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Old June 18th, 2017, 01:17 PM   #1
mikla90
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Question Cable testing with a PLC

Hello everyone! I'm Andrea, 27. I've just started working with PLC, but I know all the theory behind it from school.
First thing, I didn't find a "Welcome section" and so I couldn't introduce myself, sorry for that.

But let's begin with my problem. I have to test 30 different channels, just check if all of them are under 1/4 Ohm of resistance. I have to do it with a PLC, most probably a S7-1200. I was wondering if it's possible to directly connect via software two different outputs or an input and an output. When I say "connect" I mean phisically because otherwise the Fluke multimeter can't detect the correct resistance.
Of course, since I'm a beginner if you are open to other ideas, I'm all ears!

Thanks everybody!
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Old June 18th, 2017, 02:45 PM   #2
labeledas
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what exactly are you trying to measure. give us a description of the system.

Is it the input channel resistance or your plc of some other device, will this be a one time measurement or a continuous monitoring, since you mention 250 Ω I am assuming you are testing analog input card channels.

you also mention connect via software then connect physically. so do you want your plc to control your physical input output connection. its possible but you will require other hardware, relays, multiplexers, etc...

you mention a fluke meter but also mention you have to do it with a PLC I have never seen an actual resistance input module aside from RTD ones and I don't think those will work for what you are attempting.

Last edited by labeledas; June 18th, 2017 at 02:51 PM.
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Old June 18th, 2017, 03:23 PM   #3
mikla90
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Hi! Basically, I have to build a test bench in which I can check automatically if every single channel has a resistance that is less than 250 mΩ.
What I want to do is this: connect all the channels to the Fluke with the PLC in between them and than use it to switch between all the channels, so that the measure is done on a single channel only when the PLC closes the circuit on that channel.
To do so, I have two ideas:

1- if possible (but I don't think) connect all the I/O and then use the PLC just as a relay to directly connect an input to an output

2- connect every channel on one side to a common cable and then put it in an output. Then, connect the other channels to one output each and then "short out" one channel at a time with the common one.

It's quite tricky to explain, but really easy to understand!
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Old June 18th, 2017, 05:27 PM   #4
ASF
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I think I see what you're trying to achieve. You have 30 "channels" - for the purposes of simplicity, let's just call it 30 "devices" - and each of them, when measured with a multimeter, should read less than 0.25ohms. The Fluke multimeter will perform the test, your task is simply to create a test rig that, having connected your Fluke to it, will connect each device to the Fluke in turn. Then you watch the Fluke as it cycles through and ensure that each reading is correct. Have I got this right?

A few things you'll need to define.
- What type of outputs are you using? Relay outputs? 24VDC triac outputs?
- How does the "cycling through" need to function? Does activating a certain input (say, input 7) cause the test rig to measure a corresponding device (say, the one connected to output 7)? Or does the test rig just cycle through all 30 devices, with a 2 second pause on each?
- Are these devices just a simple load like a resistor? Or something more complex?
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Old June 18th, 2017, 05:45 PM   #5
OkiePC
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I think by "channels" the OP means "conductors". As in a 30 conductor cable or wiring harness.

Instead of connecting the meter to each end of each wire, land them all on some sort of assembly that runs the test.

I suppose you could set something up with RTD modules configured for ohms instead of an RTD standard and get a reading from that. 30 channels of RTD modules might be pretty costly but the time saved might make it worthwhile.

It would help to see a picture of the cable assembly or get more details to give better advice.
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It's not all the variables I am most concerned with, it's the undiscovered constants.
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Old June 18th, 2017, 06:01 PM   #6
mikla90
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OkiePC is almost right. The wires are just wires, there's nothing in between them. It's a rotary joint that transmit power from one end to the other of the same device.
Let me put it this way. Imagine you have to test an extension cord with 30 wires in it, ok? I've to check if every single one of this cables is under 250 mOhms. To do automatically so, I have to use a PLC. For the measure itself, I have a Fluke multimeter and I "only" need a PLC as some sort of multiplexer. My problem is if it's possible to test one channel at a time according to a schedule I made in the PLC and switching between them with the same PLC. Sorry if I'm not clear enough, but it's 1 am here and I'm very tired.
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Old June 18th, 2017, 09:12 PM   #7
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Let's assume that both ends of this cable are terminated in some sort of connector with pins 1 thru 30.

Make a connector for one end that shorts pin 1 to pin 2, pin 3 to pin 4, pin 5 to pin 6, etc... now you have 15 wires to test.

Get 2 16 channel relay output modules. On one relay output module, connect the positive side of the Fluke to the common and all the odd number pins to the outputs. On the other relay output module, connect the negative side of the Fluke to the common and all the even number pins to the outputs.

With the Fluke set to read Ohms, have the PLC turn on the first output in both relay modules. This will give you a resistance reading on wires 1 and 2.

There will be resistance added by the shorting plug and the relay modules, so you'll perhaps need to account for that.

Shawn
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Old June 18th, 2017, 09:53 PM   #8
GaryS
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I think your problem is not the PLC sequencing through the 30 lines (that’s easy to setup) but the switch itself.
.25 ohms in not much to work with. At a bare minimum you will need a relay that has bifurcated contacts that are gold plated or maybe mercury wetted contacts ( as much as I hate to see them used anymore) neither of those are available in a PLC output so you will need an interposing relay that will have a constant repeatable resistance across the contacts. Very difficult to find.

I would give some thought to setting up a constant current source switched in to each line and measure the voltage across the line. Still using the bifurcated gold plated contacts.

Set it up to use 30 analog inputs 0-10VCD ( one for each line) then switch in one line at a time give it a 1 sec delay for the current to stabilize and read each input.
Remember any contact will add resistance to the circuit and that will change with every operation of the relay so reading low ohm values through a relay contact would not be reliable a constant current would eliminate the variable resistance of the relay contacts
By using an analog input you could set up a arry in the PLC to store the results for record keeping.
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Old June 19th, 2017, 12:00 AM   #9
mikla90
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The relay solution is the best solution for my purpose. I'll let who's in charge of deciding the components what to use. My curiosity was if it was possible to use only the PLC, but apparently not.
Thanks guys, you're awesome!
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