Multiple degrees of freedom active motion control of a hydraulically actuated crane

As offshore wind farms become larger and further from the shore, there are strong economic and climate incentives to perform transfers required for operations and maintenance from floating vessels, rather than employing expensive and slow jack up rigs. However, successful transfers of heavy and sensitive equipment from a floating vessel (in all but benign sea/wind conditions) are heavily dependent on multiple degrees of freedom (DoF), high performance control. Two control design methods were employed to assess the viability of heavy lifts from floating vessels through a simulation approach using Simulink. The crane system was first modelled to operate under simulated vessel motions given by sea states with a significant wave height of 5 m and maximum wave frequency of 1 rad/s. Then, traditional control (feedback and feedforward) was designed to achieve motion compensation with steady-state position errors under 20 cm. To achieve an improved performance, a more robust controller architecture was required, thus the nonlinear generalized minimum variance (NGMV) control algorithm was chosen for this application. Due to its ability to compensate for significant system nonlinearities and the ease of implementation NGMV was a good candidate for the task at hand. Tuning the controller parameters to stabilize the system can also be based on previous classical, say PID, control solutions. Simulations showed NGMV provided an improved control performance compared to traditional control when considering model mismatch.