Wrong phase for a time delay?

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Ricardo Parreno
Ricardo Parreno on 29 Jan 2013
I have to model a time delay, I use the Pade aproximation of sixth order and multiply by the original function but when I get the Bode plot, the delay factor have always a positive angle.

Answers (2)

Jonathan Epperl
Jonathan Epperl on 29 Jan 2013
Angles aren't unique, you can add or subtract 360° as often as you want, so if you get a phase of 720° it's the same as 0°. Right click your Bode plot, choose Properties and then Options. There you'll find an option "Keep phase close to..." If you want more customization of your Bode plots:
doc bodeplot

Arkadiy Turevskiy
Arkadiy Turevskiy on 29 Jan 2013
Please note also that you can model delays exactly in Control System Toolbox, you do not have to approximate with Pade. For example, to model a first order system with time delay:
% Laplace variable
s=tf('s');
% first order transfer function
sys1=1/(s+1);
% add 2 sec time delay
sys2=sys1*exp(-2*s);
bode(sys1,sys2,'r');
legend('no delay', '2 sec delay','Location','Best');
Arkadiy

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