光纖光柵感測扭曲儀之研發

Abstract

本研究主要目的是發展光纖光柵感測系統應用於地層滑動之監測技術，以扭曲儀作為光纖感測器在地層滑動長期監測上的應用，發展一解析度高、穩定度高的自動化土木監測系統。光纖感測器有別於傳統電感測元件，其優點在於單線光纖同時作為訊號傳輸與感測器、量測數點至數十點的應變；以光波傳輸訊號，監測結果不受環境干擾，可針對電磁場強度高的地方如高壓電塔、鐵道等高危險建築物進行安全監測，在地下水面以下的區域，無須擔心線路短路發生。 傳統地滑監測是利用傾斜管與電子測傾儀，量測方式是藉由測傾儀在傾斜管內移動量測不同深度的傾斜度，累積可得地層滑動情形。傾斜管系統自動化須將多個測傾儀長時間固定在傾斜管內，如此將所費不貲。另一可行之道是將傾斜管表面貼上許多應變片，量測地滑時傾斜管上各點的應變，累加後可得到傾斜管側向變形量，這就是應變管的由來。應變管不易安裝更不易維護，因此很難被大量使用。本研究自行研發的扭曲儀融合測傾儀與應變管兩者的設計概念，可以使用在已安裝的傾斜管上，而扭曲儀量測傾斜管變形的方式，以量測彎曲應變取代測傾儀的傾斜度。基於光纖光柵感測器的優點，新儀器的重複使用避免光纖光柵成為高成本消耗品，效果較應變片更佳。本論文將闡述光纖光柵感測扭曲儀的發展與初步試驗的成果。

The main objective of this research is to develop an optic fiber bragg grating based distormeter as a means to perform long term ground movement monitoring. This new automated system has a superior resolution and higher stability than currently available electronic devices. The main advantage of a fiber optic sensor, as oppose to the conventional electronic sensors, is that it is possible to use the same optic fiber for both signal transmission and sensing physical quantities. As a result, it is conceivable to have multiple sensors on a single optic fiber. Using light as a means for data transmission, the signals are immune to the environment. Thus, they are more suitable for areas under strong electromagnetic interferences such as high voltage transmission towers or electric rails. Short cutting the circuit is not a concern when using fiber optic sensors under the groundwater. The ground movement has traditionally been monitored using the inclinometer casings buried in the ground that move with the surrounding earth mass. The amount of movement is usually measured by lowering a sensor probe to the bottom of the inclinometer casing. The sensor probe measures its inclination at a regular interval as it is pulled upward to the ground surface. The automation of a conventional inclinometer casing system would require installation of multiple sensor probes in the casing on a permanent basis, which could be prohibitively expensive. Alternatively, it is possible to attach a series of strain gages on the inclinometer casing that measure the flexural strain distribution of the casing at regular intervals. Integrating the strains would then yield the amount of casing movement. This concept is referred to as the strain pipes. The strain pipes are difficult to install and even more difficult to maintain, and thus have not been widely used. The design of the new distormeter combines the concepts of the inclinometer sensor probe and strain pipes. The new system still requires the insertion of an inclinometer casing in the ground. The amount of casing movement is now monitored with a fiber grating sensored probe that measures the flexural strains, instead of inclination of the probe, as it deforms with the casing. Because of the advantages of the optic fiber bragg grating, the new sensor probes can be multiplied at a significantly reduced cost and without the complication of strain gages. This thesis describes the development of the new fiber optic distormeter and results of its preliminary applications.