IEEE Robotics & Automation Magazine - December 2022 - 51

sense for small underwater robots. Our scheme includes an
electric sense-based hardware solution and localization
methods. Specifically, we first design a hardware solution,
including electric emitters placed in an underwater
environment and an electric receiver that can be furnished on
a small underwater robot. Then, we propose distributed
emitter architectures for large-scale localization. Finally, we
propose three localization methods to estimate the position
and orientation of the robot. We have conducted four types of
localization experiments for a small underwater robot,
demonstrating the robustness and effectiveness of our
proposed electric sense-based scheme. Our study provides a
novel solution to the localization of free-swimming
underwater robots with a limited payload and also helps
provide insights into large-scale underwater localization.
Underwater Biomimetic Perception
As one of the most popular underwater perception methods,
sonar is inspired by underwater creatures, such as dolphins.
Although sonars can take measurements across long distances
(~2 km), they are sensitive to reflections and reverberations
occurring near a wall or in turbid waters [1]. Another important
underwater perception method is vision, which is informative.
However, vision is more sensitive to lighting
conditions and can fail in dark or turbid waters. To overcome
these shortcomings, researchers and engineers keep trying to
develop new technologies, especially from the perspective of
bionics. In this process, the lateral line system, which has been
found in many aquatic vertebrates, has attracted attention. As
an important sensory system, the lateral line system is composed
of mechanoreceptors and/or electroreceptors [2]. The
mechanoreceptors are direction-sensitive neuromasts containing
hair cells, and they are more sensitive to flow [3]. The
electroreceptors are sensitive to electrical fields and have been
discovered in some fish species that we call electric fish. Further
research has revealed that electric sense is closely related
to various behaviors of electric fish, such as prey, defense, perception,
and communication [4]. Both kinds of receptors are
suitable for turbid and dark water, and both are suitable for
carrying on small underwater robots. Table 1 summarizes the
advantages and disadvantages of the biomimetic perception
technologies we have discussed.
Biomimetic Electric Sense in Underwater Robots
Among the various kinds of electric fish, electric eels (Electrophorus
electricus) are well known for their powerful discharge
capacity of hundreds of volts. They use these powerful
electric currents for catching prey or in defense. Actually,
weakly electric fish (gymnotiforms and mormyriforms) are
more common in nature. They generate only about 10 V for
short-distance electrolocalization and electrocommunication
[4]. For some underwater creatures that have electroreceptors
but cannot emit an electric field, their electric sense is used
for navigation, enabling these creatures to follow the electric
field generated by external electric sources (this is also called
electrotaxis) [6].
Inspired by the weakly electric fish in nature, various
underwater robots with electric sense have been designed for
object estimation, localization, navigation, and so on. In
2008, Solberg et al. realized electric sense-based localization
and size estimation of a sphere using a particle filter (PF) [7].
In 2015 and 2017, Bai et al. and Lanneau et al. independently
realized the estimation of the pose and the shape of an ellipsoid
[8], [9]. In 2013, Lebastard et al. fused the kinematic
model and electric measurements to estimate the robot's
pose and then used the estimated pose for navigation close to
a wall [10]. In 2014, Dimble et al. modeled the electric field
in a straight water tank based on the multiple mirror method
and then proposed a state (lateral position and orientation)
estimation method for realizing navigation in the tank [11].
In 2015, Boyer et al. designed an electric sense-based control
law for their robot " Slender probes " to move toward an
external electric emitter. They then activated multiple external
emitters in series, creating a dynamic electric field to
further realize navigation of a single robot [12]. Recently, we
proposed an electric sense-based localization method for a
small underwater robot, where a single electric emitter
based on dc is adopted. Due to the large dc noise in the
environment and the use of the single-emitter architecture,
the effective sensing distance is only twice the body length
of the signal emitter [13].
However, we observe that most existing electric sensebased
studies are realized by dragging robots on guide rails,
and there are relatively few studies based on free-swimming
robots [13], [14], [15]. Also, most of the studies are focused
on the localization of an external target based on a self-generated
electric field [7], [8], [9], rather than the localization of
the robot itself based on an external electric field.
In this article, inspired by the previous observations, we
aim to design a biomimetic electric sense-based localization
scheme for free-swimming underwater robots in a large-scale
environment (Figure 1). To this end, we extend our previous
work [13] by 1) designing an ac signal-based scheme instead
of the dc one, 2) constructing the distributed electric-emitter
architectures for large-scale localization, 3) exploring and
analyzing the contributing factors of localization in different
scenarios, and 4) verifying our localization scheme in more
complex scenarios.
Table 1. Underwater biomimetic perception
technologies.
Methods
Sonar [1]
Vision [5]
Mechano
receptors [3]
Electro
receptors [6]
Bionic Objects Pros
Dolphin
Many fish
Goldfish,
boxfish
Weakly
electricfish
Cons
Long distance Big size, turbid,
near wall
Informative
Short distance,
turbid, dark
Turbid, dark Short distance,
Flow sensitive
Turbid, dark Short distance
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