# Wireless Accelerometer Architecture for Bridge SHM: From Sensor Design to System Deployment
## 1 Introduction
### Wireless Monitoring Systems  
Wireless sensor networks (WSNs) address many of the logistical and financial drawbacks of wired systems:  
* **Rapid Installation**: The absence of physical cabling significantly reduces installation times, particularly for long-span bridges. * **Cost Efficiency**: WSNs eliminate the need for extensive hardware infrastructure, lowering both initial and ongoing costs. * **Flexibility**: Wireless systems are easier to reconfigure or expand compared to their wired counterparts. However, WSNs present unique technical challenges, particularly in achieving precise data synchronization across multiple nodes. Synchronization is critical for accurate structural health monitoring, as even minor timing discrepancies can impact data integrity and analysis. Protocols such as the time-sync protocol for sensor networks (TPSNs), reference broadcast synchronization (RBS), and flooding time synchronization protocol (FPSP) have been developed to address this challenge. While these protocols help align clocks among WSNs, factors such as software processing delays and low-quality crystals can introduce variations in sensing start times and fluctuations in sampling rates. Further complications arise during extended sensing durations with temperature variations, leading to synchronization errors that may compromise application results like system identification and damage detection [12]. In particular, for operational modal analysis (OMA) purposes, time synchronization error leads to problems in mode shape identification [9]. Several works in the literature have proposed WSN prototypes tailored for the SHM of civil infrastructure, typically utilizing micro electro-mechanical systems (MEMSs) based accelerometers. These sensors are essential for performing OMA, as acceleration measurements are required to capture the dynamic behavior of structures. These studies are reviewed below, highlighting their key strengths and limitations. [PERSON] et al. [14] developed sensor nodes that are not synchronized, and they proposed an approach for synchronization by means of offline data processing. [PERSON] et al. [15] proposed a WSN where no synchronization is mentioned and no information is provided about the possible deployment of the system in a real-world scenario. [PERSON] et al. [16] proposed a node based on ADXL345 accelerometer (noise density of 430 \(\upmu\)g), which is not suitable for OMA purposes of bridges. In addition, LoRa communication is proposed, which is not suitable for high-frequency data due to the low data rate of this technology. [PERSON] et al. [13] developed a node equipped with a solar panel by employing an accelerometer with a noise density of 70 \(\upmu\)g/\(\sqrt{\mathrm{Hz}}\), without describing how the time synchronization is guaranteed. [PERSON] et al. [17] adopted an ADXL355 accelerometer synchronized by means of the IEEE 802.11 timing synchronization function, which results in a maximum error of 2 ms. In this work, no harvesting units are considered but the authors state that the battery can last five years by considering 30 min of daily operation. This is a limitation if the objective is to both have multiple acquisitions during the day as well as long-life sensors. Moreover, modern MEMS accelerometers designed for mounting on printed circuit boards (PCBs) now offer performance levels that closely rival those of traditional wired models [18; 19]. Significant advancements have been achieved, particularly in reducing noise density, a critical factor for the modal analysis of civil structures. In this regard, [PERSON] et al. [20] showed that for carrying out operational modal analysis of civil infrastructure, by employing random white noise technique, a noise density lower than 45 \(\upmu\)g/\(\sqrt{\mathrm{Hz}}\) is mandatory to catch most of the primary natural frequencies of the structure. WSN offers a promising solution for long-term SHM by providing easier installation and lower costs compared to traditional wired systems [12].