IEEE Robotics & Automation Magazine - December 2020 - 25

(kilometer-range) mobility. We expect benthic vehicles to
complement AUV operations in small, defined areas where
more accurate exploration and intervention tasks are
required, while AUVs will remain the preferred choice for
wide-area screening. Additionally, the soft arm benefited
from operating from a stable base, and, even with real sea dis-
turbances, effectively manipulated objects within a target area.
A benthic mobile robot from which active controls can be
executed seems to be a practical enhancement for UVMSs.
The soft arm alone is able to exert significant force, with the
qualitative advantages of grasping adaptation and high dex-
terity. The gripper design [32] could be improved to exploit
the full potential of a pneumatic arm: a stiffness-changing
solution, as presented in [33], could possibly improve both
the strength and the compliance.

Our tests, although straightforward, conceptually highlight
the benefit of a continuum soft arm for underwater envi-
ronments. Contact with side walls did not impair the task
completion, and the dexterity of the arm and gripper most-
ly compensated for disturbances and inaccurate positioning
of the robot.
Furthermore, with the contribution of a mobile base, it
was possible to retrieve objects from even very deep and nar-
row apertures. In constrained environments, where the arm
alone was unable to exit, the robot's legs could effectively
retract the arm from the aperture. Unexpected contact was
damped by the passive elastic components of the arm, which
helped maintain the stability of the system as a whole. A rigid
solution would require computation and sensing capabilities
that are often not available for underwater systems.

Protocols D-E
Tasks such as collecting objects from the sea bed require
precise locomotion, arm dexterity to collect and place
items, enough force to lift and free entangled objects, and
a delicate touch from the gripper. The objects selected for
protocol D represented a small variety of heterogeneous
yet relevant categories: fragile objects, plastic litter, and
solid articles. The confined spaces prepared for protocol E
also represent possible man-made environments where
similar tasks have to be performed. Results from these
two protocols highlight the positive interaction between
the legged system and the soft arm. The thruster dynam-
ics of other systems may displace low-density target
objects and damage fragile ones. With the current
approach, we were able to select a slow operating speed
that barely affected the environment and enabled the
robot to precisely approach a target object.
In protocol D, the uncontrolled lowering motion of the
legs at the end of the walking phase influenced the arm's
final position, which suggests the adoption of other stop-
ping strategies, but, in all experiments, the robot was capa-
ble of placing the object within the arm's workspace and
retrieving objects from the seabed. There were some unex-
pected problems during the object release. We recognize
that quasi-floating, low-density objects, such as plastic bags
and fishing nets, are unique articles and poorly investigat-
ed. The dexterity required to free the gripper from ensnar-
ing objects (i.e., those that wrap around the gripper fingers)
is not present in our current solution and in other grippers.
We believe that, as robotic hands and grippers face increas-
ingly complex tasks, the release problem may be the subject
of further research.
Eventually, costly and dangerous tasks that human div-
ers currently perform, such as turning valves, replacing
components, and pushing buttons in confined spaces and
cluttered environments, will become common applications
for underwater robotic systems. However, even these
apparently simple tasks are complex for underwater robots,
and, to our knowledge, no attempts have been made to ana-
lyze underwater manipulation in confined environments.

Conclusions
To our knowledge, this article is the first to document a syn-
ergetic robotic system composed of a benthic legged plat-
form and soft continuum manipulator performing
real-world underwater mission-like experiments. We exam-
ined the workspace extensibility, force, energy consumption,
and grasping ability of the robot under different experimen-
tal scenarios. We found that the soft manipulator's workspace
could be significantly extended by adding a benthic legged
robot as a mobile base. The robot precisely approached and
collected objects without disturbing the undersea environ-
ment, which is a common challenge for traditional screw
propeller-driven ROV systems. The robot could also retrieve
objects from deep apertures in overhang environments. One
current limitation is that the soft manipulator and legged
robot are controlled by separate systems. In the future, we
aim to develop a robot with a fully integrated control system
for both the soft continuum manipulation and the benthic
legged locomotion.
Acknowledgments
This research was supported by the Blue Resolution Project,
which is funded by Arbi Dario spa; the Chinese National Sci-
ence Foundation (grants 61822303, 91848105, 61633004, and
91848206); and the National Key R&D Program of China
(grants 2019YFB1309600 and 18YFB1304600).
References
[1] J. Yuh, G. Marani, and D. R. Blidberg, " Applications of marine robot-
ic vehicles, " Intell. Serv. Robot., vol. 4, no. 4, pp. 221-231, 2011. doi:
10.1007/s11370-011-0096-5.
[2] " The 2018 annual report on EU blue economy, " Center for Coastal &
Marine Studies, Brussels, Belgium, 2018. [Online]. Available: https://op
.europa.eu/en/publication-detail/-/publication/79299d10-8a35-11e8-ac6a
-01aa75ed71a1
[3] G. Antonelli, Underwater Robots, Motion and Force Control of Vehicle-Manipulator Systems (Springer Tracts in Advanced Robotics). New
York: Springer-Verlag, 2006.
[4] B. Siciliano and O. Khatib, Springer Handbook of Robotics. Berlin:
Springer-Verlag, 2008.

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