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26 CHAPTER TWO
Open your eyes and see how well you did. Likely as not, you won’t be satisfied with
the light level because the steady state error will be too large. You will have to make a
correction in the light intensity to be comfortable reading under the light.
The corrections you have made in these two experiments by finally using your eyes
illustrates an important concept. An open-loop control system can be improved if it is
told how well its output matches the input requirements. With that somewhat broad
statement, we’ll introduce another type of control system.

Closed-Loop Control

Closed-loop control systems are also referred to as feedback control systems, because
information flows backwards at some point within the control system. Generally, this
reverse information flows from the output of the control system backward toward the
input. The information that flows backwards allows the control system to make correc-
tions in its output. Figure 2-5 is a generalized diagram of a simple closed-loop control
system.
Information flows backwards in the system, from the output signal to somewhere
near the input. I’ve labelled this reverse information flow “feedback.” In this simple ver-
sion of a close-loop control system, the output signal is sent back and directly compared
to the requirements set by the input signal. The circle shows an arithmetic computation
(subtraction). If the output does not directly match the input, the actuator will receive a
nonzero signal at its input and provide corrections at the output so its input returns to
zero. In practice, many different kinds of closed-loop control systems exist, and, as
such, one could make many variations to this diagram.
Many control systems do not have outputs that are directly comparable to the inputs;
the circle in Figure 2-5 must be much more complex than a simple subtraction element.
Often, the output signal must be transformed before it can be compared to the input sig-

Feedback

_
Input Signal Output Signal
Actuator
+

FIGURE 2-5 Closed-loop control systems use feedback.