Introduction to Programmable Logic Controller

A PLC is a solid state / computerized industrial computer that performs discrete or sequential logic in a factory environment. It was originally developed to replace mechanical relays, timers, counters. PLCs are used successfully to execute complicated control operations in a plant. Its purpose is to monitor crucial process parameters and adjust process operations accordingly. A sequence of instructions is programmed by the user to the PLC memory and when the programme is executed, the controller operates a system to the correct operating specifications.

The first PLC System evolved from conventional computers in late 60s and early 70s. These first PLCs were installed primarily in automotive plants. Traditionally the auto plant had to be shut down for up to a month at model changeover time. The early PLCs were used with other new automation techniques to shorten the changeover time. One of the major time consuming changeover procedures had been the wiring of new or revised relay & control panels. The PLC programming procedures replaced the rewiring a panel full of wires, relays, timers, counters & other components. The PLCs helped reduce the changeover time to a matter of few days.

Application of PLCs in Manufacturing and Assembly process

In an industrial set up PLCs are used to automate manufacturing and assembly processes. By 'process' we mean a step-by-step procedure whereby a product is manufactured and assembled. It is the responsibility of the product engineering dept. to plan for manufacture of new or modified products. Other processes might involve the filling and capping of bottles, the printing of newspapers, or the assembly of automobiles etc. In many such manufacturing situations, PLC plays an important role in carrying out the various processes.

Advantages of PLCs

• Reliability in operation
• Flexibility in control techniques
• Flexibility in programming and reprogramming in the plant
• Large quantity of contacts
• Online/ offline modifications
• Cost effective for controlling complex systems
• Small physical size, shorter project time
• In-house simulation & testing of project
• Speed in operation
• Ability to communicate with computer systems in the plant.
• Ease of maintenance / troubleshooting
• Documentation
• Security

The PLC is also designed to operate in the industrial environment with wide ranges of ambient temperature, vibration, and humidity and is not usually affected by the electrical noise that is inherent in most industrial locations. Troubleshooting is simplified in most PLCs because they include fault indicators, blown-fuse indicators, input and output status indicators, and written fault information that can be displayed on the programmer. PC is often used for programming and monitoring the PLC, however some PLC manufacturers also offer hand-held programming channels & dedicated programmers.

PLCs vs PCs

PCs are data processing computers. Such computers process and analyse a large amount of data, store the database, generate a report and display the data and information to operators.

There is also another type of computer, known as a process control computer or industrial computer. The main function of this type of computer is to control manufacturing and industrial processes, which are event driven.

PLCs are a type of process control computers, small, relatively inexpensive, environmentally hardened and easy to program, maintain and repair. They can handle real time data and direct I/O tasks where computers can handle non-real time tasks. PLCs are often installed close to the machine or process they control, and are often considered as an extension of a piece of industrial equipment.