Active Use of Restoring Moments for Motion Control of an Underwater Vehicle-Manipulator System

This paper proposes a framework for actively using the restoring moments of an underwater vehicle-manipulator system (UVMS) considering both kinematic and control aspects. The kinematic aspect concerns redundancy resolution of the UVMS where the redundant degrees of freedom are used to selectively optimize the restoring moments. For this, a performance index with variable gradient gain is newly proposed, in which the gain is determined by the result in the comparison of the task direction with the direction of the restoring moments. The control aspect concerns compensation of the restoring forces and moments. In this framework, the control input makes up for the difference between the performances due to the desired dynamics and the restoring moments. This is accomplished by compensation of the restoring forces and moments, which are consistently updated under certain constraints. In addition, the compensation and optimal proportional-integral-derivative (PID) control are merged into a robust adaptive control. The proposed framework requires only masses, buoyant forces, and centers of gravity and buoyancy, not any hydrodynamic parameters. Numerical simulations are presented to demonstrate the performance of the proposed framework, in which a UVMS can perform specific tasks with less control input and achieve smaller tracking errors compared to conventional control systems.