**1. Introduction**

Composite structures have been increasingly used in various aircraft structures due to their superior stiffness and weight characteristics. However, composite structures are susceptible to low velocity impacts, which can cause internal damage that will lead to a significant reduction of the local structural strength [1]. Therefore, impact monitoring has become an important task of structural health monitoring (SHM) to ensure the safety of these applications.

Recently, sensor array signal processing-based methods have emerged as a set of new promising SHM methods. Yin et al. constructed two Z-shaped clusters at different positions for acoustic source localization in anisotropic plates [2]. Xiao et al. proposed an acoustic emission source localization approach based on beamforming with two uniform linear arrays [3]. He developed a method for localizing two acoustic emission sources simultaneously based on beamforming and singular value decomposition. Agarwal and Machán proposed a statistical super-resolution technique ˇ called the multiple signal classification (MUSIC) algorithm published in Nature Communications,

and demonstrated that the MUSIC algorithm had a comparable or better performance in comparison with other localization techniques [4]. Because it has such advantages, the MUSIC algorithm was developed for the impact monitoring of plate-like structures. By processing the whole sensor array outputs using the MUSIC algorithm, the transfer function and material properties of the structure are not needed [5]. Since the MUSIC algorithm belongs to an eigen-structure and sub-space approach, it can effectively extract the key features of signals received and successfully detect the source signal under a low signal-to-noise ratio [6]. Some developments have been published by applying this method to process the sensor array signals in the structural health monitoring area. Stepinski and Engholm [7] presented a far-field MUSIC algorithm-based method to estimate the direction of arrival (DOA) of single elastic waves on an aluminum plate using a uniform circular array. Yang et al. [8] employed the far-field MUSIC algorithm to estimate the DOA of one impact on an aluminum plate. Yuan et al. presented a single frequency component-based re-estimated MUSIC algorithm to reduce the localization error caused by the anisotropy of the complex composite structure [9]. To deal with the near-field monitoring problem, the authors in previous work proposed the near-field 2D-MUSIC algorithm-based impact localization method for a composite plate using Gabor wavelet transform [10,11]. However, in previous MUSIC-based impact location methods, the narrowband signals at a particular central frequency had to be extracted from the wideband Lamb waves induced by each impact, and the specific center frequency had to be obtained after carefully analyzing the impact signal, which is time consuming.

EMD is a new time–frequency analysis technique which can decompose the complicated signal into a set of complete and almost orthogonal components named intrinsic mode functions (IMFs) [12]. The IMFs represent the natural oscillatory mode embedded in the signal and work as the basis functions, which are determined by the signal itself [13]. Thus, it is a self-adaptive signal processing method that can be applied to a nonlinear and non-stationary process to decompose it into stationary signals. Generally, EMD cannot accurately extract fault features because of the mode mixing phenomena. Wu and Huang proposed a new ensemble EMD (EEMD) method to reduce mode mixing [14]. However, the EMD and its improved version EEMD are computation intensive methods, which are not suitable for on-line detection. A noise-assisted approach in conjunction with a multivariate empirical mode decomposition (MEMD) algorithm has been proposed for the computation of EMD, in order to produce localized frequency estimates at the accuracy level of instantaneous frequency [15–17]. Leo developed Bivariate EMD to detect defective areas in composite materials [18]. Wang et al. [19] proposed a fast HHT with an optimized EEMD algorithm to speed up the computational efficiency by 1000 times. They also proved that the computational complexity of the EMD is equivalent to fast Fourier transform (FFT). Hence, the optimized EEMD method might be a good choice to be applied to the real-time impact localization of composite structures.

This paper presents an improved approach that combines the optimized EEMD and 2D-MUSIC algorithm for the on-line impact localization on composite structures without selecting the center frequency. The layout of the paper is structured as follows: In Section 2, an optimized EEMD-based 2D-MUSIC method for impact monitoring is presented. An impact monitoring experiment on a cross-ply glass fiber reinforced composite plate is performed in Section 3 to verify the proposed method. Finally, the conclusion and future works are given in Section 4.
