automatic control using plc's

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AUTOMATIC CONTROL USING P.L.CS

Incorporation of PLCs into the existing controlsystems.
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ABSTRACT

Control system is the most important part of any industry. For bigger industries like

power plants, steel plants, oil refineries etc. which are growing both in size & complexity over

decades coupled with most thrilling technical advancements in control engineering where many

parameters are to be handled simultaneously its role becomes more significant. The control of

different variables can be efficient done through the recent and more advanced technology called

Programmable Logic Controllers-PLCs.

PLCs are the control hubs for a wide variety of automated systems and processes. It

is a programmable microprocessor-based device that is used in discrete manufacturing to control

assembly lines and machinery on the shop floor as well as many other types of mechanical, electrical

and electronic equipment in a plant. Through PLC the total cost of the project and the time involved

in processing reduces drastically there by increasing the efficiency of the project.

In this paper the focus has been on PLCs, its applicatio ns and mainly on the

INDUSTRIAL AUTOMATION. A simple version of ladder logic has been introduced in the paper,

to understand the PLC programming.

INTRODUCTION:

NEED FOR SYSTEM AUTOMISATION

Any system has to function under specified norms and constraints accordance with a

directed action. As the size and complexity of the system consisting of drives, hoists, generators, auxillaries

, etc increases, the possibility of any contingency cropping up in its functioning also increases. As a result,

it becomes inevitable to have sophisticated system to monitor any discrepancy arising in the functioning of

the system and take predefined action.

These have been major reason for the development of PLC equipments that could

perform the operations, sense them and report back to its internal control center that the desired control

action had been satisfactorily effected . At the same time it is often important to sample the information

such as position of loads and condition of switchgear. In the absence of the logic system, these functions

are performed by a human operator which might become difficult to handle as the complexity of the system

grows .To overcome this problem industrial automisation using PLCs have been developed..

PLC monitors the system in real time mode using the state of art in microprocessor and microcontroller

technologies.


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A PLC system :

Unlike general-purpose computers, the PLC is packaged and designed for

extended temperature ranges, dirty or dusty conditions, immunity to electrical noise, and is

mechanically more rugged and resistant to vibration and impact.. In short a plc Is defined as small

computers, dedicated to automation tasks in an industrial environment".

PLC Operations consist of four steps

1. Input Scan: Scans the state of the Inputs

2. Program Scan: Executes the program logic

3. Output Scan: Energize/de-energize the outputs

4. Housekeeping

PLCs are the control hubs for a wide variety of automated systems and

processes. They contain many "input" terminals, through which it interprets "high" and "low" logical

states from sensors and switches. It also has many output terminals, through which it outputs "high"

and "low" signals to power lights, solenoids, contactors, small motors, and other devices lending

themselves to on/off control. . They are programmable via software interfaced via standard computer

interfaces and proprietary languages and network options.

A Programmable Logic

Controller is an industrial computer

control system used for automation of

real-world processes, such as control of

machinery,usually uses a microprocessor

that continuously monitors the state of

input devices and makes decisions based

upon a custom program that is stored in

battery backed memory or EEPROMs.
http://www.answers.com/topic/automation-3http://www.answers.com/topic/microprocessorhttp://www.answers.com/topic/eepromhttp://www.answers.com/topic/eepromhttp://www.answers.com/topic/microprocessorhttp://www.answers.com/topic/automation-3


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Features : The main difference from other computers are the special input/output arrangements.

These connect the PLC to sensors and actuators. PLCs read limit switches, temperature indicators

and the positions of complex positioning systems. Some even use machine vision. On the actuator

side, PLCs operate electric motors, pneumatic or hydraulic cylinders or diaphragms, magnetic relays

or solenoids. The input/output arrangements may be built into a simple PLC, or the PLC may have

external I/O modules attached to a proprietary computer network that plugs into the PLC.

PLCs were invented as replacements for automated systems that would use hundreds or

thousands of relays and cam timers. Often, a single PLC can be programmed to replace thousands of relays.

PLCs were initially adopted by the automotive manufacturing industry, where software revision replaced the re-

wiring of hard-wired control panels when production models changed.

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. PLCs can be programmed using structured programming languages or a graphical

programming notation called Sequential Function Charts.

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, processingpower 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

Flexibility: This feature provides tools by which an existing system could be tailored according

to the changing taking place. Thus the user can mould the system according to demands thus making it more

flexible.
http://en.wikipedia.org/wiki/Input/outputhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Machine_visionhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Pneumatichttp://en.wikipedia.org/wiki/Hydraulichttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Cam_timerhttp://en.wikipedia.org/wiki/Ladder_logichttp://en.wikipedia.org/wiki/Sequential_function_charthttp://en.wikipedia.org/wiki/Process_controlhttp://en.wikipedia.org/wiki/Distributed_control_systemhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Desktop_computerhttp://en.wikipedia.org/wiki/Desktop_computerhttp://en.wikipedia.org/wiki/Desktop_computerhttp://en.wikipedia.org/wiki/Desktop_computerhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Distributed_control_systemhttp://en.wikipedia.org/wiki/Process_controlhttp://en.wikipedia.org/wiki/Sequential_function_charthttp://en.wikipedia.org/wiki/Ladder_logichttp://en.wikipedia.org/wiki/Cam_timerhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Hydraulichttp://en.wikipedia.org/wiki/Pneumatichttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Machine_visionhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Input/output


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Programming :

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. More recently, 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. 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. Five

programming languages for programmable control systems:

FBD (Function block diagram)

LD (Ladder diagram)

ST (Structured text)

IL (Instruction list)

SFC (Sequential function chart).

LADDER LOGIC

A better understanding of the LADDER LOGIC is explained latter using

MOTOR START-STOP CONTROL example.

Ladder logic is the main

programming method used for

PLCs. As mentioned before, ladder

logic has been developed to mimic

relay logic. By selecting ladder

logic as the main programming

method, the amount of retraining

needed for engineers and

tradespeople was greatly reduced.
http://en.wikipedia.org/wiki/Computer_terminalhttp://en.wikipedia.org/wiki/Ladder_logichttp://en.wikipedia.org/wiki/Function_block_diagramhttp://en.wikipedia.org/wiki/Ladder_logichttp://en.wikipedia.org/wiki/Structured_texthttp://en.wikipedia.org/wiki/Instruction_listhttp://en.wikipedia.org/wiki/Sequential_function_charthttp://en.wikipedia.org/wiki/Sequential_function_charthttp://en.wikipedia.org/wiki/Instruction_listhttp://en.wikipedia.org/wiki/Structured_texthttp://en.wikipedia.org/wiki/Ladder_logichttp://en.wikipedia.org/wiki/Function_block_diagramhttp://en.wikipedia.org/wiki/Ladder_logichttp://en.wikipedia.org/wiki/Computer_terminal


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A AUTOMATED MOTOR START-STOP CONTROL USING PLC :

This section on programmable logic controllers illustrates just a small sample of their capabilities.

The pushbutton switch connected to input X1 serves as the "Start" switch, while the switch

connected to input X2 serves as the "Stop."

1.START BOTTON CLOSED

Another contact in the program, named Y1, uses the output

coil status as a seal-in contact, You can see the normally-

closed contact X2 appear in a colored block, showing that it

is in a closed ("electrically conducting") state.

STEP 1. If we were to press the "Start" button,input X1 would energize, thus "closing" the X1 contact in

the program, sending "power" to the Y1 "coil," energizing

the Y1 output and applying 120 volt AC power to the real

motor contactor coil. The parallel Y1 contact will also

"close," thus latching the "circuit" in an energized state:

STEP.2 Now, if we release the "Start"

pushbutton, the normally-open X1

"contact" will return to its "open"

state, but the motor will continue to

run because the Y1 seal-in "contact"

continues to provide "continuity" to

"power" coil Y1, thus keeping the Y1

output energized:


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2. AFTER RELEASE START BUTTON

3 STOP BUTTON CLOSED

4.BACK TO INITIAL STATE

In addition to input (X) and output (Y) program elements, PLCs provide "internal" coils and

contacts with no intrinsic connection to the outside world. These are used much the same as

"control relays" (CR1, CR2, etc.) are used in standard relay circuits: to provide logic signal

inversion when necessary.

From the inference of above simple example, we can able to conclude that

PROGRAMMABLE LOGIC CONTROLLERS can also be used to control more complex

systems and plays a vital role in INDUSTRIAL AUTOMATION.

STEP 3: To stop the motor, we must

momentarily press the "Stop" pushbutton,

which will energize the X2 input and "open"

the normally-closed "contact," breaking

continuity to the Y1 "coil:"

STEP :4 When the "Stop" pushbutton

is released, input X2 will de-energize,

returning "contact" X2 to its normal,

"closed" state. The motor, however,

will not start again until the "Start"

pushbutton is actuated, because the

"seal-in" of Y1 has been lost:


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PLC compared with other control systems :

PLCs are well-adapted to a certain range of automation tasks. These are typically 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.

For high volume or very simple fixed automation tasks, different techniques are used. A

microcontroller-based design would be appropriate where hundreds or thousands of units will be

produced and so the development cost (design of power supplies and input/output hardware) can be

spread over many sales, and where the end-user would not need to alter the control. Automotive

applications are an example; millions of units are built each year, and very few end-users alter the

programming of these controllers. (However, some specialty vehicles such as transit busses

economically use PLCs instead of custom-designed controls, because the volumes are low and the

development cost would be uneconomic.)

Very complex process control, such as used in the chemical industry, may require algorithms and

performance beyond the capability of even high-performance PLCs. Very high speed controls may

also require customised solutions; for example, aircraft flight controls.

Communications

PLCs usually have builtincommunications ports for at least RS232,

and optionally for RS485 and ethernet.

Modbus is the lowest commondenominator communications protocol.Others are various fieldbuses such as

Profibus.
http://www.answers.com/topic/rs-232http://www.answers.com/topic/rs-485http://www.answers.com/topic/ethernethttp://www.answers.com/topic/modbus-1http://www.answers.com/topic/communications-protocolhttp://www.answers.com/topic/fieldbushttp://www.answers.com/topic/profibus-1http://www.answers.com/topic/profibus-1http://www.answers.com/topic/fieldbushttp://www.answers.com/topic/communications-protocolhttp://www.answers.com/topic/modbus-1http://www.answers.com/topic/ethernethttp://www.answers.com/topic/rs-485http://www.answers.com/topic/rs-232


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