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'Funny' Phase Response on Matlab & Simulink (DC-motor fe

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gvanto

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motor dc + simulink

I have a model in Simulink (DC-motor velocity feedback).

When I use the Linear Analysis tool to obtain the bode plot, the phase response starts at zero and then goes positive.

(This system seems to be unstable btw)

When I derive the transfer function in the Matlab environment (from which the Simulink model is updated anyway, so they are exactly the same), and then do bode(sys), this produces the exact same Magnitude and Phase response as in Linear Analysis, only the phase response is shifted 180deg downwards (starts at -180) but still goes positive at higher frequencies.

Thus, 2 problems / questions:
a)
How can it be possible to have a positive phase 'lag' (=lead?) - does this not imply that the output knows what it's supposed to be before even given an input (time machine? :)

b) Why is phase plot diff. in Simulink than in Matlab environment?

Help much appreciated,
Gerry
 

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