A passive variable-stiffness adaptive gripper for robotic manipulation

Abstract Handling both soft and rigid objects remains a significant challenge for conventional fixed-stiffness robotic grippers. Furthermore, existing adaptive grippers typically rely on active control strategies and sensor-based feedback, which increase system complexity, energy consumption, and maintenance demand. This study presents a novel, low-maintenance adaptive gripper capable of securely grasping objects with a wide range of mechanical properties, without requiring an external active control mechanism. The proposed gripper is a cylindrical, multi-layered structure composed of four curved beams separated by interlayer gaps, enabling passive transition through five discrete stiffness states. Initially soft, the structure progressively stiffens with increasing axial displacement, reaching distinct stiffness levels at specific displacement values. Ultimately, when all interlayer gaps are fully closed, the gripper reaches its maximum stiffness, equivalent to the elastic modulus of the fabrication material. The gripper returns to its original low-stiffness state once the displacement is removed, demonstrating fully reversible passive adaptation. The effective elastic modulus range spans several orders of magnitude, from hundreds of kilopascals (kPa), suitable for handling soft and light objects, to gigapascals (GPa), enabling robust gripping of rigid and heavy ones. Finite element method simulations validate the gripper’s performance, illustrating the five-state stiffness modulation as well as corresponding stress distribution and reaction forces. The gripper is fabricated using three-dimensional printing technology and experimentally tested to validate the feasibility of the design as a proof-of-concept.