Fast Actuating Multi-Helically Heated Twisted and Coiled Polymer Actuator

Abstract

The twisted and coiled polymer actuator (TCPA), promisingly used in wearable robots, soft exoskeletons and prosthesis, retains the advantages of convenience, high energy density, scalable stroke and hysteresis-free. However, as a thermal actuator, its dynamic response is still rather low, not only because of its convection improvable long cooling time, but also due to its long heating time. PID control is used to shorten TCPAs' rising time. However, this improvement is restricted by the upper limit of the heater temperature and has not substantially improve the poor heat conduction between nylon and heater. In addition, the high working current is also an important defect limiting TCPAs' application. In this work, we have revealed that the practical thermoelectric model of a TCPA obeys the law of heat transfer with Dirichlet condition. Under the consideration that TCPAs with symmetrical Dirichlet condition can effectively improve their heating performance, we have proposed a multi-helically heated twisted and coiled polymer actuator (MHTCPA). Through physical and simulated experiments, we verify that the MHTCPA has better actuating performance, requires less current, and that the symmetry of boundary conditions is the principal factor to improve the heating performance. Given the same power limit, the MHTCPA has a 20% smaller time constant, adopting a 24% smaller current to reach a 15% higher temperature and takes 50% less time to finish actuation. With the help of closed-loop control method, the actuation of the MHTCPA takes 83% less time compared previous TCPA. For the mechanical properties, the MHTCPA is still hysteresis-free but 25.8% stronger in lifting a 100-g weight.