What's in a name?

When the name is Safety programmable logic controller (PLC), the meaning seems obvious—a special class of PLC designed for use in safety critical applications.

Yet one fact that may not flow intuitively from the moniker is among the primary reasons these products may be primed for wider use by industrial companies. Namely, that Safety PLCs and associated safety networks can help save money and boost productivity.

So far, the market is relatively small. Sal Spada, a research director at ARC Advisory Services, Dedham, Mass., estimates that worldwide sales of Safety PLCs alone this year will total less than 25 million, compared to a market of about .45 billion for standard PLCs, excluding input/output (I/O), software and services sold separately.

But with time, says Spada, that is likely to change. “There’s a large untapped market for Safety PLCs,” he contends. “Once end users start to understand the concept, and how to employ it, you’ll see Safety PLCs taking on the same adoption curve that the original PLC took on in the early 1970s,” Spada predicts. “I think it’s going to be extremely high growth.”

Money savers

One reason for the optimism is the growing influence of international safety standards such as the IEC 61508—the International Electrotechnical Commission standard that governs functional safety in programmable electronic systems. As more end users become familiar with the standards, they are beginning to recognize the savings associated with Safety PLC-based systems, say industry sources.

Safety PLCs can produce bottom-line benefits for manufacturers in a variety of ways, Spada and others point out. Just as standard PLCs emerged as programmable replacements for hardwired relay logic in the 1970s, saving oodles of money in reduced wiring and engineering costs, Safety PLCs promise to do the same for safety relays. Moreover, Safety PLC-based systems are less prone than hardwired safety systems to “nuisance trips” that can unnecessarily shut down a factory operation. And they are much easier and faster to troubleshoot, resulting in less machine or process downtime.

“If I have some big, giant panel with tons of relays that failed at 3 a.m., and I’m trying to figure out what tripped and why, a microprocessor-based (Safety PLC) system is ten thousand percent the way to go, because it’s got diagnostics,” declares Mike Scott, vice president of process safety at AE Solutions, a Greenville, S.C.-based safety systems integrator.

AE Solutions earns the majority of its revenues in the process industries, which have historically made wider use of Safety PLCs than the manufacturing industries. But some vendors note that sales and interest in Safety PLCs lately have been picking up among manufacturers—particularly automotive. “The automotive industry is one of the early adopters in implementing Safety PLC-based solutions, because they’ve recognized the cost savings and benefits associated with it,” asserts Filomena Wardzel, automation solutions business manager at the Siemens Automation and Motion division, Alpharetta, Ga.

GM buys in

One case in point is General Motors Corp. The automaker will go live with several Safety PLC-based safety networks for the first time in a production environment this fall, says Craig Ulrich, engineering group manager, control development, at the GM Technical Center, in Warren, Mich. As part of a new vehicle program at GM’s Lake Orion, Mich., assembly plant, the company is deploying about 18 Allen-Bradley Guard Safety PLCs from Rockwell Automation, Milwaukee. Configured on Ethernet safety networks, the Guard units will provide safety monitoring and “control-reliable” access for plant personnel to robotic work cells that build vehicle doors, hoods and trunk lids.

Further, the Lake Orion program is only the beginning. “Starting in 2004, we will design all of our facilities using Safety PLCs,” says Ulrich. He provides no timetable for the rollout. But eventually, says the GM engineer, the company plans to deploy around 250 Safety PLCs for use in safety critical applications in each of 44 factories—for a total of around 11,000 Safety PLCs.

That’s a lot of boxes and modules. But GM isn’t making the move because its current systems are unsafe. “We’re already the safest company in the automotive industry, and we have the data to prove that,” Ulrich declares. “So we don’t feel we’re enhancing our safety at all by going to Safety PLCs. We’re really doing it to take cost out of our systems.”

GM’s current approach to safety includes fenced equipment work cells with gated entry systems monitored using control panel-mounted safety relays. By replacing the safety relays with a safety network and a Safety PLC for each cell, GM ...