Centipedes and robots swim forward using direct body waves
Water Surface Swimming in a Centipede and its Robophysical ModeL
Robotics
Summary
Swimming multi-legged robots struggled to match their walking abilities because their bulky legs made it hard to swim efficiently. The researchers observed that both a centipede species and their robots actually moved forward on water by generating body waves that traveled in the same direction as their movement, called direct waves. They built a robot model to test how leg stiffness and movement coordination affect swimming, finding that more flexible legs improve swimming distance. This work helps explain how many-legged creatures swim and offers ideas for designing robots that can move on land and water.
What this means in practice
- •For field robot designers: Design multi-legged robots that can swim on water surface by coordinating leg movement and body waves for improved aquatic mobility.
- •For biomechanical engineers: Develop biomimetic models inspired by centipede swimming to study leg and body interactions in multi-limbed locomotion.
Authors
Zhaochen J. Xu, Delfin Aydin, Abdullah Mustafa, Margarita B. Levin, Jianfeng Lin, Tianyu Wang, Daniel I. Goldman
Abstract
Elongate multi-legged robots use coordinated body waves and distributed legs to move through cluttered terrestrial environments. However, as housing actuators for independent leg control can require bulky body segments, their non-streamlined body and limb structure makes it difficult to achieve swimming capability comparable to their terrestrial locomotor performance. At the water surface, we found that the multi-legged robots we tested unexpectedly moved backward: their body waves traveled in the same direction as their displacement, i.e., swimming with a direct wave. We found similar behavior in the centipede \textit{Lithobius forficatus}, which swims with a direct body wave and periodic leg movement. To study how distributed legs contribute to direct-wave swimming, we analyze animal kinematics and develop a multi-legged robophysical model that allows independent variation of leg morphology and stiffness, body-wave direction, and leg coordination. Robophysical experiments show that direct body waves produce consistent forward motion under the tested conditions and that swimming performance depends on body--leg coordination. Additionally, directionally compliant legs increase displacement from approximately 0.08 to 0.21 body lengths per cycle relative to rigid legs under matched anti-phase actuation. These findings clarify how distributed appendages contribute to surface swimming and establish gait and morphology design principles for extending multi-legged field robots from terrestrial locomotion into aquatic environments.