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From: "Alec Stothert" <astother@mathworks.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: sisotool
Date: Mon, 12 May 2008 07:21:23 -0400
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The 5% overshoot requirement on an rootlocus plot is exact only if the loop 
is a second order system. If the system is second order dominant the 
requirement will also be accurate but in your case the controller zero (D 
part of your PD controller) is interacting with the dominant close loop 
poles and causing additional derivative action in the response  - resulting 
in the extra overshoot.

-Alec

"Waleed Najy" <waldo90009@hotmail.com> wrote in message 
news:g02l2f$98n$1@fred.mathworks.com...
> My question is about feedback control. The open-loop process
> I'm working with is G = tf(5000,[1 1020 20000 0]). I added
> this to the "sisotool" as "G" and used a PD controller (i.e.
> added a zero), and stabilised it.
>
> Then I added a design constraint; percentage overshoot < 5%.
> And this is where the problem comes: even when all the poles
> are within the constraint, I'm still getting an overshoot
> largely in excess of 5% and I can't understand why.
>
> Could someone please try it and tell me what the problem is?
> I would be very grateful.