Responses of Carbon Nanotube/Polydimethylsiloxane Composite-Based Soft Resistive and Capacitive Strain Sensors Under Fast, Dynamic Loads

Sensors made from soft polymer composites help intelligent systems become more functional and robust, enabling new applications in which conventional rigid devices may be unsuitable. Although soft sensors have been well studied under quasi-static loading conditions, their performance and effect of sensor transduction under large-amplitude strains (≥10%) at higher frequencies (≥10 Hz) have been less explored. We carried out a comprehensive study of both piezoresistive and capacitive strain sensors made using polydimethylsiloxane (PDMS) and carbon nanotube-doped PDMS. The effects of viscoelastic stress relaxation on the sensors' responses were evaluated and captured using a generalized Maxwell model, which was then used to predict higher frequency mechanical behaviors. The sensors were then characterized under uniaxial tensile loads with three load amplitudes and 11 frequencies ranging from 0.01 to 40 Hz. Our results revealed that capacitive sensing is less sensitive to both loading conditions compared to resistive sensing due to its dependence on changing sensor geometry instead of the electromechanical properties of the soft polymer composite. New results from dynamic loads over 10 Hz also provide evidence to support previous hypotheses regarding the nonlinear behaviors of soft piezoresistive sensors. We expect these findings can serve as a framework for improved deployment of soft sensors in more dynamic robotic systems.