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Robust adaptive PID control for positioning of remotely operated vehicle working in close proximity of an underwater structure

Precise measurement of remotely operated vehicle (ROV) position is crucial for dynamic positioning close to an underwater structure. This can be achieved by using a passive arm measurement system. However, the main drawback of this measurement system is the additional forces acting on ROV due to the mechanical connection. To cope with this problem, a robust adaptive PID control scheme is proposed for dynamic positioning of ROV working in close proximity of an underwater structure in this paper. The proposed controller is a composition of a PID term, a robust term and an adaptive term. The adaptive term is adopted to estimate and compensate the additional forces caused by the passive arm, umbilical cable and uncertainties in buoyancy, and the PID and robust terms are used to eliminate the positioning errors. With respect to the existing adaptive PD controller developed for dynamic positioning of ROV, the proposed controller makes the following improvements: 1) it does not require any knowledge of the inertia matrix, Coriolis and centripetal matrix and hydrodynamic damping matrix of the vehicle dynamics in the controller design; and 2) it does not need any constraints on the control gains to guarantee the global asymptotical convergence of the position and velocity errors of the vehicle to zero except for choosing an appropriately large exponential decay rate. Finally, comparative numerical simulations are performed on an experimentally validated ROV between the standard PID controller and the proposed robust adaptive PID controller. Simulation results show that the proposed controller offers higher positioning precision and stronger robustness to the additional forces than the standard PID controller.