Design of an untethered limbless robot for aquatic locomotion in complex environments

While limbless locomotion in open aquatic environments has been well explored, there has been little work towards a robust platform capable of locomotion in cluttered hydrodynamic environments. In this work, we developed AquaMILR+, an untethered undulatory robot for locomotion in complex aquatic environments. This robot can be used as a robophysical model of anguilliform-style swimmers such as eels and sea snakes. AquaMILR+ features a bilateral actuation mechanism that models musculoskeletal actuation in organisms that allows the implementation of programmable body compliance which induces mechanical intelligence (Wang et al. 2023). Propulsion and stability are further improved with various fins and a tail allowing the robot to travel 0.31 BL/cycle in open water, and ventral rollers grant functional locomotion on the seafloor. Additionally, AquaMILR+ includes a compact depth control system that intakes and expels water in each segment, resulting in fine pitch and depth modulation to more than 1.5 m deep within 10 s. Through purely open-loop control, the robot was able to traverse a dense lattice (25 cm spacing) within 70 s and a more challenging 3D lattice within 165 s. This proficiency in cluttered environment locomotion demonstrates AquaMILR+'s capabilities and suggests a promising system for complex underwater and amphibious tasks such as search and rescue and deep-sea exploration.