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More on reverse modeling

Posted: Thu Nov 12, 1998 9:06 am
by Gerhard Werner
Jay Forrest and John Gunkler have offered some valuable comments on my
prior inquiry about reverse modeling.
I wonder whether I might pursue this thread with another question: if I am
able to represent the process (which is in my case a complex manufacturing
process) in the form of a reversible Cellular Automaton (which I think I
would be able to do), then might running the RCA in reverse be a reasonable
attempt to locate the fault ?
I am thinking here of the RCA of Fredkin, Toffoli, Margolus etc.
Has something like this ever been attempted ?
I appreciate your comments.
Thank You Gerhard Werner (gwer1@acal.corp.mot.com)

More on reverse modeling

Posted: Fri Nov 13, 1998 9:51 am
by Tom Fiddaman
Encoding a production process in an RCA is an extremely interesting idea.
Cellular automata arent really my bag, but Ive only seen examples from
physical or biological sciences where there is an obvious spatial
interpretation. Im interested to hear more about the production process
and how it could be represented as an RCA.

Unfortunately, I think you will discover that running the CA in reverse
will be frustrating, because the results will be extremely sensitive to
initial conditions. The slightest error in encoding the terminal state will
likely lead to unrecognizable results.

The same problem arises with SD-style dynamic models. When you run a model
backwards in time, all the negative loops become positive loops, and it may
be impossible to accurately determine an initial state from a terminal
state. Its easy to model a pendulum, but hard to use a model to determine
where or when a pendulum started swinging by running backwards from the
point at which it has come to rest.

In either type of model, you can use various methods (e.g. hand
calibration, brute force optimization) to improve your estimates of
initial/terminal states or parameters. My guess is that this would likely
be more successful with an SD-type model due to the fewer degrees of
freedom and the greater ability to use uderstanding of the causal structure
to perform intelligent tests.

Again, in the spirit of Jim Hines suggestion that we include reference
modes in our queries, it would help to hear a little more about the nature
of the process and the defective parts.

Best wishes,

Tom

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Thomas Fiddaman, Ph.D.
Ventana Systems http://www.vensim.com
8105 SE Nelson Road Tel (253) 851-0124
Olalla, WA 98359 Fax (253) 851-0125
Tom@Vensim.com http://home.earthlink.net/~tomfid
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