Controller by Any Other Name

How Can a PAC Solution Help Machine Builders in Ways a Traditional PLC or PC-Based Control System Can't

By Joe Feeley

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In recent years, machine builders and users have struggled to get a clear understanding of what a programmable automation controller (PAC) is, particularly since descriptions of these devices vary among automation suppliers.

More importantly, they want to know how a PAC solution might help them in ways that a traditional PLC or PC-based control system can't — or can do only with costly, messy and time-consuming add-ons.

What the hardware/software solution is called is not very important to two machine builders we'll tell you about in this article. It's the performance enhancements they enjoy compared with the older PLC or PC-based control solutions that matter. Some of this enhancement provides a competitive advantage, and some is a means to satisfy customer needs as they develop more data-collection requirements.

From PC to PAC
For 15 years, the Aagard Group has designed, engineered and built automated packaging machinery in Alexandria, Minn., including integrated and standalone flexible form fill seal packaging machines, wrap-around cartoners, endload cartoners, sleevers, retort loading/unloading systems, case packers, tray packers and palletizers/unitizers.

"We used a PC-based control system with a Sercos bus for all our product lines for about eight years, before moving to what Rockwell Automation calls one of its PAC solutions," says Daren Myren, Aagard controls engineer. Myren also is Aagard's Rockwell Automation Platform Manager across all the company's machine lines. "At that time, there weren't any PACs or PLCs out there that could handle our demand — 40 to 50 Sercos axis counts and keep the processing time reasonable — with just one controller. The driving factor to switch to a ControlLogix L7 platform about two years ago was that we now could control up to 32 axes on one controller and still maintain our performance."

Another major factor was too many failures on the PC-based system. Myren says it was a more-fragile system. "Plants wouldn't know exactly how to handle security updates, and some of them could cause issues with our control system and sometimes shut the machine down," he adds. "Since we've been using the PACs, I'm not sure we've had even one PAC failure."

When the machine automation scheme was PC-based, Aagard developed and integrated a lot of remote access connectivity technology by itself. "We wrote a lot of software for data collection," Myren recalls. "We put the PC right on the server to remotely access and control the machine. Because a lot of our customers have a Rockwell-based data-collection system, our move to ControlLogix allowed us to use the PAC data and communications capability directly with the customers' data-collection systems."

The move to a PAC architecture wasn't a big concern for Aagard's customer base. "Our customers really didn't know the PC-based system very well," Myren explains. "But at that time they knew that we couldn't handle the higher axis count with a PLC or, if we did, it was going to cost a whole lot more. The PAC is a way for us to provide that same functionality at reasonable cost, and the customers were comfortable with the Rockwell brand."

The Do-All Control Platform
The supplier community earnestly does its best to position its version of what a PAC is.

"Our PAC is a multi-discipline device that incorporates more than just PLC functions, motion, process and safety," begins Dennis Wylie, product manager for the Rockwell Automation ControlLogix platform. "It brings in more direct communication with databases, multiple field networks, and enterprise-level networks."

Wylie says it's becoming more of a server-class device that is a do-all control approach for everything except smaller, simpler installations that just don't need it all. "But we're scaling that, too," he adds. "Even the smaller CompactLogix devices have some of this capability built in for when you just don't need $25,000 worth of database integration or $25,000 of historical data logging capability."

B&R Industrial Automation's stand on what a PAC-type solution looks like these days is that the primary concern shouldn't be in the terminology. "A main difference between a PAC and PLC is that a PAC provides better control of the timing of the application programs that run on the controller," says Robert Muehlfellner, director of automation at B&R Industrial Automation. "On a PAC, the program will run in a scheduled cyclic mode. An underlying real-time operating system will schedule programs in cyclic tasks with different priorities and cycle times independent of the program execution time." This, Muehlfellner argues, allows the application to be separated into programs running in a fast cycle time for time-critical tasks such as digital I/O processing, and slower cycle times for less time-critical tasks like PID temperature control and HMI logic on the machine.

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