Wrong phase for a time delay?

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Ricardo Parreno
Ricardo Parreno 2013-1-29
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.

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回答（2 个）

Jonathan Epperl
Jonathan Epperl 2013-1-29
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 2013-1-29
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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