[Octave-bug-tracker] [bug #61355] Error on phase value of bode and margin plots

[Luiz Antonio Maccari Junior]

URL:
  <https://savannah.gnu.org/bugs/?61355>

                 Summary: Error on phase value of bode and margin plots
                 Project: GNU Octave
            Submitted by: luiz
            Submitted on: Ter 19 Out 2021 16:23:03 UTC
                Category: Octave Forge Package
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: Luiz Antonio Maccari Jr.
        Originator Email: 
             Open/Closed: Open
                 Release: 6.3.0
         Discussion Lock: Any
        Operating System: GNU/Linux

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Details:

Hello,

I am using bode function and I got wrong results when the system has two poles
in the same position. The phase should be plotted as -180 (as two poles)
however in some cases it is plotted as +180 (as two zeros). ? This behavior
leads to a wrong plot and phase margins when I call margin function. 

Above I give three examples with distinct results. 

Example 1:

First one in the file "bug_report_bode.m" is a system with 4 poles, two at the
frequency of 10^2 rad/s and two at 10^4 rad/s. 

The expected behavior is -180° and after -180°. However the bode plot
presents +180 and after -180. Bode and margin functions give the same phase
plot. I have tested the same script in matlab and there the results are
correct, as you can see on figures attached.

right result = matlab_output_bode.jpg
wrong result = octave_output_bode.jpg

Example 2: 

On this example the bode plot gives the correct plot but the margin function
does not. The example "bug_report_bode_teste3.m" has two poles at origin and
additional one at 10 rad/s.The phase plot of function bode is correct but the
margin one is wrong.


Example 3:
These wrong phase plot can lead to a problem to identify the correct gain
margin as for example in the file "bug_report_bode_ex3.m" where because the
wrong plot there is no crossover at -180 phase angle.

Best Regards



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File Attachments:


-------------------------------------------------------
Date: Ter 19 Out 2021 16:23:03 UTC  Name: octave_output_bode.jpg  Size: 56KiB 
 By: luiz

<http://savannah.gnu.org/bugs/download.php?file_id=52124>
-------------------------------------------------------
Date: Ter 19 Out 2021 16:23:03 UTC  Name: bug_report_files.zip  Size: 844B  
By: luiz

<http://savannah.gnu.org/bugs/download.php?file_id=52125>
-------------------------------------------------------
Date: Ter 19 Out 2021 16:23:03 UTC  Name: matlab_output_bode.jpg  Size: 19KiB 
 By: luiz

<http://savannah.gnu.org/bugs/download.php?file_id=52126>

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