SelTrac
From Wikipedia, the free encyclopedia
SelTrac is a digital signalling technology used to control the movements of rail vehicles.
SelTrac is a product of Alcatel.
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[edit] SelTrac technology
SelTrac is a software-based system for controlling the movement of rail vehicles (that is, trains, light rail, and trams).
The key features are
- it provides fully automatic train operation (ATO), although trains can be driven on manual override
- it provides automatic train protection (ATP) to avoid collisions, even in manual mode
- safety is guaranteed by three levels of computer control, incuding the trackside computers
- vehicles run according to a pre-planned fixed timetable
- vehicle separation is guaranteed using a "moving block" system. Vehicles can safely operate at full speed with separations down to 50 metres.
- vehicles must run on a fully segregated rail; the system cannot detect pedestrians or other vehicles, and so it is unsuitable for an integrated tram system
[edit] How it works
[edit] Older technology
The original SelTrac technology uses two radio signalling lines laid along the trackbed, one between the running rails, and one outside. At a fixed interval, the two lines swap over. The signal is transmitted with a phase difference of 180 degrees between the two lines, so the vehicle can detect the swapover, and use this to calculate its exact position on the rail network. Further positioning information is obtained by counting revolutions of the wheels.
The speed of the vehicle is set from the central control computer, and transmitted along the two control lines 50 times a second. If two consecutive transmissions are missed, the vehicle assumes there may have been a system failure, and begins an emergency braking procedure. Otherwise the vehicle will set its motors to run, brake, or idle. The idle setting allows power to be saved when the vehicle is running ahead of schedule.
On arrival at a station, the vehicle is slowed under central control until a station alignment board is detected. This is placed between the running rails. Doors can then be safely opened automatically. If the vehicle overshoots, perhaps due to impaired rail friction (leaves, oil, ice), it must be manually reversed until the alignment board is captured.
[edit] Newer technology
The two-wire system was prone to vandalism, and in newer versions of SelTrac the radio control signal is trasmitted along the running rails themselves.
[edit] SelTrac installations
SelTrac is installed in many railways around the world, including the following.
- Docklands Light Railway, London
- Vancouver SkyTrain and project news
- Thessaloniki Metro and project news
- KCR West Rail, Hong Kong
- KCR Ma On Shan Rail, Hong Kong
- MTR Disneylnad Resort Line, Hong Kong
- BART, San Francisco Bay Area
