computer/machine grounding

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computer/machine grounding

Postby omega » Mon Apr 23, 2018 5:02 pm

What is the best policy for grounding machines throughout?
I am using a uc 300 usb controller and im occasionally getting usb disconnect errors (3 times today)
all the connections are good so im hoping its noise on the usb line.
I have found it seems to run more reliably with being powered from the usb port, but im not sure what I should be doing with grounding and 0 volts.
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Re: computer/machine grounding

Postby CT63 » Mon Apr 23, 2018 5:29 pm

Try a different USB cable. I've run across more than my fair share of flaky USB cables.
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Re: computer/machine grounding

Postby Robertspark » Mon Apr 23, 2018 6:48 pm

What you want to avoid is ground loops

Although this applies to audio it is the same regardless ... loop%3famp

In simple terms... You have too many earth connections.... Yes it is possible to have too many earth's.

So your CNC machine is grounded / earthed.

Your controller box is grounded / earthed

Your pc is grounded / earthed..

Then you wire everything in multi core or screened cable and join all the cable shielding to the equipment chassis

... You have now created a ground loop system.... In theory it's all at the same potential.

Until something runs... Moves or works and currents are induced in the cable screening.... Now you have electrical noise...

If you are running usb this is more of an issue because it was never developed for noisy environments... And never for long data transmissions.... (Unlike ethernet and other protocols)

Suggestion.... Get your usb lead..... And bear the cable screening.... Cut the screening , bear it back so there is a gap.. .

Tape over the top of the beared back cable

Now try to run the system.... It should work better.

... Another suggestion....
If you have a voltmeter

Disconnect your usb from pc to uc300
Connect a voltmeter between the pc and the CNC controller / uc300 ground.... Sometimes you'll read a voltage difference... This will be because of the power supplies in the cabinet or pc.... I've seen people connect the zero volt line of the power supply to chassis earth.... I've also seen it in the power supplies.
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Re: computer/machine grounding

Postby cncdrive » Mon Apr 23, 2018 9:56 pm

Disconnection could be:

1.) The USB suspend function is enabled in the Windows power management which enables Windows to remove any USB connections whenever it thinks you don't need the USB device. It usually happens when you not moving the mouse and not pressing keyboard keys, so Windows thinks you are away from the PC and so it thinks it can remove your USB devices to save power. Make sure to disable this function.

2.) Noise problems. Which could be due to bad grounding and/or high EMI noise effecting the mains and the computer.
How noise effecting the system is it moves the ground potential (PE) which usually happens when a high power device is switching high current quickly which pulls the mains ground potential.
The issue is that USB is ground referenced communication which means that the ground of the PC and the ground of the USB device are on the same electric potential.
The grounds are connected via a wire in the USB cable. That wire ofcourse has resistance and inductance and so if the potential change rate is high enough then the wire can delay the potential change and so the USB device's ground potential will shift from the ground potential of the PC. Because the communication is referenced to the same ground point on both sides if this voltage potential shift is large enough will cause invalid datas and broken packets and then the software must disconnect if too many neighbour packets are broken or even the USB chipset of the PC could give the communication up if the noise level is high enough.

The key for grounding is to create star shape grounding scheme with all devices in the control system are grounded to a single point with thick and short wires.
And that single point is the middle of the star and that point could be then wired back to the mains ground.

With the motion controller it could also help if you will place a thick and short wire between the PC chassis (ground of the PC) and one drill of the UC300 top module.
This could not help reducing the noise, but it could help converting most of the noise from differential to common mode when it will not cause problems, because it means that the 2 ground points will be linked together with a lower impedance and so they can better follow eachother.
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Re: computer/machine grounding

Postby Battwell » Tue Apr 24, 2018 9:41 am

using the 300 usb -the last thing that balacz wrote was the only thing that stopped my usb dropping out. (thick wire from top board of uc300 to pc 0v)
all problems were solved going to uc300eth. - never dropped out or crashed.
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Re: computer/machine grounding

Postby cncdrive » Tue Apr 24, 2018 12:39 pm

Yes, ethernet works different and the possible problems which I described about the USB won't happen with ethernet, because ethernet gives an isolated connection.
How ethernet achives the isolation is that there are miniature isolation transformers (called ethernet magnetics) inside the RJ45 jack of the UC300ETH and also inside the RJ45 jack or on the motherboard of the computer. So, the connection is isolated on both sides and there are no common ground points on the PC and on the ethernet device side.
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