Page 345 - The Mechatronics Handbook

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FIGURE 16.13 Vibration of beam using piezoelectric actuators.

Coil Magnetostrictive rod

Magnetic
Field

FIGURE 16.14 Magnetostrictive rod actuator.

One application of these actuators is as shown in Fig. 16.13. The two piezoelectric patches are excited
with opposite polarity to create transverse vibration in the cantilever beam. These actuators provide high
bandwidth (0–10 kHz typical) with small displacement. Since there are no moving parts to the actuator,
it is compact and ideally suited for micro and nano actuation. Unlike the bidirectional actuation of
piezoelectric actuators, the electrostriction effect is a second-order effect, i.e., it responds to an electric
field with unidirectional expansion regardless of polarity.
Magnetostrictive material is an alloy of terbium, dysprosium, and iron that generates mechanical strains
up to 2000 microstrain in response to applied magnetic fields. They are available in the form of rods,
plates, washers, and powder. Figure 16.14 shows a typical magnetostrictive rod actuator that is surrounded
by a magnetic coil. When the coil is excited, the rod elongates in proportion to the intensity of the
magnetic field established. The magnetomechanical relationship is given as:

H
ε = S σ + dH

H
where, ε is the strain, S the compliance at constant magnetic filed, σ the stress, d the magnetostriction
constant, and H the magnetic field intensity.
Ion exchange polymers exploit the electro-osmosis phenomenon of the natural ionic polymers for
purposes of actuation. When a voltage potential is applied across the cross-linked polyelectrolytic net-
work, the ionizable groups attain a net charge generating a mechanical deformation. These types of
actuators have been used to develop artificial muscles and artificial limbs. The primary advantage is their
capacity to produce large deformation with a relatively low voltage excitation.

Micro- and Nanoactuators
Microactuators, also called micromachines, microelectromechanical system (MEMS), and microsystems
are the tiny mobile devices being developed utilizing the standard microelectronics processes with the
integration of semiconductors and machined micromechanical elements. Another definition states that
any device produced by assembling extremely small functional parts of around 1–15 mm is called a
micromachine.
In electrostatic motors, electrostatic force is dominant, unlike the conventional motors that are based
on magnetic forces. For smaller micromechanical systems the electrostatic forces are well suited as an
actuating force. Figure 16.15 shows one type of electrostatic motor. The rotor is an annular disk with
uniform permitivity and conductivity. In operation, a voltage is applied to the two conducting parallel

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