[Octave-bug-tracker] [bug #61667] tf() function gives wrong result for complex state space models

[Karl Stephan Stille]

URL:
  <https://savannah.gnu.org/bugs/?61667>

                 Summary: tf() function gives wrong result for complex state
space models
                 Project: GNU Octave
            Submitted by: kssupb
            Submitted on: Mon 13 Dec 2021 01:19:15 PM UTC
                Category: Octave Forge Package
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: 6.2.0
         Discussion Lock: Any
        Operating System: GNU/Linux

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Details:

When using the tf() function for state space models with complex entries the
result strips complex poles from the result. As can be seen in the following
example, the order decreases by two.

Example system:

A=[-0.2071,0,0,0;0,-3.7646,0,0;0,0,-0.58+4.183*i,0;0,0,0,-0.58+4.183*i];
b=[44.3;160;-39.2-38.1*i;39.2+38.1*i];
cT=[0.502,-0.075,-0.0251-0.18*i,-0.0251+0.18*i];
d=0;
sys=ss(A,b,cT,d)
tf(sys)



The result in Matlab:


ans =
 
     -(3.477-14.11i) s^3 + (89.71+35.95i) s^2 + (110.7-457.9i) s -
(1354+343.1i)
 
---------------------------------------------------------------------------------
  s^4 + (5.132-8.366i) s^3 - (11.77+38.08i) s^2 - (67.25+25.79i) s -
(13.38+3.783i)


where octave gives:


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

         10.24 s + 81.23
 y1:  ----------------------
      s^2 + 3.972 s + 0.7796


This bug also affects functions as e.g. step()

Tested with control package version 3.3.1



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File Attachments:


-------------------------------------------------------
Date: Mon 13 Dec 2021 01:19:15 PM UTC  Name: bug.m  Size: 191B   By: kssupb

<http://savannah.gnu.org/bugs/download.php?file_id=52499>

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