Mechanism and applications of elastic actuators

This PhD thesis explores the mechanics and applications of elastic actuators, with a focus on twisted and coiled polymer actuators (TCPAs) and their integration into robotic systems. TCPAs are fabricated with varying geometric parameters from polymer yarns. The fabrication process includes twisting and coiling these yarns under applied tensile load, followed by a thermal annealing step to fix the coiled shape. TCPAs are also called coiled artificial muscles because they contract when heated, similar to the contraction of biological muscles when stimulated. The thesis first describes a fundamental investigation into the mechanism of TCPAs made using nylon fishing lines. A mathematical model combining the material aspect and mechanics aspect of the coiled shape of the TCPA is proposed. The microstructure model describes the behavior of the highly strained semi-crystalline material after thermal treatment. This model captures the effect of residual strains in TCPA after annealing to fix the shape. The twisted helical geometry of the TCPA is accounted for by the modified Kirchhoff Rod Theory. A proposed strain energy term coupling the bending and coiling enables an equilibrium solution for the twisted helical shapes. The prediction from the theory is validated through tests of TCPA under different heating rates. The model provides insights into the source of the actuation of twisted and coiled structures. The observation from the parametric study highlights the importance of the geometry to the actuator performance. It also provides guidance toward the development of better artificial muscle, quantifying the importance of ductility to the actuation performance, which exploits the strain energy storage. Next, the thesis expands the basic understanding of the actuators into a comprehensive framework for characterizing and modeling elastic actuators in robotic applications. The thesis addresses the gap between the characteristics of elastic actuators and their performance in mechanisms. The commonly measured characteristics of engineering actuators are maximum stroke, blocking force, stiffness of the actuator, and work capacity. On the other hand, the commonly measured behavior of biological muscles is the passive stiffness of the relaxed muscle and the total force of the active muscle as a function of muscle stretch. The thesis reconciles these two characterization measures in a systematic way using a linearized approximation of muscle behavior. The thesis then introduces the concept of "actuator group" where multiple actuators work together while being arranged in combinations of muscles in series and in parallel. Using this framework, the thesis demonstrates the potential use of muscle groups through case studies in robotics. To illustrate the potential of these elastic actuators, the thesis describes insect-scale jumping robots that utilize TCPAs. This effort is inspired by the use of a snapping mechanism in nature to realize power amplification and hence exploits the high work capacity of natural muscles to obtain high accelerations, speeds, and impressive jumping heights. The thesis realized a nature inspired jumping robot through a "dynamic buckling cascade" mechanism, where a beam buckles and then dynamically snaps-through to the other equilibrium position and impacts the ground to launch the robot jumping. Using high-performance TCPA, this mechanism achieves remarkable performance compared to both existing robotic systems and biological jumpers. The thesis describes a systematic approach to muscle selection for such application. This work aims not only to advance our understanding of the complex mechanics involved in elastic actuation but also to provide practical tools for designing and optimizing soft robotic systems. By bridging the gap between material science, mechanical design, and engineering application, elastic actuators can bring a new generation of soft, responsive, and efficient robotic systems with potential impacts across fields ranging from bio-inspired robotics to medical devices.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Wang, Qiong

Contributors dc:contributor

• Tawfick, Sameh
• Gazzola, Mattia
• Hutchens, Shelby
• Wissa, Aimy

• Elastic Actuator
• Soft Robot

Statement dc:rights

• Copyright 2024 Qiong Wang

Language dc:language

eng, en

Handle dc:identifier

https://hdl.handle.net/2142/127326

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/127326