SPiralRoll: A Novel Adjustable-Stiffness Underactuated 3-DoF Joint with Torsion Springs for Rolling Robots

2026-06-21

AI Summary

This study addresses the limitations of conventional rolling robots, whose joint mechanisms are typically complex and lack tunable stiffness, thereby struggling to balance adaptability, safety, and energy efficiency. To overcome these challenges, this work proposes SPiralRoll, an underactuated compliant joint based on an arc-shaped torsion spring. By integrating dual-motor actuation with a nonlinear elastic deformation mechanism, SPiralRoll enables coupled three-degree-of-freedom motion—rotation, radial expansion/contraction, and axial self-rotation. Notably, this design represents the first application of arc-shaped torsion springs in rolling robot joints, offering advantages such as adjustable stiffness, full 3D printability, and low cost. Experimental results demonstrate that the full-arc configuration provides strong structural support, while the single-arc variant exhibits large deformations and pronounced actuation effects, successfully propelling a spherical robot in both forward locomotion and steering maneuvers, thus validating the feasibility and effectiveness of the proposed compliant joint for rolling robots.

Abstract

Compliant mechanisms are important in robotics because they can improve adaptability, safety, and energy efficiency while reducing hardware complexity. This paper presents SPiralRoll, a novel torsion-spring-based underactuated compliant mechanism for rolling robots and compliant robotic actuation. The mechanism uses arc-distributed elastic members and two motor inputs to realize three physically observable output motions: rotational motion, radial expansion/contraction, and axial spin induced by nonlinear compliant deformation. Two configurations, namely full-arc and single-arc designs, are developed and experimentally evaluated. Beyond benchtop validation, the mechanism is integrated into a spherical rolling robot, where proof-of-concept experiments demonstrate forward rolling and turning. The results show that the full-arc design provides better structural support and smoother deformation, whereas the single-arc design yields larger deformation and stronger inertial excitation, making it more suitable for pendulum-driven rolling locomotion. Overall, SPiralRoll provides a low-cost, compact, and fully 3D-printable solution for underactuated compliant rolling robots and adaptive robotic joints.

Problem

• compliant mechanism
• underactuated joint
• rolling robot
• adjustable stiffness
• 3-DoF

Innovation

• underactuated mechanism
• compliant robotics
• adjustable stiffness
• torsion springs
• rolling locomotion