Programmable Logic Controller (PLC)

Ahmad Shukri

Objective This training introduces the basic hardware and software

components of a Programmable Controller (PLC). It details the architecture and basic instruction set common to all PLC’s. Basic programming techniques and logic designs are covered. This training describes the operating features of the PLC, the advantages of the PLC over hard-wired control systems, practical applications, troubleshooting and maintenance of PLC’s.

IntroductionPLC according to IEC -1131 purposes

Electronic digital systems, designed for use by industry. Using a programmable memory for storing instructions, In

order to do work such as logic, sequencing, timing, counting and Arithmetic

Allows control over the digital or analoge inputs and outputs, various types of machines or proses.

Combined with the PC to enable the combination between industrial control easily done.

International

Electrotechnical

Commission

Advantages of PLC Less wiring. Wiring between devices and relay contacts are done in

the PLC program. Easier and faster to make changes. Trouble shooting aids make programming easier and

reduce downtime. Reliable components make these likely to operate for

years before failure.

Industrial Evolution Before the industrial revolution began a process that is

done with hand, for example filling, labeling confusion and packaging

Industrial Evolution At the beginning of this century, the whole process is done

automatically as filling, capping, packing and labelling. It was carried out using an electro-mechanical control.

Industrial Evolution When the computer was first introduced, there is a new

control system PLC. Introduced that will be more efficient and more flexible than

using relay control system

Type Of PLCCompact

Fixed number of input and output Input module, output module and control unit in a house

Type Of PLCModular

Module installed separately from the CPU, input and output is mounted on a rack (back plane) and can be substituted with a specific module if needed.

Module are connected to each other through the System Bus

PLC Parts PLC generally having five parts, namely: -

i) Input Sectionii) Output Sectioniii) Central Processing Unit-CPUiv) Memoryv) Programming Tools

Input Section The ability of a PLC is in accordance with the existing

number of input points signals from switches or sensors are channeled through the

input point.

Ouput Section Output modules are also important for the ability of a PLC

Central Processing Unit (CPU)

CPU is divided into three main sections Processor, Power Supply and Memory

Input and Output Device Computer System Automation System

Programming Programming equipment.

Programming data to be stored in memory to be written and incorporated into the PLC. There are many ways to do these things such as:

a) Through-PC Personal Computer        b) Hand-Held Console Programming.

             

Communication Computer to PLC PLC to PLC PLC to Touch panel

Selecting a PLCCriteria

Number of logical inputs and outputs. Memory Number of special I/O modules Scan Time Communications Software

The 5 Languages of IEC 61131-3

Ladder Diagram (LD)

Form of electric circuit diagram in which there is a switch and coil symbols.

Ladder diagrams are very popular in USA and Japan. Very suitable to be used for easy connection.

The 5 Languages of IEC 61131-3

Instruction List (IL)

Single Accumulator based execution model Based upon the German ‘Anweisungsliste’, AWL Only one operation such as storing a value in the accumulator register, is allowed per line

The 5 Languages of IEC 61131-3

Structured Text (ST)

High level language, block structured Syntax resembles PASCAL Complex statements and nested instructions possible

support for example: Iteration loops (REPEAT-UNTIL; WHILE-DO) Conditional execution (IF-THEN-ELSE; CASE) Functions (SQRT(), SIN())

The 5 Languages of IEC 61131-3

Function Block Diagram (FBD)

Graphical language, widely used in Europe Allows program elements which appear as blocks to be

"wired" together in a form analogous to a circuit diagram Used in many applications that involve the flow of

information or data between control components

The 5 Languages of IEC 61131-3

Sequential Function Chart (SFC)

Graphical language which provides a diagrammatic representation of program sequences --> flowchart

Based on the French Grafcet (IEC 848) Main structure and suitable for rapid diagnostics The basic elements are steps with action blocks and transitions Steps consist of a piece of program that is

carried out until a condition specified in the transition is met

Programming of complex tasks by dividing in smaller parts

Each element can be programmed in anyIEC-language