Ateocinico wrote:PD + PI = PID
A pure PD as Dr.Shepherd describes only amplifies noise and produces overshot.
1) Fuzzy PD and PI controller is not the same as PD+PI, at least not for most situations. But that's a topic for another discussion, and just was used for a joke (though seems it wasn't as funny as I thought
)
2) The Kd of a PD/PID controller reduces overshoot by nature, unless you put a negative coefficient (which unless you like making bombs isn't a good idea), Kp (the P in PD and PID) is what increases overshoot (Ki also does to a slightly lower degree). PD controllers are generally good at following trajectories, but don't always stop where you need them to and might have an offset throughout.
3) In the case of bullet, noise will be negligible unless you add it yourself, so he shouldn't have too much issue with it. Filtering techniques are available to deal with noise too, though they do tend to make it less effective than theory (but far more effective in real life).
4) If, now that I'm reading the initial post a third time, he meant that a moving arm doesn't end up where he wants it to at the
end of a trajectory (rather than during it as his setup implied), it is entirely due to it being a PD rather than PID, as the PD controller will have some steady state offset in the case of external forces (gravity) and "low" Kp values (depends on how close is close enough, if absolutely zero error is needed Kp goes to infinity). If the steady state position is what he cares about, he just needs to add a small Ki value to let the controller compensate for the external forces dragging the arm down.