Programmable Logic Controllers-Fourth Edition by W. Bolton
This chapter is an introduction to the programmable logic controller, its
general function, hardware forms and internal architecture. This overview
is followed up by more detailed discussion in the following chapters.
What type of task might a control system have? It might be required to
control a sequence of events or maintain some variable constant or follow
some prescribed change. For example, the control system for an automatic
drilling machine (Figure 1.1(a)) might be required to start lowering the
drill when the workpiece is in position, start drilling when the drill reaches
the workpiece, stop drilling when the drill has produced the required
depth of hole, retract the drill and then switch off and wait for the next
workpiece to be put in position before repeating the operation. Another
control system (Figure 1.1(b)) might be used to control the number of
items moving along a conveyor belt and direct them into a packing case.
The inputs to such control systems might be from switches being closed or
opened, e.g. the presence of the workpiece might be indicated by it
moving against a switch and closing it, or other sensors such as those used
for temperature or flow rates. The controller might be required to run a
motor to move an object to some position, or to turn a valve, or perhaps a
heater, on or off.
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PROGRAMMABLE CONTROLLERS HAND BOOK – L.A.BRYAN
Programmable logic controllers, also called programmable controllers or
PLCs, are solid-state members of the computer family, using integrated
circuits instead of electromechanical devices to implement control functions.
They are capable of storing instructions, such as sequencing, timing,
counting, arithmetic, data manipulation, and communication, to control
industrial machines and processes. Figure 1-1 illustrates a conceptual
diagram of a PLC application
Programmable controllers have many definitions. However, PLCs can be
thought of in simple terms as industrial computers with specially designed
architecture in both their central units (the PLC itself) and their interfacing
circuitry to field devices (input/output connections to the real world).
As you will see throughout this book, programmable logic controllers are
mature industrial controllers with their design roots based on the principles of
simplicity and practical application.
