Hello All! Hope all is we with you guys. I have a question regarding control systems. I have been reading a textbook called "Control systems Engineering" by Nise.
I have been reading about how a PID can help stabilize an unstable plant. In every picture that I see that has a PID controller, I also see a feedback loop.
My question is: Isn't the feedback loop's job to stabilize the system to begin with ? Don't we introduce the feedback loop to stabilize the entire system?Then why do we introduce a PID controller, which is just extra baggage, if the feed back loop is present? Why cant we work to make the system stable with just the feedback loop?
Thank you for your replies.

 

The feedback itself in not sufficient, you need a P.I. or P.I.D. feedback loop in order to obtain proper stable control.
Max.

 

Why cant we work to make the system stable with just the feedback loop?

Because PID in the loop is what make the loop stable.
Typically the system being controlled has elements in it that adds phase-shift to the input versus output, and this shift can vary from lead to lag with frequency.
If that phase-shift isn't corrected, the feedback can become positive at some frequency and the system will become unstable and possibly oscillate.
The basic purpose of the PID is to generate a response that cancels whatever this phase-shift is (I is integral to generate a phase lag, and D is differential which generates a phase lead), so that the feedback always remains negative.

The accuracy of this correction is usually determined by doing a Bode plot of the loop response to determine the gain and phase margin of the loop, which is measured to verify the loop stability.

Make sense?

 

Google "pid basics", lots of discussion.

Proportional Integral Derivative, characteristics of control systems
in which we care not just about the setpoint but speed at which we
reach it and eliminating error as a f(time).

https://www.csimn.com/CSI_pages/PIDforDummies.html

Some practical use -

https://www.embeddedrelated.com/showarticle/121.php

https://www.cypress.com/documentation/application-notes/an89346-psoc-4-intelligent-fan-controller

http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/


Regards, Dana.

 

My question is: Isn't the feedback loop's job to stabilize the system to begin with ? Don't we introduce the feedback loop to stabilize the entire system?.

No, this is not correct.
A system has a negative feedback loop for fulfilling some specific tasks (advantages) - however, negative feedback does NOT improve stability against unwanted oscillations. In contrary, it reduces the stability margin.
Therefore, we intoduce controllers into the feedback loop, which can give the system the desired properties: Sufficient stability margin and - at the same time - the desired behaviour in the time and frequency domain (step response, etc.).

 

Hello All! Hope all is we with you guys. I have a question regarding control systems. I have been reading a textbook called "Control systems Engineering" by Nise.
I have been reading about how a PID can help stabilize an unstable plant. In every picture that I see that has a PID controller, I also see a feedback loop.
My question is: Isn't the feedback loop's job to stabilize the system to begin with ? Don't we introduce the feedback loop to stabilize the entire system?Then why do we introduce a PID controller, which is just extra baggage, if the feed back loop is present? Why cant we work to make the system stable with just the feedback loop?
Thank you for your replies.

Good choice buying the Nise book. It's the best book on the subject by far for me.

I would recommend reading the chapter on steady-state errors where he explains how a system with feedback can be converted to a single transfer function and how you use the final value theorem to find the resulting steady-state error for step and ramp inputs. You will find that even with feedback, the system can settle at some value below the input reference.

 

All control systems require feedback. This is called "closed loop" as opposed to "open loop".

You don't need PID control. What you do with the feedback is up to the control algorithm used.

PID, pronounced "P" "I" "D", meaning proportional, integral, derivative, is only one way of designing a control system.