LESSON H5P - Programmable Logic Controllers vs Most Other Computing Devices
What are PLCs?
PLCs are at the forefront of manufacturing automation. An engineer working in a manufacturing environment will at least encounter some PLCs, if not use them on a regular basis. Electrical engineering students should have basic knowledge of PLCs because of their widespread use in industrial applications.
A programmable logic controller
(PLC), or programmable controller is an industrial digital
computer which has been ruggedised and adapted for the control of manufacturing
processes, such as assembly lines, or robotic devices, or any activity that
requires high reliability control and ease of programming and process fault
diagnosis.
They were first developed in the
automobile industry to provide flexible, ruggedised and easily programmable
controllers to replace hard-wired relays and timers. Since then they have been
widely adopted as high-reliability automation controllers suitable for harsh
environments. A PLC is an example of a "hard" real-time system since
output results must be produced in response to input conditions within a
limited time, otherwise unintended operation will result.
PLCs can range from small
"building brick" devices with tens of inputs and outputs (I/O), in a
housing integral with the processor, to large rack-mounted modular devices with
a count of thousands of I/O, and which are often networked to other PLC and
SCADA systems.
They can be designed for multiple
arrangements of digital and analog I/O, extended temperature ranges, immunity
to electrical noise, and resistance to vibration and impact. Programs to
control machine operation are typically stored in battery-backed-up or
non-volatile memory.
It was from the automotive industry
in the USA that the PLC was born. Before the PLC, control, sequencing, and
safety interlock logic for manufacturing automobiles was mainly composed of
relays, cam timers, drum sequencers, and dedicated closed-loop controllers.
Since these could number in the hundreds or even thousands, the process for
updating such facilities for the yearly model change-over was very time
consuming and expensive, as electricians needed to individually rewire the
relays to change their operational characteristics.
When digital computers became available, being general-purpose programmable devices, they were soon applied to control sequential and combinatorial logic in industrial processes. However these early computers required specialist programmers and stringent operating environmental control for temperature, cleanliness, and power quality. To meet these challenges the PLC was developed with several key attributes. It would tolerate the shop-floor environment, it would support discrete (bit-form) input and output in an easily extensible manner, it would not require years of training to use, and it would permit its operation to be monitored.
