Shielded Cable Termination Point
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Hey Everyone,
I have a question about the bonding of shielded cables. I have been chasing a noise issue on endstops that are connected to my duex. I believe it has been causing connection drop out issues as well. I have just made some screened stepper motor cables for my a and b stepper motors on my corexy to try and stop this. I have bonded them to their mounts which are aluminium and just have the other ends to bond. Where is the best place to do this. I know that really it should be the source of noise itself, however I can not find anywhere other than the negative terminal to do it. However, I don’t want to try and jam another ferrule in due to grounding issues as explained in the wiring instructions between the duet and duex. Should I just bond it to the chassis?
Kind Regards,
Sam
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Also, should I ground the end stops that I have also done as well as my magnetic filament monitor? I do not have a bonding point on the filament sensor, however one end is better than none.
Regards,
Sam
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Hi,
Normally shields are connected to ground only at one end.
But I have yet to find shielding necessary.
What sort of end stop devices are you using?
Frederick
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Hi @fcwilt
I usually have always done that, however I was advised by one of our electricans at work, that you should always bond both ends with the likes of VSD's etc so I have just made it what I do with everything I can (Transducers etc). I was having issues as per the thread below.
https://forum.duet3d.com/topic/11504/problems-after-updating-firmware/34
David believed my issues I was having with the board doing funny things like running slowly, probing in mid air, disconnections etc could be noise on an endstop on my duex upon startup. The printer would sometimes be unuseable. I basically just had a simple filament out switch. I disconnected it and the issues stopped instantly.
Kind Regards,
Sam
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For electrical work grounds are connected at both ends because they are intended to carry current in the case of a short.
Shields are not intended to do that so they should be grounded at one end only.
End stop devices are suggested to be normally closed (NC) devices. With normally open (NO) devices the wire to the Duet can act as an "antenna" and pick up enough noise to trigger the end stop input.
So shielding in that case may help but using a NC device is even better.
Frederick
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Shields are not grounds, however the only reason to connect a shield at both ends of a wire, is if one end isn't the actual end. Consider some equipment that is not electrically connected to ground, in that case you may find connectors which carry the shield through, but thats a somewhat specialized application as most industrial equipment is grounded where it is installed.
In any case it still follows that the shield is only connected to ground at one end. If you don't have connectors that have a pin or body connection for ground, the usual solution is a ground bar where you connect all your shields, and then you run a single wire to ground, easy to manage and troubleshoot.
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Hi @fcwilt
Yes i understand shields are not grounds and are always connected at both ends. I understand that ground loops can form through the shield if they are bonded at both ends and some say not to do it, however I have also read conflicting statements in my research tonight that states you should. I will try bonding it at one end and see how I go. It would be good to bond it at the source of the noise however, so it would be nice to figure out where the best place is to bond. I have earthed the 0V on the power supply so the power supply is not floating. Does that mean I can just bond it to my printers chassis?
My sensor was wired as a NC. It worked well for about 8 months until I upgraded to 2.03 which is where the issues began.
Kind Regards,
SamCustom Core-XY
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Could you perhaps be mixing up shielded and armored cables? The latter is grounded at both ends, because it needs to protect against a complete separation of a cable.
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@samlogan87 said in Shielded Cable Termination Point:
Hi @fcwilt
Yes i understand shields are not grounds and are always connected at both ends. I understand that ground loops can form through the shield if they are bonded at both ends and some say not to do it, however I have also read conflicting statements in my research tonight that states you should. I will try bonding it at one end and see how I go. It would be good to bond it at the source of the noise however, so it would be nice to figure out where the best place is to bond. I have earthed the 0V on the power supply so the power supply is not floating. Does that mean I can just bond it to my printers chassis?
My sensor was wired as a NC. It worked well for about 8 months until I upgraded to 2.03 which is where the issues began.
Kind Regards,
SamAs a practical matter bonding the shield to the chassis should be satisfactory given the way you have the chassis connected to the power supply.
But as I said I have built five machines using Duet2 WiFi/Duex5 setups and have never had a problem with noise so I suspect something else is going on, unless your wiring is really unusual.
Upgrade your firmware to 2.04 RC1 and see if that makes a difference.
Frederick
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While NO endstop switches are sensitive to interference from capacitive coupling and benefit from shielded cable, NC endstops are more likely to pick up inductive interference, especially if they are routed in the same bundle as stepper motor cables. Using twisted pair wires for the endstop or the stepper motor or preferably both will solve this.
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@bearer No he was definitely talking about shielded cables not armoured. I will talk to him about it again on Monday. He comes from an industrial automation background and was a VSD engineer as Schneider Electric as well so I don't know if that has any bearing on it whether different industries do things differently. He said that 9 times out of 10 when they went to factories that were having noise issues, it was due to inadequate bonding of shields or things not shielded at all, and they would go through and bond everything.
@fcwilt It was working perfectly as well until I updated from 2.02 rc5 to 2.03 and then 2.04 as per Davids suggestion. As I mentioned it would lag between moves, print in the air, drop the connection etc. When setting up for a print, it would home fine (including the z axis), it would also bed level fine using 4 point correction, and when it went to home the z axis again, it would probe in the air. Upon disabling and unplugging the filament runout switch it stopped almost instantly and only did it once more after that unlike the time before where it would do it all the time. Now it seems to be back to normal with the shielded cables. Unfortuantley I tried to keep all my cables in one place so I have stepper motor cables running alongside PT100, Piezo Probe, End stops and also the filament monitor for a good 500mm down the extrusion. I got enough shielded cable off aliexpress (braided with aluminium foil) to almost do every cable on the printer which I may just end up doing for the sake of it. It is a pain repinning everything but it will then eliminate any chance of noise.
I have connected the magnetic filament monitors shield and the two stepper motors shield to the aluminium frame at one end. Will see how I go.
@dc42 So even with a shielded cable, I may still have issues with NC endstops? When I set up pump stations at work etc, I set everything up NC as if a cable breaks (Usually rodents do it) the system will trip to protect against any issues. I implemented the same thing with the endstops. I do have some shielded twisted pairs somewhere I can use that is stranded instead of solid core. I need it for the Y axis as it moves with the carriage.
Kind Regards,
Sam -
Using the term bonding with shielding to me seems a bit strange as well, they usually serve two different purposes. While shielding is for signal integrity, bonding is for protecting humans against different voltage potentials from exposed conductive materials; a category a shield normally doesn't fall under. Bonding is also done with heavy gauge wires to avoid fire hazards when carrying fault currents.
Equipment and armored cables ares bonded, and shields are grounded in my world.
Just out of curiosity, does the problem go away if you revert back to 2.02 RC5?
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@bearer I guess it is different terms in different places.
I did have one event where it did it again even with 2.04. I know you commented on my other thread about it and what you can see in the video is what happens.
Kind Regards,
Sam -
That would be very surprising as the terms is used deliberately to distinguish it from grounding, as materials that are bonded for electrical purposes doesn't necessarily have to be grounded, they just have to be at the same potential eliminating the possibility of harmfull currents. More often than not that potential is also protective earth, but its not a requirement, (in fact sometimes not even a possibility, like in aviation).
If your previous statement was
@samlogan87 said in Shielded Cable Termination Point:
It was working perfectly as well until I updated from 2.02 rc5
and your response to reverting to that state is
@samlogan87 said in Shielded Cable Termination Point:
I did have one event where it did it again even with 2.04. I know you commented on my other thread about it and what you can see in the video is what happens.
It seems pretty clear that this is going to require a smaller spoons than I currently posses. I'll depart the thread with the advice to link to any previous relevant threads
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@bearer
Sorry I did not read your reply properly. Yes it did fix it by reverting back, however as I was buying a filament monitor, I could not use that firmware.
Regards,
Sam
