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 can often
control complex sequencing and is often written by engineers. The program is stored in
battery-backed memory and/or EEPROMs. Unlike general-purpose computers, the PLC
is packaged and designed for extended temperature ranges, dirty or dusty conditions,
immunity to electrical noise, and is mechanically more rugged and resistant to vibration
and impact.


The main difference from other computers are the special input/output arrangements.
These connect the PLC to sensors and actuators. PLCs read limit switches, 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.



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. The earliest PLCs expressed all decision making logic in simple ladder logic inspired from the electrical connection diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic.


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. Today, the line between a general purpose programmable computer and a
PLC is thinning. The data handling, storage, processing power and communication
capabilities of some modern PLCs are approximately equivalent to desk-top computers.
PLC-like functionality, combined with remote I/O hardware, allow a general-purpose
desktop computer to overlap some PLCs in certain applications. With the IEC 61131-3 standard, it is now possible to program PLCs using structured programming languages, and logic elementary operations. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.