Schaublin 125-CNC retrofit.

Maybe you can set up a very simple config using on a Mesa encoder to measure the position and a siggen to create a command signal, and run that in a 6kHz thread?

graph servo analog out in hal scope, open the calibration feature in axis etc. turn the P setting to zero , set ff1 to 1 make sure the other ff settings are zero. 

there 2 potential issues, 1 is gain is to high or low " most likely culprit" or the servo dampening is wrong, usually excessive from my experience. I'd suggest turning down the servo dampening to nearly ZERO, if you start to see oscillation in the motor, turn it back up, that is with ZERO commanded voltage from the mesa card and servo output and the motor in a holding position. Now, put a slight load on the servo motor until it can fight back and hold position. 

I assume your encoders go directly to your drives, so I doubt this is a tach gain issue. 

once you get your dampening settled, no oscillation even when holding TQ with the drive enabled with zero voltage commanded, connect the spindle servo out from the mesa card again, and start tuning gain.  use the jog function to traverse the drive with the carriage load on it. you should see nice square signal edges in your hal scope trace, if you don't, here's where to start. If you sea big spike on the left of the trace, then it ramps down and slowly levels off, the gain is to low, if you see a big oscillation and a ramp trending up, left to right, the gain is to high. I'd start at say 30 ipm, and jog, aggresively, then bump to 50ipm, then 100 ipm etc, each time matching the gain so that the drive output voltage looks like a nice sharp square tooth on both edges with a flat Vout once the acceleration is completed. The accel curve obviously will vary with each drive etc, but I'd expect to see a nice 45-80* climb up and down, near vertical to the held motion voltage. 

while not your drive, they do have a section about adjusting gain and compensation. and the patterns of response are representative of the behavior you need to see from the servo Vout from linuxCNC. I got my drives for 600 ipm using this method, with old brushed DC permanent magnet motors. YMMV 

www.glentek.com/download/smb7230-smb7275-smb72100-rev-c-manual/
 

There should be a way to have it jog back and forwards from within the drive. I tried to program that, bit could not get it to work. That would have been the easiest solution by far. Thanks for reminding me, I will try contacting JMC again, and ask about that function.

Thanks for that. You packed a lot of info in there. I will try this first, and see if I can follow it all.

the issue isn't the drive tuning per say, it's the relationship between what Lcnc is commanding and the drive is giving. That's what watching the Vout to the servo is so helpful. If it isn't generating nice square looking signals, it's obviously adapting, so turning the P down to say 2 or 6 something really low really helps to exagerate the problem in a nice visible way. look towards the bottom of that pdf, they give a nice example of what to look for, under and overshoot, dampened etc.

this is out of my INI file. bring the drive to Lcnc, don't try to pid tune around a gain problem. My machine runs 600ipm rapids 400ipm working with zero following issues. 

X and Z are identical

TYPE = LINEAR
HOME = 16.0
FERROR = 0.5
MIN_FERROR = 0.05
MAX_VELOCITY = 10.00
MAX_ACCELERATION = 40.00

P = 2.5
I = 0
D = 0
FF0 = 0
FF1 = 1
FF2 = 0
BIAS = 0

 

Just to clarify, with any drive in Analogue mode, the drive is completely unaware of following error, which is why it doesn't get shown in any tuning software.
In Analogue mode, the drive only receives a voltage signal from the controller, and it only uses the encoder feedback as a modern replacement for a tacho feed (and possibly initial phase timing, depending on motor/drive specifics). The drive is completely unaware of where the motor should be, so it can't calculate any following error.

Personally given the choice, I wouldn't voluntarily use analogue mode now.
Step/Dir (or even better, quadrature) is far easier to setup and tune, will give as good performance as a well setup analogue system, and out perform a less than ideal setup analogue system.