Programmable Logic Controllers


PLC History & Development

Programmable Logic Controllers (PLC) were originally invented for the U.S. automotive manufacturing industry to replace relay logic systems. The automotive industry adopted PLCs initially for software revision that replaced rewiring of control panels was used during the changing of production models. Before PLC, the process to change and update facilities for its new model each year was very expensive and time-intensive since each relay had to be individually rewired.

In 1986, GM requested a proposal for a device that would electronically change this relay system. Winning this project bid, Dick Morely of Bedford Associates is known as the “father” of the PLC with their 084 PLC as the first product of its kind. Bedford Associated founded the company Modicon to help promote and service its 084 PLC, which was later sold to the German company now known as AEG. Schneider Electric in France is now the current owner of Modicon’s 084 PLC. The PLC is now a vital part of many manufacturing industries saving hours of time and thousands of dollars, but the automotive industry continues to be perhaps the largest PLC user.

PLC Functions & System Tools

PLCs primary function is to electronically rewire the hard-wired control panels that, otherwise, require that each of the hundreds and even thousands of relays be replaced individually. PLCs accept data from switches, sensors and controls to control and drive various machines and devices. PLC functions have expanded from its basic replacement of relay logic circuits to more advanced features involving PIP implementation. The benefits and functions of the PLC are now found in industries beyond automotive manufacturing, including food processing, mining and environment control.

Early PLCs were programmed to resemble a schematic design of relay logic (known as ladder logic) to help reduce the training required for technicians, though some used more of an instruction list type of programming. PLCs today may take the form of this ladder logic or state logic, a far more complex programming language. Ladder logic is more often used today because it allows the PLC programmer to more easily detect issues with timing of the logic sequence. PLCs today are programmed used computer software which functions also as a troubleshooting tool for the PLC software.

PLC functions have evolved since the 084 to include motion control, sequential relay control, process control, networking and distributed control systems. Today, PLC data handling, storage, communication tools and processing power is equal to that of a desktop computer. In fact, there are desktop logic applications that when combined with I/O hardware can overlap some PLC functions, which may be more beneficial in smaller facilities or less critical circumstances. The advantage to PLCs in the form of a desktop logic application is generally the lower cost.

However, practicality is one area preventing some industries to still not yet fully adopt PLCs, particularly the heavy industry manufacturers due to concerns over desktop computer longevity, cost and durability in humidity, harsher temperatures and less stable environments. As expected, the PLC has transformed from its original functions to the more efficient PLCs used by industries today. Developers have also branched off from the PLC with introduction of Programmable Logic Relays (PLRs), offering similar functions as the PLC with the same goal in mind.

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