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Syafruddin Hasan

Industrial Motor Control CircuitsComplex industrial machines require

sophisticated control systemsThese control systems schematics are

referred to as motor control circuits even though other types of systems may actually be controlled, such as hydraulic, pneumatic or mechanical systems

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Relay Ladder Logic CircuitsElectrical control circuits are referred to as

relay ladder logic diagramsLadder refers to the manner in which the

diagrams are constructedLogic refers to the decision-making function of

the circuit

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Elements of a Wiring DiagramThe ladder circuit consists of four elements:

A power sourceAn input control deviceA load deviceInterconnecting wires

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Components Used as Input DevicesManual switches

Momentary switchesNormally open

push buttonsNormally-closed

push buttonsMultiple-pole push

buttonPalm operatedFoot switches

Maintained Switches

Selector Switches

Mechanical limit switches

Proximity limit switches

Pressure switchesFlow switchesLevel switchesTemperature switches

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Components Used as Output Devices

Direct DevicesMotorsPilot lights

Indirect DevicesRelaysTime delay relayContactorsolenoid

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Relay ladder Logic CircuitsRelay ladder logic

circuits perform several logical functionsANDORNOTNANDNOR

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Building a Ladder DiagramUsing the water

pumping system as a guide, the following diagram illustrates how various elements are constructed to build a ladder diagram

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Labeling Components in a Ladder Circuit Labeling is important for troubleshooting, diagnosis, and

analysis purposes The example below is a properly labeled circuit

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Motor Control Starter CircuitsThree-phase motor control circuits are

common in industrial applications

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A Programmable Logic Controller (PLC) is defined by Capiel (1982) as: (based on National Electrical Manufacturers Association (NEMA) standard ICS3-1978 Part

ICS3-304):“A digitally operating electronic system designed for use in an industrial

environment, which uses a programmable memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through analog or digital input/output modules,

various types of machines or processes”.

Definition

What Is A PLC?

A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.

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• The first Programmable Logic Controllers were designed and developed by Modicon as a relay re-placer for GM and Landis. • These controllers eliminated the need for rewiring and adding additional hardware for each new configuration of logic. •The new system drastically increased the functionality of the controls while reducing the cabinet space that housed the logic. •The first PLC, model 084, was invented by Dick Morley in 1969 •The first commercial successful PLC, the 184, was introduced in 1973 and was designed by Michael Greenberg.

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1. Easily programmed and reprogrammed, preferably in-plant, to alter its sequence of operations.

2. Easily maintained and repaired—preferably using plug-in modules.

3. More reliable in a plant environment.4. Smaller than its relay equivalent.5. Cost competitive, with solid-state and relay

panels then in use.

Initial specification was provided:

The controller must be:

Basic Stop/Start CircuitThe most basic of all industrial control circuitsDemonstrates two important functions of a

ladder diagram: stopping and interlocking

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Although PLCs are similar to ‘conventional‘ computers in terms of hardware technology, they have specific features suited to industrial control:

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Flexibility.

Implementing Changes and Correcting Errors.

Large Quantities of Contacts.

Lower Cost

Pilot Running.

Visual Observation.

Speed of Operation.

Ladder or Boolean Programming Method.

Reliability and Maintainability.

Simplicity of Ordering Control System Components.

Documentation.

Security.

Ease of Changes by Reprogramming.

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Newer Technology.

Fixed Program Applications.

Environmental Considerations.

Fail-Save Operation.

Fixed-Circuit Operation.

Introduction to PLC ComponentsAll PLC systems are comprised of the

same basic building blocksRack assemblyPower supplyProgramming unitInput/Output SectionProcessor unit

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What Is Inside A PLC?

The Central Processing Unit, the CPU, contains an internal program that tells the PLC how to perform the following functions:•Execute the Control Instructions contained in the User's Programs. This program is stored in "nonvolatile" memory, meaning that the program will not be lost if power is removed •Communicate with other devices, which can include I/O Devices, Programming Devices, Networks, and even other PLCs. •Perform Housekeeping activities such as Communications, Internal Diagnostics, etc.

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The rack assembly contains the power supply, I/O modules, processor unit and is built upon a back plane which provides the various connections

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The power supply provides the voltages necessary to operate the circuitry

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PLC Programming UnitsSeveral types of

programming units are available for PLCsHandheld

programmersDedicated terminalsMicrocomputers

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Input/Output Sections

The purpose of the I/O section is to provide an interface for the internal circuitry to the outside world

Input/Output modules serve four functions:TerminationSignal conditioningIsolationIndication

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Input modules receive signals from switching devices

The input module provides the necessary conversion to logic voltages

Three common types of input signals are:120 VACLow-level DCHigh-level DC

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The input module performs 4 tasks electronically:

It senses the presence or absence of an input signal at each of its input terminal. The input signal tells what switch, sensor, or other signal is on or off in the process being controlled It converts the input signal for on, or high, to a dc level usable by the module’s electronic circuit. For a low, or off, input signal, no signal is converted, indicating off. . The input module carries out electronic isolation by electronically isolating the input module output from its input. Its electronic circuit must produce an output to be sensed by the PLC CPU.

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A typical input module has 4, 6, 8, 12, 16 or 32 terminals plus common and safety ground terminals

Outgoing discrete signals are transmitted to field devices using output modules

Typical output modules are:120 VACLow-level DCHigh-level DC

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The output module operates in the opposite manner from the input module.

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PLC I/O module

Processor UnitThe processor unit

coordinates and controls the operation of the PLC

The PLC processor is composed of three main sectionsThe central processing

unitThe arithmetic logic

unitMemory

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There are four basic steps in the operation of all PLCs; Input Scan, Program Scan, Output Scan, and Housekeeping. These steps continually take place in a repeating loop.

How Does A PLC Operate?

Four Steps in the PLC Operations

1.) Input Scan Detects the state of all input

devices that are connected to the PLC

2.) Program Scan Executes the user

created program logic

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3.) Output Scan Energizes or de-energize all output

devices that are connected to the PLC.

4.) Housekeeping This step includes communications

with programming terminals, internal diagnostics, etc...

These steps are continually processed in a loop.

Processor FilesThe processor

contains several types of files to support the program and logic being used

In an Allen-Bradley system there are four types of program files and 11 types of data files

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To address data files, alphanumeric characters separated by delimiters are used

The type of delimiter specifies the kind of information that follows:A number that follows a colon represents

which element will be usedA number that follows a slash represents

the bit is used in a fileA number that follows a period

represents which bit is used within a word

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Address Identifiers

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Word Address

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T4:7.ACC

File Type

FileNumber

Element Delimiter

Word delimiter

Word

Element Address

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N7:12

File Type

FileNumber

Element Delimiter

Element

Input Output AddressInput addresses

are formatted as: I:e/b

I = identifier as Input: = slot delimitere = slot number of

module/ = bit or terminal

delimiterb = terminal number

of the module

Output addresses are formatted as: O:e/b

O = identifier as Input: = slot delimitere = slot number of

module/ = bit or terminal

delimiterb = terminal number

of the module

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Relationship of Data File Addresses to I/O Modules

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What Do I Need To Consider When Choosing A PLC? There are many PLC systems on the market today. Other than cost, you must consider the following when deciding which one will best suit the needs of your application. •Will the system be powered by AC or DC voltage? •Does the PLC have enough memory to run my user program? •Does the system run fast enough to meet my application’s requirements? •What type of software is used to program the PLC? •Will the PLC be able to manage the number of inputs and outputs that my application requires? •If required by your application, can the PLC handle analog inputs and outputs, or maybe a combination of both analog and discrete inputs and outputs? •How am I going to communicate with my PLC? •Do I need network connectivity and can it be added to my PLC? •Will the system be located in one place or spread out over a large area?