Driven by the need to reduce the installation cost and maintenance cost of structural health monitoring (SHM) systems, wireless sensor networks (WSNs) are becoming increasingly popular. Perfect time synchronization amongst the wireless sensors is a key factor enabling the use of low-cost, low-power WSNs for structural health monitoring applications based on output-only modal analysis of structures. In this paper we present a theoretical framework for analysis of the impact created by time delays in the measured system response on the reconstruction of mode shapes using the popular frequency domain decomposition (FDD) technique. This methodology directly estimates the change in mode shape values based on sensor synchronicity. We confirm the proposed theoretical model by experimental validation in modal identification experiments performed on an aluminum beam. The experimental validation was performed using a wireless intelligent sensor and actuator network (WISAN) which allows for close time synchronization between sensors (0.6-10 μs in the tested configuration) and guarantees lossless data delivery under normal conditions. The experimental results closely match theoretical predictions and show that even very small delays in output response impact the mode shapes.

Wireless sensor networks have attracted attention as a possible solution for applications of periodic and continuous structural health monitoring. Ensuring synchronous data acquisition across wireless nodes in large networks of sensors spatially distributed on a structure is of critical importance for many methods of structural health monitoring, especially those based on analysis of vibration. In this article we present a novel Wireless Intelligent Sensor and Actuator Network (WISAN) addressing the issue of scalability for applications of structural health monitoring. We also present a novel time synchronization algorithm that can keep the synchronization error between any number of globally distributed sensors nodes less than AE23 ms. We show proof of stability for the time synchronization algorithm. We validate WISAN in laboratory experiments, testing the actual time synchronization between randomly selected sensors in a complex network. Finally, we validate WISAN in a field experiment by reconstructing mode shapes of a highway bridge.

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The IEEE 802.15.4 and Zigbee are protocols aimed at low-duty and low-power wireless sensor networks. Continuously monitoring applications such as applications of structural health monitoring could benefit from low power consumption of the IEEE 802.15.4 radio chips. However, existing implementation of IEEE 802.15.4 has limited applicability to 'proactive', high-traffic monitoring sensor networks with constant data streams. In this paper, a Time Division Multiple Access (TDMA) scheduler was designed and implemented to resolve issues with collisions and interference, bandwidth usage and delivery of low latency data in a healthy network and to significantly minimise power consumption by sensor nodes. These improvements maintained compatibility with non-TDMA aware nodes. This scheduler was simulated using Network Simulator-2 for different network scenarios that show a remarkable improvement in the network throughput. The proposed mechanism was also successfully tested and implemented in Wireless Intelligent Sensor and Actuator Network (WISAN). intelligent sensor and actuator network.Reference to this paper should be made as follows: Krishnamurthy, V. and Sazonov, E. (2008) 'Reservation-based protocol for monitoring applications using IEEE 802.15.4 sensor networks', Int.

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