First Steps in Modeling Thermal Actuation of Twisted Polymer Actuators Using Virgin Material Properties

Abstract

Artificial muscle systems have the potential to impact many technologies ranging from advanced prosthesis to miniature robotics. Recently, it has been shown that twisting drawn polymer fibers such as nylon can result in torsional or tensile actuators depending on the final fiber configuration. The actuation phenomenon relies on the anisotropic nature of the fibers moduli and thermal expansion. They have high axial stiffness, low shear stiffness, and expand more radially when heated than axially. If a polymer fiber is twisted but not coiled, these characteristics result in a torsional actuator that will untwist when heated. During the fabrication process, these twisted polymers can be configured helically before annealing. In this configuration, the untwisting that occurs in a straight twisted fiber results in a contraction or extension depending on relative directions of twist and coiling. In these ways, these materials can be used to create both torsional or axial actuators with extremely high specific work capabilities. To date, the focus of research on twisted polymer actuators (TPAs) and twisted-coiled polymer actuators (TCPAs) has been actuator characterization that demonstrates the technologies capabilities. Our work focuses here on applying a 2D analysis of individual layers of the TPAs to predict thermally induced twisting angle and fiber length based on virgin (untwisted) material properties and actuator parameters like fiber length and inserted twist. A multi-axis rheometer with a controlled thermal environmental chamber was used to twist, anneal, and test thermally induced actuation. Experimentally measured angle of untwist and axial contraction after heating are compare the the model. In comparing the experimental results with the two dimensional model, it appears that the difference between the 2D model and experimental results can be explained by the longitudinal stresses that develop inside the material. Future work will aim to include these effects in the model in order to be able to use this model in the design of TPAs.

Cavatappi artificial muscles from drawing, twisting, and coiling polymer tubes

“…They are inexpensive, often fabricated from fishing line, and have been demonstrated in areas such as robotics (16)(17)(18), medical devices (19), and active textiles (1,20). In the past few years, our team has focused its effort on understanding the actuation mechanism of TPAs and how TPAs' actuation can be modeled using the material properties of the precursor fibers (21)(22)(23). As part of this work, we found the standard TPA materials to be highly viscoelastic (24)(25)(26) and hygroscopic, which causes their response/performance to depend on moisture content (27).…”

“…Moreover, the viscoelastic and hygroscopic properties of TPAs are strongly affected by temperature changes, and temperature changes are required for actuation. This makes TPAs challenging to model and therefore control (21)(22)(23)(28)(29)(30). Last, thermal activation is generally an inefficient and time-consuming (particularly during cooling) driver.…”

A compliant gripper with integrated twisted-coiled polymer muscles

“…Spring Mechanics Temperature TCP Stroke, Force [23] accounts for this helix angle variation in his model, as well as allowing for both axial and radial expansion of the fibre -see Fig. 3.2.…”

“…Radial (r), tangential (θ), axial (z) and molecular chain coordinates (1 and 2 directions) are shown. [23] In the virgin (untwisted) fibre axis, ε t 11 is the strain in the axial direction and ε t…”

“…Shafer et al[23] adopted a different approach; their model accounting for the axial contraction and radial expansion of the fibre. Unlike Aziz, the Shafer model allowed for differing molecular chain bias angles which vary with the fibre radius as illustrated inFig.…”