This note focuses attention on a novel approach to disturbance rejection when the µ-synthesis control procedure is applied to Unmanned Underwater Vehicles (UUVs). Environmental external disturbances simplify to ocean current for a totally submerged vehicle and greatly contributes for hydrodynamical loads and the tether cable disturbance. Our case scenario deals with the incorporation of the sea current disturbance to the plant model employed for control design. In the proposed design method, we substitute the structured unmodeled dynamics uncertainty, which is generally difficult to come up with and eventually utilized to represent external disturbances, by parametric uncertainty, relatively easier and straightforward to come by. The sea-current load parameters are, therefore, treated as parametric uncertainty and fit in the µ design framework. Assuming that both vehicle motion and current direction lie in the horizontal plane, the incoming (to vehicle) current vector sets a horizontal circumference sector in which it may vary. When in the 3D space, current uncertainty renders a cone in space. For validation purposes, the linear controller is simulated with the nonlinear vehicle model.
mobile robots; robust control; nonlinear control systems
