Description

Avitan01

opened on Dec 31, 2022

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I'm using the control package with python 3.10 in Pycharm and Jupiter lab.
I have stumbled upon a possible issue while trying to create a Nyquist plot for a simple system

import control as ct

s = ct.tf('s'))
plant_1 = 1/(s*(s + 5)*(s + 7)))
ct.nyquist_plot(plant_1)

The result I'm getting is this:

While in MATLAB I'm getting different results

clear all;
close all;
clc;


s = tf('s');
plant_1 = 1/(s*(s + 5)*(s + 7));
nyquistplot(plant_1)

Is there a limitation that might be causing this issue or am I missing something?

Activity

1. bnavigator commented on Dec 31, 2022

   

   bnavigator

   on Dec 31, 2022

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   Look at the y-axis scale. The nyquist plot from control cuts off at max_curve_magnitude and scales the "infinity" loop for display within the plot boundaries.
   https://python-control.readthedocs.io/en/0.9.2/generated/control.nyquist_plot.html#control.nyquist_plot
2. henklaak commented on Feb 2, 2023

   

   henklaak

   on Feb 2, 2023

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   Behaviour that will 'surprise' many, I guess. The documentation is not explicit about it and the default value of 20.

   How about not drawing the large magnitudes at all? Just clip the traces at the limit with a ≫to indicate direction?
3. murrayrm commented on Feb 2, 2023

   

   murrayrm

   on Feb 2, 2023

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   If you clip the traces, then it is harder to see the number of net encirclements. Especially if there is more than one. For example

   sys = ct.tf([1], [1, 3, 2]) * ct.tf([1], [1, 0, 1])
   ct.nyquist_plot(sys)
   

   which generates:
   

   Reacted by Ben Greiner
4. murrayrm commented on Feb 2, 2023

   

   murrayrm

   on Feb 2, 2023

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   We should definitely make sure all of this is well-documented, though.

   Reacted by Ben Greiner
5. henklaak commented on Feb 3, 2023

   

   henklaak

   on Feb 3, 2023

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   I'll take a stab at this (the documenting part), expect PR.

   Reacted by Ben Greiner and Rene van Paassen
6. murrayrm

   added this to the 0.9.4 milestone on Mar 26, 2023
7. murrayrm

   self-assigned this

   on Mar 26, 2023
8. murrayrm

   closed this as completedin #880on Mar 31, 2023

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