## 1 Introduction  
Modern aircraft mostly adopt composite plate structures to meet the requirements for multifunctionality, high mobility, and high reliability. However, during service, damage to the composite plate structures is unavoidable, which may abruptly reduce the strength and reliability of the plate structures and pose serious threats to aircraft safety. At present, there are two groups of methods for plate structural health monitoring (SHM): active and passive monitoring methods [1, 2]. Passive SHM methods only \"listen\" to the structure but do not interact with it. Acoustic emission (AE) is a typical passive SHM method that has been applied to the monitoring of plate damage [3, 4, 5]. However, due to a variety of factors, such as damage suddenness, damage complexity, and signal nonreproducibility, the requirements for monitoring equipment energy, signal processing methods, and sensor placement are high [6, 7]. Therefore, the application of passive monitoring methods is limited. On the other hand, active SHM methods use sensors to analyze the structure to detect damage. Active monitoring methods use actuators to excite the structure and sensors to receive the structural response signals needed for structural health monitoring. Besides, active monitoring methods are flexible, the structure response is controllable, and the sensing signal has good repeatability. Active Lamb wave technology is widely used in the health monitoring of plate structures [8, 9, 10]. A Lamb wave is an elastic wave whose propagation distance is relatively long and amplitude loss in a thin plate structure is small. Therefore, by using several sensors in combination with Lamb waves, it could be possible to achieve large-scale plate area monitoring. An active Lamb wave based monitoring of a plate structural health generates the Lamb wave by using an actuator and excites a plate structure. Then, the response signal is received by a sensor. Finally, the health information is extracted by signal processing to realize the SHM of a plate. A Lamb wave produced during the active Lamb wave based monitoring is called an active Lamb wave. Although Lamb waves have multimode and dispersion characteristics, an appropriate excitation signal and center frequency should be selected for active monitoring to control the Lamb wave waveform and mode of the plate response [8]. Accordingly, active Lamb wave based monitoring has become a research hotspot [11; 12; 13]. In active Lamb wave based plate structural health monitoring, the array sensor, which has strong anti-interference, is used to acquire enough phase information of the Lamb wave in the plate to realize the plate structure health evaluation [14; 15]. However, array sensor monitoring has directionality, and thus there are monitoring blind spots and it is difficult to arrange in the narrow space of the plate structure. There are also monitoring methods to extract damage information based on a single sensing signal. The smallest unit of these monitoring methods only needs one exciter and one sensor. It can not only form a network of multiple units, realize large area monitoring, and eliminate blind spots but also arrange a monitoring unit in a narrow space. The sensor layout is flexible. The classic method of extracting damage information by this monitoring method is based on the difference of a single sensing signal before and after damage [16]. This method of extracting damage information is intuitionistic, but its anti-interference is poor. Therefore, it is difficult to keep the starting time of each acquisition consistent, and thus there is a phase difference between the collected data. In this case, even if the plate structure is undamaged, the difference between the signals is like the scattering signals, which makes it easy to cause misjudgment. Moreover, a Lamb wave parameter model has been established in the time domain to analyze the change of the sensor signal parameter vector, which realizes the damage evaluation of aluminum plate [17].