Understanding automation selection criteria

Understanding how the programmable logic controller (PLC) has evolved and how it relates to more modern incarnations like the PAC as well as understanding servo system criteria will help a great deal on the plant floor.


When Dick Morley introduced the programmable logic controller (PLC) in 1968, I wonder if he envisioned how much it would evolve in 47 years. This evolution brought about changes in PLC form factor, functionality, and how PLCs are programmed. In 2001, market research firm ARC introduced the term "programmable automation controller (PAC)" to describe the marked shift in technology behind the ever-evolving PLC.

In this issue's cover story, the author explains the similarities and differences between PLCs and PACs, noting that "PLC-based PAC" is perhaps a better term to describe their convergence-the lines between them is blurring. "Today, it's difficult to find an industrial controller that doesn't display many of the characteristics that might be found in either a PLC or a PAC." There are similarities and differences between them. And, according to the author, the PLC-based PAC is intended to leverage the similarities and minimize the differences. He points out that users should select a controller based on the requirements of the application and not on what it's called.

The other story in this issue examines servo system selection criteria. The authors emphasize that servomotors—and the systems that control them—are not created equal. Servomotors with the same power range from different manufacturers are not necessarily equivalent. Technical aspects, such as torque, inertia, resolution, frequency response, and bandwidth determine servo system performance. In addition to providing a brief servo tutorial, the authors explain why these technical factors must be considered when selecting a servo system. As the authors state in the article, "Servo system selection criteria involves more than just wattage and price."

This article appears in the Applied Automation supplement for Control Engineering and Plant Engineering

- See other articles from the supplement below.

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