[Alan Anderson]

If you're trying to control wheel speed, consider what happens when a typical proportional controller determines that the speed is correct. The error term goes to zero, removing power from the motor. It'll slow down until the error term is high enough to add power, and eventually you'll reach a steady state with the speed much slower than you are asking for.

If you leave the P term at zero and pretend the I term is actually the P term, you can get something much closer to what you want.

(What we've done in the past is to take the output of the PID and use it to add to or subtract from the current motor control value.)