Re: control package, can't simulate complex systems

[Torsten Lilge]

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)


Torsten