Page 253 - Flexible Robotics in Medicine

242 Chapter 10

angle of 24.5 degrees. The relationship of the input current to the tentacle structure and
its maximum bending angle achieved is shown in Fig. 10.7D. The maximum bending angle
achieved by the tentacle structure increased with an increase in the input current. Although
the rate of increase saturated after a certain input current, this depends on the length and
diameter of the SMA wire and also the encasing material (silicone elastomer). A massive
increase in the maximum bending angle ( 13.1 degrees) was observed when the input
current was changed from 0.5 to 1.0 A, whereas when the input current was changed from
1.0 to 1.5 A, the maximum bending angle ( 2.2 degrees) had a tiny increase,
demonstrating the saturation has occurred.
The dynamic bending angle of the tentacle structure, when actuated at different input
currents (0.5 1.5 A) with an actuation time of 2 seconds and relaxation time of 2 seconds,
is shown in Fig. 10.7C. The bending angle increased swiftly in the first second of actuation
and slowly saturated after that, and during relaxation, the bending angle reduced rapidly in
the first second and slowly saturated after that for all three different input currents. Due to
the slow reduction in the bending angle during the relaxation period, the tentacle structure
was unable to revert to its initial position completely. It had a 0.2- and 1-degree
difference compared to the initial position when actuated for 2 seconds at 1.0 and 1.5 A,
respectively, and deactivated for 2 seconds. This is due to the thermal properties of the
silicone elastomer. Since the silicone elastomer is unable to cool the heat in the SMA wire
at a higher rate, the relaxation of the SMA wire takes a longer time. This can be overcome
by increasing the relaxation time for the tentacle structure or by utilizing materials that
allow faster cooling of the SMA wires in the tentacle. The time lapse of the actuation of the
tentacle structure in one direction, when actuated at 1.0 A, is shown in Fig. 10.7A. The
tentacle shows a similar maximum bending angle in all four directions at the same input
currents. This can be achieved by activating a different combination of SMA wires to
control the direction of the bending of the tentacle structure.

The tentacle structure is actuated at 1.0 A for 2 seconds in four directions (up, down, right,
and left), and the time lapse in the top view is shown in Fig. 10.8A. The tentacle structure
is actuated in left (2X direction) during 0 2 seconds and then relaxed to bring the tentacle
structure to its initial position (motion in 1 X direction) during 2 4 seconds. The tentacle
structure is then actuated in the right (1X direction) during 4 6 seconds and then relaxed
to bring the tentacle structure to its initial position (motion in 2 X direction) during
6 8 seconds. The tentacle structure is then actuated in the bottom (2Y direction) during
8 10 seconds and then relaxed to bring the tentacle structure to its initial position (motion
in 1 Y direction) during 10 12 seconds. The tentacle structure is then actuated in the top
(1Y direction) during 12 14 seconds and then relaxed to bring the tentacle structure to its
initial position (motion in 2 Y direction) during 14 16 seconds. The dynamic bending
angle of the tentacle structure during the actuation in all four directions are tracked in X and
Y directions and are shown in Fig. 10.8B. The 2 seconds period of actuation and relaxation