Biomedical Engineering Reference
In-Depth Information
hydrogen, safe operation (hydrogen desorption is a highly endothermic process),
rapid kinetics, and environmentally benign characteristics. Metal hydrides are tradi-
tionally used for hydrogen storage and thermal devices.
1.2.13
E
(ER) M
LECTRORHEOLOGICAL
ATERIALS
Electrorheological fluids are suspensions consisting of dielectric particles of size
0.1-100 m and dielectric base fluid. Since the dielectric constant of suspension
particles differs from the dielectric constant of the base fluid, the external electric
field polarizes particles. These polarized particles interact and form chain-like or
even lattice-like organized structures. Simultaneously, the rheological properties of
the suspension change effectively; for example, the effective viscosity increases
dramatically. ER suspensions also have a magnetic analog consisting of ferromag-
netic particles and the base liquid. Because the viscosity of the ER liquid can be
controlled with the electric field strength, the viscosity of magnetorheological (MR)
fluid is sensitive to the magnetic field.
The response time of ER fluids is of the order of 1-10 msec. In principle, this
enables the use of these liquids in such applications as electrically controlled
clutches, valves, and active damping devices. The use of ER fluids as artificial
muscles has not been reported anywhere. However, a number of publications in the
pertinent literature (Gandhi, Thompson, Choi, et al., 1989; Gandhi, Thompson, et
al., 1989; Furusha and Sakaguchi, 1999) concern the applications of ER fluid to
robotics and as intelligent materials and composites. Some other relevant references
are Huang et al. (2003) and Dwyer-Joyce et al. (1996).
1.2.14
M
(MR) M
AGNETORHEOLOGICAL
ATERIALS
Magnetorheological fluids are essentially suspensions of micron-sized, magnetizable
particles in oil. MR fluids are similar to ER fluids, but they are 20-50 times stronger.
They can also be operated directly from low-voltage power supplies and are far less
sensitive to contaminants and extremes in temperature. Under normal conditions, an
MR fluid is a free-flowing liquid with a consistency similar to that of motor oil.
Exposure to a magnetic field, however, can transform the fluid into a near-solid in
milliseconds. Just as quickly, the fluid can be returned to its liquid state with the
removal of the field. The degree of change in an MR fluid is proportional to the
magnitude of the applied magnetic field. When subjected to the field, MR fluids
actually develop yield strength and behave as Bingham solids. The change can appear
as a very large change in effective viscosity like that in ER fluids.
Applications include automotive primary suspensions, truck seat systems, con-
trol-by-wire/tactile feedback, pneumatic control, seismic mitigation, and prosthetics.
These applications are more than just a demonstration of MR fluid functionality.
Each represents a commercially field-proven MR system that embodies all of the
necessary refinements required to make it fully functional, reliable, cost effective,
and long lived. Some relevant references are Jolly et al. (1996), Jolly and Carlson
(2000), Carlson (1999a, 1999b), Carlson and Weiss (1994), and Weiss et al. (1994).
