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Include a zpk2tf function #749
Description
As it stands, a function that takes zeroes, poles, gain into numerator, denominator transfer function coefficient arrays only lives in scipy.signal or can be imported from control.matlab, and used to create a transfer function my_tf as follows:
my_tf= tf(*zpk2tf(...))
Why not instead can python-control have a method that directly takes zeroes, poles, gain into a transfer function, i.e.
my_tf=zpk2tf(..)?
Activity
sawyerbfuller commentedon Jul 16, 2022
Hi @ovega14 I agree it would be very nice to have this functionality outside of the matlab module.
Its not clear to me how we could implement that functionality inside
TransferFunctionbecause the three-argument case is already taken byTransferFunction(num, den, dt). Might be possible to include a keyword to make it explicit likeTransferFunction(zeros, poles, gain, [dt,] usezpk=True), but implementing that logic seems like it would be awkward.Probably most expedient is to just move the import of scipy's
zpk2tf(and might as well bring inzpk2sswhile you're at it) into theLTImodule rather than the matlab module, and then they will be available everywhere, including the matlab module.This might be an opportunity to also invent a more pythonic additional long name for them as well, e.g.
zeros_poles_gain_to_tf, but they are so apt and convenient I am not sure that is necessary.Feel free to contribute a pull request.
murrayrm commentedon Jul 16, 2022
Whoever takes on implementing this should make sure that
zpk2tfandzpk2sseventually create aNamedIOSystem, versus a lower levelTransferFunctionorStateSpaceobject. That will allow the output system to be compatible with the I/O system functionality (including named input/output signals for connecting things together). A diagram of the dependencies can be found here.ovega14 commentedon Jul 21, 2022
Should we import
zpk2tfandzpk2ssfromscipyinto theLTImodule? If so, then these functions will not directly return aNamedIOSystem, nor aTransferFunctionorStateSpaceobject. As a reminder, these functions return numerator and denominator coefficient arrays of a transfer function given zeroes, poles, and gain. Should we then redefine the two functions (zpk2tfandzpk2ss) to conform to this expectedpython-controlbehavior?bnavigator commentedon Jul 21, 2022
Yes functions in the
controlnamespace should return a python-control conforming system. Hence @murrayrm's request to returnNamedIOSystem. This requires a wrapper rather than a simple import.