On Wed, 2020-06-10 at 14:26 -0400, Nicholas Jankowski wrote:
> On Wed, Jun 10, 2020 at 12:40 PM Doug Stewart <doug.dastew@gmail.com>
> wrote:
> > try
> > step(ss(q))
> >
> >
>
> well that seemed to do the trick. step(ss(q)) plots quickly in
> Octave, and it matches the output produced in Matlab by step(q) and
> step(ss(q))
>
> so, thoughts on why octave needs it converted into state-space
> representation to avoid choking on it but Matlab doesn't?
I think the problem is that the discretization (which is done for the
simulation) is not sufficiently accurate for a transfer function:
# discretization of the state space model
# => stable eigenvalues
sys_ss_ct = ss (q);
sys_dt2 = c2d (sys_ss_ct, 0.01);
eig (sys_dt2.a)
# discretization of the transfer function
# unstable eigenvalues (|lambda| > 1)
sys_tf_dt = c2d (q, 0.01);
sys_dt1 = ss (sys_tf_dt);
eig (sys_dt1.a)
curious about output differences, I've attached diaries for both octave and matlab running the eig commands above. they're different. not sure how much so and if that explains Octave failing to process the tf.
diary_octave.txt