IEEE Robotics & Automation Magazine - September 2019 - 32

floating and submerging. At each time step, the optimized
slider-rocker mechanism was verified to fit the spanwise
flapping cubic curves extracted from the cownose ray with
minimized errors.
The practical motion of every point on the bionic pectoral
foil is a compound movement with a spanwise flapping
motion and a chordwise sinusoidal wave transmission. The
driving laws applied to the servo motors of the front fin ray,
middle fin ray, and rear fin are shown in Figure 4(b). In terms
of the design, this driving law follows a cownose ray to generate the desired driving waves.
The rear-fin ray used only one linkage; therefore, it duplicates the reciprocating movement of its servo motor. As discussed previously, the middle fin ray used the two-stage
slider-rocker mechanism, and the front fin ray used a onestage mechanism. If the middle fin ray can achieve the
required motion, the front fin ray will follow the same rules.
Therefore, the middle fin ray can be used as an example to
estimate the feasibility of the motion.
As shown in Figure 4(a), three key points, i.e., the two
rotating joints and the fin tip, are taken as the targets.
Motion curves calculated in a full-flapping cycle for all three
points selected are shown in Figure 4(c). The motions of the
three key points oscillated in phase with a high level of sinusoidal curves and were similar to the flapping discipline of
the cownose ray's pectoral foil. There was a low percentage
of error between the movements of the designed sliderrocker mechanism and the curve from the cownose ray at
each time step.

Tail Fin R

Tail Fin L

Tail

Rear Fin L

Rear Fin R
Middle Fin R

Middle Fin L
Middle
Body

NACA Series
Flexible Bridge

NACA Series
Flexible Bridge
Front Fin R

Head

Front Fin L

(a)

0.60 m

0.18 m

0.92 m

(b)
Figure 5. The bionic fish prototype. (a) The internal skeletal
structure of the bionic fish without the outer skin and (b) the
dimensions of the bionic fish.

IEEE ROBOTICS & AUTOMATION MAGAZINE

SEPTEMBER 2019

The physical bionic fish is composed of two main parts:
the outer skin-like cover and the inside driving skeleton. The
inside skeleton shown in Figure 5(a) possesses the functions
of propulsion and supports the outer appearance shape. The
middle body, which is a waterproof box made of aluminum
plates and plexiglass, houses the battery, control circuits, driving board, and communication apparatus. Each fin ray,
including the two at the tail part, was driven by an independent high-torque servo. These servos, which can reach a peak
torque of 0.36 Nm when the supply voltage is 7.4 V, were all
covered by a single-sealing box to make them water isolated.
The middle body and the sealing box were filled with compressed air of 0.1 MPa to test the sealing conditions and
strengthen the pressure-resistance capability of the middle
body and the sealing boxes.
The outer shape of the tip linkage of each fin ray fit the
corresponding shape profile of the referenced cownose ray.
The tip linkages are made of carbon fiber plates with a thickness of 1 mm to ensure both the elasticity and support ability
of the outer edges. The flexible bridges connecting the front
fin ray and middle fin ray were designed to support the
chordwise spatial shape and are useful in achieving smooth
foil flapping and passing wave continuity along the chordwise
direction. The bridges were formed by room-temperature
vulcanized, double-component silicon rubber. Their dimensions, including width and thickness distributions, were
determined based on the requirements of realizing a passive
flexible deformation of the pectoral FT parts, as well as the
smooth deformation of the skin covering between each of the
two neighboring fin rays. The inside skeleton was strengthened to withstand water pressure and counterbalance itself
after the sealing work was completed. The bionic fish's outer
flexible skin is made of pervious nylon cloth with an outstanding elasticity of approximately 1.5-2 times that of the
tensile ability. During the consecutive motions combined by
the fin rays, the bridges, the outer skin, and other supporting
parts, the outer skin can fill the structural gaps and make the
movements smoother.
After the second-step counterbalancing, the prototype can
suspend in water without any movement (its designed depth
is up to 30 m). The total mass of the prototype is 6.2 kg after
balancing. The basic mechanical parameters of the bionic fish
are shown in Figure 5(b). Compared to the mature cownose
ray, which is usually roughly 1 m in width, the prototype is a
successful replicate of the cownose ray in nature.
A semiautonomous control method was applied to control
the bionic fish; the general control commands, such as those
for generating linear swimming, turning, and diving, were
sent by a 72-MHz remote control transceiver system. The
specific movement control and driving strategies were integrated in an onboard controller.
Propulsion and Posture Analysis
The propulsion-force-production capability of the bionic
fish prototype as developed was theoretically evaluated by
applying a 1-D calculation method. Based on the motion

IEEE Robotics & Automation Magazine - September 2019

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2019