Page 86 - Read Online

Yu et al. Intell Robot 2022;2:180-99 https://dx.doi.org/10.20517/ir.2022.10 184

Figure 3. Soft robot fishes in BCF propulsion mode: (A) a robot fish with DEAs [20] ; and (B) Flexi-Tuna [21] . BCF: Body and/or caudal fin;
DEAs: dielectric elastomer actuators.

swimming speed of the robot fish was 0.25 BL·s at 0.75 hertz oscillation frequency. However, the authors
-1
needed to test the fish in different sizes and swimming types (e.g., turning) to figure out how much
swimming ability the fish had. Liu et al. proposed using gas-driven units to simulate muscle fibers of fish
[21]
and successfully designed the robot fish Flexi-Tuna . As shown in Figure 3B, 14 drive units were
symmetrically distributed on both sides of the robot fish’s body. Then, alternating pressure was applied to
the drive units to make the tail oscillate back and forth. According to the results, under the optimal
frequency of 3.5 Hz, the maximum swing angle of Flexi-Tuna was 20° and the maximum thrust was 0.185
N. This research realized the application of artificial muscles in robot fishes and provided new ideas for the
design of soft robot fishes. However, some optimizations, such as variable stiffness design of caudal fin, are
still needed to achieve better swimming performance of robot fishes.

2.1.2. Rigid-soft coupled robot fishes
In recent years, researchers have come up with some new ideas to improve the swimming performance of
this type of robot fish.

The headshaking of robot fishes leads to an increase of water resistance, which in turn reduces their
swimming speed. To address this issue, Liao et al. proposed using two caudal fins rather than a single caudal
fin . Caudal fins were mounted symmetrically on the tail of the robot fish, as shown in Figure 4A. They
[22]
were designed to flap oppositely to offset lateral forces, which in turn prevented the headshaking. The robot
fish had three motions: oscillatory motion, jet motion, and oscillatory and jet cooperative motion. A suitable
motion type could be chosen based on the distance between two caudal fins. This indicated that the robot
fish had great flexibility. According to the experimental results, the robot fish could reach the speed of 2.5
body lengths per second (BL·s ), demonstrating excellent swimming speed.
-1

Researchers have made great progress in mimicking the body structure of fish. Coral et al. created a robot
fish using actuators made of shape memory alloys (SMAs) . As shown in Figure 4B, these actuators were
[23]
bent into a continuous structure to resemble the fish backbone. Bio-inspired synthetic skin was used to
mimic the skin of fish. Nevertheless, the authors only verified the feasibility of this scheme. Zhu et al.
created Tunabot by mimicking the body structure of tuna and mackerel and discussed the influence of
oscillation frequency in depth . The robot fish had a streamlined shape with an elastic skin overlaid on the
[24]
actuator system. Tunabot swam at a maximum tail-beat frequency of 15 hertz, reaching 4 BL·s according to
-1