Stable Insertion of Push-Driven Slender Articulated Robots Through Sequential Unilateral Bending Using Preloaded Passive Joints
Slender robots are essential for search-and-rescue operations in disaster sites. While actively articulated slender robots can achieve follow-the-leader motions suitable for confined spaces, they are difficult to miniaturize and lighten. Conversely, soft robots with passive joints offer simpler structure but often suffer from low durability and poor self-weight compensation. This study proposes a novel design approach for slender robots utilizing passive joints with preload torque. By designing a monotonic torque distribution, stable push-driven insertion is achieved through environmental contact. This mechanism enables sequential unilateral bending, where bending propagates from the tip to the base without buckling, facilitating wall-following propulsion without the need for active control. A static model was formulated to derive the necessary conditions for this motion, demonstrating that preload torque must increase monotonically from the tip to the base. Power-law and exponential distributions were found to satisfy these requirements within specific parameter ranges. Quasi-static simulations in both single- and multiple-contact environments validated these theoretical predictions. Furthermore, prototype experiments demonstrated contact-based propulsion, confirming that a power-law torque distribution capable of compensating for self-weight ensures stable performance. The proposed method reduces actuator requirements while maintaining high operational stability, offering a promising approach for exploring confined spaces.