5 Detection 122 These technologies are mainly applied to detect occupancy of level crossings by road users (chapter 13.4.4.4) and in other cases with increased probability of objects other than rail vehicles occurring, e. g. at platform tracks, particularly in automatic metropolitan railways. In other applications, systems of this type are also suitable for indirect detection of occupancy by trains. An example is the disruption of a ray with sender and receiver on opposite sides of the track by each wagon of a train. The evaluation principle is similar to that for axle counters, but wagons are counted instead of axles. This principle is applicable mainly to metropolitan railways (Barwell 1983). By using the Doppler effect, the speed of trains and, calculated from speed and occupation time of a certain position, the train length can be measured (Fenner/Naumann/Trinckauf 2003). 5.2.4.2 Mechanical Technologies Mechanical supervision of the limiting areas of the clearance profile is applied by some railways. A net of wires or a single horizontal wire stretches outside the limits of the clearance profile and carries a low voltage current (figure 5.14). If an object of not too small extension breaks through this area with a certain minimum force, the wires break, disrupting the current. This disruption of current is evaluated by the interlocking or block system which can hold signals at red in this case. As repair works are necessary after such events, this technology is only useful to detect occurrences which seldom occur. Examples are: – Detection of avalanches and earthslides in mountainous areas. – Detection of road vehicles fallen from a bridge above a railway line, as applied on French high speed lines. – In situations where a railway is situated in proximity of an airport, to protect against an aircraft overshooting the runway and obstructing or destroying the railway line. Figure 5.14: Example of an installation to detect avalanches mechanically (France) Another mechanical technology is contact mats placed on and beside the track to detect the presence of persons, vehicles or other objects by their weight. This technology is applied on some automatic metropolitan railways, e. g. in Vancouver. 5.2.4.3 Magnetic Inductive Loops Such detectors consist of a resonant circuit with the inductivity situated in the track (figure 5.15). When a rail vehicle passes over the loop, the inductivity L changes due to the iron mass of the vehicle. According to the formula of the resonant circuit 1 (2 · f)2 = L · C (with f being the
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