Kit's Machine

[Kitwn]

Kit, your last response does concern me - if you remove the "electromechanical galvanic isolation module" from the Z-limit input to the bob, and simply connect this input to the local 0V, is it still noisy? If so you're down to BoB/Cable/PC.

I haven't actually removed the EGIM but it does strap the input to ground. I have made some interesting measurements though. Some screen grabs from the oscilloscope are very telling...

trying to accurately the voltage on the earthed case of the machine is a bit difficult since earth is meant to be the refference voltage. this is measured on the case of the LinuxCNC PC with the scope earth floating. 10mS/div. 4V/div.


Slightly less scary, this is one of the parallel port pins pins referenced to the PC case. 1mS/div. 0.2V/div. None of the motors were running so there's not controll data on the pin.


Most scary is another floating measurement of the metal frame of the CNC router itself. 1mS/div. 4V/div.


So my theory that that the intereference is going out on the mains cable of the VFD is probably wrong, it's more likely coming from the spindle itself.
The spindle is connected using screened 3-core cable with the screen connected to the mains earth inside the VFD and to the body of the spindle and hence to the frame of the machine.
So why does the mains filter on the VFD do some good? Dunno. Why does plugging the VFD into another mains socket stop all trips occuring and allow me to use the machine as normal? Dunno.

I have improved the earth bonding of the Z axis and gantry to the base frame but the above measurements are after that was done. For the present I'm just going to get on and use the machine as it is for it's intended purpose and hope for the best but, as I've said before, if this hobby turns commercial I shall want to upgrade the controller to something less flakey.

Kit