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Simulating for Safety

Manufacturers can often save big money by using simulation software to model and simulate robot work cells before actually building the cells.

“We typically tell clients that they’ll see at least a 20 percent savings,” says Jean-Pierre Rasaiah, group leader, simulation, at Applied Manufacturing Technologies Inc. (www.appliedmfg.com), an Orion, Mich.-based engineering services firm. “However, it’s really tough to measure the savings from simulation,” Rasaiah adds, “because a lot of it comes from the prevention of problems.”

For example, according to Rasaiah, the cost of moving a single misplaced robot arm runs about $1,500—an expense that can usually be avoided by simulation ahead of the fact. That’s not to mention the production time lost when problems with work cell layouts aren’t discovered until the equipment is set up on the factory floor. “I’ve seen production line start-ups delayed by up to three months because they didn’t simulate,” Rasaiah says.

While robot work cell simulations have traditionally focused on things such as robot reach distances, process verification and cycle time estimates, the consideration of safety issues has lately been getting more simulation attention. All of Detroit’s big three automakers now typically specify that simulations include safety fences, light curtains and other safety features, says Rasaiah. And other industries likewise are more frequently incorporating safety into their simulations.

Rasaiah was scheduled to discuss safety considerations for simulation as a speaker at the 2003 National Robot Safety Conference Oct. 27-30 in Cincinnati, sponsored by the Robotic Industries Association (www.robotics.org). During an interview with Automation World prior to the event, he described some of the issues he planned to address.

There is no doubt that consideration of safety issues can pay off in a robot simulation, Rasaiah says. “With a simulation, you can set up your safety fencing ahead of time and know how much floor space you’re going to use,” he observes. Layouts designed without the benefit of simulation often underestimate the extent of a robot’s reach, meaning that safety fencing must be moved out farther than anticipated. This can be costly, particularly if the extra floor space is not available and the work cell must be redesigned.

In some cases, however, the amount of floor space required for safe robot operation is an issue that should be reevaluated, Rasaiah believes. Internal safety standards used by U.S. automakers and other large manufacturers often require that safety fencing be placed three to 18 inches beyond a robot’s “wild orbit”—or the envelope that a robot arm could reach if control of the robot arm is lost, says Rasaiah. The ANSI/RIA R/15.06-1999 Robot Safety Standard includes a similar requirement, he adds. “This means that if the robot goes wild, the safety fence will contain it and the operator won’t be hurt,” Rasaiah explains.

This was a legitimate concern in the early days of robotics, when instances of “runaway robots” were known to occur, Rasaiah says. “But with new robots today, I don’t know of a case where one has run away,” he says. Modern safety features such as “collision guard” software—which stops a robot arm when it hits something—further minimize the safety risk, he points out.

Work cell designs based on robot wild orbit considerations can “easily double” the amount of floor space that would otherwise be needed, says Rasaiah. This translates to higher cost, and a competitive disadvantage for U.S. companies against some foreign manufacturers that don’t design for wild orbits, he says. “One of the things I’ll be talking about is that we should reevaluate some of the standards that are used by companies in the United States, as a way to lower our manufacturing costs.”

Among other safety related topics, Rasaiah notes that simulations that include human operators interacting with robot work cells are becoming more common. These simulations are often aimed at ergonomic issues, but can also be used to evaluate safety concerns such as trip hazards, Rasaiah says.

Tooling issues can also be considered. Clamps, for example, that hold parts in place for welding robots often require loading by human operators, Rasaiah observes. “A lot of times, if those clamps open toward the operator, they can become a stabbing hazard,” he says. “We see that a lot, and it’s something that we can correct during simulation.”

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