# Development and Field Validation of Wireless Sensors for Railway Bridge Modal Identification
## 1 Introduction
The recent technological advances in the electronic field have made it possible to create increasingly miniaturized and high-performance components, reducing production costs of innovative sensors [13]. Wireless sensors have been developed for monitoring activities in various complex fields, such as monitoring of wind-induced vibration on transmission line conductors [14], dynamics of freight wagon vehicle and the infrastructure [15; 16], and historical buildings [17]. Some applications of wireless sensors can also be found in the active vibration control field [18; 19]. The main features of sensor nodes to be suitably employed for bridges' SHM are the measurement performances, the easiness of installation, and the long autonomy to grant a permanent installation on the structure. The performance of MEMS accelerometers to be mounted on printed circuit boards (PCB) available in the market nowadays is closer to the ones of wired models [20; 21]. Great improvements have been especially made in terms of noise density, which is an aspect of crucial importance for SHM of civil structures [22; 23]. Moreover, some models of these low-cost accelerometers are characterized by low power consumption. To be mounted easily on the structure, sensor nodes must rely on wireless communication for the data transmission. Among the several available protocols (Zigbee, LoRa, Wi-Fi, etc. ), Bluetooth low energy (BLE) is drawing interest for monitoring activities thanks to the a good trade-off between data rate, low consumption, and communication range [24; 25]. In the end, sensor nodes must guarantee a long autonomy. This can be achieved by choosing very low power components to realize the electronic board and adopting a smart acquisition logic to maintain the sensor in a low-power condition (usually called \"sleep\" mode) when data gathering is not required. On the other hand, an inflow of energy must be guaranteed to recharge the sensor battery using an energy harvester [26]. Different typesof harvesting sources are described in the literature, such as piezoelectric harvesters [27], micro wind turbines [28] and thermo-electric generators (TEG) [29]. Anyway, the best solution for structures located in an outdoor environment is photovoltaic (PV) panels, since they allow harvesting solar energy, which is a reliable source characterized by a high power density [30]. Some examples of wireless sensors devoted to SHM purposes are described in [31]; anyway, they were devices with low computational power, with a short communication range and without energy harvesting sources able to provide them with a long-term autonomy. In a more recent work [32], a comparison between different wireless sensors [32; 33; 34] is reported and hereafter reproposed in Table 1.  
As can be noticed, the sensor proposed in this paper is the only one relying on an energy harvesting technique to guarantee a long-term monitoring capability. In this work, the design and development of an innovative wireless system for the continuous vibration monitoring of viaducts and bridges is described. The main advantages of the proposed system are the fact that the realized wireless accelerometers are low cost, compact, and easy to install in the point of interest on the structure. These sensors are autonomous from an energetic point of view, thanks to the sensor's low-power consumption and the use of a mini-PV panel to recharge the lithium-polymer (Li-Po) battery when an energy excess is produced [35]. This way, the realized system can be permanently mounted on the structure during its entire service life and the wireless communication helps avoid the use of any cables between sensors and the acquisition part. Moreover, the developed smart sensors are able to acquire acceleration data and perform data pre-processing activities on board, sending only significant information to the control unit to be analyzed remotely. The developed system can, therefore, represent a new tool in the field to be used for bridge modal identification purposes. The paper is organized as follows.