PCT or S-R (was Re: More Lego ev3 demos)

[Martin Taylor 2018.]

As Powers often pointed out, there is NO difference between an S-R

system with a feedback connection and a single level PCT control
loop. What you call them is your choice. The situation changes when
there is a variable reference value. The “Response” to a “Stimulus”
becomes inconsistent, though every path segment in the control loop
remains consistent. In effect, every top-level (fixed reference value) control loop is
indistinguishable from an S-R system with feedback. The relationship
between “Stimulus” and “Response” may get incomprehensibly
complicated and extend over longer and longer time stretches as more
levels are considered, but if all the inputs over all the life of
the organism are considered to be “the stimulus”, and all the
outputs over the same period as “the response”, any top-level loop
is identically an S-R system. Only the lower-level loops are
distinguishable, and then only if their reference values change.
I’m saying nothing new here. You will find this scattered through
the archives under Bill Powers’s name.


On 2018/02/15 5:37 AM, Alex Gomez-Marin


is the crux of the matter, Rupert. Thanks for bringing it out
so clearly. But then, it is ironic, because the difference in
a closed-loop system being or not being a PCT system seems to
be in the eyes of the scientist studying it: namely, if one
rewrites the input-output equations so as to have a term
called error and a controlled perception (and I am afraid this
can always be done for any system…!) then one sees it as a
PCT system, and if not, then it is not. What would Powers and
the thoughtful PCT community have to say about it? I am quite
interested. Alex

      On Thu, Feb 15, 2018 at 11:32 AM,

Rupert Young rupert@perceptualrobots.com

[From Rupert Young (2018.02.15 10.30)]

                  (Rick Marken


                      RM: I'd like to see that too!  Especially

since I think a Braitenberg
vehicle is a PCT system since it’s a
closed-loop, control system.

          Are all                 closed-loop,

control systems PCT systems? I don’t see that
Vehicles are PCT systems, though happy to be corrected.
They don’t embody a goal, or comparator, or error. Their
outputs are directly functions of the inputs; in other
words they are input-output systems. I’m not sure they are
even control systems as there is no variable that is being
controlled. Rather I’d call them iterative input-output
systems, with the outputs being continually updated based
upon the input states. They are certainly dynamic systems,
and, due to their complexity, appear to do
interesting things. But they are not purposeful, in that
they are not controlling (perceptual) variables.

          Btw, here's a good resource on Vehicles, [http://www.bcp.psych.ualberta.ca/~mike/Pearl_Street/Margin/Vehicles/](https://urldefense.proofpoint.com/v2/url?u=http-3A__www.bcp.psych.ualberta.ca_-7Emike_Pearl-5FStreet_Margin_Vehicles_&d=DwMDaQ&c=OCIEmEwdEq_aNlsP4fF3gFqSN-E3mlr2t9JcDdfOZag&r=G2rjwc9SjlT6Blyc8su_Md8P_xOsOTRMJ5teQVBC2qU&m=5BmS4VxwZlTZspM4POh4GiOMzsIKQ0Y_Gfovd-pj4AY&s=8CryfBEneaTwz8h1S6HcDhKPB64ic_9m0sPObEJl3Pk&e=).