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Re: Request for Matlab output
From: |
Lukas Reichlin |
Subject: |
Re: Request for Matlab output |
Date: |
Fri, 30 Oct 2015 20:10:00 +0100 |
> On 30.10.2015, at 18:04, Steph Bredenhann <address@hidden> wrote:
>
>
> On 29/10/2015 21:13, Lukas Reichlin wrote:
>> On 29.10.2015, at 18:50, Lukas Reichlin <address@hidden> wrote:
>>>> On 29.10.2015, at 12:36, Lukas Reichlin <address@hidden> wrote:
>>>>
>>>> Dear Octave community,
>>>>
>>>> In order to improve error handling in the control package, I would like to
>>>> know what Matlab’s output/error message looks like for each of the
>>>> following three lines of code:
>>>>
>>>> dss (1, 2, 3, 4, 0)
>>>>
>>>> feedback (tf (1), tf (-1))
>>>>
>>>> feedback (ss (1), ss (-1))
>>>>
>>>> Could someone with access to Matlab and its Control System Toolbox please
>>>> send me the resulting text output?
>>>>
>>>> Thanks in advance,
>>>> Lukas
>>>>
>>>>
>>> Gentlemen, thank you very much for your help! I’ve just pushed a file
>>> containing my conclusions:
>>>
>>> http://sourceforge.net/p/octave/control/ci/default/tree/devel/algebraic_loops.m
>>>
>>> Best regards,
>>> Lukas
>> May I ask you for another test in Matlab?
>>
>>
>> e = zeros (2, 2);
>> a = [-1, 1; 1, -1];
>> b = [1; 0];
>> c = [1, 0];
>> d = 0;
>> sys = dss (a, b, c, d, e)
>> G = tf (sys)
>> H = zpk (sys)
>>
>>
>> I think that descriptor systems with all-zero E matrix and non-invertible A
>> matrix are invalid. Now I wonder whether Matlab accepts those systems at all
>> or only returns NaN’s later on.
>>
>> Best regards,
>> Lukas
>>
>>
>>
>>
> Ubuntu 14.04 x64 with Matlab 2015b
>
>
> sys =
>
> a =
> x1 x2
> x1 -1 1
> x2 1 -1
>
> b =
> u1
> x1 1
> x2 0
>
> c =
> x1 x2
> y1 1 0
>
> d =
> u1
> y1 0
>
> e =
> x1 x2
> x1 0 0
> x2 0 0
>
> Continuous-time state-space model.
>
> Warning: The frequency response has poor relative accuracy. This may be
> because the
> response is nearly zero or infinite at all frequencies, or because the
> state-space
> realization is ill conditioned. Use the "prescale" command to investigate
> further.
> > In ltipack.ssdata/iodynamics
> In ltipack.ssdata/zpk (line 23)
> In ltipack.ssdata/tf (line 20)
> In lti/tf_ (line 283)
> In DynamicSystem/tf (line 90)
> In untitled (line 11)
>
> G =
>
> -6369051672525772
>
> Static gain.
>
> Warning: The frequency response has poor relative accuracy. This may be
> because the
> response is nearly zero or infinite at all frequencies, or because the
> state-space
> realization is ill conditioned. Use the "prescale" command to investigate
> further.
> > In ltipack.ssdata/iodynamics
> In ltipack.ssdata/zpk (line 23)
> In lti/zpk_ (line 295)
> In DynamicSystem/zpk (line 80)
> In untitled (line 12)
>
> H =
>
> -6.3691e+15
>
> Static gain.
>
> >>
>
Thank you very much! The result confirms my resolution to refuse such systems
(matrix E zero and matrix A not invertible) in control-3.0.1. The reason for
this refusal is the same as for refusing zero denominators in transfer
functions: these systems are simply invalid.
Best regards,
Lukas