Bode_plot of transfer functions has ferqueny indicated in Hz but should be rad/s?

[joe tusek]

I'm a new user of the package and wanted to check transfer function outputs between my circuit simulator and some other control simulation package. It appears that outputs from bode_plot (sympy.physics.control.control_plots) are in rad/s for frequency, which is the natural unit for control analysis. The graph produced by bode_plot  has its frequency axis shown in Hz, but analysis and comparison to other programs suggest that it should be labelled rad/s.

Is this an error, or is there some switch somewhere in the package that sets units for outputs to Hz and degrees, rather than radians and rad/s?

I have a work around by handling graphing, just would like to use the built in functions where possible. Also would like the make use of the initial and final exponent, in the units of the graphing output.

[Hanspeter Schmid]

Can you send example code, please?

[Naman Gera]

Hi,

This is not an error, per se, as the frequency is needed to be plotted in Hz at times.

Although I suppose there should be an option to allow both Hz and rad/sec for frequency, and rad / deg for phase. It is up to the user which one to choose.

If you know how to allow both, using an optional argument, please feel free to raise a pull request.

Best,

 

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[joe tusek]

Hi Naman,

This is actually an error in the plot. The plot is shifted by 2*pi on its frequency axis as it is has the wrong axis label.

[joe tusek]

See attached. Note that both x-axis are in Hz.

[Oscar Gustafsson]

Yes, it is a bug, see this much simpler example:

from sympy.abc import s

from sympy.physics.control.lti import TransferFunction

from sympy.physics.control.control_plots import bode_plot

tf = TransferFunction(100, 100+s, s)

bode_plot(tf)

Here it is clear that the -3dB-point should be at 100 rad/s, but it is at 100 Hz.

Indeed it makes sense to have some option regarding the scale.

BR Oscar

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[Hanspeter Schmid]

Confirmed; and it would also make sense to have rad and deg as options for the phase. I'll formulate an issue right away.