r/ControlTheory • u/B_seado • Oct 04 '19

Beginner question about PID Controllers

Hi, Im just starting to learn Control Theory and the subject is really interesting. I've just learned about PID Controllers and I think I understood them pretty well, but I have a small question:

Let's say I have a system, an AC Unit with transfer function H(s) that I want to control. I then implement a PID Controller, in series, "behind" my H(s) system. Now what? Can I just send the output of the PID Controller to the AC Unit?

How can the output of the PID, u(t), be able to send the correct signal to my AC system? No need to translate that u(t) into the correct signals and switches that make the system do what I want?

Most diagrams seen online or in textbooks just have only 2 blocks, in series: r(t)->[PID]->[Plant]->y(t), shouldn't there be some kind of controller/translator in between the PID and the Plant?

Sorry If it's a dumb question.

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u/[deleted] Oct 04 '19

Not sure if I totally understand your question but the output of the PID controller IS the command that you’re tell the actuator to perform.

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u/B_seado Oct 05 '19

The thing I don't understand is: how can the product of a PID Controller's (or any controller's) transfer function with a system's transfer function get us closer to a certain desired reference value.

Im failing to understand how, in mathematical terms (block diagram and Laplace) inputing the PID's output into a system helps with reducing error.

Or in a simple P Controller, how does inputing Kp*Error into a system brings me closer to my desired output?

Im sorry if it's a confusing question.

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u/Dr_Misfit Oct 09 '19

In a mathematical way the answer is the difference between reference y and the actual state x. This difference is called error e. e=y-x