Position/force operational space control for underwater manipulation

An underwater manipulator is a complex system, highly non-linear and subject to disturbances caused by underwater effects. To obtain a reliable system, robust control strategies have to be designed for the manipulator. The main contribution of this paper is the development of the low-level position/force control structure for an underwater manipulator. The proposed control strategy is planned in the operational space and combines together the parallel control structure for position/force applications with the sliding mode theory and the manipulator model information. The dynamic model of the system incorporates the hydrodynamic effects and an approximation of the end-effector force contact with the environment. This paper presents a method for computing the interaction force at the end-effector in the absence of a force-torque sensor. The control structure is validated through a Lyapunov-stability approach and experimental results. The control structure is tested on a 6 degrees-of-freedom underwater manipulator interacting with the underwater environment.