Setpoint = the desired position (or velocity or angle or whatever)
PV = the current position (or velocity or angle or whatever)
Dt = delta time (time since previous sample)
Output = what you are going to send to the actuator.
Kp, Ki, Kd = tunable constants -- you tune these to your system so that you smoothly control your actuator without too much overshoot (zig-zag around desired course) or undershoot (takes too long to get to desired course)
Dt should be calculated as the actual time between the now and the last time you checked. Ideally it is a small amount of time.
The number itself isn't necessarily small depending on the units you use to express it. Whatever units you use are bascially canceled out by your choice of Kp, Ki, Kd. If Dt is in milliseconds, then Kp is 1000x and Kd is 1/1000 of what it would be if Dt was in seconds. If you are doing all your calculations in floating point arithmetic, things should be fine no matter what units you use as long as you are consistent.
The wikipedia article gives you some guidance on how to tune Kp, Ki, and Kd to improve your system performance:
http://en.wikipedia.org/wiki/PID_controller[
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