Re: Re: dynamics: comment about MBdyn...

Tiller, Michael (M.M.) wrote:
> Although I'm not really involved in OROCOS directly, I still have a few comments...

This thread is a bit orthogonal to OROCOS, but they're
kindly hosting it because we assume some of those that
are interested in OROCOS could be also interested in
MBDyn (and, of course, other multibody and robotics
related open source simulation tools).

> I just thought I would throw those two things in since 
 > we were on the topic of Bond Graphs and robots (and since
 > Dr. Masarati mentioned Modelica in his original message).

I thin Herman forst mentioned Modelica, and I can't help
referring to it when addressing OO modeling because of the
important formalizations it contains.

> Herman mentions index reduction efforts (I assume 
 > that is what he means by reduction of sets of DAEs).
 > Symbolic manipulation gets you several other things.
 > Basic things such as equation sorting can help you
 > formulate problems in block-lower-triangular (BLT)
 > form which is computationally efficient.  In addition,
 > there are other opportunities such as equation tearing
 > that also help computational performance.  The point
 > is that symbolic information has several uses.

In our experience, when dealing with structural dynamics
and significantly with deformable bodies there is very
little symbolic model reduction to do; there is also very
limited need for index reduction in practical cases;
matrix reshaping and compuational efficiency can be
achieved also by means of traditional "numerical"
approaches; something like this is done by Umfpack, which
we use, and which is the standard sparse matrix package
for matlab, and it is used also by scilab and a lot of
other projects.  There are applications where symbolic
manipulation is key, and others where it's not.  In this
sense, we still prefer "brute force".

> 
>>>- "Nodes": generic frameworks such as Bond Graphs and Modelica offer
>>>  already well-developed terminology and definitions, so it could be
>>>  interesting to follow those. Personally, I am very interested in
>>>  discussing this in detail.
>>
> 
> Just a minor comment.  Modelica doesn't quite have "nodes" 
> in the usual sense (i.e. named entities that exist separate 
 > from the components).  Instead, Modelica has entities called
 > "connectors" that exist on the components and these connectors
 > are then in some sense merged to form a single set of variables
 > that are shared between components.

...: nodes :)  Actually, I'd call them "candidate nodes".
Note that they cannot be eliminated in a dynamic problem;
in static problems it's just a matter of static elimination,
which, I must admit, is made quite easy by symbolic
manipulation, but it can be achieved also numerically
(e.g. by looking for the nullspace of the inertia matrix).

> 
> As I mentioned earlier, there is a new multi-body library 
 > coming out in Modelica very soon (I have beta copies, but
 > I'm not sure the author wants me to circulate them).
 > I suspect you might find the structure of those libraries
 > very interesting.  It would be great to see an open-source
 > tool that could simulate the models in that library (hint,
 > hint).
> 

I'm looking forward to seeing it.

> 
> I should mention that I come from an FEM background as do 
 > a few other people involved in the Modelica effort.
 > There is interest within the group of adding some language
 > constructs to support the posing of PDEs.  It is not clear
 > at this point whether that will mean just finite difference
 > shorthand or full support for things like basis functions,
 > quadrature, etc.
> 
> It is not my intention to continually bring Modelica
 > into the mix but since Dr. Masarati mentioned Modelica,
 > I just wanted to add a few comments to his very nice
 > explanation of MBDyn where I thought it was relevant.

I think it's always good to read about Modelica's experience
because iot represents a very relevany effort in modeling
in general, with good solutions speficially for multibody.

> 
> Two final points.
> 
> First, for Dr. Masarati if you have any questions
 > or issues with Modelica, please let me know.
 > I'd be more than happy to answer any questions you have
 > and if you find that Modelica doesn't work for your,
 > that is fine by me.  The only thing I ask is that
 > you let me know why you don't think it is appropriate
 > so we can consider that for ongoing development
 > of the language.

Sure.

> 
> Second, one of the big criticisms of Modelica I have
 > seen is that there aren't any open source implementations.
 > Ideally, I'd like to see some academic groups attempt
 > an implementation.  In an effort to support this,
 > I am going to try (no promises) to get our (Ford's)
 > internal Modelica parsing and semantic processing tools
 > (very limited capability at the moment) released
 > for distribution by the Modelica Association as open source.
 >  Hopefully, this might help people like Dr. Masarati
 > who are interested in being able to read Modelica
 > models into their simulation environments.

If this is a commitment, then I'm definitely looking
forward to seeing this release.  Please let us know
any development in this sense.

Pierangelo.

-- 
Dr. Pierangelo Masarati               | voice: +39 02 2399 8309
mailto:[email protected]  | fax:   +39 02 2399 8334
http://www.aero.polimi.it/~masarati
Dip. Ing. Aerospaziale Politecnico di Milano,
via La Masa 34, 20156 Milano, Italy