Hydraulic Modulation of Silicone Knuckles for Variable Control of Joint Stiffness

Ahstract-This work describes the development of a hydraulic knuckle designed to modulate joint stiffness in an underactuated, underwater gripper. The knuckles are pressurized with water to control their stiffness. Compression and tension characterization showed that the knuckles can provide up to 34 N of resistive force in compression and 47 N of resistive force in tension. Stiffness of the knuckles was found to vary linearly with pressure. A parallel, tendon-driven underactuated gripper was fabricated to explore two relationships: finger configurations vs. knuckle hydraulic pressure and joint stiffness vs. grasp strength. This gripper demonstrated that softer knuckles enable a wrap grasp and stiffer knuckles enable a pinch grasp. Grasp strength testing showed that the planar hand can resist up to 23 N of force at 200 mA motor current, and stiffer grasps can sustain greater pull out forces.