Programmable Logic Controllers Paper (PLC) SM54

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Programmable Logic Controllers
Subhash Mahla
Electrical Engineering Department
Baldev Ram Mirdha Institute of Technology , Jaipur
subhashmahla93@gmail.com
Abstract—The analysis carried out in this technical paper high-
lights the concepts, working, advantages and practical applica-
tions of programmable logic controllers, along with a comparison
with other control systems. A PLC aids in automation of a process
by monitoring inputs and controlling outputs after making a
decision on the basis of its program. It is commonly used for
controlling many mechanical movements of heavy machinery and
to control the voltage and frequency of power supplies. PLCs
offer an array of advantages over other control systems, and
have hence evolved as an important controller in industries these
days owing to its large number of applications.
Index Terms—Automation, Ladder Logic, SCADA, Control
Loop, Scan Cycle, EEPROM
I. INTRODUCTION
In the past, humans were the main methods for controlling
a system. Over the years, control engineering has evolved to
include electricity in control systems. Previously, electrome-
chanical relay based control systems were used. These systems
obviated the use of switches for power on and power off
actions. Elementary logic control decisions are predominantly
undertaken by relays.
The advancement of economical computers has brought
about a modern revolution, the Programmable Logic Con-
troller (PLC). The PLC has its origins around the 1970s in
the automation and motor manufacturing industries.
II. DESCRIPTION
Programmable Logic Controllers (PLCs), also referred to as
programmable controllers, are a classification in computers.
It is a device that performs discrete or continuous control
logic in process plant or factory environments. PLCs are
used in commercial and industrial applications. They have
become popular since their inception in a wide variety of
operations from boiler control to robot control. The Allen-
Bradley Company (Rockwell Automation) coined the term
Programmable Logic Controllers as well as the corresponding
acronym PLC . Both of these are therefore registered trade-
marks of the company. The National Electrical Manufacturers
Association (NEMA) defines a programmable logic controller
as: A digitally operating electronic apparatus which uses a
programmable memory for the internal storage of instructions
for implementing specific functions such as logic, sequencing,
timing, counting, and arithmetic to control, through digital or
analogy input/output modules, various types of machines or
processes.[1]
Programmable Logic Controllers are hardware and software
engineered microcomputers, used to provide industrial control
operations. A PLC audits inputs, makes decisions according
to its program, and uses these decisions to administer outputs
to automate a process or machine. The PLC is proficient in
various tasks, including storage of instructions pertaining to
sequencing, timing, counting, arithmetic, data manipulation
and communication with the objective of controlling machines
and processes.
Fig. 1: Block diagram of a PLC
III. BASIC COMPONENTS OF A PLC
The basic components of a PLC are as follows:
Fig. 2: Input Module

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A. I/O Modules
I/O modules physically connect to field devices. Since there
are different types of input and output devices, the process
determines the type of input or output module selected, digital
or analog.
Inputs give the controller real-time status of variables in
the form of real world signals. These variables can be analog,
register or discrete. Typical analog inputs can be from thermo-
couples, RTDs, flow, pressure, and temperature transmitters.
These inputs are transmitted over the I/O bus to the central
processor unit after being converted into digital data. The input
module is entrusted with the operation of converting electrical
signals flowing in from input field devices like push buttons to
electrical signals that the PLC can understand.[2] Examples of
input modules include limit switches, proximity switches and
push buttons.
Fig. 3: Output Module
The output module converts the signals containing instruc-
tions of the desired status of outputs into real world outputs
after they have been transmitted over the I/O bus to the
module. There are three categories of outputs: discrete, register
and analogy. Information flowing out from the PLC is again
converted into electrical signals that can be interpreted by the
output field devices, by the output module. Examples of these
output devices are motor starters or hydraulic solenoid valves.
B. Processor
The processor is the intellect behind the PLC. It is sim-
ilar in construction to the microprocessor used in personal
computers. The Central Processor Unit (CPU) performs tasks
such as self-diagnostics, memory management, data handling
execution, scanning, peripheral and external device commu-
nications, resource sharing, program execution and I/O bus
traffic control.[3]
Execution of various logic and sequencing functions is
carried out by the CPU, which operates on the PLC inputs
to determine the appropriate output signals. Operations such
as adding, subtracting, multiplying, dividing and comparing
are performed by the Arithmetic Logic Unit which is usually
a part of the CPU (but sometimes it is separate).
Fig. 4: Central Processing Unit
The working of a processor is as follows: The signal to the
input modules is transmitted by the machine or process input
elements. The input modules then generate a logic signal that
is sent to the CPU. When it receives instructions from the
memory and the feedback on the status of the I/O devices,
the CPU produces commands for the output devices. These
commands control the devices on a machine or a process.[4]
Typical loads to be controlled are devices such as solenoid
valves, indicator lamps, relay coils and motor starters.
C. Power Supply
PLCs and their constituting modules are equipped with
power generated from a power supply. The power supply
converts power line voltages into those required by the solid-
state components. It may be integral or separately mounted
which provides the isolation necessary to protect solid-state
components from high-voltage line spikes. The power supply
also provides the heat dissipation required for the plant floor
operation. It drives the Central Processor Unit, I/O logic
signals, memory unit and some peripheral devices.[5] The
expansion of I/O has lead to increased power requirement of
some PLCs. Hence, they are furnished with additional power
supplies.
D. Memory Unit
The memory unit of a PLC is the registry where the
programs are stored. The fundamental unit of memory is the
word. Words are made up of bits. A bit is a single piece
of data. It contains information on only two states (ON/OFF
or YES/NO). Longer words contain more information within.
Programs are combination of words that produce control logic.
Memory can be volatile or non-volatile. Volatile memory
is undesirable as is results in loss of data when power is
removed. Battery backup is usually a feature of volatile
memory systems. On the other hand, power outages do not
result in change of state of the non-volatile memory. Therefore,
it is administered in cases where it is expected that the
transportation time to the job site will be long or extended
power outages will be frequent.
Memory is of three types: Read Only Memory (ROM),
Read/Write Memory (R/W) and Random-Access Memory
(RAM).[6] Another type of memory is the electrically alterable
programmable read only memory (EAPROM).

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E. Network Interface
PLCs are capable of communicating with other devices
like computers running programming software, a terminal
that lets an operator enter commands into the PLC, or an
I/O that is located in a remote location from the PLC. This
communication is carried out through a network interface.
There are two ways in which the network interface can be
a part of the PLC: it can be built directly into the PLC or it
can appear as a plug on the front of the PLC.[7]
F. Programming Units
Programming units form an interface between the require-
ment of the programmer and the understanding of the PLC.
In the course of program development, system initialisation
and troubleshooting, the programmer provides for an interface
between the PLC and the user. This programming device is
used to code and insert into memory the instructions that are
to be performed during the scan.
Programming devices vary greatly in size, ranging from
small units that are hand-held to large, astute CRT-based units
with the ability to document, compute I/O status and work in
both online as well as offline modes.[8] A personal computer
is the most commonly used programming device.
PLCs are configured using the software that runs on per-
sonal computers. Different products may require different pro-
gramming software. Software allows programs to be written
in several different languages and allows users to create, edit,
document, store and troubleshoot programs.
IV. PROGRAMMING A PLC
PLCs are programmed in two ways. One is to directly
connect the PLC to a computer and the other is to make
use of a programming panel. Logical commands are used
to program the PLC in both scenarios. The panels used in
programming a PLC differ greatly in their intricacies, ranging
from simple keypads to hand-held computers with a screen for
graphics. The programming software is operated on computers
with graphics, simulators, diagnostics and auditing. After the
software programmer develops the program, it is tested and
then sent to the PLC. This can be done using magnetic tapes,
compact disks, and communication links. However, the most
common method for the transfer of a program to the PLC is
by using a chip to which the program is written, commonly
known as an EEPROM. After this, the chip is plugged into
the PLC. The memory can be erased by exposure to UV light
and reused but it cannot be overwritten.[6]
The PROGRAM mode of the processor is used when the
program has to be entered into the PLC. The instructions
are entered sequentially, one at a time with the help of the
programming device. The controller is made to work in the
RUN state (or operating cycle) when the program has to be
carried out. As a consequence of each operating cycle, the
controller reviews the status of the input devices, executes
the program that has been entered and alters the outputs
accordingly.
V. WORKING OF A PLC
The main components of a PLC are input modules, a Central
Processing Unit (CPU), and output modules. A variety of digi-
tal or analog signals are accepted from various field devices by
the input module. They are then converted into a logic signal
that the CPU can use. The program instructions stored in the
memory form the basis upon which the CPU makes decisions.
These decisions eventually result in execution of the control
instructions as required by the program. Control instructions
from the CPU are converted into a digital or analog signal
by the output module. This signal can be used to control
various field devices. A PLC works by scrutinizing a program,
instruction by instruction, continuously. This process is also
known as scanning. One scan time is defined as the time taken
to complete the three steps. The control loop is a continuous
cycle of the PLC reading inputs, solving the ladder logic, and
then changing the outputs.[9]
Fig. 5: Scan Cycle
Control circuits have been conventionally described using
ladder diagrams. Ladder diagrams consist of rungs; each one
of these is a program statement. A program statement is
basically a condition, or a set of conditions (input). Each one
of these conditions has an action (output) associated with them
that is executed if the condition is satisfied. Elementary logic
operators like AND, OR and NOT are used to perform logical
operations required by the PLC. They combine the instructions
in the program statement (present on a PLC rung) thus making
the result of each run either true or false (1 or 0).[10]
The instructions entered using a programming device deter-
mine what action will be performed by the PLC for a particular
input. A device acting as an operator interface displays process
information and provides for the addition of new process
parameters to increase or decrease level of control.
VI. COMPARISON OF PLCS WITH OTHER CONTROL
SYSTEMS
PLCs are predominantly used in industrial processes where
the development and maintenance costs of the automation
system exceeds the total cost of automation and where it might
be necessary to alter the system during its operational life,

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examples of these include motion and positioning control.
Also, they cater to applications where the system required
is highly customized. However, PLCs lack the capability
requisite of highly complex algorithmic processes, like in the
chemical industry.
Different techniques entailing the use of microcontrollers,
relay systems, minicomputers, solid-state controls, digital logic
control systems and electronic continuous control systems,
among many others, are preferred for high volume or simple
fixed automation tasks.
Relays can construct complicated control systems by linking
an array of logic components. On the other hand, a typical
relay system has hundreds of switching contacts, presenting
the design engineer with a daunting task especially when the
control function of a panel has to be changed, eventually
resulting in complete re-wiring of the system. They do not
provide the troubleshooting flexibility offered by modern pro-
grammable control systems. Relays also take up more space
and require regular maintenance.[11] In addition, the high
cost, low speed and lack of reliability has resulted in the
replacement of relay systems by modern substitutes. Relays
remain optimal for reconstruction of small control signals to
higher current or higher voltage driving signals. This explains
their comprehensive usage as output devices for other control
systems.
PLCs have a logic called PID (proportional, integral, deriva-
tive) controller. Usually, PLCs are constructed with very less
analog loops; processes that require a large number of loops
are based on distributed control system (DCS). With time, as
the power of the PLC increases, the gap between applications
of PLCs and DCSs has lessened greatly.[12]
Digital Logic Control Systems use logic gates to pro-
cess information (exclusively encompassing binary signals).
Logic gates run at much higher speeds, cause lesser power
consumption and occupy considerably lesser space than a
corresponding relay circuit. Nonetheless, they are incapable
of switching to higher power driving signals.
Operational amplifiers, a part of Electronic Continuous
Control Systems involve the performance of mathematical op-
erations like integration, differentiation, etc. They were utilized
in the field of continuous control and lead to the simplification
of complex control functions with discrete electronic systems.
Largely built on linear integrated circuits, analog control is
efficient with high speed. The fine-tuning of such systems
during designing coupled with their fixed nature continues to
cause hindrances.
A design based on microcontrollers is pertinent in systems
where hundreds or thousands of units are to be produced
where the end-user would not have to alter the control and the
development costs are large. They can be a part of sequence
and continuous control systems. Both microprocessors and
microcontrollers simplify connection requirements due to their
small size, making them easily locatable at the point of final
control.
VII. ADVANTAGES
To implement changes in control logic of a PLC no rewiring
is required which saves considerable time. PLCs can carry out
complex functions such as generation of time relays, counting,
comparing, arithmetic operations, etc. Increased technology
makes it possible to condense more functions into smaller and
less expensive packages. The cost of a PLC is recovered in a
short period, hence making it highly economical.
When changes are made to the program circuit or sequence
design or when a programming fault has to be revised in the
ladder diagram, the necessary amendments can be made easily
by just typing it in using a keyboard. This results in faster, and
at the same time, convenient troubleshooting.
If the need arises, an instantaneous printout of the original
PLC circuit can be made available effortlessly, hence making
documentation very efficient. The PLC prints out the actual
circuit in operation at a given moment.[7] There is no need
to look for the blueprint of the circuit in remote files. Due to
their solid-state nature, PLCs are extremely compact and much
more reliable when compared with hardwired controllers.
Ergo, the costs of maintaining PLC based control systems are
low.[13]
A PLC can form networks to communicate with other
devices like controllers. They can be correlated to perform
operations such as supervisory control, data gathering, moni-
toring devices and process parameters, and downloading and
uploading of programs. PLCs consume only 10% of the power
consumed by an equivalent relay logic controller and hence
also save energy.
VIII. DISADVANTAGES
The stop button is employed to disconnect a circuit in relay
system; the system stops if the power fails. The PLC can
be programmed to perform this function; although, in certain
programs, to stop a device, one may have to apply an input
voltage. Being fail-safe is not one of the features of this
system.
Some applications are single-function applications. It does
not pay to use a PLC that includes multiple programming
capabilities if they are not needed. One of the conspicuous
features of PLC that is reprogramming would not be required
since their operational sequence is very rarely altered.[7]
In a process, the environment may experience high heat,
vibration, and direct contact with electrical appliances within
the PLC. Such events continuously disrupt the performance
of the PLC. In addition, professional expertise is required to
replace the previously used control system employing relay
ladder by a PLC computer concept as the technology is
comparatively new.
IX. APPLICATIONS
PLC is used as a widespread controller that has numerous
applications. With the program installed in its memory, it
provides the user with a simple method of changing, extending
and optimizing control processes. In a PLC, input signals are
interconnected in accordance with a specified program. If true,
the PLC jumps to the resultant output. The mathematical base
for the working is Boolean algebra that acknowledges only two
states for one variable: 0 and 1. Modern PLCs can perform
timer and counter functions, memory setting and resetting

5
and mathematical computing operations.[14] Industries have a
variety of production tasks that are highly repetitive in nature.
In spite of each stage being repetitive and monotonous, it
requires careful attention of the operator to ensure effective
production. Whenever sequential control and automation is
required, PLCs are best suited to accomplish the task.
Supervisory Control and Data Acquisition (SCADA) is a
form of widespread industrial control structure that incorpo-
rates numerous sites and large distances. Here, PLCs are of use
in industrial processes like: manufacturing, production, power
generation, fabrication and refining. Infrastructure processes
requiring PLCs are: water treatment and distribution, waste
water collection and treatment, oil and gas pipelines, electric
power transmission and distribution, wind farms, civil defence
siren systems and large communication systems.
PLCs find applications in the tire industry, steel factories,
food and beverages industry, pulp factory, petrochemical in-
dustry, mining, power, continuous bottle-filling system, 3 stage
air conditioning system, speed control of DC motor and other
similar sequential task as well as process control.[15]
X. CONCLUSION
The requirement for higher quality and reliability in control
techniques has stemmed from a constant demand for better and
more efficient and process machinery. Smart, condensed solid-
state electronic devices have made it feasible to offer control
systems that can ease maintenance, reduce downtime and
enhance productivity by a large amount. Flexible and proficient
working has been offered to the user by programmable logic
controllers. Starting out as a replacement for relays, various
math and logic manipulations were gradually added to the
PLC along with the incorporation of smaller cases, faster
CPUs, networking and various Internet technologies. PLCs
continue to develop and have become vital to a great extent of
automation processes. New values make an addition to their
potential incessantly.
XI. ACKNOWLEDGEMENT
This technical paper has been done under the support and
guidance of Mr. G.K. Dey, Manager (Engineering Services,
Instrumentation) at Godrej Industries Limited.
REFERENCES
[1] W. Bolton, Programmable Logic Controllers, Newnes Fifth Edition,
2009.
[2] Clarence T. Jones, Patrick-Turner,1996.
[3] W.N. Clare, G. T. Kaplan, D. R. Saldon, A. C. Wiktorowicz, R. A.
Gilbert, C. W. Wendt Process Control and Optimization,Volume II.
[4] James A. Rehg, Glenn J. Sartori Programmable Logic Controllers,
Prentice Hall Higher Education,2009.
[5] S.K. Singh Industrial Instrumentation and Control,Second Edition, Tata
McGraw-Hill,2007
[6]
[7] D. J. Dunn Selection and Application of Programmable Logic Con-
trollers
[8] Dr. D. J. Jackson Programmable Logic Controllers Introduction to PLCs.
[9] Sadegh Vosough, Amir Vosough, PLC and its Applications, International
Journal of Multidisciplinary Sciences and Engineering, April 2008.
[10] Greg P. Zimmerman, Logic Controllers and Ladder Logic, Vol. 2, No.
8,November 2011.
[11] Albert Falcione, Bruce H. Krogh, Design Recovery for Relay Ladder
Logic,IEEE Control Systems, First IEEE Conference on Control Appli-
cations, September 1992.
[12] Gregory K. McMillan, Douglas M. Considine (ed), Process/Industrial
Instruments and Controls Handbook Fifth Edition, McGraw-Hill, 1999.
[13] Sanjeev Gupta, S C Sharma, Selection and Application of Advance
Control Systems: PLC, DCS and PC-based system, Journal of Scientific
Industrial Research, Vol. 64, April 2005, pp. 249-255.
[14] Prof. Jose A. Rodriguez Mondejar), Programmable Logic Controller,
UPCO ICAI Departmento de Electronica y Automatica.
[15] John R. Hackworth, Frederick D. Hackworth, Jr., Programmable Logic
Controllers: Programming Methods and Applications.
[16] Vishal Kumar Alok, Ajay Goel, To study the different industrial applica-
tions of PLC through ladder diagrams, National Institute of Technology
Rourkela, May 2011.

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