Basics Of Plc: Prepared By: Guided By

BASICS OF PLC GUIDED BY:

Prepared By:

MR. SHAILESH JIVANI

JATIN PUROHIT

Contents 

History



Introduction

 PLC Programming language



Difference between PLC and DCS and PC

 Ladder logic



Data flow overview



Major components



PLC scan cycle



PLC communication

 Advantage and disadvantage  Application

History of PLC  Devloped to replace relays in late 1960’s.

 In 1970’s PLC become most common choice for manufacturing controls  Plc was first design and developed by modicon as a relay replacer for general motors

INTRODUCTION

What is PLC? 

PLC is a continuously monitor the status of devices connected as inputs. Based upon a user written program ,stored in memory it control the status of devices connected as output



A programmable logic controller (PLC) is an industrially computer-based unit that performs discrete or continuous control functions in a variety of processing plant and factory environments.

What is PLC?



It is capable of storing instructions to implement control functions such as sequencing, timing, counting, arithmetic, data manipulation and communication.



The I/O interfaces provide the connection between the PLC and the information providers (inputs like pushbuttons, sensors,…) and the controllable devices (outputs like valves, relays, lamps,…).

What is PLC?

Components of automation Sensor: Produce the input value for the controller as feedback. For example, RTD, strain gauge. Controller: Controls the process by sending the particular value according to the set value and feedback coming from the sensor. Difference between them called feedback. According to the behavior of error, control strategies is being decided. Controllers can be Digital controllers, PLCs etc. Actuators: Component which is acting on the response of the controller. And as a result process value will be effected.

comparison between PLC and PC

Difference between PLC and DCS



PLC is usually used for controlling medium or large scale application



Whereas DCS is used for controlling entire plant

Difference between PLC and DCS

Difference between PLC and DCS

Leading Brands Of PLC AMERICAN

1. Allen Bradley 2. Modicon 3. Texas Instruments 4. General Electric 5. Westinghouse 6. Cutter Hammer 7. Square D EUROPEAN 1. Siemens 2K&M 3. Festo

Selecting a PLC Criteria 

•

Number of logical inputs and outputs.

•

Memory

•

Number of special I/O modules

•

Scan Time

•

Communications

•

Software

PLC Size 1. SMALL 

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

2.

MEDIUM



It 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.

PLC SCAN CYCLE PHASE 1

Read Inputs Scan PHASE 2

Program Execution PHASE 3

Diagnostics/ Comm PHASE 4

Output Scan

CYCLE TIME DEPENDS ON THE SIZE OF THE PROGRAMS, NO. OF I/O’S AND AND AMOUNT OF COMMUNICATION REQURIED 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

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.



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 and type of instructions used in the program. The time to make a single scan can vary from 1 ms to 100 ms.

DATA FLOW OVERVIEW

DATA FLOW OVERVIEW

DATA FLOW OVERVIEW

PLC SCAN PROCESS

PROGRAMING DEVICE HAND-HELD UNIT WIH DISPLAY Hand held units are often used on the factory floor for troubleshooting, modifying programs, and transferring program to multiple machines It is not able to display long programming so its not widely used in industy

PROGRAMING DEVICE PC WITH APPROPRIATE SOFTWARE The software allows users to create ,edit and store program The plc is communicate with PC via a serial or parallel data communication

Major components of PLC POWER SUPPLY

From SENSORS

Pushbuttons, contacts, limit switches, etc.

I M N O P D U U T L E

PROCESSOR

PROGRAMMING DEVICE

O U T P U T

M O D U L E

To OUTPUT Solenoids, contactors, alarms etc.

Major components of PLC 1. Power supply : 

The power supply converted line voltage commonly 120 or 240 volts AC



AC Voltage converted into commonly 24v To power on the PLC and its components

2. I/O MODULES : 

Provides signal conversion and isolation between the internal logic- level signals inside the PLC and the field’s high level signal.

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.

INPUT MODULE 

These module act as interface between real time status of process variable and the CPU



Analog input module : typically input to these module is 4-20mA, 0-10v



Example - level Tx , RTD , thermocouple



Digital input module : input to these modules is 24V DC 115V AC 230VAC



switch , pushbuttons relays

OUTPUTS

INPUTS

MOTOR CONTACTOR

LAMP

PLC

PUSHBUTTONS

OUTPUT MODULE  These

module act as a link between the CPU and the output devices

 Analog

output module : typically output to these module is 4-20mA, 0-10v

 Ex

– control valves, speed, vibration

 Digital

output module : output to these modules is 24V DC 115V AC 230VAC

 Ex

– solenoid valve, actuator, lamp, on off control

PLC SCAN PROCESS RUNG

RAIL

PLC SCAN PROCESS

PLC SCAN PROCESS

PLC Communications Serial Communications PLC communications facilities normally provides serial transmission of information. 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.

PLC Communications Local 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.

RS 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.

PLC PROGRAMMING LANGUAGE

 Ladder

diagram (LD)  Sequential Function Charts (SFC)  Function Block Diagram (FBD)  Structured Text (ST)  Instruction List (IL)

IEC 1131-3 is the international standard for programmable controller programming languages. The following is a list of programming languages specified by this standard:

5 types of programming language used in PLC 

1. Ladder diagram (LD) 2.Sequential Function Charts (SFC) 3.Function Block Diagram (FBD) 4.Structured Text (ST) 5.Instruction List (IL)



One of the primary benefits of the standard is that it allows multiple languages to be used within the same programmable controller. This allows the program developer to select the language best suited to each particular task.

Ladder Logic 

Ladder logic is the main programming method used for PLC's. As mentioned before, ladder logic has been developed to relay logic



XIC - Examine if Closed



XIO - Examine if Open



OTE - Output Energize



OTL - Output Latch

XIC Examine if Closed

XIC Examine if OPEN

OTE Output Energize Definition Turns a bit on or off Use OTE instruction in your ladder logic to turn on a bit when rung condition is evaluated as true EXAMPLE Light Motor run signal Internal bits

Output Energize

TIMER 

TON - Timer On Delay



TOF - Timer Off Dealy



RTO - Retentive Timer

TON Timer On Delay Symbol

Definition Count time base intervals when the instruction is true. The Timer On Delay instruction begins to count time base intervals when rung conditions become true. As long as rung conditions remain true, the timer adjust its accumulated value (ACC) each evaluation until it reaches the preset value (PRE). The accumulated value is reset when rung conditions go false, regardless of whether the timer has timed out.

TOF Timer Off Delay

DEFINATION: Counts time base intervals when the instruction is false. The Timer Off Delay instruction begins to count time base intervals when the rung makes a true to false transition. As long as rung conditions remain false, the timer increments its accumulated value (ACC each scans until it reaches the preset value (PRE). The accumulated value is reset when rung conditions go true regardless of whether the timer has timed out.

RTO Retentive Timer DEFINATION: Counts time base intervals when the instruction is true and retains the accumulated value when the instruction goes false or when power cycle occurs. The Retentive Timer instruction is a retentive instruction that begins to count time base intervals when rung conditions become true.

RTO Retentive Timer 

The Retentive Timer instruction retains its accumulated value when any of the following occurs:



Rung conditions become false.



Changing Processor mode from run /Test / program mode.



The processor loses power while battery back up is still maintained.and a fault occurs.

COUNTERS

COUNTERS



CTU - Count Up



CTD - Count Down



RES - Reset

CTU Count UP Increments the accumulated value at each false to true transition and retains the accumulated value when the instruction goes false or when power cycle occurs. The CTU is an instruction that counts false to true transition. When this transition happens the accumulated value is incremented by one count. A CTU accumulation is reset by the RES instruction. If the accumulation value is over the maximum range then the overflow (OV) bit will be true.

RES Reset Resets the accumulated value and status bit of a timer or counter. Use a RES instruction to reset timers or counters. When the RES instruction is enabled, it resets the Timer On Delay, Retentive Timer, and Counter Up, Counter Down instruction having the same address as the RES instruction.

VARIOUS

SYMBOL USED IN LADDER LOGIC

Coils

Coils represent relays that are energized when power flows to them. When a coil is energized it causes a corresponding output to turn on by changing the state of the status bit controlling the output to 1. That same output status bit maybe used to control normally open or normally closed contact anywhere in the program.

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BOXES 

Boxes represent various instructions or functions that are Executed when power flows to the box. Some of these Functions are timers, counters and math operations.

Function Block Diagram 

Function Block diagram (FBD) is used for PLC programs described in terms of graphical blocks.



Functional blocks can have standard functions, such as those of the logic gates or counter or timers or have functions defined by the user, e.g. a block to obtain an average value of inputs.

Statement List 

In statement-list programming approach, an instruction set similar to assembly language for a microprocessor is used. Statement lists, available on few brands of PLCs, are the most flexible form of programming for the experienced user but are by no means as easy to follow as ladder diagrams or logic symbols.

DIGITAL OPERATION

Different Logic Gates Circuit Diagrams 

a logic gate is an idealized or physical device implementing a Boolean function; that is, it performs a logical operation on one or more logical inputs, and produces a single logical output



Logic circuits include such devices as multiplexers, registers, arithmetic logic units (ALUs), and computer memory, all the way up through complete microprocessors, which may contain more than 100 million gates. In modern practice, most gates are made from field-effect transistors (FETs), particularly MOSFETs (metal–oxide–semiconductor field-effect transistors)

Combinational Logic 

Combinational Logic Circuits are made up from basic logic NAND, NOR or NOT gates that are “combined” or connected together to produce more complicated switching circuits. These logic gates are the building blocks of Combinational Logic Circuits.

AND OPERATION A

B

C

Rung



Each rung or network on a ladder program represents a logic operation. In the rung above, both inputs A and B must be true (1) in order for the output C to be true (1).

OR OPERATION C

A Rung

B 

In the rung above, it can be seen that either input A or B is be true (1), or both are true, then the output C is true (1).

NOT OPERATION A

C

Rung 

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.

Programing example

ADVANTAGE 

Very High Accuracy



Low Power Consumption (Energy Saving)



Small in Size (Required Lased space)

 

Easily programmable Easy Maintenance

Disadvantages 

High Skilled Engineering Required for that high skilled person required



Difficulty with changes or replacements



Some high Initial Cost.

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Areas of Application 

Manufacturing / Machining



Food / Beverage



Metals



Power



Mining



Petrochemical / Chemical

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