Design and depth control of a beaver-inspired underwater robot based on center-of-mass adjustment

To enhance the adaptability and maneuverability of underwater bionic robots in complex environments, this study proposes a novel bio-inspired robotic design based on the beaver as a biological prototype, emulating its key locomotion mechanisms. The robot incorporates a flexible spine, an adjustable pitch angle mechanism, and actively actuated webbed feet. By integrating the dynamics of the pitch angle adjustment system with the propulsion actuation system, a comprehensive dynamic model is established. For precise depth control, a hierarchical sliding mode control (HSMC) strategy enhanced by a nonlinear state observer (NSO) is developed. The NSO estimates forward velocity from depth measurements, enabling the HSMC controller to cooperatively regulate propulsion frequency and pitch angle displacement. Experimental results demonstrate that the proposed method achieves high-precision depth control with a steady-state error of less than 0.05 m, significantly improving the robot's stability and robustness under varying hydrodynamic conditions.