Measuring the length of a harness' wire w/non intrusive device

[ealvarado]

Thanks for your time. I have a harness routed from a PD Box down to the chassis framerail and all the way to the rear of a truck. I want to measure the length of that harness from the PD Box terminal to any given point half way through the chassis usig non intrussive method. The harness is protected with convoluted tubing and it contains about seven wires. I thought about using a device that would send a signal, or current through one of the harness' gauge 8 wire from the terminal at the PD Box and with a non-intrussive probe to register the signal at the desired location approx. 2 400 - 2,600mm and calculate an accurate length across a sample of 50 trucks.

This measuring system will be accurate enough to pass an MSA.

I will appreciate sharing your ideas and sugestions on how to do it?

Thank you

EA

 

[Lewish]

What is an MSA?
Why do you need to measure the distance of an arbitrary point along a wire?

More info = more help.

 

[jraef]

ealvarado,

There is a product on the market that is used to detect broken wires and tell you roughly how far it is away from the connection point. I think it used a pulse transmitter and timed the reflection to calculate the distance or something like that. I used to have one for detecting a bad conductor in large bundles of control wires. I belive I saw the phone lineman using one as well for the same purpose. Unfortunately the name of the manufacturer or the trade name of the product escapes me at the moment. Check back in here later, as the name will surface in my brain and wake me up at approx. 3:00 AM on a Saturday when I could be sleeping in, as usual.

Quando Omni Flunkus Moritati

 

[peebee]

Sounds like you need a TDR -- Time Domain Reflectometer. A web search should find many for you.

 

[peebee]

By the way, you can rent them, too. If you're feeling adventurous, it's not too hard to build your own with a pulse generator and o-scope.

 

[jraef]

Never mind, I remembered it after all.

http://www.biddleinstruments.com/

Check out their Time Domain Reflectometers (TDRs). That is the device that pings the wire and tells you how far away the end is. They also have a small hand held device called a Cable Length Meter. Probably works on the same principal.

Oh, I just saw peebee's reponse. You beat me to it.

I guess I have killed too many brain cells. Must stop doing that soon.
Quando Omni Flunkus Moritati

 

[Lewish]

Sorry Guys, but a TDR will only tell you how far it is to a break. If you read his post carefully, he says nothing about a break or wanting to break the wire.

 

[jraef]

No, it will tell you how far it is to any anomally, including the end of the wire. You are right however, I overlooked the part about ANY POINT along its length, and that is not necessarilly a break.

Geologic displacement sensors work by having beads clamped onto the wire and any movement of the wire changes the reflection from the beads, giving a signal that can be interpreted for distance by virtue of the number of unrestricted beads in front of the change (I think). Those are very sophisticated versions of TDRs, (translate: expensive) but the idea is to have some sort of detectable anomally in the wire.

Magnetostrictive pulse sensors work on a similar principal and you could put a magnet over the wire at the measurement point, but I beleive that they require 2 conductors, and I'm not sure that anyone makes such a device as a portable measurement tool. This company makes a magnetostrictive LVDT (Linear Voltage Displacement Transducer) system called Gemco. Maybe they would be interested in helping come up with one.

http://www.patriotsensors.com/

For my money I would try the TDR and experiment with finding a way to create an anomally along the length non-intrusively, (i.e. magnetic or inductive). But then again, its not my money!

Quando Omni Flunkus Moritati

 

[peebee]

Lewish, the end of a wire IS a break, isn't it? Even if he had the wire terminated into a resistor which matched the impedance of the car/wire transmission line, which is unlikely, he'd still get a nice ping off the end of the wire. Unterminated, he'll get a bigger ping off the end of the wire than from any intermediate poor connection.

If you can break and then reconnect the wire at the midpoint, the TDR will work fine. Actually, if there are connectors installed throughout the run, the TDR should be able to indicate the length to each connecter, all in one shot, even with all connectors connected.

If you can't break the wire, then jraefs other ideas make sense. Another unconfirmed idea: you might even get a TDR reflectance off a standard snap-on balun, or maybe one modified with a break/resistance/impedance in the balun magnetic circuit or "secondary".

ealvarado, can you give us some more insight regarding the type of wire & signal, the accuracy required, and the reason you want to check these lengths? What's the need to measure the length to any midpoint?

And just to make sure we're not ignoring the trivial solution, are these new or existing installations? Can you just mark the harness at 6" intervals prior to installation?

 

[jbartos]

Suggestion: Frequency Domain Reflectometry (FDR) might also be applied. Visit

http://www.acterna.com/united_states/technical_resources/downloads/tech_notes/FDRdefine.pdf

etc. for more info.