Constrained control allocation for a remotely operated vehicle with collective azimuth thrusters

Remotely operated vehicles commonly have propellers with fixed position and orientation. In this paper, we highlight the advantages of equipping remotely operated vehicles with azimuth thrusters. In particular, both energy saving and increased trajectory tracking performances can be achieved with respect to the fixed orientation case. On the other hand, managing thrusters' orientation represents a further computational burden: the constraints are characterized by the presence of trigonometric functions, thus they require, in general, the use of onerous nonlinear solvers. Our proposal is to steer collectively the thrusters, in order to simplify both the optimization algorithm and the mechanical structure of the ROV. The proposed control allocation algorithm calculates, at first, the orientation of the propeller to minimize the energy consumption while taking into account saturation and rate limits. Then, single thrusts are calculated using a quadratic programming framework. Simulation results show the relevance of energy savings and tracking improvements.