Simplified Drag Modeling for the Dynamics of an Underwater Manipulator

Abstract

The dynamic modeling of an underwater vehicle–manipulator system (UVMS) is still a challenging task, mainly because of nonlinear nature of the hydrodynamic forces acting on the UVMS. This article presents a simplified modeling of the drag forces for the underwater manipulator. The drag force of an underwater body is the function of velocity distribution over the surface of the body. The basis of the proposed drag model is velocity distribution of a rigid link with uniform cross section, which can be uniquely expressed in terms of the velocities of its end points. Therefore, the drag coefficient of the link can be expressed as a function of the velocities of the end points and the ratio of length to cross-sectional dimensions. The drag force was modeled in two parts. First, the drag coefficient was identified with respect to the end point velocities of a one-link manipulator. Second, the identified drag coefficient was used to calculate the drag force. The approach was used to simulate a two-link manipulator in two different configurations. The simulation results were found in good agreement with the experimental results. The proposed drag model does not require any numerical integration or discretization of the links, as in the strip theory used repeatedly by many in the literature. The proposed concept was extended to model the drag force for a ten-degree-of-freedom UVMS and a comparative study was done with the strip theory. The efficacy of the proposed drag modeling and the one based on strip theory was evaluated in terms of accuracy of the simulation results and CPU time.