###### Abstract
Determining impact location and impact energy helps to assess structural health, especially for evaluating BVID. The strength of composite board with variable thickness is usually higher than that of common composite board, but the signal analysis under dynamic load is more complicated. However, the real-time monitoring of composite board with variable thickness of impact signal can early warning the structural low velocity impact damage [7]. In recent years, scholars from all over the world have made some research on the structure of variable thickness of composite materials. [PERSON] et al. performed the free vibration analysis of several laminated composite doubly curved shells, singly curved shells and plates, characterized by a continuous thickness variation [8]; [PERSON] et al. presented the fundamental nuclei for doubly curved structures in their explicit form for the first time, which also allow considering doubly curved structures with variable thickness [9]. At present, the research on the structure of variable thickness composite laminates mainly focuses on mechanical analysis and finite element simulation [10; 11]. Because of the severe aliasing of the impact signal of the composite with variable thickness, there are few public reports on impact monitoring. At present, there are many sensors that can be used to monitor the impact load of composite laminates. Fiber Bragg Grating (FBG) sensors have been widely used due to their advantages of light weight, small volume, corrosion resistance, interference resistance, distribution, and good insulation [12; 13]. [PERSON] et al. used impact signals obtained from four FBG sensors on a composite stiffened plate using two methods of wavelet decomposition and neural network for the impact location of two impact energy levels [14]. The feasibility of this method was verified by experiments. [PERSON] et al. studied a positioning algorithm based on strain amplitude, which locates the impact source by comparing the absolute value of the maximum strain amplitude of the impact signals measured by the FBG sensor array [15]. The algorithm can identify the position of low velocity impact and only requires the relative position information between sensors. This method requires minimal structural information and sensors, and is validated only on epoxy composite panels. [PERSON] et al. proposed a method which uses dynamic strain signals acquired by FBG sensors for impact location recognition [16]. The estimated location and strain signals were trained by neural networks to reconstruct multi-characteristic impact forces, but only for simple impact problems. [PERSON] et al. proposed an impact localization algorithm based on dynamic strain history caused by external low velocity impact [17]. They compared the RMS values between the reference signal database and the new acquisition impact signal, and used the acoustic emission signals obtained by the FBG to perform the impact localization. The proposed algorithm was applied to the composite stiffener. However, little current research is concerned with the impact energy monitoring of variable thickness composite structures. According to author's previous studies on impact location, it is found that the impact load location can only provide the basis for the impact damage location. In order to evaluate the impact damage situation more accurately, it is necessary to identify the impact energy. To monitor and identify the low velocity impact energy of composite laminates with variable thickness on aviation specimens, the fiber Bragg grating sensing technology [18; 19] with the advantages of anti-interference and distribution is applied to the low velocity impact monitoring, and the impact energy identification system is established. In addition, a new low velocity impact monitoring and identification method for variable thickness composite plate is proposed.