Nonlinear Stabilizing control for 7-DOF offshore cranes with velocity and friction estimation under complex wave-induced ship movements

Complex marine environments, including persistent waves, ocean currents, and wind disturbances, present significant challenges to offshore crane operations. In this paper, a nonlinear stabilizing controller is proposed for 7-DOF offshore cranes to simultaneously achieve payload positioning and swing suppression under external disturbances. First, considering 7-DOF motions of the cranes, the ship’s heave, roll, and yaw motions, as well as wind disturbances and mechanical friction, an offshore crane model is developed using Lagrange’s modeling method. Additionally, a vector representing the ship’s motions is introduced for model transformation, thereby simplifying the original model. Then, based on the transformed model, an observer is designed to estimate the system state variables, which are employed to design auxiliary variables. To compensate for mechanical friction, wind disturbances, and actuator input dead zones, friction estimation terms and coupling terms are designed using the auxiliary variables, improving the transient performance of the control system. Additionally, saturated PD terms are designed to limit the controller’s output amplitude, enhancing the system safety. With the proposed controller, the payload can achieve precise positioning and effective swing suppression in complex marine environments and under the system’s uncertain dynamics. The stability of the control system is proven theoretically, and the self-built hardware platform demonstrates the performance of the proposed controller in terms of effectiveness, robustness, and anti-disturbance capability.