[From Bruce Abbott (970108.1915 EST]
Tracy Harms (1997;01,08.20.00 UTC) --
I've been thinking about various prominent things which are clearly
manipulated, but which are not controlled. As an example, consider the
restfulness of a sleeping chamber when a person retires. If the area is
not sufficiently restful, action will be taken to increase that factor.
However, if restfulness of the room goes above the degree required, *no
action is taken to reduce it*. Therefore we easily conclude that it is not
controlled in the PCT meaning of the term.
I've been noticing a lot of these things. Advertising is used by
businesses to increase their favorable prominence among the public, but
counteraction is never taken. (No degree of favorable fame will produce
expenditure to lead people to ignore or think worse of the firm.) So these
factors are not controlled, but neither are they irrelevant side-effects of
control. I'd like to understand the relationship between controlled and
maximized factors. Insights, anybody?
One thing which occurs to me is that this looks akin to the difference
between a room-temperature control system which includes a furnace *and*
air-cooler, versus such a system which has only one of those items.
Lacking either, there is a threshold beyond which no responsiveness occurs.
You've hit the nail on the head, Tracy. What you're talking about is called
a "one-way control system." Such a system will take action when a given
variable deviates from its reference value in one direction, but do nothing
to compensate for deviations in the other direction. Most biological
systems are of this type; two-way control in these systems (when needed) is
achieved by separate control systems acting against deviations in opposite
directions. The heater/air conditioner system you mentioned is an example
of this type of two-way control.
For a one-way control system, the "threshold beyond which no responsiveness
occurs" is the reference value. There is control below this value but no
control above (or vice versa).