Physical Layer Diagnostic Improvements
Why is it, after weeks of seemingly trouble-free plant operation, the phone rings on the holiday weekend when the goose is in the oven and the table is set for dinner? Fortunately for me, the crew on shift was prepared to muddle through until normal business hours after the weekend. They just wanted to know whether the problem they were seeing was likely to cause a spurious trip of some production-critical pumps. They were getting an increasing number of communication errors on some instruments that shared a network—a fieldbus segment with suction pressure transmitters configured to trip the pumps on low pressure. Dinner was saved thanks to the fact that we had proven years ago that a “bad” indication from a suction pressure transmitter would “hold last value,” and the associated interlock wouldn't trip. While a persistent “bad” indication would defeat a necessary protection that prevented significant mechanical damage, the disruptions were fleeting enough to remain a mere annoyance.
When the issue was investigated on the next normal business day, it was immediately apparent that the segment was failing due to waterlogged and corroded terminals in an intermediate terminal box. The little enclosure was designed to be weather-tight, but we've all seen how simple it can be to defeat a system's accommodations for preventing moisture entry.
When a limit switch cover is removed to make an adjustment, a frail-looking O-ring flops out. It's easily misaligned or omitted when the cover is reassembled. How many NEMA 4/IP67 devices get installed with one or more plastic shipping plugs in unused conduit connections, or without thread sealant on the plugs? Conduit fittings and covers, such as those on common Form 7 fittings, have gaskets for a purpose. How many covers have you found loose, missing gaskets or simply open to the environment? In cold climates, steam hoses and tarps get applied to instruments either to remedy a freeze-up or as a prophylactic against anticipated harsh conditions; a lot of times the steam ends up condensing inside the instruments. Just condensation of moist air from the warmer seasons can be enough to begin corruption of circuit boards and terminations. In all but the most arid climates, anyone who has spent any time servicing instruments knows the experience of removing the cover of a troubled instrument and having a cup of water drain out.
The most experienced troubleshooters, such as Emerson Process Management Distinguished Technologist Marcos Peluso, agree that defects and oversights in the physical layer—twisted-pair copper, power supplies, terminals, conduit systems and enclosures—account for over 90% of the problems encountered in field device networks, even 4-20 mA. Peluso's comrades in Chanhassen have devised a solution, at least for 4-20 mA loops that connect to the top-end 3051 HART pressure transmitter: Power Advisory diagnostics for advanced loop integrity. The diagnostic is communicated to any HART-capable asset management software and alerts the end user to impending issues with moisture, cable integrity, terminal corrosion and power supplies. Since these physical layer disorders usually take some time before they affect the signal, a weekly report should allow the end user to detect and avert a problem. But what about the rest of the devices?
Profibus PA and Foundation fieldbus have had a repertoire of physical layer diagnostics for nearly a decade, ranging from simple handheld devices such as the Relcom FBT-6 to today's feature-laden Pepperl+Fuchs field diagnostics handheld and advanced diagnostics module (ADM). All these tools can alert the user to defects in the physical layer. The Pepperl+Fuchs units use an embedded expert system to suggest likely causes for the detected symptoms and recommend remedial actions. But while they can identify an ailing network, you might still have challenges determining the precise location of the defect.
Couplers providing simple short-circuit protection for each fieldbus spur substantially improved the fault tolerance of fieldbus segments. But Pepperl+Fuchs has designed a new generation of intelligent couplers that can detect and mitigate degradation or progressive failures owing to water ingress, sloppy terminations, high vibration, corrosion, cable abuse and device faults on each individual spur.
If such couplers can perform as promised, measurement and control professionals using them can count on more peaceful holidays in the future. Perhaps the goose of physical layer defects is finally cooked.