Biomedical Engineering Reference
In-Depth Information
Figure 7.8
A tactile shape display using servomotors [25] (See Plate 16)
7.3.4 Shape Memory Alloy (SMA) Tactile Display (Shape)
SMA wire shortens when it undergoes a phase transition from the Martensitic to the
Austenitic phase. This reversible phase transition can be produced by passing current
through the wire and heating it to above its transition temperature. It is well known that
SMA exhibits hysteresis when cycled through this transition [28]. It is also a relatively
slow process because it can take a long time for the wire to cool down and return to
its original length. Some researchers have suggested that it is possible to increase the
bandwidth and account for the hysteresis by developing a model of the process and
incorporating this into the controller [29, 30].
While this undoubtedly will work to some degree, Wellman
et al
. have chosen to
increase the bandwidth through careful thermal design, and have minimized the hysteresis
through position feedback control of the display [31]. Figure 7.9 shows how a single pin
is actuated by the SMA. A 3D shape display, using an array of bars actuated by SMA, is
presented in [32 - 34] and shown in Figure 7.10.
7.3.5 Piezoelectric Tactile Display (Lateral Skin Stretch)
Piezoceramic actuators constitute a practical choice to build an actuator array because they
are based on the phenomenon of deformation of a quartz crystal caused by an electrical
field. They can be operated over a large bandwidth and are relatively easy to form into a
desired miniature structure. Moreover, they are widely available from a variety of sources
and are relatively inexpensive. Unfortunately, piezoceramic actuators still require high
operating voltages although, with the advent of technology, they are steadily improving
and their usefulness is becoming more apparent with time. Hayward
et al
. worked on a
tactile display using an array of piezoceramics [35 - 38]. The objective was to create a
deformable structure upon whose exposed side there was to be an array of contactors, each
of which would contact a patch of skin in a tangential motion and produce programmable
strain fields. Among the different piezoelectric elements available, bimorphs can achieve
