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Twisted and coiled string (TCS) artificial muscles are recently discovered motor-driven compliant actuators that can consistently generate up to 70% strains. The TCS muscles’ actuation is realized in two sequential phases, namely, twisting and coiling. Actuation in the twisting phase results in smooth linear contraction along the TCS muscles’ length. This behavior is identical to that of the popular twisted string actuators. In the coiling phase, the TCS muscles are overtwisted to form coils that generate large and unique non-smooth contraction of strings. The coiling phase in actuation cycle is underexplored and exhibits unique characteristics: Firstly, at a constant motor speed, twisting of strings generates drastic contraction accompanying larger non-smooth actuation during coil formation as compared to the intermittence between adjacent coil formations. Secondly, evident hysteresis appears during the coiling-induced actuation, likely due to large friction between strings when coiled. Lastly, the muscles actuation transition between twisting to coiling is intricate and largely uninvestigated, especially when they operate under different loading conditions and different motor twisting inputs. In this study, a comprehensive experimental characterization of the coiling-based actuation of the TCS muscles is conducted. Firstly, the load dependence of the TCS muscles’ behavior is examined by applying input cycles under different loading conditions. Secondly, the non-smooth behavior is investigated by using sequences of input motor turns with different frequencies. Lastly, the hysteretic behavior and the properties of transitioning conditions are examined by applying different ranges of the input cycles under different loading conditions. The results serve as a basis for future studies on modeling and control of the TCS muscles’ coiling-based actuation.
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