Linear dynamics are virtually always assumed when designing optimal controllers for wave energy converters (WECs), motivated by both their simplicity and computational convenience. Nevertheless, unlike traditional tracking control applications, the assumptions under which the linearization of WEC models is performed are challenged by the energy-maximizing controller itself, which intrinsically enhances device motion to maximize power extraction from incoming ocean waves. In this article, we present a moment-based energy-maximizing control strategy for WECs subject to nonlinear dynamics. We develop a framework under which the objective function (and system variables) can be mapped to a finite-dimensional tractable nonlinear program, which can be efficiently solved using state-of-the-art nonlinear programming solvers. Moreover, we show that the objective function belongs to a class of generalized convex functions when mapped to the moment domain, guaranteeing the existence of a global energy-maximizing solution and giving explicit conditions for when a local solution is, effectively, a global maximizer. The performance of the strategy is demonstrated through a case study, where we consider (state and input-constrained) energy maximization for a state-of-the-art CorPower-like WEC, subject to different hydrodynamic nonlinearities.

Considerable effort has been expended on the design of control systems for wave energy converters (WECs) over the past two decades. Working from the fundamental requirement of impedance matching, a variety of conceptual and practical algorithms have emerged, which bring various levels of realism to the original complex conjugate ideal, facilitating maximum power transfer. Simplifications can be introduced, such as passive control and causal control, while some enhanced algorithms allow physical constraints to be observed, nonlinear model dynamics to be articulated, or nonideal power takeoff systems to be recognized. However, in general, model-based WEC control systems are evaluated in tandem with identical simulation models, while the sensitivity of controller performance to modeling errors is ignored. In addition, the WEC model utilized by the controller rarely, if ever, fully represents the nonlinear nature of the true WEC dynamics. This paper articulates this model sensitivity issue for different WEC control system architectures and shows that it is of potentially greater impact than for traditional regulation/servomechanism control problems. Recommendations are given on the best WEC control architecture to adopt from a sensitivity/robustness perspective.

A Linear Time Invariant (LTI) energy-maximising control strategy for Wave Energy Converters (WECs) is proposed in this paper. Using the fundamental requirement of impedance-matching, the controller is tuned to maximise the energy obtained under polychromatic wave excitation. Given the LTI nature of the proposed controller, the design and implementation procedure is significantly simpler than well-established energy-maximising controllers, including state-of-the-art numerical optimisation routines, which are predominant in this field. Additionally, a LTI constraint handling mechanism is provided. The effectiveness of both the LTI control strategy and the constraint handling mechanism are assessed using regular and irregular waves in unconstrained and constrained cases. The resulting performance is compared to those obtained using existing WEC optimal control strategies. Finally, the benefits, in terms of power production, for both the controller and the constraint handling mechanism are explicitly highlighted by means of an application case.