Twisted and Coiled Polymer Muscles and Structures for Robotics Application
New actuator technologies play a significant role to develop a lightweight, cost-effective, high performance and biomimetic robotic system. In 2014, Haines et al. [Artificial muscles from fishing line and sewing thread. science, 343(6173), pp.868-872] demonstrated the transformation of polymer fibers from fishing line and sewing thread into artificial muscles by twisting & coiling and heat treatment process, which contract in response to thermal or electrothermal stimuli. The twisted and coiled polymer (TCP) muscles can generate giant stroke, demonstrate high energy and power densities, operate silently, and are of great interest for robotics application. This work presents an extensive study of the TCP muscles using experimental methods to establish the relationship of the characteristics of the muscles in response to input parameters such as power, pre-stress, and stiffness. The experimental results in the time domain were evaluated using thermoelectric and thermo-mechanical models. A new artificial muscle mandrel-coiled fabrication apparatus was designed and developed to produce TCP muscles from fishing line and resistance wire. The new fabrication method enables twisting of the polymer fiber without adding twist into the resistance wire. Isotonic test and spring-load test characterization of TCP muscles were conducted to investigate the performance of TCP muscles. In the two test methods, the effect of power input, frequency, external load, and pre-stress on the actuation stroke and the pulling force of the artificial muscle were investigated. It was shown that a quick contraction (within 1 s) can be achieved without sacrificing actuation stroke by using short pulse with high electrical power. The full characterization of TCP muscles provides a guideline for actuator development. In this work, three novel applications of TCP muscle were demonstrated for the first time. The first one is a reconfigurable robot with icosahedral tensegrity structure that was developed using TCP muscles from sewing thread. Rolling motion of the tensegrity robot under a contact pattern 2 (contact with a ground in a non-regular, isosceles triangle) was successfully demonstrated. The second one and important contribution is a compact and low-cost humanoid hand that was powered by nylon artificial muscles made from multifilament nylon 6 sewing thread. Two different designs were presented along with the essential elements consisting of actuators, springs, tendons and guide systems. The hand design utilized a bioinspired design approach by utilizing agonist and antagonist actuation system. A kinematic model for the flexor tendons was developed to simulate the flexion motion and compared with experimental results. Grasping of various objects was demonstrated within 1 s using the robotic hand showing an array of functions similar to a natural hand. The third one is a modular musculoskeletal system based on ball and socket joint for bioinspired robotic system. For this purpose, 3D printed bone-like structure and TCP muscles were embedded within elastomeric skin to mimic natural joints. 3D printing and casting were primarily used for manufacturing the musculoskeletal system and the experimental results showed that the bio-inspired ball and socket joint could deliver a very good dynamic response, promoting TCP muscle application in musculoskeletal system and other biologically inspired robotic system.