X-Claw: An Underactuated Gripper Based on Slotted Eight-Linkage Mechanism for Pinching and Scooping with Environmental Adaptability

Abstract

Conventional rigid grippers struggle to reliably grasp thin, low-stiffness objects such as cards, coins, and metal sheets due to their inability to adapt to object geometry. Active soft grippers offer adaptability but introduce complexity, cost, and control latency. Existing passive adaptive mechanisms attempt to address these challenges, yet they suffer from structural complexity, sensitivity to assembly tolerances, and susceptibility to lateral deformation under load. This paper presents the X-Claw gripper, a passive adaptive robotic finger based on the Slotted Eight-Linkage (SEL) mechanism. The SEL mechanism integrates two nested parallelograms with a slotted rail-slider constraint, inherently constraining the fingertip to a precise vertical trajectory without requiring additional parallelogram components. This novel configuration eliminates the structural complexity and assembly sensitivity found in conventional passive linkages, while the slotted constraint ensures rigid motion transmission free from lateral deformation. The gripper achieves passive mode switching between pinching and scooping through environmental interaction alone, enabled by a beveled fingertip geometry and an extension spring for energy storage and posture recovery. Experimental validation demonstrates the X-Claw's effectiveness in pinch grasping, thin-object scooping, and adaptive grasping in cluttered environments, offering a lowcost, sensorless solution for reliable thin-object manipulation.