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Page 14 of 26 Blewitt et al. Soft Sci 2024;4:13 https://dx.doi.org/10.20517/ss.2023.49

axially that causes the pneumatic actuator to expand radially. The relatively simple robot [Figure 12B]
presented by Zhang et al. has gripping units which do not require inflation, and thus, it can reach speeds of
-1
-1
[55]
18.7 mm·s forwards and 17.7 mm·s backwards .
Pneumatically inflated robots face the same difficulties as earthworm ones, where miniaturisation and high
speeds can be difficult to achieve when there are a lot of air supplies to consider. Usually, an inchworm
mechanism will feature at least three inputs to control each unit. Yamamoto et al. reduced this by creating
such a robot made from two chambers [Figure 12C]. The two chambers can be used to inflate
[58]
corresponding grippers at low pressures and both the corresponding gripper and the elongation units at
high pressures. By switching between high and low pressures in each chamber, the robot can achieve
inchworm locomotion. It can produce speeds of 45.5 mm·s in horizontal and 23.7 mm·s in vertical 25 mm
-1
-1
pipes, albeit not as fast as the robot presented in Yamamoto et al. though within a significantly smaller
diameter .
[29]
Lim et al. managed to produce an inchworm robot using one pneumatic line with the rear gripping unit,
elongation unit and front gripping units separated by holes of different sizes . Each hole required a
[59]
threshold pressure for the air to pass through; hence, the inflation of each chamber travels from rear to front
with the deflation from rear to front the same [Figure 13]. This creates the desired peristaltic motion where
higher pressure leads to less time required for a cycle of motion.

-1
The robot developed by Lim et al. can achieve high-speed motion reaching 50 mm·s attained in a 16 mm
pipe, which is significantly smaller than the pipes used in the study of Yamamoto et al. although unlike this
robot it can only perform one directional motion . Gilbertson et al. made an inchworm robot driven by
[58]
hydraulics, similar to the study of Lim et al. [59,60] ; the robot is driven using only one supply line; however, in
this case, the supply is water. Gilbertson et al. use passive valves that release water into the next chamber
when a certain pressure threshold is reached managing a speed of 13.5 mm·s -1[60] .

From Table 1, the conclusion can be drawn that the speed at which the actuation method can be delivered
to the robot influences the overall speed of the robot. Fluid-driven robots can increase speed by introducing
passive valves, reducing tubing, and removing the reliance on solenoid valves. This means that the fluid
must travel less distance to pressurise or depressurise the actuators, reducing the overall time required to
complete a sequence of movement. Passive designs may increase the speed of an inchworm robot but
control over the individual chambers is lost. If more gripping power was required in an inchworm
mechanism where the two gripping chambers were actuated by separate supply lines, the pressure in these
chambers could be increased simply by increasing the inflation time. However, in a passive design, the
entire mechanism would need to be redesigned. This leads on to a larger implication that the robot may be
less adaptable to its environment. Even if sensing were included, the robot would not be able to change its
motion when required due to the over simplicity of its design. Hence, it is difficult to design a stable high-
speed fluid-driven inchworm robot for small-diameter pipe navigation. To overcome this imbalance
between speed and stability, one solution may be to use a different method of actuation altogether. Tang
et al. present an inchworm robot made from dielectric actuators that is considerably smaller than the
[46]
pneumatically actuated alternatives whilst maintaining a fast speed .
This suggests that electrical actuation of soft materials may be required to push the boundaries of speed and
size reduction in worm-like pipe robotics. Soft electric actuation methods are difficult and expensive to
fabricate which may be why there are only a few examples of their use in pipe inspection worm robots.