Developement of an Optic Fiber Bragg Grating sensored Distormeter
Dr. An-Bin Huang
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.
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