Adaptive Sliding-Mode Control of an Offshore Container Crane With Unknown Disturbances

In this paper, an adaptive sliding-mode control (ASMC) for offshore container cranes that load/unload containers from a mega container ship to a smaller vessel is investigated. To withstand the harsh working conditions in the open sea, such as ship motions and winds, a 4-degrees-of-freedom control model consisting of plant uncertainties and known/unknown disturbances is newly developed. After decoupling the actuated (i.e., trolley displacements) and unactuated (i.e., swing angles) joint variables, a sliding surface that incorporates the decoupled dynamics is designed. Then, a new sliding-mode control (SMC) algorithm with two adaptation laws for switching- and equivalent-control inputs is developed. The asymptotic stability to the "real" sliding surface introduced in the decoupled (actuated, unactuated) state space is proven without a priori knowledge on the bounds of unknown disturbances. For the experiment, a three-dimensional crane mounted on a Steward platform to generate the ship motions is utilized. To verify the effectiveness of the proposed ASMC method, experimental results of the proposed method are compared with two representative works: the SMC presented by Ngo and Hong and the ASMC presented by Zhu and Khayati. The vibration suppression capability of the proposed method in the presence of ship motions, large initial swings, parameter uncertainties, and sudden disturbances is superior to the two compared methods. The developed algorithm can be used for a mobile harbor system as a new tool in the modern maritime industry.