From Free Wrapping to Contact Gripping: Mechanical Response of a Vacuum-Driven Soft Actuator

Soft robots adapt to object shapes through structural compliance; wrapping actuators generate gripping force through continuous enveloping contact. This study presents a vacuum-driven pneumatic helical spring (PHS) actuator and a simplified analytical model linking manufacturing constraints, free shape, gripping force, and axial load capacity. Under quasi-static, uniform-helix, and equivalent linear-stiffness assumptions, angle mapping, geometric compatibility, and incremental energy relate constraint-tube angle, vacuum pressure, and target radius to free shape, contact onset, and total normal gripping force under cylindrical constraints. Friction and target weight provide a simplified axial-load estimate under the assumed Coulomb relation, rather than a strict physical upper bound. Tests on a single prototype show that the model captures shape-tightening and gripping-force trends over the tested range, with a root mean square error of 2.19 N between predictions and indirect estimates of total normal gripping force. Grasping and towing demonstrations show the prototype’s ability to conform to the tested target shapes. The model supports actuator design and operating-condition selection for a given target within the investigated range.