Nyquist plot with imaginary poles, semi-circles only working after 'trick'

I'm plotting the Nyquist diagram of a transfer function which has poles on the imaginary axis:


import control
import matplotlib.pyplot as plt
import numpy as np

K = 1

Gp =  control.tf([1], [1,1,0,0,0]) 
Gc = control.tf([K], [1]) 


control.nyquist_plot(Gp*Gc,plot=True,omega_num=1000)
plt.show()

Which results in the following incorrect plot, that does not show infinte semicircles, but something incorrect:

Also using indent_direction and indent_radius did not solve this.
The trick I found that makes it work is if I add omega_limits=[0.001,10000] to nyquist_plot:

Can you please fix such that the incorrect 'line' does not appear?

Yes, the algorithm to determine the default omega vector for the system starts with a lower frequency of 0.1, because the lowest non-zero pole of the system is at -1.

python-control/control/freqplot.py

Lines 1088 to 1092 in e81f648

    
           # This code looks at the poles and zeros of all of the systems that 
        
           # we are plotting and sets the frequency range to be one decade above 
        
           # and below the min and max feature frequencies, rounded to the nearest 
        
           # integer.  It excludes poles and zeros at the origin.  If no features 
        
           # are found, it turns logspace(-1, 1)

The "trick" to manually define the omega limits is the correct way to override the default omega vector determination.

Perhaps if there is a pole at the origin, the lower limit of the omega vector should be below the indent_radius ?

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Nyquist plot with imaginary poles, semi-circles only working after 'trick' #613

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	# This code looks at the poles and zeros of all of the systems that
	# we are plotting and sets the frequency range to be one decade above
	# and below the min and max feature frequencies, rounded to the nearest
	# integer. It excludes poles and zeros at the origin. If no features
	# are found, it turns logspace(-1, 1)

Nyquist plot with imaginary poles, semi-circles only working after 'trick' #613

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bnavigator commented on Apr 24, 2021

kevindekemele commented on Apr 25, 2021

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