Hello All.
I am not an electronics expert, so may I ask your advice/ pick your brains, and maybe even get some "build it yourself" diagrams.

My problem is this:

In the UK, some railway lines are layed using steel sleepers (the bits the lines sit on) See: [www.pandrol.com]

The sleepers are insulated from the rails by insulating pads (approx. 10mm thick).
To detect the passage of trains, most use a small (2-6 Volts) DC voltage fed down one rail, through a relay coil and back down the other rail. The trains axles short out the rails, the relay coil loses voltage, and an indication is sent back to the signalling centre.
Some track circuts use a frequency to achieve this, common ones are, 1700Hz, 2000Hz, 2300Hz, and 2600Hz
Some track circuits use a tuned "reed" at the frequencies from 363Hz-423Hz, going up in 3Hz steps.
In all the different track circuits, the train axles short out the rails, dropping the relay.

When we've been on a relaying job, sometimes a steel sleeper(s) get the insulating pads left out, shorting out the rails. The signalling system thinks that there is a train sitting there, and those gallant chaps from the Signalling Dept. have to go and find the offending sleeper(s).

What I was thinking was, rather than use a voltage to find the location of the short circuit, as the voltage is too supceptable to the many of the variables, particularly the length of track circuits (from 20 to 1000 meters), I could use a frequency.

I would need a transmiter putting out enough power to go 1000 meters.
My idea would be to use both the rails as the arial, and send the transmitted signal down the whole length of the track circuit. The distance between the rails in the UK is 1432mm (4foot 8 inches), so I need the receiver to be sensative, but directional, as I will be pointing it at the sleepers, from a max. height of about six inches. My idea being that where the steel sleeper is in contact with the rail, it will become part of the transmitting arial, be detected by the receiver, and BINGO!, we've found the fault. A lot quicker than the current method of clearing every sleeper of stone and physicaly checking. (Sleeper spacing about 750mm, thats 1250 sleepers checked both sides for a 1000 metre relay!)

Limitations are that with the frequencies in use on the railways, and their harmonics, namely as mentioned above,the 50Hz traction current frequency, and the following TPWS frequencies (a train protection system), that uses:

64.25, 64.75, 65.25, 65.75, 66.25, 66.75 all kHz

It's also got to be small enough to be "one-person" portable (esp. the receiver!)

Thanks in advance for your thoughts ideas,

Chris



Edited 1 time(s). Last edit at 09/27/2006 04:03PM by Chris Wink.