Modeling and design of pre-set stiffness continuum robot

Minimally Invasive Surgery (MIS) through human body cavities requires continuum robots to adapt to physiological structures while maintaining sufficient degrees of freedom for precise operations. However, their clinical adoption has been limited by complex flexible joints and actuation mechanisms. This paper proposes a Pre-set Stiffness Continuum Robot (PSCR), which employs a pair of geometrically optimized superelastic tubes with asymmetric patterns to create joint units of varied stiffness and maximum working angles, enabling pre-programmed motion trajectories. An integrated tendon structure enhances overall stiffness and bidirectional push-pull capabilities. Through mechanical analysis of a single joint unit, we established a quantitative relationship between tendon-driven force and global bending curvature, considering force balance and friction effects. This led to the derivation of mapping relationships among task space, mechanical space, kinematic space, and robot configuration, providing a theoretical foundation for motion behavior prediction. Experimental validation of a PSCR prototype confirmed its kinematic and mechanical models, stiffness, and load-bearing capacity. The single-tendon actuation PSCR enables partial extreme position locking for lumen adaptation while retaining the ability for further motion in its unlocked segments, allowing precise distal manipulation, which offers an efficient and accurate solution for MIS procedures.