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.