THE TWO GENERAL TYPES OF
SERVO-MECHANISMS
Servo-mechanisms are divided into two general types:
(1) where the target, goal, or "answer" is known, and the
objective is to reach it or accomplish it, and (2) where
the target or "answer" is not known and the objective is
to discover or locate it. The human brain and nervous
system operates in both ways.
An example of the first type is the self-guided torpedo,
or the interceptor missile. The target or goal is known—
an enemy ship or plane. The objective is to reach it. Such
machines must "know" the target they are shooting for.
They must have some sort of propulsion system which
propels them forward in the general direction of the target.
They must be equipped with "sense organs" (radar,
sonar, heat perceptors, etc.) which bring information from
the target. These "sense organs" keep the machine
informed when it is on the correct course (positive feedback)
and when it commits an error and gets off course
(negative feedback). The machine does not react or re
spond to positive feedback. It is doing the correct thing
already and "just keeps on doing what it is doing." There
must be a corrective device, however, which will respond
to negative feedback. When negative feedback informs the
mechanism that it is "off the beam" too far to the right,
the corrective mechanism automatically causes the rudder
to move so that it will steer the machine back to the left.
If it "overcorrects" and heads too far to the left, this mistake
is made known through negative feedback, and the
corrective device moves the rudder so it will steer the
machine back to the right. The torpedo accomplishes its
goal by going forward, making errors, and continually
correcting them. By a series of zigzags it literally "gropes"
its way to the goal.