Introduction to PLC:

During the Industrial Revolution of the 18th-and 19th-centuries, many traditionally manual processes were taken over by machines. These early machines relied on gears and pulleys to work and were, by our standards, extremely primitive. The first major breakthrough in the development of control systems came with the invention of electrically powered machines. The first control systems were developed in the early years of the 20th century and used sequential Relay Circuits for machine control. A major technical breakthrough in its day, and still used in some plants today, relay technology enabled machines to work faster and more safely.

Relay circuits performed their job very well, but they required large amounts of floor space, and huge amounts of energy. Adding to their drawbacks as the basis for a machine control system, relay circuits also took a long time to install, troubleshoot, and modify. Finally, in the early 1970s, a device was developed to replace sequential relay circuits: the Programmable Logic Controller (PLC).

As you will remember from reading about them in Module 24, PLCs are more reliable, faster, more flexible and more efficient than relay-based systems. For example, PLCs are cheaper and easier to wire and maintain than relays. Furthermore, when it comes to troubleshooting, PLCs are much quicker than relays at testing and debugging the program.

PLCs are used in all kinds of industries. In fact, almost any industrial process that uses electrical control needs a PLC. For example, let's assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on. We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters. With a PLC, however, we can dispense with those unwieldy timers and counters, and simply program the PLC to count its inputs and turn the solenoids on for the specified time.