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The Twisted String Actuator (TSA) is an innovative artificial muscle that can convert the rotary motion of an electric motor into linear motion by twisting one or more strings to reduce their length. This technology has several advantages for robot applications, such as high transmission ratio, intrinsic compliance, simple structure, and power transmission over long distances. However, it is challenging to obtain the contraction of TSA without external sensors. In this paper, we propose a novel strain-sensing string based on conductive polymer composites (CPCs) to address this challenge. The developed string can estimate the contraction of TSA by measuring the resistance change of the strings, eliminating the need for external sensors.
The strain-sensing string is composed of two parts: conductive polymer composites (CPCs) and a hollow pipe made of Poly Vinyl Chloride (PVC) with high tensile strength. The CPCs consist of multi-walled carbon nanotubes (MWCNT) mixed into a polydimethylsiloxane (PDMS) matrix. The liquid CPCs are injected and cured in the PVC pipe to form the string. Strings with MWCNT concentrations in CPCs of 0 vol%, 0.4 vol%, 0.8 vol%, and 1.2 vol% are prepared.
A series of experiments are conducted to evaluate the mechanical and sensing properties of the strings. The strings are subjected to monotonic stretching, continuous cyclic loading-unloading stretching, bending, and twisting strain. The results show that the resistance of the strings increases gradually with increasing strain and decreases with increasing MWCNT concentration. The strings exhibit a repeatable and stable change in resistance with strain during a continuous cyclic loading-unloading stretching process. Under bending strain, the resistance change of the strings increases with the decrease of bending radius, and a 76.5% relative change in resistance is observed when the bending radius equals the string's diameter. Finally, under twisting strain, the string demonstrates a 97% linearity of resistance change rate versus contraction.
In conclusion, the strain-sensing string based on conductive polymer composites (CPCs) has promising mechanical and sensing properties for self-sensing TSA applications. The proposed technology eliminates the need for external sensors, providing a cost-effective and efficient solution for robotics.
 

conductive polymer composites, twisted string actuator, self-sensing