A Novel Disturbance-Rejection Control Framework for Cable-Driven Continuum Robots With Improved State Parameterizations

A soft, or continuum, robot has great potential in many practical fields with confined space, such as manufacturing system, electrical industry and other, due to its flexibility and benign bending attribute. However, its innate nonlinear dynamics and infinite degrees of freedom pose challenges both in modeling and control. How to build a proper model, which can be used to analyze the characteristics of soft robots, and how to design a robustly stabilized controller, are still open issues. In this work, we build a model for cable-driven continuum robots using a piecewise constant-curvature method. An improved state parameterization is proposed to avoid singularities, which produce pathological behaviors mainly on straight configurations of continuum robots. Then, a novel disturbance-rejection control framework is proposed with a tailored sliding mode controller to achieve stabilized control. Aiming at better robustness, we lump all system uncertainties, including unmodeled dynamics, unknown external disturbances, and unknown parameter variations, together as an extended state of the system. A linear extended state observer is then proposed to estimate the total uncertainties online, enabling compensation for their negative effects. Stability analysis and numerical simulations are carried out to show the feasibility and effectiveness of the proposed approaches.