I made some further investigations (I'm writing pzmap.m for control-oo at the moment, which needs to compute the zeros of a system) and came to the conclusion that tzero.m from the control package is buggy. The attachment contains an example where the discrepancy of the results is big: Matlab gets -13.2759, -0.0155, 12.5774, while tzero gets -5.708333, -0.015561, 31.509741. Apparently the bug has been undiscovered for about 15 (!) years.
The bad thing is that I reused the code from tzero.m in the control-oo package ... Regards, Lukas
zerotest.m
Description: Binary data
> Hi Doug, Luca, and Lukas, > thanks for your responses. > >> 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'); > > yes, changing the order works for me as well :-) > >> I also found that it has something to do with the size of the >> numbers >> 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. > > Also worked in my octave installation. Thanks! > > It seems to me that the state space representation of all these systems are > equivalent, and I get the correct impulse response no matter in which way I > created the system and regardless of the result of sysout(..). So I guess the > error occurs during the conversion to the transfer function representation. > > Best regards, > Christian > > > > ------------------------------------------------------------------------------ > Come build with us! The BlackBerry(R) Developer Conference in SF, CA > is the only developer event you need to attend this year. Jumpstart your > developing skills, take BlackBerry mobile applications to market and stay > ahead of the curve. Join us from November 9 - 12, 2009. Register now! > http://p.sf.net/sfu/devconference > _______________________________________________ > Octave-dev mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/octave-dev
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