INTRODUCTION TO PROGRAMMABLE LOGIC CONTROL PLC PROGRAMMABLE LOGIC

INTRODUCTION TO PROGRAMMABLE LOGIC CONTROL

PLC PROGRAMMABLE LOGIC CONTROL (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. “

Definition of Control What is CONTROL? “ CONTROL is the process in a system in which one or several input variables influence other variables “

A Simple View of a Control System INFORMATION COMMANDS C O N T R O L S Y S T E M SENSORS ACTUATORS P L A N T

Open-loop Control System In open-loop control systems, output variables are influenced by the input variables. L N

Closed-loop Control System It is characterized by continuous comparison of the desired value (or set point) with the actual value of the controlled variable. L Xi - Required value Xi > Xs Xs - Actual value N C Xs Xi < Xs Xi

PLC and Conventional 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)

Hardwire and PLC Wiring Diagrams L 24 VDC S 1 S 2 S 1 K 1 N S 2 PLC K 1 0 V Hardwire PLC

Comparison Hardwired control systems Programmable control system » The functions are determined by the physical wiring. » The functions are determined by a program stored in the memory. » Changing the function means changing the wiring » The control functions can be changed simply by changing the program. » Can be contact-making type (relays, contactors) or electronic type (logic circuits) » Consist of a control device, to which all the sensors and actuators are connected.

Basic Structure of a PLC POWER SUPPLY INPUT MODULES PG/ PC CENTRAL PROCESSING UNIT (CPU) MEMORY (EPROM/RAM) OUTPUT MODULES

Input / 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) 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

CPU Four basic types of CPU operations: » Input and output operation » Arithmetic and logic » Reading or changing contents of memory locations » Jump operations

Program Memory Program memory RAM (Random Access Memory) ROM (Read Only Memory) • the memory contents can be read and written (modified) • the memory contents can be read, but cannot be modified • memory contents will be lost when the supply voltage fails