n the old days, industrial machine builders used a single hard-wired &ldquonetwork&rdquo for both control and safety applications. The advent of integrated digital bus technologies with their compelling advantages in diagnostics, lower wiring costs, and increased flexibility convinced many OEMs to use two networks: a digital bus for control and a hard-wired system for safety.
The two network approach never was ideal, but it was required since regulatory agencies did not allow the use of digital buses for safety systems. This no longer is the case. Both international (IEC/EN) and national (ANSI/CSA) industry standards allow digital buses for safety applications. These standards include (but are not limited to) IEC/EN 61508 (Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems), ANSI/RIA R15.06-1999 (Safety Requirements for Industrial Robots and Robot Systems), ANSI B11.19-2003 (Performance Criteria for Safeguarding Machine Tools), and CSA/Z432-04 (Safeguarding of Machinery), says Steve Freedman, director of safety systems for Sick (www.sick.com).
Digital safety buses feature the same advantages as digital buses used for control, and these advantages will spur widespread use. &ldquoSafety networks add increased reliability and safety by offering monitoring, preventive maintenance, and diagnostics not possible without digital network capability,&rdquo says Gil Guajardo, safety product marketing manager at Omron Electronics (www.omron.com/oei).
Besides better performance, another factor that will drive use of digital safety buses is regulation. &ldquoGovernmental enforcement of safety regulation will force companies to take a serious look at the lowest-cost method of compliance,&rdquo observes Steven Musick, staff product marketing specialist for Schneider Electric (www.us.schneider-electric.com). Digital networks by their nature offer lower design, startup, commissioning, operating and maintenance costs.&rdquo
AS-i bus technology, to name one, already offers this capability and DeviceNet soon will. &ldquoCertified by TÃœV in accordance with IEC 61508 up to Safety Integrity Level Three (SIL3), the safety extension to CIP [common industrial protocol—ed.] soon will be available for the CIP networks DeviceNet and EtherNet/IP,&rdquo says Katherine Voss, executive director of ODVA (www.odva.org).
Although the bus is common, the end devices are different when the application is safety related. &ldquoStandard and safety devices can coexist on the same wire because the safety protocol is located in the end nodes. Standard devices do not interfere with the functions of the safety devices and vice-versa,&rdquo adds Voss.
There are two main approaches that OEMs can take with respect to digital networks. One approach is to use two networks, one for control and one for safety. The second and preferred approach is to use one bus for both control and safety.
If your company has already standardized on one or more digital networks, then these networks should be investigated for compliance with safety standards. If compliance is not in place already, it may be best to switch to a digital network that allows both control and safety applications to function simultaneously.
Before a machine builder can standardize on a single digital network for control and safety, implementation issues must be considered. &ldquoCertain network solutions are too slow for many safety applications, while others networks such as AS-i are extremely fast,&rdquo states Helge Hornis, intelligent systems manager for Pepperl+Fuchs (www.am.pepperl-fuchs.com).
Network speed must be considered, as well as other factors. &ldquoAs with hard-wired safety systems, users must assure that minimum safety distances, response times, and control reliability conform to the requirements of their applications,&rdquo says Freedman. &ldquoSince the well-being of personnel is paramount, users must have a clear understanding of the application requirements of their safety system implementation.&rdquo
The use of a single digital bus for control and safety will create another opportunity and challenge for most OEMs with their controllers. Most machines use two control systems: one for real-time control and monitoring, and a second for safety. The safety control system often uses traditional hard-wired components such as relays and fail-safe analog instruments.
If hard-wiring now can be eliminated, can hard-wired components also be replaced by a digital controller? This would allow an industrial OEM to use one controller and one digital bus network for both control and safety.
Cost has been one hurdle to this approach. Safety-rated PLCs and other controllers typically have commanded a significant price premium as compared to standard controllers.
The use of a single control/safety digital network will increase OEM demand for a single control/safety controller. This should spur demand and decrease the price premium, resulting in more widespread adoption of the one controller/one network approach.
Dan Hebert, PE, Senior Technical Editor, email@example.com