Electroadhesion zipping with soft grippers on curved objects

Abstract

Electroadhesion soft grippers generate extremely high holding forces (> 1 kg per gram of gripper) when the fingers wrap around the object. However, adding actuators on the soft fingers to close them on the object leads to increased stiffness, which reduces the roughness-scale conforming on the fingers to the object, resulting in lower adhesion forces. With the right materials and geometry, electroadhesion forces can alone drive a soft finger to zip on an object without any added actuator. By keeping the soft fingers free from any extra device, electroadhesion zipping combines macroscopic wrapping of fingers around an object with microscopic conforming of the finger’s soft membrane to the object surface. Electroadhesion zipping shares the same mechanism with recently developed soft zipping actuators. Instead of two electrodes on polymer films zipping together and displacing a liquid, in Electroadhesion grippers the soft fingers, equipped with interdigitated electrodes, zip onto a curved object to grab it with high holding forces. In this work we investigate Electroadhesion zipping by reporting a model and a set of experimental data that relate the wrapping angle α with the applied voltage V, for given materials and geometry of object and soft fingers. We discovered that the phenomenon is governed by two voltage thresholds: a first one below which no zipping occurs (V no zip) and a second one above which the soft fingers fully collapse on the object (V full zip). We present analytical equations and design tools that quantify these voltage values for given bending stiffness, mass, shape, and capacitance of the soft finger–object pair. Our model shows that Electroadhesion zipping does not scale with V2/d2 (voltage squared over dielectric gap squared), as previously reported for Electroadhesion forces, but rather with V2/d. Our experiments fit very well with the model results, especially when zipping on dielectric objects with non-tacky surfaces (e.g., paper). We demonstrate thin electroadhesive fingers that wrap around a wide set of objects made of different materials (from conductive to dielectric) and with different geometries, reaching wrapping angles up to 90°. The new design tools and experimental data will drive the design of electroadhesion soft grippers able to conform to nearly any shape and lifting objects heavier than 1000 times their own weight.