Investigation into the feasibility of an electro-optic rail surface monitor
Cracks and surface imperfections in railway track are important when considering its lifetime. This thesis investigates the feasibility of a new electro-optic technique which records surface irregularities on a track. It is more sensitive to crack detection than the traditional trailing wheel method and approximately one thousand times less expensive than the highly sensitive ultrasonic technique used by British Rail. The electro-optic method consists of an optical projection-detection system with two mutually oblique axes. Light projected onto the rail is reflected and imaged onto a split photocell. A depression or rise in the reflective surface causes the axis of focus to shift and more light falls on one of the photocells. The difference in optical power between the two photocells is therefore a quantitative measure of displacement and consequently rail surface profile. The intention is to fix the device onto a bogey of a train so that measurements are taken as it moves over the track at speeds of up to 40ms-1. The relationship between vertical displacement and differential optical power is dependent on the shape and size of the spot. It is nonlinear for a circular spot and imperfections spanning less than 1 mm vertically can be resolved. Vertical displacements greater than 5mm cannot be detected solely using the difference in optical power and the nett power falling on both photocells must be considered in addition. An investigation into the reflective properties of rail lead to the optics being configured for specular reflection. The angles of incidence and reflection are optimally set at 35°. In addition, the use of an infra-red source is found to be an added advantage.
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