448 Electrical Power Systems Technology
mable logic controllers (PLCs). These devices have capabilities that far
exceed the older relay controllers. Programmable logic controllers are
extremely flexible, have reduced downtime when making changeovers,
occupy very little space, and have improved operational efficiency.
A programmable logic controller is very similar to a small comput-
er. In fact, most programmable logic controllers are classified as dedicat-
ed computers. This type of unit is usually designed to perform a number
of specific control functions in the operation of a machine or industri-
al process. The degree of sophistication or “power” of a programmable
logic controller is dependent on its application. Many PLCs respond like
a computer terminal and interface with a mainframe computer. Other
units are completely independent and respond only to those things that
are needed to control a specific machine’s operation.
Programmable Logic Controller Components
A programmable logic controller is basically a software-based
equivalent of the older electromagnetic relay control panel. Essentially,
the PLC is a flexible system that can be easily modified and still be used
as a general-purpose control device. Most PLCs can be programmed to
control a variety of machine functions at one time. When a production
change is necessitated, the program can be altered by a keyboard to make
the system conform to the needed changes. The new control procedure
may be entirely different from the original.
Mini-Programmable Logic Controller Systems
Recent improvements in large-scale IC and power transistor manu-
facturing technology are responsible for the development of mini-PLCs.
These systems can be used economically to control simple machine oper-
ations and numerous manufacturing processes. A number of companies
are now producing mini-PLCs.
Mini-PLCs are classified as systems that can economically replace
as few as four relays in a control application. They are capable of pro-
viding timer and counter functions, as well as relay logic, and are small
enough to fit into a standard rack assembly. Most systems of this type
have fewer than 32 I/O ports or modules. Some units can be expanded
to drive up to 400 I/O devices. Typically, the I/O of this type of system
responds to digital signals. Some units are capable of responding to ana-
log information. This makes it possible for the system to respond to tem-
perature, pressure, flow, level, light, weight, and practically any analog