## 3 Methodology
### Structural Health Monitoring Systems  
#### 3.1.1 Low-Cost Structural Health Monitoring System  
The vibration-based SHM system is composed of a WSN, made up of independent measuring nodes, and a base station. This wireless monitoring system records the response of the structure in terms of linear accelerations. When the information is collected, the vibration data are communicated towards the base station to analyze and characterize the dynamic properties of the structure. The low-cost SHM system runs short-term SHM testing, and its radiofrequency technology enables data communication. The low-cost SHM system consists of three main elements: the measuring node, the WSN, and the base station. The heart of the low-cost SHM system is the measuring node. It comprises a microprocessor platform, Raspberry Pi 3B+, and a MEMS accelerometer, ADXL355. Raspberry Pi 3B+ is a system on chip (SoC) electronic device that carries a Broadcom BCM2837B0 Cortex-A53 processor (64-bit) and 1 GB LPDDR2 RAM. The microprocessor platform holds versatile, reliable, low-power, and low-cost hardware. This computational device requires a 5.0 V input power and costs about USD 38.5 [25; 26]. The microprocessor platform Raspberry Pi 3B+ performs three primary functions: supplying power to the measuring nodes, responding to data acquisition requests, and data storage and communication. The computer module is a Linux Ubuntu MATE 18.04 LTS operating system, and it has a 16 GB SD card. The ADXL355 MEMS accelerometer is a triaxial digital device that includes a 20-bit ADC, has a 3.9 ug/LSB sensibility, and employs a \(\pm\)2.048 g full-scale range. Furthermore, the ADXL355 MEMS sensor is a low noise density, low-power, and low-cost sensor device. This digital device features a 3.3 V input power and costs about USD 35.0 [27; 28]. The triaxial sensor was used to acquire signals at 250 Hz. Figure 2 shows the composition of the proposed measuring node. The proposed WSN topology was considered to have star architecture. The star network topology has a logic map exclusively communicate information between the base station and every measuring node. Concerning this network topology, the scalability of the low-cost SHM system allows adding measuring nodes easily [9]. The local communication between the microprocessor platform and accelerometer implements an I2C communication protocol, consisting of a synchronous serial interface bus that directly connects these electronic devices [29]. For this purpose, jumpers attached to the microprocessor platform and accelerometer GPIO pin headers were needed [28]. Regarding the time synchronization of the WSN, Network Time Protocol (NTP) is employed to sync the clock of the Raspberry Pi 3B+ via the Internet with servers distributed worldwide [30]. As soon as the Raspberry Pi 3B+ gets an Internet connection, the time information request is sent to the server for clock synchronization. The functioning of the low-cost SHM system comprises two stages: data acquisition and communication, and dynamic characterization. These functioning stages occur consecutively. The Raspberry Pi 3B+ triggers the ADXL355 accelerometer through an automated request programmed in the startup file. Once the accelerometer acquires the vibration data, the Raspberry Pi 3B+ of the measuring node carries out the storage and communication processes. Afterwards, from the base station computer, it is possible to handle the signal processing and the dynamical system characterization in a pseudo-real time step. Figure 3 shows the function block diagram of the low-cost SHM system. Figure 1: Descriptive diagram of the methodology framework. For the data acquisition and communication stage, the input parameters of the measurement (such as record time, sampling frequency, and data storage) are set manually by remote connection from the base station. The wireless communication technology related to the monitoring system held to the IEEE 802.11 standard protocol on the 2.4 GHz frequency band [25].