Tension Control of Twisted String Actuators in Variation of Stiffness and Original Length of Strings

Abstract

Although several studies have been conducted on the modeling and control of twisted string actuators (TSAs), most have been limited to position control; the studies on tension control are limited. This study proposes an indirect tension control method for TSAs. Through comprehensive loading/unloading experiments involving frequency richness and a wide range of tension inputs, the results showed that the conventional static model is no longer valid with a fixed stiffness and constant original length of the string. Therefore, the proposed method allows the stiffness and original length variation in the static tension model, which is then considered in the control by introducing an estimation method. Second, the proposed method indirectly controls string tension by controlling the rotation angle of the motor. The inverse calculation of the desired rotation angle of the motor to achieve the desired tension is equivalent to adaptively changing feedback gains with respect to the nonlinear varying gear ratio of the TSAs. The proposed estimation method simulated the actual string tension from the low- to high-frequency and from the low- to high-tension regions within a tolerable error range. Therefore, the proposed tension controller demonstrated superior regulating and tracking performance compared with the best-tuned direct proportional-integral-derivative feedback controller in a wide range of desired tensions and bandwidths.