Author Topic: grounding signal grounds to chassis ground?  (Read 21 times)

OldePhart

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grounding signal grounds to chassis ground?
« on: January 02, 2020, 06:36:26 PM »
Running a PMDX-126 and -107 on a 220v 3 wire power circuit. (2 phases and ground) .
I was getting spurious signal triggers and found that signal ground from J11,12,13 float. I think it says that in the docs so I wasn't surprised.
However I find that chassis ground to signal ground measures 50Vac and I'm sure that is the reason for the spurious signal triggers.
So I tied the signal ground to chassis ground and that fixed the issue... But I'm sure you left them floating for a reason,. Do you think this is a mistake and if so what other solution is there?

In addition what about the PC ground (J1-6) which are also floating at 50VAC ref to chassis. Should I ground it as well?

You carefully designed it to not be ground referenced so how badly am I messing this up by using a common ground for all?   

Its a new build, not retrofitting anything.

OldePhart

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Re: grounding signal grounds to chassis ground?
« Reply #1 on: January 03, 2020, 01:31:34 AM »
Ok I'm finding that the IO is more resistant, but when the VFD starts, it stops right away. Certainly some effect from common grounding. removing the common grounding and the VFD starts up again just fine.  This is gonna be fun. . . .

12strings

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Re: grounding signal grounds to chassis ground?
« Reply #2 on: January 07, 2020, 12:30:43 AM »
A wiring diagram would be helpful (a quick hand sketch is OK).  What do you have connected to the J11, J12 and J13 inputs?  Depending on what you have connected, you may need/want to connect the J11/12/13 "GND" to the machine's ground.  You should not need to connect the J1 to J6 "PCGnd" to chassis GND if those signals only go to your stepper/servo drivers.

Do you know WHY the VFD stopped when you have the J11 GND tied to chassis GND?  Was it due to an EStop (i.e. was Mach disabled). Do you have an EStop switch wired to the PMDX-126?

As for the VFD, make sure your VFD power input and motor output wires are FAR away from any low-voltage wiring (EStop and/or limit switches).  Or at least do not have long parallel runs with the low-voltage wiring (i.e. in the same wiring tray).  VFDs are notoriously (electrically) noisy.

OldePhart

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Re: grounding signal grounds to chassis ground?
« Reply #3 on: January 07, 2020, 10:56:43 PM »
J11-13 are the home switches. J1-6 are indeed the stepper drivers (Clearpath SD) I removed both grounds and the problem stopped, but I did get spurious limit switch activation entries in the log with no pattern to them.
 
Later found that I didn't ground the VFD cable shield on either end (yes I am using VFD robotic cable...$$$) and had 50+ volts on it as well so that was likely a HUGE contributor to this as that cable runs parallel for a long time. That is now grounded on the VFD end, the spindle itself is grounded as well.

I haven't reconnected any of the board grounds to chassis ground and it seems to be OK... but... I built a touchplate and its on the J11 side, but when I touch the plate to the frame  I occasionally will set off the ++ limits even when nothing is moving and the spindle is off.

This gonna be one of those grounding nightmares . . .