PID behavior

[Manoel]

Originally Posted by Felipe Sagui

When the PV = SP the output starts to be constant but not 0. We guess that it has something with the I term and with the reinitialize boolean on PID .VI

Now we ask: why output doesn't decrease until 0? What we can make to do it in the better way that is possible?

Yes, it's because of the integral term. Think about this: the controller has no idea of what's going on other than, at that particular output, it drove the error to zero. Why would it want to change its output?

You are too attached to this "output must go to zero" mantra. Truth is, it's not always the case. Suppose you were controlling a motor's speed with a PID loop. When you reached the desired setpoint, would you like the controller output to go to zero? Of course not, because then the motor would stop spinning.

Manually changing the process variable value isn't really a way to test anything, as it doesn't represent anything and isn't even reproducible. You guys should follow Greg's suggestion of trying the PID example VIs - they actually simulate a process and will be a lot more helpful.


PS - It seems you already understand a bit about PID controllers, so what is the purpose of those tests?

Please ask again if you need clarifications, and best of luck!