PCL, PROGRAMMABLE LOGIC CONTROLLERS

     PROGRAMMABLE LOGIC CONTROLLERS (PLC)

Programmable Logic Controllers (PLC)

A modern controller device used extensively for sequence control today in transfer lines, robotics, process control, and many other automated systems is the Programmable Logic Controller (PLC). In essence, a PLC is a special purpose industrial microprocessor-based real-time computing system, which performs the following functions in the context of industrial operations

 • Monitor Input/Sensors

 • Execute logic, sequencing, timing, counting functions for         Control/Diagnostics

• Drives Actuators/Indicators

 • Communicates with other computers

 Some of the following are advantages of PLCs due to standardized hardware technology, modular design of the PLCs, communication capabilities, and improved development program development environment:

 • Easy to use to simple modular assembly and connection;

• Modular expansion capacity of the input, outputs, and memory;

 • Simple programming environments and the use of standardized task libraries and debugging aids; • Communication capability with other programmable controllers and computers

Evolution of the PLC

Before the advent of microprocessors, industrial logic and sequence control used to be performed using elaborate control panels containing electromechanical or solid-state relays, contactors and switches, indicator lamps, mechanical or electronic timers, and counters, etc., all hardwired by complex and elaborate wiring. In fact, for many applications, such control panels are used even today. However, the development of microprocessors in the early 1980’s quickly led to the development of PLCs, which had significant advantages over conventional control panels. Some of these are:

• Programming the PLC is easier than wiring physical components; the only wiring required is that of connecting the I/O terminals.

 • The PLC can be reprogrammed using user-friendly programming devices. Controls must be physically rewired.

 • PLCs take up much less space.

 • Installation and maintenance of PLCs is easier, and with present-day solid-state technology, reliability is greater.

 • The PLC can be connected to a distributed plant automation system, supervised, and monitored.

 • Beyond a certain size and complexity of the process, a PLC-based system compares favorably with control panels.

• Ability of PLCs to accept digital data in serial, parallel, and network modes imply a drastic reduction in plant sensor and actuator wirings since single cable runs to remote terminal I/O units can be made. Wiring only needs to be made locally from that point.

 • Special diagnostic and maintenance modes for quick troubleshooting and servicing, without disrupting plant operations.

 

Application

Programmable Logic Controllers are suitable for a variety of automation tasks. They provide a simple and economic solution to many automation tasks such as

 • Logic/Sequence control

 • PID control and computing

• Coordination and communication

• Operator control and monitoring

• Plant start-up, shut-down