Self-Sensing for Twisted String Actuators Using Conductive Supercoiled Polymers

Abstract

The twisted string actuator (TSA), as a recently discovered artificial muscle, has attracted a lot of attention as a compliant and powerful actuation mechanism. A TSA consists of two strings attached to a motor on one end and a load on the other end. The motor’s rotation twists the strings and generates linear actuation. A common challenge is to obtain TSAs’ strains using compact approaches. Previous studies exclusively utilized external position sensors that not only increased system cost, size and complexity, but also lowered actuator compliance. In this paper, self-sensing strategies are presented to estimate TSAs’ strains without external sensors. By incorporating conductive and stretchable nylon strings, called super-coiled polymer (SCP) strings, into TSAs, their strains can be estimated from the resistance values of SCP strings. Two self-sensing configurations are realized: (1) TSA with one regular string and one SCP string, and (2) TSA with two SCP strings. Experiments are conducted to show the correlation between the length and resistance of TSA under different conditions. Polynomial and Preisach hysteresis models were successfully employed to capture the Length – Resistance correlation and to estimate TSA’s length using the resistance.