Description

FeldrinH

opened on Feb 13, 2024

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I am trying to draw a phase plot with both streamlines and a vector field. Using the inverted pendulum ODE from https://python-control.readthedocs.io/en/latest/phaseplots.html, I put together the following script:

import numpy as np
import matplotlib.pyplot as plt
from control.phaseplot import phase_plot

# Define the ODEs for a damped (inverted) pendulum
def invpend_ode(x, t, m=1., l=1., b=0.2, g=1):
    return x[1], -b/m*x[1] + (g*l/m)*np.sin(x[0])

# Set up the figure the way we want it to look
plt.figure()
plt.title('Inverted pendulum')

# Outer trajectories
phase_plot(
    invpend_ode,
    X=(-10, 10, 20),
    Y=(-10, 10, 20),
    X0=[[-2*np.pi, 1.6], [-2*np.pi, 0.5], [-1.8, 2.1],
        [-1, 2.1], [4.2, 2.1], [5, 2.1],
        [2*np.pi, -1.6], [2*np.pi, -0.5], [1.8, -2.1],
        [1, -2.1], [-4.2, -2.1], [-5, -2.1]]
)

plt.show()

I would have expected this to plot a simple vector field with a grid of 20*20 arrows and streamlines from the given inital conditions. Instead, each time I run this I get a different malformed plot, often with at least one axis somehow having arrows at values like 10^281. Various RuntimeWarnings about overflow are also sometimes printed in the console. Some examples of the plots I have gotten:

This does not seem like expected behaviour. What is going on here?

Tested on Windows 10 with Python 3.10.11, matplotlib 3.8.2 and control 0.9.4 from pip.

Activity

1. FeldrinH

   changed the title [-]Strange nondeterministic behavior when using `phase_plot` with `X`, `Y` and `X0` set.[/-] [+]Strange nondeterministic behavior when using `phase_plot` with `X`, `Y` and `X0` set[/+] on Feb 13, 2024
2. FeldrinH

   changed the title [-]Strange nondeterministic behavior when using `phase_plot` with `X`, `Y` and `X0` set[/-] [+]Strange nondeterministic issues when using `phase_plot` with `X`, `Y` and `X0` set[/+] on Feb 13, 2024
3. murrayrm commented on Feb 18, 2024

   

   murrayrm

   on Feb 18, 2024

   Member

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   I ran this on the current main and got similar behavior on MacOS 14.2 with Python 3.11. There are a couple of warning messages that also appear

   [...]/matplotlib/quiver.py:646: RuntimeWarning: divide by zero encountered in scalar divide
     length = a * (widthu_per_lenu / (self.scale * self.width))
   [...]/matplotlib/quiver.py:646: RuntimeWarning: invalid value encountered in multiply
     length = a * (widthu_per_lenu / (self.scale * self.width))
   

   These could be the source of the randomness?

   I'll try to have a look and see if I can figure out what is going on and see if there is a quick fix. I'm in the process of rewriting this functionality to be a bit more Pythonic, so it may not make sense to do an interim fix to the old code. More info as I find it.

   Reacted by Ben Greiner and Juhan Oskar Hennoste
4. bnavigator

   assigned

   murrayrm

   on Feb 18, 2024
5. murrayrm

   linked a pull request that will close this issueReimplementation of 2D phase plots #980on Mar 28, 2024
6. murrayrm

   closed this as completedin #980on Mar 31, 2024

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