Nonlinear Robust Prediction Control of Hybrid Active–Passive Heave Compensator With Extended Disturbance Observer

To reduce the adverse effect of the unexpected vessel heave variation on the response of the underwater payloads, a hybrid active-passive heave compensation (HAHC) system with a nonlinear cascade controller is especially designed. An adaptive extended disturbance observer is constructed to estimate and separately compensate the external perturbations and the parameter uncertainties, which are generally regarded as lumped disturbances in previous researches. Besides, the outer position tracking loop is designed with sliding mode control to compensate disturbance estimation error by using cylinder load pressure as output, while the inner-pressure control loop provides the active heave compensator the force generator via back-stepping technique. The stability of the overall closed-loop system is proved based on Lyapunov theory. Furthermore, to compensate the time delay between the sensors and actuators, a prediction algorithm for the heave motion of the vessel is combined to improve the performance of the HAHC systems. Extensive comparative experiments based on actual irregular wave data are conducted to verify the superior performance of HAHC system with the proposed control strategy. The experimental results show that the proposed controller gives an excellent heave compensation performance in the presence of time lag, parameter uncertainties, and external disturbance.