Introduction of Soft Actuator – Twisted String Actuator (TSA)

Twisting String Actuator (TSA) is powerful and compact linear actuator, in which a string or a set of strings connected to an electric motor act as a gear. When a load is attached to the string on the other end, the rotation imposed on the string by the motor will reduce the length of the string, thus causing the translational motion of the load due to the generated pulling force. Such actuators can be very light, cheap and quiet and provide a high (though nonlinear) transmission ratio, which makes them very attractive for use in various areas of engineering, such as robotics.

Researchers: HyeonSeok Seong, JaeHyung Jang, Seung-Joon Baek, Euibin Shin, Shubhranil Sengupta, Seung-Lyeol Yi, Usman Khan.

Selected Publications

•  Gaponov, I. and Popov, D. and Jee-Hwan Ryu “Twisted String Actuation Systems: A Study of the Mathematical Model and a Comparison of Twisted Strings,” Mechatronics, IEEE/ASME Transactions on, vol.  19, pp.  1331-1342, Aug 2014. [Link]

•  Simeon Nedelchev, Igor Gaponov, and Jee-Hwan Ryu, “Accurate Dynamic Modeling of Twisted String Actuators Accounting for String Compliance and Friction,” in IEEE Robotics and Automation Letters, vol. 5, no. 2, pp. 3438-3443, 2020. [Link]

•  Simeon Nedelchev, Valeria Skvortsova, Boris Guryev, Igor Gaponov, and Jee-Hwan Ryu, “On Energy-Preserving Motion in Twisted String Actuators”, in IEEE Robotics and Automation Letters, vol. 6, no. 4, pp. 7406-7412, 2021. [Link]

TSA based Exoskeleton for lifting tasks assistance

A compliant cable-driven exoskeleton for hip assistance in lifting tasks is aimed at preventing low-back pain and injuries in the vocational setting. We used a twisted string actuator (TSA) to design a light-weight and powerful exoskeleton that benefits from inherent TSA advantages. We have noted that the nonlinear nature of twisted strings’ transmission ratio (decreasing with twisting) closely matched typical torque-speed requirements for hip assistance during lifting tasks and tried to use this fact in the exoskeleton design. Hip-joint torque and speed required to lift a 10-kg load from stoop to stand were calculated, which gave us a baseline that we used to design and manufacture a practical exoskeleton prototype. Experimental trials demonstrated that the proposed device was capable of generating required torque and speed at a hip joint while

weighing under 6 kg, including battery. In the nearest future, we are planning to conduct a more thorough experimental evaluation and verification of the exoskeleton’s performance via a cohort study. We are also looking into development of intuitive human-machine interface so that the wearer can adjust motion range and speed in real time.

Researchers: HyeonSeok Seong, Shubhranil Sengupta, Seung-Lyeon Yi.

Publications

•  Hyeon-Seok Seong, Do-Hyeong Kim, Igor Gaponov and Jee-Hwan Ryu. “Development of a Twisted String Actuator-based Exoskeleton for Hip Joint Assistance in Lifting Tasks”, 2020 IEEE ICRA. [Link]

Active-type continuously variable transmission system based on a Twisted String Actuator

external load, even during operation. Consequently, it allows an increase in the operating range of the TSA while maintaining a high contraction speed.

Researchers: JaeHyung Jang, Seung-Joon Baek, Euibin Shin.

Publications

•  JaeHyung Jang, Young-Uk Song, and Jee-Hwan Ryu, “Active-Type Continuously Variable Transmission system based on a Twisted String Actuator”, in IEEE Robotics and Automation Letters with ICRA 2022 option, vol. 7, no. 2, pp. 2605-2612, 2022. [Link]