A Modular Variable Stiffness Co-Bot System Achieving Tasks Flexibility and Contact Compliance

Enhancing contact compliance and task flexibility is essential for expanding the real-world use of robotic systems. This paper presents a modular collaborative robot system that combines antagonistic actuation with hybrid soft-rigid variable-stiffness components, including a modular variable-stiffness bending joint (mvsBJ), a modular variable-stiffness rotational joint (mvsRJ), and a modular variable-stiffness link (mvsL), supported by an integrated control framework. Experimental characterization shows that the mvsBJ achieves a 55.96° bending range and nearly threefold stiffness variation, from 9.16 to 24.72 Nm/rad, through fluidic pressure adjustment. The mvsRJ achieves 145° bidirectional rotation. The platform is benchmarked across repeated trials in healthcare assistance and industrial support scenarios. In an assistive feeding task, a 3-DoF RBBL configuration operates in a low-stiffness mode, maintaining bounded force-tracking variation with mean error drift below 0.16 N. Collision tests across 40-60 mm/s show peak interaction forces increasing predictably from 3.93 to 5.38 N. In an industrial chamfering task, the 2-DoF RB configuration achieves pressurized rigidity comparable to a conventional mechanical workbench. These results demonstrate plug-and-play reconfigurability with reliable adjustable compliance, providing a novel architecture for diverse physical human-robot collaboration applications.