Biomimetic Design of a Soft Robotic Fish for High Speed Locomotion
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greatly reducing the complexity and increasing the efficiency as compared to the
multi-link design. Using this system, a speed of 0.67 m/s could be obtained [19].
Although this system was a major step towards a viable system, the system still
had some limitations. The servo-driven system creates an almost triangular waveform movement, whereas a more sinusoidal waveform would greatly improve the
creation of reverse von Karman vortices [5]. Moreover, the maximum frequency
of the servo-motor is limited. Therefore, the fish is unable to reach the tailbeat
frequency needed to obtain a Strouhal number between 0.2 and 0.4, which is
commonly found in nature and associated with energy efficient locomotion [12].
Our system (shown in Fig. 2(f)) solves the above mentioned challenges, resulting in a higher speed and efficiency. A key innovation of our system is the use of
the continuous rotation of a DC motor to pull the cables connected to both sides
of the active tail segment, instead of the commonly used back-and-forth motion
of a servo-motor. As a result, higher frequencies can be obtained with a more
sinusoidal waveform. Our fish is able to reach speeds up to 0.85 m/s. Hereby,
it outperforms the previously reported fast soft robotic fish by Zhong et al. [19]
with a significant margin of 27%. This significant performance improvement is
an important step towards real-world applications of soft robotic fishes.
This paper is organized as follows: Sect. 2 presents the design of our soft
robotic fish, emphasizing how the various components of the design can be optimized for speed and efficiency. Section 3 focuses the methods and materials used
to fabricate and test our design. The results will be discussed in Sect. 4. Finally,
we conclude this work and discuss future work in Sect. 5.
2 Design
Our biomimetic design (Fig. 3) has a single-motor cable-driven oscillating system, in combination with a passive compliant tail segment to accurately reproduce thunniform swimming. The continuous rotation of the DC motor effectively
pulls the cables connected to both sides of the active tail segment. The use of a
DC motor allows for achieving higher frequencies with a more sinusoidal waveform, leading to improved speed and efficiency.
2.1 Motor
The soft robotic fish uses a DC-motor in combination with a gearbox system for
propulsion, creating a motion as indicated in Fig. 2(f). Two gears on opposite
sides of the motor shaft are rotated in opposite direction, pulling the left and
right cable in a half cycle delay from each other, as illustrated in Fig. 4(a). As
compared to the servo-motor driven system indicated in Fig. 4(b), a DC-motor
driven active body allows for higher oscillation frequencies, and also creates a
more cosine waveform of the active body and thus the caudal fin. In contrast, the
rapid change in direction in servo-motors leads to a triangle-like heave motion.
Hover et al. have shown that sawtooth and square angle of attack profiles are
approximately 20% less efficient than a sine profile [5].
369
greatly reducing the complexity and increasing the efficiency as compared to the
multi-link design. Using this system, a speed of 0.67 m/s could be obtained [19].
Although this system was a major step towards a viable system, the system still
had some limitations. The servo-driven system creates an almost triangular waveform movement, whereas a more sinusoidal waveform would greatly improve the
creation of reverse von Karman vortices [5]. Moreover, the maximum frequency
of the servo-motor is limited. Therefore, the fish is unable to reach the tailbeat
frequency needed to obtain a Strouhal number between 0.2 and 0.4, which is
commonly found in nature and associated with energy efficient locomotion [12].
Our system (shown in Fig. 2(f)) solves the above mentioned challenges, resulting in a higher speed and efficiency. A key innovation of our system is the use of
the continuous rotation of a DC motor to pull the cables connected to both sides
of the active tail segment, instead of the commonly used back-and-forth motion
of a servo-motor. As a result, higher frequencies can be obtained with a more
sinusoidal waveform. Our fish is able to reach speeds up to 0.85 m/s. Hereby,
it outperforms the previously reported fast soft robotic fish by Zhong et al. [19]
with a significant margin of 27%. This significant performance improvement is
an important step towards real-world applications of soft robotic fishes.
This paper is organized as follows: Sect. 2 presents the design of our soft
robotic fish, emphasizing how the various components of the design can be optimized for speed and efficiency. Section 3 focuses the methods and materials used
to fabricate and test our design. The results will be discussed in Sect. 4. Finally,
we conclude this work and discuss future work in Sect. 5.
2 Design
Our biomimetic design (Fig. 3) has a single-motor cable-driven oscillating system, in combination with a passive compliant tail segment to accurately reproduce thunniform swimming. The continuous rotation of the DC motor effectively
pulls the cables connected to both sides of the active tail segment. The use of a
DC motor allows for achieving higher frequencies with a more sinusoidal waveform, leading to improved speed and efficiency.
2.1 Motor
The soft robotic fish uses a DC-motor in combination with a gearbox system for
propulsion, creating a motion as indicated in Fig. 2(f). Two gears on opposite
sides of the motor shaft are rotated in opposite direction, pulling the left and
right cable in a half cycle delay from each other, as illustrated in Fig. 4(a). As
compared to the servo-motor driven system indicated in Fig. 4(b), a DC-motor
driven active body allows for higher oscillation frequencies, and also creates a
more cosine waveform of the active body and thus the caudal fin. In contrast, the
rapid change in direction in servo-motors leads to a triangle-like heave motion.
Hover et al. have shown that sawtooth and square angle of attack profiles are
approximately 20% less efficient than a sine profile [5].