Simple bottom-up hierarchical control strategy for heaving wave energy converters

The objective of this study was to improve the power captured in heaving wave energy converters using a simple robust hierarchical control strategy (HCS). A HCS comprises a higher level controller (HLC) and a lower level controller (LLC). The HLC provides a reference velocity for the buoy, which is in-phase with the wave's excitation force. The LLC follows the reference despite the uncertainties in the model. We propose a new HCS called bottom-up HCS (BU-HCS), where the LLC is designed before the HLC. The LLC is implemented using a feedback controlled system with a simple lead-lag compensator as its controller. The lead-lag compensator is designed using H∞ theory with the objectives of maximizing the robustness and tracking properties of the LLC while minimizing the control force of the power take-off (PTO) device. A set of optimization problem is obtained for designing the parameters of the lead-lag compensator, which are solved using a genetic algorithm. The HLC in the BU-HCS provides the velocity reference, which satisfies a constraint on the control force and the PTO's utilization index. The HLC is implemented by designing the value of an intrinsic resistance constant, which can be found using the Bode magnitude plot of a transfer function. Based on the plot, a look-up table for the intrinsic resistance constant is generated as a function of the significant height and the peak period of the wave. We tested the proposed method in various scenarios and its performance was compared with existing control techniques.