INTRODUCTION TOINTRODUCTION TO

PROGRAMMABLEPROGRAMMABLELOGICLOGIC

CONTROLCONTROL

PPROGRAMMABLE ROGRAMMABLE LLOGIC OGIC CCONTROL ONTROL (PLC)(PLC)::

“ A digital electronic device that uses a

programmable memory to store instructions

and to implement specific functions such as

logic, sequence, timing, counting and

arithmetic to control machines and process. “

PLCPLCPLCPLC

What is CONTROL?

“ CONTROL is the process in a system in which

one or several input variables influence other

variables “

Definition of ControlDefinition of ControlDefinition of ControlDefinition of Control

CCOONNTTRROOLL

SSYYSSTTEEMM

PPLLAANNTT

INFORMATION

COMMANDS ACTUATORS

SENSORS

A Simple View of a Control SystemA Simple View of a Control SystemA Simple View of a Control SystemA Simple View of a Control System

In open-loop control systems, output variables are influenced by the input variables.

Open-loop Control SystemOpen-loop Control SystemOpen-loop Control SystemOpen-loop Control System

LL

NN

It is characterized by continuous comparison of the desired value (or set point) with the actual value of the controlled variable.

CC

XsXsXiXi

Xi > XsXi > Xs

Xi < XsXi < Xs

LL

NN

Xi - Required valueXi - Required value

Xs - Actual valueXs - Actual value

Closed-loop Control SystemClosed-loop Control SystemClosed-loop Control SystemClosed-loop Control System

The essential difference between programmable control and traditional control technology may be summed up as follows:

» The functions are no longer determined by the wiring, but rather by the program

» Programming is simplified to enable symbols familiar to the control engineer to be used (contacts or logic graphic symbols)

PLC and Conventional Control SystemPLC and Conventional Control SystemPLC and Conventional Control SystemPLC and Conventional Control System

LL

NN

S1S1

S2S2

K1K1

S1S1 S2S2

PLCPLC

K1K1

HardwireHardwire PLCPLC0 V0 V

24 VDC24 VDC

Hardwire and PLC Wiring DiagramsHardwire and PLC Wiring DiagramsHardwire and PLC Wiring DiagramsHardwire and PLC Wiring Diagrams

K1K1

ComparisonComparisonComparisonComparison

Hardwired control systemsHardwired control systems

» The functions are determined by the physical wiring.

» Changing the function means changing the wiring

» Can be contact-making type (relays, contactors) or electronic type (logic circuits)

Programmable control systemProgrammable control system

» The functions are determined by a program stored in the memory.

» The control functions can be changed simply by changing the program.

» Consist of a control device, to which all the sensors and actuators are connected.

POWERPOWERSUPPLYSUPPLY

CENTRALCENTRALPROCESSINGPROCESSINGUNIT (CPU)UNIT (CPU)

INPUTINPUTMODULESMODULES

OUTPUTOUTPUTMODULESMODULES

MEMORYMEMORY(EPROM/RAM)(EPROM/RAM)

PG/PG/PCPC

Basic Structure of a PLCBasic Structure of a PLCBasic Structure of a PLCBasic Structure of a PLC

Input / Output ModulesInput / Output ModulesInput / Output ModulesInput / Output Modules

» Digital input modules adapt digital signals e.g. from proximity sensors

» Digital output modules convert the internal signal level of PLC into digital process signals e.g. relays

» Analog input modules adapt analog process signals e.g. from transducers

» Analog output modules convert internal digital values of the PLC to analog process signals e.g. temperature controller

Central Processing Unit (CPU)Central Processing Unit (CPU)Central Processing Unit (CPU)Central Processing Unit (CPU)

What is a CPU?What is a CPU?

» The “brain” of a PLC

» Controlled by a program called the executive or operating system (OS)

» The executive is a collection of supervisory programs permanently stored in memory

CPUCPUCPUCPU

Four basic types of CPU operations:Four basic types of CPU operations:

» Input and output operation

» Arithmetic and logic

» Reading or changing contents of memory locations

» Jump operations

Program MemoryProgram MemoryProgram MemoryProgram Memory

Program memory

RAM (Random Access Memory)

• the memory contents can be read and written (modified)

• memory contents will be lost when the supply voltage fails

ROM (Read Only Memory) • the memory contents can be read, but cannot be modified