A programmable logic controller (PLC) is a dedicated controller used to automate electromechanical processes. PLCs use a central processing unit to take multiple digital and analog inputs and make decisions to control multiple specified outputs. They are commonly found in industrial production lines, on machinery, amusement park rides, oil refineries and other applications.
Before PLCs, control and sequencing for manufacturing applications like automobile assembly lines were done with hard-wired relays, cam timers, drum sequencers and closed loop controllers. Because of the number of individual controllers needed for so many outputs, replacing or modifying the controllers was both time consuming and financially costly. Electricians had to individually rewire hundreds or even thousands of relays if a new product was to begin production.
The answer for this problem came in 1968 when Dick Morley and a team of engineers created the first PLC. Morley and his team knew they wanted to create a programmable controller that was rugged enough for an industrial environment, had no interrupts, and had direct mapping into memory. This idea led to the creation of the 084 (it was the 84th project for Morley’s company Bedford Associates). Bedford Associates was far from the only company working to create a similar device at this time. Many engineering teams for other companies such as GM, AEG and Schneider Electric had their hands in the development of the PLC as well. To this day Morley refutes the idea that he “invented” the PLC. Regardless, his instrumental role has led to most people referring to him as “the father of the PLC.”
Since the PLCs inception in the late 60s, the technology has evolved. Most PLCs are still based on the fundamental ladder logic programming structure, but not all. Some PLCs can now be programmed in higher-level languages such as C and BASIC. Communication options make include simple RS-232 serial communications to more advanced Ethernet protocols. With time and efficiency always driving technological advances, a PLCs scan time is always an important consideration when thinking of PLC applications. Scan time refers to the time a PLC takes to run through a program taking in data and updating outputs. This time varies depending on program length and processing speed but it is usually a few milliseconds. Higher scan times can accommodate processes with more real-time demands than traditional slower applications where scan speed is not as critical.
PLCs took the enormous and cumbersome components of the previous system and made it fit into a small customizable box. Even today, PLCs can be found in nearly every manufacturing facility in the world; they remain as one of the most important automation inventions of all time.
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