A ROBOTIC WORK CELL SAFETY PRIMER Dayton OH

Robotic systems represent a large investment for a manufacturing plant. Because the anticipated profit from their use significantly exceeds the cost, the price is considered worthwhile. Protecting robotic work cells from damage so they will remain online and functioning effectively requires protective equipment.

But the largest potential cost if that protection is inadequate has nothing to do with harming the robot. It is the plant's financial liability for accidental worker injury or death. Installation of an effective robotic work-cell safety system can be expected to reduce both on-the-job injuries and insurance premiums.

OSHA'S PERSPECTIVE

While the Occupational Safety and Health Administration (OSHA) has no specific standards regarding robotics safety, the general duty clause of the OSH Act applies. It requires employers to provide a workplace free from recognized hazards that might cause death or serious physical harm to employees. OSHA's Guidelines for Robotic Safety specify that a written robotics safety policy and worker training are necessary for effective accident prevention.

The greatest risk of harm is human error, often caused by over-familiarity with the robot's movements. This could result in a worker putting himself in a hazardous position, particularly while programming or maintaining the robot. Research done in Europe has found that worker injuries are more likely during non-routine tasks than during normal robotic operations, and this is also true in the United States.

"Contrary to popular belief, the largest number of robot accidents do not involve collisions between personnel and the robot," notes robotics safety consultant Vern Mangold, president of NAMAN (www.namanenergy.com) in Centerville, Ohio. "The self-starting of ancillary equipment, the opening and closing of fixtures, the unexpected indexing or transfer of fixtures and tools and secondary hazards such as UV radiation constitute the larger category of accidents."

For example, after changing a fixture for a new part, the technician forgets to collect a tool. When he absentmindedly reaches for it from atop a fixture, a workholding clamp automatically toggles into position, smashing his hand.

CREATING AN EFFECTIVE SAFETY SYSTEM

Before designing a robotic work-cell safety system, a company should perform a risk assessment. This can be handled by a consultant, the robot's manufacturer, the integrator who installs the robot or the user. ANSI R15.06, the industry's robot safety standard, requires consideration of the frequency of human exposure, the severity of potential injury and the possibility of avoidance, notes Steve Freedman, director of safety systems for automated control maker SICK Inc. (www.sickusa.com) in Minneapolis.

"Once we've decided what kind of risk we have, we need to decide what type of guarding is appropriate for risk reduction," explains Brian McMorris, SICK's business development manager for robotics. "Depending on the size of the robotic work cell and the nature of the work itself, there may be tasks that could be done without any workers interacting with robots. That would be fairly simple to guard by totally enclosing the cell because you just close the door and lock it."

More commonly, workers need to be near robots to load parts, do maintenance, clean or reprogram a robot or simply walk by—and all this requires appropriate safeguarding.

Mangold says the necessary components of a robotic work-cell safety system include physical barrier guarding, dual channel interlocks, request to enter controls, ANSI teach pendants and comprehensive training. "The systems must be designed with fail-safe features and computer-aided prompts and tools that remind the infrequent user of the dangers in integrated environments," he stresses. "It requires a different mindset by those who are charged with the task of designing systems."

In designing a safety system, users must take into account the potential for accidents from things like flying debris and weld flash based on what the robot does while preventing entry into harmful areas, notes Gerry Timms, product manager for Milwaukee-based Frommelt Safety Products (www.frommeltsafety.com). Users should also look at how parts are loaded or unloaded, parts' size and storage, floor space and programming.

AUTOMATED CONTROLS

When barriers alone are inadequate, automated controls can allow access when conditions are safe and prevent entry when they are not. Light curtains, for instance, are invisible beams of light that take action when the beam is broken. "If a worker goes into a robotic cell at the wrong time and breaks the beam, it knows something is where it shouldn't be, and it stops the machine's hazardous motion," says Freedman.

In designing a system using light curtains, it is important to position them properly based on how fast people might be moving when they enter the robot's space and how long it takes to stop the robot's movement.

Another technology, the laser scanner, works both horizontally and vertically and can adapt as the robot's actions change during the work cycle. "You can use them as a primary safeguard or inside the robot cell where the areas and shapes are very unusual to make sure no one restarts the robot while someone is inside the cell," comments Freedman.

Both scanners and light curtains are valuable for point-of-operation guarding because they will let someone into the robotic cell to load things during certain parts of the machine's process while keeping them out when it is not safe.

"Where workers don't interact with the robot very often, surround it with hard guarding (a physical barrier) because it's the simplest protective method," McMorris points out. "If workers are required to interact with the robot constantly, a light curtain is the best safeguarding technique because they can walk in and out of the work area freely without opening and closing doors. If there's a risk of flying parts, a fence is most desirable because light won't stop objects from hitting people."

Another way of keeping people out when robots are operating involves the interlock switch. These detect the removal of guards that should be in place, stopping the robot's function. These are typically located on doors or panels leading into the work cell. The most sophisticated is the solenoid locking switch, which keeps the door locked until the robot's motion has completely stopped. For robots that require a longer time to stop safely, this is a good choice.

COORDINATION ESSENTIAL

To protect workers from ignoring or overriding safety systems, "machines, doors and safety devices need to operate together so they cannot be bypassed," Timms emphasizes. "Machines or robots should not activate if safety systems are not operating." This can be done using safety controllers and interfaces connected to the automated devices' sensors.

While it is simple to install safety devices from an electrical and mechanical standpoint, doing it in a way that meets all safety regulations is complex, McMorris emphasizes. "The user needs to be sure the total solution is safe, not just safe components put together into an unsafe system," adds Freedman. Some users install their own systems, but others prefer to have it done by the integrator to avoid the liability risk.

Worker training should be conducted on a continual basis consistent with the requirements of the application and the plant's resources. At a minimum, Mangold says training should be done at system demonstration, installation, start-up, changeover of fixtures and tools and after major reconfigurations. Additional training is desirable as part of a machine's routine preventive maintenance, with operator orientation after 1,000 hours of machine time and maintenance training at 5,000 hours. Programming refreshers should be conducted with any major software release or annually, whichever comes first.

FOCUSING ON WORKERS

Sometimes it seems protecting the financial investment in the robot takes precedence over worker safety. "I don't think that, in every case, the focus has been deliberate," Mangold notes. "Part of the problem is the anthropomorphic image that robots have. We still slip and call the robots 'him' or 'her.' We refer to the robot 'wrist,' 'hand,' 'shoulder' and 'brain.' In addition, the old paradigm of one man operating one machine has obscured the requirements of safeguarding integrated cells. However, the climate is improving."

"We talk to our clients a lot about designing the safety into the system as early as possible," Freedman stresses. "It is often harder for the operator to do his job if safety is just added onto an existing system. People can get very comfortable with the systems they work on. If a part gets jammed, they might reach in to free up the part, not realizing that the machine may operate in an unusual way. These types of considerations should be kept in mind while designing the system."

THE PRICE OF FAILURE

When an OSHA investigation following a robotic accident finds the manufacturing plant at fault, citations will be accompanied by fines. If the report labels the event as "serious," fines can range from $1,000 for a paperwork violation to more than $100,000 if the problem results in an employee's death. "'Willful' violations and repeat offenses can result in significantly elevated fines," Mangold notes. "But it is not unusual for the fines to be reduced to a level of 10 to 30 percent of the face value after a company has met with the OSHA inspector."

Plants cited by OSHA for accidents can also expect a dramatic increase in their liability insurance premiums. That's because affected workers will sue. But because more companies are paying attention to safety standards and following them, robotics injuries have dropped significantly during the past 10 years. However, the Robotics Industries Association is currently considering the adoption of uniform robotics safety standards worldwide similar to the standards used in Europe. Should this occur, the U.S.'s existing safe practices could be diluted.

"Starting and stopping motion or getting a robot to do something is very simple from an electronic standpoint," says Freedman. "Getting it to start and stop safely can be difficult, so companies need to rely on someone who knows what they're doing to be sure the entire installation is really safe. If you do the system properly, you may even have more productive people because they won't be overly cautious around the robots. They'll know they can work safely while maintaining a high level of productivity rather than having a system that's designed just to keep them out."

Editor's Note: Jean Feingold is a freelance writer based in Gainesville, Fla. Artwork courtesy of SICK Inc.