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1 EJ501 PROGRAMMABLE LOGIC CONTROLLER (PLC) AND AUTOMATION
1EJ501
PROGRAMMABLE LOGIC CONTROLLER (PLC) AND AUTOMATION
2UNIT 3
INTRODUCTION TO PLC SYSTEM
Objective :
1. Understand basic characteristics of PLC
PLC terminologies
PLC function/ background
Type / advantages of PLC
2. Understand building structure of PLC
CPU, memory unit, display and indicator unit, input and output interface
3. Understand PLC hardware unit
Housing unit, programming unit, power supply unit, printer unit
4. Explain input and output device
3A digital device (microprocessor based controller) that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes and has been specifically designed to make programming easy.
PLCs are similar to computers but have certain features which are specific to their use as controllers:
i. They are rugged and designed to withstand vibrations, temperature, humidity and noise.
ii. The interfacing for inputs and outputs is inside the controller.iii. There are easily programmed.
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INTRODUCTION TO PLC SYSTEM
Introduction PLC (Programmable Logic Control)
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INTRODUCTION TO PLC SYSTEM
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A digitally operated electronic apparatus which uses aprogrammable memory for the internal storage ofinstructions for implementing specific functions such aslogic, sequencing, timing, counting, and arithmetic tocontrol, through digital or analog input/output modules,various types of machines or processes.
- National Electrical Manufacturers Association (NEMA)
Definition PLC
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INTRODUCTION TO PLC SYSTEM
Definition PLC
An electronic system working digitally for the use in industrial environmentwith a programmable memory for the internal storage of user-orientated controlinstructions for the implementation of specific functions, like, for examplelogic operation control, process control, timer, counting-and arithmeticfunctions, in order to control various machines and processes through digital oranalog input- , and output variables.
The logic programmable controls as well as the peripheral installations (PLC-system) are designed so that they can be easily integrated in an industrialcontrol system and be implemented in all their desired functions.
- International StandardIEC 61131-1
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INTRODUCTION TO PLC SYSTEM
PLC Background
The first Programmable Logic Controller (PLC) was developed by a groupof engineers at General Motors in 1968
The company was looking for an alternative to replace complex relaycontrol systems :
Must be programmable (and reprogrammable) Designed to operate in an industrial environment Must accept 120 V ac signals from pushbuttons, switches Outputs must switch and continuously operate loads such as motors
and relays of two amps rating
Before the 1980s, programmable controllers were called PCs.
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INTRODUCTION TO PLC SYSTEM
PLC Functions
PLC performs the functions of conventional relays, timers,counters.
Capable of storing instructions, such as sequencing, timing,counting, arithmetic, data manipulation, and communication,to control industrial machines and processes.
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INTRODUCTION TO PLC SYSTEM
Type of PLC - compact
Omron
Mitsubishi
Zelio
TECOMAT TC600
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INTRODUCTION TO PLC SYSTEM
Type of PLC - modular
Omron
Mitsubishi
Eaton
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Type of PLC plug in card
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INTRODUCTION TO PLC SYSTEM
Advantages Disadvantages
Many inputs and outputs, excellent forcontrolling and monitoring manyprocesses
Do not use the full capabilities of themicroprocessor
Designed for industrial environments,robust and reliable
Ladder logic programming style iscumbersome and prone to mistakes
Reprogrammable Only good for yes/no decisions
Modular Do not handle continuous inputs andoutputs, not good for direct control
Ideally suited to supervisory control Inflexible (compared tomicroprocessors)
Easy to set up, good for FMSenvironment
Advantages and Disadvantages of PLC
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The new control system had to be price competitive with the use of relay systems.
The system had to be capable of sustaining an industrial environment. The input and output interfaces had to be easily replaceable. The controller had to be designed in modular form, so that
subassemblies could be removed easily for replacement or repair. The control system needed the capability to pass data collection to a
central system. The system had to be reusable. The method used to program the controller had to be simple, so that it
could be easily understood by plant personnel.
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INTRODUCTION TO PLC SYSTEM
Initial Specifications
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Changes include both hardware (physical components) and software (control program) upgrades
Faster scan times are being achieved using new, advanced microprocessor and electronic technology.
Small, low-cost PLCs (Figure 1), capable of replacing four to ten relays, now have more power than their predecessor, the simple relay replacer.
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INTRODUCTION TO PLC SYSTEM
Recent PLCs
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A personal computer (PC) is the most commonly used programming device The software allows users to create, edit, document, store and troubleshoot
programs The personal computer communications with the PLC processor via a serial or
parallel data communications link
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Hand-held programming devices are sometimes used to program small PLCs They are compact, inexpensive, and easy to use, but are not able to display as much
logic on screen as a computer monitor It is often used on the factory floor for troubleshooting, modifying programs, and
transferring programs to multiple machines
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Basic PLC Structure
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INTRODUCTION TO PLC SYSTEM
Basic internal structure of PLC consists essentially of :-processing unit (CPU)-memory-input/output units
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Hardware of the PLC are onlyconsist of several components;such as Power Supply Unit,CPU Unit, Memory Unit andI/O Unit.
At the small PLC; PowerSupply, CPU, Memory and I/Ostay in the same Unit but at thebig PLC; Power Supply, CPU,Memory and I/O in the separateUnit.
Hardware of the PLC
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Power Supply
The Power Supply is used to give power to all the parts. The most PLC works with power24 VDC or 220 VAC. The big PLC has the separate power supply and the small PLC haspower supply which is one. The Power Supply is also provided by the battery back up,when happened the failure of power, automatically battery will replace the power supply tosupply the CPU, so that the program of memory user do not lose.
Memory Unit
The Memory Unit has function to save the data and program will be used by PLC. ThisMemory is divisible into two type that is ROM and RAM. ROM contains the data likefacility of the logic program, facility to edit the program, facility to monitor the program,facility for the communications, etc. The Data will be save permanently and will not losethough the power supply is off. While RAM contains the data of user program, like ladderdiagram, memory data, I/O status, etc. The Data can be writed and read. RAM has thecharacter is not permanent, if the power suplply is off the data will lose. To avoid thementioned, so the power supply is provided by the battery back up to supply if the especialpower supply is off.
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CPU Unit (Central Processing Unit)
Without seeing to type of PLC ( small, medium,big) processors and memory always in the sameunit. This unit is referred as the CPU. The CPUis the brain of a PLC. The CPU is a 8 bit, 16 bitor 32 bit microcontroller. The CPU has functionto handle communications with the externalapparatus, interconnectivity between the partsof PLC, executes the program, managementsthe memory, observing or perceiving the inputsignal and gives the output signal ( as accordingto the program or process run). The CPU is alsoprovided by the indicator lamp as indicator ofthe happening of mistake and damage.
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I/O Unit
The I/O Unit is the interface unit has function to convert the input signal and outputsignal so that the CPU can communicate with the external apparatus like the switch,stepping motor, solenoid, sensor, etc. The I/O Unit has two type; Discrete I/O andAnalog I/O.
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LG PLCOmron PLC
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Input Devices
PLCs can receive input from either automated or manual devices. The PLCcould receive data from the user via a pushbutton switch, keyboard, or similardevice. Automated input could come from a variety of devices: microswitches,timers, encoders, photosensors, and so on. Some devices, like the Limit Switchshown below, turn ON or OFF when the equipment actually makes contactwith it. Other devices, like the Photoelectric Switch and Proximity Switchshown below, use other means, such as light or inductance, in order to getinformation about the equipment being monitored.
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Output Devices
A PLC can output to a myriad of devices for use in automated control. Almostanything that you can think of could be controlled (perhaps indirectly) by a PLC.Some of the most common devices are motors, Solenoids, Servomotors, SteppingMotors, valves, switches, indicator lights, buzzers, and alarms. Some of these outputdevices; such as the motors, Solenoids, Servomotors, Stepping Motors, and valves;affect the controlled system directly. Others; such as the indicator lights, buzzers,and alarms; provide output to notify personnel.
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