Applications of Programmable Logic

Controllers

DG31 34

Purpose

• Unit purpose:• This Unit is designed to introduce candidates to

Programmable Logic Controllers (PLCs) and enablethem to understand how PLCs are applied to controlindustrial processes.

• The Unit allows candidates to develop the necessaryknowledge and skills to allow them to understand thebasic construction and operation of PLCs.

• The Unit also provides candidates with the opportunity todevelop practical programming skills to enable them toapply a PLC to simulate control of a specified industrialprocess.

Learning Outcomes

1. Explain and classify PLC hardware.2. Describe the operation of PLC

software. 3. Solve an industrial related control

problem by the safe application of PLC technology.

Assessment - 1

• The assessment for Outcomes 1 and 2 inthis Unit should be combined together intoone written assessment paper.

• This paper should be taken by candidatesat one single assessment event thatshould last one hour and thirty minutes.

• This assessment should be conductedunder controlled, supervised conditions.

Assessment - 2

• Outcome 3 should be assessed by anassignment in which candidates are asked tocomplete a series of tasks to enable them toapply a PLC to simulate the safe control of aspecified industrial process.

• The assignment tasks should involve devisingthe control strategy, writing PLC software,programming of the PLC, verifying correctoperation of the program and documentation ofthe final solution.

Assessment - 2

• Candidates should complete theassignment in eight hours.

• Candidates should have access to a PLCand be allowed to use any relevant coursenotes, textbooks and reference materialfor the PLC.

• Both the written assessment and thepractical assignment should be carried outat the end of the delivery of the Unit.

Programmable Logic Controller

What is a PLC

A Programmable Logic Controller (PLC) is a mini computer specifically designed for industrial use.

PLC Origin

Developed to replace relays in the late 1960s

Costs dropped and became popular by 1980s

Now used in many industrial designs

Purpose of PLC

• Pneumatic and Hydraulic Machines• Robots• Production Processes• Packaging Lines• Signalling Systems (Traffic Lights etc)• Refining Processes

Examples of PLC Applications are:

Purpose of PLC

• Old control room

Purpose of PLC

• Modern control room

Architecture of PLC

COMMUNICATIONINTERFACE

OU

TP

UT

MO

DU

LEIN

PU

T M

OD

UL

E

ROM

CPU

RAM

MONITOR

PROGRAMMERA/D CONVERTER

Architecture of PLC

Classification of PLC’s

• Large• Medium • Small

PLCs are Classified by :•Number of Input / Output terminals

•Available Memory

•Into three categories

Classification of PLC’s

• Classifying any microprocessor system in terms ofmemory size tends to be out of date the moment it isprinted.

• As a result use the following as a guide only.

• Overlaps are likely due to available expansion modules.

PLC Classification I/O Count

Large PLC Medium PLC Small PLC Micro PLC Nano PLC

> 1024 < 1024 < 256 < 32 < 16

Typical Classification of PLC by the Number of I/O

Memory

> 10kB RAM < 10kB RAM >1kB RAM < 1kB RAM < 1k\b RAM

Types of PLCUnitary

modular Rack Mounted

The Unitary PLC contains every feature of a basic system in one box. They are attached to the machine being controlled.

Modular use a range of modules that slot together to build up a system. The basic modules are the power supply, the main module containing the CPU, the input module and the output module. Other modules such as A/D converters may be added. The main advantage is that the number of input and output terminals can be expanded to cope with changes to the hardware system.

This is a similar concept to the modular design but the modules are on standard cards that slot into a standard rack inside a cabinet. These are flexible and allow expansion of the system.

Inputs (Sensors)•Proximity Switches•Switches•Potentiometer•LVDTs

Position (linear and angular)· Temperature· Speed· Pressure· Weight· Quantity· Flow rate· Depth· Density· Acidity· Voltage· Current· Torque· Power

Input Module Protection

Live

Neut

Outputs (Actuators)

Conveyor Belt

Linear Electric Actuator

Solenoid Valve

Steam & High Pressure Solenoid Valves

High Pressure Steam Solenoid

Water Solenoid Valve

Traffic Lights

Alarms

Heating Elements

PLC Status

For example:Power On

Program Running

Programming Mode

Fault

There are usually a number of visible status indicators on a PLC.

They do not all necessarily appear on every PLC.

Communicating with a PLC

COAXIAL CABLES

TWISTED PAIR

FIBRE OPTICS

Computer

PLC PLCPLC PLCPLC PLCPLC PLC

Make AMake BStore AStore B Inspection PackagingAssembly Warehouse

RIBBON CABLES

The Scan Cycle

HOUSE KEEPING

INPUT MODULE

PROGRAM LOGIC

OUTPUT MODULE

PLC Self test

Check I/P status

Apply current logic

Set/reset O/P status

Scan Cycle

Response Time

SELF TEST - Checks to see if all cards error free, reset watch-dog timer, etc. (A watchdogtimer will cause an error, and shut down the PLC if not reset within a short period oftime - this would indicate that the ladder logic is not being scanned normally).

INPUT SCAN - Reads input values from the chips in the input cards, and copies their valuesto memory. This makes the PLC operation faster, and avoids cases where an inputchanges from the start to the end of the program (e.g., an emergency stop). There arespecial PLC functions that read the inputs directly, and avoid the input tables.

PROGRAM LOGIC SOLVE/SCAN - Based on the input table in memory, the program is executed 1step at a time, and outputs are updated. This is the focus of the later sections.

OUTPUT SCAN - The output table is copied from memory to the output chips. Thesechips then drive the output devices.

Any Questions

PLC Faults

The PLC has certain diagnostic, monitoring and testing facilities within the software.

On most PLC’s Light Emitting Diodes (LED’s) shows the status of the inputs and outputs, some show status by way of a Liquid Crystal Display (LCD).

It is also possible to fix a bank of switches to the input side and test a programme by setting the switches to a known states and seeing if the appropriate output action is taken.

The most advanced method - connects the PLC to a computer with appropriate software and runs a complete simulation of the system being controlled showing the status of everything.

Fault Finding Guide

Shield controller from possible sources of interference: check over-voltage protection and reload program

Gross electromagnetic interference or lightning damageIntermittent fault or spurious operation

Loss battery lamp is on. Replace batter, reload program Loss of power to RAM memoryNo program

Set to RUN if safe to do so Fault lamp confirms this. Reload good copy of program and set to run. Use diagnostic functions to check scan time: if max. value has been exceeded analyze program to find a means of reducing it

Mode set to STOP by hand or by programCPU or MEM fault

Program stopped

Find out which one by reading ladder and checking LEDs or by monitoring the program Output LED is off but the destroyed output itself remains on relay contacts welded. transistor or triac faulty) Replace module

Control signals are keeping it on Module switching device

Output fails to turn off

Confirm that all signals for turning on the output are correct. Check output LED and output voltage. Replace module. etc.

Module faultSingle output device fails to turn on

Output LED is on. voltage at output is normal and voltage at device is absent Output LED is on. voltage at output is normal and voltage at device is normalFind out which one by reading ladder and checking LEDs or by monitoring the program

Wiring faultDevice faultAbsent input signal

Single output device fails to turn on

Output LEDs on: short circuit or earth fault may be the cause; check for voltage and repair or reset as necessaryOutput LEDs are off: replace module using recommended procedure

Fuse or (mcb) for that group of outputs has operatedFaulty output module

Group of outputs fail

Test the control circuit supply. using a voltmeter. Repair or reset as necessary Run lamp not on:CPU/MEM lamp may indicate an error. Secure safety functions and restart program

Control circuit fuse or miniature circuit-breaker (mcb) operatedProgram stopped

All outputs fail

Consider stopping program. Disconnect inputs one by one until the input voltage returns

Short circuit or earth fault on sensor or wiringAll inputs fail

Every area of the plant will be affectedIf power lamp is off, check the main switch or circuit- breakerInspect each emergency device and reset if it safe to do so

Power failureSupply turned offEmergency stop device activated

Total failure or stoppage of the automation system

RemarksLikely causeSymptom

PLC Faults