Hi Christian, thanks for your report.
Hi,
I have tried to create a discrete feedback system with the control
package (1.0.11 with octave 3.0.1), but got unexpected results.
I create the system with:
sysUNIT = tf(1, 1, 1);
sysB1 = tf([8e-9 -5.6e-9], [1 -1], 1);
sysB2 = tf([125e6], [1 -1], 1);
sysB3 = sysmult(sysB1, sysB2);
sysCL2 = feedback(sysUNIT, sysB3);
sysout(sysCL2, 'tf');
The resulting transfer function is
1*z^2 - 2*z^1 + 0.20215
-----------------------
1*z^2 - 1*z^1 + 0.3
However, the correct result should be
1*z^2 - 2*z^1 + 1
-----------------------
1*z^2 - 1*z^1 + 0.3
When I do not use the sysmult function but do the multiplication of
sysB1 and sysB2 by myself, I get the correct result:
sysA1 = tf([1 -.7], [1 -2 1], 1);
sysCL = feedback(sysUNIT, sysA1);
sysout(sysCL, 'tf');
1*z^2 - 2*z^1 + 1
-------------------
1*z^2 - 1*z^1 + 0.3
Am I doing something wrong here? Do you get the same results?
No, you're not ;-) I'm using Octave 3.2.3 and control-1.0.17 (get it
from
http://octave.svn.sourceforge.net/viewvc/octave/trunk/octave-forge/main/control.tar.gz?view=tar
and I get
transfer function form:
1*z^2 - 2*z^1 + 0.15744
-----------------------
1*z^2 - 1*z^1 + 0.3
which is obviously wrong.
I got several warnings from the sysgroup command during the
execution of this script, e.g.
warning: sysgroup: changed output name 2 to y_1_2
should I take these serious?
No, you shouldn't. Technically, the control package does all the
calculations in state-space form, i. e. it converts all your transfer
functions to state-space form and adds an unique name to each input,
output and state. When two systems are grouped together and have the
same names, it renames the second one and displays the warning above.
BTW: I'm currently developing a new control package which is object-
oriented and offers better Matlab compatibility. It delivers the
expected results for your calculations, doesn't convert to state-space
and doesn't show such warnings :-) However, it is not ready for daily
use because it lacks important features like bode or step. If you're
interested, take a look at
http://octave.svn.sourceforge.net/viewvc/octave/trunk/octave-forge/extra/control-oo/
Please note that I'm just an engineering student and I do this in my
free time.
Let me know if you have further questions.
Regards,
Lukas
Thank you, best regards,
Christian
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