Piezoresistive Composites in Tension and Compression Using a Cantilevered Beam for Spot Testing and Tactile Sensing

Piezoresistive soft composite materials exhibit a change in resistance when undergoing deformation. This combined with their optical, thermal, and mechanical properties makes these composites good candidates for force sensors. Tactile force sensors have long been studied for applications in healthcare, robot-human interactions, and displacement monitoring. The main goal in this work is to characterize a soft piezoresistive layer in both tension and compression to enable a model system for a piezoresistive tactile force sensor and a characterization platform. However, the mechanisms by which these composites exhibit piezoresistivity are complex and must be characterized before use not only in bulk but at the exact locations where contact is expected. In this paper, a cantilevered beam is proposed as a base-mounted force sensing mechanism. This mechanism allows for characterization of the composites at multiple locations across the sample using a two-probe technique. Multiwalled carbon nanotubes (MWCNTs) are mixed by weight with a soft polyurethane in 15,16, and 17 wt. % concentrations. Because the elastic modulus of the piezoresistive layer is not known, indentation tests using Hertz theory and numerical calculations are used to simulate the effective elastic modulus and average strain. These results are then compared with the experimental stress results. In general, these tests show a greater sensitivity in tension than in compression. However, the difference lessens as the concentration increases. A linear fit is applied to the Delta R/R versus strain graphs to calculate the gauge factors. Each sensor exhibits a positive and negative gauge factor over two different ranges. Delta R/R versus strain graphs for tension and compression show gauge factors between -19 and 24 with the range decreasing with increasing MWCNT percentage.