Design and Analysis of a Bending–Torsion Coupled Bio-Inspired Claw Attachment Mechanism for Variable-Diameter Curved Surfaces

Reliable aerial perching on variable-diameter curved surfaces requires both sufficient grasping preload and conformal interfacial contact, whereas conventional underactuated bird-claw-inspired mechanisms primarily accommodate in-plane geometric variations and therefore have limited adaptability to surfaces with spatially varying curvature. This study presents a bending-torsion coupled bird-claw-inspired attachment mechanism for variable-diameter curved surfaces. The key design combines direction-selective compliant joints with a bilateral force-balanced tendon transmission, allowing asymmetric contact reactions to passively reorient individual claw segments. This passive reorientation improves interfacial conformity during grasping and provides more favorable contact and sealing conditions for dry adhesion and vacuum suction. A grasping-stability model was employed to optimize the key structural parameters, followed by mechanical characterization of the compliant joints and attachment tests on smooth acrylic cylinders with diameters of 60-120 mm and cones with apex angles of 20°-60°. The equivalent bending stiffness of the compliant joint was tunable from 0.2 to 1.0 N·mm/°, while high torsional compliance was maintained within the small-angle working range. The maximum circumferential attachment moment reached 1.65 N·m on a 120 mm diameter cylinder and 0.60 N·m on a 60° apex-angle cone. A manually controlled flight experiment further demonstrated successful perching on a 20° conical surface. These results demonstrate that bending-torsion coupled compliance can broaden the geometric adaptation range of underactuated claw-based attachment mechanisms and provide a feasible structural approach for aerial perching on variable-diameter curved surfaces.