A Ribbed Hybrid Rigid-Flexible Tail with Graded Stiffness and Anisotropic Friction for Enhanced Robot Locomotion and Fall Damage Prevention

Lizards are capable of climbing stably on various terrains. Their tails are key to this ability. The lizard uses its flexible tail with graded stiffness as a fifth limb and climbing aid. The tail also enables soft landings, preventing injury from falls. Inspired by this, tails have been incorporated into many climbing robots to enhance their mobility, mimicking lizards. These robotic tails are generally classified as either rigid (stiff) or flexible (soft). A rigid tail can provide a large preload for pitch-back prevention but has a limited contact area for surface adhesion to avoid sliding backward on slopes. In contrast, a flexible tail conforms to the terrain’s contours, increasing the contact area and thereby improving surface adhesion. However, it provides limited preload. Therefore, in this study, we propose a novel hybrid rigid-flexible robotic tail (HIFLEX) that achieves a balanced combination of preload and contact area. The tail structure design features double-sided inclined ribs and is divided into three modular segments (base, middle, and tip), with graded stiffness decreasing progressively from the base to the tip. The asymmetric (inclined) ribbed structure allows the tail to generate anisotropic friction, resulting in high adhesion (tail-to-surface attachment) to prevent backward sliding and low friction (tail-to-surface release) to facilitate upward climbing. The proposed tail is attached to a climbing robot via an actuator capable of pressing the tail downward to generate sufficient preload. The experimental results demonstrate that this unique tail enhances the robot’s climbing performance on rough and deformable slopes while preventing damage to the robot during falls.