Fuzzy logic? Navigate blurred distinctions between micro-PLCs and full-scale PACs

Editor's Note: A variety of controller choices exist, depending on application and need. Read about how the technological gap between options has diminished as an introduction to this article.

Controller options have evolved and expanded over the decades. The line between relays, machine automation controllers (MACs), programmable logic controllers (PLCs), programmable automation controllers (PACs) and microcontrollers has blurred.

The standalone relay has a communications backplane that allows for expansion modules, and the inclusion of a common network protocol means that the micro-PLC can now talk to other devices in a control design.

The evolution of the smart relay to micro-PLC has reached the point where the micro-PLC is really a micro-PAC. A recent project I worked on allowed me to include drop-in add-on instructions that would communicate directly with variable-frequency drives (VFDs) that I would formerly have had to use with a full-on PLC or PAC as part of a control design. The hardware company has greatly enhanced its programming software, which is still free, such that I can cut and paste code directly out of the PLC or PAC into my micro-PLC software application.

Program development is greatly reduced, as I can use the library of code that I have collected over the years and adapt it, with only a few adjustments, for use with the micro-PLC.

All of this progression from smart relay to micro-PLC is creating a bit of a dilemma when it comes to selecting a controller platform for new designs because the lines of distinction between full-on PAC, a mini-PAC and the micro-PLC/PAC are quite blurred. For me, I would still use a fixed backplane PAC for a large process or material-handling operation where interaction with a higher-level supervisory control and data acquisition (SCADA) or enterprise resource planning (ERP) system might be involved.

A standalone machine would easily fit into the modular mini-PAC category where high-speed actions are required and interaction with other machines on a line might be required. However, I can see where the micro-PLC would easily be a choice for something like a tote dumper or elevator control system where the operations are relatively slow but can benefit from a controller that can be part of a plant or room network but otherwise operates independently of other processes.

The interesting part of the continuing evolution of the programmable controller is that the modular PAC was intended to offer a more economical version of the chassis-based PAC, and the micro-PLC was intended as a replacement for products like Siemens’ Simatic S5 and Rockwell Automation’s Allen-Bradley SLC-500 platforms. However, with the micro-PAC, we have a product that started out as a smart relay but has evolved into a platform that nearly equals the micro-PLC, but at a fraction of the cost of investment. Many automation companies have product lines that now rival the main brands and, in many cases, exceed the capabilities of these established product lines.

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It is worth it to thoroughly research the options available if the choice of a processor is not specified by the end user. While this is not intended to be a complete list—I do apologize if I missed anyone—many of the usual suspects, as I call them, may not be the only choices for the application.

For example, I recently came across comparable products from Keyence, which is normally a sensor company for me, Maple Systems, traditionally a human-machine interface (HMI) supplier, and Turck, sensors and cables, that popped up as considerations for programmable controllers. I have used products from AutomationDirect in the past but never gave its PLC products much of a glance and was surprised to learn how powerful they are. B&R and Wago are also steady players in the field of small/micro-PLC/PAC products. Arduino is gaining a lot of notice and is a favorite on many of the social-media platforms. Unitronics has PLCs with built-in HMIs, and this is certainly a consideration that reduces cost and development time. Schneider Electric, Eaton, Beckoff, Phoenix Contact, Opto22, Raspberry Pi and Mitsubishi Electric also offer products that fit into the small PLC/PAC realm.

The point to listing all of the above brands is to encourage a wider view when it comes to automation design. We don’t necessarily need to stick to our favorite brands, and we might even discover that going with what we might think of as an off-brand may end up being a new favorite and open up opportunities to work with new clients who might have a tighter budget but still require some horsepower when it comes to an application.

Best of all, not all PLCs restrict the programmer to ladder logic. There are many more options available with structured text languages gaining strength and offering the ability to migrate into HTML or Python languages as an alternative to the traditional programming languages.