01 September 2003
Stacking chips in system control game
Do PCs or PLCs hold the winning hand?
By Ellen Fussell
Most people think about PC control as all the information coming into the personal computer (PC) rather than the programmable logic controller (PLC). For instance, a company such as National Instruments might provide circuit boards to put into a PC. That interfaces with a customer's hardware on the outside. Now the PLC's function is in software instead of hardware, said Erwin Icayan, president of Aces Inc. in Richland, Wash.
In a typical application, such as maintaining a certain amount of flow, manufacturers can control a valve based on feedback from the PC data. "The software in the PC says you have a set point of, say, 30 gallons per minute. If the flow is too low, then the algorithm, the calculation done in the PC, will send a signal to the valve to open a bit more so it will bring the flow rate back up," Icayan said. "You can actually manipulate it as you're watching the progress."
"Typically a user will implement PLCs for the lion share of their automation needs. They'll augment or add PC-based control for niche-oriented portions of their applications where it makes sense," said Mike Miclot, a product marketing manager at Rockwell in Mayfield Heights, Ohio. In one case, Trane Manufacturing used PC-based control to augment a portion of their process (see accompanying article on Trane Manufacturing).
One of the things PC-based control does more elegantly than PLC control is taking up residence in places where it makes sense to integrate a client's operator interface with some other type of software application. "If you have man/machine interface [MMI] and automation control needs, you can do all that in one piece of hardware," Miclot said. "You can run your operator interface using your display or computer. With PC-based control you can actually control out of that piece of hardware."
But many times customers have proprietary code and need something specific to meet their needs. "That code could have been written a week or a decade ago. It could be some type of special algorithm, perhaps some secret recipe," Miclot said. Customers might want to maintain that because "either it is near and dear to what they do (top secret) or is legacy code—the guy who wrote it retired, but they need to move it forward because it does something particular," he said. With off-the-shelf PC control you can run PLC control, which is more understandable for the operator, but you can also run that special code. It reduces maintenance.
PC-based control's good points are on the front end, Miclot said. "Acquisition costs are smaller, because you can go to Wal-Mart and buy a computer and piece meal it out. But you wind up paying on the commissioning and maintenance side. In most cases the total cost of ownership is more expensive with PC-based control than PLC. Another advantage is it's flexible—without boundaries. I can install my own software and write my own code. It gives me lots of flexibility and familiarity."
PC-based control also serves niches, Miclot said. The niches are places where it makes sense to combine operator interface and control, proprietary customer software you need to execute. PLCs make sense in a material-handling application, "but at the very end of the conveyor system, you'll do a bar-code scan to determine what shipping lane it's diverted to, based on the mailing address—an enormous database of zip codes," he said. "So crunching data to read a zip code, look up where it is, and figure out what stations downstream to divert it to might be best handled by a PC. Zip codes translate well into computer speak but not PLC speak," he said.
WHEN PLCS TRUMP PC CONTROL
Although PC control works well in some situations, it might not be the ultimate solution for everything. PC acquisition costs are lower, but you could end up paying more in the implementation phase—or the commissioning phase—and for maintenance with PC-based control, Miclot said. "When we sell a PLC it's a piece of hardware dedicated to doing control," he said. "The boundaries are set, but the amount of user interface to get the thing running is small." A PC of course was created to do everything from desktop publishing to operations on a small palm-held device. It is a very universal device, so the boundaries are limited. "That means the guy who is going to use it for control has to create his own boundaries. So you're sourcing your computer equipment from one vendor and applications from another vendor. It's kind of a buffet of vendors—all designed to work together, but rarely can you get it all installed and up and running in seconds," he said. A lot of rules are enforced to make the process run smoothly. "But if you need to be doing control by a certain date, you have to plan for that and iron out kinks."
PC-based control will also incur higher maintenance costs, Miclot said. "If you expose your PC-based controller to an environment that causes a video card in your computer system to fail—a year after you've started the system up, you won't be able to find the exact replacement, because the technology on the card has changed. You get a new video card, and suddenly it's 3.2 volts instead of 5.5 volts. Now you have to change the whole PCI bus of your computer system. So any other PCI card in your system better support the 3.2 volts. Then you find out that change requires you to be on Windows 2000 or XP, and you were running Windows NT," he said. "Now you have an operating system upgrade. And two or three of your software applications need to be upgraded. It's a huge snowball effect."
Security is also a big issue these days, Miclot said. People are worried about hackers. "So IT [information technology] institutes a corporate-wide 'thou shalt install a new firewall and antivirus software on all computers' rule," he said. In such a case, PC-based control falls under IT's domain. "And they now have authority to install Norton AntiVirus; which slows down a computer's performance. Now you're not executing control like you used to, so you need a faster computer to take on the burden." But with a faster computer you might have to upgrade two or three packages of software and drivers. So again there is a snowball effect.
PLCS BRING PEACE OF MIND
With a PLC, sensors can tell you something is present; you can read that input and create an action—do something with it—"say, count the number of beer bottles for a six pack—real simplistic actions," Miclot said. PLCs are perfect for that. Another benefit is its operating system isn't universally known, he said. As a result, it is much more difficult to hack into. "For instance, Rockwell's is proprietary to Rockwell, Siemens' is proprietary to Siemens," he said.
PLCs are much more robust from an environmental standpoint; they can tolerate greater temperature extremes, greater shock or vibration, and more environmental testing than a PC can. Plus, they're more durable. "Our operating systems tend to not lock up. If I'm manufacturing diapers or razor blades and paid on the thousands of output per minute, downtime is expensive. So the number of occurrences and the duration thereof is expensive time," Miclot said.
Yet with all the downsides of PC-based control, Miclot still believes it will continue to play a part in automation well into the future. "I think there are enough applications out there where it makes sense to use computers, but we need to deploy them judiciously," he said. "For example, Trane may use a combination of both. If they're doing data collection, where the PC makes sense, it is functionality above and beyond what PLCs do today. I think there'll be some kind of convergence. You're seeing it now. We're adding PC-like functionality to our PLCs. We do trigonometry on our PLCs. We're embedding Ethernet on our PLCs. Those are things we provide today—Web services out of our PLCs, and Web pages out of a standard browser. I think that's what customers want. They'll keep asking us for PC attributes, but they'll still want the robust, secure, environmentally sound devices they've had for years, because they want peace of mind." IT