programable logic controller

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PROGRAMMABLELOGIC

CONTROLLER


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Control Systems Types

Programmable Logic Controllers

Distributed Control System PC- Based Controls


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PROGRAMMABLELOGIC

CONTROLLER

Aprogrammable logic controller

(PLC), orprogrammablecontrolleris a digital computerusedfor automationof industrial

processes, such as control ofmachinery on factory assembly lines.
http://en.wikipedia.org/wiki/Digital_computerhttp://en.wikipedia.org/wiki/Automationhttp://en.wikipedia.org/wiki/Assembly_linehttp://en.wikipedia.org/wiki/Assembly_linehttp://en.wikipedia.org/wiki/Automationhttp://en.wikipedia.org/wiki/Digital_computer


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PLC

Sequential logic solver

PID Calculations.Advanced Subroutines

BIT Operations.

Data Transfer.

Text Handling.


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Applications : Machine controls, Packaging, Palletizing, Material handling, similar

Sequential task as well as Process control

Advantages of PLC :

They are fast and designed for the rugged industrial environment. They are attractive on Cost-Per-Point Basis.

These Devices are less Proprietary ( E.g.. Using Open Bus Interface.)

These Systems are upgraded to add more Intelligence and Capabilitieswith dedicated PID and Ethernet Modules.

Disadvantages of PLC : PLC were Designed for Relay Logic Ladder and have Difficulty with

some Smart Devices.

To maximize PLC performance and Flexibility, a number of OptionalModules must be added



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PLC Types Nano (Small)

Micro (Medium)

Large

Basic criteria for PLC Types

Memory Capacity I/O Range

Packaging and Cost per Point



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Components

Central Processing Unit (CPU)

Input Output Modules

Power Supply

Bus system



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Central Processing Unit

It is a micro-controller based circuitry. The CPU consists offollowing blocks :

Arithmetic Logic Unit (ALU), Program memoryProcess image memory (Internal memory of CPU)

Internal timers and counters & Flags

CPU performs the task necessary to fulfill the PLCfunctions. These tasks include Scanning, I/O bus trafficcontrol, Program execution, Peripheral and External devicecommunication, special functions or data handlingexecution and self diagnostics.



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Input module

These modules act as interface between real-time status ofprocess variable and the CPU.

Analog input module : Typical input to these modules is

4-20 mA, 0-10 V

Ex : Pressure, Flow, Level Tx , RTD (Ohm), Thermocouple(mV)

Digital input module : Typical input to these modules is 24 VDC, 115 V AC, 230 V AC

Ex. : Switches, Pushbuttons, Relays, pump valve on offstatus



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Output module

These modules act as link between the CPU and the outputdevices in the field.

Analog output module : Typical output from thesemodules is 4-20 mA, 0-10 V

Ex : Control Valve, Speed, Vibration

Digital output module : Typical output from these modulesis 24 V DC, 115 V AC, 230 V AC

Ex. : Solenoid Valves, lamps, Actuators, dampers, Pumpvalve on off control



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Power Supply

The power supply gives the voltage required forelectronics module (I/O Logic signals, CPU, memoryunit and peripheral devices) of the PLC from the linesupply.

The power supply provides isolation necessary toprotect the solid state devices from most high voltageline spikes.

As I/O is expanded, some PLC may require additionalpower supplies in order to maintain proper powerlevels.



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Bus System

It is path for the transmission of the signal . Bu system isresponsible for the signal exchange between processorand I/O modules

The bus system comprise of several single line ie wires /tracks



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PLC Cycle

Outputs

Machine

or

Process

Programmable

controller

Inputs

Sense the Input

Process the Logic

Give Output


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PLC Signal Flow

Programming Terminal

O:0/7

O:0/7

O:1/5

Output Devices

Output ModulesProcessor MemoryInput Module

Input Devices

Ladder Program

O:0/7

O:1/5

I:0/6

I:1/4

O:1/5

I:0/6

I:1/4

I:0/6

I:1/4

DataInput

Image Table

Output

Image Table


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PLC Architecture Evolution

Mid - 1970s : Discrete Machine Control

Programming Language :

- Relay ladder logic- Flexibility in altering

Control system operation

Programming

Terminal

PLC

I/O

Connection is Point to Point

Connection is Point to Point


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Early - to - Mid 1980 : Discrete and Process Control


Programming Language :

- Ladder Program

- PID

- Data Storage

Reasonable Computer

Running PLC

Programming Software

PLC

I/O

MS - DOS


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Late 1980s to early 1990s : Discrete and ProcessControl

PLC became a part of the

developing enterprise resource

system

PC running

PLC Programming Software

PLC

I/O

Connection in networked allowing

Multiple PLC

Windows

PLC


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Today: Distributed I/O Modules

Distributed I/O modules

PLC

Distributed I/O scanner

Data Communication Bus



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Remote

I/O Network

SPLITTERS

FIBER OPTIC LINK

TAPS

Today : Hot Redundant System



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Controller Controller

Controller

Controller

Workstation Workstation Workstation Workstation

Switched Hub


Today : Ethernet Technology in PLCs


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Remote

Platform

Wireless Modem

Wireless Modem

PLC

H M IDisplay PC


Today : Wireless communication

PLC


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PLC Systems of various vendors

Siemens S5 -110U, 115U, 135U

S7 - 200, 300, 400

Allen Bradley Micrologix 1000, 1200, 1500

SLC 5/01, 5/02, 5/03

PLC 5/10, 5/25 and 5/40

Modicon Nano Micro

Premium

Quantum


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8 Analog Inputs 1Analog Output

Up/Down FastCounter

Up Counter

Programming Terminal PC Connection

Unitelway Port for connectionof up to 5 Slaves

PCMCIA memory expansion port

PCMCIA communications port

TSX37-22

Built in display for I/O(in-rack, AS-i) and Diag

I/O Modules

Configuration of PLC : Modicon


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Configuration of PLC : Siemens

CPU

External Power

Supply

I/O Modules


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Configuration of PLC : Allen Bradley

CPU

Power SupplyI/O Modules


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Configuration of PLC : GE FANUC

CPU

I/O Modules Back plane


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PLC Programming Standards

The open, manufacturer-independent programmingstandard for automation is IEC 61131-3. You can thus choosewhat configuration interface you wish to use when writing your

application :

Ladder Diagram

Instruction List

Function Block Diagram Sequential Function Chart

Structured Text


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Cost of hardware, software, Integration Engineering,Design, Installation, Start-up and Commissioning,Validation documentation and Execution, Training, Spareparts, Maintenance, System service contract and system life

cycle. Reliability, Flexibility, Scalability and Validatability.

Ease of Database configuration, Graphics development,Interlocks and Batch processing.

Integration of High-level Application. Control Philosophy for Centralized versus Remote

Operator Console or both.

Compliance with an Industry batch standard such as ISA

SP88 and new Communication Protocol.

PLC DCS Selection Criteria


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Introduction to

IEC1131-3 Ladder Diagram


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CPU

Origins of Ladder Diagram The Ladder Diagram (LD) programming

language originated from the graphicalrepresentation used to design an electricalcontrol system Control decisions were made using relays

After a while Relays were replaced by logiccircuits Logic gates used to make control decisions

Finally CPUs were added to take over thefunction of the logic circuits I/O Devices wired to buffer transistors

Control decisions accomplished through

programming Relay Logic representation (or LD) was

developed to make program creation andmaintenance easier Computer based graphical representation of

wiring diag. that was easy to understand

Reduced training and support cost

ORAND


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What is a Rung?

A rung of ladder diagram code can containboth input and output instructions Input instructions perform a comparison or test

and set the rung state based on the outcome Normally left justified on the rung

Output instructions examine the rung state andexecute some operation or function

In some cases output instructions can set the rung state Normally right justified on the rung

Input Instruction Output Instruction


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Series Vs Parallel Operations Ladder Diagram input instructions perform logical AND and OR

operations in and easy to understand format

If all Input Instructions in series must all be true for outputs toexecute (AND)

If any input instruction in parallel is true, the outputs will execute

(OR) Paralleling outputs allows multiple operations to occur based on the

same input criteria

OR

AND

A

B

C D

IF ((A OR B) AND (NOT C) AND D) THEN E=1; F=1 END_IF

E

F

Branches


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Ladder Logic Execution

Rungs of Ladder diagram are solved fromLeft to right and top to bottom

Branches within rungs are solved top left to

bottom right

A D

B

F

G

I J

Left Power Rail

R

K

P S

E

H

Branch

Right Power Rail

Ladder Rung


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Non Retentive Coils The referenced bit is reset when processor power is

cycledCoil -( )-

Sets a bit when the rung is true(1) and resets the bit when therung is false (0)

PLC5 calls this an OTE Output Enable

Negative coil -( / )- Sets a bit when the rung is false(0) and resets the bit when the

rung is True(1)

Not commonly supported because of potential for confusion

Set (Latch) coil -(S)- Sets a bit (1) when the rung is true and does nothing when the

rung is false

Reset (Unlatch) Coil -(R)- Resets a bit (0) when the rung is true and does nothing when

the rung is false


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Contacts

Normally Open Contact -| |- Enables the rung to the right of the instruction if the rung to

the left is enabled and underlining bit is set (1)

Normally Closed Contact -|/|- Enables the rung to the right of the instruction if the rung to

the left is enabled and underlining bit is reset (0)

Positive transition contact -|P|- Enables the right side of the rung for one scan when the

rung on left side of the instruction is true

Allen Bradley PLC5 uses -[ONS]-

Negative transition contact -|N|- Enables the right side of the rung for one scan when the

rung on left side of the instruction is false


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Retentive Vs Non-retentive

OperationDefinitions

Retentive values or instructions maintain their laststate during a power cycle

Non-retentive values or instructions are reset tosome default state (usually 0) after a power cycle

IEC1131 permits values to be defined asretentiveA contradiction to this is ladder diagram where 3

instructions are classified as retentive

In most PLCs only timer and coil instructionsoperate as non-retentive


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Retentive Coils

The referenced bit is unchanged whenprocessor power is cycledRetentive coil -(M)-

Sets a bit when the rung is true(1) and resets the bit whenthe rung is false (0)

Set Retentive (Latch) coil -(SM)- Sets a bit (1) when the rung is true and does nothing

when the rung is false

PLC5 uses OTL Output Latch

Reset Retentive (Unlatch) Coil -(RM)- Resets a bit (0) when the rung is true and does nothing

when the rung is false

PLC5 uses OUT Output Unlatch


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Transition Sensing Coils

Positive transition-sensing coil -(P)- Sets the bit bit (1) when rung to the left of the

instruction transitions from off(0) to on(1)

The bit is left in this state PLC5 use OSR (One Shot Rising)

Negative transition-sensing coil -(N)-Resets the bit (0) when rung to the left of the

instruction transitions from on(1) to off(0) The bit is left in this state

PLC5 uses OSF (One Shot Falling)


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IEC Comparison Instructions in Ladder

If the rung input (EN) is enabled, the instructionperforms the operation and sets the rung output(ENO) based on the comparison

Example: when EN is true, EQ (=) function

compares In1 and to In2 and sets ENO

Comprehensive instruction set

EQ(=), GT (>), GE (>=), LT (


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Timers in Ladder Diagram There three timer instructions

in IEC1131 TP - Pulse timer

TON - Timer On Delay

TOF - Timer Off Delay

Time values

Time base is 1msec (1/1000 ofa sec)

Values entered using durationliteral format

Two possible visualizationsDepending on use of EN/ENO

1st method requires extraprogramming if timer donestatus needs to be referencedon other rungs

2nd method sets a bit with Qwhich can be referenced by

other logic, ENO=EN

TON

T#200ms

Pump_Tmr

PT ET 178

Q

IN ENO

Pump_Tmr_DN

TONIN

T#200ms

Pump_Tmr

PT ET 178

Q


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Timer Operation

IN

Q

ETPT

|

0

Pulse (TP) Timing

IN

Q

ETPT

|

0

On-Delay (TON) Timing

IN

Q

ETPT

|

0

Off-Delay (TOF) Timing

IN = Rung inputcondition

Q = Comparison

output results Varies with timer types

PT = Preset Time

ET = Elapse Time


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Counters in Ladder Diagram

There three counter instructionsin IEC1131 CTU - Count Up Counter

CTD - Count Down Counter

CTUD - Count Up/DownCounter

All three count rung transitions

Two possible visualizationsDepending on use of EN/ENO 1st method requires extra

programming if timer donestatus needs to be referenced onother rungs

2nd method sets a bit with Qwhich can be referenced byother logic, ENO=EN

CTU

200

Load_Cnt

PV CV 178

Q

IN ENO

Load_Cnt_DNR

CTU

200

Load_Cnt

PV CV 178

QIN

R


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Counter Operation

Parameters CU/CD = Count up/Down

Q/QU/QD = Comparison Output

R = Reset to Zero

LD = Load CV with PV

PV = Preset Value

CV = Count Value

...

...

CVPV

|

0

CU

QUCD

QD

LD

R

Count Up/Down (CTUD) Counter

...IN

Q

CVPV|0

LD

...Count Down (CTD) Counter

...IN

Q

CVPV

|

0

R

Count Up (CTU) Counter

...


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CAL

RET RET

CAL

Execution Control Elements Jump / Label

Instructions Jump to a label skips a

block of code without it

being scanned LBL - Named target for a

jump operation

JMP - Performs a jumpwhen the rung

conditions are true

CALL / RETURNInstructions Used to encapsulate logic

and call it as a subroutine

Causes execution to change

between functions orsubroutines

CAL - Passes control toanother named function

PLC5 uses JSR

RET - Exits a function and

returns control back to thecalling routine

| Skip_Calc |

|-| |-------------(JMP)--|

| ... |

| Skip_Calc |

|---[LBL]---...


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The look and feel of IEC 1131-3 is somewhatdifferent from the 1Million+ PLCs that AllenBradley has running in factories throughoutthe world

IEC places the input parameters on theoutside of the instruction block vs the PLC5where they are presented inside of the block

TON

Timer

Preset

Pump_Tmr

200.000

Accum 178.251

(EN)

(DN)

ADD

Source A

Source B

Tank1_In

Offsetr

Destination Tank_Level

178.251

78.251

100.000

+EN

100.000 178.251

ENO

78.251Offsetr

Tank1_In Tank_Level

Different Instruction Presentations

TON

T#200ms

Pump_Tmr

PT ET 178

Q

IN ENO

Pump_Tmr_DN

Extending the IEC1131-3 Instruction Set


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Extending the IEC1131 3 Instruction Set

IEC1131-3 Provides a very basic set of instructions to do simple operations (81Ladder Diagram Instructions)

Data Type Conversion - Trunc, Int_to_Sint, Dint_to_Real, Bcd_To_Int

Boolean Operations - Bit Test, Bit Set, One Shot, Semaphores Timers / Counters - Ton, Tp, Ctu, Ctd, Ctud

Simple Math - Add, Sub, Mul, Div, Mod, Move, Expt

Misc. Math - Abs, Sqrt, Ln, Log, Exp, Sin, Cos, Tan, Asin, Acos, Atan

Bit Shift - Shl, Shr, Ror, Rol

Logic - And, Or, Xor, Not Selection - Sel, Max, Min, Limit, Mux

Compare - GT, GE, EQ, LE, LT, NE

String - Len, Left, Right, Mid, Connect, Insert, Delete, Replace, Find

Control - JMP, LBL, JSR, RET

All complex operations are left to the user or vendor to define File Operations, PID, Diagnostic, For/Nxt Loop, Search, Sort are not in

IEC1131-3

Extensions to the instruction set are permitted so that vendors can addinstructions that their customers need

All vendors have defined their own set of extensions

Rockwell Automation controllers have significantly more capabilitywith over 130 Ladder Instructions

Extensions to IEC provide code


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=

Rockwell Automation FIFO Load Instruction

IEC1131-3 Load FIFO Logic

1 Rung of Logic

1 InstructionMinutes to code and debug11 Rungs of Logic

17 Instructions

Hours to code and debug

Extensions to IEC provide codeoptimization and ease of use


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Instruction Extension to

IEC1131-3 FIFO & LIFO - FFL, FFU, LFL, LFU File math and search - FAL, FSC

Table operations - SRT, STD, AVE

Sequencers - SQI, SQL, SQO, SDS

Diagnostics - DDT, DFA, FBC

Compare - CMP, MEQ

Compute - CPT, NEG

Data moves - MVM, COP, BTD

Program Control - AFI, NOP, MCR, TND Interrupt Services - UID, UIE

Retentive Timer - RTO

Ladder Loop Instruction - FOR, NXT

Process - PID

programable logic controller

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