At the University of Bristol, a coil-powered robot fish has been developed that could make underwater exploration highly accessible.
Robot fish. Image Credit: Tsam Lung You.
The robot fish was fitted with a twisted and coiled polymer (TCP) to drive it forward, a lightweight, low-cost device that is reliant on temperature change to generate movement, which also limits its speed.
A TCP contracts like muscles while subjected to heat, transforming the energy into mechanical motion. In this work, the TCP utilized is warmed by Joule heating — the pass of current via an electrical conductor generates thermal energy and further heats the conductor.
Allowing the robot fish to reach new speeds is made possible by reducing the distance between the TCP on one side of the robot fish and the spring on the other, which triggers the fin at the rear.
The undulating flapping of its rear fin was measured at a frequency of 2 Hz, which is equivalent to two waves per second. The frequency of the electric current is the same as the frequency of the tail flap.
The study outcomes, reported at the 6th IEEE-RAS International Conference on Soft Robotics (RoboSoft 2023), offer a new route to raising the actuation - the action of causing a machine or device to operate - frequency of TCPs through thermomechanical design and highlight the possibility of using TCPs at high frequency in aqueous environments.
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.
Tsam Lung You, Study Lead Author, Department of Engineering Mathematics, University of Bristol
You added, “They can be made from very easily assessable 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.”
Maximizing the TCP-spring antagonistic muscle pair’s structural design and bringing their anchor points closer collectively enabled the posterior fin to swing at a larger angle for a similar amount of TCP actuation.
Although this requires a greater force, TCP is a powerful actuator with high work energy density and is still able to drive the fin.
So far, TCPs have mainly been utilized for applications like robotic hands and wearable devices. This study opens the door for more areas of application where TCP could be used, like marine robots for underwater monitoring and exploration.
Our robotic fish swam at the fastest actuation frequency found in a real TCP application and also the highest locomotion speed of a TCP application so far. This is really exciting as it opens up more opportunities of TCP application in different areas.
Tsam Lung You, Study Lead Author, Department of Engineering Mathematics, University of Bristol
As part of their next steps, the team plans on expanding the scale and developing a knife fish-inspired TCP-driven ribbon fin robot that can swim in water.