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By Dan Hebert, senior technical editor
Fiber optic cabling is better for plant process control networks than copper, but it costs more. The extra expense can often be justified by designing a combined process control and IT network.
Benefits of fiber over copper are detailed by Rick Pennavaria, the sales manager for fiber optic products at Weed Instrument. “Copper can’t be used at distances over 100 meters for Ethernet, so fiber is virtually a must in long-distance applications. Other advantages of fiber over copper include optical isolation, resistance to lightning and other power spikes, better common mode isolation, higher bandwidth, a wider range of operating temperatures, less space requirements and better EMI/RFI immunity.”
Another big advantage of fiber is security. “It is extremely difficult to tap or eavesdrop on fiber-optic connections without detection, guaranteeing a high degree of security,” observes Paul Wacker, the product manager for industrial communication in the eAutomation Group at Advantech.
So fiber has many advantages over copper, but it costs more. Some of the added cost can be justified by better performance, but more compelling justification can come from using the fiber network to carry more than just process control traffic. “New fiber installations often use wavelength division multiplexing over single-mode fiber which allows for multiple network signals to be overlaid on the same fiber optic cable simultaneously at different wavelengths. These signals can include Ethernet, voice, video or more traditional analog/discrete signals,” according to Pennavaria.
Kris Dhupar, a consulting engineer at Invensys Process Systems, adds, “In-plant fiber optic networks also can carry plant asset management, operator action, configuration and system alarm data. Management data can be handled via TCP, IP and UDP protocols. They can also be used to transmit plant performance measures to corporate networks as well as enterprise-level information. With proper controls, plant fiber networks can also be used to transmit video.”
Potential uses for fiber optic networks seem to multiply along with available bandwidth. “Within the confines of the redundant process control system network, other types of data that the network may carry are CCTV communications, phone system data and plant information management system communications,” says Steve Lazok, the technical systems support manager at Yokogawa.
That said, installing a fiber optic network that can safely handle many types of traffic is not without challenges.
“The toughest requirements for the network arise from process control needs in terms of real-time execution, availability, security and safety,” according to Michael Connaughton, the manager of fiber-optic cable products at Belden.
“To ensure real-time execution, there are several solutions, such as the IEEE 1588 Precision Time Protocol.
Availability is typically ensured by redundant ring structures and meshed networks. In mission-critical applications all components are at least doubled. For network components such as switches, the reconfiguration time after a fault is an important parameter,” adds Connaughton. “Non-process control traffic is always kept outside of the process area. This means that as soon as other traffic is introduced anywhere on the same network, layer-3 routing, firewalls and security policies must be implemented. These responsibilities are often an issue between production and IT.”
Dhupar agrees with the necessity to prioritize process control traffic. “The network should be divided into various parts so that the highest importance is given to the process control system.”
At the plant-floor level, unique installation requirements for fiber must also be met. “While fiber optic cables look very similar to copper cables from the outside, additional care is required during installation,” instructs Wacker.
“Fiber is very strong when pulled straight but breaks easily when bent too sharply, so care should be taken to not exceed the cable bend radius. Similarly, excessive twisting during installation can stress the fibers. It is best to install cable before attaching connectors, and as compared to copper cable, fiber termination takes special training and particular tools. Cleanliness in very important, and while a simple continuity test can be done with a flashlight, an optical-loss meter set should be used to measure the optical loss of each individual fiber,” concludes Wacker.
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