Capacitive Stretch Sensing for Robotic Skins

Abstract

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.

Nonlinear Poisson's Ratio for Modeling Hyperelastic Capacitive Sensors

“…It is well documented that capacitance and strain are linearly related for elastomeric capacitive sensors. [ 6,7,10,15 ] At high strains, inaccurate measurements of the capacitance may occur due to an increasing resistance in the electrodes in combination with the chosen measurement frequency, [ 15,34 ] resulting in a seemingly nonlinear relationship. The nonlinearity is, however, a measurement artifact, and if it occurred in the current work the linear part of the data was extrapolated to 150%.…”

“…Capacitive sensors measure strain based on changes in the geometry of the sensor during stretch. A very common sensor design is the parallel‐plate capacitor, [ 6,7,10,15–17 ] where thin electrode and dielectric layers are stacked. For ideal plate capacitors, the capacitance C is related to the sensor dimensions by where ∈ 0 and ∈ r are the free space and relative permittivity, respectively; L and w are the length and width of the active area of the sensor, respectively; and t is the thickness of the dielectric layer.…”

“…Electrode layers can be made from a composite of elastomer and conductive filler, such as graphite [ 1,6 ] or carbon black. [ 7,15,18 ] The addition of fillers generally reduces the Poisson's ratio of the composite compared to neat silicone [ 19–22 ] due to a mismatch between the Poisson's ratio of the elastomer and of the filler material. [ 19,23 ] Similarly, a mismatch between the Poisson's ratio of the electrode and the dielectric layers is expected to result in a Poisson's ratio of the sensor as a whole in between those of the separate layers.…”

Triboelectric nanogenerator sensors for soft robotics aiming at digital twin applications

“…Based on triboelectric mechanism, Lai et al 20 have developed a self-powered stretchable robotic skin to help soft robot sense and interact with environment via self-generated signals, indicating another solution that has high compatibility with soft robots. Till now, the proposed solutions to achieve perception for soft robotics include photo detection 19 , 21 , triboelectricity 22 , 23 , piezoelectricity 24 , electromagnetic effect 25 , and conductive nanocomposites 18 , 26 . However, some of the above methods are restrained by several drawbacks, e.g., the electromagnetic field is required for Hall sensor 25 and nonlinearity or creep is a material concern in the nanocomposites 18 .…”

Specialized CNT-based Sensor Framework for Advanced Motion Tracking

“…In such cases, stretchable sensors are deemed more appropriate, and active research is continuing in this field. Currently, stretchable sensors are built employing either capacitive [19,20,21], or resistive [22,23,24] technologies. Following this line of research, our team is developing rapid-response, widely stretchable sensors based on carbon nanotubes specialized for the detection of human motions [25,26,27].…”