From: [email protected]
To: [email protected]; [email protected]
Date: Tue, 20 Oct 2009 13:43:14 +0200
Subject: Re: [OctDev] Discrete feedback loop
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
fromhttp://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
athttp://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
I took a look at this and found that If you change the order in sysmult then
you get the correct answer
try
sysUNIT = tf(1, 1, 1);
sysB1 = tf([8e-9 -5.6e-9], [1 -1], 1);
sysB2 = tf([125e6], [1 -1], 1);
sysB3 = sysmult(sysB2, sysB1);
sysCL2 = feedback(sysUNIT, sysB3);
sysout(sysCL2, 'tf');
I also found that it has something to do with the size of the numbers
try this
sysUNIT = tf(1, 1, 1);
sysB1 = tf([8e-9 -5.6e-9].*1000, [1 -1], 1);
sysB2 = tf([125e6]./1000, [1 -1], 1);
sysB3 = sysmult(sysB1, sysB2);
sysCL2 = feedback(sysUNIT, sysB3);
sysout(sysCL2, 'tf');
What i did is make sysB1 1000 bigger and sysB2 1000 smaller so that the answer
is still the same, and now we get the correct answer either way we multiply it.
I do not know why this is happening but with this start someone should be able
to track it down faster.
Doug Stewart
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