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Prehensile tail robot for robust cylindrical grasping
The tail of the spider monkeys (Ateles) functions as an additional limb that can wrap around branches to support body weight, keep the torso stable during dynamic swings, and maintain secure anchoring while moving from tree to tree. Even though it looks simple, the prehensile tail is remarkably effective at grasping cylindrical objects, able to handle a wide range of branch diameters and orientations, not just "easy-to-grasp" shapes.
Inspired by this capability, this research investigates capstan-effect–based wrapping mechanics to achieve robust cylindrical grasping in a prehensile-tail robot. We aim to identify stable, high-load wrapping principles and validate them through a soft-tail prototype. To generate the desired wrapping shapes, we designed a Frenet–Serret–based actuation framework that uses both bending and twisting inputs (curvature–torsion profiles), enabling a wide range of 3D wrapping curves. Ongoing experiments confirm that hanging/grasping performance varies significantly with the wrapping path.
Hanging on an Inclined Bar Is Harder
It is usually harder to hang from an inclined bar than from a horizontal bar. When using a horizontal bar, it only needs to hold up the weight of the body. When using an inclined bar, the system also needs to fight against tangential forces that cause slipping. If the friction coefficient isn't high enough, the normal reaction force needs to be raised by increasing the grasping force. Therefore, performance on inclined contacts can vary significantly depending on the interaction conditions, such as surface friction, contact alignment, and normal force.
Spider monkeys (Ateles), on the other hand, can hang from branches across a wide range of diameters and orientations. Their tails have a hairless, high-friction pad that is similar to a palm. They actively wrap while aligning this pad with the target surface through muscular control, enabling robust anchoring under diverse contact conditions.
Frenet–Serret–based actuation for automatic pad–object alignment
Conventional bending-only tail mechanisms usually have multiple segments with multiple bending tendons per segment to steer the bending direction. As a result, similar to a universal joint (U-joint), the bending plane varies with cylinder orientation and wrapping path, causing the object-contact region (i.e., where curvature is formed) to shift.
Video at 2× speed
In contrast, the Frenet–Serret–based framework keeps the tail's bending plane fixed, similar to a revolute joint (R-joint), and adjusts the direction of the curvature by twisting it, like rotating the base frame. This keeps the contact side of the tail consistent during wrapping, enabling automatic alignment of a one-sided friction pad and contact sensors to the object, similar to the prehensile tail of a spider monkey.
We made a soft robotic tail mechanism that uses two motor actuators—one for bending and one for twisting—to create different wrapping motions. We're using it to figure out which wrapping paths are most effective for hanging on inclined bars.
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