Depth control of a biomimetic mantis shrimp underwater robot using a gas-driven variable buoyancy system and ESO-BSMC

Biomimetic underwater robots have broad application prospects in marine resource exploration, environmental monitoring, and operations in complex waters. However, the compact structure, limited payload capacity, and restricted energy reserves of small biomimetic underwater robots make conventional thruster-based depth control prone to flow-field disturbance, which is unsuitable for low-disturbance depth-holding tasks. Inspired by the slender and flexible body of mantis shrimp, we design a biomimetic mantis shrimp underwater robot integrated with a gas-driven variable buoyancy sinking–floating regulation system. A nonlinear coupled dynamic model incorporating gas-pressure variation, airbag-volume variation, buoyancy change, and vertical motion is established, and an extended state observers-backstepping sliding-mode control (ESO-BSMC) depth controller is designed to compensate for model uncertainty and external disturbances while reducing sliding-mode chattering. The closed-loop stability is further analyzed by considering disturbance-estimation error and actuator input saturation, and global ultimate boundedness is proved via Lyapunov theory. Simulation and prototype experiments show that the proposed robot and its buoyancy regulation system can realize multi-depth switching and stable depth keeping, while the proposed control method exhibits favorable tracking performance, disturbance rejection, and engineering applicability. The study provides a feasible solution for the design of low-disturbance buoyancy-regulation systems and depth control of small biomimetic underwater robots.