help-octave
[Top][All Lists]

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

--------------------------------------------------------------------
| 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 |
--------------------------------------------------------------------

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).
>
>
>
> -------------------------------------------------------------
> Octave is freely available under the terms of the GNU GPL.
>
> Octave's home on the web:  http://www.octave.org
> How to fund new projects:  http://www.octave.org/funding.html
> Subscription information:  http://www.octave.org/archive.html
> -------------------------------------------------------------
>

-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------

reply via email to