Plc Basics

1. PLC Basics Muhammad Atta ul Ghaffar Senior Electrical Engineer CENTURY PAPER & BOARD MILLS PAKISTAN 1

2. Programmable Logic Controller 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. “ A PLC is a computer designed to work in an industrial environment. It uses a programmable memory to store the instructions and specific functions that include On/Off control, timing counting, sequencing, arithmetic and data handling. PLCs are equipped with special input/output interfaces . 2

3. PLC System 3

4. What is Control? “ CONTROL is the process in a system in which one or several input variables influence other variables “ 4

5. A Simple Overview of Control System C O N T R O L S Y S T E M P L A N T INFORMATION COMMANDS ACTUATORS SENSORS 5

6. Types of Control System 6 CONTROL SYSTEM OPEN-LOOP CONTROL SYSTEM CLOSED-LOOP CONTROL SYSTEM

7. Open Loop Control System In an open loop system, the controller is told what the desired process output is, and the controller sends a command to the actuator to try and achieve this. The process is shown below as a block diagram: 7

8. Closed Loop Control System Systems that utilize feedback are called closed-loop control systems. The feedback is used to make decisions about changes to the control signal that drives the plant. An open-loop control system doesn't have or doesn't use feedback. 8

9. 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) 9

10. Hard Wire & PLC Wiring diagram L 10 N S1 S2 K1 S1 S2 PLC K1 Hardwire PLC 0 V 24 VDC K1

11. Comparison Hardwired 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) 11

12. Comparison Programmable 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. 12

13. History of PLC During the late 1960s, General Motors (USA) was interested in the computer application to replace the hardwire systems. Bedford Associates ( Modicon ) and Allen Bradley responded to General Motors. The name given was “Programmable Controllers” or PC. Programmable Logic Controller or PLC was a registered trademark of the Allen Bradley. Later, PC was used for “Personal Computer” and to avoid confusion PLC for “Programmable Controller” and PC for a personal computer 13

14. PLC History 14

15. PLC History 15

16. PLC Timeline 16

17. Advantages of PLC compared to Hardwire Implementing changes and correcting errors Pilot run - trial / test run Visual observation - online monitoring Speed of operation Reliability Documentation 17

18. Basic Structure of PLC 18 POWER SUPPLY CENTRAL PROCESSING UNIT (CPU) INPUT MODULES OUTPUT MODULES MEMORY (EPROM/RAM) PG/ PC

19. PLC Inputs & Outputs (I/Os) 19 PLC USER PROGRAM (LOGIC) Input Devices Output Devices

20. 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 20

21. 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 21

22. CPU Four basic types of CPU operations: Input and output operation Arithmetic and logic Reading or changing contents of memory locations Jump operations 22

23. CPU 23 PROCESSOR ACCUMULATOR TIMERS, COUNTERS, Memory PII INTERNAL PROGRAM MEMORY (RAM) PIQ SERIAL INTERFACE MEMORY SUBMODULE (EPROM/ EEPROM/ RAM)

24. CPU The CPU reads in input signal states, processes the control program and controls the outputs. The CPU provides internal Memory, timers and counters. Restart procedure can be preset and errors can be diagnosed using the CPU’s LEDs. The overall Reset on the CPU is used to delete the contents of the RAM. A PG or a Memory sub-module is used to transfer the control program to the CPU. 24

25. Power Supply Module The power supply module supplies the operational voltage for the PLC and provides backup for the RAM with a battery Backup battery The backup battery maintains the program and data when the PLC is switch off The backup battery has a service life of approximately 2 years 25

26. How does a Programmable Controller Works? 26 Power Supply Program Memory Processor Input modules Output modules 24 VDC GND Sensors Actuators / Annunciators

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