Page 107 - Flexible Robotics in Medicine

92 Chapter 4

4.3.5.6 Accuracy of prototype
The accuracy of the prototype largely depended on the amount of friction between the
individual cylindrical segments and the spherical ball bearing-like structures, as well as the
mechanical properties of the control cables. Two types of control cables were tested: metal
and nylon (several nylon cables of different diameters). Nylon cables gave the best results
and were thus chosen for the prototype because they gave minimal “whiplash effect,” where
the sudden slippage of the cables with some of the individual segments and spherical ball
bearing-like structures resulted in uncontrolled motion.

4.3.5.7 Total bending angle and individual bending segment
The number of segments has a direct implication on the bending angle, as with the size of
each individual segment. As shown in Fig. 4.12, several standalone sections were put
together to observe the segment-to-segment bending. The larger the size of the cylindrical
segments, the less space exists between each segment. This results in a smaller gap and,
thus, a smaller segment-to-segment angle (which then translates to poorer bending angle at
a macro scale). However, the dimensions are dependent on the size of the spherical ball
bearing-like structures (made of necklace beads in the prototype), and the sizes/ratios have
to be carefully calculated (refer to engineering drawings report).

Figure 4.12
(Left) Total bending angle with overlayed photographs for the prototype. (Right) Three segments,
where (a) indicates the neutral, unactuated position between the top and middle segments, and
(b and c) indicate a bent position between the bottom and middle segments, with (b) showing
the outer radius of bending curvature and (c) showing the inner radius of bending curvature. The
middle segment slid along the smooth curved surface of the spherical ball bearing-like structure to
reach the bent position. The limit was reached when the edges of the bottom and middle
segments came into contact.