Robust control algorithm using time delay estimation for speed mode of twisted string actuator

Twisted string actuators (TSAs) have been used where conversion of the rotational motion of a motor into a translatory motion by twisting two strings to control the length of the actuator is needed, e.g., in robot applications. Speed mode TSA (SM-TSA) improves the translatory motion achieved in previous TSAs by adding a shaft between two strings. However, the nonlinear response of the translatory displacements remains a problem. Modeling has been one approach, but payload changes or disturbances make it difficult to solve the nonlinear response of SM-TSA through modeling. Here, proportional–integral–derivative (PID) control and time delay control (TDC) in SM-TSA are evaluated as feedback mechanisms during translatory displacements. By following the desired trajectory through PID control and TDC under conditions of payload changes and spring disturbances in SM-TSA and evaluating tracking results, we show that both control methods can provide somewhat precise positional control in SM-TSA even if there are payload changes or spring disturbances. However, TDC shows smaller tracking errors and yields more robust performance against payload changes and disturbances compared to PID control.