Understanding piezoresistive soft composites as applied to multi-directional tactile sensing arrays
An improvement in tactile force sensing has far reaching implications in many fields. To enable human robot interaction machines must be able to mimic human dexterity. Piezoresistive soft composite materials are one transduction mechanism of great interest to researchers for this purpose. Flexible piezoresistive films are often used as skin analogs and integrated into complex array sensors for tactile sensing. Each mechanism has distinct advantages and disadvantages, but the optical, thermal, and mechanical properties - combined with cheap fabrication cost - make these composite materials good candidates for force sensors. 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. In this paper a cantilevered beam is proposed as a base mounted force sensing mechanism. The uniformity of the sensor characteristics heavily depends on the homogeneity of the composite. However, the processes are complex and must be characterized before use. Tests carried out at multiple locations yielded consistent sensitivity values, making these types of composites suitable for array type force sensors. Therefore, the ability to locally characterize a film that will be integrated into a complex force sensor could be critical. Here, several methods to characterize the local sensitivity of flexible piezoresistive films is presented. Multi-walled carbon nanotubes and carbon black are mixed by weight with soft polyurethanes in 13% to 18% concentrations. Numerical calculations are used to simulate several aspects of the materials and the sensors. These simulations are then compared to the experimental tests. Results show that lower weight percentage composites exhibit a higher rate of change of resistivity and gauge factor than films of the same thickness with higher percentages. On the other hand, thicker films exhibit higher gauge factors for the two tested carbon black contents. A linear fit is applied to the ∆ R⁄R vs strain graphs to calculate the gauge factors. ∆ R⁄R vs strain graphs for tension and compression show gauge factors between -8 and 10.3 with the range decreasing with increasing MWCNT percentage.
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