Optically Powered Electrical Accelerometer and Its Field Testing

Abstract

This paper introduces an optically powered electrical accelerometer recently developed by the author for measuring dynamic response of civil structures and describes a field test using its prototypes installed on a 310-m tall concrete TV tower, together with the results of system identification analysis based on the field measurement. This sensor employs an electric circuit, LC oscillator, in the sensor head. The resonant frequency of the LC oscillator is modulated by the movement of a pendulum installed inside the sensor head, indicating the acceleration of the pendulum support. The oscillatory electric voltage is transformed to an optical signal and transmitted through an optical fiber. The LC circuit is optically powered as well. Therefore, there are no electrical cables involved in this sensor, which brings many advantages to the sensor, such as immunity to an electromagnetic field and significantly less troublesome cabling problems. Prototype accelerometers were fabricated and installed on the TV tower to measure its ambient vibration. Based on the measurements, dynamic characteristics of this tower, including the natural frequencies, damping ratios, and mode shapes, were successfully identified. They are essential for the assessment of the dynamic response of the tower under wind and earthquake loads. This field test has not only provided valuable information of the tower's dynamic characteristics, but also demonstrated the excellent performance and great potential of this optically powered sensor for civil structure applications.