Bill Powers (980101.0754 MST), responding to Martin Taylor 971231 17:45 --
Martin:
One way of studying any
feedback structure is to break the loop somehwere and to look at the
input-output relations among the components of the loop. That's what
a psychophysical experiment (other than method of adjustment) does.
Bill:
Breaking the loop works fine for an artificial nonadaptive control
organization. It is much harder to use with a living control system,
because breaking the loop means loss of control, and loss of control
usually leads to an immediate switch to a different mode of control. So
when you think you're measuring the same system open-loop, you're probably
looking at a different system.
Martin's original post almost enticed me into responding...
Alternatively, controlling for a perception of "knowledgeable," Martin's
post offered me an opportunity to maintain that perception (at least in my
own eyes, figuratively speaking)...
In any event, I agree with Martin and Bill (surprise, surprise).
What Martin's post reminded me of was my second generation of fire control
computer, the MK 47. My first generation fire control computer was the old
MK 1A, an electro-mechanical analog beast filled with cams and gears and
resolvers, and miles of brightly colored spaghetti-like wiring. In the case
of the old MK 1A, the emphasis was clearly on the mechanical portion of
electro-mechanical. The MK47, like the MK 1A, was from the Ford Instrument
Division of the Sperry-Rand Corporation, however, it placed the emphasis on
the electro portion of electro-mechanical. Gone were the cams and most of
the gears. The motor-driven time line remained but almost all computations
were performed by a combination of resolvers and little solid-state plug-ins
referred to as "Z" networks (quite literally, little black boxes). Many of
these Z networks were embedded in what we called "rat race" loops, that is,
closed loops consisting of still more closed loops (a hierarchy?) in which
outputs were part of the inputs used to calculate the outputs.
When something went wrong, as it did from time to time, troubleshooting the
MK47 was a bitch! Once you got into those rat-race loops, about the only
way you could isolate the fault was to begin "breaking the loop," as Martin
terms it. This entailed disconnecting the actual wiring. It proved far
easier and quicker to keep on hand a ready stock of the various types of Z
networks and simply "easter-egg" our way to the faulty component. That flew
in the face of accepted troubleshooting practice and also contradicted stern
Navy policy about not keeping "battle spares" in the working space. As a
practical matter, we technicians ignored both restraints and, mindful of our
mission to "keep the weapons systems in peak operating condition at all
times and, if it goes down, bring it back up as quickly as possible," we (or
at least I) kept a suitable supply of "battle spares" on hand. Further
advances in technology led to a generation of digital computers and a
generation of technicians whose troubleshooting skills were "raised a level"
to where all they had to find was the faulty module, not the faulty
component. The era of plug-in-a-new-module as the central process of fault
isolation had arrived. Old timers, like me, were convinced that the new
generation of technicians couldn't troubleshoot their way out of a wet paper
bag -- but then "old timers" all feel that way, don't they?
But, as Bill observes, "breaking the loop" applies to artificial systems
such as the MK47 computer, not to living control systems -- although, I
wonder if sensory deprivation experiments might not qualify as "breaking the
loop."
Anyway, a Happy New Year to one and all...
Regards,
Fred Nickols
The Distance Consulting Company
nickols@worldnet.att.net
http://home.att.net/~nickols/distance.html