Determination of speed is an important requirement in a wide variety of
technical and industrial applications. The measurement of speed has
evolved from pure mechanical to magnetoelectric and radio frequency
(RF) based devices. There is however a demand for low-cost sensors
capable of operating in potentially hazardous environments in which
optical fibre sensors are more suited. In this thesis the development of a
non-intrusive speed measurement sensor, based on optical technology is
described.
The optical fibre sensor described herein gives an almost instantaneous
indication of the speed and is compared to a commercial speed sensor
(Cateye, 1993) which relies on a magnet attached to a rotating surface
yielding an averaged speed reading over several revolutions.
The optical speed sensor requires incoherent light, conveniently provided
by two low cost 780 nm CD-type laser diodes. The light was directed
through a pair of spatially displaced fibres illuminating the moving
surface. Some of the back-scattered light from the moving surface was
collected by a second pair of fibres leading to two photodiodes (where
the light information was converted into an electrical signal). An
analogue to digital converter in conjunction with a PC (386SX-33) was
used to digitise the signals from the photodiodes. The numeric data was
then used to compute the speed of the moving surface using a cross-correlation
technique.
The optical fibre speed sensor was tested successfully on a large
number of surfaces at speeds up to 30 km/h. It showed excellent
agreement (better than 3%) with the commercial speed sensor. The
absolute accuracy (about 9% at 30 km/h) is limited only by the quality of the analogue to digital converters and indirectly by the data processing
capability of the PC.