Safety in the context of a factory or piece of equipment requires that several considerations be addressed: How often are workers at risk for injury from a piece of machinery? How easily could a person avoid injury in such a situation, and how badly would the individual be hurt if they could not avoid it? What's required to render a system benign?
The risk assessment process EN 1050, updated in Europe last year with ISO 14121-1, uses these and other questions to quantify the level of danger in manufacturing environments. “Then there are detailed lists of safety precautions, components, and levels of redundancy to help sufficiently abate risk in such systems,” adds Lyle R. Masimore, Business Manager, Safety, Rockwell Automation.
In addition to helping improve safety, risk assessments are often a designer's best protection against litigation — but they must be documented. Consultants can help here; they visit plants to provide safety risk analyses as well as train personnel on safety, to help them avoid occupational injuries, which in turn helps reduce safety incidents, improve employee morale and increase production, and support lean manufacturing.
Continuum of responses and systems
Tailored responses are possible because of the continuum of safety products available today. “At one end there are dedicated-function relays and at the other, safety PLCs — with electromechanical, multi-function relays, application-specific controllers, and solid state electronic relays in between,” says Dan Hornbeck, Manager, Safety Business Development, Rockwell Automation. In many installations today, hard-wired safety devices are used in addition to the standard PLC doing the control. A more cost-effective alternative for some newer systems is the use of integrated safety PLCs capable of both standard and safety control. So, yesterday's solutions are not necessarily the best anymore. “In fact, 60% of our safety offerings are products that have come out in the last five years,” Hornbeck adds.
For example, newer configurable safety relays and safety PLCs allow zone control, where controls and networks allow individual manufacturing sections on a machine to be slowed, while allowing the rest of the line to continue work. This can provide a safer situation for employees without shutting down the entire machine. It also helps improve productivity because it can reduce lengthy startup cycles.
Wiring
A chain's only as strong as its weakest link; similarly, a safety system is only as safe as its least-safe element. Often, sensors, controllers, and actuators must all meet the same level of safety. So products are shifting toward the more sophisticated end of the continuum.
Over the last few years, open networks such as DeviceNet and EtherNet/IP have implemented safety extensions that allow safety and standard devices to reside on the same network. “Safety devices are offering the same cost benefits of distributing standard devices over a single network — including reduced wiring and eliminating disparate networks, both of which help save costs,” says Hornbeck. “As a result, we are seeing many customers move entirely to EtherNet/IP. Another benefit EtherNet/IP offers is its ability to handle the safety and standard needs for the control part of the application, and provide a connection into the information systems of the facility.”
In addition to leveraging single networks, the latest frontier in safety is to allow for safe speed — as opposed to safe off — for some applications where motion is present. When a risk assessment indicates it is acceptable to run equipment at slower speeds or in certain directions while someone is performing procedures, such as jogging equipment, the latest safe-speed monitoring relays and drive products help meet safety requirements while maintaining efficiency.
Treating all safety incidents as if they pose the same risk and require the same response often decreases productivity and results in unnecessary emergency stops. It's a bit like slamming the brakes on your car for every situation in which you need to slow down. That's no good. Similarly, when an operator e-stops a machine, it's hard on moving systems. Everything screeches to a halt, causing mechanical wear from shock and impact. In contrast, newer systems can help improve safety while minimizing factory worker reliance on e-stops because they're capable of more sophisticated, appropriate responses.
The standing tower of Babel
An industry movement is afoot to consolidate local standards as well as to base them, at least in part, on global standards, such as IEC and ISO. For example, China may soon adopt or reference IEC standards, and Japan already is using those. This is good news for designers as it's becoming easier to supply and create machines and even facilities in other countries with minimal changes to designs that meet global standards. In addition, because more manufacturers use the same products, designers can benefit from the cost-savings that result from higher volumes.
Even with the movement toward adopting, or at least mirroring, global standards, differences remain. In the United States, for example, the responsibility for providing a safe place to work rests primarily on the manufacturer, while in Europe the responsibility falls primarily on machine builders, who are required to make machines safer. So how do you know if you are providing a safe and productive solution? “The answer lies in performing a risk assessment on the application,” says Hornbeck. “Risk assessments are a requirement to determine the best solution for protecting people as they interact with equipment. When risk assessments are done right, they can balance the need for a safer system and a productive system.”
Most standards make recommendations on how to go about risk assessments. For example, RIA outlines them for using robots, PMMI has another for packaging applications, and ANSI TR3 is one standard for automotive and press-control-type applications.