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Topic: TDR (Time-domain reflectometer) concept. On the right track? (Read 9950 times) previous topic - next topic

t00158713

Thanks for the advice. I will have to use a ground spike(the fence is single wire and is earthed near the energiser/fencer with a long probe going into the ground).
The joinings in the fenceline include 'junctions' where the path splits in two. If there is a fault in one or both of the paths after a junction then the TDR would not be able to tell which path the fault(s) come from. Therefore you would have to test each path individually to find out where the fault is.
I wonder if it would be possible to map out all distances between the junctions on the fenceline and then pinpoint the location of the fault on the map using the following method.
Say you would expect a junction on 'path 1' 50 metres after the first junction and junction on 'path 2' 30 metres after the first junction.
Then if the TDR reads a fault 20 metres out from the first junction and then a junction 50 metres out from the first junction then you know the fault is on 'path 2'. Then you coud display on an LCD the map of the fence with 'path 2' flashing in red to show there is a fault there. The TDR would probably have to be fixed near the energiser for this unless you could tell it where on the map you are.
The main problem with this idea however is that after a few junctions the signal might be dissipated. I will first build the TDR and see if it would be possible after that.

dc42

You will also get a reflection from every junction, which will make it difficult to locate a fault after a junction.

What sort of faults are you looking for: open circuit wire, short circuit to ground, or both? A junction will look like a short circuit (i.e. the reflected pulse is inverted), but with lower amplitude than a dead short. So detecting an open circuit after a junction may be easier than detecting a short circuit after a junction.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

t00158713

I will be looking for both opens and shorts but a short circuit is the most likely.
From what gathered from research TDR manuals, the reflections at connections are a negative reflection followed by a positive reflection. This would mean that not much of the signal is lost (because the two reflections would cancel each other out).
If I

t00158713

I will be looking for both opens and shorts, but a short circuit is the most likely in the event of a fault.
From what I gather from TDR manuals, the reflections at connectors are a negative reflection followed by a positive reflection. This would mean that not much of the signal is lost(because the two reflections cancel each other out).
If I'm going to be able to distinguish between junctions and faults,  I expect I'll need a higher resolution and possibly to change the code a bit (to be able to see the two reflections instead of just one). Would your TDR be able to do this?

dc42


From what I gather from TDR manuals, the reflections at connectors are a negative reflection followed by a positive reflection.


At a junction of 3 wires, I would expect you to see just a negative reflection. At the end of the wire (i.e. open circuit), you will get a positive reflection.


This would mean that not much of the signal is lost(because the two reflections cancel each other out).


You are bound to get a significant signal loss, because the energy you sent down the wire is being split. I think the manual refers to a very short discontinuity in impedance cause by a connector.


If I'm going to be able to distinguish between junctions and faults,  I expect I'll need a higher resolution and possibly to change the code a bit (to be able to see the two reflections instead of just one). Would your TDR be able to do this?


The software can already see multiple positive reflections, or multiple negative reflections, or multiple positive reflections followed by multiple negative reflections. To see other combinations of multiple reflections, you would need to change the software to ignore the earlier reflections. This would only be possible if the reflections are well-separated in time. Alternatively, you could use external gating to hold off the comparator output for a while.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

t00158713

How much of a signal loss would you expect? If the tdr is going to tell me I have a short at every junction and then nothing after that, I may have to abandon the idea.

dc42

After looking at http://s2629002012.files.wordpress.com/2012/11/huitransmissionlines.pdf, I believe that at a junction of 3 wires, you will get 2/3 of the signal voltage propagating along the two branches, and 1/3 reflected back. If one of those branches contains a fault (i.e. completely open or short circuit), then 2/3 of the reflected signal would pass back through the junction. So you should see a negative reflection of 1/3 the initial amplitude (from the junction), followed by a positive or negative reflection of 4/9 the initial amplitude from the fault. Of course, this ignores the losses in the line itself.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Riva

#37
Nov 08, 2013, 12:56 pm Last Edit: Nov 08, 2013, 12:59 pm by Riva Reason: 1
Just a small update on this project. After much waiting/problems getting solder mask stencil to solder the GP22 chip (first one disappeared in the post but managed to get another sent after a lengthy delay) I finally got it soldered down about a week ago and have been writing test code and doing hardware debugging ever since.
Problem was the Arduino was talking to the chip fine over SPI but after initial time reading it continually timed out according to the status register. I found the cause was a problem with the oscillator circuit so after a bit more soldering I am now getting ever changing reading as I expect.
Code: [Select]
Starting...
Setup Interrupt. Current Pin State = 1
Starting SPI MODE1...
Taking Chip Out Of RESET
Configure GP22 Measure Mode 2
End Of Setup
delayMicroseconds = 1
Interrupt = 1
RES_0 = 26418 Difference from last reading = 26418
RES_1 = 466000
RES_2 = C00E4020
RES_3 = 26418
STAT = 10001
PW1ST = 0
delayMicroseconds = 2
Interrupt = 2
RES_0 = 5E54E Difference from last reading = 38136
RES_1 = 26418
RES_2 = C00E4020
RES_3 = 5E54E
STAT = 10001
PW1ST = 0
delayMicroseconds = 3
Interrupt = 4
RES_0 = 7E626 Difference from last reading = 200D8
RES_1 = 5E54E
RES_2 = C00E4020
RES_3 = 7E626
STAT = 10001
PW1ST = 0
delayMicroseconds = 4
Interrupt = 6
RES_0 = 9E716 Difference from last reading = 200F0
RES_1 = 7E626
RES_2 = C00E4020
RES_3 = 9E716
STAT = 10001
PW1ST = 0
delayMicroseconds = 5
Interrupt = 8
RES_0 = BE7CA Difference from last reading = 200B4
RES_1 = 9E716
RES_2 = C00E4020
RES_3 = BE7CA
STAT = 10001
PW1ST = 0
delayMicroseconds = 6
Interrupt = 10
RES_0 = DE8C6 Difference from last reading = 200FC
RES_1 = BE7CA
RES_2 = C00E4020
RES_3 = DE8C6
STAT = 10001
PW1ST = 0
delayMicroseconds = 7
Interrupt = 12
RES_0 = FE992 Difference from last reading = 200CC
RES_1 = DE8C6
RES_2 = C00E4020
RES_3 = FE992
STAT = 10001
PW1ST = 0
delayMicroseconds = 8
Interrupt = 14
RES_0 = 11EA46 Difference from last reading = 200B4
RES_1 = FE992
RES_2 = C00E4020
RES_3 = 11EA46
STAT = 10001
PW1ST = 0
delayMicroseconds = 9
Interrupt = 16
RES_0 = 13EB44 Difference from last reading = 200FE
RES_1 = 11EA46
RES_2 = C00E4020
RES_3 = 13EB44
STAT = 10001
PW1ST = 0
delayMicroseconds = 10
Interrupt = 18
RES_0 = 15EC1C Difference from last reading = 200D8
RES_1 = 13EB44
RES_2 = C00E4020
RES_3 = 15EC1C
STAT = 10001
PW1ST = 0


dc42

I'm glad to see that you are making progress. I haven't done any work on my design for a while, due to getting distracted by other projects. My 40-year old homebuilt bench supply developed a short after a bakelite capacitor support broke, taking several components out; so I've been designing a digitally-controlled voltage regulator to replace the existing one.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Riva

Yes, getting distracted by other projects is a problem. What with the 6 week delay in getting the solder stencil. A new engineer at work needed bringing up to speed (I try to do my soldering at work when I'm not busy) and teaching soldering/re-work skills I have not had much time. At least I finished the Arduino DMX tester for work. My problem now is finding the rest of the components for this project as they were built and tested weeks ago and now I cannot find them. Hopefully they will be in one of my toolboxes at work.
Hope you get your PSU up and running again.


lomanas123

This threat is very interesting and i believe with a very high level of know - how.
So thank you all for that.

I would like to ask if this circuit can be modified to create a TDR sensor for water level sensing.

Thanks in advance

dc42

Water level sensing using ultrasonic transducers has been done with an Arduino, as a search will reveal.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Riva


I would like to ask if this circuit can be modified to create a TDR sensor for water level sensing.

The design I'm working on using the GP22 chip could be modified to use a laser/opto detector in place of the dual comparators but not to sure it the most elegant solution as you would need to be some distance away from surface of water to allow suitable time of flight for the reflected laser signal.

dc42

To expand on Riva's reply and my earlier short reply:

TDR can be used with various types of wave, including electricity (not applicable in this case), light, and ultrasonics. Using ultrasonics is relatively easy, but the available resolution is limited by the wavelength of the ultrasonic sound, which is about 8.5mm using standard 40kHz ultrasonic transducers. Maybe you could get a resolution of 1/10 of that (about 1mm) by comparing the phase of the reflected signal as well as the time of flight. Using light is more difficult, because you need a light source (e.g. laser) that can be turned on an off with a delay that is stable to a fraction of a nanosecond, and similarly for the receiver. So I think you are better off using ultrasonics.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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