The Establishment of Distributed Fiber Brillouin Scattering Measurement System
|關鍵字:||健康監測;光纖;布里淵散射;分佈式感測;自動量測;health monitoring;optical fiber;Brillouin scattering;distributed sensing;automatic measuring||公開日期:||2007||摘要:||由於結構物的大型化、複雜化，使得結構健康的監控變得越來越重要。目前已有很多監控系統實際應用的資料，但由於結構未來損害的位置很難預測，而傳統式感應器僅能在特定位置測量到結構的反應狀態，所以很難反映出結構體的真實狀況。最近光纖感測技術已經廣泛地被運用在橋梁和公共工程的監控方面，有些甚至已具有分佈式的感測能力。
As structures become bigger, taller, and more complex, the health monitoring of structures becomes more and more important. Many monitoring systems have been proposed in the literature. However, due to the characteristics of conventional sensors, structural response can only be measured at specific locations. Since it’s very difficult to predict the future damage locations of the structure, the monitoring systems usually fail to reflect the conditions of the structures. Recently, optical fiber sensing techniques have been widely adopted in the monitoring of bridges and public works, and some of them have the capacity of distributed sensing. The goal of this dissertation is to establish an automatic optical fiber sensing system based on the Brillouin optical correlation domain analysis (BOCDA) such that static and dynamic response can be measured continuously at any point along the fiber with high spatial resolution. By the aid of GPIB and locating schemes, a quick start (about 2 minutes) measurement system has been developed. Three major hardware modifications were made to the BOCDA system to improve the performance. First, an attenuator was added to the measurement system to keep the power level of the probe at a suitable level such that a better signal to noise ratio can be attained and pump depletion can be avoided. Second, a polarization controller was applied to keep the polarizations in optimal states such that the fluctuation of the polarizations of the lightwaves can be prevented. Third, in order to enlarge the measurement range, a polarization isolator was developed. With this isolator, the measurement range could be multiplied several times. A simple beam test, a material property test, and a crack detection test were conducted to verity the performance of the measurement system. It is seen that the system can provide not only strain and temperature but also the location information of the measurand. The strain and temperature measurements of the system are accurate up to 100me and 1°C, respectively, with a spatial resolution of 2 cm. The spatial resolution was further improved by a signal processing technique to reconstruct the broadened Brillouin spectrum. A limiting spatial resolution of about 6mm was realized. The monitoring system is also applicable to dynamic measurement. A measurement scheme was developed in this study to overcome the problem of low response rate. With the new measurement scheme, the system is able to measure the strain accurately in a dynamic test with a vibration frequency up to 7 Hz. Finally, several sensors were devised in this study other than strain and temperature sensors, including the displacement sensor, rotation sensor, and inclination sensor. Such multi-sensor architecture provides more flexibility and meets the requirements of structural health monitoring in real applications.
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