Setting up an industrial network and getting it running is one of the most critical aspects of modern factories. But this complex task can be easier to manage when it’s broken down into manageable tasks.
In most applications, the job can be segmented using the five steps common to many industrial projects. Design is the first, and it’s generally the most time-consuming since it sets the stage for those that follow. Once it’s done, the job continues with configuration, installation, commissioning and maintenance.
During design, a central focus is to establish the level of determinism that’s needed and set up a system that will provide the necessary performance reliably. The critical functions of the overall plant will dictate which topology should be utilized. “The deterministic aspects inform you so you can select which topology your application needs,” says Carl Henning, deputy director of PI North America, formerly the Profibus Trade Organization.
Regardless of the topology, it’s important to place equipment in the best spot. If devices that send and receive many signals are near the end of the line, they can also overload switches. That’s because it takes time to send data packets through switches and along cables.
“You don’t want to put a vision system or human machine interface at the end of the line, they need to be close to the PLCs,” says Carl Henning “They have a lot of traffic. When it goes through a lot of switches, it ties up the available bandwidth.”
The need for redundancy is another factor that will determine the architecture. Many installations can benefit from using a ring architecture, which provides what Henning calls “bumpless redundancy.” Nothing needs to be added when a switch or cable malfunctions. That’s because signals are transmitted around the ring in both directions, so they’ll get there quickly either way. That eliminates bumps for redundant components, simplifying the topology diagrams and trimming equipment costs.
Just as importantly, there are no bumps in timing when cables break. “It takes zero time to recognize problems with cables and correct for the problem,” Henning says. “Devices get two messages, they use the latest one. It’s elegantly simple.”
The messages that are time-sensitive, such as motion control, get special priority so they arrive on time. Profinet provides bandwidth reservation, setting aside a portion of bandwidth that’s used only for isochronous communications. That prevents other network traffic from consuming all the available capability and delaying critical data packets.
Another important step in the design process is the selection of equipment. Profinet is so flexible that any Ethernet switch can be used, even one sold by retailers like Best Buy. But most users will want something that’s been ruggedized for industrial use, so PI North America helps users see what’s available by providing an on-line product guide.
Within these ruggedized switches, there’s one basic question: managed or unmanaged. Unmanaged switches are less expensive, but they don’t have the capabilities of managed hardware. For example, their diagnostic information consists only of LEDs that show whether or not a module is working.
Managed switches store some data and many of them have features such as Web browsers. Storing data such as the number of retries provides important information.
“That data is essential for diagnostic purposes,” Henning says. “To reduce downtime, you need to troubleshoot problems. Scanning the wealth of data that’s stored in managed switches is vastly better than scouring the racks looking for red LEDs on unmanaged switches.”
In many environments, wireless is becoming an important aspect of networking. It makes it much simpler to install hardware, eliminating the costs of routing wires. Most Profinet users opt for Wi-Fi, a member within the IEEE 802.11 that includes Ethernet. Many of the concepts used to build reliable wired networks carry over to the unwired world. But there are some differences.
“One caveat is that you need to see things like how walls and equipment like welders impact the signals. Another is that wireless is a shared network so timing can be an issue,” Henning says. These timing issues typically mean that time-critical messages are better suited for wired connections, he adds.
Both wired and wireless networks need to be protected to keep intruders from stealing information or creating problems. A tradeoff for gaining the many benefits of an open architecture like Ethernet is the need for greater security.
Many of these steps are simple, such as installing a firewall and isolating the industrial portion of the network from the corporate architecture. Limiting access to certain portions of the network using passwords is also important. Many companies have several layers of access that ensure that neither operators nor managers can go outside their area of expertise.
Once the system is designed, the remaining steps are fairly straightforward. Configuration is similar to Profibus and other networks. PSD files are used to tell the system the characteristics of each device. In many installations, Profinet isn’t the only network. Existing devices may use Profibus, ASi-Bus, DeviceNet and other field buses, so these architectures must be linked to the main network.Installation is one of the most important steps, since it involves real world issues that may have been overlooked in the earlier steps. PTO offers installation guidelines at www.profibus.com to help users through this critical step. Choices made during installation can impact performance over the lifetime of the facility.
“We urge everyone to use shielded cables. If they don’t, noise from drives, welder and other equipment can interfere with the transmission of data, Henning said.
As the five-step process nears its end, it’s time to commission the equipment. This phase is important, but commissioning is greatly streamlined by the overall selection of Profinet. “The real benefit of using Ethernet comes into play here, users gain access to a suite of built-in tools such as SNMP,” Henning said. He noted that diagnostics should be built into the HMI during commissioning. When the proper steps are taken, maintenance can be easier.
Much of the diagnostic data used during maintenance is readily available in managed switches. There’s a wealth of diagnostic data in the transport-oriented layers of the protocol stack. Much of this can be accessed even if a data line wire is broken. When these diagnostic programs are constantly monitored, technicians can often troubleshoot and spot problems before they occur.
Have an Inquiry for Siemens about this article? Click Here >>