Tracking control for underwater vehicle-manipulator systems with velocity estimation

In this paper, the problem of tracking a desired motion trajectory for an u nderwater vehicle-manipulator system without using direct velocity feedback is addressed. For this purpose, an observer is adopted to provide estimati on of the system's velocity needed by a tracking control Law. The combined controller-observer scheme is designed so as to achieve exponential converg ence to zero of both motion tracking and estimation errors. In order to avo id representation singularities of the orientation, unit quaternions are us ed to express the vehicle attitude. Implementation issues are also consider ed and simplified control laws are suggested, aimed at suitably trading off tracking performance against reduced computational load. Simulation case s tudies are carried out to show the effectiveness of the proposed controller -observer algorithm. The obtained performance is compared to that achieved with a control scheme in which the velocity is reconstructed via numerical differentiation of position measurements. The results confirm that the chat tering on the control commands is significantly reduced when the controller -observer strategy is adopted in lieu of raw numerical differentiation; thi s Leads to lower energy consumption at the actuators and increases their li fetime.