Twisted and Coiled Polymer Actuators (TCPAs) are a new type of thermally driven fiber artificial muscles with many attractive advantages, including high power-to-weight ratio, large contractions, lightweight, and low cost. However, one drawback is a low force that one TCPA can generate. To fully implement TCPAs in robotic systems, TCPAs must be combined into larger structures to achieve higher forces. A common force amplification strategy, arranging actuators in parallel, can lead to bulky structures, especially for TCPAs. This design strategy differs from that in nature. Nature arranges muscle fibers in angular orientation, which is a space-saving strategy, to build powerful muscles in a slender muscle-like form factor. Subsequently, control of TCPA structures requires a model. Closely packing multiple TCPAs potentially introduces a thermal interaction between the TCPA fibers in a structure that could affect their force production.
This work investigates whether the behavior of a bioinspired TCPA structure can be predicted given a force model of single TCPA fiber. To that end, this work studies the influence of the neighboring thermal effects between TCPAs. The Joule-heated TCPAs were modeled using a Standard Linear Solid model in combination with a proportional contribution by temperature. Experiments were conducted to determine the influence of neighboring effects, by varying (1) the distance between fibers and (2) the number of fibers. Planar structures composed of up to 8 TCPAs were experimentally investigated. The scaled and geometrically-translated Joule-heated TCPA force model was used to compare the mechanical behavior of structures with different conditions.
The tested structures showed no notable difference in force measurements. It was concluded that the varied number of fibers and the distance between fibers in the TCPA structures did not affect force production of individual TCPA fibers, meaning that neighboring effects can be neglected. When heated, however, the thermodynamics of the structures could not be predicted by the TCPA force model, which was estimated in a separate experiment. Adaptations to convection coefficient in the TCPA force model resulted in sufficient prediction of the TCPA structures. The difference in model convection coefficient between experiments highlighted the TCPAs' sensitivity to conditions in the working environment. It was suggested that the TCPA structures up to 8 fibers could be adequately predicted given TCPA force model estimated under the exactly same ambient conditions as used in the structure.
This work provides the basis for TCPA structure modeling to promote the use of TCPAs in the wider force range robotic applications.