Antiswing Control of Offshore Boom Cranes With Ship Roll Disturbances

Offshore cranes often work in harsh sea conditions. As a result, they may suffer from various external disturbances, especially the persistent and unpredictable ship motion caused by sea waves, which will introduce severe disturbances to the crane dynamics, including the unactuated part. Due to this reason, the control problem for offshore boom cranes presents great challenges, for which only boundedness or stability, instead of the generally desired asymptotic stability, of the closed-loop system can be usually guaranteed by currently available methods. For this problem, a novel nonlinear control strategy for an offshore crane is proposed in this brief, which ensures that the closed-loop system's equilibrium point is asymptotically stable even in the presence of the persistent ship roll disturbances. The proposed control law is further extended to be an output feedback controller to deal with the situations when the velocities are unavailable, still guaranteeing asymptotic stability. Finally, some experimental results are provided to validate the efficiency of the proposed controllers.

I. Introduction

Offshore cranes are a special kind of transportation tools mounted on ships, which are widely utilized for the building and maintenance of offshore installations, at-sea replenishment, loading/unloading of the cargoes on board, and so on. As the ocean-related human activities become more and more frequent, the usage for offshore cranes is always keeping increasing during the past decades. Consequently, the modeling and control problem for an offshore boom crane has recently attracted world-wide attention. On the other hand, with less control inputs than their degrees of freedom (DOFs), offshore boom cranes belong to the class of underactuated systems, whose control problem has been a research focus within the mechatronics and control community [1]–[5]. Similar to traditional land-fixed cranes, the main objective for offshore cranes is to convey cargoes to the desired location precisely while suppressing the payload swing simultaneously. During the transportation process, the payload will swing back and forth undesirably, which may cause danger to the surrounding people. To improve the transportation efficiency and reduce the risk of accidents, many researchers have put much effort on the automatic control of offshore cranes [6]–[13]. However, the control of offshore cranes is much more difficult than that of land-fixed cranes. Specifically, offshore cranes often work in harsh sea conditions where severe external disturbances usually cause unpredictable ship motion, and, subsequently, not only will the swing angle of the payload be amplified by the ship motion, but the payload positioning process will also be badly disturbed.