Satoshi Ishizaki
Development of sensor node using MEMS-based technologies for bridge vibration monitoring
Masatsugu Nagai
Recently, corrosion of bridges has progressed and induced the breakings of members in steel truss bridges in Japan. At present, Structural Health Monitoring (SHM), which diagnoses the condition of a structure using sensors, has been paid to attentions. Among them, vibration-based SHM is quite promising since the changes of stiffness or mass resulted from deterioration or damage in the structure lead to the changes of dynamic characteristics such as natural frequencies, mode shapes and so on. In order to capture the changes of dynamic characteristics, it is necessary to measure vibration in the structure periodically and densely in space. However, it is difficult to achieve this situation due to the expensiveness of sensors.
Therefore, this study aims to develop a wireless sensor node using MEMS-based technologies, which have been a low-cost and high performance rapidly. At first, components of the sensor node satisfying the requirements of bridge vibration monitoring are selected and implemented on a substrate after testing their performance. Next, a laboratory experiment is carried out to evaluate the developed sensor node. Herein, MEMS-based accelerometer is compared with a high accurate servo-type accelerometer. Then, the developed sensor nodes are applied to multiple point vibration measurement in a real bridge in order to evaluate wireless communication and synchronization. Lastly, they are applied to a demolished bridge to catch the changes of dynamic characteristics under progressive damage
The results obtained from this study can be summarized as follows.
1. Although cheap components are used in the developed sensor node, it can sample accelerations below 1 gal from a tri-axial accelerometer in 200Hz and store measured data in a memory card such as SD card for a long time.
2. Laboratory experiment revealed that the implemented tri-axial accelerometer had almost same measurement accuracy as the servo-type accelerometer. Moreover, it was confirmed from a field measurement that the developed sensor node could be applied to a tensile force measurement in a cable-stayed bridge.
3. The developed sensor nodes were applied to multiple point vibration measurement in a real bridge. As a result, although it was difficult to realize time synchronization in hardware, new software synchronization method was developed. Herein, harmonic wave corresponding to the first symmetric mode is utilized as a common signal in sensor nodes. However, phase shift between sensor nodes appears as the amplitude of acceleration is smaller or the order of natural frequency is higher.
4. The developed sensor nodes were applied to vibration measurement in a demolished bridge under progressive damage. As a result, it was found that natural frequency in the first anti-symmetric mode decreased significantly although there were no changes in mode shapes. Clarification of this mechanism is a future study
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