This is clearly wrong: there is no way that kind of signal won't result in a residual oscillations that will dampen over time. Interestingly enough, when the driving signal repeats, the output is different. But do you notice something weird?

Yup! The oscillations start only at the third descending edge: there is no way this can be the correct solution.

So where did it go wrong?

Yup! The oscillations start only at the third descending edge: there is no way this can be the correct solution.

So where did it go wrong?

Let's change the start time for the driving oscillation.

When the first falling edge is at t=1, t=3 or t=5, the simulation shows no oscillation. When the first falling edge occurs at t=2 or t=4, the model seems to behave correctly, and when the first falling edge is at t=6, the first two falling edges are missed. This looks like the time step is variable, and if it falls on an "unchanging" output, it doesn't bother computing the intermediate values.

Octave's

*lsode_options()*allows to get or set the options used by

*lsode()*. These options include the tolerances, the integration method used, but also the minimum and maximum step size. By default the minimum is set to 0 (no minimum) and the maximum is set to -1 (no maximum): the step size can take any value and skip has many points as needed.

Here is the output from

*lsode_options()*right after Octave was started.

Options for LSODE include:

keyword value

------- -----

absolute tolerance 1.4901e-08

relative tolerance 1.4901e-08

integration method stiff

initial step size -1

maximum order -1

maximum step size -1

minimum step size 0

step limit 100000

keyword value

------- -----

absolute tolerance 1.4901e-08

relative tolerance 1.4901e-08

integration method stiff

initial step size -1

maximum order -1

maximum step size -1

minimum step size 0

step limit 100000

What if we change the maximum step size to be half of the pulse duration? Let's try that with the command

*lsode_options("maximum step size",0.5)*. The options are now

Options for LSODE include:

keyword value

------- -----

absolute tolerance 1.4901e-08

relative tolerance 1.4901e-08

integration method stiff

initial step size -1

maximum order -1

maximum step size 0.5

minimum step size 0

step limit 100000

And my test set looks like this.

Which is way better: this is consistent with the spring oscillations starting at the first falling edge. Redoing the first example with the new options, I have:

And that is consistent with the real life experience.

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