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Programmable Logic Controllers( Definition according to NEMA standard ICS3-1978)

A digitally operating electronic apparatus which uses a programming 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 modules, various types of machines or process.

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Advantages of PLCs

• Less wiring.• Wiring between devices and relay contacts are done in the PLC program.• Easier and faster to make changes.• Trouble shooting aids make programming easier and reduce downtime.• Reliable components make these likely to operate for years before failure.

INTRODUCTION TO PLCS

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Major Components of a Common PLC

PROCESSOR

POWERSUPPLY

I MN O P D U UT L E

O M U OT DP UU LT E

PROGRAMMING DEVICE

From SENSORS

Pushbuttons,contacts,

limit switches,etc.

ToOUTPUT

Solenoids, contactors,

alarmsetc.

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The Microprocessor or CPU Module• Is the brain of a PLC system.

• It consists of the microprocessor, memory integrated circuits, and circuits necessary tostore and retrieve information from memory.

• It also includes communications ports to other peripherals, other PLC's or programmingterminals.

• Today's processors vary widely in their capabilities to control real world devices.

• Some control as few as 6 inputs and outputs (I/O) and others 40,000 or more.

• One processor can control more than one process or manufacturing line.

• Processors are often linked together in order to provided continuity throughout the process.

• The number of inputs and outputs PLCs can control are limited by the overall capacity ofthe PLC system hardware and memory capabilities.

• The job of the processor is to monitor status or state of input devices, scan and solve the logic of a user program, and control on or off state of output devices.

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RAM• Random Access Memory is a volatile memory that would lose it's information if power

were removed.This is why some processor units incorporate a battery back up.

• The type of RAM normally used is CMOS or Complementary Metal Oxide Semiconductor.

• CMOS RAM is used for storage of the user's program (ladder logic diagrams) and storage memory.

ROM or Read Only Memory is a non-volatile type of memory. • This means you don't need an external power source to keep information. • In this type of memory, information can be read, but not changed. • For this reason the manufacture sometimes calls this firmware. • It is placed there for the internal use and operation of processor units.

EEPROM or Electrically Erasable Programmable Read Only Memory• Is usually an add-on memory module that is used to back up the main program in

CMOS RAM of the processor. • In many cases, the processor can be programmed to load the EEPROM's program

to RAM if RAM is lost or corrupted.

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Input Module• There are many types of input modules to choose from. • The type of input module used is dependent upon what real world input to the PLC is

desired. • Some examples of inputs are limit switches, electric eyes, and pushbuttons.• DC inputs, such as thumbwheel switches, can be used to enter integer values to be

manipulated by the PLC.• DC input cards are used for this application. • Since most industrial power systems are inherently noisy, electrical isolation is

provided between the input and the processor. • Electromagnetic interference (EMI) and Radio Frequency Interference (RFI) can

cause severe problems in most solid state control systems. • The component used most often to provide electrical isolation within I/O cards is

called an optical isolator or Opto-coupler. • The wiring of an input is not complex. • The object is to get a voltage at a particular point on the card. • Typically there are 8 to 32 input points on any one input module. • Each point will be assigned a unique address by the processor. • Analog input modules are special input cards that use analog to digital conversion

(A to D) to sense variables such as temperature, speed, pressure, and position. • The external device normally is connected to a controller (transducer) producing an

electrical signal the analog input card can interpret. • This signal is usually (4 to 20) mA or a (0 to 10) volt signal.

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Output Module

Output modules:• can be used for ac or dc devices such as solenoids, relays, contractors,

pilot lamps, and LED readouts. • Output cards usually have from 6 to 32 output points on a single module. • The output device within the card provides the connection from the user

power supply to the load. • Usually silicon controlled rectifiers (SCR), triac, or dry contact relays are use

for this purpose. • Individual outputs are rated most often at 2 to 3 Amperes. • Output cards, like input cards have electrical isolation between the load

being connected and the PLC.• Analog output cards are a special type of output modules that use digital to

analog conversion (D to A). • The analog output module can take a value stored in a 12 bit file and

convert it to an analog signal. • Normally this signal is (0 -10) volts dc or (4 to 20) m A.• This analog signal is often used in equipment such as motor operated

valves and pneumatic position control devices.

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NAND Gate

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NOT OPERATION

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.

RungA

C

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INPUTS

• These are the physical connections from the real world to the PLC.

• They can be limit switches, push buttons, sensors, anything

that can "switch" a signal on or off.

• The voltage of these devices are usually, but not always,  24 Volt DC.

• PLC Manufacturers make inputs that can accept a wide range of voltages both ac and dc.

• It should be remembered that an input will be ON, "status 1", when the voltage is present at the input connection and OFF, "status 0", when the voltage is no longer present at the input connection.

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OUTPUTS

• These are the connections from the PLC to the real world. They are used to switch solenoids, lamps, contactors, etc. ON and OFF.

• Again they are usually 24 Volt DC, either relay or transistor, but can also be 115/220 Volt AC.

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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.

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Common Industrial Hardware Representing the Binary Concept

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I/O Module

DC INPUT MODULE

OPTO-ISOLATOR

IS NEEDED TO: Prevent voltage transients from damaging the processor.Helps reduce the effects of electrical noise

CurrentLimitingResistor

FROM INPUTDEVICE

USE TO DROP THE VOLTAGE TO LOGIC LEVEL

Buffer, Filter, hysteresis Circuits

TOPROCESSOR

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I/O Module

AC INPUT MODULE

OPTO-ISOLATOR

IS NEEDED TO: Prevent voltage transients from damaging the processor.Helps reduce the effects of electrical noise

Rectifier,ResistorNetwork

FROM INPUTDEVICE

CONVERTS THE AC INPUT TO DC AND DROPS THE VOLTAGE TO LOGIC LEVEL

Buffer, Filter, Hysteresis Circuits

TOPROCESSOR

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PLC

INPUTS

OUTPUTS

MOTOR

LAMPCONTACTOR

PUSHBUTTONS

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L1 L2

P. B SWITCH

INPUT MODULE

WIRING DIAGRAM

LADDER PROGRAM

I:2

0

I= Input Moduleslot # in rack

ModuleTerminal #

Allen-Bradley 1746-1A16

Address I:2.0/0

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Discrete Input

A discrete input also referred as digital input is an input that is either ON or OFF are connected to the PLC digital input. In the ON condition it is referred to as logic 1 or a logic high and in the OFF condition maybe referred to as logic o or logic low.

Normally Open PushbuttonNormally Closed Pushbutton

Normally Open switch

Normally Closed switch

Normally Open contact

Normally closed contact

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OUT

PLC

AnalogOutputModule

An analog output is an output signal that has a continuoussignal. Typical outputs may vary from 0 to 20mA, 4 to 20mAor 0 to10V.

Analog Output

EP

Pneumatic control valve

Supply air

Electric to pneumatic transducer

0 to 10V

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PLC Operation

Basic Function of a Typical PLC

Read all field input devices via the input interfaces, execute the user program stored in application memory, then, based on whatever control scheme has been programmed by the user, turn the field output devices on or off, or perform whatever control is necessary for the process application.

This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.

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While the PLC is running, the scanning process includes the following four phases, which are repeated continuously as individual cycles of operation:

PHASE 2Program

Execution

PHASE 3Diagnostics/

Comm

PHASE 4Output

Scan

PHASE 1Read Inputs

Scan

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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

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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.

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Selecting a PLC

Criteria

• Number of logical inputs and outputs.• Memory• Number of special I/O modules• Scan Time• Communications• Software

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A Detailed Design Process

1. Understand the process2. Hardware/software selection3. Develop ladder logic4. Determine scan times and memory requirements

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Specifications

OUTPUT-PORT POWER RATINGS

Each output port should be capable of supplying sufficient voltage and current to drive the output peripheral connected to it.

SCAN TIME

This is the speed at which the controller executes the relay-ladder logic program. This variable is usually specified as the scan time per 1000 logic nodes and typically ranges from 1 to 200 milliseconds.

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PLC Status Indicators

•Power On

•Run Mode

•Programming Mode

•Fault

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Examples of PLC Programming Software:

1. Allen-Bradley – Rockwell Software RSLogix5002. Modicon - Modsoft3. Omron - Syswin4. GE-Fanuc Series 6 – LogicMaster65. Square D- PowerLogic6. Texas Instruments – Simatic6. Telemecanique – Modicon TSX Micro

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What is a PLC?

A special computer for logic controls using ladder logic programming.

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PLC a Real World Relay Replacement

Real world Parts:

• Switch

• Bell

• Relay

Switch controlling AC circuit or bell

Relay

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Example of a Motor Control

STARTSTOP

output

START STOP

output

EQUIVALENT DIAGRAMS

Normally Open Contact

NormallyClosed Contact

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How to program PLCs?

• Using a specialized Ladder Logic Software– Allen-Bradley uses RsLogix 500 software to program

SLC, Micrologix and so on

– Omron uses Cx Programmer software for its SYSMAC PLCs

• Other manufacturers has their own software

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Ladder Logic

• Ladder logic is a method of drawing electrical logic schematics

• A graphical language popular for PLCs Controllers

• Invented to describe logic made from relays• Language resemble ladders, with two vertical

"rails" and a series of horizontal "rungs" between them

• Rule-based language, rather than a procedural language

• Executed sequentially by software, in a loop

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Examples

Relay Logic

Ladder Logic

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Binary Data Representation

• We communicate to others using groups of letters arranged into words.

• The PLC uses groups of bits called words.

• Different bit patterns represent different information.

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PLC Words

• One measure of a computer’s capabilities is the length of the data words on which it can operate.

• Current PLCs use 16-bit words.

• Newer PLCs use 32-bit words.

• SLC 500 family PLCs are 16-bit computers.

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Information Represented as Combinations of Bits

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Parts of a 16-Bit Word

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Bytes, Nibbles, and Bits

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16-Point Module’s I/O Points Represented in a

Word

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Physical Input Conditions and the Corresponding Input Data

Word

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8-Point Input Module Represented in a Word

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24-Point I/O Module Represented in Two Words

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Two Words Representing Inputs for a 32-Bit Module

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Data Table Format

• Words are 16 bits.– Bits 0 through bit 15

• First word or bit is always 0.

• SLC 500 data tables can contain up to 256 words (0 to 255).

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Words Arranged in a Data Table

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Input Data File (1 of 2)

• Each input screw terminal has one memory location to store on or off status.

• Input data is stored in the input data file. – Also called the input status file

• Input status file holds input status information, which is used to solve ladder program.

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Input Data File (2 of 2)

• Identified as an I-type data file

• Only one input status file allowed per project

• Only has words created for actual modules in system

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Output Data File (1 of 3)

• Each output screw terminal has one memory location to store on or off status.

• Output data is stored in the output data file, also called the output status file.

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Output Data File (2 of 3)

• Output status file holds output status information to update outputs

• Is the result of solving the ladder program.

• Output data sent to modules during output update portion of scan

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Output Data File (3 of 3)

• Identified as an O-type data file

• Only one output status file allowed per project

• Only has words created for actual modules in system