[Octave-bug-tracker] [bug #43586] tf function outputs wrong results

[Arnaud Miege]

URL:
  <http://savannah.gnu.org/bugs/?43586>

                 Summary: tf function outputs wrong results
                 Project: GNU Octave
            Submitted by: am304
            Submitted on: Wed 12 Nov 2014 10:57:13 GMT
                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
         Discussion Lock: Any
                 Release: 3.8.0
        Operating System: Microsoft Windows

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Details:

The  tf  function from the control package can output the wrong result if the
transfer function is not correctly entered, and it does not error out. The
details can be found at this StackOverflow question
<http://stackoverflow.com/questions/26869827/octave-control-package-tf> but in
essence, it boils down to the following:


>> a = tf(1,[1 1])

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

        1
 y1:  -----
      s + 1

Continuous-time model.
>> a(1)
ans =  0.50000 - 0.50000i
>> b = 1/(tf('s')+1)

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

        1
 y1:  -----
      s + 1

Continuous-time model.
>> b(1)
ans =  0.50000 - 0.50000i
>> c = tf('1/(s+1)')

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

 y1:  1/(s+1)

Continuous-time model.
>> c(1)
ans =  0 + 1i
>> s = tf('s')

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

 y1:  s

Continuous-time model.
>> d = 1/(s+1)

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

        1
 y1:  -----
      s + 1

Continuous-time model.
>> d(1)
ans =  0.50000 - 0.50000i


 c  is incorrectly defined, but Octave does error out when doing so, and it
then reports the wrong result.






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