Industrial networks have been replacing hard-wiring in manufacturing applications for decades now, but until recently, safety applications have either remained hard-wired or have been handled by special certified safety networks. All that changed a few years ago when safe versions of several common networks received safety certification. Now that Ethernet has penetrated the device level in many applications, safe versions of industrial Ethernet are now either available or in the works.
Communication technology using standard industrial networking protocols is not adequate to achieve the required levels of reliability to satisfy safety standards according to the International Electrotechnical Commission’s IEC 61508, the international standard for functional safety using programmable electronic safety systems, as well as related standards IEC 62061 and ISO 13849. Messages communicated over standard networks can get lost, become corrupted or delivered out of sequence. To ensure data integrity to meet safety requirements, safety networks add additional data checks at the application layer that monitor watchdog timers, check telegram numbering, verify sequencing and signatures, and perform additional data consistency checks.
SafetyBus p, developed by Pilz (www.pilzusa.com), may be the “original” safety bus, but it is no longer the only solution available. Profibus, DeviceNet, Ethernet PowerLink and AS-i now offer safe versions of their networks that use enhanced protocols to meet safety risk requirements up to Safety Integrity Level 3 (SIL 3) as defined in IEC 61508. Safe versions of industrial networks marked the beginning of a new era in which a single bus can now handle both standard and safety-related messages.
With the increasing acceptance of industrial Ethernet at the device level, several networking organizations have ported, or are in the process of porting, network safety to their industrial Ethernet solutions.
Profibus International (www.profibus.com) introduced Profisafe, a TÜV-certified safety application profile for Profibus, in 1999, and recently obtained additional certification for use with Profinet—the Ethernet implementation of Profibus. Profisafe is designed to work with all possible Profibus and Profinet architectures, including seamless bridging from one network to the other. Volkswagen AG recently completed a successful pilot project using Siemens (www.siemens.com) safety programmable logic controllers communicating via Profisafe on Profinet, as well as with other safety devices on Profibus.
The Open DeviceNet Vendors Association (ODVA, www.odva.org), the industrial consortium behind DeviceNet, ControlNet and EtherNet/IP, developed DeviceNet Safety in 2002. Early in 2005, ODVA published the specification for “CIP Safety” that includes functional safety extensions to the Common Industrial Protocol (CIP). The beauty of the CIP solution is that all three ODVA networks use the same CIP layer, implying that applications developed in general for CIP can be deployed with a relatively small effort on any of the three networks.
“EPLsafety” is an alterative Ethernet-based safety solution that adds functional safety to Ethernet Powerlink (EPL). The specification for EPL was developed by Austrian supplier B&R Automation (www.br-automation.com) in 2001, then turned over to an independent consortium. EPLsafety preserves the integrity of the TCP/IP stack while delivering fine-tuned, real-time performance for high-end motion control.
For users with existing legacy network systems, AS-i Safety at Work may be a viable alternative for adding low-level safety components such as emergency-stops and position switches via a gateway to an existing network. Besides being a low-cost solution, the AS-i Safety at Work solution is finished.
Users should carefully observe the development and certification of safe industrial networks, as these can lower hardware costs, increase machine flexibility and boost overall productivity.
David W. Humphrey, firstname.lastname@example.org, is a Senior Analyst at ARC Advisory Group Inc., in Dedham, Mass.