Page 26 - Read Online

Page 8 of 26 Blewitt et al. Soft Sci 2024;4:13 https://dx.doi.org/10.20517/ss.2023.49

active elastomers which expand radially and contract when exposed to a magnetic field. Though the robot
manages to navigate straight pipes without tethering, it does require a magnetic field to be created and
moved to create actuation which is impractical in most pipe inspection environments.

Seok et al. determined the speed of a mesh-based earthworm robot using Equation (3), supposing that if all
other variables were kept constant, speed would be proportional to the wavelength (l) . However,
[22]
-1
experimentally, they found that the shorter wavelength motion was faster at 3.47 mm·s . It was observed
that for longer wavelength (l), fewer units are in a gripping state, so less friction is being created which can
result in slippage. Equation (3) may be considered an accurate calculation of speed for the robot if the
number of units gripping at a moment is enough to keep the robot stable. Hence, it may be argued that the
speed of a robot may be most adjustable if the gripping force of a single unit is sufficient to prevent slippage.
This is particularly difficult in robots made from mesh material as their lattice-like structure does not create
significant contact area with the pipe when actuated, and they tend to be made of low-friction materials
[23]
such as nylon further reducing their wall grip ability.

Many worm-like pipe robots do not have any active bending actuation facility. These platforms rely on their
compliance and the constraints of the pipe environment to guide their movement. However, this can only
work in pipe structures that do not contain complex junctions such as Y and T-bends where the robot is
required to actively steer itself. To create active steering in the robot, soft bending actuators need to be
integrated into the design. Soft bending actuators are made from a variety of materials ranging from
[31]
[32]
[33]
elastomers to electroactive polymers and shape memory alloys . Their actuation can be driven
by pressurised fluids, electric stimuli, or even chemical reactions. Pneumatic bending actuators tend to be
the most popular in worm robots due to their low cost and high power-to-weight ratio.
As earthworm robots tend to be modular or are made from a single piece of material, it is difficult to
integrate steering into the modular design without adding a separate steering unit. Omori et al. designed a
pipe inspection earthworm robot made from two plates connected by flexible belts . Two motor crank
[34]
systems inside the units were used to extend and contract the motor creating expansion and elongation.

To create steering, one side of the robot is extended more than the other [Figure 7], creating a planar
turning motion. While the active turning method was not practically tested in a pipe environment, the robot
was capable of navigating bends of up to 90 degrees by using forward peristaltic motion alone. Multiple
actuators were required for a single unit to enable steering. This design makes miniaturisation difficult;
consequently, the robot had a contracted width of 82.7 mm. In pneumatic earthworm mechanisms, steering
may be integrated by splitting the units into chambers to create a force imbalance. Tang et al. also developed
an earthworm robot with turning capabilities built into the modules by fabricating a single earthworm unit
[13]
from three pneumatic chambers . Three modules were used to create a full robot, and when contracted,
the Tang et al. platform had a diameter of 32 mm . However, when placed vertically, each module could
[13]
only produce a bending angle of 7-9° which could affect its suitability in pipe networks with sharp turns.

The integration of full body steering into the actuation units of an earthworm robot may not always be
required. Zhang et al. demonstrated that a mobile pipe robot can successfully traverse multiple bends,
including vertical T-junctions, using a steering mechanism at the head of the robot . Hence, if there is
[35]
enough driving force from the robot body, steering at the head may be sufficient. This was demonstrated by
Liu et al. who added a soft bending unit onto the top of a modular inchworm robot . The robot was then
[36]
able to navigate more complex junctions such as Y-junctions.