Convert to Digital Safety

Though less messy than changing them, manufacturing disposable diapers is certainly more complicated, especially when the machines that make them need to combine control and safety functions.

Many of these 120-ft-long machines are made by Curt G. Joa of Sheboygan Falls, Wis., which builds its devices by assembling them from a dozen or more 10-ft machine sections (Figure 1). The firm’s users produce diapers, bandages and absorbent pads, and so Curt G. Joa reports its applications require a lot of additional customization.

“We’re doing control and safety on the same wire, even though we don’t see a lot of other people doing it,” says Kevin Zeinemann, PE, Joa’s electrical engineering manager. “This is because one of our diaper manufacturers that previously bought a hardwired machine wanted to add safety PLCs. Over the years, we had experience with proprietary lockdown safety systems, and they had a lot of initial expenses and many added costs to customize. These earlier machines had to be updated constantly, and it was a real hornet’s nest.”

Figure 1: Control + Safety = Modularity Curt G. Joa’s 120-ft disposable diaper machine is assembled from 10-ft machine sections that use PLCs with safety functions to perform control and safety communications via DeviceNet. Source: CURT G. JOA

When Joa sought a safety PLC, it ran across Rockwell Automation’s GuardLogix PLC with safety functions, which is supported by RSLogix 5000 software and runs via DeviceNet and EtherNet/IP network protocols.

“The user was prepared to pay for a safety PLC, and we were sold on GuardLogix because we’d already standardized on its core logic, so it was easier for us to implement,” says Zeinemann. “We did two machines for this customer, and, though there weren’t initial total savings on equipment, the cost of these improvements was much less than what it would have been otherwise because of savings on labor, common hardware and software and troubleshooting efforts. The individual control modules in our machine sections were easier to build, ship and plug and play, and so we had much easier modularity and less downtime. And, even though our new machines are more complex and have new capabilities, our installation and checkout time is the same as before. We’re just looking to build our machines faster, and this helps us do it, even though we still have a lot of customization.”

Flirting with Fieldbus? Seek Standards, Too

It’s these potentially huge savings in network hardware, labor and time that attract users, integrators and developers to digital networks, fieldbuses and wireless in the first place. However, despite all the gains that digital controls networks have made in recent years, most potential users still stop short when safety gets mixed into the equation. Because they don’t want to injure colleagues, damage equipment or halt costly operations, users and integrators often default to traditional, hardwired networks when safety is needed or required. 

To gain some reassurance, most users also rely on industry-defined standards for networks and other components used in discrete and process applications. These standards include rules and best practices for networks that vary widely in their specificity and requirements. For instance, mostly for discrete applications, the two most recent National Fire Protection Assn. updates to its NFPA-79 standard in 2003 and 2007 allow use of safety-certified PLCs for safety and control communications on the same physical network. As a result, many suppliers developed and launched safety PLCs and other devices able to combine safety and control and have sought safety certifications for them from TÜV Rheinland or other independent, safety-testing organizations.

“NFPA-79 is the common standard for control system development in North America, and all machine specification includes it and other ANSI standards and UL requirements as needed,” says Kelly Schachenman, Rockwell’s safety business marketing manager. “The NFPA-79 update in 2003 allows solid-state safety PLCs without an air gap in their output string as long as they’re properly safety-certified and part of an overall risk-assessment program.”

Likewise, mostly for the process side, the International Electrotechnical Commission’s 61508 and 61511 standards cover safety requirements for process applications. IEC 61511 is designated by the International Society of Automation as ISA S84.00.01-2004.

“IEC 61508 calls for separate control and safety systems, but it doesn’t prescribe how the user should do it,” says Luis Duran, safety and critical control marketing director for Invensys Process Systems. “When you try to mix safety and control on the same box, there are provisions you must make to be sure your application is safe, and this is why a risk assessment is necessary to determine what level of separation is needed and how to achieve it. Essentially, developers and users need to ensure that any issue affecting control on one layer won’t affect safety on another layer.”