Experiments in the hydrodynamic modeling of an underwater manipulator

In this paper, the hydrodynamic modeling of a cylindrical single-link manipulator undergoing typical robotic slews is investigated. The basis of the model presented here is a 2D potential-flow theory analysis of a cylinder undergoing unsteady motions. This analysis was extended semi-empirically to three dimensions using strip theory. The analysis demonstrated that drag and added-mass coefficients for a single-link arm swinging from one position to another are not constant, but are dependent on how far the cylinder has rotated. Experiments were conducted both to characterize the state-dependent behavior of the hydrodynamic coefficients and to validate the hydrodynamic model for a variety of motions. For these experiments, a circular cylinder (length/diameter = 9.1) was swung about its end through moderate angles (<120 degrees) in a start-stop fashion common to manipulation tasks. By using coefficients identified under the assumption of state-dependent behavior in the single-link arm model, a significant improvement in modeling accuracy over results from standard constant-coefficient models was achieved.