7i77 isol

I'm having some issues adding a 4th servo drive to a 3 axis mill. I'm mixing a bl setup with 3 brushed dc motors and I get interference on the 4th bl axis. I run a 5i25 with 7i77 and have found that the isolated 5v external power is sharing it's negative with the machine chassis ground. I think I need it isolated as motion from any of the 3 dc drives feeds back to the brushless drive and induces movement on the 4th axis  I think the ground loop passes through mesa's DB25 cable. When I unplug the cable, I have no ground loop. To solve this, do I need to get the 7i77 isol board? I tried sharing the signal grounds between drive and 7i77 with no improvement. Analog signal cable has its shield grounded on one end and has worked well on the 3 axis setup for a long time now. The servo drives are AMC and minimally connected to mesa hardware. Only connecting +/-10v analog to 7i77 on the on the 3 axes, connecting analog and sharing encoder signal with mesa on the 4th. Any suggestions?

A 7I77ISOL (or 7I92T perhaps) may help though the 4 drives will
still share a ground.

Is the 4th drive using a differential encoder connection?

 

The 4th drive is using single ended encoder connection and the 7i77 is configured for single now as well.  
All drives are cassis grounded but I do not have the drives signal ground connected. Other than the 4th drive connecting signal negative to 7i77 encoder negative. What's troubling is, the high voltage neg terminal on the drive shows continuity to chassis ground when the low voltage connectors are in place (encoder on the 4th drive). Continuity stops when the db25 cable is removed.  
I don't have a ferrite bead. I'll consider that as well once I sort out the main ground loop. 

Hi JT,

I already ordered one from Peter but thanks for letting me know you have them in stock.

I might want to isolate at the source instead.
I think the main problem is that the drive is not isolated. The power ground is shared with it's internal low voltage ground.
One workaround I'm looking at is running the drive in current mode and not splitting the encoder back to the drive. The catch with that is the hall sensor ground is still shared at the motor encoder. Could I add a simple opto isolator board to isolate the encoder from the 7i77 and drive/motor? 

Any idea if this could play nicely with the 7i77 encoder inputs? Motor encoder has 1500 count running at 3000 rpm. I didn't see a spec on timing.

Donker optocupler:

www.amazon.com/Optocoupler-Isolation-Mod...erter/dp/B08LYQQ7X6/


 

I tried the donker optocoupler for starters. It can't keep up with the speed of the servo. With the isolation, oddly feedback still occurs.
The 7i77 isol I have to shoehorn into my enclosure. I'm holding off on installing it for now.
When switching the drive to current/torque mode, where the drive no longer reads in the encoder, the feedback problem goes away. Probably still there just not traveling back / assuming influencing velocity feedback through the encoder in the drive. How is that possible?
I struggle tuning in current mode. I can't really get the motor shaft locked and slow speed seems very coarse/rough running. 


Here is a link to what's going on prior to switching to current mode. The analog feedback from mesa is disconnected shown at the end to illustrate that the interference is not coming through the 7i77. 

 

 

Sounds like maybe the  the issue is the common ground
between encoders (because you are sharing the encoder
output from the new drive with the drive and the 7I77)

Have you tried a common mode choke on the new drives
encoder connection to the 7I77?

Not yet. What do you recommend on the common mode choke for the encoder? Snap on filter and pass through or wrap around?
AMC also suggest suppression core in one of their docs for the motor leads at 2-5 turns for each phase. If my rings are too small to fit all leads, can I use one ferrite per phase or do all leads have to pass through one?

 

Snap-on or toroid type should be about the same
Snap-on's have the advantage that they are better if
you have larger connectors so don't need such huge
toroids if you don't want to remove connectors.

Its usually better to pass all 3 UVW through one larger core
as this allows larger inductance (higher permeability) without
saturating. (since the net U,V,W current is 0, only the unbalanced
currents to ground that cause the issues are filtered)