bugGNU Octave - Bugs: bug #49433, Bode plot is totally inaccurate

 
 

bug #49433: Bode plot is totally inaccurate

Submitted by:  None
Submitted on:  Tue 25 Oct 2016 01:26:03 AM UTC  
 
Category: Octave Forge PackageSeverity: 3 - Normal
Priority: 5 - NormalItem Group: Inaccurate Result
Status: InvalidAssigned to: None
Originator Name: Jay AyersonOriginator Email: -unavailable-
Open/Closed: ClosedRelease: other
Operating System: Other

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Wed 26 Oct 2016 04:05:34 AM UTC, comment #2:

I was confused, but not about what you think. Due to the multiple poles, it should in fact slope up at the left, for an open loop system.

Plot it on Matlab, or Octave, and you will find this to be the case.

However, the plot I produced - and the one with major discrepancies from Octave's plot - is the closed loop transfer function.

I am withdrawing the error report.

-Jay

Anonymous
Tue 25 Oct 2016 01:46:52 AM UTC, comment #1:

I think you are confused.
The system that you show has a pole ate the origin
and therefore has an integration term and the left side of the plot should slope down.

The plot from Matlab that you show is horizontal at the left
and therefore no integration term

Anonymous
Tue 25 Oct 2016 01:26:03 AM UTC, original submission:

Using Octave 3.6.4, on fedora core 20.
Octave-control installed 25 October 2016.

Compared bode plot of transfer function (obtained on Matlab) to Octave plot and found little correlation. Also sketched by hand for reference and found Octave to be wrong. Both magnitude and angle plots are higher than they should be in the low frequency region (left side of plot), indicating a systematic error.

I suspect that either the low-frequency cutoff is being ignored, or the integrator has been discarded.

The transfer function is entered using
s=tf('s');
and matches the polynomial given by Matlab.

Transfer function:
G=50(1+s/5)(1+500s)/[(1+s/2)(1+s/30)(1+s/50)(1+1000s)s];

Is this a version compatibility fault or a software bug?

Anonymous

 

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Attached Files
file #38804:  Bode4.PNG added by None (47KiB - image/png - The attached file is the expected result from Matlab.)

 

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  • -unavailable- added by mtmiller (Updated the item)
  • -unavailable- added by None (Submitted the item)
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    Follow 3 latest changes.

    Date Changed By Updated Field Previous Value => Replaced By
    Thu 27 Oct 2016 04:17:51 PM UTCmtmillerStatusNone=>Invalid
      Open/ClosedOpen=>Closed
    Tue 25 Oct 2016 01:26:03 AM UTCNoneAttached File-=>Added Bode4.PNG, #38804

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