Programmable logic controller

A programmable logic controller, PLC, or programmable controller is a small computer used for automation of real-world processes, such as control of machinery on factory assembly lines. The PLC usually uses a microprocessor. The program is usually created by a skilled technician at an industrial site, rather than a professional computer programmer. The program is stored in battery-backed memory.

Related Topics:
Computer - Automation - Microprocessor

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The main difference from other computers are the special input/output arrangements. These connect the PLC to a process's sensors and actuators. PLCs read limit switches, dual-level devices, temperature indicators and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs drive any kind of electric motor, pneumatic or hydraulic cylinders or diaphragms, magnetic relays or solenoids. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a proprietary computer network that plugs into the PLC.

Related Topics:
Input/output - Sensor - Electric motor - Pneumatic - Hydraulic - Relay - Solenoid

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PLCs were invented as less-expensive replacements for older automated systems that would use hundreds or thousands of relays and cam timers. Often, a single PLC can be programmed to replace thousands of relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels.

Related Topics:
Relay - Cam timer

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The functionality of the PLC has evolved over the years to include typical relay control, sophisticated motion control, process control, Distributed Control Systems and complex networking.

Related Topics:
Process control - Distributed Control System - Networking

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The earliest PLCs expressed all decision making logic in simple ladder logic inspired from the electrical connection diagrams. The electricicians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This was chosen mainly to reduce the apprehension of the existing technicians.

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Recently, inspired from Grafcet, the PLC have integrated the Sequencial Function Charts : a new graphical language which allows now to directly program the sequencial nature of processes.

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Today, the line between a programmable computer and a PLC is thinning. With the IEC-1138 standard, it is now possible to program these devices using structured programming languages (such as C), and logic elementary operations.

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