PLCs, PCs, and PACs: When the Lines in Motion Control Become Blurred

Historically, motion controllers, programmable logic controllers (PLCs), and industrial personal computers (PCs), which have clearly defined functions in a control system, were separate components. With the rise of programmable automation controllers (PACs), motion controllers are increasingly difficult to distinguish from PLCs.

Essentially, a PLC is a ruggedized control device made up of a microprocessor, memory, and a variety of peripherals. PLCs typically use IEC 61131-3, an industry-standardized set of programming languages, including Ladder Diagram. Traditional PLCs typically rely on peripheral devices such as smart drives and stand-alone motion controllers to provide advanced functionality.

Industrial PCs, first introduced in the mid-1980s, are high-reliability computers with hardware and operating systems engineered to withstand the constant vibrations, temperature extremes, and wet or dusty conditions common in industrial environments. Using an industrial PC increases flexibility, giving users the freedom to communicate to any device using either pre-built APIs or by writing their own communication drivers.

PACs merge PLCs, PCs, and motion controllers into a single device. Rather than requiring a separate stand-alone motion controller, PACs provide multi-axis motion trajectories over a bus—such as EtherCAT—while drives close the local PID loop around the motor. This architecture not only allows the entire system to be programmed with IEC 61131-3, but also within a singular development environment—reaping all the benefits of standardized programming.

After understanding each system architecture and their pros and cons, it is critical to weigh a variety of considerations to avoid ending up with a less-than-optimal motion control solution.

Motion control technology diagram

In response to the demand for more connectivity options, today’s motion controllers increasingly offer support for multiple communication protocols. For example, a modern PAC controller provides EtherCAT communication for real-time motion, I/O, and third-party device connectivity as well as EtherNet/IP, PROFINET, and an OPC Server for machine-to-machine and plant level communications.

>Read more by Tucker and Wiley, Machine Design, May 3, 2016