Abstract:
Closed loop control of an unmanned underwater vehicle (UUV) in the dynamically
difficult environment of shallow water requires explicit consideration of the highly
coupled nature of the governing non-linear equations of motion. This coupling between
an UUV's six degrees of freedom (6 DOF) is particularly important when attempting
complex maneuvers such as coordinated turns (e.g. simultaneous dive and heading
change) or vehicle hovering in such an environment. Given the parameter and modelling
uncertainties endemic to these equations of motion, then a robust 6 DOF sliding controller
employing six-element vector sliding surfaces provides a framework in which satisfactory
UUV control can be achieved in shallow water.
The vehicle equations of motion are developed and cast in a form that is amenable
to non-linear sliding control design. A complete 6 DOF sliding controller with vector
sliding surfaces is then formulated via a Lyapunov-like analysis. The sliding controller
is then modified via a weighted least-squares approach to work with a particular UUV
which has only 4 DOF control authority available. The modified controller is shown to
work well for a variety of commanded UUV maneuvers in the presence of significant
environmental disturbances and vehicle hydrodynamic parameter uncertainties via
numerical simulation. Use of the signals generated by the controller are shown to be of
utility in vehicle buoyancy control.
