Also on the Mentor drive there are pots for tuning..I used them for setting the rpm after I had my motor re built..worth a try?
So does that drive has cascade contoller:
1) Inner loop for curren or torque i.e. motor "native" possibly analog control loop
2) Outer loop in PLC for position control
Cascade controller are pretty sensitive that way. When I had to tune somewhat similar system, we did it shis way:
1) Make sure that inner loop HW stop works with limitswitces or such (would be drag to crash it)
2) Remove PLC control loop totally from inner loop, we used programmed step respose generator for inner loop SP.
3) Use step response to tune inner loop as tight as possile, but make sure that it has enough stability to live with wear, changing load etc.
4) After inner loop step response is good , connect inner loop to outer loop and adjust outer loop.
Gain ratio is critical, inner loop should be least 10x faster or controllers start fighting. If cascaded controller outer loop is too tight to inner controller it will cause bad behavour.
Now, if the outer loop gain has to be too high for confort (it will try to ocilate in certain ocasions) there are few paramaters to play with like dead band (When position is "good enough" and correction would send the controller to hunt near SP) and derivate type parameter (how small error you want the controller to react).
We also used "Forward" controller type: Depending the difference on measured value and setpoint, the controller would lock the outer loop AND shoot the inner loop with precalculated (massive) step to jerk the system "close" and when "nearly there" would release the outer loop controller that would do it's job gently to control exact position. This needes the system to be modelled or tables of mass and transfer distance - like you change the moving mass and then your step response to moving distace had to be different.
Hope I did not confuse anything further. We did not made machine tools, but machines that sometimes had a lot in common with machine tools. I.E. linear rails, torque/speed/position control loops with electro hydraulics or electrical servos.
Pekka