Magnetorheological fluid damper with multiple annular flow gaps

Details Magnetorheological fluid damper with multiple annular flow gaps Magnetorheological fluid damper with multiple annular flow gaps

1999-09-13

2001-08-28

Oberleitner, Robert J. (Department: 3613)

Brakes

Internal-resistance motion retarder

Magnetic fluid or material

Reexamination Certificate

active

06279701

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to damping devices used in controlled damping applications such as semi-active vehicle suspension systems. More particularly, the present invention relates to high performance controlled damping devices using magnetorheological (MR) fluid.
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 electro-magnetomechanical interactive properties for advantageous use in a variety of magnetorheological (MR) damping devices, such as rotary devices including brakes and clutches, and linear-acting devices for damping linear motion or for providing controllable dissipative forces along the damper's axis.
High performance controlled damping applications, such as those used in passenger vehicle suspension systems, preferably provide a relatively low damping force at low speeds for comfort, and provide relatively high damping force at higher speeds for safe handling of the vehicle. Thus, continuously variable real-time damping (CV-RTD) actuators have become increasingly popular. The damping performance of a MR fluid based CV-RTD is largely dependent on the force-velocity characteristics of the damper.
FIG. 1
illustrates the optimum force-velocity characteristics of a damper used in automotive applications. The slope of the off-state force-velocity curve should be as low as possible for a smooth ride, with a desirable value of approximately 600 N-s/m. The on-state force-velocity curve preferably has an initial slope in the range of 5-30 kN-s/m up to a velocity of 0.1 to 0.4 m/s and a final slope similar to that in the off-state. The desirable maximum on-force should be limited to a suitable value (e.g., 4500 N) at 2 m/s. The ratio of the damping force when the damper is in the on-state (on-force) to the damping force when the damper is in the off-state (off-force) at a given velocity is known as the turn-up ratio. It is desirable to have a turn-up ratio of at least 3 to 6 at a velocity of 1 m/s for good control of the vehicle chassis dynamics.
FIG. 2
shows a known monotube MR damper
10
having a piston
12
sliding within a hollow tube
14
filled with MR fluid. The piston
12
is attached to a hollow rod
18
that slides within a sealed bearing
20
at one end of the body of the damper
10
. The piston
12
contains a coil
22
carrying a variable current, thus generating a variable magnetic field across a flow gap
24
between an inner core
26
and an outer shell or flux ring
28
of the piston
12
. A bearing
30
having relatively low friction is disposed between the flux ring
28
and the tube
14
. The flux ring
28
and the inner core
26
of the piston
12
are held in place by spoked end plates
32
. Terminals
34
of the coil
22
extend through the hollow rod
18
and are provided with suitable insulation for connection to a source of electricity. One end portion
36
of the tube
14
is filled with inert gas which is separated from the MR fluid by a floating piston
38
. The floating piston
38
and inert gas accommodate the varying rod volume during movement of the piston. U.S. Pat. No. 5,277,281 discloses a similar MR damper.
FIG. 3
illustrates the force-velocity characteristics of the type of MR damper disclosed in FIG.
2
. Clearly, in comparison to the preferred curves of
FIG. 1
, improvements in the force-velocity characteristics of conventional MR dampers are desirable. Although the above-described conventional MR dampers may perform adequately is certain applications, these devices do not achieve the required turn-up ratio and substantially stiction free performance near zero velocity for realistic automotive applications. Conventional monotube dampers do not provide sufficient tuning capability to effectively control the damping characteristics as represented, for example, by the slope of the force-velocity curves. Also, conventional dampers have an unnecessarily long length for a given performance.
Therefore, there is a need for a more compact MR damper capable of more effectively and controllably damping motion.
SUMMARY OF THE INVENTION
It is an object of the present invention, therefore, to overcome the disadvantages of the prior art and to provide a magnetorheological (MR) fluid damper capable of approximating ideal performance requirements by effectively and predictably providing a desired damping effect, while minimizing the damper size.
This and other objects are achieved by providing a damper comprising a cylinder containing a 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 plurality of substantially annular flow gaps positioned concentrically to one another wherein each flow gap is formed between opposing surfaces of magnetic material. The piston assembly further includes a magnet assembly adapted to generate a magnetic field extending through the plurality of substantially annular flow gaps to cause magnetorheological fluid flowing through the plurality of substantially annular flow gaps to experience a magnetorheological effect affecting the flow of the magnetorheological fluid through the plurality of substantially annular flow gaps. The plurality of substantially annular flow gaps may include three substantially annular flow gaps. The piston assembly may further include a plurality of annular flux rings positioned concentrically to form the plurality of substantially annular flow gaps. The damper may also include a first end plate secured to one end of the piston assembly and a second end plate secured to a second end of the piston assembly. The plates are formed of a non-magnetic material and include radial extensions connected to the plurality of annular flux rings. Each of the radial extensions preferably includes grooves for receiving the plurality of flux rings. A central portion of each of the flux rings may include a magnetic flux barrier formed of a non-magnetic material to prevent shunting. The piston assembly may further include a piston bearing mounted on the assembly and positioned axially along the assembly entirely between an axial center of the assembly and one of the first and the second chambers. The piston assembly may further include a piston core and a rod connected to the first end plate. The first end plate preferably extends axially between the rod and the piston core to isolate and position the rod a spaced axial distance from the piston core while covering an entire axial end face of the piston core.


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“Magnetorheological Effect As A Base Of New Devices and Technologies”, W.I. Kordonsky, Journal of Magnetism Materials, 122 (1993) 395-398.
“MagneShock™ Scores First Race Win”, Carrera Racing Shocks, Jun., 1999.

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