Dynamics and control of tethered underwater-manipulator systems

In this work, the dynamics modelling strategy of a tethered underwater remotely operated vehicle (ROV) coupled with a and spatial manipulator have been studied. With regards to the cable dynamic modelling, it is considered to be a series of lumped point masses connected by linear, massless, visco-elastic springs. In addition, the model accounts for the tether bending and twisting effects. Regarding the manipulator dynamics, the Articulated-Body Algorithm is employed due to its computational efficiency. In order to control the ROV motion under disturbance forces and moments caused by the tether and the manipulator motion, a series of Model-based SISO sliding-mode controllers are implemented and the ABA is used to predict the dynamic coupling force expressions based on the feedback of ROV and the manipulator states. The control gains of the sliding-mode controllers are defined as a closed function of the articulated inertias of the ABA algorithm; leading to time varying gains as opposed to constant as in conventional methods. As a case study, a Saab-Seaeye FALCON™ ROV with a modified Hydrolek™ HLK 43000 manipulator is presented. Numerical simulations are performed to reveal the extent to which the tether dominates the Falcon-manipulator dynamics. It is shown that disturbance forces and moments created by tether motion must be actively compensated using while the ROV is held stationary during manipulator operation. It is also shown that the use of force sensors at the FALCON™´s tether termination can dramatically improve the performance of the series of SISO sliding mode controllers.