bugGNU Octave - Bugs: bug #62292, [octave forge] (control) Generic...

 
 

bug #62292: [octave forge] (control) Generic close loop formula doesn't show same result than feedback function

Submitter:  Pablo Scinardo <pabloscinardo>
Submitted:  Mon 11 Apr 2022 02:49:41 PM UTC
 
Category:  Octave Forge Package Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Incorrect Result
Status:  Works For Me Assigned to:  None
Originator Name:  pabloscinardo Open/Closed:  * Closed
Release:  * 6.4.0 Operating System:  * Microsoft Windows
Fixed Release:  None Planned Release:  None
* Mandatory Fields

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Tue 06 Sep 2022 01:55:43 PM UTC, comment #3: 

Found this bug was still marked as open. Closing as per the intent of comment #2.

Arun Giridhar <arungiridhar>
Project Member
Tue 17 May 2022 08:12:01 PM UTC, comment #2: 

one month without further comment. closing report.

Nicholas Jankowski <nrjank>
Project Member
Mon 11 Apr 2022 07:12:32 PM UTC, comment #1: 

The both transfer functions of the closed loop are describing the same system. It's just, that in the longer form, the common roots and zeros are not canceled.

You can check that the transfer functions are equivalent by

GD1 = G1/(1+G1*G2);
GD2 = 1/(1/G1+G2);
% Bode plot
bode (GD1, GD2)
% tf -> ss -> tf
tf (ss (GD1))
tf (ss (GD2))

Btw: Matlab is giving the same result.

Torsten Lilge <ttl>
Project Member
Mon 11 Apr 2022 02:49:41 PM UTC, original submission:  

Generic loop back formula:

             G
Sys =  --------------
         1 + G * H

It shows wrong result.

But changing the loop back formula:
             1
Sys =  --------------
         1/G + H

The result is right, the same than feedback(G,H) funcion

Follow some example on Octave 6.4.0

>> pkg load control
>> num1 =[10];


>> den1 =[1 2 10];


>> G1=tf(num1,den1)


Transfer function 'G1' from input 'u1' to output ...

            10
 y1:  --------------
      s^2 + 2 s + 10

Continuous-time model.

>> num2 =[0 5];


>> den2 =[1 5];


>> G2=tf(num1,den1)


Transfer function 'G2' from input 'u1' to output ...

            10
 y1:  --------------
      s^2 + 2 s + 10

Continuous-time model.

>> feedback(G1,G2)


Transfer function 'ans' from input 'u1' to output ...

             10 s^2 + 20 s + 100
 y1:  ---------------------------------
      s^4 + 4 s^3 + 24 s^2 + 40 s + 200

Continuous-time model.

>> G1/(1+(G1*G2))


Transfer function 'ans' from input 'u1' to output ...

             10 s^4 + 40 s^3 + 240 s^2 + 400 s + 1000
 y1:  -------------------------------------------------------
      s^6 + 6 s^5 + 42 s^4 + 128 s^3 + 520 s^2 + 800 s + 2000

Continuous-time model.

>> 1/((1/G1)+G2)


Transfer function 'ans' from input 'u1' to output ...

             10 s^2 + 20 s + 100
 y1:  ---------------------------------
      s^4 + 4 s^3 + 24 s^2 + 40 s + 200

Continuous-time model.

Pablo Scinardo <pabloscinardo>

 

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    Follow 4 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2022-09-06 arungiridhar Open/ClosedOpen Closed
    2022-05-17 nrjank StatusNone Works For Me
    2022-04-11 mmuetzel Carbon-CopyRemoved 102357 -
    2022-04-11 mmuetzel SummaryGeneric close loop formula doesn't show same result than feedback function [octave forge] (control) Generic close loop formula doesn't show same result than feedback function

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