  Underwater robots play a crucial role in exploring aquatic environments. The
ability to flexibly adjust their attitudes is essential for underwater robots
to effectively accomplish tasks in confined space. However, the highly coupled
six degrees of freedom dynamics resulting from attitude changes and the complex
turbulence within limited spatial areas present significant challenges. To
address the problem of attitude control of underwater robots, this letter
investigates large-range pitch angle tracking during station holding as well as
simultaneous roll and yaw angle control to enable versatile attitude
adjustments. Based on dynamic modeling, this letter proposes an adaptive
integral sliding mode controller (AISMC) that integrates an integral module
into traditional sliding mode control (SMC) and adaptively adjusts the
switching gain for improved tracking accuracy, reduced chattering, and enhanced
robustness. The stability of the closed-loop control system is established
through Lyapunov analysis. Extensive experiments and comparison studies are
conducted using a commercial remotely operated vehicle (ROV), the results of
which demonstrate that AISMC achieves satisfactory performance in attitude
tracking control in confined space with unknown disturbances, significantly
outperforming both PID and SMC.
