8000 Stability margins for discrete time systems by bnavigator · Pull Request #469 · python-control/python-control · GitHub
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Stability margins for discrete time systems #469

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Merged
merged 4 commits into from
Dec 21, 2020
Merged

Stability margins for discrete time systems #469

merged 4 commits into from
Dec 21, 2020

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bnavigator
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My attempt to fix #465

If someone has an idea how to solve min |1 + num(z) / den(z) | with z=exp(i w dt) analytically, that would be great. I opted for a numerical optimization instead.

sawyerbfuller
sawyerbfuller approved these changes Nov 17, 2020
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Very nice, Looks good to me. Not sure why the previous author used an arcane incantation rather than just numpy.polyval in _poly_iw(), but maybe there is a numeric justification when working with imaginary arguments that I don’t know about.

I don’t have any good ideas for an analytic solution to the optimization problem.

ilayn
ilayn reviewed Nov 17, 2020
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Not sure why the previous author used an arcane incantation rather than just numpy.polyval in _poly_iw(),

Both yours / @repagh's and @swaldherr's (is there a more current GitHub handle?) implementations are very similar. They split the polynomials up into real and imaginary parts after applying iω, and just store the result in a different variable structure. I just refactored to remove the "duplicate" code.

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coveralls commented Nov 17, 2020 •
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Coverage Status

Coverage increased (+0.2%) to 86.319% when pulling d0f333e on bnavigator:discrete-margins into 2d7aad0 on python-control:master.

@python-control python-control deleted a comment from coveralls Nov 18, 2020
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I think this should go into 0.8.4. It enhances functionality, but does not change the API and it fixes the bug that it returns wrong results with discrete input.

@bnavigator bnavigator added this to the 0.8.4 milestone Nov 19, 2020
@bnavigator bnavigator merged commit e525ff6 into python-control:master Dec 21, 2020
@bnavigator bnavigator deleted the discrete-margins branch January 5, 2021 20:13
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control.margin() gives wrong result
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