## 2 Materials and Methods
### System Description
#### 2.3.1 Sensor Node and Gateway Device  
The sensor node utilized in the system incorporates an RPi0-2W device, which is a cost-effective solution. It is equipped with an ADXL345 MEMS accelerometer as depicted in Figure 1a, offering a 12-bit resolution for measurements ranging from \(\pm\)2 g to \(\pm\)16 g. The accelerometer demonstrates a sensitivity of 3.9 mg/LSB [57]. Additionally, to ensure precise timekeeping within the system, a DS3231 RTC module as depicted in Figure 1b is integrated into the sensor node. This choice is motivated by the fact that the RPi0-2W lacks an internal clock and experiences time loss during power outages. The DS3231 RTC module is a well-established component that has been employed in numerous prior research studies [58; 44; 59].  
The selection of the ADXL345 accelerometer is motivated by its widespread use and proven effectiveness in various vibration applications. It has established itself as a versatile and reliable sensor. [PERSON] et al. conducted a study [16] highlighting the ADXL345's capability to accurately measure vibrations within a frequency range below 45 Hz. This sensor has been successfully employed in spectral analysis and machine vibration monitoring due to its high-resolution measurements and accurate capture of vibration data [60; 61]. Moreover, the ADXL345's fast response and high accuracy make it valuable for achieving stable flight dynamics in drones [62]. It has also found extensive use in SHM for detecting structural changes, predicting failures, and advancing SHM techniques [63; 64; 65; 66]. The study by [PERSON] et al. [67] demonstrated the superior capabilities of the ADXL345 accelerometer compared to a commercial accelerometer in terms of accuracy and reliability. Leveraging the availability of numerous open-source libraries and building upon previous research, this study employs the ADXL345 accelerometer for wireless vibration-based SHM, showcasing its suitability and effectiveness in this specific application.  
The communication between the ADXL345 accelerometer and DS3231 RTC module with the RPi0-2W device is established using the Inter-Integrated Circuit (I2C) communication protocol as depicted in Figure 1c. The I2C protocol utilizes only two wires: the Serial Data Wire Line (SDA) and the Serial Clock Wire Line (SCL). Unlike parallel communication where multiple wires are used to transmit data simultaneously, I2C transfers data bit by bit sequentially along a single wire. It is worth mentioning that while I2C may be slightly slower compared to Serial Peripheral Interface(SPI), in the case of the ADXL345 sensor which supports both I2C and SPI communication, the use of I2C is adequate for the vibration-based SHM applications. In this study, the sampling rate was set at 100 Hz, which  
Figure 1: Sensor node and gateway devices used in this study: (**a**) ADXL345 (**b**) DS3231 RTC module (**c**) i2c wiring RPi02W, ADXL345, and DS3231, and (**d**) RPi4 as gateway.  
does not require high-speed data transfer. Therefore, the I2C communication protocol is sufficient for the intended purposes.  
The selection of the RPi4, as depicted in Figure 1d, as a gateway or server in this study is based on its notable characteristics. The RPi4 is equipped with a robust 64-bit Quad-core Cortex-A72 processor, which offers advanced processing capabilities suitable for demanding tasks. Its dual Wi-Fi and Bluetooth connectivity further enhance its communication capabilities, allowing seamless interaction with other devices. Moreover, the inclusion of a micro HDMI port, supporting dual display functionality, adds to its versatility and suitability for the experimental test. These features enable efficient data transfer and convenient integration with monitors, facilitating the visualization. Therefore, considering its robust processing power, versatile connectivity options, and compatibility with monitors, the RPi4 serves as an optimal choice for the designated gateway or server role.