P L C - Gen

PROGRAMMABLE LOGIC CONTROLLER

Prolific Systems and Technologies Pvt Ltd

History

Early control systems consisted of huge control boards consisting of hundreds to thousands of electromechanical relays. An engineer would design the system logic, electricians would receive a schematic outline of logic then implement the logic with relays. The schematic was commonly called Ladder Schematic The ladder displayed all switches, sensors, motors, valves, relays etc in the system. Problems: Long commissioning time, dependency on mechanical reliance, Any system logic design change required the power to the control board to be isolated stopping production.


History

General Motors was among the first to recognize a need to replace the systems wired control board Hydramatic Division of GM specified the design criteria for the programmable controller in 1968. Goal Eliminate the high cost associated with inflexible, relay controlled systems.


History

New Controller Specifications:

Solid State System Computer Flexibility Operate in Industrial Environment (vibrations, heat, dust etc.) Capability of being reprogrammed Easily programmed and maintained by electricians and technicians.


HistoryIn 1969 Gould Modicon developed the first PLC. Strength Programmed with Ladder Logic Initially called Programmable Controllers PCs Now - PLCs, Programmable Logic Controllers PLCs have evolved from simple on/off control to being able to communicate with other control systems, provide production reports, schedule production, diagnose machine and process faults.


Advantage of PLC Over Relay StyleRELAY1-Hard wiring 2-Changes difficult 3-More power 4-More maintenance 5-Difficult to expand

PLC1-Less wiring 2-Easy modification 3-Low power 4-Less maintenance 5-Ease of expansion


Control Systems Types

Programmable Logic Controllers Distributed Control System PC- Based Controls


NEMA Definition year 1978National Electrical Manufacturers Association

The PLC, also known as programmable controller is defined by the National Electrical Manufacturers Association (NEMA) in 1978 as: "a digitally operating electronic apparatus 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 digital or analog input/output, various types of machines or process".


Programmable Logic Controllers

Sequential logic solver PID Calculations. Advanced Subroutines BIT Operations. Data Transfer. Text Handling.


Programmable Logic ControllersApplications :

Machine controls, Packaging, Palletizing, Material handling, similar Sequential task as well as Process control 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 Capabilities with dedicated PID and Ethernet Modules. PLC were Designed for Relay Logic Ladder and have Difficulty with some Smart Devices. To maximize PLC performance and Flexibility, a number of Optional Modules must be added PLC holds only one copy of program

Advantages of PLC :

Disadvantages of PLC :

Programmable Logic ControllersPLC Types

Nano (Small) Micro (Medium) Large

Basic criteria for PLC Types

Memory Capacity I/O Range Packaging and Cost per PointProlific Systems and Technologies Pvt Ltd

Sizing of PLCMicro PLCs: I/O up to 32 points Small PLC: I/O up to 128 points Medium PLC: I/O up to 1024 points Large PLC: I/O up to 4096 points Very Large: I/O 4096 Onwards


Most Basic of PLC SystemsIn the most basic of PLC systems, a self contained (shoe box) PLC has 2 terminal blocks, one for Inputs and one for Outputs Today, most PLCs in this category are know as Micros. Typically they provide front panel LED status indication of I/O and Processor states

Outputs Inputs

C R


Modular Chassis Based PLCsThe vast majority of PLCs installed today are modular chassis based systems consisting of: 1. Processor Module (CPU) 2. Input & Output Modules 3. Chassis 4. Power Supply


Modular Chassis-less PLC Systems ChassisAlso available from many vendors are Chassis less but still Modular PLC systems. These systems still require a Processor, I/O Modules, and Power Supply, but in place of a chassis these components mount directly onto a panel, din rail, and many use a tongue and grove system to allow easy insertion and removal


P L C Components

Central Processing Unit (CPU) Input Output Modules Power Supply Bus system Programming DeviceIN CPU OUT

m

MODULE PROGRAM DEVICE

MODULE


Basic PLC Schematic

CPU Power Supply Memory Input Blocks Output Blocks Communications Expansion Connections


P L C : Central Processing Unit

It is a micro-controller based circuitary. The CPU consists of following blocks : Arithmatic Logic Unit (ALU), Timing / Control ckt, Program memory, Process image memory (Internal memory of CPU) Internal timers and counters and Flags, Address stack and instruction registers The Central Processing Unit (CPU) Module is the brain of the PLC.


CPU Module

Self CheckPLC Program SCAN

Scan Inputs

Update Outputs

Execute Code

Primary role to read inputs, execute the control program, update outputs.

CPU performs the task necessary to fulfill the PLC functions. These tasks include Scanning, I/O bus traffic control, Program execution, Peripheral and External device communication, special functions or data handling execution and self diagnostics.


Memory

The memory includes pre-programmed ROM memory containing the PLCs operating system, driver programs and application programs and the RAM memory. PLC manufacturer offer various types of retentive memory to save user-programs and data while power is removed, so that the PLC can resume execution of the user-written control program as soon as power is restored.


Memory

Many PLCs also offer removable memory modules, which are plugged into the CPU module. Memory can be classified into two basic categories: volatile and nonvolatile. Volatile memory loses state (the stored information) when power is removed. Nonvolatile memory, maintains the information in memory even if the power is interrupted.


MemorySome types of memory used in a PLC include:

ROM (Read-Only Memory) RAM (Random Access Memory) PROM (Programmable Read-Only Memory) EPROM (Erasable Programmable Read-Only Memory) EEPROM (Electronically Erasable Programmable Read-Only Memory) FLASH Memory Compact Flash Can store complete program information, read & write text files


I/O Modules

Input and output (I/O) modules connect the PLC to sensors and actuators. Provide isolation for the low-voltage, low-current signals that the PLC uses internally from the higher-power electrical circuits required by most sensors and actuators. Wide range of I/O modules available including: digital (logical) I/O modules and Analog (continuous) I/O modules.


PLC : Input module

These modules act as link between field input sensors and the CPU. Analog input module : Typical input to these modules is 4-20 mA, 0-10 V, Ohms, mV Ex : Pressure, Flow, Level Tx, RTD (Ohm), Thermocouple (mV) Digital input module : Typical input to these modules is 24 V DC, 115 V AC, 230 V AC Ex. : Switches, Pushbuttons, Relays, pump valve on off status


PLC : Input module

Transfer of data:-I/P sensor to CPU Conversion:- 24vdc/230vac to 5vdc Isolation :- By Opto Coupler


Input Devices

Pushbuttons Selector Switches Limit Switches Level Switches Photoelectric Sensors Proximity Sensors Motor Starter Contacts Relay Contacts Thumbwheel Switches


Digital Inputs ModulesThe list below shows typical ranges for input voltages.

5 Vdc used when Microcontroller based sensors are used as I/P 12 Vdc Now not much used 24 Vdc Widely used (Standard for I/P)

48 Vdc - Was used high power devices pneumatic / hydraulic cylendors / limit switches

12 Vac not being used 24 Vac discontinued 120 Vac Sensors are at long distance from controller 240 Vac Not much used


SOURCING vs. SINKING DC InputsSINK SOURCE

Field Device

+VDC IN1

+DC Power Supply

VDC IN1

+DC Power Supply

DC Input ModuleDC COM C

Field Device

-

DC Input ModuleC

-


Analogue Inputs/Outputs

Analogue input cards convert continuous signals via a A/D converter into discrete values for the PLC Analogue output cards convert digital values in then PLC to continuous signals via a D/A converter. Resolution can be important in choosing an applicable card Example, for a temperature input of 0 to 100 degrees C

For 8 bit resolution the value in the PLC is 0 to 255 For 12 bit resolution the value in the PLC is 0 to 4095 For 12.5 bit resolution the value in the PLC is 0 to 6000 For 13 bit resolution the value in the PLC is 0 to 8192 For 16 bit resolution the value in the PLC is 0 to 32768


Analogue Cards

Typical Analogue Input signals are:

Flow sensors Humidity sensors Load Cells Potentiometers Pressure sensors Temperature sensors Vibration

Analogue Output signals control: Analogue Valves Actuators Chart Recorders Variable Speed Drives Analogue Meters Typical Analogue Signal Levels 4-20mA 1-5 Vdc 0-10 Vdc -10 10Vdc


PLC : Input module


PLC : Output module

These modules act as link between the CPU and the output devices in the field. Analog output module : Typical output from these modules is 4-20 mA, 0-10 V Ex : Control Valve, Speed, Vibration Digital output module : Typical output from these modules is 24 V DC, 115 V AC, 230 V AC Ex. : Solenoid Valves, lamps, Actuators, dampers, Pump valve on off control


PLC : Output module - Types

Relay type -For AC or DC Transistor Type Logic(TTL) - For DC Triac (Triode AC) type - For AC Isolated common type -For different device


PLC : Output module


Output Devices

Valves Motor Starters Solenoids Control Relays Alarms Lights Fans Horns

Relays 120 VAC/VDC 240 VAC 24 VAC/VDC Triac 120/230 VAC Transistor MOSFET 24 VDC


Relays

The most important consideration when selecting relays, or relay outputs on a PLC, is the rated current and voltage. For transistor outputs or higher density output cards relay terminal blocks are available. Advantage of individual standard replaceable relays


I/O Specifications

INPUT VOLTAGE Magnitude and type of voltage ON-STATE INPUT VOLTAGE RANGE voltage at which signal is recognized Nominal current per input Min. current to operate input circuit AMBIENT TEMP RATING Max temp of surrounding the I/O module INPUT DELAY Time duration for input signal to be on before known as valid input. ( 9-ms to 25ms) NOMINAL OUTPUT VOLTAGE It is min and max o/p operating voltage.e.g. Rated 120 v ac o/p ckt. Works in 92 to 138 v range. MAX O/P CURRENT RATING Max current a single o/p or module can safely carry under load OFF STATELEAK CURRENT PER O/P Max value of leak current flows through the o/p in OFF position ELECTRICAL ISOLATION Max volts between I/o and logic ckt.


P L C : Power Supply

The power supply gives the voltage required for electronics module (I/O Logic signals, CPU, memory unit and peripheral devices) of the PLC from the line supply. The power supply provides isolation necessary to protect the solid state devices from most high voltage line spikes. As I/O is expanded, some PLC may require additional power supplies in order to maintain proper power levels.


P L C : Bus System

It is path for the transmission of the signal . Bus system is responsible for the signal exchange between processor and I/O modules The bus system comprise of several single line ie wires / tracks Types of Bus Address bus - Location Data bus - Carries Data Control bus - Synchronization


Special Modules

RF ID Voice Gas Flow Calculation Weigh Cell Hydraulic Servo ASCII Fuzzy Logic Temperature Sensor Temperature Control Heat/Cool Control Field Bus Cards DeviceNet, Profibus etc Lonworks, BACNet

Fast Response (Interrupt) PID Loop Controller BASIC Cards RS232 Comms Modbus ASCII/RTU Ethernet Comms High Speed Counters Position Control Cards Peer to Peer Comms Controller Link DH+ Modbus Plus


PLC Operating CycleSTART

HousekeepingInternal checks on memory, speed and operation. Service any communication requests, etc.

Input ScanThe status of external inputs (terminal block voltage) is written to the Input image (Input file).

Output ScanThe Output Image data is transferred to the external output circuits, turning the output devices ON or OFF.

Program ScanEach ladder rung is scanned using the data in the Input file. The resulting status (Logic being solved) is written to the Output file (Output Image).

This scan cycle can be interrupted if required using interrupts


PLC Signal FlowInput Module Processor Memory Data Output Input Image Table Image Table I:0/6 I:0/6 I:1/4 Input Devices Ladder Program I:0/6 I:1/4 I:1/4 O:0/7 O:1/5 O:1/5 O:1/5 Output Devices O:0/7 O:0/7 Output Modules

Programming Terminal

PLC Architecture Evolution

Mid - 1970s : Discrete Machine Control Programming TerminalConnection is Point to Point

PLCProgramming Language : - Relay ladder logic - Flexibility in altering Control system operation

I/O



Early - to - Mid 1980 : Discrete and Process ControlMS - DOS

Reasonable Computer Running PLC Programming Software

PLCProgramming Language : - Ladder Program - PID - Data Storage

I/O



Late 1980s to early 1990s : Discrete and Process ControlWindows

PC running PLC Programming SoftwareConnection in networked allowing Multiple PLC

PLC I/O

PLCPLC became a part of the developing enterprise resource system


PLC Architecture EvolutionToday : Distributed I/O ModulesPLC

Distributed I/O scanner

Data Communication Bus

Distributed I/O modules


PLC Architecture EvolutionToday : Hot Redundant SystemLevel of redundancy Power Supply CPU I/O CommunicationTAPS SPLITTERS Remote I/O Network FIBER OPTIC LINK


PLC Architecture EvolutionToday : Ethernet Technology in PLCs

Workstation

Workstation

Workstation

Workstation

Switched Hub

Controller Controller Controller


Controller

PLC Architecture EvolutionToday : Wireless communicationPLC Wireless Modem / GSM Communication

Remote Platform Wireless Modem / GSM Communication PLC

Prolific Systems and

HMI Display Technologies Pvt

PC

Ltd

Configuration of PLC : ModiconBuilt in display for I/O (in-rack, AS-i) and Diag 8 Analog Inputs 1 Analog Output Programming Terminal PC Connection

I/O Modules Up/Down Fast Counter

Up Counter Unitelway Port for connection of up to 5 Slaves

TSX37-22 PCMCIA communications port PCMCIA memory expansion port


Configuration of PLC : Siemens

CPU I/O Modules External Power Supply


Configuration of PLC : Allen Bradley

CPU I/O Modules Power Supply


Configuration of PLC : GE FANUCI/O Modules Back plane

CPU


PLC Programming StandardsThe open, manufacturer-independent programming standard for automation is IEC 61131-3. You can thus choose what configuration interface you wish to use when writing your application :

Ladder Diagram Statement List Instruction List Function Block Diagram Sequential Function Chart Structured Text


Scan Rate

The completion of one cycle of the sequence is calledSCAN Time required for one cycle is called SCAN TIME


Binary System

BIT Each digit of a binary number BYTE Group of 8 bit WORD Group of one or more byte LSB Least significant bit or smallest value 7 6 5 4 3 2 1 0 MSB Most significant bit or largest value 8 BIT ( 1 BYTE )LSB BYTE BYTE 16 BIT (1 WORD)

MSB


PLC : Terminology

INPUT is referred by I OUTPUT is referred by Q / O TIMER is referred by T FLAGS are referred by M /B COUNTERS are referred by C


PLC : Communication Protocol

It is a set of rules for data transmission when PLC is connected to network RS-232 (Recommended standard) RS-485 MPI(Multi point Interface) Profibus DH(Data Highway) Ethernet Controlnet Devicenet


Baud rate (Communication Speed)

It is rate of data transmission on network Unit is bits/second Range:120 bits /sec to 100 Mega bits per second


Selecting a PLC

Number of logical inputs and outputs Memory Number of special I/O modules Expansion Capabilities Scan Time Communication Software Support Cost


Manufactures Major Brands

OMRON Allen Bradley Schneider GE Fanuc Siemens Automation Direct (Koyo) Toshiba Mitsubishi Hitachi Keyence VIPA


PLC StandardizationIEC 61131 Based on IEC 1131 (1992) standard, developed to be a common and open framework for PLC architecture. IEC 61131-1 Overview IEC 61131-2 Requirements & Test Procedures IEC 61131-3 Data Types & Programming IEC 61131-4 User Guidelines IEC 61131-5 Communications IEC 61131-7 Fuzzy Control IEC 61131-7 Guidelines for the application and implementation of programming languages


PLC Programming

The purpose of a PLC Program is to control the state of PLC outputs based on the current condition of PLC Inputs

Different PLCs support different languages, but the most popular PLC language is know as Ladder Logic.

PLC Ladder Logic purposely resembles Relay Logic


IEC 61131-3 61131-

IL (Instruction List) mnemonic programming LD (Ladder Diagram) Relay logic ST (Structured Text) A BASIC like programming language FDB (Functional Block Diagram) Graphical dataflow programming language SFC (Sequential Flow Chart) Graphical method for structured programs


Ladder Logic ConceptsRead / Conditional Instructions Start (Rung #1) Write / Control Instructions

|| || || || || || |/| |/| |/|

() () () ()

End (Rung #5)

||

|/|

()


Ladder Logic ConceptsRead / Conditional Instructions Write / Control Instructions

|/|True

||False

()False

No Logical Continuity

|/|True

|/|True

()True

Logical Continuity


Logical AND ConstructionIF input 4 AND input 5 have power THEN energize output 0I/4 I/5 O/0

On

||T

||TLogical Continuity

()T


Logical OR ConstructionIF input 4 OR input 5 have power THEN energize output 0I/4

TLogical Continuity

O/0On

||F I/5

()

|| ||

I/4

FLogical Continuity

O/0

On

()

T I/5

||


Typical Construction

I/4

I/0

I/1

I/9 I/10

||I/5

|/|I/1

||I/7

||I/8

|| ()

O/0

||

|/|I/2

|/|I/3

|/|

|| ||I/11 I/1

|/|

||


Addressing Examples

L1 PB1 LS1 PS2 SOL6

L2I/5 I/6 I/7 O/0

||

||

||

()

DEVICE PB1 LS1 PS2 SOL6

ADDRESS HHP Logix I/5 I:0/5 I/6 I/7 O/0 I:0/6 I:0/7 O:0/0


Relay Logic to Ladder LogicPB1 LS1 SOL2 I/4 I/6 O/0

||LS1 LS3 B/0 CR3 |/| LS4 M1 PB2 CR3 I/7 I/5

|| ||I/8

()B/0

|| || |/|

()

I/9

O/1

||

()

INPUT Address Assignment: PB1- I/4 PB2- I/5 LS1- I/6 LS2- I/7 LS3- I/8 LS4- I/9 OUTPUT Address Assignment: SOL2- O/0 M1- O/1


Available Instructions

Sequence Input Output Control Logic Timer and Counters Comparison Range Comparison Data Movement Data Shift Step / Step Next Serial Communications Text String Processing File Manipulation

Increment/Decrement Conversion ASCII Number Systems Math Floating Point Math Statistics Scaling PID PID with Auto tune Clock / Date Block Processing IF,THEN,ELSE,LOOP Table Processing LIFO, FIFO


Few more Instructions

SEQUENCERS SHIFT REGISTERS DATA HANDLING HIGH SPEED COUNTER SUBROUTINES


Thanks .


P L C - Gen

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