PROGRAMMABLE LOGIC

CONTROLLERS
 LECTURE 1 - CHAPTER ONE

INTRODUCTION

This chapter is an introduction to the programmable logic controller (PLC) and its
general function, hardware forms, and internal architecture. This overview is
followed by more detailed discussion in the following chapters.
 Controllers
WHAT TYPE OF TASK MIGHT A CONTROL SYSTEM HANDLE?
It might be required to control a sequence of events, maintain some
variable constant, or follow some prescribed change.
 For example, the control
system for an automatic
drilling machine 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.
 WHAT FORM MIGHT A CONTROLLER HAVE?

1. Hard-wired control
 The automatic drilling machine
task can be performed by
contactors, control relays, and
other electromechanical devices.
This is often referred to as hard-
wired control.
 Circuit diagrams had to be
designed, electrical components
specified and installed.

 Electricians would then wire the components necessary to perform a specific

task. If an error was made, the wires had to be reconnected correctly . Such
electrical circuits would have to be specific to the automatic drilling
machine.
 A change in function or system expansion requires extensive
component changes and rewiring.
 2. Microprocessor-Controlled Systems
 Instead of hardwiring each control circuit for
each control situation, we can use the same
basic system for all situations (different
tasks) if we use a microprocessor-based
system and write a program to instruct the
microprocessor how to react to each input
signal from, say, switches and give the required
outputs to, say, motors and valves.
 As an illustration, the modern domestic washing machine uses a
microprocessor system. Inputs to it arise from the dials used to select the
required wash cycle, a switch to determine that the machine door is
closed, a temperature sensor to determine the temperature of the water,
and a switch to detect the level of the water. On the basis of these inputs
the microprocessor is programmed to give outputs that switch on the drum
motor and control its speed, open or close cold and hot water valves,
switch on the drain pump, control the water heater, and control the door
lock so that the machine cannot be opened until the washing cycle is
completed.
 Microcontrollers are not user friendly. It requires well trained
engineers to deal with.
3 The Programmable Logic Controller
 A programmable logic controller (PLC) is a
special form of microprocessor-based
controller that uses programmable
memory to store instructions and to
implement functions such as logic,
sequencing, timing, counting, and
arithmetic in order to control machines
and processes.

 It is designed to be operated by engineers

with perhaps a limited knowledge of
computers and computing languages.
 The designers of the PLC have
preprogrammed it so that the control
program can be entered using a simple,
rather intuitive form of language.
 PLCs have the great advantage that the
same controller can be used with a wide
range of control systems with no need to
rewire.
 PLCs are similar to computers, but whereas computers are optimized for
calculation and display tasks, PLCs are optimized for control tasks and the
industrial environment. Thus PLCs:
 Are rugged and designed to withstand vibrations, temperature, humidity,
and noise.
 Have interfacing for inputs and outputs already inside the controller.
 Are easily programmed and have an easily understood programming
language that is primarily concerned with logic and switching operations.
 Hardware
Typically a PLC system has the basic components of processor unit,
memory, power supply unit, input/output interface section,
communications interface, and the programming device.

 The processor unit or central processing unit (CPU) is the unit

containing the microprocessor. This unit interprets the input signals and
carries out the control actions according to the program stored in its
memory, communicating the decisions as action signals to the outputs.
 The power supply unit is needed to convert the mains AC voltage to the
low DC voltage (5 V) necessary for the processor and the circuits in the
input and output interface modules.
 The programming device is used to enter the required program into the
memory of the processor. The program is developed in the device and then
transferred to the memory unit of the PLC.

 The input and output sections are where the processor receives
information from external devices and communicates information to external
devices. The inputs might thus be from switches or other sensors. The
outputs might be to motor starter coils, solenoid valves, or similar things.
 The memory unit is where the program containing the control actions to be
exercised by the microprocessor is stored and where the data is stored from
the input and output units for processing.
 The communications interface is used to receive and transmit data on
communication networks from or to other remote PLCs.
 The CPU
 The central processing unit contains the system microprocessor. It controls
and processes all the operations within the PLC.
 It is supplied with a clock that has a frequency of typically between 1 and 8
MHz. This frequency determines the operating speed of the PLC and
provides the timing and synchronization for all elements in the system.

The Memory
There are several memory elements in a PLC system:
 System read-only-memory (ROM) gives permanent storage for the
operating system and fixed data used by the CPU.
 Erasable and programmable read-only-memory (EPROM) is used to
store programs permanently.
 Random-access memory (RAM) is used for data. This is where
information is stored on the status of input and output devices and the
values of timers and counters and other internal devices. The data RAM is
sometimes referred to as a data table or register table.
 The programs and data in RAM can be changed by the user. All PLCs will
have some amount of RAM to store programs that have been developed by
the user. However, to prevent the loss of programs when the power
supply is switched off, a battery is used in the PLC to maintain the RAM
contents for a period of time. After a program has been developed in RAM it
may be loaded into an EPROM memory chip, and so made permanent

Input/Output Unit
The input/output unit provides the interface between the system and the
outside world.

 The input/output channels provide

isolation and signal conditioning
functions so that sensors and actuators
can often be directly connected to them
without the need for other circuitry.
 Input
 The digital signal that is generally compatible with the microprocessor in
the PLC is 5 V DC. However, signal conditioning in the input channel, with
isolation, enables a wide range of input signals to be supplied to it. A range
of inputs might be available with a larger PLC, such as 5 V, 24 V, 110 V, and
240 V digital/discrete, that is, on/ off, signals. A small PLC is likely to have
just one form of input, such as 24 V.

Output
 The output from the input/output unit will be digital with a level of 5 V.
However, after signal conditioning with relays, transistors, or triacs, the
output from the output channel might be a
 24 V, 100 mA DC;
 110 V , 1 A DC;
 240 V, 1 A AC;
 240 V, 2 A AC.
 PLC Systems
There are two common types of mechanical design for PLC systems:
 single box type (brick)
 modular/rack type

Single box type

 The single-box type (or, as it’s

sometimes called, a brick) is
commonly used for small
programmable controllers.
 It is supplied as an integral
compact package complete with
power supply, processor, memory,
and input/output units.
 Typically such a PLC might have 6, 8, 12, or 24 inputs and 4, 8, or 16
outputs.
 A memory can store some 300 to 1000 instructions.
 Some brick systems have the capacity to be extended to cope with more
inputs and outputs by linking input/output boxes to them.
 Modular type
 The modular type consists of separate modules for power supply,
processor, and the like, which are often mounted on rails within a metal
cabinet.
 The mix of modules required for a particular purpose is decided by the user
and the appropriate ones then plugged into the rack.
 Thus it is comparatively easy to expand the number of I/O connections by
simply adding more input/output modules or to expand the memory by
adding more memory units.

Programming PLCs
 Programs for use with PLCs can be written in a number of formats. To
make it easier for engineers with no great knowledge of programming to
write programs for PLCs, ladder programming was developed.
 A programming device can be:
 a handheld device (can be carried
from one place to another),
 a desktop console (are likely to
have a visual display unit with a
full keyboard and screen display),
 or personal computers.
When the program has been
designed on the programming
device and is ready, it is transferred
to the memory unit of the PLC.