Kflop USB ground loop

[curtiskelowna]

Not sure if anyone has had this problem,

I am currently having an issue with a ground loop on my kflop device. I have a PC and a 5V switching power supply connected to earth ground (AC side). The DC output is routed to the KFLOP 5V and GND. The problem is that USB cable also has a GND - same as earth GND on the PC and also the 5V power supply.

If I run KFLOP from 5V USB with J3 installed I have no problems. If I remove J3 and power KFLOP using an isolated 5V power supply I get noise related issues. If I use a plug in style power supply with no EARTH GND everything also works.


The way I see it, my options are to:

1. use the 5V usb connection but use a powered HUB so I am not limited to 0.5A
2. dont hook up earth GND on my 5V power supply (seems unsafe)
3. Use a USB isolator https://www.amazon.ca/iFi-iDefender3...60717904&psc=1
4. Modify the USB cable and disconnect the GND somehow (likely will affect noise on USB cable)

Any advice is appreciated,

-Curtis

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[TomKerekes]

Hi Curtis,

Strange that connecting earth GND to an isolated 5V supply would cause a problem.

I don't think that would normally be called a ground loop as there aren't multiple ohmic connections to ground.

I'm guessing your earth ground noise is very high and is somehow being coupled into the KFLOP +5V/GND lines.

Are you getting Read Failed - Auto Disconnect Errors?

Have you checked the USB cable shield? Is it less than 1 ohm shell to shell? Does it have ferrite rings?

Do you happen to have an oscilloscope to look for noise?

Have you tried turning off components, such as the Spindle Power, to see if you can isolate a main source of noise?

You might try connecting the 5V Supply's Earth GND to some other point away from sources of noise.

Otherwise I'd suggest a USB Isolator. Even though I don't believe you have a normal ground loop issue, its always good to be isolated.

[curtiskelowna]

Hi Tom, thanks for replying so quickly.

I think you are correct, I dont think it is technically a ground loop. I do get a disconnect failure only when first powering up my VFD (aka plugging in the 220V chord). I just checked my USB cable and it was reading 3.5 ohms, not sure how bad that is. I grabbed another cable and it is reading 1.1 ohms but it only has a ferrite choke on the PC side. Should I switch them out moving forward?

The source of noise is 100% the spindle, I have been running for months already with a porter cable router and have had no noise issues as far as i can tell. When I run the spindle and send step/direction signals to the servos, axis position "grows" due to noise being perceived as step pulses. However, the machine is working totally fine despite the EMI spewing spindle when I use the 5V USB connection or a non grounded walwart supply. I have tested the repeat-ability for over 30 minutes at different spindle rates and it is solid. As soon as I introduce earth gnd to kflop GND I get rogue steps.

What are your thoughts on a powered USB hub? Obviously my IO amperage requirements are within 0.5 amps because it is working on 5V USB. I am sure I am close to the limit though. A powered USB hub should fix that assuming it is also isolated from earth GND.

I will also order a USB isolator, I think they are rated for high speed/low speed operation - is there anything I should be looking for when choosing an isolator?

-Curtis

[TomKerekes]

Hi Curtis,

I just checked my USB cable and it was reading 3.5 ohms, not sure how bad that is. I grabbed another cable and it is reading 1.1 ohms but it only has a ferrite choke on the PC side. Should I switch them out moving forward?

The USB 2.0 Specification is that the shield should be less than 0.6 ohms. 3.5 ohms seems quite high. That isn't the one we provided is it?

The source of noise is 100% the spindle, I have been running for months already with a porter cable router and have had no noise issues as far as i can tell. When I run the spindle and send step/direction signals to the servos, axis position "grows" due to noise being perceived as step pulses. However, the machine is working totally fine despite the EMI spewing spindle when I use the 5V USB connection or a non grounded walwart supply. I have tested the repeat-ability for over 30 minutes at different spindle rates and it is solid. As soon as I introduce earth gnd to kflop GND I get rogue steps.

Strange. What kind of Drives do you have? How are they wired? Shielded?

What are your thoughts on a powered USB hub? Obviously my IO amperage requirements are within 0.5 amps because it is working on 5V USB. I am sure I am close to the limit though. A powered USB hub should fix that assuming it is also isolated from earth GND.

I'm not sure. Normally I would think a USB hub would complicate things and add opportunities for problems. But then if it avoids earth ground connections it might be a solution.

I will also order a USB isolator, I think they are rated for high speed/low speed operation - is there anything I should be looking for when choosing an isolator?

Actually USB 2.0 supports low, full, and high speeds. KFLOP only uses full speed and works with most isolators. See also here.