Re: FSM vs. Petri nets for component modelling...

On Wed, 5 Dec 2001, Daniel Simon wrote:

> >- do you make distinction between signals and events?
> no, but events are classified in pre-conditions, post-conditions, exceptions
> (3 sub-classes) and synchro-signals.

A `pre-condition' is an event? Strange. How does this work?

[...]
> >- what is the determinacy of the Orccad transition implementation?
>
> It is under control of the automaton and of the OS, and it is as
> deterministic as the OS! A problem is the avoidance of transient cpu
> overload. It is currently done by relaxing the deadline, and a
> better solution is studied.

What is ``transient cpu overload''?

[...]
> The two difficult steps are:
> i) design, checking and encoding a supervisor (FSM or others)  which meets
> your requirements (strl is nice for that)
> ii) interfacing the supervisor with the control code through system calls
> (Orccad binds the I/O functions of the automaton with system calls, and it is
> easier to do that from the standard interface provided by a compiler).

Okay, I think I understand what you say here. This implementation can
not be used in a real-time context, unless we provide deterministic
``system calls''; in my current view on the hard real-time design of
the Orocos motion control kernel, these system calls would correspond
to the generation of events. What are the system calls in your
current implementation?

> >- any ideas about the `best' specification language? I mean: go for a
> >  specialised custom-made language, or use already existing(?) XML
> >  based functional specification languages?
> Some of my colleagues have design Maestro, a "robotics-oriented" language
> which handles our model of control tasks and compile into Esterel (in
> particular because directly using a synchronous language like Esterel can be
> harmfull).

Can you explain in what sense it can harm? Can you share some of your
experience and thoughts on this Maestro approach?

> IMO most of the semantics of the robot controller resides in the
> control algorithms (on which the user must have access, not only
> through a blind Move() statement). Thus I consider that the control
> tasks are activities which must be started, stopped, suspended...
> upon the occurence of events, and a general purpose language is able
> to specify that.

So, do I understand you correctly if I say that you think there is no
need for something like Maestro??

[...]
> >- what are the fundamental differences between FSM/Petri-nets and the
> >  new vogue UML?
> tools for implementing probably. and the real-time extensions of UML seems
> very weak.

Indeed. And my impression is that the efforts to extent current UML
with real-time features boils down to re-inventing FSM/Petri-nets; do
you think I am oversimplifying here, and forgetting some fundamental
issue?

[...]
> Hope this help

It surely does, thanks! (At least, for the time being :-)

Herman