PID Control

[Tom Saxton]

Originally Posted by gnormhurst

For auton, there is a fundamental problem with the code as is, and I have written about it elsewhere. The S_DRIVE thing sets the position target at the final goal and lets the PID system chase it. This leads to the motor drive values being pegged at their max, so small hanges can't get to the motors, so the loop is not closed -- it's open.

This means that both motors will push as hard as they can, but if one goes faster than the other, there is nothing the PID system can do about it until it gets closer to the goal, so it tends to turn in a circle.

For autonomous, what you want to do is run it in velocity mode to drive both sides at the same speed, then manually monitor encoder ticks until one or both sides reach the target.

We had really good results using the PID code in position mode to control arm segments. We can command a position and have it hold pretty well even when adding or removing a tetra. It would sag a little, but not enough to drop the tetra if it started with some backwards tilt. The only problem we had was integration wind-up when we requested a large movement. I've since read about that issue - there are easy solutions like zeroing the integration term until the arm gets close to the target position.

[dm0ney]

We used the 'kit' pid code for the first regional. Since then, we have migrated back to pid code written last year by our team. The switch is very easy as we have macroed 'KIT_PID.'

We only used the position loop during autonomous and we also split gains and max/min for rotate and drive.

The pid code was quite easy to tune for linear distances but until we split gains, we had a heck of a time getting spins with the same gains.

The reason we abandoned the kit pid code was eventually a robot will not drive straight (assuming it drove straight to begin with). Last year we had implemented 'go straight' code that would compensate for mechanical imperfections by engaging encoder counts and running it through pid. We tried implementing this into the new code but decided that since we already had code that works, it wasnt worth rewriting it and inserted the macros instead.