Off-Axis Compliant RCM Joint with Near-Isotropic Stiffness and Minimal Parasitic Error

TL;DR

This paper introduces a novel off-axis compliant RCM joint with near-isotropic stiffness and minimal parasitic motion, validated through modeling, finite-element analysis, and benchtop experiments for neuroendoscopic applications.

Contribution

It presents a new monolithic compliant RCM joint design that improves line of sight, sterilization, and tool release, with validated performance and fatigue analysis.

Findings

Achieved a stiffness-ellipse PAR of 1.37 and PRR of 0.63%.

RCM drift remains below 0.2 mm under 4.5° rotation.

Fatigue analysis indicates a usable rotational workspace of 12.1°-34.4°.

Abstract

This paper presents an off-axis, monolithic compliant Remote Center of Motion (RCM) joint for neuroendoscopic manipulation, combining near-isotropic stiffness with minimal parasitic motion. Based on the Tetra II concept, the end-effector is placed outside the tetrahedral flexure to improve line of sight, facilitate sterilization, and allow rapid tool release. Design proceeds in two stages: mobility panels are sized with a compliance-based isotropy objective, then constraining panels are synthesized through finite-element feasibility exploration to trade stiffness isotropy against RCM drift. The joint is modeled with beam elements and validated via detailed finite-element analyses, including fatigue-bounded stress constraints. A PA12 prototype is fabricated by selective laser sintering and characterized on a benchtop: a 2 N radial load is applied at the end-effector while a 6-DOF electromagnetic sensor records pose. The selected configuration produces a stiffness-ellipse principal axis ratio (PAR) of 1.37 and a parasitic-to-useful rotation ratio (PRR) of 0.63%. Under a 4.5{\deg} commanded rotation, the predicted RCM drift remains sub-millimetric (0.015-0.172 mm). Fatigue analysis predicts a usable rotational workspace of 12.1{\deg}-34.4{\deg} depending on direction. Experiments reproduce the simulated directional stiffness trend with typical deviations of 6-30%, demonstrating a compact, fabrication-ready RCM module for constrained surgical access.