J. David Carlson
Edited by Lawrence Kren
Magnetorheological (MR) fluids can rapidly change from a free-flowing liquid to a semisolid when exposed to magnetic fields. This property makes them useful in a variety of mechanical systems. For example, rotary MR-fluid brakes act as variable-resistance elements in exercise equipment such as stair climbers and recumbent cycling machines. Active-vibration-control seats for large, Class 8 trucks reduce driver fatigue, adjustable-rate shock absorbers improve race car handling, and pneumatic actuators fitted with feed-back-controlled rotary brakes and linear dampers move loads more precisely than previously possible.
Despite the benefits, potential users are often reluctant to switch from passive devices. One reason is the limitations associated with complex hardware and seals needed to contain the fluid. Another is the high cost of the fluid itself.
But equipment built with a new MR-fluid sponge eliminates these concerns. Here, capillary forces hold a small amount of MR fluid absorbed in a porous matrix material. Such devices require much less fluid than conventional types and need no seals, bearings, and other high-precision parts. The approach could make feasible cost-sensitive applications including active vibration control for washing machines, force-feedback (haptic) joysticks and mice, variable-rate automotive steering systems, and furniture latches and locks.
MR sponge is particularly appropriate for these moderate-force applications. Nevertheless, it may also scale to build larger-capacity dampers (over 200 kN) that install in building foundations to mitigate damage from earthquakes.
Consider a simple MR-fluid sponge damper. A layer of open-celled, polyurethane foam saturated with MR fluid surrounds a steel bobbin fitted with an electromagnetic coil. Together, these elements form a piston on the end of a shaft that is free to move axially inside a steel housing. The housing also provides a magnetic flux return path.
Electric current applied to the coil induces a magnetic field that aligns metallic particles in the MR fluid perpendicular to piston motion. This alignment causes the fluid to develop yield strength and resist shear. Shear or damping force is proportional to the area of sponge exposed to the magnetic field or typically about 5 N/cm 2 . These MR fluids have the consistency of light grease (relatively low viscosity) in the off state, so dampers can have a broad, dynamic load range. Because some designs also expose the fluid to atmosphere, low-vapor pressure base oils, such as synthetic hydrocarbons, are used to help minimize evaporation.
The sponge itself tends not to wear because the fluid carries most of the shear stresses. Further, performance is largely unaffected by sponge condition provided it can hold sufficient fluid. Candidate open-cell polyurethane foams have a pore volume of about 95% or more, and a 250 to 500µm pore size. Other suitable materials include felts, fabrics, and metal meshes and foams.
Matrices built from polymeric foams are sized to compress lightly against walls in the working space. Fabrics, on the other hand, should fit without compression. The sponge's wicking action not only traps the fluid but also helps prevent gravitational settling or sedimentation of the magnetic particles, a problem plaguing some conventional MR-fluid devices. It also makes possible systems with multiple degrees of freedom and novel geometries.
Linear dampers, for instance, can be tubular, flat, or planar, while rotary brakes may include a magnetic caliper and a thin disc rotor. In another configuration, the disk of the caliper brake is "unrolled" into a long steel strip to create a linear brake. The strip (piston) rides in a yoke-shaped steel housing fitted with a coil and MR-fluid sponge elements on both inner faces. It's designed primarily for controlling motion along the shaft axis but can also handle limited perpendicular and rotary motion.
Smoother-running washing machines
Tubs in conventional washing machines suspend from a housing with springs and fixed-rate dampers. The springs allow the tub to move easily at low frequencies while quelling high-frequency vibrations. The problem is conventional dampers, able to control tub excursions and possible damage during low-speed resonance, severely limit isolation at higher rotational speeds. MR-fluid sponge dampers, in contrast, energize for maximum effort at tub resonance, then power down other times. Better resonance control and isolation allows the use of bigger drums, smaller housings, and higher spin speeds. Such systems may, in fact, be necessary for emerging horizontal-axis, front-loading washing machines and their higher spin speeds (near 2,000 rpm).
One design uses a pair of MR-fluid-sponge dampers each providing 50 to 150 N when energized, and less than 5 N powered down. As a test of durability, the damper was pneumatically driven through a stroke of 2 cm at a 2-Hz rate and energized for five of every 30 sec with sufficient force to halt the pneumatic actuator. After 2 million cycles — corresponding to 5,000 loads of wash or six loads weekly for 15 years — damping force remained above the 60-N design threshold.