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Blewitt et al. Soft Sci 2024;4:13 https://dx.doi.org/10.20517/ss.2023.49 Page 17 of 26

Table 2. Comparison of bending abilities in worm-like pipe robots and demonstrated ability
Robot Bending method Actuation Angles Demonstrated ability in pipes
Integrated inchworm [50] Three vacuum actuators Pneumatic 40 Navigation of straight pipes
[13]
Earthworm Three pneumatic Pneumatic 7-9 degrees per Navigation of straight pipes
chambers are arranged segment
axially
Inchworm with bending Semi-circular bending Hydraulic 90 degrees Navigation of straight pipes and two unconnected
actuator at the top [64] actuator (Safe in medical Also presented a planar tubes placed at 90 degrees to each other
applications) PBA that could do 85
Earthworm [34] Length control from Electric, motor n/a Navigation of straight pipes and L Bends
servo driven
Inchworm Integrated [55] Parallel Bellows Pneumatic Not recorded Navigation of straight pipes and L Bends using
Actuator trajectory planning
Inchworm with bending Three pneumatic Pneumatic 25 Navigation of straight pipes, pipes with gaps,
[36]
actuator at the top chambers pipes of variable diameter and Y junctions
[65]
Inchworm Integrated PBA Pneumatic 120 degrees (two Navigation of straight pipes and a pipe with
actuated chambers) multiple branches, but no evidence of a 90-
90 for one degree sharp turn
It should be noted that “Navigation” here simply means the robot was able to move through and does not suggest autonomy. PBA: Parallel
Bellows Actuator.

There are few examples of worm robots that can navigate two-directional junctions, and though some of the
robots are able to turn L-junctions, there are also many examples of worm-like robots without active
turning also being able to achieve this by simply relying on compliance alone. Moreover, there is yet to be a
robot that demonstrates autonomous sensing to guide turning.

COMPARISON OF MECHANISMS
As shown in Figure 15, there is much similarity between the capabilities of Earthworm and Inchworm
mechanisms. Earthworm mechanisms offer a solution for small-diameter pipe inspection for straight and
bent pipelines in any orientation with little complexity. They have demonstrated a variety of speeds and an
ability to travel in two directions. Their relatively simple design makes them easy and cheap to fabricate and
control, as more than one unit can be actuated at a time. Moreover, sensors such as cameras could easily be
added to the head of the robot to aid inspection purposes. These could be used to identify typical distress
[68]
indicators required in pipe inspection . These devices could further be used to inspect underground gas
pipes which are currently inspected using manually operated instrumentation such as ground penetrating
radar inspection and walkover beacon systems . Earthworm robots offer a remote semi-autonomous cheap
[69]
alternative to this manual method. Without steering, straight pipe mapping is relatively simple for a
tethered robot as the distance travelled can be indicated by the length of the tether. Alternatively, a visual
SLAM algorithm could be used on the camera head. The use of earthworm mechanisms in more complex
pipe networks is limited by difficulties in steering, as the units are optimised for both contraction and
extension; thus, steering is difficult to work into the design. Inchworm robots demonstrate a similar ability
to traverse small-diameter straight pipes vertically and horizontally. However, unlike earthworm robots,
they have the advantage of superior turning ability as the extension unit in an earthworm can be created in
the form of a three-bellow actuator. Nevertheless, there are only a few examples of inchworm robots
navigating pipe networks with sharp turns, and it is unclear in these experiments whether the steering
motion is precalculated given knowledge of the setup or preprogrammed for a known environment. Both
mechanisms rely on the control of expanding or contracting soft actuators in sequence. Though the motion
sequence differs, the control strategies can be considered similar enough to discuss together.