Re: Filtering options for reflected signal with noise

Does anyone think a multi-ton engine running on steel wheels on steel
tracks does *not* short the rails? They don't mount wheels
individually. Two wheels and the axle are one solid piece. It would
be nearly impossible for the engine to not short the rails. In fact,
nearly all train detectors operate on the shorting of the rails... at
least they did some 30 odd years ago. Railroads change about as much
in 30 years as other companies do in a month!

But I suppose that anyone not familiar with railroads would not know
any of this.

On Apr 27, 1:42 pm, "John121" <adriane...@xxxxxxxxxxx> wrote:
Considering the track as a transmission line, the characteristic
impedance and propagation velocity depend only on the geometry of the
rails and the material of the ties. The location of the short that you
hope the train represents determines the round-trip time of your pulse.
The basic principles are used to locate faults in transmission cables.
The subject is called "time-domain reflectometry".
The wavelength of a 1 KHz signal in free space is 30 Km, and more than
half that on an open-wire transmission line. Don't expect finer
resolution that about a tenth of a wavelength. You need a much higher
frequency (narrower pulse).

I am not aware of any reason to consider the frequency rather than the
pulse width and edge rate. There are both radars and sonars which
send single single pulses and watch for the return. The rep rate is
very low and therefore the frequency. But the pulses are short with a
fast rise time. This allows the round trip time to be measured very
accurately.


These are the reasons we have changed the method of finding the location
of the train. After researching the attenuation and velocity of travel of
different frequencies it was seen that the original method would not work
so well.

What sort of distances are you trying to measure? Typically train
detectors are measuring distances in the range of a couple of miles or
less. For crossings and even for track sections, this is done with a
DC current. But this only measures the presence of the train, not the
distance. What data do you have for attenuation of signals on rails?


If the probe and power signals have different frequencies, a bandpass
filter seems like the obvious way. What is the difficulty?

Yes I understand if our signal is in the kHz and the traction noise is
lower at 50Hz then all I need is a bandpass or highpass filter.

One thing I am unsure of would the following:
If we are measuring the input impedance via measuring the voltage at
different points (here is a rough idea of how we intend to measure input
impedance:http://www.mitedu.freeserve.co.uk/Theory/inzoz.htm) and then
seeing the difference in amplitude and phase to calculate the resistance
and reactance which we can then use to work out input impedance.

As we are sending the signal down the line, it is going to be reflected
back towards the source (either via an O/C or S/C). This reflected signal
is going to alter the readings at the 2 points. Would this not affect our
measurements?

If so then I would need to 'filter' the reflection. What types of filters
can be used for this? Also the reflection is going to be different
depending on S/C or O/C so I cant seem to think of a way of 'filtering'
this. When I say filtering for the reflection I don't mean filtering but
rather some subtraction or addition.

I thought you wanted the reflected signal to alter your measurement?
If not, how will the train be detected?

If you are trying to measure long distances, you might be able to
measure DC resistance. But tracks are often cut and joined with links
to allow for expansion and contraction of the rail. The joint is
typically bridged with a wire welded the to rails just to allow the
detectors to work. If this is always true in your case, then a DC
measurement should suffice.

To be honest, I don't see how the "impedance" measurement will work or
why you need a low frequency AC signal. Do you have an idea of what
the impedance of the rails will be for a 1 kHz signal? How do you
expect the impedance to vary with the position of the train?

One thing that does make this more complex is that you obviously are
working with electric trains running off of a powered third rail. One
of the rails will be the return path which means you could find some
very high potentials on that rail. You would need to do a lot more
than just "filter" the incoming signal. I would suggest that you use
transformer coupling to connect to the rail for safety reasons.
.

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