Tuesday, 7 September 2010

The ALR track detection system

After 18 months development, the ALR Track Detection system is up and running, although there is still some fine tuning to do. The basics are that the track detects when a train is present by a short circuit being detected across the two rails via the metal wheels and axles of the engines and carriages. This triggers a low impedence relay which is housed in the trackside distribution boxes (in batches of four- one relay for each track section - see photo).

We spent some time experimenting with different relays until we found one that had suitable impedence and drop out voltage characteristics. If these were wrong we found it difficult to achieve relay switching in wet weather conditions because the resistance across 200 wood sleepers drops dramatically when wet, and holds the relays on. Also the length of track sections had an effect.
The track is divided into 16 sections. The switched contact of the relay is fed to the input of a 555 electronic timer circuit which is designed to give a 5 second consistent high on its output as soon as it detects a train. The reasoning behind this is to give a constant output even if the track is dirty in places resulting in relay judder. This 5 seconds also allows the next section of track to be detected and displayed in the signal box before the last carriage leaves the previous section.
The output of the timer circuit is then fed via another PO type relay to the display board in the signal box which lights up relevant LED's on the display board. As you can just see in the photograph, there are three sections lit, 'Bamboo', 'Home straight' and the first ''Lake' section which indicate there are 3 trains running.

At this month's Open Day we had 4 trains running simultaneously for the first time and the track detection system enabled the signalman to keep trains evenly spaced ( most of the time!) around the circuit.