Discussion on the mode mixing in wave energy control systems using the Hilbert-Huang transform

Abstract

A great improvement in the absorption of energy of a wave energy converter (WEC) is obtained with a time-varying power take-off (PTO) damping over a constant damping. In a passive control scheme based on the Hilbert-Huang transform (HHT), the PTO damping is time-varying and tuned to the instantaneous frequency of the wave excitation force. The HHT method relies on the use of the empirical mode decomposition (EMD) method to decompose the wave signal into a number of components (IMFs) from the highest to the lowest frequency component. However, the decomposition process is not always perfect and may result in mode mixing, where an IMF will consist of signals of widely disparate frequency scales, or different IMFs will consist of signals with similar frequency scales. Mode mixing can be caused by intermittent/noisy signals, and by specific amplitude and frequency relations of the original modes in the signal. The aim of this paper is to extend the studies on the use of the HHT for WEC tuning purposes by revealing how the EMD mode mixing problem affects the WEC performance. A comprehensive study using firstly synthetic two-tone waves (i.e., superposition of two sinusoidal waves) is performed. Then, the observations from the two-tone studies are used to further improve the energy absorbed by WECs using the HHT in real ocean wave scenarios resembling the analytic scenario.