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Re: Bode weirdness
From: |
Peter Gawthrop |
Subject: |
Re: Bode weirdness |
Date: |
Fri, 27 Jun 2003 08:51:05 +0100 (BST) |
Control engineers like smooth Bode plots. Matlab uses (if I recall
correctly) something called fixphase.m which has a simple test for
large jumps in phase and then adds or subtracts the appropriate angle.
But it is just a matter of taste as to whether this is regarded as
important or not.
Peter
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| Prof. Peter J Gawthrop | Tel: +44 141 330 4960/2528 |
| Centre for Systems and Control & | Fax: +44 141 330 4343 |
| Dept. of Mechanical Engineering | Room: James Watt 653 |
| University of Glasgow | Email: address@hidden |
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From: Przemek Klosowski <address@hidden>
Subject: Re: Bode weirdness
Date: Thu, 26 Jun 2003 09:06:25 -0400 (EDT)
>
> What you see is probably the phase jumping from -180 to 180 or visa
> versa. There is no difference between a phase angle of alpha and
> alpha+360, so it plots everything in a comfy range on your screen. It
> also at a glance shows you where possible stability snags might be in a
> feedback loop.
>
> Not an error.
>
> Unfortunately, in the examples brought out, the phase jumps from X to
> X+180, as you can check by replotting the recalculated phase:
>
> [mag,phase,w]= bode(tf2sys([1],[1,1,1,1]));
> plot(phase)
> phase(phase<0)+=180
> plot(phase)
>
> I don't know enough about the bode analysis to say whether such phase
> reversal has physical consequence or not (if it does, then either
> Octave or Matlab is giving a wrong result; if it doesn't then it is
> just a matter of presenting the result, just like in the case of a phase
> shift by 360 degrees).
>
>
>
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