How do you "map" the output of a PID controller to a process?
Say you have a resistor, a thermometer, and a PID controller. The thermometer provides feedback, and the PID controller can control the electric current of the resistor. You want the resistor to run at X degrees, so you use the PID to regulate the current going through the resistor.
Do you connect the PID such that the controller sets the current to a certain level, or do you connect the PID such that the controller adds/subtracts current to the instantaneous current to reach the desired level?
That is, does a PID's output directly control the process output or does the controller's output keep changing a processes output to reach a desired level?
EDIT: Whooo man, did I write this? I asked this question 2 years ago when I was first fiddling around with a PID. Now, I can barely understand what I was asking. At any rate, @spehro pefhany's answer provided what I think I was looking for back then: a comparison of position- vs velocity-based control.control.