# Development and Field Validation of Wireless Sensors for Railway Bridge Modal Identification
## 2 Description of the Wireless Monitoring System
### Wireless Sensor Nodes
#### 2.2.2 Acquisition Procedure  
The proposed monitoring system based on wireless sensor nodes implements a power saving logic, which ensures a long battery life. The goal is to provide the sensor with the longest possible autonomy, the only problem being battery ageing [38]. The power available on the photovoltaic panel is limited, and it is not constant over time, since it depends on many variables (day/night, environmental conditions, irradiation, season, dirt). For this reason, the power consumption of the node must be as low as possible.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Full-Scale Range & \(\pm 2\) g, \(\pm 4\) g, \(\pm 8\) g and \(\pm 16\) g \\ Sensitivity & 16,384 LSB/g (for \(\pm 16\) g range) \\ Noise Density & 70 \(\mu\)g \(\surd\)Hz \\ Output Data Rate & up to 8 kHz \\ Supply Voltage & 1.8 \(\div\) 3.6 V \\ Supply Current & 0.3 mA \\ Operating Temperature Range & \(-40\div 105\)\({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 2: MEMS accelerometer (TDK IIM-42351) main features.  
To do so, the sensor node always works in a low-power condition in which the clock frequency is reduced, and all functions are disabled, except for the wake-on-threshold feature of the accelerometer and the BLE pairing with the gateway. The first function represents the automatic trigger for the acquisition of the acceleration data. In this condition, the accelerometer is not communicating with the microcontroller via the SPI protocol. Once the acceleration module exceeds a given threshold parameter, a logic signal (detected by an interrupt pin) is sent to the CPU to start the acquisition. The second function, instead, is kept active because the sensor node can be woken up by the gateway anytime, both to trigger an acquisition without exceeding the acceleration threshold and to change the configuration parameters stored into the flash memory (as mentioned in Section 2.1). The parameters that can be modified are the sampling frequency, the measurement range, the wake-up acceleration threshold, the acquisition period, the number of axes to sample, and the axis that triggers the wake-up condition. The useful aspect of this approach is that such parameters can be modified remotely by directly connecting to the gateway, without downloading a new firmware on each sensor node.  
So, when a significant disturbance force excites the whole structure, like the one generated when a train crosses the bridge, the accelerometer triggers the node as described before. Then, the sensor node switches to the full-operating condition and samples the acceleration values with a sampling frequency equal to 200 Hz for 10 s, both on the vertical and the lateral axis. When the acquisition is over, the data are divided into some packages and then sent to the gateway, together with some other information (such as the timespan, the battery voltage, the voltage on the PV panel, the power consumption, and the temperature). Once all the data have been properly transmitted, the sensor node switches again to the low-power condition and the gateway uploads the collected data on the cloud, as shown in Figure 3.