Accurate modeling and nonlinearity compensation in the speed mode of a twisted string actuator

A twisted string actuator (TSA) is an effective method that can change the rotational motion of a motor into a linear motion, as well as control the speed and stiffness of the actuator. In particular, because the speed mode of the TSA (SM-TSA) can adjust the rotation–linear motion ratio by changing the diameter and length of the twisting shaft, it is a good to increase the usability of the TSA. However, the SM-TSA has a significant limitation in that it demonstrates a nonlinear translatory motion with respect to the constant rotational motion of the motor in terms of its operating principle. To solve this problem, a more accurate modeling method of the SM-TSA should be applied to predict the nonlinearity and compensate for the nonlinearity. Herein, we analyze the tendency of the twisted strings of the SM-TSA and propose a more precise modeling method of the nonlinear SM-TSA. In addition, a nonlinearity compensation algorithm using the proposed modeling of the SM-TSA is developed to linearize the nonlinear translatory motion. Through various experiments of the SM-TSA, we validated that the proposed model exhibits the nonlinearity of the SM-TSA more precisely when compared to the previous model. Additionally, we confirmed that the nonlinearity compensation algorithm using the proposed model can perform more accurate linearization of the translatory motion of the SM-TSA.