2-3
THE CLOSED-LOOP CONTROL SYSTEM
A closed-loop control system is another name for a servo system. To be classified as a servo, a
control system must be capable of the following:
1. Accepting an order that defines the desired result
2. Determining the present conditions by some method of feedback
3. Comparing the desired result with the present conditions and obtaining a difference or an error
signal
4. Issuing a correcting order (the error signal) that will properly change the existing conditions to
the desired result
5. Obeying the correcting order
We have discussed the open- and closed-loop control systems and defined a servo system as a
closed-loop control system. Although not technically accurate by definition, open-loop control systems
are also often referred to in the Navy and many publications as servo systems even though they lack one
of the five basic requirements, that of feedback.
Q-2. In an open-loop control system, what action reduces the input to zero so the load is stopped at the
desired position?
Q-3. What basic requirement of a closed-loop system (not present in open-loops) enables present load
position to be sensed?
OPERATION OF A BASIC SERVO SYSTEM
For the following discussion of a servo system, refer to figure 2-2, view (A), view (B), view (C) and
view (D).This closed-loop servo system is the most common type in the Navy today. It is normally made
up of electromechanical parts and consists basically of a synchro-control system, servo amplifier, servo
motor, and some form of feedback (response).
Figure 2-2A.—A basic servo system (closed-loop).
