## 2 Materials and Methods
### Hardware Architecture  
In this section, an accurate description of the hardware used for the implementation of the proposed system is provided. A single sensor node is composed of a microcontroller central unit (MCU), two accelerometers for acceleration tracking, a global positioning system (GPS) for node position and synchronization, a Long Range (LoRa) unit for data transferring, an RS485 interfacing unit for communication with other nodes and the master unit, a 1 Gb NAND flash memory for data storage and a specific power path for the supply of each unit. The ESP32-WROOM-32 UE MCU from Espressif Systems [26] is used in this system. This microcontroller, having a 3.3 V supply, is predisposed to wide-range IoT applications due to the internal 2.4 GHz combo chip integrating Wi-Fi and Bluetooth connectivity. This chip, in particular, supports the 802.11 b/g/n Wi-Fi standards, Bluetooth 4.2 and 5.0 with BLE (Bluetooth Low Energy). In addition, this chip presents good computing performance due to the integrated 240 MHz dual-core processor and an internal 4 MB flash memory. The number of supported interfaces comprises UART, SPI, I2C, and several ADC-oriented ones, making this chip suitable for a wide range of applications, including home and industrial automation, smart agriculture, and beyond. Node position is provided by a Global Positioning System (GPS) unit, the FGPM-MOPA6H [27]. This device communicates with the MCU through the National Marine Electronics Association (NMEA) 0183 standard, used in satellite communications. This unit has a power supply ranging from 3 to 4.3 V and is a high-performance GPS module with a \(-\)165 dBm signal sensitivity, instant Time-to-First Fix (TTFF), low power consumption (82 mW during signal acquisition and 62 mW during tracking), 10 Hz update rate, and a multi-tone active interference canceller. Tri-axial node displacement is provided by two accelerometers: the first is used as the main device, and the second, with less resolution but also less current consumption, is used either with an aggressive filtering to determine modifications in the static trim of the structure, or as a backup solution in the case of a failure of the first element. The main accelerometer is an ADXL355 [28], while the backup accelerometer is an ADXL345 [29]. Both accelerometer units need a 3.3 V power supply. The main differences between these two devices can be listed in terms of:  
1. Sensitivity, as the ADXL355 25 \(\upmu\)g/\(\surd\)Hz noise density is less than the 150 \(\upmu\)g/\(\surd\)Hz ADXL345 noise density, making the first device more sensitive to changes in acceleration;
2. Accuracy, as a consequence of the previous point, and due to the ADXL355 2 mg bias instability, with respect to the ADXL345 20 mg;
3. Resolution, because despite the selectable measurement range between \(\pm\)2 g, \(\pm\)4 g, or \(\pm\)8 g for both accelerometers, ADXL355 provides 20-bit strings, while ADXL345 has a 10-bit resolution;
4. Power consumption, smaller for ADXL345 (23 \(\upmu\)A in measurement mode and 0.1 \(\upmu\)A in standby mode) with respect to ADXL355 (200 \(\upmu\)A in measurement mode and 21 \(\upmu\)A in standby mode);
5. Communication with the MCU, as ADXL355 uses the SPI interface, while ADXL345 uses the I2C protocol. Structural data retrieved from the sensors are stored in a 1 GB NAND flash memory, the IS37 SML01G1-LLI [30], which interfaces with the MCU through the SPI protocol and requires 3.3 V to work correctly. As reported in the previous section, RS485 and LoRa are used for data interchange, so data transceivers for each protocol are used. The RS485 transceiver, a MAX13488 EESA+T [31], has a proprietary AutoDirection control, which manages data direction automatically from hardware without the need for additional control signals. This device has an error-free transmission of up to 500 kbps and transmit speeds of up to 16 Mbps.