Non-Causal Linear Optimal Control of Wave Energy Converters With Enhanced Robustness by Sliding Mode Control

Sea wave energy converter control is a non-causal optimal control problem, and the control performance relies on the accuracy of wave prediction information. However, the existing wave prediction methods, such as Auto-Regressive (AR) method, extended Kalman Filter (EKF), Artificial neural network and deterministic sea wave prediction (DSWP), inevitably introduce prediction errors. This paper presents a robust non-causal linear optimal control of wave energy converters to explicitly cope with the prediction error of sea wave prediction and simultaneously compensate the modelling uncertainty caused by wave force approximations. This is achieved by designing a non-causal linear optimal control (LOC) to maximize the energy output and a sliding mode control (SMC) to compensate unmodeled WEC dynamics and wave prediction error. The parameters of both SMC and non-causal LOC are calculated off-line, which significantly enhances the real-time implementation of the proposed controller with reasonably low computational load. Simulation results demonstrate the efficacy of the proposed control strategy.

I. Introduction

Sea waves provide an enormous source of renewable energy with high energy density and continuous power supply [1], [2]. To harness wave energy [3], many wave energy converters (WECs) have been developed, including oscillating water columns, overtopping WECs, point absorbers and attenuators. It has also been recognized that WEC control is a non-causal optimal control problem, which means the current control decision is based on the prediction of the incoming sea waves [4]. Recent studies show that wave prediction can play an important role in improving WEC control performance and maintaining safe operations compared to the counterpart of causal control, see [5]–[9].