THCAD Environmental Fluctuations

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03 Feb 2021 13:56 #197549 by robertspark
my 2 pence / cents...

Check you do not have multiple earth / ground connection points at both ends of the cable.

This is just my opinion (former electrician / consulting engineer).

Suggest connecting the THCAD "Shield" connection to the cable shield, (directly) and in turn connect to the ground pin on the CPC at the plasma cutter.

connect the THCAD (+) to the plasma cutter (+) terminal at the CPC at the plasma cutter

connect the THCAD (-) to the plasma cutter (-) terminal at the CPC at the plasma cutter

CPC = circular plastic connector {CNC interface}

if you have a THCAD300, then (+) to plasma cutter clamp and (-) to plasma cutter torch (cutting current, not start current!!!) connection.

I would NOT suggest connecting the shield to your cabinet "GND" or EARTH coonection point, and then sub feeding it somewhere else.

If you look at the circuit of the THCAD it has an RC (resistor capacitor) filter arrangment between (-) and (shield) and (+) and (shield) connections.

If you are using the THCAD for hypersensing then... connect the shield to the cable shield but DON'T terminate it at the other end.

The question is where you take the (+) and (-) connections to....

for the THCAD with a Plasma Cutting Voltage Sense application, I have bought (awaiting delivery) of some 2-core shielded cable so that the 2-cores voltage differential are given as much chance as possilble to prevent noise (I am using a ~50:1 divided input (100k + 2K) hypertherm 45 (non XP) std) fed into a THCAD5

thee shield will "hopefully" filter out all potential external noise to this very small differential voltage signal.

(I also have available a 10:1 and raw output too from previous THC devices, but want to use what hypertherm gave it as the others have RCL filter arrangements built in).

I am not going to use hypersensing (yet) as I have an adjustable ohmic device that works fine and I dont have space for a water table.

But if I was.... I would probably either get some 1 core shielded cable, or use the 2-core shielded calble and join the 2 cores together to form a single cable with a shield and route each one to the ohmic sense point and the table material clamp. I would terminate each cables shield core AT ONE END ONLY, to the THCAD and use a piece of heat shrink over the other end so that there was no chance of it connecting to the table or some other earth / ground point. That way the shield would work as intended to filter out any potential induced voltage via the THCAD (-) <> shield and (+) <> shield RC connection network.

Just a suggestion .... each to their own... if it works for you it works.... if not, try something else. (Thomas Edison didn't get the light bulb design right first time.... but then again he did end up with > 1000 patents .... I like Joseph Whitworths ingenuity more though [ hexagonal bullet].

tea break over, back on my head!
www.sjacob.org/home/humour/BackOnYourHeads.html
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03 Feb 2021 17:05 #197572 by tommylight
FYI, i use cheap "camera" cable for THCAD with a single coaxial shielded wire plus 4 very thin wires and everything is inside another shield, both shield touch each other so they are not separate:
coax = encoder input on 7i92 from THCAD
shield to GND, grounded on the control box
red wire to +5V from a small DC-DC converter used to power both Mesa and the relays
black wired to ARCOK reed switch on the plasma source/transformer
yellow and blue to torch on on the plasma from the relay in the control box.
This is as simple as it gets, costs about 38 cents/m, it is the only shielded cable i can find here (can not find STP any more).
The big machine, the one most used here has a length of 13 meters of such cable running all over the place, the plasma has a 12m long torch cable that is not shielded and sometimes can cause the drives (Lam Technologies) to fault, causes proximity sensors to trigger occasionally, but never causes any issues with THC.
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03 Feb 2021 17:53 - 03 Feb 2021 17:59 #197578 by robertspark
interesting, not used that cable before (didn't even know it existed):

cpc.farnell.com/concordia-technologies/5...Ea0eT4BoCr9cQAvD_BwE
Last edit: 03 Feb 2021 17:59 by robertspark.
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03 Feb 2021 17:59 #197580 by rodw
Thanks guys, What I did was really something quite artificial on a test bench without any need for shielded cable but it showed similar behaviour to Stefan's wonky cuts so I shared it. I think the correct approach here in the absence of any shielded cables to the outside world that need terminating is to leave the THCAD ground unconnected.

The behaviour I observed is not present on my table where I have paid careful attention to noise management.

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03 Feb 2021 18:19 #197581 by tommylight
The cable on the link is 10.4mm in dia, mine is under 7! Cheap cr@p ! :(
But it gets the job done nicely.
Not sure if i mentioned this, but Stefan should monitor the situation in his shop and check what equipment is being used when the issue happens, the ground interference can be caused even by your neighbours with faulty equipment, but since it is in Australia i am assuming desert, no neighbours and lots of animals that can kill. !:)

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03 Feb 2021 20:22 #197591 by thefabricator03

Not sure if i mentioned this, but Stefan should monitor the situation in his shop and check what equipment is being used when the issue happens, the ground interference can be caused even by your neighbours with faulty equipment, but since it is in Australia i am assuming desert, no neighbours and lots of animals that can kill. !:)



I will do Tom, its an odd problem because it only happens when the temperature gets over 32 degrees, otherwise it works properly.
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03 Feb 2021 21:05 #197593 by tommylight
Temperature over 32 is a nice starting point for troubleshooting, but in this case it just makes the suspect pool grow a lot, still there are things to test and eliminate the usual suspects:
-if there is access to the grounding rod, watering it plentifully will help in eliminating the dry ground as a suspect,
-unplugging equipment in the shop while monitoring the THCAD, one by one, this is the most probable cause although it does not seem so, reason being all the equipment in these type of shops have big motors and big transformers = a lot of varnish isolated coper wire wound around metal parts, that isolation fails with time by cracking due to thousands of heat cycles and opens a less restricted path for the current to leak to the ground. I would add this to the "humidity" related issues, not to the "temperature" side of things, but i have seen temperature issues caused by the dust collected and rammed in by the fans and the consequent holding of moisture by said dust.
-having a separate ground to the cutting table that is directly wired to the grounding rod or as close as possible.
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04 Feb 2021 07:32 #197621 by Mike_Eitel
The best ways of getting rid oof the 50hz is either over sampling and surpressing with the square root of samples, but means you loose detection speed.
Or do it analog. Have a floating wire parallel to your measurement cables and adjust that signal in hight, plus after add it with 180 degrees to jour analog signal.
Both old fashioned but work.
Mike
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04 Feb 2021 10:16 #197628 by rodw
Mike thanks. Actually when I think about it, when I use this signal, I will be using a moving average.

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