Servo PID Tuning - can't clamp down on overshoot

Howdy all,

We are having a bit of trouble getting our first servo motor (starting with the X-axis) on our latest retrofit tuned properly. This is a Mazak VQC 15/40 with Mitsubishi TRA-31 servo drives and HD81-12S-TT 0.8kW brushed DC servo motors.
The drive operating manual is at: TRA-31 manual
and the maintenance manual for the old Mitsubishi Meldas YM2 / Mazatrol M2 control (with various wiring info, etc) is at: YM2 Manual

We followed John Thornton's basic servo tuning guide, which worked well on our last mill (a CNC knee mill retrofitting old Anilam Crusader GXM controls). We started with P = 5, increased it to about 15-25 where it started oscillating, and then tuning FF1 to about 0.36 to center the cruising velocity, and increasing FF2 to try and clamp down on the starting/stopping lag.

Currently, max velocity is 700IPM (11.67 - in/sec - in the INI file), and 23.3333 in/s^2 acceleration, to reach max velocity in 0.5s. The max velocity is the specs we found on several websites for this mill, and the acceleration was a best guess. We did find a research article that had empirically determined maximum 'safe' acceleration for this model, and it was 78.74 in/s^2, so we SHOULD be safely in acceleration capabilities. These are pretty big servo motors for a fairly small table - 0.8kW motors for a 25 x 15.75" table.

PROBLEM: We can't get rid of the artifacts during starting/stopping. It seems to lag by 0.002-0.005", then overshoot by approximately the same amount, before settling in properly.
We have to drop P down to about 5 to get rid of noticeable oscillations altogether, and then it takes a LONG time to correct. We have tried various combinations of P, I, and D with basically no success.
Most of the adjustment pots on the drive say to leave them alone, as they are factory tuned, but we did try adjusting VR3 (Speed Loop Compensation) to try and increase response. It smoothed up the curve after the over-compensation, but didn't reduce the magnitude at all.

Any ideas??? Thanks to PCW, cradek, KimK, tjtr33, and others that have been helpful on IRC in getting this far!

SRDC wrote: PROBLEM: We can't get rid of the artifacts during starting/stopping. It seems to lag by 0.002-0.005", then overshoot by approximately the same amount, before settling in properly.

What is the I-term doing during this phase? Can you halscope it?

I should be able to - I'll try again this morning and post back.

Here is a screenshot with the same values as last time, with Igain on the halscope.
I also increased the jog speed to the max rapid of 700ipm, and attached the screenshot. Pretty ugly!

I was actually meaning pid.N.errorI but your plots are interesting.

Why does the Igain change? Is it connected to something in HAL? Is it meant to be?

The I gain thing is weird but a couple of things:

You have too much FF2, This s clear from the 700 IPM plot
( dont try to fix the initial error spike with excess FF2, but rather try to zero the
bulk of the error during long accel/decel phases )

There is a noticeable delay somewhere:

Possibilities:

1. Drives are slow and you are running higher accel than expected for the machine (lower accel)
(and or)

2. Resolver response is slow because of low signal (measure with a good voltmeter/scope)
is the 7I49 a 7i49HV model?

3. Driver velocity loop has low gain and you have not backed off on the (too high)
FF2 when raising the drives velocity loop gain, leading to more overshoot