Experimental Validation of a Nonlinear Wave Encounter Frequency Estimator Onboard a Wave-Propelled USV

Abstract

Abstract Knowledge of the amplitude and frequency of sea waves is useful to characterize the sea state, and increases the situational awareness as perceived by unmanned marine crafts. The ability to adapt navigation control parameters according to the current sea state is an essential feature for vehicles whose motion is mainly governed by environmental forces. From a control perspective, an estimate of the wave encounter frequency (WEF) can be used to filter out first-order wave-induced motions that cause undesired rudder control action and servo wear and deterioration. In this work we consider a nonlinear second-order observer capable of computing the approximate frequency of a sinusoidal signal with unknown amplitude and phase. This work presents the first experimental validation of the nonlinear estimator, which is tested for the first time on a five meters long wave-propelled unmanned surface vehicle (USV). Moreover, this article shows the estimator’s ability to characterize the dominant frequency of wave spectra across different sea states and discusses two applications of the WEF estimation: sea state estimation and wave-filtered steering control. Theoretical considerations are supported with both simulation and experimental results.