Wire-Storage Pantograph Mechanism for Strain and Force Amplification of a Twisted and Coiled Polymer Fiber

A twisted and coiled polymer fiber (TCPF) is a soft thermal actuator increasingly used in current robotics applications. When implementing TCPF in robotics applications, amplifying the TCPF strain and force is necessary to obtain the desired performance. Therefore, this study proposes a mechanism to amplify the strain and force of a TCPF. Considering the TCPF as a wire, the fixed and movable pulley mechanisms are available to amplify the TCPF strain and force, respectively. However, the amplification mechanism becomes bulky when the fixed pulleys are installed separately from the movable pulleys; thus, the TCPF utility reduces. This study focuses on two characteristics of the pantograph to design a compact amplification mechanism. The first characteristic is that the pulleys on the pantograph diagonal axes can be used as a movable pulley mechanism. The other characteristic is that the pantograph circumference can be a fixed pulley mechanism, as the circumference is always invariant. Based on these characteristics, the proposed mechanism can amplify both the TCPF strain and force. The experiments show that the force is amplified based on the number of loops around the diagonal axes. Although the load and temperature affect the strain amplification, the experiments confirm that the strain is amplified. In particular, depending on the loop number, the strain becomes 5.8 times the TCPF strain while maintaining the force level. Furthermore, changing the loop number, the strain and force are amplified 2.8 times and 2 times, respectively.