Programmable Automation Controllers
In the beginning there was the relay and the timer. And in the process industries, there was the hardwired controller. A generation ago, digital electronics made the hardwired relay and controller obsolete in favor of the PLC, the programmable logic controller. Made to be fully deterministic and have limited control functions, PLCs were able to sweep hardwiring into the dustbin because they were easy to program in the "ladder" style of electrical wiring diagrams, and they were easily reprogrammable so that they could accommodate changes on assembly lines and in batch processes.
But PLCs are limited. Ladder-logic programming cannot be used for complex mathematical formula, such as the basic PID algorithm found in every single loop controller in a process plant, for example. Then, along came the PC. Inexpensive computing power became ubiquitous and easily affordable. PCs were tried early on in industrial control, but early operating systems and hardware were not up to the stresses and standards of the industrial workplace.
One of the biggest issues was determinism. In many industrial automation environments, especially in motion control and other discrete automation applications, it is absolutely required that an instruction get where it is supposed to go, when it is supposed to get there. PC operating systems are non-deterministic and have variable latency, depending on processor loading or the requirements of the operating program.
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