Magnetorheological fluid damper tunable for smooth transitions

Details Magnetorheological fluid damper tunable for smooth transitions Magnetorheological fluid damper tunable for smooth transitions

2001-01-30

2001-11-20

Schwartz, Christopher P. (Department: 3613)

Brakes

Internal-resistance motion retarder

Using magnetic flux

C188S322220, C188S267200

Reexamination Certificate

active

06318520

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a magnetorheological fluid damper and more particularly, to a linear acting fluid damper for a vehicle suspension employing magnetic tuning in connection with a magnetorheological working fluid to effect desired damping levels.
BACKGROUND OF THE INVENTION
Magnetorheological fluids that comprise suspensions of magnetic particles such as iron or iron alloys in a fluid medium are well known. The flow characteristics of these fluids can change by several orders of magnitude within milliseconds when subjected to a suitable magnetic field due to suspension of the particles. The ferromagnetic particles remain suspended under the influence of magnetic fields and applied forces. Such magnetorheological fluids have been found to have desirable electromagnetorheological interactive properties for advantageous use in a variety of controllable coupling and damping devices, such as brakes, clutches, and dampers.
In particular, linear acting MR dampers are proposed for suspension systems, such as a vehicle suspension system and vehicle engine mounts. PCT patent application 10840, published Jan. 8, 1998 (the '840 application), discloses a conventional linear acting controllable vibration damper apparatus that includes a piston positioned in a magnetorheological fluid-filled chamber to form upper and lower chambers. The piston includes a coil assembly, a core, i.e. pole pieces, and an annular ring element positioned around the pole pieces to form an annular flow passage for permitting flow of the magnetorheological fluid between the chambers. When the piston is displaced, magnetorheological fluid is forced through the annular flow passage. When the coil is energized, a magnetic field permeates the channel and excites a transformation of the magnetorheological fluid to a state that exhibits increased damping forces.
The damping performance of a suspension damper is largely dependent on the force-velocity characteristics of the damper. In standard suspension dampers of the prior art that do not use MR fluid, the force-velocity curve typically has a steeper slope at low velocities and desirably passes through the zero point of damping force at zero velocity, thus producing a smooth transition between damper movements in compression and extension directions. Without special design considerations, however, a suspension damper using MR fluid tends to have a force-velocity curve that intersects the force axis at a value above zero from the positive velocity side, as seen in curve
50
of
FIG. 4
, and a value below zero from the negative velocity side, thus producing a jump in force between finite positive and negative values with each change in the direction of damper movement. These jumps in force tend to provide a harshness to the vehicle ride which may be felt by the vehicle occupants. Conventional MR dampers attempt to solve the zero intersect problem by including one or more fluid bypass passages through the piston or on the outer surface thereof, in an area of weak or no magnetic flux and not open to the main, magnetic flux controlled fluid path through the piston: for example in the outer surface of the flux ring. The relatively unimpeded flow of MR fluid through the outer bypass passages permits the damping curves to intersect zero. However, this design also results in an undesirable steep rise in the damping curve from the zero point followed by a sharp transition into higher velocities. In addition, the steep rise may often result in the damper overshooting the desired force at the transition. The steep slope and overshooting, as seen in curve
52
of
FIG. 4
, results in discontinuities that are generally undesirable in vehicle suspensions. Specifically, the use of a totally separate bypass passage impairs the ability to achieve noise control and smooth load transfer. Also, the MR fluid flowing through the outer bypass passages is not within the magnetic flux path, is not exposed to magnetic flux and therefore, does not experience an MR effect. As a result, the outer passages represent a pure loss in pressure in the system that disadvantageously reduces the maximum force achievable.
Therefore, there is a need for an MR damper capable of effectively providing a smooth and controllable transition, without a sharp break in the damper force/velocity curve, between very low damping forces near zero damper piston velocity to higher damping forces at higher damper piston velocities while maintaining desirable maximum force levels.
SUMMARY OF THE INVENTION
The present invention is aimed at providing an MR damper capable of effectively providing a smooth transition between very low damping forces near zero damper piston velocity to a higher damping forces at higher camper piston velocities without sacrificing maximum force levels.
One aspect of the invention provides a damper, comprising a cylinder containing magnetorheological fluid and a piston assembly slidably mounted for reciprocal movement in the cylinder to form a first chamber positioned on one side of the piston assembly and a second chamber positioned on an opposite side of the piston assembly. The piston assembly includes a flow gap extending through the piston assembly to permit fluid flow between the first and the second chambers. The piston assembly further includes a magnet assembly adapted to generate a magnetic filed extending through the flow gap to cause magnetorheological fluid flowing through the gap to experience a magnetorheological effect affecting the flow of magnetorheological fluid through the gap. The piston assembly further includes a passage positioned to permit fluid to flow through the passage between the first and second chambers. The passage is positioned within the magnetic field and sized and shaped to permit fluid flowing through the passage to experience a reduces magnetorheological effect less than a magnetorheological effect compared to the MR effect experienced by fluid flowing through the gap.
The magnet assembly may include a core having an outer surface and a flux ring positioned around the core. The flux ring includes an inner surface that at least partially forms the flow gap. At least one of the outer surface of the core and the inner surface of the flux ring includes the passage, i.e. a groove, extending longitudinally along the piston assembly. The groove is open to the main portion of the flow gap. Preferably, the flux ring and flow gap are annular and the groove extends axially along the piston. Preferably, the groove extends from a first end to a second end of the flux ring to permit fluid to flow through the groove between the first and second chambers.
The passage or groove forms a force/velocity optimization feature for effectively smoothing the transition between very low damping forces near zero damper piston velocity to higher damping forces at higher damper piston velocities without sacrificing maximum force levels. The groove may be formed in a variety of cross-sectional shapes, including a curvilinear, triangular and modified rectangular. In addition, the groove may be formed so as to extend linearly and axially along the piston assembly.
Another aspect of the invention provides first and second grooves. The first and second grooves may be a pair of opposed grooves. In the damper a flow gap includes an annular, axially directed passage within the piston assembly. The first groove is axially directed along one of an inner wall and an outer wall defining the annular, axially directed passage and the second groove is axially directed along the other of an inner wall and an outer wall defining the annular, axially directed passage.
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 thereo

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