Re: matrix exponential used by the control package

On Sat, Jul 25, 2020 at 10:28 AM Torsten Lilge <[email protected]>
wrote:

> On Tue, 2020-07-21 at 07:22 +0200, Torsten Lilge wrote:
> > On Mon, 2020-07-20 at 17:22 -0400, Doug Stewart wrote:
> > >
> > > On Mon, Jul 20, 2020 at 4:38 PM Torsten Lilge1960 Blue Heron Drive,
> > > <
> > > [email protected]> wrote:
> > > > Hi everyone,
> > > >
> > > > the control package uses the SLICOT function mb05nd [1]
> > > > (__sl_mb05nd__()) instead of the octave function expm() for the
> > > > matrix
> > > > exponential. The problem with mb05nd is that when discretizing a
> > > > static
> > > > gain in state space results in
> > > >
> > > > > > [a,b,c,d] = ssdata (c2d (ss (tf (1,1)),1))
> > > > a = [](1x0)
> > > > b = 0
> > > > c = [](1x0)
> > > > d = 1
> > > >
> > > > Instead, using expm() leads to the correct result
> > > >
> > > > > > [a,b,c,d] = ssdata (c2d (ss (tf (1,1)),1))
> > > > a = [](0x0)
> > > > b = [](0x1)
> > > > c =
> > > > [](1x0)
> > > > d = 1
> > > >
> > > > Are there any reasons against using expm()?
> > > >
> > > > Torsten
> > > >
> > > >
> > > > [1] http://slicot.org/objects/software/shared/doc/MB05ND.html
> > > >
> > > >
> > >
> > > Is there a difference in speed?
> > > If it is slower then we should look and see where the '0" is coming
> > > from.
> >
> > __sl_mb05nd__ is a compiled function and - at least on my system -
> > approx. 20 times faster.
> >
>
> Hello Doug,
>
> The routine mb05nd does not accept input dimensions 0x0 for an empty
> matrix but it accepts (1x0) which is forced by the wrapper sl_mb05nd for
> empty matrices. I have therefore  pushed changeset 7a8204f62fe1 to the
> control package which adds the following test to sl_mb05nd:
>
> +        if (a.numel () == 0)
> +          {
> +            // given matrix is empty, just return the empty matrix,
> +            // otherise mb05d would make a [](1x0) matrix from it
> +            retval(0) = a;
> +            retval(1) = a;
> +            return retval;
> +          }
>
> I have also prepared a patch for adding the first order-hold method
> 'foh' to c2d. This enables us to use 'foh' in lsim() or ramp(). Should I
> push this to the default branch or should I create a feature branch for
> this?
>
> Torsten
>
> I am sorry but I ave been sick (nothing too bad) and did not look at this
> yet :-(
>
I am better now!

I would push this to the default branch .
After you push this I hope to do more testing using 6.1
Doug


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