Prepared By:J S Shekhawat

Sr. Engg. C&I

(Programmable Logic Controller)

Prepared By:J S Shekhawat

Sr. Engg C&I

What is PLC?

PLC is a Programmable Logic Controller and also

a kind of a digital computer which can be

Programmed as per the process which is needed

to be controlled, designed for multiple inputs

and output arrangements, having immunity to

electrical noise and resistance to vibration and

impact.

History of PLC

PLC was introduced in late 1960’s

First commercial & successful Programmable Logic

Controllers was designed and developed by Modicon

as a relay replacer for General Motors.

Earlier, it was a machine with thousands of electronic parts.

Later ,in late 1970’s,the microprocessor became reality & greatly enhanced the role of PLC permitting it to evolve form simply relay to the sophisticated system as it is today.

Major Components of a PLC

CENTRAL PROCESSING

UNIT (CPU)

POWERSUPPLY

I MN O P D U UT L

E

O M U OT DP UU LT E

PROGRAMMING DEVICE

From SENSORSPushbuttons,

contacts,limit switches,

etc.

ToOUTPUT

Solenoids, contactors,

alarmsetc.

Handheld Device or Computer

etc.

Major Components of a PLC

Major Components of a PLC Central Processing Unit It is a micro-controller based circuitry. The CPU consists of

following blocks :Arithmetic Logic Unit (ALU), Program memory, Process image

memory (Internal memory of CPU), Internal timers and counters.

CPU performs the task necessary to fulfill the PLC funtions. 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.

Input module

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

Analog input module : Typical input to these modules is 4-20 mA, 0-10 V

Example : 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

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

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.

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 i.e. wires / tracks

PLC operation sequence

1) Self test: Testing of its own hardware andsoftware for faults.

2) Input scan: If there are no problems, PLC will copy all the inputs and copy their valuesinto memory.

3) Logic solve/scan: Using inputs, the ladderlogic program is solved once and outputs are updated.

4) Output scan: While solving logic the outputvalues are updated only in memory when ladder scan is done, the outputs will be updated using temporary values in memory.

Self test

Input scan

Logic scan

Outputscan

Programming Languages of PLC

Most common languages encountered in PLC programming are:

1) Ladder Logic

2) Functional Block Diagram

3) Sequential Function Chart

Ladder Logic

The ladder logic is the oldest programming language for PLC.

It is well suited to express Combinational logic.

The main ladder logic symbols represent the elements :

make contact

break contact

relay coil

Block diagram of a PLC

Outputs

LEDs

Inputs

Programming Example:Ladder Logic Program for Start/Stop of Motor :

X2X1

Y1

Y1

Programming PLC:

Starting of Motor:

Continuous Running of motor when Start Button is released:

To Stop the Motor :

PLC Selection Criteria Cost of hardware, software, Integration Engineering, Design, Installation,

Start-up and Commissioning, Validation documentation and Execution, Training, Spare parts, 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.

Advantages of PLCs: Reliability.

Flexibility in programming and reprogramming.

Cost effective for controlling complex systems.

Small physical size, shorter project time.

High speed of operation.

Ability to communicate with computer systems in the plant.

Ease of maintenance /troubleshooting.

Reduced space.

Energy saving.

Disadvantages of PLCs

PLC devices are proprietary it means that part or software of one manufacturer can’t be used in combination with parts of another manufacturer.

Limited design and cost option

Fixed Circuit Operations.

PLCs manufacturers offer only closed architectures.

Applications:

Wherever automation is desired the PLCs are best

suited to meet the task.

Few examples of industries where PLCs are used :

1) Robots : manufacturing and control

2) Car park control

3) Train control station system

4) Food processing

5) Materials handling

6) Machine tools

7) Conveyer system etc.

(Distributed Control System)

Prepared By:J S Shekhawat

Sr. Engg C&I

Distributed control system (DCS) DCS is a system of dividing plant or process control into

several areas of responsibility, each managed by its own controller, with the whole system connected to form a single entity, usually by means of communication buses. Distributed Control System (DCS) refers to a control system usually of a manufacturing system, process or any kind of dynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. The entire system of controllers is connected by networks for communication and monitoring.

The entire system of controllers is connected by networks for communication and monitoring.

Why it is called DISTRIBUTED???As is apparent from the abbreviation, the word ‘Distributed’supports following functionality’s

Physical Distribution - Nodes or Subsystems can beDistributed i.e. located physically apart

Functional Distribution - Specific Functionality is impartedfor a Node basing on the combination of hardware andsoftware used. For e.g. Application work-processor withHistorian, Application work-processor with controlconfiguration software

Structural Distribution - Different Structural hardwareplatforms (Application Workstation processor, Workstationprocessor, Control processor etc.) are used to achieve therequired functionality.

What does DCS System consists of?DCS System consists minimum of the following components.

Field Control station (FCS): It consists of input/output modules, CPU and communication bus.

Operator station: It is basically human interface machine with monitor, the operator man can view the process in the plant and check if any alarm is presents and he can change any setting, print reports..etc...

Engineering station: It is used to configure all input & output and drawing and any things required to be monitored on Operator station monitor.

Process Control Systems

Process Control Systems

Digital Control System

Drawbacks Of CCS:

If the CPU fails the entire plant gets affected.

Redundancy concept is not available.

Redundancy is having two controllers. One would be active and the other would be standby. If the active controller fails, the standby controller takes over.

Digital Control System

How does it work?

Example of simple control mechanism

Process measured value (PV) Manipulated value (MV) Set Value (SV).

Operation WindowsInformation regarding the process is gathered as well asmonitored by the following Standard Operation windows : Tuning Window Control Group Window Trend Window Process Alarm Window Operator guide Message Window Graphic Window Overview Window Process Report Window Historical Report Window

Control function is distributed among multiple CPUs (Field Control Stations). Hence failure of one FCS does not affect the entire plant.

Redundancy is available at various levels. Instruments and interlocks are created by software. Generation and modifications of the interlocks are very

flexible and simple. Information regarding the process is presented to the

user in various formats. Field wiring is considerably less. Maintenance and trouble shooting becomes very easy. Cost effective in the long run.

Advantages of DCS

WHY DCS ? For Total Plant Automation

For Higher Productivity

For Optimal Process Control

For Advance Process Control

For Regulatory Compliance

For Management Information System

In Tune With Global Requirement

DCS MANUFACTURERS ASEA BROWN BOWERI - ABB TATA HONEYWEL FOXBORO - INVENSYS YOKOGOWA BLUE STAR - YBL ALLEN BRADLEY GE FANUC FISHER ROSEMOUNT TOSHIBA METSO Automation

(Basic Difference between PLC & DCS)

Prepared By:J S Shekhawat

Sr. Engg C&I

Difference Between PLC & DCS A PLC is cost-effective up to a certain I/O count, and so is the

DCS. But the difference is in their starting points: the PLC iscost-effective from 0 to a few thousand I/O points; the DCSbecomes cost-effective starting from a few thousand points andbeyond.

A PLC becomes a subsystem of the DCS in rare occasions whenthe situation calls for it, i.e., purchase of huge package systemswith engineering schedules incompatible with the DCS schedule(I/O lists cannot be submitted on time before the DCS hardwarefreeze date). Note that this package system is a process systemusing continuous control, not discrete. Based on this, a PLC cannever be larger than a DCS in terms of I/O count.

In large plants the DCS is king because most owners want a singlesource of hardware support and service, and this mentalitynaturally denies the PLC a foothold. Package vendors are nolonger required to provide PLC for their system. Everything isconnected to the DCS.

Difference Between PLC & DCS Cont… Generally, PLCs are stand alone and perform a particular task,

where a DCS is a network of PLCs that communicate in somefashion to accomplish a particular task. For example, in a waterfiltration plant, there might be a PLC that is used to perform abackwash of a particular filter, in that same water plant a DCSmay be communicating with 14 filter PLCs and starting thebackwash routine when required.

PLC only handle sequential process than DCS can handled bothContinue process and large loop control.

If we see from security angle, PLC doesn't have dongle so peoplecan crack the software easy. DCS have a dongle so it's onlylicense to industry which have it.

PLC is a programmable Logic controller which is used mainly forinterlocking different equipment’s. DCS is Distributed ControlSystems which is used to control very big plants by using simpleGUI screens. PLCs are interfaced with DCS for interlocking.

Thank You!!!