Integrating Safety into Control Contributes to Competitiveness

For most people, competitiveness means reliability and speed at low cost. Although these parameters make a good beginning at packaging machine builder Ropak Mfg. Co., they are not the end of its story about competing in the international packaging-machine industry. Because the builder’s machines work at extremely high speeds, safety also must figure prominently in its strategy. The engineering team at company headquarters in Decatur, Ala., therefore, takes every opportunity to incorporate the latest safety technology into its designs.

Consequently, Ropak became one in a growing number of manufacturers in the discrete-parts industries taking advantage of modern networking technology to integrate their safety and control systems. “The thinking in the United States has been that any safety circuitry must be hard-wired,” notes Andrew Fenton, one of the builder’s electrical engineers. “Back in the ’90s, though, some of the standards agencies decided that certain networks meet safety criteria and would be allowed.” Although using the networks to integrate safety and process control has not caught on among users as quickly here as it has in Europe, he predicts that Ropak will be using the technology on new machines that it builds for the U.S. market.

他的信念源于成功的他和他的贴画agues had on a Ropak IV packager that they had configured for an Irish company that supplies Europe with packets of white and brown sugar. Because the machine fills as many as 3,000 packets a minute, keeping people away from its moving members was a top priority. Of particular concern was the large filling wheel and the metal spouts hanging from it. They could pose a significant safety hazard if people were allowed near the mechanism as it spins to fill and move the wrapping paper to the cutting station. The machine must shut itself off and stop the wheel and other mechanisms quickly—and reliably—if someone were to open any of the guards designed to keep people away.

To get the necessary responsiveness and reliability at a competitive cost, Ropak’s engineers collaborated with Siemens Energy & Automation, of Elk Grove Village, Ill., and its local technology center, Nashville-based Edison Automation Inc. Rather than hard-wiring each safety device to the programmable logic controller (PLC) running the machine, they connected the emergency stop buttons and coded magnetic interlock switches to ASIsafe, an input/output (I/O) and safety network based on the Actuator Sensor-Interface (AS-Interface or AS-i) network. A monitoring unit supervises the safety devices and sends feedback information to the PLC, and ultimately to a human-machine interface (HMI), without any additional hardware.

This system responds within five milliseconds to any signal from the emergency stops or the paired-magnet switches on the guard panels. If the machine is in motion when the monitoring unit detects a problem, then the safety system tells the Siemens S7-200 Micro PLC to perform a hard stop. After a brief duration to achieve zero velocity, the AS-i safety monitor then disengages the motor from its power source to ensure that the machine halts until the safety triggers are reset.

除了反应,这种方法降低了the time and cost for building the machine. “Not only can you use it for safety, but you also can use it for general input and output,” says Fenton. “So instead of running a wire from each device to the PLC, we used the AS-Interface.” He estimates that wiring took nearly 40 percent less time because the network relies on a trunk line consisting of two cables running the length of the machine. Both safety and general I/O devices plug directly into the cable through two kinds of interfaces along it: M12 connector interfaces and viper-tooth metal plugs for devices like emergency-stop buttons.

Because the AS-i safety monitor operates independently of the PLC, Fenton also reports that the AS-i network saves the expense of installing a safety-rated PLC. “There’s nothing wrong with these PLCs, but the AS-i system saves the expense of putting one on a machine that really didn’t require it,” he says. It also saves on the redundant programming that would be necessary for a safety PLC. Fenton needed only a day to program the safety monitor, a process that would have taken three or four days on a safety PLC.

Standards Allow Integration

Success stories such as the one at Ropak are poised to multiply now that standards organizations are permitting users to put their safety devices on networks. A key standard in the United States is the NFPA 79: Electrical Standard for Industrial Machinery, promulgated by the National Fire Protection Agency, of Quincy, Mass., in 2001. The Occupational Safety and Health Administration, in Washington, D.C., recognizes this standard for monitoring industrial equipment and initiating emergency shutdowns.

In the intervening four years, industry groups have been striving to bring emergency stops, gate switches, safety light curtains, pressure mats, area scanners and other safety devices into their networks. “Customers are always asking us to come up with better and cheaper mousetraps,” says Helge Hornis, intelligent systems manager at Pepperl + Fuchs Inc., of Twinsburg, Ohio.

解释如何将安全设备集成到process control networks satisfies demand, he compares the networks to roads. Once you build the infrastructure, you can allow various modes of transportation to travel on it. Because of the new standard and the priority it gives to safety data, control networks that have been transporting traditional standard I/O data from input devices to PLCs will be able to carry emergency-stop and other safety data that had to travel on dedicated wires in the past.

The approval process for networks and the devices connecting to them are in various stages. So far, the only network that has full approval in the United States is the bit-level AS-i network, according to Hornis. The reason for its unique status is that the more than 300 manufacturers promoting this network were the ones who lobbied for the change in the standard, says Hornis. So they were prepared to apply for approval of their AS-i products through TÜV Rheinland in Germany as soon as NFPA released its update. Approval for other networks and their devices is pending.

One of them is the Common Industrial Protocol, the standard that drives EtherNet/IP, DeviceNet and ControlNet. “Within the last year, the Open Device Net Vendors Association (ODVA) added safety extensions to its specs so that DeviceNet can do safety and process control communication on the same wire,” says Jeff Gellendin, at Milwaukee-based Rockwell Automation. “It’s coming for EtherNet/IP as well.” The definitions for porting safety information to EtherNet/IP are at least a year away.

A Tale of Two Networks

Although these standards allow or will allow users to send both safety and process control data on the same network, most manufacturers in the discrete-parts industries are integrating safety and process control by creating separate networks for each function and connecting the central controllers for the networks. One way is the technique of connecting a monitoring unit to the PLC used by Ropak. Another is to have a PLC supervise each network and connect the PLCs. “Integration is much easier because you simply tie a communication cable between the safety and standard PLCs,” says Gellendin, at Rockwell.

“The next [step in the] evolution is an integrated network that can run safety and process control on the same network,” he continues. “Instead of pulling two network cables and integrating that way, you can only pull one network cable. It’s easier and less expensive.”

Nevertheless, the practice of tying more than one network together will probably persist for a while to guarantee rapid response times when the networks are large. Gellendin recalls one large Tier One automotive body shop in Canada that subdivided its large networks into four smaller ones for this reason. Some of Rockwell’s supervisory Guard PLCs communicate with lower level Guard PLCs and handle traffic between the networks. Because of the small networks, local response time is always fast enough to stop the robots in a welding station if someone were to break a light curtain.

The practice of using safety PLCs also allows users to reap the another benefit of integration—diagnostics. Especially when systems contain more than 16 safety devices, not only is the old method of wiring them to safety relays expensive, but it also consumes a large amount of cabinet space and is difficult to troubleshoot. “In the past, when a robotic system with 50 to 100 safety relays went down, users would be in there with a hammer tapping on every relay to figure out which one caused the shutdown,” says Tina Hull, an applications engineer at Pilz Automation Safety L.P., in Canton, Mich.

This method was simply too slow for troubleshooting an eight-robot cell containing 32 safety devices at an automotive plant that Hull helped to replace safety relays with safety PLCs. “The company had large transfer lines,” she recalls. “If one portion went down, they would start losing other sections of their line as well.”

By replacing the relays with a Pilz PSS safety PLC, she and her team gave the maintenance technicians there the ability to troubleshoot developing problems and failures with the diagnostics software inside the controller. The software identifies which safety device failed and gives the reason, allowing the technicians to contribute to the bottom line by solving problems much faster and keeping the production line running longer. Besides replacing all of those relays, the upgrade also eliminated about four miles of wires, which reduces the number connections and potential points of failure.

It also simplified installation. One way is by eliminating the need to create wire troughs and string the wires. “It took 40 percent less time for the electrical contractor to wire the system,” says Hull. “The engineering programming time went down about 30 percent to 40 percent because we have pre-designed safety blocks that have already been certified and approved.” Consequently, integrating safety devices into the control network contributed to the company’s bottom line and its competitiveness in more than one way.

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