IEEE 802.11-Based Wireless Sensor System for Vibration Measurement

Network-based wireless sensing has become an important area of research and various new applications for remote sensing are expected to emerge. One of the promising applications is structural health monitoring of building or civil engineering structure and it often requires vibration measurement. For the vibration measurement via wireless network, time synchronization is indispensable. In this paper, we introduce a newly developed time synchronized wireless sensor network system. The system employs IEEE 802.11 standard-based TSF-counter and sends the measured data with the counter value. TSF based synchronization enables consistency on common clock among different wireless nodes. We consider the scale effect on synchronization accuracy and evaluated the effect by taking beacon collisions into account. The scalability issue by numerical simulations is also studied. This paper also introduces a newly developed wireless sensing system and the hardware and software specifications are introduced. The experiments were conducted in a reinforced concrete building to evaluate synchronization accuracy. The developed system was also applied for a vibration measurement of a 22-story steel structured high rise building. The experimental results showed that the system performed more than sufficiently. 1. Introduction Rapid progress of wireless network technology and embedded sensor technology has been integrated into wireless sensor network and various prospective applications are expected to emerge. Among the many sensing network applications, particularly promising one is the structure health monitoring, which monitors the structural health of buildings and civil engineering structures [1]. Since measuring objects such as a bridge and a building are usually huge and installing very long signal cables requires high installation cost. Additionally, long cables leave wires vulnerable to ambient signal noise corruption, thus wireless data transmission is highly beneficial. Structure health monitoring often requires measuring vibration data such as acceleration and velocity. Measured data are analyzed by the modal analysis method to obtain the resonance frequency, damping ratio and spectrum response [2]. For wireless vibration measurements, time synchronization is very important because the vibration measurement for the modal analysis requires simultaneous multipoint sensing data which are often transmitted via multihop relayed wireless devices. Due to the queuing process and stochastic media access method, the data transmissions are randomly delayed. As a result, even