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PRESENTATION ON PROGRAMMABLE LOGIC CONTROLLER

HIMSHEKHAR DAS 11-1-3-042GNYANDEEP BISWAS 11-1-3-048

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OBJECTIVES

1. What is a PLC and Why was it developed 2. Familiarization with major components of

PLC3. PLC Operation4. Ladder Logic5. Simple Ladder Logic Program6. PLC Features7. Ladder Diagram of a DC Motor Control Circuit

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INTRODUCTION

PLC is a digital computer used for automation of electromechanical processes such as control of machinery on factory assembly lines , amusement rides.

PLC’s have multiple inputs and outputs and are designed for extended temperature ranges, immunity to electrical noise etc

The control program is stored in non-volatile memory

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WHY PLC WAS DEVELOPED

Before the PLC, control was done using relays, drum sequencers, closed loop control system. Since they could number in thousands the process of updating such facilities was time consuming and expensive.

Digital computer were now used .But these computers required the following attributes: tolerating the shop floor environment , require less training for use ,have fast response time

Hence the PLC was developed and it came into use

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DEVELOPMENT

PLC’s were designed to replace relay logicPLC’s were programmed using ladder logic

(similar to schematic diagram representation of relay logic) or instruction list programming

Mordern PLC’s are programmed using State logic ,BASIC,C

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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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Major Components of a Common PLC

POWER SUPPLY

Provides the voltage needed to run the primary PLC components

I/O MODULES

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

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Major Components of a Common PLC

PROCESSOR

Provides intelligence to command and govern the activities of the entire PLC systems.

PROGRAMMING DEVICE

used to enter the desired program that will determine the sequence of operation and control of process equipment or driven machine.

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PLC

INPUTS

OUTPUTS

MOTOR

LAMPCONTACTOR

PUSHBUTTONS

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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– Diagnostics · Once the program is executed, the CPU performs diagnostics and

communication tasks

• 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.

PHASE 4 - Output Status Scan

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LADDER LOGIC

Each device in the relay rack is represented by a symbol on the ladder diagram with connections between those devices

Used to develop software for programmable logic controllers used in control applications

Used in conjunction with HMI interface Ladder logic can be thought of as a rule based

language rather than procedural language. A “rung” in the ladder represents a rule

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A SIMPLE LADDER LOGIC PROGRAMPre-requisite

Ladder diagram is read from top to bottom or from left to right

The vertical lines are called rails . They provide power to the circuitry. AC or DC power may be connected

The horizontal lines are called rungs .They are composed of input or output.

Output from a PLC is called coil. A coil may be a motor , pump etc.

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Inputs/Sensors to a PLC are referred to as Contacts and may consist of switches, buttons, etc. Contacts begin in one of two states normally open or normally closed.

These contacts have an initial and follow-on state

Stop buttons are given a higher order of importance and therefore be shown ahead of other devices.

The normal condition of electrical diagrams is the circuit de-energized and with no external forces such as pressure, flow, etc. acting on the device.

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L1L2

Ladder Rail

Ladder Rung

Schematic Ladder Diagram.

INPUT OUTPUT

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OFFLogic 0

IN

PLC

InputModule

24 V dc

ONLogic 1

IN

PLC

InputModule

24 V dc

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All contacts associated with a device change state when

the device is energized Devices that perform a STOP function are normally

placed in series on a rung Devices that perform a START function are normally

placed in parallel or in a branch configuration.Some commonly used ladder symbols : -( )- : A regular coil energized when rung is closed -(\) : A “not” coil , energized when rung is open -[ ]- : A regular contact, closed whenever its

corresponding coil is energized -[\]- :A “not” contact , closed whenever its coil is not

energized -- -- :Boxes represent various functions such as

timers ,counters , etc.

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Logical AND : |--------[ ]----------[ ]-----------( )------ key 1 key 2 dc motor when the normally open contacts of both the switches close ,

electricity is able to flow and the dc motor starts Logical AND with NOT : |------------[ ]------------[\]-------------( )-------- close door obstruction door motor The above realizes the function : Door Motor The circuit shows a pushbutton that closes a door and a

obstruction detector that senses if something is in the way of closing the door. When the normally open pushbutton closes and there is no obstruction the normally closed obstruction detector is closed and current is able to flow.

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CIRCUIT 1

CIRCUIT 2

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PROGRAMMING

Early PLC’s were programmed using proprietary programming panels which had dedicated function keys representing the various logical elements of PLC programs

Recently PLC’s are programmed using application software on personal computers, the software allows entry and editing of the ladder style logic

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FUNCTIONALITY

Primary functionality includes relay control , motion control , process control , distributed control systems and networking

Programmable logic relay are used in light industry where only a few points of I/O are involved and low cost is desired

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More About PLC

FEATURES SCANSYSTEM SCALEUSER INTERFACE COMMUNICATION PLC PROGRAMMING

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Tank Used to Mix Two Liquids

A

B

C

FS

MOTOR

TIMER

FLOAT SWITCH

SOLENOIDS

SOLENOID

1 -MINUTE

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DC MOTOR STARTING CIRCUIT

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