應用布拉格光纖光柵感測器於結構件承受撞擊之暫態應變量測

Abstract

光纖具有優良的機械性質、高靈敏度，以及不受電磁干擾等優點，促使了光纖感測技術的研發在近年來蓬勃發展。尤其以布拉格光纖光柵感測器最為受到重視，其在靜態與低頻的動態量測能力已受到肯定，且有大量應用在各種工程結構量測的成功實例，不過對於高頻及暫態方面的量測能力，還有待進一步的研究與開發。 在布拉格光纖光柵的理論分析上，本文利用模態耦合理論探討反射頻譜響應，模擬布拉格光纖光柵在靜態與動態應變場下的行為特性，顯示感測靈敏度隨著頻率增加而下降，以及增加應變頻率與振幅會造成感測訊號失真的現象，並建立失真臨界的概念，探討布拉格光纖光柵的動態感測範圍與限制，可作為實際應用時的參考依據。 在布拉格光纖光柵的實驗量測上，本文利用本實驗室開發的能量調變感測系統，針對各種結構件承受動態撞擊負載而產生的暫態波傳訊號進行應變量測，包括直立圓桿構件、中空圓管構件及懸吊鋼索構件，也和應變規進行同步量測並相互比較。此外再分析實驗結果的頻譜響應，並和理論及有限元素法相互對照，多方驗證了布拉格光纖光柵感測器優異的動態應變量測能力，也對結構件的暫態波傳特性有所了解並建立相關應用。

Recently, the optical fiber sensing technologies, especially fiber Bragg grating (FBG) sensors, have been rapidly developed due to the advantages such as excellent mechanical properties, high sensitivity and electromagnetic immunity. The measurement ability of FBG sensors in static and low frequency responses has been demonstrated, and there are many successful engineering applications. However, further investigations are needed for FBG to improve the measurement ability in high frequency and transient responses. In the theoretical analysis of FBG, the coupled-mode theory is used to present the reflected spectra and to simulate the responses under static and dynamic strain fields. The results indicate that the sensitivity declines with the increase of input frequency and the increase of frequency and amplitude of strain result in signal distortion. The concept of distortion that limit the practical applications is established to discuss the dynamic sensing range of FBG. In the experiments of FBG, the power modulated sensing system is established to measure the transient responses of dynamic strain for structural components under impact loadings, which include solid rod, hollow tube and suspended cable. The strain gage is also used to measure simultaneously with the FBG sensors to compare the results obtained from two different techniques. In addition, the frequency spectra of experimental results are analyzed and compared with theory and finite element method. According to the available results, it is proved that FBG sensors have excellent ability for dynamic strain measurement. At the same time, the transient wave propagation properties of structural components are analyzed in detail and the associated applications are established.