Through the ’90s and the past decade, PCs became indispensable to every business, and naturally, many people and vendors started looking at using them to control machines and processes on the factory floor. Now, some automation vendors and systems integrators are saying PCs can be used for machine and process control in an increasing number of applications.

For example, in 2004, McCain Foods of New Brunswick implemented a PC-controlled refrigeration system in its new frozen foods plant in Maine to precisely control temperature and optimize energy consumption of multiple compressors. Systems integrator TechCold International, based in Keswick Ridge, N.B., wrote a sophisticated control system in a computer language easily processed by a PC but not by even a top-of-the-line PLC. The software also communicates with almost every PLC from every manufacturer on the market, and developing interfaces and drivers is relatively straightforward — not true for PLCs. Not only does the system reduce costs, energy consumption and greenhouse gas emissions, says TechCold president Ernie Adsett, it also can be duplicated in McCain’s other plants around the world. The project attracted a lot of attention in the automation world.

In the ’90s, Siemens, a longtime player in the PLC and automation field, brought out “soft PLCs,” software that runs on a Windows PC and emulates sophisticated PLCs. Its Simatic line reduces control costs by 20 to 30 percent compared to traditional PLCs, says product manager Ehab Rofaiel.

The idea of using a PC to drive automated systems was originally met with considerable skepticism. PCs, after all, crash, which could lead to production downtime and critical data loss. Siemens’ solution was its RTX real-time operating system, an OS that runs alongside Windows in the PC that keeps going even if Windows stops for some reason. Siemens has been demonstrating it in operation for years and is making headway in convincing the market that it’s viable. “Every month, we see more acceptance of PC-based controls in the market,” Rofaiel says.

Some, however, maintain that PCs just aren’t cut out for real-time manufacturing automation control.

Very early on, some manufacturers noticed PCs weren’t always performing as quickly or reliably as their old-fashioned PLCs.

“Some customers who switched from PLCs to PC control have gone back to PLCs,” says Bill Black, controller product manager with GE Fanuc, a major supplier of PLCs. “They’ve found that it’s more difficult to get support for PC control.” One reason is that regular updates to the operating system or control software often requires updates to all the drivers. And costs of PLCs have fallen recently, as well.

“Controllers have gotten more sophisticated, faster and able to handle larger programs today,” he says. Today, manufacturers who want to automate can choose between PLCs, PCs and a hybrid of the two to control processes and machines, monitor factory floors and ensure safety. But how do they choose?