Thermo-Mechanical Modeling and the Application of Coiled Polymer Actuators in Soft Robotics and Biomimetics

Coiled polymer actuators (CPA) are a recently discovered smart material. Due to their large tensile stoke and power densities they are often used as actuators or artificial muscles. CPA’s are fabricated from a polymer fiber, typically nylon, mechanically twisted into a coil or coiled around a mandrel and annealed. When heated over the glass transition temperature they can contract, expand or exhibit torsional actuation, depending on the fabrication method and end conditions. The fabrication and application of CPA is well documented and has made many innovations in the fields of smart materials, soft robotics and the likes. However, there is a lack of knowledge in the modeling of CPA, this is partly due to the novelty of the actuator. To address this problem, a theoretical and experimental investigation of thermo-mechanical response is proposed. An energy and variational methods and continuum mechanics approach is utilized with numerical methods to describe the actuation response. To verify the model, the numerical simulation displacement response is compared to CPA samples that are fabricated and experimentally tested in lab using a dynamic machine analyzer (DMA). The results indicate the proposed model accurately predict the actuation response of the CPA under thermal loading. The numerical simulation and experimental comparison is in good agreement and helps to further understand the underlining cause of the actuation behavior of the coiled polymer actuator. Furthermore, the model can be used in application purposes where the results of the model can be used in designing and optimizing soft robotics using CPA as an artificial muscle. In addition to the numerical and experimental investigation of the CPA’s thermomechanical response, an application in biomimetics is being studied. Biomimetics is an interdisciplinary field in engineering and sciences used to overcoming complex human challenges by designing and fabricating materials and systems modeled after nature. Applications of biomimicry can be seen in many technological advancements such as catheters, hearing devices, and artificial appendages such as arms, legs and fingers. The inspiration for this study is the hydrofoil like structured pectoral fin of the Harbor Porpoise whale. Studies will be focused on understanding the fluid forces acting on the pectoral fin. First and foremost, a highly accurate pectoral fin is fabricated from CT scans of a Harbor Porpoise whale fin. 3D models are obtained using Simpleware ScanIP and post-processed in Autodesk for 3D printing components, which were used to assemble to artificial whale fin. An array of thermally driven Coiled Polymer Actuators (CPA) fabricated from Nylon and heated with Nichrome are used as artificial muscles for actuating the pectoral fin. CPA’s were used for their similarity to biological muscles and are of great interest due to its high specific power and large actuation stroke. A simple control circuit for supplying power to the Nichrome heating wires is developed using an Arduino and motor drivers. The displacement over time of the fin is tested and captured using a laser distant sensor. The fin shows a great displacement response, largely deflecting in both direction relative to its size. The artificial fin was then be further utilized in our studies. The fluid forces imposed on the fin while in motion was measured in a laboratorycontrolled setting. A low-velocity belt driven tow tank was used to displace the artificial fin through water. The tow velocity was varied, and the drag force measurements were taken with and without fin actuation using a cantilever beam load cell. A theoretical derived drag force was compared to the experimental drag data and showed good comparison for the non-actuated fin. Increased drag was exhibited with actuation in both directions when towed through water. This demonstrates the ability of the fin to manipulate is geometry to change the drag force on itself serving as a controllable hydrofoil. We hope to elaborate on this ability and apply it to mechanical designs such as under and above water vehicles.