Electroadhesion for Soft Robotics: advancements in Soft Gripper applications

This thesis advances the field of electroadhesion applied to soft robotics, with focus on electroadhesion soft grippers. The main contribution of my work regards novel insights into the influence of mechanics on electroadhesion. In the first part of the Thesis, I explored how mechanics plays a critical role in the performance of current designs of electroadhesion soft grippers. In the second and last part of Thesis, I investigated an alternative approach for the realization of a novel electroadhesion soft robotic gripper embedding hydrogel into its structure. Despite extensive research on electroadhesion based devices has been conducted in the past, only recently the role of the mechanical features of the system has been discovered and outlined. This is particularly important for robotic devices such as electrostatic and electrohydraulic actuators, electrostatic clutches and electroadhesion grippers. In this Thesis, I focused mainly on grippers. First, I explored the relationship between mechanical and electrical parameters of the grasping system and how it influences the wrapping capabilities of an electroadhesion gripper. Despite some current designs involve the use of external actuators to ease the grasping of objects with complex shapes, passive wrapping around objects can be reached under certain conditions. In the latter case, the wrapping is based on the phenomenon of zipping, already exploited in soft electrostatic actuators. Our work allowed us to discover that the zipping of gripper’s fingers on a curved object is ruled by two voltage thresholds, depending on the characteristics of the system. We experimentally validated the theoretical model and observed that actual behavior is predicted by the model, even if further investigations are needed to clarify what happens under certain conditions. The outcomes of this investigation are the starting point for determining how even more complex shape influence the object wrapping and to design grippers able to reliably grasp even the most complicated objects. The results can be useful for the advancement of electrostatic and electrohydraulic actuators as well. The shape of the object also influences the maximum holding force of an electroadhesion gripper. When it grasps curved objects, the geometry of the object produces an exponential increase in the adhesion force. The effect has been observed in previous works, but no theoretical or systematical experimental investigations were conducted. I produced a model that considers the shape of the object and dramatically reduces the error in force prediction if compared with previous models. We also conducted a preliminary validation of the model. The first results confirm the validity of our hypotheses. Further work is required to fully validate the model. The results of the study are valuable not only for determining the capabilities of an electroadhesion soft gripper. They can easily be employed to guide the design of novel soft robotic devices such as electrostatic clutches. We aim at producing electrostatic clutches able to reduce requirements in terms of applied voltage or increase the output force, being equal other parameters. The final part of the thesis presents the preliminary results conducted on the modulation of hydrogel friction obtained by applying voltage to it. Previous works demonstrated that hydrogel friction and adhesion respond to the application of relatively low voltages. We aim at exploiting this effect to produce the first electroadhesion soft gripper made by hydrogels. I started by investigating methods to increase water retention of hydrogel and to obtain bonding to external surfaces. Fulfilling these requirements is crucial to integrate hydrogels into a soft robot. I then produced preliminary experiments on the modulation of friction of hydrogel with applied voltage, that confirmed the hypotheses. Finally, I produced some tests on archaic prototypes of soft gripper’s finger embedding hydrogel into their structure. The first experiments did not produce the expected results. The work will be carried on by improving the designs of the prototypes. Further investigations for the refinement of bonding and water retention methods will be also conducted.