General Overyiew of Plc

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GENERAL OVERYIEW OF PLC

INDUSTRIAL AUTOMATION ;

Industrial Automation is the use of Control Systems to control Industrial Machinery and

Processes, reducing the need for human intervention.

If we compare a job being done by human and by Automation, the physical part of the

job is replaced by use of a Machine, whereas the mental capabilities of the human are

replaced with the Automation.

The human sensory organs are replaced with electrical, mechanical or electronic

Sensors to enable the Automation systems to perform the job.

For example, a grinding wheel driven by a human can be replaced by a motor (which is

a machine). But starting and stopping the grinding, which were done by the human by

looking at the output, will be replaced by the control of the motor by Automation.

Higher level of human intelligence like planning, analysis, prediction and intuitive

decision making is not done by this Level of Automation.

AUTOMATION COMPARED TO HUMAN ACTIVITY

CONTROL SYSTEMS USED IN INDUSTRIAL AUTOMATION :

Micro-controllers

Programmable (Logic) Controllers orPLC

Distributed Control System or DCS

Computers, etc.


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WHAT IS PROGRAMMABLE (LOGIC) CONTROLLER ;

Programmable (Logic) Controller orPLC is a type of computer commonly used incommercial and industrial control applications.

The prominent manufacturers of PLC are Allen-Bradley (Rockwell), Siemens, GE

Fanuc, Schneider Electric, Mitsubishi, etc.

HOW IS PLC DIFFERENT FROM COMMON COMPUTERS -

The PLC and computers are different in their hardware and software as their target

application and solution areas are different. While the computer is required to do many

activities simultaneously or compute and handle huge amount of data, the PLC is

required to interact with the process and focus on controlling a physical process reliably,

which means, common PC syndromes like hang, crash or reboot can not be tolerated for

a PLC.

Hence, the difference can be summarized as follows:

The number of simultaneous processes handled by PC is more than PLC PC is faster in terms of processor speed and data handling capacity

PLC has higher capability of handling external input output signals, specially

discreet input and outputs

The PLC is more reliable, repetitive and precise for a particular job

PLCs are very rugged and designed to work under harsh ambient conditions

PLC hardware is generally modular and expandable

PLC is essentially a Real Time System

REAL TIME SYSTEM ;

A system is a Real Time System when it can respond to an external event within a

specified window of time.

Responses can be periodic, time-initiated, input driven or interrupt driven. The time is

determined by the time constant of the dynamic process.

Example of time constants for external processes:

Milliseconds for machining or electric power systems

Seconds for flow / pressure processes

Minutes for temperature controlled processes

Weeks/months/years for social / economic processes


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LEVELS OF AUTOMATION ;

Automation can be categorized in various levels based on the required intelligence level.

In relation to the human capabilities, both physical and mental, each level relates to a

particular capability.

COMPONENTS OF PLC ;

A PLC consists of minimum the following components-

Central Processing Unit(CPU)

Memory

Rail/Rack

Input Module

Output Module

Components of PLC:

CPU :

The CPU is the Brain of the PLC. It essentially performs the following functions:


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Updating the Input / Output status

Scanning the application program

Perform all binary (logical) and arithmetic functions as per the inputs and the

application program and generate output

Communicate with Memory and Programming Terminal to read or write programor data

The CPUs are categorized primarily based on their Input/Output handling, special

functions and communication capabilities.

The operating system of the CPU is permanently written in the same. Unlike

personal computers, the hardware and the OS, as well as the programming

software, are always from the same manufacturer.

Memory :

The Memory stores the operating system, the application program and sometimes user

data. Depending on the type of data to be stored, memory can be Read-Only type or

ROM. Operating systems of the CPU are stored in such memory areas. The user

program is generally stored in a Read-Write memory or RAM.

Memory can be built in the CPU or can be external. External memory is normally of

Flash EPROM type. The RAM is normally backed up by the Battery provided in the PLC

in case of power failure.

Power Supply :

The Power Supply provides power to the CPU, Memory and the Input / Output modules.This is the power required by the electronics of the modules to function and NOT the

field power required for the input / outputs.

Rail / Rack :

The modules of a PLC are either mounted on a Rail or a Rack. While the Rail serves

only as a mechanical support for the modules, the Rack serves both as a mounting

support as well as a backplane. The backplane is the PCB board, similar to the

motherboard of a PC, on which modules are connected.


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Input / Output Modules :

If CPU is the Brain of the PLC, Inputs are the Sensors. The information acquired by

the inputs is used by the CPU to generate outputs. Input / Outputs are of two

types:

Digital

Analog

Digital Modules :

Though the digital input and output modules vary through a wide range in terms of size

and type, the following are their characteristics.

Digital input modules are used to read signals from selector switches, push

buttons, limit switches, proximity switches, etc Digital output modules switch on/offrelays, contactors, lamps, etc.

Digital inputs and outputs can accept 24V DC, 48V DC, 120V AC or 240V AC.

The entire module is for a particular voltage.

The number of inputs / outputs or channels available on a single module can be

8, 16, 32 or 64.

The most commonly used signal voltage level is 24V DC.

Digital input modulescan be either Source Type or Sink Type. For

source type modules, the channel connected to the contact in the field supplies the

Interrogation Current. For the sink type modules, the interrogation current issupplied by else and sent to the channel via the contact. Hence, for source type

modules, the contacts in the field normally have a common return path and for the

sink type input modules, the contacts have a common supply.

Digital outputmodulesalso can be either Source Type or Sink Type. For

source type modules, the channel connected to the device in the field supplies the

LoadCurrent. For the sink type modules, the load current is supplied by else and

sent to the channel via the device. Hence, for source type modules, the devices in

the field normally have a common return path and for the sink type output modules,

the contacts have a common supply.

DIGITAL

OUTPUTS

DIGITAL

INPUTSPLC

CPU


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Types of Digital Inputs and Outputs

Analog Modules :

The Analog Input modules are used to read signals from analog transducers,

potentiometers, etc

Analog Output modules are used to give setpoints to drives or other processes or

for displaying values on meters.

The principle of operation of an analog module is based on the analog to digital

converters or vice versa.

Foranalog input modules, the analog value connected to the terminals of the analog

channel is converted into digital value by means of Analog to Digital Converter (ADC).

The digital value is further processed for diagnostic data like overflow, wire breakage,

etc. before being sent to the CPU via the backplane bus.

For analog output modules, the digital value generated by the CPU is sent to the

analog module via the backplane bus for conversion into analog value by means of

Digital to Analog Converter (DAC). The digital value is processed for diagnostic data like

overflow, etc. before being sent to the DAC.

ANALOGO

UTPUTS

ANALOG

INPUTSPLC

CPU


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Types of Analog Inputs and Outputs

Analog Input Types :

Analog Inputs are available in four types

Voltage 0-10V, +/-10V

Current 0-20mA, 4-20mA

Resistance Pt100, standard resistance (linear)

Thermocouple J, K, N, L, etc.

Voltage Inputs :

Voltage inputs are the one of the most common type of analog inputs. Typical

examples are potentiometer signal and signals from drives.

The voltage signals are used where the distance between the source and

destination of the signal are at distance.

Current inputs :

Current inputs are commonly available from field mounted transducers like

pressure transducer, flow transducer, etc.

The current signals are used where the distance between the source and the

destination is high and thus used for field devices.

The preferred type of current signal is 4-20mA. In these transducers, 4 mA

represents the 0 signal and the 20mA represent the maximum value. The

represents loss of signal or wire breakage. This distinction of fault and normal

signal 0 is not possible for 0-20mA signals.

Resistance inputs :


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Resistance inputs can be of two types

Linear resistance

Non-linear resistance

Linear resistance :

Linear resistance can be a simple resistance or a strain gauge type device.

Non-linear resistance :

Non-linear resistance signals are primarily used for measurement of temperature.

Normally a Pt100 thermometer is used for such measurement.

Thermocouple inputs :

Thermocouples are used for measurement of temperature. Selection of eitherPt100 or thermocouple is done depending on the temperature range required and

the application type.

For various ranges of temperature range, different types of thermocouples are

used. The type names being B, E, J, K, L, N, R, S, T, U, C.

CONFIGURATION OF PLC


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PLC is a modular system can be configured in different ways:

Centralized : In Centralized configuration, the I/Os are located near the CPU or withina short distance from it. The input/outputs, irrespective of their physical location, arewired up to the PLC panel. This type of configuration is useful when the process is

compact and concentrated and not spread across a long distance.

Remote I/O :

In the Remote I/O configuration, the I/Os are distributed in smaller groups and locatednear the devices the signals are connected to. The communication between the CPU

and the I/Os are established with the help of BUS system. The protocol of thecommunication is specific to the manufacturer of the PLC.

The advantages of the Remote I/O systems are as follows:

Savings in wiring cost as the I/Os are wired near the field devices and the major

length of the cable is one single twisted pair cable.

Troubleshooting in field cable problems become easier.

Remote I/O stations are rugged and can be housed in control desks, control

posts, MCC or other similar enclosures.


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PROGRAMMING BASICS

The user develops the Application Program on a Programmer and transfers the

program to the CPU for execution.

Programming is done through a Programming Software Package, which is

dependant on the manufacturer and the type of PLC used.

Almost all prominent PLC vendors have PLCs in Small, Medium and Large

category. The categorization is done on the I/O handling and communicationcapacity.

Some of the CPUs come with a mode switch on it. At PROG position, only

programming can be done, but outputs are not transferred. At REM position, bothprogramming and execution is done. At RUN position, only program execution isdone, no program transfer to the CPU is allowed.

The way of execution of the program in the CPU is based on the Type of

Program it is written into.

Types of Programs :

The application program running in a CPU can be categorized as per their type of

execution

Cyclic Program

Time Interrupt Program

Event Interrupt Program

Startup Program


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The Cyclic Program is the main program of the PLC and is executed cyclically. The

status of inputs from the field and the status of outputs by the PLC is updated once

every cycle.

1) The Time Interrupt Program is a section of the program which is triggered based on

a time value. E.g., this part of the program is executed every 100 mS.

2) The Event Interrupt Program is triggered by an event, which can be either internal

or external. It can be triggered by a hardwired signal or a fault or other internal event.

3) Startup Programs are executed during startup only and are not processed during

normal running.

General Overyiew of Plc

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