Servo Tuning

I've followed John Thorntons servo tuning guide, and my X and Y are okay, but I don't know what to do with the image on my Z.  How do i shift the whole ferror plot up?  If I change FF1 then my FError positive and negative movements move away from each other.  Also the spike at the beginning...

Can you please share a link with John Thorntons tuning guide?

It's under the "OLD" section, so it may no longer apply perfectly, but it has helped me.

The FF1 value is directly influenced by the output scale.
FF1 can be over 1 if the outputs scale is greater than machine units per second
at full output. This is why I suggest for analog system tuning, that the output scale
(and limits) be set to machine  units per second at 10V. This means FF1=1.0.

I would not change this now if the tuning is close.

I can't seem to get rid of the spike with FF2, is there anything else I can do?

Can you plot the current error along with velocity?
(position is not really useful for this kind of tuning)

FF2 is use to reduce the error during acceleration
if the "spike" duration is the same as the period of
acceleration you should be able to tune this out.
(if you add too much FF2 you can make the spike reverse)

If the spike is only at the beginning and end of the acceleration
phases, this represents delays in the command --> feed back
in the system and the physical limitations in the drive/motor
hardware (its not possible to have a step acceleration function)

Delays may be cured by drive setup (for example any input filtering
should be minimized)

The delay and drive/motor physical limit issues can be worked
around a bit by lowering acceleration. A better solution is to use a
jerk limited trajectory planner. There is some ongoing work on this:

forum.linuxcnc.org/38-general-linuxcnc-q...ner-using-ruckig-lib
 

I reduced the acceleration, then could tune out the spike with FF2. Makes sense the head needs a lower accel than the table.

Yes, it may be that you did not have enough torque
for the original acceleration settings, given the Z axis
mass and unbalanced weight.