## 1 Introduction
Impact energy is also used to perform system identification to calculate the magnitude and location of impacts as studied in [19]. Hardware for impact detection is often related to measuring the charge signal of PZT sensors, which can be lost quickly due to the input resistance of measurement equipment. Therefore, special equipment is necessary. A charge amplifier is a traditional solution for detecting the full impact signal. [PERSON] et al. [20] use a different approach in which they convert the analog impact signal to a digital form where a reference voltage for digitalization is needed. This new design can quickly and steadily process the impact signal, but an accurate method of choosing the reference voltage is still undetermined. In addition, digitalization may cause loss of the impact information. Another solution is a wireless sensor network. One of its advantages is that it can process numerous sensors. Moreover, it can reduce the weight and energy cost of the whole system when there is a significant amount of wire [21]. These hardware systems used to be applied in a library environment, and thus are not suitable for real applications. To date, researchers [22; 23; 24; 20; 25] tend to focus on a single SHM method rather than a combination of several methods. However, the integrations of several SHM methods have many advantages. First, the use of different SHM methods can share their merits to provide a complete and more robust SHM system. Second, SHM combinations can also reduce the weight of the total system, and by combining the active and passive SHM methods, the energy consumption can also be reduced. The active method, which requires more energy, will only be active when the passive method detects an issue, and the passive method requires a low level of power consumption and will inspect the structures continuously [8; 26]. These advantages have led to many studies that directly combine different SHM methods to achieve a more robust damage detection strategy. [PERSON] et al. [27] developed a damage detection strategy of combining EMI methods and GW methods. [PERSON] et al. [28] developed a baseline-free damage detection strategy that combines Lamb waves and EMI methods. [PERSON] and [PERSON] [29] designed a system, called the integrated impedance and guided wave (IIG) system, which combines EMI-based and GW-based methods through a baseline-based damage indicator for damage detection. They also proposed a circuit design for the IIG system where a wave generator, a multiplexer (MUX), and a digitizer are involved. However, these only focused on the combination of strategies or only proposed the prototype of the circuit design. Recently, [PERSON] and [PERSON] [30] achieved damage detection through the combination of the EMI method and GW methods, and [PERSON] et al. [8] presented an integrated SHM system that includes acoustic emission, EMI, and guided ultrasonic wave. [PERSON] et al. [31] proposed a wireless hardware scheme for implementing the GW method and measuring EMI signals. The hardware systems involved here are more about the layout of sensors rather than the design of hardware systems and they only utilized disjoint hardware and software parts. Thus, they need to be improved for engineering applications. Nevertheless, limited research has been undertaken to propose a design in which all functions can be integrated into one printed circuit board (PCB). This paper aims to develop a small, lightweight, low-power consumption PZT-based SHM system which can be applied on real engineering structures. The features of the proposed SHM system are compared with three other systems as illustrated in Table 1 in Appendix A. This system has a three-layer architecture (sensors, hardware, and software). Based on the architecture, an integrated SHM system with impact monitoring and electromechanical impedance measurement is developed. In addition, a scheduling module is developed to manage the two functions of the system. To transfer the data and the command, an integrated interface is developed.