International Journal of Advancements in Research & Technology, Volume 3, Issue 8, August-2014
ISSN 2278-7763
25
Conveyor Control Using Programmable Logic Controller
*
Chitra.S,* Lecturer,
Department of Electronics and
Communication Engg, Dr.TTIT,
K.G.F,
Email:chitra.sd10@gmail.com
**Vijaya Raghavan. **Assistant
professor, Department of Mining
Engg, Dr.TTIT, K.G.F,
Email:raghavan_pp@rediffmail.
com
Abstract
A programmable Logic Controller (PLC) is a
specialized computer used for the control and
operation of manufacturing process and
machinery. It uses a programmable memory to
store instructions and execute functions including
on/off control, timing, counting, sequencing,
arithmetic, and data handling. Programmable
Logic Controllers (PLC) is used in almost every
aspect of industry to expand and enhance
production.
Where older automated systems
would
use
hundreds or thousands of
electromechanical relays, a single PLC can be
programmed as an efficient replacement. The
functionality of the PLCs has evolved over the
years to include capabilities beyond typical relay
control. Sophisticated motion control, process
control, distributive control systems, and complex
networking have now been added to the PLC’s
Functions. Therefore, PLCs provide many
advantages over conventional relay type of control,
including increased reliability, more flexibility,
lower cost, communication capability, faster
response time and convenience to troubleshoot.
The paper is based on systematic conveyor
controller programming by programmable logic
controller using omron software which is a world
smallest plc, offers variety of expansion options
and has user-friendly software.
as temperature and pressure), and the positions of
complex positioning systems. PLCs operate electric
motors, pneumatic or hydraulic cylinders, magnetic
relays or solenoids, or analog outputs. The
input/output arrangements may be built into a
simple PLC, or the PLC may have external I/O
modules attached to a computer network that plugs
into the PLC. PLCs were invented as replacements
for automated systems that would use hundreds or
thousands of relays, cam timers, and drum
sequencers. Often, a single PLC can be
programmed to replace thousands of relays.
Programmable controllers were initially adopted by
the automotive manufacturing industry, where
software revision replaced the re-wiring of hardwired control panels when production models
changed. Many of the earliest PLCs expressed all
decision making logic in simple ladder logic which
appeared similar to electrical schematic diagrams.
The electricians were quite able to trace out circuit
problems with schematic diagrams using ladder
logic. This program notation was chosen to reduce
training demands for the existing technicians. Other
early PLCs used a form of instruction list
programming, based on a stack-based logic solver.
The functionality of the PLC has evolved over the
years to include sequential relay control, motion
control, process control, distributed control systems
and networking. The data handling, storage,
processing power and communication capabilities
of some modern PLCs are approximately
equivalent to desktop computers. PLC-like
programming combined with remote I/O hardware,
allow a general-purpose desktop computer to
overlap some PLCs in certain applications. PLCs
can be programmed using standards-based
programming languages. A graphical programming
notation called Sequential Function Charts is
available on certain programmable controllers. The
primary reason for designing such a device was
eliminating the large cost involved in replacing the
complicated relay based machine control system.
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1. Introduction
The PLC is a microcontroller based device with
input/output circuitry that monitors the status of
field connected sensor (inputs) and controls the
attached (output) actuators (motor -starters,
Solenoids, Speed drives, Valves etc.) according to
a user created logic program stored in the memory.
The main difference from other computers is that
PLCs are armored for severe condition (dust,
moisture, heat, cold, etc) and have the facility for
extensive input/output (I/O) arrangements. These
connect the PLC to sensors and actuators. PLCs
read limit switches, analog process variables (such
Copyright © 2014 SciResPub.
1.1 PLC system
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International Journal of Advancements in Research & Technology, Volume 3, Issue 8, August-2014
ISSN 2278-7763
•
26
Modules are built into the PLC. The
modules come together in one physical
block. The backplane in this case is
transparent to the user.
Figure 1.1: Block diagram of PLC system.
There are five basic components in a PLC system:
• The PLC processor or controller
• I/O (Input /Output) modules
• Chassis or backplane
• Power supply
• Programming software that runs in a PC
• Network Interface
Fig 1.2: Backplane in a chassis based system
1.1.4. Power Supply. A power supply is needed to
provide power to the PLC and any other modules.
Power supplies come in various forms:
•
Power supply modules that fit into one
of the slots in a chassis.
External power supplies that mount to
the outside of a chassis.
Stand alone power supplies that
connect to the PLC or I/O through a
power cable.
Embedded power supplies that come as
part of the PLC block.
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1.1.1. The PLC processor. It stores the control
program and data in its memory. Reads the status
of connected input devices.
Executes the
control program. Commands connected outputs to
change state based on program execution For
example: Turn a light on, start a fan, adjust a speed,
or temperature and Comes in various physical
forms.
1.1.2 I/O Modules. They Physically connect to
field devices. Input modules convert electrical
signals coming in from input field devices such as
pushbuttons, to electrical signals that the PLC can
understand. Output modules take information
coming from the PLC and convert it to electrical
signals the output field devices can understand,
such as a motor starter, or a hydraulic solenoid
valve. I/O comes in various forms.
•
•
•
1.1.5 Programming Software. Software that runs
on a PC is required to configure and program
PLCs. Different products may require different
programming software. Software allows programs
to be written in several different languages.
1.1.3 Chassis/Backplane. All PLCs need some
method of communicating between the controller,
I/O and communications modules. Here are three
ways used to accomplish this communications
between the various components that make up the
PLC system.
•
•
Modules are installed in the same
chassis as the PLC and communicate
over the chassis backplane.
Modules are designed to “plug” into
each other. The interconnecting plugs
form a backplane. There is no chassis.
Copyright © 2014 SciResPub.
Fig 1.3: PLC program
1.1.6. Network Interface. Most PLCs have the
ability to communicate with other devices. These
devices include computers running programming
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International Journal of Advancements in Research & Technology, Volume 3, Issue 8, August-2014
ISSN 2278-7763
software, or collecting data about the
manufacturing process, a terminal that lets an
operator enter commands into the PLC, or I/O that
is located in a remote location from the PLC. The
PLC will communicate to the other devices through
a network interface.
27
PLC architecture design can be an open
architecture design or a closed architecture design.
An open architecture design allows the system to
be connected easily to any device and also to
programs developed by other manufacturers. A
closed architecture which is also known as
proprietary system is one whose design makes it
more difficult to connect with other devices or
programs developed by other manufacturers. When
working on a proprietary PLC system, all hardware
and software that is used should be compatible
with the PLC.
2.1 Programming in PLCs
Fig 1.4: Network connecting other devices
2. PLC Architecture
Early PLCs, up to the mid-1980s, were
programmed using proprietary programming panels
or special-purpose programming terminals, which
often had dedicated function keys representing the
various logical elements of PLC programs.
Programs were stored on cassette tape cartridges.
Facilities for printing and documentation were very
minimal due to lack of memory capacity.PLC
programs are typically written in a special
application on a personal computer, then
downloaded by a direct-connection cable or over a
network to the PLC. The very oldest PLCs used
non-volatile magnetic core memory but now the
program is stored in the PLC either in batterybacked-up RAM or some other non-volatile flash
memory. Early PLCs were designed to replace
relay logic systems. These PLCs were programmed
in "ladder logic", which strongly resembles a
schematic diagram of relay logic. Modern PLCs
can be programmed in a variety of ways, from
ladder logic to more traditional programming
languages such as BASIC and C. Another method
is State Logic, a Very High Level Programming
Language designed to program PLCs based on
State Transition Diagrams. The International
standard IEC 61131-3 defines five programming
languages for programmable control systems: FBD
(Function block diagram), LD (Ladder diagram),
ST (Structured text, similar to the Pascal
programming language), IL (Instruction list) and
SFC (Sequential function chart). These techniques
emphasize logical organization of operations.
While the fundamental concepts of PLC
programming are common to all manufacturers,
differences in I/O addressing, memory organization
and instruction sets mean that PLC programs are
never perfectly interchangeable between different
makers. Even within the same product line of a
single manufacturer, different models may not be
directly compatible.
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Fig 2.1: PLC architecture
The PLC architecture is divided into three parts:
a. CPU : It is the brain of PLC system .It
consists of the microcontroller, Memory
IC and necessary circuit to store and
retrieve information from the memory.
The Job of CPU is to monitor status or
state of input device, scan and solve the
logic of a user program and control ON
or OFF state of output device.
b. Memory: The type of RAM (Random
Access Memory) normally used is CMOS
(Complementary
Metal
Oxide
Semiconductor) to store the program.
c. Input/output: Input is the one through
which signal is send and result is observed
at the Output.
2.2. User Interface
PLCs may need to interact with people for the
purpose of configuration, alarm reporting or
Copyright © 2014 SciResPub.
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International Journal of Advancements in Research & Technology, Volume 3, Issue 8, August-2014
ISSN 2278-7763
everyday control. A Human-Machine Interface
(HMI) is employed for this purpose. HMI's are also
referred to as MMI's (Man Machine Interface) and
GUI (Graphical User Interface).A simple system
may use buttons and lights to interact with the user.
Text displays are available as well as graphical
touch screens. Most modern PLCs can
communicate over a network to some other system,
such as a computer running a SCADA (Supervisory
Control And Data Acquisition) system or web
browser.
2.3. PLC Software
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continuity of each rung by referring to the input
image table to see if the input conditions are met. If
the conditions controlling an output are met, the
processor immediately writes a 1 in its memory
location, indicating that the output will be turned
ON; conversely, if the conditions are not met a 0
indicating that the device will be turned OFF is
written into its memory location. And the final
step of the scan process is to update the actual
states of the output devices by transferring the
output table results to the output module, thereby
switching the connected output devices ON (1) or
OFF (0).
The PLC software is manufacturer dependent and
even when the manufacturer is the same, it may
vary for the different models of the same brand.
For, instance for a manufacturer like Allen Bradley
the software may vary for its PICO Controller
models and other models. For example, the
software used for these controllers is PICO Soft
whereas for its higher models it is RSLogix.
Moreover, the HMI Interface may also vary for the
different controllers.
2.4. Program Scan
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When a PLC executes a program, it must know in
real time when external devices controlling a
process are changing. During each operating cycle,
the processor reads all the inputs, takes these
values, and energizes or de-energizes the outputs
according to the user program. This process is
known as a program scan cycle. Figure 2.2
illustrates a single PLC operating cycle consisting
of the input scan, program scan, output scan.
Fig 2.3 Scan process applied to a multiple rung
program.
2.5. Ladder programs
Ladder programs process inputs at the beginning of
a scan and outputs at the end of a scan, as
illustrated in Figure 2.3 . For each rung executed,
the PLC processor will perform the following
steps: The first step is to update the input image
table by sensing the voltage of the input terminals.
Based on the absence or presence of a voltage, a 0
or a 1 is stored into the memory bit location
designated for a particular input terminal. The
second step is to solve the ladder logic in order to
determine logical continuity. The processor scans
the ladder program and evaluates the logical
Copyright © 2014 SciResPub.
Fig 2.4: Typical micro PLC.
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International Journal of Advancements in Research & Technology, Volume 3, Issue 8, August-2014
ISSN 2278-7763
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2.6. Latching Relays
Electromagnetic latching relays are designed to
hold the relay closed after power has been removed
from the coil. Latching relays are used where it is
necessary for contacts to stay open and/or closed
even though the coil is energized only
momentarily.
The Figure 3.1 illustrates a bottle-filling motion
control process. This application requires two axes
of motion: the motor operating the bottle filler
mechanism and the motor controlling the conveyor
speed. The role of each control component can be
summarized as follows:
2.7 Ladder Logic
3.1. Programmable Logic Controller
Before a PLC can perform any control task, it must
be programmed to do so. The most popular
language used to program a PLC is ladder logic. It
is a graphical, problem oriented programming
language which replicates electronic switching
blueprints or circuits. In a conveyor system, we
have several “requirements” to accomplish; for
example, timing and counting parts on the
conveyor. Each of these requirements must be
programmed into the PLC so that it knows how to
respond to different events. The programmer
develops the program, and connects their personal
computer to the PLC through a network or cable
and then downloads the program to the PLC.
Ladder logic rungs are basically IF-THEN
statements. Each individual rung is executed from
the left to the right.
• The controller stores and executes the user
program that controls the process.
• This program includes motion instructions that
control axis movements.
• When the controller encounters a motion
instruction it calculates the motion commands for
the axis.
• A motion command represents the desired
position, velocity, or torque of the servo motor at
the particular time the calculations take place.
3.2. Motion Module
The motion module receives motion commands
from the controller and transforms them into a
compatible form the servo drive can understand. In
addition it updates the controller with motor and
drive information used to monitor drive and motor
performance.
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3.3. Servo Drive
The servo drive receives the signal provided by the
motion module and translates this signal into motor
drive commands. These commands can include
motor position, velocity, and/or torque. The servo
drive provides power to the servo motors in
response to the motion commands.
Fig 2.5: Example of Ladder Logic diagram
3. Block diagram of conveyor Controller
3.4. Servo Motor
• The servo motors represent the axis being
controlled.
• The servo motors receive electrical power from
their servo drive which determines the motor shaft
velocity and position.
• The filler motor must accelerate the filler
mechanism in the direction the bottles are moving,
match their speed, and track the bottles.
• After the bottles have been filled, the filler motor
has to stop and reverse direction to return the filler
mechanism to the starting position to begin the
process again.
Fig 3.1: Bottle filling control process
Copyright © 2014 SciResPub.
IJOART
International Journal of Advancements in Research & Technology, Volume 3, Issue 8, August-2014
ISSN 2278-7763
4. Programming PLC for Conveyor
30
The figure 4.2 shows the program for the conveyor:
The first line of code turns on the motor and the
light when a box is detected by photoeye1.
Likewise, the motor and light are turned off when
photoeye2 detects the box in the second line of
code.
Fig 4.1: Example of conveyor
The simulation is done using omron which ensures
that the program can be successfully implemented
in PLC. Today delta PLC is available in market
which helps in easier implementation of the above
required purpose. It uses simple omron motion
language for programming. A PLC program
consists of rules that make logic relation between
inputs and outputs of the controller. Basically it
uses logic operands: AND, OR, negation. The
structure of the rules is IF…THEN…ELSE. The
PLC reads all field input devices via the input
interfaces. Executes the user program stored in
application memory, then, based on whatever
control scheme has been programmed by the user.
Turn the field output devices on or off, or perform
whatever control is necessary for the process
application. The PLC resolves the program rule by
rule (sequential execution). The PLC operates in a
synchronous way i.e. inputs does not change under
a scan cycle. And the figure 4.1 shows that when a
box is placed on the conveyor in front of
Photoeye 1, Light 1, and Motor 1 will turn on,
causing the box to move down the conveyor to the
left. When the box passes in front of Photoeye 2,
Motor 1 and Light 1 will turn off, stopping the
conveyor.
Fig 4.3: Relay ladder logic
The third line begins a timer when the box passes
by photoeye1, and if the box does not pass by
photoeye2 in 30 seconds (the timer counts in
milliseconds), the motor and light are shut off by
line 4. This is the indication of a jam condition.
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Fig 4.2: Relay Ladder Logic Example
Copyright © 2014 SciResPub.
4.1. The control system
A complete control system is made up of a
combination of PLCs, networks, I/O, terminals and
software. All the components work together to
form a complete control system. The control
system is the system that is responsible for the
control of the process. This is the system that
includes the PLC, all of the I/O and any Human
Machine Interfaces (HMI).
Fig 4.4: The complete control system
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International Journal of Advancements in Research & Technology, Volume 3, Issue 8, August-2014
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4.2. Data Acquisition System
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6. PLC Applications
•
•
•
•
•
•
Automotive Industry
Beverage Industry
Marine Industry
Packaging Industry
Intelligent Motor Control
Motor Control Applications
7. Conclusion
Fig 4.5 : Data acquisition system
The Data Acquisition system is generally
responsible for collecting data about the control
system, and storing it on master computers or
servers, or displaying it on terminals. The data is
often used later for reporting or charting purposes.
They are made up of devices and networks which
are responsible for acquiring data about the process
but are not responsible for direct control of the
process. The network used for data acquisition is
often Ethernet. While data acquisition devices can
exist directly on the control network, a gateway is
often used to separate network traffic between the
data acquisition system and the control system.
PLCs are well-adapted to a range of automation
tasks. These are typically useful in industrial
processes in manufacturing where the cost of
developing and maintaining the automation system
is high relative to the total cost of the automation,
and where changes to the system would be
expected during its operational life. PLCs contain
input and output devices compatible with industrial
pilot devices and controls; little electrical design is
required, and the design problem centers on
expressing the desired sequence of operations in
ladder logic (or function chart) notation. PLC
applications are typically highly customized
systems so the cost of a packaged PLC is low
compared to the cost of a specific custom-built
controller design. On the other hand, in the case of
mass-produced goods, customized control systems
are economic due to the lower cost of the
components, which can be optimally chosen
instead of a "generic" solution, and where the nonrecurring engineering charges are spread over
thousands of places.
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8. References
Fig 4.6: Omron PLC with modbus-RTU
network
5. Advantages of PLC
•
•
•
•
•
Cost effective for controlling complex
system.
Flexible and can be reapplied to control
other system quickly and easily.
Computational abilities allow more
sophisticated control.
Trouble shooting aids make programming
easier and reduce downtime.
Reliable components make these likely to
operate for longer life.
Copyright © 2014 SciResPub.
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Programmable Logic Controller (PLC) Course in
Electrical Engineering Technology”, ASEE Annual
Conference & Exposition, 2008
[2] Johnson, C. D., Process Control Instrumentation
Technology, Prentice Hall, 2006
[3] Petruzella, F. D., Programmable Logic Controllers,
McGraw Hill, 2005
[4] Yang, G., Rasis, Y., “Teaching PLC in Automation –
A Case Study”, ASEE Annual Conference &
Exposition, 2003
[5] URL: http://read-out.net/signpost/oid.html; Education
and Research in the area of Measurement, Control and
Automation.
[6] URL: http://www.plcs.net; Your Personal PLC Tutor
Site - The BEST Place to Learn PLC Programming.
[7] URL: http://www.me.ua.edu/ME360/plc; ME 360 Programmable Logic Controller Module.
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