Gaozhang, Wenlong; (2025) Modular variable stiffness joints: towards inherent safe cobots. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Collaborative robots (Co-bots) have undergone significant advancements, allowing for safe human-robot interaction in shared workspaces. Despite this progress, ensuring safety in situations involving sensory limitations and motor control failures during accidents remains a challenge. In contrast, soft robotics provide inherent advantages in safety, stiffness control, and flexibility due to the unique properties of materials like silicone rubber. When combined with modular design, these features make soft robotics a promising avenue for developing safer, more adaptable co-bots. Such advancements could extend co-bots applications across various domains, including domestic tasks, healthcare, industry, and academic research. This thesis addresses the research problem of how to deliver the next generation of collaborative robots that are reconfigurable and stiffness controllable. The focus is on the design and optimisation of critical components, including joints and links, through the use of soft materials, modular architectures, and antagonistic actuation principles. It aims to enhance compliance at human-robot interaction and reduce the force exerted by co-bots upon human-robot collision to reduce damage while offering high adaptability to diverse task requirements. Beginning with the design of Variable Stiffness Joints (VSJs), which incorporate two soft reinforced chambers and a central hinge, variable stiffness is achieved by inflating both chambers, mimicking the motion of traditional joints. This compact, soft structure endowed the joint with the capability of stiffness variation, and a prototype was tested, with theoretical models developed to analyse both motion and stiffness. Next, Variable Stiffness Links (VSLs) were developed, consisting of an inner silicone rubber cylinder and an outer reinforced fabric layer. When combined with the VSJs, these links form Joint-Link Units (JLUs). The JLU's performance was validated through comparison with traditional components, providing evidence of its feasibility as a replacement and offering valuable insights for the soft robotics field. Finally, a modular design was applied to refine these components, enabling flexible configurations of co-bots for a variety of tasks. A complete co-bot system, featuring modular joints and links (modular variable stiffness bending joints - mvsBJ, modular variable stiffness rotational joints - mvsRJ, modular variable stiffness links - mvsLs), was constructed and tested for workspace range and trajectory control. This system was then successfully applied in both healthcare and industrial support scenarios, demonstrating the advantages of compliance at human-robot interaction, the capability of damage deduction upon human-robot collision and multi-task adaption.

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