Chief Delphi - Arm Position Control

Quote:

Originally Posted by Jared (Post 1339367)

It's really tough to get a D term to compensate for the inertia of an arm. In order for the D to really affect the system the way you want it to kD must be fairly large, which requires that kP and kI be pretty much perfect.

I know. I have never gotten a good cal set with a high enough D gain to be meaningful by hand, it always has stability issues all over the place.

I did, however, autotune two arms. I was surprised at the time, but the D gains were around 5x the P gain. It worked marvously, and happily jittered very slightly to hold position perfectly (even as game pieces were added) and moved to position and stabilized very quickly.

Some links on the methods I used:
http://en.wikipedia.org/wiki/Ziegler...Nichols_method
https://controls.engin.umich.edu/wik...Nichols_Method

I made a spreadsheet which calculated all of the gains, using all of the tables from both the Wikipedia and Umich site, tried all of them, and liked the resulting gains from the Umich tables more.

Note: Time is whatever time unit the control loop uses. If the control loop is time-compensated, use that unit for time. If the control loop is not time-compensated, then use the number of iterations for time, AND the iteration time must be constant and stable.

Edit Note: Even with autotuning, I still had to gain schedule 33's 2012 FRC arm, as the linkage changed the motion ratio quite a bit near the end of stroke, and I needed really fine control in all places. I ended up autotuning at the far end and in the middle, then linearly interpolating them (past the middle, it would just hold the middle gain set).