Active Stiffness Control of a Supportive Continuum Robot

2026-08-04

AI Summary

This study addresses the challenge of online task-space stiffness adjustment in supportive continuum robots, whose passive stiffness is fixed and constrained by closed-chain kinematics, limiting adaptability to varying payloads. To overcome this limitation, the work proposes an active stiffness control framework that, for the first time in such robots, enables virtual Cartesian spring stiffness modulation based on position error feedback. A geometrically varying strain model captures the closed-chain dynamics and projects them onto the constraint-consistent motion subspace. By integrating sliding-mode control with Lyapunov-based stability analysis, the approach simultaneously achieves precise end-effector positioning and strict enforcement of closed-chain constraints. Simulation and experimental results demonstrate that increasing the stiffness gain significantly reduces payload-induced end-effector deflection, thereby enhancing directional stiffness, disturbance rejection, and positioning accuracy.

Abstract

Supportive continuum robots (SCRs) enhance the load-bearing capability of an operative continuum robot by mechanically coupling it with a supportive arm. However, their passive stiffness is determined by the mechanical configuration and cannot be adjusted online for varying payloads or interaction forces. Active stiffness control is therefore needed to regulate the load response and maintain positioning accuracy. Meanwhile, the closed-chain structure introduces kinematic constraints that complicate task-space regulation and stiffness control. This paper presents an active task-space stiffness control framework for a tendon-driven SCR. An existing geometric variable strain model describes the closed-chain dynamics, which are projected onto the constraint-consistent motion subspace. A projected sliding mode controller regulates the operative arm tip while preserving the constraints, and closed-loop stability is established through Lyapunov analysis. After position regulation, active apparent stiffness is introduced through a virtual Cartesian spring based on position-error feedback to shape the force--displacement response. The framework is evaluated in simulation and experimentally validated under prescribed external loads and different desired configurations. Results show that increasing the commanded stiffness gain reduces load-induced tip deflection and increases apparent directional stiffness, thereby improving load resistance and positioning robustness under external loading.

Problem

• continuum robot
• active stiffness control
• closed-chain kinematics
• load-bearing capability
• task-space regulation

Innovation

• active stiffness control
• continuum robot
• closed-chain dynamics
• sliding mode control
• virtual Cartesian spring