Design and Modeling of a Compound Twisted and Coiled Actuator Based on Spandex Fibers and an SMA Skeleton

Abstract

Twisted and Coiled Actuators (TCAs) are a class of new artificial muscles for flexible actuations. However, conventional TCAs based on nylon fibers commonly require high driving temperature, which limits their applications. Although the TCAs based on spandex fibers can produce high strain under low driving temperature, their output force is low. In this work, a Compound TCA (CTCA) based on spandex fibers and an SMA skeleton is developed. The spandex fibers are spirally wound on the surface of the SMA wire with large number of spiral turns, and the SMA skeleton forms a coil, which can produce additional contraction force for the CTCA owing to the shape memory effect of the SMA wire. To obtain the designated CTCA configuration, a new double twisting fabrication process is proposed, i.e., twisting of spandex fibers followed by twisting of the pre-twisted spandex fibers and an SMA wire together. The CTCA will form a compound twisted fiber configuration, which has the merits of low driving temperature, high strain, and high output force. The influence of the fabrication parameters on the stroke and output force of the CTCA is investigated experimentally. The effect of temperature on the thermal expansion coefficient of spandex fibers is quantified through experiments. It is found that pre-stretching of spandex fibers significantly enhances its thermal expansion behavior. Based on the geometrical parameters and the materials properties of the CTCA, a static model that describes the displacement of CTCA with respect to the temperature and load is established. Experiments under different loads are conducted to verify the accuracy of the static model. Simulation and experimental results show that the proposed CTCA achieves superior performance, with the maximum contraction strain of 46.8% and the maximum output force of 4 N at 80 °C driving temperature.