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Chapter 9 Related motors and actuators 233
FIG. 9.5 The characteristic of a piezoelectric material. (A) Domains in the unpolarised material. (B) Domains align
when the material is polarised. (C) The application of a voltage causes displacement,d, along the axis of
polarisation
Fig. 9.6 shows the basic concepts and operation of a piezoelectric motor. Two
piezoelectric crystals are preloaded against a flat wear surface, by way of the motor head,
to produce a normal contact force. The friction is important in the design of the motor,
since the friction force is used to translate the motion of the piezoelectric ceramic into
the motor’s output. As a positive sinusoidal voltage waveform is applied which increase
its thickness, the axial motion imparts a frictional force along the wear strip. When the
drive voltage goes negative, the same crystal thickness contacts. This action creates a
separation between the motor head and the wear strip, allowing the motor to return to
its original position without dragging the wear strip backward. As the drive voltage
swings positive again, the crystal stroke cycle repeats and the wear strip moves another
incremental step to the left.
9.4 Shape-memory alloy
Shape-memory alloy (SMA) materials have the unique ability to return to a
predetermined shape when heated, leading to their uses in a wide range of applications,
particularly when micro-actuation is required. This property arises due to a reversible
crystalline phase transformation that occurs between the low temperature martensite
and high temperature austenite phases. Although the phases have the same chemical
composition and atomic order, the two phases have different crystallographic structures.
Austenite has a body-centred symmetric structure that exists at high temperature, while
martensite has a low symmetric monoclinic structure that stabilises at relatively low
temperature (Jones, 2001 [p148]). When an SMA is cooled from a high temperature, the
material undergoes a martensitic transformation from the high temperature austenite.
Since the bond energy in the martensite is low, this phase can be easily deformed.
In martensite, even after removal of the stress, the strain remains. This residual strain