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From: "Craig Buhr" <cbuhr@mathworks.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: sisotool
Date: Mon, 12 May 2008 07:27:46 -0400
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Hi Waleed,

The overshoot design constraint in the SISOTOOL is based on a second order 
system with no zeros
1/(s^2+2*zeta*wn+wn^2). It uses the relationship between damping ratio and 
overshoot for this system.
So the constraint is rigorous for this class of systems but is an 
approximation for higher order systems.

-craig

"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.