Page 356 - Control Theory in Biomedical Engineering

322 Control theory in biomedical engineering

Through this mechanism, we theoretically analyzed and tested our design
for a variable stiffness device.
From the tests, we observed that the created vacuum was not sufficient
to hold the robot in place. This is because there is a physical limit to the
amount of resistance by the membrane as the internal pressure cannot go
below zero. Hence, the negative pressure is limited to the atmospheric
pressure. Additionally, the coefficient of static friction between the mem-
brane and the robot vertebrate is not large enough to resist motion even
when an almost complete vacuum is created. As shown in Fig. 2, without
negative pressure (A) and (B), the robot mostly returned to its original
shape and was not stiff enough for load-bearing applications. We also
encountered many sealing problems, as it was almost impossible to get a
perfect vacuum and there were many microscopic holes near the end junc-
tions where leakage was evident. For these reasons, we did not continue
pursuing this method.
Based on our concept evaluation, we find that most of the existing
methods cannot be readily applied as solutions for a variable stiffness module
of tubular surgical instruments because they do not meet our design criteria
or because they require complex modifications to achieve them. Hence,
there is a need for us to come up with a fast mechanism that can better meet
our specific needs of tunable stiffness of tubular flexible manipulators in
medical robotics.

3 Concept combining jamming and continuum
metamaterials with negative Poisson’s ratio materials
(auxetics)

While the method of negative pressure did not yield satisfactory results,
the method of jamming using friction to resist motion is a promising
direction for exploration. We are exploring tunable stiffness approaches
through the means of auxetic continuum materials (also known as neg-
ative Poisson’s ratio (NPR) materials), folding/unfolding and jamming
mechanisms.
Poisson’s ratio indicates transverse strain relative to longitudinal strain.
Most common materials display a positive Poisson’s ratio (PPR) as they
get shorter in the transverse direction when elongated in the longitudinal
direction, and vice versa. An auxetic material displays the opposite charac-
teristics in Fig. 3, and they elongate in the transverse direction upon being
elongated in the longitudinal direction.