annular flow gap  60  between the inner annular surface  59  of the flux ring  58  and an outer surface  62  of piston core  46  and coil  50 . The piston assembly  28  divides the volume of MR fluid within the damper body tube  32  into a compression chamber  64  and an extension chamber  66 .  
         [0023]    If, for example, the damper body tube  32  moves upward relative to the piston assembly  28 , the MR fluid flows from extension chamber  66 , through flow gap  60  and into compression chamber  64 . The flux ring  58  is designed with an outer diameter size to form a sliding fluid seal with an inner surface  68  of damper body tube  32 . Therefore, as the damper body tube  14  slides over flux ring  58 , MR fluid does not leak past the flux ring  58 .  
         [0024]    The MR fluid within damper body tube  32  is a conventional MR fluid that has magnetic particles such as iron or iron alloys. The magnetic particles are controllably suspended within the fluid by controlling a magnetic field through the flow gap  60 . Thus, a desired damping effect between the sprung and unsprung masses of the vehicle is achieved by controlling the application of an electric current to coil  50  in order to vary the magnetic field and hence, the flow characteristics of the MR fluid in the flow gap  60 .  
         [0025]    A gas cup  70  is also carried in the damper body tube  32  between the piston assembly and the end. The gas cup  70  carries a dynamic seal  72  and slides along the inner surface  68  of damper body tube  32 , separating a compensation chamber  74  from the compression chamber  64 . While the extension chamber  66  and compression chamber  64  carry a supply of MR fluid, the compensation chamber  74  carries a compressible nitrogen gas supply. During extension and compression directed travel of the damper body tube  32  relative to the piston assembly  28 , a decreasing or an increasing volume of the piston rod  30  is contained within the damper body tube  32  depending on the stroke position of the strut  10 . In order to compensate for this varying volumetric amount of the piston rod  30  within the fluid-filled chambers  64 ,  66 , the gas cup  70  slides, compressing or expanding the compensation chamber  74 .  
         [0026]    The predominate means of supporting the damper body tube  32  within the housing tube  14  is provided by a bearing system  76 . The bearing system  76  includes a bearing sleeve  78  slip-fit near the open end  16  of the housing tube  14 . The bearing sleeve  78  is maintained in position by a retaining cap  80  that is pressed onto the open end  16  of housing tube  14 . The bearing system  76  also includes a pair of plain bearings  82 ,  84  that are fixed by a press-fit within the bearing sleeve  78 . A fluid-tight chamber  86  is formed between the bearings  82 ,  84  which is filled with a lubricating oil. The bearings  82 ,  84  contact the damper body tube  32  and guide linear motion of the damper body tube  32  with respect to the piston assembly  28 .  
         [0027]    Referring to FIG. 2, an enlarged cross section view of the bearing system  76  shows a sleeve  88  made of a ferromagnetic material, for example, a low carbon steel, disposed within the housing tube  14 . In particular, the ferromagnetic sleeve  88  is fixed within the bearing sleeve  78 , for example by a press-fit, adhesive, or other known means. The ferromagnetic sleeve  88  is registered with, that is, is located immediately adjacent to, the piston assembly  28 . The sleeve  88  is normally secured in that position by the friction of the press-fit; however, as will be appreciated, the sleeve  88  may be fixed in its desired position by other known means, for example, welding, adhesives, fasteners, etc. The ferromagnetic sleeve  88  effectively increases the wall thickness of the damper body tube  32  only immediately adjacent the piston assembly  28 . Thus, the ferromagnetic sleeve  88  provides a desired magnetic field return path without having to increase the thickness of the damper body tube  32  itself or change the location of the flow gap  60 .  
         [0028]    In one alternative embodiment, the flux ring  58  of FIG. 2 can be eliminated; and a magnetic field return path provided as shown in FIG. 3. A magnetic return path  90  passes through the damper body tube  32 , damper body tube  32 , ferromagnetic sleeve  88 , bearing sleeve  78  and housing tube  14 . In particular, the ferromagnetic sleeve  88  enhances the magnetic return path  90 , compensating for the corresponding reduction in material in the magnetic flux return path  90  of having no flux ring  58 . Consequently, the increased performance of a larger flow gap  60  adjacent to the housing tube  14  may be realized without a weight penalty of a thick housing tube  14 .  
         [0029]    Although piston assembly  28 ′ is depicted as laterally supported by two bearing plates at each end of the piston core  46 , it should be appreciated that the piston assembly  28 ′ may be laterally supported by an interrupted bearing in the flow gap  60  or only one bearing plate.  
         [0030]    In another alternative embodiment, one of the bearings, for example, bearing  82 , is constructed of a thin layer of bearing material on a soft steel base. The bearing  82  is then press-fit within the bearing sleeve  78  to an axial position registered with, that is, immediately adjacent, the piston assembly  28 . With this embodiment, the ferromagnetic bearing  82  functions as a magnetic flux return path. Therefore, with this embodiment, a separate ferromagnetic sleeve  88  and its associated assembly step is eliminated.  
         [0031]    In use, referring to FIG. 1, a linearly-acting magneto-rheological (MR) fluid damper, such as a strut  10 , includes a ferromagnetic member, such as a sleeve  88 , that is located outside the damper body tube  40  at an axial position adjacent the piston assembly  28 . Referring to FIGS. 2 and 3, an electric current is applied to the coil  50  that is representative of the desired damping effect between the sprung and unsprung masses of the vehicle. The electric current creates a magnetic field that sets the flow characteristics of the MR fluid in the flow gap  60 , thereby providing the desired damping effect between the sprung and unsprung masses of the vehicle. The magnetic field has a return path through the ferromagnetic member, for example, the sleeve  88 , that is independent of the damper body tube  32 .  
         [0032]    The MR fluid damper  10  having the ferromagnetic sleeve  88  can be designed to provide the desired magnetic flux return path without adversely influencing the cost or performance of other components of the MR fluid damper  10  or diminishing its damping capacity. Thus, the MR fluid damper  10  of the present invention has increased performance and without a substantial increase in cost or weight.  
         [0033]    While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, struts  10  consistent with aspects of the invention may be based on a twin tube cylindrical reservoir having an outer tube surrounded by a ferromagnetic member. Further, the invention may also be applied to shock absorbers.