[SciPy-Dev] Pythonic Simulink clone... - how much interest?

[Joshua Holbrook]

On Tue, Nov 16, 2010 at 2:48 AM, burley <burley at zonnet.nl> wrote:
> Visual modeling support as offered by the Simulink/Matlab and Xcos/Scilab
> suites is very powerful, for the modeler as well as in communicating
> concepts and ideas to others. Neither is Pythonic or extendable by Python,
> unfortunately (disregarding the prohibitively expensiveness of Sim/Matl).
> Wouldn't it be *very* nice to have a free (LGPL++?), user extensible, Python
> based Simulink alternative, based on Numpy, SciPy,  Wx and/or Qt-like, and
> the rest. All (e.g. most of) the algorithms are there, many would benefit. I
> know of Viper (Tcl/Tk), Elefant (Bayesian Learning), ... but none of them is
> general purpose or flexible in its use. Only a collaborative effort may make
> that happen I would believe. What's the community interest in such an
> effort? (I for myself am not a GUI programmer...).
>
> -- Burley
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>

I've done some thinking on this problem, though I haven't had the time
to pursue anything directly. I started working on a graphical
front-end for dataflow-family languages in javascript, but didn't get
very far.

I would say that there are two major issues that come in trying to
build something simulink-esque.  The first, obviously, is the issue of
being able to build block diagrams in some sort of GUI and then
translate those to a computer representation of a directed graph.
While I think it's not something to be ignored, it's definitely been
done before, and I think a nice way to describe these sorts of
directed graphs in some sort of text format (python would work ;) )
anyway--so, working from a computer representation first and making a
gui tool tool to build said representations second could be the way to
go. In fact, I think a generalized block diagram component would be
pretty awesome, since there are many worthwhile variations of the
graphical dataflow format besides Simulink (Yahoo! Pipes/Pypes,
LabVIEW, PD, Excel).

The other part, and the one that I think makes this actually
difficult, is converting the block diagram into a solution. My
understanding of what Simulink actually does to generate a solution is
something like this:

1. Convert everything to be in the time domain (For example, transfer
functions would need to be transformed).
2. Convert the graph to a state-space representation suitable for use
with a RK-like method--that is, dx/dt = f(x) where x is a vector, and
y = g(x), where y is a vector of all the things you want to put
virtual O-scopes onto.
3. Apply an RK-like method to the state-space representation.
4. Profit!

Matlab/Simulink give you lots of options for step (3), and this part
can be looked up in any numerical methods textbook, so it's relatively
easy to get a handle on. In fact, I'm sure scipy has RK methods in it
somewhere if it can claim to be anywhere near complete. Similarly, I
would hope part (1) to be fairly straightforward.  The part that I
didn't have much luck on is step (2), but my guess is that you would
have to traverse the graph and try to unroll any loops using rules
specific to 'special' blocks, such as "1/s" blocks. I'm not sure it
can get any more general than that.

My $0.02,

--Josh