Modeling of One-Ply and Two-Ply Twisted and Coiled Polymer Artificial Muscles

Twisted and coiled polymer (TCP) muscles are recently introduced thermal actuators that offer unprecedented performance. Merits such as low cost, high load capacity, and light weight make them suitable for robotics and biomechanics. Using these actuators for practical applications requires an understanding of their behavior in response to the electrical input. In this article, a physics-based model is proposed to predict the displacement of TCP muscles with respect to the input electrical energy and the applied load. We quantified effects of temperature on coefficients of thermal expansion and elastic modulus. It is found that the change of properties above the polymer glass transition temperature has a crucial effect on the actuator performance. A stiffness model is proposed, which include the effect of change in geometry and elastic modulus of TCP muscles. Finally, a set of equations for simulating the TCP muscles are formulated in discrete time representation. The model is generalized for the two-ply case by considering the difference in geometry. The accuracy of the model is tested in comparison with experimental data, acquired at several actuation levels and applied loads, for both one-ply and two-ply configurations. The model showed a good accuracy in predicting the behavior of the actuators both in temperature and displacement. Our model was also compared with other representative models and revealed improvement in the prediction accuracy.