Programmable Logic controller

Control engineering has evolved over time. In the past, humans were the resource for controlling a system manually. Nowadays, advanced technology is used for controlling the equipment and electricity is the resource for running most of the equipment. The advent of PLC began in the 1970s, and has become the most common choice for manufacturing controls. PLCs have been an integral part of factory automation and industrial process control for decades. It performs many functions, such as providing analog and digital input/output interfaces, signal processing, data conversion, and various communication protocols.

Programmable means ‘able to provide coded instructions for the automatic performance of a task’. Logic means ‘a system or set of principles underlying the arrangements of elements in a computer or electronic device so as to perform a specified task’. Controller means ‘a thing that directs or regulates something’. Hence, PLC is a set of instructions in an electronic device that help peform the regulatory task. It is an industrial computer that contains hardware and software used to perform control functions. PLC is used for automation of industrial processes to control machinery placed on assembly line of manufacturing units. and food processing. They can accept digital and analog inputs. A PLC consists of two basic sections, input/output (I/O) interface system, and central processing unit (CPU).

Input and output are the physical interface between a processing unit and user. There are four basic steps in the operation of all PLCs: input scan, program scan, output scan, and housekeeping. These steps take place in a repeating loop. Input scan detects the state of all input devices that are connected to the PLC. Program scan executes the user created program logic. output scan energises or de-energises all output devices that are connected to the PLC. Housekeeping includes communicationi with programming devices and performing internal diagnostics. It is easier to create and change a program in a PLC than to wire and rewire a circuit. End-users can modify the program in the field, for example a PLC used for controlling speed of rotation.

Processor- Processor is the brain of the PLC. It consists of microprocessor for implementing logic and controlling communication among the modules. The processor accepts input data from various sensing devices, executes stored user program and sends appropriate output commands to control devices. The structure of a PLC is based on the same prinicples as those employed in computer architecture.

In programmable logic controller, electronic components are used, These electronic components work on digital signal. This digital code such as '0100101101010' is not understood by a human, In such a condition, there is a gap between the machine and the human. To voer this gap, human machine interface is used. Human machine interface is user interface that connects an operator to the controller for an industrial system. There are three basic types of HMIs, push- button replacer, data handler and overseer

To connect human machine interface and programmable logic controller, USB, RS-232, RS-485 are used. A wiring schematic from the production line to the PLC is highly recommended.