High Precision Structural Health Monitoring System using Wireless Sensor Networks

Some advances in extensive bridge monitoring using low cost dynamic characterization Structural Health Monitoring (SHM) systems have excellent potential to improve the regular operation and maintenance of the structures. Wireless Sensor Networks (WSN) have been used to avoid high cost of traditional generic wired systems. The most important limitations in SHM wireless systems are time synchronization accuracy, scalability and reliability. In this article, a complete wireless system for structural identification under environmental load is presented. Our contribution ranges from the hardware to the graphical front-end. The system is paying attention in avoiding the main limitations of WSN for SHM specially focused in reliability, scalability, and synchronization. In order to validate the system we have carry out different tests: check synchronization at the lab, compare our operational modal analysis over a real bridge with a numeric analysis and analyze the results of a comparison between couples of signals which belong the same position over the bridge. Obtained results have been excellent and the system has been validated. tion is modal identification and reconstruction of mode shapes. The historical records of the bridge are also important to calibrate the parameters of structural FEM with the theoretically undamaged state of the bridge. That calibration is relevant because it is usual for the constructed bridges may be different from theoretically projected ones. If the same FEM model is modified to fit the second monitoring campaign the changed FEM parameters can be used to trace the structural bridge evolution. As much historical records the structure have less uncertainty levels of some variables will influence the modal parameters (Peeters & De Roeck 2001). The best approximation to the theoretical FEM parameters is to make the first monitoring at the reception stage to get the 'birth DNA' of the structure without any kind of erosion or damage. During the last decades the most common monitoring systems are compound of a large number of sensors. Each one is wired to a central data acquisition system. That makes the initial cost very high and also very difficult installation and maintenance for the complicated mesh of cables that is implicit in the system. Mixing the both exposed ideas it is easy to understand that one of the objectives for SHM is to significantly reduce the money spent on maintenance. But the use of expensive wired systems and the cost in time to deploy them makes that the cost for maintenance in civil infrastructures will rise till the estimated $8.2 trillion in 2010 in United States. According to the study in (Straser et al 1998), up to 25% of the total system cost and 75% of the installation time can be attributed exclusively to the installation of system cables. For that reason, wireless sensor networks (WSN) are becoming more and more important in this field. In (Lynch 2006), Lynch point out that the combination of smart sensor with wireless communication capability reduce the installation effort and add the capability to extent and help to create a dense array of sensors. Some of the advantages that WNs offers are that no cables are required for data transfer, which is one of the reasons that reduce the cost of the global system setup and maintenance. Also data processing can be distributed across the network nodes and that makes the system more fault-tolerant. On the other hand, WNs have also some limitations to be considered. For example timesynchronization accuracy, packet loss, battery life and communication bandwidth are, between others, some of the relevant topics a research have to face. That is why our goal is the design of a WSN for SHM that solved the problems of reliability, scalability and synchronization. A complete wireless system for structural identification under environmental load is presented in this paper. It had been developed paying special attention on reliability, scalability and synchronization to avoid the main limitations. To summarize this introduction is essential to understand that the main goals of SHM systems are damage detection, damage localization, damage quantification, and assessment of the remaining lifetime of the structure. For that reason is desirable an economic affordable and easy to deploy system with high accuracy. The rest of this paper is structured as follows. Section 2 will talk about related work in the SHM field. Section 3 will make a global view of the developed wireless system. Section 4 describes the achievement of a measurement and the experimental results are showed in Section 5. And the end of the paper, in Section 0, the conclusions are given.