Tailoring the viscoelastic properties of soft pads for robotic limbs through purposely designed fluid filled structures

The majority of soft pads for robotic limbs studied so far were made by visco-elastic polymeric solids whose behavior is significantly influenced by the rate of application of the external loads or displacements. In particular, contact interfaces which are intrinsically visco-elastic are found, for instance, in human fingers and feet or in various robotic devices covered by a compliant surface. An outstanding instance are anthropomorphic hands where time-dependent phenomena profoundly affects the stability and sustainability of the grasp. Alternatively to homogenous solid pads, this paper proposes the use of fluid filled soft structures with differentiated layer design that is the adoption of a single solid material, dividing the overall thickness of the pad into a continuous skin layer coupled with an internal layer having communicating voids. The voids are then hermetically sealed and, in case, filled with fluid. Given the allowable pad thickness, the purpose is to tailor the pad properties to the specific application by 1) selecting a skin material characterized by proper tribological features, 2) designing an inner layer geometry so as to obtain a specific static compliance, 3) filling the pad with a viscous fluid chosen so as to modify time-dependent phenomena and increase damping effects. The proposed concept is validated by designing artificial pads whose viscoelastic properties are either similar or more pronounced when compared to those of the human fingertip.