A Novel Twisted-Winching String Actuator for Robotic Applications: Design and Validation

Abstract

This paper presents a novel actuator system combining a twisted string actuator (TSA) with a winch mechanism. Relative to traditional hydraulic and pneumatic systems in robotics, TSAs are compact and lightweight but face limitations in stroke length and force-transmission ratios. Our integrated TSA-winch system overcomes these constraints by providing variable transmission ratios through dynamic adjustment. It increases actuator stroke by winching instead of overtwisting, and it improves force output by twisting. The design features a rotating turret that houses a winch, which is mounted on a bevel gear assembly driven by a through-hole drive shaft. Mathematical models are developed for the combined displacement and velocity control of this system. Experimental validation demonstrates the actuator's ability to achieve a wide range of transmission ratios and precise movement control. We present performance data on movement precision and generated forces, discussing the results in the context of existing literature. This research contributes to the development of more versatile and efficient actuation systems for advanced robotic applications and improved automation solutions.

Figures (11)

• Figure 1: The TSA/winch hybrid actuator. (A) shows the actuator without all supporting brackets, standoffs, and fasteners, with the blue region highlighting the turret mechanism for twisting the string. (B) shows the same actuator without the turret gear transmission to better see the through-hole shaft that drives the bevel gear train and winch (highlighted orange).
• Figure 2: Cross section of the actuator and experimental setup. Motors (1) and (2) drive the winch and the turret, respectively. Motor 2 rotates a turret (3) via gears to twist the string (4). Motor 1 drives a drive shaft (5) that goes through the gears to rotate bevel gears which in turn rotates the winch (6). The string pulls a linear carriage (7), the position of which is measured by a linear potentiometer (8). The string is redirected over the edge of a table at (9) to a hanging adjustable set of masses that acts as a load force. For the force-torque experiments, (9) is replaced with a load cell that the string is fixed to.
• Figure 3: Line diagram showing all relevant variables.
• Figure 4: Experimental setup for displacement and velocity tests.
• Figure 5: Experimental setup for measuring force.