The hybrid approach
However, PCs and PLCs are not an either-or choice. Almost every systems integrator advocates using them side by side.

Even Simatic’s Rofaiel says PCs cannot completely replace PLCs. “PLCs are best suited, for example, in fault-tolerant applications, where you have two PLCs running side by side so that if one goes down, the second kicks in instantly,” he says.

Systems integrator Arel says there is a place for both PLC and PC control systems. “The PLC’s computer power is not as good as a PC’s,” he says. “Their speed is limited.”

He worked on an installation for a printing company that prints numbered lottery tickets. Quality control is essential, so the company needed a fast and precise way to track all production and any wasted paper or tickets due to paper breaks or other production problems. Managers also wanted to link all the different equipment for a real-time, accurate picture of productivity in their plant. “Printing goes at 1,000 feet per minute, and they want to take it up to 2,000 feet per minute,” he says. That means they need very fast data exchange between the plant floor and the front office systems. “The amount of data is humongous.”

The solution was using PLCs on the equipment for tracking of lottery numbers, with a PC system on top of that for the data exchange and quality control applications.

TechCold’s Adsett agrees with this hybrid approach. “When we go into a customer’s plant, we don’t say, ‘You have to replace all your PLCs with PCs.’ Instead, we add a layer of control on top of the PLC system.”

PC in a PLC
Luckily, manufacturers don’t really need to choose between PLCs and PCs anymore. “The evolution of the programmable automation controller (PAC) has removed the barriers between PCs and PLCs,” says GE Fanuc’s Black. PACs combine PCs and PLCs, adding the newest high-speed and flexible microprocessors to the rugged, reliable operation of PLCs and using newer, reliable, customized operating systems.

Combining the two types of controllers means PACs are being used not only for process and machine control but also data acquisition, machine vision and remote monitoring. They can handle multiple communications protocols, including PC protocols from TCP/IP to OLE for process control (OPC), and PLC Fieldbus networks such as Profibus, RS-485 and others. They can also have USB ports and Ethernet connections.

“PACs can use more sophisticated programming languages like structured text, which makes programming them for math functions more intuitive. And they can handle tasks that can’t be programmed with ladder logic,” Black adds.

Simply put, choosing a side is not a necessity. With ever-evolving technologies and hybrid approaches, it’s just a matter of settling on an approach that fits the application.

Scott Bury is a freelance writer based in Kanata, Ont.