14 Platformone A coil-powered robot fish could make underwater uncrewed systems easier to develop and deploy (writes Nick Flaherty). The robot fish, developed at the University of Bristol in the UK, is fitted with a twisted and coiled polymer (TCP) to drive it forward. The TCP is a lightweight low-cost device that relies on temperature changes to generatemovement, but it has been limited in its speed. A TCP works by contracting like muscles when heated, converting the energy into mechanical motion. It is warmed by Joule heating, where the passing of current through an electrical conductor produces thermal energy and heats up the conductor. Minimising the distance between the TCP on one side of the robot fish and the spring on the other activates the fin at the rear, enabling the robot fish to move faster. The undulating flapping of the rear finwas measured at a frequency of 2Hz, or two waves per second, moving the fish forward at 25.7mm/s. The frequency of the current is the same as the frequency of tail flap. A TCP actuator is designed with light weight, low cost and high energy density, and can be fabricated using a simple process. However, the coiled polymer actuators have low non-resonant actuation frequencies because of the time needed for the heat to be dissipated lines, and they contract and provide linear actuation when heated up,” said lead researcher Tsam Lung You from the Department of Engineering Mathematics at Bristol. “However, because of the time needed for heat dissipation during the relaxation phase, this makes them slow. “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,” he said. The team now plan to expand the scale and develop a knifefish-inspired TCP-driven ribbon fin robot that can swim autonomously. Underwater vehicles Robot fishwith a twist June/July 2023 | Uncrewed Systems Technology during the relaxation phase, limiting the speed to under 0.5 Hz. The antagonistic TCP-spring design comes fromminimising the distance between the TCP and the spring. Although this approach requires greater force than other, simpler, designs, TCP is a strong actuator with enough energy density to drive the fin. This is a new route to raising the actuation frequency of TCPs through thermomechanical design, and shows the possibility of using them at high frequency for underwater robots “TCPs can be made from very easily assessable materials such as fishing Propulsion for the robot fish comes from a twisted and coiled polymer, shown as TCP above and running along the side of the fish, below
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