Magnetorheological or MR fluids have been successfully used to enable highly effective semi-active control systems in automobile primary suspensions to control unwanted motions in civil engineering structures and to provide force-feedback in steer-by-wire systems. A key to the successful use of MR fluids is an appreciation and understanding of the balance and trade-off between the magnetically controlled on-state force and the ever-present off-state viscous force. In all MR fluid applications, one must deal with the fact that MR fluids never fully decouple or go to zero force in their off-state. Magnelok devices are a magnetically controlled compliment to traditional MR fluid devices that have been developed to enable a true force decoupling in the off-state. Magnelok devices may be embodied as linear or rotary dampers, brakes, lockable struts or position holding devices. They are particularly suitable for lock/un-lock applications. Unlike MR fluid devices they contain no fluid yet they do provide a variable level of friction damping that is controlled by the magnitude of the applied magnetic field. Magnelok devices are low cost as they easily accommodate relatively loose mechanical tolerances and require no seals or accumulator. A variety of controllable Magnelok devices and applications are described.
High-strength, stable, magnetorheological (MR) fluids and devices such as rotary brakes and linear vibration dampers that enable the benefits of controllable fluid technology are now commercially available. Recently, a new way of using MR fluids in which the fluid is contained in an absorbent matrix has been developed. Such MR fluid sponge devices enable the benefits of controllable MR fluids to be realized in cost sensitive applications. Most of the high-cost components normally associated with a fluid-filled device can be eliminated with this approach. Low-cost, controllable MR sponge dampers are particularly appropriate for moderate-force vibration control problems where a high degree of control authority is desired. One attractive area for their application is in next-generation high-performance home washing machines. These new machines operate under varying agitation rates, load imbalances, and often at very high speeds, and as such require varying damping charateristics. Test results have shown the MR sponge damper to be an ideal way to control vibration in these new washing machines.
One of the most exciting new applications for magnetorheological fluid technology is that of real-time controlled dampers for use in advanced prosthetic devices. In such systems a small magnetorheological fluid damper is used to control, in real-time, the motion of an artificial limb based on inputs from a group of sensors. A 'smart' prosthetic knee system based on a controllable magnetorheological fluid damper was commercially introduced to the orthopedics and prosthetics market in 2000. The benefit of such an artificial knee is a more natural gait that automatically adapts to changing gait conditions.
The rheological and magnetic properties of several commercial magnetorheological (MR) fluids are presented and discussed. These fluids are compared using appropriate figures of merit based on conventional design paradigms. Some contemporary applications of MR fluids are discussed. These applications illustrate how various material properties may be balanced to provide optimal performance.
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