Stretchability is a property that brings versatility and design freedom to human interface devices. We present a soft, flexible and stretchable keyboard made from a dielectric elastomer sensor sheet. Using a multi-frequency capacitance sensing technique based on a transmission line model, we demonstrate how this keyboard can detect touch in two dimensions, programmable to increase the number of keys and into different layouts, all without adding any new wires, connections or modifying the hardware. The method is efficient and scalable for large sensing systems with multiple degrees of freedom.
Various types of artificial skins have been developed to provide robots with a sense of touch. Because of their compliance, dielectric elastomer (DE) capacitive sensors are particularly suitable for soft robots. Although the electrodes of DE sensors exhibit nonlinear effects such as transient resistance changes and resistance peaks, this does not affect the capacitance readout representing stretch, as long as the frequency of the excitation voltage used for capacitance measurement is sufficiently low. At higher frequencies, however, the approximation of a DE sensor with an ideal capacitor and a series resistor accounting for electrode resistivity leads to an underestimation of capacitance in static sensors. We demonstrate how this effect is amplified by peaks and transient changes of electrode resistance caused by periodic stretching. At high frequencies, distinctive capacitance undershoots occurred that correlated with the change of electrode resistance. The close match between a simulation of the DE sensor as an R-C transmission line and recorded data supports the hypothesis of the undershoot having been caused by dynamic electrode resistance changes and the lumped parameter approximation. Our results show that nonlinear responses in DE sensors can be avoided by appropriately adjusting the excitation frequency.
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