Mill Retrofit with Fanuc DC Drives

I’m working on a retrofit and I’m having trouble with oscillation on an analog velocity-controlled axis using existing Fanuc DC motors and drives.I have a Mesa 7i77EU for the FPGA, 7i85 for the encoders, and 7i83 for the analog outputs. I am using two outputs for each drive. one for a velocity command and one for a virtual tachometer. This is similar to how the original Fanuc controller was doing it.Current status:

• I have the analog outputs scaled correctly as far as I can tell. I measured the voltages going to the drive and they match the drive specs. 7V/2000RPM for velocity and 3V/1000RPM for the tachometer signal.
• With the velocity command wires disconnected from the drive, the system appears stable. The motor holds position well and resists manual twisting of the shaft.
• As soon as I connect the analog command wires to the drive, the axis begins oscillating even when all PID values are set to 0.(I was expecting this to act the same as when the wires were disconnected. The motor oscillates and drifts until i get a following error.
• Increasing the PID P gain to about 0.5 prevents the following error, but the oscillation is still present. 
  
  Does anyone have any suggestions on why the drive would be stable without the velocity command wires hooked up but unstable with them connected even with no movements?Any suggestions on where to start troubleshooting would be appreciated.
  

  File Attachment:

  File Name: AnalogConfig.hal
  File Size:9 KB

  File Attachment:

  File Name: AnalogConfig.ini
  File Size:3 KB

Thanks for the response! I’ve followed that outline pretty carefully but get stuck at setting P. It doesn’t matter what I set it to I can’t get rid of the vibration in the motor. If it’s too high or too low I get following errors but no value reduces the vibration to an acceptable level. The screenshot in the first post shows the minimum level of vibration I can get by changing P. I’ve tried increasing the dead band but that doesn’t seem to help unless it’s so big that it gives following errors.

I am in the US. That vibration on analogout3 is the million dollar question. If I disconnect the wires on analogout0 the vibration completely goes away on analogout3.

I am using original wiring except for 8-10inches of twisted pairs for both analog signals to connect the original wiring to the mesa boards.

I suspect its something with the tachometer emulation or connections to the Fanuc drive

How did the original drive get velocity feedback?

Thanks again for help. I swapped both analogoutput0 and analogoutput3 to RG58 cable that I had laying around and had the same results. I also double checked the ground and shielding on the original wiring. i had disconnected one of the grounds but reconnecting it also did not change anything.It looks like the oscillation is at about 33Hz so I don't think it is from the main power. Another note, I change the base period on the servothred from 1000000ns to 800000ns to see if that changed anything. it seems to change the frequency slightly but it no affect on the amplitude.