Dynamic characteristic analysis and hybrid control for an underwater flexible joint-flexible link robotic manipulator considering grasping mass variation

An underwater flexible joint-flexible link robotic manipulator (UFJFLRM) mounted on the autonomous underwater vehicles (AUV) can significantly enhance operating ranges, facilitating precise underwater grasping and inspection operations. However, in flowing water environments, the UFJFLRM is subject to hydrodynamic disturbances, dynamic parameters variation caused by the changes in grasping different object, and inherent joint/link flexibility, resulting in rotational angle fluctuations that degrade control performance. To address this rotational angle fluctuation, this study uses the hybrid control strategy in the manipulators, thereby reducing the UFJFLRM's output angle fluctuation. The UFJFLRM dynamic model in a flowing water environment is established using the Morison formulation and the assumed mode method (AMM), allowing for an in-depth analysis of the manipulator's deformation during the process of grasping objects. To address hydrodynamic disturbances and dynamic parameters variation, the hybrid control strategy is developed that combines the fractional-order disturbance observer (FODOB) and the fuzzy-tuning PI (FTPI) controller. The efficacy of the hybrid control strategy is confirmed via simulations and prototype experiments of the UFJFLRM. Experimental outcomes attest that the hybrid control strategy attains high-precision control for the UFJFLRM. The hybrid control strategy reduces the UFJFLRM mean absolute error (MAE) by 61.3% compared with the PI control strategy.