[Mr. Lim]

Okay, last post on the topic form me, as I think we've found a solution, although not ideal.

The workaround is to tune the P-term to attain 50% of the setpoint, then bump up the I to get you the rest of the way. Add D to smooth out oscillations and overshoot, if needed (it wasn't needed for us). After an evening of tuning, we have something workable, but not great. I is slow to react to get you from 50% of the setpoint to the setpoint, and it makes the controls feel delayed and sluggish. Because it is slow, there is also a tendency to wind-up and overshoot. Increase the I, the system reacts a bit faster, but ends up overshooting more. Bottom line, the I is responsible for making up too much error.

As an aside, I'm happy to report that the I term is capped - we felt a bit of wind-up and overshoot, but it was obviously capped, as we had no real runaways. It doesn't seem to get reset, as we were running pretty smoothly around the setpoint once we finished our tuning.

Now, if a TI / Luminary Micro person is reading this:

I think the REAL solution is that a simple linear feed-forward (ff) feature should be added, where (Kff * setpoint) is added directly to the PID's output.

The rationale is that the linear feed-forward can be roughly tuned to get us close to the desired speed, open loop. Then the PID is responsible for making up the difference between the feed-forward open loop result, and the set-point.

If you feel really enterprising, you can give us non-linear feed-forward too, and allow us to specify an exponential response.

This would make me very happy...