WorMa: An Undulatory Robot With Center of Mass Regulation via Internal Fluid Redistribution for Amphibious Locomotion

Undulatory robots are capable terrestrial locomotors, yet none have demonstrated locomotion that includes both water entry and exit. We present WorMa, an amphibious undulatory robot that uses internal fluid redistribution as the sole terrain adaptation mechanism, shifting the center of mass between head‐biased, balanced, and tail‐biased configurations through a pump‐driven latex balloon system. Operating with a single open‐loop undulatory gait throughout, the robot traverses flat ground, inclined surfaces up to 19.5°, steps up to 15 cm, and open water. We characterize how discrete center‐of‐mass configurations determine locomotion performance across terrain types through traction experiments, which are informed by kinematic simulations. On inclined terrain, head‐biased mass placement is the only configuration that enables successful slope climbing and is the only configuration that completes water‐to‐land transition. Step clearance, which fails under any fixed mass configuration, succeeds through dynamic mass transfer synchronized with the undulatory wave. The results show that internal fluid redistribution can serve as a mechanically simple and effective terrain adaptation strategy in undulatory robots, enabling amphibious capability without dedicated reconfiguration actuators or gait switching.