Abstract:
An improved magneto-rheological (“MR”) fluid damper includes a damper cylinder containing a volume of MR fluid. The cylinder includes an inner surface. A piston assembly is disposed in the cylinder and has an outer surface in slidable contact with the cylinder inner surface. The piston assembly includes a flow gap formed thereabout and an external coil surrounding a portion of the cylinder, the external coil capable of generating a magnetic field across at least a portion of the flow gap.

Description:
TECHNICAL FIELD 
     The present invention relates to a magneto-rheological (“MR”) fluid damper, and more particularly, to a linear-acting fluid damper suitable for vibration damping in a vehicle suspension. 
     BACKGROUND OF THE INVENTION 
     Magneto-rheological fluids are materials that respond to an applied magnetic field with a change in rheological behavior (i.e., change in formation and material flow characteristics). The flow characteristics of these 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 chainlike structures parallel to the applied magnetic field, thus increasing the viscous characteristics, or apparent viscosity, of the MR fluid. 
     Devices, such as controllable dampers and struts, benefit from the controllable viscosity 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 all to the MR fluid. At high levels of vehicle vibration, the amounts of damping can be selectively increased by increasing the density of the magnetic field and by applying control integration into vehicle suspension systems that sense and respond to vehicle load, road surface condition, and driver preference by adjusting a suspension performance accordingly. 
     Generally, current linearly acting MR fluid dampers are based on a monotube design with a coil positioned in a piston of the damper. In the monotube design, the piston moves within the fixed length cylindrical reservoir in response to force from a piston rod that extends outside of the cylinder. The monotube approach simplifies sealing of the MR fluid within the monotube reservoir; however, monotube dampers may experience reliability problems arising from the electrical wiring leading to the coil, etc., necessary for generating a magnetic field in or around parts of the piston. Typically, the electrical wiring passes up through a passage in the piston rod to a coil in the piston. Elaborate assembly procedures are required to seal this passage. Even if adequately sealed, the electrical wiring may flex with the movement of the piston, sometimes resulting in breakage of the wires. 
     In some dampers, it is known to reduce failure from wire flexing by holding the coil stationary with respect to a portion of the reservoir of (e.g., either the inner or outer tube). In particular, in U.S. Pat. No. 5,277,281, a reduced diameter piston moves within a reduced diameter inner tube. A coil, separate from the piston, acts as a valve control for a flow passage between the inner and outer tubes, rather than a coil integral to the piston controlling flow past the piston. Although wire flexing is reduced, the reduced piston diameter correspondingly reduces damping. Also, leaks due to introducing wiring into the reservoir are not avoided. 
     Consequently, a significant need exists for an MR fluid damper that is more reliable and inexpensive to manufacture while being tolerant of side loads on the damping components and furthermore, reduces the likelihood of pressure leaks from the MR fluid reservoir. 
     SUMMARY OF THE INVENTION 
     The present invention provides an MR fluid damper that is of a simpler construction then known dampers and can be manufactured for less cost. However, the MR fluid damper design of the, present invention provides an improved, more reliable performance and substantially increases the reliability of the electrical connection to the coil. One aspect of the invention provides an improved magneto-rheological (“MR”) fluid damper including a damper cylinder containing a volume of MR fluid. The cylinder includes an inner surface. A piston assembly is disposed in the cylinder and has an outer surface slidably contacting with the cylinder inner surface. The piston assembly includes a flow gap formed therein and an external coil surrounding a portion of the cylinder, the external coil capable of generating a magnetic field across at least a portion of the flow gap. A pair of ferromagnetic rings are provided, one of which is positioned above and the other of which is positioned below the external coil for directing the magnetic field or flux through the flow gap. 
     Other aspects of the invention provide a damper wherein the piston assembly includes a first portion having a first diameter and a second portion having a second diameter, the first diameter being less than the second diameter, the second portion including the outer surface in contact with the cylinder inner surface. The MR damper flow gap can be formed along the first portion of the piston assembly. The second portion of the piston assembly can include a plurality of openings. The MR damper can further include a piston rod, a major portion of which is disposed in the cylinder and wherein the piston assembly is secured to an inner end of the piston rod. The piston assembly can be secured to the rod by a pin. The pin can secure the first portion of the piston assembly to the inner end of the piston rod. The outer surface of the second portion of the piston assembly may include a wear resistant coating. The wear resistant coating can include a nickel plating. The wear resistant coating can include an iron alloy including from about 27-50% cobalt and alternately, about 2% vanadium. The wear resistant coating can be sprayed onto the outer surface of the second portion of the piston assembly. The outer surface may be turned and roller burnished. The MR damper may further include a first pair of retaining members positioned in grooves formed in the piston rod at positions above and below the piston assembly and a Belleville spring positioned between one of the first pair of retaining members and the piston assembly to secure the piston assembly to the piston rod. The retaining members may be retaining rings. The extending end of the piston rod opposite the inner end is secured to a housing of the damper by a threaded member. The extending end of the piston rod opposite the inner end may be secured to a housing of the damper by a second pair of retaining members positioned on the inside and the outside of the housing and a Belleville spring can be positioned between one of the pair of retaining members and the housing to secure the piston rod to the housing. The piston rod can be a solid rod. The cylinder can be made of a material that saturates at about 0.5 to about 2 Tesla. The MR damper can further include a gas cup slidingly contained within the cylinder, the gas cup defining a gas chamber containing a gas in one portion of the cylinder, the gas cup configured to seal the MR fluid from the gas chamber. The ferromagnetic rings may include a pair of inner bearings for allowing the ferromagnetic rings and the coil positioned therebetween to slidingly contact the cylinder. The vertical span of the coil and ring assembly may be a length at least equal to a vertical span of the piston. 
    
    
     The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of one embodiment of a magneto-rheological damper in accordance with the present invention; and 
     FIG. 2 is a sectional view of another embodiment of a magneto-rheological damper in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For purposes of this description, words such as “upper”, “lower”, “right”, “left” are applied in conjunction with the drawing for purposes of clarity. As is well known, dampers may be oriented in substantially in any orientation, so these directional words cannot be used to imply the particular absolute directions for damper consistent with the invention. 
     Referring to the drawings, illustrated in FIG. 1 is a linearly acting magneto-rheological (MR) fluid damper, in particular, a strut generally illustrated at  10 . In general, the strut is designed for operation as a load bearing and shock-absorbing device within the vehicle suspension system. The strut  10  can be connected between the sprung (body) and unsprung (wheel assembly) masses (not shown) as is known in the art. The strut  10  may include housing  12  including a housing tube or cylinder  14  with an open end  16  and a closed end  18 . The closed end  18  includes an opening  20 . A mounting bracket  22  adjacent the closed end  18  is secured to cylinder  14  by any 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 piston assembly  28  is connected to a piston rod  30  and is positioned within the housing tube  14 . Any suitable means may be used to fix the assembly  28  to the rod  30 . In the illustrated embodiment, the piston  28  is connected to the rod by pinning the piston to the rod with a transverse cross-dowel or pin  90  or the like. The piston rod  30  extends through and is attached to the housing  12  at the opening  20 . In the illustrated embodiment, the rod  30  is secured to the housing portion  18  by way of a threaded nut  92 . The piston assembly  28  is slidingly received within a damper body tube  32  that 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. A rod guide  38  supports the second end  36  of the damper body tube  32  about the piston rod  30 . An opening  40  in the rod guide  38  allows the damper body tube  32  to move longitudinally inboard and outboard with respect to the housing  12 . The damper body tube  32  thus forms a fluid-tight cylindrical reservoir  41 . 
     The piston assembly  28  includes a solid piston core or stepped cylinder  42  containing ferromagnetic material, such as soft steel or sintered iron. The piston core  42  preferably includes a narrowed portion  44  and an extended portion  46 . An annular flow gap  43  is formed about the narrow portion  44 , between piston portion  44  and cylinder  32 . A plurality of openings or orifices  56  are formed through extended portion  46  to permit fluid to pass from a compression chamber  52  and an extension chamber  54  of reservoir  41 . 
     A non-magnetic cap (not shown) may be provided the piston  42  at the end near the narrow portion  44  as is known in the art to reduce flux leakage to the damper tube  14 . Any magnetic flux leakage from the rod  30  to the tube  14  that may occur only improves performance by increasing flux density in the flow gap  43 . The outer surface of the extended portion or outer step portion  46  of the piston  42  can be coated with a thin wear resistant coating such as electroless nickel plating. The coating may be a thicker coating such as a thermal spray coating, provided such coating is hard enough to withstand the wear and has “soft” magnetic properties to minimize residual magnetization. One of such coatings could be a 27-50% Co, 2% V, Fe bal. Alloy that is sprayed on the surface, turned and roller burnished to increase the hardness and improve the surface finish. An outer surface of the piston portion thus prepared bears on inner surface  58  of tube  32 . 
     The cylinder  32  can be made from medium or low carbon steel and allowed to saturate at a value of about 2 Tesla. In the alternate, a material may be chosen to saturate at a lower flux density from about 0.8 to about 1.5 Tesla thereby decreasing the amount of flux “lost to the tube” and further improving magnetic performance. The optimum saturation value should be such that the portion of tube  36  in contact with the piston  42  is nearing saturation. The magnetic field energy that is dissipated through other portions of the damper body tube  36  is referred to as “lost to the tube” since it does not interact with MR fluid contained between shear surfaces of the piston assembly  28  and damper body tube  32 . 
     The MR fluid may be any conventional fluid including magnetic particles such as iron or iron alloys which can be controllably suspended within the fluid by controlling a magnetic field, thereby varying the flow characteristics of the MR fluid through flow gap  43  defined in piston portion  46 . Varying the magnetic field thereby controls the flow characteristics of the MR fluid to achieve a desired damping effect between the sprung and unsprung masses of the vehicle for a given application. 
     Fundamentally, during damping, MR fluid present in one of the chambers  52 ,  54  of the damper body tube  32  flows through flow gap  43  from, for example, extension chamber  54  to compression chamber  52 , as the tube  32  moves outboard with respect to the housing  12 . 
     A gas cup  62  may also be carried in the damper body tube  32  between the piston assembly  28  and the first (outboard) end  34 . The gas cup  62  slides along the inner surface  58  of damper body tube  32 , separating out a compensation chamber  64  from compression chamber  52 . While the extension chamber  54  and compression chamber  52  carry a supply of MR fluid, the compensation chamber  64  may carry 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 strut position of the strut  10 . In order to compensate for this varying volumetric amount of the piston rod  30  within the fluid filled chambers  52 ,  54 , the gas cup  62  slides, compressing or expanding the compensation chamber  64 . 
     An external coil  70  generates the magnetic field across the flow gap  43  to the piston assembly  28 . The external coil  70  encompasses a portion of the damper body tube  32  corresponding to, and stationary with respect to, the piston assembly  28 . To concentrate the magnetic field, the external coil  70  is longitudinally placed between a pair of ferromagnetic rings  72 ,  74 , forming an external coil assembly  76 . 
     The external coil assembly  76  is advantageously contained within an external coil crimp casing  78  that provides structural support when the open end  16  of the housing  12  is deformed around the external coil assembly  76  to form an attachment. Any suitable method of fixing the coil assembly  76  may be used to attach the assembly  76  in place about the tube  32 . 
     An internal surface of the external coil assembly  76  laterally supports the damper body tube  32 . In particular, the assembly  76  includes a pair of plain bearings  84 ,  86  that are pressed into the external coil assembly  76  and bear against the damper body tube  32 . The bearings  84 ,  86  concentrically support the damper body tube  32  with respect to the external coil assembly  76 . This provides a fluid-tight chamber  88  between the bearings  84  and  86 , which is filled with a lubricating oil. The fluid tight chamber  88  and bearings  84 ,  86  can be protected by scraper seals (not shown) on each axial end of the assembly  76  and are in contact with the damper body tube  32 . 
     An advantage of placing the external coil  70  outside of the cylindrical reservoir  41  is that electrical wiring (not shown) to the external coil  70  is readily installed through the housing tube  14 . In addition, the electrical wiring is secured to the housing  12  so that wire flexure and failure is reduced or prevented. 
     Referring to FIG. 2, an alternate means of securing the rod and piston is illustrated with the same elements as shown in the above illustration being referred to with the same reference characters. This embodiment uses a number of retaining rings and Belleville springs or washers to retain the rod  30  to the housing  14  and the piston assembly  28  to the rod  30 . 
     In particular, the piston assembly  28  is secured to the free end or upper end  110  of the rod  30 . A first retaining ring or circlip  112  is positioned in a groove of the rod  30 . The piston assembly  28  is slid onto the end  110  of the rod  30 . A Belleville spring or washer  114  is positioned upon the end of the assembly  28  adjacent the upper end  110 . A second retaining ring  116  is positioned in a groove of the rod  30  to trap the Belleville spring  114  between the assembly  28  and the ring  116 . It will be understood that the spring or washer  114  will be provided with a pre-load or bias to secure the assembly  28  to the rod  30  and prevent movement or misalignment of the assembly with respect to the bore  58  of the tube  32 . 
     The rod  30  may be held to the housing  14  at a lower end  102  of the rod. A first (lower) retaining ring or circlip  104  is held in a groove of the rod outside the lower end  18  of the housing  14 . A second (lower) retaining ring  106  is held in a groove of the rod  30  just inside the lower end  18 . Between the second ring  104  and the lower end  18  of the housing  14  a Belleville spring  108  is positioned to bias the first ring  104  against the lower end  18  and secure the rod  30  thereto in a similar preload manner as above. It will be understood that maintaining the rod  30  in a secure fashion with respect to the housing  14  helps to align the piston assembly  28  concentrically within the cylinder  32 . 
     The use of an external coil improves the reliability of the electrical connection thereto and allows higher flux densities to be generated, improving performance of the damper. The stepped piston assembly includes both a flow gap and a bearing surface, lowering complexity and cost of the assembly. Since the bearing surface is magnetic, (previously provided by a stainless steel plate or the like) stainless steel (non-ferrous) components are eliminated from the MR damper. 
     While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.