Today's programmable logic controllers (PLCs) answer to many descriptions: smaller, smarter, faster, more flexible, expandable, scalable, lower cost, highly reliable.

Units ranging from simple to sophisticated address virtually any control need a manufacturer might have. Increasingly they are also being called on to serve as data-gathering devices, while at the same time, suppliers are expanding communications capabilities and improving migration paths.

As their capabilities increase, their availability also grows. Lower price points and compact sizes make it possible to install PLCs on machines and lines where it was previously impractical. In many cases, PLCs are replacing standard controls like relays. "PLC actuation means less point-to-point wiring," explains Tim Roberts, product manager at Schneider Electric (Palatine, IL).

Functionality is substantially broader, too, especially on lower-end models. For example, the CompactLogix platform from Rockwell Automation (Milwaukee, WI) now includes integrated motion control, a feature previously available only on the higher-level ControlLogix platform. Indeed, high-end PLCs like the ControlLogix have become so sophisticated, ARC and some vendors describe them as "programmable automation controllers" (PACs) rather than PLCs.

Many top-end PLCs are multidisciplinary. Rockwell combines control, sequential, process, motion, drive, and safety in its flagship controller, the ControlLogix. "One platform gives users a single common development environment," explains Mike Miclot, director of Rockwell's Commercial Programs. The single platform also improves asset management, since spares and training are the same for each functionality.

In addition, integrated safety control permits warm shutdowns while still protecting the operator. Eliminating the cold shutdowns required by all-mechanical, electromechanical, and standalone safety controllers reduces downtime and improves productivity.

In 2004, GE Fanuc introduced the PAC RX3i controller, based on Intel's Celeron and M-class processors. It supports multiple programming languages and possesses 10 MB of memory, allowing documentation, diagnostics, and other information to reside in the controller. An RX3i Universal Analog Input module (configurable on a per channel basis for voltage, current, RTD, Thermocouple, strain gage, or resistive) delivers the functionality of four modules in one, reducing input/output (I/O) module space requirements, decreasing the number of spare parts and inventory, and simplifying wiring and configuration.

A newer model, the PAC RX7i, is even more powerful, boasting 65 MB of user memory and a high-speed 1.8 GHz processor. It's designed for applications like paint booths, where hundreds of recipes need to be stored in the controller. "Certain applications want to store as much process information as possible at the controller level rather than relying on the network server to have all the data," explains Bill Black, controllers product manager at GE Fanuc Automation Americas, Inc. (Charlottesville, VA).

Despite the increased capabilities, some features haven't changed: PLCs continue to rely primarily on ladder logic programming, which is familiar to plant personnel. And PLCs remain one of the most reliable components on the factory floor. In fact, today's units are more reliable than ever due to the replacement of hardware with software and improved quality in the PC board supply chain. Failure rates of less than 0.5% are not uncommon, meaning that time before failure numbers can reach into the tens of thousands of hours.

With PLCs gaining functionality expanding, the worldwide market will grow 5.9% per year, rising from $7 billion in 2004 to $9 billion in 2009, according to a report from ARC Advisory Group titled "Globalization Drives PLC Market Growth."