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 exerted 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.
Generally, linearly-acting MR fluid dampers are based on either a monotube or a twin tube cylindrical reservoir design. In the monotube cylindrical reservoir design, a piston moves within the fixed length cylindrical reservoir in response to force from a piston rod that extends outside of the cylinder. In the twin tube cylindrical reservoir approach, an open end of an outer tube slides over an open end of an inner tube to form the twin tube cylindrical reservoir, which has an adjustable length.
Both monotube and twin tube cylindrical reservoirs experience reliability problems arising from the electrical wiring 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 flexes with the movement in the piston, sometimes resulting in wire breaks.
In twin tube cylindrical reservoirs, it is known to reduce failure from wire flexing by holding the coil stationary with respect to a portion of the reservoir (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 path between the inner and outer tubes, rather than a coil integral to the piston controlling flow past the piston. Although wire flexure is reduced, the reduced piston diameter correspondingly reduces damping. Also, leaks due to introducing wiring into the reservoir are not avoided. In addition, moving parts like check valves wear, reducing the service life of the damper.
Consequently, a significant need exists for an MR fluid damper having a reduced the likelihood of pressure leaks from the MR fluid reservoir, yet does not suffering from reduced performance.
The present invention provides an MR fluid damper that is of a simpler construction than 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.
According to the principles of the present invention and in accordance with the described embodiment, the present invention provides an MR fluid damper having a damper body tube containing a volume of MR fluid. A piston assembly is disposed in the damper body tube to form a flow gap between an outer surface of the piston and an inner surface of the damper body tube. An external coil surrounds a portion of the damper body tube. The external coil is capable of generating a magnetic field across at least a portion of the flow gap. With the external coil, wires leading to the coil do not have to pass through a fluid seal, nor do they experience flexing or bending during the operation of the MR fluid damper. Therefore, such wires are much less susceptible to breakage.
In another aspect of the invention, an MR fluid damper has a piston assembly disposed in a damper body tube containing a volume of MR fluid. The piston assembly forms a flow gap between the piston assembly and the damper body tube and includes a bearing in contact with the inner surface of the damper body tube. The bearing maintains the piston assembly concentric within the damper body tube while permitting a flow of the MR fluid through the flow gap. The bearing makes the combination of the damper body tube and the piston assembly much more tolerant of side loading, thereby minimizing fluid rheology changes during operation and reducing the wear resistance requirements of those parts.
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.