What is a PLC?
A Programmable logic controller or PLC is an industrial digital computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices. It has been adapted for the control of manufacturing processes such as assembly lines, robotic devices or anything that requires great reliability and ease of programming.
Almost any production line, machine function, or process can be greatly enhanced using this type of control system. They were first used in the automobile industry to replace hard-wired relays and timers. Since then, they have been widely employed in environments considered harsh and when high-reliability automation controllers have been required. Another advantage of a PLC system is that it is modular, meaning you can mix and match the types of Input and Output devices to best suit your application.
In 1968, General Motors requested a relay replacement and the first PLCs were designed and developed by Modicon, which stood for Modular Digital Controller. The automotive industry remains one of the biggest users of PLCs today. For Electrical Control Components manufacturers, visit http://www.osmelectrical.com/.
The benefits included:
- Eliminating the need for rewiring and adding additional hardware for each new configuration of logic.
- The new system drastically increased the functionality of the controls while reducing the cabinet space that housed the logic.
The Central Processing Unit, the CPU, contains an internal program that tells the PLC how to perform the following functions:
- Execute the Control Instructions contained in the User’s Programs. This program is stored inside something called ‘nonvolatile’ memory, meaning that the program will not be lost if power is removed
- Communicate with other devices such as I/O Devices, programming devices, networks and even other PLCs.
- Perform Housekeeping activities such as communications and internal diagnostics.
These are the four basic steps in the operation of all PLCs- Input Scan, Program Scan, Output Scan and Housekeeping which take place continually a repeating loop. Ladder logic is the most commonly used programming language but there are others.
The main difference from other computers is that PLCs are armoured for severe conditions including dust, moisture and extremes of heat and cold They also have the facility for extensive input/output arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables such as temperature and pressure and the positions of complex positioning systems. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.
PLCs often need to interact with people for everyday controls purposes, programming or alarm reporting. An HMI or human-machine interface is used for this. These can also be called GUIs (graphical user interface) or MMIs (man-machine interface). Text displays are available as well as touch screen devices or a simple system may use buttons and lights to interact with the user. A more complex set up could use programming and software installed on a PC and connected to the PLC via a communication interface.