Distributed temperature sensing and non-contact torsion measurement with fibre bragg gratings

M.Ing.This thesis demonstrates the measurement of temperature distribution across a metal plate and the non-contact measurement of torsion in a rotating shaft. After a thorough theoretical study of the origin of fibre Bragg gratings, methods of fabrication and erasing of gratings, methods of demodulating the signals from gratings, and methods of discriminating between strain and temperature variations in gratings, simulations and experiments were done to prove the feasibility of both temperature distribution measurement and non-contact torsion measurement. For the measurement of temperature distribution, a 450 x 450 x 1.6 mm type 304 stainless steel plate was used. Two 4,7Ω 25W resistors were used as heat sources, and five fibre Bragg gratings measured the temperature at strategic points on the plate. The measured values were interpolated appropriately and a three-dimensional representation of the temperature distribution was shown on a personal computer monitor. The measurement of torsion was first done on a non-rotating 25 mm diameter solid shaft with torque applied. Both resistive strain gauges and fibre Bragg gratings were attached to the shaft and the measurement results compared well. Up to 90 Nm of torque was applied to the shaft. The non-contact measurement of torsion on a rotating shaft was done with fibre Bragg gratings and involved the use of graded index (GRIN) lenses for transmitting optical information between the light source, the gratings, and back to the spectrum analyser through free space. A special Böhler steel with high yield strength was used, and the set-up was mounted on a lathe. The one end of the shaft was clamped in the lathe chuck, and the other was fixed to an automobile drum brake system, allowing the application of torque on the rotating shaft. Up to 97 Nm of torque was applied to the shaft rotating at 190 rpm, and the results are within 10% of the expected values. As far as we are aware, this is the first time that torsion is measured by fibre Bragg gratings without the need to have physical contact with the shaft, i.e. slip rings, to transmit the information to and from the sensors. The experimental results of this thesis confirm that fibre Bragg grating sensors can be applied usefully in temperature distribution measurement, as well as in the measurement of torsional stress in rotating shafts