The present invention relates to a magneto-rheological (xe2x80x9cMRxe2x80x9d) fluid damper, and more particularly, to a linearly-acting MR fluid damper suitable for vibration damping in a vehicle suspension system.
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.
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.
In some applications, linearly-acting MR fluid dampers use a piston assembly that moves within a damper body tube having a cylindrical reservoir that separates a volume of MR fluid into a compression chamber and an extension chamber. The piston assembly has a piston core positioned within a flux ring to form an annular flow gap therebetween. Relative motion between the damper body tube and the piston assembly is dampened by a flow of the MR fluid through the flow gap from one chamber to another caused by the relative motion.
Alignment of the flux ring is critical for optimum performance. Ideally, the piston assembly should move freely within the reservoir in the damper body tube without friction or binding. In addition, the radial width and concentricity of the annular flow passage must be precisely set and maintained along the axial length of the passage throughout the operation to ensure optimum, predictable control of the damping. Consequently, the flux ring must be correctly aligned with the piston core.
Attachment elements have been suggested to provide flux ring alignment with nonmagnetic bridge elements. In particular, perforated end plates are aligned above and below the flux ring and piston core. These attachment elements have several potential problems. First, the attachment elements increase the length of the piston assembly. Consequently, less travel distance is available for the piston to move within the cylindrical reservoir of the damper body tube. Second, the attachment elements require tight manufacturing tolerances in order to correctly align the flux ring to the piston core. Third, such attachment elements often include tabs or other projections that increase the drag as the piston moves, which may be undesirable. Fourth, the attachment elements have numerous components and require manufacturing operations such as spot welding. Therefore, such attachment elements are costly to manufacture and time consuming to assemble.
Consequently, there is a need for an improved piston assembly suitable for use in a magneto-rheological (MR) fluid damper.
The present invention addresses the above need by providing an improved piston assembly for a linearly-acting MR fluid damper. The piston assembly of the present invention confines a flux ring within the functional length of the piston assembly without significantly restricting fluid flow, thereby providing optimum performance with minimal piston length. Further, the part count of the piston assembly is reduced; and the piston assembly is easier to assemble in a desired alignment. Thus, the piston assembly of the present invention is of a simpler construction than known damper pistons that can be manufactured for less cost.
According to the principles of the present invention and in accordance with the described embodiment, the present invention provides a piston assembly for use with an MR fluid damper. The piston assembly has a flux ring positioned in a desired alignment with a piston core to form an annular flow gap between the flux ring and the piston core. The piston core is secured to the flux ring in the desired alignment by a plurality of projections extending across the flow gap between an inner surface of the flux ring and an outer surface of the piston core. Thus, the flux ring is secured on the piston ring without using expensive, high precision attachment components; and the piston assembly is able to utilize its full length, thereby providing optimum performance with a minimum of length piston.
In one aspect of the present invention, the projections are molded through attachment passages in the flux ring. In a further aspect of the invention, the attachment passages are holes intersecting the inner and outer surfaces of the flux ring.
In another embodiment of the invention, a method is provided for making a piston assembly for use with an MR fluid damper. The method comprises first, fixing a piston core and a flux ring in a desired alignment forming a flow gap therebetween; and then, forming a plurality of projections in the flow gap between the flux ring and the piston core to secure the flux ring on the piston core in a desired alignment.