PLC - Introduction

What does PLC stand for?

PLC - programmable logic controller

PLC implements logic control functions by means of a program

PLC - Introduction

An application example 1: Gate Control

PLC can sense a vehicle at the entrance or exit, and open and close the gate automatically

The current vehicle count is easily determined by programming a simple counter

An application example 2: Conveyor System

PLC can be used to start/stop latching logic for motor controlCounters can be used for monitoring product amounts

PLC - Introduction

Comparing traditional and programmable control systems - 1

PLC - Introduction

In traditional control, the switches S1, S2 and S3 must close for K1 to be turned on - the wiring makes the rule

In PLC systems, the program is written to perform the logic “when S1 is closed AND S2 is closed AND S3 is closed, THEN turn on K1” - the program makes the rule

PLC - Introduction

How does a PLC differ from a computer?

A computer is optimized for calculation and display tasks

A computer is programmed by specialists

A PLC is designed for (logic) control and regulation tasks

A PLC is programmed by non-specialists

A PLC is well adapted to industrial environment

PLC - Introduction

Why are PLCs so common?

They are cost-effective

They are flexible, reliable and compact

They have significant advantages over traditional control systems based on relay or pneumatics

PLC - Introduction

Where are PLCs used?

In every industry where automation is involved, from individual machines to whole processes

PLC - Introduction

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What tasks do PLCs perform?

The logic control tasks such as interlocking, sequencing, timing and counting (previously undertaken with relays or pneumatics)

In addition, PLCs can perform a variety of calculation, communication and monitoring tasks

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Inputs

Outputs & Power Supply

Com

mun

icati

onPo

rts

(RS-

485)

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Structure of a PLC

Level of Liquidin Tank

The PLC processor

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PLC Input/Output Devices

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PLC Input Devices Push buttons Switches (limit switches, level switches, etc.) Sensors ...

PLC Output Devices

Relay contacts

Solenoid valves

Signal devices (such as lamps, alarms, etc.)

Motors

...

PLC(Programmable Logic Controller)

Inputs Output Devices

CR

• A solid state (electronic) device that controls output devices based on input signals and a user developed program.• Originally developed to directly replace relays used for

discrete control.

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Understanding the PLC Operating Cycle

House Keeping

STARTInput Scan

Program Scan

Output Scan

PLC OPERATINGCYCLE

The status of external inputs (terminal block voltage) is written to the Input image (“Input file”).

Each ladder rung is scanned using the data in the Input file. The resulting status (Logic being solved) is written to the Output file (“Output Image”).

The Output Image data is transferred to the external output circuits, turning the output devices ON or OFF.

Internal checks on memory, speed and operation. Service any communication requests, etc.

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What you must consider when selecting a PLC• Inputs/Outputs

– How many Inputs/Outputs? including embedded, local expansion, and networked I/O

• 10, 16, 20, 32, 138, 156, >256

– How many Discrete vs. Analog

• Type of I/O• AC, DC, Analog,

Thermocouple, sourcing, sinking, etc.

• Communications Networks• DF1 Full Duplex, DF1 Half

Duplex, DF1 Radio Modem, DH485, ModBus Master / Slave

• DeviceNet, Ethernet

• Functions required– PID– PTO/PWM (Pulse Train Output/Pulse Width Modulated)– Data Logging– Messaging between PLC’s– Math Calculations

• Memory Size• 1k, 6k, 8k 12k, 14k,

Ladder diagrams are specialized schematics commonly used to document industrial control logic systems

They are called "ladder" diagrams because they resemble a ladder, with two vertical rails (supply power) and as many "rungs" (horizontal lines) as there are control circuits to represent.

The "L1" and "L2" designations refer to the two poles of a 120 VAC supply,

Digital Logic Functions

OR

AND

NOT

XOR

Permissive and interlock circuits

A practical application of switch and relay logic is in control systems where several process conditions have to be met before a piece of equipment is allowed to start

e.g. 01.

A burner control for large combustion furnaces. In order for the burners in a large furnace to be started safely, the control system requests "permission" from several process switches, including

high and low fuel pressure, air fan flow check, exhaust stack damper position, access door position, etc.

Each process condition is called a permissive, and each permissive switch contact is wired in series, so that if any one of them detects an unsafe condition, the circuit will be opened

e.g. 02.

Application of relay logic is in control systems where we want to ensure two incompatible events cannot occur at the same time.

An example of this is in reversible motor control, where two motor contactors are wired to switch polarity (or phase sequence) to an electric motor, and we don't want the forward and reverse

Take note of the normally-closed "OL" contact, which is the thermal overload contact activated by the "heater" elements wired in series with each phase of the AC motor. If the heaters get too hot, the contact will change from its normal (closed) state to being open, which will prevent either contactor from energizing.

This control system will work fine, so long as no one pushes both buttons at the same time. If someone were to do that, phases A and B would be short-circuited together by virtue of the fact that contactor M1 sends phases A and B straight to the motor and contactor M2 reverses them; phase A would be shorted to phase B and visa-versa. Obviously, this is a bad control system design!

To prevent this occurrence from happening, we can design the circuit so that the energization of one contactor prevents the energization of the other. This is called interlocking, and it is accomplished through the use of auxiliary contacts on each contactor

Can you explain following ladder diagrams ?

Product packaging is one of the most frequent cases for automation in industry

Industry Application:

Cxxx is the name of the counter

yyyyy is the number of pulses we want to count before doing something

Counters

: . Timers

An accumulating timer

Case1:Ensure two incompatible events cannot occur at the same time. An example of this is in reversible motor control, where two motor contactors are wired to switch polarity (or phase sequence) to an electric motor, and we don't want the forward and reverse contactors energized simultaneously

When contactor M1 is energized, the 3 phases (A, B, and C) are connected directly to terminals 1, 2, and 3 of the motor, respectively.

However, when contactor M2 is energized, phases A and B are reversed, A going to motor terminal 2 and B going to motor terminal 1.

This reversal of phase wires results in the motor spinning the opposite direction. Let's examine the control circuit for these two contactors:

Take note of the normally-closed "OL" contact, which is the thermal overload contact activated by the "heater" elements wired in series with each phase of the AC motor. If the heaters get too hot, the contact will change from its normal (closed) state to being open, which will prevent either contactor from energizing.

system will work fine, so long as no one pushes both buttons at the same time

BUT If someone were to do that ????

To prevent this occurrence from happening, use interlocking….

Try………………

when M1 is energized, the normally-closed auxiliary contact on the second rung will be open, thus preventing M2 from being energized, even if the "Reverse" pushbutton is actuated. Likewise, M1's energization is prevented when M2 is energized

Explain this….

When the "Forward" pushbutton is actuated, M1 will energize, closing the normally-open auxiliary contact in parallel with that switch.

When the pushbutton is released, the closed M1 auxiliary contact will maintain current to the coil of M1, thus latching the "Forward" circuit in the "on" state. The same sort of thing will happen when the "Reverse" pushbutton is pressed.

Exercise: Explain the following ladder diagram

Answer:

If the motor has been running in the forward direction, both M1 and TD1 will have been energized.

This being the case, the normally-closed, timed-closed contact of TD1 between wires 8 and 5 will have immediately opened the moment TD1 was energized.

When the stop button is pressed, contact TD1 waits for the specified amount of time before returning to its normally-closed state, thus holding the reverse pushbutton circuit open for the duration so M2 can't be energized.

When TD1 times out, the contact will close and the circuit will allow M2 to be energized, if the reverse pushbutton is pressed. In like manner, TD2 will prevent the "Forward" pushbutton from energizing M1 until the prescribed time delay after M2 (and TD2) have been de-energized.

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Leading Brands Of PLC

AMERICAN 1. Allen Bradley2. Gould Modicon3. Texas Instruments 4. General Electric5. Westinghouse6. Cutter Hammer7. Square D

EUROPEAN 1. Siemens2. Klockner & Mouller3. Festo 4. Telemechanique

JAPANESE 1. Toshiba2. Omron3. Fanuc4. Mitsubishi

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PLC Size

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

- these PLC’s are capable of providing simple to advance levels or machine controls.

2.MEDIUM - 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.

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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 0 or logic low.

Normally Open PushbuttonNormally Closed Pushbutton

Normally Open switch

Normally Closed switch

Normally Open contact

Normally closed contact