Page 185 - Flexible Robotics in Medicine

172 Chapter 7

7.2 Explored concepts for actuation

7.2.1 Cable-driven actuation

Actuators driven by cables can relay forces from external actuators to far-off locations,
which can be small and found deep within the human body. These cables can be made of
different materials, ranging from metals [such as stainless steel and shape memory alloy
(SMA)] to plastics (such as polyethylene).

There are two main types of actuation configurations using cables. They are the tendon-
sheath mechanism (TSM) and tendon routed mechanism (TRM). In typical applications, one
of the ends of the cable/tendon connects to an actuator (e.g., motor), while the other end
connects to the actuated joint. When tension acts on the cable, it will be transmitted through
the pulleys or sheaths to the distal end. TSM comprises of a hollow pipe or coiled wire as a
sheath that encloses the cables inside. TRM, on the other hand, uses pulleys to bear and
transfer the force from the tendon to the site of interest. TRM has a distinct disadvantage
over TSM as there is a need for the tendon to have high pretension to reduce backlash and
hysteresis. As a result, TRM is not suited for applications requiring frequent changes in the
direction of motion. Furthermore, TRM cables are not constrained within an enclosed shell
and, in the case of snapping, may damage the surrounding tissue. TSM, on the other hand,
is safer as it is constrained within an external sheath.

7.2.2 Pneumatic/fluidic actuators

Flexible fluidic actuation comprises elastic materials that can deform under the action of
forces that results from pneumatic or hydraulic pressure. They can be further classified into
three subgroups—elastic fluidic, piston-cylinder fluidic, and drag-based fluidic actuators.
For our study, we only considered elastic fluidic actuators as we want our backbone stem to
be flexible.
Elastic fluidic actuators contain diaphragms that can flexibly enlarge under the action of
pressure. They can be mainly classified into bending/deflection due to differential pressure
and anisotropic rigidity [1,4 6].
• Bending due to differential pressure: A conventional construction of such flexible
devices is to have a fiber-reinforced rubber containing separate chambers that are sealed
from each other. The pressure inside the chambers are self-contained and regulated by
compressed fluids with the help of valves and tubes. The fibers are circularly embedded
in the rubber to prevent radial deformation. The device will bend based on the
differential pressure inside the three chambers, leading to a net force on the chamber
walls. It is difficult to fabricate such a device properly due to challenges in embedding
the fibers in the right direction.