PLC, PAC and IPC standoff

Selecting the Best Type of Controller Depends on how you Define the Technology as well as the Application,

By Leslie Gordon

1 of 3 < 1 | 2 | 3 View on one page

Are there a few key things that OEMs or system integrators say they've discovered programmable automation controllers (PACs) do better than programmable logic controllers (PLCs) or industrial personal computers (IPCs)? Perhaps not so surprisingly, the answer lies in a grey area, depending on how those in industry distinguish between the technologies.

James Ingraham, software development team leader at systems integrator Sage Automation in Beaumont, Texas, says, "I'm not 100% convinced that PACs are actually a thing because any supplier that sells a PAC sells it to end users that call it a PLC.

Historically, PLCs were, in fact, logic controllers, so they were really good at Boolean on/off, 'check-a-photo-eye, turn-on-a-motor' kind of stuff. The addition of servos, PID loops, analog control, communications and database access have complicated the issue. Yet, out here in the real world, we still call it a PLC."

The IEC 61131-3 standard implemented some years ago defines five languages for controlling devices with a PLC or PAC, explains Ingraham. "The languages are ladder logic, which we all know and love, structured text, sequential function charts, function block diagrams and instruction lists. In addition, a lot of PACs also provide more traditional programming languages like C. That said, an advantage of IPCs is their processing power is vastly more than that of a PLC or PAC. A 12- or 16-core Intel processor with essentially unlimited RAM runs circles around a PLC or PAC. So if you have to scale up, that's probably your best bet."

In an example of one project, Sage used a PAC to combine motion control and database integration with traditional logic control for the just-in-time order fulfillment of tires (Figure 1). "When it comes to end users that are running a bunch of conveyors, for instance, whether they have an Allen-Bradley shop or a Siemens shop down the street matters a whole lot more than a particular processor's technical specs," adds Ingraham. "Most of today's technology is of relatively equal high quality. More important questions to ask are: where is the support coming from, and which supplier do we have the best relationship with?"

PACs Handle Tilt-Tray Sorters
Another system integrator has a slightly different take. Pyramid Controls in Cincinnati, Ohio, specializes in developing automation solutions to optimize flow, increase accuracy, increase machine uptime and improve the productivity of product distribution centers (DCs)."Over the years, we started calling controllers PACs because they have more capability than traditional PLCs. A PAC or ControlLogix processor, for instance, can handle servos, all kinds of motion controls and so forth," says Pyramid's engineering director, Mark Hegge. "But in our minds, the biggest difference between PC-based controls, PLCs and PACs comes down to the reliability of the equipment."

Also Read: The father of the PLC explains the difference between the PLC and the PAC

In 2012, the firm replaced an outdated PC-based industrial control at a major retailer's distribution center (DC) with a system driven by Allen-Bradley ControlLogix PACs from Rockwell Automation. Many goods sold in major department stores such as dresses, suits, appliances and accessories arrive at the DC in bulk shipments from manufacturers. There, they're sorted using four tilt-tray sorters and then shipped to hundreds or thousands of retail locations. Packing-sorter machines carry items from the area where workers unload wholesale merchandise from palletized cartons and place individual items on tilt-tray machines, which feed the items into different chutes, depending on where they'll be shipped.

The retailer's legacy PC-based control system was aging and outdated. It couldn't properly control the tilt-tray sorters, which are highly sensitive to the timing at which internal switches needed to fire, within 50 ms or under 12 ms. Missing this window could shut the entire sorter down. The retailer also needed a technology that could synchronize more than 70 motors on each of the four loop sorter lines, as well as track products to eliminate the problem of lost inventory.

The biggest difference between PC-based controls, PLCs and PACs comes down to the reliability of the equipment.

1 of 3 < 1 | 2 | 3 View on one page
Show Comments
Hide Comments

Join the discussion

We welcome your thoughtful comments.
All comments will display your user name.

Want to participate in the discussion?

Register for free

Log in for complete access.


No one has commented on this page yet.

RSS feed for comments on this page | RSS feed for all comments