Continuous Integral Sliding Mode Control of an Offshore Container Crane with Input Saturation

An offshore container crane is expected to realize high productivity and economic feasibility in the maritime logistics due to its versatile functions for container transportation. However, severe working conditions resulting from the oceanic environment may cause difficulty in its accurate and safe cargo transfer. Specifically, the presence of ship-motion-induced disturbances, wind disturbances and parameter uncertainties in the payload mass and rope length result in positioning errors and large swings during ship-to-ship transportation. For efficient cargo transportation, such control problems should be solved with consideration of physical constraints like control input saturation. Therefore, a continuous integral sliding mode control is developed in this study using a 4-degrees-of-freedom (4-DOFs) dynamic model with input saturation. The control scheme consists of two parts: i) A super-twisting control for disturbance rejection, and ii) a nominal control to drive the state vector to the desired equilibrium point. Therein, the nonlinear integral-type sliding surface is decoupled into two sub-systems, so that the control laws for the lateral and longitudinal trolley dynamics are independently developed. The asymptotic stability of the closed-loop system in consideration of the input saturation is assured by the Lyapunov method. Moreover, simulation results are provided to demonstrate the effectiveness of the proposed control strategy compared with the conventional sliding mode control.