Kevin Colloton
Senior Project Engineer
Bob Hirschinger
Jeff Faris

Motion Control Product Managers
Rockwell Automation, MicroLogix Div.
Milwaukee, Wis.

While large PLCs continue to offer greater capacity and extended motion instruction sets, smaller microPLCs are increasingly edging into territory once reserved for larger controllers and should not be dismissed immediately when considering new systems.

For example, microPLCs can handle many of the general motion-control functions built into large PLCs, but are much smaller and less expensive. These microPLCs, with simple instruction sets, stepper-drive outputs, and increased computing power, now also provide advanced control capabilities such as S-curve acceleration control for smoother jerk-free motion.

In addition, microPLCs are widely used for relatively simple tasks with dedicated, repetitive motion such as in pick-and-place or assembly machines where a platform moves in and out or from side to side when positioning products. Other common motion-control applications for microPLCs include indexing and conveyor controls where the product moves from one station to another.

The microPLC's smaller size is a major benefit in equipment where machine or panel space is at a premium. Also, built-in drive interface and motion features make the microPLC an affordable and simple solution for many motion-control applications with up to two axes.

The decision to select a larger PLC usually centers on more complex applications that require continuous process tracking and close synchronization of multiple axes, or when numerous servodrive axes are controlled in a single machine or process. These include the cam-type motions often programmed into high-speed packaging machines such as used in horizontal flow wrapping operations.

These applications also need more I/O and network connections that make the larger PLC a more efficient choice. Moreover, large PLCs can execute multiple tasks concur-rently and integrate motion and ladder logic into one function, providing extended multiaxis motion-control instructions while increasing data throughput on complex processes.

Weighing the benefits of larger PLCs versus those of microPLCs is imperative to finding the right combination of function and size, and the most reliable and efficient solution for an application at the lowest cost.