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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
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