Design of a Passive Variable-Stiffness Compliant Joint Based on a Flexible Hinge

Abstract

Variable-stiffness compliant joints are key components for enhancing robot safety and environmental adaptability. Existing compliant joints face challenges in applications such as precision assembly, including fixed stiffness that cannot be adjusted, structurally complex and costly active stiffness-variation solutions. Accordingly, a compliant joint is proposed whose intrinsic structure alone yields adaptively adjustable dual stiffness — low stiffness at small deflection angles and high stiffness at large ones. This joint employs a cantilevered auxiliary-contact flexible hinge as its core elastic component, with the hinge’s deformation guided by geometric constraints imposed by guide slots. The structural dimensions of the hinge were determined through optimization design based on a chained pseudo-rigid body model (CPRBM) and a preset angle-torque curve. Subsequently, the hinge’s stiffness characteristics were verified through simulation, and the shape of the guide slot was determined. To validate the effectiveness of this design scheme and methodology, a physical prototype was fabricated. The joint’s bending characteristics, as well as the hinge’s deformation and stress under bending loads, were obtained through strain gauges on the hinge surface and bending tests. The experiments demonstrated that the measured angle-torque curve of the physical prototype closely matched the preset target curve, confirming the effectiveness of this design method and the rationality of the joint design.