Abstract
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.