Machine safety is moving from the outside to the inside. Traditional guarding and other protections are being replaced or at least supplemented by intelligent, automated safety PLCs, better-coordinated networks and other supporting devices. Integrated into machines at their earliest design and assembly stages, these safety components and software can establish safe zones, guarantee safe speeds and non-injurious motion, and allow operators to stay safe even as they interact more closely with their machines. Likewise, domestic and international machine safety standards are harmonizing to better help builders and users apply common, uniform safety components (Figure 1).
"The Occupational Safety and Health Administration's regulations have been around for more than 40 years, so most companies have some safeguards in place on their machinery, but most of these safeguards are expensive, physical barriers that most likely limit productivity," says Jeff Winter, safety business manager for North America at Grantek Systems Integration (www.grantek.com) in Oak Brook, Ill. "However, recent changes in international safety standards have plowed the road for the rest of the world to integrate safety technology into standard automation functionality. Compared to conventional guarding, we now can reduce hardware costs, simplify control architecture, reduce design and engineering time, increase diagnostics and ultimately make a safer work environment. So even if a machine is conventionally safe, overhauling its safety system could improve its safety and overall equipment effectiveness (OEE)."
For instance, Sandvik Materials Technology is a worldwide developer and producer of advanced stainless steels, alloys, titanium and high-performance materials, and its cold-rolling mill in Sandviken, Sweden, has been a key part of its precision-strip-steel production line since it was built in the 1930s. The mill is 20 meters long and processes high-carbon steel and stainless grades, and produces strips up to 400-mm wide and 1 to 6 mm thick (Figure 2).
Understandably, the mill's mechanical, electronic and control systems were renovated over the years, and Sandvik recently added servomotors, standard PLCs and touchscreen HMIs from ABB. However, the company's latest effort to migrate its hard-wired safety systems to zoned safety guarding required it to switch out even more equipment. "We needed to expand the plant and production line's safety, so we decided to install a new control system with a safety central processing unit (CPU) on the cold-rolling mill, but this also meant replacing its safety-related electronics and controls and adding some automatic functions too," says Torbjörn Pettersson, Sandvik's engineering development specialist.
Put Risks into Zones
To find the most appropriate safety and control solution for their cold-rolling mill, Sandvik's engineering and production staffs conducted a risk assessment (RA) in accordance with Swedish directive 2006:4, användning av arbetsutrustning (use of work equipment), and determined the mill needed six different safety zones based on its inlet parts, rolling and removal parts sections.
Read more: Help Machine Safety Break Free of the Past
"It's important to have a risk assessment to start with,"Pettersson explains. "After that, you must work through function descriptions and operating instructions. For us, it took a lot of time to find solutions for our new safety functions so they would be safe and productive. Once the instructions are finished, it's much easier to write the safety program. This is important because it can take lot of time to do a new RA and change safety functions and programs during commissioning if the initial solutions don't work out. It's also important to separate safety and the non-safety programs in the controls. This will make it easier to commission and test the safety functions, and these systems will be much easier to maintain because a clear, readable program will minimize the risk of any misunderstandings.”