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Until recently, machine and robot builder OEMs needed two automation systems. One of them controlled the machine or robot, while the second dealt specifically with machine safety. Typically, the machine safety system required a separate safety PLC and a dedicated hard-wired I/O network.Separate hard-wired safety systems were required for a number of reasons. First, many suppliers simply charged too much for their safety controllers and I/O, restricting use to safety functions. Second, safety-rated versions of many digital communication networks were still in the regulatory approval stages. Third, many OEM customers were not quite ready for change in the sensitive area of safety.
All that changed in the past few years, and integrated safety is fast becoming a viable solution in many OEM applications. Today, you can put control and safety functions into the same automation system, and run machine and safety I/O signals over the same wired or wireless safety-rated network. The price difference between standard and safety-rated controllers has narrowed, meaning that it's often cost-effective to use one automation system for both control and safety, especially in systems with a high percentage of safety I/O compared with standard I/O. Of equal importance, OEM customer acceptance grows more widespread.Integrated Safety I/O is Faster/Easier
Brent Lekx-Toniolo is the director of the Automation Division at Toniolo Research & Development, an automation and robotics systems integrator in Oxford Mills, Ontario. He has experience with old, separate, safety systems and with new, integrated alternatives, and he prefers the new.
Toniolo built a control and safety system for a spot weld assembly cell with 11 robots. The safety system included emergency stops, access control to safeguarded spaces, robot-to-human interference detection (a combination of robot zone switches and light curtains), and general detection of operators entering work stations via light curtains.
"This was a very large safety implementation that included fail-safe over EtherCat (FSoE), 380 TwinSafe inputs, and 144 TwinSafe outputs across the welding system on 15 EtherCat I/O stations," Lekx-Tonilo explains. "On top of the significant safety requirements of the cell, the systems also needed to control more than 600 standard I/O points, 12 pneumatic manifolds and two servo drives, while interfacing with 11 robot controllers.
Distributed Safety Next?
The next step for some applications could well be distributed safety, with safety functions separated from the main controller via distributed safety components, but still tightly integrated to the main controller via a high-speed safety-rated network.
A distributed safety component can perform safety functions independently of the main controller, continuing operation even if all communications with the main controller are lost.
Some machines are built in modules, with each module performing a specific function. The builder mixes and matches modules to create the machine, with interconnections among modules typically via a digital network.
Lekx-Toniolo found, to his surprise, that Beckhoff Automation's TwinSafe system could perform both the control and safety functions, and it was faster than the old, dedicated safety system. "The typical deactivation time of a standard safety relay is 20 ms and most safety relay systems require cascading of safety relays to build safety logic," he notes. "Many older safety networks have system response times that exceed 120 ms, and frequently exceed 200 ms. Currently, the PLC task, the entire EtherCat network and all safety in the welding cell is updated every 20 ms, which is much faster than a traditional PLC and relay-based system."