Magnetorheological fluid devices exhibiting settling stability

Details Magnetorheological fluid devices exhibiting settling stability Magnetorheological fluid devices exhibiting settling stability

2000-01-18

2002-08-06

Rodriguez, Pam (Department: 3683)

Brakes

Internal-resistance motion retarder

Magnetic fluid or material

C267S140140

Reexamination Certificate

active

06427813

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the area of controllable fluid devices. Specifically, it relates to controllable fluid devices which utilize a magnetically controllable fluid.
BACKGROUND OF THE INVENTION
Dampers are known which use a hydraulic fluid as the working medium to create damping forces to control motion, shock, and/or vibration. One special class of these devices include a Magnetorheological (MR) fluid. MR fluid devices are of particular interest because they only require small electrical currents (typically several amps or less) and do not present a potential shock hazard, because they operate at low voltage (typically 12 volts or less). MR fluid devices, such as dampers, brakes, and clutches employ a controllable Magnetorheological (MR) fluid comprised of small soft-magnetic particles dispersed within a liquid carrier. Typical particles include carbonyl iron, or the like, having various shapes, but which are preferably spherical, and which exhibit mean diameters of between about 0.1 &mgr;m to 500 &mgr;m, and more preferably between about 1 &mgr;m and 100 &mgr;m. The carrier fluids include various known hydraulic oils, and the like. These MR fluids exhibit a thickening behavior (a rheology change), sometimes referred to as an “apparent viscosity change”, upon being exposed to a magnetic field of sufficient strength. The higher the magnetic field strength exposed to the MR fluid, the higher the resistance (damping force, braking or locking torque) that can be achieved within the particular MR device. Examples of MR fluid may be found in U.S. Pat. Nos. 5,578,238 and 5,382,373.
In particular, MR fluid devices provide controllability through simple fluctuations in the electrical current supplied to the magnetic field generator (generally a wound-wire coil). Descriptions of prior art MR mounts and dampers may be found in U.S. Pat. Nos. 5,492,312, 5,398,917, 5,284,330, and 5,277,281 and copending U.S. Ser. No. 08/674,179 now U.S. Pat. No. 5,878,851, entitled “Controllable Vibration Apparatus”. Descriptions of controllable brakes may be found in copending U.S. Ser. Nos. 08/674,371, now U.S. Pat. No. 5,842,547, entitled “Controllable Brake” and 08/304,005, now U.S. Pat. 5,816,372, entitled “Magnetorheological Fluid Devices and Process of Controlling Force in Exercise Equipment Utilizing Same”.
Recently, the use of MR dampers has been proposed for control of civil engineering structures for earthquake hazard mitigation. MR dampers, because of their low power requirements, are highly adaptable to battery power operation. Notably, loss of the main electrical power source during a seismic event is a reality that must be contended with, making MR dampers attractive. In these applications, the MR fluids used must be able to withstand relatively long stationary periods (as much as 20 years or more). In particular, settling of the particles in the MR fluid may be detrimental to the device's performance. Much formulation work has been done regarding various means for minimizing settling in the fluids, some more effective than others. For example, PCT/US97/02743 entitled “Magnetorheological Fluid Seismic Damper” describes a “licorice custard” fluid which has a custard like consistency which prevents settling. PCT/US97/02322 entitled “Controllable Fluid Rehabilitation Device Including A Reservoir of Fluid” teaches applying a low-level magnetic field to a large reservoir of fluid to prevent settling of the particles in an exercise device into which the user inserts a body part. Notably, settling of particles in the MR fluid still presents a problem which is difficult to overcome through formulation alone. Further, settling of the particles within damper and brakes is a problem which has been largely unsolved. Therefore, there is a need for a device which is time stable, and in which, the fluids will not settle.
SUMMARY OF THE INVENTION
In light of the advantages and drawbacks of prior art systems, the resent invention is a magnetorheological fluid device comprising a housing including a hollow, a moving element contained within said hollow, said housing and said moving element oriented to form at least one working section and at least one chamber within said hollow, a magnetorheological fluid contained within said at least one working section and said at least one chamber, means for generating a magnetic field to act upon said magnetorheological fluid contained within said at least one working section to cause a rheology change therein, and means for generating a low-level magnetic field to act upon a substantial portion of said magnetorheological fluid contained in said at least one chamber to minimize settling of said magnetorheological fluid contained therein. The device may be a damper including a damper piston assembly having first and second ends and first and second chambers spaced on either end thereof. The invention has particular application in combination with civil and other structures which include first and second relatively moveable structural elements wherein said housing interconnects to said first structural element and said moving portion interconnects to said second structural element.
According to one novel aspect, the means for generating the low-level magnetic field includes at least one electromagnet. The electromagnet may be mounted to a wall portion of said housing or mounted to said first and second ends of said piston assembly. According to another aspect, the means for generating the low-level magnetic field includes at least one permanent magnet. The permanent magnet(s) may be housed within each of said first and second chambers, attached at said first and second ends of said damper piston assembly, or mounted to a wall portion of said housing, or combinations thereof. According to another aspect, the permanent magnet(s) is a strip magnet arranged on an outer wall portion of the housing or a portion of the wall itself may be magnetized. The invention also has applicability to brakes and clutches wherein said housing is an outer member and said moving element is a rotor received therein. Accordingly, the means for generating a low-level magnetic field may be mounted on said rotor or to a wall portion of said housing.
It is an advantage of the present invention MR device that it is time stable, in that it can be placed in service for many years without settling of the particles contained in the working MR fluid.
It is an advantage of the present invention MR device that the fluid within the device does not settle, yet off state resistance is not appreciably affected.
The above-mentioned and further features, advantages, and characteristics of the present invention will become apparent from the accompanying descriptions of the preferred embodiments and attached drawings.


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