Trends Affecting the PLC vs. PC Control Decision

The increasing integration of IT and operations technologies is reviving the PLC versus PC-based control decision for many companies. The key to making the right decision is based more on application than functionality.

The list of non-industrial technologies changing industrial automation seems to grow longer each year (see the list at the bottom of this article highlighting the IT-sector technologies currently impacting automation the most). Ever-present in the background of this ongoing transformation is the PC versus PLC decision. This decision is more often faced by OEMs than end users, but the reasons surrounding the decision are having a bigger impact for both sides in the industrial machinery equation.

For years now, PCs have played an increasing role in various automation applications. And though the use of PLCs is in no way endangered, two sets of trends could start to sway the pendulum toward greater use of PC-based control.

One of these trends is the increasing integration of IT and operations technologies. As can be seen throughout the article links at the end of this article, the influence of IT on automation is not fad—it’s a clear trend that’s been developing for some time and is being applied in industries of all types. A PC’s ability to interact with another PC—and other information technologies in general—make it more adaptable for the various IT and operations linkages we’re seeing more of these days. A PLC can, of course, be used in much the same manner as a PC-based controller, but gateways, switches and/or servers are often needed as an intermediary connection step.

The other trend is cost. PC-based controllers are often cheaper than their PLC counterparts. Some of the reasons for this cost difference can attributed to supplier pricing (as I have heard several sources mention over the years), but it’s also due to the costs associated with the ruggedized factors common to PLCs.

Speaking with Joe Martin of Beckhoff Automation (a proponent of PC-based automation) at the ONS event in Norway, he equated the decision of whether to use a PLC or PC-based controller today to the decisions faced by OEMs when it comes to using NEMA or IEC products. NEMA is the PLC counterpart in this comparison, as NEMA products feature more robust designs for broad applicability and, thus, are typically more expensive than IEC products. Like their PC-based control counterparts in this discussion, IEC products are viewed as more flexible than NEMA products but require specific application knowledge for successful use.

This application-specific use is a key point for Martin, who noted that use of redundant controls in industry has been the standard approach because that’s how PLCs operate. “They’re designed to run the same programs simultaneously and be able to switch over quickly if one fails,” he said. “Two PCs can’t run the same program at the same time. However, by using a fast switch between two PC-based controllers, I can download all the variables from one controller to another and have it running four times faster than a standard PLC unit can switch over.”

Martin mentioned a large oil and gas customer who has recently made the move from PLC to PC-based controllers for some its key valve operations. He said the PLC-based valve controllers cost the company about $7,000 each. Using a PC-based controller brought the cost down to $3,000 each. “Plus the new PC-based controller can talk directly to the control PC located in the control room,” he added.

With all the price pressures facing the oil and gas industry from the continued low price of oil, more companies in this industry are looking to other industries for lessons on how to automate and save money, said Martin. “They’re seeing that they’re maybe not as unique as they once thought they were and that these proven industrial technologies can work just as well in their industry as they do in others,” he said.

PLCs are still preferred in certain zones of oil and gas operations, Martin noted, particularly in hazardous/explosion-proof areas. Beyond that, however, the discussion around PC-based control versus PLCs is really becoming like the decisions made around NEMA or IEC. “It’s application specific,” Martin said, “not capability specific.”

Recent Automation World coverage of IT technologies affecting industrial control:

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