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Description

This chapter 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 unit 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.

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Objectives

At the end of this chapter we should be able to:

Describe the major components of a common PLC. Interpret PLC specifications. Apply troubleshooting techniques. Convert conventional relay logic to a PLC language. Operate and program a PLC for a given application.

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Contents

History of Programmable Controllers Relay Ladder Logic Central Processing Unit Input/output System Programming and Peripheral Devices Programming Concepts Applications Troubleshooting and Maintenance

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Advantages of PLCs

1. Less wiring.2. Wiring between devices and relay contacts are done

in the PLC program.3. Easier and faster to make changes.4. Trouble shooting aids make programming easier and

reduce downtime.5. Reliable components make these likely to operate for

years before failure.

INTRODUCTION TO PLCS

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PLC Origin

- Developed to replace relays in the late 1960s

- Costs dropped and became popular by 1980s

- Now used in many industrial designs

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Historical BackgroundThe Hydramatic Division of the General Motors Corporation

specified the design criteria for the first programmable controller in 1968.

Their primary goal was to eliminate the high costs associated with inflexible, relay-controlled systems.

The controller had to be designed in modular form, so that sub-assemblies could be removed easily for replacement or repair.

The control system needed the capability to pass data collection to a central system.

The system had to be reusable.

The method used to program the controller had to be simple, so that it could be easily understood by plant personnel.

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Programmable Controller Development

1968 Programmable concept developed1969 Hardware CPU controller, with logic instructions, 1 K of memory and 128 I/O points 1974 Use of several (multi) processors within a PLC - timers and counters; arithmetic operations; 12 K of memory and 1024 I/O points1976 Remote input/output systems introduced1977 Microprocessors - based PLC introduced

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Programmable Controller Development

1980 Intelligent I/O modules developedEnhanced communications facilitiesEnhanced software features

(e.g. documentation)Use of personal microcomputers asprogramming aids

1983 Low - cost small PLC’s introduced1985 on Networking of all levels of PLC, computer

and machine using SCADA software.

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Comparison between PC and PLC The main difference from other computers is that PLCs are armored for

severe conditions (dust, moisture, heat, cold, etc) and have the facility for extensive input/output (I/O) arrangements.

Advantages over PC :1. Cost effective for controlling complex systems.2. Flexible and can be reapplied to control other systems quickly and

easily.3. Computational abilities allow more sophisticated control.4. Trouble shooting aids make programming easier and reduce downtime.5. Reliable components make these likely to operate for years before

failure.Disadvantages over PC :6. Too much work required in connecting wires.7. Difficulty with changes or replacements.8. Difficulty in finding errors; requiring skillful work force.

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Programmable Logic Controllers( Definition according to NEMA standard ICS3-1978)

A digitally operating electronic apparatus which uses a programming memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through digital or analog modules, various types of machines or process.

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Programmable Logic Controller: Definition

Definition: “small computers, dedicated to automation tasks in an industrial environment"

cabled relay control (hence 'logic'), analog (pneumatic, hydraulic) “governors”

real-time (embedded) computer with extensive input/output

Function: Measure, Control, Protect

Formerly:

Today:

Distinguish Instrumentation

flow meter, temperature, position,…. but also actors (pump, …)Control

programmable logic controllers with digital peripherals & field bus

Visualization

HMI* in PLCs (when it exists) is limited to service help andcontrol of operator displays

*Human Machine Interface

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Simple PLC

networkbinary inputs

binary outputs

analog inputs / outputs

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PLC in a cabinetCPU1

redundant field bus connection

CPU2

inputs/outputs

serial connections

PLC in a cabinet

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example: turbine control (in the test lab)

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PLC: functions

Measure

Control (Command and Regulation)

•

•

• Event Logging

• Communication

• Human interface

Protection•

PLC = PMC: Protection, Measurement and Control

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PLC: Characteristics

• large number of peripherals: 20..100 I/O per CPU, high density of wiring, easy assembly.

• binary and analog Input/output with standard levels

• operate under harsh conditions, require robust construction, protection against dirt, water and mechanical threats, electro-magnetic noise, vibration, extreme temperature range (-30C..85C), sometimes directly located in the field.

• programming: either very primitive with hand-held terminals on the target machine itself, or with a lap-top able to down-load programs.

• network connection allows programming on workstations and connection to SCADA

• primitive Human-Machine-Interface for maintenance, either through LCD-display or connection of a laptop over serial lines (RS232) or wireless.

• economical - €1000.- .. €15'000.- for a full crate.

• the value is in the application software (licenses €20'000 ..€50'000)

• field bus connection for remote I/Os

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PLC: Location in the control architectureEnterprise Network

Control Bus(e.g. Ethernet)

Engineerstation

I/O

I/O

I/O

I/O

CP

U

Sensor Bus (e.g. ASI)

Field Bus

gateway

Field Stations

Control Station

with Field Bus

direct I/O

I/O

Field DevicesFBgateway

gateway

I/O

I/O

I/O

I/O

CP

U

CO

M

I/O

I/O

I/O

CO

M

CP

U

CO

M

CO

M

CO

M

I/O

Field Bus

CP

U

CO

M 2

I/O

I/O

I/O

CP

U

CO

M1

CO

M 2

I/OCP

U

Operatorstation

largePLCs

small PLC

PLCPLC

CO

M1

CO

M1

SupervisorStation

data concentrators,not programmable,

but configurable

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Why 24V / 48 V supply ?

… After the plant lost electric power, operators could read instruments only by plugging in temporary batteries…[IEEE Spectrum Nov 2011 about Fukushima]

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Leading Brands Of PLC

AMERICAN 1. Allen Bradley2. Gould Modicon3. Texas Instruments 4. General Electric5. Westinghouse6. Cutter Hammer7. Square D

EUROPEAN 1. Siemens2. Klockner & Mouller3. Festo 4. Telemechanique

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Leading Brands Of PLC

JAPANESE 1. Toshiba2. Omron3. Fanuc4. Mitsubishi

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Areas of Application

Manufacturing / Machining

Food / Beverage

Metals

Power

Mining

Petrochemical / Chemical

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PLC Size

1. SMALL - it covers units with up to 128 I/O’s and memories up to 2 Kbytes.

- these PLC’s are capable of providing simple to advance levels or machine controls.

2. MEDIUM - have up to 2048 I/O’s and memories up to 32 Kbytes.

3. LARGE - the most sophisticated units of the PLC family. They have up to 8192 I/O’s and memories up to 750 Kbytes.

- can control individual production processes or entire plant.

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Types of PLCMonolithic constructionMonoprocessorFieldbus connection

Fixed casing

Fixed number of I/O (most of them binary)

No process computer capabilities (no MMC)

Typical product: Mitsubishi MELSEC F, ABB AC31, SIMATIC S7

(1)

Modular construction (backplane)One- or multiprocessor systemFieldbus and LAN connection

3U or 6U rack, sometimes DIN-rail

Large variety of input/output boards

Connection to serial bus

Small MMC function possible

Typical products: SIMATIC S5-115, Hitachi H-Serie, ABB AC110

(2)

Compact

Modular PLC

(3) Soft-PLCWindows NT or CE-based automation productsDirect use of CPU or co-processors

Basic PLC Compact PLC

Monolithic (one-piece) constructionFixed casingFixed number of I/O (most of them binary)No process computer capabilities (no MMC)Can be extended and networked by an extension (field) busSometimes LAN connection (Ethernet, Arcnet)Monoprocessor

Typical product: Mitsubishi MELSEC F, ABB AC31, SIMATIC S7

costs: € 2000

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courtesy ABB

Modular PLC

RS232

CPU CPU Analog I/O Binary I/O

backplaneparallel bus

• housed in a 19" (42 cm) rack (height 6U ( = 233 mm) or 3U (=100mm)

• concentration of a large number of I/O

Power Supply

• high processing power (several CPU)

• primitive or no HMI

• cost effective if the rack can be filled

• tailored to the needs of an application

• supply 115-230V~ , 24V= or 48V= (redundant)

fieldbus

LAN

• large choice of I/O boards

• interface boards to field busses

• requires marshalling of signals

fieldbus

developmentenvironment

• cost ~ €10’000 for a filled crate

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Small modular PLC

mounted on DIN-rail, 24V supplycheaper (€5000) not water-proof, no ventilatorextensible by a parallel bus (flat cable or rail)

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Soft-PLC (PC as PLC)• PC as engineering workstation• PC as human interface (Visual Basic, Intellution, Wonderware)• PC as real-time processor (Soft-PLC)• PC assisted by a Co-Processor (ISA- or PC104 board)• PC as field bus gateway to a distributed I/O system

2122

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234

I/O modules

Basic PLC PLC evolution

A

B

P2

P1

I1

Analog WorldBinary World

C

continuous processes

Regulation, controllers

discrete processes

combinatorial sequential

relay controls, Relay controlpneumatic sequencer

Pneumatic and electromechanical controllers

Programmable Logic Controllers

Basic PLC Continuous Plant Example: traction motors, ovens, pressure vessel,...

The time constant of the control system must be at least one order of magnitude smaller than the smallest time constant of the plant.

F(s) = yx

The state of continuous plants is described by continuous (analog) statevariables like temperature, voltage, speed, etc.

Continuous plants are normally reversible and monotone. This is the condition to allow their regulation.

There exist a fixed relationship between input and output , described by a continuous model in form of a transfer function F.

This transfer function can be expressed by a set of differential equations. If equations are linear, the transfer function may expressed as Laplace or Z-transform.

time

y

(1+Ts)

(1+T1s + T2 s2)

the principal task of the control system for a continuous plant is its regulation.

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Discrete Plant

Examples: Elevators, traffic signaling, warehouses, etc.

The plant is described by variables which take well-defined, non-overlapping values.The transition from one state to another is abrupt, it is caused by an external event.

Discrete plants are normally reversible, but not monotone, i.e. negating the event which caused a transition will not revert the plant to the previous state.

Example: an elevator doesn't return to the previous floor when the button is released.

Discrete plants are described e.g. by finite state machines or Petri nets.

the main task of a control system with discrete plants is its sequential control.

ec + ¬d

1

2 3

6 5

4

7

a

bc + d

e

init

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Example of Discrete state system:Tank Used to Mix Two Liquids

A

B

C

FS

MOTOR

TIMER

FLOAT SWITCH

SOLENOIDS

SOLENOID

1 -MINUTE

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Tank Used to Mix Two LiquidsA tank is used to mix two liquids. The control circuit operates as follows:

1. When the start button is pressed, solenoids A and B energize. This permits the two liquids to begin filling the tank.

2. When the tank is filled, the float switch trips. This de-energizes solenoids A and B and starts the motor used to mix the liquids together.

3. The motor is permitted to run for one minute. After one minute has elapsed, the motor turns off and solenoid C energizes to drain the tank.

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4. When the tank is empty, the float switch de-energizes solenoid C.

5. A stop button can be used to stop the process at any point.

6. If the motor becomes overloaded, the action of the entire circuit will stop.

7. Once the circuit has been energized it will continue to operate until it is manually stopped.

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Tank Used to Mix Two Liquids

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Block Diagram of PLC

PROCESSOR CPU ALU Memory

POWERSUPPLY

I MN O P D U UT L E

O M U OT DP UU LT E

PROGRAMMING DEVICE

From SENSORS

Pushbuttons,contacts,limit switches,etc.

ToOUTPUTSolenoids, contactors, alarmsetc.

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Major Components of a Common PLC

POWER SUPPLY

Provides the voltage needed to run the primary PLC components

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Major Components of a Common PLC

I/O MODULES

Provides signal conversion and isolation between the internal logic level signals inside the PLC and the field’s high level signal. Performs the following functions:1. Termination: Provides terminals for connection to field devices2. Isolation: Provides isolation between the high power field

devices and low power controller generally by making use of optical couplers.

3. Signal Conditioning: Performs signal conversion like A to D or D to A.

4. Indication: Makes use of low power LEDs to indicate the absence (low state) or presence (high state) of the field devices.

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Major Components of a Common PLCPROCESSOR Provides intelligence to command and govern the activities of the entire PLC systems. Consists of :1. CPU2. ALU3. Memory: RAM, ROM and Files ( Program and Data)

PROGRAMMING DEVICE External device used to enter the desired program that will determine the sequence of operation and control of process equipment or driven machine. Different types of programming devices used with PLC are:4. Hand held terminals5. Dedicated terminals6. Microcomputer (PC)

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Programming Device

Also known as:

Industrial Terminal ( Allen Bradley )

Program Development Terminal ( General Electric )

Programming Panel ( Gould Modicon )

Programmer ( Square D )

Program Loader ( Idec-Izumi )

Programming Console ( Keyence / Omron )

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Programming Device1. Hand held unit with LED / LCD display : It is a small self contained

unit in which the ladder diagram is displayed one rung at a time in a special liquid crystal display. The user can enter a program, perform diagnostic tests, run the program through the programmable controller and perform editing of the installed program. The installed program is stored in a temporary memory that will be lost without ac power or battery back up. The program can be permanently burned into a ROM for final installation.

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2. Dedicated terminals: These are used with only one type and make of PLC and is used when programming has to be done in mass for the same type of the controller.

3. Microcomputer (PC) : It is able to display many rungs of the ladder Diagram. The advantage of PC is that it can be used for programming Different makes of PLC by running their respective loaded software and when not on the network can be used for other applications such as design or accounting.

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I/O Module The I/O interface section of a PLC connects it to external field

devices.

The main purpose of the I/O interface is to condition the various signals received from or sent to the external input and output devices.

Input modules converts signals from discrete or analog input devices to logic levels acceptable to PLC’s processor.

Output modules converts signal from the processor to levels capable of driving the connected discrete or analog output devices.

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Classification of I/O Module

1) Serial I/O

2) Parallel I/O

3) Discrete I/O : AC , DC Discrete I/O

4) Analog I/O : Analog Input (0-5 V,0-10 V,1-5 V,4-20 mA) Analog Output (4-20 mA, 0-10 V)

5) Special purpose I/O : ASCII communication module, Stepper motor module, Thermocouple module, Bar code module, Vision system module, PID Controller module.

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Discrete Input ModuleMost common input interface used with PLCs.

DISCRETE DC INPUT MODULE

OPTO-ISOLATOR

IS NEEDED TO:· Prevent voltage

transients from damaging the processor.

· Helps reduce the effects of electrical noise

CurrentLimitingResistor

FROM INPUTDEVICE

USE TO DROP THE VOLTAGE TO LOGIC LEVEL

Signal ConditioningBuffer, Filter, hysteresis

Circuits

TOPROCESSOR

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DISCRETE AC INPUT MODULE

OPTO-ISOLATOR

IS NEEDED TO:· Prevent voltage

transients from damaging the processor.

· Helps reduce the effects of electrical noise

Rectifier,ResistorNetwork

FROM INPUTDEVICE

CONVERTS THE AC INPUT TO DC AND DROPS THE VOLTAGE TO LOGIC LEVEL

Signal ConditioningBuffer, Filter, hysteresis

Circuits

TO PROCESSOR

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DISCRETE OUTPUT MODULEThey are most widely used and simply act as switches to control output field devices.

DISCRETE DC OUTPUT MODULE

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DISCRETE AC OUTPUT MODULE

OPTO-ISOLATOR

IS NEEDED TO:· Prevent voltage

transients from damaging the processor.

· Helps reduce the effects of electrical noise

FROM PROCESSOR

TTLCircuits

AmplifierRELAYTRIACX’SISTOR

TOOUTPUTDEVICE

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Operation flow of an Output Module

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Analog Input Module It is used to interface a PLC to analog input signals. The module converts analog input signals to 16-bit binary values

for storage in processor’s input image table. It accepts signals 0 to 10 V DC,-10 V to +10 V DC, 1 to 5 V DC, 4 to 20 mA, 0 to 20 mA, -20 to +20 mA.

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Analog Output Module It accepts a 16 bit output status word which they convert into an

analog value through digital to analog converter.

Typical analog signals 0 to 10 V DC,-10 V to +10 V DC, 1 to 5 V DC, 4 to 20 mA, 0 to 20 mA, -20 to +20 mA.

Analog output modules are selected to send out either a varying

current or voltage signal.

For Ex: If the speed of the DC motor is to be varied over a range of say 1000-3000 rpm, the voltage of an output module of range 0 – 1 V DC will represent a specific speed over the range.

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General PLC architecture

CPUReal-Time

Clockflash

EPROMROM

buffers

signal conditioning

power amplifiers relays

signalconditioning

serial portcontroller

Ethernet

parallel bus

ethernetcontroller

RS 232

analog-digital

converters

digital-analog

convertersDigital Output

DigitalInput

field buscontroller

externalI/Os

extensionbus

field busdirect Inputs and Outputs

Basic PLCThe signal chain within a PLC

analogvariable

(e.g. 4..20mA)

filtering&

scaling

analog-digital

converter

processing

digital-analog

converter

analogvariable

e.g. -10V..10V

time

y

time

y(i)

sampling

binaryvariable

(e.g. 0..24V)filtering sampling

time

y

transistoror

relay

binaryvariable

amplifier011011001111

counter

1

non-volatilememory

0001111

time

y(i)

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Example: Signal chain in a protection device

A/D CPUU/I

Trip

Digitalfilter

Sample and holdA/D conversion

Inputtransformer

Anti aliasing

filterProtectionalgorithm

Outputdriver

f = 1 MHz

f = 200 kHz

f = 100 kHz

f = 300 -1200 Hz

reaction < 10 ms

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I/O Circuits

DIFFERENT TYPES OF I/O CIRCUITS

1. Pilot Duty OutputsOutputs of this type typically are used to drive high-current

electromagnetic loads such as solenoids, relays, valves, and motor starters.

These loads are highly inductive and exhibit a large inrush current.

Pilot duty outputs should be capable of withstanding an inrush current of 10 times the rated load for a short period of time without failure.

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I/O Circuits

2. General - Purpose OutputsThese are usually low- voltage and low-current and are used to drive indicating lights and other non-inductive loads. Noise suppression may or may not be included on this types of modules.

3. Discrete InputsCircuits of this type are used to sense the status of limit switches, push buttons, and other discrete sensors. Noise suppression is of great importance in preventing false indication of inputs turning on or off because of noise.

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I/O Circuits4. Analog I/O

Circuits of this type sense or drive analog signals.

Analog inputs come from devices, such as thermocouples, strain gages, or pressure sensors, that provide a signal voltage or current that is derived from the process variable.

Standard Analog Input signals: 4-20mA; 0-10V

Analog outputs can be used to drive devices such as voltmeters, X-Y recorders, servomotor drives, and valves through the use of transducers.

Standard Analog Output signals: 4-20mA; 0-5V; 0-10V

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I/O Circuits5. Special - Purpose I/O

Circuits of this type are used to interface PLCs to very specific types of circuits such as servomotors, stepping motors PID (proportional plus integral plus derivative) loops, high-speed pulse counting, resolver and decoder inputs, multiplexed displays, and keyboards.

This module allows for limited access to timer and counter presets and other PLC variables without requiring a program loader.

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PLC

INPUTS

OUTPUTS

MOTOR

LAMPCONTACTOR

PUSHBUTTONS

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L1 L2

P. B SWITCH

INPUT MODULE

WIRING DIAGRAM

LADDER PROGRAM

I:2

0

I= Input

Module/ Elementslot # in rack

ModuleTerminal #/ Bit

Allen-Bradley 1746-1A16

Address I:2.0/0

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N.O

C

L2 L1

L1 L2

OUTPUT MODULEWIRING

MOTOR

CONTACTOR

O:4

0CONTACTOR

LADDER PROGRAM

L1 L2

FIELD WIRING

• SOLENOID

• VALVES• LAMP• BUZZER

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Discrete Input A discrete input also referred as digital input is an input that is

either ON or OFF are connected to the PLC digital input. In the ON condition it is referred to as logic 1 or a logic high and in the OFF condition maybe referred to as logic o or logic low.

Normally Open Pushbutton

Normally Closed Pushbutton

Normally Open switch

Normally Closed switch

Normally Open contact

Normally closed contact

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OFFLogic 0

IN

PLC

InputModule

24 V dc

OFFLogic 1

IN

PLC

InputModule

24 V dc

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IN

PLCAnalogInputModule

Tank

Level Transmitter

An analog input is an input signal that has a continuoussignal. Typical inputs may vary from 0 to 20mA, 4 to 20mAor 0 to10V. Below, a level transmitter monitors the level of liquid in the tank. Depending on the level Tx, the signal to thePLC can either increase or decrease as the level increases or decreases.

Analog Input

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OUT

PLC

Digital OutputModule

Lamp

A discrete output is either in an ON or OFF condition. Solenoids, contactors coils, lamps are example of devices connected to the Discrete or digital outputs. Below, the lamp can be turned ON or OFF by the PLC output it is connected to.

Digital Output

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OUT

PLC

AnalogOutputModule

An analog output is an output signal that has a continuoussignal. Typical outputs may vary from 0 to 20mA, 4 to 20mAor 0 to10V.

Analog Output

EP

Pneumatic control valve

Supply air

Electric to pneumatic transducer

0 to 10V

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ProcessorThe processor module contains the PLC’s microprocessor,

its supporting circuitry, and its memory system.

The main function of the microprocessor is to analyze data coming from field sensors through input modules, make decisions based on the user’s defined control program and return signal back through output modules to the field devices. Field sensors: switches, flow, level, pressure, temp. transmitters, etc. Field output devices: motors, valves, solenoids, lamps, or audible devices.

The memory system in the processor module has two parts: a system memory and an application memory.

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Memory Map Organization

SYSTEM

• System memory includes an area called the EXECUTIVE, composed of permanently-stored programs that direct all system activities, such as execution of the users control program, communication with peripheral devices, and other system activities.

• The system memory also contains the routines that implement the PLC’s instruction set, which is composed of specific control functions such as logic, sequencing, timing, counting, and arithmetic.

• System memory is generally built from read-only memory devices.

APPLICATION• The application memory is divided into the data table area and

user program area.• The data table stores any data associated with the user’s

control program, such as system input and output status data, and any stored constants, variables, or preset values. The data table is where data is monitored, manipulated, and changed for control purposes.

• The user program area is where the programmed instructions entered by the user are stored as an application control program.

• Data Table• User Program

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Memory Designs

VOLATILE. A volatile memory is one that loses its stored information

when power is removed.

Even momentary losses of power will erase any information stored or programmed on a volatile memory chip.

Common Type of Volatile Memory

RAM. Random Access Memory(Read/Write) Read/write indicates that the information stored in the

memory can be retrieved or read, while write indicates that the user can program or write information into the memory.

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Memory Designs

The words random access refer to the ability of any location (address) in the memory to be accessed or used. Ram memory is used for both the user memory (ladder diagrams) and storage memory in many PLC’s.

RAM memory must have battery backup to retain or protect the stored program.

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Memory DesignsSeveral Types of RAM Memory:

1.MOS 2.HMOS 3.CMOS

The CMOS-RAM (Complimentary Metal Oxide Semiconductor) is probably one of the most popular. CMOS-RAM is popular because it has a very low current drain when not being accessed (15microamps.), and the information stored in memory can be retained by as little as 2Vdc.

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Memory Designs

NON-VOLATILEHas the ability to retain stored information when power is

removed, accidentally or intentionally. These memories do not require battery back-up.

Common Type of Non-Volatile Memory

ROM, Read Only MemoryRead only indicates that the information stored in memory

can be read only and cannot be changed. Information in ROM is placed there by the manufacturer for the internal use and operation of the PLC.

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Memory DesignsOther Types of Non-Volatile Memory

PROM, Programmable Read Only MemoryAllows initial and/or additional information to be written into

the chip.

PROM may be written into only once after being received from the PLC manufacturer; programming is accomplish by pulses of current.

The current melts the fusible links in the device, preventing it from being reprogrammed. This type of memory is used to prevent unauthorized program changes.

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Memory DesignsEPROM, Erasable Programmable Read Only Memory

Ideally suited when program storage is to be semi-permanent or additional security is needed to prevent unauthorized program changes.

The EPROM chip has a quartz window over a silicon material that contains the electronic integrated circuits. This window normally is covered by an opaque material, but when the opaque material is removed and the circuitry exposed to ultra violet light, the memory content can be erased.

The EPROM chip is also referred to as UVPROM.

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Memory Designs

EEPROM, Electrically Erasable Programmable Read Only Memory

Also referred to as E2PROM, is a chip that can be programmed using a standard programming device and can be erased by the proper signal being applied to the erase pin.

EEPROM is used primarily as a non-volatile backup for the normal RAM memory. If the program in RAM is lost or erased, a copy of the program stored on an EEPROM chip can be down loaded into the RAM.

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PLC OperationBasic Function of a Typical PLC

Read all field input devices via the input interfaces, execute the user program stored in application memory, then, based on whatever control scheme has been programmed by the user, turn the field output devices on or off, or perform whatever control is necessary for the process application.

This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.

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While the PLC is running, the scanning process includes the following four phases, which are repeated continuously as individual cycles of operation:

PHASE 2Program

Execution

PHASE 3Diagnostics/

Comm

PHASE 4Output

Scan

PHASE 1Read Inputs

Scan

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PHASE 1 – Input Status scan

· A PLC scan cycle begins with the CPU reading the status of its inputs.

PHASE 2– Logic Solve/Program Execution

· The application program is executed using the status of the inputs.

PHASE 3– Logic Solve/Program Execution · Once the program is executed, the CPU performs

diagnostics and communication tasks.

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PHASE 4 - Output Status Scan

• An output status scan is then performed, whereby the stored output values are sent to actuators and other field output devices. The cycle ends by updating the outputs.

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As soon as Phase 4 are completed, the entire cycle begins again with Phase 1 input scan.

The time it takes to implement a scan cycle is called SCAN TIME. The scan time composed of the program scan time, which is the

time required for solving the control program, and the I/O update time, or time required to read inputs and update outputs.

The program scan time generally depends on the amount of memory taken by the control program, type of instructions used in the program and the clock frequency of the processor. The time to make a single scan can vary from 1 ms to 100 ms.

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PLC CommunicationsCommon Uses of PLC Communications Ports

Changing resident PLC programs - uploading/downloading from a supervisory controller (Laptop or desktop computer).

Forcing I/O points and memory elements from a remote

terminal. Linking a PLC into a control hierarchy containing several

sizes of PLC and computer.

Monitoring data and alarms, etc. via printers or Operator Interface Units (OIUs).

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PLC CommunicationsSerial Communications

PLC communications facilities normally provides serial transmission of information.

Common Standards

RS 232

Used in short-distance computer communications, with the majority of computer hardware and peripherals.

Has a maximum effective distance of approx. 30 m at 9600 baud.

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PLC CommunicationsLocal Area Network (LAN)

Local Area Network provides a physical link between all devices plus providing overall data exchange management or protocol, ensuring that each device can “talk” to other machines and understand data received from them.

LANs provide the common, high-speed data communications bus which interconnects any or all devices within the local area.

LANs are commonly used in business applications to allow several users to share costly software packages and peripheral equipment such as printers and hard disk storage.

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PLC CommunicationsRS 422 / RS 485

Used for longer-distance links, often between several PCs in a distributed system. RS 485 can have a maximum distance of about 1000 meters.

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PLC CommunicationsProgrammable Controllers and Networks

Dedicated Network System of Different Manufacturers

Manufacturer Network

Allen-Bradley Data Highway

Gould Modicon Modbus

General Electric GE Net Factory LAN

Mitsubishi Melsec-NET

Square D SY/NET

Texas Instruments TIWAY

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SpecificationsSeveral factors are used for evaluating the quality and

performance of programmable controllers when selecting a unit for a particular application. These are listed below.

NUMBER OF I /O PORTS

This specifies the number of I/O devices that can be connected to the controller. There should be sufficient I/O ports to meet present requirements with enough spares to provide for moderate future expansion.

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Specifications

OUTPUT-PORT POWER RATINGS

Each output port should be capable of supplying sufficient voltage and current to drive the output peripheral connected to it.

SCAN TIME

This is the speed at which the controller executes the relay-ladder logic program. This variable is usually specified as the scan time per 1000 logic nodes and typically ranges from 1 to 200 milliseconds.

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Specifications

MEMORY CAPACITY

The amount of memory required for a particular application is related to the length of the program and the complexity of the control system. Simple applications having just a few relays do not require significant amount of memory. Program length tend to expand after the system have been used for a while. It is advantageous to a acquire a controller that has more memory than is presently needed.

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Selecting a PLC

Criteria

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

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A Detailed Design Process

1. Understand the process2. Hardware/software selection3. Develop ladder logic4. Determine scan times and memory requirements

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PLC Status Indicators

• Power On

• Run Mode

• Programming Mode

• Fault

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Troubleshooting

1. Look at the process2. PLC status lights

HALT - something has stopped the CPURUN - the PLC thinks it is OK (and probably is)ERROR - a physical problem has occurred with the PLC

3. Indicator lights on I/O cards and sensors4. Consult the manuals, or use software if available.5. Use programming terminal / laptop.

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List of items required when working with PLCs:

1. Programming Terminal - laptop or desktop PC.2. PLC Software. PLC manufacturers have their own specific software and license key.3. Communication cable for connection from Laptop to PLC. 4. Backup copy of the ladder program (on diskette, CDROM, hard disk, flash memory). If none, upload it from the PLC.5. Documentation- (PLC manual, Software manual, drawings, ladder program printout, and Seq. of Operations manual.)

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Examples of PLC Programming Software:

1. Allen-Bradley – Rockwell Software RSLogix5002. Modicon - Modsoft3. Omron - Syswin4. GE-Fanuc Series 6 – LogicMaster65. Square D- PowerLogic6. Texas Instruments – Simatic6. Telemecanique – Modicon TSX Micro

Basic PLC The five types of PLC Programming languages

Structured Text (ST)VAR CONSTANT X : REAL := 53.8 ;Z : REAL; END_VARVAR aFB, bFB : FB_type; END_VAR

bFB(A:=1, B:=‘OK’);Z := X - INT_TO_REAL (bFB.OUT1);IF Z>57.0 THEN aFB(A:=0, B:=“ERR”);ELSE aFB(A:=1, B:=“Z is OK”);END_IF

Ladder Diagram (LD)

OUT

PUMP

Function Block Diagram (FBD)

PUMP

AUTO

MAN_ON

ACT

DO

V

Instruction List (IL)A: LD %IX1 (* PUSH BUTTON *) ANDN %MX5 (* NOT INHIBITED *) ST %QX2 (* FAN ON *)

Sequential Flow Chart (SFC)

START STEP

T1

T2

D1_READY

D2_READY

STEP A ACTION D1N

D ACTION D2

STEP B D3_READY

D4_READY

ACTION D3N

D ACTION D4T3

DI

V

CALC1

CALC

IN1

IN2

OUT >=1

graphical languages

textual languagesAUTO

MAN_ON

ACT

CALC1

CALC

IN1

IN2

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PLC Ladder diagram

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The most widely used language for PLC programming.

Rules for writing PLC Ladder1. The vertical lines called as rails represents the power lines. Where the left rail

represents positive lead and the right rail represents negative lead.2. The horizontal lines are called as rungs and are labeled in numerical order from left to

right and top to bottom.3. The ladder is read like a book from left to right and from top to bottom.4. Whenever possible the components are labeled in numerical order from left to right

and from top to bottom.5. The components are shown in their normal condition which means they are de-energized.6. Contacts will have the same letter and number designation as the device that controls

them. These control devices include relay coils, timers or motor starters.7. A normally open contact closes when the device that controls it is energized. A

normally closed contact opens when the devices that controls it is energized.

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Addressing

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Each instruction written on the ladder diagram has to be given a specific addresses.

One such simple scheme for addressing is given in the following table.

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Types of PLC ladder instructions

1) Relay logic instructions,

2) Timer and counter,

3) Program control instructions,

4) Arithmetic instructions,

5) Data manipulation instructions,

6) Data transfer instructions, and

7) Advanced instructions such as sequencer, subroutine, shift etc.

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Logic states

ON : TRUE, contact closure, energize, etc.

OFF: FALSE, contact open , de-energize, etc.

Do not confuse the internal relay and program with the externalswitch and relay.

Internal symbols are used for programming.

External devices provide actual interface.

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Relay logic Instructions Relay logic instructions consists of two parts, the coil (output condition

instruction) and the contacts ( input condition instruction).

Contacts:

a. Normally open -| |-

b. Normally closed -|/|-

c. Off-on transitional -||-d. On-off transitional -| |-

Coil:

a. Energize Coil -( )-

b. De-energize -(/)-

c. Latch -(L)-

d. Unlatch -(U)-

( )

01 08

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AND and OR Logic

PB1 R1PB2

R2

R1 = PB1.AND.PB2 R2 = PB2.AND.~PB4

PB3 PB4

PB1 R1

PB2

R1 = PB1 .OR. PB2

AND

OR

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Combined AND & OR

R1 = PB1 .OR. (PB2 .AND. PB3)

PB1 R1

PB2 PB3

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NOT Operation

In the rung above, it can be seen that if input A is be true (1),then the output C is true (0) or when A is (0), output C is 1.

RungA

C

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Timers These are output condition instructions which are used to provide

specific time delay. They also have their associated normally open and normally closed

contacts which are used to control the outputs in other rungs. At the time of programming the user needs to select time base value

and enter preset value. When the accumulated value = preset value timer times out and the

contacts change their status.

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Counters These are also output condition instructions and are used to

count the no. of events. They also have associated normally open and normally closed

contacts used to control the devices in other rungs. They also reset instruction associated with them used to reset

the count accumulated by the counter.

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Programming Examples1. Develop a ladder diagram for the elevator system shown in the figure. The objective is

to move the platform up and down. The global objective is to move the platform up when the UP button is pressed and to move it down when the down button is pressed. The following hardware is associated with the system.

OUTPUT ELEMENTS:

M1: Motor for up movement

M2: Motor for up movement

INPUT ELEMENTS:

LS1: NC Limit switch to indicate UP position

LS2: NC Limit switch to indicate DOWN position

START: NO push button for START

STOP : NO push button for STOP

UP : NO push button for UP Command

DOWN: NO push button for DOWN Command

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Solution : Here first we will have to develop narrative statements to describe sequence of events1. When the START button is pushed the platform is driven to down

position.2. When the STOP button is pushed the platform is halted at whatever

position it occupies at that time.3. When UP button is pushed the platform if not in the downward motion

is driven UP.4. When the DOWN button is pushed the platform if not in the upward

motion is driven DOWN.

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The ladder logic program for this system is as follows.

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2. For the oven shown in the figure all the inputs and outputs are two state variables and the relation of the states and the variables is indicated. Construct Boolean equations that implements the following events and then construct the ladder diagram from that.1. The heater will be on when the switch is activated ,the door is closed and the

temperature is below the limit.2. The fans will be turned on when the heater is on or when the temperature is above the

limit and the door is closed.3. The light will be turned on if the light switch is on or whenever the door is opened.

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The Boolean equation for the above description is given as follows from which we can construct the ladder diagram.

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3. A Bidirectional movable arm is as shown below. Develop a ladder diagram to control it as per the conditions mentioned.Conditions: Both start and stop switches are push buttons. When the system is turned ON the motor should rotate continuously alternating between the counter clockwise and clockwise directions as the movable arm touches two limit switches RLS and LLS.

Solution: Lets use the following addressing scheme for the systemInputs:Start switch I:0/11Stop switch I:0/12LLS I:0/13RLS I:0/14

Outputs:CCW Rotation O:0/2CW Rotation O:0/3 Based on the above addressing schemethe ladder diagram is as constructed below.

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4. When the system of the following figure is turned ON the motor is to produce alternate rotation CW and then CCW, cycling as the shaft extension contacts the two limit switches RLS and LLS. All the four switches have only normally closed positions. Prepare a PLC Ladder program with the following requirements:1. When the ON button is pushed the system motor moves the arm to the right limit

switch position and waits 30 seconds.2. The system then cycles 75 times between right and left limit switches and stops.3. The OFF button stops the system at any time or after the 75 cycles have been made

resets the system.

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5.Prepare the programmable ladder diagram for the control problem shown in the following figure. The global objective is to heat a liquid to a specified temperature and keep it there for 30 minutes.The hardware has the following characteristics:1. START push button is NO, STOP is NC.2. NO and NC are available for the limit switches.

The event sequence is 3. Fill the tank.4. Heat and stir the liquid for 30 minutes.3. Empty the tank4. Repeat from step 1.

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