Meet this robotic fish with fins which may help in underwater exploration

Researchers used an artificial muscle which moves in response to change in the temperature around it.
Sejal Sharma
The robotic fish.
The robotic fish.

University of Bristol 

Scientists in the UK have developed a soft robot fish that mimics the locomotion of the aquatic animal and can be used in underwater exploration and monitoring purposes.

In a press release, the scientists explained that the robot fish was fitted with a twisted and coiled polymer (TCP), which is kind of like an artificial muscle that moves in response to changes in the temperature around it. TCPs are lightweight and low-cost devices, which can generate large strokes and output high power.

In the study, the researchers from the University of Bristol optimized the structural design by bringing the TCP at one end of the robot fish closer to the spring on the other side. This allowed the fin at the rear end of the robot to swing at a larger angle and reach new speeds.

Speed is one of the key parameters for soft robotic actuators

TCPs contract upon heating and their contraction and relaxation are limited by the rate of heating and cooling. Meaning, this limits its speed. Its contraction rate can be scaled up by increasing the heating power.

The team ran several static tests and free-swimming tests with the robotic fish prototype. The robot reached a speed of 25.7 mm/s, outperforming previous TCP-driven aquatic robots.

Lead author Tsam Lung You said: “They can be made from very easily accessible materials such as a fishing line and they contract and provide linear actuation when heated up. However, because of the time needed for heat dissipation during the relaxation phase, this makes them slow.”

The team also did a preliminary investigation for higher swimming efficiency by using flexible fin materials. Apart from this, they are also working on exploring alternative designs to increase TCP actuator frequency. 

In the press release, the researchers point out that until now, TCPs were mostly used in wearable devices and robotic hands, but the robot fish opens up more areas of TCP application, such as marine robots for underwater exploration and monitoring.

Study abstract:

Twisted and coiled polymer (TCP) actuator is a promising novel actuator, exhibiting attractive properties of light weight, low cost, high energy density and simple fabrication process. However, coiled polymer actuators have low non-resonant actuation frequencies because of the time needed for heat dissipation during the relaxation phase. This restricts them to applications where frequencies are less than 0.5 Hz. In this paper, we present a robotic fish driven by a novel TCP–spring antagonistic pair at high frequencies in water. By minimizing the distance between the TCP and the spring, the robot achieved a maximum swimming velocity of 25.7 mm/s (11.5\% body length/s) by undulatory flapping of its caudal fin at a frequency of 2 Hz using periodic Joule heating. This demonstrates the highest frequency and swimming speed achieved for a TCP actuator in a practical aquatic application. The design, fabrication and verification of the fish robot, including characterisation of the TCP actuators in air and water, are presented. A study on different fin stiffness is also presented. This paper provides a new route to raising the actuation frequency of TCPs through thermomechanical design and shows the possibility of using TCPs at high frequency in aqueous environments.

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