This thesis investigates the use of a novel optical fibre sensor (OFS) for structural monitoring in remote or inaccessible areas. OFSs are desirable for this application because they are intrinsically safe, add little weight and are free from electrical interference. Fibre Bragg gratings (FBGs) in particular are attractive since they act as optical strain gauges, converting fibre strain to a wavelength-encoded signal. This project uses FBGs written into three or four cores of a novel four-core fibre at the same point along the fibre length. This configuration allows the local curv~ture of the fibre to be obtained by measuring the strain difference between cores ·at an applied bend. The multicore fibre (MCF) eliminates any temperature sensitivity and strain transfer issues ofthe sensor. Dynamic curvature measurement was achieved by interrogating each of the MCF cores with an arrayed waveguide grating (AWG). This was demonstrated at interrogation speeds >11 kHz for a stainless steel cantilever vibrating at -30 Hz to achieve a curvature resolution of 0.09 m-I. Quasi-static tests found a curvature resolution of 0.02 m-I. This dynamic curvature measurement technique was applied to create an accelerometer from a cantilever formed from a short length ofMCF vibrating at frequencies up to 3 kHz. The accuracy of the acceleration measurement was better than 5 % at frequencies below 300 Hz. A commercial interrogator based on a tunable laser technique was used for several quasi-static applications. The curvature of a 240.8 mm diameter aluminium and Perspex cylinder was found to be resolved by the MCF sensor to 0.01 m-I. Four MCF FBGs were spliced together to form a multiplexed array in order to investigate shape measurement using several curvature measurements. The array was attached to a 33 cm long deformable Perspex rod and the MCF FBGs were found to measure curvatures that matched very closely with values predicted by a mathematical model. This was demonstrated for both horizontal and vertical deformations. Finally, an MCF FBG sensor was embedded in a short strip of compliant material (Sylgard) in order to create a sensor that could easily be wrapped around a small test object without the need for permanent bonding. This was used to measure the change in radius of a 2 cm diameter sample as it was compressed. This was found to be capable of measuring a radius change of< 30 Jlm.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:491408 |
| Date | January 2008 |
| Creators | Fender, Amanda |
| Publisher | Heriot-Watt University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10399/2058 |
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