Page 246 - Flexible Robotics in Medicine

Soft-bodied flexible bending mechanism with silent shape memory alloys 235

the dimension of the wire and its transition temperature. The SMA wires with a transition

temperature of 70 C are chosen over the wires with 90 C transition temperature, although the

wires with higher transition temperatures have a higher cooling rate. The lower transition
temperature is preferred to reduce the heating/actuation time of the tentacle. Also, frequent
exposure of the elastomeric material to high temperatures could induce permanent damages.
The diameter and the length of the SMA wires play a significant role in terms of power
consumed for the actuation and the force exerted by the tentacle, which is necessary to control
the bending angles of the tentacle. Wires with larger diameters exert higher pulling force and
have lesser resistance compared to the smaller diameter wires. The higher pulling force
induces larger bending angles and the lesser resistance improves power utilization of the
tentacle. Longer wires tend to produce larger bending angles, but as the length of the wires
increases, the resistance increases, resulting in higher power consumption. Despite several
advantages of larger diameter SMA wires, the high pulling force exerted might damage the
elastomeric material surrounding the SMA wires and also lengthen the cooling time during
the relaxation phase, increasing the actuation cycle time. Based on these factors, 250 μm
diameter, 100 mm long SMA wires with a martensite start and finish temperature of 42 C and

52 C and an austenite start and finish temperature of 68 C and 78 C, respectively, are found

to suit the best for this application.
The SMA wire can be programmed to memorize a specific conformation by fixing the wire in

the desired shape in a mandrel of a fixture [31] and heated at 500 C for a short period of
time. This high temperature arranges the atoms into a compact and regular manner, causing a
rigid cubic arrangement, known as the austenite phase [32]. In this application, the SMA wire
is programmed in two steps, as shown in Fig. 10.2. The SMA wire is bent 180 degrees with
equal lengths (50 mm) on both sides and programmed in the first step, and then the equal
lengths are curved 90 degrees together and programmed as the second step (Fig. 10.2).

Figure 10.2
Schematic showing the steps involved in thermomechanical programming of the SMA wire in the
tentacle.