[Octave-bug-tracker] [bug #45334] control package 2.8.1 - incorrect results on repeated MIMO tf multiplies

[Nick Jankowski]

URL:
  <http://savannah.gnu.org/bugs/?45334>

                 Summary: control package 2.8.1 - incorrect results on
repeated MIMO tf multiplies
                 Project: GNU Octave
            Submitted by: nrjank
            Submitted on: Tue 16 Jun 2015 09:26:08 PM 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: 4.0.0
        Operating System: Microsoft Windows

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Details:

using control-2.8.1. performing a multiple MIMO tf multiplication produces
erroneous output. (haven't checked this under 2.8.2, as that version produces
a different error)

using a simple 2x2 tf, the following commands reproduce the problem:


>> a1=tf({[1],[2];[3],[4]},{[1 3],[1 4];[2 3],[1 1]});
>> a2=a1*a1
>> a3=a1*a2
>> a4=a1*a3
>> a5=a1*a4


a2 calculates as expected, and a minreal(a2) produces the expected minimized
form. the a3 and a4 results get huge, and then the a5 result drops and zeros
out almost all of the numerator coefficients.  using minreal between
multiplies for a3, a4 and a5 make little difference.

As the output gets messy, I attached a text file of the Octave output with and
without minreal.

Note that the order of multiplication (whether it's a1* or *a1) in the
definition of a2-a5 seems to matter on whether the coefficients match the
expected analytical result, but it produces zeros on a5.

Checking Matlab output for the same (ML 2015a) produces sensible results,
although as expected they don't minimize as nicely as Maxima. A minreal
between multiply steps gets it fairly close through a4.

I attached a png of a maxima output showing what the minimized output of those
should be. granted I expect Maxima to be better at handling the higher order
polynomials. 



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File Attachments:


-------------------------------------------------------
Date: Tue 16 Jun 2015 09:26:08 PM GMT  Name: Octave_output.txt  Size: 32kB  
By: nrjank
outputs from Octave (txt), Maxima (png) and Matlab (txt files)
<http://savannah.gnu.org/bugs/download.php?file_id=34237>
-------------------------------------------------------
Date: Tue 16 Jun 2015 09:26:08 PM GMT  Name: MLoutput-withminreal.txt  Size:
17kB   By: nrjank
outputs from Octave (txt), Maxima (png) and Matlab (txt files)
<http://savannah.gnu.org/bugs/download.php?file_id=34238>
-------------------------------------------------------
Date: Tue 16 Jun 2015 09:26:08 PM GMT  Name: MLoutput.txt  Size: 29kB   By:
nrjank
outputs from Octave (txt), Maxima (png) and Matlab (txt files)
<http://savannah.gnu.org/bugs/download.php?file_id=34239>
-------------------------------------------------------
Date: Tue 16 Jun 2015 09:26:08 PM GMT  Name: Octave_output_withminreal.txt 
Size: 21kB   By: nrjank
outputs from Octave (txt), Maxima (png) and Matlab (txt files)
<http://savannah.gnu.org/bugs/download.php?file_id=34240>

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