A Novel Under-Actuated Gripper Based on Passive-Locking Mechanism for Stable Gripping Under Environmental Constraints

This paper presents a novel under-actuated twofinger gripper that passively adapts to various environments and maintains its grip posture using a passive-locking mechanism. The proposed mechanism features fingers with three phalanges, each incorporating four-bar and eight-bar linkages arranged in parallel. These linkages perform crucial functions, including maintaining the grip angle and ensuring passive characteristics during pinch grips. Previous grippers with passive mechanisms and three-phalanx fingers faced issues with gripping instability, particularly when changes in the passive joint angle were caused by object inertia or external lateral forces. To address this problem, we propose a new passive-locking mechanism utilizing an eight-bar linkage. This innovative design is engineered to adapt to environmental conditions, establish a secure grip, and maintain the grip angle of the passive joint after the grip is achieved. To demonstrate the advantages of the proposed mechanism, this paper conducts a fingertip force vector analysis and a mobility analysis according to the pinch sequence. It also details the derivation process and principles of the mechanism. The gripper's operational range and gripping force are examined through kinematic analysis and verified by simulation. Furthermore, the study shows that the proposed mechanism effectively responds to environmental constraints, even in environments with obstacles surrounding the object. Comparative experiments with and without a contact bar indicate that the proposed gripper can stably secure an object in scenarios involving swing motions and external forces of approximately $5 N$.