Programmable Logic Controller (PLC)

Programmable logic control or PLC is the most commonly used industrial automation technique in the world. It is universally applied for factory automation, process control and manufacturing systems. Programmable logic control originated from the creation of computerized versions of relay control systems used to control manufacturing and chemical process systems. The programming is done using a special technique called ladder logic, which allows sequences of logical actions to be set up, inter-linked and timed. A standard task in logic control is batch control and sequencing in a process system.

PLC Introduction

A PLC or Programmable Logic Controller is a user friendly, microprocessor specialized computer that carries out control functions of many types and levels of complexity. Its purpose is to monitor crucial process parameters and adjust process operations accordingly. It can be programmed, controlled and operated by a person unskilled in operating computers. Essentially, a PLC's operator draws the lines and devices of ladder diagrams with a keyboard onto a display screen. The resulting drawing is converted into computer machine language and run as a user program.

PLC will operate any system that has output devices that go on and off (Discrete, or Digital, outputs). It can also operate any system with variable (analog) outputs. The Programmable Logic Control can be operated on the input side by ON/OFF devices or by variable (analog) input devices.

Control engineering has evolved over time. In the past humans was the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls.

Programmable Logic Controllers have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer.