As a special class of programmable logic controllers (PLCs) that are designed for use in safety-critical applications, Safety PLCs account for just a fraction of overall PLC sales. But some think the market for Safety PLCs may be positioned for strong growth.
One primary reason: Safety PLCs and associated safety networks can help save money and boost productivity. 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.
One manufacturer that is buying into the idea 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.
Safety plus savings
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 contends. “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 expects to reduce hardware and cabling costs by about 14 percent, with an additional 15 percent to 20 percent savings in engineering costs, says Ulrich. Without the safety relays, “the hardware design for the panels is minimized significantly,” he notes.
An added benefit will be better productivity, Ulrich confirms. “When we look at the mean time between failures and mean time to repair on safety relays, we know that our uptime is going to improve with Safety PLCs,” he says. “Compared to safety relays, it’s a lot easier to troubleshoot the Safety PLC circuits.”
