Programmable Logic Controllers

Programmable Logic Controllers or PLCs were developed in the late sixties by Bedford Associates of Bedford, Massachusetts to replace the hard-wired relay control systems used at the General Motors Hydra-Matic automatic transmission plant (Programmable Logic Controller, 2014). While they are still used extensively, relays have some problems. The biggest problem being they are mechanical devices and like all things mechanical they wear out and have to be replaced quite often. In addition, relays take up quite a bit of space, especially in larger applications where it would not be uncommon to use thousands of them. This, along with other considerations like wiring and programming difficulties led to the development of PLCs. As technology improved, the size of PLCs was reduced and the ability for PLCs to interact with one another was developed.
A PLC accomplishes three basic tasks: it scans the input modules, runs through the program and solves logic, and submits changes to the outputs as needed. The PLC’s program then loops back around and the process is repeated. This all seems like a simple task, but it can get quite complex with a large number of inputs and outputs combined with various timers and counters. The time it takes for the PLC to go through these three basic steps is called scan time and is an important factor when programming as it affects how quickly the inputs can be read.
PLCs are very good at controlling outputs based on the inputs they receive. They are quite durable, and able to withstand harsh industrial environments. They are built to last and will give trouble-free service for many years. Unlike relays, they do not have contacts to wear out and they can switch quickly without getting too hot. This means that cooling costs are decreased. Because of their communication abilities, they can be place away from the object they are controlling and it takes a lot less wiring to install PLCs as opposed to a hard-wired relay control system.
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