How to Choose the Best Controller for Each Application

Consider the features and typical applications for industrial controllers to understand what type of controller fits each application. Do you need a programmable logic controller (PLC), programmable automation controller (PAC), or industrial PC (IPC)? See comparison tables for PLCs, PACs, and IPCs.

Automation controller applications are widespread-as are the overlapping capabilities of programmable logic controllers (PLCs), programmable automation controllers (PACs), and industrial PCs (IPCs). Most of these controllers will work in discrete, process, and hybrid control applications to varying degrees, but what criteria should be used to choose the right controller for an application? (See Fig. 1.)

To help understand what controller fits each application best, there are merging features of automation controllers to identify. Examining typical applications can highlight similarities and differences among these controllers.

Understanding the Types of Control Systems

With the many controller choices, a basic understanding of the different types of controllers is important. Even within each controller system type, such as the PLC, there can be several families of controllers from low-end to high-end, with a wide range of functionality among families. The following are among controller types:

The PLC—the original relay replacer—is what started it all nearly 50 years ago. It's suitable for controlling a wide variety of applications. Although available in many fixed input/output (I/O) formats with minimal expansion, (often called brick PLCs) the most common form factor is a rugged, modular, rack-based design that allows flexible configuration of the I/O based on the system requirements. The central processing unit (CPU) of a PLC is typically a purpose-designed controller with limited serial and Ethernet communications capabilities. The PLC commonly uses ladder logic programming, although other options are often available, and is a very competitively priced machine control option.

The PAC is the next-generation PLC. While these controllers are similar in form factor and design to the PLC, the PAC's newer technologies, borrowed from innovations in consumer PCs and mobile device design innovations, have advanced their capabilities. PACs typically have expanded communications and data-logging capabilities compared to PLCs.

PACs also have a variety of programming options, typically centered around the International Electrical Commission (IEC) 61131-3programming standard, which extends control capability into high-end applications. But even with high-end programming available, the PAC's roots are still in ladder logic.

The IPC is a PC built to operate reliably in an industrial environment. But with newer and smaller component designs and more compact operating systems, the IPC no longer looks like a desktop PC or even a panel-mount PC, two of the most common form factors.

The IPC is now being designed for DIN-rail or rack mounting, which expands the application space. Because an IPC is a PC at heart, its theoretical maximum processing power and communications and data storage capabilities are unmatched by PLCs or PACs.

Some early versions of the IPC received some strong criticism because they were not as rugged as they should have been and because their operating systems were very unstable, but today's versions have improved.

Controller Feature Comparison