INTRODUCTION TO PLC:

A programmable logic controller, PLC or programmable controller is a digital computer used for automation of typically industrial electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines. PLCs are designed for multiple analogue and digital inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory. A PLC is an example of a "hard" real-time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result.

A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.

Almost any production line, machine function, or process can be greatly enhanced using this type of control system. However, the biggest benefit in using a PLC is the ability to change and replicate the operation or process while collecting and communicating vital information.

Hardware Components of a PLC System:Processor unit (CPU), Memory, Input/Output, Power supply unit, Programming device, and other devices.

Central Processing Unit (CPU)CPU – Microprocessor based, may allow arithmetic operations, logic operators, block memory moves, computer interface, local area network, functions, etc.CPU makes a great number of check-ups of the PLC controller itself so eventual errors would be discovered early.

System BussesThe internal paths along which the digital signals flow within the PLC are calledbusses.The system has four busses:– The CPU uses the data bus for sending data between the different elements,– The address bus to send the addresses of locations for accessing stored data,– The control bus for signals relating to internal control actions,– The system bus is used for communications between the I/O ports and the I/O unit.

MemorySystem (ROM) to give permanent storage for the operating system and the fixed data used by the CPU.

RAM for data. This is where information is stored on the status of input and output devices and the values of timers and counters and other internal devices. EPROM for ROM’s that can be programmed and then the program made permanent.

I/O SectionsInputs monitor field devices, such as switches and sensors.Outputs control other devices, such as motors, pumps, solenoid valves, and lights.

Power SupplyMost PLC controllers work either at 24 VDC or 220 VAC. Some PLC controllers have electrical supply as a separate module, while small and medium series already contain the supply module.

Programming DeviceThe programming device is used to enter the required program into the memory of the processor.The program is developed in the programming device and then transferred to the memory unit of the PLC.

Working Principles Of PLC

There are four basic steps in the operation of all PLCs; Input Scan, Program Scan, Output Scan, and Housekeeping. These steps continually take place in a repeating loop.

Four Steps In The PLC Operations

1.) Input Scan

Detects the state of all input devices that are connected to the PLC

2.) Program Scan

Executes the user created program logic

3.) Output Scan

Energizes or de-energize all output devices that are connected to the PLC.

4.) Housekeeping

This step includes communications with programming terminals, internal diagnostics, etc...

Types of PLCs

Programmable Logic Controllers (PLCs) are integrated as either single or modular units.

An integrated or Compact PLC is built by several modules within a single case. Therefore, the I/O capabilities are decided by the manufacturer, but not by the user. Some of the integrated PLCs allow to connect additional I/Os to make them somewhat modular.

Integrated or Compact PLCs

A modular PLC is built with several components that are plugged into a common rack or bus with extendable I/O capabilities. It contains power supply module, CPU and other I/O modules that are plugged together in the same rack, which are from same manufacturers or from other manufacturers. These modular PLCs come in different sizes with variable power supply, computing capabilities, I/O connectivity, etc.

A modular Types of PLC

Modular PLCs are further divided into small, medium and large PLCs based on the program memory size and the number of I/O features.

Small, Medium and Large sized Types of PLCs

Small PLC is a mini-sized PLC that is designed as compact and robust unit mounted or placed beside the equipment to be controlled. This type of PLC is used for replacing hard-wired relay logics, counters, timers, etc. This PLC I/O module expandability is limited for one or two modules and it uses logic instruction list or relay ladder language as programming language.

Medium-sized PLC is mostly used PLC in industries which allows many plug-in modules that are mounted on backplane of the system. Some hundreds of input/ output points are provided by adding additional I/O cards – and, in addition to these – communication module facilities are provided by this PLC.

Large PLCs are used wherein complex process control functions are required. These PLCs’ capacities are quite higher than the medium PLCs in terms of memory, programming languages, I/O points, and communication modules, and so on. Mostly, these PLCs are used in supervisory control and data acquisition (SCADA) systems, larger plants, distributed control systems, etc.

SIMATIC S7-200: Micro PLC for basic automation tasks

Compact and powerful: The SIMATIC S7-200 Micro PLC is fast, communication-capable and highly productive in real-time mode. The consistently modular design facilitates the creation of tailor-made, expandable solutions in the low-end performance range. The S7-200 Micro PLC from Siemens can be used as either a stand-alone Micro PLC solution or in conjunction with other controllers.

SIMATIC S7-300: the modular universal controller for the manufacturing industry

The SIMATIC S7-300 universal controller is specially designed for innovative system solutions in manufacturing, specifically the automotive and packaging industries.This modular controller serves as an ideal universal automation system for centralized and decentralized configurations. Safety technology and motion control can be integrated with standard automation into this universal controller.

SIMATIC S7-400 process controller: a powerful solution

Part of the SIMATIC process controller series, the S7-400 has been designed for system solutions in the fields of manufacturing and process automation. This process controller is ideal for data-intensive tasks that are especially typical for the process industry.High processing speeds and deterministic response times ensure short machine cycle times on high-speed machines in the manufacturing industry.