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​Photoelectric Sensors in Public Transport

​Photoelectric Sensors in Public Transport

Issue

When a trolley bus reaches a road junction, wire-switches, installed on the overhead conductor cables, control its direction of travel. Switches may be in the default “straight-through” position or in “turnout” position. As a bus passes a switch in turnout position, a sensor mounted on the overhead cables detects the presence of the current collector. The control system then resets the switch to the default position.

Application

In many countries, public-transit operators run fleets of trolley buses in major towns and cities. At road junctions, wire-switches, installed on the overhead conductor cables, control the direction of travel. Switches default to the straight-through position. When a bus approaches a turn, the driver uses a remote trigger to change the switch to the turnout position; after the bus completes the turn, the switch reverts to the straight-through position.

If a triggered switch reverts to the straight-through position prematurely, the current collector may become disengaged, damaging the switch. Operators install safety systems to prevent premature switching. Typically, these utilize timer circuits that introduce a predetermined delay or mechanical sensors that rely on physical contact with the current collector. Neither system is sufficiently reliable.

A Swiss manufacturer of trolley-bus control systems requires a highly reliable, rugged sensor system that will detect the presence of a current collector as it passes a switch. The system must operate with minimal maintenance and withstand the varied weather conditions of an outdoor environment.

Solution

The molded-resin sensor casings, fitted with glass sensing faces, are rated to IP67, making them virtually impervious to the harsh, outdoor environment. Physically small housings (30x30x15mm) eliminate the risk of accidental damage arising from a mechanical collision. Additionally, the bracket, complete with sensor, occupies a minimal amount of space in the crowded overhead area. Rugged construction and vacuum-encapsulated electronics minimize the effects of vibration.

Each unit is connected to the trolley-bus control systems via a PVC-sheathed cable with a hermetically sealed entry. The receiver has an industry-standard PNP dark-on interface. The one-piece mounting bracket allows easy removal and replacement of the complete assembly during routine maintenance. Operational results have been excellent. The new Contrinex sensors are highly reliable, eliminating switching errors and reducing maintenance costs significantly.

16th Dec 2018 S.Abedi

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