Wrong phase for a time delay?

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.

Answer 1

[Jonathan Epperl]

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

Answer 2

[Arkadiy Turevskiy]

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