Robotic Fish driven by Twisted and Coiled Polymer Actuators at High Frequencies

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.