Modeling and Inverse Compensation of the Non-Smooth Coiling-Induced Actuation in Twisted and Coiled String Actuators

Twisted and coiled string (TCS) actuators are recently discovered motor-driven compliant actuators which generate actuation in two sequential phases, namely, twisting and coiling. TCS actuators' coiling-induced actuation is under-explored and exhibits unique characteristics: At a constant motor speed, twisting of strings generates contraction at increasing rate as the coil evolves from the coil initiation stage to the coil formation stage. As the coiling process is discrete with individual coils forming sequentially, the aforementioned behavior generates non-smooth input-output correlation. To facilitate the application of TCS actuators, modeling and control of their behavior needs to be established. As a first step towards this objective, in this study, modeling and compensation for the unique non-smooth behavior of the TCS actuators in the coiling phase is presented. The proposed strategies were formulated by relating the bias angle of each coil to the corresponding motor twist inputs. The proposed strategies were experimentally validated to perform satisfactorily: The model validation and control errors with the proposed strategies were 1.27 %, and 1.11 % of the overall actuation range, respectively.