A novel stiffness-controllable joint using antagonistic actuation principles

The inherent safety of collaborative robots is essential for enhancing human–robot interaction. The primary challenge in creating soft components for these robots is achieving sufficient force and stiffness. This paper presents a joint design for collaborative robots that addresses this challenge by incorporating an antagonistic actuation principle, allowing for adjustable stiffness. The novelty of our variable-stiffness joint lies in achieving a wide range of stiffness variation at any bending angle through antagonistic actuation. This bio-inspired principle results from the activation of two opposing actuation chambers. Compared to existing joints, our proposed joint is compact and utilises a high percentage of soft materials, enabling safer human–robot interaction. The paper outlines the joint’s design and fabrication process, highlighting the feasibility of our innovative concept. Kinematic and stiffness models are introduced to analyse the bending and stiffness characteristics, which are further validated through experimental testing. The stiffness experiments demonstrate significant stiffness changes achievable through the antagonistic actuation principle. Additionally, force experiments reveal our joint can generate 20 N force at a 1.5×105 Pa pressure. A constant force output experiment confirms the joint’s advantages in providing consistent force compared to motors. Finally, a case study showcases how our proposed joint can be embedded in serial robots with variable stiffness capabilities under loading and safe human–robot interaction.