lti.evalfr and sys._evalfr have different behavior

The evalfr function in the lti module expects a complex number and evaluates the system at that complex frequency. The method sys._evalfr, on the other hand, where sys is a transfer function or state space system, expects a real argument and multiplies it by j before evaluating the frequency response.

Is this dichotomy intentional?

• matlab’s evalfr function expects a complex number, which seems like a more general-purpose solution. sys.freqresp expects real-valued omega arguments, making it perhaps a better interface for users
• also, right now, only sys._evalfr correctly performs the z-plane transformation for discrete-time systems.

[sawyerbfuller]

a __call__ method makes a lot of sense to me. It always struck me as odd how hard it is in matlab to find the frequency response of a transfer function – I didn’t know about evalfr until very recently.

Presumably a change moving evalfr to the matlab module and changing freqresp to frequency_response would entail a version or two in which those were wrappers accompanied by deprecation warnings?

[murrayrm]

We definitely need to remove the inconsistency, but I would encourage us to think more broadly and use a more Pythonic approach:

• We can override the __call__ function so that P(s) will represent the “value” of a transfer function at a complex value s. This can replace the evalfr function. It should work for all LTI classes (including transfer functions, state space systems, FRD systems).

• We can provide a class method frequency_response that takes a variety in input arguments representing a single frequency or list of frequencies. We could shorten this to freqresp, but I think that using the longer name is more consistent with other usage. We could also provide an “alias” for freqresp (both as a function and as a method?).

• We can use the MATLAB compatibility module to provide functions that match MATLAB’s standard usage, but I don’t think we need to represent those in the main python-control module.

It would be great to get thoughts from others.