An Inflatable Eversible Finger Pad for Variable-Stiffness Grasping with Parallel-Jaw Grippers

We present an inflatable finger pad that allows regular parallel-jaw grippers to vary their grasp stiffness while maintaining a contact force and contact to non-planar surfaces. An eversible radial bellows structure made of silicone rubber allows the pad to extend to four times its original height and to retract into a rigid pod when not needed. The bellows act as passive universal joints when everted, enabling aerial contact with surfaces inclined by up to 45 degree. The bellow geometry is intentionally nonlinear but avoids bistable configurations to facilitate control. We find that the nonlinear stiffness behavior allows a pair of opposing pads to increase the compliance of a grasp twenty fold, resulting in a total of two orders of magnitude difference between the most stiff and most compliant configuration. Crucially, high compliance can be achieved while exerting a contact force between 0.7 N and 5 N, allowing for compliant but firm grasps. Pads are manufactured using printed molds and sacrificial-mold casting.