B.Tech Industrial Training Report

Babu Banarsi Das Institute of Technology, Ghaziabad
INDUSTRIAL TRAINING REPORT ON
Automation using PLC
SUBMITTED IN PARTIAL FULFILLMENT FOR THE AWARD OF THE
DEGREE OF
BACHELOR OF DEGREE
IN
ELECTRONICS AND COMMUNICATION ENGINEERING
SUBMITTED BY
Rohit Singh
B.Tech VI SEMESTER
0803531072
(2008-12)
Training taken under
Moserbaer, A-164, Sector-80
Phase –II, Noida,
G.B Nagar, India
JULY 2011

AUTOMATION USING PLC
Submitted by
Rohit Singh
0803531072
Under the Guidance of
Vikas Goyal
&
Anil Nagar
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
Babu Banarsi Das Institute of Technology, Ghaziabad

DECLARATION
I hereby declare that the project work entitle (“Automation using PLC”) is an authentic
record of my own work carried out at MOSERBAER, PHASE- II, NOIDA as
requirement of six weeks project for the award of degree of B.Tech in Electronics &
Communication Engineering, Babu Banarsi Das Institute of Technology, Ghaziabad,
Under the guidance of Mr. Vikas Goyal & Anil Nagar (Industry coordinator), during 15
June to 27 July 2011.
Rohit Singh
0803531072
27 July, 2011
It is certified that the above statement made by the student is correct to the
best of our knowledge and belief.
Mr. Vikas Goyal
(Industry Coordinator)
Mr. Anil Nagar
(Industry coordinator)

CONTENTS
1. INTRODUCTION OF MOSERBAER
2. COMPACT DISC
3. STEPS INVOLVED IN CD MANUFACTURING
4. PNEUMATIC CYLINDER
5. SENSORS
6. PLC

1. INTRODUCTION OF MOSER BAER
Moser Baer, headquartered in New Delhi, is one of India's leading technology companies.
Established in 1983, Moser Baer successfully developed cutting edge technologies to
become the world's second largest manufacturer of Optical Storage media like CDs and
DVDs. The company also emerged as the first to market the next-generation of storage
formats like Blue-ray Discs and HD DVD. Recently, the company has transformed itself
from a single business into a multi-technology organization, diversifying into exciting
areas of Solar Energy, Home Entertainment and IT Peripherals & Consumer Electronics.
Moser Baer has a presence in over 82 countries, serviced through six marketing offices in
India, the US, Europe and Japan, and has strong tie-ups with all major global technology
players.
Moser Baer has the distinction of being preferred supplier to all top global OEM brands.
Moser Baer stands committed to supplying highest quality fully licensed media to its
customers.
Moser Baer's products are manufactured at its three state-of-the-art manufacturing
facilities. It has over 6,000 full-time employees and multiple manufacturing facilities in
the suburbs of New Delhi.

2. COMPACT DISC
A Compact Disc (also known as a CD) is an optical disc used to store digital data. It was
originally developed to store sound recordings exclusively, but later it also allowed the
preservation of other types of data. Audio CDs have been commercially available since
October 1982.
HISTORY OF THE COMPACT DISC:
Figure 1
Starting in the mid 1980's, compact discs (CD) began to take over both the audio and
computer program market. Much of this can be attributed to a general acceptance of
certain specifications regarding compact discs, known as the "Color Books." Originally
designed and developed by both Sony and Phillips, the concept of the Color Books was
patented and standards were developed. These are a collection of five books that describe
the specifications and standards CD technology follows. This led eventually to the
current audio CD technology.

TYPES OF CD:
There are three main types: standard manufactured CDs (CD-DA), CD-R recordable and
CD-RW rewriteable.
 Standard manufactured CDs can be played on any CD digital audio player.
 CD-Rs can be played on CD-R machines and many but not all CD digital audio
players.
 CD-RWs can only be played on CD-RW compatible machines.
Both writeable types can be burned to play in audio machines and PCs.

3. STEPS INVOLVED IN CD MANUFACTURING
THE COMPLETE PROCESS FROM COLER BUFFER TO RECEIVER HANDLER
1. Moulder handler: handling over the freshly moulded discs to the cooler.
▼
2. Moulder take over handling: transporting the discs from the moulder to the
handover point.
▼
3. Sample handler: transporting discs from the sampler position to the sample out feed
position.
▼
4. Distributor handler: handling the discs from the inter handler to the sampler
handler.

▼
5. Cooler indexing table: moving the discs from the in feed position to the handler
position towards the cooler.
▼
6. Conveyor cooler: cooling the hot discs coming from the moulder.
▼
7. Buffers: in case the material flow piles up at the areas behind.
▼
8. NSK pick and place handler: moving the discs between conveyors cooler, buffer
and dye-coating.
▼
9. Dye coating: coating the discs coming from the coolers with the data carrying
dye.
▼
10. Bottom edge washing process stations: removing the dye hump from the outer
edge of the discs by dissolving via solvent.
▼
11. quality control: marking the dye coated discs via ink jet printer . It is done by
scanners buffering of good discs and batching of bad discs.
▼
12. Dye inspection: 1. scanning the discs for faulty coating.

▼
13. Out feed handler: handling the discs between the output handler of the indexing
unit and the reject spindle.
▼
14. Disc lift: lifting the discs batch being on the spindle up to working position.
▼
15. Hot air dryer: drying the discs coming from the dye cooler.
▼
16. Sputter: metalizing the discs. The sputter is a separate unit with a control of its
own.
▼
17. Solvent top edge cleaning: removing the dye hump from the outer edge of the
discs by dissolving with the solvent
▼
18. Solvent media supply system
▼
19. U V lacquering and drying: coating the sputtered and cleaned discs with a
transparent UV coating protective layer.
▼
20. 2-arm NSK process handler

▼
21. Final inspection and receiver: inspecting the manufactured discs for overall
quality.
▼
22. Receiver handler: handling the discs from the UV o/p from the final scanner to the
spindle.
▼
23. Receiver/ Reject unit: malfunctioning may occur using wrong spindles.
▼
24. Rotary indexing unit, Pneumatically: the step wise rotation movement is initiated
by a pneumatic control pulse and varies automatically until reaching the next stop
position.

SEQUENCE OF OPREATION OF A ROBOTIC ARM
STEP FUNCTION COMPPONENT
USED
MOTION
1. First arm waits for CD to
be free
-------------- At rest
2. Arm moved to get hold of
CD-R
Motor Rotary motion
3. Suction cup moves to pick
the CD-R
Pneumatic linear
cylinder
Linear motion
4. Then arm picks the CD-R Vacuum technique Grasping a CD
5. Arm again comes to the
initial position
Motor Rotary motion
6. Arm again comes to final
position to transfer the CD-
R
Pneumatic rotary
cylinder
Rotary motion
7. CD-R is placed at the disc
putting place to be carried
by the second arm
Vacuum technique Leaving the disc
8. Second arm senses if CD-R
is free to be carried from
that place
Sensors --------------
9. Second arm carries CD-R
to place it on the conveyor
belt.
Step 1-8 Step 1-8

PROCESS INFEED COOLER:
Taking over discs offered by the three moulders cooling and feeding them to the dye
process unit, extraction of the production samples directly from the moulder unit
MOULDER HANDLER:
Handling over the freshly moulded discs to the cooler.
Functions :
 Arm drive is actuated and swivels the gripper arm from the safety position to the
end position at the moulder, pneumatic cylinder of the gripper head is actuated
and turns the gripper head so that the freshly mounted discs can be sucked.
 Moulder opens and takes the blank discs off the moulder and holds it such that the
blank discs can be sucked.
 Vacuum is switched ON, the moulder handler releases the disc and retracts.
 The pneumatic cylinder of the gripper head is reversely actuated such that disc is
lying horizontally.
 When arm reaches the next station the disc is released and now the disc is lying in
the deposit ends of the next station.
 Handler swivels back to the actual position.
 The reference points and the safety areas are detected.
MOULDER TAKE OVER HANDLING:
 Deposits the discs on the sucker of the take over handler and retracts.
 Lift moves the handler up. The overhead drive swings the handler arm to the outer
end position.
 The lift lowers the handler, thus the discs is deposited on the deposit fork.
 The lift moves up and the handler arm swings back to take the next disc.
 The deposit fork is swung to the handover position towards the distributors
handler.

SAMPLER HANDLER:
 The pneumatic turns the cylinder with the turn axis, mechanical stops, shock
absorbers at the turn axis and position sensors.
 The handler arm with the sucker at the turn axis.
Functions:
 Distributor handler deposits a disc on the deposit pin of the sampler handler.
 The swivel arm swings to the out feed position and offers the disc to the
operation.
 The end positions of the pneumatic drives are monitored by the sensors.
DISTRIBUTOR HANDLER:
Handling the discs from the inter handling to the sampler handler or distributing the discs
to the cooler indexing table.
COOLER INDEXING TABLE:
Moving the discs from the in feed position to the handler position towards the cooler.
Indexing unit: stepping-in-circle of the depositing spider.
CONVEYOR COOLER:
Cooling the hot discs coming from the molders.
Discs are loaded, where they can cool down while the conveyor is stepping to the end of
the track.
 Conveyor track: Transporting and holding the discs while cooling.
 Conveyor drive: The conveyor belt system is moved by a geared motor. An angle
gear transmits the motor rotation to the output drive tooth pulley which is in
permanent contact with the drive shaft of the distributor gear.

 Conveyor loading/unloading handler: Mounted at the end points of the conveyor
track. Ht e handler consists of vertical pneumatic slides at which the pneumatic
turn drive is mounted. On the turn axis of the drive the vacuums sucker for
holding the discs.
BUFFERS:
 It delivers discs in case the material flow piles up at the areas behind.
 Re feeds the stored discs to the material flow.
 Buffering indexing table: stepwise rotation is initiated by the pneumatic control
pulse and runs automatically until reaching the next stop position.
 Disc lift: lifting the discs batch on the spindle into the working position.
 This position is detected as that height as which the upper most discs is detected
by the disc sensor.
DYE COATING:
Coating the discs coming from the coolers with the data carrying dye.
Processing is performed in 5 processing stations. Discs are moved through process area
by a conveyor transport system.
1. Conveyor transport system
2. 5 process handler
3. 5 Dye coating and bottom edge cleaning process.
Functions:
 Motor drives the angular gear which moves the conveyor tooth belt.
 When the rest pin on the belt reaches the light barrier at the process station, the
motion is stopped so that the process handler can pick and place discs from the
pin.

SENSOR ICS:
Stop position for the belt is determined by the light barriers at the in feed position.
Presence of discs is monitored by the reflective light barriers at the in feed / out feed
position and the coater handler position.
PROCESS HANDLERS:
Loading/ unloading discs of the dye coating process module.
DYE COATING PROCESS STATIONS:
Coating the discs with the data carrying dye.
 Discs are put on the chuck and held by the vacuum.
 Dye is dispensed onto the surface at the center of the disc. At the same time the
disc which is being dispensed rotates slowly.
 Disc is spun with 5000 rpm, dye spreads uniformly.
 Spun-off dye is caught in the process cup.
 Particles and dye vapors are sucked off at the edge of the process cup.
Components: 1. process cup
2. Motor
3. Chuck
DYE DISPENSE UNIT:
 Bottom edge cleaning process station.
 Recovering the dye heap (from below) from the outer edge of the disc by
dissolving via solvent.

N S K DRIVE:
Mega torque motor system is a unique actuator with special capabilities. System consists
of almost all servo motor systems.
Incorporated in two units:
1. motor
2. driver unit
1. Motor: Motor consists of a high torque brushless actuator, high resolution brushless
resolver and a heavy duty precision NSK bearing.
The high torque eliminates the need for gear reduction, while the built in resolver usually
makes feedback components such as encoders or techno meters unnecessary. The heavy
duty mechanical support since the motor case can very often support the load directly in
most applications.
2. Driver Unit: It consists of a power amplifier, resolver interfaces and the digital motor
control circuits.
The driver units provide everything that is needed to control the motor torque, velocity
position for interface to any standards motor position controller or to act as a standalone
digital motion control system with its built-in zero backlash position control capabilities.
High speed: It features higher speed than ever before with less torque drop-off at the
immediate speeds. Smaller motors may be used for high speed indexing applications
when torque management is primarily for accelerations.
EASE OF USE:
1. The circuit parameters can be changed by the command, rather than by alternating
to adjust a multi-turn pot or changing capacitor values.

2. Significant changes can be made with little or no hazel wave changes.
HIGH REPEATABILITY:
With zero backlash direct drive this system offers repeatability as high as app. 2.1”.
 Easy to maintain.
 Login diagnosis outputs identity the nature of any error condition quickly and
accurately.
FUNCTIONAL PRINCIPLE:
Motor by virtue of its unique design it is capable of producing extremely high torque at
low speeds suitable for direct applications. It can produce these torque levels without
using an undue amount of power, so it can sustain these torques levels indefinitely under
most condition without overheating.

4. PNEUMATIC CYLINDER
Operation diagram of a single acting cylinder.
The spring (red) can also be outside the cylinder, attached to the item being moved.
Operation diagram of a double acting cylinder
Pneumatic cylinders impart a force by converting the potential energy of compressed gas
into kinetic energy. This is achieved by the compressed gas being able to expand, without
external energy input, which itself occurs due to the pressure gradient established by the
compressed gas being at a greater pressure than the atmospheric pressure. This air
expansion forces a piston to move in the desired direction. Once actuated, compressed air
enters into the tube at one end of the piston and, hence, imparts force on the piston.
Consequently, the piston becomes displaced (moved) by the compressed air expanding in
an attempt to reach atmospheric pressure.
Types:
 Single acting cylinders: Single acting cylinders (SAC) use the pressure imparted
by compressed air to create a driving force in one direction (usually out), and a
spring to return to the "home" position.
 Double acting cylinders: Double Acting Cylinders (DAC) use the force of air to
move in both extends and retract strokes. They have two ports to allow air in, one
for outstroke and one for in stroke.
 Rotary air cylinders: actuators that use air to impart a rotary motion
 Rod less air cylinders: These have no piston rod. They are actuators that use a
mechanical or magnetic coupling to impart force, typically to a table or other
body that moves along the length of the cylinder body, but does not extend
beyond it.

5. SENSORS
A sensor is a device that measures a physical quantity and converts it into a signal which
can be read by an observer or by an instrument. For example, a mercury-in-glass
thermometer converts the measured temperature into expansion and contraction of a
liquid which can be read on a calibrated glass tube. A thermocouple converts temperature
to an output voltage which can be read by a voltmeter.
Uses:
 Sensors are used in everyday objects such as touch-sensitive elevator buttons
(tactile sensor)
 Lamps which dim or brighten by touching the base.
A sensor differs from a transducer in the way that a transducer converts one form of
energy into other form whereas a sensor converts the received signal into electrical form
only.
RESOLUTION:
The resolution of a sensor is the smallest change it can detect in the quantity that it is
measuring. Often in a digital display, the least significant digit will fluctuate, indicating
that changes of that magnitude are only just resolved. The resolution is related to the
precision with which the measurement is made.

PROXIMITY SENSOR:
A proximity sensor is a sensor able to detect the presence of nearby objects without any
physical contact. A proximity sensor often emits an electromagnetic or electrostatic field,
or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in
the field or return signal. The object being sensed is often referred to as the proximity
sensor's target. Different proximity sensor targets demand different sensors.
The maximum distance that this sensor can detect is defined "nominal range". Some
sensors have adjustments of the nominal range or means to report a graduated detection
distance.
Proximity sensors can have a high reliability and long functional life because of the
absence of mechanical parts and lack of physical contact between sensor and the sensed
object.
TYPES OF SENSORS:
 Inductive
 Capacitive
 Capacitive displacement sensor
 inductive sensor
CAPACITANCE SENSORS:
Capacitance sensors detect a change in capacitance when something or someone
approaches or touches the sensor. The technique has been used in industrial applications
for many years to measure liquid levels, humidity, and material composition. Integrated
circuits specifically designed to implement capacitance sensing in human-machine
interface applications are now available from Analog Devices.
WORKING OF CAPACITANCE SENSOR:
A basic sensor includes a receiver and a transmitter, each of which consists of metal
traces formed on layers of a printed-circuit board (PCB). As shown in Figure 1, the

AD714x has an on-chip excitation source, which is connected to the transmitter trace of
the sensor. Between the receiver and the transmitter trace, an electric field is formed.
Most of the field is concentrated between the two layers of the sensor PCB. However, a
fringe electric field extends from the transmitter, out of the PCB, and terminates back at
the receiver. The field strength at the receiver is measured by the on-chip sigma-delta
capacitance-to-digital converter. The electrical environment changes when a human hand
invades the fringe field, with a portion of the electric field being shunted to ground
instead of terminating at the receiver. The resultant decrease in capacitance—on the order
of femto farads as compared to Pico farads for the bulk of the electric field—is detected
by the converter
.
In general, there are three parts to the capacitance-sensing solution, all of which can be
supplied by Analog Devices.
 The driver IC, which provides the excitation, the capacitance-to-digital converter,
and compensation circuitry to ensure accurate results in all environments.
 The sensor—a PCB with a pattern of traces, such as buttons, scroll bars, scroll
wheels, or some combination. The traces can be copper, carbon, or silver, while
the PCB can be FR4, flex, PET, or ITO.
 Software on the host microcontroller to implement the serial interface and the
device setup, as well as the interrupt service routine. For high-resolution sensors
such as scroll bars and wheels, the host runs a software algorithm to achieve high
resolution output. No software is required for buttons.

ADVANTAGES OF USING CAPACITIVE SENSORS:
 Capacitance sensors are more reliable than mechanical sensors—for a number of
reasons.
 There are no moving parts, so there is no wear and tear on the sensor, which is
protected by covering material,
INDUCTIVE SENSOR:
An inductive sensor is an electronic proximity sensor, which detects metallic objects
without touching them.
The sensor consists of an induction loop. Electric current generates a magnetic field,
which collapses generating a current that falls asymptotically toward zero from its initial
level when the input electricity ceases. The inductance of the loop changes according to
the material inside it and since metals are much more effective inductors than other
materials the presence of metal increases the current flowing through the loop. This
change can be detected by sensing circuitry, which can signal to some other device
whenever metal is detected.
Applications of inductive sensors :
 metal detectors, traffic lights,
 car washes,
 host of automated industrial processes.

Elements of a simple inductive sensor..
 Field sensors
 Oscillator.
 Demodulator.
 Flip-flop.
 Output
PHOTOELECTRIC SENSOR:
A photoelectric sensor, or photo eye, is a device used to detect the distance, absence, or
presence of an object by using a light transmitter, often infrared, and a photoelectric
receiver.
The object is detected when the beam light is blocked.
FUNCTIONAL TYPES:
 Opposed arrangement: Receiver is located within the line os sight of the
transmitter. Object is detected when the beam is blocked from getting at the same
location.
 Retro reflective: Transmitter and receiver are at the same location and uses
reflector to bounce light bean back to the receiver. Object id sensed when the
beam is interrupted.
 Proximity sensing arrangement: Object is detected when the receiver sees the
transmitter source rather than when it fails to see it.
Some photo eyes have two different operational types, light operate and dark operate.
Light operate photo eyes become operational when the receiver "receives" the transmitter
signal. Dark operate photo eyes become operational when the receiver "does not receive"
the transmitter signal.

The detecting range of a photoelectric sensor is its "field of view", or the maximum
distance the sensor can retrieve information from, minus the minimum distance. A
minimum detectable object is the smallest object the sensor can detect. More accurate
sensors can often have minimum detectable objects of minuscule size.
REED SWITCH:
The reed switch is an electrical switch operated by an applied magnetic field.
The contacts can be:
 Normally open, closing when a magnetic field is present.
 Normally closed and opening when a magnetic field is applied.
The switch may be actuated by a coil, making a reed relay, or by bringing a magnet near
to the switch. Once the magnet is pulled away from the switch, the reed switch will go
back to its original position.
The reed switch contains a pair (or more) of magnetizable, flexible, metal reeds whose
end portions are separated by a small gap when the switch is open. The reeds are
hermetically sealed in opposite ends of a tubular glass envelope.
A magnetic field (from an electromagnet or a permanent magnet) will cause the reeds to
come together, thus completing an electrical circuit. The stiffness of the reeds causes
them to separate, and open the circuit, when the magnetic field ceases
USES:
 Reed switches are used in reed relays.
 Reed switches are widely used for electrical circuit control, particularly in the
communications field.
 Reed switches actuated by magnets are commonly used in mechanical systems as
proximity switches as well as in door and window sensors in burglar alarm
systems and tamper proofing methods.
 Reed switches are used in modern laptops which puts the laptop on
sleep/hibernation mode when the lid is closed.

LASER SENSORS:
A laser sensor is a device which uses a laser beam to determine the distance to an object.
It operates on the time of flight principle by sending a laser pulse in a narrow beam
towards the object and measuring the time taken by the pulse to be reflected off the target
and returned to the sender.
Due to the high speed of light, this technique is not appropriate for high precision sub-
millimeter measurements, where triangulation and other techniques are often used.
PULSE:
The pulse may be coded to reduce the chance that the rangefinder can be jammed. It is
possible to use Doppler effect techniques to judge whether the object is moving towards
or away from the rangefinder, and if so how fast.
RANGE:
Despite the beam being narrow, it will eventually spread over long distances due to the
divergence of the laser beam, as well as due to scintillation and beam wander effects,
caused by the presence of air bubbles in the air acting as lenses ranging in size from
microscopic to roughly half the height of the laser beam's path above the earth.
VACUUM SWITCHES:
Prior to effective engine control unit computers, engine vacuum was used for many
functions in an automobile. Vacuum switches were employed to regulate this flow, and
were commonly controlled by temperature, solenoids, mechanically, or directly. They
operated vacuum motors, other vacuum switches and other devices.
The internal combustion engine in a common automobile produces almost 20 inches (51
cm) of vacuum, and this pressure differential may be harnessed for many uses. Engine
vacuum is also the best direct source of information on the engine. Most delay valves
have a one-way function, where there is either no restriction or no movement in one
direction.

TYPES
 Check valve: A valve that only allows the vacuum signal to move in one
direction. Often used with vacuum reservoirs.
 Delay valve: A vacuum delay valve is a valve with a small orifice, which delays a
vacuum signal. These are commonly used in automobiles to alter the behavior of a
vacuum signal. Delay valves are usually color-coded to their function
SUCTION CUP:
A suction cup, also sometimes known as a sucker is an object that uses negative fluid
pressure of air or water to adhere to nonporous surfaces. They exist both as artificially
created devices, and as anatomical traits of some animals such as octopi and squid.
The working face of the suction cup has a curved surface. When the center of the suction
cup is pressed against a flat, non-porous surface, the volume of the space between the
suction cup and the flat surface is reduced, which causes the fluid between the cup and
the surface to be expelled past the rim of the circular cup.
One cup suction lifter
When the user ceases to apply physical pressure to the centre of the outside of the cup,
the elastic substance of which the cup is made, tends to resume its original, curved shape.
Because all of the fluid has already been forced out of the inside of the cup, the cavity
which tends to develop between the cup and the flat surface has little to no air or water in
it, and therefore lacks pressure. The pressure difference between the atmosphere on the
outside of the cup, and the low-pressure cavity on the inside of the cup, is what keeps the
cup adhered to the surface.

6. PLC
INTRODUCTION:
A programmable logic controller (PLC) or programmable controller is a computerized
for automation of electromechanical processes, such as control of machinery on factory
assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and
machines. Unlike general-purpose computers, the PLC is designed for multiple inputs
and output arrangements, extended temperature ranges, immunity to electrical noise, and
resistance to vibration and impact. Programs to control machine operation are typically
stored in battery-backed or non-volatile memory. A PLC is an example of a hard real
time system since output results must be produced in response to input conditions within
a bounded time, otherwise unintended operation will result.

HISTORY:
The PLC was invented in response to the needs of the American automotive
manufacturing industry. Programmable logic controllers were initially adopted by the
automotive industry where software revision replaced the re-wiring of hard-wired control
panels when production models changed.
Before the PLC, control, sequencing, and safety interlock logic for manufacturing
automobiles was accomplished using hundreds or thousands of relays, cam timers,
and drum sequencers and dedicated closed-loop controllers. The process for updating
such facilities for the yearly model change-over was very time consuming and expensive,
as electricians needed to individually rewire each and every relay.
Digital computers, being general-purpose programmable devices, were soon applied to
control of industrial processes. Early computers required specialist programmers, and
stringent operating environmental control for temperature, cleanliness, and power quality.
Using a general-purpose computer for process control required protecting the computer
from the plant floor conditions. An industrial control computer would have several
attributes: it would tolerate the shop-floor environment, it would support discrete (bit-
form) input and output in an easily extensible manner, it would not require years of
training to use, and it would permit its operation to be monitored. The response time of
any computer system must be fast enough to be useful for control; the required speed
varying according to the nature of the process.
DEVELOPMENT:
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. 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.

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.
Many early PLCs did not have accompanying programming terminals that were capable
of graphical representation of the logic, and so the logic was instead represented as a
series of logic expressions in some version of Boolean format, similar to Boolean
algebra. As programming terminals evolved, it became more common for ladder logic to
be used, for the aforementioned reasons and because it was a familiar format used for
electromechanical control panels. Newer formats such as State Logic and Function Block
(which is similar to the way logic is depicted when using digital integrated logic circuits)
exist, but they are still not as popular as ladder logic. A primary reason for this is that
PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows the
programmer (the person writing the logic) to see any issues with the timing of the logic
sequence more easily than would be possible in other formats.
FUNCTIONALITY:
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. Regarding the practicality of
these desktop computer based logic controllers, it is important to note that they have not
been generally accepted in heavy industry because the desktop computers run on less
stable operating systems than do PLCs, and because the desktop computer hardware is
typically not designed to the same levels of tolerance to temperature, humidity, vibration,
and longevity as the processors used in PLCs. In addition to the hardware limitations of
desktop based logic, operating systems such as Windows do not lend themselves to
deterministic logic execution, with the result that the logic may not always respond to

changes in logic state or input status with the extreme consistency in timing as is
expected from PLCs. Still, such desktop logic applications find use in less critical
situations, such as laboratory automation and use in small facilities where the application
is less demanding and critical, because they are generally much less expensive than
PLCs.
In more recent years, small products called PLRs (programmable logic relays), and also
by similar names, have become more common and accepted. These are very much like
PLCs, and are used in light industry where only a few points of I/O (i.e. a few signals
coming in from the real world and a few going out) are involved, and low cost is desired.
These small devices are typically made in a common physical size and shape by several
manufacturers, and branded by the makers of larger PLCs to fill out their low end product
range. Popular names include PICO Controller, NANO PLC, and other names implying
very small controllers. Most of these have between 8 and 12 digital inputs, 4 and 8 digital
outputs, and up to 2 analog inputs. Size is usually about 4" wide, 3" high, and 3" deep.
Most such devices include a tiny postage stamp sized LCD screen for viewing simplified
ladder logic (only a very small portion of the program being visible at a given time) and
status of I/O points, and typically these screens are accompanied by a 4-way rocker push-
button plus four more separate push-buttons, similar to the key buttons on a VCR remote
control, and used to navigate and edit the logic. Most have a small plug for connecting
via RS-232 or RS-485 to a personal computer so that programmers can use simple
Windows applications for programming instead of being forced to use the tiny LCD and
push-button set for this purpose. Unlike regular PLCs that are usually modular and
greatly expandable, the PLRs are usually not modular or expandable, but their price can
be two orders of magnitude less than a PLC and they still offer robust design and
deterministic execution of the logic.
SCAN TIME:
A PLC program is generally executed repeatedly as long as the controlled system is
running. The status of physical input points is copied to an area of memory accessible to
the processor, sometimes called the "I/O Image Table". The program is then run from its

first instruction run down to the last rung. It takes some time for the processor of the PLC
to evaluate all the ladders and update the I/O image table with the status of outputs. This
scan time may be a few milliseconds for a small program or on a fast processor, but older
PLCs running very large programs could take much longer (say, up to 100 ms) to execute
the program. If the scan time was too long, the response of the PLC to process conditions
would be too slow to be useful.
As PLCs became more advanced, methods were developed to change the sequence of
ladder execution, and subroutines were implemented. This simplified programming and
could also be used to save scan time for high-speed processes; parts of the program used,
for example, only for setting up the machine could be segregated from those parts
required to operate at higher speed.
Special-purpose I/O modules, such as timer modules or counter modules, could be used
where the scan time of the processor was too long to reliably pick up, for example,
counting pulses from a shaft encoder. The relatively slow PLC could still interpret the
counted values to control a machine, but the accumulation of pulses was done by a
dedicated module that was unaffected by the speed of the program execution.
SYSTEM SCALE:
A small PLC will have a fixed number of connections built in for inputs and outputs.
Typically, expansions are available if the base model has insufficient I/O.
Modular PLCs have a chassis (also called a rack) into which are placed modules with
different functions. The processor and selection of I/O modules is customised for the
particular application. Several racks can be administered by a single processor, and may
have thousands of inputs and outputs. A special high speed serial I/O link is used so that
racks can be distributed away from the processor, reducing the wiring costs for large
plants.

PROGRAMMING:
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 program is
stored in the PLC either in battery-backed-up RAM or some other non-volatile flash
memory. Often, a single PLC can be programmed to replace thousands of relays.[4]
Under the IEC 61131-3 standard, PLCs can be programmed using standards-based
programming languages. A graphical programming notation called Sequential Function
Charts is available on certain programmable controllers. Initially most PLCs utilized
Ladder Logic Diagram Programming, a model which emulated electromechanical control
panel devices (such as the contact and coils of relays) which PLCs replaced. This model
remains common today.
IEC 61131-3 currently defines five programming languages for programmable control
systems: function block diagram (FBD), ladder diagram(LD), structured text (ST; similar
to the Pascal programming language), instruction list (IL; similar to assembly language)
and sequential function chart (SFC). These techniques emphasize logical organization of
operations.[4]
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.
USER INTERFACE:
PLCs may need to interact with people for the purpose of configuration, alarm reporting
or everyday control. A human-machine interface (HMI) is employed for this purpose.
HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interface
(GUIs). A simple system may use buttons and lights to interact with the user. Text
displays are available as well as graphical touch screens. More complex systems use

programming and monitoring software installed on a computer, with the PLC connected
via a communication interface.
COMMUNICATION:
PLCs have built in communications ports, usually 9-pin RS-232, but optionally EIA-
485 or Ethernet. Modbus, BAC net or DF1 is usually included as one of
the communications protocols. Other options include various field buses such as Device
Net or Profibus. Other communications protocols that may be used are listed in the List
of automation protocols.
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.
PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between
processors. This allows separate parts of a complex process to have individual control
while allowing the subsystems to co-ordinate over the communication link. These
communication links are also often used for HMI devices such as keypads or PC-type
workstations.

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