Design Improvement of a 5-Bar Linkage-Based Needle Insertion Robot for Pain Intervention Procedures

Accurate needle positioning in robot-assisted pain intervention requires kinematic accuracy, structural stiffness, and stable motion transmission under external loading. Our earlier needle insertion robot, based on a spherical five-bar remote-center-of-motion mechanism, exhibited structural deformation and rotational play during repeated bidirectional actuation. This study redesigned the rotational joint while preserving the architecture and degree-of-freedom configuration. Joint geometry was modified to enlarge the supporting contact area by approximately 4.2-fold, and a friction-based keyless shaft coupling was introduced to improve shaft fastening and suppress rotational clearance. An equivalent stiffness model related vertical needle guide loading to structural displacement and rotational joint compliance. Comparative experiments evaluated structural stiffness, rotational lost motion, and phantom insertion stability. The improved design exhibited smaller steady-state displacement under vertical loads of 0.5–2.0 N, reducing displacement to approximately 40–52% of the previous design; linear force–displacement analysis yielded stiffness values of 0.839 and 1.345 N/mm, an approximately 1.60-fold increase. Rotational lost-motion tests showed an increased mean output-to-input peak ratio, indicating improved transmission. Phantom insertion tests, performed under matched insertion-force conditions, showed significantly reduced peak lifting distance for the improved design across all tested insertion angles. Overall, the redesigned interface reduced structural compliance and improved needle guide stability without altering the fundamental robot architecture.