Wireless Networks and Machine Safety: Lessons Learned in a Non-Industrial Environment
Designing a system to control a myriad of large hydraulic gates at the Elephant Community Center at the Smithsonian National Zoological Park provides lessons learned in an application that involves hydraulics, solenoids, wireless, and safety PLCs.
Making sure he doesn’t scare elephants isn’t normally in Barry Stringer’s list of challenges for the day. But when his team was designing a control system for hydraulic gates in a zoo’s elephant house, avoiding unexpected door movements that could spook the elephants was a major safety issue.
Stringer is president of Solvere LLC, a Belmont, N.C. system integrator that won a contract to design the door controls for the Smithsonian National Zoological Park. There are fifty-six doors that guide elephants from one section to another and isolate them on occasions such as quarantining an elephant or segregating the bull from the herd. One set of doors in what’s called the Elephant Community Center holds the pachyderms in a small pen used for medical care.
“The controls are very critical. You can’t spook the elephant by something like a door that works improperly. If they’re startled, a zookeeper or the elephant itself could be put in danger,” Stringer says. “These animals are smart. If one is scared around a door, he’ll never go through it again.”
While the setting was unusual, many aspects of this unusual project were quite familiar. The system has common industrial functions: actuating hydraulic controls using a wireless link.
“The doors are huge, so they’re hydraulically driven. They need to be controlled wirelessly. Whether staff are outside the pen or walking around inside, they have to be able to control the gates,” Stringer says.
“There’s one hydraulic system for all the doors, we control solenoids for the individual doors. We use a Siemens safety PLC to manage all the inputs and outputs that control the doors. There are two parts, controlling the door movement, open or close, and controlling the speed,” Stringer adds.
There were, of course, some aspects that weren’t even close to common industrial issues. When the technology experts who worked on the project worked on-site, they walked amongst the elephants. Accompanied by experienced zookeepers, of course.
“There’s only one bull in the facility. His nature is to show everyone he’s boss and to protect the herd. When I came into the pen, I was new to him. I noticed that he picked up a drain cover, and the zookeeper’s casual attitude immediately changed. She said I needed to get out of there because the bull was getting ready to throw it at me,” Stringer says.
Another engineer was isolated from the herd by one of the gates, so he was left alone to do some programming. The animals are constantly moving and making noise, so the programmer didn’t notice that one had come to the gate, which was only a few feet from where he sat.
“All of a sudden, the elephant did a full-on trumpet. The roar is loud enough any time, but it was amplified because they were in a concrete building,” Stringer says.
Given the potential for problems when humans work with a herd of animals that can weigh over three tons, reliability was paramount. Stringer and his team picked a Siemens safety PLC, selected for its reliability. Siemens designs these units to safety integrity level (SIL) 3, ensuring that there’s no single point of failure. The controls for this highly-specialized installation doesn’t have any mandatory regulations, but the zookeepers are very focused on reliability.
“We went through some of our basic questions when we were initially working on the design. One of them is ‘in the event of failure, what are the negative results?’ The zookeeper looked at me very seriously and said one word. ‘Death.’”
That meant the focus on reliability extended to the wireless network connections. Bandwidth requirements are very low, so designers could focus on highly reliable coverage. Zookeepers moving amidst a herd of elephants don’t want to be thinking about dead zones.
Ensuring that wireless routers provided full coverage wasn’t easy. Concrete walls for structures that house multi-ton mammals are thick, as are the metal bars of the doors. The access points have to be mounted quite high – ceilings in the elephant house are 24 feet high. The ceilings are protected by a metal grate that prevents elephants from getting into the wiring.
“We worked with Siemens to model coverage. They ran modeling software that took everything into account, the size of the structure, the materials and other factors,” Stringer says.
Though many of the controls are wireless, there is one central controller that’s wired. Emergency buttons located throughout the elephant enclosure will shut down the system if necessary.
Now that the project’s finished, Stringer looks back with a sense of amusement and awe.
“It was really something to work there and be that close to those elephants. I learned a great deal a lot about them, and the zoo has been very happy with the way the system works,” Stringer says.
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