Extending feeling past the e-skin surface: adding proximity sensing to a soft capacitive force sensor

A focus on ‘human-like’ robotics over the past decade has resulted in a wide variety of novel soft sensors, commonly called ‘E-skins’. Skin-like sensors which sense single- or multi-axis force are common, but the inclusion of alternative modalities such as proximity have yet to be thoroughly explored. The addition of proximity to the inside of a robotic hand may enhance the ability to perceive object position and shape for grip. In this work, proximity sensing is incorporated into a three-axis capacitive E-Skin force sensor intended for use on a robotic hand. This sensor is fabricated out of Ecoflex 00-30© with carbon nanoparticle-infused elastomer used to form conductive elements and has a 14 x 14 x 2.5 mm³ total volume without disrupting the underlying sensor functionality; the best-performing design achieved a maximum effective object detection distance of 25.2 mm. The functionality of these conformable proximity sensor designs was also demonstrated on curved surfaces mimicking those expected in E-skin application. The designs were observed to maintain general function down to curvatures of 20 mm inner radius with a 23-69% reduction in maximum signal change compared to mounting on a flat surface but no other significant change to overall approach signal curvature. Each proximity sensor was also demonstrated to act as a sufficient capacitive shield for the underlying tactile sensor when switched to a grounding mode, allowing for a thinner overall multimodal sensor with proximity and 3-axis force detection.