Highly Durable Bidirectional Joint With Twisted String Actuators and Variable Radius Pulley

Abstract

Twisted string actuation is an actuation concept which uses the twisting motion of strings to convert the rotational motion of a motor into a linear motion. By replacing the heavy, high friction, and high-cost gearheads with light, silent, compliant, and low-cost strings, twisted string actuators (TSAs) can provide powerful tendon-based transmission, making it a unique solution for human–robot interactive tasks. Despite such advantages, TSAs exhibit highly nonlinear force/displacement characteristics when compared to conventional geared motors, thus limiting their performance in bidirectional joints. In order to implement TSAs in a bidirectional joint, two motors had to be used to compensate for the nonlinearities, which eventually increased the control complexity, weight, and overall cost of the robotic system. In addition, TSAs show poor durability, which also limits its applicability. To address these issues, a high-performance bidirectional joint has been proposed by implementing optimized TSAs with lubrication, and a variable radius pulley (VRP). By implementation of specially designed TSAs, we achieved 1) extended service time of the strings. Furthermore, by replacing the conventional circular pulley with a VRP, we achieved 2) improved range of motion; 3) increased payload; and 4) better tracking performance, even with a single motor.