# control by perception, continued

**URL:** <http://discourse.iapct.org/t/control-by-perception-continued/13416>\
**Category:** CSG1996\
**Created:** [July 2, 1996, 1:36pm UTC](http://discourse.iapct.org/t/control-by-perception-continued/13416 "1996-07-02T13:36:07Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![William\_T\_Powers4](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/w/977dab/32.png) [@William\_T\_Powers4](http://discourse.iapct.org/u/William_T_Powers4)\
**Post date:** [July 2, 1996, 1:36pm UTC](http://discourse.iapct.org/t/control-by-perception-continued/13416/1 "1996-07-02T13:36:07Z")

</div>

[From Bill Powers (960702.0700 MDT)]

James Nord (960702. 5p.m. Japan) --

Just an example of a misapprehension:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A heat control system consists of a thermometer sensor  
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(communication system) and a furnace (power system). The  
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;thermometer sensor can detect a difference, that makes it a sensor.  
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;But the thermometer is coupled with a furnance such that it  
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;requires a specific difference to actually trigger any action by  
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;the furnance (either on or off), and that difference, "the  
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;difference that makes a difference" is information in the view of  
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;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.

> **···**
>
> -----------------------------------------------------------------------  
> Best,
> 
> Bill P.