Re: Bode weirdness

[Peter Gawthrop]

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 |
| GLASGOW G12 8QQ, Scotland, UK    | URL: www.mech.gla.ac.uk/~peterg |
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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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