Gogoasa, Ion (1996) Fibre optic sensor for speed measurement. Research Master thesis, Victoria University of Technology.

Abstract

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.

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