[From Bill Powers (960702.0700 MDT)]
James Nord (960702. 5p.m. Japan) --
Just an example of a misapprehension:
A heat control system consists of a thermometer sensor
(communication system) and a furnace (power system). The
thermometer sensor can detect a difference, that makes it a sensor.
But the thermometer is coupled with a furnance such that it
requires a specific difference to actually trigger any action by
the furnance (either on or off), and that difference, "the
difference that makes a difference" is information in the view of
Bateson (one of the members of Wiener's Think Tank group)
In a thermostat with a bimetallic sensor, the "perceptual signal" is the
position of a movable contact mounted on one end of the sensor coil.
This position represents not a "difference," but an amount. It is
proportional to temperature. As temperature rises and falls, this
contact moves back and forth in an arc. In many thermostats, there is a
pointer attached to the same part of the bimetallic coil, which
indicates temperature on a scale. This is the present temperature as the
bimetallic coil senses it.
There is also a contact that can be positioned by the user of the
thermostat; this is the "reference signal." This reference signal is
also in the form of a position, the same kind of variable that
represents temperature. A pointer attached to the user-settable contact
indicates temperature on (another) scale. This temperature is not the
actual temperature, but the desired temperature.
The "Comparator" of this control system is simply the spatial relation
between the two contacts. As the room cools, the bimetallic element
uncoils and the movable contact approaches the reference contact. When
the two positions are the same, the contact closes, activating a relay
that brings the flame in the furnace to a high level and starts a
circulating blower. The hot air circulates through the house, warming
the air and ultimately warming the bimetallic coil. This causes the coil
to tighten and makes the movable contact move away from the reference
contact, opening the circuit and turning the furnace off.
So the temperature of the bimetallic coil rises and falls, opening and
closing the contact and turning the furnace on and off. If someone opens
a window and cools the room, the bimetallic coil will close the contact
for a little longer on each cycle, adding more heat to the room at a
rate sufficient to counteract the losses through the window. So the
action of this system (the duty cycle of the furnace on-time) varies
automatically to oppose any disturbances of the temperature sensed at
the bimetallic coil.
By moving the reference contact to a new desired temperature, the user
changes the point where the movable contact will be maintained, and thus
changes the temperature at which the coil itself will be maintained.
Since the temperature of the coil depends on the air temperature in the
room, the air temperature will be brought to a corresponding level. Of
course the control system can seem to be fooled; if you locate a lamp
right near the sensor, you can make the room air get cooler by turning
on the lamp. However, the system is still keeping the coil temperature
at the same reference level; the only difference is that some of the
heat is coming from the lamp, and less of it from the air (because the
furnace spends less time on).
Now, you can say all sorts of interesting-sounding things about this
system. You can talk about a "communication channel" and a "power
device." You can say that the bimetallic coil is extracting
"information" about room temperature, and express this information in
bits or bits per second. You can say that the sensor detects a
"difference that makes a difference." You can say that the cycling
furnace exhibits "variety," and that this variety has to be "matched to
the variety of the environment." You can talk about autonomy and
abstraction and snakes eating their own tails. None of that has anything
to do with what actually makes this system work. I have just described
what makes it work. These other ideas are just metaphors and analogies;
they do not make the system work. These other notions, all too often,
are the sorts of things people say when they really don't understand
what makes a system work, but still want to say something that sounds
profound about it.
When you understand how control works, you don't have to say vague
abstract metaphorical things about it. You just talk about how it works,
and that will answer most questions about it.
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Best,
Bill P.