Page 344 - The Mechatronics Handbook

PROGRAMMED SHAPE
AUSTENITE PHAE

AT ROOM TEMPERATURE
STRAIGHTENED
MARTENSITE PHASE

REGAINS SHAPE
WHEN HEATED
AUSTENITE PHASE
FIGURE 16.11 Phase changes of Shape Memory Alloy.

+
V
_

_
V
+
FIGURE 16.12 Piezoelectric actuator.

Smart Material Actuators
Unlike the conventional actuators, the smart material actuators typically become part of the load bearing
structures. This is achieved by embedding the actuators in a distributed manner and integrating into the
load bearing structure that could be used to suppress vibration, cancel the noise, and change shape. Of
the many smart material actuators, shape memory alloys, piezoelectric (PZT), magnetostrictive, Electrorheo-
logical fluids, and ion exchange polymers are most common.
Shape Memory Alloys (SMA) are alloys of nickel and titanium that undergo phase transformation
when subjected to a thermal field. The SMAs are also known as NITINOL for Nickel Titanium Naval
Ordnance Laboratory. When cooled below a critical temperature, their crystal structure enters martensitic
phase as shown in Fig. 16.11. In this state the alloy is plastic and can easily be manipulated. When the
alloy is heated above the critical temperature (in the range of 50–80∞C), the phase changes to austenitic
phase. Here the alloy resumes the shape that it formally had at the higher temperature. For example, a
straight wire at room temperature can be made to regain its programmed semicircle shape when heated
that has found applications in orthodontics and other tensioning devices. The wires are typically heated
by passing a current (up to several amperes), 0 at very low voltage (2–10 V typical).
The PZT actuators are essentially piezocrystals with top and bottom conducting films as shown in
Fig. 16.12. When an electric voltage is applied across the two conducting films, the crystal expands in
the transverse direction as shown by the dotted lines. When the voltage polarity is reversed, the crystal
contracts thereby providing bidirectional actuation. The interaction between the mechanical and elec-
trical behavior of the piezoelectric materials can be expressed as:

T = c S - eE
E

E
where T is the stress, c is the elastic coefficients at constant electric field, S is the strain, e is the dielectric
permitivity, and E is the electric field.

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