Page 88 - Read Online

Yu et al. Intell Robot 2022;2:180-99 https://dx.doi.org/10.20517/ir.2022.10 Page 186

Figure 5. Rigid-soft coupled robot fishes in MPF propulsion mode: (A) a robot fish with soft tissue and cartilages. [27] ; and (B) bionic
cownose ray robot fish [28] . MPF: Median and/or paired fin.

2.3. Robot fishes in BCF-MPF propulsion mode
2.3.1. Soft robot fishes
The caudal fin of this robot fish mainly serves a steering function, and the pectoral fins mainly provide
propulsion. Zhang et al. first tried to build a robot fish . The dielectric elastomers (DEs) were attached to
[29]
the elastic frame, and the variable voltage was applied to drive the pectoral fins up and down to generate
forward thrust. A steering electrical servo drove the caudal fin deflection angle for turning. Figure 6A
depicts the position of the pectoral and caudal fins. Unfortunately, the authors did not test the performance
of this robot fish. Li et al., inspired by manta rays, designed a soft electronic robot fish driven by DEAs, as
shown in Figure 6B . The speed of the robot fish was 0.69 BL·s . It could use the surrounding water as an
-1
[30]
electric ground and swim for up to 3 h on a single charge. This thoroughly illustrated the robustness of this
robot fish.

2.3.2. Rigid-soft coupled robot fishes
The caudal fin of this robot fish is able to oscillate significantly and rapidly, allowing for high propulsion
power. Simultaneously, the pectoral fins have multiple degrees of freedom, allowing for great
maneuverability. As a result, the excellent swimming performance of these robot fishes has attracted the
interest of many researchers. This was attempted by Li et al., who created the robot fish shown in
[31]
Figure 7 . The caudal fin of the robot fish had three rigid joints, which ensured its high flexibility. The
pectoral fins could perform rotary motion and forward-backward motion, and the two motions were
completely independent. This robot fish could reach a turning speed of 0.6 radians per second (rad·s ) with
-1
the coordinated propulsion of the caudal and pectoral fins. This provided the robot fish with more turning
[32]
options and higher maneuverability. Zhong et al. designed a new type of robot fish . The caudal fin of the
robot fish was driven by wires, which could be deformed along a chordwise direction or both chordwise and
spanwise directions. Flapping and rowing motions were possible with the pectoral fins. The results show
that, without using the pectoral fins, the turning radius of the robot fish was 0.6 BL; with the pectoral fins,
the turning radius was reduced to 0.25 BL. This clearly had higher maneuverability. These experiments only
tested the turning performance of these robot fishes and did not test their performance in other swimming
types (e.g., straight swimming).