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PLCAn Introduction

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Basic Component of PLC

i. The PLC Processor & controller

ii. I/O module

iii. Chases or backplane

iv. Power supplyv. Programming Software

In addition to these 5 components, most PLCs also have

Network Interface.

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Processor, Controller or CPU

Stores the control program and data in its memory

Reads the status of connected input devices

Executes the control program

Commands connected outputs to change state based on programexecution

For example: Turn a light on, start a fan, adjust a

speed, or temperature

Comes in various physical forms.

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I/O Modules

Physically connect to field devices

Input modules convert electrical signals coming in from input field

devices such as pushbuttons, to electrical signals that the PLC can

understand.

Output modules take information coming from the PLC and convert it

to electrical signals the output field devices can understand.

For example: a motor starter, or a hydraulic solenoid valve.

I/O comes in various forms

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Input Modules

Input modules interface directly to devices such as switches and

temperature sensors.

Input modules convert many different types of electrical signals such

as 120VAC, 24VDC, or 4-20mA, to signals which the controller can

understand.

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Input Modules

Input modules convert real world voltage and currents to signals the

PLC can understand. Since there are different types of input devices,

there is a wide variety of input modules available, including both

digital and analog modules.

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Discrete vs. Analog Modules

Discrete modules use only a single bit to represent the state of the

device. For example, a switch is either open or closed. Therefore, the

bit is either a 0 (switch is open) or a 1 (switch is closed). Discrete

modules are also known as Digital modules.

Analog modules use words to represent the state of a device. An

analog signal represents a value.. For example, the temperature could

be 5, 9, 20, 100, etc degrees. Analog modules use a value, such as 52,

rather than a 0 or 1 to represent the state of the device.

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Output Module

Output modules interface directly to devices such as motor starters

and lights

Output modules take digital signals from the PLC and convert them to

electrical signals such as 24VDC and 4 mA that field devices can

understand

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Output Module

Output modules take a signal from a PLC and convert it to a signal

that a field device needs to operate. Since there are different types of

output devices, there is a wide variety of output cards available,

including both digital and analog cards.

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Chassis/Backplane

All PLCs need some method of communicating between the controller,

I/O and communications modules. Here are three ways used to

accomplish this communications between the various components that

make up the PLC system.

I. Modules are installed in the same chassis as the PLC and

communicate over the chassis backplane.

II. Modules are designed to pluginto each other. The interconnecting

plugs form a backplane. There is no chassis

III. Modules are built into the PLC. The modules come together in one

physical block. The backplane in this case is transparent to the user.

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Chassis/Backplane

Below is an example of a backplane in a chassis based system. You

can see the backplane in the area where the modules are not inserted.

The modules have connectors that plug into the black connectors on

the backplane.

All of the connectors on the backplane are connected together

electrically.

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Chassis/Backplane (example)

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Backplane & Chassis

Great flexibility in choice of modules.

Modules can be easily installed orremoved without affecting other modules

Great flexibility in choice of modules.

In some cases modules cannot be

removed without breaking the chain

and affecting all modules downstream.

No chassis cost.

Low cost solution but limited flexibility.

Generally used in smaller, simpler

systems.

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Power Supply

A power supply is needed to provide power to the PLC and any other

modules. Power supplies come in various forms:

Power supply modules that fit into one of the slots in a chassisExternal power supplies that mount to the outside of a chassis

Stand alone power supplies that connect to the PLC or I/O through a

power cable

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Programming Software

Software that runs on a PC is required to configure and program PLCs.

Different products may require different programming software.

Software allows programs to be written in several different languages

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Network Interface

Most PLCs have the ability to communicate with other devices.

These devices include computers running programming software, or

collecting data about the manufacturing process, a terminal that lets an

operator enter commands into the PLC, or I/O that is located in a

remote location from the PLC.

The PLC will communicate to the other devices through a network

interface.

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PLC Programming

Every PLC has associated programming software that allows the user to

enter a program into the PLC.

Software used today is Windows based, and can be run on any PC.

Different products may require different software: PLC5, SLC, andControl-Logix each require their own programming software.

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Example

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Example

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Linking PETRA to ML1500

Petra Output Symbol PLC Input Wire Color Comment

Hole detection sensor 3 of 6. H3 I0 Black Aligned to detect a hole. ONif materialis in b/w the sensors.

Hole detection sensor 6 of 6. H6 I1 Brown Aligned to detect a hole. ONif material

is in b/w the sensors.

Cut out length Detector 1 L1 I2 Red Aligned to detect cut out. ON if

material is in b/w the sensors.

Cut out length Detector 2 L2 I3 Orange ONif Material Below the sensor.

Part Thickness Sensor T I4 Yellow ON if Component is of correct

thickness.

Slot sensor S I5 Green Aligned to detect slot. ON if material

below the sensor.

Carriage Position Status CS I6 Blue ONif the carriage is not at the pick and

pace position. Determined by CPL &

CPH

Arm Position AP I7 Purple OFFarm reaches belt 2. stays ON until

arm return to belt 1.

Dispenser Empty DE I8 Gray ONwhen there is no component in thedispenser.

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Connection b/w PETRA control I/P and PLC O/P

Petra Input Symbol PLC Output Wire Color Comment

Hole detection sensor 3 of 6. PV O4 Brown When energized, It applies vacuum to the

plunger pad.

Hole detection sensor 6 of 6. PA O5 Red Move the Plunger down when energized.

Carriage Position Low CPL O6 Orange CPL & CPH determined Position of

carriage.

Carriage Position high CPH O7 Yellow 00 = Dispenser 01 = Belt 1

10= Reject Bin 11 = Belt 2

Conveyor Belt 1 C1 O8 Green Belt 1 moves when energized.

Conveyor Belt 2 C2 O9 Blue Belt 2 moves when energized.

Arm Activate AA O10 Purple

Arm moves from belt 1 to belt 2 when

energized.

Gripper Activate GA O2 White Gripper hold the component whenenergized.

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Basic Instruction

Ladder logic Inputs XIC- Examine if closed

XIO- Examine if open

Ladder logic Outputs OTE - Output Energized

OTLOutput Latch

OUTOutput Unlatch

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LADDER LOGIC I/P

XIC / Normally Open:Symbol:

Definition:

Examine a bit for ON condition. Use XIC bit in your ladder logic to determine if a bit is ON

0 = False

1 = ON

Devices: Start / Stop Push Buttons

Limit Switches Selectors

Proximity Switches

Sensors

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LADDER LOGIC I/P

XIO/ Normally Closed:Symbol:

Definition: Examine a bit for an OFF condition.

Use an XIO instruction in your ladder logic to determine if a bit isOFF.

1 = True

0 = FalseDevice:

Start/ Stop push button.

Selectors

Limit Switches

etc.

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LADDER LOGIC O/P

Output Energize (OTE):Symbol:

Definition: Turns a bit ONN or OFF

Use OTE instructions in Ladder logic when rung condition is

evaluated as True.

Devices: Light

Motor

Internal bits

Actuators

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LADDER LOGIC O/P

OUTPUT Latch (OTL)Symbol:

Definition:

Turns a bit on when a rung is executed, & a bit is retain its stateWhen the rung is not executed.

Once an OTL bit has been set "on" (1 in the memory) it will

remain "on" even if the rung condition goes false. The bit must

be reset with an OTU instruction.

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LADDER LOGIC O/P

OUTPUT Unlatch (OTU)Symbol:

Definition: Turns a bit OFF when the rung is executed.

Use this output instruction to unlatch (reset) a latched (set) bit

which was set by an OTL instruction. The OTU address must

be identical to the OTL address which originally set the bit.

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Example

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Example

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The Slot Sensor

Detects whether there is a slot in a component or not.

Principle:

Optical-reflection wherein it gives a 1 whenever light

reflects back from a smooth surface into the sensor (i.e.when the solid part of the component is beneath the sensor)

and gives a 0 when light does not reflect back to the

sensor (i.e. when the belt or any non-reflective material is

beneath the sensor).

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LAB TASK

1- Create a program that does the following:

When Dispenser is empty, Belt 2 in PETRA should run.

When Dispenser is not empty, Belt 1 in PETRA should run.

Save your program. We need a mechanism to stop bothbelts if required. We will employ

the Slot sensor for this.

When there is a part beneath the Slot sensor it comes ON

Use Table 3.1 and 3.2 for the addresses of inputs and outputs.

2-Amend the program with the following condition:

Neither belt should be driven when the slot sensor is detecting

material beneath the sensor.

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TIMERS

Three basic types of Timers are.

Timer ON Delay

Timer OFF Delay

Retentive Timer ON

Retentive Timer OFF

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Timer ON Delay

This types of timers simply delay turning on.i.e. After our sensor (I/P) turns on we wait X-sec before activating

a solenoid valve (O/P)

Symbol:

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Timer ON Delay

It will count the time base interval when the instruction is true. The Timer on delay instruction begins to count time base interval when the rung

condition remains true.

The Timer adjust its accumulator value until it reaches its preset value.

Time = Preset * Time base

The accumulator value goes reset when rung condition got false. Timer on delay is consist of 3 words element.

Control word

Bit 0-12: Internal use

Bit 13: DN (true when ACCUMULATOR >= Preset Value )

Bit 14: TT (This bit is on when the timer is timing)

Bit 15: EN (On when TT is energized)

Store the Preset value

Usually this bit is from 0-32767

It cant be negative

Accumulator

This is the time elapsed since the timer was last reset.

TIME base is the timing update interval vary from 0-1 sec

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Timer ON Delay

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TIMER ON DELAY (Example-1)

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Timer ON Delay (Example-2)

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Timer OFF Delay

Counts time base intervals when the instruction is

false.

TOF instruction begins to count time base intervals

when a rung makes true to false instruction.

As long as the condition will remain false , the timer

increments its accumulator value .

The accumulator value reset when the rung

condition goes true regardless of weather the timer

has timed out.

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Timer OFF Delay

Symbol:

Timer address:

Control Address

Bit: 0-12 (internal use)Bit: 13 DN (Done)

Bit: 14 TT (Timer Timing)

Bit: 15 EN

Store the Preset Value

When accumulated value become =/> the presetvalue the done bit is set.

Preset value is from 0-32767

If the Timer preset value isev an error will occur.

Accumulated value

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Timer OFF Delay

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Timer OFF Delay

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Recursive Timer

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Counters

Types of counters:1. Count up (CTU)

2. Count Down (CTD)

Count up: (CTU)

It increments the accumulated value at each false to true transition

and retain the accumulator value when the instruction goes false.

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Count up: (CTU)

CTU is an instruction that counts false to true transition.

The transition causes the accumulated value to increment by

one.

It is reset by the RES instruction.

If the accumulation value is equal to Preset value then

DN = 1

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Count up (CTU)

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Count Down

Decrements the accumulator value at each false to true

transition and retain the accumulated value when instruction

goes false.

CTD is an instruction that counts false to true transition.

Transition causes the accumulated value by one count.

CTD is reset by the RES instruction.

If accumulator value is below the Preset value the DN bit gets

low.

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Count Down (CTD)

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