2013-02-17

electrical noise issues with CNC


Starting yesterday I got terrible electrical noise issues with my CNC milling machine.

Update:

  • I was able to see the noise on my super-cheap pocket-oscilloscope
  • Replacing the RS485-cable with a shielded one and connecting the shield to GND on the USB2RS485 converter helped a bit
  • I'm investigating some USB isolators one, two and RS485 isolators
  • It was pointed out to me that the VFD is not connected to PE. PE ends in the line-filter. I should change that.
  • Connecting VFD-GND to the Spindle case reduced the noise as meassured on the Z-stepper case a lot but did not eleminate it.
  • Noise is greatest on the VFD line filter case, second on the spindle and steppers, least on the USB cable shield and RS485 cable shield now. 
  • Currently revisiting CNC Noise
  • Connecting VFD-GND to mains PE (protective earth) didn't change a thing. At best it made the noise much worse. 

The Players

  1. Touchscreen PC running MACH3
  2. Huanyang VFD inverter - controller and power supply for the spindle (motor that does the milling)
  3. powered USB hub
  4. USB to RS485 interface
  5. Control box containing:
  6. USB control board -  creating opto-isolated STEP+DIR signals for the
  7. 4x Stepper Driver Boards

 The Problem

Whenever the spindle is running and USB from the USB control board is connected to either the USB hub or the PC, the steppers twitch and jog around wildly.
This issue did not happen since october and just started yesterday.
4 hours earlier I dan 6+hour milling jobs without even loosing a single step.
Changes made:
  • coiled up cable to some lamps
  • coiled up power+USB cable to PC (undone, no change)
  • coiled up DC power cable of USB hub (undone, added ferrit core, no change)
  • switched oin a refrigator in the room (undone, no change)

The Setup

All share the same power socket.

PC connected to  Powered USB Hub.
USB Hub has ferrite core on it's DC supply power.

Powered USB Hub is connected to USB-RS485 converter.
RS485 converter connected to VFD via TX+ and TX- but not GND

VFD connected to spindle via shielded cable.
Shield on the cable not connected to anything.
VFD connected to mains via line filter element.

Powered USB Hub is connected to USB control board via USB cable with 2 ferrite cores.


USB control board connected to Stepper Driver Boards via +5V, STEP and DIR.

Stepper Driver Boards connected to their own 24V? 48V?  power supply.
Stepper power supply connected to mains via filter element.

Page 17 of the manual of the USB control board (link below):
 Photo of the inside of the Control Box containing the USB control board, the 4 stepper driver boards and their power supply:

Possible solutions

  • Should I connect GND on the USB control board to GND on the stepper power supply even though the manual doesn't mention it?
No.
The stepper driver boards get A+ A- B+ B- as STEP+DIR. So they clearly expect a differential input independent from GND. getting A+=B+=+5V and A-=STEP B-=Dir that is exactly what they get.
  • Should I add a capacitor between USB control board GND and it's +5V to filter anything but DC?
No.
  since the stepper driver boards get a differential input and don't care for either.
  • Should the GND contact on the VFD be connected to anything?
Manual of the VFD only mentions to use that to give muiltiple spindles a single connection to ground.
  •  Maybe I should connect the VFD GND to the spindle case as mentioned here?
Original wiring didn't do this.
DONE, did not solve the problem
  • ????? 
The PC power cable and the RS485 cable seem to have a large influence on the frequency of these interferences.

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