IEEE Robotics & Automation Magazine - December 2022 - 53

localization of a stationary robot or a robot under towing, the
position tracking problem for an autonomous swimming
robot, and the kidnapped robot problem, which is much
more complicated. This article may provide insights into the
localization of small underwater robots in a large-scale environment
and help explore the contributing factors of this
new localization technology.
Our main contributions are summarized as follows.
1) We construct an electric sense-based localization scheme,
including the ac-based receiver, ac-based emitters, and distributed
emitter architectures. The localization scheme
enables a small underwater robot to locate itself in a largescale
environment.
2) We propose three localization methods to explore the contributing
factors of biomimetic electric sense-based perception.
The best one is successfully applied to a more
complex localization issue (the kidnapped robot problem).
3) As far as the authors are aware, few studies have considered
the electric sense for free-swimming robots or the localization
of the robot itself by the external electric field.
Hardware Solution to Biomimetic Electric Sense
To realize the underwater electric sense, we first design a hardware
solution, including an electric emitter and an electric
receiver (Figure 2). The emitter located in the underwater environment
generates signals, while the
receiver equipped on a robot continuously
measures the electric field.
According to the receiver's measurements,
the robot can obtain its relative
position and orientation to the emitter.
When there is more than one emitter,
the receiver detects the frequencies of
the signals to tell which emitter(s) the
signals come from. Then, based on the
single emitter, we construct electricemitter
architectures suitable for a largescale
environment and demonstrate
three typical architectures (Figure 3).
Electric Emitter
The electric emitter located in the
underwater environment consists of
two copper electrodes and a circuit. To
generate signals at a certain frequency,
we adopt a direct digital synthesizer
(DDS) in the circuit for the emitter.
The DDS is a low-power, programmable
waveform generator capable of producing
sine, triangular, and square
wave outputs. The output frequency
and phase are software programmable,
allowing easy tuning. In our circuit, we
use three DDSs (the chip model is
AD9833 from Analog Devices), each
of which can generate a sinusoidal
wave with a set frequency. Therefore, an emitter is able to generate
single-, dual-, or three-frequency signals when one, two,
or three DDSs are activated, respectively. In Figure 2, an
example of the single-frequency case is shown, where two
emitters generate single-frequency signals at the frequencies
of f1 and f2, respectively.
Electric Receiver
The electric receiver on a robot is composed of four copper
electrodes and a receiving circuit. To eliminate the influence
of the electric potential reference on the measurements, we
take the electric potential differences between each pair of
electrodes as the measurements. Since the strength of the
original signal measured by the receiver attenuates with
the third power of the distance between the receiver and the
emitter [16], it needs to be amplified and filtered.
To this end, first, the original signal is processed through a
bandpass filter with a maximum gain of 100, which includes a
high-pass second-order Chebyshev filter, a low-pass fourthorder
Butterworth filter, a low-pass second-order Chebyshev
filter, and a high-pass second-order Chebyshev filter. Figure 5
shows the amplitude-frequency response characteristics of
the bandpass filter. It has a stable gain in the frequency range
of 2-7 kHz and suppresses the indoor 50/60-Hz ac signal
very well. Table 2 summarizes the main parameters of the
Simple Form
Figure 3. Three typical distributed electric-emitter architectures. Columns from left to
right denote each configuration's simple form, configuration, and the schematic diagram
of the distribution of electric field intensity. In the schematic diagrams, the yellow
(respectively, blue) area represents the high (respectively, low) electric field intensity.
DECEMBER 2022 * IEEE ROBOTICS & AUTOMATION MAGAZINE *
53
Architecture 3
Architecture 2
Architecture 1

IEEE Robotics & Automation Magazine - December 2022

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