Programmable Logic Controllers

Electromechanical relays are versatile and reliable devices, and it is possible to perform virtually any control function given enough relays and wiring. However, they are subject to wear due to their moving parts, and re-configuring a relay-based control system entails disconnecting and re-connecting wires: a tedious task at best. Solid-state logic gates may replace electromechanical relays to avoid problems of wear (as well as vastly increase speed of response), but once again re-configuration would require disconnecting and re-connecting many wires between the gates.

A very practical alternative to relay- or gate-based control logic circuits is to use a programmable computer to perform the same functions. A computer equipped with the necessary input and output peripheral circuits, configured to be easily programmed by technical personnel for virtually any logical task, is called a programmable logic controller or PLC.

1. PLC examples

Programmable logic controllers are essentially nothing more than special-purpose, industrial computers. As such, they are built far more ruggedly than an ordinary personal computer (PC), and designed to run extremely reliable operating system software1. PLCs as a rule do not contain hard disk drives, cooling fans, or any other components with moving parts. This is an intentional design decision, intended to maximize the reliability of the hardware in harsh industrial environments where the PLC chassis may be subjected to temperature extremes, vibration, humidity, and airborne particulates (dust, fibers, and/or fumes).

Large PLC systems consist of a rack into which circuit “cards” are plugged. These cards include processors, input and output (I/O) points, communications ports, and other functions necessary to the operation of a complete PLC system. Such “modular” PLCs may be configured differently according to the specific needs of the application. Individual card failures are also easier to repair in a modular system, since only the failed card need be replaced, not all the cards or the whole card rack. Small PLC systems consist of a monolithic “brick” containing all processor, I/O, and communication functions. These PLCs are typically far less expensive than their modular cousins, but are also more limited in I/O capability and must be replaced as a whole in the event of failure.

 

The photograph is of a Siemens (Texas Instruments) 505 series PLC, installed in a control panel of a municipal wastewater treatment plant. This is an example of a modular PLC, with individual processor, I/O, and communication cards plugged into a rack. Three racks appear in this photograph (two completely filled with cards, and the third only partially filled).

Article resource from :

Lessons In Industrial Instrumentation
By Tony R. Kuphaldt
Version 1.4 – Released April 11, 2010