Motion System Design

MR fluids

Mix a designer magnetic powder into oil, water, or glycol, and what do you get? A fast-acting mechatronic fluid perfect for damping or moving variable loads.

Jacob Rabinow of the U.S. National Bureau of Standards first developed the slurry in the 1940s and dubbed it magnetorheologic (MR) fluid for the way it changes properties under the influence of an electromagnetic field. Since then, the technology has been refined and put to use in a variety of applications.

In 2002, MR shock systems became standard equipment on the Cadillac Seville. Offering extraordinary ride comfort, the technology is also available on Corvettes and the Buick Lucerne in the form of real-time-controlled monotube shock absorbers with single-stage, axisymmetric MR piston valves.

Elsewhere, MR technology is finding use in industrial brakes, locking devices, and short-stroke actuators; in fact, half a million MR devices are now employed in manufacturing, with more on the way.

Select strength

After just milliseconds under a magnetic field, “liquidy” MR fluids stiffen into a semisolid. What makes the stiff version useful is that its yield strength is controllable. So as a liquid, MR fluids are Newtonian, but under a magnetic field, they behave according to a Bingham-plastic model that defines their total yield stress as:

where tMR(H) is the yield stress caused by the applied magnetic field H, hp is field-independent plastic viscosity, and g is the shear rate.

Pushing the limit

The ultimate strength of a solidified MR fluid depends on the square of the field strength at which the fluid becomes fully saturated. Particles with high saturation magnetization (like iron-cobalt alloys with about 2.4 Tesla) make the strongest MR fluids. However, pure elemental iron (with saturation magnetization of 2.15 Tesla) is more economically practical. Depending on the percent volume of iron particles, maximum yield strengths range from 30 to 80 kPa for applied magnetic fields of 150 to 250 kA/m.

What it takes

MR fluids operating near their maximum yield strength need a magnetic field energy density of about 0.1 J/cm3 to function. To establish the required magnetic field H within a time interval 3t — say, to negate cable vibrations of respectable frequency — the MR power source (usually a dc current supply) must be capable of producing electrical energy at a rate of 0.1 J/cm3 times fluid volume, divided by 3t.

Now that's intense

An MR fluid's properties aren't the only things that affect response time. Responsiveness of practical MR devices is also determined by the time it takes the current source to establish a magnetic field — in turn dependent on the electromagnet's resistance and inductance, Eddy currents nearby, and current amplifier output. Roll-off in MR response is under one millisecond. As for transient response time, dampers can reach rheological equilibrium within about six milliseconds after step voltage input to the current driver. Driven by a current amplifier with higher voltage compliance, it's under two milliseconds.

Marching to a new beat

MR fluids control smart prosthesis knees that adapt and respond in real-time to changing conditions, giving above-knee amputees a natural gait.

A small MR fluid damper semi-actively controls knee motion based on inputs from sensors in the prosthesis. An embedded microprocessor interprets input signals (axial force, bending moment, knee angle, and speed) to determine what the person is attempting: fast or slow walking, slope or stair navigation, or fleeing an ill-tempered 140-lb dog on a hot August day. By comparing these commands to measured data from knee angle and force sensors, the controller is able to adjust damper current and compensate the actual gait profile so that it matches an ideal profile stored in memory.

This month's information provided by J.D. Carlson and Jim Toscano of Lord Corp., Cary, N.C. For more information, visit

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