Take MR fluids, for instance. Under normal conditions, magneto-rheological fluids are free-flowing with a viscosity akin to motor oil. In the presence of a magnetic field, the fluid can become a near-solid in milliseconds. It can return instantly to its fluid state when the magnetic field is withdrawn.
Jacob Rabinow invented MR fluid while he was at the National Bureau of Standards in the 1940s. An early picture of a demonstration of the material's capabilities showed an MR fluid device supporting a 117-pound woman suspended on a swing. For decades, until the technical infrastructure grew up around it, MR fluid did not get past the point of novelty. 
Sophisticated algorithms, fast control circuits, and sensors eventually brought its possibilities closer to usefulness, said Lynn Yanyo, manager of marketing and sales at Lord Corp.'s materials division in Cary, N.C. Lord Corp. has developed a number of commercial applications for its Rheonetic MR fluid, which has been under development by the company since 1991. Yanyo said that it was a challenge to use the fluid in systems that were stable and would not abrade. She said it took six years to resolve the issues. Lord's magneto-rheological fluids are suspensions of micron-size iron particles in a carrier fluid, which is often some kind of oil. In 1997, the company put MR fluid into a seating shock absorber system for trucks. The suspension uses the controllable magnetic fluid to provide varying levels of firmness and adapts to changing levels of shock and motion 500 times per second, according to the company. The system reacts in real time, eliminating compromises of passive systems, which essentially try to satisfy many conditions with one setting, Yanyo said. Delphi Corp. in Troy, Mich., a supplier of automotive components to General Motors, also had a long-standing interest in MR fluid and was investigating it around the same time, according to Yanyo. In 1999, Delphi contacted Lord to supply MR fluid for its new MagneRide semi-active automotive suspension system. General Motors has specified the MagneRide on the 2002 Cadillac Seville STS and the 50th Anniversary edition of the Chevrolet Corvette for 2003. It also specified the system on two 2004 Cadillac models: the SRX sport utility and XLR roadster. Darin Dellinger, senior vehicle development engineer at Delphi, said that the MagneRide system resulted in at least a 40 percent part reduction, mostly of valve parts, and also eliminates traditional shock absorber fluid. In its place is essentially a cylinder filled with MR fluid and surrounded by a coil. 
The MR fluid system requires the setting of a few physical parameters, and most settings are made electronically, with a laptop computer. Dellinger added that there is a flexibility in setting performance characteristics that could not have been accomplished with traditional shocks.Dellinger said that advances in sensor technology and rugged controllers helped to open the way for MR fluid technology. The MagneRide system is coupled to an array of sensors, including four that monitor wheel-to-body position, and to a control module. Damping is adjusted on each corner of the car about every millisecond. Dellinger's biggest initial concern with the technology was durability, he said. Computer modeling concepts developed in the early 1990s addressed many of those issues, he said.Dellinger described MagneRide as an active suspension system in a passive package. That is, it does not supply active power to lift or push down a wheel, but when it goes over bumps, it dissipates energy intelligently. "We are able to use selective damping to get a lot of the benefits of active damping without all of the terrible packaging and mass," he said. Lord Corp. has expanded MR fluid applications into other areas as well, including tactile feedback in control-by-wire systems, and seismic and wind mitigation systems for buildings and bridges.

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