Operational Power Control Systems 447
PROGRAMMABLE LOGIC CONTROLLERS (PLCS)
For a number of years, industrial control was achieved by electro-
mechanical devices, such as relays, solenoid valves, motors, linear ac-
tuators, and timers. These devices were used to control large production
machines where only switching operations were necessary. Most control-
lers were used simply to turn the load device on or off. In addition, some
basic logic functions could also be achieved. Production line sequencing
operations were achieved by motor-driven drum controllers with tim-
ers. As a rule, nearly all electromagnetic controllers were hardwired into
the system, and responded as permanent fixtures. Modification of the
system was rather difficult to accomplish and somewhat expensive. In
industries where production changes were frequent, this type of control
was rather costly. It was, however, the best way-and in many cases the
only way-that control could be effectively achieved with any degree of
success.
In the late 1960s, solid-state devices and digital electronics began to
appear in controllers. These innovations were primarily aimed at replac-
ing the older electromechanical control devices. The transition to solid-
state electronics has, however, been much more significant than expect-
ed. The use of solid-state electronic devices, digital logic integrated cir-
cuits (ICs), and microprocessors has led to the development of program-Figure 16-21. A 60 hertz to 25 hertz frequency conversion system